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

Abstract

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.