導電劑對碳-矽負極表現影響與固態複合電解質對鋰離子電池之研究

Abstract

本研究分為兩大部分，第一部分是將導電劑導入碳包覆程序以製備鋰離子電池之碳矽負極。第二部分為複合高分子電解質之研究。 有別於以往於配製漿料時額外添加導電劑，本研究將導電劑導入碳前驅物與回收矽混合並共同參與無氧燒結進行碳包覆程序，利用燒結過程使導電劑與活性材料之間有更緊密的接觸，進而建立良好的電子傳遞網路，其中木質素:回收矽:導電石墨KS-6以重量比1:1:2.91所製成的複合材料其首圈嵌入、嵌出電容量可達1055.5 mAh/gC-Si+KS-6、879.3 mAh/gC-Si+KS-6，庫倫效率為0.833，經過101圈充放電循環後，電容量保留率為62.6%，平均電容量衰退率為0.370%/cycle，於首圈便能展現其高電容量，而木質素:回收矽:導電碳黑Super-P以重量比1:1:2.91所製成的複合材料，其首圈嵌入、嵌出電容量為1042.9 mAh/gC-Si+Super-P、767.3 mAh/gC-Si+Super-P，庫倫效率為0.737，經過101圈充放電後，其電容量保留率為65.1%，平均電容量衰退率為0345%/cycle，可見透過導入導電劑參與燒結過程，能夠建立有效的導電網路利於電子轉移並減緩電容量衰退。 本論文的第二部分為複合高分子電解質之研究。首先比較不同鋰鹽濃度製備的聚氧乙烯(PEO)高分子電解質，隨著鋰鹽濃度的上升，其高分子結晶性下降，結晶區域減少，其中[EO](ethylene oxide)/[Li+] = 15之高分子電解質於60°C下，離子導電度可達2.65×10-4 S/cm，隨後以此比例混合成固態電解質，再摻雜不同重量比之陶瓷氧化物Li7La3Zr2O12。透過差式掃描分析(DSC)顯示隨著陶瓷氧化物的添加量增加，其熔點與玻璃轉化溫度呈先降後升的趨勢，並且於50 wt%添加量時達到最低點，顯示摻雜陶瓷氧化物能夠有效抑制高分子結晶，然而陶瓷氧化物表面容易與空氣中的微量水氣以及二氧化碳反應，透過拉曼分析顯示為Li2CO3之鈍化層，導致鋰離子傳遞受阻，因此隨著陶瓷氧化物的摻雜量上升，造成離子導電度下降，然而離子導電度並非影響其應用的唯一因素，摻雜陶瓷氧化物能有效的改善鋰金屬與電解質界面阻抗之問題，並且隨著添加量的提升，電解質之氧化裂解電位由4.42V上升至6.0V，顯示添加陶瓷氧化物於高分子中能夠有效改善其電化學穩定性。

This thesis contains two parts which are the carbon-silicon composite negative electrode for lithium-ion battery employing the conductive agent into calcination process and composite polymer-ceramic solid electrolytes. Besides adding conductive agents in paste formulation stage of negative electrode materials, the conductive agents employed together with carbon precursor and recycled silicon in calcination for the carbon-coated-silicon process. In this study, conductive agents and active materials have a closer contact in calcination process to build an efficient conductive network. It shows the lithiation and delithiation capacities of the first cycle 1055.5 mAh/gC-Si+KS-6 , 879.3 mAh/gC-Si+KS-6 with the coulomobic efficiency of 83.3% for the composition of lignin:recycled silicon:conductive graphite in mass ratio 1:1:2.91. The composite electrode exhibits capacity retention of 62.6% and average capacity decay rate of 0.370%/cycle over 101cycles. The electrode shows the lithiation and delithiation capacities of the first cycle 1042.9 mAh/gC-Si+Super-P , 767.3 mAh/gC-Si+Super-P with the coulomobic efficiency of 73.7% for the composition of ligin:recycled silicon:conductive carbon black in mass ratio of 1:1:2.91. The composite electrode of carbon-coated silicon with carbon black exhibits capacity retention of 65.1% and average capacity decay rate of 0.345%/cycle over 101cycles. It can be found that introducing the conductive agents in calcination stage helps to build a conductive network for charge transfer and reduce the rate of capacity decay. In the study of composite polymer-ceramic solid electrolyte, we compared the polymer electrolyte in various lithium salt concentration. With the increase of lithium salt concentration, crystallinities of the polymer electrolyte decreased. The ionic conductivity of the polymer electrolyte with [EO](ethylene oxide)/[Li+] = 15 is 2.65×10-4 S/cm at 60C. Then different amount of LLZO was added based on this ratio of polymer electrolyte. It can be found from DSC analysis that adding ceramic oxide inhibits the polymer crystallization effectively. However the surface of ceramic oxide react with the moisture and carbon dioxide to form Li2CO3 easily. This insulating layer can be seen by Raman spectroscopy. So the ionic conductivity decreases as the addition of the ceramic oxide increases. The addition of the ceramic oxide improves the interfacial resistance between lithium metal and polymer electrolyte. The oxidative decomposition voltage increases from 4.42V to 6.0V with increasing the addition of ceramic oxide. So the addition of ceramic oxide into polymer electrolyte can improve the electrochemical stability effectively.