Mechano‐Electrochemical and Structural Insights into Composition‐Driven Behavior of LiNi _1‐x‐y Co _x Mn _y O ₂ (0.3 < 1‐x‐y ≤ 0.8) Cathodes in Sulfide‐Based All‐Solid‐State Batteries

Abstract Sulfide‐based all‐solid‐state batteries (ASSBs) offer enhanced safety and high energy density but face challenges from mechano‐electrochemical degradation at the cathode–electrolyte interface. This study systematically investigates the composition‐dependent interfacial behavior of layered LiNi 1–x–y Co x Mn y O 2 (NCM) cathodes – NCM811, NCM523, and NCM111 – in contact with the sulfide electrolyte Li 6 PS 5 Cl (LPSCl). Two primary degradation pathways are considered: (i) lattice volume changes in NCM during cycling and (ii) electrochemical decomposition of LPSCl at the interface. Surprisingly, lower‐Ni NCMs, despite exhibiting smaller volume changes and higher structural stability, show inferior electrochemical performance compared to higher‐Ni NCMs under high‐pressure conditions. Comprehensive electrochemical and spectroscopic analyses reveal that interfacial decomposition of LPSCl is the dominant degradation route, increasing in severity with lower Ni content. Furthermore, in situ high‐temperature X‐ray diffraction demonstrates that contact with LPSCl induces comparable structural instability and oxygen release in delithiated NCMs, irrespective of their composition. These findings underscore that the principal limitation of LiNi 1‐x‐y Co x Mn y O 2 (0.3 < 1‐x‐y ≤ 0.8) in sulfide‐based ASSBs under high‐pressure conditions arises from the interfacial chemical reactivity between LPSCl and NCM, which is strongly governed by transition metal composition. Tailored interfacial engineering based on NCM composition is therefore essential for the development of high‐performance sulfide‐based ASSBs.