Studies of fast-ion conducting Li3V2(PO4)3 coated LiFePO4 via sol–gel method

Abstract

To improve the electrochemical performance and energy density, LiFePO4 powders are firstly coated with the fast-ion conducting Li3V2(PO4)3 using a sol–gel process. X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), energy dispersive spectroscopy (EDS) and electrochemical measurements are used to study the structure and properties of the materials. The results show that a uniform coating layer of Li3V2(PO4)3/C exists on the surface of LiFePO4/C particles. The modified sample contains olivine LiFePO4 and monoclinic Li3V2(PO4)3 phases. Some V3+ and Fe2+ ions are doped into the lattices of LiFePO4 and Li3V2(PO4)3 separately, resulting in the lattice contraction and the formation of lattice defects. Compared with the commercial LiFePO4, the diffusion coefficient of lithium ion and exchange current density of modified LiFePO4 is both improved by one order of magnitude. Electrochemical measurements indicate that the rate capability and cycle performance of LiFePO4 are significantly enhanced by coating with Li3V2(PO4)3, especially at high current rates. At 5°C and 10°C rates, the modified LiFePO4 exhibits the initial discharge capacities of 134.85 and 104.99mAhg−1 and capacity retentions of 93.79% and 94.53% after 190cycles, respectively, whereas the commercial LiFePO4 shows much lower capacities of 100.61 and 40.58mAhg−1 at the same current rates.