Improved performance of LiFePO4F cathodes via V/Na-ion co-doping for application in lithium-ion batteries

The quest for better cathode materials for lithium-ion batteries continues due to its burgeoning application in electric vehicles. In this context, an effort has been made to improve the electrochemical performance of tavorite-structured LiFePO4F with co-doping of vanadium and sodium ion. A solid-state reaction with FePO4 and LiF as precursors has been used for the synthesis. Powder X-ray diffraction (XRD) and electron microscopy techniques have been used for product characterization. Addition of dopants leads to a reduction in the particle size of doped LiFePO4F. The aliovalent substitution of vanadium ion has been ascertained by X-ray photoelectron spectroscopy (XPS). The electrochemical properties have been investigated using charge-discharge testing and electron impedance spectroscopy (EIS). The vanadium- and sodium-doped LiFePO4F cathodes delivered improved reversible capacities, more stable cycle performance, better rate capability and decreased charge transfer resistance along with enhanced ionic conductivity compared to the pristine sample. The Li0.95Na0.05Fe0.9V0.1PO4F cathode delivers a specific capacity of ~100.7 mAh g−1 at a moderate current rate of 1C after an extended run of 2000 cycles with ~94% retention of the initial specific capacity. Sodium addition proves to be beneficial for improving the charge transfer kinetics in the lithium vanadium fluorophosphate–based cathodes for lithium-ion battery.