Recent Progress on Conversion of LiFePO4 to Triphylite NaFePO4 Cathode Material for Sodium-Ion Batteries

Abstract

Sodium iron phosphate (NaFePO₄, or NFP) is an attractive cathode material for sodium-ion batteries due to its combination of excellent stability, good operating voltage (2.9 V) and high theoretical specific capacity (154.2 mAh g⁻¹). NaFePO₄ exists in two polymorphic forms, known as maricite and triphylite. The synthesis of NFP, which can achieve the desired electrochemical performance, is challenging due to conventional synthesis methods leading to the formation of the thermodynamically stable, but electrochemically inactive, maricite-NaFePO₄. To tackle this issue, the use of spent battery-grade LiFePO₄ as a precursor in the synthesis of NaFePO₄ has been suggested as an avenue to the formation of the desired triphylite-NaFePO₄. This study reviews electrochemical and chemical methods reported for the conversion of LiFePO₄ to triphylite-NaFePO₄. Finally, the sodium ion extraction/insertion mechanisms during charge/discharge and the triphylite-NaFePO₄ electrochemical performances reported to date are reviewed in detail.