Direct Mechanochemical Synthesis, Phase Stability, and Electrochemical Performance of α-NaFeO2

To better understand polymorph control in transition metal oxides, the mechanochemical synthesis of NaFeO₂ was explored. Herein, we report the direct synthesis of α-NaFeO₂ through a mechanochemical process. By milling Na₂O₂ and γ-Fe₂O₃ for 5 h, α-NaFeO₂ was prepared without high-temperature annealing needed in other synthesis methods. While investigating the mechanochemical synthesis, it was observed that changing the starting precursors and mass of precursors affects the resulting NaFeO₂ structure. Density functional theory calculations on the phase stability of NaFeO₂ phases show that the α phase is stabilized over the β phase in oxidizing environments, which is provided by the oxygen-rich reaction between Na₂O₂ and Fe₂O₃. This provides a possible route to understanding polymorph control in NaFeO₂. Annealing the as-milled α-NaFeO₂ at 700 °C has resulted in increased crystallinity and structural changes that improved electrochemical performance in terms of capacity over the as-milled sample.