Direct observation of Degradation in LiNi0.8Mn0.1Co0.1O2-Li6PS5Cl0.5Br0.5 Composite Electrodes for All Solid-State Batteries

The interfacial stability between cathodes and solid electrolytes remains a critical challenge limiting the performance of all-solid-state batteries (ASSBs). In this work, we elucidate the degradation mechanisms of LiNi _0.8 Mn _0.1 Co _0.1 O ₂ (NMC811)-Li ₆ PS ₅ Cl _0.5 Br _0.5 composite electrodes during electrochemical cycling using advanced electron microscopy. In the pristine state, spherical NMC811 particles are uniformly embedded in the Argyrodite electrolyte, forming intimate interfacial contact with the LiNbO ₃ coating layer. With cycling, progressive interfacial separation and crack formation occur, accompanied by a structural transformation of the electrolyte from an Argyrodite-type polycrystalline phase to β-Li ₃ PS ₄ . Further decomposition leads to the precipitation of Li ₂ S nanocrystals, preferentially localized at the cathode-electrolyte interface. Concurrently, chlorine migrates out of the electrolyte and segregates on the NMC811 surface as Cl-rich domains, while sulfur diffuses into NMC811 particles through cracks and accumulates as Li ₂ S. These findings demonstrate that interfacial degradation, electrolyte decomposition, chlorine segregation, and sulfur diffusion cause the long-term instability of NMC811-Argyrodite composite electrodes. This study provides mechanistic insights into interfacial degradation pathways and offers design guidelines for enhancing interfacial stability in future solid-state batteries.