Role of Ligand-to-Metal Charge Transfer in O3-Type NaFeO₂–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-NaFe0.3Ni0.7O2 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.