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 (O2-) 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 Na1-x/12FePO4 with different sodium compositions. Deformation charge density plots and magnetization for NaFePO4 are also calculated to help understand the redox reaction processes.