Degradation mechanisms of composite cathodes in argyrodite-based all-solid-state batteries

All-solid-state batteries (ASSBs), which replace flammable liquid electrolytes with inorganic solid electrolytes (SEs), have attracted considerable attention as safer and higher-energy storage technologies. Among various candidates, sulfide-based SEs such as argyrodite-type Li₆PS₅Cl combine high ionic conductivity with excellent processability, facilitating the fabrication of dense and well-integrated composite cathodes. Nevertheless, the electrochemical performance of ASSBs remains largely constrained by interfacial and structural degradation occurring within the composite cathode. Understanding the degradation phenomena at the interface between the SE and the cathode active material is thus essential to improving the electrochemical performance and cycle life of these systems. Specifically, these degradation pathways can be broadly divided into (electro)chemical mechanisms—such as electrochemical decomposition of SE, transition-metal dissolution, oxygen-involving interfacial reactions, and surface reconstruction—and mechanical mechanisms, including particle cracking and interfacial contact loss driven by anisotropic stress and volume fluctuations. Acting synergistically, these processes reduce Li⁺ mobility, increase interfacial impedance, and cause irreversible capacity loss. In this review, we systematically examine these degradation modes in sulfide-based ASSBs, with a particular focus on Ni-rich layered oxide/argyrodite composite cathodes. By organizing recent experimental findings, we aim to provide a comprehensive understanding of the limitations of current ASSB systems and to guide future efforts toward more stable and durable solid-state battery technologies.