Stack Pressure Research Articles

Dependence of Solid-State Metal Battery Thermodynamics on Interfacial Mechanics

Next generation batteries seek to utilize metal electrodes due to their increased gravimetric and volumetric energy density compared to intercalation electrodes. Single-ion conducting solid electrolytes are viewed as a promising candidate to aid in the adoption of metal electrodes as they are mechanically stiff, possess high ionic conductivity, and may improve safety relative to conventional liquid electrolytes. Solid-state batteries are often constructed with an external stack pressure, and plating and stripping of the metal electrode leads to variable stress states in the cell during operation. The mechanical state of a material affects its thermodynamic state and can thereby lead to variations in the current distribution in the cell. This talk will focus on the effect of interfacial mechanics on the thermodynamic state of metal electrodes and single-ion conducting electrolytes. Experimental results and mechanics models will be shown to demonstrate the relationship. Disagreements in the literature regarding the appropriate stress terms to use when assessing the effect of mechanics on interfacial thermodynamics will be discussed. The talk will address how our new results can help to guide the development of future electro-chemo-mechanical plating and stripping stability models.

Pressure-Driven and Creep-Enabled Interface Evolution in Sodium Metal Batteries.

All-solid-state batteries (ASSBs) using an alkali metal anode and a solid-state electrolyte (SE) face several problems due to poor physical and electrical contact. Recent experiments have shown that applying a stack pressure can improve the interface contact and suppress void formation. The mechanical properties of Na metal are different from those of Li metal, leading to differences in the mechanisms of the pressure-dependent interface evolution. Herein, we report a three-dimensional time-dependent model for tracking the evolution of interfaces formed between Na metal and Na-β″-alumina SE. Our results show that Na metal contacts more conformally with the SE, providing a lower interfacial resistance, compared with Li metal, assuming equal resistance due to contamination. The differences due to contact elastoplasticity are larger than the differences in metal creep effects. In fact, we show that increased stack pressure can lead to lower creep because the contact is more conformal at high pressures. Our excellent agreement with recent experiments determines an effective hardness of Na in the Na-SE batteries to be 15 MPa. The results further reveal that the pressure dependence of void suppression is dominated by contact elastoplasticity.

Unraveling the mechanical origin of stable solid electrolyte interphase

Summary The capability of a solid electrolyte interphase (SEI) in accommodating deformation is critical to the electrode integrity. However, the nanoscale thickness and environmental sensitivity of SEI make it challenging to characterize multiple mechanical parameters and identify an appropriate predictor for such capability. Here, we develop a feasible atomic force microscopy (AFM)-based nanoindentation test that circumvents the interference of the anode substrate and allows accurate probing of Young's modulus and elastic strain limit of SEI. The "maximum elastic deformation energy" (U) that an SEI can absorb is proposed to predict the stability of the SEI, as successfully demonstrated in Li/K metal anodes. We show that another asset of U is to provide a rapid and effective means to screen proper electrolytes for the stabilization of Sn and Sb microparticle anodes through building high-U SEIs. Overall, this new indicator, U, offers future directions toward rational design of robust SEIs for advanced anodes.

Optimizing multistage reverse electrodialysis for enhanced energy recovery from river water and seawater: Experimental and modeling investigation

• Fair comparison between single-stage and multistage reverse electrodialysis. • New “saving the gradient” electrical control strategy improves process performance. • To harvest energy from river water and seawater, applying two stages is optimal. • Reducing stack pressure drops favors multistage application. Reverse electrodialysis has been established as a promising method to harvest salinity gradient energy. To achieve market viability, an optimum process configuration is needed, in addition to material and stack development, to increase energy efficiency without compromising power density. Multistage reverse electrodialysis is a practical strategy providing several degrees of freedom, such as independent electrical control of the stages, asymmetric staging, and different configurations. This study tests a two-stage configuration experimentally, using seawater and river water (NaCl only), at several residence times and changing the electrical control. Furthermore, the results are compared with a numerical model that is subsequently used to predict the behavior of alternative multistage configurations. The results show that multistage reverse electrodialysis yields higher gross power density and energy efficiency than a single-stage configuration fed with the same salinity gradient. A new strategy named “saving the gradient” (i.e., lowering the discharge current in the first stage) increased the gross overall performance of the two stages up to 17% relative to single-stage and up to 6% relative to a sequentially optimized two-stage system. Modeling different configurations revealed that only two stages are needed when feeding seawater and river water. When retrieving 40% net energy efficiency, the net power density for a single stage is 0.86 W∙m −2 and 0.94 W∙m −2 for a two-stage system, representing an improvement of 9%. Multistage reverse electrodialysis is therefore a viable concept to enhance power and energy efficiency, and benefits from optimization through electrical control.

Coupled crack propagation and dendrite growth in solid electrolyte of all-solid-state battery

Lithium metal all-solid-state batteries (ASSBs) are promising candidates for future lithium batteries thanks to their high safety performance and energy density. However, Li dendrite growth and interfacial failure are the two fatal issues deteriorating the cyclability that hinders the wide commercialization of ASSBs. To further understand the underlying mechanisms, a coupled electrochemical-mechanical phase-field model for grain crack propagation and lithium dendrite growth is proposed from the energy conservation perspective in this study. We discover that longer defect with sharp edge and angle ( θ ≥ 45 ° ) causes more severe crack propagation and leads to larger dendrite growth area due to the increased strain energy density. We observe that the initial defect within grain plays an irrelevant role in the dendrite growth within the grain boundary. Stacking pressure greater than 10 MPa significantly speeds up crack propagation as well as dendrite growth due to the nontrivial mechanical driving force. Mechanical stress-induced strain-energy would contribute to more than 15% of the total dendrite growth once the stacking pressure exceeds 20 MPa, while it is trivial if the stacking pressure is below 10 MPa. Large enough fracture threshold strain can prevent the crack propagation. Results provide a fundamental tool for the design and evaluation of ASSB safety and cyclability from a more comprehensive perspective and clear the barrier for the development of next-generation ASSBs. • A coupled model for crack propagation and Li dendrite growth is proposed. • Longer defect with sharp edge and angle promotes crack propagation and dendrite growth. • Stacking pressure above 10 MPa greatly assists crack propagation and dendrite growth. • Strain-energy contributes more than 17% of the dendrite growth if the stacking pressure is above 20 MPa.

The Stack Pressure Dilemma in Sulfide Electrolyte Based Li Metal Solid‐State Batteries: A Case Study with Li6PS5Cl Solid Electrolyte

AbstractLithium–solid electrolyte (SE) improper contact and morphological instabilities at the interface during Li plating and stripping are two critical problems that may hinder the realization of Li metal solid‐state batteries (SSBs). While the application of a substantial stack pressure during battery operation can potentially overcome these issues, its proper metering is challenging, especially, when Li metal is employed as the anode, as demonstrated here with Li6PS5Cl as the SE. The low yield strength of Li leads to its extrusion through the micropores of a considerably dense (≈92%) SE even under a moderate stack pressure, resulting in mechanically induced short circuit, which is expedited with decreasing SE thickness. This predicament is avoided at low stack pressures, but inhomogeneous Li plating/stripping triggers dendritic failure at higher currents. Besides highlighting the significance of assembly pressure on the Li‐SSB performance, a comprehensive understanding of the too much versus too little stack pressure dilemma is presented here, which can potentially guide the development of strategies to overcome the electrochemomechanical issues at the Li–SE interface.

Analysis and improvement of flow distribution in manifold for proton exchange membrane fuel cell stacks

The flow distribution in manifold has significant effect on the output performance of the stack according to the Cannikin Law, especially for multi-cell proton exchange membrane fuel cell (PEMFC) stacks. This study presents an analytical model which simultaneously considered local pressure losses, pressure recovery phenomenon, electrochemical reactions, and liquid water to calculate the flow distribution using the flow network method. Both U-shape and Z-shape flow configuration stack are investigated. The analytical model is simultaneously and quantitatively validated with three distinct variables (the dimensionless mass flow rates, the stack pressure drop and the pressure distribution in manifold) to ensure the accurateness of modelling results. Effects of reactant consumption and two-phase flow in unit cell on the flow distribution are quantitatively investigated. Results show that the flow maldistribution is overestimated when ignoring gas consumption and two-phase flow for both 200-cell U-shape and Z-shape stack. Balance the pressure drop of inlet and outlet manifold significantly improves the flow distribution for U-shape stack. Finally, the differentiated assembly strategy is first proposed to improve the flow distribution in manifold for multi-cell PEMFC stacks. Compared to the original design, the ratios of the improvements for 200-cell U-shape and Z-shape stack are 85% and 25%, respectively.

Effect of stacking pressure on the performance of polymer electrolyte membrane fuel cell with various channel types

Polymer electrolyte membrane fuel cell (PEMFC) stacks are assembled via the sequential stacking of a number of PEMFC unit cells. In such a process, stacking pressure is applied to the PEMFC stack. This compression pressure causes a change in mass transport and electrical conduction characteristics. In this paper, various stacking pressures were applied to three types of PEMFC unit cells with serpentine, parallel, and interdigitated cathode channels to understand the effect of stacking pressure on PEMFC performance. Numerical analyses were conducted with a validated 3-D PEMFC model. Deformation of the gas diffusion layer (GDL) by compression was not influenced by the channel type. However, the results of the electrochemical simulation showed different influence of stacking pressure depending on the operating voltage and cathode channel type. To improve PEMFC performance, it was concluded that the stacking pressure should be determined by considering the channel type and operating voltage.

Building airtightness measurement uncertainty due to steady stack effect

Current building pressurization test methods considerably reduce the scope of validity of the test results when stack effect is significant without clear justification. This work attempts to fill this gap based on physical modelling and numerical simulations. In steady and no-wind conditions, it gives the error in the test result with leaks discretely distributed over the building height assuming the flow exponent is the same for all leaks. It also gives an analytical expression of the error when the building can be modelled with two leaks, one at the bottom and one at the top. The results show that the maximum absolute value of the error in the two-leak case has a very high probability to be the upper limit of the absolute value of the error obtained with any leakage distribution of more than two leaks. Consequently, one can use the two-leak model to obtain the minimum induced pressure to generate to contain the error below a given limit. For example, using an induced pressure greater than twice the stack pressure should be sufficient to contain the error in absolute value below about 5%; the error should be below 14% if the induced pressure is greater than the stack pressure. In turn, this paper gives recommendations for testing together with tables and charts to be used by field technicians.

A stack pressure based equivalent mechanical model of lithium-ion pouch batteries

The stack level mechanical pressure will be inevitably generated among the battery cells, which dramatically affects the battery properties. To accurately characterize the battery performance with such aspects, an equivalent mechanical model with full consideration of stack pressure is proposed in this paper. Consisting of a small amount of simple mechanical components, the proposed model is simple but accurate, which is suitable for real time applications. By measuring the stack pressure, the battery thickness can be predicted by the proposed model, and accordingly, the battery states can be estimated. So the proposed equivalent mechanical model provides a potential use of the force signal based battery state estimation. The characteristics of the battery thickness variations caused by the applied force are first analyzed and the equivalent mechanical model is proposed. Furthermore, the coupling of stack pressure and the battery thickness is incorporated to the proposed model. Then a reliable parameter identification method is proposed to obtain the model parameters under different conditions. On this basis, the accuracy and the adaptability of the proposed model are experimentally validated.

The initiation of void growth during stripping of Li electrodes in solid electrolyte cells

We analyse the initiation of void growth in the Li electrode during the stripping phase of an Li-ion cell with a solid (ceramic) electrolyte. We first show that standard Butler-Volmer kinetics fails to predict the observed void formation. This motivated us to recognise that void initiation/growth involves power-law creep of the Li electrode that is linked to the motion of dislocations. We show, via thermodynamic considerations, that dislocations significantly affect the interface kinetics and use variational principles to develop a modified form of Butler-Volmer kinetics for the interface flux that is associated with a deforming Li electrode. Numerical solutions are presented for the coupled flux of Li+ in a single-ion conductor solid electrolyte and the associated creep deformation of the Li electrode for an imposed stripping current. This involves solution of a Laplace equation for flux in the electrolyte and the nonlinear Stokes flow equations for a power-law creeping solid in the electrode. These two domains are coupled together via the modified Butler-Volmer relation. The calculations predict that an increasing stack pressure needs to be exerted with increasing cell current to avoid the initiation of void growth and are in excellent quantitative agreement with measurements for an Li/LLZO/Li cell.

Compressive creep deformation of lithium foil at varied cell conditions

Lithium metal rechargeable batteries have great potential for higher energy density storage systems. Current configurations of all solid-state and anode-free cells depend on significant internal mechanical pressure (typically 0.5–1 MPa) to reduce interfacial resistance and suppress lithium dendrite formation. Mechanical stack pressure can increase over time and with use as cell components degrade. Lithium is soft at room temperature and may not be physically strong enough to support the required stack pressures in standard lithium-ion operating conditions. Compressive creep deformation of low aspect ratio lithium metal foils was measured in hermetically sealed, rigid but flexible cells at temperatures between 30–60°C and applied pressures between 0.6–3.6 MPa. Creep rates were typically on the order of a few μm h−1, depending on pressure, temperature, lithium thickness, and grain size. While difficult to notice during fast cycling these creep rates could have a pronounced effect during storage or practical use. Results suggest that pure lithium may not be strong enough for use in high energy density all solid-state devices, and that alternatives (e.g.lithium-rich alloys) should be considered.

Microstructure and Pressure-Driven Electrodeposition Stability in Solid-State Batteries

Interfacial deposition stability at the Li-metal-solid electrolyte interface in all solid-state batteries is governed by the stress-transport-electrochemistry coupling in conjunction with the polycrystalline/amorphous solid electrolyte architecture. In this work, we delineate the optimal solid electrolyte microstructure comprising grains, grain boundaries, and voids possessing desirable ionic conductivity and elastic modulus for superior transport and strength. An analytical formalism is provided to discern the impact of external “stack” pressure-induced mechanical stress on electrodeposition stability; the stress magnitudes obtained are in the megapascal range, considerably diminishing the stress-kinetics effects. For experimental stack pressures ranging up to 10 MPa, the impact of stress on reaction kinetics is negligibly small, and electrolyte transport overpotentials dictate electrodeposition stability. High current density operation with stable deposition can be ensured with ample external pressure, high temperature, and low surface roughness operation for a low shear modulus ratio of the solid electrolyte to Li-metal. Solid electrolyte architecture is screened via virtual generation and characterization Large grain size, low void size, and porosity provide favorable conduction and stiffness Stack pressure <10 MPa has negligible impact on stress-kinetics coupling High operating pressure, temperature, and low roughness can aid stable deposition Lithium electrodeposition is dependent on solid electrolyte microstructure, stack pressure, and operating temperature. Verma et al. suggest a method to delineate the architecture and operating conditions that lead to stable deposition.

Gibbs energy minimization using Lagrange method of undetermined multipliers for electrochemical and thermodynamic modeling of a MCFC with internal steam reforming

Precise determination of the chemical equilibrium state is important for electrochemical and thermodynamic modeling of a molten carbonate fuel cell (MCFC). One method for determining the equilibrium state of a MCFC with internal reforming is to compute the total Gibbs free energy of the system (G) and minimize it by adjusting the molar amounts of each substance. Multi-dimensional optimization algorithms can be used for this purpose. The Lagrange method of undetermined multipliers as a powerful analytical solution is used in the present work to estimate chemical equilibrium state. Under this condition, the effects on voltage losses, output voltage, stack power, efficiency and molar portions of the exhaust chemical species at the anode and cathode are investigated, separately of such parameters as current density, reaction temperature, stack pressure as well as fuel and carbon dioxide utilization ratios. A simulation of a MCFC stack performance by the Lagrange method at a specified state, results in an efficiency of 50.3% and output power of 554.3 kW.

Creep Deformation of Lithium Foil at Moderate Pressures (< 2 MPa) and Temperatures (30-50°C)

Lithium metal is receiving significant research attention as a potential next generation negative electrode material in lithium ion batteries because it can offer a ~50% increase in cell energy density compared to conventional graphite electrodes. There are, however, a number of mechanical challenges using lithium metal electrodes. Mechanical stack pressure is required to ensure good contact between solid-state electrodes and electrolytes. Moderate to high stack pressures (e.g. < 10 MPa) can also supress void formation during fast charging and discharging.1 Creep, the plastic deformation of a material under constant pressure, improves battery performance by preventing and filling voids, but can also lead to decreased performance if lithium is unable to sustain the required stack pressure, and potentially cell shorting if lithium is e.g. extruded beyond the current collectors.Creep deformation rates are highly dependent on homologous temperature and pressure. Lithium metal has a relatively low melting point (181°C) / high homologous temperature at common cell temperatures which should result in substantial lithium creep deformation. Previous studies have characterized both the tensile creep properties of lithium foils2 and compressive creep properties of bulk lithium rods.3 Significant barrelling of the high aspect ratio lithium rods (initial height:width of 4:1 to 1:1) during compression made it difficult to determine the applied pressure on the lithium, obscuring the relationship between pressure and creep rate.3 These important studies increased our collective understanding of the mechanical properties of lithium, but are not necessarily reflective of the compressive creep of lithium foils in a cell environment. Here, we report on the compressive creep of lithium foil within an electrochemical cell at commercially relevant stack pressures and temperatures.Compressive creep deformation of low aspect ratio (1:40 to 1:10; initially 0.3 – 2.5 mm thick, 13 mm diameter) lithium metal foils was measured in hermetically sealed, rigid but flexible cells at temperatures between 30 - 110°C and applied pressures between 1 - 2 MPa. Creep testing was performed by integrating a Conflat-style electrochemical cell4 featuring welded bellows and an optical window with an Instron 5966 Universal Testing System (which measures / controls force and displacement). Creep rates were determined using the time-dependent displacement data after reaching a constant applied force. Two methods to convert force to pressure will be described, including the use of an inline camera to measure the lithium-compressive piston contact area.Creep deformation was observed at all tested temperatures, and all pressures above 1 MPa. Creep rates were on the order of a few tenths of a micron per hour, which while small, implies very limited device lifetime with thin lithium layers. Lithium metal may not be physically strong enough for use in high energy density all solid-state devices. Figure 1

Influence of External Pressure in Fasting-Charging Li-Ion Batteries

Lithium-ion batteries (LIBs) capable of fast-charging are essential in the popularization of electric vehicles. Typical fast-charge requirements are charging to 80% state of charge within 10 mins. However, such high rates cause active material degradation and undesired lithium plating, leading to capacity fading and safety hazards. These issues are impacted by charging conditions, e.g. charging protocol, temperature, external pressure, etc. Here, we focus on the pressure-dependence of fast-charge batteries using an in-house designed gas bladder cell configuration. Through a systematic study, we revealed correlations between externally applied electrode stack pressure and capacity fade. Additionally, we propose possible capacity fade mechanisms during fast charge and a way to mitigate it.We utilized single-layer pouch cell batteries with a graphite anode, LiNi0.5Mn0.3Co0.2O2 (NMC532) cathode, and organic liquid electrolyte solution. Electrode stack pressure is achieved by opposing pairs of external flexible gas bladders. These gas bladders are formed by airtight sealing of a Kapton dome against each side of the flexible battery pouch. (Fig. (a)). By controlling the gas pressure within the bladders, the external pressure is precisely set and the flexible bladders locally conform to provide uniform pressure across the electrode stack. This combination bypasses issues with rigid plates where electrode thickness variations and internal electrical connections prevent uniform pressure. Additionally, the pressure is stable during battery cycling process regardless of material expansion/contraction or gassing. Furthermore, the X-ray transparent Kapton domes allow operando X-ray characterization.Using external pressures of 10, 50, and 125 psi, and on a control group with pressure applied by rigid plates, we performed a series of electrochemical experiments. All batteries were charged at 6C using CC-CV protocol and discharged at C/2 at room temperature for 140 cycles with a voltage range of 3 – 4.1 V. We found that the capacity fade decreases with increasing pressure (Fig. (b)); the capacity fade of rigid-plate cells is larger than gas-bladder cells. Additionally, the fast charge capacity increases with increasing pressure, indicating that the depth of charge/discharge in the entire cell is greater with higher external pressure (Fig. (c)). In contrast, the difference between the open circuit voltages at the end of charge and discharge is smaller at higher pressure. These two observations imply that the depth of reaction within the active material decreases with increasing pressure (Fig. (d)). Combining these two findings, we infer that there is more active material loss with lower external pressure, which is the main reason for capacity fading. The active material loss may come from the loss of electrical contact when particles crack or pulverize during fast charge, while higher external pressure can help preserve the electrical contact.Our study sheds new light on the influence of external pressure on fast-charge performance of LIBs. The use of gas-bladder cells helps to eliminate non-reproducibility in non-uniform cell thickness and material expansion/contraction. We plan to conduct operando X-ray diffraction experiments using the X-ray transparent AP cell to monitor the evolution of anode and cathode materials and Li plating during fast charge to investigate the underlying mechanism of the pressure-dependent behaviors. Figure 1

Quantifying Slow Diffusion in High Capacity, Multi-Phase Electrode Materials

The use of metal alloys as negative electrode materials in lithium-ion batteries offers the potential for high lithium storage capacities at low cost, two key aspects for competitive energy storage technologies. Recent work shows that a single piece of aluminum foil can be used as both the negative electrode active material and current collector, and in a full cell configuration, support over 100 cycles without appreciable capacity fade.1 This breakthrough is due to control of the nucleation and growth of the β-AlLi phase. Many other (higher capacity) Al-Li phases are predicted and can be detected when formed via thermal methods, but are generally not accessible via electrochemical methods. We demonstrated the reversible electrochemical formation of four Li-Al phases (β-AlLi, Al2Li3, AlLi2-x, Al4Li9) by cycling at slightly elevated temperatures (>= 40°C) to overcome nucleation and diffusion barriers.2 There is a strong need to decouple kinetic (e.g. diffusion) and thermodynamic limits to understand and improve the practical performance of metal alloy electrodes.Reliable quantification of diffusion rates can be challenging. Many of the techniques commonly used (e.g. potentio-or-galvanostatic titration, electrochemical impedance, cyclic voltammetry) depend on questionable assumptions or are not always applicable to materials with co-existing phases, as found in most lithium-metal alloys.3 We will report on our use of the lithiation of thin aluminum foil as a model (i.e. diffusion limited) system to compare and contrast apparent (i.e. technique-dependent) lithium diffusion rates in the various Al-Li phases. In addition, as the samples are in effect planar, experimental diffusion measurements will be complemented with numerical simulations of one dimensional Fickian diffusion in the same conditions, to validate the calculated diffusion values against experimentally measured galvanostatic potential-capacity data sets. All electrochemical experimental data was collected using Conflat-style electrochemical cells, which we designed for repeatable electrochemical testing over a wide temperature range (30 - 150°C).4 Repeatability is enabled by engineered alignment of electrodes, adjustable control of stack pressure, and ultra-high vacuum tight Conflat seals. Cell-to-cell variability (including all factors) is <5% at intermediate temperatures (<90°C) and <10% at higher temperatures (120-150°C) as we approach the limits of liquid electrolyte stability and the melting point of Li (181°C). Galvanostatic data was collected over a wide range of current densities (from dynamic to near-equilibrium rates, e.g. C/9 to C/280) and a wide range of tightly controlled temperatures (30 ± 1 to 150 ± 3°C). Examples of the dramatic effects of current density and temperature on phase formation / cell potential and final capacity are provided in the attached figure. Nucleation of β-AlLi did not occur at low temperatures and moderate current densities. Nucleation of Al2Li3 only occurred at temperatures above 40°C, even at very low rates. Measured potentials of the lithiation of β-AlLi (to form Al2Li3) varied by almost an order of magnitude, presumably due to very slow diffusion at moderate temperatures. Insight in to technique-dependent diffusion measurements and robust estimates of diffusion coefficients validated against a broad experimental data set will improve our analysis of, and the potential for, metal alloy electrode materials.[1] H. Li et al., Nature Communications, 11, 1584 (2020).[2] M.Z. Ghavidel et al., Journal of The Electrochemical Society, 166, A4034-A4040 (2019).[3] Y. Xu et al., International Journal of Hydrogen Energy, 35, 6366 (2010).[4] M.D. Fleischauer et al., Journal of The Electrochemical Society, 166, A398-A402 (2019). Figure 1

Modelling Intercalation Induced Stresses in All Solid State Batteries Using Discrete Element Method

Recently, all-solid-state lithium-ion batteries (ASSLiB) has emerged as one of the most promising candidates for next-generation power sources for electric vehicles (EVs) and energy storage. The main reason is the introduction of non-flammable solid electrolyte (SE) which substantially reduces the risk of battery fires. However, more research is needed in order to improve the durability and the overall performance.High charge capacity active material (AM) such as Si have been developed but their high expansion during charging and discharging reduces the durability. 1 During charge, AM in the cathode will absorb Lithium by intercalation and expand. A reverse contraction process occurs in the anode. This is known as Vegards expansion/contraction and cause buildup of high stresses, so called intercalation induced stresses. Although materials with less expansion/contraction have been developed, it is unclear how much expansion/contraction the materials in AM and SE can tolerate.Previous studies that model intercalation induced stress can be found in the literature. Most of the research is focused on the intercalation induced stresses either inside the AM or inside SE material. In the study of Bucci et al., 2 the stress in the SE due to Vegards expansion of AM was modelled by the finite element method (FEM) and the location of cracks could be predicted. However, according to Sun et al., 3 the critical region of failure is not within AM and SE but at the interface. Moreover, in FEM it is difficult to resolve the grained boundaries at the interfaces and the continuity assumption may not be valid as there are often void spaces between SE particles. These issues can be solved by modelling AM and SE as particles as in the Discrete Element Methods (DEM).In this study we perform DEM simulations and utilize the multisphere method for the AM particles. In the multisphere method, non-spherical particles are represented as a cluster of spherical sub-particles as illustrated in Figure 1 (a). We assume elliptical shapes which is in agreement with TEM image of graphite AM. In the DEM solver, the particle motion is determined from the forces acting on each particle. The elastic contact force between two particles with radii Ri and Rj is calculated by the Herzian contact law: F elastic = 4/3∙E eff δ 3/2 R eff 1/2 (1)where R eff = RiRj /(Ri + Rj ) is the effective radius and E eff is the effective elastic modulus given by1/E eff = (1-ν i 2)/E i +(1-ν j 2)/E j (2) where E i and E j are the Young modulus of elasticity of the particles. In the DEM solver, collision damping force and the tangential forces due to static and kinetic friction are also calculated. The motion and rotation of each particle was calculated from the sum of forces and torques acting on the particles.A series of DEM simulations were conducted. Different aspect ratios of the elliptical shapes were generated and the relationship between the aspect ratio and circularity agreed well with the experimental results. A gravity driven packing simulation was conducted followed by a press simulation at 500 MPa and a release simulation to stack pressure at 25 MPa. In order to solve for the intercalation induced stresses, expansion of AM particle was calculated by gradually increasing the particle radius. The results of the spatial distribution of the stresses of the SE material is shown in Figure 2 in the case of a volume expansion of 10%.During cold pressing of ASSLiB electrode, the contact force is not perfectly elastic but plasticity effects of SE plays a role large role and is an important design parameter 4. In cold pressing applications the normal force is often modeled as follows 5 F normal = min(F elastic , F plastic ) (3)where F plastic is the plastic force. A model is currently under development that can simulate the effect of plasticity on stress and damage during cold pressing and intercalation. The model can aid in the development and selection of favorable of materials for ASSLiB electrode.This research was supported by Grants-in-Aid for Scientific Research on Innovative Areas, “Science on Interfacial Ion Dynamics for Solid State Ionics Devices” MEXT, Japan FY2019-2023. References M. N. Obrovac, L. Christensen, D. B. Le, and J. R. Dahn, J. Electrochem. Soc., 154, A849 (2007).G. Bucci, T. Swamy, Y.-M. Chiang, and W. C. Carter, ArXiv170300113 Cond-Mat (2017) http://arxiv.org/abs/1703.00113.C. Sun, J. Liu, Y. Gong, D. P. Wilkinson, and J. Zhang, Nano Energy, 33, 363–386 (2017).K. Nagao et al., Solid State Ion., 308, 68–76 (2017).C. L. Martin, D. Bouvard, and S. Shima, J. Mech. Phys. Solids, 51, 667–693 (2003). Figure 1

Decoupling Electrolyte and Electrode Reactions Using in-Operando Electrochemical X-Ray Powder Diffraction

Lithium-ion batteries are the dominant energy storage technology – steady and significant progress toward lower costs, better safety, and better performance is being realized by an improved understanding and control of new materials and system engineering. Various novel materials for lithium and lithium-ion battery electrodes are emerging, many of which depend on alkali-metal alloys. Among them, Aluminum electrodes offer the potential for high lithium-ion capacities at low costs, for example, capacities of 2000 mAh/g (six times higher than graphite, the current commercial standard) can be accessed at moderate temperatures (above 40°C),1 and recent studies show that Aluminum foil electrodes can act as both the active material and current collector.2 Nevertheless, challenges with capacity fade, electrolyte breakdown, slow diffusion and nucleation barriers still exist and alkali-metal electrochemical reactivity is not fully understood. Elevated temperature operation can be used to overcome nucleation barriers and slow diffusion, but at the cost of increased rates of liquid electrolyte breakdown. Here, we combine controlled temperature electrochemical lithiation and delithiation with x-ray powder diffraction for in-situ studies of electrode and electrolyte reactions to shed light on these problems.Our in-operando cell is based on commercially available ultra-high vacuum compatible Conflat flanges with Beryllium windows and electrical feedthroughs to maintain commercially-relevant stack pressure and cell sealing.3 We will report on cell design and mechanical, electrochemical, and x-ray diffraction performance, including full-powder-pattern refinement of the Li-Al electrode material during electrochemical lithiation and delithiation. Such refinement of the diffraction patterns allows us to quantify the fractions of the phases formed as well as details of the crystal structure (e.g. solid solution vs. new phase). The evolution of the measured cell potential (red line) and the powder diffraction pattern (heat map) are shown in sections a) and b), respectively, of the attached figure. We confirm the formation of AlLi, Al2Li3 and AlLi2 and follow their fractions in-operando during charge and discharge of the cell. In section a) of the attached figure, electrode composition determined from pattern refinement (black dots) is compared to the composition predicted from electrochemical methods (blue solid lines) to separate the effects of electrode phase transitions and electrolyte breakdown. Electrode composition determined from XRD matches well the predicted composition at the beginning of the lithiation process, however it starts to deviate significantly by the time the delithiation is finished. In-operando XRD allows us to quantify this deviation and calculate the rate of liquid electrolyte breakdown. Better understanding of phase formation and electrolyte breakdown in Li-ion battery electrode materials and electrolytes will enable more robust production and improved performance of the batteries on the market.[1] M.Z. Ghavidel et al., Journal of The Electrochemical Society, 166 (16) A4034-A4040 (2019).[2] H. Li et al., Nature Communications, 11, 1584 (2020).[3] M.D. Fleischauer et al, Journal of The Electrochemical Society, 166 (2) A398-A402 (2019). Figure 1

Interfaces and Interphases of All Solid State Battery

All-solid-state batteries (ASSBs) have attracted enormous attention as one of the future critical technologies for safe and high energy batteries. With the emergence of several highly conductive solid electrolytes (SEs) in recent years, the bottleneck is no longer Li ion diffusion within the SEs. Rather, most ASSBs still suffer from low Coulombic efficiency, poor power performance, and short cycling life, due to the high resistance from the interfaces in ASSBs. Due to the varied chemical/physical/mechanical properties of different solid components in ASSBs as well as the nature of solid-solid contact, many kinds of interfaces are present in ASSBs: these involve loose physical contact, grain boundaries, chemical and electrochemical reactions, to name a few. All of these contribute to increasing resistance at the interface. Here, we will present the distinctive features of the typical interfaces and interphases in ASSBs and summarize the recent work on identifying, probing, understanding and engineering them. We will highlight the complicated but important nature of interphases, namely the composition, distribution, electronic, and ionic properties of the CEI on the cathode side and SEI on the anode; understanding these properties are the key to achieving a stable interface. In addition, conformal coatings to prevent side reactions and their selection criteria are reviewed. We emphasize the significant role of the mechanical properties of the interface as well as the mechanical property of all components, especially when the soft Li metal anode is used under constant stacking pressure. Finally, we provide full-scale (energy, spatial, and temporal) characterization methods to explore, diagnose and understand the dynamic buried interfaces and interphases.