| In this thesis, LiFeP04 has been intensively investigated as the most promising cathode material for power Li-ion batteries and its large-scale production has been realized. The synthesis mechanism of LiFePO4 is carefully studied that is important for the preparation of high-performance cathode material. A synthesis mechanism from FePO4·2H2O to LiFePO4 and a Li-ion diffusion model for one-step solid phase synthesis of LiFePO4 using FePO4·2H2O as raw material have been proposed according to the thermal analyses for starting materials and precursor. LiFePO4/C composites with excellent electrochemical performance has been prepared by controlling the crystallinity and carbon coating. A pilot plant test with a production capacity of 100 kg per day has been carried out.A three-step reaction mechanism for the synthesis of LiFePO4 has been found using FePO4·2H2O as the raw material. The mechanism is as follows:first, FePO4 is obtained from the dehydration of FePO4·2H2O; second, FePO4 is reduced to Fe2P2O7 by the reducing gases from the pyrolysis of the carbon source with the simultaneous formation carbon coating; finally, LiFePO4 is formed by the diffusion of Li-ion into Fe2P2O7 cores. The conversoion from Fe2P2O7 to LiFePO4 is a critical step which is limited by the Li-ion diffusion. The factors that affect the Li-ion diffusion include Li segregation and Li-ion diffusion path which are determined by the degree that Li source is uniformly dispersed in the precursors.A new carbon coating route with a high crystallinity of carbon has been proposed. Fe(OH)3 formed by the reaction of FePO4·2H2O and LiOH·H2O catalytically grows the carbon fibers in C2H2. The in situ formed carbon fibers with a high crystallinity enhance the electrical conductivity of LiFePO4, leading to significant improvement of the electrochemical performance. The sample prepared at 450℃for 0.5 h CVD reaction shows the best electrochemical performance. The capacities at of the sample at 0.1,1,5, and 10 C are 168,156,135, and 123 mAh/g, respectively. After cyclied at 1 C for 1200 times, the capacity fade rate is only 5.5% for this sampleA new approach been proposed to improve carbon coating. FePO4-2H2O is reduced to Fe2P2O7 by the reducing gases from polypropylene. At the same time, Fe2P2O7 is coated by a layer of carbon to form the Fe2P2O7/C composite. In this way, a good carbon coating can be realized for Fe2P2O7. The Fe2P2O7/C composite then reacts with Li source to form LiFePO4/C with a high-quality carbon coating. It is found that the LiFePO4/C composite using LiOH·H2O as Li source shows the best electrochemical performance. The capacities of the sample at 0.1,1, and 5 C reach 151,136 and 109 mAh/g, respectively. After cyclied at 1 C for 100 times, the capacity fade rate is only 1.3%for this sample.Based on the lab results, a pilot plant test with a production capacity of 100 kg per day using FePO4·2H2O, LiOH·H2O and (C6H10O5)n as the starting materials has been carried out. The capacity of the sample at 0.1 C 157 mAh/g. After cycled 100 times at 1 and 5 C, capacities of 126 and 93 mAh/g can be miantianed. The pilot plant test has been optimized according to the proposed synthesis mechanism for LiFePO4. |
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