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研究生： 賴惠慈
Hui-Tzu Lai
論文名稱： TTE作為稀釋劑對碳酸鹽電解質溶劑化結構和還原行為的影響：藉由 AIMD 模擬之研究
The influence of TTE as a diluent on solvation structure and reduction mechanisms of carbonate-based electrolytes: Insights from AIMD Simulations
指導教授： 江志強
Jyh-Chiang Jiang
口試委員: 林昇佃
Shawn D. Lin
蔡明剛
Ming-Kang Tsai
學位類別： 碩士
Master
系所名稱： 工程學院 - 化學工程系
Department of Chemical Engineering
論文出版年： 2023
畢業學年度： 111
語文別： 英文
論文頁數： 104
中文關鍵詞： 局部高濃度電解質 、溶劑化結構 、鋰電池
外文關鍵詞： AIMD simulation, solvation structure, TTE
相關次數： 點閱：145  下載：3 
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    • 電解液對高壓鋰離子電池（LIBs）的性能和循環穩定性至關重要。相較於傳統電電解液在高壓鋰離子電池（LIBs）中的性能和循環穩定性至關重要。相對於傳統電解液（通常為 > 1~2 M），高濃度電解液（HCEs）（通常為 > 3M）在嚴苛條件下改善陽極材料的循環穩定性方面展現出潛力。然而，HCEs在實際應用中面臨著多個挑戰，例如高黏度、差濕潤性、低離子導電性、低溫性能差和高成本，其主要是由其獨特的溶劑結構引起的。為了進一步提升HCEs，目前已開發出局部高濃度電解液（LHCEs），通過在HCEs中引入稀釋劑製成。本研究考慮了一種先進的電解液，該電解液在鹽/碳酸鹽溶劑基電解液（LiTFSI/EC-EMC）中添加了1,1,2,2-四氟乙基-2,2,3,3-四氟丙基醚（TTE）作為稀釋劑。通過密度泛函理論（DFT）和分子動力學（AIMD）模擬，本研究探討了不同鹽濃度下的溶劑結構、還原行為和固體電解質界面（SEI），包括傳統電解液（2M LiTFSI/EC/EMC）、高濃度電解液（6M LiTFSI/EC/EMC）和局部高濃度電解液（2M LiTFSI/EC/EMC/TTE）。本研究首先考慮了TTE對電解液系統之溶劑結構的影響，然後研究了這些溶劑結構在銅電流收集器界面的反應。

    計算結果顯示在純電解液系統中，TTE與鋰離子之間的相互作用是研究的所有物種中最弱的。它在LHCE系統中位於第二層次的溶劑化結構，有效地保持了HCE的團簇結構和其特徵。然而，添加TTE部分改變了每個鋰離子周圍的配位環境，從而影響了電解液的還原/氧化性質。此外，三維網絡結構被破壞，導致導離率增加。TTE分子的加入，也顯著影響固體電解質界面（SEI）的形成和穩定性。TTE促進了氟化鋰（LiF）的加速形成並增加其含量。通過將TTE作為稀釋劑，可以促進更穩定的SEI成分的生成，並且能夠承受多次還原反應。這項理論研究探討了以TTE作為稀釋劑對LHCE電解液的溶劑化結構和動力學影響。

    The electrolyte is critical to the performance and cycling stability of high-voltage lithium-ion batteries (LIBs). High-concentration electrolytes (HCEs) (typically > 3M) have shown promising potential in improving the cyclic stabilities of anode materials under stringent conditions compared to conventional electrolytes (typically > 1~2 M). However, HCEs face several challenges in practical applications, including high viscosity, poor wettability, low ionic conductivity, poor low-temperature performance, and high costs, which are attributed to a unique solvation structure. To further advance HCEs, localized high-concentration electrolytes (LHCEs) have been developed by introducing diluent into HCEs. Herein, an advanced electrolyte is considered that includes the addition of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) as a diluent in salt/carbonate-solvent-based electrolytes (LiTFSI/EC-EMC). The solvation structure, and the reduction behaviors and SEI formation are explored with different salt concentrations such as conventional electrolyte referred to here as dilute (2M LiTFSI/EC/EMC), HCE (6M LiTFSI/EC/EMC), and LHCE (2M LiTFSI/EC/EMC/TTE) using density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. This study first considers the role of TTE on the solvation structures of systems with only electrolytes (dilute, HCE, LHCE) and then investigates when they are exposed to the Cu current collector surface.

    In the pure electrolyte system, TTE exhibits the weakest interaction with lithium ions among all the species studied. It occupies the second shell solvation structure in LHCE system, effectively maintaining the salt-solvent clusters of HCE and the characteristic features of HCE. However, the addition of TTE partially alters the coordination environment surrounding each lithium ion, thereby influencing the reductive/oxidative processes of the electrolytes. Additionally, the three-dimensional (3D) network structure is disrupted, resulting in a decrease in the population of aggregate species (AGG) and an increase in ionic conductivity. The inclusion of TTE molecules significantly affects the formation and stability of the solid-electrolyte interphase (SEI) components. TTE facilitates the accelerated formation of lithium fluoride (LiF) and enhances its abundance. By incorporating TTE as a diluent, the generation of more stable SEI components is promoted, capable of enduring multiple reduction reactions. This theoretical study offers a detailed atomic-level understanding of the effects of TTE as a diluent on the solvation structure and dynamics of LHCE electrolytes.

    List of Figures vii List of Tables x Chapter 1 Introduction 1 1.1 Lithium-Ion Battery 1 1.2 The Working Principle of Lithium-ion Battery 4 1.3 Main Components of Li-ion Battery 5 1.3.1 Anode 7 1.3.2 Cathode 9 1.3.3 Electrolyte 11 1.3.3.1 Lithium-salt 13 1.3.3.2 Solvents 16 1.4 Conventional electrolyte 20 1.5 High-Concentration Electrolytes 21 1.6 Localized High-Concentration Electrolytes 23 1.7 Diluent 25 1.7 Present Study 27 Chapter2 Computation Details 28 2.1 Model 28 2.2 AIMD simulation 30 2.3 Binding energy 32 Chapter 3 The influence of TTE as diluent on solvation structure and reduction behavior of electrolytes 33 3.1 Introduction 33 3.2 Coordination Behavior Analysis 34 3.2.1 Radial Distribution Function 34 3.2.2 The Coordination Site Analysis 35 3.2.3 The Coordination Number Analysis 41 3.2.4 Aggregate network structure 45 3.3 Reductive Behavior Analysis 55 3.4 Binding Energy Analysis 57 3.5 Conclusions 59 Chapter 4 The influence of TTE as diluent on the reactivity on the copper anode surface 61 4.1 Introduction 61 4.2 Reactivity on the copper anode surface 63 4.2.1 Conventional electrolyte 63 4.2.2 High concentration electrolyte 68 4.2.3 Localized high concentration electrolyte 73 4.2.4 Trajectories of lithium ions 78 4.3 Conclusions 80 Chapter 5 Summary 82 Reference 83

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