Synthesis of Li3V2(PO4)3/C Composites as Cathode Materials for Lithium Ion Batteries via a Sol-Gel Method

Li3V2(PO4)3/C composite cathode materials were successfully synthesized via a sol-gel route based on LiOH·H2O, V2O5, NH4H2PO4 and C6H8O7·H2O (citric acid) as raw materials. Citric acid acts as a chelating reagent and a carbon source during the synthesis process, which can improve the conductivity of the composite materials and hinder the growth of Li3V2(PO4)3 particles. The thermal stability, crystal structure and morphology of the as-prepared powders were characterized by thermogravimetric analysis (TG-DTG), X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM). The results indicate that the sintering temperatures have a remarkable effect on the crystal structure, morphology and particle size of Li3V2(PO4)3/C composites. Demonstrated by XRD patterns, Li3V2(PO4)3/C composites have a monoclinic structure indexed to the P21/n space group and well crystallized single phase calcined at 800 °C. TEM images show Li3V2(PO4)3/C particles are roughly nanospheres in the diameter of about 50 nm embedded in carbon networks. Electrochemical tests show that Li3V2(PO4)3/C composite synthesized at 800 °C for 8 h displays the best electrochemical performances among these Li3V2(PO4)3/C composites. Li3V2(PO4)3/C composite synthesized at 800 °C for 8 h shows that the discharge capacities in the first and 100th cycle are 169.65 and 131.72 mAh g–1, respectively, in the voltage of 3.0−4.8 V at the current rate of 30 mA g–1. When the current rate is raised to 120 mA g–1, Li3V2(PO4)3/C composite calcined at 800 °C for 8 h delivers the high initial capacity of 156.04 mAh g– and still maintains a capacity of 123.5 mAh g–1 after 100 cycles, indicating that Li3V2(PO4)3/C composite with high discharge capacity and good rate capability can be obtained via this sol-gel method.