Green chemical delithiation of lithium iron phosphate for energy storage application

Heterosite FePO4 is usually obtained via the chemical delithiation process. The low toxicity, high thermal stability, and excellent cycle ability of heterosite FePO4 make it a promising candidate for cation storage such as Li+, Na+, and Mg2+. However, during lithium ion extraction, the surface chemistry characteristics are also affected by chemical agents. In this work, heterosite FePO4 was synthesized via the delithiation of LiFePO4 by acetonitrile solution with nitronium tetrafluoroborate (NO2BF4/AN) or by peracetic acid (PAA) to extract lithium ion from the olivine structure. As a host of lithium and sodium ion, heterosite FePO4 prepared by PAA exhibits better capacity, cycle retention, and high rate capability than that delithiated by the conventional NO2BF4/AN method. The superior electrochemical performance of using PAA could be attributed to the better integrity of the carbon coating on the pristine LiFePO4 surface. The delithiated samples were further tested as a cathode in a sodium ion battery with capacity retention > 85.4% after 70 cycles. This work provides a simple and eco-friendly method to synthesize heterosite FePO4 without organic solvent and demonstrates its viability for lithium and sodium battery applications.