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Synthesis and Electrochemical Properties of LiVxFe1-xPO4/C Cathode Materials

Author: ZhouKeQin
Tutor: ZhengJunWei
School: Suzhou University
Course: Physical and chemical
Keywords: LiVxFe1-xPO4/C LiFePO4 Cathode material High-temperature solid-phase reaction method Li3V2（PO4）3 microwave-assistant high-temperature solid reaction method sol-gel method
CLC: TM910.4
Type: Master's thesis
Year: 2010
Downloads: 93
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Abstract


LiVxFe1-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 LiVxFe1-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 LiVxFe1-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 LiVxFe1-xPO4/C, it was found that the crystallinity of LiVxFe1-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 LiVxFe1-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.

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