Phase Diagram of Olivine NaxFePO4 (0 < x < 1)

The composition-temperature phase diagram of cathode material NaxFePO4 in the olivine phase for sodium-ion battery has been determined. Samples were prepared by solid-state synthetic, electrochemical and chemical oxidation/reduction methods. Powder X-ray diffraction and Mossbauer spectroscopies were employed to investigate the phase behavior. At room temperature, despite the resemblance to the LixFePO4 structure, the solubility limit of alkali metal (sodium) vacancy in NaxFePO4 is found to be large. In the range 2/3 < x < 1 NaxFePO4 is found to be a solid-solution phase. Two distinguished Fe2+ sites in the solid-solution phase were found by Mossbauer spectroscopy. Ab initio calculations reproduce the quadrupole splitting constants and suggest large distortion of the Fe2+ octahedra. High-temperature in situ X-ray diffraction suggests a completely different phase diagram of sodium olivine with a much more stable nature of the intermediate at x = 2/3 in comparison to the lithium one.