Sol–gel synthesis and electrochemical properties of 9LiFePO4·Li3V2(PO4)3/C composite cathode material for lithium ion batteries

9LiFePO4·Li3V2(PO4)3/C composite cathode material is prepared by a sol–gel method, using ferric citrate, V2O5, Li2CO3, NH4H2PO4 and citric acid as raw materials. The composite material is composed of the olivine LiFePO4 and monoclinic Li3V2(PO4)3 phases. XRD results indicate that most of the iron and vanadium in the raw materials tend to form the LiFePO4 and Li3V2(PO4)3 phases, and only small amounts of Fe and V as the dopants enter into the lattice of Li3V2(PO4)3 and LiFePO4, respectively. The electronic conductivity and Li+ diffusion coefficient of 9LiFePO4·Li3V2(PO4)3/C are 6.615 × 10−3 S cm−1 and ∼10−10 cm2 s−1, which are three orders of magnitude and one order of magnitude larger than those of the LiFePO4/C, respectively. The composite material shows a first discharge specific capacity of 131.3 mAh g−1 and capacity retention of 95.1% after 200 cycles at 10 C rate. Compared with the LiFePO4/C, its rate capability and cycle performance are both remarkably improved.

► Nano-sized 9LiFePO4·Li3V2(PO4)3/C powders are prepared by a sol–gel method.
► Mutual doping in 9LiFePO4·Li3V2(PO4)3/C can improve its electronic conductivity.
► The addition of Li3V2(PO4)3 can improve the ionic diffusivity of LiFePO4.
► LiFePO4, Li3V2(PO4)3 and LiFePO4–Li3V2(PO4)3 unit cells coexist in the composite.