Mitigating Electrochemical Isolation in Ni‐Rich Layered Cathodes for Durable Solid‐State Batteries

ABSTRACT Cathode–solid electrolyte (SE) interfacial instability poses a major challenge for achieving stable and high‐power operations in all‐solid‐state batteries, which promise superior energy density, thermal stability, and safety over the current Li‐ion technology. For technologically important Ni‐rich NMCs (LiNi x Mn y Co z O 2 or NMCxyz; x/y/z: Ni/Mn/Co stoichiometry) paired with sulfide SEs, redox‐mediated instability of the SE is often blamed for rapid cathode deterioration. Here, in‐depth spectroscopic and electrochemical analyses of Ni‐rich NMCs with a promising sulfide SE reveal hitherto unrecognized electrochemical isolation of active NMC particles driven by rapid interfacial degradations, sparking accelerated capacity fading and poor thermal stability. Introducing a functionalized conductive carbon into the cathode suppresses sulfide SE degradation into reactive polysulfides that drive NMC deterioration. Consequently, NMC622 and NMC811‐based cells display high active material utilization, enhanced stability, attractive rate capability and thermal resilience – illustrated by 1C (1C: 160 mA g −1 ) capacity of ∼150 mAh g −1 , 5C rate retention of 95% after 500 cycles with high active loading (≥12 mg cm −2 ), and an average Coulombic efficiency of 99.8% even for high‐temperature cycling. This study uncovers a critical performance degradation pathway in a key cathode‐SE pairing and presents a scalable strategy for its in situ regulation, enabling significant performance gains.