Decoupling stage-dependent reaction heterogeneity caused by air-induced surface degradation in single-crystal Ni-rich cathodes

Single-crystal Ni-rich layered oxides provide mechanical robustness for high-energy batteries but remain vulnerable to air-induced surface degradation. The high-temperature synthesis required for their monolithic morphology inevitably enriches the surface with residual lithium species, which rapidly evolve into insulating compounds upon air exposure. While the degradation of electrochemical performance is well-documented, the microscopic link connecting surface chemistry to electrochemical and structural heterogeneity during charging remains largely unexplored. Here, we systematically investigate the mechanistic roles of air-induced surface degradation in single-crystal LiNi_0.92Co_0.06Mn_0.02O₂ during the charging process. We reveal that the non-uniform distribution of residual lithium compounds triggers severe interparticle heterogeneity during the early H1–M transition. On the other hand, the unevenly developed NiO-like rock-salt phase persistently suppresses the H2–H3 transition, resulting in pronounced intraparticle heterogeneity. These findings elucidate the stage-dependent impact of air-induced surface degradation and highlight the importance of strict environmental control for achieving high performance in ultra-high-Ni single-crystal cathodes.