| Summary: | 碩士 === 國立成功大學 === 機械工程學系 === 102 === In the recent years, LiFePO4 materials with the olivine structure have become a promising cathode material for the lithium ion battery. LiFePO4 has a lot of advantage, such as high operation voltage, long operational life, low materials cost, environmental friendliness. However, disadvantage of low electronic conductivity and poor ionic conductivity greatly restricts the commercial applications of LiFePO4. Metal doping is one of the effective way to improve materials properties of the LiFePO4. In this study, the materials properties of LiFePO4 after doping metal ions were performed by first-principles calculation. It was found that doping metal atoms to LiFePO4 can significantly reduce the volume variation during the lithiation/ delithiation cycles. Consequently, the working life of cathode materials can be improved. The metal doping in LiFePO4 leads to the increase of hopping distance. This expansion effect would benefit the Li ion diffusion. The effects of metal doping on the electronic structures were performed by the investigation of band structure. The results show that doping metal ion into LiFePO4 induces a narrowing of the band gap, which could benefit to improve the electronic conductivity. From the analysis of the density of states, we can find the energy bands near the Fermi energy were mainly attributed to the doping metal atom. This result leads to the decrease of energy gap between the valence band and conduction band. In this work, V-ion doping shows an optimum effect than other elements under study. The band gap of V-ion doping (0.2068eV) is much smaller than the band gap of un-doped LiFePO4 (0.9245eV).
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