A Strategic Phosphate Coating for Constructing a Robust Self-Forming Cathode–Electrolyte Interphase Toward Ultra-Stable LiNi0.9Co0.05Mn0.05O2 Cathodes

High-nickel layered oxide cathodes, exemplified by LiNi0.9Co0.05Mn0.05O2 (NCM90), exhibit high specific capacity but suffer from severe interfacial degradation and structural instability during electrochemical cycling. Herein, we present a phosphate-based in situ modification approach that forms a durable, self-established cathode–electrolyte interphase (CEI), thereby resolving these key challenges from the root. We employ a controlled (NH4)2HPO4 coating and optimized thermal treatment to fabricate a thin, dense layer of crystalline lithium phosphate on the NCM90 surface. This coherent layer serves as an artificial CEI precursor, which electrochemically evolves into a highly stable and ionically conductive interfacial shield during operation. It effectively suppresses parasitic reactions, mitigates transition metal dissolution, and alleviates mechanical strain induced by phase transitions. Comprehensive optimization of calcination temperature and coating content identifies 760 °C and 1 wt% as the optimal conditions, yielding a well-preserved layered structure and effectively suppressed Li+/Ni2+ mixing compared with the pristine NCM90. When tested at 0.1 C in the potential range of 2.75–4.3 V, the coated electrode delivers a high initial discharge specific capacity of 204.08 mAh g−1. After 100 charge–discharge cycles at 1 C, it retains 89.24% of its capacity, and its rate capability is also significantly improved. Collectively, these findings verify that forming a customized CEI via precursor coating successfully suppresses interfacial degradation and improves structural integrity, thus representing a viable, scalable pathway toward advanced lithium-ion batteries with exceptionally stable cathodes.