| Abstract: | 由於磷酸亞鐵鋰(LiFePO4)之污染低、成本低、熱穩定性高，以及循環壽命長，且可耐大電流放電等優點，使其成為前景看好之陰極材料之一。然而導電度低、量產不易、振實密度低與鋰離子擴散速率慢，則為此材料固有之缺點。近年來，許多學者紛紛使用各種碳源進行表面改質，以改善LiFePO4導電度不佳之缺點，並增進其電化學性能。本論文利用不同粒徑之聚苯乙烯(Polystyrene, PS)微球為碳源，對LiFePO4進行表面改質，以探討碳源粒徑對LiFePO4/C複合陰極材料之影響。 由SEM下觀察LiFePO4/C粉體，可發現以最小粒徑0.22 μm PS球為碳源之複合陰極材料，遏止粒子間團聚之效果最佳，LiFePO4/C粒子分布介於100－600 nm之間。此材料於充放電截止電壓分別為4.0與2.8 V，0.2 C下，第一次放電電容量為145 mAh g-1，為所有材料中最高者。然而其循環壽命僅為289次，為所有材料中最短者。相反的，以粒徑最大之2.75 μm PS球為碳源，所合成之LiFePO4/C複合材料，則具有較高導電度(4.44 S cm-1)及表面積(20.58 m2 g-1)。此材料於相同充放電條件下之初始放電電容量僅有132 mAh g-1，但循環壽命卻長達755次。吾人亦利用XRD、SEM、TEM/EDS/SAED、拉曼光譜與慢速循環伏安法等鑑定，以釐清碳源粒徑對LiFePO4/C複合材料之物化性與電池性能影響。 LiFePO4 is a promising alternative cathode material because of its low material cost, environmental friendliness, superior thermal safety, and long operational life. However, the main problem with LiFePO4 is its hard to scale up, low electronic conductivity, low tap density, and slow lithium ion mobility. In recent years, many researchers have used carbon coating to overcome the low electronic conductivity. They have tried various techniques and organic materials to optimize the effect of carbon coating on the cell performance of LiFePO4/C. In this thesis, different sizes of polystyrene (PS) spheres were employed as carbon sources to synthesize the carbon-coated LiFePO4 and to study how sphere size impacts the electrochemical properties of LiFePO4/C composites. From SEM images, PS sphere with smallest size of 0.22 μm were more effective at preventing the aggregation of LiFePO4 particles. LiFePO4/C with 0.22 μm PS sphere as a carbon source delivered the highest first cycle discharge capacity of 145 mAh g-1 at a 0.2 C. However, it just maintained 289 cycles 80 % capacity retention. On the other hand, the composite prepared using the largest size (2.75 μm) PS sphere, with greater electronic conductivity (4.44×10-4 S cm-1) and surface area (20.58 m2 g-1), showed the lowest initial discharge capacity of 132 mAh g-1, but longest cycle life of 755 cycles. In order to understand how carbon coating influences the electrochemical properties of LiFePO4/C composite cathode materials, we analyzed them using several kinds of instruments, such as XRD, SEM, TEM/EDS/SAED, TOC, BET, Raman spectroscopy, four-point probe conductivity measurements, cyclic voltammetry, and so on. |
