Structure and Electrochemistry of Vanadium-Modified LiFePO₄

Abstract

Doping LiFePO4 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 LiFePO4 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 LiFePO4 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 LiFePO4 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 FeO6 octahedron. This structural modification improves the cycling rate performance by increasing the Li+ effective cross-sectional area of the LiO6 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 LiFePO4 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).