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

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_xMn_yCo_zO₂ 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.