Realizing High-Performance Cathodes with Cationic and Anionic Redox Reactions in High-Sodium-Content P2-Type Oxides for Sodium-Ion Batteries

P2-type layered transition-metal oxides with anionic redox reactions are promising cathodes for sodium-ion batteries. In this work, a high-sodium content P2-type Na7/9Li1/9Mg1/9Cu1/9Mn2/3O2 (NLMC) cathode material is prepared by substituting Li/Mg/Cu for Mn sites in Na2/3MnO2. The Li/Mg ions trigger the anionic redox reaction, while the Cu ions enhance the structure stability during electrochemical cycling. As a result, the oxide has a high reversible capacity of 225 mAh g-1 originating from both cationic and anionic redox activities with a capacity retention of 77% after 100 cycles. The migration energy barrier and Na ion diffusion kinetics are studied using density functional theory (DFT) calculations and the galvanostatic intermittent titration technique. Furthermore, X-ray diffraction, DFT, scanning electron microscopy, and transmission electron microscopy are applied to reveal the structural evolution and charge compensation of NLMC, providing a thorough understanding of the structural and morphology evolution of Na-deficient oxides during cycling. The results are inspiring for the design of a high-Na content P2-type layered oxide cathode for sodium-ion batteries.