Chemo‐Mechanical Behavior of High‐ and Mid‐Ni Cathodes in Sulfide‐Based All‐Solid‐State Batteries: Who Will Prevail?

Achieving high energy densities in sulfide‐based all‐solid‐state batteries (ASSBs) is fundamentally constrained by the coupled chemo‐mechanical degradation of layered oxide cathodes, driven by lattice instability and interfacial chemical reactivity with solid electrolytes. Herein, by reinterpreting conventional lithium‐ion battery design principles, we systematically evaluate two contrasting strategies: voltage‐restricted operation of a high‐nickel cathode (NCM811 at 4.2 V) and high‐voltage operation of a mid‐nickel cathode (NCM622 at 4.4 V). Using pressure‐resolved electrochemical measurements, operando mechanical analysis, and interfacial spectroscopic characterization, we show that limiting the NCM811 upper cut‐off voltage effectively suppresses the H2–H3 phase transition, minimizing lattice volume fluctuations and preserving the intrinsic mechanical and chemical stability even without surface protection. In contrast, NCM622 high‐voltage operation induces deep delithiation accompanied by pronounced lattice contraction and accelerated sulfide electrolyte oxidative decomposition, leading to severe interfacial degradation and capacity fading. Applying a boron‐based (B‐based) surface coating to NCM622 significantly suppresses parasitic interfacial reactions, enabling substantial performance recovery and energy densities comparable to those of voltage‐restricted high‐nickel systems. These results decouple the roles of mechanical instability and chemical interfacial degradation, establishing a flexible cathode design framework that combines voltage window optimization for high‐nickel cathodes with targeted interfacial engineering for high‐voltage mid‐nickel cathodes in ASSBs.