Tuning anionic redox activity to boost high-performance sodium-storage in low-cost Na0.67Fe0.5Mn0.5O2 cathode

Na-based layered iron-manganese oxide Na0.67Fe0.5Mn0.5O2 containing only low-cost elements is a promising cathode for Na-ion batteries used in large-scale energy storage systems. However, the poor cycle stability restricts its practical application. The capacity decay of Na0.67Fe0.5Mn0.5O2 mainly originates from the irreversible anionic redox reaction charge compensation due to the high-level hybridization between oxygen and iron. Herein, we rationally design a surface Ti doping strategy to tune the anionic redox reaction activity of Na0.67Fe0.5Mn0.5O2 and improve its Na-storage properties. The doped Ti ions not only enlarge the Na migration spacing layer but also improve the structure stability thanks to the strong Ti–O bond. More importantly, the d0-shell electronic structure of Ti4+ can suppress the charge transfer from the oxidized anions to cations, thus reducing the anionic redox reaction activity and enhancing the reversibility of charge compensation. The modified Na0.67Fe0.5Mn0.5O2 cathode shows a reversible capacity of 198 mA h g−1 and an increased capacity retention from 15% to 73% after about 1 month of cycling. Meanwhile, a superior Na-ion diffusion kinetics and rate capability are also observed. This work advances the commercialization process of Na-based layered iron-manganese oxide cathodes; on the other hand, the proposed modification strategy paves the way for the design of high-performance electrode materials relying on anionic redox reactions.