Synthesis And Electrochemical Properties Of LiV_xFe_1-xPO₄/C Cathode Materials

LiV_xFe_1-xPO₄/C cathode materials were synthesized by high-temperature solid-phase reaction method. The X-ray diffraction (XRD) studies show that the crystal structure of LiV_xFe_1-xPO₄/C is similar to that of LiFePO₄; The fact that the redox peaks of vanadium can not be observed in cyclic voltammogram indicates that the vanadium atoms partially replace the Fe2+ in the LiFePO₄ skeleton, such a replacement have no effect on the skeleton structure. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis show that the surface of LiFePO₄ particles is coated with carbon generated from the organics during the high temperature treatment in the inner atmosphere.The charge-discharge studies have shown that the LiV_xFe_1-xPO₄/C possesses a higher charge-discharge capacity and better cycling performance than LiFePO₄/C. Further examination of the different calcination conditions and the different amount of V doped in the LiV_xFe_1-xPO₄/C, it was found that the crystallinity of LiV_xFe_1-xPO₄/C increased as the calcination temperature increased, however, the discharge capacity and cycle performance was degraded. Under the same calcination temperature, it was found that prolonging calcination time results in a better crystallinity of the sample, but worse discharge capacity and cycle performance.The discharge capacity increases with increasing of the amount of doped vanadium. Vanadium-doping seems to improve the cycle performance of LiFePO₄/C. The best cycling performance of material was obtained when the amount of the doped V was x=0.05. A further increase of the amount of doped vanadium lead to the degrade of the cycle performance of LiV_xFe_1-xPO₄/C.In this paper, Li3V(2PO₄)3/C composite materials and pure Li3V(2PO₄)3 were also synthesized by microwave-assistant high-temperature solid reaction method and sol-gel method, respectively. ?In the process of the microwave-assistant high-temperature synthesis, the effects of experimental conditions, such as the reaction temperature and reaction time, on crystal structure and electrochemical performance were investigated. Charge-discharge test results show that a better electrochemical properties of the samples could be obtained by using excess of 5% ascorbic acid as carbon source and under the microwave sintering at 700℃for 10min. In the voltage range of 3.0-4.3 V, the first reversible capacity of pure Li₃V₂(PO₄)₃/C is 112 mAh/g at 0.1 C rate. After 40 cycles, the discharge capacity still remained 106.5 mAh/g. For comparison, we also studied the crystal structure and electrochemical properties of pure Li3V2 ( PO₄ ) 3 prepared by sol-gel method. Charge-discharge test results showed that the samples synthesized with sol-gel precursor under the same experimental conditions reached 106.4 mAh/g in the first discharge capacity of the sample in the voltage range of 3-4.2 V at 0.05C rate.