Structure and Electrochemistry of Vanadium-Modified LiFePO₄

Doping LiFePO 4 with vanadium has proven to enhance electrochemical performance, but the underlying reasons for this improvement are not well understood. To better comprehend the relationships between the electrochemical performance, crystal structure, and surface carbon layer, we prepared vanadium-modified LiFePO 4 by three different methods. The electrochemical performance of each sample was determined via a series of cycling studies, the detailed crystal structures of the doped samples were identified by X-ray diffraction and absorption spectroscopy, and the surface carbon coating was examined by high resolution transmission electron microscopy. In V-modified LiFePO 4 prepared by a modified solid-state reaction, the vanadium is present in an impurity phase at the surface, which improves conductivity but has only a slight improvement in the electrochemical properties. The V-modified LiFePO 4 samples prepared by the conventional solid-state reaction method and a solution method revealed that the vanadium was substituted into the lattice occupying iron sites in the FeO 6 octahedron. This structural modification improves the cycling rate performance by increasing the Li + effective cross-sectional area of the LiO 6 octahedral face and thereby reducing the bottleneck for Li + migration. In addition, analysis of the carbon coating revealed that the material prepared by the solution method forms a uniform carbon coating with a thin, well-ordered interface between the LiFePO 4 and the carbon. The surface properties improve the electronic and ionic conductivities (with respect to the other samples), resulting in a high rate capability (87 mAh g –1 at 50 C).