鋰離子電池具有體積小、工作電位廣且無記憶效應的優點，被廣泛應用在便於攜帶的電子產品，且充電速度快、能量密度大及環保等特性，使其在電動車的電源供應上受到重視。但其仍存在著安全性問題，有待改善。碳酸丙烯酯 (Propylene carbonate, PC) 及六氟磷酸鋰 (Lithium hexafluorophosphate, LiPF6) 為常用的電解液組成，正極材料及六氟磷酸鋰具高吸濕性，水分子會促使氫氟酸 (HF)的生成，進而腐蝕正極材料，使電池衰退速度加快。因此，水分子在氧化分解機構中，所扮演的角色值得探討。
此研究利用理論計算軟體 Gaussian09 中的密度泛函理論（Density Functional Theory, DFT），計算不同系統的最佳結構，比較其氧化電位及吉布斯自由能 (Gibbs Free Energy) 等性質，所有計算皆考慮溶劑效應；同時，亦使用計算軟體VASP，進行第一原理分子動力學(Ab-initio MD) 的模擬，來探討水分子在系統中，與其它組成的作用力方式。從Ab-initio MD的計算結果，發現不同時間點的計算結構，與DFT的計算結構相似，確認DFT的計算結構合理。最後進一步探討PC的氧化分解機構，了解水分子如何影響PC分解。我們發現水分子的涉入，會大幅降低PC的氧化電位，使系統更容易產生二氧化碳，對鋰離子電池的性能，具有負面影響。計算結果顯示丙酮自由基及丙醛自由基為初步產物，最終會得到丙酮、丙醛及聚碳酸酯等，這些產物皆已在實驗上被觀測到。

Lithium-ion rechargeable batteries are widely used in portable electronic devices as compared to the conventional batteries due to their high energy density and good environmental properties. However, there are still many challenges concerning the safety requirements. Propylene carbonate (PC) and lithium hexafluorophosphate (LiPF6) have been extensively studied as electrolyte compositions for lithium ion battery. At open-circuit voltage and during cell charging, the high potentials of several cathode materials may result in electrolyte oxidation. These highly exothermic electrolyte decomposition reactions are usually accompanied by the decomposition of the cathode material and gas evolution that will cause potential danger and reduced cyclic stability. The presence of trace amount of water is also believed to enhance the electrolyte decomposition reactions.
In this study, we investigate the oxidative decomposition mechanisms of PC/LiPF6 with water using density functional theory methods. The solvent effect is also included using the implicit solvation model with density method. Based on the calculation, there is a significant difference in the oxidative decomposition mechanism of [H2O-PC-PF6] - compared to [PC-PF6]-. For the [PC-PF6]-, HF is formed at the initial oxidation step; whereas, in [H2O-PC-PF6] - the elementary oxidation reactions, such as H3O+ formation and C-C bond-breaking happened depending on the position of water. The presence of H3O+ which could act as an acidic catalyst would accelerate the PC ring open process. The energy barrier for the ring-opening step of [PC-H2O-PF6] was found to be much lower than that of [PC-PF6]. Our calculations suggest that, the initial oxidative decomposition products of PC in the presence of water are acetone radical, propanal radical and carbon dioxide. We also performed the molecular dynamic simulations to confirm the results from DFT calculation.

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