Screening Na-Excess Cation-Disordered Rocksalt Cathodes with High Performance

Abstract

The practical application of Na-ion cathode materials is currently restricted by their low energy density and sluggish dynamics, while the cation-disordered rocksalt (DRX) structures offer a possible solution to the challenge. In this study, among the 24 candidates containing d⁰ elements, we use mixing temperature as a descriptor to screen the synthesizable Na-excess DRX, and we have identified Na_1.2Mn_0.4Mo_0.4O₂ as the most promising candidate that exhibits a Na percolating fraction of 53%, which is higher than that of Li_1.2Mn_0.4Ti_0.4O₂ (35%) proposed in the previous study due to the larger lattice constant in Na-excess DRX cathodes. More importantly, Na_1.2Mn_0.4Mo_0.4O₂ is predicted to have a capacity of 228 mAh/g with an energy density of 552 Wh/kg derived from percolation theory and cluster-expansion Monte Carlo simulations, which is higher than that of Na_1.3Nb_0.3Mn_0.4O₂ and Na_1.14Mn_0.57Ti_0.29O₂ synthesized recently. For a better understanding, the redox mechanism is explored, which involves Mo⁴⁺/Mo⁶⁺, Mn³⁺/Mn⁴⁺, and O^2–/O^n– (0 < n < 2), indicating the participation of anionic redox. Meanwhile, the Na⁺ diffusion prefers a divacancy mechanism via an o-t-o diffusion channel with a low diffusion barrier of 0.29 eV. This study expands the family of DRX for the cathode of Na-ion batteries with enhanced performance.