| This paper mainly adopts the research of vanadium doped LiFePO4 by liquid chromate graphy. Firstly LiFe1-xVxPO4/C was prepared by co precipitation, based on co precipitation. Hydrothermal method was used for V doped research, and exploring the feasibility of V doped by liquid phase method and studying the suitable doping amount and the mechanism.Using FeSO4·7H2O, H3PO4 and LiOH·H2O as raw material, pure phase LiFePO4 was prepared by a co precipitation method with protection of ascorbic acid. The more appropriate mole ratio of Li:Fe:P was 3:0.8:1. With ammonium metavanadate as vanadium source, investigating the effects of doping methods on product phase, finding doping mode that the vanadium source and lithium source was mixed, dropped into the iron sourceand by which XRD card of powder kissed well. Lattice parameters decreased gradually with increasing of the doping amount, which causing the volume contraction of the unit cell. Particles were spherical and the particle diameter is about 5μm. Tap density of V doped powder increased, and greater than the theoretical value. Among them, LiFeo.97V0.o3P04/C had maximum tap density that was 1.229 g·cm-3.Charge discharge test of LiFe1-xVxPO4/C was conducted, and the results show that the V doped material increased the initial charge discharge performance; among, first discharge specific capacity of LiFe0.95V0.05PO4/C was maximum to 143.3 mAh-g-1; the first charge discharge capacity at 0.1,0.2, 0.5,1 and 5 C were 142.7,139.1,131.7,120.1 and 60.9 mAh-g-1; after 100 cycles at 1 C, the discharge specific capacity of the sample was 110.3 mAh·g1, and the capacity retention rate was 91.5%.The suitable temperature of LiFePO4/C prepared by hydrothermal method was 200 ℃, and the time was 6 h. Choosing inorganic dispersant six sodium metaphosphate modified LiFePO4/C; when the amount of six sodium metaphosphate was 0.2 wt‰, smaller powder particle about 300-500 nm could beobtained with good dispersion. About 10 nm uniform carbon layer could be seen on surface of the particle. The different V doping did not change the crystal structure, and had little effect on the particle morphology. The electrical performance tests on LiFe1-xVxPO4/C showed that V doping improved the electrochemical performance of the material; among which the first discharge specific capacity of LiFe0.95V0.05PO4/C was maximum with value of 157.2 mAh·g-1. Vanadium doping improves the rate performance of materials, in which LiFe0.95V0.05PO4/C showed the best; even at a high rate 5 C charge and discharge, the first discharge specific capacity could reach 76 mAh·g-1. the initial discharge capacity of sample at 1 C was 133.6 mAh·g-1, and at 100 cycles was kept at 129.5 mAh·g-1, and the capacity retention rate was as high as 96.9%.The electrochemical properties are better than the co precipitation method, which is mainly attributed to the hydrothermal method reduced the particle diameter and increased the dispersion of powder. Small particles reduced the diffusion path of Li+; at the same time, the increased specific surface area of particles was favorable forreaction between contact particles. At the same time the effect of doping V on the electrical properties of could be proved by the impedance and cyclic voltammetry. |
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