Study on Vanadium Substitution to Iron in Li₂FeP₂O₇ as Cathode Material for Lithium-ion Batteries

A series of Li₂Fe_1-3x/2V_xP₂O₇ (x = 0, 0.025, 0.05, 0.075, and 0.1) cathode materials for LIBs were prepared by the sol-gel method. Structural characterization of Li₂Fe_1-3x/2V_xP₂O₇ (x = 0, 0.025, 0.05, 0.075, and 0.1) samples was conducted by synchrotron X-ray diffraction. The morphology and oxidation states of Fe²⁺ and V³⁺ in the Li₂Fe_1-3x/2V_xP₂O₇ samples were confirmed by scanning electron microscopy and magnetic susceptibility measurements, respectively. The electrochemical measurements indicated that Li₂Fe_1-3x/2V_xP₂O₇ (x = 0.025) delivered the higher reversible capacity of 79.9 mAh g⁻¹ at 1 C in the voltage range of 2.0 - 4.5 V with higher 77.9% capacity retention after 300 cycles than those of Li₂FeP₂O₇ (48.9 mAh g⁻¹ and 72.6%). Moreover, the rate capability of Li₂Fe_1-3x/2V_xP₂O₇ (x = 0.025) were also significantly enhanced through vanadium substitution to iron of Li₂Fe_1-3x/2V_xP₂O₇. The vanadium substituted to Fe2 site of Li₂FeP₂O₇ decreases Li occupying the Li5 position in the FeO₅ unit, leading to a low degree exchange between Li and Fe in the MO₅ (M = Li and Fe). The low degree cation disorder was beneficial to lithium-ion extraction/insertion during the charge-discharge process and hence enhances the capacity and rate capability