Fine phase adjustment optimizes structure stability and sodium ion diffusion ability of Na0.6Mn0.67–xNi0.22Fe0.11+xO2 via chemical environment variation

In this work, Na0.6Mn0.67−xNi0.22Fe0.11+xO2 (x = 0, 0.05, 0.1, 0.15) cathode materials were synthesized by a simple solid-state method, and the influence of Fe/Mn ratio on its fine phase structure and electrochemical performance was emphatically investigated. Firstly, XRD Rietveld refinement demonstrates that proportion of P2 phase and the distance between Na layers increases via increase of Fe/Mn ratio, which is beneficial to improve the diffusion rate of sodium ions. Secondly, reduced proportion of Mn3+ and Ni3+ in the material with the increase of Fe/Mn ratio is verified via XPS data analysis, which weakens Jahn-Teller effect and improves the cyclic stability of the material. Among them, the designed Na0.6Mn0.62Ni0.22Fe0.16O2 exhibits the initial discharge specific capacity 139 mAh g-1 at 0.5 C. The reversible specific capacity of 95 mAh g-1 can be maintained even at a higher current density of 2 C. The high electrochemical performance can be contributed to the lower charge transfer resistance and the higher sodium ion diffusion coefficient, which are evidenced by EIS and GITT analysis.