||Effects of metal doping on the material properties of LiFePO4 cathode material by first-principle calculation
||Department of Mechanical Engineering
本研究利用VASP量子化學計算軟體模擬摻雜金屬離子後LiFePO4的材料性質變化，模擬結果顯示，摻雜金屬離子可有效的抑制鋰離子在脫嵌過程中產生的體積變化，有助於提升材料的使用壽命。鋰離子的擴散與晶格的間隙有關，量測摻雜金屬離子後的晶格間隙皆呈現變大的趨勢，代表摻雜金屬離子可提升鋰離子的擴散速率。由能隙結構圖發現摻雜金屬離子可縮小LiFePO4的能隙大小，表示摻雜金屬離子可有效的提升LiFePO4的導電性質，由電子狀態密度圖發現能隙縮小的主因，在於摻雜的金屬離子在費米能附近有局域性的電子分佈，使價電帶與導電帶電子的能量差距縮小，其中以摻雜V離子的改善效果最佳，能隙約為0.2068 eV遠小於未摻雜LiFePO4的能隙(0.9245 eV)。
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).
第一章 緒論 1
第二章 文獻回顧 10
2.2 LiFePO4電化學性質的改善方法 16
2.3 LiFePO4的製備方法 25
第三章 基本理論 29
3.3 Born-Oppenheimer近似 32
3.4 Thomas-Fermi模型 34
3.5 Hohenberg-Kohn方法 34
3.6 Kohn-Sham方法 35
第四章 結果與討論 45
4.2 LiFePO4的材料性質 48
4.3 LiFePO4的電化學性質 58
第五章 結論與未來展望 72
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