分子動力學探討鋰金屬電池中負極形變對於固態電解質界面生長機制之影響

Abstract

因為鋰金屬具有很高的能量密度,許多研究將其視為下一代電池的負極材料。然而,鋰金屬負極跟電解液之間的電化學反應非常地不穩定,鋰原子在充放電的過程中會在負極表面不均勻的沉積,稱之為鋰枝晶(Dendrite)。過去的研究發現固態電解質界面(Solid Electrolyte Interphase, SEI)的組成和機械性質與枝晶有很大的關係。固態電解質界面是一層長在負極表面的結構,主要是由於電解液的分解和電極的氧化還原反應而產生的,因此不同的電解液所產生的固態電解質界面組成成分也會不同。過去的研究發現在充放電的過程中,負極會受到應力影響而變形甚至造成電極和固態電解液界面的破裂,破裂後電解液會重新跟變形的負極反應重新生長固態電解質界面,而鋰原子也會沉積進而使枝晶產生。因此,了解固態電解質界面在受變形的負極上的生成機制的和其機械性質十分重要。 為了研究負極形變對固態電解質界面生長機制的影響,本論文使用分子動力學之反應力場(Reactive Force Field, ReaxFF)模擬固態電解質界面的生成。本論文首先討論了以下兩種電解液對固態電解質界面的影響:碳酸乙烯酯(Ethylene Carbonate, EC)和碳酸二乙酯(Diethyl Carbonate, DEC)的混合溶液,以及添加氟代碳酸二乙酯(Fluoroethylene Carbonate, FEC)和六氟化磷酸鋰(Lithium Hexafluorophosphate, LiPF6)的電解液。研究發現含氟電解液可以大幅的減少氣體的生成,並且增加無機物的產量。藉由分析其中碳原子的分布,我們發現固態電解質界面可以被大致分為兩層,外層包含較多的有機物,而內層則較少。接著為了模擬在多次充放電後的電池,我們讓固態電解質界面在受變形的電極上生長,發現在受到擠壓的電極中,固態電解質界面的氣體含量下降,在後續的拉伸試驗中,也發現氣體含量較少的固態電解質界面相比於氣體含量交高的固態電解質界面有較高的韌性。本論文發現形變後的電極對於固態電解質界面的影響甚鉅,並且對於充放電循環後的固態電解質界面有更進一步的了解。

Lithium metal batteries (LMB) with lithium metal as anode have much higher specific capacity compares to the lithium-ion batteries; however, the chemical reaction at the anode/electrolyte interface is unstable and often leads to the non-uniform growth of lithium on the anode surface, namely dendrites. Studies suggest that the structure and the mechanical properties of the SEI, protective thin layer consisting of electrolyte decomposition products between the anode and the electrolyte, is the key to the dendrite formation. Studies suggest that the anode deformation during cycles could deform and eventually break the surface of the anode, including SEI, and the reformation of the SEI and dendrites take place at these rupture sites. We perform reactive MD simulation to study the effects of anode deformation on the SEI composition and its mechanical stability. Two electrolyte systems are used in this study: a mixture of EC and DEC with or without FEC and LiPF 6 as an additive, which is able to suppress the dendrite growth according to previous studies. We first find out that with fluorine additive in the electrolyte, SEI contains less gas but more inorganic components. Also, the spatial distribution of the carbon atom indicates that the SEI can be divided into inner and outer regions. To mimic the anode deformation after cycles, we grow the SEI with a deformed anode and the result shows that fewer gas molecules are observed in the SEI with the compressed anode. The further tensile test suggests that the SEI have larger toughness in the inner region and compressed anode. We believe this study provides a protocol for understanding the mechanism of SEI formation after cycles.