本論文主要研究以雙重碳源，高溫固相法製備橄欖石結構LiFePO4/C 複合陰極材料，期能利用碳塗佈於LiFePO4 表面的方式，改善該材料導電度不佳之缺點，增進其大電流充放電能力。首先針對不同煆燒溫度、煆燒時間、碳源添加量及鍍碳距離等變因進行電池性能測試，求出最佳製程條件；並利用各項材料鑑定，對添加單一碳源及雙重碳源合成之樣品進行各種物化性質探討與比較。 利用高溫固相法，添加20 wt.% 碳源A 及50 wt.% 碳源B 為雙重碳源，以250 oC/hr 之升降溫速率，600 oC 下煆燒12 h 合成LiFePO4/C，為本研究之最佳製程。LiFePO4/C 在充放電速率0.2 C 及充放電截止電壓分別為4.0 和2.8 V 時，其初次放電電容量為151 mAh/g，至電荷維持率80 % 時，循環壽命為481 次；於特徵曲線硬幣型電池測試中，於10 C 的高速充放電速率下，仍可得21 mAh/g 的電容量，顯示本材料於大電流充放電的情況下仍然具有良好表現。 在合成LiFePO4的過程中加入碳源，可抑制LiFePO4晶粒成長並提升電池性能，綜合樣品碳含量、比表面積及導電度的分析結果可知，加入碳量越多，可使材料得到更高的比表面積及導電度，但過多的碳反而會降低材料電容量，故添加最適量的碳源，使材料減少團聚，並均勻分散碳的分佈，為影響材料電容量的主要原因。此外，由拉曼光譜、四點探針導電度計及比表面積分析結果發現，添加雙重碳源之樣品具有較高的導電度與比表面積；由慢速循環伏安法計算材料鋰離子擴散係數，得知添加雙重碳源之樣品具有更佳的鋰離子傳導度，故可得到較佳的大電流充放電性能。 In this thesis, olivine LiFePO4/C composite cathode materials were synthesized by a solid state method using two different organic materials as dual carbon sources. In addition to disadvantages of low electronic conductivity and low lithium ion motion ability, LiFePO4 only reveals its good electrochemical properties at low current density and at room temperature. To overcome this problem, the LiFePO4 cathode material was coated with highly conductive carbon using dual carbon sources so that high power LiFePO4/C cathode materials could be obtained. The electrochemical studies concluded the optimum calcinations process for the synthesized LiFePO4/C composite material were 600 oC for 12 h in Ar/H2 (vol. 95:5) atmosphere with the addition of 20 wt.% carbon A and 50 wt.% carbon B as double carbon sources. The LiFePO4/C delivered a discharge capacity of 151 mAhg-1 in the first cycle at a 0.2 C-rate between 4.0 to 2.8 V, and the number of cycles sustained was 481 cycles at a preset cut-off value of 80% capacity retention. In the cyclability tests of LiFePO4/C electrodes at different charging and discharging rates, the capacity retention remained very good for all different rates, and it showed capacity about 21 mAhg-1 at a high charge/discharge rate of 10 C at room temperature. We have shown that carbon as a conductive additive plays an important role in improving the performance of LiFePO4 cathode materials. The results of BET and the four-point probe conductivity meter revealed that the specific surface area and the conductivity of LiFePO4/C both improved with increased levels of carbon addition. However, too much carbon coating could reduce the capacity of LiFePO4/C because lowers the ratio of active material in the composite and raises the resistance of lithium ion diffusion on the surface of the material. Adding an optimum amount of carbon increases the utilization of the active material and the electrical conductivity of electrode. Furthermore, Raman spectroscopy and the four-point probe conductivity meter both showed that the LiFePO4/C coated with dual carbon sources had greater conductivity than the cathodes coated with a single carbon source. Cyclic voltammetry also showed that we could obtain higher lithium ion diffusivity using dual versus a single carbon source.
