Ti-Gradient Doping to Stabilize Layered Surface Structure for High Performance High-Ni Oxide Cathode of Li-Ion Battery

High-Ni layered oxide cathodes are considered to be one of the most promising cathodes for high-energy-density lithium-ion batteries due to their high capacity and low cost. However, surfice residues, such as NiO-type rock-salt phase and Li₂CO₃, are often formed at the particle surface due to the high reactivity of Ni³⁺, and inevitably result in an inferior electrochemical performance, hindering the practical application. Herein, unprecedentedly clean surfaces without any surfice residues are obtained in a representative LiNi_0.8Co_0.2O₂ cathode by Ti-gradient doping. High-resolution transmission electron microscopy (TEM) reveals that the particle surface is composed of a disordered layered phase (≈6 nm in thickness) with the same rhombohedra structure as its interior. The formation of this disordered layered phase at the particle surface is electrochemically favored. It leads to the highest rate capacity ever reported and a superior cycling stability. First-principles calculations further confirm that the excellent electrochemical performance has roots in the excellent chemical/structural stability of such a disordered layered structure, mainly arising from the improved robustness of the oxygen framework by Ti doping. This strategy of constructing the disordered layered phase at the particle surface could be extended to other high-Ni layered transition metal oxides, which will contribute to the enhancement of their electrochemical performance.