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研究生： 吳宥甫
Wu, You-Fu
論文名稱： 以第一原理計算及分子動力學模擬研究鋰離子電池中含六氟磷酸鋰電解液之熱分解反應
Thermal decomposition reaction of LiPF6-based electrolyte in lithium ion battery：First-principle calculation and Molecular dynamics simulations
指導教授： 許文東
Hsu, Wen-Dung
學位類別： 碩士
Master
系所名稱： 工學院 - 材料科學及工程學系
Department of Materials Science and Engineering
論文出版年： 2019
畢業學年度： 107
語文別： 中文
論文頁數： 124
中文關鍵詞： 鋰離子電池 、六氟磷酸鋰 、熱分解反應 、第一原理分子動力學模擬
外文關鍵詞： Lithium ion batteries, LiPF6, thermal decomposition reaction, first-principle calculation, ab initio molecular dynamics simulation
相關次數： 點閱：77  下載：1 
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    • 　　本研究針對LiPF6與DEC、DMC、GBL和EC等四種溶劑所形成之電解液溶液在不同氧化還原環境中的熱分解反應，以僅將溶劑視為環境因子的第一原理靜態計算及實際考慮溶質與溶劑分子間交互作用的第一原理分子動力學模擬，分別進行能量計算與分解反應過程中電荷轉移情形。
    　　在第一部分的靜態計算中，首先考慮LiPF6及其分解產物在真空及不同溶劑環境中的幾何結構與反應能量計算。LiPF6分解為PF5及LiF的反應能在真空及溶劑中雖會隨溫度及介電常數增加而下降，但依舊是需吸收能量來進行的非自發反應。即便加上後續PF5與水反應生成POF3及氫氟酸的化學反應能計算結果，總反應仍然需要額外吸收能量才可進行，但與實驗文獻並不相符。
    　　第二部分的第一原理分子動力學模擬部分一併考慮氧化還原環境、水分子、鋰及鋰鹽濃度對鋰鹽及電解液有機溶劑分子的分解反應所造成的影響，其中具有多餘電子的還原環境及鋰離子為電解液內分子分解反應的主要原因。模擬系統中的額外的鋰離子會形成局部的還原環境，且進一步影響與之鍵結或在其周遭的分子而導致分解反應發生。相較之下，電解液中的水分子雖亦可能加速分解反應的進行，但不是最主要造成電解液分解反應的因素，而鋰鹽濃度較高的電解液也將大幅提高發生分解反應的可能。

    Nowadays, lithium-ion batteries are widely used in portable devices and electrical or hybrid vehicles due to their high energy density, long cycle life, high voltage and low cost. They, however, still have some shortcomings needed to be improved. For example, the safety, durability in high-temperature, performance in low temperature and stability. In commercial electrolyte, there include about ten ppm of water, and some literature pointed that the existence of water would affect the high-temperature performance and stability of electrolyte. In this study, the reaction energy of LiPF6 decomposition is calculated via static density functional theory calculations. The dynamic reaction process is simulated by ab-initio molecular dynamics simulations. The static calculations included the molecule optimization and reaction energy calculation, they were calculated by Gaussian03 with HF/6-31g(d) and B3LYP/6-31g(d,p). Solvent effect was considered by Onsager model. The results show that the implicit solvent model cannot correctly predict the decomposition of LiPF6 observed in experiments. In order to reflect the explicit interactions between solvent and solute molecules in electrolyte, AIMD simulations was used to further investigate the how the interaction between LiPF6 and solvent molecules affected the decomposition of LiPF6. And both of the redox environment in electrolyte and the presence of water were considered to realize the decomposition mechanism of LiPF6. Bader charge analysis were used to analyze the electron transfer process of LiPF6 in DEC, DMC, GBL and EC solution, respectively. From the results, the decomposition mechanism of LiPF6 in different solvents and redox environments can be inferred.

    摘要 I Abstract II 誌謝 XII 目錄 XIII 表目錄 XVI 圖目錄 XVII 第一章 前言 1 第二章 文獻回顧 4 2.1 鋰離子電池 4 2.2 有機溶液電解液發展現況 6 2.3 六氟磷酸鋰熱分解行為 10 2.4 電解液之模擬相關文獻 14 第三章 原子級模擬基礎理論 17 3.1 第一原理計算(First Principles) 17 3.2 密度泛函理論(Density Functional Theory, DFT)[41, 42] 17 3.2.1 Hartree-Fock方程式[44, 45] 18 3.2.2 Hohenberg-Kohn定理[46] 18 3.2.3 Kohn-Sham 方程式[47] 19 3.2.4 交換關聯函數(Exchange-correlation Function)[49] 20 3.2.5 基底函數組(Basis Set)[55] 21 3.3 分子動力學模擬(Molecular dynamics simulations)[61] 24 3.3.1 第一原理分子動力學模擬(Ab-initio Molecular Dynamics simulations) 26 3.3.2 Verlet algorithms[64] 27 3.3.3 Nosé-Hoover thermostat[66, 67] 28 第四章 物理模型與模擬設計 29 4.1 電解液有機溶劑分子與鋰鹽 29 4.1.1 碳酸二乙酯(Diethyl carbonate, DEC) 29 4.1.2 碳酸二甲酯(Dimethyl carbonate, DMC) 30 4.1.3 碳酸乙烯酯(Ethylene carbonate, EC) 30 4.1.4 γ-丁內酯(gamma-Butyrolactone, GBL) 31 4.1.5 六氟磷酸鋰(Lithium hexafluorophosphate, LiPF6) 32 4.2 模擬方法 33 4.2.1 分子結構優化與能量計算 33 4.2.2 溶劑化效應(Solvent Effect) 34 4.2.3 平衡修正(Counterpoise Correction) 34 4.2.4 第一原理分子動力學模擬 35 4.3 分析方法 40 4.3.1 反應能(Reaction energy) 40 4.3.2 徑向分布函數(Radial Distribution Function, RDF)[75, 76] 40 4.3.3 Bader電荷分析(Bader charge analysis)[77-80] 41 第五章 結果與討論 43 5.1 結構優化與能量變化 43 5.1.1 LiPF6分子及其分解產物分子的結構優化結果 43 5.1.2 LiPF6熱分解反應之反應能計算 47 5.1.3 PF5與水反應之反應能計算 50 5.2 第一原理分子動力學模擬 55 5.2.1 1 M LiPF6在有機純溶劑中 55 5.2.2 1 M LiPF6在含水之有機溶劑中 65 5.2.3 1 M LiPF6在含額外鋰之有機純溶劑中 74 5.2.4 1 M LiPF6在含額外鋰及水之有機溶劑中 86 5.2.5 2 M LiPF6在有機純溶劑中 97 5.2.6 2 M LiPF6在含水之有機溶劑中 104 5.2.7 不含鋰鹽的純有機溶劑 109 5.2.8 不含鋰鹽的含水有機溶劑 112 第六章 結論 115 第七章 參考文獻 116 第八章 附錄 122 8.1 各帶電系統的總價電子數 122

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