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.