Delithiation of LiFePO4 with Cl2 Gas: Preparation of FePO4 for Sodium-Ion Batteries with High Li Recovery

Triphylite NaFePO4 is considered a promising positive electrode for sodium-ion batteries due to its exclusive composition of abundant elements, high thermal stability, and theoretical capacity (154 mAh g–1). However, the low thermodynamic stability of triphylite NaFePO4 precludes direct synthesis techniques such as the solid-state route, thus hampering its commercial viability. Herein, we report a novel preparation of heterosite FePO4, a triphylite NaFePO4 desodiation product, through the delithiation of triphylite (olivine) LiFePO4 using Cl2 gas. In addition to the scalability prospects, the present technique efficiently recovers the lithium source in a utilitarian form. The prepared FePO4 electrode yields a high reversible capacity of 127 mAh (g-FePO4)−1 at 0.05C (1C = 154 mA g–1) and good cycling performance (capacity retention of 98.5% after 250 cycles at 1C). X-ray diffraction revealed the reversible phase transition between heterosite FePO4 and triphylite NaFePO4 during charge–discharge.