Recent Progress on Conversion of LiFePO 4 to Triphylite NaFePO 4 Cathode Material for Sodium‐Ion Batteries

ABSTRACT Sodium iron phosphate (NaFePO 4 , 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 ). NaFePO 4 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 ‐NaFePO 4 . To tackle this issue, the use of spent battery‐grade LiFePO 4 as a precursor in the synthesis of NaFePO 4 has been suggested as an avenue to the formation of the desired triphylite ‐NaFePO 4 . This study reviews electrochemical and chemical methods reported for the conversion of LiFePO 4 to triphylite ‐NaFePO 4 . Finally, the sodium ion extraction/insertion mechanisms during charge/discharge and the triphylite ‐NaFePO 4 electrochemical performances reported to date are reviewed in detail.