Origin of initial capacity loss in Ni-rich cathodes for sulfide-based all-solid-state batteries

All-solid-state batteries (ASSBs) provide safer alternatives to liquid-electrolyte-based lithium-ion batteries (LIBs). Sulfide solid electrolytes (SEs) offer high ionic conductivity and mechanical compliance, but their electrochemical instability induces interfacial reactions with cathode materials. This leads to rapid performance loss with substantial initial capacity loss (ICL), which hinders commercialization of sulfide-based ASSBs. In this study, we identify two key contributors to ICL in Ni-rich cathodes of sulfide-based ASSBs. Recoverable ICL arises from kinetic hindrance of Li⁺ reinsertion, whereas irrecoverable ICL stems from permanent loss of Li⁺ storage sites and side reactions. During the first charge, unprotected cathode material–electrolyte interfaces degrade, thereby increasing resistance to Li⁺ reinsertion and thus recoverable ICL. A LiNbO₃ surface coating mitigates interfacial degradation and reduces recoverable ICL to LIB-like levels. Irrecoverable ICL originates from capacity contributions unrelated to Li⁺ extraction from the active material and is dominated by irreversible Li⁺ extraction from SE oxidation and interfacial reactions at cathode material–SE interface. These unavoidable side reactions drive the inherently higher ICL of sulfide-based ASSBs versus LIBs. This work provides a diagnostic framework that differentiates recoverable and irrecoverable components and identifies the dominant factors governing each, guiding efforts to address ICL challenges in Ni-rich sulfide-based ASSB cathodes.