Structural and Electrochemical Characterization of Vanadium-Doped LiFePO4 Cathodes for Lithium-Ion Batteries

A series of Li1-xFe1-xVxPO4/C (with 0≤x≤0.1) samples have been synthesized using a wet chemistry method and characterized via Rietveld structure refinement of powder X-ray diffraction data. The amount of impurities is negligible up to x = 0.07, whereas for higher V content also Li3V2(PO4)3 phase is formed in minor quantities. The unit cell parameters of the olivine phase undergo anisotropic variations that cause an overall decrease of the unit cell volume with increasing Vanadium content. Structural data suggest that V enters the olivine lattice substituting for Fe. Compared with those of pure LiFePO4, V doped compounds have higher specific capacities especially at high rates delivering about 100 mAhg−1 at 10C rate. Local geometry and oxidation state of Fe and V in cycled electrodes was determined by X-ray absorption spectroscopy at the Fe and V K-edge. The data demonstrate that V is trivalent in both the oxidized and reduced electrodes meaning that V does not participate to the redox process. However, with increasing the vanadium content in the LiFePO4 lattice, the amount of Fe that reversibly oxidizes and reduces during battery cycling increases with an enhancement of the electrochemical performances.