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Development of novel, high performance LiFePO₄ cathode materials for Li-ion batteries

Authors Oh, Seiwoon
Issue Date 2012
Summary LiFePO4 (LFP) has been extensively studied as one of the most promising cathode materials of Li-ion batteries because of its low cost, acceptable environmental characteristics, and good safety. In spite of these attractive features, LFP requires further modifications to overcome limitations, such as poor electronic conductivity and slow Li-ion diffusion through the LFP/FePO4 interfaces. In this thesis, graphene oxide (GO) particles are mixed with LFP nanocrystals based on a polyol method to increase the electrical conductivity of the active material without further annealing steps. As a result, an effective 3D conducting network is formed by GO sheets that bridge the LFP crystals, which in turn facilitate electron transport and thus improve the kinetics and rate performance of LFP. The GO/LFP (1:9 wt.%) composites exhibite a discharge capacity of 164 mAh/g, 156.7 mAh/g and 121.5 mAh/g at 0.1, 1 and 10 C, respectively. The crystal size and their arrangement influence the capacity and rate performance of LFP. While the crystal size can be minimized by modifying the synthesis conditions, too small a crystal may reduce the energy density due to the very large surface area. The retention rate of bulk crystals with a porous structure can be a good solution to the low energy density. A method is developed to synthesize porous LFP spheres using sugar spheres as template based on a layer-by-layer self-assembly process. The porous spheres are deposited with a carbon coating via electropolymerization of aniline monomer followed by carbonization, producing a porous core-shell structure. A high capacity of 158.5 mAh/g at 1C achieved due to the porous core structure, while both a high specific capacity and high rate capabilities of 158.5, 121, 101.5, 77 and 79.7 mAh/g at 1, 10, 20, 30 and 40 C, respectively, are obtained due to the carbon shell. Carbon free LFP porous structure has good cycleability, but it has a marginally low specific capacity of 147 mAh/g at 0.1 C.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2012
Language English
Format Thesis
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