Studies of fast-ion conducting Li3V2(PO4)3 coated LiFePO4 via sol–gel method
Abstract To improve the electrochemical performance and energy density, LiFePO 4 powders are firstly coated with the fast-ion conducting Li 3 V 2 (PO 4 ) 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 Li 3 V 2 (PO 4 ) 3 /C exists on the surface of LiFePO 4 /C particles. The modified sample contains olivine LiFePO 4 and monoclinic Li 3 V 2 (PO 4 ) 3 phases. Some V 3 + and Fe 2 + ions are doped into the lattices of LiFePO 4 and Li 3 V 2 (PO 4 ) 3 separately, resulting in the lattice contraction and the formation of lattice defects. Compared with the commercial LiFePO 4 , the diffusion coefficient of lithium ion and exchange current density of modified LiFePO 4 is both improved by one order of magnitude. Electrochemical measurements indicate that the rate capability and cycle performance of LiFePO 4 are significantly enhanced by coating with Li 3 V 2 (PO 4 ) 3 , especially at high current rates. At 5 °C and 10 °C rates, the modified LiFePO 4 exhibits the initial discharge capacities of 134.85 and 104.99 mAh g − 1 and capacity retentions of 93.79% and 94.53% after 190 cycles, respectively, whereas the commercial LiFePO 4 shows much lower capacities of 100.61 and 40.58 mAh g − 1 at the same current rates.