| New alternative energy sources were forced to be found due to oil crisis during the period of1960s and1970s. Since lithium is the lightest metal, its redox potential is the minimum, and the quality energy density is the largest. As a result, the lithium ion battery has become one of the alternative sources of energy. Lithium battery with large capacity and high performance will be the ideal energy source for environmentally friendly electric vehicles. Lithium battery has become the commercialization in the early1970s. Olivine structure LiMPO4(M=Mn, Co, Ni or Fe) with high theoretical capacity and the large anion has been a popular study point.The basic reaction processes of hydrothermal synthesis were systematically studied. Characterizations of X-ray diffraction, Fourier transform infrared absorption spectroscopy, scanning electron microscopy, field emission-high resolution transmission electron microscopy, thermal analysis, cyclic voltammetry, charge-discharge testing and electrochemical impedance spectroscopy have been carried out to investigate the phase composition, surface morphology and electrochemical properties of LiFePO4.According to the results, the structure for precursor prepared by a hydrothermal process is amorphous. The pure LiFePO4with an olivine structure was formed at700℃. For LiFePO4, and the cell parameters are such as follows, a=10.4081A, b=6.0076A, c=4.7120A, the cell volume is V=294.6305A3. The mean particle size is330-510nm. The Fe2+is located in the Z chain of octahedral. Li+is located in the linear chain of the alternate planar octahedral. All of Li+is insertion and deintercalation, obtained the layered structure of FePO4-1. In addition, the electrochemical measurements showed that the first discharge capacity of LiFePO4material is106.8mAh·g-1, the corresponding charge capacity is110.2mAh·g-1. After50cycles, the discharge capacity is45.4mAh-g"1. So the cycling stability during charge/discharge processes of pure LiFePO4is undesirable, and capacity fading is serious.The best reaction process, microstructure and electrochemical properties of hydrothermal synthesis were systematically studied regarding improving the poor inherent poor conductivity and low lithium ion diffusion rate. Carbon was used to coat and high valence metal ions were doped resepectively to improve the electrochemical performance. The influence of the amount of coating carbon, the calcined temperature/time on the structure and electrochemical performance were studied. The results show that when the amount of coated carbon is5wt%, and the sintering temperature and calcining time are700℃and10h, the first charge and discharge capacity LiFePO4/C material are160mAh·g-1and147.8mAh·g-1, respectiveiy. Ater50cycles, the discharge capacity is136.5mAh·g-1. These incated that the carbon coated LiFePO4material behaved better charge/discharge capacity and better cycling stability. Moreover, the carbon coated is mainly amorphous on the particle surface. Active carbon reduced the particle size and shortened the Li+diffusion path, which significantly reduced the electrochemical reaction resistance and electrode polarization of charge and discharge process, and improved the Li+diffusion rate and charge and discharge capacity of LiFePO4cathode material. In addition, according to the results of the Nb5+doping, when the amount of ion doping0.05, the first charge and discharge capacity were improved. The first charge and discharge capacity LiFe0.95Nb0.05P04material were136mAh·g-1and118mAh·g-1, respectively. This may be due to the addition of Nb5+, which can cause the microstucture defects inside the LiFePO4material and change the lattice constant and crystalline state. A small amout addition of Nb5+caused the shrinkage of LiFe1-xNbxPO4unit cell and refinement of crystal grain. |
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