Effects of the microstructure of solid-electrolyte-coated LiCoO2 on its discharge properties in all-solid-state lithium batteries

Li-ion conduction in electrolyte materials and its intercalation properties in active materials are key factors that determine the electrochemical performances of batteries. Sulfide electrolyte coating onto active materials was done to form a favourable electrode-electrolyte interface and to increase the electrochemically active surface area. In this study, to obtain higher packing density pellets, amixture of LiCoO2 particles with different grain sizes was used as a host material during the electrolyte deposition. The cell performance enhancement was achieved by the formation of a denser environment within composite electrodes. However, the influence of electrode microstructures on ion-conductive pathways has not been elucidated clearly and completely because of difficulties in conducting the experiments. In this context, a simulation model would provide insightful information about the potential of appropriate electrode structural designs. We proposed the implementation of the phase-field method, where the temporal evolution at electrode microstructures is calculated based on a free-energy function to construct realistic 3D images of the electrode structure. This study theoretically evaluated the constant current discharge properties of positive electrodes consisting of LiCoO2 with sulfide electrolyte coatings. The simulation results were able to reproduce an experimentally obtained correlation between the electrode potential and discharge capacity. The present simulation provided an estimation of the material properties required under specific discharge conditions. The preliminary examination demonstrated that the mutually connected fine network of ion-conductive pathways benefits from sulfide electrolyte coatings, and that it affects the discharge properties at higher rates than a conventional microstructure of mixed powder electrodes.