Achieving High-Performance Defect-Free LiCoO2 Cathode via a Dopant-Free Approach

Elevating the charging voltage of layered oxide cathodes to achieve higher capacity induces phase transitions associated with transition metal slab gliding, which significantly impacts the material's structural stability. Doping with inert elements is commonly employed to delay such phase transitions to higher voltages. However, these electrochemically inactive elements do not participate in redox reactions, thereby compromising lithium storage capacity. This compromise raises a critical and underexplored issue regarding whether doped materials with reduced capacity still maintain an advantage in energy density. In this study, using LiCoO₂ as a model material, it was observed that an increase in the concentration of Al dopant indeed delayed the onset voltage of the H1-3 phase transition. However, the extent of delithiation associated with this phase transition remains largely unchanged. When the discharge capacity is controlled to just below the threshold for the global H1-3 phase transition, the undoped material demonstrates even superior capacity retention and rate performance compared to the doped samples, at a lower charging cutoff voltage. Comprehensive experimental characterizations and theoretical calculations reveal that the doping-induced structural defects hinder Li⁺ conduction and promote oxygen release, consequently accelerating performance degradation. This study suggests that in the development of high-voltage layered oxide cathodes, it is crucial to prioritize enhancing material capacity. Additionally, it is imperative to meticulously assess the adverse effects of doping, as industrial preparation methods often lead to nonideal dopant incorporation, causing undesirable structural defects that are particularly harmful to the reversibility of high-voltage phase transitions.