| With the intensifying of environmental pollution and energy crisis, humans are forced to develop and utilize of renewable energy and more strict requirements are put forward for energy storage devices. Nowadays developing the lithium-ion battery which has some advantages such as high-capacity, high-power and high-safety has become the main tendency. In many cathode materials of lithium-ion battery, LiFePO4 cathode material has become a hot research, because of the advantages such as low-cost, long cycle life, high-safety. However, there are still some issues that remain to be resolved, such as bad low-temperature electrochemical performance and poor rate performance. In this thesis, we prepare the LiFePO4 by solvothermal method, and then we study on its rate performance and the electrochemical performance of some materials at the low-temperature. So we can to achieve the aim of research about low-temperature electrochemical performance and rate capability. The results of the experiments are as follows:In the research of rate capability, we use solvothermal method with ethylene glycol as solvent to prepare the LiFePO4 by controlling different reaction time(10h, 14 h, 18 h, 22h). The X-ray diffraction studies show that the materials are pure phase, preserving the olivine structure. The scanning electron microscope results show the morphology of the materials is similar and their particle sizes are small, but there is a phenomenon of agglomeration. The electrochemical performance tests show that material which is synthesized by 14 hours reaction shows the best rate capability. The discharge capacities are 141.8mAh/g, 135.8mAh/g, 132.1mAh/g, 127.1mAh/g, 118.7mAh/g, 110.5mAh/g at 0.1C, 0.5C, 1C, 2C, 5C, 10 C.So the reaction time should be 14 hours. We understand that the different solvents have different forces acting on the crystal of the materials so that the crystal of the materials can be preferentially grown on the different crystal planes in the process of preparation these materials. Therefore, we use solvothermal method to prepare the LiFePO4 by different solvent(ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol) as solvent. After analyzing the result of X-ray diffraction and the scanning electron microscope, we know that the materials are pure phase, preserving the olivine structure and the material’s morphology is sheet which is synthesized with ethylene glycol as solvent with its particle size smaller than the others. And the materials’ morphology is dumbbell which is synthesized with the other solvent as solvent. The electrochemical performance tests show that material which is synthesized with triethylene glycol as solvent shows the best rate capability at low rate capability. The discharge capacities are 151.7mAh/g, 140.2mAh/g, 134.9mAh/g at 0.1C, 0.5C, 1C.And the material which is synthesized with ethylene glycol as solvent shows the best rate capability at high rate. The discharge capacities are 118.7mAh/g, 110.5mAh/g at 5C, 10 C.In the research of low temperature performance, we study on commercial LiFePO4 cathode materials to test and analyze the low temperature electrochemical performance. The X-ray diffraction studies show that materials are pure phase, preserving the olivine structure with well crystalline and they are preferential growth in(010) crystal plane. The scanning electron microscope and transmission electron microscope results show that materials’ particle sizes are small and they have been coated with carbon. And materials’ particle size is smaller, carbon layer coating is more uniform and the thickness is moderate(~2nm), the low temperature electrochemical performance of materials are better. The electrochemical performance tests show that material of LFP-A shows the best low temperature electrochemical performance which is preferential growth in(010) crystal plane and the particle size is smaller, carbon layer coating is uniform and the thickness is moderate. The discharge capacities are 159.9mAh/g, 117.6mAh/g, 89.1mAh/g, 71.1mAh/g and the capacities are 73.5%, 55.7%, 44.5% of normal temperature capacity at-20℃,-30℃,... |
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