Role of ligand-to-metal charge transfer in O3-type NaFeO ₂-NaNiO₂ solid solution for enhanced electrochemical properties

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 _0.3Ni_0.7O₂ 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.