First Observation and Mitigation of Ni-Leaching Mediated Degradation to Enhance Stability of Ni-rich Layered Cathodes in Solid-State Batteries

Cathode–solid electrolyte (SE) interfacial stability issues pose a significant challenge to stable and high-power operations of all-solid-state batteries, which promise greater energy density, thermal stability, and safety than the current liquid electrolyte-based Li-ion technology. For technologically important Ni-rich NMCs (LiNixMnyCozO2 or NMCxyz; x/y/z: Ni/Mn/Co stoichiometry) 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 rapid interfacial degradations underpinned by hitherto unknown selective Ni leaching from the NMC particles, sparking accelerated capacity fading and poor thermal stability. We demonstrate that introducing a functionalized conductive carbon in the cathode subdues the oxidative degradation of the sulfide SE into reactive polysulfides that trigger the NMC degradation. Consequently, NMC622 and NMC811-based cells display attractive active material utilization, enhanced stability, and excellent rate capability and thermal stability – together with a very high average Coulombic efficiency of 99.8% even for high-temperature cycling, which otherwise compounds cycling instability. This study unveils a previously unforeseen interfacial degradation of a technologically critical cathode-SE combination and presents a scalable approach to its in situ regulation, enabling remarkable performance improvement.