分子動力學探討溶劑殼對碳酸酯電解液中離子傳輸性質的影響

Abstract

近年，科技對於各式儲能設備的續航力要求飛速成長，傳統以石墨作為負極主要材料的鋰離子電池(lithium ion battery, LIB)已難以滿足需求，因此許多團隊轉而研究開發穩定的高電容量電池。以鋰金屬作為負極的鋰金屬電池(lithium metal battery, LMB)成為近期的熱門話題，由於其理論電容量高出鋰離子電池的10倍之多，達到3860mAh/g之譜。儘管如此，LMB尚未有良好的商業化，由於其潛在的問題仍需解決，如在鋰金屬負極表面經過多次的充放電循環後產生的枝晶(dendrite)問題，將造成庫倫效率(Coulombic efficiency, CE)降低，甚至枝晶持續生長後將可能刺穿隔離膜(separator)而導致短路的風險，因此如何良好抑制枝晶生長是LMB朝向商業化的一大關鍵。 枝晶的生長，可歸因於電解液與鋰金屬負極表面自發性還原反應所生成的固態電解質介面(solid electrolyte interphase, SEI)，其機械性質不穩定所致，進一步使得鋰原子不均勻的沉積。因此電解液的配方將會大幅影響SEI生成後的成分與性質。近年已有許多研究表明在電解液中加入不同的鋰鹽或鹽添加劑可以改變鋰原子沉積時的型態，其原因在於電解液中的鋰離子溶劑殼結構發生改變，使得SEI成分由溶劑分子主導轉為以鋰鹽為主導，達到抑制枝晶生長的效果。本研究使用反應分子動力學(reactive molecular dynamics, RMD)模擬由碳酸乙烯酯(ethylene carbonate, EC)為電解液，當中添加六氟磷酸鋰(lithium hexafluorophosphate, LiPF6)與硝酸鋰(lithium nitrate, LiNO3)兩者在電解液中的動態表現，並探討不同配方之下所形成的鋰離子溶劑殼結構。此外，溶劑殼結構的改變，意味著鋰離子在電解液中的移動行為會受到影響，其牽涉到電解液中重要的性能表現---離子擴散係數與離子電導率。因此本研究也開發了一套針對不同溶劑殼結構進行分類並量化的篩選模型，最終以數值化的方式來探討不同鋰鹽所形成之溶劑殼結構對於鋰離子擴散行為的影響。 結果顯示在EC+LiPF6的電解液中鋰離子周圍的EC分子會形成一個主要與兩個次要的特徵結構，主要特徵結構來自於溶劑分離離子對(solvent-separated ion pairs, SSIP)的形成，兩個次要的特徵結構是由反應分子動力學模擬參數表造成EC分子分解並與鋰離子結合後產生的副產物所造成。溶劑殼結構隨著鋰鹽濃度產生變化，低濃度下溶劑殼中溶劑分子較多，隨後隨著濃度上升而降低，由於負離子進入溶劑殼當中而替代了溶劑分子；EC+LiNO3的電解液中則並不具有明顯的溶劑結構，由於LiNO3於本研究的模擬系統的電解液中並未解離，因而無法形成溶劑殼。儘管如此，透過本團隊開發的篩選模型，可將不同鋰鹽濃度下的各個溶劑結構型態量化成比例，並且將比例、離子擴散係數、離子電導率三者進行比較。結果表明在低濃度下，儘管鋰鹽解離比例較高，也具有較高的離子擴散係數，但最佳的離子電導率並不在越低的濃度，這是因為離子電導率是由電荷載子數量、帶電量以及遷移能力三者最佳化後的結果。本研究以鋰離子溶劑殼結構的角度出發，探討不同配方所造成的現象，從而理解電解液的組成對於SEI成分或是離子電導率的影響，並對於電解液中鋰鹽濃度最佳化有更進一步的了解。

The development of stable high-capacity lithium metal battery (LMB) become hot topic since its anode is composed of lithium metal with a theoretical capacity of up to 3860mAh/g. However, LMB has not yet been commercialized well due to safety issues such as dendrite problem generated on the surface of the lithium metal anode, which will result in a decrease in Coulombic efficiency (CE) and high possibility that may pierce the separator and cause the risk of short circuit. Therefore, inhibiting the dendrite growth is a key to the commercialization of LMB. The growth of dendrites can be attributed to unstable mechanical properties of the solid electrolyte interphase (SEI). Previous studies have shown that adding different Li-salts or salt additives can change deposition of lithium ions on anode. The reason is that the structure of the solvation shell changes, which also affects the composition of SEI and achieves the goal which inhibiting dendrite growth. In this study, reactive molecular dynamics (RMD) was used to simulate two system: ethylene carbonate (EC) with lithium hexafluorophosphate (LiPF6) and lithium nitrate (LiNO3) separately, and discuss the lithium ion solvation shell structure formed under two kind of electrolyte condition. In addition, the change of solvation shell structure will affect the ion diffusivity & conductivity. Therefore, this study also developed a model to classify and quantify solvation shell structure, and numerically explored the influence of solvation shell structure on the transport properties of lithium ions. The results show that there is a main characteristic structure formed by solvent-separated ion pairs (SSIP) and two minor structures caused by the byproducts of EC molecules decomposition in EC+LiPF6 electrolyte, and the structure changes with the concentration of Li-salt due to exchanging of anions and solvent molecules inside solvation shell. EC+LiNO3 electrolyte did not have an obvious solvation shell owing to LiNO3 has not been dissociated. Through the developed model, the various solvation shell types are quantified into proportion, and further compared with ion diffusivity & conductivity. The results show that ion transport properties are optimized by the number of charge carriers, charge capacity and mobility. From the perspective of solvation shell effects, this study discussed different solvation shell structure and the influence on ion transport properties, and have better understanding of optimizing the concentration of Li-salt in electrolyte.