Structure modulation and performance optimization of P2-Na0.7Mn0.75Fe0.25-x-yNixCoyO2 through a synergistic substitution of Ni and Co for Fe

The poor cycling stability and inferior rate capability of the cathodes critically restrict the application of sodium ion batteries (SIBs). Herein, we propose a strategy to optimize the electrochemical performances of P2-Na0.7Mn0.75Fe0.25-x-yNixCoyO2 cathode materials for SIBs through modulating the crystal structure with a synergistic substitution of Fe by Ni and Co. The structure-performance relationship and the synergistic improvement mechanism have been unraveled by means of x-ray diffraction, neutron diffraction and electrochemical techniques. The synergistic substitution of Fe by Ni and Co leads to the enlargement of the interlayer spacing and the Na-O bond length, and shrinks the TM-O and O-O bonds, which enhances the sodium ion diffusion coefficient, the rate capability and the cycling stability. In addition, Jahn-Teller effect is also alleviated owing to the reduction of Mn3+, which can further enhance the stability of the layered structure. Ni and Co co-doped Na0.7Mn0.75Fe0.15Ni0.05Co0.05O2 (FNC) cathode shows a relatively smooth charge-discharge curve in 1.5-4.2 V and demonstrates a much better rate performance in compared to Na0.7Mn0.75Fe0.25O2 (MF), Na0.7Mn0.75Fe0.15Ni0.1O2 (FN) and Na0.7Mn0.75Fe0.15Co0.1O2 (FC). At a high current density of 1C, 2C, 5C and 10C, FNC can still deliver a reversible capacity of 127, 109, 83 and 58 mAh g−1, respectively. FNC also provides a higher reversible capacity of 181 mAh g−1 at 0.1C with a high cycling stability. This study offers some new insights into designing high performance cathode materials for SIBs through cooperatively modulating the crystal structure with multi-elements doping.