Symposium F.EN06—Advancement of Lithium-Based High-Energy Density Batteries at Multiple Scales, Factoring in Safety

In the past few years, due to rapidly increased global demand for energy storage devices such as advanced electronics, electric vehicles (EVs), and implantable medical devices, it is essential to develop novel materials for energy storage technologies. Lithium-ion batteries (LIBs) have been the most popular and commercially available energy storage devices. However, anodes of LIBs still suffer poor theoretical capacity of graphite (372 mAh g^-1). Recently, phosphorus as an anode material has attained much attention owing to its high theoretical capacity (2596 mAh/g) via Li₃P alloying formation. White P (WP), black P (BP), and red P (RP) are the main three allotropes of solid phosphorus. WP is not stable in the air, flammable, and very toxic, which impose safety concerns for LIBs. BP is the most thermodynamically stable form of phosphorus with an orthorhombic crystal structure. However, the synthesis of BP via high-pressure and high-temperature process is expensive, which restricts its practical application. Compare to WP and BP, RP is non-toxic, low cost, eco-friendly, and easy to handle, making RP a promising anode for LIBs. Nevertheless, the application of RP in LIBs is obstructed by two serious issues: low electrical conductivity of RP (∼ 10^-14 S cm^-1), and huge volume change (more than 300 %) during lithiation and delithiation causing the pulverization of the active materials which leads to significant capacity fading, and poor cycling performance. Compositing phosphorous with carbonaceous materials and construing stable architecture are the two main methods that have been evaluated to resolve the issues and improve the electrochemical performance of P-based electrode materials. Moreover, the chemical bond of P to the conductive matrix could contribute to the integrity of the electrode during cycling In this research, for the first time we developed a binder-free P-based anode materials with sulfurized polyacrylonitrile (SPAN) as functional polymer composite fabricated through ball-milling and electrostatic spray deposition (ESD) method. SPAN-based electrodes have shown attractive electrochemical performance such as good cycling life and Coulombic efficiency due to its chemical compatibility for energy storage devices. In addition, due to the low bonding energy of S-S (265 kJ mol-1) and its high reactivity to form additional chemical bonds with RP, the conductive SPAN could be an ideal composite to increase the conductivity and control the volume expansion of RP in LIBs. The electrochemical performance of the fabricated anode materials evaluated via electrochemical impedance spectroscopy, cyclic voltammetry, and galvanostatic charge/discharge tests. The RP-SPAN hybrid electrode exhibited excellent specific capacity up to 1605 mAh g^-1 at 0.1 A g^-1 at 100 cycles, which is one of the best reported performances among the RP-based composite electrode in LIBs. Moreover, a specific capacity of 320 mAh g^-1 was maintained at 3 A g^-1. Due to the SEM images, the hybrid electrode still maintains its microstructural integrity without obvious degradation after cycling, indicating that the volume change didn’t affect the structural stability. Our results show that the ESD method is an effective approach to prepare RP-based composite with favorable morphology and composition in order to buffer mechanical stress and alleviate crack formation during cycling. The RP-SPAN hybrid delivered high specific capacity, most likely due to the synergetic effect of chemical bonding and improved kinetics. The developed anode material and its material and electrochemical characterizations will be discussed in detail at the conference.

Lithium metal plating on graphite anodes has been identified as a mechanism contributing to diminished capacity retention of Li-ion batteries under conditions of repeated fast charge. Due to mass transfer limitations, instead of lithium ion insertion in the graphite anode Li metal can deposit on the graphite anode surface when the electrode is highly polarized. The result is inactive Li metal which limits cell cycle life and raises risk of internal short-circuits due to dendrite growth. The objective of this research was to mitigate Li-plating during fast charging by increasing the overpotential of Li-deposition via interfacial surface modification. The hypothesis tested was that deposition of Cu or Ni metal on the surface of graphite electrodes can increase the Li nucleation overpotential due to crystalline mismatch and suppress Li-plating.
Thin metal films of Cu or Ni were deposited on graphite anodes. The films were characterized by a variety of methods including atomic force microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray absorption near edge structure spectroscopy. Lithium ion cells with the treated graphite anodes as well as control cells were assembled and evaluated using galvanostatic cycling. Benefit in capacity retention for the treated electrodes was demonstrated. Further, Li-plating was quantified over a series of experiments and supported the electrochemical results. The findings of this study establish that with rational design of an electrode interface, the overpotential for Li-deposition can be modulated, providing a new conceptual approach for reducing Li-plating on graphite anodes.

Cerium oxide (CeO₂) nanocomposite material for Li-ion battery is synthesized using hydrothermal method. To fabricate a hybrid nanostructure electrode, CeO₂ is coated on tin oxide (SnO₂) nanorods (NR), which is grown using a vapor-liquid-solid (VLS) technique on a steel substrate. The structure of SnO₂/CeO₂ nanocomposite was confirmed by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM). The surface morphology of the hybrid nanostructure electrode confirms that, single-crystalline SnO₂ nanorods are grown vertically with spine-like structures, a few microns in length, as evident from FESEM. The electrochemical performance of SnO₂/CeO₂ shows an enhanced charge storage capability. The coin cell shows improved capacity with a higher number of charging and discharging cycles. This SnO₂/CeO₂ hybrid composite electrode shows better specific capacitance value as compared to the pristine SnO₂ electrode and pristine CeO₂ electrode.

The use of silicon (Si) in the anodes of next-generation lithium ion batteries (LIBs) has the potential to dramatically improve electrochemical performance over current LIBs with graphite anodes, due to the higher specific capacity of silicon.¹ However, widespread implementation of Si-containing anodes is inhibited by issues such as significant volume expansion of Si during lithiation and an unstable solid-electrolyte interphase (SEI), resulting in unreliable performance and poor cycle life. Currently, composite anodes with both Si and graphite active materials are used to increase capacity and mitigate some of the limitations associated with Si. In composite electrodes with a heterogeneous distribution of components with varying electrical properties (including Si, graphite, conductive carbon additive, and binder), it is important to understand the local distribution of each component to correlate with electrochemical processes, particularly localized degradation and heterogeneous aging, and to optimize performance. The heterogeneous distribution of these components leads to localized lithiation, nonuniform SEI formation and material degradation, and complex electron transfer pathways, which all impact anode cycling and performance.

To investigate Si-containing composite anodes in the nanoscale, we employ scanning spreading resistance microscopy (SSRM), a form of scanning probe microscopy (SPM) that probes local electronic resistivities. By examining the contrast in intrinsic electronic resistivity between the anode components, separate phases can be distinguished and understood within the composite structure. Previous work has been conducted on composite Si-graphite electrodes using SSRM, as well as with X-ray tomography, to study degradation after cycling.^2,3 SSRM is able to measure composite electrode components over a wide range of relative intrinsic resistivities with nanometer-scale resolution and a smaller sampling volume than X-ray based methods, making it ideal for distinguishing anode components and determining their spatial positions. This work studies the effects of electrochemical cycling on component distribution and aging by comparing the electrical and structural evolution of composite Si-graphite electrodes before and after charge-discharge cycling. Additionally, we compare the resistivity maps to STEM-EDS and EELS results for both pristine and cycled electrodes to understand chemical changes that occur within the observed structure. By analyzing structural and chemical changes that occur during cycling, we identify degradation mechanisms and potential design improvements to increase capacity and cycle life next-generation composite anodes.

1. W. J. Zhang. A review of the electrochemical performance of alloy anodes for lithium-ion batteries J. Power Sources 196 13–24 (2011)
2. S.H. Kim, Y.S. Kim, W.J. Baek, S. Heo, D.J. Yun, S. Han and H. Jung. Nanoscale Electrical Degradation of Silicon-Carbon Composite Anode Materials for Lithium-Ion Batteries ACS Appl. Mater. Interfaces 10 24549–53 (2018)
3. O.O. Taiwo, M. Loveridge, S.D. Beattie, D.P. Finegan, R. Bhagat, D.J.L. Brett and P.R. Shearing. Investigation of cycling-induced microstructural degradation in silicon-based electrodes in lithium-ion batteries using X-ray nanotomography Electrochim. Acta 253 85–92 (2017)

Ni-rich Li[Ni-Mn-Co]O₂ is a promising cathode material due to its high discharge capacity, high voltage and low cost, but its application is hindered by the poor thermal stability and low capacity retention. To guide the design of Ni-rich cathode materials, multiscale modeling analysis was used to determine its atomic structure, band structure, open-circuit voltage, and mechanical properties and to predict the critical size of Ni-rich cathode single particles that are mechanically stable. First, density functional theory (DFT) calculations were performed to identify the base structure of pure LiNiO₂ layered compounds based on three different space groups, namely Rm, C₂/m, and P₂/c. It was found the calculated band structures of the widely applied Rm structure of LiNiO2 is metallic, contrary to the experimentally reported insulating behavior. P₂/c structure, which allows a disproportionation of the oxidation state of Ni (Ni²⁺ and Ni⁴⁺), is the most energetically stable and insulating layered structure of LiNiO2. Therefore, model structures representing Ni-rich NMC cathode materials were built by partially replacing Ni with Co or Mn in the base P₂/c LiNiO₂ structure. The electronic structure of model Ni-rich NMC cathode shows that Ni tends to be Ni³⁺ when neighboring with Co³⁺, while becoming Ni²⁺ next to an Mn⁴⁺. Based on the model Ni-rich cathode structures, the elastic constants and fracture energy were also computed from DFT. The information served as input to a continuum diffusion induced stress model to predict when a crack starts to propagate and leads to the fracture of a single Ni-rich NMC particle. The predicted results show good agreement with single-particle observations.

Lithium metal anode holds great promise in the next-generation battery but it suffers from great challenges in efficiency and safety. Previous research has developed various methods to mitigate these issues but most of them are still based on tradition flammable electrolyte systems. Here we demonstrate that reversible Li plating/striping process, which cannot be implemented in non-flammable phosphate electrolyte previously, can be realized by introducing oxygen as an additive in triethyl phosphate (TEP) electrolyte. The symmetric Li||Li cell can achieve more than 300 cycles at current density of 0.5 mA/cm² with O₂ existence. Mechanistic study reveals that the electrochemically induced chemical reactions between reactive O₂ species and TEP molecules alter the decomposition pathway of TEP. The possible products, Li₃PO₄/poly-phosphate thereby serve as beneficial SEI components to enable stable Li metal operation in non-flammable electrolyte. Demonstration of Li-O₂ and Li-ion battery also shows that it is promising to deploy phosphate-based electrolyte in future high-energy density batteries efficiently and safely.

There is a global effort to develop safer lithium-ion batteries with a high energy density and long lifetime. Material properties, their mutual interactions, and active interfaces are key factors in battery performance. Popular methods involve the mixing of different ingredients and development of a suitable fabrication process. Significant effort has been spent to understand the reaction mechanism of battery materials, but this is very challenging due to high system complexity. Different techniques have been developed to follow these reactions, and most significantly, the reactions between Li and electrode materials. Our team has developed a new method to characterize Li thermal diffusion and structural interaction on the surfaces of potential anode and cathode materials. This method is based on surface crystallographic and compositional characterization of deposited ultra-thin film of Li on potential single crystal anode material. For surface crystallography we are using Low Energy Electron Diffraction (LEED) and for composition Auger Electron Spectroscopy (AES). This characterization monitors Li diffusion into the electrode surface and changes in surface crystalline structure and composition providing data on Li interaction with the crystalline anode material structure. This dynamic interaction of Li is monitored at the nanoscale level and gives the direct response how Li atoms interact with the substrate. The diffusion of evaporated Li on single crystals of HOPG, Si(111)-(211)-(100), SiC-6H, Diamond and LiNbO3 have been performed. The results shows that HOPG is the excellent reference substrate for this method as Li diffuses at room temperature and surface crystallography of HOPG remains unchanged. The structural reaction of Li with Si is very strong causing LEED pattern to disappear and there is no Li diffusion at room temperature. The data for SiC-6H and LiNbO3 shows better Li diffusional behavior than on Si but not as good as on HOPG. This method of using a single crystal substrate and basic surface characterization techniques is providing simple nanoscale access into the chemical reactions between Li and the substrate material that will result in better understanding of diffusional Li respond to high capacity anodes with new chemistry content.

Li-ion intercalation compound enlisting multi-electron redox is a promising strategy to maximize the storage capacity of LIBs. Lithium vanadyl phosphate, LiVOPO₄, is one such material that can intercalate up to two Li⁺ ions per vanadium ion through the change in vanadium valence from V⁵⁺ to V³⁺, which translates to a theoretical capacity of 305 mAh/g and an energy density above 900 Wh/kg. However, Li-ion intercalation in the high voltage region (V⁵⁺/V⁴⁺) is kinetically limited by the sluggish Li-ion diffusion, and thus impedes the use of LiVOPO₄ in practical applications. Transition metal substitution has been a technique used to enhance the performance of battery cathodes by changing its intrinsic properties and stabilizing the crystal structure. The goal of this work is to explore experimentally if substitution with chromium and niobium can enhance the electrochemical performance of LiVOPO₄. X-ray diffraction and inductively coupled plasma atomic emission spectroscopy indicate the substituent is successfully incorporated into the structure. The effects of transition metal substitution on the crystal structure and the electrochemical performance of LiVOPO₄ will be discussed.

Development of a lithium-ion battery (LIB) is of great interest with an anode based on transition metal dichalcogenide nanosheets due to their intrinsically high capacity. The stability of a LIB is, however, detrimental with the nanostructured MoS₂ anode resulting from the gradual decrease of electrical conductivity with time due to the re-stacking of the nanosheets. In this work, we present a novel MoS₂ anode with molecularly architecture graft polymer binder having functionalized groups, enabling the high stability of a LIB. The fluorinated graft polymer synthesized by one-step atomic transfer radical polymerization allows not only for exfoliating MoS₂ nanosheets in the liquid phase to guarantee the stable sheet-to-sheet separation by preventing re-aggregation but also for providing self-assembled ionic channels through which lithium ions in electrolyte readily reach close to the surface of the nanosheets. The electrochemical reduction of lithium ions efficiently occurs at the layered nanostructure of MoS₂ facilitated by the functionalized polymer binder, giving rise to a high-performance LIB with the high rate capability and stability. Our binary anode of MoS₂ nanosheets self-assembled with the graft polymer enables high cell capacity even without the requirement of conductive additives which are frequently employed for ensuring high conductivity and thus comparable cell capacity, making our approach one-step closer to commercial energy applications.

Future applications for energy storage will demand high level function with long lifetime. As batteries increase in size and use requirements increase in complexity, safety takes on expanded importance for multiple applications including transportation and grid-level storage. Examples of electrode design approaches leading to improved safety based on materials chemistry will be highlighted in this presentation. Complementary ex-situ, in-situ and operando characterization approaches to understand energy storage material/composite degradation modes and relationship to cell level safety will also be described.

Driven phase separation in ion intercalation materials is known to result in different non-equilibrium phase morphologies, such as intercalation waves and shrinking-core structures, but the mechanisms of pattern selection are poorly understood. Here, based on the idea that the coarsening of the slowest phase is the rate limiting step, we introduce a scaling law that quantifies the transition from quasi-equilibrium intercalation-wave to diffusion-limited shrinking-core behavior. The scaling law is validated by phase-field simulations of single Li_xCoO₂ particles, in situ optical imaging of single Li_xC₆ particles undergoing transitions between stage 1 (x = 1) and 2 (x = 0.5) at different rates, and all the available literature data for single-particle imaging of Li_xCoO₂, Li_xC₆ and Li_xFePO₄. The results are summarized in operational phase diagrams to guide simulations, experiments, and engineering applications of phase-separating active materials. Implications for Li-ion battery performance and degradation are discussed.

Thick electrode with high-areal-capacity is a practical and promising strategy to increase the energy density of batteries, but development towards thick electrode is limited by the electrochemical performance, mechanical properties, and manufacturing approaches. In this talk, we overcome these limitations and report an ultra-thick electrode structure, called fiber-aligned thick or FAT electrode, which offers a novel electrode design and a scalable manufacturing strategy for high-areal-capacity batter electrodes. The FAT electrode uses aligned carbon fibers to construct a through-thickness fiber-aligned electrode structure, with feature of high electrode material loading, low tortuosity, high electrical and thermal conductivity, and good compression property. The low tortuosity of FAT electrode enables fast electrolyte infusion and rapid electron/ion transport, exhibiting a higher capacity retention and lower charge transfer resistance than conventional slurry-casted thick electrode design.

Fast-charging batteries typically employ electrodes capable of accommodating lithium continuously via solid-solution transformation because they have few kinetic barriers apart from Li⁺ diffusion.[1, 2] One exception is lithium titanate, an anode that can cycle without the Li plating observed in graphite [3] and exhibits extraordinary rate capability seemingly inconsistent with its two-phase reaction and the sluggish diffusion within its endmembers [4]. Through first-principles calculations combined with nudged-elastic band calculations, we reveal that the kinetic pathway that enables facile ionic transport in lithium titanate consists of distorted face-sharing Li polyhedra in metastable intermediate states. The importance of the face-sharing Li polyhedral is further confirmed by real-time tracking of Li⁺ local environments during migration using operando electron energy-loss spectroscopy. This study highlights that the rate capability of fast-charging electrodes may be controlled not solely by the intrinsic ionic diffusivity of macroscopic phases, but also by the transport pathways available via kinetically accessible low-energy states. Findings from this study, particularly on the improved kinetics originating from the face-sharing Li motifs in intermediate lithium titanate configurations, may open new directions for designing electrode materials for fast-charging batteries.

References:
[1] H. Liu, F. C. Strobridge, O. J. Borkiewicz, K. M. Wiaderek, K. W. Chapman, P. J. Chupas, C. P. Grey, Capturing metastable structures during high-rate cycling of LiFePO₄ nanoparticle electrodes. Science 344, 1252817 (2014).
[2] R. Malik, F. Zhou, G. Ceder, Kinetics of non-equilibrium lithium incorporation in LiFePO₄. Nat. Mater. 10, 587-590 (2011).
[3] T.-F. Yi, S.-Y. Yang, Y. Xie, Recent advances of Li₄Ti₅O₁₂ as a promising next generation anode material for high power lithium-ion batteries. J. Mater. Chem. A 3, 5750-5777 (2015).
[4] W. Schmidt, P. Bottke, M. Sternad, P. Gollob, V. Hennige, M. Wilkening, Small change-great Effect: steep increase of Li ion dynamics in Li₄Ti₅O₁₂ at the early stages of chemical Li insertion. Chem. Mater. 27, 1740-1750 (2015).

Composite electrodes that incorporate oxidized silicon are a cost-effective way to accommodate stresses and extend cycle life. To obtain fundamental information about chemo-mechanical phenomena in these composite structures, several different types of materials are being investigated through tuning the microstructures of Si nanoparticles and composite SiOx particles. The evolution of internal stresses in all of these structures was monitored with precise in-situ curvature in conjunction with parallel electrochemical measurements. Ex-situ characterization with electron microscopy, x-ray diffraction, and XPS provide important complementary information about changes in the materials. The different types of materials used for this work make it possible to systematically investigate key length scales, by independently varying the microstructures such as oxide layer thicknesses and particle/nano-cluster sizes. Analysis of these results requires assessments and models of both the chemical and mechanical effects of oxide surface layers and silicon encapsulation. The implications for optimizing these composite electrode structures will also be presented.

One of the critical factors for determining the success of lithium-ion batteries (LiBs) in electric vehicles is their resilience to a wide range of environmental conditions. As batteries with higher energy densities are being developed, ensuring that they can withstand elevated temperatures without severe capacity fade is critical. Silicon (Si), which delivers higher specific capacity compared to graphite (Gr), is a promising anode material toward increasing the energy density of LiBs. Thus, evaluating the response of Si anodes to harsh temperatures that an electric vehicle might experience is crucial to ensure the development of safe, high energy density, and long life LiBs.
In this work, the performance of Si-Gr electrode is examined in half-cell (Si-Gr/Li), full cell (Si-Gr/LiNi_0.5Mn_0.3Co_0.2O₂ and Si-Gr/LiFePO₄), and symmetric cell (Si-Gr/Si-Gr) configurations at varying temperatures (25, 45, and 70°C). By examining the three different cell configurations, contributions from different cell components are deconvoluted and physical mechanism for each electrode (i.e., loss of lithium inventory or active material) are evaluated. In addition, the effect of fluoroethylene carbonate (FEC) electrolyte additive on the thermal stability of Si is examined by testing two different electrolyte compositions (Gen2 (1.2 M LiPF6 in ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3:7, w/w)) and GenF (Gen2 + 10 wt% FEC)). The degradation mechanisms in each system is further analyzed via incremental capacity analysis. As dQ/dV analysis of Si-Gr composites represent challenges not accounted for in Gr electrodes (i.e., hysteresis and path dependency), dQ/dV methods for Si-Gr electrodes are developed in this work. Detailed analyses of the electrochemical performance of Si-Gr electrodes, along with incremental capacity analysis and post-mortem analysis, provide insights into the critical factors affecting the performance of Si-Gr electrodes at elevated temperatures. Based on the results, we propose the appropriate cell configuration and protocol for testing the thermal stability of Si-based cells.

The ever-increased demand in energy and consumable goods have led to a high necessity for sustainable catalysts^[1,2] and innovative energy storage systems^[3,4,5] with high energy densities, such as Li-ion batteries (LiBs). To maximize the anodic charge storage capacity of LiBs, alloying-type anode materials such as Sn and Sb have attracted considerable interest because of their high theoretical capacity of 992 and 660 mAh g⁻¹ and a suitable lithiation/delithiation voltage window of 0.01-1.5 V vs. Li⁺/Li. Recent advances in nanostructuring of the alloying-type anodes provide an effective way of mitigating the challenges of their volume expansion upon lithiation that severely hinder the cycling stability.^[6] Besides, one of the prevailing approaches toward stabilization of such electrodes is the embedding of Sn or Sb in the form of nanoparticles (NPs) in a matrix. The matrix helps to buffer the volume changes, impart better electronic connectivity, and prevent particle aggregation upon lithiation/delithiation. In this context, polymer-derived ceramics, namely, silicon oxycarbide (SiOC), is an appealing candidate for stabilizing Sn and Sb inclusions. This matrix features a high Li-ion storage capacity ranging from 600 to 700 mAh g⁻¹, low volume expansion upon lithiation of about 7%, and high electronic conductivity. In this study, we report a facile synthesis of Sn^[7] and Sb^[8] NPs embedded in a SiOC matrix via the pyrolysis of a cross-linked preceramic polymer. The NPs are formed in-situ via a carbothermal reduction during the pyrolysis. We determined that a key element to obtain a homogeneous distribution of NPs in SiOC is the employment of precursors with compatible polarities, allowing for an intimate blending of the functionalized polysiloxanes with the Sn or Sb precursors. This approach proved to be effective for buffering volume changes while retaining practical energy densities and high rate capabilities. For instance, SiOC/Sn nanocomposite exhibited a high rate capability, delivering a high capacity of 553 mAh g^-1 at a current density of 2232 mA g^-1 (≈ 6C for a graphite anode). Additionally, the electrochemical performance of SiOC was assessed in full cell configuration using areal capacities of 1.3 mAh cm^-2 being relevant for high-power industrial applications. The mechanism of Sn lithiation in SiOC was evaluated by in-situ powder X-ray diffraction.
References:
[1] R. Dubey, R. Comito, Z. Wu, G. Zhang, A. Rieth, C. Hendon, J. Miller, M. Dinca, J. Amer. Chem. Soc. 2017, 139, 12664.
[2] E. Metzger, R. Comito, Z. Wu, G. Zhang, R. Dubey, W. Xu, J. Miller, M. Dinca, ACS Sust. Chem. Eng. 2019, 7, 6654.
[3] R. Dubey, J. Nüssli, L. Piveteau, K. Kravchyk, M. Rossell, M. Campanini, R. Erni, M. Kovalenko, N.Stadie, ACS Applied Mater. Interfaces 2019, 11, 17686.
[4] K. Kravchyk, R. Widmer, R. Erni, R. Dubey, F. Krumeich, M. Kovalenko Sci. Rep. 2019, 9, 1-8.
[5] R. Dubey, T. Colijn, M. Aebli, E. Hanson, R. Widmer, K. Kravchyk, M. Kovalenko, ACS Applied Mater. Interfaces 2019, 11, 39902.
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[7] R. Dubey, P. Vallachira, F. Krumeich, G. Blugan, J. Kuebler, K. Kravchyk, T. Graule, M. V Kovalenko, Adv. Sci. 2019, 6, 1901220. (featured as Frontispiece Cover)
[8] R. Dubey et al. 2020, in revision.

Semiconductor physics mainly discuss the energy band structures and the carrier behaviors in semiconductors. The carrier dynamics and the resulting conduction properties of semiconductors. As a semiconductor, the carrier transporting characteristics of Si has been well studied and adjusted via doping (n type, N, P; p type, B), which has been widely applied in electronics, solar cell and photocatalysis. In recent years, Si has been regarded as the most promising anode material for its highest theoretical specific capacity, and natural abundance. However, the poor conductivity of intrinsic Si still limits the rate performance of Si materials.
Previously, Yi and me found boron doping can obviously enhance the rate capability of Si–C composite. Recently, phosphorus-doped yolk-shell Si@C materials (P-doped Si@C) with balanced electron and Li+ transportation dynamics are rational designed and prepared, which deliver high specific capacities (2709 mAh g^-1 @ 0.4 A g^-1), great rate capability (510 mAh g^-1@35 A g^-1) and good retention of ~95% over 800 cycles at 4 A g^-1. As a result, P-doped Si@C anode pairs with commercial activated carbon to assemble lithium-ion capacitor (high power density of ~61080 W kg^-1 at 20 A g^-1).
Junction is also an important concept in semiconductor physics, which refers to the interface region with rectification characteristics formed by the contact of two different semiconductor phases. The formation of semiconductor junction will change the charge state of atoms near the junction interface, and then regulate the adsorption and catalytic activity of active sites in the region. Our previous research reveals the space charge of p-n junction of p-type CoP and n-type FeNi LDH can be utilized to improve the adsorption of OH^- and promote the OER process. With this in mind, The in-situ formed VS_x@VO₂ junctions can designed to promote the absorption of polysulfide and result high volume specific Li-S batteries. Furthermore, a n-n-type CoS₂-CoS heterojunction were designed to facilitate the electrocatalytic conversion of I⁻/I₃⁻ and polysulfides, which can significantly improve the power density and lifetime of flow batteries. Therefore, it is possible to modulate the electronic configuration of active site by designing proper semiconductor junctions following the theories of solid physics.
In all, the semiconductor physics can offer useful theoretical guidance for rational design of high performance materials for energy storage.
Ref
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Alloys and intermetallics are of great interest as anode materials due to their high energy density, but they generally suffer from poor cycling life due to large volume expansion that leads to cracking. In this work, we demonstrate a new and facile route using cryogenic milling (cryomilling) to fabricate stable and high energy density anode materials. Because a ductile-to-brittle transition occurs for most metals at a low temperature, cryomilling can efficiently reduce the grain/particle size while adding a small amount of well-dispersed nanocarbon to stabilize the resulting nanostructures. Adding a minimum amount (1.2 wt%) of graphite during cryomilling produces SnSb anodes that demonstrate an initial coulombic efficiency of 83%, averaged efficiency >99.5%, and capacity retention of 90% over 100 cycles. Scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and various electrochemical characterization were employed to understand this high stability. The refined grain size (~10nm) and well-dispersed carbon matrix can alleviate the volume expansion and prevent particle cracking after cycling. Cryomilling is a cost-effective manufacturing method that is widely used in the food industry, plastic powder synthesis, and fabrication of nanostructured alloys. This work demonstrates the successful application of cryomilling to battery electrode materials for the first time and shows much-improved cycle life compared with conventional ball milling.

Of the thousands of materials that have been investigated as prospective rechargeable lithium-ion battery electrodes, only a handful have successfully traversed the journey toward commercialization. Following the award of the 2019 Nobel Prize in Chemistry to Whittingham, Goodenough, and Yoshino “for the development of lithium-ion batteries”,¹ and owing to the global effort to develop advanced battery materials, we seek to chronicle a modern commercialization pathway via the tale of high-power and high-energy density titanium niobium oxide (TNO, TiNb₂O₇) a crystallographic shear structure.

The state-of-the-art material in the high-power and fast-charging performance space is LTO, commercialized by Toshiba as the SCiB™ battery and now used globally in applications including mild-hybrid electric passenger cars, electric buses and trolleys, electric trains, and grid energy storage systems. Relative to graphite, Li₄Ti₅O₁₂ provides only 50% of the gravimetric capacity and sacrifices approximately 40% of the voltage when paired with a 4.0 V cathode. Despite their limited energy density, Li₄Ti₅O₁₂-based cells reached commercial success because they can be cycled over 20,000 times and operated safely at current densities and temperatures that are not suitable for graphite-based batteries.

Owing to the favorable properties of niobium, TiNb₂O₇ is being developed as a high-energy-density alternative anode material to Li₄Ti₅O₁₂. It was first discovered by crystallographers in the 1950s and 1960s,^2,3 reported as a lithium-ion battery material by Bell Labs in 1983,⁴ and “rediscovered” in the last decade by a team led by John Goodenough^5,6. Since then, it has been the most studied crystallographic shear or Wadsley–Roth phase, the subject of numerous applied and fundamental studies on characteristics such as the mechanism of structure evolution,^7–9 ionic and electronic conduction,⁹ and battery performance^10,11. The lithiation reaction may proceed all the way to Li₅Ti^IIINb^III₂O₇ with a theoretical capacity of 387.7 mAh×g^–1. The multielectron redox chemistry of TiNb₂O₇ puts it on par with the gravimetric capacity of graphite (371.9 mAh×g^–1). It is worth mentioning that the crystal density of TiNb₂O₇ (4.33 g×cm^–3) is twice that of graphite (2.22 g×cm^–3). Thus, the volumetric energy density of this oxide anode compares even more favorably than its gravimetric energy density. The high voltage of TiNb₂O₇ (above 0.6 V vs. Li⁺/Li) enables safe charging under high current densities even in a heterogeneous electrode with large overpotentials and may be considered a necessary feature despite the cell voltage penalty. The high operating voltage may also mitigate surface–electrolyte interphase (SEI) formation as the voltage window is largely within the stability limits of standard electrolytes.
We will present the development, the state-of-the-art, and the future of TiNb₂O₇ and Wadsley–Roth crystallographic shear structures as fast-charging and high-energy-density lithium-ion battery materials.

(1) Press release: The Nobel Prize in Chemistry 2019 https://www.nobelprize.org/prizes/chemistry/2019/press-release.
(2) Roth, R. S.; Coughanour, L. W. J. Res. Natl. Bur. Stand. 1955, 55, 209–213.
(3) Wadsley, A. D. Acta Crystallogr. 1961, 14, 660–664. https://doi.org/10.1107/S0365110X61001996.
(4) Cava, R. J.; Murphy, D. W.; Zahurak, S. M. J. Electrochem. Soc. 1983, 130, 2345–2351.
(5) Han, J.-T.; Huang, Y.-H.; Goodenough, J. B. Chem. Mater. 2011, 23, 2027–2029.
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The unique combination of high energy and power density made lithium-ion batteries the state-of-the-art energy storage technology to power small consumer electronics, as well as large-scale applications like electric vehicles.^[1] However, especially the latter application, requires batteries that cannot only provide high energy density, but also the possibility for rapid recharging.^[2] This is maybe the greatest challenge for the presently used graphite-based anodes, since the low lithiation potential (~0.1 V vs. Li/Li⁺) in combination with the sluggish lithium transport within the graphite structure and across the solid electrolyte interphase (SEI) provides the risk of lithium plating and dendrite formation during fast charging, especially at low temperatures.^[3]
To solve this issue, alternative anodes, following, e.g., a conversion or an alloying mechanism, are being investigated.^[4] These alternatives frequently possess higher capacities and rate capabilities; however, conversion materials still suffer from low energy efficiencies, due to a significant voltage hysteresis, and the extensive volume variation of alloying materials leads to rapid capacity fading and low coulombic efficiencies. Conversion/alloying-materials (CAMs), such as Zn_0.9Fe_0.1O, are a relatively new material class, that combines both, the conversion and alloying mechanism in a single material.^[5] During the initial lithiation of CAMs, nanograins of an alloying element and a percolating conductive network of transition metal nanoparticles are formed in situ by the (reversible) conversion reaction. The metallic nano-network leads to fast de-/lithiation kinetics, making them a promising candidate for high-power applications. Yet, the achievement of sufficiently high energy efficiencies and the volume variations occurring upon cycling are still remaining obstacles and there is still a lack of knowledge about how to potentially tackle these issues.
Herein, we present our findings towards a detailed mechanistic understanding of the de-/lithiation of (carbon-coated) Zn_0.9Fe_0.1O. By combining in situ microcalorimetry, with in situ XRD, ex situ ⁷Li NMR and ex situ ⁵⁷Fe Mössbauer spectroscopy we were able to propose a refined model for the initial de-/lithiation reaction. Furthermore, the volume variations of Zn_0.9Fe_0.1O electrodes upon cycling were studied by complementary in situ dilatometry and ex situ cross-sectional SEM analysis. These measurements reveal a continuous volume variation at the electrode level in the range of 10%. This is much lower than the theoretically predicted expansion when considering bulk densities only – even though the electrodes where cycled within a 3-V potential window. On the basis of these results we highlight the favorable effect of limiting the operational voltage window. We finally confirm this for Zn_0.9Fe_0.1O/LiNi_0.5Mn_1.5O₄ full-cells, providing an energy and power density of 284 Wh kg^-1 and 1105 W kg^-1, respectively, accompanied by an excellent energy efficiency of >93%.

[1] N. Nitta, F. Wu, J. T. Lee, G. Yushin, Mater. Today 2015, 18, 252–264.
[2] M. Li, J. Lu, Z. Chen, K. Amine, Adv. Mater. 2018, 30, 1800561.
[3] J. Asenbauer, T. Eisenmann, M. Kuenzel, A. Kazzazi, Z. Chen, D. Bresser, Sustain. Energy Fuels 2020.
[4] N. Loeffler, D. Bresser, S. Passerini, M. Copley, Johnson Matthey Technol. Rev. 2015, 59, 34–44.
[5] D. Bresser, S. Passerini, B. Scrosati, Energy Environ. Sci. 2016, 9, 3348–3367.

While the transport of ions and electrons in conventional Li-ion battery cathode materials is well understood, our knowledge of the phonon (heat) transport is still in its infancy. We present a first-principles theoretical investigation of the chemical trends in the phonon frequency dispersion, mode lifetimes, and thermal conductivity in the series of layered lithium transition-metal oxides Li(Ni_xMn_yCo_z)O₂ (x + y + z = 1). The oxidation and spin states of the transition metal cations are found to strongly influence the structural dynamics. Calculations of the thermal conductivity show that LiCoO₂ has highest average conductivity of 45.9 W.m^-1.K^-1 at T = 300 K and the largest anisotropy, followed by LiMnO₂ with 8.9 W.m^-1.K^-1, and LiNiO₂ with 6.0 W.m^-1.K^-1. The much lower thermal conductivity of LiMnO₂ and LiNiO₂ is found to be due to 1-2 orders of magnitude shorter phonon lifetimes. We further model the properties of binary and ternary transition metal combinations and show that the thermal conductivity of NMC is suppressed with decreasing Co content and increasing Ni/Mn content. The thermal conductivity of commercial NMC622 and NMC111 compositions are substantially larger than NMC811. These results serve as a guide to ongoing work on the design of multi-component battery electrodes with more effective thermal management.

[Ref 1] Yang, H., Yang, J.Y., Savory, C.N., Skelton, J.M., Morgan, B.J., Scanlon, D.O. and Walsh, A., J. Phys. Chem. Lett. 2019, 10, 18, 5552–5556
[Ref 2] Yang, H., Savory, C., Morgan, B., Scanlon, D., Skelton, J. and Walsh, A., Chemrxiv, 2020, https://t.co/KNArzKhsdq

The recent success of Li-excess cation-disordered rocksalt (DRX) cathodes is providing an avenue to develop high energy density cathodes with abundant and low-cost metals, such as Mn, Fe and Ti. As they have high energy density, these cathode materials are currently the most viable strategy to address the resource issues of Co / Ni that will arise as conventional layered-type Li-ion cathodes are scaled towards multiple TWh of annual production. In DRX cathodes, facile Li transport relies mostly on the so-called “0-TM” 3D percolation channel, in which the intermediate tetrahedral sites are coordinated only by Li (no transition metals, TMs). The Li migration barrier along this “0-TM” channel is significantly lower compared to that of “1-TM” channel, where the intermediate tetrahedral sites are coordinated by 3 Li and 1 TM. The complexity of DRX cathodes lies in the fact that in most cases, they present various types of cation short-range order (SRO), which influences the frequency and connectivity of “0-TM” channels, thus controls the capacity and rate capability of DRX cathodes. Monte Carlo simulations have suggested that the presence of SRO in DRX cathodes generally leads to reduced Li percolation, when compared to that of a random arrangement of TM species.
Inspired by the recent observations of nearly-random cation distribution in several high-entropy metal and oxide compounds, we have found that increasing the number of TM species in DRXs will improve Li transport properties by preventing the formation of a single dominant SRO type, resulting in improved capacity and high rate capability. We show that SRO in DRX cathodes systematically decreases as more TM components are added, and as a consequence, energy density and rate capability systematically increase. A DRX cathode with six TM species achieves 307 mAh g⁻¹ (955 Wh kg⁻¹) at 20 mA g⁻¹ and retains more than 170 mAh g⁻¹ when cycling at 2,000 mA g⁻¹. The high compositional flexibility of these materials can also enable the use of less pure precursor materials.

High nickel content cathodes (Ni >90%) are promising candidates for lithium ion batteries (LIBs) due to their high energy capacity and low cost. But they are still suffering from poor cycling stability and potential thermal run away since lattice structures would become unstable upon cycling. Incorporation of Al atom by dry coating into structure has been found to be beneficial to the electrochemical and thermal stability because Al doping not only can decrease the Li/Ni mixing ratio in the Li layer but also enhance the thermal resistance to oxygen evolution due to strong Al-O bonding. The Al doping by dry coating NCM (DC_NCM_Al(OH)₃ and DC_NCM_Al₂O₃ ) reserve 87.8% and 87.1 % capacity respectively after 200 cycles while NCM only remains 70.7 %. Additionally, DC_NCM_Al(OH)₃ at charged state exhibits an exothermic peak at 212.4 °C which is 10 °C higher than NCM. The Al doping NCM samples reveal improved properties for high nickel content cathode, which provides a strategy for next generation LIBs.

To achieve the high energy densities demanded by emerging technologies, lithium-ion batteries (LIBs) require cathode electrodes possessing high volumetric and specific capacities¹. While recent developments in Ni-rich cathode material composition have significantly increased the volumetric and specific capacities of cathode active materials^{2, 3}, the other inactive components, such as binders and carbon additives, still need to be minimized in order to approach theoretical volumetric and specific capacity limits of the entire cathode electrode. Towards this end, here we report conformal, ultrathin, and conductive graphene coatings on LIB active material particles using a scalable Pickering emulsion method⁴. Since this approach employs high-quality, liquid phase-exfoliated graphene that possesses exceptional electrical conductivity⁵, only 0.5 wt% graphene is required to achieve high electrochemical performance in electrodes. Furthermore, this minimal amount of conductive additive also allows the binder loading to be reduced to 0.8 wt%, even for industrially relevant active material areal loadings (~11 mg cm^-2). Due to its exceptionally low inactive material loading, the resulting LiNi_0.8Co_0.15Al_0.05O₂ (NCA) electrodes achieve specific and volumetric capacities that reach ~98% of the theoretical limits for NCA. The efficient electron conduction path from the graphene coating and the shortened lithium diffusion path due to a simplified electrode structure also imply superlative rate capability, while the conformal graphene coating mitigates electrochemical degradation mechanisms and consequently enhances cycle-life. Since this Pickering emulsion coating scheme is agnostic to the cathode material composition, it can be widely applied to other active material particles, thus providing a general pathway to theoretical specific and volumetric capacities in practical LIB electrodes.

1. Armand, M. & Tarascon, J. M. Building Better Batteries. Nature 451, 652-657 (2008).

2. Liu, W. et al. Nickel-Rich Layered Lithium Transition-Metal Oxide for High-Energy Lithium-Ion Batteries. Angew. Chem., Int. Ed. 54, 4440-4457 (2015).

3. Li, W., Erickson, E. M. & Manthiram, A. High-Nickel Layered Oxide Cathodes for Lithium-Based Automotive Batteries. Nat. Energy 5, 26-34 (2020).

4. Park, K.-Y. et al. Concurrently Approaching Volumetric and Specific Capacity Limits of Lithium Battery Cathodes via Conformal Pickering Emulsion Graphene Coatings. Adv. Energy Mater. DOI: 10.1002/aenm.202001216 (2020).

5. Liang, Y. T. & Hersam, M. C. Highly Concentrated Graphene Solutions via Polymer Enhanced Solvent Exfoliation and Iterative Solvent Exchange. J. Am. Chem. Soc. 132, 17661-17663 (2010).

Nickel-rich layered lithium transition metal oxides are the most attractive cathode materials for the next generation lithium-ion batteries for automotive application. However, they suffer from crystal and interfacial structure instability under aggressive electrochemical and thermal driving force, leading to rapid performance degradation and severe safety concerns.In this talk, we reported a transformative approach by using an oxidative chemical vapour deposition technique to build a protective conductive polymer (PEDOT) skin on layered oxide cathode materials. The ultra-conformal PEDOT skin facilitates the transport of lithium ions and electrons, significantly suppresses the undesired layered to spinel/rock-salt phase transformation and the associated oxygen loss, mitigates intergranular and intragranular mechanical cracking, and effectively stabilizes cathode-electrolyte interface. This approach remarkably enhances the capacity and thermal stability under high-voltage operation. Building a protective skin at both secondary and primary particle levels of layered oxides offers a design strategy for the promising Ni-rich cathodes towards high-energy, long-life and safe lithium-ion batteries.

We have devised an approach to fabricate dense textured V₂O₅ thin films, which allows us to scrutinize the root cause of capacity fade in V₂O₅ cathodes of Li-ion batteries. Specifically, we performed in-situ measurements of stress of V₂O₅ thin films during 50 electrochemical cycles. Surprisingly, electrochemical cycling appears to induce elastic and rate-independent deformation over a voltage range relevant to battery operation (4 - 2.8 V). However, the compressive stresses gradually increase with cycle number during the first few cycles, likely due to side reactions and/or residual Li left in the V₂O₅, even after delithiation (to 4 V). Further cycling leads to accumulated mechanical (e.g., fracture, delamination) and structural changes which ultimately result in severe capacity fade.

The charge transfer reaction is the fundamental reaction for rechargeable batteries. The energy landscape of this reaction depicts the equilibrium and kinetics of the electrochemical process. Typically, a Li-metal electrode is always covered by a thin layer of solid electrolyte interphase (SEI), forming a complex Li/SEI/electrolyte interface. We first introduced a new half-cell model to predict the energy landscape of the electrochemical reaction of at the complex Li/SEI/electrolyte interface, with combined density functional theory (DFT) and tight-binding (DFTB) calculations. The simulated atomic details revealed that the Li-metal surface is negatively charged at the electrochemical equilibrium condition (the experimentally defined zero voltage for Li⁺/Li⁰) due to the large solvation energy of Li⁺ in the EC-based electrolyte. The electric field created by the negatively charged surface can reorient the electrolyte into an ordered structure, lower the Li⁺ ion desolvation energy barrier, and help the Li⁺ ion transport through the SEI.
Further, the calculated energy landscape can be used to describe the electrochemical reaction kinetics with a Butler-Volmer equation, enabling a multiscale modeling approach. In this study, the DFT and DFTB predicted interfacial structures, de-solvation processes, and the ion transfer energy barrier, were inputted into a non-linear phase-field model to capture the morphological evolution during the electroplating process. Specifically, this model captured the morphology difference between the dendritic Li plating in carbonate (EC)-electrolyte and faceted Mg plating in tetrahydrofuran(THF)-electrolyte under the similar current density. Furthermore, through systematical parametric analyses, we conclude that the cation desolvation-induced exchange current difference between Li and Mg is mainly responsible for their deposition morphological differences. This study provides a strategy of connecting the phase-field method and atomistic calculations and predicts that increasing the desolvation energy and lowering the equilibrium exchange current can help to achieve dendrite-free Li-metal anode.

Lithium (Li) metal offers high theoretical capacity, lowest negative potential and light weight, making it the ideal anode material for high-energy-density batteries. Porous electrodes resulting from unregulated Li growth is believed to be the major cause of the low Coulombic efficiency (CE) and potential safety hazards of rechargeable Li metal batteries. Strategies aiming to achieve large granular Li deposits have been extensively explored; yet, the ideal Li deposits, which consist of large Li particles seamlessly packed on the electrode and can be reversibly deposited and stripped, have never been reported. Here, by using cryo-FIB-SEM, 3D reconstruction and MD simulation, we quantitatively revealed the role of stack pressure on Li nucleation, growth and stripping, and discovered that optimal pressure for approaching 100% dense Li deposition by establishing a pressure-morphology-performance correlation. Unprecedented, a dense Li deposition (99.49% electrode density) with an ideal columnar structure has been achieved and can be reversibly plated and stripped during extended cycles at room temperature. The tailored Li deposition and stripping by optimizing stack pressure opens new opportunities to rationally mitigate inactive Li formation, improve cycle life and enable fast charging for rechargeable Li metal batteries.

Imaging the microstructure of a battery cell at different length scales is important to understand the degradation processes through the battery cycle life. For example, investigating the formation of defects such as dendrites at the Li anode interface requires both high resolution imaging and large field of view. Ideally, imaging techniques cover the entire electrode thickness at submicron resolution with a method that preserves the sample interface. Here, we present an approach to probe the defects at the Li-metal battery interface. A coin cell has been sliced under cryo conditions with a laser inside the vacuum of the scanning electron microscope (SEM) chamber so a pristine surface is obtained and defect formation can be observed across the entire thickness of the cell.
In another example, transport properties for a thick NMC 811 cathode were obtained from focused ion beam (FIB) milling in combination with SEM. Because of the cathode thickness, an alternative to traditional Ga FIB milling was used. With a Xenon plasma beam, it was possible to obtain a 3D data set with more than 100 µm field of view on a 70 µm thick cathode. The microstructural characteristics extracted from such representative 3D volume help the understanding of the correlation between electrode microstructural evolution and performance deterioration.

All solid-state batteries (ASSB) promise high energy density, by enabling the use of Li metal anode, and safe operation, thanks to their non-flammable solid-state electrolyte. Degradation over the course of the ASSB lifecycle, however, remains a limiting issue for the technology, as the aging process constrains energy storage capabilities and performance. Better understanding of the degradation mechanisms is critical to improving ASSB functionality and performance, [1].
Scanning electron microscopy (SEM) analytical studies, such as energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) analyses are important tools that give researchers the ability to characterize battery materials and understand their structures, as well as degradation. The role of the Li microstructure (grain size, crystallographic preferential orientation, etc.) on battery performance is not well understood; few reports on the microstructural characterization of lithium metal are to be found in the literature. In addition, further investigation is needed into the cathode/anode solid electrolyte interface, such as solid electrolyte decomposition, intermediate transition layer formation, and Li metal dendrite formation [2, 3].
To complete accurate and informative SEM analytical studies, the samples must be well-prepared and represent the native microstructure of study element without any oxidation, contamination and deformation or cracking induced by preparation. An artifact-free sample preparation technique is a critical key to better understand battery functionality and its ageing process. Preparing Li metal ASSB samples is particularly difficult because Li is highly sensitive to oxygen, nitrogen, water, and carbon dioxide; requiring Ar protective atmosphere. Lithium is also a very soft material and its crystallographic structure can be easily damaged by mechanical sample preparation techniques. The inherent characteristics of Li metal demand a controlled environment workflow from sample preparation through SEM analyses [4].
In this work we report on EBSD characterization of pure Li metal using a unique controlled environment workflow: argon broad ion beam milling, sample transfer to the SEM, and EBSD analyses. We present structural EBSD results on pure Li metal used as an anode in an ASSB, with special focus on texture and grain size. We will also present SEM imaging and EDS studies that document the interface layers formed during battery cycling.

References
[1] Abhik Banerjee, Xuefeng Wang, Chengcheng Fang, Erik A. Wu, and Ying Shirley Meng, Interfaces and Interphases in All-Solid-State Batteries with Inorganic Solid Electrolytes, Chemical Reviews 120 (14), 2020
[2] Kaihui Nie, Yanshuai Hong, Jiliang Qiu, Qinghao Li, Xiqian Yu, Hong Li, and Liquan Chen, Interfaces Between Cathode and Electrolyte in Solid State Lithium Batteries: Challenges and Perspectives, Frontiers in Chemistry, Volume 6, 2018
[3] Cheng Lin, Aihua Tang, Hao Mu, Wenwei Wang, and Chun Wang, Aging Mechanisms of Electrode Materials in Lithium-Ion Batteries for Electric Vehicles, Journal of chemistry, Volume 2015, 2015
[4] Cecile Bonifacio, Pawel Nowakowski, Mary Ray, Paul Fischione, Multi-length Scale Cryogenic Sample Preparation to Electron Microscopy of Battery Materials, Microscopy and Microanalyses, 2020

The potential safety hazards of lithium ion batteries (LIBs), as well as their capacity fade and long charging time, are major challenges that currently prevent the widespread adoption of electric vehicles. These effects are strongly correlated with lithium plating, a parasitic reaction on graphite anodes that competes with lithium intercalation when LiBs are charged at high rates or under low temperature. Li plating is known for depletion of the lithium inventory, cell shorting, and thermal runaway.

Addressing Li plating problem relies on material engineering and battery operation optimization. The efficacy of these approaches lies in the fundamental understanding of Li plating mechanism. Despite Li ion battery has been invented for 30 years, a systematic understanding on the onset and growth of Li in graphite anode is still lacking. Thermodynamics criterion requires the voltage of graphite to be negative vs. Li metal for Li plating to happen. However, in practice graphite is usually able to tolerate certain degree of negative voltage without plating. Such discrepancy necessitates the incorporation of kinetic factors. Several kinetic mechanisms have been proposed, which include electrolyte transport limitation and solid diffusion limitation. However, neither of them are validated experimentally on graphite electrode.

In this work, we test these hypotheses and elucidate the mechanism of Li plating, by investigating the interplay between Li intercalation and Li plating on a single graphite particle, using in-situ optical microscopy coupled with electrochemical test. The results show Li plating happens when the surface of graphite saturates, which shuts down the intercalation reaction. The saturation can happen much earlier than full lithiation of graphite particle due to solid diffusion limitation, resulting in severe under-utilization. The physical picture was discussed by analyzing the energy landscape of the two competing reactions and their kinetics. A theory was proposed, followed by a 1D simulation of the dynamics of Li insertion and Li plating. The discovery sheds light on directions and guidelines of materials innovation or electrode design for reducing the risk of Li plating and extreme fast charging.

Lithium-metal anode holds tremendous potential for enabling next generation of rechargeable batteries due to their high storage capacity (3,861 mAh g^-1) and low reduction potential (-3.04 V vs SHE). When paired with a high-capacity cathode, such as high-nickel NMC or sulfur, lithium-metal anode can help deliver energy-dense storage systems with ~ 500 Wh kg^-1. However, the poor efficiency associated with lithium plating and stripping severely limits the cycle life. For realizing practical viability, reversible lithium deposition needs to be achieved by stabilizing the lithium-electrolyte interphase (SEI) layer and mitigating the extreme volume changes accompanying the cycling.

The chemistry of the lithium-electrolyte interface is rendered distinct by the choice of the cathode in a full cell, viz., high-nickel NMC or sulfur. This is not only due to the different electrolyte formulations used, but also due to the crossover of species between the cathode and anode. The characteristics of deposited lithium can fundamentally vary across different cathode systems, and unique strategies must be employed to stabilize lithium deposition in each case. Hence, an in-depth fundamental study is necessary to develop a thorough understanding of the lithium-electrolyte interfacial chemistry, while creative engineering solutions are necessary to achieve highly efficient lithium plating and stripping. This presentation will focus both on a fundamental understanding and offering engineering solutions.

When paired with a high-nickel NMC cathode, the lithium-metal anode is affected by transition-metal dissolution at the cathode and crossover to the anode. However, we find that during initial cycling, crossover of electrolyte decomposition products from the anode to the cathode is more severe and plays a more critical role. Not only does it alter/thicken the cathode SEI layer, it also affects the anode itself as the lithium SEI components could continually dissolve. The magnitude of this crossover is expected to depend on the nickel content in the cathode.

When paired with a sulfur cathode, the lithium-metal anode is affected by polysulfide dissolution at the cathode and crossover to the anode. We use anode-free full cells, which contain no excess lithium at the anode, to investigate the dynamics of lithium deposition. With advanced surface characterization methodologies, e.g., time-of-flight secondary ion spectrometry, we find that the depletion of cyclable lithium inventory is due to the formation of “dead” metallic lithium, which itself is caused by severe electrolyte decomposition on lithium-metal surface with cycling.

The presence of polysulfide intermediates is found to have an intrinsically stabilizing effect on lithium deposition in Li-S batteries. This motivates our strategies towards engineering further improvements in cycle life. With tellurium as a cathode additive, tellurium-substituted polysulfides are formed, which reduce on lithium surface to form Li₂TeS₃ as a thin, Li⁺-ion conducting SEI layer. This enables uniform deposition of lithium and mitigates electrolyte decomposition. A remarkable improvement in cyclability is achieved in energy-dense anode-free full cells and lean-electrolyte pouch cells. Similarly, using organosulfide molecules as electrolyte additives opens the possibility of favorably tuning the properties of lithium SEI layer by simply modifying the organic functional group.

The development of three-dimensional current collector matrices is also critical to achieving reversible lithium deposition. The highly conductive frameworks provide efficient electron/ion transport and guides a homogenous deposition of lithium, while the porous 3D architecture helps mitigate severe volume changes during cycling. The presentation will demonstrate various lithiophilic host materials that show considerable promise in improving the cyclability of lithium-metal anode when paired with both high-nickel NMC and sulfur cathodes.

One step towards more environmentally friendly and safer batteries is to eliminate fluorinated electrolytes, as they are susceptible to release toxic and corrosive compounds such as HF. Yet, fluorinated electrolytes are still dominating the research and market of lithium ion batteries.¹ Alternatively, replacing the state-of-the-art LiPF₆ with fluorine-free salt would reduce cost, increase safety and decrease toxicity, both in the manufacturing and recycling processes.² In addition, additives able to form a stable solid-electrolyte interphase (SEI) are another common source of fluorine, for example fluoroethylene carbonate (FEC), which is particularly important for silicon-based electrodes as they suffer high volume changes upon cycling.³ Despite this advantage of FEC, it has been also shown that it accelerates the degradation of the electrolyte containing LiPF₆ upon storage.⁴

Herein, we investigate the effects of electrolyte fluorination, from a highly fluorinated electrolyte based on the conventional LiPF₆ salt with FEC and vinylene carbonate (VC) as SEI-forming additives to a fluorine-free electrolyte based on lithium bis(oxalato)borate (LiBOB) salt and VC additive. These electrolytes were tested in full cells combining silicon-graphite composite electrodes with LiNi_1/3Mn_1/3Co_1/3O₂ cathodes, and characterized by means of electrochemical testing and post-mortem analysis. Our results show that the SEI composition on the anode is very different, being F-rich in the fluorinated electrolyte and O-rich in the fluorine-free electrolyte. However, both electrolyte formulations are able to stabilise silicon-based electrodes and support long cycle life in full cells. Therefore, fluorine-free alternatives are good candidates for high-energy-density full cells opening new possibilities towards safer, more sustainable and less toxic batteries.<sup>5

1</sup> von Aspern, N.; Röschenthaler, G.-V.; Winter, M.; Cekic-Laskovic, I. Fluorine and Lithium: Ideal Partners for High-Performance Rechargeable Battery Electrolytes. Angew. Chem. Int. Ed. 2019, 58, 15978-16000.

² Xu, K. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104, 4303-4418.

³ Etacheri, V.; Haik, O.; Goffer, Y.; Roberts, G. A.; Stefan, I. C.; Fasching, R.; Aurbach, D. Effect of Fluoroethylene Carbonate (FEC) on the Performance and Surface Chemistry of Si-Nanowire Li-Ion Battery Anodes. Langmuir 2012, 28, 965-976.

⁴ Xu C.; Hernández G.; Abbrent S.; Kobera L.; Konefal R.; Brus J.; Edstrom K.; Brandell D.; Mindemark J. Unraveling and Mitigating the Storage Instability of Fluoroethylene Carbonate-Containing LiPF₆ Electrolytes To Stabilize Lithium Metal Anodes for High-Temperature Rechargeable Batteries. ACS Appl. Energy Mater. 2019, 2, 7, 4925-4935.

⁵ Hernández G; Naylor A. J.; Chien Y-C.; Brandell D.; Mindemark J.; Edstrom K. Fluorine-Free Electrolytes on the Performance of LiNi_1/3Mn_1/3Co_1/3O₂/Silicon–Graphite Li-Ion Battery Cells. ACS Sustain. Chem. Eng. 2020, DOI 10.1021/acssuschemeng.0c01733.

Incorporation of organosilicon (OS) compounds in modest concentrations as low as several percent in conventional carbonate electrolytes lead to substantial improvements in the thermal and electrochemical stability of the resulting lithium ion batteries. We have been investigating the fundamental chemical degradation pathways of organosilicon compounds in an effort to understand the chemical and electrochemical origins of these remarkable properties. The combination of NMR spectroscopy, in situ FTIR, AFM, and XPS provides unique insights into how organosilicon electrolytes reduce or eliminate autocatalytic chemical pathways associated with PF6 and its degradation products. OS compounds further alter the chemical nature of SEI layers on silicon, leading to improvements in overall performance. In this talk I will summarize some of our recent work characterizing organosilicon thermal stability and electrochemical stability against different materials, including silicon-based anodes and NMC-based cathodes.

Lithium (Li) ion batteries have made a great impact to society, recognized by the 2019 Nobel Prize in Chemistry. Albeit commercialized for decades, Li ion batteries are rapidly approaching their theoretical limit in energy density, thus motivating the revival of the Li metal chemistry. Nevertheless, the implementation of Li metal batteries is plagued by their poor cycle life. Uncontrollable reactions between Li metal and electrolytes form a chemically unstable and mechanically fragile solid-electrolyte interphase (SEI). The SEI easily cracks during cycling, leading to dendritic growth, “dead Li” formation and irreversible Li inventory loss. By tuning the SEI structure and quality, electrolyte engineering is a critical and pragmatic approach. For a promising electrolyte, several key requirements are marked: (1) consistently high Coulombic efficiency (CE) to minimize Li loss, including that in the initial cycles; (2) functionality under lean electrolyte and limited-excess Li conditions for maximized energy density; (3) oxidative stability towards high-voltage cathodes; (4) rmoderate salt concentration for cost-effectiveness; (5) high boiling point and low flammability for safety and processability.

Several reports on electrolyte engineering improved the cyclability of Li metal batteries, such as additive optimization, solvent ratio modification, and liquified gas electrolytes. Particularly, the high concentration electrolytes (HCEs) and localized high concentration electrolytes (LHCEs) were acknowledged to be the most effective methods. Take the state-of-the-art version, LHCEs, as an example. The fluoro-diluents used in LHCEs were Li metal compatible yet hardly able to solvate Li⁺ ions by itself. As a consequence, small amount of unstable solvents were necessary for salt dissolution, reducing but not eliminating undesirable parasitic reactions. These side reactions with the solvent molecules led to low CE in the initial cycles. Therefore a detailed methodology of rational electrolyte design is still highly demanded.

In our work, we target new solvent molecules that not only dissolve Li salt but also stay compatible when confronted with both Li metal anodes and high-voltage cathodes. Thus we design a first-time synthesized liquid molecule, fluorinated 1,4-dimethoxylbutane (FDMB), solely as the electrolyte solvent. It is paired with 1 molar lithium bis(fluorosulfonyl)imide (1 M LiFSI) in a single-salt, single-solvent formulation (1 M LiFSI/FDMB) to enable stable and high-energy-density Li metal batteries. Theoretical simulations show a unique Li⁺-solvent interaction in 1 M LiFSI/FDMB and thus higher anion-solvent ratio in the Li⁺ solvation structure, endowing the electrolyte with both Li metal compatibility and high-voltage tolerance. Cryogenic transmission electron microscope of Li metal plated in 1 M LiFSI/FDMB reveals an ultrathin SEI (~6 nm), accompanied by a high CE (~99.52%) and fast activation (CE ramps up to >99% within 5 cycles) for Li metal anodes. The Li|NMC full cells with limited-excess Li retain 90% capacity after 420 cycles with an average CE of 99.98%. Industrial anode-free Cu|NMC 811 pouch cells achieve ~325 Wh kg^-1 single-cell energy density while Cu|NMC532 ones show 80% capacity retention after 100 cycles, which is one of the best performing anode-free cells. Our rational design concept for electrolyte provides a promising path to high-energy, practical Li metal batteries with high cyclability and processability.

Recently, layered oxides LiNi_1-x-yMn_xCo_yO₂ (NMC), have found wide use in the automotive industry as positive electrode materials for lithium-ion batteries, due to their high energy density given by the nickel content¹. Unfortunately, their surface composition is readily compromised by the chemical reactivity towards H₂O and CO₂. The instability towards H₂O limits the aqueous processing of the electrodes² and the surface degradation towards H₂O and CO₂ into surface species (typically Li₂CO₃ and LiOH) induces slurry gelation during the preparation of the electrodes³, increases the cell’s impedance⁴, provokes early capacity fade⁵ and heavier gassing is produced during battery operation⁶. Many research efforts have been devoted to demonstrating the negative consequences of Li₂CO₃ and LiOH to the electrochemical performance of the battery. Nonetheless, the whole NMC surface degradation mechanism upon storage under H₂O/CO₂ atmosphere is still controversial.

Therefore, in this work⁷, the complementary acid-base titration and inductively coupled plasma techniques were implemented as accurate methods to identify and quantify the amount of soluble species in four nickel-rich NMC: 532, 622, 811 and 901; before and after exposure under a specific H₂O/CO₂ atmosphere. In addition to the typical Li₂CO₃, and LiOH, we unveiled the presence of lithium and sodium sulfate salts. Moreover, thanks to thermogravimetric analysis coupled to mass spectrometry of the as-received and exposed to H₂O/CO₂ materials, we propose a novel NMC surface degradation mechanism that involves the formation of transition metal-based surface species. Lastly, the chemical reactivity of the surface species towards a commercial LiPF₆-based liquid electrolyte was studied through nuclear magnetic resonance (¹⁹F and ³¹P NMR), at room and high temperature. In addition to discriminating between LiOH and Li₂O as surface species, these experiments evidenced the presence, as major decomposition product, of lithium difluorophosphate, well-known for its beneficial effect as electrolyte additive.

1. Myung, S. T., et al (2017). “Nickel-Rich Layered Cathode Materials for Automotive Lithium-Ion Batteries: Achievements and Perspectives.” Acs Energy Letters, 2(1), 196-223.
2. Bichon, M., et al (2019). “Study of Immersion of LiNi_0.5Mn_0.3Co_0.2O₂ Material in Water for Aqueous Processing of Positive Electrode for Li-Ion Batteries.” Acs Applied Materials & Interfaces, 11(20): 18331-18341.
3. Kim, D., et al (2012). “Positive electrode slurry composition for lithium secondary battery, lithium secondary battery comprising the same and method of making the lithium secondary battery.” Patent
4. Zhuang, G. V., et al (2004). “Li₂CO₃ in LiNi_0.8Co_0.15Al_0.05O₂ cathodes and its effects on capacity and power.” Journal of Power Sources, 134(2): 293-297.
5. Liu, H. S., et al (2006). “Investigation and improvement on the storage property of LiNi_0.8Co_0.2O₂ as a cathode material for lithium-ion batteries.” Journal of Power Sources, 162(1): 644-650.
6. Renfrew, S. E. and McCloskey, B. D. (2017). “Residual Lithium Carbonate Predominantly Accounts for First Cycle CO₂ and CO Outgassing of Li-Stoichiometric and Li-Rich Layered Transition-Metal Oxides.” Journal of the American Chemical Society, 139(49): 17853-17860.
7. Martinez, A. C., et al (2020). “High reactivity of the nickel-rich LiNi1-x-yMnxCoyO2 layered materials surface towards H₂O/CO₂ atmosphere and LiPF6-based electrolyte.” Journal of Power Sources, 468, 228204.

Lithium ion batteries (LIBs) are wildly used for portable electronics and electric vehicles due to the favorable performance compared with other rechargeable batteries. However, their degradation deteriorate their service life remarkably. Fortunately, charge/discharge curves can provide quantitative information for understanding how such degradation correlates with usage conditions and environment of LIBs.
In general, the charge/discharge curves can be divided into two parts, i.e. the open circuit voltage (OCV) and the polarization in the cell. The OCV provides a thermodynamic fingerprint of the electrode which could be roughly estimated by charge/discharge the cell at very low C-rate. With regards to a commonly used 2430 coin cell, the difference between charge/discharge curve and theoretical OCV is less than 30mV at the full range of SOC when a 1/25C current rate is used. In this presentation, the changes in OCV during long cycles will be exhibited, which offers a quantified information on active material decay as well as the side reactions in the cell. The polarization of the cell is strongly affected by the electron and lithium ion transport inside the electrode microstructure. In this work, both the time-domain and the frequency-domain electrochemical methods ware applied to the analysis to distinguish each process onto the polarization in the cell, i.e. the contact problem between the solid phases, the mass transport through the interface and in the solid phase of the electrodes, along with the slow electrochemical reactions. Furthermore, the thermodynamic fingerprints of the positive and negative materials have strongly relation with the polarization in the cell. These relations generally becomes very complex, and thus a mathematical modeling is also used to understand and explain the charge/discharge curve for LIB in detail. Overall, it is proposed that more collaborations could be needed between the academia and industry so that the lithium ion battery degradation could be fully investigated.

Li-ion batteries are being widely used in planetary space missions, owing to their impressive performance characteristics, including high specific energy, long life, and the ability to operate over a wide temperature range. Historically, batteries with custom large-format Li-ion cells of 10-55 Ah have been used in various Mars missions, e.g., Mars Exploration Rovers (Spirit and Opportunity), Mars Curiosity Rover, Mars landers (Phoenix and InSight), Mars and Jupiter orbiters and the upcoming Mars 2020 Rover. However, in several other robotic and human space missions, e.g., Europa Clipper, Artemis missions to moon, and Portable Life Support System (PLSS) for Extra-Vehicular Activities, commercial 18650 Li-ion cells are being used due to their improved specific energies of ~250 Wh/kg (20-50% over custom cells), improved safety from built-in safety devices, modularity, and excellent uniformity, which obviate the need for cell-level control during charging. Several cell types with different chemistries and from different manufacturers are being evaluated at JPL. The cell chemistries differ mostly in cathodes, i.e., either lithium nickel cobalt aluminum oxide (NCA), Li-rich nickel manganese cobalt oxide (Ni-rich NMC) or blends with lithium manganese spinel oxide (LMO). Various performance tests, including cycle life at different depths of discharge, radiation tolerance and high-power characterization at different temperatures are being carried out in conjunction with Electrochemical Impedance Spectroscopy (EIS). In addition, destructive physical analysis of the cells have been performed to analyze the electrode materials, and correlate them with the performance characteristics. Finally, low temperature charge characterization studies have been carried out to ascertain the propensity of lithium plating as a function of charge current and temperature.

Separators are an important component within a Li-ion battery cell. They need to mechanically separate anode and cathode within a cell while allowing maximum ionic conductivity of the Li-ion containing electrolyte. Commercially available separators are usually made from polyolefine membranes composed of pores in 10s to 100s of nanometers. The shape, size and distribution of porosity in the separator membrane is a key parameter influencing the performance of the Li-ion battery cell. Due to its small size, scanning electron microscopy is currently the preferred method to directly image and investigate the separators.
However, the separator membrane is highly insulating, and easily gets damaged by the electron beam. Typical SEM imaging condition usually causes significant shrinkage and melting of the porous structure, leading to the wrong measurement of pore size and distribution. In this contribution, we demonstrate a method to image the separator membrane at very low beam energy (< 1 kV) to reduce charging artifacts. Additionally, we use either a Peltier cooler or a liquid nitrogen cooled sample holder to stabilize the specimen against beam damage. The resulting images shows significantly different pore size distribution and surface morphology compared to typical SEM images obtained at room temperature. By optimizing the electron beam energy, scanning method and specimen temperature, an optimum strategy to image the separator membrane is developed to reveal its pristine surface morphology and pore structure.

Li-ion batteries offer excellent energy conversion rate and high energy storage density in electric vehicles, aircraft, consumer electronics and renewable energy storage. However, these batteries suffer from several safety related problems, as evidenced by recent product recalls and incidents of fires in electric cars and aircraft. Overheating due to poor thermal transport is at the heart of several of these challenges, and therefore, a fundamental understanding of heat transfer in Li-ion battery materials is critically needed. While Li-ion battery materials have been well optimized from the perspective of electrochemistry, the nature of thermal transport in these materials remains relatively less known.

This talk will summarize ongoing research on understanding and optimizing heat transfer in Li-ion battery materials through experiments and theoretical analysis. Measurement of orthotropic thermal conductivity of a Li-ion cell will be described. Measurements of thermal conductivity of the separator of a Li-ion cell will be discussed. Measurements that identify poor thermal transport across the cathode-separator interface as the fundamental root cause of the low thermal conductivity of Li ion cells will be discussed. A molecular bridging approach that reduces this rate-limiting thermal contact resistance by 4X through improved interfacial thermal transport will be discussed. This approach is expected to significantly enhance thermal conductivity of a Li-ion cell. More recent work on enhancement of thermal transport in Gel Polymer Electrolytes (GPEs) through inclusion of BN/Al₂O₃ nano/microparticles will also be discussed. While this approach is shown to improve thermal conduction in general, a careful optimization of thermal and ionic transport in GPEs will help fully utilize this promising class of materials for next-generation solid-state batteries. Related research on developing predictive models for onset of thermal runaway based on thermal properties of battery materials and kinetics of underlying electrochemical processes will also be discussed briefly.

With increasing energy density safety and thermal management of Li-ion cells is becoming more and more important because the thermal runaway can cause an ignition or even explosion with simultaneous release of toxic gases. In order to prevent such hazards, extensive research is undertaken to determine the causes of thermal runaway, as well as the influencing factors. In the last nine years, we have established battery calorimetry as a versatile characterization technique, which allows advancements for the thermal management and the safety of batteries. With six adiabatic Accelerating Rate Calorimeters (ARC) of different sizes and two sensitive Tian-Calvet calorimeters combined with cyclers we operate Europe’s largest battery calorimeter center. This enables the evaluation of thermodynamic, thermal and safety data on material, cell and pack level under quasiadiabatic and isoperibolic environments for both normal and abuse conditions (thermal, electrical, mechanical).
The primary purpose of this work was to determine the correlation between cell aging and the thermal runaway of commercial Sony VTC6 18650 cells with a nominal capacity of 3 Ah, a Li-containing graphite anode and a high-Ni LiNi_0.9Co_0.075Al_0.025O₂ (NCA) cathode as determined by inductive coupled plasma optical emission spectroscopy (ICP-OES). Therefore, a comprehensive ageing study was performed with 116 cells.
First, fresh cells were stored in temperature chambers at different temperatures and states of charge and then characterized at fixed time intervals in order to study the influence.of this storage on the performance (calendar ageing). Second, cells of the same type were aged in temperature chambers at different charge/discharge rates (cyclic aging) and comprehensively characterized every 100 cycles. The ageing procedure was stopped at a state-of-health (SOH) of 80%. Information on the causes of the aging was obtained using electrochemical impedance spectrometry (EIS), X-ray diffraction and measurement of the entropy changes using the potentiometric method, where the change in the open circuit voltage is monitored against the temperature. The change in entropy showed a capacity-dependent behavior. It was found that the cathode has no signficant influence on the entropy changes and that the anode represents the dominating factor. This allows the determination of the loss of lithium inventory (LLI) in the anode and to correlate the LLI to the total capacity loss. The cathode only showed changes in the charge transfer resistance that was extracted from the EIS spectra.
After these characterizations, the cells were thermally abused using the Heat-Wait-Seek (HWS) test in an ARC. The cyclic aged cells showed increased exothermic reactions in the low temperature range. It is unclear whether these indicate lithium plating or decomposition of the SEI. The absence of these increases with calendar aging, in which plating can be ruled out, means that a larger SEI layer has a small influence on an increased temperature rate. Accordingly, it is assumed that the increase in cyclic aging in the low temperature range is due to a small amount of plating. The aged cells show a vent opening at higher temperatures than the fresh cells and minimal deviations in the temperature rates, which can be attributed to the lower capacity of the aged cells. Thus, it can be concluded that for these cells the safety level is not negatively affected when they reach a SOH of 80%.
The next step is to perform a similar analysis for cells aged until SOH 60% and 40% and to extract electrodes from the aged cells from which coin half cells can be prepared. These cells will then be studied using electrochemical impedance spectroscopy, CV measurements and both isothermal and adiabatic calorimetry.

Thermal and electrochemical behavior of a Li-ion battery throughout thermal runaway (TR) is investigated with a goal of understanding the conditions that cause this phenomena in an effort to improve battery safety testing. TR can be caused by multiple methods, but one that is nearly impossible to detect is one triggered by an latent event such as internal short circuit (ISC). This type of initiation mechanism has a low probability, but is still a severe risk when considering the safe use of Li-ion batteries because of the likelihood of catastrophic damage. During TR, the cell temperature increases which initiates and catalyses additional exothermic chemical reactions. These reactions become self sufficient, and if heat cannot be removed quickly enough, the cell fails and can result in ignition. The danger of a single cell TR is that it can propagate through an entire battery pack because of the close packed arrangement of the cells.
Currently, research on alternative designs to prevent propagation as well as attempts to mitigate TR are being conducted. They are based on information gathered from studies that trigger TR through the use of external heaters which can affect the kinetics of the decomposition reactions in the trigger cell and surrounding cells and exacerbate the TR event and the propagation, respectively. It is clear that in order to improve upon the fidelity of Li-ion safety research, the TR method must be further characterized. This project aims to study the trigger mechanisms relating to ISC caused by point heat source through multi-physics modeling. The results will be applied in future investigations and development of a TR trigger device to be used in Li-ion safety testing.
Initially, the focus will be on thermal modeling to identify the dimensional constraints and power requirements for a local heat source to initiate TR. Specifically regarding heat generation results due to an ISC from anode active material to the positive current collector. Previous research in the field has determined acceptable values for initiating temperatures for electrochemical reactions in the solid electrolyte interface (SEI). These conditions are used as guidelines for the thermal models. The cell and heat source is studied as 1D, 2D, and/or 3D models. Cell layers are represented with accurate material properties, cell geometries for multiple electrode configurations (pouch and cylindrical), and will be paired with realistic cell voltage and impedance. The heat source size, shape, and power conditions will be varied in order to understand where the heat transfers throughout a Li-ion cell, and how long it takes for the temperature of the various components to reach initiation temperature for their respective electrochemical reactions. Simulations are adjusted accordingly until the hot spot results in TR while mimicking real world conditions within the cell. The results will give a more detailed understanding of how an ISC is formed and how it can lead to TR. The generic heat source will later be replaced with the properties of a resistive heating element to be used in the trigger device.
Characterization through simulation is not only convenient, but it also ensures safety during initial research. Thermal modeling will be paired with future electrochemical models for a more complete and accurate representation of a Li-ion cell throughout TR. Future work also includes experiments to further validate these findings. The knowledge gained from this research is vital for understanding the effects of geometry and power on spontaneous TR. Understanding what causes this phenomenon can lead to design innovations for tools meant to identify TR or prevent propagation and the development of mitigation techniques within Li-ion batteries.

The evaluation of lithium ion battery safety has traditionally focused on the performance of single cells exposed to abusive conditions. As lithium ion batteries are fielded in increasingly large and complex systems, the system level safety performance is becoming of greater concern. Testing laboratories and battery end users have begun to consider the hazards associated with a localized thermal runaway event, a single cell or small group of cells, propagating to the bulk of a large battery system. Evaluating this propagating failure requires initiating thermal runaway of a single cell or group of cells and observing how that failure affects the surrounding system.

Ultimately, testing cell to cell propagation requires a method for initiating or simulating single cell runaway while adding a minimum of excess energy into the system. Traditionally single cell failure is initiated with standard battery abuse tests, such as overcharge, thermal ramp and nail penetration tests. However, nail penetration tests can be difficult to perform in fully developed battery systems while overcharge and thermal ramp add a significant amount of excess energy into the battery system. Recent work at Sandia has looked at non-traditional thermal runaway triggers that can be applied to off-the-shelf cells. Single cells were initiated using a fiber coupled laser to create a localized failure on a cell. The ability to initiate single cell failure in both pouch and cylindrical cells is in multiple orientations is demonstrated. Limited cell to cell propagation initiated by the laser is performed as well, demonstrating the technique as a low energy initiation source for propagation testing.

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

Thermal runaway in lithium-based cells and batteries have been a significant concern during transportation. The ICAO had set temporary restrictions on transportation to limit the SOC to 30% or less. In parallel, the ICAO commissioned the Society of Aerospace Engineers (SAE) to write a standard that would have test procedures that confirmed that li-based cells in the package as shipped will not become a fire hazard. The test procedure and the verification of test protocols have been looked into in this study. The results obtained as part of this test program will be presented.

All-solid-state Li batteries (ASLBs) have been emerged as the enabler to achieve significantly enhanced safety and energy densities, as compared to conventional lithium-ion batteries (LIBs). Several classes of inorganic solid electrolyte (SE) materials have garnered much attention as they could be integrated with electrode active materials into ASLBs by a scalable process such as cold-pressing. Sulfides (or thiophosphates, e.g., Li₆PS₅Cl) and halides (e.g., Li₃YCl₆) satisfy critical features for scalable high-performance ASLBs; those are mechanically sinterability and high Li⁺ conductivity. However, they show different comparative advantages in multiple aspects. For example, while halides are heavier than sulfides, they show superior electrochemical oxidation stability to sulfides.
As is the case for advanced LIBs based on liquid electrolytes, Ni-rich cathode materials are the key to the practical ASLB technologies. Unfortunately, they suffer from par below electrochemical performances in ASLBs, which are attributed to the interfacial instabilities in terms of both electrochemical and electrochemo-mechanical effects.
In this presentation, we report on our recent findings on the electrochemo-mechanical effects for Ni-rich layered cathode materials for ASLBs. Microstructural effects of NCM particles on the performance of ASLBs are revealed. Moreover, the performances of NCM using halide SEs are extensively compared. These results shed light on a design strategy for high-performance practical ASLBs.

References
[1] K. H. Park, Q. Bai, D. H. Kim, D. Y. Oh, Y. Zhu, Y. Mo, Y. S. Jung, Adv. Energy Mater. 2018, 8, 1800035.
[2] S. H. Jung, U.-H. Kim, J.-H. Kim, S. Jun, C. S. Yoon, Y. S. Jung, Y.-K. Sun, Adv. Energy Mater. 2020, 10, 1903360.

Polymers are playing already an important role for lithium batteries, serving as the main component for the separator and as binder to ensure the mechanical integrity of the electrodes. Recently, the use of polymer-based electrolytes has also gained an increasing momentum – not least due to the successful commercialization of lithium-metal polymer batteries employing poly(ethylene oxide) as ion conducting matrix.^[1] Nonetheless, the need for adding a lithium salt in combination with the relatively low ionic mobility at ambient temperatures raises concerns about (reversed) charge carrier concentration gradients, rather low limiting current densities, and inhomogeneous lithium deposition at the negative electrode.^[1–3] Single-ion conducting polymer electrolytes are considered beneficial in this regard – especially if the Li⁺ transport occurs along well-defined conduction pathways.^[2,3] The limited ionic conductivity of such systems, however, calls for alternative conduction mechanisms – e.g., employing molecular “vehicles” that facilitate Li⁺ transport.^[3–5]
Herein, the most recent findings in this field will be presented, covering fundamental insights about the charge transport mechanism, the potential safety of such systems, and their suitability for high-energy lithium-metal batteries incorporating Ni-rich Li[Ni_1-x-yMn_xCo_y]O₂ as active material for the positive electrode. Moreover, it will be shown that such single-ion conducting polymers can dramatically improve the interfacial stability at the cathode – also for hybrid systems.

References
[1] J. Kalhoff, G. G. Eshetu, D. Bresser, S. Passerini, ChemSusChem 2015, 8, 2154–2175.
[2] D. T. Hallinan, N. P. Balsara, Annu. Rev. Mater. Res. 2013, 43, 503–525.
[3] D. Bresser, S. Lyonnard, C. Iojoiu, L. Picard, S. Passerini, Mol. Syst. Des. Eng. 2019, 4, 779–792.
[4] H.-D. Nguyen, G.-T. Kim, J. Shi, E. Paillard, P. Judeinstein, S. Lyonnard, D. Bresser, C. Iojoiu, Energy Environ. Sci. 2018, 11, 3298–3309.
[5] V. Bocharova, A. P. Sokolov, Macromolecules 2020, DOI 10.1021/acs.macromol.9b02742.

With growing demand for energy storage technologies in emerging electric vehicles and flexible electronics, a combination of research on high-performance rechargeable batteries (i.e. enhanced thermal safety and longer cycle life) and smart manufacturing techniques (i. e. additive manufacturing) is becoming increasingly crucial. In order to face the challenges, research has been going on to improve batteries from three aspects: electrochemistry, material, and device architecture. New material candidates with improved properties are being tested for different battery components: electrode, electrolyte, and separator. While conventional electrolytes suffer from high flammability and parasitic reactions due to its liquid form and result early battery failure, solid polymer electrolytes using polymer materials are gaining attention to overcome these challenges (Zhao et al., 2019). However, the main challenges associated with solid polymer electrolytes (SPEs), particularly for the so called “metal battery”, include finding SPEs that can: (i) offer fast Li⁺ ion transport at room temperature, (ii) accommodate volume changes at the anode by reversible deformation and high flexibility, (iii) suppress the Li dendrite growth by having mechanical rigidity, and (iv) increase battery reliability by reducing thermal runaway events.
In this work, boron nitride nanosheets (BNNSs) incorporated SPEs (composite SPEs) were fabricated utilizing our lab-developed 3D printing technology. A robotic deposition system was used to extrude PVdF-based SPE inks onto electrode directly. This robotic deposition system is equipped with three-axis direction stage, control system, pneumatic extrusion dispenser, fine cylindrical nozzle, and self-designed heating block to facilitate printing of electrolytes continuously. This 3D printed composite SPE was further characterized, and performances were compared with SPE without BNNS by evaluating electrochemical, mechanical, and thermal properties. Electrochemical impedance spectroscopy study indicates 10 times improvement in ionic conductivity for composite SPE because of BNNSs addition. This increased Li⁺ ion conduction was observed because of the Lewis acidic characteristic of BNNS which facilitates to trap the anions in the electrolytes (Shim et al., 2017). In addition, the Young’s modulus and yield strength are increased by 100 and 168 %, respectively, because of BNNS reinforcement. This improvement in mechanical properties may help to suppress Li dendrite growth as well as accommodation of volume changes (Monroe and Newman, 2005). Moreover, high thermal conductivity of BNNS is likely to enable homogenous heat distribution in composite SPEs. As a result, the hot-spot formation during parasitic thermal runaway events will be reduced, resulting in enhanced reliability of the battery. This increased mechanical robustness, homogenous heat distribution, and improved ionic conductivity will pave the way for longer battery cycle life.

Rechargeable battery technology awaits further advances to meet future demands for large scale and safe storage of energy from renewable but intermittent sources. Solid polymer electrolytes based on poly(ethylene oxide (PEO) have been under investigation for some four decades as an safer alternative to flammable organic electrolytes.¹ It still lack sufficiently high ionic conductivity for practical use at ambient temperature.² Recent reports of high ionic conductivity and an expanded electrochemical window of aqueous electrolytes enabled by very high salt concentration (~ 20m)^3,4 as well as earlier work on the “salt-in-polymer” concept⁵ have inspired this investigation of a solid PEO-salt-water system, where the dissolved salt (LiTFSI) concentration approaches its solubility limit and in combination with water association succeeds in inhibiting crystallization of the PEO matrix leading to higher conductivity. Room temperature ionic conductivity in these solid-like polymer electrolytes is about 2 mS-cm-¹, which is sufficiently high for battery application.⁶ We have further characterized the ion transport process by nuclear magnetic resonance (NMR) pulsed field gradient diffusion measurements over a wide range of temperature for ⁷Li and ¹⁹F as the cation and anion, respectively. The diffusion measurements in concert with electrochemical impedance spectroscopy show that the degree of salt ion dissociation in these electrolytes is also sufficiently high. It is found that the lithium transport number, which is related to the fraction of current carried by the Li ions, exceeds 0.6 for the whole temperature range, which is unusually high for polymer electrolytes.
References
1. M. B. Armand, J. M. Chabagno, M. J. Duclot, in International Conference on Fast Ion Transport in Solids, Electrodes, and Electrolytes., P. Vashishta, J. N. Mundy, G. K. Shenoy, Eds. (North Holland, Lake Geneva, Wisconsin, U.S.A., 1979), pp. 131.
2. M. Armand and J.-M. Tarascon, “Building Better Batteries,” Nature, Vol. 451, 2008, pp. 652-657.
3. L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C.Wang, K. Xu, Science 2015, 350, 938−943.
4. O. Borodin, L. Suo, M. Gobet, E. Gobrogge, X. Ren, M. Olguin, M. S. Ding, M. Schroeder, A. von Cresce, J. Peng, A. Faraone, S. Munoz, J. Dura, S. Greenbaum, C.Wang, K. Xu, ACS Nano 11, 10462-10471 (2017).
5. C.A. Angell, C. Liu, E. Sanchez, Nature 362, 137–139 (1993).
6. S. Bhattacharyya, M. Garaga, S. Greenbaum, APS March Meeting 2019 abstract ID:E47.001

Anodic stability of solid-state electrolyte in full-solid-state lithium ion batteries (SSLIBs) is attracting more attention recently as SSLIB is receiving increasing attention from both industry and academia. Yet neither the definition of electrochemical stability of solid-state electrolyte nor the determination of such stability has been thoroughly investigated. The most common characterization technique for oxidative stability of solid-state materials, linear sweep voltammetry (LSV), lacks validity and reproducibility while applied on solid-state systems, as its result is greatly affected by cell configuration, sample preparation and data processing method. For example, independent research groups have reported oxidation potentials spanning from 4.0 V–5.5 V vs. Li/Li⁺ for PEO electrolytes, 3.7 V–9.0 V vs. Li/Li⁺ for LLZO electrolytes and 2.1 V–4.0 V vs. Li/Li⁺ for LGPS electrolytes using LSV methods. In practice, cycling of PEO with cathodes having charge potentials above 4 V are usually unstable.

In this work, we revisited the open question of understanding the oxidative decomposition behavior of polymer electrolyte using electrochemical, spectroscopy and chromatographical tools. A capacity-based method was developed to replace current-based LSV method. A reversibility factor (R), defined as the ratio between the Faradic (decomposition) charge versus non-Faradic (capacitive) charge during cycling, was established as a reliable reference for the degree of oxidative decomposition below certain cutoff potential. To reproduce the environment in solid-state cathode, a large amount of conductive additive was added into the polymer electrolyte sample and nuclear magnetic resonance (NMR) data suggests the sensitivity of the test is greatly improved. Using poly(ethylene oxide) (PEO) as sample material, the polymer was found gradually decomposing above 3.7 vs. Li/Li⁺ and reaches its practical stability limit around 4.0V vs. Li/Li⁺ with R-based test. The result was further confirmed with gel permeation chromatography (GPC), where the PEO sample was allowed to decompose at given voltage. Molecular weight-oxidation potential relation of the partially decomposed PEO shows good correlation with R-cutoff potential relation, providing direct support of the validity of the electrochemical test. The method is further verified with a nitrile-based, more oxidatively stable polymer electrolyte, hydrogenated butadiene nitrile rubber (HNBR), and the universality of such test for other polymer electrolytes was proved. HNBR was found decomposing above 3.9 V vs. Li/Li⁺ and reaches stability limit above 4.3 V vs. Li/Li⁺.

All-solid-state Lithium-Sulfur Battery (ASSLB) is one of the protagonists for next-generation energy storage system. The high theoretical capacity of sulfur, the nonflammable characteristics of solid electrolyte and the elimination of shuttle effect endow ASSLSBs with supreme safety, high energy density and long cycle life. However, due to the insulating nature of sulfur, the utilization of active material during cycling is usually low which leads to lower capacity and practical energy density. To resolve this issue, highly efficient ionic and electronic transportation framework needs to be constructed. In all-solid-state sulfur cathode, the weight of solid electrolyte is usually above 40 wt%. While serving as lithium ion conductor, solid electrolytes generally possess low electronic conductivity (<10^-8 mS cm^-1) which may hinder electron transport in all-solid-state sulfur cathode. Therefore, to improve electronic transportation within sulfur cathode, we have prepared inorganic solid electrolytes with mixed ionic-electronic conductivity and controlled electronic conductivity for ASSLSBs. Specific capacity of the sulfur cathode could increase from below 900 mAh g^-1 to over 1300 mAh g^-1 (based on the weight of sulfur) at 0.1 C after employing the prepared mixed ionic-electronic conductive solid electrolyte. Besides, various aspects of the synthesized material and the fabricated sulfur cathode have also been investigated, including material structure and electrochemical evolution, cathode cycling stability, rate capability and etc.

Advances in hybrid and electric vehicle technologies combined with a demand for green initiatives have motivated necessary diversification in energy storage research. To achieve customer expectations for hybrid and electric vehicles, new battery systems with higher energy densities, power densities and cycle life than the current state-of-art Lithium (Li)-ion battery are needed. Post Li-ion battery systems, especially those focused on the utilization of Li metal have recently come to the forefront of research. The ability to directly utilize Li metal anodes in rechargeable batteries presents itself as an ideal situation via the accessibility of a maximum possible theoretical specific capacity (3860 mAh/g) in comparison to commercially used anodes (e.g. graphite – 380 mAh/g). Hence, significant efforts in recent literature have targeted the development of robust Li metal anode systems.
One such system is Li-sulfur which has attracted attention due to its high theoretical capacity (1673 mAh/g) and potential low cost. However, this system is hindered by polysulfide dissolution and electrolyte decomposition at the Li metal anode. Among the various strategies which have been employed to overcome these hinderances, the use of solid-state electrolytes (inclusive of polymers, gels and conducting ceramics) stands out since the implementation of solid-state electrolytes can also serve as a mechanical barrier towards Li dendrite formation. However, solid-state electrolytes exhibit relatively lower ionic conductivities and also display poor interfacial stability towards Li metal anodes. While advances in solid-state electrolyte materials continue to improve their ionic conductivities, little is known about the interfacial interactions between sulfide-based solid-state electrolytes and Li metal. Hence, studies into understanding the interactions between Li metal and these solid-state electrolytes remains essential towards realizing all-solid-state Li battery technologies.
Here, we present a study into the chemo-mechanical transformations within lithium thiophosphates and at their solid-solid interfaces. Tandem analytical ex-situ and in-situ studies via X-ray tomography and transmission electron microcopy are used to reveal the interfacial interactions and failure modes between Li metal and lithium thiophosphates, the deposition and dissolution properties of Li metal from these electrolytes, and the effects of the deposition and dissolution properties on the bulk electrolyte structure. The presented studies allow for comparisons of Li deposition and dissolution properties below and above the critical current densities for each lithium thiophosphate electrolyte material and stand to help clarify interfacial, morphological and failure evolution mechanisms during Li cycling from them.

Reactive metal anodes like lithium are known to electrodeposit in the form of irregular morphological features on planar substrates. Formed during the earliest stages of deposition, these features are thought to seed non-planar, mossy structures that proliferate in the electrode spacing, hampering electrode reversibility. A growing body of work suggests that the mechanics, structure, ion transport properties, reductive stability, and interfacial energy of interphases formed spontaneously on the metal electrode play important, but differentiated roles in regulating nucleation, growth, and reversibility of these non-planar structures. Fundamental understanding of the formation mechanisms, mechanical stability, ion transport characteristics, and interfacial properties of solid-electrolyte interphases (SEI), though in its infancy is considered a requirement for progress.
Several approaches have been investigated to mitigate or, in rarer cases completely prevent the growth of lithium dendrites. These include salt additives to improve the properties of the SEI,concentrated electrolytes that change the solvation structure of the ions and resulting SEI formation,single ion conductors that prevent concentration gradients in the bulk electrolyte phase<span style="font-size:10.8333px"> </span>and high modulus electrolytes that prevent dendrite formation via mechanical pressure forces. Tailoring the lithium metal/electrolyte interface with polymer coatings that serve as a protective barrier against chemical and physical instabilities has been gaining a lot of attention in the past few years.
In this talk, we elucidate design rules for elastic interphases formed on any generic metal anode and utilize experiments based on Li metal anodes to evaluate their effectiveness in arresting the various instabilities (e.g. morphological, chemical, mechanical/orphaning, and hydrodynamic) known to lower lifetime and reversibility of metal anodes. By combining the experiments with a theoretical stability analysis of metal electrodeposition across elastic interphases, we find that interphase thickness, mechanics, ion transport and interfacial properties all play precise, differentiated roles in setting the optimal interphase design.

The introduction of lithium-ion battery technology in 1991 has led to the prevalence of personal electronic devices and the burgeoning electric vehicle space. Advancements in energy density and safety of battery technology will enable further adoption of electric vehicles. Solid-state (SS) battery technology is a leading candidate for the next generation of energy storage in which the liquid electrolyte and polymer separators are replaced with a ceramic ionically conducting solid. While this technology replaces the flammable liquid electrolyte of lithium-based technology, the highly reactive lithium metal and brittle separator materials found in SS battery technology may, ostensibly, introduce other unknown considerations to safety.

Lithium metal is known to be reactive when exposed to water. With the potential application of SS battery technology in electric vehicles, an understanding of lithium metal water exposure in the context of battery architecture is necessary for the adoption of the technology. The reactivity of lithium metal in water has been assessed in various in-situ architectures in the context of existing international battery safety test standards.

Semiconductor physics mainly discuss the energy band structures and the carrier behaviors in semiconductors. The carrier dynamics and the resulting conduction properties of semiconductors. As a semiconductor, the carrier transporting characteristics of Si has been well studied and adjusted via doping (n type, N, P; p type, B), which has been widely applied in electronics, solar cell and photocatalysis. In recent years, Si has been regarded as the most promising anode material for its highest theoretical specific capacity, and natural abundance. However, the poor conductivity of intrinsic Si still limits the rate performance of Si materials.
Previously, Yi and me found boron doping can obviously enhance the rate capability of Si–C composite. Recently, phosphorus-doped yolk-shell Si@C materials (P-doped Si@C) with balanced electron and Li+ transportation dynamics are rational designed and prepared, which deliver high specific capacities (2709 mAh g^-1 @ 0.4 A g^-1), great rate capability (510 mAh g^-1@35 A g^-1) and good retention of ~95% over 800 cycles at 4 A g^-1. As a result, P-doped Si@C anode pairs with commercial activated carbon to assemble lithium-ion capacitor (high power density of ~61080 W kg^-1 at 20 A g^-1).
Junction is also an important concept in semiconductor physics, which refers to the interface region with rectification characteristics formed by the contact of two different semiconductor phases. The formation of semiconductor junction will change the charge state of atoms near the junction interface, and then regulate the adsorption and catalytic activity of active sites in the region. Our previous research reveals the space charge of p-n junction of p-type CoP and n-type FeNi LDH can be utilized to improve the adsorption of OH^- and promote the OER process. With this in mind, The in-situ formed VS_x@VO₂ junctions can designed to promote the absorption of polysulfide and result high volume specific Li-S batteries. Furthermore, a n-n-type CoS₂-CoS heterojunction were designed to facilitate the electrocatalytic conversion of I⁻/I₃⁻ and polysulfides, which can significantly improve the power density and lifetime of flow batteries. Therefore, it is possible to modulate the electronic configuration of active site by designing proper semiconductor junctions following the theories of solid physics.
In all, the semiconductor physics can offer useful theoretical guidance for rational design of high performance materials for energy storage.
Ref
[1] R. Yi, J. Zai; F. Dai, et al., Electrochem Commun, 2013, 36,
[2] M. Chen, B. Li, X. Liu, et al., J Mater Chem A, 2018, 6, 3022-3027.
[3] K He, T. Tadesse Tsega, X. Liu, et al., Angew. Chem. Int. Ed., 2019, 58, 34, 11903-11909.
[4] L. Zhou, L. Yao, S. Li, et al., J Mater Chem A 2019, 7 (8), 3618-3623.
[5] D. Ma, B. Hu Bo, W. Wu, et al., Nat Commun, 2019, 10, 1, 3367-3367.

Lithium (Li) metal offers high theoretical capacity, lowest negative potential and light weight, making it the ideal anode material for high-energy-density batteries. Porous electrodes resulting from unregulated Li growth is believed to be the major cause of the low Coulombic efficiency (CE) and potential safety hazards of rechargeable Li metal batteries. Strategies aiming to achieve large granular Li deposits have been extensively explored; yet, the ideal Li deposits, which consist of large Li particles seamlessly packed on the electrode and can be reversibly deposited and stripped, have never been reported. Here, by using cryo-FIB-SEM, 3D reconstruction and MD simulation, we quantitatively revealed the role of stack pressure on Li nucleation, growth and stripping, and discovered that optimal pressure for approaching 100% dense Li deposition by establishing a pressure-morphology-performance correlation. Unprecedented, a dense Li deposition (99.49% electrode density) with an ideal columnar structure has been achieved and can be reversibly plated and stripped during extended cycles at room temperature. The tailored Li deposition and stripping by optimizing stack pressure opens new opportunities to rationally mitigate inactive Li formation, improve cycle life and enable fast charging for rechargeable Li metal batteries.

All-solid-state Li batteries (ASLBs) have been emerged as the enabler to achieve significantly enhanced safety and energy densities, as compared to conventional lithium-ion batteries (LIBs). Several classes of inorganic solid electrolyte (SE) materials have garnered much attention as they could be integrated with electrode active materials into ASLBs by a scalable process such as cold-pressing. Sulfides (or thiophosphates, e.g., Li₆PS₅Cl) and halides (e.g., Li₃YCl₆) satisfy critical features for scalable high-performance ASLBs; those are mechanically sinterability and high Li⁺ conductivity. However, they show different comparative advantages in multiple aspects. For example, while halides are heavier than sulfides, they show superior electrochemical oxidation stability to sulfides.
As is the case for advanced LIBs based on liquid electrolytes, Ni-rich cathode materials are the key to the practical ASLB technologies. Unfortunately, they suffer from par below electrochemical performances in ASLBs, which are attributed to the interfacial instabilities in terms of both electrochemical and electrochemo-mechanical effects.
In this presentation, we report on our recent findings on the electrochemo-mechanical effects for Ni-rich layered cathode materials for ASLBs. Microstructural effects of NCM particles on the performance of ASLBs are revealed. Moreover, the performances of NCM using halide SEs are extensively compared. These results shed light on a design strategy for high-performance practical ASLBs.

References
[1] K. H. Park, Q. Bai, D. H. Kim, D. Y. Oh, Y. Zhu, Y. Mo, Y. S. Jung, Adv. Energy Mater. 2018, 8, 1800035.
[2] S. H. Jung, U.-H. Kim, J.-H. Kim, S. Jun, C. S. Yoon, Y. S. Jung, Y.-K. Sun, Adv. Energy Mater. 2020, 10, 1903360.

Lithium ion batteries (LIBs) are wildly used for portable electronics and electric vehicles due to the favorable performance compared with other rechargeable batteries. However, their degradation deteriorate their service life remarkably. Fortunately, charge/discharge curves can provide quantitative information for understanding how such degradation correlates with usage conditions and environment of LIBs.
In general, the charge/discharge curves can be divided into two parts, i.e. the open circuit voltage (OCV) and the polarization in the cell. The OCV provides a thermodynamic fingerprint of the electrode which could be roughly estimated by charge/discharge the cell at very low C-rate. With regards to a commonly used 2430 coin cell, the difference between charge/discharge curve and theoretical OCV is less than 30mV at the full range of SOC when a 1/25C current rate is used. In this presentation, the changes in OCV during long cycles will be exhibited, which offers a quantified information on active material decay as well as the side reactions in the cell. The polarization of the cell is strongly affected by the electron and lithium ion transport inside the electrode microstructure. In this work, both the time-domain and the frequency-domain electrochemical methods ware applied to the analysis to distinguish each process onto the polarization in the cell, i.e. the contact problem between the solid phases, the mass transport through the interface and in the solid phase of the electrodes, along with the slow electrochemical reactions. Furthermore, the thermodynamic fingerprints of the positive and negative materials have strongly relation with the polarization in the cell. These relations generally becomes very complex, and thus a mathematical modeling is also used to understand and explain the charge/discharge curve for LIB in detail. Overall, it is proposed that more collaborations could be needed between the academia and industry so that the lithium ion battery degradation could be fully investigated.

Incorporation of organosilicon (OS) compounds in modest concentrations as low as several percent in conventional carbonate electrolytes lead to substantial improvements in the thermal and electrochemical stability of the resulting lithium ion batteries. We have been investigating the fundamental chemical degradation pathways of organosilicon compounds in an effort to understand the chemical and electrochemical origins of these remarkable properties. The combination of NMR spectroscopy, in situ FTIR, AFM, and XPS provides unique insights into how organosilicon electrolytes reduce or eliminate autocatalytic chemical pathways associated with PF6 and its degradation products. OS compounds further alter the chemical nature of SEI layers on silicon, leading to improvements in overall performance. In this talk I will summarize some of our recent work characterizing organosilicon thermal stability and electrochemical stability against different materials, including silicon-based anodes and NMC-based cathodes.

Lithium-metal anode holds tremendous potential for enabling next generation of rechargeable batteries due to their high storage capacity (3,861 mAh g^-1) and low reduction potential (-3.04 V vs SHE). When paired with a high-capacity cathode, such as high-nickel NMC or sulfur, lithium-metal anode can help deliver energy-dense storage systems with ~ 500 Wh kg^-1. However, the poor efficiency associated with lithium plating and stripping severely limits the cycle life. For realizing practical viability, reversible lithium deposition needs to be achieved by stabilizing the lithium-electrolyte interphase (SEI) layer and mitigating the extreme volume changes accompanying the cycling.

The chemistry of the lithium-electrolyte interface is rendered distinct by the choice of the cathode in a full cell, viz., high-nickel NMC or sulfur. This is not only due to the different electrolyte formulations used, but also due to the crossover of species between the cathode and anode. The characteristics of deposited lithium can fundamentally vary across different cathode systems, and unique strategies must be employed to stabilize lithium deposition in each case. Hence, an in-depth fundamental study is necessary to develop a thorough understanding of the lithium-electrolyte interfacial chemistry, while creative engineering solutions are necessary to achieve highly efficient lithium plating and stripping. This presentation will focus both on a fundamental understanding and offering engineering solutions.

When paired with a high-nickel NMC cathode, the lithium-metal anode is affected by transition-metal dissolution at the cathode and crossover to the anode. However, we find that during initial cycling, crossover of electrolyte decomposition products from the anode to the cathode is more severe and plays a more critical role. Not only does it alter/thicken the cathode SEI layer, it also affects the anode itself as the lithium SEI components could continually dissolve. The magnitude of this crossover is expected to depend on the nickel content in the cathode.

When paired with a sulfur cathode, the lithium-metal anode is affected by polysulfide dissolution at the cathode and crossover to the anode. We use anode-free full cells, which contain no excess lithium at the anode, to investigate the dynamics of lithium deposition. With advanced surface characterization methodologies, e.g., time-of-flight secondary ion spectrometry, we find that the depletion of cyclable lithium inventory is due to the formation of “dead” metallic lithium, which itself is caused by severe electrolyte decomposition on lithium-metal surface with cycling.

The presence of polysulfide intermediates is found to have an intrinsically stabilizing effect on lithium deposition in Li-S batteries. This motivates our strategies towards engineering further improvements in cycle life. With tellurium as a cathode additive, tellurium-substituted polysulfides are formed, which reduce on lithium surface to form Li₂TeS₃ as a thin, Li⁺-ion conducting SEI layer. This enables uniform deposition of lithium and mitigates electrolyte decomposition. A remarkable improvement in cyclability is achieved in energy-dense anode-free full cells and lean-electrolyte pouch cells. Similarly, using organosulfide molecules as electrolyte additives opens the possibility of favorably tuning the properties of lithium SEI layer by simply modifying the organic functional group.

The development of three-dimensional current collector matrices is also critical to achieving reversible lithium deposition. The highly conductive frameworks provide efficient electron/ion transport and guides a homogenous deposition of lithium, while the porous 3D architecture helps mitigate severe volume changes during cycling. The presentation will demonstrate various lithiophilic host materials that show considerable promise in improving the cyclability of lithium-metal anode when paired with both high-nickel NMC and sulfur cathodes.

Nickel-rich layered lithium transition metal oxides are the most attractive cathode materials for the next generation lithium-ion batteries for automotive application. However, they suffer from crystal and interfacial structure instability under aggressive electrochemical and thermal driving force, leading to rapid performance degradation and severe safety concerns.In this talk, we reported a transformative approach by using an oxidative chemical vapour deposition technique to build a protective conductive polymer (PEDOT) skin on layered oxide cathode materials. The ultra-conformal PEDOT skin facilitates the transport of lithium ions and electrons, significantly suppresses the undesired layered to spinel/rock-salt phase transformation and the associated oxygen loss, mitigates intergranular and intragranular mechanical cracking, and effectively stabilizes cathode-electrolyte interface. This approach remarkably enhances the capacity and thermal stability under high-voltage operation. Building a protective skin at both secondary and primary particle levels of layered oxides offers a design strategy for the promising Ni-rich cathodes towards high-energy, long-life and safe lithium-ion batteries.

Li-ion batteries offer excellent energy conversion rate and high energy storage density in electric vehicles, aircraft, consumer electronics and renewable energy storage. However, these batteries suffer from several safety related problems, as evidenced by recent product recalls and incidents of fires in electric cars and aircraft. Overheating due to poor thermal transport is at the heart of several of these challenges, and therefore, a fundamental understanding of heat transfer in Li-ion battery materials is critically needed. While Li-ion battery materials have been well optimized from the perspective of electrochemistry, the nature of thermal transport in these materials remains relatively less known.

This talk will summarize ongoing research on understanding and optimizing heat transfer in Li-ion battery materials through experiments and theoretical analysis. Measurement of orthotropic thermal conductivity of a Li-ion cell will be described. Measurements of thermal conductivity of the separator of a Li-ion cell will be discussed. Measurements that identify poor thermal transport across the cathode-separator interface as the fundamental root cause of the low thermal conductivity of Li ion cells will be discussed. A molecular bridging approach that reduces this rate-limiting thermal contact resistance by 4X through improved interfacial thermal transport will be discussed. This approach is expected to significantly enhance thermal conductivity of a Li-ion cell. More recent work on enhancement of thermal transport in Gel Polymer Electrolytes (GPEs) through inclusion of BN/Al₂O₃ nano/microparticles will also be discussed. While this approach is shown to improve thermal conduction in general, a careful optimization of thermal and ionic transport in GPEs will help fully utilize this promising class of materials for next-generation solid-state batteries. Related research on developing predictive models for onset of thermal runaway based on thermal properties of battery materials and kinetics of underlying electrochemical processes will also be discussed briefly.

Li-ion batteries are being widely used in planetary space missions, owing to their impressive performance characteristics, including high specific energy, long life, and the ability to operate over a wide temperature range. Historically, batteries with custom large-format Li-ion cells of 10-55 Ah have been used in various Mars missions, e.g., Mars Exploration Rovers (Spirit and Opportunity), Mars Curiosity Rover, Mars landers (Phoenix and InSight), Mars and Jupiter orbiters and the upcoming Mars 2020 Rover. However, in several other robotic and human space missions, e.g., Europa Clipper, Artemis missions to moon, and Portable Life Support System (PLSS) for Extra-Vehicular Activities, commercial 18650 Li-ion cells are being used due to their improved specific energies of ~250 Wh/kg (20-50% over custom cells), improved safety from built-in safety devices, modularity, and excellent uniformity, which obviate the need for cell-level control during charging. Several cell types with different chemistries and from different manufacturers are being evaluated at JPL. The cell chemistries differ mostly in cathodes, i.e., either lithium nickel cobalt aluminum oxide (NCA), Li-rich nickel manganese cobalt oxide (Ni-rich NMC) or blends with lithium manganese spinel oxide (LMO). Various performance tests, including cycle life at different depths of discharge, radiation tolerance and high-power characterization at different temperatures are being carried out in conjunction with Electrochemical Impedance Spectroscopy (EIS). In addition, destructive physical analysis of the cells have been performed to analyze the electrode materials, and correlate them with the performance characteristics. Finally, low temperature charge characterization studies have been carried out to ascertain the propensity of lithium plating as a function of charge current and temperature.

Future applications for energy storage will demand high level function with long lifetime. As batteries increase in size and use requirements increase in complexity, safety takes on expanded importance for multiple applications including transportation and grid-level storage. Examples of electrode design approaches leading to improved safety based on materials chemistry will be highlighted in this presentation. Complementary ex-situ, in-situ and operando characterization approaches to understand energy storage material/composite degradation modes and relationship to cell level safety will also be described.

Driven phase separation in ion intercalation materials is known to result in different non-equilibrium phase morphologies, such as intercalation waves and shrinking-core structures, but the mechanisms of pattern selection are poorly understood. Here, based on the idea that the coarsening of the slowest phase is the rate limiting step, we introduce a scaling law that quantifies the transition from quasi-equilibrium intercalation-wave to diffusion-limited shrinking-core behavior. The scaling law is validated by phase-field simulations of single Li_xCoO₂ particles, in situ optical imaging of single Li_xC₆ particles undergoing transitions between stage 1 (x = 1) and 2 (x = 0.5) at different rates, and all the available literature data for single-particle imaging of Li_xCoO₂, Li_xC₆ and Li_xFePO₄. The results are summarized in operational phase diagrams to guide simulations, experiments, and engineering applications of phase-separating active materials. Implications for Li-ion battery performance and degradation are discussed.

The charge transfer reaction is the fundamental reaction for rechargeable batteries. The energy landscape of this reaction depicts the equilibrium and kinetics of the electrochemical process. Typically, a Li-metal electrode is always covered by a thin layer of solid electrolyte interphase (SEI), forming a complex Li/SEI/electrolyte interface. We first introduced a new half-cell model to predict the energy landscape of the electrochemical reaction of at the complex Li/SEI/electrolyte interface, with combined density functional theory (DFT) and tight-binding (DFTB) calculations. The simulated atomic details revealed that the Li-metal surface is negatively charged at the electrochemical equilibrium condition (the experimentally defined zero voltage for Li⁺/Li⁰) due to the large solvation energy of Li⁺ in the EC-based electrolyte. The electric field created by the negatively charged surface can reorient the electrolyte into an ordered structure, lower the Li⁺ ion desolvation energy barrier, and help the Li⁺ ion transport through the SEI.
Further, the calculated energy landscape can be used to describe the electrochemical reaction kinetics with a Butler-Volmer equation, enabling a multiscale modeling approach. In this study, the DFT and DFTB predicted interfacial structures, de-solvation processes, and the ion transfer energy barrier, were inputted into a non-linear phase-field model to capture the morphological evolution during the electroplating process. Specifically, this model captured the morphology difference between the dendritic Li plating in carbonate (EC)-electrolyte and faceted Mg plating in tetrahydrofuran(THF)-electrolyte under the similar current density. Furthermore, through systematical parametric analyses, we conclude that the cation desolvation-induced exchange current difference between Li and Mg is mainly responsible for their deposition morphological differences. This study provides a strategy of connecting the phase-field method and atomistic calculations and predicts that increasing the desolvation energy and lowering the equilibrium exchange current can help to achieve dendrite-free Li-metal anode.

Imaging the microstructure of a battery cell at different length scales is important to understand the degradation processes through the battery cycle life. For example, investigating the formation of defects such as dendrites at the Li anode interface requires both high resolution imaging and large field of view. Ideally, imaging techniques cover the entire electrode thickness at submicron resolution with a method that preserves the sample interface. Here, we present an approach to probe the defects at the Li-metal battery interface. A coin cell has been sliced under cryo conditions with a laser inside the vacuum of the scanning electron microscope (SEM) chamber so a pristine surface is obtained and defect formation can be observed across the entire thickness of the cell.
In another example, transport properties for a thick NMC 811 cathode were obtained from focused ion beam (FIB) milling in combination with SEM. Because of the cathode thickness, an alternative to traditional Ga FIB milling was used. With a Xenon plasma beam, it was possible to obtain a 3D data set with more than 100 µm field of view on a 70 µm thick cathode. The microstructural characteristics extracted from such representative 3D volume help the understanding of the correlation between electrode microstructural evolution and performance deterioration.

Thermal runaway in lithium-based cells and batteries have been a significant concern during transportation. The ICAO had set temporary restrictions on transportation to limit the SOC to 30% or less. In parallel, the ICAO commissioned the Society of Aerospace Engineers (SAE) to write a standard that would have test procedures that confirmed that li-based cells in the package as shipped will not become a fire hazard. The test procedure and the verification of test protocols have been looked into in this study. The results obtained as part of this test program will be presented.