Optimizing Li-Excess Cation-Disordered Rocksalt Cathode Design Through Partial Li Deficiency

Li-excess disordered rocksalts (DRXs) are emerging as promising cathode materials for Li-ion batteries due to their ability to use earth-abundant transition metals. In this work, a new strategy based on partial Li deficiency engineering is introduced to optimize the overall electrochemical performance of DRX cathodes. Specifically, by using Mn-based DRX as a proof-of-concept, it is demonstrated that the introduction of cation vacancies during synthesis (e.g., Li_1.3-xMn²⁺_0.4-xMn³⁺_xNb_0.3O_1.6F_0.4, x = 0, 0.2, and 0.4) improves both the discharge capacity and rate performance due to the more favored short-range order in the presence of Mn³⁺. Density functional theory calculations and Monte Carlo simulations, in combination with spectroscopic tools, reveal that introducing 10% vacancies (Li_1.1Mn²⁺_0.2Mn³⁺_0.2Nb_0.3O_1.6F_0.4) enables both Mn²⁺/Mn³⁺ redox and excellent Li percolation. However, a more aggressive vacancy doping (e.g., 20% vacancies in Li_0.9Mn³⁺_0.4Nb_0.3O_1.6F_0.4) impairs performance because it induces phase separation between an Mn-rich and a Li-rich phase.