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.

Automated summary

Triphylite NaFePO4 is a promising positive electrode material for sodium-ion batteries due to its abundant elements, high thermal stability, and theoretical capacity. However, its low thermodynamic stability hinders its commercial viability. In this study, a novel preparation method for heterosite FePO4 was developed by delithiating triphylite LiFePO4 using Cl2 gas. The prepared FePO4 electrode exhibits a high reversible capacity and good cycling performance, showing a reversible phase transition between heterosite FePO4 and triphylite NaFePO4.