Simultaneously Enhancing Structural Stability and Cationic Redox in Na0.67Mn0.75Fe0.25O2 through a Synergy of Multisite Substitution

P2-type Na0.67Mn0.75Fe0.25O2 cathode materials with low cost and high specific capacity have aroused much interest for sodium-ion batteries. Nevertheless, the cyclic and structural stabilities need to be improved for practical application. Herein, we tune the activity and reversibility of the cationic redox and the structure stability in Na0.67Mn0.75Fe0.25O2 by doping Mg/Ca/F into multi-Na/TM/O sites, respectively. The capacity performance at a high rate and cyclic stability are significantly enhanced. The mechanism of the synergy has been revealed by means of an X-ray photoelectronic spectrometer, neutron powder diffraction, ex situ/in situ X-ray diffraction, and so on. The improvement of the rate performance is mostly due to the broadening of the interlayer spacing caused by the element doping and the decrease of the Na-ion diffusion barrier, which facilitates Na-ion diffusion. The reduced transitional metal (TM)-O bond length demonstrates that the bonding energy has been strengthened, which benefits the structural stability and the cycling stability. Moreover, the synergy of multisite substitution suppresses the P2-OP4 phase transition at a high voltage and further improves the cycle stability. The Mg doping also activates Mn3+/Mn4+ redox and increases the intensity of Mn3+/Mn4+ redox. The ratio of Mn3+/Mn4+ is reduced by Ca substitution. Therefore, the Jahn-Teller effect, which is harmful to the structural stability, is restrained. The insights into the synergy of multisite substitution proposed in this study may also be helpful in the design of other cathode materials.

Automated summary

Doping Mg/Ca/F into multi-Na/TM/O sites in P2-type Na0.67Mn0.75Fe0.25O2 cathode materials enhances the capacity performance at a high rate and cyclic stability. The synergy of multisite substitution improves the rate performance by broadening the interlayer spacing, decreases the Na-ion diffusion barrier, and strengthens the TM-O bond length for enhanced structural stability. The insights from this study could be beneficial for designing other cathode materials.