Na-ion batteries have been the subjects of intensive studies for grid-scale energy storage recently. O3-type NaFeO2 is a promising candidate for the Na-ion cathode materials, though the irreversibility during Na-ion extraction/insertion seriously hinders its practical application. The present work demonstrates that partial replacement of Fe in O3-NaFeO2 with Ni leads to the significant improvement of the electrochemical properties. The 57Fe Mössbauer and X-ray absorption spectra show that O3-type NaFeO2-NaNiO2 solid solution forms hybridized frontier orbital of a Fe-O-Ni bond via ligand-to-metal charge transfer, which plays a dominant role in the charge-discharge process. The resulting O3-NaFe 0.3Ni0.7O2 delivers an initial discharge capacity of 135 mA·h·g-1, most of which is in a high-voltage region of 2.5-3.8 V, with a high initial Coulombic efficiency of 93%, and shows enhanced cycle stability.
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