Symposium EE6 : Research Frontiers on Liquid-Solid Interfaces in Electrochemical Energy Storage and Conversion Systems

Understanding and controlling the reactivity at the electrode/electrolyte interface (EEI) is one of the key parameters for the development of high capacity and efficient lithium-ion batteries. The heterogeneous and partially catalytic reaction of the electrode with the electrolyte triggers the formation of surface films on the electrode surface which can cause degradation of the cell performance. Whereas the EEI layer properties have been heavily studied for negative electrodes such as lithium metal and graphite [1,2], the EEI layer on positive electrode materials is still puzzling. In particular the interface layers on high voltage and high capacity positive electrodes, whose potentials approach the limit of electrolyte stability against oxidation [3] and which can release oxygen species from their lattices, is quite unexplored. One of the challenges in understanding the reactions at the surface of the electrode is the complicated composition of the positive electrodes, containing not only the active material but also conductive agents and polymeric binders, that can modify the EEI layers on the electrode. To bypass these ambiguities, there is a need for study model electrodes such as thin films or pure active material electrodes, which allow for investigating solely the reactivity of the electrolyte at the active material surface. Here, combining X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption and Emission Spectroscopy (XAS/XES), of model electrodes, we will show how the species formed at the electrode/electrolyte interface are affected by change in charging potential and the structure and nature of the transition metal in the material. XES, XAS and Density Functional Theory (DFT) will be used to shed light on the changes of electronic structure upon delithiation, that can trigger the formation of oxygen species. With the help of DFT calculations we will show the possible reaction mechanisms by which oxygen species such as O₂ gas or superoxide can react with the solvents commonly used in Li-ion batteries and discuss the stability requirements for the electrolytes to be used in combination with high-voltage positive electrodes.



[1] Peled, E., J. Electrochem. Soc. 126, 2047–2051 (1979).
[2] Aurbach, D. et al., J. Power Sources 81–82, 95–111 (1999).
[3] Xu, K. et al., Chem. Rev., 114, 11503-11618 (2014).

In this talk, I will present several operando x-ray microscopy techniques that can be used to gain insights into the kinetic limitations facing lithium ion battery performance. The first example in my talk will focus on operando X-Ray tomographic microscopy (XTM). We have shown that absorption-contrast XTM can be performed during electrochemical cycling to visualize and quantify lithiation dynamics as well as degradation phenomenon [1]. This approach works well for a range of alloying and conversion systems. Here, I will present new results on silicon-based electrodes, showing that we can track the lithiation front through the battery electrode.

However, most commercial lithium ion batteries today use graphitic-based anodes and transition metal oxide based cathodes, where the difference in absorption between the lithiated and delithiated states is too small to reliably use absorption-contrast XTM to track the lithation dynamics. Graphitic materials and transition metal oxides require a different approach for operando imaging of lithiation dynamics. Therefore, as a second example, I will show how phase-contrast XTM can be used to image lithiation dynamics in graphite and silicon-graphite composite electrodes. During lithiation and delithiation of transition metal oxides, the transition metals change oxidation state, an effect that can be tracked by detecting shifts in their x-ray absorption near edge structure (XANES). X-ray absorption spectroscopy (XAS) microscopy can be used to visualize lithiation kinetics in these materials [2, 3]. As a third example, I will present an approach that enables us to decrease the measurement time for operando XAS microscopy and track the lithiation dynamics in electrodes with lithium nickel cobalt aluminum oxide (NCA) and lithium manganese oxide (LMO) during operation.

References:
[1] M. Ebner, F. Marone, M. Stampanoni, V. Wood, Science, 342 (2013) 716-720.
[2] J. Wang, Y.K. Chen-Wiegart, and J. Wang, Nature Comm., 5 (2014) 4570.
[3] F. Yang, et al., Nano Lett., 14 (2014) 4334-4341.

Ion insertions from a liquid electrolyte into a solid crystal is the fundamental reaction underpinning intercalation battery electrodes. Quantifying the reaction rates as functions of position and time have proved challenging because the Li insertion reaction is extremely heterogeneous, particularly for phase-separating materials like LiFePO₄. To address this challenge, we developed an in situ synchrotron X-ray nanoimaging platform which to track the distribution of Li in LiFePO₄ platelets while the particles cycle in an organic liquid electrolyte. By tracking the Li distribution with high spatial (~ 50 nm) and temporal (~ 30 s per particle) resolution, we directly visualize and quantify the nanoscale reaction rates within a particle as a function of the state-of-charge. We observe that a domain-by-domain intercalation pathway is the predominant mechanism at most cycling rates, in contrast to the conventional phase separation and solid solution pathways. Such domains arise from sub-particle-level heterogeneities in reaction rate, which is at a length scale previously inaccessible for in situ microscopy. Furthermore, we quantified the reaction kinetics and observed a skewed dependence of the exchange current density as a function of the Li concentration, explaining the differences between charging and discharging reactions. This nanoimaging platform provide a new frontier in measuring ion insertion kinetics at the solid/liquid interface with high spatial and temporal resolution.

The performance characteristics of Li-ion batteries are intrinsically linked to structural and chemical changes that occur within the electrode and along the electrode-electrolyte interface. In situ electrochemical scanning transmission electron microscopy (in situ ec-STEM) is a TEM-based characterization technique that allows for high spatial resolution imaging and spectroscopic analysis, whose signals are acquired simultaneously with quantitative electrochemical measurements. Here, we utilize this technique to probe the mechanisms and kinetics of solid electrolyte interphase formation and Li electrodeposition on a glassy carbon working electrode from a 1M LiPF₆ EC:DMC electrolyte. We demonstrate how annular dark field (ADF) STEM imaging is used to quantify the thickness of the SEI and account for the dark contrast observed for Li during electrodeposition. In addition to the high spatial resolution imaging, we also apply electron energy loss spectroscopy (EELS) to identify metallic Li and components of the SEI (e.g. LiF, Li₂O, LiOH) while still intact within the liquid electrochemical cell. Furthermore, we provide a framework for obtaining and analyzing quantitative EELS measurements to directly determine the oxidation state of battery electrodes (LiMn₂O₄ and Li₄Ti₅O₁₂) using the “white-lines” of the L_2,3 core loss ionization edges of Mn and Ti. By simultaneously acquiring low-loss and core-loss EEL spectra, we are able to remove plural scattering effects caused by scattering through the electrolyte and quantify the oxidation state of Mn and Ti using the white-line intensity ratio method. The information obtained from these studies can help provide a deeper understanding of how batteries function at the nanoscale.

Research supported by the Fluid Interface Reactions Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the Department of Energy’s Office of Basic Energy Sciences Division and by the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

The reaction between the aprotic liquid electrolyte and a lithium ion battery electrode material is arguably the most important process for rechargeable batteries. This reaction occurs when an aprotic electrolyte is reduced, or oxidized, on the surface of an electrode at a given potential forming the so-called solid-electrolyte interphase (SEI). This reduction/oxidation reaction forms a passivating layer, which has been shown to be a mixture of inorganic (e.g. LiF, POF) and organic/polymeric (e.g. C-O-C, Li-ROCO2) species that build up on the surface. A properly formed SEI prevents additional reduction/oxidation reactions from occurring and enables long term cycling. A poor SEI layer leads to safety issues, such as fires and gassing, as well as lifetime and power limitations due to consumption of electrolyte and the resistance of the SEI to both ionic and electronic transport. Understanding these reactions in situ is difficult since they occur at the liquid-solid or solid-solid interface of optically absorbing materials that hinder the use of traditional spectroscopic techniques. Furthermore, since some interfaces involve liquids it is necessary to use an analytical technique that can “see” through structural materials required to contain the liquid. Neutron reflectometry (NR) is a neutron scattering technique highly sensitive to morphological and compositional changes occurring across surfaces and interfaces, including buried interfaces and those occurring at the boundary between a liquid and a solid. Neutrons, by virtue of their nature, are deeply penetrating and therefore ideally suited as a probe to study materials in complicated environments, such as electrochemical cells. NR can be used to study thin film morphology and composition over lengths scales extending 1 nm to hundreds-of-nanometers. We will present results of the application of NR to the study of SEI formation on anode and cathode materials recently carried out on the Liquids Reflectometer at the Spallation Neutron Source at ORNL.

While known for more than a century, ionic liquids – commonly defined as salts with a melting point around or below room temperature – have only become a staple in material research over the course of the last quindecinnial. It is their unique properties, including a large electrochemical window and negligible vapor pressure, that makes these substances particularly interesting for the development of novel applications including, among others, catalysis, lubrication and, most notably, electrochemistry. While downsizing technology in an attempt to build smaller, yet more powerful devices, interface effects start to become more and more dominant in these systems and a sound understanding of occurring structural phenomena is a prerequisite for engineering applications.
Using an unprecedented complementary approach, combining experimental X-ray scattering data and atomistic simulation, we study the behavior of an archetypical family of aprotic ionic liquids, dialkylimidiazolium–bis(trifluormethylsulfonyl)imide ([C_nMIm][NTf₂]), at the sapphire (001)–liquid interface. X-ray reflectivity (XRR) allows us to reveal an interface-normal layering profile of the buried solid–ionic liquid interface. We report a strong excess of cations at the interface, followed by alternating anion/cation layers, slowly decaying towards the bulk over a region of about 40 Å. Moreover, the experimental data can be employed to parametrize and verify force fields used in our molecular dynamics simulations (MD). Reaching a good agreement between XRR and MD puts us in a position to extract reliable information on an atomic level that are otherwise inaccessible by the experiment alone. We find that both cations and anions in the vicinity of the substrate tend to assume a very specific orientation/conformation, enabling them to efficiently form hydrogen bonding with the substrate. The anions remaining close to the interface take a well-defined lateral order, intercalating the network of cations.

Nanoporous carbon materials have become increasingly adopted for use as electrodes in electrochemical double layer capacitors, owing to their high specific and volumetric surface area and good electrical conductivity. However, a widely scattered spectrum of both experimentally-measured energy and power densities have been reported, which can be related to the accessibility and transport of ions into and through these nanoporous channels. As the performance of these devices is primarily related to the evolution of the electrode-electrolyte interfacial microstructure, atomistic insights from molecular simulations can help provide the guidance needed to design materials with better and more reliable performance. To this end, we model the electrochemical interface using several disordered nanoporous structures containing structural and chemical motifs, including edge defects, topological defects, and chemical functional groups. First, we use density functional theory to discover the large electrode capacitances of these materials, as well as identify the origins of their metallic nature. More importantly, the charge accumulation and redistribution behavior is discussed, which in turn noticeably influences the electrode-electrolyte interactions. Using classical molecular dynamics, our attention turns toward the triggering of distinct ordering in the electric double layer due to the presence of these aforementioned motifs. We will discuss the relative influence of these motifs on the double layer capacitance, in addition to the ionic transport behavior. Finally, we will discuss strategies to utilize these insights for the improved design of electrode materials for supercapacitors.

Understanding the nature and stability of cathode surface and cathode-electrolyte interface (CEI) are of fundamental importance for future generation electrochemical cells where higher cell voltage will be applied for more Li extraction. In this work, we use LiNi_0.8 Co_0.15 Al_0.05 O₂ (NCA) as a model cathode system in LiPF₆ electrolyte to study the structure, composition, morphology, and valence state of TM ions at the interface using high-resolution scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The chemical and structural evolution are studied in different charge voltage conditions, temperature and time scales to understand the stability of layered NCA and cathode electrolyte reactions in a comprehensive way. The results are compared with x-ray absorption spectroscopy and electrochemical studies to correlate the change in properties with that in structure/composition. The funding for this work is provided by NECCESS, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001294.

Photo (electro) catalysts, used in energy applications, rely on the solid-liquid interface to carry out productive chemistry. This interface is generally less amenable to standard surface-science characterization methods, making the investigation of the surface activity, catalyst structure and chemical evolution at the nanoscale very challenging. Bulk measurements have been applied, but these lack sufficient resolution to identify conclusively which structures (protrusions, flat film surface, or cracks) are responsible for chemistry in these materials. In order to address these questions, we have designed and fabricated scanning probe tips for combined atomic force microscopy (AFM) and scanning electrochemical microscopy (SECM) of photoelectrocatalysts. AFM combined with SECM provides a direct correlation of topological information with the chemical surface reactivity, at a resolution defined by the probe radius. The special probe fabrication starts with the preparation of SECM tips using the established method of sealing a metal wire into a glass capillary and pulling with a pipette puller. The probe ends are then cut polished using Focused Ion beam (FIB) to expose the Pt surface. The SECM probes are subsequently cut and attached to a quartz tuning fork to complete the fabrication of the combined SECM/AFM probe. We report here on the characterization of these tips using an in-house fabricated calibration sample and a model anode.

Nickel hydroxide incorporated with 10-20 at% Fe is arguably the most active electrocatalyst for oxygen evolution reaction (OER) in alkaline media. However, the mechanism of Fe doping for developed performance remains largely unclear until now. X-ray absorption spectroscopy (XAS) is a powerful technique that can be applied to electrochemical systems and look at the electrocatalysts under reaction conditions. Herein we investigated the Ni-Fe hydroxide catalyst by using in-situ x-ray spectroscopy to probe the electronic structure of Ni-Fe hydroxide and related interaction between Fe and Ni centers during OER. Information about the evolution of their chemical states from Ni and Fe K-edge XAS spectra may provide critical insights into the OER reaction mechanism.

Reverse electrodialysis (RED) is one of the promising processes for generating electricity from the salt concentration gradient between river and sea water. A RED stack contains alternately arranged anion and cation exchange membranes (AEMs and CEMs, respectively) which separate salt solutions of different concentrations. The power generation performance of RED significantly depends on the characteristics of ion exchange membranes (IEMs), which are selective for cations or anions. When the IEMs with different polarities are stacked alternately, with compartments for seawater or river water in between, the Donnan potentials over each membrane result in a voltage that can be used for electricity generation. The important membrane properties dominating the power generation in RED processes are ion-exchange capacity, water swelling degree, electrical resistance, permselectivity, membrane thickness, and surface morphology etc. Recently, pore-filled IEMs have been receiving great interests due to the extraordinary dimensional and chemical stabilities since the report by Yamaguchi et al. In the pore-filling method, an inert porous substrate provides both mechanical and chemical stabilities while a filling polymer enables selective ion transport through the membrane. Moreover, modification of a membrane surface is one of the significant issues to improve the selectivity, thus, enhancing the salt rejection. In this work, we have successfully prepared high performance pore-filled anion-exchange membranes. In addition, the optimization of membrane design parameters has been systematically investigated using the membranes via various electrochemical analyses in terms of the enhancement of RED performances. Finally, the membrane surface has been successfully modified with acid-doped polypyrrole through a facile method for the improvement of the membrane properties. (This work was supported in part by the Material Technology Development Program funded by the Ministry of Trade, industry & Energy (MOTIE) (No. 10047796) and by the Ministry of Education (MOE) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (No. 2015H1C1A1034436))

Supercapacitors are probably the most promising candidate to be used in electric vehicle industries, with the potential to replace batteries in many other suitable electronic applications. Having the ability to be charged at much faster rate than a battery, supercapacitors are the preferred candidates in many situations, given that there is a significant improvement in their energy densities. Despite of the fact that recently there are a number of scientific developments reporting outstanding performances for new electrode materials, most of them are not yet suitable for real applications. As in some instances, complex and time consuming synthesis processes have reduced the feasibility of commercialization of those materials, on the other hand absence of true performance matrices has hindered the realization of real devices. During performance evaluations, one of the noticeable trend has been the characterisation of thin / ultra-thin electrodes with mass loadings limited to only few mg per cm², and in many instances even less than 1 mg per cm², and they are characterized based on active material’s mass only, as a result of which those results are to be scaled down by at least a factor of 3-4.^[1]
Here in our work we have presented a unique and novel method to fabricate a highly dense graphene foam like structure with precisely controlled porosity, with a suitable combination of micropores and mesopores, which resulted in a hierarchical porous electrode with a very high surface area of ~ 1140 m² g^-1, which has enabled us to fabricate electrodes with extremely high mass loading, as high as 40 mg cm^-2. This has resulted in a very high energy density of ~40 Wh kg ^-1 (device's energy density) at very impressive power density of (1.1 x 10⁴) W kg^-1, the result has been obtained after taking total weight of the device under consideration, i.e., two current collectors, electrolyte, separator and packaging. This excellent performance can be attributed to the excellent pore accessibility of the electrode material. We are also capable of have fabricating these electrodes free standing, and also directly on the current collector (Cu - foil) in one single step in less than 30 minutes of time. We also prepared these electrodes without any binders, also we have not used any other chemicals, and thus the electrodes are free of any chemical impurities, and electrode materials thus keeping its high conductivity, as high as ~ 10⁴ S m^-1. This single-step, quick and simple process, along with extremely high energy density and power density from ultra-high mass loading, makes our unique approach has the true potential to enable real applications of supercapacitors.

References:
[1] Y. Gogotsi, P. Simon, Science 334, 917 (2011).

Eumelanin (EuM) is a natural pigmentary macromolecule found throughout the biosphere based on dihydroxyindole and dihydroxyindole carboxylic acid monomeric units and their various redox forms.
From the point of view of its electrical properties, EuM is an electronic-ionic hybrid conductor, of interest for applications in bioelectronics and sustainable electronics.
The efficient and reversible charge storage properties of EuM in aqueous electrolytes permit to exploit it as electrode material in electrochemical energy storage/conversion devices, such as batteries and supercapacitors.

Recently we explored an aqueous micro-supercapacitors making use of EuM-based electrodes. EuM drop casted on a current collector made of carbon paper (EuM/CP) was performed by cyclic voltammetry (CV) at different scan rates. Ammonium acetate buffer at pH 5.5, EuM electrodes featured a specific capacitance values as high as 150 F g^-1, which is of great interest for supercapacitor applications. The catechol-quinone moieties present in the EuM and the proton conducting properties of EuM are likely responsible for the reversible pseudocapacitive current observed. Other functional groups present in the monomeric units, such as COOH and aromatic amines, also influence on the ion storage property of EuM.
EuM/CP supercapacitors in ammonium acetate buffer at pH 5.5 where tested by repeated galvanostatic charge/discharge cycles between 0 V and 0.75 V at various current densities, with the highest current corresponding to ~90 A/g of EuM. The EuM/CP supercapacitors displayed a maximum energy density of ~0.1 mWhcm^-2 and a maximum power of ~20 mWcm^-2. Based on these promising results we are now opening new directions in our research on the ion storage properties of EuM.
We explore the use of electrolytes including Zn(II), Cu(II), Fe(III) for (i) their multivalency, of interest for energy storage, and (ii) the biological role EuM plays in their binding. In the latter case, EuM would help, as a model molecule for neuromelanin, to better understand the antioxidant action of neuromelanin (related to the sequestration of redox-active metal ions).
We are considering the photoconductivity properties of EuM to enhance its ion storage properties, in other words to demonstrate a photo-supercapacitor. Preliminary results exposing the EuM electrodes to simulated solar light at 1kW/m2 show a significant enhancement of the storage properties. In principle, EuM photo-supercapacitors may be used as biodegradable self-powered sources for implantable medical devices and their study might yield new insights into the mechanisms of photoconduction in EuM based architectures.

Manganese oxide is one of the ideal materials for high power supercapacitive device with high energy density. With its high specific capacity, natural abundance and non-toxic nature, tremendous research efforts in MnO₂-based electrode have shown promising charge-discharge and cycling ability. Despite the recent progress in enhancing the performance of MnO₂ electrode, the low electrical conductivity of MnO₂ remains the major challenge limiting its rate capability for practical application. Here, we demonstrate a PDDA functionalized three-dimensional (3D) Graphene/Nanotube/MnO₂ hybrid nanostructure with exceptional high specific capacity. The hybrid has been successfully synthesized by a template-free, one-step and one-phase method. The positively charged PDDA not only serves as the reducing agent in KMnO₄ reduction, it also provides anchoring sites that effectively prevents the negatively charged MnO₂ from agglomeration during synthesis and cycling. By the synergistic interaction among PDDA, MnO₂, nanotube and graphene, the hybrid exhibits cycling ability with high columbic efficiency after long-term operation, and binder-free thin films can be formed on various substrates. Moreover, the large surface area of interconnected 3D graphene facilitates fast electron transfer during faradaic redox reaction, which greatly improve the utilization of poorly conductive MnO₂. The resultant MnO₂ NTs/graphene nanostructure possess superior specific capacitance (~1050 Fg^-1). We also investigate the practical performance of the prepared hybrid materials using both symmetric and asymmetric capacitors. The high specific energy density (~46 Whkg^-1) and power density (~58 kWkg^-1) confirm its excellent supercapacitive performance. The assembled supercapcitors also show strong durability with high capacitive retention after 10000 charge-discharge cycles at 10 Ag^-1.

Multi-electron redox material is an effective way to develop new battery systems with higher energy density [1]. Among the candidates, V₂O₅ attracts great attentions as one of promising cathode materials due to its low cost, relatively low toxicity, better safety and the potential to insert 3 electrons, which corresponds to a theoretical capacity of 440 mAh g^-1 [2, 3]. However, its moderate electrical conductivity and poor diffusion coefficient of lithium ions in the matrix greatly limits its practical applications. Porous structure materials have been becoming one of the most powerful means to improve electrochemical performance of electrode materials [4] as it involves the combination of both void space and nano-scale particles. In the present work, we develop a novel spray pyrolysis (SP) route to prepare porous nanostructure V₂O₅. The effect of pore structure, namely dense, mesoporous and mesoporous combining macroporous, on the electrochemical properties was investigated. In SP process, the precursor solution was prepared by dissolving the required amounts of ammonium metavanadate (NH₄VO₃) and ammonium nitrate (NH₄NO₃) in distilled water with heating. The concentrations of NH₄VO₃ was 0.068 mol L^-1 and the concentration of NH₄NO₃ varied from 0 to 0.408 mol L^-1. A schematic diagram of the experimental apparatus used has been provided elsewhere [5]. The precursor solution was atomized at a frequency of 1.75 MHz using the ultrasonic nebulizer. The generated droplets were carried to the reactor by air at a flow rate of 3 L min^-1 and the reactor temperature varied from 400 to 700 C. The obtained samples could be identified as orthorhombic V₂O₅ phase when the synthesis temperature ranged between 500 and 700 C. The morphology and V₂O₅ synthesized at 500 C could be remarkably changed with increase the concentration of NH₄NO₃ to 0.408 mol L^-1. The pore structure analysis based on N₂ adsorption-desorption isotherm measurements indicated that the nanostructure V₂O₅ powders with pore size less than 200 nm could be prepared by the novel SP and the increase of NH₄NO₃ concentration in precursor solution could enlarge the porosity in V₂O₅ powders, especially the pore size between 20 to 80 nm. The porous nanostructure V₂O₅ powders prepared at a NH₄NO₃ concentration of 0.272 mol L^-1 delivered a first discharge capacity of 400 mAh g^-1 at 20 mA g^-1. The unique porous structure of V₂O₅ powders significantly enhanced the rate performance and delivered 180 mAh g^-1 at 1200 mA g^-1, which is much higher than that of dense V₂O₅ electrode (70 mAh g^-1)

[1] Gao et. al., Energy Environ. Sci., 3 (2010) 174-189.
[2] Liu et. al., Energy Environ. Sci., 4 (2011) 4000-4008.
[3] Zhou et. al., J. Power Sources, 238 (2013) 95-102.
[4] Li et. al., Adv. Funct. Mater., 22 (2012) 4634-4667.
[5] Shao et. al., J. Power Sources, 199 (2012) 278-286.

Electrochemical capacitors (ECs) use a very different charge storage mechanism than batteries, relying on the physical adsorption of ions on a high surface area material instead of chemical reactions. ECs can yield a power density of more than 10x that of batteries, however, they provide about 10x less energy density. Since the energy density of the device is proportional to the square of operating potential window, an effective way to increase energy density of the device is by increasing this voltage window. Theoretically, ionic liquid electrolytes can operate at up to 6 V, though experimentally, the value is between 3-4 V, depending on the properties of electrode materials. Though they boast a large operating potential window, ionic liquids are known to contain large and bulky ions, which often limits the charging rates of the devices. This can make it difficult to use an ionic liquid on a porous carbon with a range of pore sizes, even though the specific surface area of the electrode material is higher. It has also been shown that the capacitance of porous electrodes is maximized when the ion size and pore size are equal.¹ In this case, the ion is small enough to fit inside the pores while still large enough to take advantage of the surface area within the pore walls.
Therefore, we have studied the relationship between the electrode and electrolyte when designing a high performance EC, specifically focusing on the interface of the IL electrolyte at the carbon surface. By designing an electrolyte based on mixed ionic liquids,² we can match the ions in the mixture to the multiple pore sizes of the electrode material. Different porous carbons with varying pore size distributions are used to illustrate the effect of ionic liquid mixture electrolytes. While it is possible to tune the carbon electrode to the electrolyte³, such as in carbide-derived carbons, it is much simpler to design an electrolyte mixture that will optimize the performance of a conventional electrode material, such as activated carbon. We show that by tuning the electrolyte to the electrode material, the operating voltage window of the device can be expanded by 1 V, thereby increasing the energy density. New results indicate that the mixture electrolyte can enhance the capacitive performance of the porous carbon materials as well.
1. Lin, R. et al. Solvent effect on the ion adsorption from ionic liquid electrolyte into sub-nanometer carbon pores. Electrochim. Acta 54, 7025–7032 (2009).
2. Van Aken, K. L., Beidaghi, M. & Gogotsi, Y. Formulation of Ionic-Liquid Electrolyte to Expand the Voltage Window of Supercapacitors. Angew. Chemie 127, 4888–4891 (2015).
3. Largeot, C. et al. Relation between the Ion Size and Pore Size for an Electric Double-Layer Capacitor. 2730–2731 (2008).

The enhancement of catalyst for oxygen reduction reaction (ORR) is still a huge challenge. Recently, the incorporation of PtNi alloy with a hydrophobic, protic ionic liquid shows good electrocatalytic activity, due to the high oxygen solubility of the ionic liquid which can induce the increase of oxygen concentration at the catalyst surface. Thus, the residence time of oxygen on the catalyst surface increases, resulting in high frequency and high mass activity of ORR. However, the PtNi-ionic liquid composites in previous reports were synthesized through several complicated steps after just mixing ionic liquid with PtNi alloy, finally ended up with unstable composites. Herein, we report a facile one-pot strategy of synthesizing Pt-based alloy–ionic liquid composite dispersed on carbon black in room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, under one atmospheric pressure plasma. The PtNi-NPs with the size of 2–3 nm were stably and uniformly hybridized on the surface of carbon blacks, which were covered by the ionic liquid supramolecules layer. The obtained PtNi-ionic liquid/carbon black composite catalyst shows high ORR activity, which is better than that of commercial Pt/C catalysts. The PtNiRu-ionic liquid/carbon black catalyst is also more stable than the commercial Pt/C under the same ORR condition. The results prove that the Pt-based alloy–ionic liquid composites dispersed on carbon black push the ORR towards completion and can indeed be one of promising candidates for many electrocatalytic applications.

Compact, pin-hole-free zinc oxide (ZnO) is a promising electrode material as blocking layer to hinder electron back transfer in dye-sensitized solar cells (DSCs) [1] and in perovskite solar cells (PSCs) [2,3]. Here, the electrochemical deposition of crystalline ZnO out of aqueous Zn(NO₃)₂ solutions under pulses of controlled current were studied on gold substrates in micrometer dimensions [4]. The observed potential-time curves showed three significant stages in the established potential for all depositions. Scanning electron and confocal laser microscopy analysis revealed a correlation between a successively completed coverage of the gold electrode by ZnO and an abrupt transition of the deposition and rest potential to less negative values. The transition time t_trans at which the transition was detected depended on the current density during electrodeposition, pointing at a different growth mode of ZnO, and on the electrode geometry, pointing at the influence of different diffusion profiles of the reacting ions. In order to elucidate the role of different participating redox reactions in this electrodeposition, measurements were carried out in various reference electrolytes. Pulsating times were varied which revealed that the three stages in the potential were only seen for pauses which were equal or longer than the current pulses. The significance of each stage and the corresponding reaction at the electrode/electrolyte interface will be discussed with regard to the preparation of ZnO films completely covering an electrode surface to optimize the deposition for future applications in electronic devices.
[1] H. Minoura and T. Yoshida, Electrochem. 76, 109 (2008); [2] J. Zhang, P. Barboux and T. Pauporté, Adv. Engery Mat. 4, 1400932 (2014); [3] H.J. Snaith, J. Phys. Chem. Lett. 4, 3623 (2013); [4] M. Stumpp, T.H.Q. Nguyen, C. Lupo and D. Schlettwein, Electrochim. Acta 169, 367 (2015).

Due to their high cost-efficiency and the possibility to produce large-scale systems, organic light emitting diodes (OLEDs) are intensively researched for display and lightning applications. Extensive research has been devoted to the synthesis of new materials, the implementation of interlayers and the development of device-structures to improve the long-term-stability and efficiency of the device. The optimization of morphology and supra-molecular geometry plays an important role to establish new active layers for organic electronics.
Here we introduce amphiphilicity to dyes to enable self-assembling of dye layers [1] and, thus, to control the morphology of active layers. The amphiphilic dyes can form flat mono- and bilayers with well-defined and stable interfaces. These layers could be used for model-designs for organic light emitting diodes. As chromophores we use 4-hydroxy-1,3-thiazoles as they are photochemical stable and exhibit a fluorescence quantum yield up to 95%. [2-4] We present in-situ monitoring of self-assembly by UV-vis spectroscopy und photo thermal deflection spectroscopy on layers of amphiphilic thiazoles, growth-kinetics and thermodynamic stability of dye layers for organic light emitting diodes.

References:
[1] M. Hupfer, F. Herrmann, S. Fischer, M. Kaufmann, R. Beckert, B. Dietzek, M. Presselt, to be submitted (2015).
[2] R. Menzel, D. Ogermann, S. Kupfer, D. Weiss, H. Görls, K. Kleinermanns, L. González, R. Beckert, Dyes Pigm. 94 (2012) 512.
[3] E. Täuscher, D. Weiss, R. Beckert, H. Görls, Synthesis 2010 (2010) 1603.
[4] L.K. Calderón-Ortiz, E. Täuscher, E. Leite Bastos, H. Görls, D. Weiss, R. Beckert, Eur. J. Org. Chem. 2012 (2012) 2535.

Acknowledgements:
We thank the Deutsche Forschungsgemeinschaft (DFG Grant No. PR 1415/2-1) for the financial support.

Imaging interfacial reactions between liquid and solid by transmission electron microscopy (TEM) is still a challenging issue. Here, we report focused-ion-beam (FIB)-deposited Pt wire structures coupled with liquid cells, and characterizing electrical properties with simultaneously visualizing structural evolutions of metal-water interface by TEM. We fabricated a device in which Pt wires are deposited upon silicon-nitride membrane (50 nm thickness). The Pt wires are connected to electrode-lead structures for electrical characterizations. The Pt wires placed upon the silicon-nitride membrane are encapsulated by another silicon-nitride membrane sample while the two membranes are separated by 100-200 nm thick silicon-oxide spacer layers. The space between two silicon-nitride membranes was filled with water solution by forming a liquid cell. Before supplying water solution, we examined an in-situ electrical test of the as-fabricated Pt wire (of dry state). As external electric-voltage biases are applied to the Pt wire, Joule heating leads the pristine amorphous Pt wire to be crystallized, along with a pronounced increase in electrical conductance. We monitored the crystallization behaviors by both bright-field TEM and scanning tunneling electron microscopy (STEM). During the crystallization, TEM image-contrast of the edge of the Pt wires becomes sharper, probably due to de-wetting of Pt materials during Joule heating process. Inside the membrane-cell incorporating metal layers, we filled water solution by creating a metal-water interface. Electron beams irradiated the laterally contacted metal-water interface, and we monitored its reactions. We identified dissolutions of metal particles and their motions in the water solution. As the dose of electrons is increased, the dissolution rate of metal particles becomes faster. Our demonstration of liquid cells with metal wires inside TEM offers a platform to study interfacial reactions between liquid and solid materials.

Electrodeposition is used in various manufacturing processes for creating metal, colloid, and polymer coatings on conductive electrode substrates. The process also plays an important role in electrochemical storage technologies based on batteries, where it must be carefully managed to facilitate stable and safe operations at low operating temperatures, high rates and over many cycles of charge and discharge. A successful electrodeposition processes requires fast transport of charged species (e.g. ions, particles, polymers) in an electrolyte and stable redox reactions and transport at the electrolyte/electrode interface at which the deposition occurs. This talk considers the stability of electrodeposition of metals on planar electrodes with an emphasis on its role in enabling next-generation secondary batteries based on lithium and sodium metal anodes. Such batteries promise substantial improvements in electrochemical energy storage over todays’s state-of-the art lithium ion technology and are under active investigation worldwide.

Development of a practical rechargeable lithium metal battery (LMB) remains a challenge due to uneven lithium electrodeposition and formation of ramified denderitic electrodeposits during repeated cycles of charge and discharge. Known consequences of unstable electrodeposition in LMBs include accumulation of electrically disconnected regions of the anode or “dead lithium”, thermal runaway of the cell, and internal short circuits, which limit cell lifetime and may pose serious hazards if a flammable, liquid electrolyte is used in a LMB. Lithium-ion batteries (LIBs) are designed to eliminate the most serious of these problems by hosting the lithium in a graphitic carbon substrate, but this configuration is not entirely immune from uneven lithium plating and dendrite formation. Specifically, the small potential difference separating lithium intercalation into versus lithium plating onto graphite, means that a too quickly charged or overcharged LIB may fail by similar mechanisms as a LMB.

Using a continuum transport analysis for electrodeposition in a structured electrolyte in which a fraction of the anions are fixed in space, the talk will show that electrodeposition at the lithium anode can be stabilized through rational design of the electrolyte and salt. Building upon these ideas, the talk will explore structure and transport in novel nanoporous hybrid electrolyte configurations designed to stabilize metal anodes against dendritic electrodeposition and premature failure. Finally, the talk will explore an application of these electrolyte designs for LMBs to evaluate stability conditions deduced from theory.

Successful strategies of silicon (Si)-based anode materials for lithium ion batteries (LIBs) have numerously reported during past decade, such as nano-designed Si structure, stronger new binder system, Si composite with other materials, and so on. However most of those strategies provided only specific energy or power density with low volumetric energy density or area capacity. The superior stability of specific energy or power density couldn’t represent the higher electrochemical performance in practical application for LIBs. To investigate the practical use of anode materials, initial area capacity of anode should be higher than 3.7 mAh/cm² which is commercial level of graphite. It appears in high loading level that the more critical problems due to volume expansion which doesn’t appear in low loading level. Moreover, high loading level causes fast degrading of lithium metal in half-cell test. Thus electrochemical test of high loaded electrode should be conducted in a full-cell test. Therefore, here we conducted full-cell electrochemical test of Si oxide-based anode with high loading (also using only 3wt% of CMC/SBR binder) while physical mixing of LiCoO₂ and Ni-rich cathode as cathode material, also known as commercialized cathode material for LIBs, and investigated its detailed fading mechanisms out to 1000 cycles.
Our investigation scope of the fading mechanism is from electrode level to atomic level. To verify the effect of electrode volume change, the thickness of electrode and solid-electrolyte-interphase (SEI) layer was measured after cycle test and chemical compositions of SEI layer were analyzed by X-ray photoelectron spectroscopy (XPS). To observe the atomic structure of electrode materials, high resolution-transmission electron microscopy (HR-TEM) were operated after long-term cycling. On the evidence of ex-situ analysis and electrochemical result, we created the algorithm for the possible fading mechanism of Si-based anode. We also separated the reasons of fast and gradual degrading respectively and suggested the behaviors of idealized Si-based anode. We believe that our findings provide a foundation to clearly verify fading mechanism of Si-based anode for LIBs and envision the considerations of future Si-based anode for practical use.

Silicon clathrates contain cage-like structures that can encapsulate various guest atoms or molecules. Here we present an electrochemical evaluation of type I silicon clathrates based on M₈Y_xSi_46-x (M = Ba, Sr; Y = Al, Cu, Ni) as the anode material for lithium-ion batteries. For the Ba-Al-Si system, post-cycling characterization with NMR and XRD show no discernible structural or volume changes even after electrochemical insertion of 44 Li (~1 Li/Si) into the clathrate structure. The observed properties are in stark contrast with lithiation of other silicon anodes, which become amorphous and suffer from large volume changes. The electrochemical reactions are proposed to occur as single phase reactions at approximately 0.2 and 0.4 V vs. Li/Li⁺ during lithiation and delithiation, respectively, distinct from diamond cubic or amorphous silicon anodes. Reversible capacities as high as 499 mAh g^-1 at a 5 mA g^-1 rate were observed for silicon clathrate with composition Ba₈Al_8.54Si_37.46, corresponding to ~1.18 Li/Si. These results show that silicon clathrates could be promising durable anodes for lithium-ion batteries. Changing the composition of the clathrate, namely replacing the Ba guest atom and Al framework substitution with other metals, was found to have a strong effect on the number of Li reversibly inserted into the structure and the shape of the voltage profile.

Si has been considered as a promising alternative anode for next-generation Li-ion batteries (LIBs) because of its high theoretical energy density, relatively low working potential, and abundance in nature. However, Si anodes exhibit a rapid capacity decay and increase in the internal resistance, which are caused by the large volume changes upon Li insertion and extraction. This unfortunately obstructs their practical applications. Therefore, managing the total volume change remains a critical challenge for effectively alleviating the mechanical fractures and instability of solid-electrolyte-interphase products. In this regard, in spite of many new ideas being published, all of them are still from practical implantation in the Li-ion batteries. Accordingly, it is inevitable to composite with the graphite to minimize the volume change and to balance with the cathode material. In this talk, I am going to present new advanced results of the Si and graphite composites with reversible capacity of < 600 mAh/g, which can be immediately implanted in the full cell.

Silicon is considered as a potential next-generation anode material for lithium ion battery (LIB). Experimental reports of up to 40% increase in energy density of silicon anode based LIBs have been reported in literature. However, such increase in energy density is achieved when silicon anode based LIB is allowed to swell more than graphite based LIB and beyond permissible limits. For practical applications such as in automotive or mobile devices, one cannot have any volume expansion. We determine the theoretical bounds of silicon composition in a silicon – carbon composite (SCC) based anode to maximize the volumetric energy density of LIB by assuming no increase in the external dimensions of the anode during charging. The porosity of SCC anode is adjusted to accommodate the volume expansion during lithiation. The determined threshold value of silicon was then used to calculate the volumetric energy densities of SCC anode based LIBs and improvement over graphite anode based LIBs for three types of cathodes - lithium cobalt oxide (LCO), lithium manganese cobalt oxide (NMC) and lithium nickel cobalt aluminum oxide (NCA), and at a constant cathode thickness of 70 μm. The maximum improvement in the volumetric capacity of SCC anode based LIB over graphite anode based LIB for LCO, NMC and NCA cathodes was determined to be ~20%, ~22%, and ~24%, respectively. Theoretical maximum in the volumetric capacity and energy density is obtained when it is assumed that there is zero porosity in the lithiated anode and that the displaced electrolyte does not need additional volume. The level of practically achievable improvements in capacity and energy density of silicon anode based LIBs is expected to be between 5-15% for lithiated anode porosities of 10-30% to ensure the battery has similar life and power characteristics of conventional LIB.

Metal fluoride conversion electrodes have been of fundamental interest as high energy density electrodes for lithium batteries for over 40 years, however, the theoretical electrochemical activity of such materials remained elusive as a result of their high bandgap and poor ionic and electronic charge transport characteristics. Well over a decade ago, electronically and mixed conducting matrices to form metal fluoride nanocomposites resulted in the revelation of the theoretical voltages, high energy densities, and minimal reversibility of some of the most promising fluorides and oxyfluorides which operate over 2V. Since this time many in our community have investigated these materials and advanced the state of the science significantly. This paper will discuss a sampling of the scientific, technological, and practical questions that still stand today as supported by examples of research from the community and our laboratories.

Achieving solid state batteries that operate at room temperature is an elusive, but compelling goal, one that researchers have been working on decades. Solid state batteries hold the promise of much safer and robust energy storage, with potentially higher energy density as well. But the challenges for a thin, stable solid electrolyte with adequate transport and mechanical properties, plus a practical route for large scale manufacturing is daunting. Perhaps more worrisome is the challenge of a stable interface with electrodes being single phase or solid composites. How can the electrodes cycle many times while still maintaining good physical contact and low resistance to ion transport with the solid electrolyte? Should we perhaps compromise with an “almost all” solid state battery? What energy densities can we reasonably expect? Can advanced manufacturing methods provide solutions?
This presentation highlights recent research at ORNL and with our collaborators, as well as reports from other groups, that clearly point out the challenges facing solid state battery development, where we lack fundamental understanding of materials and interfaces, and pragmatic approaches that might move us toward a near term success.
Acknowledgements: The presenter thanks co-editors, William E. West and Jagjit Nanda, and the contributing authors of the Handbook of Solid State Batteries, 2015, for their insights. Research conducted at ORNL was supported by the U.S. Department of Energy through the Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division (for inorganic solid electrolytes) and through the Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office, Advanced Battery Materials Research program (for polymer and composite electrolytes).

Achieving significant gains in energy density and specific energy beyond lithium ion battery technology will require the use of alkali (lithium) and alkaline earth (magnesium) metals as anodes. Power requirements over a practical temperature range necessitate the use of these reactive metals in direct contact with a liquid electrolyte resulting in parasitic reactions yielding solid electrolyte interphases that control the accommodation and removal of metal during energy release (discharge) and energy storage (charging cycles). Where Li dendrite formation is the most readily recognized form of loss of dimensional control leading to safety concerns, structural changes that lead to Coulombic efficiency loss are far more common for Li anodes in oxygen and sulfur cells and for Mg anodes coupled with insertion and sulfur cathodes. We explore the origins of loss of dimensional control due to film formation within the Mg system starting with ether-based Mg chloro complex forming electrolytes. Well faceted Mg deposits form in these electrolytes as step-flow growth dominates deposition. Using a combination of chronopotentiometric trace analysis and operando imaging and spectroscopic analysis of the interface during metal addition and removal, we show that interfacial films are responsible for guiding localized dissolution phenomenon that result in cumulative morphology evolution leading to Mg loss with repeated cycling. Interfacial films also play an important role when weakly coordinating anions are used in the place of chloride to deliver the cation to the Mg surface. In these systems, re-nucleation of Mg onto itself plays a dominant role in defining structure and in dictating subsequent efficiency loss. We compare interfacial film composition for several weakly coordinating anions, including bis(trifluoromethylsulfonyl)imide, and contrast film identity and role when chloride anion is present. Lastly, we focus on recent work that explores the role of lithium fluoride – lithium imide salt combinations in ethers at concentrations that yield a solvate electrolyte. Within this system of electrolyte, we probe the role the fluoride film plays in directing Li accommodation and removal from the Li substrate. Loss of dimensional control is probed as a function of local ionic transport within and mechanical properties of the film.

This work is supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science. Sandia is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. DOE’s NNSA under contract DE-AC04-94AL85000.

Interphase has been the central component that enables battery chemistries of high voltage to reversibly operate, the prominent example of which is the very successful Li-ion ^{1, 2}. The possibility of such a protective interphase has been confined to non-aqueous electrolytes thus far, where the carbonate solvents serve as the main contributor of chemical building blocks of interphase. Recently, we found that by manipulating the inner solvation sphere of Li-ion, one could form such interphase in aqueous electrolytes ³. The expanded electrochemical stability window of such new electrolytes opens new possibilities of aqueous electrochemical devices. In this talk we will examine the criteria for electrolyte components that enables the formation of aqueous SEI as well as the formation mechanism involved.

The goal of surface science research is to provide atomic level understanding of the structural and dynamic properties of surfaces, a goal particularly relevant for chemical applications, including catalysis, photochemistry, batteries and fuel cells. With X-ray Photoemision Spectroscopy (XPS) and X-ray absorption Spectroscopy (XAS) we can determine composition and electronic structure. With Scanning Tunneling Microscopy (STM) we can image atoms and molecules as they adsorb, diffuse and react on surfaces. To carry out these studies in the presence of gases in the Torr to Atmospheres range, which is relevant to practical catalysis, new instrumentation is needed. Over the last years my group has developed an array of techniques, including high pressure STM, XPS and XAS, for studies of surfaces in equilibrium with gases and liquids. Using combinations of these techniques I will show how the structure of surfaces can be very different from that of the pristine bulk termination or at low coverages. I will present also recent results on the solid-liquid interface, part of our research effort to understand the nature of the electrodes and the electrochemical double layer with examples of Graphene, Au and Pt under aqueous solutions of salts and acids.

Understanding the atomic structure of layered oxides and nanosheets is complicated by the inherently large surface to volume ratio, extensive stacking disorder, and complex surface chemistry. The paper will highlight the links between these structural complexities and the electrochemical function, with focus on atomic-scale defects in the disordered 2-D nanomaterials. Our recent results demonstrate doubling the electrochemical capacity – to 300 F/g - by application of soft chemical reduction reactions that introduce cation defects into the MnO2 nanosheets.

Robust characterization of disordered layered materials is of broad interest in electrochemical applications, and we report a new approach to modeling distributions of stacking defects to access the nano and mesoscales using X-ray diffraction and X-ray pair distribution functions. Application of large supercells for modeling the XRD data via Rietveld analysis has been successful, and these models can be improved using X-ray PDF studies. PDF data can be modeled using distributions of stacking defects within the formalisms of the software DISCUS and DiffaX – resulting in statistical defect models that provide the foundation for defect engineering in oxide nanosheets that are disordered on the mesoscale. We demonstrate the application of our new methods to MnO2 nanosheet electrodes in two forms: as 3-D porous solids in working electrodes; and as free nanosheets in aqueous suspension before incorporation into electrodes. The results show that the electrochemical charge capacity can be directly linked to the cation defects that take the form of out-of-plane [MnO6] octahedra that disrupt the perfection of the nanosheet.

Charge transfer is the most important functionality of electrodes. Typically, charge transfer is assed with electroanalytical methods. With increasing complexity, particular chemical complexity of electrode materials and architectures, it become smore and more difficult to comprehend the charge transfer of electrodes with their immediate environment, be it the electorolyte or be it the source/sink of redox partners. Hence, an "integrated" signal like from electrical measurements permits only limited information on the structural origin of charge transfer. This holds particularly in systems where the electrode meets a liquid.
I will show how x-ray and electron spectroscopy have emerged i the recent years so wel that it is possible to virtually "deconvolute" the charge transfer between electrode and liquids or gases with element specific and even orbital specific sensitivity.
In the beginning it was not so easy to design targeted experiments where "living" systems such as batteries or fuel cells or photoelectrochemical cells could be invrstigated and monitored "operando", means in operation. But it works now. we can look into electrochemical interfaces with very high spatial resolution and energy and momentum transfer resolution.
Therefore I will shows examples from b attery research and PEC cell research, specifially with soft- ray spectroscopy, where the valence band characteristic can explain at the quantitative level where the charge transfer actually originates from, such as from O2p orbitals or from 3d orbitals, depending on case-to-case.

Fossil fuels have become increasingly scrutinised due to their environmental impact, extraction risks, and the depletion of resources. An alternative to fossil fuels is presented by the usage of CO₂ as source for the production of carbon based fuels, including methanol. The development of an efficient CO₂ reduction catalyst necessary for fuel production faces many research challenges, in particular the development of a catalyst able to direct reactions through stable intermediates, e.g. CO.

Nanoscale copper is an ideal candidate; however, high overpotentials have to be used to overcome the competition with H₂O reduction to H₂. Recently, so-called “oxide-derived” copper has been shown to overcome this problem, working at moderate overpotentials of around -0.2 V vs. RHE. This system is complicated by the fact that most oxide-derived copper samples still show a considerable amount of copper oxide (Cu₂O) present at the surface. Therefore, it is not clear if the increase in catalytic activity originates solely in the change in surface morphology of the copper or also in the presence of the oxide. As this system promises to be an excellent catalyst for the reduction of CO₂ a detailed understanding of the basis of its catalytic activity is essential and absolutely necessary for any further development.

Photoelectron spectroscopy (PES) is used widely in the solid state sciences but due to its nature as a UHV technique, a study of the solid-gas interface, which is intrinsic to the CO₂ reduction catalysis, is not possible. High-pressure x-ray photoelectron spectroscopy (HPXPS) is a recent advanced method, which allows the measurement of solid samples at elevated pressures of between 1 and 30 mbar (in comparison to 10^-9 mbar in conventional XPS). Using this method a study of the surface chemistry of the solid state catalysts as a function of sample preparation, the presence of oxide, temperature, and the influence of co-adsorbates (O₂/H₂O) becomes possible.
This work presents results on the interaction of CO₂ with the surface of oxide-derived Cu foil and nanoparticles, and in particular the behaviour of CO₂ in the presence of a thin film of pre-adsorbed water on the catalyst surface. This is possible due to the ability of accurate control of gas environment in the high-pressure reaction cell of the HPXPS system. The C 1s and O 1s core levels are used to track free gas passing over the sample surface as well as subsequent CO₂ adsorption. Furthermore, valence band spectra of the as-presented samples and samples under the presence of CO₂ and H₂O are compared and contributions of them both will be identified.

Ultimately, the presented results provide a starting point for the detailed understanding of hydrated catalyst surfaces and lead to the identification of possible ways to further improve and develop their catalytic characteristics.

Despite the pivotal role of electrode-electrolyte interfaces in the performance and lifetime of electrochemical devices, the complexity shown in their structure and chemical composition poses great challenges in gaining fundamental understanding. Advanced characterization techniques with high sensitivity in examining the surface species, specificity in identifying chemical phases, and selectivity in distinguishing surface from bulk are particularly valuable to address critical questions in this area of research. With advanced magnetic resonance techniques, including in situ and ex situ spectroscopy and imaging, the electrode-electrolyte interfaces involving high-energy intercalation-type cathodes and solid-state electrolytes have been examined. The effects of surface coatings of different types and thickness are also evaluated. This study demonstrates the unique capabilities of combined magnetic resonance techniques in probing the structure and chemical composition at the electrode-electrolyte interfaces and also provides insights regarding the origins of battery degradation and interface impedance.

Very few interfaces in lithium batteries are stable at the extreme electrochemical conditions imposed on them during operation of a lithium battery. When modern layered cathode materials are charged to high voltage in liquid electrolytes they tend to lose oxygen near the surface leading to a transformation to spinel and rocksalt phases. These phases significantly increase the cathode impedance, and lead to loss of capacity and rate performance. By presenting a detailed study of the surface phases in Ni-based layered materials I will show how first principles methods can be used to model this transformation and study the growth of impedance resulting from them.
Similarly, interfacial stability is one of the most limiting issues in solid-state batteries where the liquid electrolyte has been replaced by a solid state Li-ion conductor. Most compounds that display very high Li mobility are not stable at the extreme anode and/or cathode potentials and require passivation or the use of buffer layers. I will show how these interfacial reactions can be predicted with first principles calculations. Our results confirm that very few conductors will be stable at the high voltage of the cathode material.

The talk focuses on investigating mechanical stresses on the interface between the cathode and the electrolyte (i.e., a half-cell system). LiFePO4 is used as a cathode material and the combination of several different kinds of electrolyte is considered. Multiphysics finite element models incorporating a Fluid Flow module (CFX), a Transient Thermal module, a Static Structural module, and additional Application Customization Toolkits (ACTs) in ANSYS are developed to study mechanical stresses in the half-cell battery system during discharging. Our results provide a better understanding of mechanical stresses on the interface between the electrode and the electrolyte in lithium-ion batteries in several conditions. We explore (i) the impact of the porosity of electrode-electrolyte interface, (ii) the impact of porous electrodes in a lithium-ion half-cell, and (iii) the impact of phase transformation in porous electrodes. Specifically, we study effects of C-rate, porosity (volume fraction), viscosity, particles size, and lithiation stage during a half-cell system discharging. Moreover, our simulations demonstrate that both electrode and electrolyte material properties have greater effects when investigating mechanical stresses on the electrode-electrolyte interface. These computational models would aid on mitigating higher stresses in cathode particles to ensure longer battery cycle life.

Lithium ion batteries have been the dominant energy storage device used in consumer electronics for nearly two decades. However, conventional lithium ion batteries have not been able to cope with the ever increasing demands of electric vehicles. Lithium – sulfur (Li-S) battery, with theoretical capacity comparable to that of gasoline, is a promising candidate to meet these demands. Abundance of sulfur and environmental friendliness are other major factors which make Li-S batteries interesting and important. However, low electrical conductivity of pure sulfur cathode and loss of active material due to dissolution of intermediate polysulfides from the cathode during discharge limit the performance of Li-S batteries. Development of superior cathode in order to mitigate these problems has been a major area of research in recent times. Carbon-based materials embedded with sulfur are being explored as cathodes for Li-S batteries. In particular, graphene, which has superior electrical conductivity and mechanical flexibility, has been studied as potential component for cathodes in Li-S batteries. However, synthesis of pure graphene is very expensive and would hinder the commercialization of Li-S batteries.
Graphene oxide – sulfur composite based structures are viable alternatives that would provide required electrical conductivity, while the functional groups present in graphene oxide could act like anchors to sulfur and help reduce dissolution of polysulfides, leading to significantly improved performance of Li-S batteries. However, leveraging the advantages of both electrical conductivity and reducing polysulfide shuttle requires tuning the concentration of functional groups present in the graphene oxide. In an effort to determine the most favorable graphene oxide structure, we performed molecular dynamics (MD) simulations to calculate the mobility of polysulfide species in the vicinity of different graphene oxide structures with varying concentration of functional groups. Diffusion coefficients of lithium polysulfides were calculated along the surface of these graphene oxide sheets using MD simulations. Initially graphene oxide sheets with only epoxy, hydroxyl and carboxylic acid functional groups were simulated. Partial charge on carbon atoms and optimized structures were obtained using Density Functional Theory (DFT). A standard electrolyte DME – DOL in 2:1 and 1:1 v/v ratio was used in all the MD simulations. The density of equilibrated solvent was within 5% of the experimental value. Systems comprising graphene oxides solvated with electrolyte containing lithium polysulfides were simulated at 300K. The surface diffusion coefficients of lithium polysulfide was evaluated in the vicinity of graphene oxide and compared to that near pure graphene. In addition to providing mechanistic insight on how graphene oxides reduce diffusivity of polysulfides, our simulations also compare the effectiveness of various groups in reducing dissolution of polysulfides.

The solid|electrolyte interface is considerably more complex than the solid|gas interface. Solvated ions, solvent, and the electrode potential are all very challenging to treat theoretically at the atomistic level. In the past decade, we have applied the computational hydrogen electrode model [1], which allows us to determine the energies of reaction intermediates without explicitly treating the electrons and ions in solution. This thermochemical approach has been shown to correlate well with experimental onset potentials [2-4] and has been successfully applied to computational screening of new catalysts [5,6]. However, an understanding of charge transfer barriers, kinetics, selectivity, and pH effects all require explicit consideration of solvent and charge. In this talk, we will discuss new developments in a fully ab initio treatment of the electrochemical interface: barriers at constant potential [7], charge delocalization, and applications to hydrogen evolution and CO2 electroreduction.

[1] Nørskov, J. K., Rossmeisl, J., Logadottir, A., Lindqvist, L., Kitchin, J. R., Bligaard, T. & Jonsson, H. Origin of the overpotential for oxygen reduction at a fuel-cell cathode. The Journal of Physical Chemistry B 108, 17886– 17892 (2004).
[2] Peterson, A. A., Abild-Pedersen, F., Studt, F., Rossmeisl, J. & Nørskov, J. K. How copper catalyzes the electroreduction of carbon diox- ide into hydrocarbon fuels. Energy & Envi- ronmental Science 3, 1311–1315 (2010).
[3] Shi, C., Hansen, H. A., Lausche, A. C. & Nørskov, J. K. Trends in electrochemical CO2 reduction activity for open and close-packed metal surfaces. Physical Chemistry Chemical Physics 16, 4720–4727 (2014).
[4] Kuhl, K. P., Hatsukade, T., Cave, E. R., Abram, D. N., Kibsgaard, J. & Jaramillo, T. F. Electrocatalytic Conversion of Carbon Dioxide to Methane and Methanol on Transi- tion Metal Surfaces. Journal of the American Chemical Society 136, 14107–14113 (2014).
[5] Greeley, J., Jaramillo, T. F., Bonde, J., Chorkendorff, I. & Nørskov, J. K. Computational high-throughput screening of electrocatalytic materials for hydrogen evolution. Nature materials 5, 909–913 (2006).
[6] Greeley, J., Stephens, I., Bondarenko, A., Jo- hansson, T. P., Hansen, H. A., Jaramillo, T., Rossmeisl, J., Chorkendorff, I. & Nørskov, J. K. Alloys of platinum and early transition metals as oxygen reduction electrocatalysts. Nature chemistry 1, 552–556 (2009).
[7] Chan, K. & Nørskov, J. K. Electrochemical Barriers Made Simple. The Journal of Physical Chemistry Letters 6, 2663–2668 (2015).

Gaining a thorough understanding of electrochemical interfaces is critical for the development of next-generation energy storage/conversion devices with high energy density and long-life operation. Direct observation of structural changes provides invaluable information about charge transfer, ionic diffusion, and side reactions in electrochemical reaction processes. However there remains a challenge in detecting lithium ions in the interfacial region in operando. Recently, we have introduced neutron reflectometry [1,2] with epitaxial-film model electrodes [3-5] to elucidate the behavior of lithium ions between electrodes and electrolytes. It is well known that neutrons are powerful probes for determining structural information about light elements. Furthermore, the reflectometry analysis can provide us the concentration gradient of ionic species from the electrode to the electrolyte on battery operation. In this presentation, we focus on Li₄Ti₅O₁₂ anode and Li₂MnO₃ cathode materials due to their high lithium storage capacities in the interfacial regions [3,4,6]. Epitaxial films of Li₄Ti₅O₁₂(111) and Li₂MnO₃(001) were synthesized on Nb:SrTiO₃ substrates by pulsed laser deposition. In situ neutron reflectometry measurements were conducted on a time-of-flight reflectometer (J-PARC, BL16 SOFIA) with a Li counter electrode and a deuterated propylene carbonate electrolyte containing 1 M LiPF₆ [2]. The neutron reflectometry analyses demonstrate that the lithium concentrations in the electrode surface change just after the cell-construction (prior to electrochemical cycling). The lithium ions participate in a space charge layer formation to equilibrate the electrochemical potentials between the electrode and the electrolyte materials. The surface structures are reconstructed at the first cycle based on the initial structures, which deliver high lithium storage capacities. The change in the lithium ion distribution is also observed at the electrolyte-side interface. The depth-profile of the lithium ion distribution could give insight into research directions for electrode materials with highly-functional interfaces.
References
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6. Wagemaker, et al., J. Am. Chem. Soc., 131 (2009) 17786.

The development of new electrodes for next-generation Li-ion batteries requires fundamental understanding of how electrodes function by real time tracking the electrochemical reactions in individual particles and interfaces in composite electrodes. In-situ X-ray scattering based techniques, are powerful for real time probing charge/discharge dynamics of bulk electrodes in liquid electrochemical system, with spatial resolution limited to tens of nm scale, whereas in-situ transmission electron microscopy (TEM) can be used to identify the local reactions at interfaces and/or in individual nanoparticles with resolutions below 1 nm. However, due to the use of an open-cell configuration in these in-situ TEM platforms, only solid or ionic liquid electrolytes are allowed in the TEM measurements, due to the high vacuum of TEM column. The latest development of liquid electrochemical cells, with thin membrane windows to seal the liquid electrolyte, provides new opportunities to study the electrochemical reactions in standard liquid electrolyte. Here, we present our recent results from the development and application of the liquid electrochemical cells for correlative in-situ TEM and synchrotron X-ray studies of lithium-ion transport and reaction in individual nanoparticles during lithiation and delithiation. By using single crystalline FeF₂ nanoparticles as a model electrode system in this study, we were able to identify the anisotropic electrolyte-electrode interactions, leading to different solid-electrolyte interfaces at different crystal surfaces. The lithium-ion transport and electrochemical dynamics in individual FeF₂ nanoparticles, along with complex reactions at the electrode-electrolyte interfaces, will be discussed.

Secondary zinc-air batteries are a promising technology for portable and large-scale energy storage systems. Their major advantage over standard Li-ion batteries is that Zn-air batteries have a much higher energy density making them ideal for transportation applications. The formation of dendrites on the Zn anode during electrochemical cycling, has been a major issue for their development which has been shown to result in loss of cell performance and overall loss in device functionality when the cell short-circuits. Efforts to inhibit dendritic growth are hindered by a lack in understanding of why dendrites form during Zn deposition fundamentally. In situ electrochemical STEM (ec-STEM) characterization can directly observe a physical phenomenon that occurs on the nanoscale level as the result of an electrochemical process while simultaneously gathering the relevant electrochemical data. In situ ec-STEM studies were conducted to directly observe nucleation and growth during electrodeposition from .1M sulfuric acid. Image analysis of the videos captured during in situ ec-STEM gave quantitative information about deposition behavior. COMSOL modeling was used to better understand cell behavior such as global and local overpotential changes, solute concentration, and current distributions. During cyclic voltammetry cathodic sweep underpotential deposition was observed at -0.4V (vs pseudo Pt reference) and then normal deposition onset at -0.76V (vs pseudo Pt reference) and the peak at -0.86V (vs pseudo Pt reference). During the anodic sweep of the CV a dissolution peak was observed at 0.2V(vs pseudo Pt Reference). During deposition growth rates during the first 30 seconds of deposition as the cell reached steady were not seen to be size dependent with all deposits growing by at least 100% of area observed. The largest growth was a 300% increase in observed area on the second smallest deposit studied with an initial area of 136 mm². After steady state was achieved growth rate was dependent on the size a particle achieved, where a deposit of 552 mm² after achieving steady state grew at an average 1.2 mm²/sec over the next 90 seconds while a deposit that was 293 mm² at the beginning of steady state grew at an average 0.57 mm²/sec during the same time. The size of the deposits and their extension from the electrode being directly correlated to growth rate in steady state suggests that diffusion limited growth is the dominate force in deposition and dendrite formation. Ex situ experiments corresponding to in situ experiments were done to confirm the results seen in situ. This study has established a framework for further studies of Zn deposition out of other aqueous electrolytes.
Acknowledgements
Research was supported by following three sources: [1] The Bredesen Center for Interdisciplinary Research. [2] The DOE Office of Electricity. [3]The Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

Dynamic development of supercapacitors technology especially in terms of electrode materials design requires a novel and more in-depth approach for their investigation. Apart from numerous materials synthesis and characterization methods proposed to date, there is a need for their investigation during device operation, in order to recognize the major aspects of charge accumulation and ageing phenomena as well as the performance failure. Conventional electrochemical techniques used allow the typical parameters (capacitance, resistance) to be determined, however, the mechanism of performance degradation requires a novel insight on the overall chemistry in the system.
On one hand, the cycle life of electrochemical capacitors (especially in Electric Double Layer ones) is by definition unlimited, as there is no structural change of the electrode material and charge is accumulated only on the electrostatic manner. On the other hand, several additional processes occurring during device operation cause that cycle life is somewhat limited.
This study is majorly focused on the employment of in-situ techniques such as Raman spectroscopy or Quartz Crystal Microbalance (EQCM) for determination of charge storage phenomena and recognition of ageing factors in activated carbon-based supercapacitors.
In-situ Raman investigation for activated carbon electrodes operating in neutral aqueous media like Li₂SO₄ or LiNO₃ solutions indicated that there is a mild oxidation of positive electrode during cycling (vibration modes from oxygen-based functionalities found) whereas the surface chemistry of negative electrode is rather stable. Extended voltage, i.e. above 1.4V caused serious oxidation of the positive electrode and hydrogen storage in negative one followed by its further recombination. EQCM study confirmed significant frequency/mass variation on positive side, whereas negative electrode remained almost constant. More interesting results were obtained for carbon electrodes operating in redox active electrolytes, like KI or KBr solutions. It has been confirmed that iodide anion undergoes several redox processes and strongly interacts with activated carbon surface; formation of -I bond as well as polymeric forms of iodine/iodide species (triiodides, pentaiodides, etc.) have been observed. Moreover, oxidation of carbon surface has been identified near to iodide/iodine redox activity potentials. EQCM study confirmed the presence of various iodine specimen in the electrolyte solution with strong dependence of the potential and polarization-exposure time. Carbon ‘corrosion’ has been observed especially for more concentrated iodide solutions, however, we have proved that IO₃^- anion does not contribute significantly in this process; it has significant influence on the cyclability. In case of bromide-based solutions, it has been observed that bromide has similar affinity to carbon surface as iodide, but typical -Br bonds have not been found to date.

When a lithium-ion battery is charged, the non-aqueous electrolyte in the battery is exposed to a both strong oxidative environment at the surface of the cathode and a strong reducing environment at the surface of the anode. A good chemical/electrochemical stability of the electrolyte at both extreme ends is critic to ensure long life and safe operation of the battery. However, the dominant solvent, carbonates, is well known for their electrochemical instability at both ends. The slow reaction of the solvent with charged electrode materials at ambient condition causes the continuous loss of reversible capacity upon normal cycling/storage. The accelerated of such reactions at elevated temperatures can lead to rapid release of a large amount of heat that can drive the battery into thermal runaway mode. Hence, the thermodynamics and kinetics of the interfacial reactions between the electrolyte and the charged electrode materials are crucial knowledge for designing long-life and safe lithium batteries. In this talk, a homebuilt high precision electrochemical testing system will be introduced and case study using this system will also be discussed.

High lithium transference number (t_Li⁺) electrolytes are greatly beneficial as they could solve the problem of ion concentration gradients within the lithium cell which limit its lifetime and practical energy density. In “soggy sand” electrolytes (insulating oxide nanoparticles dispersed in typically organic lithium salt solution), lithium conductivity is enhanced (and anion conduction depressed) in the space charge zones due to the coupled effects of anionic adsorption and association-dissociation equilibrium.¹ When mesoporous silica particles are used as a solid phase, lithium can be efficiently transported through the interconnected pores.² A convenient way of circumventing particle network reproducibility and stationarity issue is infiltration of solid mesoporous silica pellets (sol-gel synthesis, ≈0.5 mm thick, S_BET≈500 m² g^-1) with liquid electrolyte, for example lithium triflate/poly(ethylene glycol) dimethyl ether. Such composite liquid/solid electrolytes exhibit high lithium transference numbers (t_Li⁺≈0.9) linked with high ionic conductivities (σ_m=0.5-0.8 mS cm^-1) and remarkably stable solid electrolyte interphases.³ Charge carrier chemistry at liquid/solid interface will be discussed in terms of various silica surface modifications. Additionally, the importance of indirect (vehicular) lithium transport mechanism will be shown using a multi-technique approach (impedance spectroscopy, DC polarization, pulse field gradient NMR).⁴ The pertinence of composite liquid/solid electrolyte materials is remarkable as they could make lithium battery separators dispensable offering excellent contact/adhesion with high power nanostructured electrodes.

1. C. Pfaffenhuber, M. Goebel, J. Popovic, J. Maier, Phys. Chem. Chem. Phys., 2013, 15(42), 18318.
2. C. Pfaffenuber, F. Hoffmann, M. Froeba, J. Popovic, J. Maier, J. Mater. Chem. A, 2013, 1(40), 12560.
3. J. Popovic, G. Hasegawa, J. Maier, in preparation
4. J. Popovic, C. Pfaffennhuber, J. Melchior, J. Maier, Electrochem. Comm., DOI: 10.1016/j.elecom.2015.09.009

Conversion electrodes (e.g. metal oxides, metal fluorides, and metal sulfides) can be lithiated through a reaction which forms a composite of metal nanoparticles embedded in lithium compounds (e.g. Li₂O, Li₂S, LiF). Through Li-induced reduction of the metal in M_aX_b (where M = metal, X= S, O, F…, etc.,) to its metallic state, and subsequent formation of Li rich compounds, conversion materials can be discharged to 3-5X the specific capacity found in intercalation materials. However, these complex conversion reactions present intrinsic limitations to the mechanical stability and reversibility of the system. Mechanically, expansion (lithiation) and shrinkage (delithiation) of the materials induces the formation of cracks and voids in the active material, leading to electrode pulverization and loss of active material. Electrically, the formation of insulating lithium compounds (e.g Li₂S, Li₂O, LiF) during the conversion reaction impedes electron pathways between active materials and the current collector, blocking electrical accessibility to the insulating domains formed in the active material and causing high overpotentials in the system.

Using a newly developed protocol for atomic layer deposition (ALD) of lithium phosphous oxynitride (LiPON)¹, we have fabricated model 3D heterostructured conversion electrodes (MWCNT@RuO₂) protected by an ALD LiPON solid electrolyte. The LiPON serves as a nano-cladding layer to provide (1) high conductivity for the Li ion transport and (2) mechanical constraints that accommodate volume and compositional changes in the conversion electrode during lithiation and delithiation. We show that the ion-conducting LiPON protective layer stabilizes this “core double-shell electrode” (MWCNT@RuO₂@LiPON), decreasing overpotentials of the system and enhancing capacity retention during cycling, i.e. enabling reversibility.
Reference:
[1] Kozen, A. C.; Pearse, A. J.; Lin, C.–F.; Noked, M.; Rubloff, G. W., Atomic Layer Deposition of the Solid Electrolyte LiPON. Chem. Mater. 2015, 27 (15), pp 5324–5331

The design of safe, high-energy batteries with prolonged lifetime greatly requires understanding the interface reactions on the electrodes and the properties of the formed solid electrolyte interphase (SEI) film. Most of the naturally formed SEI films are unstable and prone to have continuous side reactions that induce capacity decay of Si-based Li-ion batteries (LIBs). A cost-effective way to modify the surface chemistry in LIBs is to use electrolyte additives, rather than modify electrode architecture. The objective of the present study is to understand the influence of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) additives on SEI formation mechanism and mechanical properties, which may substantially affect the fracture behavior of electrode.

A significant improvement in the cycling performance of Si electrodes can be achieved under operation conditions of optimum VC and FEC concentration. To better understand the additive concentration effect, the reduction mechanisms of VC and FEC are investigated for both Si nanoparticle and Si(100) single crystal working electrodes. Ex-situ ATR-FTIR spectra of Si electrodes for 5% VC additive show a broad peak at a higher wavenumber (1831 cm^–1) compared to the EC residual electrolyte (1799 and 1769 cm^–1). Based on synthesized poly-VC standard spectra, the VC additive reduction compound is assigned to poly-VC or its oligomer, which is consistent with previous reports. However, for Si electrode and FEC additive, the original C=O stretch (1829 and 1802 cm^–1) disappears or shifts to a lower wavenumber, indicating alkyl carbonate structure. The reduction path of VC additive is polymerization and the reduction product remains a “ring” structure (C=O peak shifts to a higher wavenumber). This polymer reduction product effectively fill into tiny surface cracks and bridge the crack faces, thus increase the fracture toughness and prevent crack further propagation. The reduction path of FEC is close to that of ethylene carbonate (EC), leading to reduction product of open “ring” structure, similar to that produced from EC reduction. This organic salt reduction product stuff the surface cracks and passivate fresh faces, although these discrete and small molecules are not effective in inducing very tough SEI film. The current work shows different additives affect not only the chemical composition, but also mechanical properties of SEI film, which in turn influences the electrode failure. While less reported in previous literature, we believe this study will bring more researchers' attention to enhance electrode robustness by engineering additives.

Acknowledgement:
This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Freedom CAR and Vehicle Technologies of the U.S. Department of Energy under contract No. DE-AC02-05CH11231.

Tailoring electrolyte electrochemical properties is critical for stabilizing the electrode – electrolyte interfaces and enabling novel electrochemical couples in lithium batteries. The rational design of an electrolyte requires not only the knowledge of the limits of electrolyte electrochemical stability and electrolyte decomposition reactions that might or might not occur at the surface of the electrode but also understanding of the structure and transport properties of the passivation film formed at the anode and/or cathode surfaces.
I will discuss application of the distributed multi-scale computing framework¹ for high-throughput screening of solvent and salt electrochemical stability and decomposition reactions in the multicomponent electrolyte. When combined with accurate predictions of the electrolyte structure and transport obtained from molecular dynamics simulations, such calculations provide insight into the mechanism of the in-situ formation of anode passivation layer and electrolyte stabilization at the cathode. A particular example of the protective anode passivation layer the aqueous electrolytes will be discussed. An unusual structure of the highly concentrated aqueous electrolytes predicted from molecular dynamics simulations will be related to its transport properties.

All-solid-state Li-ion batteries based on solid electrolyte materials potentially provide intrinsic safety, high energy density, and enhanced cyclability. The key problem to enable this new battery technology is the high interfacial resistance and interfacial degradation at the electrolyte-electrode interfaces. In this presentation, I will demonstrate computational modeling to provide unique insights into the fundamental mechanisms at these buried interfaces, which are difficult to access in experiments. Our computation results suggest the formation of decomposition interphase layers and solid-electrolyte-interphases in all-solid-state Li-ion batteries. The properties of the interphase layers were found to significantly affect the electrochemical performance of the all-solid-state Li-ion batteries. The mechanisms of artificial interfacial layers to improve the interfacial properties have been revealed by our computation. In addition, the strategies for interfacial engineering are also suggested to address various problems at the electrolyte-electrode interfaces.

Owing to its ease of preparation, low cost and non-toxicity, LiMn₂O₄ (LMO) has become a promising candidate to replace LiCoO₂ as a cathode material in high power Li-ion batteries. However, the performance application of LiMn₂O₄ cathode is limited by capacity fade, particularly at high temperatures. It is believed that the capacity fade of LiMn₂O₄ is driven by the loss of Mn resulting from the disproportionation of surface Mn³⁺ with the resultant divalent Mn ions dissolving into stray moisture and electrolyte. In this work, we investigate possible candidate LMO dopants that can inhibit or reduce Mn dissolution. Several dopant-selection criteria are considered, and the electronic structure of the doped LMO will be discussed.

There is increasing interest in designing and manufacturing high-performance batteries for a wide variety of applications; however, selecting the best electrochemical system (i.e. electrode and electrolyte materials) and operational parameters is a challenging task. In emerging metal-air batteries, choosing an optimal electrolyte is particularly daunting given the complexity of molecular interactions and transport in these multiphase systems. Herein we propose a computational approach to model the influence of liquid electrolyte properties on electrochemical function in porous air electrodes. Through a combination of techniques (e.g. ab initio calculations which provide fundamental parameters to molecular dynamics which in turn provide transport coefficients to a full-scale model battery) we outline a methodology for predicting battery performance as a function of electrolyte choice that correlates well with data obtained from experimental Li-air cells. Differences between the proposed battery model and standard polagraphic relationships like that of Nicholson and Shain^[1] will be highlighted. While the concept of computational screening is not new, here we focus on connecting electrolyte properties directly to battery function.

[1] R. Nicholson, I. Shain, Anal. Chem. 1964, 36, 706–723.


Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Sulfide materials have been established as the treasure of fast lithium conducting materials, and Li₃PS₄ seems promising being part of the thio-LISICON family. Although has been recognized far back then in 1982, this material does not receive much attention due to the phase transformation from the highly ionic conducting β-Li₃PS₄ phase to the γ-Li₃PS₄ phase with relatively low conductivity when cooling down to room temperature from high temperature.^[1] Until recent Zengcai Liu et al.^[2] managed to synthesize room temperature stabilized nanoporous β-Li₃PS₄ with nanometer-sized framework which shows having conductivity even higher than the primitive bulk phase β-Li₃PS₄ and good cycling performance against lithium metal electrodes. The much improved performance is credited to the reduction of dimensions and increase of surface area by the author and later It triggered broad discussions about the actual mechanism. N. D. Lepley et al.^[3] made comparative first principle calculation study between Li₃PO₄ and Li₃PS₄, and ascribed the stabilization to the kinetic barriers and enlarged surface considering the surface energy effect. After that Mallory Gobet et al.^[4] carefully studied the structural evolution and Li dynamics in the synthesis procedure using the Pulsed-Field Gradient NMR technique and pointed out the possible existence of (PS₃O)^3- units which raised the question whether the enhancement of stability and conductivity of nanoporous β-Li₃PS₄ is caused by the O dopant.
In this report with the aid of first principle DFT computation and quasi-empirical bond-valence calculation, the effect of O atoms doping at S site is revealed. A single O dopant can not only lower the local vacancy hopping barriers a lot but also equal the vacancy formation energy of Li ions at neighboring 8d site and 4b site. Thus an O dopant can open a portal for the 8d site Li ions hopping to the fast transportation path consisting of the 4b and 4c sites. Besides, thermodynamic calculation shows that the β-Li₃PS₄ phase can become energetic favorable with more O atoms doping. Without undermining the originally wide electrochemical window of β-Li₃PS₄ the O atoms doping benefits the stability of electrolyte-Li metal interface. Based on all these simulation results hopefully our research can bring a new possible explaination for the modified properties of nanoporous β-Li₃PS₄ to the researchers.


[1] K. Homma, M. Yonemura, T. Kobayashi, M. Nagao, M. Hirayama, R. Kanno, Solid State Ionics 2011, 182, 53.
[2] Z. Liu, W. Fu, E. A. Payzant, X. Yu, Z. Wu, N. J. Dudney, J. Kiggans, K. Hong, A. J. Rondinone, C. Liang, Journal of the American Chemical Society 2013, 135, 975.
[3] N. D. Lepley, N. A. W. Holzwarth, Y. A. Du, Physical Review B 2013, 88.
[4] M. Gobet, S. Greenbaum, G. Sahu, C. D. Liang, Chemistry of Materials 2014, 26, 3558.

Understanding structure-property relationships at critical interfaces in such applications as energy storage and photocatalysis represents a significant challenge for theory. For aqueous interfaces, important examples include the alignment of electrochemical redox levels with the semiconductor band edges, the identification of catalytic active sites at the interface and pathways for electron and proton transfer. I will describe the approaches we have developed to go from structure discovery to evaluation of specific characteristics.

We investigate the interface structure using ab initio molecular dynamics (AIMD), acknowledging limitations this imposes on the degree of structure reorganization [1]. Our initial studies focused on GaN, ZnO and their alloys, motivated by experiments showing high efficiency for photocatalytic water oxidation under visible light for alloys. At the non-polar (10-10) facet of GaN, a full monolayer of water at the interface spontaneously dissociates forming -N-H and -Ga-OH interface motifs. For ZnO, a mixed state is found, with some -Zn-OH₂ motifs as well. We have extended these studies to the stable TiO₂ anatase (101) and rutile (110) facets. The AIMD calculations reveal interface water dissociation and reassociation processes through several distinct pathways, both direct at the interface and engaging spectator water molecules. Further, the energy landscape for these pathways depends on the interface structure.

We calculate the semiconductor band edge alignment to the electrochemical levels by starting from the average interface dipole deduced from the AIMD simulations. Then the GW approach from many-body perturbation theory, which corrects for errors associated with Kohn-Sham eigenvalues, is used to calculate the energy band offset to the centroid of the occupied 1b₁ energy level of water and thus, the alignment to electrochemical levels [2]. We find that the degree of interface water dissociation and structure-controlled dynamical effects make significant contributions.

Finally, we use both slab and cluster models to study reaction pathways [3]. For GaN, an injected hole tends to localize on the -N-H, but study of reaction pathways shows that the first step of the water oxidation process goes through the -Ga-OH motif, leaving a hole localized on -Ga-O. Local water dissociation affects the localization of injected holes at the interface for the TiO₂ interfaces, with implications for photocatalyzed water oxidation.

Work performed in part at the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.

[1] N. Kharche, et al., Phys. Chem. Chem. Phys. 16, 12057 (2014).
[2] N. Kharche, et al., Phys. Rev. Lett. 113, 176802 (2014).
[3] M. Z. Ertem, et al., ACS Catal. 5, 2317 (2015).

Electrochemical interface has attracted considerable attention in material science as a key to understanding the energy conversion problems. Contrary to typical solid-solid and solid-vacuum interfaces, of which modern ab initio simulations have predicted the properties with increasing success, the electrochemical interface is still challenging theoreticians to adapt the simulation tool to this non-equilibrium and open interface. To attack this problem, we have been developing methods to apply the bias potential to the interface [1] and to control the bias, or the excess interface charge, within the ab initio molecular dynamics scheme [2]. With the method, we have performed large-scale simulation on the platinum-solution interface, to understand the electric double layer (Helmholtz layer) and electrochemical reactions occurring therein, including the hydrogen oxidation reaction [3] and the oxygen reduction reaction [4].
Reaction mechanism research, however, requires a large number of simulations to quantitatively predict the free-energy, which is not affordable yet. In addition, the density functional theory used in our simulation is not accurate enough in the energy-scale corresponding to the room temperature (kT). Yet, by combining most recent experimental and theoretical data, it is possible to improve understanding on the mechanism [5]. We will demonstrate some of the related topics in this talk.

References
[1] M. Otani and O. Sugino, Phys. Rev. B 73 (2006) 115407(1-11).
[2] N. Bonnet, T. Morishita, O. Sugino and M. Otani, Phys. Rev. Lett. 109 (2012) 266101(1-5).
[3] M. Otani, I. Hamada, O. Sugino, Y. Morikawa, T. Ikeshoji and Y. Okamoto, J. Phys. Soc. Jpn. 77 (2008) 024802.
[4] Y. Okamoto and O. Sugino, J. Phys. Chem. C 114 (2010) 4473.
[5] N. Bonnet, M. Otani and O. Sugino, J. Phys. Chem. C 118 (2014) 13638.

The doping density dependence of photocurrents has been experimentally measured at single crystal rutile TiO₂ electrodes sensitized with the N3 chromophore and a thiacyanine dye. As the doping density of the electrodes was varied from 10¹⁵ cm^-3 to 10²⁰ cm^-3, three different regimes of behavior were observed for the magnitude and shape of the dye sensitized current-voltage curves. Low-doped crystals produced current-voltage curves with a slow rise of photocurrent with potential. At intermediate doping levels, Schottky barrier behavior was observed producing a photocurrent plateau at electrode bias in the depletion region. At highly doped electrodes tunneling currents played a significant role especially in the recombination processes. These different forms of the current-voltage curves could be fit to an Onsager-based model for charge collection at a semiconductor electrode. The fitting revealed the role of the various physical parameters that govern photoinduced charge collection in sensitized systems.

We reveal the general mechanisms of partial reduction of multivalent complex cations in conditions specific for the bulk solvent and in the vicinity of the electrified metal electrode surface and disclose the factors affecting the reductive stability of electrolytes for multivalent electrochemistry. Using a combination of ab initio techniques we clarify the relation between the reductive stability of contact-ion-pairs
comprising a multivalent cation and a complex anion, their solvation structures, solvent dynamics, and the electrode overpotential. We found that for ion-pairs with multiple configurations of the complex anion and the Mg-cation whose available orbitals are partially delocalized over the molecular complex and have anti-bonding character, the primary factor of the reductive stability is the shape-factor of the solvation sphere
of the metal cation center and the degree of the convexity of a polyhedron formed by the metal cation and its coordinating atoms. We focused specifically on the details of Mg (II) bis(trifluoromethanesulfonyl)imide in diethylene glycol dimethyl ether (Mg(TFSI)₂)/diglyme) and its singly charged ion-pair, MgTFSI⁺. In particular, we found that both stable (MgTFSI)⁺ and (MgTFSI)^o ion pairs have the same TFSI con-
figuration but drastically different solvation structures in the bulk solution. This implies that the MgTFSI/dyglyme reductive stability is ultimately determined by the relative time scale of the solvent dynamics and electron transfer at the Mg-anode interface. By examining other solute/solvent combinations, we find that the electrolytes with highly coordinated Mg cation centers are more prone to reductive instability due to the chemical decomposition of the anion or solvent molecules. The obtained findings disclose critical factors for stable electrolyte design and show the role of interfacial phenomena in reduction of multivalent ions.

Chemical and electrochemical interactions between positive electrode materials and electrolyte solutions and the evolution of the transport properties of subsequent products represent some of the most studied but still least understood lifetime determining phenomena occurring in batteries. This stems from a combination of characterization difficulty, inhomogeneity of products, and the real world deviation from ideal surfaces. The latter presents a further degree of challenge to theoretical modelling of such interactions. Despite the lack of a comprehensive understanding, experimental data from many groups over the past 20 years have consistently shown that the control of the positive electrode surface chemistry and the management of deleterious reactions through the chemistry of the electrolyte have resulted in truly extraordinary stabilization of the electrochemical performance. This talk will explore some of the relevant examples in contrasting electrochemical systems we have explored in our group and collaborators over the past two decades. The intent is to extract some of the common performance defining motifs and address some of the distinct differences observed. Examples from LiMn₂O₄, LiCoO₂, metal fluoride conversion, LiNi_0.5Mn_1.5O₄, and LiNi_0.8Co_0.15Al_0.05O₂ based positive electrodes will be presented.

Nuclear Magnetic Resonance (NMR) methods have contributed much to battery and fuel cell materials science. Because it is a radiofrequency spectroscopy with a consequently relatively small Boltzmann factor at room temperature, NMR is most often employed as a bulk materials characterization technique. Nonetheless, recent advances in detection sensitivity enhancement and the use of isotopic enrichment have made possible the study of solid interfaces and interphases on electrode surfaces. Several examples of these investigations from our lab as well as others, using ¹³C-enriched carbonate solvents, will be reviewed. Solution phase NMR can also yield insight into SEI formation by elucidating the nature of the Li⁺ or Na⁺ ion solvation sheath in binary or tertiary solvents. Several cases of this will be presented, in particular involving the use of natural abundance ¹⁷O NMR.

The efficient electrocatalysis of the oxygen evolution and reduction reactions (respectively, OER and ORR) remains a serious challenge for the development of electrochemical energy conversion and storage devices such as fuel cells, electrolyzers, and metal-air batteries. New non-noble metal catalysts and improved understanding of their interfacial mechanisms are necessary. This presentation will describe a new class of alkaline OER electrocatalysts based on layered lithium metal oxides. In this class of materials, the catalytic activity can be tuned by careful design of the metal composition and content of the layered oxide. First, the results of a systematic screening study reveal that within this class of materials, the highest activity is obtained in nickel-rich phases (Ni ≥ 0.7). This activity can be dramatically improved by substituting ~10% of the Ni with Fe, possibly due to the flexibility of Fe to adopt different surface coordination geometries. The ability to tune the oxidation state of the transition metal via lithium content is also tantalizing. The Li content can be varied during the solid-state synthesis as well as after synthesis by either chemical or electrochemical delithiation. During the solid-state synthesis, Li deficiency introduces Ni²⁺ into the structure which is correlated with increased cation mixing and poor electrocatalytic activity. Electrochemical delithiation occurs in situ during electrocatalytic cycling of these materials in an alkaline electrolyte and at highly oxidizing potentials. This electrochemical delithiation is followed by the emergence of pseudocapacitive redox peaks which indicate the presence of an in situ transformation process. Chemical delithiation also leads to surface transformation, in both the surface area and structure. These results highlight both the promise and challenges of lithium metal oxides for catalyzing the OER.

The application of Li-ion batteries for vehicle-electrification and grid-storage is deterred by their safety, environmental and cost concerns, which are mainly imparted from the non-aqueous electrolytes used therein. These obstacles could be circumvented by an aqueous alternative; however, narrow electrochemical stability window (1.23 V) of the latter, imposed by hydrogen and oxygen evolutions at anode and cathode, respectively, sets an intrinsic limit on the practical voltage and energy output of an aqueous Li-ion cell. Here, we report a new aqueous electrolyte, whose electrochemical stability window was expanded to ~3.0 V via super-concentration and concomitant interphasial chemistry. A full Li-ion battery of 2.3 V was demonstrated to cycle over 1000 times in this electrolyte, with nearly 100% Coulombic efficiency at both low (0.15 C) and high (4.5 C) rates. For the first time, breaking Pourbaix-limits makes it possible for an aqueous Li-ion chemistry to deliver energy density over 100Wh/kg.

The motion of free charges in solids at solid-electrolyte interfaces plays a role in how charge transfers and the efficiency of the transfer. In order to better understand this role, ultrafast transient optical reflection and transient optical grating methods, sensitive to both charge diffusion and trapping, are applied to investigate n-doped gallium nitride (n-GaN). n-GaN, a wide bandgap semiconductor with a high ultraviolet absorption coefficient and stable photocatalytic activity for water oxidation, serves as a useful model for extracting the fast photo-excited carrier diffusion and recombination kinetics at the n-GaN/electrolyte interface during heterogeneous catalysis. In particular, with enhanced surface selectivity via heterodyne transient grating reflectivity spectroscopy, we relate how the interfacial material properties are modified by chemical adsorbates and the electrostatic Helmholtz double layer. Although the recombination kinetics remain similar when altering the n-GaN surface (i.e. in air vs. in 0.1 M HBr (pH = 2)), we observe a change in the surface hole diffusion by a factor of two. This suggests the intrinsic mobility also plays a role in catalytic processes. By following not only trapping mechanisms but also diffusion at the catalytic surface, we hope to identity active intermediate sites and elucidate the initial water oxidation mechanism following hole injection to the electrolyte.

The cubic garnet-type solid electrolyte Li₇La₃Zr₂O₁₂ with aliovalent doping exhibits a high ionic conductivity, reaching up to ~10^-3 S/cm at room temperature^[1,2]. Fully understanding the Li⁺ transport mechanism including Li⁺ mobility at different sites is a key topic in this field, and Li_7-2x-3yAl_yLa₃Zr_2-xW_xO₁₂ (0 ≤ x ≤ 1) are selected as target electrolytes. X-ray & neutron diffraction as well as AC impedance results show that a low amount of aliovalent substitution of Zr with W does not obviously affect the crystal structure and the activation energy of Li⁺ ion jumping, but it does noticeably vary the distribution of Li⁺ ions, electrostatic attraction/repulsion, and crystal defects, which increase the lithium jump rate and the creation energy of mobile Li⁺ electrostatic attraction/repulsion, and crystal defects, which increase the lithium jump rate and the creation energy of mobile Li⁺ ions. For the first time, high resolution NMR results show evidence that the 24d, 96h, and 48g sites can be well resolved. In addition, ionic exchange between the 24d and 96h sites is clearly observed, demonstrating a lithium transport route of 24d-96h-48g-96h-24d. The lithium mobility at the 24d sites is found to dominate the total ionic conductivity of the samples, with a diffusion coefficient of 10^-9 m² s^-1 and 10^-12 m² s^-1 at the octahedral and tetrahedral sites, respectively.
The work carried out here sets up a direct relationship between the macro-scale ionic conductivity and micro-scale lithium dynamics, i.e., lithium jump rates and/or lithium diffusion coefficients, at different sites, and provides a deeper understanding of lithium transport in the lithium-stuffed Li_7-2x-3yAl_yLa₃Zr_2-xW_xO₁₂ structure. It is anticipated that a combination of AC impedance testing (at high and low temperature ranges) and the use of the ss - NMR technique would be a powerful way to study the lithium transfer performance and could be helpful in understanding the lithium dynamics in all solid electrolytes.
References
1. Allen, J. L.; Wolfenstine, J.; Rangasamy, E.; Sakamoto, J. Effect of Substitution (Ta, Al, Ga) on the Conductivity of Li₇La₂Zr₂O₁₂. J. Power Sources 2012, 206, 315-319.
2. Dhivya, L.; Janani, N.; Palaniverl, B.; Murugan, R. Li⁺ Transport Properties of W Substituted Li₇La₂Zr₂O₁₂ Cubic Lithium Garnets. AIP Advances 2013, 3, 082115.

Reliable and low cost energy storage systems are widely recognized as critically important to address the grand energy challenges for a sustainable society, and currently substantial efforts are devoted to this area. Among the post lithium ion technologies, batteries that based on the Mg metal anode hold particular promises due to the divalent nature of Mg²⁺ ions and the intrinsic safety and wide availability of Mg materials. Mg/Li Hybrid batteries that combined magnesium and lithium chemistry are able enable the advantages of different materials and delivered outstanding performance, but currently demonstrated systems suffer from low voltage. In this work we explored the design of high voltage hybrid batteries through rational control of the electrolyte chemistry and electrode materials. We were able to construct cells with voltage around 3.0V and excellent cyclic stability.

A general route to fabricate all solid micro-lithium ion batteries was proposed and developed by s3D printing technology. A typical printed devices was composed of the printed Ag electrode, active materials and PVA/H₃PO₄ solid electrolyte. we report the high performance and excellent cycling stability for the printable energy storage devices. The devices could be used as on-chip power, on-spot power, or being part of the 3D printable devices. Furthermore, new carbonaceous and metal-oxide/carbonaceous nanomaterials have been used as active materials to provide novel electrochemical mechanism for the devices. The results demonstrate a general route and promising future for 3D printing technology for various devices and compatible with different industiral process.

Electrochemical Double Layer Capacitors (or EDLCs) attracted considerable attention since they combine high energy and power density with an unmatched stability¹. Moreover, the relative ease of miniaturization of EDLCs opens a wide horizon of applications such as autonomous sensor networks, active RFID², etc.

Although carbon is the most studied electrode material for micro-EDLCs, silicon remains the material of choice for on-chip integration, provided its surface area is sufficient to allow high capacitance values. Among the various strategies to design high surface area silicon nanostructures, the bottom-up CVD approach permits a fine tuning of the electrical and morphological parameters of the nanostructures in order to obtain the highest values of capacitance, energy and power densities, while retaining high electrode conductivity and electrochemical robustness.

EDLCs based on highly doped CVD silicon nanowires (Si-NWs)³ and nanotrees (Si-NTrs)⁴ were recently designed and proved extremely beneficial in terms of electrochemical window (4 V)⁵, maximal power densities (225 mW.cm^-2) and cyclability (>10⁶ charge/discharge cycles). Adding branches to Si-NWs to yield Si-NTrs also largely improved capacitance values, proving the versatility and potentiality of the bottom-up approach. However, previously published work rarely studied the influence of the morphological parameters on the supercapacitive behavior of the SiNTr-based electrodes and several steps are still required to permit on chip integration.

In the present work, we improved previously published performances by designing advanced Si-NTrs morphologies using a simple, selective and cost effective electroless gold deposition for the CVD catalyst deposition steps. The highly conductive Si-NTrs forests were optimized concerning trunks and branches densities, length and diameters, yielding specific capacitance value as high as 1.7 mF.cm^-2 with excellent cyclability and large cell voltage (using a EMI-TFSI as the electrolyte).

These optimized Si-NTrs were successfully grown on microstructured interdigitated electrodes, paving the way to future integrated all-silicon on chip micro-EDLC. Moreover, solid state micro-EDLC were obtained combining silicon nanotrees and ionogel electrolyte, thus leading to highly stable, solder reflow resistant devices able to withstand exceptional temperature conditions with a quasi-ideal capacitive behavior. Finally, the electrode/electrolyte interface was drastically improved by highly conformal surface coatings, leading to extremely large electrochemical windows over 4 V, improved Coulombic efficiency and unprecedented cycling ability.

(1) Simon, P.; Gogotsi, Y. Nat. Mater. 2008, (11), 845-54
(2) Beidaghi, M.; Gogotsi, Y. Energy & Environ. Sci. 2014, 7 (3), 867-884
(3) Thissandier, F. et al., Nano Energy 2014, 5, 20-27
(4) Thissandier, F. et al., J. of Power Sources 2014, 269, 740-746
(5) Berton, N. et al., Electrochem. Commun. 2014, 41, 31-34

Within the current technological context, the demand on miniaturized power sources is growing. A trendy way to meet the need on power sources is the elaboration of micro devices such as microbatteries or microsupercapacitor. Hence, a lot of efforts have been focused on the development of thin film technologies. Among them, sputtering is one of the most widely used technologies to deposit thin film materials for large scale and one pot synthesis of planar power sources (electrodes, solid electrolyte, and current collectors).
In order to fulfill the performance requirements, positive electrodes with high storage capacity and high operating voltage as well as solid state electrolyte with a large electrochemical window are necessary to achieve high power and high energy densities all-solid-state Li-ion microbatteries.
Among high voltage positive electrode candidates, the use of Ni substituted LiMn₂O₄ spinel and their derivatives are a really promising candidate. Indeed, the spinel LiMn_1.5Ni_0.5O₄ (LMNO) exhibits an operating voltage of 4.75 V vs Li/Li⁺, a theoretical specific capacity of 147 mAh.g^-1 which corresponds to a volumetric capacity of 51.7 µAh.cm^-2.µm^-1 assuming an experimental bulk density of 4.4 x 80% g.cm^-3, and a good electrochemical behavior despite the decomposition of liquid electrolytes at such a high operating voltage [1].
In the present study, we focused on the elaboration of thin film LMNO positive electrode by means of RF magnetron sputtering deposition. Actually, this material has been extensively studied in the literature at macro and micro-scale as powder. But only three groups have successfully developed the LMNO deposition by sputtering [2,3,4]. A preliminary study has evidenced that the deposition pressure within the chamber and the afterwards annealing temperature were key parameters to meet the electrochemical performance requirements. In a first step, the microstructure, the surface morphology and the stoichiometry of the films were characterized as a function of the deposition pressure. Then the ex-situ annealing under air atmosphere has been tuned in order to reach good electrochemical performances: 40 µAh.cm^-2µm^-1 at 1C between 4.4 and 4.8 V vs Li/Li⁺ for 50 cycles. The synthesized LMNO electrode exhibits a remarkable electrochemical stability upon cycling, and good retention capability. Furthermore, the last results dealing with the structural change of the LMNO upon cycling as well as the development of the LMNO thin film by atomic layer deposition for 3D Li-ion solid state on chip microbattery will be shown.

Corresponding authors: [email protected], [email protected] and [email protected]

[1] Santhanam, R. et al. (2010), Journal of Power Sources 195(17): 5442-5451.
[2] Baggetto, L. et al. (2012), Journal of Power Sources 211: 108-118.
[3] Matsui, M., et al. (2010), Journal of The Electrochemical Society 157(2): A121-A129.
[4] P. Soudan,et al. (2003), ECS spring meeting

The Mg-ion battery technology is attractive merely because the 2+ charge carried by each Mg-ion can potential increase the energy storage density that will supersede the current monovalent Li-ion batteries. However, since the report of the first Mg-ion battery cell, further development of Mg-ion battery technology is hindered by the difficulties of finding new, in-expensive, high-voltage cathode material that can reversibly intercalate Mg and of establishing the corresponding electrolyte that are not corrosive and are electrochemically stable at high potential. From the literature, only a handful of cathode materials are proved applicable to intercalate Mg and the layered Molybdenum trioxide is one of them. It is recently shown that by partially replacing oxygen with fluorine, an order of magnitude higher Mg capacity can be achieved. Since there is no fundamental structural change upon F substitution, the increase of capacity maybe related to the improved Mg-ion mobility in the F-substituted MoO₃ lattice. In this work, we use density functional theory to examine the impact of F substitution on the electronic structure of MoO₃. The diffusion behavior of Mg-ion is studied using the climbed nudged elastic band method. It is found that the layered MoO₃ structure can provide a 3-dimensional diffusion channel and upon F substitution, certain Mo-anion bonds are softened thus facilitates Mg diffusion.

This work is supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.