利用臨場穿透式X射線顯微鏡探討鋰金屬於高濃度電解液電鍍及剝除機制之研究

In situ transmission X-Ray microscopy studies of Li plating and stripping mechanism in highly concentrated electrolyte

王俊堯(Wang Chun-Yao)

指導教授 ： 陳家俊 ； 陳貴賢 ； 吳恆良

國立臺灣師範大學/理學院/化學系/碩士(2019年)

https://doi.org/10.6345/NTNU201900516

摘要

由於能量的需求日漸上升，鋰金屬被預期是下一世代高能量密度存儲系統中重要的陽極材料之一。然而，以鋰金屬作為陽極的二次電池，在充放電的過程中會產生枝晶結構的鋰金屬，導致循環效率低落和安全上的疑慮，導致鋰金屬至今仍然很難被應用於商業化的鋰二次電池。
近年來，高濃度電解液被認為是能提升鋰金屬二次電池充放電效能及循環效率的一大關鍵，因此開發新型態的電解質系統是儲能領域中的熱門題目。其中高濃度的電解液系統廣泛被用於鋰離子電池中。高濃度電解液系統除了可以提升電池的循環穩定性及提供優異的性能外，還可以抑制鋰金屬電池中陰極材料產生的許多問題(例如:增加陰極材料的充放電速率、電容值及穩定性；鋰硫電池中多硫化物的穿梭效應)，促使高濃度電解液系統廣泛的被進行開發及探討。儘管高濃度電解液系統對陰極材料之效能有顯著提升，但目前較少研究探討超高濃度的電解液系統對鋰陽極的效應與鋰金屬電鍍和剝除之間的關係。
本研究中，我們建立了臨場的X射線穿透式顯微鏡的技術，並利用該技術探討以乙腈、四氫呋喃的和二甲氧基乙烷作為溶劑的高濃度電解液系統，對鋰金屬電鍍和剝除機制的影響。從影像結果顯示，以乙腈作為溶劑的高濃度電解液在充放電循環後，表面鋰金屬呈現針狀；然而，在四氫呋喃的和二甲氧基乙烷系統中，表面鋰金屬則呈現球狀或棒狀。此結果也對應到以乙腈為溶劑之高濃度電解液系統相對低落的充放電循環庫倫效率。此外，為了探究不同高濃度電解液系統導致鋰金屬生長機制改變的原因，我們進一步利用電化學阻抗分析以及表面元素組成分析儀進行一系列的探討。最後，我們得到這三種高濃度電解液在充放電循環過程中，固態電解質界面之分子組成差異（C-F及Li-F之空間分布及元素比例）是造成表面鋰金屬生長機制不同的關鍵因素。

關鍵字

鋰陽極 ； 鋰晶枝成長 ； 高濃度電解液 ； 電鍍剝除 ； 固態與電解質介面

並列摘要

Lithium metal has been strongly considered as one of potential anode materials in lithium battery for high-energy-density storage devices in the future. However, dramatical lithium dendrite formation during charge/discharge leads to poor cycling efficiency and safety concerns which cause the difficulties to commercialize lithium metal batteries with lithium anode.
Highly concentrated electrolyte is widely utilized in lithium secondary batteries in recent years, because it can provide higher cycling stability and superior Columbic efficiency; it could also suppress many problems generated from cathode side in lithium metal battery. Although highly concentrated electrolyte could significantly improve the efficiency and capacity of cathode materials, only few studies have paid their attention on the effect of lithium plating and stripping in highly concentrated electrolyte.
In this study, we report an in-situ transmission X-ray microscopy (TXM) technique to study the mechanism of lithium plating and stripping in various highly concentrated electrolytes. According to our TXM images, different lithium metal formation behaviors were observed in acetonitrile-based, tetrahydrofuran-based and 1,2-dimethoxyethane-based highly concentrated electrolytes. The TXM images could also be correlated to battery performance including Columbic efficiency and galvanostatic results. In order to figure out the key reason guiding lithium formation mechanism, we further explore the surface solid electrolyte interphase composition and electrochemical impedance by thickness-dependent X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy, respectively. Eventually, we demonstrate that the unique spatial and elemental composition of solid electrolyte interphase derived from three individual highly concentrated electrolyte systems is the crucial factor.

並列關鍵字

lithium anode ； lithium dendrite formation ； highly concentrated electrolyte ； lithium plating and stripping ； solid electrolyte interface

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