Abstract
碳極表面鈍性膜的存在，對於鋰離子二次電池的性能、循環壽命與電容量均有相當程度的影響，也可以說是鋰離子二次電池成敗的關鍵部份。因此本文首先利用FTIR光譜分析與GC-MS質譜分析，探討碳極表面的鈍性膜生成機構，鑑定有機電解液還原在碳極表面之產物，分析結果顯示在EC(ethylene carbonate)單一溶劑中，碳極表面所產生的鈍性膜結構為(CH2OCOOLi)2，在DEC(diethyl carbonate)或DMC(dimethyl carbonate)單獨使用時，其還原在碳極表面的鈍性膜結構分別為C2H5OCOOLi與Li2CO3。而GC-MS的分析結果發現EC分解時產生CO2的氣體，DEC分解時產生CO與C2H6氣體，DMC分解時產生CO與CH4的氣體。此外，我們進一步發現在EC溶劑為主的二元有機電解液中，碳極表面所產生的鈍性膜結構仍為(CH2OCOOLi)2，和使用純EC的有機電解液系統所產生的鈍性膜結構完全相同。本文並利用交流阻抗分析技術探討碳極表面與有機電解液之界面現象，採用五個RC電路的模型來說明碳極與有機電解液間的界面性質。分析結果發現在單一有機溶劑EC或DMC的電解液中，界面總阻抗與鈍性膜厚度隨著嵌入電位的降低而增加；相反的DEC電解液系統卻隨嵌入反應的進行，其界面總阻抗與鈍性膜厚度越來越小。同時我們也發現在單一有機溶劑系統中，鈍性膜外側的多孔性質對界面總阻抗有決定性的影響，此外當第一次嵌入反應完成後，界面總阻抗與鈍性膜厚度就不再大幅變化，顯示鈍性膜的性質已經相當穩定。當二元有機溶劑EC/DEC與EC/DMC做為電解液系統時，鈍性膜外側的多孔性質對界面總阻抗之影響不大，其界面總阻抗與鈍性膜厚度遠較單一溶劑DEC (或DMC)系統為低。同時EC/DEC之界面總阻抗與鈍性膜厚度之變化趨勢接近DEC的行為。同樣的EC/DMC電解液系統之界面總阻抗與鈍性膜厚度的變化也較接近DMC的行為。最後我們還利用旋轉電極分析鋰離子在鈍性膜與碳電極中的擴散行為。This work examines the formation of passive film on the carbonelectrode of lithium batteries. According to those results,with a single solvent of EC (ethylene carbonate), thestructure of the passive film was found to be (CH2OCOOLi)2on the carbon electrode's surface. In the DEC (diethylcarbonate) or DMC(dimethyl carbonate) system, C2H5OCOOLi and Li2CO3 were formed on the carbon electrode's surface. According tomass spectra, CO2 gas is the main product when EC isdecomposed. In addition, DEC is decomposed into CO and C2H6and DMC into CO and CH4 as well. Those results suggest thecomposition ofpassive film depends on the solvent's property. Ina binary solvent system which contains EC, the passive filmstill contains chiefly (CH2OCOOLi)2, which is identical toa single EC solventsystem. Thiswork examines the interface between carbon electrode and organicelectrolyte of lithium ion battery by AC impedance technology. Afive RC circuits model was adopted to investigate theinterfacial property. It was found in EC or DMC electrolytesystem, the total interfacial resistance and the passive film's thickness increase with a decreasing intercalationpotential. However, in DEC system, the total interfacialresistance and the passive film's thickness decrease asintercalation proceeds. The porous layer of the passive filmobviously affects the total interfacial resistance in a singlewhich organic electrolyte. Furthermore, the total interfacialresistance and the passive film's thickness remain constantafter the initial intercalation. This finding suggests that thepassive film's property is steady in a single electrolytesystem. In the binary electrolyte system such as EC/DEC or EC/DMC system, the effects of porous parts on the totalinterfacial resistance are slight. However, the totalinterfacial resistance and the passive film's thickness aresmaller than that in a single electrolytesystem. In addition,the variation of the total interfacial resistance and thepassive film's thickness in EC/DEC (or EC/DMC) electrolyte aresimilar to DEC (or DMC) electrolyte during intercalationprogress, respectively.