Improving Li percolation and redox reversibility of Li-rich disordered rocksalt cathode with Mn^2+/4+ double redox via inactive d⁰ Nb⁵⁺ substitution

Disordered rocksalt (DRX) cathodes are promising for next-generation lithium-ion batteries due to its high discharge capacity. Benefiting from the high degree of freedom in compositional design, cationic multi-electron redox is realized through the stabilization of d⁰ transition metals (TMs), and irreversible oxygen loss is alleviated by increased TM capacity contribution. However, such materials reported so far are almost weakly crystalline (fluorine)oxides prepared by mechanochemical methods. Herein, we prepare highly crystalline Mn(II)-based DRX oxides Li_1.15Mn_0.275Ti_0.575O₂ (LMTO), and Li_1.23Mn_0.275Ti_0.255Nb_0.24O₂ (LMTNO) with partial d⁰ Ti⁴⁺ replaced by d⁰ Nb⁵⁺. Increased lattice spacings of LMTNO are confirmed due to the larger ionic radius of Nb⁵⁺, which is conducive to Li⁺ diffusion. In combination with bond-valence sum method and electrochemical impedance spectroscopy, lower Li⁺ percolation energy and better rate performance of LMTNO are demonstrated kinetically. Density functional calculations reveal that d⁰ Nb⁵⁺ is superior to d⁰ Ti⁴⁺ for the stabilization of Mn-O bonds, and relieves MnO₆ octahedral distortion to alleviate the Jahn-Teller effect. In-situ X-ray diffraction indicates that the structural evolution and redox reaction are more reversible in LMTNO during the electrochemical process. This work understands the effect of d⁰ TMs on the electrochemical performance of DRX cathodes and provides new insights into multi-electron redox.