Long-term cycling studies on 4 V-cathode, lithium vanadium fluorophosphate

Abstract

Lithium vanadium fluorophosphate, LiVPO₄F is prepared by two-step carbothermal reduction method and characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), density and surface area. Its cathodic behaviour is examined by galvanostatic charge-discharge cycling up to 1260 cycles, cyclic voltammetry (CV) and impedance spectra using Li-metal as the counter and reference electrode. When cycled between 3.0 and 4.5 V at 15 mA g⁻¹ (0.12C), a reversible and stable capacity of 130 (±3) mAh g⁻¹ is observed in the range 20-200 cycles. Slow capacity fading occurs between 200 and 360 cycles. When cycled at the 0.92C rate (1C = 130 mA g⁻¹), a reversible and stable capacity of 122 (±3) mAh g⁻¹ is obtained at 200-800 cycles. The capacity degrades slowly over 800-1260 cycles and the total loss is ~14%. Coulombic efficiency increases to 96-98% after the first 10-15 cycles. The CV data show that the charge-discharge process, a two-phase reaction, occurs between 4.2 and 4.5 V, in agreement with literature data. Impedance spectra, up to 90 cycles are fitted to an equivalent circuit and the variation of impedance parameters is interpreted.