Effect of Vanadium Incorporation on Electrochemical Performance of LiFePO4 for Lithium-Ion Batteries

A series of LiFe1-xVxPO4/C samples have been successfully prepared using a two-step solid-state reaction route. The effect of vanadium incorporation on the performance of LiFePO4 has systematically been investigated with X-ray diffraction, Raman spectroscopy, charge/discharge measurements, and cyclic voltammetry tests. It is found that V incorporation significantly enhances the electrochemical performance of LiFePO4. Particularly, the LiFePO4/C sample with 5 at. % vanadium doping exhibits the best performance with a specific discharge capacity of 129 mAh g(-1) at 5.0 C after 50 cycles; the capacity retention ratio is higher than 97.5% at all C rates from 0.1 to 5.0 C. X-ray absorption spectroscopy results show that the valence of V in LiFe0.95 V0.05PO4/C is between +3 and +4. It is confirmed that the samples with x <= 0.03 are in single phase, whereas the samples with 0.05 <= x < 1.00 contain two impurity phases: Li3V2(PO4)3 and LiVOPO4. A clear feature of vanadium incorporation in LiFePO4 has been specified.