利用分子動力學研究鋰金屬電池中固態電解質界面之機械性質

Abstract

鋰金屬電池與現今被廣泛地使用在3C產品上的鋰離子電池不同的地方在於將電池中的負極材料由石墨轉換為能夠提供更高能量的鋰金屬，因此理論上相較於鋰離子電池能夠提高2∼5倍的能量密度。然而，鋰金屬電池至今仍未商業化的關鍵點在於電池反覆充放電時鋰在負極表面上的不均勻沈積。此種現象將會嚴重影響電池中固態電解質界面(Solid Electrolyte Interphase, SEI)的穩定性，導致在負極表面上會產生樹枝狀的枝晶（dendrite），甚至可能造成電池內部短路而有爆炸的危險。現今鋰離子電池經常使用碳酸乙烯酯(ethylene carbonate, EC)和碳酸二乙酯(diethyl carbonate, DEC)作為電池內部的電解液系統，但是當我們將此電解液系統沿用在鋰金屬電池上會發現電池的固態電解質界面並不穩定，因此找尋適用於鋰金屬電池的電解液系統成為了迫切的問題。近年來，有研究團隊發現當電解液系統中含有氟代碳酸乙烯酯(fluoroethylene carbonate, FEC)與硝酸鋰(lithium nitrate, LiNO3)能使電池在充放電過程中形成穩定的固態電解質界面，進而有效地抑制枝晶的生長。本論文使用能夠考慮化學反應的反應力場(Reactive Force Field, ReaxFF)模擬固態電介質界面(SEI)在傳統EC/DEC、EC/DEC/LiNO3、EC/DMC/LiNO3以及FEC/DMC/LiNO3此四種不同的電解液系統中之生成情形，並且將四種樣本分為前二與後二兩組以便有效地研究LiNO3與FEC如何個別影響固態電解質界面的機械穩定度。我們針對這四種固態電解質界面(SEI)分別進行拉伸試驗並量測其韌性值來驗證電解液系統中的成份與固態電解質界面的材料行為之間確實存在著關聯性。經過模擬結果顯示加入LiNO3添加物將會使得所生成之固態電解質界面具有更多有機物成份，使其延展性提升。而若是將電解液系統中之EC分子替換為FEC分子將能夠同時提升固態電解質界面有機物與無機物之成份，使其機械強度與延展性獲得大幅提升。本論文提供了一個新觀點來探討鋰金屬電池內固態電解質界面的機械穩定度與其對於枝晶生長的抑制機制。

When choosing lithium metal as the anode material instead of graphite, the lithium metal batteries have in theory 2∼5 times energy density of today’s widely used lithium ion batteries. However, the lithium deposition in lithium metal batteries is often nonuniform during cycles, which could affect the stability of the Solid Electrolyte Interphase (SEI), leading to the growth of dendrites or even a short circuit. The formation of the SEI layer is unstable in the ethylene carbonate (EC) and diethyl carbonate (DEC) electrolyte, which is one of the often-used electrolyte systems in today’s lithium ion batteries. Recent experiments found that the formulation of electrolyte containing fluoroethylene carbonate (FEC) and lithium nitrate (LiNO3) can effectively inhibit dendrite growth during cycling and this is attributed to the improvement of the mechanical strength and flexibility of the SEI layer. This study uses reactive MD simulations to investigate the formation of the SEI layer in four kinds of electrolyte systems including conventional EC/DEC electrolyte, EC/DEC/LiNO3, EC/DMC/LiNO3 and FEC/DMC/LiNO3. From the first two and latter two formulations, we want to find out how LiNO3 and FEC influence the mechanical stability of SEI respectively. We impose tensile test on these SEI layers and measure their toughness to show that there is a relation between the electrolyte components and the mechanical behavior of the SEI layers. We find out that adding LiNO3 additives could generate more organic species in SEI and thus increases its ductility. Also, when we change EC molecules into FEC in the electrolyte system could generate more organic and inorganic species in SEI at the same time and increases both mechanical strength and ductility. This study provides insight into the mechanical stability of the SEI layer and the suppression mechanism of dendrite formation in lithium metal batteries.