Anionic Redox Processes in Maricite- and Triphylite-NaFePO of Sodium-Ion Batteries.

In recent years, NaFePO has been regarded as one of the most promising cathode materials for next-generation rechargeable sodium-ion batteries. There is significant interest in the redox processes of rechargeable batteries for high capacity applications. In this paper, the redox processes of triphylite-NaFePO and maricite-NaFePO materials have been analyzed based on first-principles calculations and analysis of Bader charges. Different from LiFePO, anionic (O) redox reactions are evidently visible in NaFePO. Electronic structures and density of states are calculated to elaborate the charge transfer and redox reactions during the desodiation processes. Furthermore, we also calculate the formation energies of sodium extraction, convex hull, average voltage plateaus, and volume changes of NaFePO with different sodium compositions. Deformation charge density plots and magnetization for NaFePO are also calculated to help understand the redox reaction processes.