Degradation Processes in Positive Electrode Composites for All‐Solid‐State Lithium‐Ion Batteries Visualized by Scanning Spreading Resistance Microscopy

All-solid-state lithium-ion batteries (ASSLIBs) are promising next-generation energy storage devices because of their high safety and energy density. However, the performance of ASSLIBs is restricted by various degradation factors in positive electrode composites, including the structural changes in positive electrode materials, associated volume changes, and formation of solid-electrolyte decomposition products. Despite their importance, the individual effects of these factors are not well understood. Herein, the chemical and mechanical degradation processes in positive electrode composites with LiNi_0.5Co_0.2Mn_0.3O₂ (NCM) and argyrodite-type sulfide solid electrolytes are revealed using scanning spreading resistance microscopy. The results of local resistance analysis based on this technique demonstrate the appearance of electronically isolated NCM particles due to their volume shrinkage and the electronically conductive decomposition products generated from solid electrolytes at high potentials. These processes cause different degradation scenarios, namely rapid capacity loss during initial cycling and continuous capacity loss, respectively. The results of X-ray photoemission spectroscopy analysis indicate that the electronically conductive decomposition products are lithium thiophosphates with long-chain crosslinked sulfur. The study unveils the effect of contact loss between NCM particles and offers insight into the dynamic evolution of electrolyte decomposition layers within positive electrodes for ASSLIBs.