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 Na1.2Mn0.4Mo0.4O2 as the most promising candidate that exhibits a Na percolating fraction of 53%, which is higher than that of Li1.2Mn0.4Ti0.4O2 (35%) proposed in the previous study due to the larger lattice constant in Na-excess DRX cathodes. More importantly, Na1.2Mn0.4Mo0.4O2 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 Na1.3Nb0.3Mn0.4O2 and Na1.14Mn0.57Ti0.29O2 synthesized recently. For a better understanding, the redox mechanism is explored, which involves Mo4+/Mo6+, Mn3+/Mn4+, and O2-/On- (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.