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

To better understand polymorph control in transition metal oxides, the mechanochemical synthesis of NaFeO was explored. Herein, we report the direct synthesis of α-NaFeO through a mechanochemical process. By milling NaO and γ-FeO for 5 h, α-NaFeO 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 NaFeO structure. Density functional theory calculations on the phase stability of NaFeO phases show that the α phase is stabilized over the β phase in oxidizing environments, which is provided by the oxygen-rich reaction between NaO and FeO. This provides a possible route to understanding polymorph control in NaFeO. Annealing the as-milled α-NaFeO at 700 °C has resulted in increased crystallinity and structural changes that improved electrochemical performance in terms of capacity over the as-milled sample.