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

Abstract

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.