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

A series of Li2Fe1-3x/2VxP2O7 (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 Li2Fe1-3x/2VxP2O7 (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 Li2Fe1-3x/2VxP2O7 samples were confirmed by scanning electron microscopy and magnetic susceptibility measurements, respectively. The electrochemical measurements indicated that Li2Fe1-3x/2VxP2O7 (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 Li2Fe1-3x/2VxP2O7 (x = 0.025) were also significantly enhanced through vanadium substitution to iron of Li2Fe1-3x/2VxP2O7. 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.