Mitigating Surface Irreversible Layer‐To‐Spinel Phase Transition for Stable and Ultrahigh‐Capacity LiCoO ₂ Cathodes

Abstract Currently, the surface structure collapse issues have long hindered the utilization of ultrahigh‐capacity LiCoO ₂ (LCO) cathodes. Herein, a dual‐optimized LCO (D‐LCO) is proposed with a surface rocksalt (RS) phase (10 nm) and a subsurface layered phase reinforced by Al/F doping (>50 nm). Unlike solely surface modifications, subsurface Al/F doping drastically suppresses lattice O ⁿ⁻ (0<n<2) migration at high states of charge (SOC), preventing the irreversible layer‐to‐spinel transitions and RS‐phase thickening in the near‐surface region. Consequently, D‐LCO achieves an ultrahigh capacity of 236 mAh g ⁻¹ at 4.6 V versus Li ⁺ /Li, with 90.3% capacity retention after 200 cycles at 1 C and 81.2% after 1000 cycles at 4 C. This work demonstrates that coordinated subsurface/surface stabilization is essential for unlocking the ultrahigh capacity of layered oxide cathodes.