Application of Electrospinning and Electrospray to the Fabrication of Anode Materials for Lithium-Ion Batteries

在過去的十年當中，由於矽相較於石墨而言，具有絕佳的比電容量（~3500 mAh/g），而受到重視。然而矽在充放電的過程當中，伴隨劇烈的體積膨脹收縮與本身所具有的低導電度特性，使得矽負極材料在商業化應用上受到了阻礙。 本論文之目的在於開發以矽為主體的鋰離子二次電池負極材料。根據前人的研究，我們可以知道矽碳複合材料可以改善上述矽應用在鋰離子電池所遭遇到的問題。本研究主要探討利用靜電紡絲和電噴霧技術分別製備出矽碳微米纖維和多孔性矽碳複合物。 矽碳微米纖維負極顯現出比純矽負極更好的循環壽命，這主要是因為一維碳纖維構成三維電極時有許多空間可以用來減緩矽在充放電時極大的體積變化。另一方面，多孔性矽碳複合物負極有更佳的循環壽命，充放電圈數可達到五百圈沒有衰退，這是由於它的多孔性結構提供很多預設的孔洞可以降低體積膨脹，且網狀似的碳包圍住奈米矽粒子，可避免矽粒子的聚集或剝落。 此外，本研究也利用兩種不同方法把鎳加入多孔性碳矽複合物中，這是由於鎳本身不會和鋰反應且它可以提高複合物的導電性，其結果顯示披覆鎳層的矽碳複合物有較好的庫倫效率和充放電能力。 本研究也利用預置鋰遷入法來改善第一圈的庫倫效率，實驗結果顯示，此方法確實有效地提高第一圈庫倫效率，因此它未來十分有潛力被實際應用在鋰離子電池的製程中。In the last decade, silicon has received much attention due to its highest specific capacity (~3500 mAh/g) compared to graphite (372 mAh/g). However, silicon is known for its intrinsic low conductivity while possessing a dramatic volumetric variation upon cycling, which results in structural instability and poor cycle life that hinders its further application. The main objective of this research is to explore new Si-based anode materials for lithium-ion battery. According to the previous studies, the Si/C composites are believed to overcome the inherent problems of silicon. This study focuses on using electro-hydrodynamic techniques (electrospinning and electrospray) to fabricate Si/C microfibers and porous Si/C composites. The Si/C microfibers electrode showed the improved cycle performance compared to the pure Si electrode because carbon fiber matrix offers the room to alleviate the huge volume expansion of nano-Si particles upon discharge and charge processes. On the other hand, the porous Si/C composites electrode exhibited the outstanding cycle life (500 cycles without decaying) due to its combination of unique porous structure and carbon matrix. The preset voids in the composites provide more available space to buffer the volume expansion of Si particles, while the net-shaped carbon matrix enclosing Si particles prevents them from aggregating and being detached off the composite upon cycling. Moreover, the nickel was incorporated into the porous Si/C composites by two different methods because nickel is inactive to lithium and could increase the conductivity of the composites. The results showed that Ni-coated Si/C composites electrode demonstrated the better overall coulombic efficiency and rate capability compared to the porous Si/C composites electrode. Finally, the pre-lithiation approach was adopted to enhance the coulombic efficiency in the first cycle. The result indicated that this approach is effective and has the potential for practical use