Modulating Surface Structural Evolution of LiCoO₂ for Enhanced Extreme Fast-Charging Durability

The applied cathodes in lithium-ion batteries usually suffer from severe structural degradation upon fast charging, and the correlated mechanism still remains vague. Here, we reveal the surface structural evolution of LiCoO₂ (LCO) during cycling at 4.6 V vs Li/Li⁺ with an extreme high fast-charging current of 10 C. Fast charging induces surface heterogeneous delithiation, promoting nonuniform surface phase transitions and resulting in the formation of a triphase hybrid on the charged surface. The triphase hybrid consists of the layered, spinel, and rock-salt (RS) phases. As cycling proceeds, this triphase hybrid propagates gradually toward the bulk, accompanied by a progressive thickening of the surface RS phase, leading to deteriorated Li⁺ transport kinetics and accelerated capacity fading. Thus, suppressing the heterogeneous Li⁺ delithiation of LCO is crucial for enhancing fast-charging durability. By applying a uniform and robust surface coating, the surface delithiation homogeneity upon extreme fast charging is significantly improved, and the thickening of the surface Li⁺-blocking RS phase is greatly reduced, thereby achieving enhanced cycling stability of LCO. This work benefits the development of more advanced LCO cathodes tailored for fast-charging applications.