Regulating the electrochemical activity of Fe-Mn-Cu-based layer oxides as cathode materials for high-performance Na-ion battery

Fe-Mn based layer oxides cathode materials have attracted widespread attention as a potential candidate for sodium-ion batteries (SIBs) owing to the earth abundance, cost-effectiveness and acceptable specific capacity. However, the irreversible phase transition often brings rapid capacity decay, which seriously hinders the practical application in large-scale energy storage. Herein, we design a nickel-doped Na0.70Fe0.10Cu0.20Ni0.05Mn0.65O2 (NFCNM-0.05) cathode material of SIBs with activated anionic redox reaction, and then inhibit the harmful phase transition. The ex-situ X-ray diffraction patterns demonstrate the NFCNM-0.05 always keeps the P2 phase during the sodiation/desodiation process, indicating a high structure stability. The ex-situ X-ray photoelectron spectroscopy implies the redox reactions between O2− and O− occur in the charging process, which offers extra specific capacity. Thus, the NFCNM-0.05 electrode delivers a high initial discharge capacity of 148 mA h g−1 and remains a prominent cycling stability with an excellent capacity retention of 95.9% after 200 cycles at 1 C. In addition, the electrochemical impedance spectroscopy and galvanostatic intermittent titration technique show the NFCNM-0.05 electrode possesses fast ion diffusion ability, which is beneficial for the enhancement of rate performance. Even at 10 C, the NFCNM-0.05 can offer a reversible discharge capacity of 81 mA h g−1. DFT calculation demonstrates the doping of appropriate amount of Ni ions is benefit for the enhancement of the electrochemical performance of the layer oxides. This work provides an effective strategy to enhance the electrochemical performance of Fe-Mn based cathode materials of SIBs.