Anionic Redox Processes in Maricite- and Triphylite-NaFePO₄ of Sodium-Ion Batteries

In recent years, NaFePO₄ 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-NaFePO₄ and maricite-NaFePO₄ materials have been analyzed based on first-principles calculations and analysis of Bader charges. Different from LiFePO₄, anionic (O^2-) redox reactions are evidently visible in NaFePO₄. 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 Na_1-FePO₄ with different sodium compositions. Deformation charge density plots and magnetization for NaFePO₄ are also calculated to help understand the redox reaction processes.