反應分子動力學探討鋰鹽對固態電解質界面機械性質之影響

Abstract

鋰金屬電池因為擁有較大的能量密度，所以有望在未來能取代鋰離子電池成為攜帶式裝置中的儲能方式。然而，在鋰金屬電池正式取代鋰離子電池之前仍然有許多問題等待克服，例如：負極的體積變化與枝晶的生長等，其中，當負極體積發生變化時會使固態電解質界面(solid electrolyte interphase, SEI)產生形變最終造成SEI破裂，進而引起電解液的過度消耗與枝晶的生長，因此SEI是否擁有足夠的機械強度以抵抗負極體積的變化是鋰金屬電池能否取代鋰離子電池的關卡之一。

SEI為電解液在鋰金屬負極表面發生還原反應形成的鈍化層，使用不同的電解液會使其組成成分與結構改變，進而影響到SEI的機械性質，因此本研究選用碳酸乙烯酯(ethylene carbonate, EC)與碳酸二乙酯(diethyl carbonate, DEC)作為溶劑；LiPF6作為鋰鹽；並選用能抑制枝晶生長的功能性鋰鹽LiNO3作為添加劑。最終，我們以EC與DEC作為溶劑，分別探討添加LiPF6與LiNO3對於SEI組成造成的變化，同時使用拉伸試驗與奈米壓痕試驗(nanoindentation)檢測不同配方對於SEI機械性質的影響。

研究結果表明，在以EC與DEC作為溶劑的電解液配方中添加LiPF6或LiNO3會使得生成的SEI變薄，同時也會使其孔洞增加。為了檢測SEI之機械性質，我們使用拉伸試驗與奈米壓痕試驗對其進行檢測，在拉伸試驗與奈米壓痕試驗的結果中顯示，當在電解液裡添加LiPF6與LiNO3時均會使SEI之機械強度下降。為了對SEI機械性質有更深入的研究，本團隊在未來將持續優化奈米壓痕試驗之進行方式，以提供不同的SEI之機械性質檢測方法，並期望能加速鋰金屬電池商業化的過程。

Lithium metal batteries have a higher energy density, and are expected to replace lithium-ion batteries as the energy storage device in the future. However, before lithium metal batteries can become a viable alternative, there are still many challenges to overcome, such as volume changes of anode and dendrite growth. The volume changes of the anode can cause deformation on the solid electrolyte interphase (SEI), eventually leading to SEI rupture, excessive consumption of electrolyte, and a decrease in cycle life. Therefore, whether the SEI can resist the volume changes of the anode is one of the main obstacles for lithium metal batteries to replace lithium-ion batteries.

SEI is a passivation layer that forms on the surface of the lithium metal anode through the reduction reaction of the electrolyte. The composition and structure of the SEI can be influenced by the choice of electrolytes. For this study, we choose ethylene carbonate (EC) and diethyl carbonate (DEC) as solvents. LiPF6 is chosen as the lithium salt, while LiNO3 is selected as an additive to inhibit dendrite growth. Our objective is to investigate the effects of LiPF6 and LiNO3 on the composition of the SEI generated from EC and DEC solvents, and to evaluate the mechanical properties of the resulting SEI using tensile testing and nanoindentation.

The study indicates that after adding LiPF6 or LiNO3 to the electrolyte, the thickness of SEI decreases. Additionally, we found that adding LiPF6 and LiNO3 to the electrolyte increases the porosity of the SEI. To measure the mechanical performance of the SEI, we use tensile testing and nanoindentation. In the results, the addition of LiPF6 and LiNO3 to the electrolyte causes a decrease in the Young's modulus and toughness of the SEI. In order to gain deeper insights into the mechanical performance of the SEI, we will continue to optimize the methodology of nanoindentation. This will enable the development of diverse approaches to assess the mechanical performance of different SEI.