Oxygen Evolution along Grain Boundaries of LiNi0.5Co0.2Mn0.3O2 Particles as an Early Stage Degradation of High-Voltage All-Solid-State Batteries

Operating sulfide-based all-solid-state batteries (SSBs) at high voltages increases energy density but induces O2 release from cathode materials, degrading battery performance. To elucidate this early stage degradation, SSBs were fabricated using LiNi0.5Co0.2Mn0.3O2 (NCM523) particles with distinct grain-boundary (GB) densities: single-crystalline (SC) or polycrystalline (PC). Gas analysis of SSBs during charging up to 5.0 V (vs Li/Li+) reveals that the PC-cell produces 27 times more O2 than the SC-cell despite having identical total surface areas. STEM-EELS analysis reveals 2–5 nm thick rock-salt phase formation across particle surfaces and along GBs regardless of the crystallinity, indicating that O2 evolution in the PC-cell occurs mainly along GBs of NCM523 particles. Because surface coating on the PC substantially suppresses O2 evolution, oxygen-vacancy formation initiated at the particle surface propagates O2 release along the GBs. Consequently, suppressing surface oxygen-vacancy formation represents a critical strategy for improving the long-term stability of high-voltage SSBs.