High-energy P2-type Na-layered oxide cathode with sequentially occurred anionic redox and suppressed phase transition

Abstract

Although anionic-redox-based layered oxide materials have attracted great attention as promising cathodes for Na-ion batteries because of their high practical capacities, they suffer from undesirable structural degradation, resulting in the poor electrochemical behavior. Moreover, the occurrence of stable anionic-redox reaction without the use of expensive elements such as Li, Co, and Ni is considered one of the most important issues for high-energy and low-cost Na-ion batteries. Herein, using first-principles calculation and various experimental techniques, we investigate the combination of vacancy ((square)) and Ti4+ cations in the transition-metal sites to enable outstanding anionic-redox-based electrochemical performance in the Na-ion battery system. The presence of vacancies in the P2-type Na-0.56[Ti0.1Mn0.76 square 0.14]O-2 structure suppresses the large structural change such as the P2-OP4 phase transition, and Ti4+ cations in the structure result in selectively oxidized oxygen ions with structural stabilization during Na+ deintercalation in the high-voltage region. The high structural stability of P2-type Na-0.56[Ti0.1Mn0.76 square 0.14]O-2 enables not only the high specific capacity of 224.92 mAh g(-1) at 13mA g(-1) (1C = 264.1mA g(-1)) with an average potential of similar to 2.62V (vs Na+/Na) but also excellent cycle performance with a capacity retention of similar to 80.38% after 200 cycles at 52mAg(-1) with high coulombic efficiencies above 99%. Although there are some issues such as low Na contents in the as-prepared state, these findings suggest potential strategies to stabilize the anionic-redox reaction and structure in layered-oxide cathodes for high-energy and low-cost Na-ion batteries. Published under an exclusive license by AIP Publishing.