Symposium ET06 : Advanced Materials and Chemistries for High-Energy and Safe Rechargeable Batteries

Over the last decade the energy density of lithium-based batteries has gradually increased, but commercially available cells have now topped out at around 200-250 Wh/kg at the cell level. I will describe several materials-centered approaches that will allow in excess of 300-350 Wh/kg and 700 Wh/l. First the carbon-based anode must be replaced by a higher capacity material, preferably lithium metal itself; we have found that tin-based anodes can achieve 50-100% greater capacity than carbons with coulombic efficiencies over 99.5%. Second on the cathode side, I will describe two options: two-electron systems using lithium, and high nickel NMCAs, both of which have theoretical energy densities of around 1 kWh/kg and have the capability of attaining 1 kWh/liter. The safety aspects will also be covered. This work is supported by US DOE, BES-EFRC and EERE-VTO-BMR.

Li-ion batteries are expected to play a vital role in stabilizing the electrical grid as solar and wind generation capacity becomes increasingly integrated into the electric infrastructure. In this work, different commercial Li-ion batteries based on LiNi_0.8Co_0.15Al_0.05O₂ (NCA), LiNi_xMn_yCo_zO₂ (NMC) and LiFePO₄ (LFP) cathode chemistries have been tested under the grid duty cycle protocols recently developed for frequency regulation (FR) and peak shaving (PS) with and without being subjected to electric vehicle (EV) drive cycles. The lifecycle comparison derived from capacity, round trip efficiency (RTE), resistance, charge/discharge energy and total utilized energy of the battery chemistries will be presented. Furthermore, degradation mechanisms of different battery chemistries will be discussed. The results can be used as a guideline for selection, deployment, operation and cost analyses of Li-ion batteries used for different applications.

Overcharge testing has long been used as a standard abuse test evaluation for lithium ion batteries, which have notably poor tolerance for overcharge and overvoltage conditions. While this has historically been strictly an abuse test, large potential gradients created by high rate charge and discharge operations in electric vehicle and stationary storage applications may lead to areas of localized overcharge or overpotential on the electrodes. This work looks at high capacity (10 AH) prismatic pouch cells, applying overcharge from 105 – 200% total State of Charge (SOC), up to and including energetic thermal runaway of cells. The mechanisms of overcharge failure are investigated using electrochemical techniques including EIS and differential capacity measurements to evaluate the degradation and failure mechanisms that occur during the overcharge condition. This is supported with materials evaluations to further evaluate the impact of overcharge on the constituent materials.
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

The skyrocketing price of cobalt pushes battery manufacturers to look back into low cost manganese containing positive electrodes. However, manganese dissolution has been considered as a critical problem for the majority of manganese containing positive electrodes. Although many efforts have been devoted to stabilizing the crystal structure and exploring new electrolyte additives, less progress has been reported so far. In this work, we have developed a novel multifunctional separator, targeting the root cause of manganese dissolution. A lithium-ion exchanged zeolite has been coated on polymer separator as the ceramic coating, which provides multi-functions to mitigate the issues arose from sequential scenario associated with manganese dissolution, including: 1. Trapping trace water: the high surface area in zeolite traps the trace water in the electrolyte, mitigating the hydrolysis of lithium salt and HF generation. 2. HF scavenger: in case of the HF already existing in the electrolyte, Al₂O₃ in the zeolite can preferentially react with HF as the scavenger due to the high surface area, therefore protecting the oxides in positive electrodes 3. Trapping Mn ions: in the worst scenario of Mn dissolved into electrolyte, the Li ion in zeolite will have the ion exchange with Mn ions in the electrolyte and trap Mn ions in the separator to avoid its damage to the SEI layer on anode side. In addition, lithium-ion exchanged zeolite separator can improve the wettability and thermal stability of the plain separator on which zeolite is coated. Based on this technology, we have demonstrated that Lithium-ion exchanged zeolite separator leads to the enhanced cycle performance (high capacity and Coulombic efficiency) of graphite/ (LiNi_0.5Mn_0.3Co_0.2O₂+LiMn₂O₄) full cells at both room temperature and elevated temperature, comparing with the plain separator and commercial alumina coated separator. The coin cell with Li-zeolite coated separator exhibits an average Coulombic efficiency of 99.89% and achieves a capacity retention rate of 78.3% after 500 cycles at 25 °C. We find out that a lower amount of manganese is present on the cycled graphite electrode when Li-zeolite coated separator is used, suggesting less side reactions resulted from Mn.

Solid electrolytes with low area specific resistance must be developed to enable high energy density lithium metal batteries. In addition to high ionic conductivities and large electrochemical stability windows, solid electrolytes also require robust mechanical properties to allow for large-scale production and successful integration into conventional lithium battery cell designs. To achieve these features, a novel solid electrolyte separator design was developed, in which a 50 - 100 nm fully dense solid electrolyte layer was coated onto microporous Celgard separators. The supporting Celgard made the solid electrolyte more robust and flexible, which enabled integration into coin cells. Due to its thinness, the resistance of the solid electrolyte layer was 5 - 10 Ω-cm². The solid electrolyte also showed low interfacial resistance with liquid electrolyte. The total resistance of solid electrolyte-Celgard membrane was determined to be 40 Ω-cm² in alkyl carbonate electrolytes, which is much lower than Garnet and Ohara solid electrolytes. The solid electrolyte membrane also was also shown to inhibit the crossover between anode and cathode in Li-S cells. The Li-S cell enabled by the new solid electrolyte membranes showed high coulombic efficiency and stable cycling performance. The advent of the new robust solid electrolyte paves the way for the commercialization of high energy density lithium metal batteries.

This work was supported by the ARPA-E IONICS program, U.S. Department of Energy, award DE-AR0000775.

With the increased penetration of electrified vehicles in the automotive market, requirements pertaining to battery durability are becoming increasingly stringent. To meet the requirements of the automotive industry, lithium-ion batteries must exhibit extensive life before reaching a terminal state of capacity loss and impedance growth. For battery designers and manufacturers, it is therefore of paramount importance to understand and predict long-term battery cell degradation based on a necessarily limited set of accelerated degradation tests.

Long-term calendar aging, defined as the temperature-induced cell degradation in the absence of current, is especially difficult to predict at relevant battery operating temperatures (25^oC – 45^oC). Unlike cycling tests, which can be rapidly conducted to the end-of-life by removing rest periods between cycles, calendar tests cannot be directly accelerated. To this end, a variety of empirical and physics-based models have been developed to predict the long-term storage behavior of battery packs based on a set of accelerated storage tests conducted at high temperatures. However, the validity of these calendar predictions has not, to the best of our knowledge, been extensively studied against actual long-term storage data surpassing the 4 year mark.

In this presentation, we present a set of long-term storage experiments performed over the course of 4-to-6 years on LiFePO₄/graphite cells, at various states of charge and temperatures. Analysis of the storage data sheds light on the long-term degradation mechanism in the cell, and demonstrates a transition between a reaction-controlled to a diffusion-controlled growth of the anodic solid electrolyte interphase (SEI). The dependence of state of charge and temperature on the degradation rate is clarified, and the predictive performance of empirical calendar life models is assessed. This work provides a mechanistic analysis of the nature of long-term degradation mechanisms in Li-ion batteries and paves the way towards an improvement of the predictive ability of empirical calendar life models. The conclusions of this study can also serve to understand long-term calendar degradation in higher-voltage NMC/graphite batteries, in which both the anode and the cathode may experience calendar degradation at high states-of-charge.

High energy density rechargeable batteries are critical for the widespread adoption of EVs, however their high energy density leads to an inherent safety risk if an internal short circuit (ISC) forms and releases all the energy in the battery in seconds. When an ISC occurs – from dendrite formation, cell deformation/damage, or a manufacturing defect – the entire battery capacity rapidly discharges. This release of energy leads to extremely high temperatures near the short that can induce thermal runaway, cell rupture or venting, and fire. While the use of shutdown separators can help mitigate ISCs in smaller cells, they are often ineffective in the larger, high-energy batteries used in EVs and grid storage applications. Rather than relying on thermally-induced shutdown that may fail to shut down regions of the battery far from the ISC, it is preferable to directly detect and stop the internal current that flows during an ISC. In this presentation, I will introduce our work on adaptive current-collectors, which allow for direct control over the local current that flows between the current-collector and the active material by simple printed electronic circuits. I will show how the electrical properties of printed electronics can be tuned to meet the demanding requirements of adaptive current-collectors, including low resistance during normal operating currents and high resistance under abuse conditions. Our simulations indicate that adaptive current-collectors can reduce the current flowing through ISCs by more than 90%, converting this catastrophic failure mode into a graceful one. In addition to enabling safe high-energy rechargeable batteries, the development of adaptive current-collectors will give battery users finer control over current flow at the sub-cell level, which is expected to improve battery reliability and rate capability.

Lithium metal constitutes an attractive anode material mainly due to its high theoretical specific capacity of 3860 mAh g−1, ten times higher than graphite (372 mAh g−1). The use of lithium metal in rechargeable batteries with typical liquid organic solvent based electrolytes suffers so far from severe safety problems associated with the formation of high surface area metallic lithium (HSAL) upon repeated charge/discharge.[1] Solid polymer electrolytes (SPEs) designed to be compatible with lithium metal are able to mechanically suppress HSAL formation and are considered as viable alternative. Solvent-free SPEs exhibit advantages in terms of mechanical stability, operational safety and simplicity of cell design.[2] However, application of polymer electrolytes to all-solid-state lithium ion batteries (ASS-LIBs) and all-solid-state lithium ion batteries (ASS-LMBs), requires improvements in respect to lithium ion conductivity, especially at ambient temperature.
Although high ionic conductivities can be achieved by high chain mobility linked to low molecular weight polymers, they are mostly too soft and therefore cause deterioration in mechanical stability of the SPE. In order to use low molecular weight polymers for fast lithium ion transport with sufficient mechanical strength at the same time, one strategy is related to utilization of a hyperbranched co-polymer where one segment represents a stable, hard backbone while the second segment is derived from a soft polymer with high ionic conductivities.[3] With this in line, a new generation of Li+-conducting SPEs obtained from supramolecular self-assembly of PEO, cyclodextrin (CD) and lithium salt was designed and thoroughly investigated for application in lithium metal batteries (LMBs) and LIBs. When mixing an aqueous solution of PEO together with an aqueous solution of CD, a precipitate forms where the CD is threaded onto a PEO chain.[4] The channel-type structure formed by self-assembly of PEO and CD can be used as the backbone structure whereas the hydroxyl groups of CD rings can be modified. Here, we use the ability of CD being the initiator for ring-opening polymerization of cyclic carbonates. This strategy enables synthesis of grafted polycarbonate side chains with low molecular weight. The obtained inclusion complexes show impressive ionic conductivity up to 1 mS cm-1 at 60 °C, together with high oxidative stability and allow for application in LFP/Li cells at 40 °C for more than 200 charge/discharge cycles. Post mortem XPS and SEM studies confirm that the polymer/LiTFSI penetrates the cathode upon cycling, facilitating improved contacts. This new system provides a platform for further modifications of the polymer side-chains.

Lithium metal combines the lowest reduction potential in the electrochemical reactivity series with a high theoretical specific capacity, and using metallic Li as anode would therefore significantly improve the energy density of the Li-battery. There exist, however, some challenges in the application of the Li metal electrode, such as safety risks and low coulombic efficiency [1]. In recent years, there has been a growing interest to find more stable electrolytes when in contact with the reactive Li electrode in Li-metal batteries. It has in this context been found that solid polymer electrolytes (SPEs), formed by doping a polymer with a lithium salt, are promising candidates, which can provide both high mechanical stability and better battery safety [2,3]. The major disadvantage of SPEs – their low inherent ion conductivity – can be resolved by a somewhat higher operational temperature.
We have, in a number of studies [4-6], modelled the Li metal/electrolyte interface using different simulation techniques. In this current study, we apply computational materials modelling to investigate the interface between the ion-conductive polymeric systems and Li metal surfaces by first principle calculations. To this end, Density Functional Theory (DFT) have been used to study several potential SPE host polymers such as poly(trimethylene carbonate) (PTMC), poly(vinyl alcohol) (PVA) and polycaprolactone (PCL), in order to get insights into their electronic structures and their stability when in contact with the Li metal surface. Using this knowledge, conclusions are drawn on which ion-conductive polymers are stable at the Li-metal surface, and which can adhere well to it.

References:

[1] X. B. Cheng, R. Zhang, C.-Z. Zhao, F. Wei, J.-G. Zhang, Q. Zhang, Adv. Sci. 3 (2015) 1500213.
[2] J. Kalhoff, G. G. Eshetu, D. Bresser, S. Passerini, ChemSusChem 8 (2015), 2154.
[3] J. Mindemark, M. J. Lacey, T. Bowden, Daniel Brandell, Prog. Polym. Sci (2018), doi.org/10.1016/j.progpolymsci.2017.12.004
[4] M. Ebadi, D. Brandell, C.M. Araujo, J. Chem. Phys. 145 (2016) 204701.
[5] M. Ebadi, L.T. Costa, C.M. Araujo, D. Brandell, Electrochim. Acta. 234 (2017) 43.
[6] M. Ebadi, M. J. Lacey, D. Brandell, C. M. Araujo, J. Phys. Chem. C, 121 (2017) 23324.

More than forty years after the first report of a rechargeable lithium battery, electrochemical cells that utilize metallic lithium anodes are again under active study for their potential to provide more energy dense storage in batteries. Electrolytes based on small-molecule ethers and their polymeric counterparts are known to form stable interfaces with alkali metal electrodes and for this reason are among the most promising choices for rechargeable lithium batteries. Uncontrolled anionic polymerization of the electrolyte at the low anode potentials and oxidative degradation at the working potentials of the most interesting cathode chemistries have led to a quite concession in the field that solid-state or flexible batteries based on polymer electrolytes can only be achieved in cells based on low- or moderate-voltage cathodes. In this work, we show that cationic chain transfer agents in an ether electrolyte provide a fundamental strategy for limiting polymer growth at the anode, enabling long term (at least 2000) cycles of high-efficiency operation of asymmetric lithium cells. Building on these ideas, we also report that cathode electrolyte interphases composed of anionic polymers and the superstructures they form spontaneously at high electrode potentials provide as fundamental a strategy for extending the high voltage stability of ether-based electrolytes to potentials well above conventionally accepted limits. Through computational chemistry, we discuss the mechanistic processes responsible for the extended high voltage stability and on this basis report Li||NCM cells based on a simple diglyme electrolyte that offer unprecedented stability in extended galvanostatic cycling studies.

All-solid-state Li-ion battery is a promising next-generation energy storage technology, providing intrinsic safety and higher energy density. Currently, high interfacial resistance and interfacial degradation at the solid electrolyte-electrode interfaces are the critical issues limiting the cycling and rate performance of all-solid-state battery. Fundamental understanding about the interfaces is yet lacking due to the difficulties of direct experimental characterizations. In this presentation, I will show how we use first principles computation to bring new understanding about these buried interfaces. Using our developed computation approach based on large materials database, we calculated the true electrochemical stability window of solid electrolytes and predicted interphase decomposition products, which are verified by in-situ experiments at solid electrolyte-electrode interfaces. I will discuss the critical role of decomposition interphase layers at electrolyte-electrode interface and their effects on the battery performance. From these insights, we are able to classify different interface types for different solid-electrolyte and cathode pairs and to estimate their impacts on battery performance. Moreover, specific interfacial engineering strategies are proposed to address potential issues at these interfaces in all-solid-state Li-ion batteries. I will present the predicted novel chemistry and strategies to stabilize lithium metal anode, which is greatly impeded by the lack of knowledge about lithium-stable materials chemistry. With first-principles calculations based on large materials database, we found that most oxides, sulfides, and halides, which were commonly studied as protection materials, are reduced by lithium metal due to the reduction of metal cations. New materials chemistry that are stable against Li metal are predicted, as promising candidates for lithium metal anode protection to achieve long-term stability. This series of computational studies provides novel insights and general design principles for interface engineering in all-solid-state Li-ion batteries.

The electrodeposition of lithium metal is a core process in next-generation, high energy density energy storage devices. However, the high reactivity of the lithium metal causes short cycling lifetimes, and there are safety issues due to the growth of dendrites. Recently, a number of approaches have been pursued to stabilize the lithium metal interface, including soft polymeric coatings that have shown the ability to enable high-rate and high-capacity lithium metal cycling, but a clear understanding of how to design and modify these coatings has not yet been established. In this work, we studied the effects of several polymers with systematically varied chemical and mechanical properties as coatings on the lithium metal anode. By examining the early stages of lithium metal deposition we determine that while global morphology depends on the coating quality and mechanics, the local morphology of the lithium particles is strongly influenced by the chemistry of the polymer coating. We have identified polymer dielectric constant and surface energy as two key descriptors of the lithium deposit size. Low surface energy polymers were found to promote larger deposits with smaller surface areas. This is consistent with a reduction of the coatings interaction with the lithium surface and thus an increase in the interfacial energy. On the other hand, high dielectric constant polymers were found to increase the exchange current and gave larger lithium deposits, suggesting improved Li⁺ ion solvation in the coating and decreased nucleation rate. We also note that the thickness of the polymer coating should be optimized for each individual polymer, and that polymer reactivity is an important parameter to be considered as it was found to strongly influence the Coulombic efficiency. Overall, this work offers new fundamental insights into lithium electrodeposition processes and provides direction for the design of new polymer coatings to better stabilize the lithium metal anode.

Rechargeable battery technology based on the lithium (Li) metal anode is plagued by the unstable solid-electrolyte interphase (SEI), which grows upon cycling and is associated with dendritic/mossy Li deposition. A key challenge in improving SEI stability lies in regulating its chemical composition and nanostructure. Here we report a new approach that enables the design of SEI layers with tunable structure and stable properties. This involves the use of a reactive polymeric composite, which can generate a stable SEI layer in situ by reacting with Li to occupy surface sites and then electrochemically decomposing to form nanoscale SEI components. Cryo-TEM shows that the resulting SEI layer is composed of organic polymeric Li salts, nanoparticles of inorganic Li salts, and two-dimensional nanosheet components. This conformal nanocomposite SEI layer exhibits excellent passivation, homogeneity, ionic conductivity, and mechanical strength and stabilizes the interface for dendrite-free Li deposition in a conventional carbonate electrolyte. 950-cycle life was achieved in a full cell paired with a LiNi_0.5Co_0.2Mn_0.3O₂ cathode. Moreover, under lean electrolyte conditions, the full cells also show significantly extended cycle lives, owing to the excellent stability of the polymeric nanocomposite SEI.

Despite the development of new technologies, such as lithium-sulfur or sodium-ion cells, lithium-ion devices remain the most used batteries: they are found in a majority of electronic devices and the demand for electric vehicles keeps growing. Their performance and characteristics vary according to the chosen electrode but their main advantages are their high energy density and their low self-discharge. However, lithium-ion batteries suffer safety risks which are mainly due to the use of liquid electrolytes. These electrolytes are based on a lithium salt dissolved in a mix of organic solvents and therefore are highly flammable. A safer alternative is to replace this liquid electrolyte by a solid one. Ceramic electrolytes are a first possibility but they are often complex to synthesize and too rigid. Thus, solid polymer electrolytes are good candidates regarding their flexibility but they have a rather low conductivity and poor mechanical properties.
This work focuses on the conception of a solid hybrid electrolyte. The organic part is composed of a mix of polyethylene oxide (PEO), known for its lithium conductivity, and polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP), known for its mechanical properties. The inorganic part is a silica network formed in situ via a sol gel reaction. This network is functionalized with an immobilized ionic liquid in order to increase the conductivity of the electrolyte. These components are dissolved in N,N-dimethylformamide altogether with a lithium salt, LiTFSI, and a first set of solid electrolyte is accomplished by solvent casting. Conductivity and lithium transference number are measured in temperature while the structure and the transport at different scales are analyzed by NMR, SAXS-WAXS, and Differential Scanning Calorimetry (DSC). Electrochemical performances at different rates are assessed in full cell using LiFePO₄ as the cathode and metallic lithium as the anode. Then, the importance of microstructure is investigated by achieving a second set of solid electrolyte. A skeleton of PVDF-co-HFP and functionalized silica is electrospun, producing a mat of fibers of 100nm in diameter. The space between fibers is then filled by concentrated LiTFSI or PEO/LiTFSI salts. The structure as well as their performances will be discussed using the same techniques.

Electrolytes have become key players in the design of modern high-energy solutions as they can be leveraged to enhance various important aspects of the device such as recharge time and efficiency, anode/cathode stability, and safety. However, conductivity and general transport properties of the cation/anion pair(s) dissolved in the electrolyte often pose a technological limit to the viability of the battery, and progress in fundamental insights into the origin of transport limitations are challenging yet extremely valuable. In our group, molecular modelling techniques are adopted in order to shine light on transport properties and correlation effects in electrolyte system.

Poly(ethylene) oxide (PEO) -based solid polymer electrolytes have a long history of research due to the easy processability and good transport properties, yet new observations that challenge our conventional understanding are still reported, especially at high salt concentrations relevant for technological applications. Our molecular dynamics simulation study of such regimes for one of the most prominent materials, PEO-Li-TFSI, reveals the central role of the anion in coordinating and hindering Li ion movements. In particular we observed significant competition between the anion and the polymer backbone to coordinate lithium atoms and surprising formation of asymmetric cation-anion clusters. In particular, the latter novel observation resonates well with recent experimental findings, where negatively-charged Li-anion clusters were speculated to exist to justify the surprising negative lithium transference number observed in this system for high salt concentrations.

Ionic liquids (ILs) -based electrolytes are attractive candidates as they generally exhibit better diffusion properties then polymer-based electrolytes, and several recent studies focus on assessing the performance of different mixtures. Given the highly-correlated nature of these systems, understanding the role of species correlation is non-trivial yet crucial for a rational design of future solutions. As a case study we investigated the promising NaFSI-C3C1PyrrFSI system at different Na concentrations and focus on highlighting the correlations in this system as well as how they effect the transference number for high NaFSI concentrations.

Understanding the surface reactivity between electrode and electrolyte is critical for cycle life of Li-ion batteries. The composition, properties and mechanisms behind electrolyte/electrolyte interface (EEI) on positive electrode are still unidentified for most lithium ion battery positive electrode materials [1,2]. Especially at high potentials, EEI layer on positive electrode becomes more critical since it approaches electrolyte instability limit for oxidation. The EEI layers on carbon-free, binder-free and thin film LiCoO₂ electrodes were investigated by using X-ray photoelectron spectroscopy (XPS). The growth of oxygenated and carbonated species was observed together with salt decomposition starting at 4.1 V_Li [3]. By DFT calculations, we correlated the EEI composition to the thermodynamic tendency of the EC solvent for dissociative adsorption on the Li_xCoO₂ surface, which can have a role in the salt decomposition on oxide surfaces [4]. The salt decomposition products had been also observed by solution ¹⁹F-NMR measurements. Finally, we demonstrated that the addition of diphenyl carbonate to the electrolyte has a strong impact on EEI layer and salt decomposition on LiCoO₂ surface. With this study, we also highlight the strength of using the carbon-free, binder-free electrodes to get fundamental insights in the reactivity of the positive electrode with the electrolyte.

[1] Xu, K. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104 (10), 4303–4418.
[2] Gauthier, M.; Carney, T. J.; Grimaud, A.; Giordano, L.; Pour, N.; Chang, H.-H.; Fenning, D. P.; Lux, S. F.; Paschos, O.; Bauer, C. Electrode–electrolyte Interface in Li-Ion Batteries: Current Understanding and New Insights. J. Phys. Chem. Lett. 2015, 6 (22), 4653–4672.
[3] Gauthier, M.; Karayaylali, P.; Giordano, L.; Feng, S.; Lux, S. F.; Maglia, F.; Lamp, P.; Shao-Horn, Y. Probing Surface Chemistry Changes Using LiCoO2-Only Electrodes in Li-Ion Batteries. J. Electrochem. Soc. 2018, 165 (7), A1377–A1387.
[4] Giordano, L.; Karayaylali, P.; Yu, Y.; Katayama, Y.; Maglia, F.; Lux, S.; Shao-Horn, Y. Chemical Reactivity Descriptor for the Oxide-Electrolyte Interface in Li-Ion Batteries. J. Phys. Chem. Lett. 2017, 8 (16), 3881–3887.

Recently, aqueous energy storage systems (AESSs), such as Na-ion battery & capacitor, have demonstrated their uniqueness compared with their non-aqueous counterparts due to the excellent safety performance in nature. Furthermore, the advantage of their low cost derived from the high abundance of sodium and the simplified assemble process in ambient endows AESSs the possibility of application in large-scale power grid. However, restricted by the narrow electrochemical window (~1.23 V) of water, the common used electrode materials in non-aqueous batteries/capacitors such as transition metal oxides/sulfides/selenides are directly excluded from the scope due to the high voltage platform, other than limited kinds of materials like NASICON-type NaTi₂(PO₄)₃ with appropriate voltage platform. Nevertheless, conventional NaTi₂(PO₄)₃ materials with irregular morphology and large size prepared by solid-state reaction still hinder the application of AESSs. Herein, a newly structured porous single-crystal NaTi₂(PO₄)₃ with uniform sizes was fabricated via a well-designed novel liquid transformation of ultrathin TiO₂ nanosheets, followed by coating a conductive carbon sheath. To best of our knowledge, this is the first report of the porous single-crystal structure of NaTi₂(PO₄)₃ materials. Examined in a three-electrode cell, this NaTi₂(PO₄)₃ electrode demonstrates an outstanding rate capability of 80-102 mA h g^-1 at varied current densities of 0.5-3 A g^-1 due to the synergistic effect between porous nanostructure and outstanding stability originated from single-crystal structure. The high-quality NaTi₂(PO₄)₃ was also assembled with N-doped carbon to fabricate an aqueous Na-ion battery with robust flexibility. This work paves the way for designing advanced NASICON based materials for aqueous energy storage systems.

Ni-rich cathode materials for lithium-ion batteries, commonly in the forms of LiNi_1-x-yCo_xMn_yO₂ (NCM) and LiNi_1-x-yCo_xAl_yO₂ (NCA), are of great interests in commercial applications due to their high reversible capacity and relative low cost compared to LiCoO₂. Due to the well-known Ni/Li cation mixing problem, such a multicomponent (high-entropy) compound cannot be synthesized at elevated temperatures, the more so the higher Ni concentration. This temperature doctrine suppresses the inter-diffusion kinetics between transitional metal ions (Ni, Co, Mn, Al etc.) during material synthesis, hindering a homogeneous distribution and electrochemical properties, and limits the achievable particle size, impairing electrode packing and volumetric energy density. This is part of the reason why solid state synthesis of NCM/NCA materials was not successful. The conundrum is tentatively solved by introducing a specialized co-precipitation technique and self-agglomerated secondary particles (~10 micron) for NCM/NCA systems. Yet the secondary particles would still crack along grain boundaries during electrode pressing and electrochemical cycling. Therefore, micron-size single-crystalline NCM/NCA with good electrochemical properties is a great treasure. In our recent work, we revisited the solid state synthesis of such single-crystalline Ni-rich cathodes and obtained competing capacities with the state-of-the-art co-precipitation technique, thus re-opening the doors to process Ni-rich cathodes in the same synthesizing route as classical LiCoO₂ and genius control of doping and coating at the primary-particle level.

Reactive superoxide triggers side reactions during discharge of lithium-oxygen batteries (LOBs) and therefore affects seriously harmful effects on LOB performances. In living organisms exposed to oxygen, superoxide dismutase (SOD) manages superoxide in a way of converting the reactive species to less reactive oxygen and peroxide. Herein we adopted a functionalized fullerene molecule (MA-C₆₀ where MA = maleic acid) that mimicked dismutation or disproportionation function of SOD. Superoxide-triggered side reactions during discharge were significantly reduced by MA-C₆₀ so that desired oxygen evolution reaction was dominantly encouraged during charge with less CO₂ evolution. Toroidal lithium peroxide particles were generated, which indicated that solution mechanism for peroxide formation was favored. Resultantly, MA-C₆₀-containing LOB cells exhibited tremendously improved capacities especially at high rates.

K-ion battery (KIB) is a new-type energy storage device that possesses potential advantages of low-cost and abundant resource of potassium. To develop advanced electrode materials and electrolytes for accommodating the relatively large size and high activity of potassium is of great interests. In order to address the fast capacity decay caused by severe volume expansion of Sb anode, a novel segment-like antimony nanorod encapsulated in hollow carbon nanotube electrode material (so called Sb@N-C) was prepared by hydrothermal synthesis, polymerization coating and followed by an in-situ pyrolysis and reduction process. The potassium storage performance and mechanism of Sb@N-C in rechargeable KIB were also investigated.
Beneficial from the virtue of abundant nitrogen doping in hollow carbon nanotube, one-dimensional nanotube structure and hollow structure advantages, Sb@N-C exhibited excellent potassium storage properties: based on potassium hexafluorophosphate (KPF₆) electrolyte, the reversible capacity could be maintained 318.6 mA h g^-1 at a current density of 0.5 A g^-1, whereas the cycle stability and rate performance were unsatisfactory. Electrolyte optimization strategy was further used to boost its potassium storage performance, and the optimization mechanism was also disclosed. In the potassium bis(fluorosulfonyl)imide (KFSI) electrolyte, Sb@N-C displayed a reversible capacity of up to 453.4 mA h g^-1 at a current density of 0.5 A g^-1. The rate performance (a reversible capacity of 316.9 mAh g^-1 could be achieved at a current density of 2 A g^-1). Additionally, Sb@N-C demonstrated excellent long-cycle stability at ultra-high current of 5 A g^-1 over 600 cycles, its reversible capacity could be retained at 234.7 mA h g^-1.The results confirm that Sb@N-C nanocomposite is an advanced and superior promising electrode material for KIB. Besides, electrolyte chemistry optimization is an effective strategy for greatly improving electrochemical performance.

Reference:
1. Advanced Energy Materials, 2018, DOI: 10.1002/aenm.201703237.
2. Advanced Energy Materials, 2018, DOI: 10.1002/aenm.201703288.
3. Nano Letters, 2016, 17(1): 544-550.

Multivalent battery chemistries are seen as a promising alternative to energy storage systems based on lithium-ions. Among those, aluminum-based systems have the potential to offer higher energy densities at a lower cost. Aluminum is the most abundant metal in the Earth's crust and the extraction/processing of its ores is one of the most cost-effective for transition metals.¹ Added to this, its ions are trivalent and bear a small ionic radii, which allows for extremely high theoretical gravimetric and volumetric capacities, 2980 mAh g^-1 and 8040 mAh cm^-3, respectively. In fact, the latter figure represents the highest value attained by metal-ion batteries.²

Research in the field of aluminum batteries has focused heavily on electrodes made of carbonaceous materials. Accordingly, it is believed that the high structural quality and low defect density of graphitic carbons are crucial to obtain superior performance and cycling stability in these batteries.^{3, 4} Still, and despite all effort, the capacities reported for these systems remain stubbornly low, particularly when compared to the >300 mAh g^-1 attained by commercial lithium-ion batteries.

We wish to communicate an Al-chloride battery where reduced graphene oxide powder, dried under supercritical conditions, is used as the active cathode material. This system boasts a gravimetric capacity of 171 mAh g^-1 at 100 mA g^-1 and remarkable stability over a wide range of current densities. These properties are thought to be the consequence of the cathode's tailored porosity.⁵ On one hand, its microporosity assists in breaking down the Coulombic ordering of the electrolyte; on the other, the mesoporosity (originated from the drying conditions) facilitates the movement of the large chloroaluminate ions within the active material.

REFERENCES
1. CRC Handbook of Chemistry and Physics 90th Edition, CRC Press, 2010.
2. G. A. Elia, K. Marquardt, K. Hoeppner, S. Fantini, R. Lin, E. Knipping, W. Peters, J.-F. Drillet, S. Passerini and R. Hahn, Advanced Materials, 2016, 28, 7564-7579.
3. H. Chen, F. Guo, Y. Liu, T. Huang, B. Zheng, N. Ananth, Z. Xu, W. Gao and C. Gao, Advanced Materials, 2017, 29, 1605958s.
4. D.-Y. Wang, C.-Y. Wei, M.-C. Lin, C.-J. Pan, H.-L. Chou, H.-A. Chen, M. Gong, Y. Wu, C. Yuan, M. Angell, Y.-J. Hsieh, Y.-H. Chen, C.-Y. Wen, C.-W. Chen, B.-J. Hwang, C.-C. Chen and H. Dai, Nature Communications, 2017, 8, 14283.
5. A. Alazmi, O. El Tall, S. Rasul, M. N. Hedhili, S. P. Patole and P. M. F. J. Costa, Nanoscale, 2016, 8, 17782-17787.

Lithium-sulfur batteries have been extensively considered as a promising alternative for lithium–ion batteries (LIBs) owing to their high theoretical energy density (2500 Wh kg^-1) based on the reaction of lithium with sulfur to form Li₂S. In addition, S is inexpensive, abundant and nontoxic. However, the practical application of lithium–sulfur batteries is hindered by the low conductivity and huge volume expansion of S during the charge-discharge process. Recently, selenium, a congener of sulfur, has also been used as a cathode material with a theoretical capacity of about 675 mA hg^-1. Compared with sulfur, selenium has a better conductivity and it is found that the polyselenides are insoluble in carbonate based electrolytes, indicating that the shuttle effect could be suppressed effectively. Combing the merits of sulfur and selenium, selenium–sulfur solid solution (like SeS₂) incorporated with carbon materials display an excellent performance when using as the cathode materials lithium–sulfur/selenium batteries. Herein, we designed a MOF derived N–doped microporous carbon nanofibers, which was encapsulated with SeS₂. The resultant cathode materials possesses a free standing structure, which could avoid the utilization of non–active additives, thus ensuring a higher energy density. As a result, the Li–S batteries with C–SeS₂ composites deliver a high specific capacity (950 mA hg^-1 @ 0.5C) and outstanding rate performance. The nanofibers could buffer the volume expansion of the S/Se species as well as increase the contact area between the cathode and electrolyte to decrease the ion transmission distance. Furthermore, the in–situ Raman technology was applied to observe the changes during charge/discharge process.

Silicon (Si) is a high energy, low cost anode material for lithium-ion batteries. With a theoretical specific capacity that is almost tenfold higher than graphite, Si has been the focus of extensive research recently for its potential use in electric vehicles and other devices. Unfortunately, Si anode technology has been held back by its lack of cyclability, largely attributed to the structural disintegration of Si electrodes associated with large volume changes during electrochemical cycling. This work aims to address this problem by stabilizing the electrode structure with a robust polymeric coating, allowing for reversible electrochemical reactions. Moreover, spatial molecular layer deposition (MLD) has been developed and applied here to realize the distinct all-organic polymer film chemistry deposited. The aromatic polyamide film chosen for this work has the desired mechanical properties of strength and elasticity that help stabilize Si anodes upon cycling. A very thin film polyamide coating on a Si anode enabled a stable and reversible capacity of 2186 mAh g^-1 over 100 electrochemical cycles. Using a spatial MLD configuration further enables fast growth rates on the order of minutes, thereby permitting ease of future scale-up. This spatially deposited, all-organic thin film coating is a promising material that may enable high energy, low cost lithium-ion batteries.

<span style="font-family:times new roman,serif; font-size:12pt; line-height:115%; margin:0px"><font color="#000000">The upsurge in the market for electric vehicles (EVs) as well as high-power portable electronics demands batteries with higher energy and higher power densities with longer cycle life. Among the available secondary batteries, lithium-sulfur (Li-S) batteries have become quite attractive for next-generation rechargeable batteries; they are known for the high-energy density (~2600 Wh kg^-1) which is five times higher than that of the commercial lithium-ion batteries. Lithium (Li) metal has been considered an ideal anode material for the next-generation high-capacity batteries. However, the practical use of Li metal for Li-S batteries is inhibited due to the parasitic growth of Li dendrites and high reactivity of Li with electrolyte and other active species of polysulfides. Here, we introduce an atomic layer two-dimensional MoS₂ as a passivation layer for Li metal anode. With the Li-intercalated atomic layer of MoS₂ formed, stable Li electrodeposition is realized with the nucleation sites for dendrite growth inhibited. The deposition/dissolution process of a symmetric cell for the MoS₂ coated Li metal operates at a current density of 10 mA cm^-2 with low voltage hysteresis; it shows three-fold improvement in cycle-life than that of the bare Li metal. Using Li-S full cell configuration, the MoS₂ coated Li anodes assembled with 3D carbon nanotube-sulfur cathodes provide superior electrochemical performance demonstrating specific energy density of over 652 Wh kg^-1 and capacity retention (~84%) for up to 1200 cycles with a nominal Coulombic efficiency of ~98%. These exceptional results open a new pathway towards the realization of high energy density and safe Li-metal based batteries.</font></span>

Demand with higher energy density LIB is increasing in many areas such as automobile and group IV elements, such as Si, Ge and Sn, have been received much interest for this purpose. However, the use of these elements is limited due to the large volumetric change during long term charge-discharge cycle which results in undesirable rapid capacity fading, low initial coulombic efficiency and poor rate performance.
Among various approaches to avoid this limitation, chemically modified graphene and SnO₂ has been hybridized to accommodate volume change and improve the capacity and cycling stability of the electrode material. Hybridization of SnO₂ with carbonaceous materials has been used to circumvent this limitation [1-3]. Reddy et al studied hybridization of SnO₂ with long chain alkylamine rafted graphene oxide and specific capacities decreased more with increasing alkylamine chain length [4].
Compared with traditional polymer binders, the self-healing chemistry is designed to enable spontaneous repair of the mechanical damage in the electrode and enhance the lifetime of the anode materials.
In the present study it is attempted to synthesize SnO₂ nanopillared carbon structures using dodecylamine grafted graphene oxide as templates. Self-healing polyurethane is synthesized with disulfide and it is compared with commercial PVDF as a binder.
Structural and morphological characterizations of self-healing polyurethane and electrode were done using FT-IR, XRD, SEM and TEM. Electrochemical studies, such as charge-discharge, cyclic voltametry abd impedence, were carried by fabricated 2032 type coin cells using RGO-SnO₂ as electrode.
Self-healing characteristics of prepared polyurethane is confirmed from restored tensile property and SEM photographs of cutted samples. Electrochemical measurements revealed that the SnO₂ pillared carbon based anode materials with self-healing polyurethane binder showed improved cycling performances with excellent reversible capacity relative to the electrode prepared by poly(vinylidene difluoride).
References
1. C.C. Chang, S.J. Liu, J.J. Wu, C.H. Yang, J. Phys. Chem. C111 (2007) 16423-16427.
2. Y. Fu, R. Ma, Y. Shu, Z. Cao, X. Ma, Mater. Lett. 63 (2009) 1946-1948.
3. X. Zhu, Y. Zhu, S. Murali, M.D. Stoller, R.S. Ruoff, J. Power Sources 196 (2011) 6473-6477.
4. M. Jeevan Kumar Reddy, Sung Hun Ryu, A. M. Shanmugharaj, Nanoscale 8 (2016) 471-482.

Since the introduction of lithium-ion batteries (LIBs) to market in 1991, their performance has improved significantly, which has been achievable through development in materials technologies. However, further breakthroughs are still needed to ameliorate cycle-life, safety and energy density of LIBs. This requires new electrode materials and a detailed understanding of the electrochemical mechanisms during cycling. Transition metal phosphates are interesting candidates as cathode materials for LIBs [1]. In this work, we report the electrochemical performance of FPHH/C and FPHH/CNT composites where FPHH represents Fe_1.19(PO₄)(OH)_0.57(H₂O)_0.43 while carbon black and carbon nanotubes (CNT) were used as precursors in the one-pot hydrothermal synthesis, respectively. We show that the addition of conducting carbon black into the solution has a strong influence on reducing the particle size and tailoring their morphology, but does not interfere with the formation of the FPHH phase. Thanks to its favorable microstructural characteristics, the FPHH-10 wt% C and FPHH-20 wt% C materials exhibited good performance [2]. The CNT also improve the performance of FPHH such as capacity retention (500 cycles at 1 C).
The mechanisms of lithiation-delithiation were investigated by combining operando X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy. FPHH undergoes a monophasic reaction based on Fe³⁺/Fe²⁺ redox process. However, the variations of the lattice parameters and ⁵⁷Fe quadrupole splitting indicate a more complex mechanism than a random occupation of the vacant sites within FPHH. This can be related to the peculiar structure of FPHH formed by chains of face sharing (Fe_{0.6 0.4})O₆ octahedra connected by PO₄ tetrahedra and by channels for Li diffusion along [100] and [010] directions. The existence of Fe vacancies provide interconnections between the one-dimensional channels, improving lithium diffusion within FPHH. This mechanism, combined with the addition carbon black or nanotubes in the solution prior to hydrothermal treatment as a simple and effective way to reduce particle size and improve electronic conductivity, provides good cycle life and rate capability for FPHH.

Acknowledgements
A. Mahmoud is grateful to University of Liege and FRS-FNRS for the grants and thanks to the Walloon region for a Beware Fellowship Academia 2015-1, RESIBAT n° 1510399. Part of this work was supported by the Walloon Region under the “PE PlanMarshall2.vert” program (BATWAL – 1318146).

References
1. C. Karegeya, A. Mahmoud, F. Hatert, B. Vertruyen, R. Cloots, P.E. Lippens, F. Boschini, Journal of Power Sources 388 (2018) 57-64.
2. C. Karegeya, A. Mahmoud, R. Cloots, B. Vertruyen, F. Boschini, Electrochim. Acta 250 (2017) 49-58.

Li-ion batteries (LIBs) took over the lead in rechargeable battery technologies since their introduction in the early 1990s. However, the price and abundance of lithium element are still the issues and alternative cell chemistries based on abundant elements might become important especially when stationary energy storage reaches its market breakthrough. Therefore, alternatives to “lithium-ion technology” are being examined. Sodium-ion batteries (SIBs) are recently being revisited as attractive alternative. The hope to realize more cost-effective batteries rises due to the large abundance of sodium. Although SIBs often have lower energy densities and cell voltages compared to their lithium analogues, the lower polarization of the sodium-ion might enable cells with peculiar advantages over conventional lithium-ion technology^[1].

Due to its low cost, safety and good cycling performance, graphite is currently the preferred choice as an anode material for LIBs. Graphite is also favored in SIBs but storage of sodium-ions is only possible by formation of ternary intercalation compounds ^[2]. Instead of the naked ion, the solvated ion is intercalated in between graphene layers. The co-intercalation of the ether based solvent molecule causes an enormous volume change in graphite lattice and so in the electrode. In order to observe this change, in situ electrochemical dilatometry (ECD) can be used to measure changes in the electrode thickness during charging and discharging. Phase transitions during de/insertion of ions, irreversible reactions such as solid electrolyte interphase (SEI) formation or structural changes such as delamination of graphite can be followed on-line. Overall, the studied electrode reaction behaves very different from conventional intercalation reactions of graphite. Finally, and maybe most intriguingly, the reaction is possibly the first case of an SEI-free graphite anode material ^[3]. Moreover, formation of gasses is examined during cycling by online electrochemical mass spectrometry (OEMS).

Moreover, was also present a systematic study on temperature effects related to the intercalation of solvated sodium ions into graphite. For this, a series of glymes (mono- to pentaglyme) and several crown ethers are used.

[1] P. K. Nayak, L. Yang, W. Brehm, P. Adelhelm, Angewandte Chemie International Edition 2017.
[2] B. Jache, P. Adelhelm, Angewandte Chemie International Edition 2014, 53, 10169-10173.
[3] M. Goktas, C. Bolli, E. J. Berg, P. Novák, K. Pollok, F. Langenhorst, M. v. Roeder, O. Lenchuk, D. Mollenhauer, P. Adelhelm, Advanced Energy Materials 2018, 1702724.

High theoretical specific energy density (2600Wh/kg) and high specific capacity (1675mA/g) along with natural abundance and low toxicity of sulfur have been attracting significant attention for development of an alternative battery system to replace traditional lithium ion batteries which suffer from safety and capacity/energy density limitations. However, challenges such as polysulfide dissolution and shuttling prevent mass commercialization of sulfur cathode batteries. Here we show a practical yet comprehensive approach for development of high performance metal sulfur batteries. Aramid nanofiber (ANF) based composite asymmetric separator [1] not only prevent dendrite formation[2] but also confine polysulfides on the cathode side. ANF composite battery separators provide diverse and opposing properties including high mechanical properties, high ionic conductivity and high thermal/chemical stability. These separators therefore provide a safe and long cycle life as well as high performance metal sulfur batteries. Fabrication of such safe, affordable, flexible and high energy density battery is quite crucial in powering next generation electronics including but not limited to portable, wearable and implantable biomedical devices.
References:
[1] M. Yang et al., “Aramid nanofiber-reinforced transparent nanocomposites,” J. Compos. Mater., vol. 49, no. 15, pp. 1873–1879, 2015.
[2] S.-O. Tung, S. Ho, M. Yang, R. Zhang, and N. A. Kotov, “A dendrite-suppressing composite ion conductor from aramid nanofibres.,” Nat. Commun., vol. 6, no. 2015, p. 6152, 2015.

Lithium-rich layer oxide, Li_1.2Ni_0.16Mn_0.56Co_0.08O₂ (NMC), is a potential cathode candidate for high-energy density batteries. Issues such as cycling stability, rate performance, and cost are yet to be overcome before successful commercialization of the material. Here, we report on the synthesis of Cr-doped lithium-rich phases Li_1.2Ni_0.16Mn_0.56Co_0.08-xCr_xO₂ (where x=0.0, 0.01 & 0.02) (NMC-Cr) by the sol-gel technique. Cr is homogeneously distributed in the crystal structure evidence by XRD, XPS and elemental mapping measurements. The Cr-doped materials exhibit much better cycling stability with 100% capacity retention versus 44% for the undoped sample after 50 cycles. The Cr-doped samples show excellent electrochemical performance at higher C-rate in comparison with the undoped NMC. The latter shows rapid capacity fading from 220 to 50mAhg^-1 at the 0.1 to 1C rates, respectively. Moreover, the Cr-containing materials do not show significant signs of voltage fading during cycling owing to the stabilization of the crystal lattice by chromium. The electrochemical impedance spectroscopy measurements also indicate the stable cell resistance on cycling for the Cr-doped phases compared to the undoped phase.

As Li-ion batteries are long advanced for its electrochemical properties in application to both electric vehicles and devices, many ongoing researches are focusing on the improvement of energy density, capacity, cycling stability and rate performance. These can divide into three main parts in Li-ion batteries; cathode, anode, and electrolyte. One of the solution for this enhancement is to find novel cathode materials. This work focuses on the synthesis and characterization of Li,Mn-rich cathode, LiCo_0.1625Ni_0.1625Mn_0.675O₂, for Li-ion batteries. The layered Mn-rich transition metal (Mn, Co, Ni) hydroxide precursor was synthesized via facile co-precipitation method in a continuous stirred tank reactor (CSTR). Under proper control of pH, temperature, time, concentration of the reactants, and feeding rates in the reactor, uniform spherical particles were obtained. The hydroxide precursor was then undergone two-step heat treatment process to achieve Li,Mn-rich cathode powder. Morphology and structure were examined using SEM and XRD, respectively, showing plate-like primary particles which intercalated into spherical secondary particle shape (average size ~ 17 μm). Chemical stoichiometry was confirmed by ICP-OES technique and electrochemical performance was studied to examine the reliability of this cathode for large scale production and its future commercial use in Li-ion batteries. Drawback of Li,Mn-rich, such as capacity fading and voltage decay, were discussed with an effort to minimize these issues.

The VO₂(M/R) system undergoes a structural change from monoclinic [VO₂(M)] to rutile [VO₂(R)] phase at a temperature that is easily accessible and corresponds to an electrical conductivity increase two orders of magnitude. The ability to exploit electrical conductivity makes the system attractive for study as a lithium ion battery electrode considering uniform electron access can be a limiting factor in producing electrodes that deliver high capacities. In the work presented, several forms of characterization were employed to gain insight on the relationship of structure and electrochemical function. Synchrotron based x-ray powder diffraction (XPD) data was used to monitor the structural changes as a function of temperature. Electrochemical impedance spectroscopy was utilized to track impedance as a function of temperature. Further, the VO₂ system was then tested in two electrode cells to determine the impact of the structural transition on functional electrochemistry. The results from the compliment of experiments provides a foundation for investigating charge transport properties in polymorphic materials and sets a precedent for understanding the impact of phase changes on electrochemistry in a complex energy storage system.

A critical challenge for electrical energy storage is to achieve more useful work (w) and minimize the generation of waste heat (q). Batteries have often been approached at the macro level, where bulk parameters are identified and manipulated, with optimization as an ultimate goal. However, such a strategy may not provide insight toward the complexities of electric energy storage, especially when addressing multiple length scales in application and demands on devices. Beginning from a fundamental approach of identifying and reducing sources of localized resistance facilitates the understanding of the inherent heterogeneity of ion and electron flux both at multiple interfaces and length scales.At a fundamental level, it is necessary to identify and reduce sources of localized resistance and to understand the inherent heterogeneity of ion and electron flux at numerous interfaces found at several scale lengths within a battery. Benefits from experimentation and characterization over multiple length scales will be highlighted in this presentation.

The suppression of lithium dendrite is critical to the application of lithium metal batteries. Many approaches have been applied previously to demonstrate the improved cycling performance with lithium mental anode, which include the 3D conductive framework, lithium metal surface protection and the formation of special SEI protection layer, etc. In this talk a combined simulation and experiment approach is used to understand the new principles behind these approaches. Innovative synthesis procedures, electrochemical battery tests and morphology and spectroscopy characterizations are used to demonstrate the advanced dendrite proof capabilities. Specifically, new approaches to construct the general 3D conductive framework from nonconductive materials and to apply the surface protection layers are demonstrated. Moreover, combined thermodynamic modeling, DFT simulation and new modeling approach is used to understand the dendrite growth thermodynamics and kinetics down to the atomic scale. We further propose some new design principles behind these technological approaches based on our experiment and theory.

Enabling stable and safe reversible Li metal anode is essential to achieve a specific energy density of 500 Wh/kg from a cell level in next-generation Li batteries. The low Coulombic efficiency and dendrite growth issues significantly hinder the commercialization of Li metal anode. It is well accepted that after electrochemical cycling, formation of "inactive" Li, consisting of Li ions that form SEI (Li⁺) and SEI wrapped Li metal (Li⁰), is a direct reason for capacity loss. Differentiating and quantifying Li⁺and Li⁰ after cycling is one of the most critical yet challenging problems that impedes the thorough understanding of the failure mechanism of Li metal anode. A new chemical analytic method has been introduced in this work and provides a solution enabling the quantitative measurement of Li⁰ content in cycled Li metal cells at microgram (µg) level for the first time. Combining with Cryo-FIB-3D reconstruction, Cryo-TEM and XPS, a correlation among mass content, microstructure, SEI nanostructure and chemcial composition has been established to investigate the properties of "inactive" Li generated in different electrolytes.

Lithium metal is known as the “Holy Grail” of anode materials, as it has the highest theoretical capacity, lowest density, and most negative electrochemical potential of known anode materials for rechargeable batteries. Unfortunately, dendrites of lithium form during repeated cycling, posing a safety hazard and deterring commercialization of lithium metal batteries. Previous studies, each with different methods of sample preparation and testing methods, show that Lithium’s yield strength may vary by more than 2 orders of magnitude (from ~1 MPa to 100-300 MPa). However, comprehensive knowledge of the mechanical behavior of Li remains a key obstacle to understanding how to engineer anode-separator interfaces that can mitigate or suppress dendrites. Through a combination of in-glovebox tensile testing and nanoindentation in as-received lithium ribbon, we probe the mechanical properties of Li metal at different length scales.

Formation and growth of dendrites on Li metal surfaces is a well-known problem and is one of the main reasons that prevent the successful operation of Li-metal batteries. Numerous mitigation strategies are continuously being developed such as electrode coatings, artificial solid electrolyte interphase (SEI) films, electrolyte design for specific SEI compositions, and use of solid electrolytes among others. In some of these processes, mitigation is induced by chemical or mechanical (or both) modifications of the environment surrounding the sites where Li ions are plated. Here we use first principles calculations first to investigate why dendritic nucleation occurs and what properties should be tuned to mitigate such phenomena. We then analyze the microscopic mechanisms behind some of the proposed strategies and determine which ones have the best probabilities of a long-term solution to this problem.

Li metal anode has been deemed as the ultimate choice of rechargeable Li batteries with high energy density due to highest capacity and lowest electrochemical potential. However several challenges, such as severe dendrite formation, poor Coulombic efficiency, and drastic volume expansion, impeded its application in practice. Herein, we utilize the lignin, one of the most abundant but underutilized biomaterial on earth, to fabricate a layered and holey carbon. This novel structure owns enlarged interlayered gap in micrometer scale and holes in each layer, which can decrease the area current density, accelerate the ion transference and stabilize the Li metal to some extent. The Li anode with novel host exhibits high Coulombic efficiency (~97% over 350 cycles), large areal capacity (20 mA h cm^-2), and long life-span in cycling (>500 h, 250 cycles) at a high current density of 4 mA cm^-2. In the full cell with LiFePO₄ cathode, a high capacity of 90 mA h g^-1 was achieved and kept stable for 1800 cycles with a high capacity retention (>92%) under a high current density of 10 C (corresponding to 4.5 mA cm^-2).

Next-generation energy storage devices, such as Li-ion batteries (LIBs) and Li-sulfur batteries (LiS), demand high energy, power and better safety. Conventional graphite anode in Li-ion batteries falls short of fulfilling all these necessities. Carbon nanostructural materials have gained the spotlight as promising active materials for energy storage; they exhibit unique physico-chemical properties such as large surface area, short Li⁺ ion diffusion length, and high electrical conductivity, in addition to their long-term stability. Carbon-nanostructured materials have issues with low areal and volumetric densities for the practical applications in electric vehicles, portable electronics, and power grid systems, which demand higher energy and power densities. One approach to overcoming these issues is to design and apply a three-dimensional (3D) electrode accommodating a larger loading amount of active materials (e.g., sulfur) while facilitating Li+ ion intercalation. Furthermore, 3D nanocarbon frameworks can impart a conducting pathway and structural buffer to high-capacity non-carbon nanomaterials, which results in enhanced Li⁺ ion storage capacity. Recent advance of two-dimensional (2D) materials enables us to design/fabricate atomic layer deposition on electrode materials for high-efficiency active electrode materials: atomic layered 2D MoS₂ - coated Li metal demonstrates a stable Li electrodeposition with the suppression of nucleation sites for dendrite growth. The MoS₂ coated Li anodes assembled with 3D carbon nanotube-sulfur cathodes provide superior electrochemical performance in Li-S batteries ever reported to date. The superior performance of 3D carbon nanotubes and 2D materials coated Li-metal in energy storages will be presented along with their mechanistic analysis.
References:
1. High performance rechargeable Li-S batteries using binder-free large sulfur-loaded three- dimensional carbon nanotubes, M Patel, E Cha, C Kang, B Gwalani, W Choi, Carbon http://dx.doi.org/10.1016/j (2017)
2. Recent development of 2D materials and their applications, Wonbong Choi, Nitin Choudhary, Juhong Park, Deji Akinwande, Younghee Lee, Materials Today, 116-130, 20, (2017).
3. 2D MoS₂ as an efficient protective layer for lithium metal anodes in high performance Li-S batteries, Cha, E., Patel, M.D., Park, J., Hwang, J., Prasad, V., Cho, K., and Choi, W., Nature Nanotechnology, 13, pages337–344 (2018).

Engineering the surface chemistry of lithium ion battery materials is necessary for the development of safe, stable, high energy-density cells. The application of ultrathin film coatings is a widely explored strategy to design surface chemistries and mediate electrochemical reactions. The preponderance of work in this field has investigated inorganic thin film coatings, very often metal oxide materials. This talk will describe work in our group to engineer organic surface chemistries using polymeric thin films. We have developed two approaches to apply polymer thin films on battery electrodes. The first approach is surface-initiated atom transfer radical polymerization. Polymerization initiators are tethered to the surface of a model thin film SI anode and then used to synthesize poly(methyl methacrylate) (PMMA) brushes of tunable thicknesses from approximately 20nm to several hundred nanometers. It was shown that the presence of 75nm brushes reduces the first cycle irreversibility on thin film Si anodes to 23.7%. The irreversibility in untreated Si is 37.6%. Post-mortem FTIR-ATR confirmed that less ethylene carbonate is reduced on the PMMA-coated Si in the first cycle, and electrochemical impedance spectroscopy showed that the PMMA brushes inhibit the growth of a resistive surface layer during extended cycling. In the second approach, initiated chemical vapor deposition (iCVD) is used to apply ultrathin conformal polymer layers on conventional lithium ion battery electrodes prepared by slurry casting. The conformality and coverage of the polymer coatings was confirmed by scanning electron microscopy and x-ray photoelectron spectroscopy. For coatings on lithium ion battery anodes, polymer compositions with high crosslinking densities were developed to exclude liquid electrolyte from the electrochemical interface. Three distinct polymer chemistries have been applied by iCVD: crosslinked poly(lithium methacrylate) (PLiMA), poly(ethylene glycol diacrylate) (PEGDA), and poly(1,3,5-trimethylcyclotrisiloxane) (PV3D3).With these crosslinked films, there is a trade-off between the reduction in irreversible side reactions and an increased area specific resistance. We have also demonstrated that capacity retention of full cells (NMC vs. graphite in 1M LiP6 in EC:DMC:DEC) at 55°C is improved by coating both cathode and anode with these ultrathin films. Our efforts to understand how polymer film chemistry and morphology influences the electrochemical reactions in lithium ion cells will be discussed.

The demand for large-scale renewable energy generation and electric mobility is rising the need for high capacity and safe energy storage systems. Utilizing metal anodes are gaining momentum, owing to their very high energy densities compared to conventional intercalation-based electrodes. Nevertheless, considering the hostless nature of the metal anodes and their interfacial instability, the practical utilization of such systems has been restricted. Inhomogeneous metal electrodeposition (dendrites) and unwanted byproduct formation during cycling limit the cycle life and safety of the metal anode-based batteries. Therefore, research community has focused on designing innovative approaches to regulate the deposition behavior of metal anodes. In this context, one of the most reliable methods is the use of ultra-thin and ultra-stable materials on the metal anode to both prevent the side reactions at the interface and also suppress the formation of dendritic deposits. Among the proposed solutions, 2D materials are promising candidates, owing to their ultrahigh mechanical strength, superflexibility and chemical stability. However, it is not truly clarified that how these approaches affect the nucleation and growth modes of the metal during the electrodeposition.
In this work, we have studied the nucleation and growth mechanism of lithium (Li) and zinc (Zn), as examples of metal anodes, in the presence of high-quality graphene (Gr) layer. Interestingly, addition of an ultra-thin layer of carbon is able to significantly regulate the morphology and electrochemical performance of these metal anodes. Utilizing electrochemical potential tests and scanning electron microscopy (SEM) the nucleation mechanism was explored. Accordingly, Li forms homogeneous spherical nucleation electrodeposits all over the Gr-coated electrode surface, being different from randomly whisker like deposition in case of bare electrode. Utilizing transmission and scanning electron microscopy we detected that upon further electrodeposition, in contrast to the expected highly dendritic Li deposition, Li spheres can develop into a uniform and compact structure composed of vertically aligned Li nanorods. Moreover, despite the randomly oriented inhomogeneous deposition of multi-crystalline Zn on the bare substrate, a planar Zn deposition, composed of single crystalline flat flakes, was observed in case of Gr-coated sample. Therefore, we can conclude that high quality graphene not only provides a homogenous metal-ions nucleation, but also regulates the morphology and growth orientation of the final deposition products, significantly. Further experimental and computational efforts are being carried out to provide comprehensive explanations for our observations is this research. Overall, we believe that such systematic studies can pave the way in the evolution of such surface engineering approaches into the industrial scale applications of rechargeable metal batteries.

A stable and high efficiency metal anode (such as Li, Na, Zn, Mg) is critical for all rechargeable metal batteries, including the batteries with various cathodes such as ion intercalation compounds, conversion materials, sulfur, and oxygen. An ideal metal anode not only needs to have a very high Coulombic efficiency, but also need to have a low concentration, high conductivity and low cost. Recently, we have developed a series of “localized high-concentration electrolytes (LHCE)” by diluting high-concentration electrolytes with electrochemically “inert” solvents or poorly solvating diluents. Unlike the high concentration electrolytes reported before, the electrolyte reported in this work exhibits low concentration, low cost, low viscosity, improved conductivity, and good wettability to separator and electrodes. With selected Li salt, solvent, and the diluent, we demonstrated a fire-retardant LHCE that enables stable, dendrite-free cycling of LMAs with high coulombic efficiency of up to 99.2%. Moreover, this electrolyte exhibits excellent anodic stability even up to 5.0 V and greatly enhances the cycling performance of LMBs. A Li|| LiNi_0.6Mn_0.2Co_0.2O₂ battery using this electrolyte can retain > 97% capacity after 600 cycles at 1C rate (ca. 1.6 mA cm^-2), corresponding to a negligible capacity decay of < 0.005% per cycle. Similar concept of “localized high-concentration electrolytes (LHCE)” have also been used to stabilize Na metal anode for more than 40,000 cycles. In addition, this concept was also used to expand the electrochemical windows of water based electrolytes in a salt concentration much less than those reported before. Therefore, this new approach opened a window for further development of novel electrolyte for practical high-energy metal batteries.

With the fast-growing demands for high energy storage, lithium (Li)-ion batteries (LIBs) can no longer satisfy the application needs due to their relatively low energy densities. Li metal batteries (LMBs) are regarded as one of the most promising next-generation energy storage systems. The key to enable long-term cycling stability of high-voltage LMBs is the development of functional electrolytes that are stable against both Li anodes and high-voltage (>4 V vs. Li/Li⁺) cathodes. Due to their limited oxidative stability (< 4 V), ethers have so far been excluded from being used in high-voltage batteries, in spite of their superior reductive stability against Li metal compared to the conventional organic carbonate electrolytes. Here we design a concentrated dual-salt/ether electrolyte that can form stable interfacial layers on both the high-voltage LiNi_1/3Mn_1/3Co_1/3O₂ cathode and the Li metal anode, thus realizing an unprecedented capacity retention >90% over 300 cycles and ~80% over 500 cycles with a charge cut-off voltage of 4.3 V. Various characterization techniques were used to reveal the detailed mechanisms for the enhanced electrode stabilities. All these fundamental findings extend the conventional knowledge on ether-based electrolyte systems, and provide an effective approach to achieve high energy density LMBs.

Solvate ionic liquids (SILs) are classified as one of the subclasses of ionic liquids, composed of long-lived complex ions. ¹ Equimolar molten complexes consist of an appropriate combination of oligoethers (glymes) and Li salts (LiY), [Li(glyme)][Y], have shown to yield a highly Li-ion conductive and non-flammable SILs, which can be employed as an electrolyte for lithium secondary batteries. ² Furthermore, recent studies have shown that SILs consist of the complex anions with reversible redox behavior can be used as liquid cathode materials (catholyte) for various energy conversion devices including redox flow batteries ^3,4, sodium-iron batteries ⁵, and lithium secondary batteries. ⁶ In this work, we obtained series of novel redox active SILs consist of an symmetric ([Li(G3)]⁺) and asymmetric ([Li(G3Bu)]⁺) triglyme-Li complex, and redox active tetrahalogeno-ferrate ([FeX]^- (X =Br₄, Cl₃Br, Cl₄)). The physicochemical properties of [Li(G3/G3Bu)][FeX] were investigated by differential scanning calorimetry (DSC), Raman spectroscopy, electrochemical impedance spectroscopy, and thermogravimetry (TG). Cyclic voltammetry confirms the reversible redox property of the [FeX]^-/[FeX]^2- couple for all SILs. Among the SILs tested, [Li(G3Bu)][FeCl₄], consist of the complex cation with asymmetric structure and small complex anion, showed the lowest melting point (T_m) of <30 °C. The lithium secondary batteries utilizing [Li(G3/G3Bu)][FeX] as a catholyte exhibits a high coulombic efficiency of over 90 % after 50 cycles as well as stable discharge capacities of approximately 80 % of the theoretically predicted value, suggesting a new and promising path to enhance the performance of lithium secondary batteries.

(1) C. A. Angell, Y. Ansari and Z. Zhao, Faraday Discuss., 154, 9-27 (2012).
(2) K. Ueno, Electrochemistry, 84, 674-680 (2016).
(3) K. Takechi, Y. Kato and Y. Hase, Adv. Mater., 27, 2501-2506 (2015).
(4) M. A. Miller, J. S. Wainright, and R. F. Savinell, J. Electrochem. Soc., 163, A578-A579 (2016).
(5) L. Xue, T. G. Tucker, and C. A. Angell, Adv. Energy Mater., 5, 1500271 (2015).
(6) Y. Wang and H. Zhou, Energy Environ. Sci., 9, 2267-2272 (2016).

Low cycle efficiency and dendrite growth are two critical barriers for rechargeable batteries using Li metal as negative electrodes, mainly due to the coupled mechanical/chemical degradation of the SEI layer formed on Li metal surface. We have developed a comprehensive set of in situ diagnostic techniques combined with atomic/continuum modeling schemes to investigate and understand the coupled mechanical/chemical degradation of the SEI layer/Li system including fundamentally understanding the surface and interface phenomena. We have found that the mechanical incompatibility between SEI and soft Li leads to the complicated mechanical behaviors of the lithium metal electrode during the plating and stripping process. We systematically investigated the relationship between surface morphology and current density distribution which results in an inhomogeneous Li plating/stripping process. Based on this understanding, we have developed a new coating design strategy to achieve high cycle efficiency/dendrite free and extend the cycle life of lithium rechargeable batteries.

The physiochemical properties of the solid-electrolyte interphase (SEI), primarily governed by electrolyte composition, have a profound impact on the electrochemical cycling of metallic lithium (Li). Herein, we discovered that the effect of nitrate anions on regulating Li deposition previously known in ether-based electrolytes can be extended to carbonate-based electrolytes, which can dramatically alter the morphology of Li nuclei from dendritic to spherical, albeit extremely limited solubility. The effect can be attributed to the preferential reductive decomposition of nitrate anions during SEI formation that modifies the interfacial environment. And the mechanistic origins behind the phenomenon were investigated based on the structure, ion-transport property and charge transfer kinetics of the modified SEI utilizing advanced characterization techniques such as cryo transmission electron microscopy and ultramicroelectrode. Furthermore, to overcome the solubility barrier, a solubility-mediated sustained release methodology was introduced, in which nitrate anions were encapsulated in porous polymer gel and can be steadily dissolved during battery operation to maintain a high concentration at the electroplating front. As such, effective Li dendrite suppression and remarkably enhanced cycling stability can be achieved in both half- and full-cell configurations in corrosive carbonate electrolytes, significantly outperforming conventional electrolyte additives. The proposed approach is generally applicable in various carbonate-based electrolyte systems and can improve the reversibility of Li metal anode without sacrificing the stability, ionic conductivity, or the cost of electrolytes.

One way to achieve high energy density batteries is to use Li metal as the anode material due to its very high theoretical specific capacity of 3860 mAh g^-1. This approach however bring many challenges. Since over 40 years scientists spent important efforts to find the right strategies to overcome the high reactivity of Li metal and the formation of lithium dendrites during cycling. So far, only technologies based on solid polymer electrolytes (e.g. poly(ethylene oxide)) have been commercialized. One recent approach to employ Li metal anodes relies on the use of superconcentrated electrolytes in which the amount of solvent molecules reaches that of ions. It has been proposed that, under such conditions, the absence of free (i.e. non coordinating to the ions) solvent molecules prevent thermodynamically favourable decomposition at Li. With such system, acetonitrile (ACN) have been used with lithium metal. Because of is high oxidative stability and is high dielectric constant, ACN represent an interesting option as a solvent for the electrolyte.

Here we propose a different approach consisting in the formation of an artificial solid electrolyte interface (SEI) on the lithium to hinder the reaction between ACN and the metallic anode. To do so, we studied the spontaneous decomposition of neat fluoroethylene carbonate (FEC) on lithium metal (Li-FEC)^[1] to form a protective layer. This film have been characterized by AFM, XPS and by SEM to determine its composition and its morphology and a dense film made of a mixture of LiF with a polymeric phase have been found. This artificial SEI is porous to the lithium ions but impermeable to ACN, allowing the use of this solvent with conventional Li salt concentrations which maximise electrolyte conductivity. We demonstrate the protective properties of such artificial SEI using a 1 M LiPF₆ in acetonitrile (ACN) electrolyte into a symmetric cell (both electrodes Li-FEC). This cell was cycled with a current density of 0.1 mA/cm² during 30 min per half-cycle and for a total of 1500 hours. The electrochemical performance of a full cell made of c-LiFePO₄ as cathode and Li-FEC as anode with the 1 M LiPF₆ in ACN electrolyte have been analysed and shown a capacity of 162 mAh/g at 0.2C (theoretical capacity is close to 170 mAh/g). This approach opens new possibilities for the use of metallic anodes to improve energy storage in batteries.

1) N. D. Trinh, D. Lepage, D. Aymé Perrot, A. Badia, M. Dollé, D. Rochefort, Angewandte Chemie International Edition 2018, 57, 5072.

The ability to directly utilize Lithium (Li) metal anodes in rechargeable batteries presents itself as an ideal, albeit challenging, situation. Li metal anodes could provide a maximum possible theoretical specific capacity (3860 mAh/g) in comparison to commercially used anodes (e.g. graphite – 380 mAh/g). However, Li metal anodes remain absent in commercial devices due to inherent safety concerns associated with the formation of Li dendrites during high rate cycling, as well as Li metals’ susceptibility to exhibit high reactivity towards commercially used organic electrolytes. Due to the possibility of thermal runaway, such concerns have adversely affected the potential use of Li metal in commercially available batteries. Additionally, Li metal reacts vigorously with water or passivates in the presence of small quantities of moisture rendering it unusable. This presents a significant hurdle for systems where water-tolerant Li cycling is of practical necessity (e.g. Li-air). Hence, significant efforts in recent literature have targeted the development of robust systems, capable of use with Li metal.
To date, various strategies have been employed to overcome such hurdles; the use of solid electrolytes as a mechanical barrier, or the use of specific organic solvent-based electrolytes which control the properties of the solid-electrolyte interphase (SEI), being noted observations. Amongst the available classes of Li battery electrolytes, ionic liquids (ILs) have been shown to facilitate enhanced Li cycling efficiencies and favorable Li plating morphologies while being inherently non-volatile/non-flammable alternatives to commercially available organic electrolytes. Through the capability to combine various cations, anions and salts; the use of such ILs could produce robust SEIs, resulting in the improved cycling behaviors reported in literature.
Recently, we reported that certain ILs allowed for successful Li metal cycling in the presence of water mixed into the IL electrolyte. To our knowledge, no reports have shown the capability to sustain morphologically friendly Li deposition upon application of high rates while sustaining stable cycling in a water containing electrolyte. In recognition of this unique capability, we now introduce a new method to artificially form these SEIs on Li metal via the screening of various IL electrolyte (cation, anion and salt) combinations. The electrochemical results, along with fundamental analytical analyses of the ILs capable of water-tolerant interphase formation and transplantation on Li metal, while sustaining commercially feasible Li morphologies at practical cycling rates in the presence of water containing electrolytes will be presented and discussed.

Sodium (Na) metal is an attractive anode for next-generation energy storage systems due to its high specific capacity, low cost and high abundance. Nevertheless, uncontrolled Na dendrite growth caused by the formation of unstable solid electrolyte interphase (SEI) leads to poor cycling performance and severe safety concern. Herein, we first reveal that sodium polysulfide (Na₂S₆) alone can serve as a positive additive or pre-passivation agent in ether electrolyte to improve the long-term stability and reversibility of Na anode, while Na₂S₆-NaNO₃ as co-additives has an adverse effect on Na anode, which is contrary to the prior findings in lithium (Li) metal anode system. A superior cycling behavior of Na anode is first demonstrated at a high current density up to 10 mA cm^-2 and a capacity up to 5 mAh cm^-2 over 100 cycles. As a proof of concept, we present a high-capacity Na-S battery via pre-passivating Na anode with Na₂S₆. Our study gives new insights into understanding the differences between Li and Na systems.

Rapid growth of electric vehicles has stimulated the development of high-energy storage systems, especially the lithium–sulfur and lithium–air batteries that employ lithium metal anodes. However, the wide deployment of Li-metal batteries has been hindered by its poor cycling efficiency and safety concerns, both of which stem from the uncontrollable Li deposition-stripping process. While various theories have been raised in the past, the underlying mechanisms of lithium redox couple are still controversial.
Here, we report the texturing behavior of lithium metal in batteries and reveal the mechanism of lithium electro-deposition from crystallographic perspective. Additives in electrolytes and the cross-over molecules from the cathode play crucial roles on the crystallographic texture because they inhibit the cathodic process and adsorb/react on different crystal planes selectively. Exchange current density has been identified as an indicator for the additive adsorption, which can only be accurately measured with microelectrode. Nano-void formation at interface of Li and SEI is found during the lithium stripping, which is attributed to the accumulation of lithium metal vacancies. High rate dissolution of lithium drives vigorous growth, subsequent aggregation of voids, and eventually collapse of the SEI layer, i.e. pitting. The polarization behavior and pitting potential of lithium are systematically measured by 3-electrode cell. Metallurgical factors, such as grain boundaries and slip lines, are found greatly accelerate the local dissolution of lithium. The understanding of the electro-crystallization and stripping beneath interface process of lithium will shed light on future lithium anode and electrolyte design.

Acknowledgement:
The work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the US Department of Energy under the Battery 500 Consortium program.

Vanadium diselenide (VSe₂) is a transition metal dichalcogenide with metallic conductivity, which makes it a potentially promising electrode material for electrochemical applications. However, the developments of VSe₂ electrodes for such applications has been severely hampered by the difficulty of preparing nanosized products. In this work, a new facile solvothermal synthesis process is developed to synthesize ultrathin VSe₂ nanosheet assemblies. To obtain the ultrathin nanosheets, N-methyl pyrrolidone, which has the similar surface energy to many transition metal dichalcogenides, was used as the solvent to limit the crystal growth along the c-axis direction. The resulted ultrathin VSe₂ nanosheets exhibit good performance toward the alkaline ions (Li⁺ and Na⁺) storage, yet the performance can be further significantly enhanced by carbon coating. Specifically, the carbon-coated VSe₂ nanosheets can deliver high capacities of 768 mA h g^-1 (Li⁺ storage) and 571 mA h g^-1 (Na⁺ storage) along with outstanding stability. This work presents general strategies to solution grow ultrathin VSe₂ and transition metal dichalcogenides nanosheets, and to further enhance the alkaline metal ion storage performance by in-situ carbon coating.

The demand for energy storage with higher energy density is increasing day by day and intensive researches are being carried out. Many kinds of research have been concentrated on the anode of lithium-ion battery with high energy density. However, the practically usable anode electrode material is very limited for a number of reasons. And commercial cathode still has low capacity, the effect of full-cell configuration using a high capacity anode is insignificant, which is an obstacle to the development of the entire energy storage device. The newly proposed conversion type cathode has a storage capacity of 10 times that of the existing intercalate cathode. Sulfur is an interesting cathode material due to its high theoretical capacity of 1,673 mAh / g, large quantities, very low cost and low toxicity. Theoretically, S-based cathodes are configured full cell battery with a Si or lithium metal anode can achieve four or five times the theoretical specific energy of a commercial C-LiCoO2 system. Despite the theoretical capacity of the sulfur, the practical steps mentioned above could not be reached because of insulating properties and large volume expansion of sulfur, and dissolution of the intermediate reaction product in the electrolyte results in the poor lifetime.



In this study, we use a polymer with a channel to enclose sulfur particles to prevent volume expansion and dissolution and try to approach to impart mass transfer capability through hybridization with conductive graphene. The polymer with a channel is synthesized through a low-temperature heat treatment based on the difference of the vaporization temperature according to the degree of polymerization of the polymer. Through the channel, the lithium ion and the electron to be delivered to the sulfur contained in the polymer. All of the synthesis processes attempted in this study are scalable and very easy to mass-produce.



The polymer used in this study is an aliphatic rubbery synthetic polymer and has a vaporization temperature of 60 ° C or less when it is in a monomer state. However, the vaporization temperature rises sharply according to the degree of polymerization, and when it is completely cured, it has a vaporization and decomposition temperature of 300 degrees or more. But in this work, the polymer in the hybrids mostly had a very low degree of polymerization in the MALDI-TOF analysis of the synthesized whole hybrids. This is because of the polymerization reaction, which originated from sulfur, terminated very quickly and the reaction was complete when the polymer met the sulfur and graphene particles. Due to the very low degree of polymerization of the polymer, the polymer can be vaporized at temperatures below 90 degrees, which results in many pores and channels on the surface and inside of the polymer from low-temperature heat treatment. Such a pore-forming polymer has superior mass transfer characteristics and high lifetime performance while offsetting the disadvantages of sulfur.

The Na₃V₂(PO₄)₂F₃ is highly studied material as a sodium ion battery cathode in recent years. One of the major drawbacks of the phase is high price of vanadium. Here we report on the electrochemical properties, interfacial kinetics and ionic diffusivity of Fe- substituted Na_3-x Fe_0.3V_1.7(PO₄)₂OF₂ as a function of temperature _. It is seen that the electrochemical performance at lower temperature interval is very poor and the cell resistance increases exponentially with decreasing temperature. This indicates that electrochemical performance of the phase is limited either by charge transfer kinetics or mass transport properties. In order to validate this statement we measured the impedance spectra at a particular sodium concentration (x=1) at different temperatures and derived the activation energy of different electrochemical processes. The resistance of charge transfer process at Na_3-xFe_0.3V_1.7(PO₄)₂OF₂/electrolyte is higher than other processes and activation energy is 0.56eV. The obtained sodium ion diffusivity is in the order of D = 10^-12 cm²/s and does not vary significantly with the changing of sodium concentration at the electrode material except at x=1. The activation energy due to sodium ion diffusion is 0.55 eV which is consistent with the available data for the parent phase Na₃V₂(PO₄)₂F_3. The obtained cell resistance as a function temperature and ionic diffusivity data indicate that the rate performances of the material Na_3-xFe_0.3V_1.7(PO₄)₂OF₂ is most likely limited by interfacial charge transfer kinetics particularly at higher cycling rate. Particle morphology thus needs to be engineered in order to obtain higher surface area and shorter diffusion length for practical C-rate. On the other hand, the electrolyte composition has important impact on the cycling stability of sodium ion battery at different temperatures due to the change of composition of SEI (solid electrolyte interface) layer. Based on that, three different types of electrolyte compositions were formulated and the charge-discharge behavior is tested at 25 °C and 45 °C temperature. It reveals that the cell with EC-PC combination exhibits lower capacity than DMC-EC-PC and DEC-EC compositions at room temperature. The DMC-EC-PC electrolyte combination shows best electrochemical performances. This material shows the capacity close to119, 130 and 125mAh g^-1 at 0.1C with EC-PC, DMC-EC-PC and DEC-EC, respectively and at 1C rate performance is not much promising. Whereas at 45°C with the same electrolyte compositions exhibit above 100mAh g^-1 at 1C rate. This observation also suggests that electrochemical rate performance of the material is limited by the interfacial charge transfer kinetics and the ionic diffusivity of the material.

In recent years, an interesting charge compensation mechanism by anions, also called anionic redox, has attracted a lot of attention in lithium ion battery society because it has considered as promising strategy to overcome the capacity limitations of classical cathode materials. Another important implication of this research trend is that it triggered the interest on anionic contribution in charge compensation of cathode materials. However, recent studies dealing with anionic redox are all concentrated on overlithiated materials. Here, we tried to unveil the anionic redox behavior in LiNi_1/3Co_1/3Mn_1/3O₂ (NCM111), one of the representative layered oxide cathode materials, by reexamining the charge compensation mechanism of NCM111. Even though charge compensation mechanism of NCM111 in the practical voltage range (< 4.3 V) have revealed in detailed by a variety of studies, the charge compensation mechanism in the high voltage region still remains unclear. Finally, through the detailed analysis, it is confirmed that the oxygen in the metal-oxygen bond contributes to the charge compensation in the low-voltage range, and the lone-pair oxygen participates in charge compensation in the high-voltage range. This finding is expected to not only provide a new perspective for anion redox researchers but also help establish a high voltage stabilization strategy for layered oxide materials.

Electrochemical oxygen reduction and oxygen evolution reactions are critical reactions in many energy conversion and storage system including metal-air batteries and fuel cells. Efficient and sustainable oxygen catalysts have been required to replace noble metal based catalysts. In this regards, application of biomass for catalysts have attracted increasing attention due to abundance and low cost. In this study, N, S-co-doped porous carbon were prepared from pyrolysis of bamboo stems following doping process. As zinc-air battery cathodes, the N, S-co-doped carbon exhibit extremely high maximum power density, based on the superior catalytic activity when compared to the previously reported biomass-based catalysts. These excellent performances can be attributed to adequate micro/mesoporosity and the presence of sufficient active sites. This work will provide sustainable and efficient strategies to design high-performance cathode materials for zinc-air batteries.

The development of a low-cost, highly active, and durable catalyst for oxygen electrocatalysis is crucial for water electrolyzer and rechargeable Zn-air batteries. Here we report the carbonaceous material, graphitic carbon nitride-carbon nanofiber material (g-CN-CNF), as bifunctional catalyst of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in alkaline media. g-CN-CNF catalyst exhibits enhanced catalytic performances for oxygen reactions, with high electron transfer number, low Tafel slopes, and low overpotentials. We applied the synthesized g-CN-CNF catalyst on primary and rechargeable Zn-air battery. The g-CN-CNF showed the comparable performance of primary cell with Pt despite the same catalyst loading. Furthermore, rechargeable Zn-air battery revealed that g-CN-CNF had a good retention of capacity for 156^th cycling, resulting in the catalytic activity and stability of bifunctional catalyst. Therefore, g-CN-CNF is a good candidate for carbonaceous bifunctional catalyst which can be applied on practical electrochemical devices.

The development of potassium-ion batteries as new energy storage systems using potassium ions as ionic shuttles has recently attracted significant attention [1]. It has been discovered that graphite has an ability to intercalate K⁺ ions reversibly, which results in a series of intercalation compounds and a theoretical capacity of 278 mAhg^-1. This has greatly stimulated the field to develop further. Apart from graphite and carbonaceous materials, families of inorganic compounds have been extensively studied as high capacity anodes in lithium-ion and sodium-ion batteries [2]. However, these inorganic phases have not been significantly explored as potassium-ion battery anodes yet.

Some of the candidate inorganic phases of metalloids and sulphides are studied by this team as prospective anode materials [3]. This includes germanium as well as sulphides of tin and antimony. These inorganic phases have been showed to deliver capacities of 350 - 1024 mAhg^-1 in lithium-ion and sodium-ion batteries [4, 5]. Here, we present an overview of potassium electrochemistry of these compounds. In order to prepare model materials, the phases of interest (Ge, SnS₂ and Sb₂S₃) are dispersed on sheets of reduced graphene oxide. The electrochemical activity and gravimetric capacities of anode materials are evaluated. Phases formed during the discharge and charge are probed via ex-situ X-ray diffraction measurements. It is demonstrated that capacities in excess of that of graphite are possible in the inorganic phases studied here. Conclusions on possible reaction mechanisms in these materials are made and are presented in this contribution as well.

References

[1] X. Wu, D. P. Leonard, X. Ji, “Emerging non-aqueous potassium-ion batteries: challenges and opportunities,” Chem. Mater., vol. 29, pp. 5031–5042.
[2] Y. Zhao, L. P. Wang, M. T. Sougrati, Z. Feng, Y. Leconte, A. Fisher, M. Srinivasan, Z. Xu, “A review on design strategies for carbon based metal oxides and sulfides nanocomposites for high performance Li and Na ion battery anodes,” Adv. Energy Mater., vol. 7, 1601424.
[3] V. Lakshmi, Y. Chen, A. A. Mikhaylov, A. G. Medvedev, I. Sultana, M. M. Rahman, O. Lev, P.V. Prokhodchenko, A. M. Glushenkov, “Nanocrystalline SnS₂ coated onto reduced graphene oxide: demonstrating the feasibility of non-graphitic anode with silfide chemistry for potassium-ion batteries,” Chem. Comm., vol. 53, pp. 8272–8275.
[4] L. Baggetto, J. K.Keum, J. F.Browning, G. M.Veitha, “Germanium as negative electrode material for sodium-ion batteries, Electrochem. Comm, vol. 34, pp. 41- 44.
[5] X. Li, Z. Yang, Y. Fu, L. Qiao, D. Li, H. Yue, D. He, “Germanium Anode with Excellent Lithium Storage Performance in a Germanium/Lithium–Cobalt Oxide Lithium-Ion Battery, ACS Nano, vol. 9, pp. 1858-1867.

Due to its excellent cyclic ability and high energy density, lithium-ion batteries (LIBs) are playing a crucial role in today’s technology. Unlike the technological advancement on LIBs anode electrodes, where they have high capacities and low manufacturing cost; cathode materials are facing weighty drawbacks like lower capacity, volume changes and distortions affecting the mechanical integrity of cathode and as a consequence of these limitations quite high manufacturing cost. LiNi0.80Co0.15Al0.05O2 (NCA) is one of the promising cathode materials that has been comprehensively utilized with various synthesis methods. The objective of the study is to synthesize NCA via solid-state reaction method and investigate the influence of sintering temperature of 750, 800 and 850 °C on the morphology of the powders by SEM. In this work, we started the NCA synthesis with a sol-gel method. The powder obtained from the dried sol was calcined as the initial thermal treatment. After that, the calcined samples were sintered with a stoichiometric amount of lithium acetate at different temperatures. XRD analysis was performed in order to observe the phase analysis of the samples sintered at several temperatures. Electrochemical tests including cycle performance and cyclic voltammetry were also performed. Results indicated that the synthesized NCA cathode that was sintered at 750 °C had a favorable electrochemical performance and the capacity retention of NCA sintered at 750 °C is better than the ones sintered at 800 and 850 °C without capacity loss at various high C rates. The correlation between the cathode performance and the final microstructure was developed and this was directly related to the specific surface area and porosity of the materials.

In recent years, two-dimensional (2D) layer structure metal dichalcogenides and their composites with 1D and 2D carbonaceous materials are considered as promising anode materials for lithium-ion battery due to their higher theoretical capacitance and conversion reaction with Li. Among several 2D metal chalcogenide SnS₂ has appeared as an anode material for Li-ion battery applications due to optimistic high efficiency at lower redox potential. However, in practice the SnS₂ as an anode material in Li-ion battery is suffering from its low intrinsic electric conductivity, structural instability and large volume expansion during charging reaction.
In this study, we have fabricated hierarchical SnS₂-CNTs composite on carbon cloth (CC) as an anode by binder free direct growth method for Li-ion application. We adopted two-step fabrication methods: CNTs was prepared by a microwave plasma-enhanced chemical vapor deposition (MPECVD) and followed by solvothermal process to synthesize uniform tin disulfide (SnS₂) nanosheet on CNTs (SnS₂-CNTs). X-ray diffraction, Raman's spectroscopy, FE-SEM and HR-TEM studies showed that the as synthesized SnS₂-CNTs possessed a hierarchical composite nanostructure anode. The optimized hybrid SnS₂-CNTanode showed superior electrochemical performance in Li-ion battery, indicating highest performance of 1250 mAh/g at 645 mA g^-1 after 120 cycles with high rate capability. The direct growth SnS₂-CNT hybrid hierarchical architectures showed the synergistic effect for superior electrochemical performance due to improved surface reactivity, electrical conductivity and strong mechanical strength of insulating SnS₂ in composite anode material. We believe our hierarchical SnS₂ nanosheet grown on CNT makes this a promising anode material for Li-ion battery application.
Reference
Hu, Shan et al. Advances in Nanomaterials pp 61-75. Springer International Publishing AG 2018.
Luo, B.; Fang, Y.; Wang, B.; Zhou, J.; Song, H.; Zhi, L. Energy Environ. Sci. 2012, 5, 5226.

Lithium-air batteries are an active area of research because of their potential to have a much higher energy density than traditional lithium-ion batteries. However, they are not yet commercially viable due to poor efficiency, high charging voltages, and low cycle lifetimes. Experimental studies of Li-air batteries with aprotic solvents have shown that the O₂ reduction starts when superoxide (O₂^-) forms in solvent and reacts with Li⁺ to form lithium superoxide (Li⁺-O₂^-) [1]. Solid Li₂O₂ then forms as the final discharge product on the cathode. Recent experimental work has suggested that a better understanding of the factors governing the behavior of the lithium superoxide in solvent could help control the discharge at the cathode [1]. We are therefore modeling the interactions between LiO₂ molecules in various common solvents such as dimethylsulfoxide (DMSO) and 1,2-dimethoxyethane (DME) and studying properties such as the clustering behavior of these molecules using density functional theory calculations and ab-initio molecular dynamics simulations. Results from these explicit solvent calculations will be presented and discussed. [1] D. G. Kwabi, V. S. Bryantsev, T. P. Batcho, D. M. Itkis, C. V. Thompson, Y. Shao-Horn, Angew. Chem. Int. Ed. 2016, 55, 3129.

Laser scribing is used to directly form expanded and highly doped (≈13 at% N) 3D graphene anodes on Cu foil without the need for a binder or conductive fller. The simultaneous graphitization and doping of the 3D graphitic anodes in this process result in exceptional electrochemical storage of Na-ions. Specifically, an initial coulombic efficiency (CE) up to 74% was achieved. In addition, Na-ion capacities up to 425 mAh g^-1 at 0.1 A g^-1 have been achieved with excellent rate capabilities. Further, a capacity of 148 mAh g^-1 at a current density of 10 A g^-1 was obtained with excellent cycling stability.

The invention of rechargeable batteries has dramatically changed our landscapes and lives, underpinning the explosive worldwide growth of consumer electronics, ushering in an unprecedented era of electric vehicles, and potentially paving the way for a much greener energy future. Unfortunately, current battery technologies suffer from a number of challenges, e.g., capacity loss and failure upon prolonged cycling, limited ion diffusion kinetics, and a rather sparse palette of high-performing electrode materials. Mechanistic understanding of compositional and electronic structure heterogeneities spanning from atomistic to mesoscale dimensions is imperative to facilitate the rational design of novel high-performing electrode chemistries and architectures. Scanning transmission X-ray microscopy (STXM) observations indicate the formation of lithiation gradients in individual nanowires of layered orthorhombic V₂O₅ that arise from electron localization and local structural distortions. Electrons localized in the V₂O₅ framework couple to a local structural distortion, giving rise to small polarons, which are observed to trap Li-ions and represent a major impediment to Li-ion diffusion. In addition, I will discuss the first direct visualization of patterns of compositional inhomogeneities within cathode materials during electrochemical discharge. Two distinct patterns are evidenced: core—shell separation and striping modulations of Li-rich and Li-poor domains within individual particles. 3D compositional maps have been developed and translated to stress and strain maps, providing a hitherto unprecedented direct visualization of stress and strain inhomogeneities. Furthermore, a cluster of interlaced Li_xV₂O₅ nanoparticles is evaluated by scanning transmission X-ray microscopy. Increased heterogeneity at the interface between particles suggests the exchange of Li-ions, implying a “winner-takes-all” behavior (corresponding to particle-by-particle lithiation of an ensemble of particles). Such behavior portends the creation of localized hot-spots and provides insight into a possible origin of failure of Li-ion batteries. Finally, I will discuss prospects for “beyond-Li” batteries given the safety and earth abundance issues that have assumed great significance in recent years.

The success of LiFePO₄ as a cathode material for lithium-ion batteries (LiB) has generated considerable interest in various polyanion-type materials including Li₂MSiO₄ and Li₂MPO₄F (M = Fe, Mn or Co). In the case of Li₂FeSiO₄, its operating voltage vs. Li/Li⁺ (2.8 V) and specific capacity of 166 mAh g^-1 lead to energy densities in the range of 500 Wh kg^-1, which is only slightly less than that of LiFePO₄.
Recently, another lithium iron silicate, Li_2.6Fe_0.7SiO₄, has been investigated. Li_2.6Fe_0.7SiO₄ has a similar structure to lithium super ionic conductor (LiSICON) Li_2+2xZn_1-xGeO₄ and exhibits better kinetics than Li₂FeSiO₄ at higher discharge rates. However, iron silicate-based materials have an operating potential of 2.8 V vs. Li/Li⁺, and compounds possessing the features of Li_2.6Fe_0.7SiO₄ with higher operating voltages are of considerable interest.

The similarity in structure between the LiSICON Li_2+2xZn_1-xGeO₄ and Li_2.6Fe_0.7SiO₄ has motivated us to investigate the electrochemical properties of compounds with the chemical composition of Li_2.6M_0.7GeO₄ (M = Mn, Co). While Li_2.6Co_0.7GeO₄ has already been studied, to the best of our knowledge, this is the first report of the synthesis and properties of the Mn isomorph, Li_2.6Mn_0.7GeO₄. In contrast to the Co compound that can be synthesized at 900°C in air, the Mn compound must be synthesized under argon at 700°C to obtain the proper Mn valence. The Co and Mn materials show very different electrochemical properties. Whereas the Li_2.6Co_0.7GeO₄ exhibits very little redox activity, the Li_2.6Mn_0.7GeO₄ can be charged and discharged between 1.5 and 4.8 V vs Li/Li⁺ with lithium capacities in excess of 100 mAh g^-1, which is close to the capacity of 97 mAh g^-1 for one electron redox reaction. Interestingly, the (dis)charge curves vary monotonically with lithium content, suggesting a single phase (de)insertion mechanism. Additional details regarding the kinetics of Li_2.6Mn_0.7GeO₄ and comparisons to Li_2.6Fe_0.7SiO₄ will be presented.

Investigation of novel electrode materials with high areal capacity, faster electron transport kinetics, and high Li ion diffusion are currently one of the most active frontiers for improving energy density of lithium ion batteries (LIBs). In order to fulfill these demands of LIB’s, herein, we developed 3D structure CZTS flowers by hydrothermal method and followed by simple and facile carbon coating in ex-situ step. A systematic electrochemical performance behavior has been carried out at different amount of carbon coating. The developed nanoscale carbon coating enhanced conductivity of CZTS and buffer the mechanical stress/strain induced over subsequent cycles. As a results, carbon-coated 3D structured CZTS showed high areal capacity about 1-2 mAh/cm^-2 as well as high gravimetric capacity (1210 mAh/g) combined with excellent stability (100 cycles) and high rate capability (1500 mAh/g, 920 mAh/g and 425 mAh/g at 500 mA/g, 2000 mA/g and 5000 mA/g) is superior to other metal chalcogenide materials. This study highlights the potential importance of conformal carbon coating over electrode material as an effective strategy for enhancing the columbic efficiency and charge storage kinetics of active material.

Storage of electrical energy in media and protocols with high energy and power densities have received a revived interest due to high power mobile electrical devices, electric vehicles, and other disconnected from grid but electrically powered technologies such as drones and robots. As a result, many new battery concepts such as sodium batteries, lithium-air, and lithium-sulphur are currently under intensive research as a replacement for traditional lithium ion batteries. One of the promising replacement for batteries is supercapacitors owing to their potentially higher power density (<5 kW kg^-1) and longer life cycle (>100,000) compared to that of batteries; however, they have an unimpressive energy density, which is at least an order of magnitude lower than that of batteries. Allotropes and polymorphs of carbon from diverse sources are a universal choice to fabricate supercapacitor electrodes; owing to their renewability carbons from waste biomass is now an active area of research. Most of these biomass-derived carbons process a large volume of passive pores which do not contribute to the final functionality of the devices. In this paper we show that the passive pores could be effectively filled using 3D hierarchical ceramic nanostructures such as flowers and the resulting composite offer charge storage capabilities several fold higher than its constituents or when they are filled using conventional nanoparticles. Finely powdered kernels of oil palm seeds are used as the source of carbon; nanoparticles and 3D flower-shaped ceramic nanostructures are integrated into the pores of carbon via a simple wet impregnation method. The pure carbon showed a specific surface area of ~500 m² g^-1; and when used as an electrode for supercapacitive charge storage gave a specific capacitance (C_S) of 150 F g^-1 with cyclic stability (97% after 5000 cycles) in 1 M Na₂SO₄ electrolyte (achievable potential ~1 V). When the flowers are impregnated, the C_S increased five-folds whereas the carbons impregnated with nanoparticles did not even doubled. In addition to the enhanced charge storage, flowers-filled carbons showed improved potential window also; carbon with MnCo₂O₄ flowers showed the highest (530 F g^-1) with a potential window of ~1.2 V. The reason for many fold enhancement in C_S is systematically studied and shown that this increment resulted from efficient ion transport through pore channels. Newly prepared carbon composites could retain ~97 % of their initial capacitance even after 5000 cycles. The adopted methodology is simple, scalable as well as applicable for a broad range of pseudocapacitive materials.

Li ion battery (LIB) is a most famous secondary battery because of its superior characteristics such as high working voltage, large specific capacity, lightweight and so on. However, capacity of LIB fades under applying large current, therefore the charge speed(current) is still restricted at least below 2C. For example, we needs approximately 2-3 hour to fully charge the smartphone. Realization of high-speed rechargeable LIB will make us possible to eliminate the charging time. We have reported that BaTiO₃ dot supported LiCoO₂ cathode exhibits better chargeability under high-rate charge/discharge current.[ref] In addition, in the case of thin film battery, very high-rate performance of 75% and 50% charge at 50C and 100C, respectively, was achieved by BaTiO₃ nanodots deposited LiCoO₂ cathode thin film. In this study, we would like to explain the mechanism of this enhancement using dot supporting of various materials on LiCoO₂ cathode thin film. The key point of high-rate performance might be permittivity of supported material. Computing simulation by Finite element method supported our results. To deeply understand this mechanism, we tried to deposit various supporting materials for controlling relative permittivity. For instance, we fabricated LiCoO₂ epitaxial thin film supported with low permittivity material CeO₂ (ε_r = 7) by pulsed lase deposition method. The enhancement of high-rate performance under applying larger charge/discharge current was also observed in dot-CeO₂/LiCoO₂ cathode thin film. However, the degree of enhancement was worse than in the case of dot-BaTiO₃/LiCoO₂ thin film. Additionally, cell resistivity of dot-CeO₂/LiCoO₂ thin film was evaluated by charge curves, resulting that higher resistivity was observed as comparison with that in dot-BaTiO₃/LiCoO₂ thin film. As a result, it is revealed that supporting materials having higher permittivity assisted a formation of lower resistivity in the battery cell. We will discuss about the relation between relative permittivity of another supporting material and high-rate performance, and also detailed mechanism in this system.

Lithium-ion batteries have enjoyed great success and have outperformed other rechargeable battery system since 1980. However, Li-ion batteries face many challenges and limitations: safety, the low abundance of lithium in the Earth’s crust. Recently, Sodium-ion batteries attracted a lot of interest as a potential alternative to lithium-ion batteries for large-scale energy storage applications, due to the large natural abundance and lower cost of sodium. In recent years, fluorophosphates with the NASICON (Na Super-Ionic Conductor) type structure are considered among the most interesting series of cathode materials for Li/Na-ion batteries, because they exhibit rich chemistry, attractive lithium/sodium insertion properties and thus offer promising electrochemical properties [1]. Na₃V₂(PO₄)₂F₃ (NVPF) attracted high attention thanks to its promising electrochemical properties. The inductive effects of both PO₄^3- and F^- allow for a high working potential combined with a high theoretical specific capacity due to the multiple oxidation states of vanadium[1-2]. One of the key drawbacks of Na₃V₂(PO₄)₂F₃ electrodes is their low intrinsic electronic conductivity.

NVPF and NVPF/carbon composite materials were prepared by spray-drying method using the same conditions used in our previous work [2]. Spray drying is a cost-effective and easily up-scalable route to prepare homogeneous multi-component powders, thus making it a suitable method to incorporate carbon in the composite powder. We used different carbon sources like conductive carbons (MWCNTs, Carbon Black, etc) and organic sources (PVA, Citric Acid, Ascorbic acid, etc) to prepare NVPF/carbon composite powders.

the structural, electrochemical, and morphological properties of the synthesized Na₃V₂(PO₄)₂F₃/C samples were systematically investigated in order to understand the influence of carbon source on structural and morphological properties and most importantly electrochemical performance of NVPF and NVPF/carbon composite cathode materials for Na-ion batteries. The chemical diffusion of Na ions was studied using results obtained by varying scan rates in cyclic voltammetry measurements. Raman spectroscopy is used to evaluate the quality in disordered carbon materials and its electronic conductivity [3] and compared the results with the results from EIS and cycling performance of different samples.

Silicon has emerged as the most promising component in anode materials for next-generation lithium-ion batteries (LIBs) owing to its natural abundance, relatively low working potential, and its high theoretical storage capacity of 3579 mAh/g. However, the practical application of Si-based anodes is severely hindered by its low intrinsic electrical conductivity and its large volume change (>300%) during charging and discharging. The resulting mechanical stress causes rapid pulverization of the silicon, and insulation and disconnection of the active material from the current collector. These failure events can cause rapid degeneration of the Si electrode and is especially prominent for silicon particles exceeding the size of a few hundred nanometres. Thus, recent research mainly focusses on nanostructures and nanocomposites that tolerate the volume change.

A very promising way to stabilize silicon in LIB anodes is the incorporation of nitrogen, which has been shown to significantly improve the cycle performance. We therefore developed a scalable gas-phase synthesis method based on the pyrolysis of monosilane in ammonia-rich atmosphere. Therefore, we are able to synthesize Si/SiN_x core/shell nanoparticles for high-performance anodes. Their electrochemical performance can be increased by adjusting the synthesis parameters, thus affecting stoichiometry, Si/SiN_x ratio, morphology, particle size, and crystallinity. FTIR measurements confirm the synthesis of high-purity silicon nitride coatings avoiding Si-H, Si-O, or Si-OH functional groups that might increase parasitic surface reactions. First results show significantly enhanced cycling performance of LIB-electrodes comprised of Si/SiN_x nanoparticles. Electrodes made of materials with low nitrogen content (approx. 5 at.%) show an initial specific discharge capacity as high as 720 mAh/g and a highly stable cycle performance with a capacity retention of 92% after 100 cycles. LIB electrodes comprised of Si/SiN_x nanoparticles with high nitrogen content (approx. 56.5 at.%) show much lower initial specific discharge capacities of 350 mAh/g. However, repeated electrochemical cycling seems to increasingly activate the material leading to a highly stable cycle performance and a retention of 125% after 100 cycles. These results imply that Si/SiN_x based LIB electrodes are promising candidates for high-performance lithium-ion batteries with very high durability.

Demands for rechargeable batteries with higher energy density have continued to rise as the amount of the energy required for next-generation portable electronic devices and electric vehicles increases. Rechargeable lithium sulfur (Li/S) batteries have been intensively studied over the past several years to fully utilize their high theoretical energy density (2600 Wh kg-1). However, the fundamental challenge to the commercialization of Li/S batteries is the practical design of sulfur cathodes that enable competitive electrochemical performance while maintaining the sulfur content and the loading high and the electrolyte/sulfur ratio and the cost of inactive components low. This paper presents a novel solvent drying method that is designed to help the wet-dry surface of the slurry endure the local tension stress from the shrinkage rate difference. Using our scaffold-supported drying approach, we have fabricated a crack-free sulfur electrode of ultrahigh loading (16 mg cm^-2) with a high sulfur content of 65 %. The compact structure without excessive carbon interlayers or 3D architectured scaffolds allows the electrolyte/sulfur ratio to be as low as 7 and exhibits a good capacity retention of 11 mAh g^-1 over 80 cycles. Our results show that the crack-free and compact structure enhances the electrical network, the uniform reaction throughout the whole active mass, and thus the reversibility of the active mass. We also confirmed the applicability of the scaffold-supported drying approach to other types of scaffolds. When coupled with a thoroughly-designed scaffold, the facile scaffold-supported drying approach provides limitless directions for high energy density batteries, closing the gap between research and commercialization of Li/S.

The operation of lithium ion batteries (LIB) at elevated temperatures is a unique chemical challenge as the electrolyte materials must be thermally, electrochemically, and chemically stable at temperatures from 60 to 150 ^oC. Currently, carbonate based electrolytes do not satisfy these requirements and new materials are needed to fabricate high temperature operational LIBs. To that end, we have synthesized three phosphonium and three piperidinium based ionic liquids (ILs), and prepared 1.0 M lithium bis(trifluoromethylsulfonyl) imide (LiTFSI) salt electrolytes and characterized the thermal stability, viscosity, conductivity, and electrochemical stability window of each compound. An alkyl ether chain was introduced into the structure of both the phosphonium and piperidinium cationic structures and analogues containing one or two ether atoms were prepared. With these structural changes, we hypothesize: (1) that the distinct structural differences between the phosphonium and piperidinium cationic center will have substantial impacts on the conductivity and viscosity; and (2) the incorporation of the alkyl ether will have a concurrent reduction in viscosity and increase in conductivity. The phosphonium and piperidinium based ILs displayed large differences in all measureable outcomes; the phosphonium ILs were more conductive, less viscous and have larger electrochemical stability windows. For example, phosphonium ILs with one alkyl ether incorporated were electrochemically stable from -1 to 5 V at 25 ^oC while the analogues piperidinium ILs were only stable from -1 to 2 V at 25 ^oC. Furthermore, inclusion of the alkyl ether affected the viscosity and conductivity of the ILs; at 25 ^oC the piperidinium based ILs displayed varied conductivities with values of 1.40, 2.29, and 1.99 mS/cm with zero, 1 and 2 alkyl ether substitutions, respectively. These findings support further exploration of the phosphonium based ILs for high temperature operations of LIB.

Spinel zinc ferrite (ZnFe₂O₄) is a candidate anode material for Lithium-ion batteries (LIBs), owing to its large theoretical capacity of 1000 mAh g^-1. Although the structure of ZnFe₂O₄ has been well studied, the origin of the high performance in LIBs materials is not well understood, in particular the fundamental understanding of the discharge mechanism is lacking. Here, we report a density functional theory (DFT) study of the discharge process at early stage from ZnFe₂O₄ up to Li_xZnFe₂O₄ (x = 2), where both bulks and various relevant surfaces are into consideration. The estimated open-circuit voltages based on the stable intermediate bulk and surface structures identified by the DFT calculations are in good agreement with the experimental values, which enables the in-depth understanding of the discharge mechanism at the atomic level. Our study not only highlights the importance of the interplay among Li, O^2-, Fe³⁺ and Zn²⁺ in enabling the high performance as LIBs materials, but also provides a design strategy for more stable particle morphologies with enhanced discharge performance.

Crystallite size reduction of an electroactive material is a tool to increase deliverable capacity of a lithium ion battery by decreasing the path length for lithium ion diffusion. However, the reduction of crystallite size may also negatively impact the cycle life of the battery by promoting unfavorable reactions. The impact of crystallite size on the capacity retention, reversibility and rate capability of silver containing α-MnO₂ will be presented in light of two contributing mechanisms. First, delivered capacity may be hindered by the irreversible loss of the electroactive material to electrolyte. The transition metal may deposit on the anode, forming a layer that prevents ion migration and increases internal resistance of the cell. Quantitative dissolution studies conducted as a part of this work will be discussed. Second, capacity losses in α-MnO₂ materials may result from structural distortion of the 2 x 2 MnO₆ tunnels that make up the crystal structure. Observations regarding structural evolution as a function of (de)lithiation as determined by several complementary characterization modalities, including synchrotron based x-ray absorption spectroscopy, will also be discussed. In summary, this work quantitatively dissects the effect of structural distortion and transition metal dissolution on capacity retention of electrodes with different crystallite sizes, facilitating rational future development of long life, high capacity energy storage systems based on tunneled crystallographic motifs.

The primary alkaline Zn/MnO₂ battery has had a ubiquitous presence in portable energy storage systems. Yet, current technological demands require energy storage capable of extended rechargeability and high-power delivery. The inherent cost and safety advantages associated with zinc and manganese oxide (MnOx) electrodes can be translated to next-generation rechargeable “zinc-ion batteries” by replacing the traditional alkaline electrolyte with even safer neutral-pH Zn²⁺-containing electrolytes. The functionality of such “zinc-ion cells” can be extended further when incorporating a nanoarchitectured form of MnOx, well-known for pseudocapacitance, and tapping this high-rate charge–storage mechanism by adding Na⁺ to the electrolyte. The cells reported in this study comprising a mixed Na⁺:Zn²⁺ aqueous electrolyte and a nanoarchitectured MnOx@carbon nanofoam cathode paired with a zinc foil anode (Zn||MnOx) exhibit high specific capacity at slow C-rates (1C; 300 mAh g^–1) and reasonable capacity at more challenging C-rates (20C; 100 mAh g^–1). The fundamental charge-storage mechanisms of the redox-active nanostructured MnOx are further investigated by a fleet of electroanalytical and ex situ materials characterization techniques, which suggest that a number of electrochemical processes are responsible for the combined pseudocapacitive and battery-like processes of MnOx cycled in the mixed Na⁺:Zn²⁺ electrolyte.

Metallic zinc has been regarded as an ideal anode material for the aqueous batteries systems for its high theoretical capacity (820 mAh/g), low negative potential (0.762 V vs. SHE), abundance, low toxicity and the intrinsic safety advantages that arise from nonflammable aqueous electrolytes. Recently, rechargeable batteries using zinc metal anode have been investigated extensively.
However, an important barrier of the Zn based batteries is the poor cycle life. The cyclability of the traditional alkaline Zn based batteries is mainly restricted by dendrite growth, high solubility of discharge product (i.e. zincate) in the electrolytes, water loss from the liquid electrolyte, electrolyte depletion caused by the narrow electrochemical window. In most previous studies, the zinc-based aqueous batteries suffered from low columbic efficiency (CE) even using the high rate to minimize the side reaction. Significant excessive zinc has to be used to keep the cycle stability, results in the suboptimal utilization of the zinc theoretical capacity, as in the case of the lithium metal anode. The goal of achieving high CE in aqueous zinc metal batteries remained elusive.
The poor reversibility and low CE of the Zn metal anode are closely related with the Zn (II) cation solvate structure in the aqueous electrolyte. The hydration effects of the Zn (II) cation in water is so significant that the zinc hydroxide is easily formed. The slight but nonignorable water decomposition caused by the narrow stability window produces more hydroxyl ion and certainly aggravates the formation of zinc hydroxide. Zinc hydroxide converts into insoluble zinc oxide (ZnO) when the solubility limit of the hydroxide species is reached. Formation of solid ZnO can be a difficult process to reverse during recharge.
We report the development of a highly concentrated neutral electrolyte that alters Zn(II) solvation structure resulting in the dendrite-free plating of Zn metal with high CE. The suppression of the zinc hydration was achieved through the formation of [ZnTFSI] solvation structure instead of the [Zn(H₂O)₆] ²⁺ solvation structure. The solvation structure change is ascribed to the introduction of the TFSI anion, which has the strong coordination to the Zn (II) cation in concentrated electrolytes. The solvation structure change was investigated via a combination of IR spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, density functional theory (DFT) calculations and molecular dynamics (MD) simulations using polarizable force fields. We demonstrate an exceptional performance of zinc metal cells containing Zn-based aqueous electrolyte that delivered an unprecedented high practical energy density of 300Wh/kg (based on both the cathode and anode electrodes). This study opens an avenue for the highly efficient utilization of zinc metal electrodes for advanced energy storage applications while the fundamental knowledge gained can also be applied to other metal anodes.

Our team at the U.S. Naval Research Laboratory has taken the lessons from 20 years of probing the operational and design characteristics of catalytic and energy-relevant nanoarchitectures to create a zinc sponge—a monolithic, 3D-wired anode that improves current distribution within the electrode structure during charge–discharge cycling, thwarts dendrite-formation, and can challenge the energy density of Li-ion battery packs, all while using safer aqueous-based chemistry. Rechargeable zinc-based batteries (e.g., Zn–air, Ag–Zn, Ni–Zn, MnO₂–Zn, Zn-ion) now have the potential to overcome some of the limitations of current Li-ion batteries: safety concerns associated with toxic and flammable electrolytes, high materials/manufacturing costs, and high single-cell specific energy decremented by weight and volume additions to control thermal events in Li-ion stacks. Because the aperiodic architecture ensures three-dimensionally (3D) wiring of all of the transporting reactants (electrons, ions, molecules), the entire volume of the 3D electrode becomes a more uniformly reacting phase, which lowers the kinetics of charge transfer, which minimizes the local current density, which thereby prevents any one region of the zinc electrode launching a dendrite, thereby physically ensuring more uniform charge–discharge reactions. The road beyond the drama still too common with Li-based batteries is paved with zinc.

Monolithic nanoporous metals are usually fabricated by dealloying, the selective dissolution of a homogeneous alloy. These metallic structures comprise networks of nanometer-wide ligaments and pores, which suit them well for electrochemical applications that desire high specific area and high, uniform conductivity. However, dealloying has difficulty in achieving nanoporous reactive metals like Zn, for the challenging selection and fabrication of its alloy precursor, and for the oxidative nature of dealloying. Our previous work developed an alloy-free alternative towards nanoporous metals; we used chemical reduction to selectively dissolve anions out of a solid compound, and rendered a nanoporous structure identical to that from dealloying. Here, we further develop this method of reduction-induced decomposition (RID) of compound for the fabrication of nanoporous Zn. We deliberately designed the RID reaction, including the selection of the Zn compound and the reductant, to synthesize a large piece of monolith in a short period. The resulted nanoporous Zn comprises ~100 nm wide ligaments, and a specific area of 45 m²/g. We also successfully formed a composite of nanoporous Zn and carbon fibers, which improved the strength of the 400 μm thick Zn electrode. The electrode achieved a high depth of discharge in an alkaline cell, with considerable cycle life.

The greatest remaining challenges facing the development of electrochemical energy storage – such as durability, safety and fast charging – lie in the development of appropriate materials. We present a concept where redox-active conjugated polymers are specially designed to operate in safe electrolytes (e.g. salty water).¹ After identifying suitable organic conjugated polymer backbones which can be reversibly be charged and discharged in aqueous solutions, we studied the role of the side chain on the electrochemical redox reactions. Polar side chains were attached to the backbone to increase ion conduction to allow for a reversible and fast charging and discharging of the materials in pH neutral sodium chloride water based electrolytes. In particular, ethylene glycol based side chains were attached for the polymer which becomes oxidized (p-type polymer) and zwitterion side chains for the polymer which becomes reduced (n-type polymer). This strategy enables the reversible charging and discharging of non-porous, single phase films (additive free) with non-toxic aqueous electrolytes on timescales of seconds. Finally, we demonstrate the use of the developed concept in a battery device combining the p- and n-type conjugated polymers as the cathode and the anode of a water based electrochemical storage cell that can be operated under a nearly unipolar potential window of 1.4 V. We believe that our findings will play a central role for the development of fast electrochemical charging electrodes with potentials for use with safe and environmental friendly electrolytes.

1. D. Moia, A. Giovannitti, A. A. Szumska, M. Schnurr, E. Rezasoltani, I. P. Maria, P. R. F. Barnes, I. Mcculloch, and J. Nelson, arXiv:1711.10457 [physics.app-ph], 2017, 1–62.

Lithium ion batteries have reshaped our life with their omnipresence in portable electronics. However, increasing the specific energy of these batteries is reaching its limit and high-profile fire accidents (e.g. cell phones spontaneously combusting) cast doubt of their applications in electric vehicles and large-scale energy storage. Intrinsically safe batteries such as aqueous batteries and all-solid-state batteries are being actively studied in the battery community. Aqueous batteries use water-based electrolytes and offer robustness and environmental friendliness over lithium-ion batteries that feature flammable organic electrolytes. However their adoption is plagued by the poor cycle life due to the structural and chemical instability of the anode materials. In the first part of talk, I will report several redox-active quinones (oxidized derivatives of aromatic compounds) as anodes for aqueous batteries by exploiting their structurally stable ion-coordination charge storage mechanism and chemical inertness towards aqueous electrolytes. We demonstrate three systems that coupled with industrially established cathodes and electrolytes exhibit long cycle life (up to 3,000 cycles/3,500 h), fast kinetics (320C),
high anode specific capacity (up to 200-395 mAh g^-1), and state-of-the-art specific energy/energy density for several operational pH values (-1 to 15), charge carrier species (H⁺, Li⁺, Na⁺, K⁺, Mg²⁺), and atmosphere (with/without O₂). Reversible proton-coupled electron transfer process is also first demonstrated in organic crystals. In the second part, I will discuss the application of quinones in all-solid-state batteries. One main challenge is the mismatch between low anodic decomposition potential of solid-state sulfide electrolytes and high operating potentials of cathodes which leads to a volatile cathode–electrolyte interface. I will show how molecular engineering of quinone molecules leads to high-capacity cathode materials in all-solid-state batteries that is chemically and electrochemically compatible with sulfide electrolyte.

Electrolytes are an essential component of energy storage devices. Electrolyte composition has a significant impact on the safety, price and performance of the battery. Intrinsically nonflammable aqueous electrolytes can offer safer battery operation and decreased associated toxicity, but suffer from a smaller electrochemical stability window (and hence energy density) compared to traditional organic electrolytes. To circumvent the small electrochemical stability window, highly concentrated “water-in-salt” lithium organic imide systems which demonstrate significantly wider stability windows were recently proposed. However, the toxicity often associated with organic imides and very high price make the practical implementation of current water-in-salt electrolyte chemistries into commercial energy storage devices challenging.
Herein, we address the challenge of developing new formulations of water-in-salt electrolytes caused by the lack of lithium salts having water solubility high enough to satisfy the water-in-salt condition. The proposed mixed cation strategy is whereby cheaper (by at least an order of magnitude) and more soluble salts featuring alkali cations beyond lithium, such as potassium, are used to create the water-in-salt condition. Co-dissolved lithium salts enable compatibility with traditional intercalation battery electrodes. We show that such highly concentrated electrolytes can provide the same benefits of the extended voltage window as imide-based electrolytes and, once combined with lithium salt, demonstrate compatibility with traditional Li-ion battery electrode materials while being low-cost and environmentally benign.

Developing Li-O₂ batteries into a practical electrochemical energy storage technology hinges on the availability of a stable electrolyte. Because of the high reactivity of oxygen species in the Li-O₂ battery system, no known organic electrolytes satisfy the requirements for stable cell operations. The search for a compatible electrolyte system remains a significant challenge in Li-O₂ battery research. Here, we show that the water-in-salt electrolyte system, which is essentially super-concentrated aqueous LiTFSI solution, is stable against parasitic chemical reactions with reactive oxygen species for Li-O₂ battery operations. This electrolyte provides the necessary functionalities to support aprotic Li-O₂ chemistries via reversible Li₂O₂ formation and decomposition. The lack of organic solvent molecules is a key advantage shown here. It eliminates the known decomposition pathways that would result in by-product formation from organic solvent degradations. Qualitative as well as quantitative product analysis show no measureable by-products formation in the WiS system. When the conventional carbon cathode is used, greatly improved cyclability of over 70 cycles can be achieved with the WiS electrolyte compared with the organic ones. When the carbon cathode is replaced with a stable carbon-free material, Ru catalyst decorated TiSi₂ nanonets, up to 300 cycles of stable Li-O₂ battery operations are measured. The result sets a new benchmark in Li-O₂ battery research with quantitative product detection. It presents the stage for future studies to achieve the full potentials held by Li-O₂ battery as a stable, high-capacity electrochemical energy storage technology.

Solid electrolyte interphases (SEI) enable the Li-ion intercalation chemistries to operate reversibly beyond the thermodynamic stability limits of non-aqueous electrolytes. The chemical building blocks of SEI mainly come from solvents decomposition products. Exceptions arise, when salt anion are reduction-labile, or when salt concentration exceeds certain thresholds, where anion starts to participate in the interphasial chemistry. In those latter cases, unusually high F-content were often found in the interphases, and unexpected benefits from such intrephasial chemistries arose. However, high F-content in interphases were not always welcomed, thus, the morphology and structure of interphases should play an equally important role as their chemical compositions.
In this work, we explore the different manners that interphases could be fluorinated via the electrolyte sources, and the various battery chemistries that could benefit from such fluorination.

There is a great need to significantly increase the energy and power density of Li-ion batteries while ensuring battery safety. Nonflammable electrolytes have been studied for a long time, but their compatibility with common electrode materials, particularly anode materials, remains an obstacle. Recently electrolytes with high Li salt concentrations have demonstrated great promise for improved electrolyte stability. In this talk, we will discuss the key role of the salt to solvent ratio in nonflammable phosphate electrolytes. At a high Li salt-to-solvent molar ratio (~1:2), the phosphate solvent molecules are mostly coordinated with the Li⁺ cations and the reactivity of the solvent molecules toward the graphite anode can be effectively suppressed. High cycling coulombic efficiency (99.7%), good cycle life, and safe operation of commercial 18650 Li-ion cells with these electrolytes are demonstrated. In addition, these nonflammable electrolytes show significantly reduced reactivity toward Li-metal electrodes. Non-dendritic Li-metal plating/stripping in the Li/Cu half-cells is demonstrated with high coulombic efficiency (>99%) and good stability. Electrolytes developed using this approach significantly improved the cycling stability in high energy Li metal pouch cells.

Particle/polymer electrospinning is a cost-effective and robust technique for the fabrication of high performance nanofiber electrodes for batteries and fuel cells. For Li-ion battery electrode applications, the advantages of fiber mats over conventional slurry cast electrode designs include: (i) a large electrode/electrolyte interfacial area for enhanced electrochemical reaction kinetics, (ii) a controllable interfiber void volume to ensure good electrolyte infiltration into the electrode, and (iii) micron/sub-micron diameter fibers with high nanoparticle content and short Li⁺ transport pathways in the radial fiber direction. Pintauro and coworkers have created and evaluated a number of electrospun electrodes for Li-ion batteries; C particles, mixtures of TiO₂+C particles, or mixtures of Si+C particles for the anode and a LiCoO₂+C mixture for the cathode, where the polymer binder was either poly(acrylic acid) (PAA) or poly(vinylidene fluoride). More recently, a new dual fiber design has been developed/investigated to prepare Si-based nanofiber anodes, where separate fibers of Si particles with PAA binder and carbon powder with polyacrilonitrile (PAN) binder are electrospun simultaneously onto a common collector surface at controlled solution flow rates. Such an anode worked well in half-cell tests, due to the presence of numerous intersection cross-points between electrochemically active but non-conducting Si/PAA fibers and electrically conductive C/PAN fibers. In this presentation, methods for preparing these internally nano-wired fiber composite anodes will be discussed, along with their performance and capacity during charge/discharge cycling. The effects of fiber composition (particle/binder weight ratio) and mat configuration (the relative amounts of Si and C fibers) on gravimetric, volumetric, and areal capacities at different C-rates will be presented.

SiO_x is a promising Li-ion battery negative electrode material because of its high capacity and unique microstructure that leads to good cycle life^[1,2]. However, SiO is typically made by high temperature methods that are expensive and difficult to realize, especially at lab scale. Here, SiO_x negative electrode materials were synthesized using a simple and scaleable method by controlling the air exposure time during ball milling at room temperature^[3]. This method allows efficient control of oxygen content, and results in a similar microstructure as a commercially purchased SiO.
XRD and TEM results show that the SiO_x negative electrode materials prepared by ball milling in air are composed of nanocrystalline Si embedded in an amorphous silicon oxide matrix. The very low initial coulombic efficiency (ICE) of conventionally made SiO (measured here to be ~55% for Aldrich SiO) is one of its major drawbacks. The ball milled SiO_x samples synthesized here have much higher reversible capacities (>1500 mAh/g) and higher ICE values (>70%). The advantages of the synthesized SiO_x are many: an inexpensive and simple synthesis process, high capacity, high ICE, and a special microstructure that protects Si from reaction with electrolyte, resulting in excellent cycling performance.
Here, a detailed study will be presented describing SiO_x synthesis and how SiO_x electrochemical performance in Li-ion cells is related to its composition and microstructure.
References
[1] Y. Hwa, C. M. Park, and H. J. Sohn, J. Power Sources, 222, 129 (2013).
[2] X. Zhao, R. J. Sanderson, M. A. Al-Maghrabi, R. A. Dunlap, M.N. Obrovac. J. Electrochemical Soc., 164 (6) A1165-A1172 (2017).
[3] Yidan Cao, J. Craig Bennett, R.A. Dunlap, Congxiao Wei, and M.N. Obrovac, submitted to Advanced Energy Materials (2018).

The solid electrolyte interphase (SEI) has been well acknowledged as a key component of the stability of Si anodes. During cycling, the fresh surface of lithium silicide is exposed because of the volume expansion and contraction of silicon. The exposed lithium silicide surface chemically reduces electrolyte and forms SEI. However, due to the lithium silicide being buried under SEI during formation, it has been difficult to characterize the SEI formed by chemical reduction of lithium silicide. In this work, lithium silicide (prelithiated silicon) thin films were prepared and chemically reacted with electrolyte. XPS was used to characterize the SEI formed on surface. This is the first time SEI formed by chemical reduction has been characterized directly. It was found out that the SEI is composed of LiF, Li₂O, Li₂CO₃and organic species which are commonly found in SEI layers. The ratio between organic species and inorganic species is dependent on the additives. When Fluoroethylene carbonate (FEC) was added into electrolyte, more organic species formed, and less LiF formed. Early stage SEI formation was investigated by looking at the voltage profile and the irreversible capacity. It was found that at the first cycle, the Si anode only forms SEI by electrochemical reduction while the lithium silicide anode only forms SEI by chemical reduction. Cycling performance shows the SEI formed by chemical reduction is possibly better SEI than the SEI formed by electrochemical reduction. Prelithiated Si not only increases the columbic efficiency by reducing the electrochemical reduction, but also forms beneficial SEI by chemical reduction.

Because of its high capacity (3579 mAh g^-1_, based on Li₁₅Si₄) and proper delithiation voltage (~ 0.4 V vs. Li/Li⁺), silicon (Si) is considered one of the most promising negative electrode materials for future lithium ion batteries (LIBs). However, rapid capacity fading due to the huge volume change (~300 %) of Si during lithiation/delithiation hinders practical applications of Si electrodes. To address the problem of the volume change and to improve the performance of Si composite electrodes, polymeric binders are required to buffer the volume change of Si particles, maintain electrical conductivity, and improve mechanical integrity of Si composite electrodes. Desirable binders should be electrochemically stable, mechanically robust, and have high adhesion strength with Si particles. Nevertheless, limited attention has been paid to the role of binders in the degradation of Si composite electrodes from the viewpoint of mechanics, which is crucial for the design of polymeric binders for Si composite electrodes. In this study, we investigated the mechanical degradation of Si composite electrodes made with different polymeric binders, including polyvinylidene fluoride (PVDF), sodium-carboxymethyl cellulose (Na-CMC), and sodium-alginate (SA). The porosity, at both lithiated and delithiated states, and irreversible thickness change of all electrodes increased with increasing cycle number. Si/PVDF electrodes have larger volume change than Si/Na-CMC and Si/SA electrodes. Environmental nanoindentation measurements (in liquid electrolytes) showed that stiffness and hardness of Si composite electrodes decrease as the cycle number increases [Adv. Energy Mater., (doi.org/10.1002/aenm.201702578)]. Compared with Si/Na-CMC and Si/SA electrodes, Si/PVDF electrodes have poor mechanical integrity after 100 cycles. The mechanical degradation of composite electrodes correlated well with mechanical properties of binders and mechanical interactions between polymeric binders and Si particles. Our results provide insights into designing effective polymeric binders to improve mechanical integrity, microstructure stability, and electrochemical performance of Si composite electrodes.

Lithium ion batteries containing silicon as anodes have gained much attention due of their potential high energy density. The electrolyte reduction and interaction with the surface of silicon anodes result in the formation of solid electrolyte interphase (SEI) at the interface, which determines the cycling performance and the battery reversibility. The electrolyte reduction and its interaction involve in the early stage of SEI formation for silicon anodes. The chemical properties of the early-stage SEI is absolutely vital in stabilizing the surface of silicon anodes, and determines the following electrochemical cycling performance. In this research, two questions will be addressed: Can the early-stage SEI layer prevent further reduction of electrolyte? Is the early-stage SEI stable in electrolyte? In order to better understand the early-stage SEI chemistry, a new methodology has been developed here to decouple the lithiation-induced mechanical deformation from the SEI formation, to directly investigate the interphase formation and evolutions. Galvanostatic reduction was performed on the silicon anode with a specific cut off voltage before silicon lithiation voltage, which allows the formation of an early-stage SEI without the mechanical deformation. This presentation will discuss the composition of early-stage SEI and its evolution under electrochemical condition by combining structural and spectroscopic studies.

Lithium ion batteries (LIBs) are widely used in several electric devices as the prominent energy storage system. The performance of LIBs with organic liquid electrolyte is closely related to the formation of the solid electrolyte interphase (SEI) film on the electrode surfaces, which is essentially composed from the decomposition products of both electrolyte and electrolyte additives [1]. In particular, it is widely accepted that electrolyte additives have strong influence on the capacity retention of LIBs as a result of improving and altering the resulting SEI structure.
Organosilicon compounds have attracted recent attentions as a new candidate for electrolyte additives [2]. Intriguingly, it has been reported that the added organosilicon compounds improve the thermal and electrochemical stability of the electrolyte. However, since it is still difficult to experimentally observe the transient process of the SEI growth on the electrode surface, it has not shown how the organosilicon additive affects the SEI formation process.
In order to gain physical insights into this mechanism, we theoretically investigated the SEI growth on the graphite anode surface by means of the atomistic reaction technique, called the hybrid Monte Carlo (MC)/molecular dynamics (MD) reaction method. As denoted by Takenaka et. al., this reaction simulation allows us to numerically study the submicroscopic structure produced by a number of complex chemical reaction processes, such as the electrolyte reduction reaction on the electrode surface [3].
Employing the MC/MD method, we firstly investigated changes in the SEI film structures in the ethylene carbonate (EC)-based electrolyte systems with and without the organosilicon additive. Remarkably, the obtained results revealed that the excessive growth of the SEI film was suppressed by adding the organosilicon additive, which was consistent with the experimental observations. It was further elucidated that the products derived from the organosilicon molecules was stably aggregated in the vicinity of the anode surface, and protected the electrolyte solvents and lithium salts from the reduction decomposition reaction. These findings indicate that the organosilicon additive possibly improve the cycle performance of LIBs owing to the formation of the effective SEI film.

[1] K. Xu, Chem. Rev. 104, 4303, (2004).
[2] X. Chen, M. Usrey, A. P. Hueso, R.West, and R. J.Hamers, J. Poewr Sources, 241, 311, (2013).
[3] N. Takenaka, Y. Suzuki, H. Sakai, and M. Nagaoka, Phys. Chem. C, 118, 10874, (2014).

In Lithium-ion (Li-ion) batteries, transition metal oxides have reached their capacity limits. The growing demand for high energy batteries has inspired great interest in exploring high-capacity cathode materials. In this regard, Li-S and Li-Se batteries are promising because elemental sulfur and selenium have high theoretical specific capacities of 1672 mAh g^-1 and 679 mAh g^-1, respectively. Although sulfur has an insulating nature (5×10^-28 S m^-1), selenium below sulfur in the periodic table has a much higher electronic conductivity (1×10^-3 S m^-1) and it also has a high volumetric capacity density of 3253 mAh cm^-3 because of its high mass density (4.8 g cm^-3). Recently, our group has demonstrated that organopolysulfides (RS_nR) are a class of high-capacity cathode materials for rechargeable lithium batteries. The unique character of linear organopolysulfides is that short sulfur chains are chemically capped by R groups, therefore high-order polysulfides are significantly reduced initially and upon cycling. The obvious benefit is their low dependence on liquid electrolyte, in another word, it can enable high energy densities of batteries.

In this contribution, we select phenyl diselenide (PDSe, PhSeSePh) as a model compound which has a low specific capacity and poor cycling performance. The Se-Se bond can break in a 2e^- reduction reaction, but it has limited capacity as a cathode material. To increase its capacity, redox active species (e.g., sulfur) could be added in the middle of the selenium atoms. Herein, we mix PDSe with sulfur to form two hybrid compounds with 1:1 and 1:2 molar ratios, which almost double and triple the capacity of PDSe, respectively. Differential scanning calorimetry (DSC), Mass spectrometry (MS), scanning electron microscopy (SEM) and X-ray diffraction (XRD) are applied as characterizations. The experimental study is combined with the first-principles calculations based on the density functional theory (DFT) to reveal the electrochemical phenomenon.

Theoretical calculations suggest that phenyl selenosulfide (PDSe-S, PhSe-S-SePh) and phenyl selenodisulfide (PDSe-S₂, PhSe-SS-SePh) could form via addition reactions, which is supported by mass spectrometry analysis. These hybrid compounds show interesting electrochemical behavior in lithium batteries with three reversible discharge voltage plateaus involving frequent Se-S and S-S bond break and formation. PDSe-S and PDSe-S₂ show initial capacities of 252 mAh g^-1 and 330 mAh g^-1, respectively, followed by stable cycling performance with a capacity retention of >73% after 200 cycles at C/5 rate. In addition, they show steady rate capabilities. This study reports a novel strategy to increase the electrochemical performance of organo-diselenide by addition of sulfur. This strategy adds new members to the family of high-capacity cathode materials and provides a new way to explore Se/S containing hybrid compounds, which are valuable for rechargeable lithium batteries and beyond.

Wet-chemical synthesis of nanosilicon with dimeters in the range of 1-5 nm has been challenging due to the lack of appropriate precursors that decompose in mild conditions. Conventional synthetic procedures use extreme temperatures. For example; carbothermal synthesis occurs at 1700 °C, molten salts synthesis takes place between 200-850 °C and alumino/magnesiothermal synthesis typically takes place above 600 °C. On the other hand, bottom up wet-chemical synthesis employs unsafe reagents such as sodium naphtalide, lithium hydrides, sulfuric acid, while top-down synthesis uses dangerous reagent such as HF-etching (of silicon wafers) followed by an elaborate post-annealing treatment to temperature exceeding 900 °C to mitigate surface oxidation problems.

We will discuss our recent synthesis of metallic nanosilicon by thermal decomposition (< 500 °C) of organometallic precursors in the absence of any reducing agent, molten salts or HF-etching process. This mild temperature synthesis prevents excessive growth leading to nanoparticles in the sub-5 nm ranges. The particles size can be increased by gradual sintering.

These nanoparticles are smaller than silicon Bohr Exciton radius (4.5 nm), luminescent and potentially useful for a variety of applications spanning from energy storage, displays, medical imaging to tunable LED lighting.

Li-alloy anodes have long been recognized as a promising substitute for commercial graphite anode in lithium-ion batteries in terms of high specific capacity and safety. Alloying the active anode material (element A) with a buffer matrix material (element B) to alleviate the large volume change of the former during lithiation/delithiation cycle is a promising strategy to solve the major issue impeding the commercialization of Li-alloy anodes. However, while several Li-A-B alloys have been investigated previously, many more other combinations are waiting to be explored. Moreover, even in those reported combinations, the investigated compositions are limited. Thorough investigation on these materials using the traditional material synthesis strategy is more of a labor-intensive and time-consuming task than a scientific investigation. This then offers a great opportunity to the combinatorial material screening technique characteristic of high throughput exploration, which could be employed as a high efficient screen tool to narrow down the interesting material combinations and compositions for further investigation in a real battery condition.
Here, based on two model ternary alloy combinations, i.e., Li-Si-Al and Li-Co-Sn, we successfully demonstrated the efficacy and accuracy of the combinatorial material screening technique on exploring potential Li-alloy anode materials for lithium-ion batteries. Specifically, the lithium-free binary alloy thin film composition spreads were firstly deposited on silicon wafer using combinatorial co-sputtering. The crystallinity was tuned by varying the sputtering temperature. The as-deposited thin film was then lithiated in pouch cell to form Li-alloy ternary thin film for further characterization. XRD and Synchrotron XRD were used to identify the structural variation before and after lithiation. XPS was used to precisely determine the lithium concentration within the films.

Silicon (Si) has been extensively studied as an anode material in lithium ion batteries (LIBs) due to the high theoretical capacity of 4,200 mA/g. However, Si is structurally unstable during lithium (Li) insertion into Si due to a 400% volume expansion and it acts as main cause of capacity fading. Herein, we report carbon coated silicon nanosheets (SiNSs) as anodes for LIBs. First, SiNSs are synthesized on grafoil current collectors with thickness and diameter of < 15 nm and > 10 μm, respectively. Our previous work demonstrated that SiNSs have great mechanical flexibility to accommodate the structural instability of Si during Li insertion. It is also ideal materials for fast Li storage by the high specific surface area and short diffusion length in SiNSs. Parylene is then coated on SiNSs as carbon source and thermally decomposed to carbon. The process provides excellent homogeneous coating of carbons on the SiNSs that have highly anisotropic morphology on a nanometer scale. The carbon coated on SiNSs (C-SiNSs) showed high capacity and stable cycle performance by the carbon layer that works as buffer for the Li insertion into Si and volume expansion. The C layer also provide high electrical conductivity that stabilize the electrochemical performance during cycles. These findings can contribute to fabricating high performance Si based anodes for LIBs.

Silicon is a promising anode material for lithium-ion batteries due to its large theoretical capacities [1]. The large volume expansion upon lithiation results in cracks, accompanied with a phase transformation from crystalline to amorphous silicon [2]. In this study, we investigate the surface morphology and microstructure of initial lithiated crystalline Si wafer by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Cracks propagate along (100) and (110) directions. Interestingly, we find a crack-direction dependence of the amorphous Si layer. The distribution of Li is presented by STEM-EELS. We further use a coated Si wafer by MLD with ~ 5nm thickness coating layer, and this Si wafer is free of cracks after lithiation. The thickness of amorphous Si layer is significantly reduced to ~ 15nm, which is one magnitude thinner than that of lithiated Si wafer without coating. How the amorphous Si layer develops during the following lithiation/delithiation cycles is also studied.
[1] U. Kasavajjula, C. Wang, A.J. Appleby, J. Power Sources 163 (2007) 1003.
[2] M.N. Obrovac, L. Christensen, Electrochem. Solid State Lett. 7 (2004) A93.

Electrochemical stability windows of multi-component electrolytes, both solid polymer and organic liquid, largely determine the limitations of operating regimes of Li-ion batteries. In order to increase energy densities and lifetimes of batteries, new electrolyte materials need to be discovered and optimized. Achieving higher operating voltages requires better understanding of electrolyte degradation and oxidation. Electrolyte degradation is difficult to probe experimentally, but computational tools allow one to study the relevant phenomena at the atomistic level, and thus obtain insights into oxidation mechanisms and possible design rules, as well as the ability to screen for better materials. However, reliable computational studies of the complex oxidation mechanisms remain challenging, considering the difficulty in obtaining material properties with greater accuracy than density functional theory (DFT) for large systems.

We present new insights into the oxidation mechanism that governs stability of multi-component polymer and liquid electrolytes and introduce an efficient computational method and a simple general model predicting the overall electrolyte stability. We find that explicitly including solvent molecules in the computation of the anion stability has a strong impact, and we show that this effect stems from electrostatic interactions between the molecules. Particularly, we find that across all chemistries studied, only one molecule in the system is oxidized. Building on this, we construct a model where two oxidation scenarios lead to different stability behaviors for the anion-solvent pairs, depending on their relative strength and geometry. Thus, we are able to provide a simple model that accurately predicts the stability of the pair depending on the ionization potential of the isolated anion and solvent. This model is not only useful for predicting the ionization potential (and thus the breakdown voltage) of the electrolyte species, but also gives insight into the atomistic details of bulk oxidation. This understanding of the microscopic details of oxidation allows one to formulate design rules as well as it provides better grounds to study subsequent degradation mechanisms and reactions. This simple model also allows for very simple screening of solvent and salts to find stable electrolytes.

With the increasing demand for renewable energy resources, sodium based-batteries have attracted enormous interest to realize the cost-effective and grid scale electrochemical energy storage (EES) devices because of high abundance and low cost of sodium in contrast to lithium worldwide. In the available sodium based-batteries, aqueous sodium batteries have fundamental advantages over non-aqueous due to safe and cost-effective aqueous electrolyte system. However, low operating voltage of aqueous Na-ion battery is a great challenge to realize high energy storage system. Here, we firstly report a high performance rechargeable battery that utilizes a metallic Na anode and a redox couple cathode of hierarchically nanostructured NiCoAl-layered double hydroxide (NCA-LDH) cathode with enhanced performance by adopting trivalent atomic doping (Co³⁺, Al³⁺) into the Ni(OH)₂ layer. In this design, the wide potential range of the Na metal anode and the high capacity of NiCoAl-LDH enable an aqueous rechargeable Na/Ni battery with excellent energy storage performances. We employed hybrid electrolyte system using both non-aqueous and aqueous electrolytes, which are separated by the alkali-ion solid electrolyte (NASICON, Na₃Zr₂Si₂PO₁₂). Furthermore, the binder-free strategy was implemented to create the efficient cathode material of NiCoAl-LDH/carbon microfiber which eventually improve the electrochemical performance. The Na/Ni battery exhibits a stable operating voltage of ~3.1 V during discharge which outperforms the low cell voltage (~1.23 V) of aqueous rechargeable battery, a high capacity of ~350 mA h g⁻¹, and a resulting energy density of ~1085 W h kg⁻¹.

Recently, Na-ion batteries (SIBs) are attracting the attention of the scientific community as an alternative to the already well established Li-ion batteries, due to the more abundant reserves and lower price of Na resources. However, the larger radius of the Na⁺ than that of Li⁺ results in huge volume variation of electrode materials during Na⁺ insertion/extraction, leading to poor cyclability, which inhibits the NIBs widely application in the energy storage. Metal phosphides as potential anodes for SIBs have recently been demonstrated owing to their higher specific capacities compared with those of carbonaceous materials. Unfortunately, most reported metal phosphides showed irregular particle sizes ranged from several hundred nanometers to tens of micrometers, leading to limited cyclic stability. Herein, polypyrrole (PPy), which is one of the representative conducting polymers, encapsulated cobalt phosphide (CoP) nanowires (NWs) grown on carbon paper (CP), finally realizes 1D core–shell CoP@PPy NWs/CP. The CoP core is connected to the PPy shell via strong chemical bonding, which can maintain a Co–PPy framework during charge/discharge. It also possesses bifunctional features that enhances the charge transfer and buffers the volume expansion. Consequently, 1D core–shell CoP@PPy NWs/CP demonstrates superb electrochemical performance, delivering a high areal capacity of 0.521 mA h cm⁻² at 0.15 mA cm⁻² after 100 cycles, and 0.443 mA h cm⁻² at 1.5 mA cm⁻² even after 1000 cycles. Even at a high current density of 3 mA cm⁻², a significant areal discharge capacity reaching 0.285 mA h cm⁻² is still maintained. The outstanding performance of the CoP@PPy NWs/CP free-standing anode provides not only a novel insight into the modulated volume expansion of anode materials but also one of the most effective strategies for binder-free and free-standing electrodes with decent mechanical endurance for future secondary batteries.

As the widespread emergence of modern technologies combined with human life, lithium ion battery (LIBs) has become one of the most important power supplier for mobile electronic devices, electric vehicles and stationary applications. However, the current LIBs provide a low energy density with approaching to the capacity limits, which emphasize the urgent need for high energy density battery systems.
Herein, we have prepared the amorphous silicon nanolayer-embedded graphite/carbon (SGC) hybrids by chemical vapor deposition (CVD) method with developing the cost-effective and scalable pyrolysis system. With developing an industrial-relevant modified CVD process, sophisticated structure of optimum SGC hybrids achieved high reversible capacity (523 mAh g-1) and unprecedented coloumbic efficiency (92%) at a 1^st cycle in the industrial standard electrode density (> 1.6 g cc^-1) and areal capacity loading of > 3.3 mAh cm^-2. Moreover, fabricated SGC electrode confirmed rapid increase of cycling efficiency upward of 99.5% over only 6^th cycles and exactly allowed favorable cyclability of 96% capacity retention after 100 cycles and high rate capability comparable to the industrial graphite anode. In addition, the electrode composed of SGC hybrids entirely overcame the detrimental effects of the volume variation problems, exhibiting 23% of additional expansion excepting for graphite counterpart. This, in turn, completely preserved the electrical interconnectivity and mechanical integrity without any cracks and contact losses. Finally, a prototype full cell device is demonstrated with high voltage lithium cobalt oxide (LCO) cathode through the coin cell configuration, which achieved 92% of capacity retention after 100 cycles with considerable potential toward next generation target as practical devices. Consequently, this successful SGC hybrid anodes could be proposed for commercial extension to the next generation high-energy battery systems as a major breakthrough for electric vehicle or grid energy storage applications.

Flow lithium batteries (FLB) represent an emerging technology that bring in a unique solution the advantages of the high specific energy of lithium batteries and the design flexibility of redox flow batteries that in turn permits to decouple energy and power.
Different kinds of FLBs have been proposed, including batteries featuring semi-solid anolytes and/or catholytes with Li-ion intercalation powders, like LiFePO₄ or Li₄Ti₅O₁₂, dispersed in organic electrolyte, and Li/S and Li/O₂ flow batteries.
In order to increase energy and power of FLB advancements in materials, design and concepts are required.
Semi-solid slurries need to be properly formulated to provide high electronic and ionic conductivity for fast electrochemical processes and suitable rheological features for an efficient flow. Current collectors and electrolyte have to be properly selected to control solid electrolyte interface formation and enable electron transfer between the fluid electrode and the current collector.
Furthermore, FLBs inherently need a smart cell design capable to maximise the power output and minimize the power loss related to the flow. This is particularly important in the case of viscous fluidic electrodes.
A study on FLB semi-solid slurries that highlights the mutual effect of the half cell components on electrochemical performance is here reported. The nature of active material, conductive additive and electrolyte impacts on the electronic percolating network, and then on the cycleability of the flowable electrodes.
Superconcentrated electrolytes based on tetraethylene glycol dimethyl ether-bis(trifluoromethane) sulfonimide electrolyte and slurries with different carbon content and type are investigated for an use in flow Li-ion and Li/O₂ batteries. The electrochemical results are presented and discussed and related to the morphology, rheology and electrical conductivity of the slurries.
A semi-empirical study on the evaluation of the pressure drops through laboratory cell prototypes of flow-Li/O₂ cells is reported. The experimental work and the fluid dynamic analyses of the cell flow frame provide indications on design strategies that maximize power output of FLBs.

References:
[1] M. Park, J. Ryu, W. Wang, and J. Cho, Nature Reviews Materials, 2 (2016) 16080.
[2] K. B. Hatzell, M. Boota, and Y. Gogotsi, Chem. Soc. Rev., 44 (2015) 8664.
[3] E. Ventosa, G. Zampardi, C. Flox, F. La Mantia, W. Schuhmann, and J. R. Morante, Chem. Commun., 51 (2015) 14973.
[4] I. Ruggeri, C. Arbizzani, F. Soavi, Carbon, 130 (2018) 749

Replacing graphite anodes with lithium metal in Li-ion batteries can significantly boost achievable anode capacities by more than 10-fold (3860 mAh/g_Li vs. 372 mAh/g_graphite) and also pave the way for other “beyond Li-ion” batteries such as Li-O₂ and Li-S batteries. However, the use of Li metal as the anode faces several challenges including the virtually infinite volume expansion, parasitic reactivity with electrolyte, and dendrite growth during the Li plating-stripping cycling process.¹ One promising strategy to overcome the latter two issues is to design an artificial solid electrolyte interface (SEI) which can circumvent the compositional inhomogenity and mechanical instability of the native SEI. Among multiple studies,² lithium fluoride (LiF) has been suggested as the key component to prevent dendrite formation and enable efficient cycling of Li metal; however, a safe and facile approach to grow a LiF-only SEI layer on Li still remains lacking. In this study, we report the formation of an artificial LiF-only SEI layer on Li metal via the reaction between Li and perfluorinated gases under controlled conditions. After systematically employing different reaction conditions including gas pressure, reaction temperature, and reaction time, we are able to control the formation of the LiF layer with different crystallinity and spatial homogeneity, as confirmed by combined X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy analyses. We also invesitigate the effect of this LiF layer on the chemical stability of Li in common Li-ion electrolytes via electrochemical impedance spectroscopy, and probe the behavior of this LiF layer and identify its eventual failure mode under repeated Li plating-stripping cycling. This study sheds light on the controlled growth of an artificial inorganic SEI layer on Li, and its intimate link with the Li chemical stability and cycling behavior. Moreover, it presents a strategy to finely tailor the properties of artificial SEIs using safe and scalable SEI-forming reactants, which can potentially impart clean, well-characterized films that can address some of the major safety risks of previous approaches.
References
(1) Lin, D.; Liu, Y.; Cui, Y. Reviving the Lithium Metal Anode for High-Energy Batteries. Nat. Nanotechnol. 2017, 12, 194–206.
(2) (a) Lin, D.; Liu, Y.; Chen, W.; Zhou, G.; Liu, K.; Dunn, B.; Cui, Y. Conformal Lithium Fluoride Protection Layer on Three-Dimensional Lithium by Nonhazardous Gaseous Reagent Freon. Nano Lett. 2017, 17 (6), 3731–3737. (b) Zhang, X.-Q.; Chen, X.; Xu, R.; Cheng, X.-B.; Peng, H.-J.; Zhang, R.; Huang, J.-Q.; Zhang, Q. Columnar Lithium Metal Anodes. Angew. Chemie Int. Ed. 2017, 56 (45), 14207–14211. (c) Fan, L.; Zhuang, H. L.; Gao, L.; Lu, Y.; Archer, L. A. Regulating Li Deposition at Artificial Solid Electrolyte Interphases. J. Mater. Chem. A 2017, 5 (7), 3483–3492.

Nickel-rich cathodes such as LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) have received considerable attention as a result of their high capacity and relatively low cost. However, owing to high nickel content NCM622 cathodes suffer from reduced cycle life and thermal stability. (1) Traditional approaches for modifying cathode materials have been employed in the past to improve the performance of Ni-rich cathodes, this includes incorporating a surface coating (Al₂O₃, AlF₃, SiO₂) or dopant (Al³⁺, Mg²⁺, F^-) species which acts as a passivating layer between the cathode surface and reactive electrolyte or stabilizes the structure of the material, respectively. (2-9)
The work reported in this poster illustrates NCM622 doped with Al³⁺ during two distinct stages of the materials synthesis, either during co-precipitation of the M(OH)_x species (M = 6:2:2 ratio of Ni, Mn, and Co) using a CSTR (continuous stirred tank reactor) or during the calcination process. The resulting product from both synthetic processes demonstrates the distribution of Al³⁺ throughout the entirety of the NCM622 particles. The presence of Al³⁺ in NCM622 was established via EDX/SEM mapping of a cross-sectioned particle and slight expansions of the crystal lattice observed in XRD which is consistent with the inclusion of Al³⁺ atoms in the transition metal layer. Significant differences are observed in the capacity, rate performance, and thermal stability of the NCM622 cathode depending on whether doping occurred during co-precipitation or calcination. In particular, Al³⁺ doping during the co-precipitation process results in substantial improvements in all measured categories relative to both the pristine NCM622 control and materials doped via calcination. Since there are only trivial changes in particle size, morphology, and composition, this difference in performance appears to be the result of the synthesis methodology.

References
(1) Liu, W.; Oh, P.; Liu, X.; Lee, M.-J.; Cho, W.; Chae, S.; Kim, Y.; Cho, J. Angew. Chem. Int. Ed. 2015, 54, 4440-4457.
(2) Mohanty, D.; Dahlberg, K.; King, D. M.; David, L. A.; Sefat, A. S.; Wood, D. L.; Daniel, C.; Dhar, S.; Mahajan, V.; Lee, M.; Albano, F. Sci. Reports 2016, 6, 26532.
(3) Han, B.; Paulauskas, T.; Key, B.; Peebles, C.; Park, J. S.; Klie, R. F.; Vaughey, J. T.; Dogan, F. ACS Appl. Mater. Interfaces 2017, 9, 14769-14778.
(4) Liao, J.-Y.; Manthiram, A. J. Power Sources 2015, 282, 429-436.
(5) Du, K.; Xie, H.; Hu, G.; Peng, Z.; Cao, Y.; Yu, F. ACS Appl. Mater. Interfaces 2016, 8, 17713-17720.
(6) Zhou, P.; Zhang, Z.; Meng, H.; Lu, Y.; Cao, J.; Cheng, F.; Tao, Z.; Chen, J. Nanoscale 2016, 8, 19263-19269.
(7) Woo, S.-U.; Yoon, C. S.; Amine, K.; Belharouak, I.; Sun, Y.-K. J. Electrochem. Soc. 2007, 154, A1005-A1009.
(8) Hu, G.; Zhang, M.; Liang, L.; Peng, Z.; Du, K.; Cao, Y. Electrochimica Acta 2016, 190, 264-275.
(9) Krishna Kumar, S.; Ghosh, S.; Ghosal, P.; Martha, S. K. J. Power Sources 2017, 356, 115-123.

The current organic electrolytes used for energy storage applications suffer from volatility, flammability, and limited cyclability at elevated temperatures. Ionic liquids being non-flammable and non-volatile are an ideal replacement for the unsafe organic solvents used today. The performance of supercapacitors containing piperidinium and phosphonium based ionic liquids (ILs) with an alkyl ether chain introduced into its structure are compared to the traditional propylene carbonate (PC) electrolyte at 25 ^oC and 100 ^oC. The phosphonium ILs are relatively stable with an electrochemical window of -1 to 5 V at room temperature while the piperidinium ILs are stable only up to 2 V. An activated carbon electrode based supercapacitor with the piperidinium IL electrolyte with 1 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) at 100 ^oC cycles for more than 10000 cycles. There is a decrease of 30% capacitance after 10000 cycles at 100 ^oC for the piperidinium based electrolyte whereas a supercapacitor with PC having 1 M LiTFSI fails at 3200 cycles. A 3.5 X improvement in energy density for piperidinium based supercapacitor is observed upon going from room temperature to 100 ^oC. The wider electrochemical stability window makes the phosphonium IL a better candidate as an electrolyte in lithium ion batteries. Studies on phosphonium ILs as electrolytes in supercapacitors and lithium ion batteries at 25 ^oC and 100 ^oC are ongoing. These successful findings support the basis for the replacement of organic electrolytes with IL based electrolytes in energy storage devices at high temperatures.

Observing Li₂O₂ nucleation and oxidation on various cathode surfaces in Li-oxygen batteries was challenging with three-dimensional porous electrodes. In particular, examining the effect of a catalyst on preventing cathode clogging in porous cathode was hard to investigate visually. Here we study the use of CVD grown graphene in two-dimensional configurations as Li-oxygen battery cathodes to easily examine the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on various cathode surface structures. The composite structure made of metal oxides and carbon surface is created by electron-beam lithography or simple wet transfer method to investigate the Li₂O₂ distribution during Li-oxygen battery discharging and charging on composite cathodes. Furthermore, since CVD graphene can be transferred to different substrates, the Li-oxygen battery cathode can be built by coating the graphene layer on top of structural components. We investigate few possible materials as a substrate for Li-oxygen battery graphene electrodes to demonstrate the easy incorporation of CVD graphene cathode to an existing device architecture. After investigating polymer, ceramic, or metal substrates, we learned that this structural material needs to be electrochemically inactive, and does not attenuate the ORR activity of graphene. In particular, we maximized the Li-oxygen battery discharge capacity of this planar graphene electrode by using the electrolyte formulations that promote the solvation of intermediate discharge products. Thus, not only served as a model cathode, but 2D graphene also presented an improved storage capacity as a potential micro-battery cathode compared to one of the most common lithium-ion insertion based cathodes (e.g., LiCoO₂). We obtained 20 % higher areal cathode energy density and 2.7 times higher cathode specific energy than that can be derived from the same volume or mass of LiCoO₂. In summary, we investigate the Li₂O₂ distribution during ORR and OER and demonstrate the fabrication of micron scale devices where CVD grown graphene structures are used as 2D Li-oxygen battery cathodes. We believe that two dimensional CVD graphene cathode can facilitate the microscale device fabrication by simply transferring or coating the target device structure with flexible graphene layers to integrate the energy storage component without complications.

Lithium-sulfur (Li-S) batteries have attracted considerable attention as one of the most promising next-generation energy storage system owing to the high theoretical specific capacity (1675 mAh g^-1), low cost, abundance and environmentally benign nature of sulfur. However, the poor electrical conductivity of elemental sulfur results in low utilization of sulfur specific capacity, and hence, insufficient specific energy of Li-S batteries. Selenium disulfide (SeS₂), with a theoretical specific capacity of 1342 mAh g^-1, is a promising cathode material as it has attractive merits over elemental sulfur. Because of the better conductivity of SeS₂ than S, the electrochemical reaction kinetics of S/SeS₂ are expected to be remarkably improved. Herein, to combine the high conductivity and higher density of SeS₂ and high specific capacity of elemental sulfur, we investigated hierarchical architectures of carbon as effective hosts for S and SeS₂. By a facile one-step melt infiltration method, we synthesized S/SeS₂@carbon nanofibers (CNFs) composites in which S and SeS₂ were uniformly distributed over the CNFs. The hierarchical S/SeS₂@CNFs (S:SeS₂, 1:2 w/w) electrodes shows good initial discharge capacity of 940 mAh g⁻¹ at 0.1C rate with high mass loading of material (~6-8 mg/cm² of composites) and > 90% initial coulombic efficiency. The composite electrode shows more than 830 mAh g^-1 specific capacity after 100 charge-discharge cycles at 0.1C rate.

Silicon (Si) is one of the most promising materials for next-generation Li-ion battery anode since Si has a high gravimetric capacity (3570 mAh g^-1) and a relative low lithiation and de-lithiation voltage (0.4 V vs Li/Li⁺). However, the major challenge when applying Si as anode material in Li-ion battery is its poor cycle performance. It can be attributed to drastic volume expansion of lithiated silicon (300% theoretical expansion). Approaches such as yolk-shell silicon-carbon or silicon-titanium dioxide composites have been proposed to improve the cycle stability of Si-based electrode. Basically, void space was created by forming a carbon or titanium dioxide layer with larger volume, which accommodates the expansion of Si during lithiation and improves cycle performance. However, these approaches do not fundamentally reduce the intrinsic lattice expansion of the Si. The larger volume in fact reduces the overall volumetric capacity of the electrode. Therefore, there is a need for establishing an approach that reduces the intrinsic Si expansion while keeping the high volumetric capacity of Si. Herein, we study the effect of Titanium (Ti) addition to the volume change of Si electrode during lithiation and de-lithiation.
Si-Ti thin films with different Ti contents (ranging from 43-100 at%) were made by RF magnetron sputter deposition (co-sputtering) under vacuum condition on copper foil. No binder and conductive materials were added into the electrode. The resultant Si-Ti thin film is dense and is firmly deposited onto the foil. Therefore, the intrinsic electrode thickness variation can be monitored by in-situ dilatometer.
100 at% Si film electrode shows a 1^st discharge capacity of 3415 mAh g^-1 which is close to the theoretical capacity of Si (3570 mAh g^-1). However, its cycle stability is poor and the capacity fades linearly for the first 10 cycles. At the 15^th cycle, no capacity can be obtained. Addition of Ti into the thin film reduces available capacity but improves cycle performance. A film with 20 at% Ti gives a capacity of 1500 mAh g^-1, and the capacity retention is about 95% after 50 cycles with a high average coulombic efficiency of 99.58% (from 3^rd to 50^th cycle).
To understand the role of the Ti inside the Si matrix, the thickness change of the thin film during lithiation and de-lithiation is monitored by in-situ dilatometry. While 100 at% Si film electrode exhibits a thickness increase of 345% after full lithiation, the thickness change is reduced significantly to 130% when 20 at% Ti is added. Furthermore, Ti addition allows full mechanical reversibility after Li removal. Moreover, Raman spectroscopy shows that the interaction between Si and Ti is still observed after cycling, suggesting that Ti is playing an active role to maintain the integrity of the Si electrode. More results on Si-Ti thin films will be presented at the meeting.

There is a pressing need of new and advanced and cost effective stationary energy storage systems in concomitance with the fast development of solar, wind and other types of renewable sources of energy. The capability of high-rate cycling and environmental benignity at less expensive defines the ingress of upcoming energy storage systems. Multivalent ion batteries are emerging as a promising alternative of Li ion technology due to their natural abundance, safety, low cost proposition and higher volumetric capacity. But these batteries are still challenged by sluggish cation diffusion in the electrode material and high polarisation of the respective cations (Mg²⁺ Zn²⁺ Al³⁺). Calcium ion on the contrary have low polarisation compare to Li-ion, thus enjoying the benefits of overcoming the kinetics issues in large. Here, we introduce a reversible electrochemistry of Ca-ion cell in conjunction with inexpensive aqueous electrolyte, 1 M aqueous Ca(ClO₄)₂. Prussian Blue analogue barium hexacyanoferrate (BaHCF) cathode half-cell provides a capacity of 70 mAh/g with around 93% reversibility and 97% discharge capacity retention after 200 cycle was observed. In full cell, carbon cloth, BaHCF and meso-carbon microbeads (MCMB) have been explored as the current collector, cathode and anode material, respectively. The full cell provides 40 mAh/g (based on the mass of cathode active material) capacity at 5C rate till 100 cycles. We believe, the investigation of this simple full cell at the early stage of Ca-ion battery that will pave a fast transition in the forthcoming energy storage systems.

Electrochemical energy storage is highly sought-after for mankind owing to increasing concerns regarding renewable energy resources.¹ Albeit, invincible in the domain of battery research, Li−ion based materials possess some major drawbacks regarding the limited abundance, thermal runaway, expensiveness and environmental issues which refrains their inordinate usage.² To unravel these intricacies, researchers have successfully employed sodium as a potential alternative which has significantly revitalized the field of battery research. Still, the discovery of Na−ion electrodes is basically centralized on the imported “know−how” knowledge from Li-ion chemistry. Primarily the layered oxides and polyanionic compounds have shown a long-standing impact on Na-ion battery (SIB) cathodes.^3,4 Among all those myriads of compounds, the transition metal fluorides scintillate the structural, as well as electrochemical aspects due to the rich crystal chemistry, lies within, especially when it comes to the case of Iron-based fluorides which are inexpensive and consisted of earth−abundant precursors. For instance, ReO₃−type FeF₃ and NaFeF₃−based electrodes were reported to have good reversible capacity in SIBs.⁵ But this rich crystal chemistry can only be probed if suitable synthetic strategies are performed, by careful modulation of the spatial arrangement inside the crystals. The extreme challenge is to manipulate the homogeneous connectivity between the anions without destroying the main crystalline matrix itself. This can be achieved by the unique methodology known as ‘ Topochemical synthesis’ which allows the transformation of the precursor attaining topological resemblance with the final product.
In this work we focus on designing a successful direct bottom-up pathway to synthesize both the 2D monoclinic MFeF₄ and 3D orthorhombic M₂Fe₂F₇ (M = alkali metal) from 1D−fluoride precursor at low temperatures, utilizing a blend of topochemical approaches. Notably, the linear 1D chains originating from the precursor have been preserved throughout the topochemical conversion of 1D → 2D → 3D frameworks. In fact, the as−synthesized orthorhombic weberite M₂Fe₂F₇ has shown exquisite cycling stability and also the reversible capacity of ∼55 mAh/g with low polarization. Hence, we have designed a new topochemical strategy by which multidimensional fluoride structures can be developed. This study is expected to open a new avenue on the topochemical synthesis where more than one approaches can be simultaneously used to convert low dimensional solids to a higher one.
References
1. Armand, M. et al. Nature 2008, 451, 652.
2. Ye, H. et al. Adv. Energy Mater. 2017, 7, 1700530.
3. Palomares, V. et al. Energy Environ. Sci. 2013, 6, 2312.
4. Masquelier, C. et al. Chem. Rev. 2013, 8, 6552.
5. Li, C. et al. Chem. Mater. 2013, 25, 962.

Rechargeable aqueous zinc ion batteries have been considered as reasonable substitutes for current primary batteries, since zinc manganese dioxide batteries were proved to be reversible in mild-acidic electrolyte. Recently, manganese dioxide polymorphs have attracted extensive attention as cathode materials due to zinc intercalation in their host tunnel or layer structures. Although phase-transfers from tunnel to layer or spinel have been reported, the complex intercalation chemistry remains ambiguous, especially the effect of relative physical properties, such as electronic conductivity, tunnel size, morphology and porosity. The focus of this study will be a variety of porous nanostructured manganese dioxide with tunnels ranging from 1 X 1 (pyrolusite), 1 X 2 (ramsdellite), 2 X 2 (cryptomelane), 2 X 3 (romanechite), 2 X 4 (RUB-7). Our study shows zinc ion selectively yield an appropriate tunnel structure (1 x 2 or 2 X 2) based on current density. Urchinlike morphology and mesopore are the key to enhance the battery performance as well.

Recently, rechargeable graphene-based aluminum-ion batteries (AIBs) have been reported. Owing to the requirements such as high conductivity and low defects, the graphene used in AIB is typically fabricated using chemical vapor deposition (CVD). Here, we utilize solution processable microwave reduced graphene oxide (MWrGO) that has been shown to be of very high quality and low defect density [1] as cathodes in AIBs. Our results show that it is possible to increase the AlCl₄^– ion storage with MWrGO relative to thermally reduced graphene oxide. In particular, we have fabricated binder-free MWrGO film cathodes by directly microwaving the slightly reduced graphene oxide film. These MWrGO cathodes for AIBs show stable discharge capacity of ~88 mA h g^–1 at 200 mA g^–1 with a coulombic efficiency of > 96% and rate capability that is able to withstand more than 4500 cycles without capacity decay. We have also examined the relationship between microwave time and AlCl₄^– ion storage capability to elucidate the fundamental relationship between AlCl₄^– intercalation and defects in graphene.

Reference
[1] Voiry, D.; Yang, J.; Kupferberg, J.; Fullon, R.; Lee, C.; Jeong, H. Y.; Shin, H. S.; Chhowalla, M., Science 2016, 353 (6306), 1413.

Lithium-sulfur batteries are considered as a promising alternative to commercial Li-ion batteries due to their unique high specific energy of ~2680 Wh/kg and cost-effectiveness, environmentally friendliness and natural abundance of sulfur. These distinctive features make them suitable for hybrid/electrical vehicles. However, there are various practical challenges such as shuttling of intermediate polysulfides, insulating nature of Li₂S₂/Li₂S/S and volume expansion (~80%) during conversion reactions which are plaguing the commercial development of Li-S batteries. Over the years, various approaches striving to deal with such issues are reported. These approaches include the use of different conducting host materials, additives for electrolytes, sulfur confinement into micropores, interlayers and lithium protective layers.

In this work, we first report the use of various non-stoichiometric polar metal monoxide nanofibers as host materials in Li-S batteries. It is shown that these oxides have a significant amount of oxygen and Mⁿ⁺ (M: metal) vacancies in their both sub-lattices due to their NaCl-type cubic crystal structures. The presence of vacancies in their lattices provides more active sites for polysulfide trapping through Lewis acid-base interactions which minimize the shuttle effect in Li-S batteries. Second, we will discuss the critical role of porosity and surface area of the host matrix in additive-electrolyte based Li-S batteries, which is generally overlooked. It is shown that Li-S batteries with non-porous host matrix will inevitably involve the unsought diffusion of intermediate polysulfide-additive complex species into the electrolyte which result in short cycle life. Third, we will discuss how sulfur can be organically modified and serve as an active material for achieving long-term cycling in Li-S cells. The critical role of the organic component in modified sulfur for attaining long cycle life will also be discussed in details.

Oxygen redox has garnered intense interest as a means to increase the energy density of transition metal (TM) oxide positive electrodes in lithium ion batteries, as it enables additional lithium (de)intercalation capacity at high voltages beyond the usual TM redox capacity. However, most oxygen-redox-active materials discovered to date suffer from large charge-discharge voltage hysteresis and irreversible voltage fade over extended cycling, limiting their practical use. Several hypotheses have been proposed to explain the nature of the oxidized oxygen species in order to guide improvements to the electrochemical properties of oxygen redox. The general consensus is that in more ionic oxides (e.g. those with certain 3d TMs), reactive and unstable O^– species are created when oxygen is oxidized, whereas in more covalent systems (e.g. those with 4d and 5d TMs) the increased hybridization allows for the formation of stabilizing, long (~ 2.3 Å) O₂^n– dimers. Accordingly, recent work has attempted to tune the covalency of the TM–O interactions to improve the reversibility of oxygen redox in 3d materials, but has found limited success. In this presentation, I propose that consideration of only electronic structure properties is insufficient to explain the electrochemical behaviors associated with oxygen redox, and that the intrinsic coupling of oxygen redox to defect formation and cation disordering is in fact more significant in determining its reversibility. I first show that in the commercially promising lithium-rich Ni/Mn/Co oxides, oxygen oxidation occurs simultaneously with migration of TMs into Li sites, forming TM_Li defects in the material bulk. By drastically altering the local oxygen coordination environment, these defects lower the oxygen redox voltage, which we observe experimentally to fall by ~ 1V after the first charge. This redox voltage shift is a major driver of the charge-discharge voltage hysteresis in oxygen-redox-active materials and cannot be explained without considering the link between oxygen redox and structural evolution. By then investigating two model systems, I reveal the origin of the correlation between oxygen redox and TM_Li formation, wherein the oxidation of oxygen is not sufficiently stabilized by either long (~ 2.3 Å) O₂^n– dimers or O^– species, and instead promotes the formation of short (~ 1.4 Å) O₂^n– dimers and short TM=O π bonds. These species require the decoordination of oxygen to single-TM-coordinate, which is realized in the layered structure through the formation of TM_Li defects. I show spectroscopically that these defect-localized oxidized oxygen species exist in 3d, 4d, and 5d systems, suggesting that the electrochemical properties of oxygen redox in TM oxides may depend more on the structural mechanism of oxygen decoordination than the identity of the TMs. Finally, I use this understanding to propose a new pathway to achieving reversible oxygen redox by employing new structures outside the layered framework.

Significant research efforts have focused on improving the specific energy of lithium-ion batteries for emerging applications, such as electric vehicles and renewable energy resources. A rocksalt-type nanostructured Li₄Mn₂O₅ cathode material has been recently discovered that exhibiting a large discharge capacity of >350 mAh g^-1, involving cationic (Mn³⁺/Mn⁴⁺/Mn⁵⁺) and anionic (O^2-/O^-) redox processes. However, the detailed structure of Li₄Mn₂O₅ and its corresponding phase transformation, as well as the origins of higher capacities are poorly understood. In this work, we use first-principles density functional theory (DFT) calculations to investigate both the disordered rocksalt-type Li₄Mn₂O₅ structure [using the special quasi-random structure method (SQS)] and also the ordered ground state structure. The ionic ordering in the ground state structure is determined via a DFT-based enumeration method, by exploring symmetrically-distinct Li/Mn and O/vacancy ionic configurations on the cation and the anion sites in the rocksalt structure, respectively. We use both the ordered and disordered structures to interrogate the delithiation process, and find that it occurs via a three-step reaction pathway involving the complex interplay of cation and anion redox reactions: i) an initial metal oxidation, Mn³⁺ → Mn⁴⁺ (Li_xMn₂O₅, 4 > x > 2), ii) followed by anion oxidation, O²⁻ → O¹⁻ (2 > x > 1), and iii) finally, further metal oxidation, Mn⁴⁺ → Mn⁵⁺ (1 > x > 0). This final step is concomitant with Mn migration from the original octahedral site to the adjacent tetrahedral site, introducing a kinetic barrier to reversible charge/discharge cycles. Armed with this knowledge of the charging process, we utilize high-throughput DFT calculations to study metal mixing in this compound, screening potential new materials for stability and kinetic reversibility. We predict that mixing with M = V and Cr in Li₄(Mn,M)₂O₅ will produce new stable compounds with substantially improved electrochemical properties. Our discoveries should generate considerable interest in the experimental battery community and give guidance to experimental studies of simultaneous cation/anion redox in high-energy-density electrodes.

1. Yao, Z., Kim, S., He, J., Hegde, V. I. & Wolverton, C. Interplay of Cation and Anion Redox in Li4Mn2O5 cathode material and Prediction of Improved Li4(Mn,M)2O5 Electrodes for Li-ion Batteries. Sci. Adv. 4, eaao6754 (2018).

Ni-rich NMCs (LiNixMnyCozO2; x≥y+z) are emerging as the next generation cathodes of choice for lithium-ion batteries intended for electric vehicle battery applications. While these materials have the potential of raising energy density due to higher practical capacities than NMCs with lower Ni content, they also suffer from abbreviated cycle life and thermal stability issues, particularly at high states-of-charge. To better understand what determines the thermal characteristics, we have prepared Ni-rich NMC materials with varying amounts of lithium content using the chemical oxidant NO2BF4 and heated them. Our in situ XRD studies show that there is a general trend to lower thermal stability with respect to phase conversion as lithium content decreases or nickel content increases, as well as a tendency to form products containing progressively more reduced transition metal ions and lower oxygen contents. While hard XAS Ni, Co and Mn K-edge experiments confirm this trend, showing that reduction occurs upon heating, soft XAS, which probes surfaces, portray a more complicated picture. In some cases, metals on surfaces oxidize although they are reduced in the bulk. Transmission X-ray microscopy (TXM) experiments also show evidence of Ni migration upon heating for some of the materials.
While modifications such as partial substitution or coatings may improve the thermal stability of Ni-rich NMCs in conventional cells, a longer term strategy may be to design completely solid state cells containing non-flammable ceramic electrolytes such as variants of LLZO (Li7La3Zr2O12), to ameliorate safety hazards associated with thermal stability issues. In this talk, we will briefly discuss the properties of LLZO solid electrolytes and our strategies for the design of high-energy all solid-state lithium cells using LLZO and Ni-rich NMC cathode materials.

Commercialization of lithium-ion batteries began in 1991 by Sony with development of LiCoO2 cathode coupled with graphite anode. Since then cobalt constitute an important role in stabilizing current commercial cathodes for lithium-ion whether it is LiNixMnyCo1-x-yO2 (NMC) or LiNi0.85Co0.15Al0.05O2 (NCA). Apart from battery application, cobalt is widely used in electronics and magnetic recording industry with limited reserves and supplies worldwide. With growing market demand for advance lithium-ion for both automotive and stationary storage, there is an increasing research activity in developing cathodes with minimal or no cobalt content. The talk will highlight some recent progress in stabilizing Ni and Mn based lithium excess composition using early transition metal such as Mo and Cr. Early stage electrochemical results and oxygen stability at high voltage (> 4.4 with respect to Li+/Li) of these composition will be presented. Recent progress related to compositional and interfacial modifications such as creating cation disorder, doping/substitution, surface coating to attain high oxygen stability and reversible anion redox will be discussed.
Acknowledgement This research performed at Oak Ridge National Laboratory, is managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) under contract DE-AC05-00OR22725, is funded by Asst. Secretary, Energy Efficiency and Renewable Energy (EERE) Vehicle Technologies Office (VTO).

Among the various cathode materials pursued over the years for lithium-ion batteries, layered LiMO₂ (M = Mn, Co, Ni, and their solid solutions) oxides offer the highest energy density with an operating voltage of around 4 V. Each of the three transition-metal ions (Mn, Co, and Ni) involved in LiMO₂ has its advantages and disadvantages with respect to structural stability, chemical stability, cost, electronic conductivity, and environmental impact. The position of the metal:3d redox energies with respect to the top of the oxygen:2p band determines the chemical stability with respect to oxygen release from the lattice. On the other hand, the relative stability of the transition-metal ions in the octahedral vs. tetrahedral sites in the cubic close-packed oxygen array controls the structural stability. Since Ni³⁺ can be oxidized fully to Ni⁴⁺ without releasing oxygen from the lattice unlike Co³⁺, which can be oxidized only to ~ 3.5+ to avoid oxygen release from the lattice, and Ni^3+/4+ ion exhibits high stability in octahedral sites, there has been tremendous interest to develop layered LiMO₂ cathodes with high nickel contents. However, cathodes with high Ni contents suffer from poor cycle life, thermal stability, and air stability.

This presentation will focus first on developing a fundamental understanding of the complexities that control the capacity fade, thermal stability, and air-reactivity of high-nickel layered oxide cathodes, employing samples with secondary particle sizes of ~ 10 microns and advanced bulk and surface characterization methodologies. In-depth understanding obtained with high-nickel layered oxide cathodes with Ni contents of as high as 94% and graphite anodes retrieved from full cells before and after thousands of cycles based on a combination of characterization techniques, viz., X-ray photoelectron spectroscopy, time-of-flight – secondary ion mass spectroscopy, and high-resolution transmission electron microscopy, will be presented. Based on the profound understanding gained, the presentation will then concentrate on the design and development of layered oxide compositions with controlled bulk and surface structures as well as novel electrolyte solutions that are compatible with both the high-nickel layered oxide cathodes and graphite anodes to realize a robust electrode-electrolyte interphase. Based on the findings, viability to realize lithium-ion cells with cathode capacities of > 220 mA h g^-1, high energy density, high power capability, and long cycle life will be discussed. Finally, approaches towards designing low-cobalt or cobalt-free cathode compositions will be pointed out.

The ever-increasing demand for renewable energy storage and electrifying transportation vehicles calls for developing high energy density electrode materials for rechargeable alkali-ion batteries. As one of the most important components of energy storage devices, cathode materials, such as alkali-ion containing layered oxide compounds play a crucial role in determining the energy density of batteries. Major efforts have been invested to push the cathode capacity approaching its theoretical limit while reducing the cost of raw materials. However, novel design strategies must be considered to reach the envisioned goal. Three-dimensionally homogeneous or gradient distribution of transition metals has been acclaimed the design principle for obtaining good performing layered oxide cathode materials. Counterintuitively, we discovered that highly heterogeneous cathode materials at multiple length scales can give rise to unexpected good battery performance. Such phenomenon is equally pertinent for multiple types of layered alkali-ion cathode materials (Li-ion, Na-ion, and K-ion). The heterogeneous nature of the cathode materials was characterized through variously advanced spectroscopy and imaging techniques (STEM, TXM, SEM, EDX, and XAS). Batteries assembled using such cathode materials in half/full cells with high-mass loading delivered unexpected excellent and stable electrochemical performance. The results show the promise of heterogenizing the elemental distribution of multicomponent alkali-ion layered oxide materials and open up a new pathway to further develop the layered oxide cathode for high energy and stable alkali-ion batteries.
Reference:
1. Mu et al. Multiscale Heterogeneous Stoichiometric Layered Cathode Materials for Stable and High Energy Lithium Batteries, submitted;
2. Rahman et al.Empowering Multicomponent Cathode Materials for Sodium Ion Batteries by Exploring Three-Dimensional Compositional Heterogeneities, Energy Environ. Sci. 2018, accepted.

The instability of LiCoO₂ layered structure at >0.5 Li extraction has been considered as an obstacle for the reversible utilization of its near theoretical capacity at higher voltage cut-offs in lithium-ion batteries (LIBs). To date, much of previous researches have been focused on the surface engineering of LiCoO₂ to extend its usable state-of-charge (SOC) range, which has proven to be effective in suppressing the phase transformation. Herein, in contrast to conventional belief, we verify the superior reversibility of bulk LiCoO₂ with extended lithium extraction by ruling out the effect of damaged surface. A high-voltage cycling of uncoated LiCoO₂ at 4.8 V (vs. Li/Li⁺) cut-off offers unexpectedly better cycle stability and lower polarizations than those at 4.6 V (vs. Li/Li⁺). In detail, it is revealed that the rapid cycle degradation at high-voltage cycling is mostly caused by the formation of a surface resistive layer composed of spinel phase, however these damaged surfaces are leached out at 4.8 V enabling its superior cycleability to 4.6 V-cycling. In the absence of the resistive surface, the capacity retention of LiCoO₂ electrode with 4.8 V cut-off cycling could be remarkably high. This work decouples the effects of the intrinsic instability of highly delithated LiCoO₂ and the surface degradation on the capacity fades toward practical high-voltage cycling and proposes that a rational strategy against the formation of the resistive phases would be a critical step for the full utilization of LiCoO₂ with higher cut-off voltage cycling.

Severe safety concerns are impeding the large-scale employment of lithium/sodium batteries. Conventional organic electrolytes are highly flammable and volatile, which may cause catastrophic fires or explosions. Efforts to introduce flame-retardant solvents into the electrolytes have generally resulted in poor charge-discharge cycleability, because those solvents do not suitably passivate carbon-based negative electrodes. Here we report salt-concentrated electrolytes^1-5 to resolve this dilemma. Our previous works demonstrate that concentrated electrolytes can form a salt-anion-derived passivation film via the downward shift of anion’s LUMO level resulting from the extensive anion coordination to Li⁺.^1,2 Applying this strategy to a popular flame-retardant solvent (trimethyl phosphate), we demonstrate that fire-extinguishing concentrated electrolytes, without any additive or soft binder, allow stable charge-discharge cycling of hard-carbon and graphite negative electrodes for more than 1,000 cycles (over one year) with negligible degradation; this performance is comparable or superior to that of conventional flammable carbonate-based electrolytes.⁶ The unusual interphasial character of the concentrated electrolytes coupled with their fire-extinguishing property contributes to developing safe and long-lasting batteries, unlocking the limit toward development of much higher energy-density batteries.

1. Y. Yamada et al., J. Am. Chem. Soc., 136, 5039 (2014).
2. K. Sodeyama and Y. Yamada et al., J. Phys. Chem. C, 118, 14091 (2014).
3. Y. Yamada et al., J. Phys. Chem. C, 114, 11680 (2010).
4. Y. Yamada et al., Nat. Energy, 1, 16129 (2016).
5. J. Wang and Y. Yamada et al., Nat. Commun., 7, 12032 (2016).
6. J. Wang and Y. Yamada et al., Nat. Energy, 3, 22 (2018).

Anionic redox reactions in the cathodes of lithium-ion batteries are enabling opportunities to double or even triple the energy density. However, it is still challenging to develop a cathode exploring the anionic redox for real-world applications using earth-abundant materials, due to the limited strategy to intercept the oxygenates from further irreversible oxidation to O₂ gas. Here we report simultaneous iron and oxygen redox activity in a Li-rich anti-fluorite Li₅FeO₄ electrode. During the removal of the first two Li ions, the oxidation potential of O^2- is lowered to approximately 3.5 V vs. Li⁺/Li⁰, at which potential the cationic oxidation occurs concurrently. These anionic and cationic redox reactions show high reversibility without any obvious O₂ gas release. Moreover, this study provides an insightful guideline to design high-capacity cathode with reversible oxygen redox by simply introducing oxygen ions coordinated exclusively by Li⁺.

The promising application of high Ni-content layered oxide materials in Li-ion batteries has attracted increasing attention from the research community. Compared to traditional layered oxide materials such as LiCoO₂ (LCO) or LiNi_1/3Mn_1/3Co_1/3O₂ (NMC333), high Ni-content materials can provide higher specific capacity and energy density. However, capacity fading and thermal runaway remain the major issues for high Ni-content layered oxides. In order to overcome these challenges, the in-depth understanding of the relationship between the structural changes and perform deterioration of high-Ni-content materials during charge-discharge cycling are critically needed. Using multi-modal and multi-scale TEM and TXM imaging techniques, the structure stabilization mechanism has been thoroughly studied to provide valuable guidance for future material design. The results obtained from our newly developed multi-scale characterization techniques will be reported, including the state-of-the-art aberration-corrected scanning transmission microscopy (STEM) imaging, STEM- electron energy loss spectroscopy (EELS), transmission X-ray imaging(TXM), X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS). These results show that such multi-modal and multi-scale characterization tools can help us to study the structural changes of high Ni content materials during cycling. The structure degradation is highly correlated to the irreversible cation migration and intermixing at atomic level during extensive cycling. Also, we found that the surface plays a vital role in these materials.
Acknowledgement: Dr. Z. Shadike, Dr. S. Bak, and Dr. X.-Q. Yang were supported Vehicle Technologies Office, Office of Energy Efficiency and Renewable Energy, the U.S. Department of Energy, through the Advanced Battery Materials Research (BMR) Program, including Battery500 Consortium, under contract number DE-SC0012704. The TEM studies were supported by the Center for Functional Nanomaterials, which is a U.S. Department of Energy Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. This research used the Hard X-ray Nano-probe Beamline of the National Synchrotron Light Source II, a U.S. Department of Energy Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. Dr. Y. Shin was supported by the Vehicle Technologies Office, Office of Energy Efficiency and Renewable Energy, the U.S. Department of Energy, under Contract No. DE-AC02-06CH11357.

Multi-modal correlative microscopy has been used extensively through the last decade in various research fields in order to overcome optimization challenges of complex materials systems. The multiscale aspects of these new functional or structural materials, such as nanostructured energy storage systems, and the complexity of their physical interactions, require new multi-modal and multiscale characterization tools in order to shed light on pertinent parameters controlling the process at a given scale, and the link between various scales.

3D Correlative microscopy allows simultaneous quantification of structural, morphological and compositional spatial distributions throughout scales, from which constitutive governing laws and effective properties can be derived.
Main obstacles to its wide range usage are the simultaneous tracking of the sample within various instruments and the correlation between data sets obtained on dispersed zoomed-in ROIs ( region of interest ) within the sample. As a result, 3D correlative microscopy remains a time consuming, cumbersome , non-reproducible and limited-precision methodology.

In this abstract, an innovative procedure for multi-scale µCT ( X-ray microtomography ) and multiple volume-based ROIs with FIB/SEM is proposed. The procedure is applied to cutting edge NMC type ( LiNi_xMn_yCo_1-x-yO₂ ) Li Ion batteries in order to study the effects of both defects and morphology on the batteries transport properties such as grain connectivity, porous media percolation and effective diffusivity.
This will allow nearly automatized data correlation and fusion between structural information provided by µCT with the morphological and compositional information obtained by FIB/SEM imaging at both micro and nano scales.

Two battery samples have been considered for this correlative study. First sample consists of a micro-layer of NMC type cathode material on an aluminum substrate. Micro-nano delamination features and cathode phase grain connectivity morphologies have been studied.
Localized electric conductivity and effective diffusivity have also been calculated. Second sample considered is a LG MJ1 cell with the NMC type cathode and a Graphite/Si hybrid anode. Both structural and compositional anode/electrolyte and cathode/electrolyte interfacial distributions have been studied in micro and nano scales.

It is commonly recognized that utilization of oxygen redox is an intriguing route for obtaining higher capacity of Li-ion batteries (LIBs). Despite numerous experimental and theoretical attempts to unravel the mechanism of oxygen redox behavior, the electronic origin of oxygen activities in energy storage of Li-rich LIB materials remains under intense debate. In this work, the onset of oxygen activity was examined using a Li-rich antifluoride material that has been reported to exhibit oxygen redox, namely Li₅FeO₄. Ab-initio Molecular Dynamics (AIMD) simulations were performed to investigate the structural response of oxygen matrix to delithiation. The oxygen K-edge X-ray absorption near-edge spectra (XANES) were modeled using Bethe-Salpethe Equation (BSE) approach and compared with experiments, from which the oxygen redox mechanism was uncovered.

There is increasing interest in utilizing lithium-excess cathode materials as new generation energy storage material instead of traditional layered materials such as LiCoO₂ and LiNi_xMn_yCo_1-x-yO₂ because of their ultra-high charge capacity. However, lithium-rich materials still suffer problems such as low coulombic efficiency for the first cycle, voltage and capacity fading with extended cycling. Here we report the investigation of a promising high-capacity lithium-rich 3d-4d transition-metal layered compound. The incorporation of 4d transition metals here offers an uncharted phase space for mechanistic exploration as compared to the well documented 3d transition metal (TM) oxides. Utilizing state-of-the-art tools, we found that that a three-dimensional porous structure is formed in this material and the 3d and 4d transition metals can segregate at the sub-micron scale after extended cycling. More surprisingly, at the nanoscale, we found that the 4d metal is expelled from the surface. In conjunction with ab initio thermodynamics calculations, this study for the first time reveals the intricate connection between the instability of the surface and the degradation of the layered cathode materials. More importantly, the revealed mechanism allows us to provide predictive guidance for future design of lithium-rich as well as stoichiometric layered cathode materials.

This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.

The growth of the electrical vehicle market has led to a significant interest in developing safe high-capacity batteries with good cycling properties. Lithium-rich layered oxides such as Li₂MnO₃ have shown great potential as cathodes in Li-ion batteries, mainly because of their large capacities which have been demonstrated to be caused by a cumulative cationic and anionic redox activity. However, these materials still suffer from structural degradation as the battery is cycled, reducing the average voltage and capacity of the cell. This voltage fade is believed to be related to the migration of transition metals into the lithium layer, which results in a transformation of the structure to a spinel-type phase. Recent theoretical works have linked the migration of transition metals to the formation of O-O dimers with a short bond length, driven by the presence of oxygen holes due to the participation of oxygen in the redox process. However, so far such studies have been limited to fully charged structures, which are inherently very unstable and difficult to achieve in practice. Moreover, very few results in the literature have studied the O1 stacking of the layered structure, to which Li₂MnO₃ is expected to transform when the cathode is delithiated by 75%. We investigate the formation of O-O dimers for partially charged O1-Li_0.5MnO₃ using a first-principles density functional theory approach by calculating the thermodynamic driving force for dimer formation, as well as the kinetic barriers. Next, we perform similar calculations for partially charged Li₂IrO₃, a Li-rich material for which the voltage fade was not observed during cycling. When we compare the stability of the oxygen framework, we conclude that the formation of O-O dimers is both thermodynamically and kinetically viable for O1-Li_0.5MnO₃. For O1-Li_0.5IrO₃, on the other hand, we observe that the oxygen lattice is much more stable, either returning to its original state when perturbed, or resulting in a structure with an O-O dimer that is much higher in energy. This can be explained by the lower participation of oxygen in the redox process for Li₂IrO₃, which is also clear from the calculated magnetic moments on oxygen. The lack of O-O dimer formation in O1-Li_0.5IrO₃, as well as its connection to the migration of transition metals, provides valuable insight as to why Li₂IrO₃ does not demonstrate a voltage fade as the battery is cycled, which can be used to design Li-rich battery cathodes with an improved cycling performance.

Lithium-ion batteries have emerged as dominant power sources for portable electronics and vehicular applications. However, the ever-growing market has put higher demands in terms of energy and power needs, and hence alternative rechargeable battery systems, for example sodium-ion batteries, are being explored. Recently, there has been growing interest in rechargeable batteries based on an anion shuttle, for example, chloride or fluoride battery that operates at room temperature. With a high theoretical energy density (~ 2500 WhL^-1), chloride ion battery show a noticeable potential as future power source and several proof-of-principles have already been reported. The electrode dissolution of metal chlorides in the electrolyte causes stability issues and thus limits its performance as an efficient cathode material for chloride ion battery. Though several metal chlorides have been studied as cathode material for chloride ion batteries, developing an efficient cathode material that is stable in electrolyte and with lower volume expansion than metal chlorides, still remains a challenge. Metal oxychlorides such as FeOCl, BiOCl, VOCl have recently been explored as cathode material and show better electrochemical performance and stability compared to metal chloride based cathodes. Here, we demonstrate antimony oxychloride as a new cathode material for chloride ion battery. Sb₄O₅Cl₂ microstructures synthesized through hydrothermal route undergoes reversible redox reactions when cycled as cathode for chloride ion battery. With an aim to address the poor conductivity and huge volume expansion of metaloxychloride based cathodes upon on cycling, we prepared Sb₄O₅Cl₂ - graphene aerogel composite (Sb₄O₅Cl₂-GAG) which exhibited improved electrochemical performance, with a stable capacity of ~ 80 mAh g^-1 after 100 cycles. The electrochemical reaction mechanism of antimony oxychloride electrode as cathode material in chloride ion battery has been studied through ex-situ XRD and XPS characterization techniques. The obtained results are promising and demonstrate 3D networked antimony oxychloride/graphene aerogel composite as a potential cathode material for Chloride ion batteries.

Layered Ni-rich Li(Ni_xMn_yCo_z)O₂(x≧0.5, x+y+z=1) or NCM has been considered as one of the most promising candidates for the next-generation Li-ion battery cathodes because of its high energy density. However, its commercial usage is still hindered partly due to the following two critical issues: (1) safety issue that closely relates to significant NCM lattice oxygen release during operation, (2) capacity fading due to the irreversible layered to spinel/rock-salt phase transformation triggered by Ni/Li mixing defect formation [1].

Herein, by means of density functional theory (DFT)-based calculations we study the aforementioned oxygen release phenomena by examining the oxygen vacancy (O_vac) formation energy, taking Li(Ni_0.8Mn_0.1Co_0.1)O₂ or NCM811 as the representative of Ni-rich NCM. We also investigate the Ni/Li mixing defect formation at surface and near-surface regions of NCM811.

Based on the energetics analysis, we found that the bulk and the three low-index surfaces considered in this work viz. (104), (110) and (012) are stable against spontaneous oxygen evolution in the fully lithiated state. Further, our results also suggest that the O_vac formation in the bulk has a high energy while there is a large variation of O_vac formation energy at the surfaces. These results can be mainly attributed to the difference of transition metal (TM) oxidation states distribution at each surface facets and bulk. While the pristine bulk of NCM811 is exclusively composed by Ni²⁺ and Ni³⁺, we observed presence of less stable Ni⁴⁺ ions at the surface and sub-surface regions. Our results suggest that one way to stabilize this unstable Ni⁴⁺ is releasing Oxygen nearby (i.e. the O_vac formation) that subsequently lower the oxidation state to Ni^2+/3+. In the case of Ni/Li mixing defect, we found that the polar (012) surface has more resistance against the Ni/Li mixing as compared to the non-polar (104) and (110) surfaces in which Ni/Li mixing is predicted to spontaneously occur. This can be attributed to the existence of surface Ni²⁺ that can interchange with Li⁺ without further reduction at (104) & (110) surfaces, while on the contrary pristine polar (012) has only Ni³⁺ at its outermost TM layer. Our results indicate that the choice of surface orientation and termination may play an important role in determining the formation of both O_vac and Ni/Li mixing defects.

References
[1] W. Liu, P. Oh, X. Liu, M.-J. Lee, W. Cho, S. Chae, Y. Kim, J. Cho, Angew. Chem. Int. Ed. 54 (2015) 4440.

The maximum power output and minimum charging time of a lithium-ion battery – key parameters for its use in, for example, transportation applications – depend on mixed ionic– electronic diffusion. While the discharge/charge rate and capacity can be tuned by varying the composite electrode structure, ionic transport within the active particles represents a fundamental limitation. Thus, to achieve high rates, particles are frequently reduced to nanosize dimensions despite this being disadvantageous in terms of volumetric packing density as well as cost, stability, and sustainability considerations. As an alternative to nanoscaling, we show that complex niobium tungsten oxides with topologically frustrated polyhedral arrangements and dense μm-scale particle morphologies can rapidly and reversibly intercalate large quantities of lithium. Multielectron redox, buffered volume expansion, and extremely fast lithium transport approaching that of a liquid lead to extremely high volumetric capacities and rate performance as very recently reported in both crystallographic shear structure and bronze-like niobium tungsten oxides[1]. The active materials Nb₁₆W₅O₅₅ and Nb₁₈W₁₆O₉₃ offer new strategies toward designing electrodes with advantages in energy density, scalability, electrode architecture/complexity and cost as alternatives to the state-of-the-art high-rate anode material Li₄Ti₅O₁₂. The direct measurement of solid-state lithium diffusion coefficients (D_Li) with pulsed field gradient NMR demonstrates room temperature D_Li values of 10^–12–10^–13 m²×s^–1 in the niobium tungsten oxides, which is several orders-of-magnitude faster than typical electrode materials and corresponds to a characteristic diffusion length of ~10 μm for a 1 minute discharge. Structural and chemical analysis of high-rate and multi-electron energy storage will be discussed with insights from operando X-ray diffraction and multi-edge X-ray absorption spectroscopy. Materials and mechanisms that enable lithiation of μm particles in minutes have implications for high power applications, fast charging devices, all-solid-state batteries, and general approaches to electrode design and material discovery.

[1] Griffith, Kent J.; Wiaderek, Kamila M.; Cibin, Giannantonio; Marbella, Lauren E.; Grey, Clare P. Niobium Tungsten Oxides for High-Rate Lithium-ion Energy Storage. Nature, 2018, 559, 556–563.

Rechargeable Mg-ion batteries boast a high theoretical volumetric capacity and an absence of dendrite formation, making them a promising candidate for energy storage applications in the electric grid and in transportation. However, many electrolytes which have been developed for the Mg-ion battery chemistry thus far passivate the Mg anode surface or suffer from a low anodic stability or ion conductivity. We report on a Mg-ion electrolyte, based on the weakly coordinating anion [Al(HFIP)4]- in DME, which can reversibly and efficiently deposit magnesium metal and is stable at voltages greater than 3V vs. Mg/Mg2+ on various current collectors. The electrolyte is characterized via a variety of electrochemical methods, including cyclic voltammetry and impedance spectroscopy. The kinetics of Mg-ion transport in the electrolyte vary with its concentration, with optimal ion conductivity achieved between 0.45M and 0.50M. In this range, the electrolyte displays its lowest deposition overpotentials. These overpotentials decrease with cycling, signaling the ability of the electrolyte to break down passivating layers and activate the anode surface. The properties of this electrolyte, which has a simple, scalable, Grignard-free synthesis, suggest interesting directions for furture research.

Because of high capacity and high energy density, lithium air batteries have been studied as next generation secondary batteries. However, reported current density is relatively low and it is necessary to increase the rate of electrochemical reaction inside the battery. At the cathode, an oxygen reduction reaction (ORR) occurs. In the beginning of ORR, oxygen from the gas phase adsorbs to the surface of the catalyst, and further reacts with lithium ions to produce lithium peroxide. In general, a carbon materials are used for the cathode electrode, however, it is desired to add catalysts on it to improve the reaction rate. Therefore, our research aims to select for the optimum ORR catalyst to be used in combination with the carbon material. Focusing on the relationship between the d-band center of catalysts and an adsorption energy of oxygen molecule, a novel material suitable for cathode electrode ORR catalyst in lithium air battery was investigated. In addition, by considering the correlation between electronic state of catalytic materials and ORR, a method for screening better catalyst for any specific reaction was discussed.
Model of oxides was prepared with various surface indices. As a unit cell, the thickness of the vacuum part was set about 40 Å in order to eliminate the interaction between the slabs. For these surface models, structural optimization and energy calculation were performed using a density functional theory (DFT). Oxygen molecule was placed on the surface of the models to determine a stable adsorption structure. From the result of the density of states (DOS), d-band center of the metal oxides was calculated and ΔE, which is the energy difference between 2p orbital of the oxygen molecule and a fermi energy of the oxide is evaluated.
The Fermi levels of the metal oxide, the O-O bond distances in the adsorbed O₂, and the charges of the oxygen molecule were calculated from the DFT. For CdO(101) and CrO₂(001), the distances between O₂ don't change so much, and the change of molecular charge are negligible. On the other hand, the bonding distance of oxygen greatly change for WO₂(111), and the change of the molecular charge of adsorbed O₂ is large. Similar results are observed for IrO₂(010), the oxygen bonding distance and the molecular change also increase. Therefore, it is suggested that these material can be expected as better ORR catalyst. The bonding distances and adsorption energies of oxygen are correlate with ΔE. It was revealed that there is a goal correlation between ΔE and the bonding distance of oxygen and adsorption energy. Consequently, it is suggested that the ORR is predictable by the estimation of ΔE under the assumption that when the bond distance between oxygen is elongated, ORR tends to occur. Similar calculations were carried out for solid models without surface, and similar trends were observed. Other oxide materials are also presented in the conference.

Li metal batteries have been projected to outperform the current graphite anode batteries in both storage capacity and potential (3860mAh/g Li vs. 372mAh/g graphite). However, significant research needs to be performed in order to alleviate potential materials problems of dendrite formation and associated capacity loss and cell shorting before they can be commercially available. Integral to the development of not only Li metal anodes, but all metallic electrodes, is an understanding of the mechanisms of dendrite formation. In this work, the effect of patterned electrodes on the nucleation and growth of dendrites will be examined by atomic force microscopy (AFM), with specific focus on surface potential variations due to patterned geometry. These surface potential variations can then give insight into nonuniform solid electrolyte interphase (SEI) formation.

There are two factors that contribute to the SEI formation that are unique to patterned electrodes: (i) local curvature of the electrode, and (ii) nonuniform electrolyte depletion. Curved conductive surfaces produce higher electric field magnitudes than flatter surfaces for a given applied voltage, and so produce higher local current densities, leading to inhomogeneous Li deposition. This can be inhibitied by the formation of a thicker SEI as a result of this higher current, leading to a suppression of nonuniform current. The second factor, nonuniform electrolyte depletion, also directly affects SEI formation. In a flat, parallel electrode system the concentration of Li ion at every point in the system can be calculated with relative ease for sufficiently low current using Fick’s law. The inhomogeneities associated with a patterned electrode break this simple derivation, often requiring numerical simulation of the diffusion-supplied flux of Li delivered to the electrode surface. In regions where diffusion is insufficient to supply the electrode with Li, a large overpotential (and consequently, electric field) develops; this accelerates electrolyte decomposition, even in “low current” limits, promoting SEI formation and consequently limiting nonuniform deposition. A combination of these effects can lead to suppression of dendritic growth of Li.

We will present results using Kelvin probe microscopy, and Electrostatic force microscopy to visualize variations in the conductivity and/or thickness of the SEI as a function of patterning. Both of these measurements will be conducted on a series of scratches with varying aspect ratio in Li metal anodes. The nucleation and growth of the Li dendrites will be monitored using an in-situ electrochemical cell for the AFM. Together, this information about inhomogeneous or nonuniform SEI formation can be used to inform future efforts in nanopatterned electrodes for Li batteries.

Polymeric binders are a critical component of all composite electrodes used in today's lithium ion batteries (LIB). Binders also play an important role in ensuring the cycling stability of Li-alloy forming anode materials such as Si which are known to degrade due to expansion and contraction during cycling. Meanwhile, the formation of solid electrolyte interphase (SEI) can also exacerbate capacity loss in novel anode materials, however the contribution of the binder to SEI formation and stability of alloying anodes has not been emphasized. It has been suggested that elastomeric binders such as polyvinylidene fluoride (PVDF) could be utilized in order to mitigate the high stress that occurs between particles during cycling. However, experimental studies have shown that the use of carboxymethyl cellulose (CMC) improved both coulombic efficiency as well as cyclic life when compared to elastomeric binders. This result was counterintuitive because CMC is a stiffer material then PVDF and from these results it has been speculated that CMC may alter the Si particle surfaces in order to form an SEI that improves the cyclic life[1]. This phenomenon has not been fully investigated. The objective of this study is to understand the effect different polymer binders on the nature, formation, and location of the SEI layer of silicon based anodes. In this study Si wafers coated with thin films of CMC and PVDF binders were utilized as the active material. Half cells containing either bare crystal Si or a crystal wafer with a thin film of CMC or PVDF were cycled in a standard lithium-ion electrolyte (1 molar LiPF6 in 1:1:1 vol. ratio of EC:DC:DMC) to form a stable SEI. After cycling, the cells were opened in an inert atmosphere and x-ray photoelectron spectroscopy (XPS) was carried out to analyze and compare the surface chemistries. In addition, the morphology of the substrates and binder films were characterized by SEM. Preliminary results indicate that binder layers on the order of hundreds of nm thick do not impede the lithiation and delithiation of Si, but do play a role in the observed coulombic efficiency and SEI composition. These results can help inform the optimization of Si containing anodes in commercial LIB, which are expected to enter the market in the near future.
[1] Chem. Soc. Rev., 2018, 47, 2145

The low energy densities associated with Li-ion batteries compared to Li-metal batteries has necessitated a revisit to the latter. Li metal barriers have remained the attraction since their discovery and early commercialization attempts, mainly because of highest gravimetric capacity (3860 mAh g^-1) of metallic Li, which translates to higher energy density. While as the state of art Li-ion battery delivers the energy density in the range of 200-250 Wh kg^-1, the Li-metal cell (with transition metal oxides as cathodes) can deliver an energy density of 400-450 Wh kg^-1. Moreover, the Li metal anodes form the heart of promising high energy systems like Li-air and Li-S which can go up to 650 Wh kg^-1 to 950 Wh kg^-1 on energy density scale. However, working with metal anodes is a difficult ploy owing to rampant underlying chemistries. The processes like Li dendrite growth, uneven deposition, non-facile Li/electrolyte charge transfer and huge volume changes upon metal reduction (charging), has hampered the progress of this technology towards targeted commercialization.

Herein we demonstrate direct laser writing of interconnected porous network of fluorine-doped carbon nano-onion film (F-CNOF) on copper current collector to stabilize Li metal anode by preventing dendrite formation. The unique morphology of interconnected carbon nano-onions leads to high lithium intake and long-term cyclic stability due to the availability of uniform lithiophilic sites that control the lithium nucleation. The F-CNOF electrode shows a high Li plating capacity of 10 mAh cm^-2 at low overpotential -25 mV. An impressive Coulombic efficiency ~100% was achieved for long 1500 hours corresponding to more than 300 cycles capacity. The first principles DFT calculations show that the high curvature of the nano-dimensional carbon onions (~ 20 nm) achieved herein can significantly enhance the binding energy of Li to the carbon surface, which helps to improve lithiophilicity and long-term stability. A full cell fabricated using Li₄Ti₅O₁₂ as the positive electrode showed cyclic stability of 700 cycles. This impressive performance exhibited by F-CNOF electrodes indicate laser scribing as an efficient tool for direct writing of porous network of carbon nanostructures that can be used as an efficient and stable scaffold to stabilize Li metal anode.

The maximum power output and minimum charging time of a lithium-ion battery – key parameters for its use in, for example, transportation applications – depend on mixed ionic– electronic diffusion. While the discharge/charge rate and capacity can be tuned by varying the composite electrode structure, ionic transport within the active particles represents a fundamental limitation. Thus, to achieve high rates, particles are frequently reduced to nanosize dimensions despite this being disadvantageous in terms of volumetric packing density as well as cost, stability, and sustainability considerations. As an alternative to nanoscaling, we show that complex niobium tungsten oxides with topologically frustrated polyhedral arrangements and dense μm-scale particle morphologies can rapidly and reversibly intercalate large quantities of lithium. Multielectron redox, buffered volume expansion, and extremely fast lithium transport approaching that of a liquid lead to extremely high volumetric capacities and rate performance for both crystallographic shear structure and bronze-like niobium tungsten oxides. The active materials Nb₁₆W₅O₅₅ and Nb₁₈W₁₆O₉₃ offer new strategies toward designing electrodes with advantages in energy density, scalability, electrode architecture/complexity and cost as alternatives to the state-of- the art high-rate anode material Li₄Ti₅O₁₂. The direct measurement of solid-state lithium diffusion coefficients (D_Li) with pulsed field gradient NMR demonstrates room temperature D_Li values of 10^–12–10^–13 m²×s^–1 in the niobium tungsten oxides, which is several orders-of-magnitude faster than typical electrode materials and corresponds to a characteristic diffusion length of ~10 μm for a 1 minute discharge. Materials and mechanisms that enable lithiation of μm particles in minutes have implications for high power applications, fast charging devices, all-solid-state batteries, and general approaches to electrode design and material discovery.

Besides transition-metal redox, anionic solid-state redox has become a crucial constituent in boosting capacities of Li-rich layered cathodes more than 300 mAh/g. Compared with a thorough study of the intercalation mechanism involving oxygen redox in canonical oxides Li₂MO₃ (M = 3d/4d/5d transition metal), Li₃MO₄ series offer a possibility exploring the detailed intercalation chemistry of a compound with high O/M ratio and the role of Li/Ru ordering in triggering oxygen redox. Here two distinctive phases, disordered and ordered, of Li₃RuO₄ (O/M = 4) with various polymorphs in terms of temperature was synthesized to probe the intercalation chemistry and structural changes. A comprehensive study by spectroscopic methods coupled with computations of the electronic structure revealed that the 2.5-3.9 V voltage window evidences an exclusive oxygen redox for delithiation process with irreversible structural evolution followed by Ru^5+/4+ redox while solo reversible Ru^5+/4+ redox accounts for the Li uptake-removal mechanism of 1.5-2.5 V. Furthermore, the ordered polymorph exhibits better electrochemical properties due to the regular ordering of Li/Ru within the transition-metal layer. The investigation demonstrates that the high O/M ratio and inerratic ordering of Li/M could promote the occurrence of the anionic redox, further guiding the design of high capacities Li-rich cathode materials.

As the importance of renewable energy grows, Sodium/β”-alumina(BASE) cell has been recognized as one of the most effective energy storage device because of its high specific energy, high efficiency of charge/discharge and long cycle life. For better operation of Sodium-BASE cell, poor wettability of electrolyte on liquid sodium anode should be enhanced.
Oxygen is a highly reactive element and can reacts rapidly with Alkali metals. Using high affinity between sodium and oxygen, we fabricated ‘sodiophilic’ r-GO sheet for use as a Na-wetting layer in the anode of Na-BASE cell. Thermal reduction and expanding of interlayer gaps of GO sheets triggered by contact with liquid sodium carefully controls the density of oxygen-functional group and produce nano and microscale gaps that host liquid sodium. Liquid sodium can infuse into the interlayer spacing of the r-GO sheets and rapidly cover the entire surface area of the sheet due to the high affinity between Na and residual oxygen-functional group of r-GO and capillary force produced by the nano and microgaps. These characteristics give rise to stable cycling with low overpotential.of Na-BASE battery.

Li-S batteries, one of the most promising candidates of electrochemical energy storage systems due to their advantages of high energy density, low cost and good capacity and so on, have attracted high attention and intense research in last two decades. Despite tremendous efforts, the commercial use of Li-S batteries is still blocked by their poor stability caused by polysulfide shuttle effects and large volume expansion during the insertion of lithium ions¹. Herein, we report a facile strategy to prepare submicron sulfur particles which are coated by phytic acid shell directly via ball-milling method and further, confined by the complexation effect accompanying with modified carbon nanotubes and graphene oxides (GO) and forming a three-dimensional (3D) gel as cathode materials for Li-S batteries. The porous 3D architecture can provide excellent conductivity and buffer volume change as well as suppress dissolutions of polysulfide owing to its continuous electron pathways, large reinforced porous structure and strong physical and chemical absorptions produced by carboxylic and hydroxyl groups, respectively²; Meanwhile, the P hybrid carbon can be further used to improve the reaction kinetics and enhance ionic conductivity³, which can achieve a good long-term life span, high reversible energy density and good rate performance with a very low cost and straightforward approach for Li-S batteries.
Acknowledgements
This work was financially supported by Special Funds for the Cultivation of Guangdong College Students' Scientific and Technological Innovation (pdjhb0450).
Reference
1. Manthiram A, Fu Y, Chung SH, Zu C, & Su YS. (2014). Rechargeable lithium-sulfur batteries. Chemical Reviews, 114(23), 11751-11787.
2. Lyu F, Sun Z, Nan B, Yu S, Cao L, & Yang M, et al. (2017). Low-cost and novel si-based gel for li-ion batteries. Acs Appl Mater Interfaces, 9(12), 10699-10707.
3. Zhang, J., Shi, Y., Ding, Y., Peng, L., Zhang, W., & Yu, G. (2017). A conductive molecular framework derived li2s/n,p-codoped carbon cathode for advanced lithium–sulfur batteries. Advanced Energy Materials, 7(14), 1602876.

The practical application of next generation high energy density storage systems has long been delayed by the unstable lithium (Li) metal anode with problems like uncontrolled Li dendrites growth, low Coulombic efficiency (CE) or short cycle life. Here, we demonstrate a multifunctional and chemically cross-linked composite film that can provide excellent protection for Li metal anode. By a chemical and electrochemical process, this film can help Li metal anode in-situ form a stable organic/inorganic hybrid SEI layer at the interface between the film and deposited Li, which is mainly composed of polymer based organosulfides, inorganic sulfides (LiPS_4+n, Li₂S, Li₂S_n) and Li salts (LiCl, LiF, etc.). Furthermore, this film as a robust and solid layer can provide extra mechanical protection or acting as a material inventory for this in-situ formed robust SEI layer, so that guarantee its long-term cycling stability. Half cells with this composite film show dendrites free Li deposition, high CE and excellent cycling performances at different cycling conditions (e.g. 98.5% CE for over 460 cycles at 2 mA cm^-2 and 2mAh cm^-2). Full cells with either sulfur (S) or LiFePO₄ as cathode and composite film protected Li as anode show clearly enhanced capacity retention and superior cycle life. This study will inspire new pathway to design functional SEI layer for Li metal anode and promote the practical application of rechargeable Li metal batteries.

Lithium sulfur (Li-S) battery, with specific capacity several times that of state of the art lithium ion battery, has become one of the hottest topics of next generation energy storage systems. However, due to the harsh environment inside the Li-S battery and the multi-steps reaction mechanism, the degradation of battery components are more serious than lithium ion batteries. For the sulfur cathode part, the insulating nature of charge/discharge products (sulfur/lithium sulfide) and the dissolution of intermediate reaction products (lithium polysulfides (LiPSs)) in electrolyte with subsequent parasitic reactions lead to low sulfur utilization and poor cycle life. As for the lithium anode side, the uncontrollable lithium dendrite growth is easy to penetrate the separator and cause internal short circuits triggering security issues of the batteries.
In this presentation, to solve the problems of LiPSs dissolution and low cathode conductivity, we rationally design a sulfur cathode structure by depositing few-layer phosphorene nanosheets on a conductive carbon scaffold as LiPSs immobilizer and catalyst to effectively trap LiPSs, improve the cycle life, lower the polarization, and accelerate the redox reaction of Li-S battery. Further, based on the electrochemical tests and first-principles density functional theory, an outlook will be given on how to choose LiPSs' immobilizers in the Li-S battery with consideration on the balance of battery performance and the loading fraction of the immobilizers. ^[1]
To solve the lithium dendrite issues in the Li-S battery, we start with the Li/Li symmetric cells to study the nucleation and growth of Li dendrites related to the current densities. Unlike the general opinion that the dendrite problem is exacerbated at high current densities, what we find is when the current density is high enough passing through the Li electrode, there is a significant self-heating phenomenon for the dendrites. The local heating caused by current gives flux and flow to Li, triggering extensive surface diffusion, which smoothens the dendrites and enables the equilibrium flat configuration to be established quicker. And this repeated doses of high current density healing treatment enables the safe cycling of Li-S battery with high Coulombic efficiency.^[2]


[1] Li, L.; Chen, L.; Mukherjee, S.; Gao, J.; Sun, H.; Liu, Z.; Ma, X.; Gupta, T.; Singh, C. V.; Ren, W., et al., Phosphorene as a Polysulfide Immobilizer and Catalyst in High-Performance Lithium–Sulfur Batteries. Adv. Mater. 2017, 29, 1602734
[2] Li, L.; Basu, S.; Wang, Y.; Chen, Z.; Hundekar, P.; Wang, B.; Shi, J.; Shi, Y.; Narayanan, S.; Koratkar, N., Self-Heating–Induced Healing of Lithium Dendrites. Science 2018, 359, 1513-1516.

Recent research has focused on developing layered oxide cathode materials such as lithium-rich 3d-transition-metal oxides for high-energy-density lithium-ion batteries. However, practical applications of these materials are challenging due to a continuous voltage decay on cycling, which is associated with internal and interfacial material degradation. Here we show that the formation of a stable surface structure on layered lithium transition-metal oxide particles can prevent the further structural and chemical evolution of the lithium-ion diffusive surface layers. This is demonstrated by creating a few atomic layers with a cation-disordered structure on the specific facet of the layered oxide nanoparticles during the synthesis. Through the advanced atomic-scale analysis by high-resolution electron microscopy, we investigate the atomic arrangement and valence states of transition metals from the bulk to the surface region. In addition, combined analysis with in-situ X-ray characterization reveals the understanding of the origin of the bulk and surface chemistries and cationic redox mechanism upon electrochemical reaction. Our findings will provide new insights for designing and preparing future layered oxide cathode materials.


This work is supported by the National Research Foundation of Korea (NRF-2011-C1AAA001-0030538).

Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid) (PEDOT:PSS) thin films have been considered for use as the electrode material in electrochemical capacitors due to their high conductivity. Despite their high conductivity, conducting polymers, including PEDOT:PSS, usually suffer from a low capacitance because only a fractional number of electrons are obtainable from the monomer units of the polymer. Recently, screen-printing has provided a simple, effective way to obtain defined patterns of conducting materials with various shapes and sizes. In general, conducting inks for screen-printing require a relatively high viscosity (~10³ centipoise, cp) to achieve good adhesion between the patterns and the substrate. Despite the high conductivity of PEDOT:PSS, most PEDOT:PSS solutions usually have viscosities of less than 10² cp to facilitate transparent thin-film formation. Graphene, consisting of sp²-hybrid carbon atoms with a honeycomb crystal structure, is a promising candidate for constructing conductive composites with PEDOT:PSS, due to its outstanding charge-transport properties, mechanical strength, and flexibility. Accordingly, a PEDOT:PSS/graphene system with an optimal amount of graphene can induce a higher solution viscosity and enhanced charge-transport properties compared with pristine PEDOT:PSS. Ruthenium (IV) oxide (RuO₂) nanoparticles (NPs), especially the hydrous and amorphous forms, offer great potential as a pseudocapacitor component due to their fascinating virtues (e.g., high capacitance, fast redox behavior, and good electrical conductivity). Both PEDOT:PSS and graphene sheets provide functional groups that promote uniform dispersion of RuO₂ NPs. For this reason, RuO₂ NPs are promising for hybrid electrode systems consisting of PEDOT:PSS and graphene sheets. Thus, more effective and facile ways for combining the advantages of these three components are highly desired.
In this presentation, we report a ternary screen-printed electrode system, composed of aqueous PEDOT:PSS, graphene, and hydrous RuO₂ nanoparticles for use in high-performance electrochemical capacitors. As a polymeric binder, PSS allows stable dispersion of graphene and hydrous RuO₂ NPs in an aqueous PEDOT:PSS system through electrostatic stabilization, ensuring better utilization of the three components. Additional PSS molecules were added to optimize the solution viscosity to obtain screen-printed electrodes. The effects of graphene and hydrous RuO₂ NPs on the electrical and electrochemical properties of PEDOT:PSS were systematically investigated. The resulting RuO₂/PEDOT:PSS/graphene screen-printed electrode system exhibited significantly higher conductivity (1570 S cm⁻¹), a larger specific capacitance (820 F g⁻¹), and better cycling performance (81.5% after 1000 cycles) compared with PEDOT:PSS (680 S cm⁻¹, 195 F g⁻¹, and 50.2% after 1000 cycles), making this system suitable for electrochemical capacitors.

Polypyrrole (PPy) is a p-doped conducting polymer composed of repetitive bonds of five-membered heterocyclic rings including amine (N-H) groups, thus allowing the formation of hydrogen bonding forces between PPy chains. When PPy is made of nanomaterials, it has following advantages over bulk materials: (1) higher surface area; (2) better reactivity; (3) enhanced electrical conductivity. Among the various nanomaterials, the core-shell is very suitable as a stable electrode material because the Si core can protect the CP shell from swelling and shrinkage problems. In addition, high surface area of the Si cores enable rapid adsorption/desorption of electrolyte ions within the electrodes. On the other hand, the CP shell plays roles in reinforcing the poor electrical properties of Si cores. Thus, the synergistic effect from the PPy shell and the Si core would be advantageous for making a supercapacitor electrode that provides robustness and high electroactivity. In conventional synthesis, the SiO₂ templates limit the growth of polymer chains, resulting in undesirable α,α’-linkages inside the PPy chains. In addition, the low electrical properties of SiO₂-PPy composites are highly related to the low efficiency of core-shell formation in the in-situ syntheses. Therefore, there is a need for optimization and development for producing SiO₂-PPy core-shells with high efficiency of core-shell formation, which may result in the improved electrical and electrochemical performances.
In this presentation, we report the preparation and optimization studies of PPy-encapsulated silica nanoparticles (SiO₂ NPs) using ex-situ method. The SiO₂-PPy core-shell NPs prepared by the ex-situ method are well dispersed in water and facilitate the mass production of thin-film electrodes with improved electrical and electrochemical performances using a simple solution process. By using the ex-situ method, the PPy shell can be produced without the influence of SiO₂ NPs, and the resultant PPy will have higher doping level and better electrical performance than the conventional ones. As-prepared SiO₂-PPy core-shell NPs with different particle sizes were applied to electrode materials for two-electrode supercapacitors based on coin cell batteries. It was confirmed that the areal capacitance (73.1 mF/cm²), volumetric capacitance (243.5 F/cm³), and cycling stability (88.9 % after 5000 cycles) of the coin cell employing the ex-situ core-shell was superior to that of the conventional core-shell (4.2 mF/cm², 14.2 mF/cm³, and 82.2%). Considering these facts, the ex-situ method enables a facile way to produce highly-conductive thin-film electrodes with enhanced electrical and electrochemical properties for the coin cell supercapacitor application.

The growing demand for high-performance portable electronic devices and large-scale electrical energy storage systems has promoted lots of efforts to develop up-to-date lithium-ion batteries (LIBs) with greater capacity and cycle stability. Along this requirements, silicon (Si) is the material of choice for anode materials because of its high theoretical capacity (~4200 mA/g), which is almost 10 times higher than that of conventional graphite anodes (372 mA/g). However, the practical application of Si has been frustrated by rapid capacity fading, due to the unavoidable large volume change during charge-discharge processes, which results in particle fracture and loss of electrical contact between active materials, consequently causes an unstable solid electrolyte interphase (SEI) growth on the Si interface. Therefore, a SiO_x has aroused interests as the most promising alternatives for practical application for the lithium-ion batteries (LIBs) as a modification for Si. Compared to Si, the formation of inactive components, Li₂O and/or lithium silicates, coming with generation of Si anodes during the initial lithiation process at SiO₂ matrix can buffer the volume variation and maintain the structural stability. Nonetheless, some disadvantages of SiO_x such as low initial coulombic efficiency (ICE) and unstable SEI layer result in unsatisfactory electrochemical performance. To overcome these problems, there were many efforts to develop coating methods to protect the anode surface as an artificial SEI layer. However, it is still limited to improve ICE and long lifetime of SiO_x because these treatments were only for electrode surface.
In this study, the formation of electrode has modificated with the stable SEI by applied potentiostatic which is inspired by electrodeposition. In electrodeposition, the film made by AC has less porosity than by DC and formed uniformly. The electrochemical impedance spectroscopy (EIS) measurements were performed to understand electrical and ionical conductivities. The surface microstructure and components of the electrode were characterized by using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM). As a result, the as-prepared electrode delivered an initial charge and discharge capacity of 2603 mAh/g and 1140 mAh/g, respectively with an initial coulombic efficiency (ICE) of 42.3 % at 0.1 A/g. In contrast, the electrode charged by potentiostatic method showed initial charge and discharge capacities of 1180 mAh/g and 1105 mAh/g, respectively with an ICE of 93.6 %. Also, the electrode charged by potentiostatic method at 1 A/g was a capacity retention of 85.1% maintained over 1000 cycles, whereas the as-prepared electrode was a capacity retention of 70.1% under the same conditions. These results have been noted that it is important to make the compact electrode structure to improve electrochemical properties such as ICE, cycling, and capacities.

Lithium-sulfur (Li-S) batteries suffer from shuttle reactions during electrochemical cycling, which cause the loss of active material sulfur from sulfur-carbon cathode, and simultaneously incur the corrosion and degradation of the lithium metal anode by forming passivation layers on its surface. These unwanted reactions therefore lead to the fast failure of batteries. The preservation of the highly reactive lithium metal anode in sulfur containing electrolyte has been one of the main challenges for Li-S batteries. In this study, we systematically controlled and optimized the formation of smooth and uniform solid electrolyte interphase (SEI) layer through electrochemical pretreatment of the Li metal anode under controlled current densities and cycle number. A distinct improvement of battery performance in terms of specific capacity and power capability was achieved in charge-discharge cycling for Li-S cells with pretreated Li anodes compared to pristine, untreated ones. Importantly at higher power density (1 C rate, 3mA cm^-2), the Li-S cells with pretreated Li anodes protected by controlled elastomer (LPE) show the suppression of the Li dendrite growth and exhibit 3-4 times higher specific capacity than the untreated ones after 100 electrochemical cycles. The formation of such controlled uniform SEI was confirmed and its surface chemistry, morphology and electrochemical properties were characterized by X-ray photoelectron spectroscopy (XPS), focused-ion beam (FIB) cross-sectioning, and scanning electron microscopy (SEM). Adequate pretreatment current density and time are critical in order to form the continuous and uniform SEI, along with good Li ion transport property.

Lithium-sulfur (Li-S) batteries gain more attention as a secondary battery because of low cost, abundant and non-toxic sulfur as an active cathode material. As the sulfur possess high specific capacity (1672 mA h g^-1) and energy density (2600 W h kg^-1) which enables to reach the high scale energy demands like hybrid electric vehicles and power grid applications. The lower cyclic stability, the poor electrical conductivity of sulfur, and dissolution of intermediate lithium polysulfide into the electrolyte and its migration towards the anode (shuttle effect) are the major challenge for the commercialization of Li-S batteries. To address these challenges, many porous carbon materials and polar metal oxides and sulfides have been studied as a lithium polysulfide trapping materials. In this work, we focus on the polar and conductive metal carbides for polysulfide confinement. For this, we have synthesized Fe₃C filled nitrogen doped carbon nanotube (Fe₃C@NCNT) and NCNT by an easy single step process. Sulfur infiltrated NCNT (NCNT/S) have been synthesized by melt diffusion technique. The high surface area and large pore volume of NCNT can enhance the diffusion of Li⁺ ions and electrolyte during the charge-discharge process. Fe₃C@NCNT has used an interlayer where polar Fe₃C can adsorb lithium polysulfides through polar-polar interactions and therefore immobilized lithium polysulfides by minimizing the shuttle effect. NCNT/S/Fe₃C@NCNT cathode shows the enhanced specific capacity at different C-rates (1C 1672 mA g^-1) and long cyclic stability than the cathode without Fe₃C@NCNT. This signifies the Fe₃C@NCNT can minimize the active material loss in the cathode and enhance the sulfur redox kinetics.

The lithium-sulfur (Li-S) battery offers a high theoretical energy density of ~2600 Wh/kg and low cost, positioning it as a promising candidate for next-generation battery technology. However, problems including disastrous Li polysulfides dissolution and irreversible Li2S deposition have severely retarded the development of Li-S batteries. To solve these issues, we have reported a functional dimethyl disulfide (DMDS)-containing electrolyte that promoted an alternate electrochemical reaction pathway for sulfur cathodes by a formation of dimethyl polysulfides and Li organosulfides as intermediates and reduction products, leading to significantly boosted Li-S cell capacity with improved cycling reversibility and stability. Recently, we further investigated using dimethyl trisulfide (DMTS), a primary discharge-charge intermediate in the DMDS-containing electrolyte, which is also a commercially available reagent, as a co-solvent in functional electrolytes for Li-S batteries. We found that, DMTS, with higher theoretical capacity (851 mAh g-1) than DMDS (570 mAh g-1), functions similarly to DMDS to promote an alternate electrochemical reaction pathway for sulfur cathodes and to enable good cycling performance for cathodes with high sulfur content by way of an automatic discharge shutoff mechanism. In addition, owing to the better reactivity of DMTS with Li2S to form lithium polysulfides, the irreversible capacity loss caused by insoluble Li2S deposition and separation of active material from conductive networks could be efficiently mitigated. Thanks to all these advantages, a 25 vol% DMTS-containing electrolyte enables Li-S batteries with even higher cell capacity and improved cycling performance than using previous optimal 50 vol% DMDS-containing electrolyte. And capacity fading rate as low as 0.11% per cycle was achieved for cathodes with high sulfur content of 70 wt% in carbon/sulfur composite using an optimal 25 vol% DMTS-containing electrolyte.

Compact energy storage is of great significance especially in portable electronics and electric vehicles. Lithium-sulfur battery represents an advanced energy storage system because of its environmental benignity, high theoretical energy density (2600 Wh kg^-1) and natural abundance of sulfur. However, the cathodes of lithium-sulfur battery usually contain low-density porous carbon hosts for sulfur to alleviate polysulfides dissolution and improve the conductivity, which will lead to the low volumetric energy density (~500 Wh L^-1). Although enormous efforts have been devoted to improve the specific capacity and cycling stability of lithium-sulfur batteries, little attention has been payed to the aspect of high volumetric capacity simultaneously, which plays a key role in practical use.

Here, we for the first time fabricated the monolithic hybrid of metal-organic framework (MOF) and carbon nanotube (CNT) sponge, which was embedded within the MOF monolith to achieve the high-volumetric-energy-density lithium-sulfur batteries. Specifically, we first synthesized the ZIF-8 methanol suspension (~50 mg mL^-1), and dropped the suspension into the CNT sponge afterwards. The CNT sponge filled with ZIF-8 suspension was finally dried at the room temperature. Previously, MOFs or hybrids of MOFs and CNTs were always dried at the condition of vacuum and high temperature, which results in the rapid evaporation of the solvent, leaving a loose structure with a lot of void space among MOF particles. Room temperature drying in our study can make the solvent evaporate slowly and provide enough time for MOF particles aggregating into a dense monolith. Although the hybrid is highly dense, the large amount of micropores from ZIF-8 can still guarantee the high and safe sulfur loading. Besides, the embedded CNT networks act as conductive agent and supportive scaffold to improve the sulfur utilization and mechanical properties, respectively. Furthermore, superior rate performance can be achieved through optimizing the density of the monolith, which was controlled by the time period of room temperature drying. As a result, our novel free-standing electrodes exhibit a much superior volumetric energy density (~1533 Wh L^-1) than previously reported MOF-based electrodes in recent literature without compromising the specific capacity, rate performance and cycling stability.

In this talk, we report a new safe eutectic solvent composing of amides as an electrolyte for lithium-sulfur batteries. Such eutectic solvents are safe, highly stable and low-cost. It shows strong resistance against fire, which even cannot be ignited. In contrast, conventional ether-based electrolyte catches fire immediately after ignition. Moreover, it can dissolve the whole sulfide family, including Li₂S₂/Li₂S which are not dissolved in the conventional ether-based electrolyte. This unique property help address the dead sulfur issue caused by insoluble Li₂S₂/Li₂S in Li-S batteries, which is a major reason for low cycling lifetime of Li-S batteries. By using the eutectic solvent which can dissolve all sulfides species, a high specific capacity of 1360 mAhg^-1 and a capacitance retention of 88% over 40 cycles were achieved at 0.1 C without any electrode modification. With the further addition of TiO₂ nanoparticles on carbon electrode, stable capacity retention of 81% over 100 cycles is achieved, which prove the effectiveness of the strategy. More importantly, its cost is only ~$2-3/kg, even lower than traditional ether-based electrolyte, not to mention those expensive ionic liquids or solid electrolytes. Thus it will have great practical application in lithium-sulfur batteries. The development of eutectic solvent as electrolyte will improve lithium-sulfur batteries from aspects of safety, performance, and costs.

Lithium-sulfur (Li-S) batteries are a next generation Li battery technology that provides large theoretical capacity (1672 mAh g^-1) while also being earth-abundant and low cost¹. Li-S delivers its high capacity via a chemical transformation mechanism rather than Li intercalation as in Li-ion. Elemental sulfur (S₈) is reduced to a final solid discharge product lithium sulfide (Li₂S) through a series of soluble lithium polysulfides (Li₂S_x, 2 ≤ x ≤ 8); upon charging, this reaction is reversed. Many of these reactions occur in the electrolyte solution phase, but the interactions of the polysulfides, lithium salts, and solvent molecules are not well-established, making effective electrolyte development challenging.

In this study, we investigated the behavior of lithium polysulfides (LiPS; Li₂S_x, x = 4,6,8) and lithium salt (LiTFSI; Li bis(trifluoromethane)sulfonimide) in solutions using small angle X-ray scattering (SAXS). SAXS probes the nanoscale structure of the electrolyte and determines the size and shape of scatterers in solution due to their differences in electron density. Isolated LiPS have a chain-like configuration but may form dimers, aggregates, or other structures in solution². A concentration range (200-1000 mM) of polysulfides in two solvents - 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME; 1:1 by volume), which freely solvates LiPS, and acetonitrile (ACN), which sparingly solvates LiPS³ - were examined.

Some aggregation of LiPS was observed at high concentrations (1000 mM based on the mass of sulfur) in both solvents, although shorter chain polysulfides exhibited larger populations of smaller scatterers. Polysulfide solutions of lower sulfur concentration showed multiple aggregate populations below about 10 nm in size. A greater population of larger scatterers on the order of 10 nm were observed in higher concentrations in DOL/DME. Most of the features in the SAXS pattern for ACN-based polysulfide solutions were below 10 nm, indicating isolated LiPS structures, and these features remained at roughly the same location, even at higher concentrations. Agglomeration of polysulfides was both solvent and chain length dependent; however, at higher concentrations, the chain length differences present in the SAXS disappear. Addition of 1 M LiTFSI to DOL/DME, which is the conventional electrolyte used in Li-S batteries, resulted in an additional weak peak at 0.7 Å^-1, representing a change in the small scatterers on the order of 1 nm, indicating that salt ions interfere with LiPS clustering. These results add molecular level insight into Li-S electrolytes, aiding in understanding electrolyte-salt interactions.

References
1. L. F. Nazar, M. Cuisinier, and Q. Pang, MRS Bull., 39, 436–442 (2014).
2. M. Vijayakumar et al., Phys. Chem. Chem. Phys., 16, 10923–32 (2014).
3. L. Cheng et al., ACS Energy Lett., 1, 503–509 (2016).

Layered oxide cathode materials with P2 structure and general formula A_xMO₂ (0.4<x<0.8, M=transition metals) demonstrated high reversible capacities in sodium ion batteries [1,2]. However, commonly the capacity retention of the P2 compounds are not ideal when deeply cycled, majorly due to the mechanical damage caused the by the irreversible phase transitions during the sodium intercalation/deintercalation processes. Previously we developed a novel strategy to dope lithium into the transition metal sites in the P2 structure with precisely controlled synthesis. The lithium in the transition metal layer helps to stabilize the P2 structure and eliminate the P2-O2 transition in the model compound Na_0.66(Li_0.2Mn_0.2)O₂, which results its high reversible capacity[3]. Recently, we designed another compound P2-Na_0.66Li_0.18Fe_0.12Mn_0.7O₂, in order to improve the capacity retention and the high voltage stability. This compound was successfully synthesized and electrochemically tested. The reversible capacity was as high as 190 mAh/g and the capacity retention after 80 cycles was 87%. In-depth structure characterizations with using in situ and ex situ synchrotron X-ray diffraction, neutron diffraction and solid state NMR were performed. The results showed that the P2-O2 transitional was also eliminated in this compound. Meanwhile, more Li was kept in the transition metal layer after long term cycling, compared with that of Na_0.66(Li_0.2Mn_0.2)O₂, implying the doping of Fe at the transition metal layer effectively helps to lock lithium in the lattice. This result provides new insights of the cycling mechanism of P2 structured cathodes, as well as new opportunities in designing next generation cathode materials for sodium ion batteries with ultrahigh reversible capacity and excellent capacity retention.

[1] Billaud J, Singh G, Armstrong AR, Gonzalo E, Roddatis V, Armand M, et al. Na0.67Mn1-xMgxO2 (0 <= x <= 0.2): a high capacity cathode for sodium-ion batteries. Energy Environ Sci 2014;7:1387-91.
[2] Xu J, Lee DH, Clement RJ, Yu XQ, Leskes M, Pell AJ, et al. Identifying the Critical Role of Li Substitution in P2-Na-x LiyNizMn1-y-z O-2 (0 < x, y, z < 1) Intercalation Cathode Materials for High-Energy Na-Ion Batteries. Chem Mat 2014;26:1260-9.
[3] Yang LF, Li X, Ma XT, Xiong S, Liu P, Tang YZ, et al. Design of high-performance cathode materials with single-phase pathway for sodium ion batteries: A study on P2-Na-x(LiyMn1-y)O-2 compounds. J Power Sources 2018;381:171-80.

Hard carbon (HC) is the state-of-the-art anode material for sodium-ion batteries (SIBs). However, its performance has been plagued by the limited initial Coulombic efficiency (ICE) and mediocre rate performance. Here, we combined experimental and theoretical studies to demonstrate the application of lithium-pretreated HC (LPHC) as high-performance anode materials for SIBs by manipulating the solid electrolyte interphase (SEI) in tetraglyme (TEGDME)-based electrolyte. The LPHC in TEGDME can 1) deliver >92% ICE and ~220 mAh g-1 specific capacity, twice of the capacity (~100 mAh g-1) in carbonate electrolyte; 2) achieve >85% capacity retention over 1000 cycles at 1000 mA g-1 current density (4C rate, 1C=250 mA g-1) with a specific capacity of ~150 mAh g-1, ~15 times of the capacity (10 mAh g-1) in carbonate. The full cell of Na3V2(PO4)3-LPHC in TEGDME demonstrated close to theoretical specific capacity of ~98 mAh g-1 based on Na3V2(PO4)3 cathode, ~2.5 times of the value (~40 mAh g-1) with non-treated HC. This work provides new perception on the anode development for SIBs.

Nanostructured materials are important for boosting the performance of energy storage devices. Here, we report our recent efforts on designing nanocomposites for rechargeable Na- and Li-ion batteries and Li-S batteries. For Na-ion battery anodes, we constructed ultra-fine SnO₂ nanocrystals anchored on a unique reduced graphene oxide (rGO) porous matrix and showed excellent rate capability and long cycle life. The improved capacity and superior rate capabilities were rooted in the enhanced transport kinetics of both electrons and ions within the electrode structure because of the well-interconnected, macro-porous rGO matrix. Besides 2D graphene supported materials, we also explored yolk-shell antimony nanoparticles as high-performance Na-ion battery anodes. The unique low-dimensional nanostructure rendered stable cycling and high rate capability, despite ~300% volume change at full sodiation/desodiation. For Li-ion battery anodes, we designed a sandwich-structured N-doped graphene anchored with nearly monodisperse Fe3O4 nanoparticles. The unique sandwich structure enabled good electron conductivity, Li⁺ accessibility and accommodated a large volume change upon cycling. Hence, it delivered good cycling reversibility and rate performance. Carbon nanotube aerogels (CNA) with controlled nitrogen and phosphorus dopants also are introduced as a promising cathode host for rechargeable Li-S batteries. The combined theoretical study and electrochemical evaluation revealed the role of heteroatom dopants in enhancing chemical interaction of CNA towards various polysulfide intermedia species.

Recently, the rapidly increasing demands for electric vehicles (EVs) and smart-grid systems further require high-performance batteries with sustainability and high safety. In this regard, organic electrode materials which consists of only earth abundant elements have been considered to be a very promising alternatives to the conventional inorganic ones because of their great potential in high energy and power density, low cost, and environmental friendliness. Although a lot of organic cathode materials showing high cyclability and rate capabilities have already been developed, most of them have redox potentials below 3.0 V vs Li/Li+, which is considerably inferior to the commercial metal oxide cathodes. This is due to that their redox mechanisms are mostly relied on the reduction reactions; however, stabilizing the LUMO energy level of the organic molecules, which is responsible to the reduction, below 5.0 eV is difficult to achieve.
Here, we report a series of novel p-type organic electrode materials bearing redox-active heterocyclic azine compounds such as phenoxazine and phenothiazine, which exhibit high discharge voltage above 3.5 V vs. Li/Li⁺. They delivered large specific capacity more than 120 mAh/g with high specific energy as high as 450 Wh/kg at even high rate in Li-ion coin cell. We found out that 3-dimensional molecular design but maintaining π-conjugation could facilitate the redox reactions and capacity utilization with increased redox potential.

Identifying safe, low-cost, effective solutions to evolving electrical energy storage challenges remains a national priority, essential to meeting rapidly growing global energy demands. Here, we describe a promising approach to using low-intermediate temperature molten sodium batteries as a candidate technology offering high performance with long cycle life and inherent, engineered improvements in battery safety. Here, we describe molten salt-based batteries that integrate molten sodium anodes, solid state ceramic electrolytes, and sodium iodide-based molten salt catholytes to create high performance battery constructs that operate below 150^oC. The inherent nature of the material chemistries in these all-inorganic systems further eliminates common hazards associated with runaway exothermic reactions, polymer separators, and organic electrolytes that plague other battery systems. Here, we specifically highlight recent advances in materials chemistry of developing NaI batteries. By refining the composition and chemistry of the molten salt catholyte, we influence catholyte performance while optimizing electrocchemical interactions at both the separator and current collector interfaces. These refinements to catholyte chemistry stand to improve battery performance, reduce effective operating temperatures, and makes the batteries fundamentally safer as well. Accelerated rate calorimetry reveals neither runaway exothermic reactions, nor hazardous pressurized gas generation that make other battery systems inherently more hazardous in nature. Continued improvement in battery performance through optimization of the materials chemistry in these molten-salt sodium batteries promises exciting new solutions to impact a growing national need for safe, robust rechargeable electrical energy storage.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

For electrochemical device applications such as in high energy density rechargeable batteries, fuel cells, supercapacitors, electrochromic displays, etc. polymer based electrolytes are favorable materials.Previous research on most electrolytes, were made compatible for diffusion of Li ions only, due to its wide usage. Though Li-ion batteries are most abundantly used today, they have drawbacks such as being inherently toxic to the atmosphere, limited availability compared to other metals and its high costs. Here, we focus on preparing an electrolyte compatible to diffuse Al ions. Al is chosen due to its abundant availability, potentially dendrite free deposition and high capacity. In addition to gel electrolyte preparation, we also try to improve the room temperature ionic conductivity (σ_i) of the synthesized electrolyte layer.
We use a nanofiber cellulose (NFC) hydrogel as the base constituent for preparing the electrolyte. NFC hydrogel is favored due to its inherent ability to be strong, excellent flexibility, and lightweight. Initially, σ_i of pristine hydrogel was measured to be 10^-6 - 10^-7 S/cm. In an effort to improve σ_i of the pristine hydrogel, varying amounts of KOH were added to hydrogel as an additive and casted to form films, but during preparation we had difficulties with coagulation of fibers. Also, the resulting films were flaky, brittle and not stable enough to be characterized. In order to make hydrogel more flexible and stable various additives such as Gelatin, Polyvinyl alcohol (PVA), Polyacrylic acid (PAA) were added in different ratios individually and tested. Various weight ratios of Gelatin (1, 0.75, 0.5, 0.25): Hydrogel (1): PAA (0.25): KOH (0.1, 0.2, 0.3, 0.4) were added respectively individually into vials and sonicated for 90 minutes to obtain a homogeneous mixture. The mixture was then drop casted into silicon molds and dried in a vacuum oven for about 30 hours. The obtained gel electrolytes turned out to be much more stable and flexible as compared to hydrogel with KOH. These samples were then tested against Stainless steel (SS) and Aluminum (Al) block electrodes. The best average σ_i recorded was 1.086 mS/cm for 1:1:0.25:0.3 sample against Al electrodes, which are at least 3 magnitudes higher than the pristine hydrogel. The results obtained were also compared against σ_i values of samples made with PVA as an additive. PVA (1, 0.75, 0.5, 0.25): Hydrogel (1): KOH (0.1, 0.2, 0.3, 0.4) ratio gel samples were made similar to gelatin samples and characterized. An average highest ionic conductivity of 0.564mS/cm was obtained for 1:1:0.4 sample against SS electrodes and 0.115 mS/cm for 1:1:0.3 sample against Al electrodes respectively. When comparing the results, Gelatin along with PAA and KOH turned out to be better with hydrogel rather than PVA and KOH. These results are comparable to that of Li ion batteries and are twice for gel polymer electrolytes for Al-based batteries.

A new method is introduced for determining unknown concentrations of major components in typical lithium-ion battery electrolytes. The method is quick, cheap, and accurate. Machine learning techniques are used to match features of the Fourier transform infrared (FTIR) spectrum of an unknown electrolyte to the same features of a database of FTIR spectra with known compositions. With this method, LiPF₆ concentrations can be determined with similar accuracy and precision as an inductively coupled plasma optical emission spectrometry (ICP-OES) method. The ratios of organic carbonate solvent species can be determined with more rapidity than gas chromatography (GC). This FTIR method is faster and less expensive than GC and ICP-OES, and has the added benefit of being able to determine LiPF₆ concentration and solvent fractions simultaneously. It will be shown how the application of this tool facilitates electrolyte analysis of aged lithium-ion cells, and helps elucidate mechanisms for cell degradation.

MXene are a recently discovered family of two-dimensional, 2D, transition metal carbides, nitrides and carbonitrides that have shown a lot of promise in the field of energy storage with applications ranging from high capacity anodes for lithium, sodium, potassium, and aluminum ion batteries, supercapacitors and catalyst for hydrogen evolution among several others.
Typically, to form MXene films, neutral, aqueous colloidal suspensions are vacuum filtered. Even though this method produces free-standing films, the process is slow and the MXene films obtained are densely packed which hampers ionic mobility resulting in low capacities when tested as anodes in Na-ion batteries.
Herein, we show that by simply decreasing or increasing the pH of a Ti₃C₂T_x (where T_x are the terminations on the MXene sheet) colloid suspension, the 2D nanosheets crash out into crumpled flakes, resulting in randomly oriented mesoporous powders. Electrodes made with the crumpled powders deflocculated using an acid or alkali hydroxides have Na ion capacities of ≈ 180 mAh g^-1 and ≈ 230 mAh g^-1 at 100 mA.g^-1 respectively. As importantly, we also found that when deflocculated with alkali hydroxides, the intercalation of the alkali cations that occurs resulted in smaller first cycle capacity losses and suggest a method to potentially solve this pesky problem.

A lot of phosphate-type polyanion-type compounds, such as Na₃V₂(PO₄)₃, NaTi₂(PO₄)₃ et al. have been widely researched as electrode materials for Na-ion batteries (SIBs), and they are usually endowed with enhanced structural stability through modification.^{1,<span style="font-size:13px"> </span>2, 3,4} However, anode materials with low operation potential lower than 2.0 V and high cyclic stability based on polyanion-type phosphate materials are hardly reported.⁵ Owning to the chemical versatility of vanadium element, vanadium-based phosphates generally have multiple redox couples at a wide potential range.⁶ Therefore, it is possible to explore a kind of vanadium-based phosphate compound with multiple electron transfer as high stable anode for SIBs. Herein, a type of NaV₃(PO₄)₃/C as high-performance anode material is reported. The obtained NaV₃(PO₄)₃/C composite shows high capacity (152 mAh g^-1 at 75 mA g^-1 and 113 mAh g^-1 at 750 mA g^-1), excellent cyclability (145 mAh g^-1 after 200 cycles at 150 mA g^-1 and 124 mAh g^-1 after 1200 cycles at 750 mA g^-1) and outstanding rate performance (88 mAh g^-1 at 4.5 A g^-1). According to the achieved results, it can be seen that the as-prepared NaV₃(PO₄)₃/C is a promising anode of SIB for large-scale energy storage applications.
Acknowledgements: The authors greatly acknowledge the financial support from National Natural Science Foundation of China (No. 21405053), and the National Thousand Talents Program of China.
References:
1. Feng, P.; Wang, W.; Wang, K.; Cheng, S.; Jiang, K. J. Mater. Chem. A 2017, 5, (21), 10261-10268.
2. Zhou, W.; Xue, L.; Lu, X.; Gao, H.; Li, Y.; Xin, S.; Fu, G.; Cui, Z.; Zhu, Y.; Goodenough, J. B. Nano letters 2016, 16, (12), 7836-7841.
3. Zhang, Q.; Wang, W.; Wang, Y.; Feng, P.; Wang, K.; Cheng, S.; Jiang, K. Nano Energy 2016, 20, 11-19.
4. Fang, Y.; Xiao, L.; Qian, J.; Cao, Y.; Ai, X.; Huang, Y.; Yang, H. Advanced Energy Materials 2016, 6, (19),1502197.
5. Hu, P.; Wang, X.; Ma, J.; Zhang, Z.; He, J.; Wang, X.; Shi, S.; Cui, G.; Chen, L. Nano Energy 2016, 26,382-391.
6. Fang, Y.; Zhang, J.; Xiao, L.; Ai, X.; Cao, Y.; Yang, H. Advanced Science 2017, 1600392.