Synthesis And Properties Of Li₃V₂(PO₄)₃ Cathode Materials For Lithium-ion Batteries

As polyanion cathode materials for lithium ion batteries, monoclinic lithium vanadium phosphate Li3V2(PO4)3 is attractive for stable structure, excellent cycle performance and safety. However, their low electronic conductivity impedes their use as electrode materials. In this paper, Li3V2(PO4)3 was respectively prepared by high temperature carbothermal reduction reaction, low temperature carbothermal reduction, sol-gel method reaction. The physical properties of Li3V2(PO4)3 were investigated by XRD and SEM, and their electrochemical performances were investigated by galvanostatic current charge-discharge, cyclic voltammetry, electrochemical impedance spectroscopy. The effects of preparation conditions on the physical properties and electrochemical performances of active materials were investigated.The cathode material Li3V2(PO4)3 was prepared by high temperature carbothermal reduction reaction. The effect of synthesis conditions on the physical performance and electrochemical behavior of Li3V2(PO4)3 was studied and the synthesis conditions were optimized. The results showed that with the increase of sintering temperature, the growth of crystal was quickened and the crystal becomes more perfect. It was also found that the morphology of samples was affected by the specific capacity and cycling performance of Li3V2(PO4)3 varied with different synthesis conditions, while the charge-discharge curves were similar. The initial charge and discharge capacity of Li3V2(PO4)3 synthesized on the optimized condition was 131,122 mAh-g-1, respectively, and the capacity retained 113 mAh-g'1 after 30 cycles.The modification of Li3V2(PO4)3 by doping was studied. The effect of different content of doped-Li, V, Li and F on the structure, morphology and electrochemical properties was investigated. The results indicate that doping doesn't affect the structure of samples. Appropriate amount of doping can make the particle size of samples smaller. It was clear that the initial charge and discharge capacity of samples increased and the corresponding coulombic efficiency and cycling performance were enhanced. The discharge capacity of Li3.04V2(PO4)3, Li3V2.04(PO4)3 and Li3.o2V2(PO4)Fo.o2 were 123,125 and 127 mAh·g-1, and the capacity retained 114,115 and 118 mAh·g"1 after 30 cycles. The potential difference between the oxidation potential and the reduction potential of samples become samller than those of Li3V2(PO4)3, indicating the enhancement of the reversibility of electrode reaction due to doping. Though modification by doping can enhance the cycling performance of the materials, their capacity fading was still notable.Li3V2(PO4)3 cathode material was prepared by low temperature carbothermal reduction reaction. Compared with high temperature carbothermal reduction reaction, low sintering temperature and short sintering time are involved. The results showed that the sintering temperature and time played an important role in the crystal structure and morphology. The optimized sintering temperature and time were 600℃and 20 h. Electrochemical test showed that the initial discharge capacity of Li3V2(PO4)3 powder synthesized at the optimized conditions was 126 mAh g-1, the capacity retained 120 mAh-g-1 and the capacity fading was 4.76% after 30 cycles.Synthesizing technologies of Li3V2(PO4)3 was first exploited by sol-gel method in aqueous solution. Its advantages included low sintering temperature and short sintering time. Electrochemical test showed that the initial charge and discharge capacity of Li3V2(PO4)3 powder synthesized on the optimized condition was 135,129 mAh-g-1, respectively, the capacity retained 106 mAh-g'1 and the capacity fading was 17.82% at 0.2 C after 100 cycles. And the capacity retained 112 mAh-g-1 and the capacity fading was 13.17% at 1 C after 30 cycles.The modification of Li3V2(PO4)3 by doping was studied. The effect of different content of doped-Li and V on the structure, morphology and electrochemical properties was investigated. The results indicated that doping didn't affect the structure of the material. Appropriate amount of doping can make the particle size of samples smaller. It was clear that the initial charge and discharge capacity of samples increased and the corresponding coulombic efficiency and cycling performance were enhanced. The discharge capacity of Li3.04V2(PO4)3 and Li3 were 129 and 128 mAh·g-1, and the capacity retained 113 and 115 mAh·g-1 at 1 C after 100 cycles, and the capacity retained 120 and 122 mAh·g-1 at 1 C after 30 cycles. The potential difference between the oxidation potential and the reduction potential of samples become samller than that of Li3V2(PO4)3, indicating the enhancement of the reversibility of electrode reaction due to doping.The kinetics behaviors of Li3V2(PO4)3 were studied by means of linear sweep voltammetry and the potentiostatic intermittent titration technique (PITT). It was found that the exchange current density and diffusion coefficient (DLi+) were increased with Li interaction into Li3V2(PO4)3 material. And the magnitude level of diffusion coefficient (DLi+) in Li3V2(PO4)3 material is about 10-11-10-14 cm2·s-1 respectively. Doping and synthesized method can affect the exchange current density and diffusion coefficient (DLi+).