- 學位論文
碳-矽複合負極材料的製備應用於鋰離子電池之研究
The Preparation and Applications of Carbon-Coated Silicon Composites as Negative Electrodes for Lithium Cells
摘要
矽基材是新一代鋰離子電池負極材料的發展重點，目前廣泛使用的石墨電極其理論電容量僅372mAh/g，石墨較低的理論電容量成為未來發展高電容鋰離子電池的瓶頸，然而，矽具有非常高的比電容量(理論比電容量: 4200 mAh/g)，非常符合未來電池的發展趨勢，極具取代傳統石墨電極的潛力，但矽基材在電池循環的過程中會伴隨劇烈的體積膨脹效應(~350%)，多次循環後電極材料容易龜裂使電性接觸變差，加上矽基材本身導電度不高，這些原因造成純矽負極材料在發展上受到限制。 本研究將導電石墨導入碳矽複合材料的開發，以天然的蜂蜜作為碳前驅物，進行奈米矽基材與石墨的披覆程序，披覆在矽基材外的不定型碳，可以保護矽基材，解決在循環過程中的體積效應，石墨的高導電性也能彌補矽原料基材不導電的缺點。經過電池循環測試後，蜂蜜、奈米矽基材與導電石墨KS-6混和比例為5:1:0.25燒結製成的複合材料，表現最佳，首次去鋰化(delithiation)比電容量高達1341mAh/g，經過51次循環後去鋰化(delithiation)比電容量仍有711mAh/g，電容量保持率約50%，此外，每圈循環的庫倫效率都有95%以上的水準；另一方面，蜂蜜與奈米矽基材為5:1，但未添加導電石墨的複合材料，其51次循環後的電容量保持率僅有23%，故加入導電石墨一起燒結後能有效降低不可逆(irreversibility)電容量及極化(polar-ization)現象等提升電池效能，另一方面，導入導電石墨後能夠遏止在碳矽複合材料在電池循環中的體積效應，51次電池循環後。材料的體積增加率僅有189 %。這項研究對未來鋰離子電池負極材料的發展，提供很好的研究方向，導入導電石墨於碳矽複合材料中，能夠顯著提升電池循環過程中的穩定性，提高電池的使用壽命。
並列摘要
Silicon is regarded as one of the most promising anode materials for Li-ion batteries due to its high theoretical capacities (~4200mAh/g), which is 10 times higher than conventional graphite (372mAh/g) used in commercial Li-ion batteries. However, Si-based Li-ion battery exhibits low intrinsic electrical conductivity and dramatic volumetric variation (~350%) during the lithiation and delithaition process. To improve the stability of Si-based anodes, carbon-coated Si composite materials have been developed. In this study, we prepare a graphite/carbon-coated Si nanoparticles hybrid to overcome the poor cycling stability and enhance the intrinsic electrical conductivity. Efficient and low-cost thermal treatment is introduced to prepare new anode materials. Initially, honey, which is adopted as carbon precursor, is mixed with Si nanoparticles and graphite via ultrasonication. Next, amorphous carbon coats Si nanocomposites prepared by pyrolysis at 1000 oC for 2 hr in Ar atmosphere. Within the materials, Si nanoparticles are wrapped evenly among amorphous carbon layers and firmly attached to graphite, which indicate the strong interactions among Si nanoparticles、amorphous carbon and graphite. This structure effectively suppress the huge volumetric variation (189% after 50 cycles ) and improve the cycling stability. In addition, it keep the electrode materials highly conductive. Consequently, the composites (honey:nano-Si: KS-6 ,5:1:0.25 in weight) exhibit outstanding specific capacity for cycling performance of 711mAh/g after 51 cycles at 100mA/g . The graphite/carbon-coated Si nanoparticles hybrid with distinguished electrochemical performance is prepared by a low-cost, simple pyrolysis method, making it greatly promising for a large-scale production of high-performance Si-based anode materials.