Editors’ Choice—Visualizing the Impact of the Composite Cathode Microstructure and Porosity on Solid-State Battery Performance

The kinetics of composite cathodes for solid-state batteries (SSBs) relies heavily on their microstructure. Spatial distribution of the different phases, porosity, interface areas, and tortuosity factors are important descriptors that need accurate quantification for models to predict the electrochemistry and mechanics of SSBs. In this study, high-resolution focused ion beam-scanning electron microscopy tomography was used to investigate the microstructure of cathodes composed of a nickel-rich cathode active material (NCM) and a thiophosphate-based inorganic solid electrolyte (ISE). The influence of the ISE particle size on the microstructure of the cathode was visualized by 3D reconstruction and charge transport simulation. By comparison of experimentally determined and simulated conductivities of composite cathodes with different ISE particle sizes, the electrode charge transport kinetics is evaluated. Porosity is shown to have a major influence on the cell kinetics and the evaluation of the active mass of electrochemically active particles reveals a higher fraction of connected NCM particles in electrode composites utilizing smaller ISE particles. The results highlight the importance of homogeneous and optimized microstructures for high performance SSBs, securing fast ion and electron transport

Influence of NCM Particle Cracking on Kinetics of Lithium-Ion Batteries with Liquid or Solid Electrolyte

In liquid electrolyte-type lithium-ion batteries, Nickel-rich NCM (Li$_{1+x }$(Ni$_{1−y−z}$Co$_{ y}$Mnz)$_{1−x}$O$_{2}$) as cathode active material allows for high discharge capacities and good material utilization, while solid-state batteries perform worse despite the past efforts in improving solid electrolyte conductivity and stability. In this work, we identify major reasons for this discrepancy by investigating the lithium transport kinetics in NCM-811 as typical Ni-rich material. During the first charge of battery half-cells, cracks form and are filled by the liquid electrolyte distributing inside the secondary particles of NCM. This drastically improves both the lithium chemical diffusion and charge transfer kinetics by increasing the electrochemically active surface area and reducing the effective particle size. Solid-state batteries are not affected by these cracks because of the mechanical rigidity of solid electrolytes. Hence, secondary particle cracking improves the initial charge and discharge kinetics of NCM in liquid electrolytes, while it degrades the corresponding kinetics in solid electrolytes. Accounting for these kinetic limitations by combining galvanostatic and potentiostatic discharge, we show that Coulombic efficiencies of about 89% at discharge capacities of about 173 mAh g$_{1+x }$NCM$^{-1}$ can be reached in solid-state battery half-cells with LiNi$_{0.8}$Co$_{0.1}$Mn$_{0.1}$O$_{2}$ as cathode active material and Li$_{6}$PS$_{5}$Cl as solid electrolyte

Designing Cathodes and Cathode Active Materials for Solid‐State Batteries

Solid-state batteries (SSBs) currently attract great attention as a potentially safe electrochemical high-energy storage concept. However, several issues still prevent SSBs from outperforming today\u27s lithium-ion batteries based on liquid electrolytes. One major challenge is related to the design of cathode active materials (CAMs) that are compatible with the superionic solid electrolytes (SEs) of interest. This perspective, gives a brief overview of the required properties and possible challenges for inorganic CAMs employed in SSBs, and describes state-of-the art solutions. In particular, the issue of tailoring CAMs is structured into challenges arising on the cathode-, particle-, and interface-level, related to microstructural, (chemo-)mechanical, and (electro-)chemical interplay of CAMs with SEs, and finally guidelines for future CAM development for SSBs are proposed

Correlation between Surface Reactions and Electrochemical Performance of Al2O3‐ and CeO2‐Coated NCM Thin Film Cathodes

Abstract Depositing ultrathin oxide coatings has been proven a successful approach to stabilize the surface of LiNixCoyMnzO2 active cathode material in lithium‐ion batteries (LIB). The beneficial effect of Al2O3 coatings arises at least partly from spontaneous reactions between coating and liquid electrolyte. However, it remains unclear if comparable surface reactions occur for other oxide coatings. One difficulty is the characterization of reaction products at the cathode–electrolyte interface due to the multi‐phase properties of composite cathodes. Here, thin films are utilized as model systems to correlate surface reactions with the performance of Al2O3‐ and CeO2‐coated nickel cobalt manganese oxides (NCM). Electrochemical characterization confirms that an Al2O3 coating improves long‐term cycling stability, while CeO2‐coated thin films perform even worse than uncoated counterparts. The analysis of the surface reaction products using X‐ray photoelectron spectroscopy shows that both coatings are fluorinated upon contact with liquid electrolyte in agreement with thermodynamic considerations. The fluorinated Al2O3 coating is stable during cycling, resulting in the improved cell performance. In contrast, the fluorinated CeO2 coating changes chemical composition, facilitating corrosion of the NCM surface. The results demonstrate the importance of a detailed analysis of surface reactions to evaluate the suitability of ultrathin oxide layers as protective coatings for LIBs

Correlation between Surface Reactions and Electrochemical Performance of Al2O3- and CeO2-Coated NCM Thin Film Cathodes

Depositing ultrathin oxide coatings has been proven a successful approach to stabilize the surface of LiNixCoyMnzO2 active cathode material in lithium-ion batteries (LIB). The beneficial effect of Al2O3 coatings arises at least partly from spontaneous reactions between coating and liquid electrolyte. However, it remains unclear if comparable surface reactions occur for other oxide coatings. One difficulty is the characterization of reaction products at the cathode–electrolyte interface due to the multi-phase properties of composite cathodes. Here, thin films are utilized as model systems to correlate surface reactions with the performance of Al2O3- and CeO2-coated nickel cobalt manganese oxides (NCM). Electrochemical characterization confirms that an Al2O3 coating improves long-term cycling stability, while CeO2-coated thin films perform even worse than uncoated counterparts. The analysis of the surface reaction products using X-ray photoelectron spectroscopy shows that both coatings are fluorinated upon contact with liquid electrolyte in agreement with thermodynamic considerations. The fluorinated Al2O3 coating is stable during cycling, resulting in the improved cell performance. In contrast, the fluorinated CeO2 coating changes chemical composition, facilitating corrosion of the NCM surface. The results demonstrate the importance of a detailed analysis of surface reactions to evaluate the suitability of ultrathin oxide layers as protective coatings for LIBs.Deutsche Forschungsgemeinschaft (DFG); ROR-ID:018mejw6

State of Charge-Dependent Impedance Spectroscopy as a Helpful Tool to Identify Reasons for Fast Capacity Fading in All-Solid-State Batteries

Thiophosphate-based all-solid-state batteries (ASSBs) are considered the most promising candidate for the next generation of energy storage systems. However, thiophosphate-based ASSBs suffer from fast capacity fading with nickel-rich cathode materials. In many reports, this capacity fading is attributed to an increase of the charge transfer resistance of the composite cathode caused by interface degradation and/or chemo-mechanical failure. The change in the charge transfer resistance is typically determined using impedance spectroscopy after charging the cells. In this work, we demonstrate that large differences in the long-term cycling performance also arise in cells, which exhibit a comparable charge transfer resistance at the cathode side. Our results confirm that the charge transfer resistance of the cathode is not necessarily responsible for capacity fading. Other processes, such as resistive processes on the anode side, can also play a major role. Since these processes usually depend on the state of charge, they may not appear in the impedance spectra of fully charged cells; i.e., analyzing the impedance spectra of charged cells alone is insufficient for the identification of major resistive processes. Thus, we recommend measuring the impedance at different potentials to get a complete understanding of the reasons for capacity fading in ASSBs

Fast Charging of Lithium‐Ion Batteries: A Review of Materials Aspects

Fast charging is considered to be a key requirement for widespread economic success of electric vehicles. Current lithium-ion batteries (LIBs) offer high energy density enabling sufficient driving range, but take considerably longer to recharge than traditional vehicles. Multiple properties of the applied anode, cathode, and electrolyte materials influence the fast-charging ability of a battery cell. In this review, the physicochemical basics of different material combinations are considered in detail, identifying the transport of lithium inside the electrodes as the crucial rate-limiting steps for fast-charging. Lithium diffusion within the active materials inherently slows down the charging process and causes high overpotentials. In addition, concentration polarization by slow lithium-ion transport within the electrolyte phase in the porous electrodes also limits the charging rate. Both kinetic effects are responsible for lithium plating observed on graphite anodes. Conclusions drawn from potential and concentration profiles within LIB cells are complemented by extensive literature surveys on anode, cathode, and electrolyte materials—including solid-state batteries. The advantages and disadvantages of typical LIB materials are analyzed, resulting in suggestions for optimum properties on the material and electrode level for fast-charging applications. Finally, limitations on the cell level are discussed briefly as well