Effects of vanadium substitution on the cycling performance of olivine cathode materials.

Abstract: V-substituted LiFePO4 (LiFe1−x V x PO4, 0 ≤ x ≤ 0.10) powders are prepared via a solution method. The compositions, crystalline structure, morphology of the prepared powders are systemically investigated with inductive couple plasma-option emission spectrometry (ICP-OES), synchrotron radiation X-ray diffraction (s-XRD), X-ray Absorption Spectroscopy (XAS) and field emission-transmission electron microscopy (FE-TEM). Results of s-XRD and Rietveld analysis reveal that olivine phase is observed exclusively for LiFe1−x V x PO4 (x ≤ 0.05) samples, whereas a minor amount of monoclinic Li3V2(PO4)3 is detected in the prepared LiFe0.93V0.07PO4 and LiFe0.90V0.10PO4 samples. It is also found that the lattice parameters of olivine structure and average Li–O and M–O (M = Fe or V) bond lengths slightly increase and decrease with the amount of V substitution, recommending an enhanced lithium diffusivity in the structure. Results of XAS study suggest that V ions occupy octahedral sites (4c) of Fe in LiFePO4 structure with valence of 3+, showing good agreement with shortened M–O bonds determined in Rietveld analysis. As a result of V substitution, LiFe1−x V x PO4, (0 ≤ x ≤ 0.10) cathodes exhibit better electrochemical performance, such as discharge capacity, rate capability, and cycling performance at both room and elevated temperatures. [Copyright &y& Elsevier]