Dissertation Submitted to Hebei University of Technology for The Master Degree of Materials Physics and Chemistry STUDY ON CARBON COATING FOR HYDROTHERMALLY SYNTHESIZED LiFePO4 by Wu Yichen Supervisor: Prof. Ou Xiuqin March 2013 河北工业大学硕士学位论文 i 水热合成磷酸铁锂的碳包覆工艺研究 ii 水热合成磷酸铁锂的碳包覆工艺研究水热合成磷酸铁锂的碳包覆工艺研究 摘摘 要要 碳包覆是提高水热合成磷酸铁锂电导率的重要步骤 碳源及包覆工艺对碳包覆层和 LiFePO4/C 的物化性能有较大影响选取葡萄糖、蔗糖和 PEG6000（PEG6000）等碳源对磷酸铁锂进行碳包覆利用热分析仪（TG-DTA）研究碳源及其在磷酸铁锂表面的热行为采用 X 射线衍射仪（XRD） 、透射电子显微镜（TEM）和电化学性能测试仪研究碳包覆对 LiFePO4/C 性能的影响 磷酸铁锂浆料的洗涤是碳包覆的前序工艺， 考察洗涤用水量及洗涤次数对磷酸铁锂中硫酸根含量和电化学性能的影响。
结果表明，单次洗涤用水量 m1:浆料质量 m2=2:1，洗涤两次，洗涤效果最好 碳源单独碳化和在磷酸铁锂表面的碳化过程明显不同 磷酸铁锂对碳源碳化起催化作用，使起始碳化温度降低 8%以上对于三种选定的碳源（葡萄糖、蔗糖、PEG6000） ，失重主要发生在 400 ℃之前依据 TG-DTA 曲线确定了关键的煅烧温度点研究了煅烧温度、升温速度和保温时间对 LiFePO4/C 电化学性能的影响，制定出了最佳的煅烧工艺：以 8 ℃/min 的升温速度升至 350 ℃后保温 2小时再以同样的速度升至 700 ℃并保温 4 小时，自然冷却到室温煅烧温度是影响 LiFePO4/C 电化学性能的最主要因素， 它不仅影响碳层的石墨化程度还提高了磷酸铁锂的结晶度 研究了碳包覆量对 LiFePO4/C 电化学性能的影响结果表明，LiFePO4/C 中碳含量为 2 wt.%左右时是最佳的碳层厚度， 以 PEG6000 为碳源制备的 LiFePO4/C表面碳网络均匀致密，表现出最高的容量和优异的倍率性能、循环性能、低温性能 通过改变样品中和气氛中的水分含量考察气氛还原性对水热合成磷酸铁锂的碳包覆效果的影响结果表明，煅烧过程中气氛还原性对 LiFePO4/C 电化学性能有一定的影响。
水分含量越高气氛还原性越强，并影响磷酸铁锂材料表面的活性和包覆碳层的均匀性，导致 LiFePO4/C 电导率降低，电化学性能变差 本文通过实验优化和理论分析探索出了最佳的碳包覆工艺制备的LiFePO4/C 复合材料表现出优异的电化学性能，即 161.6（0.2 C） 、156.6（1 C）和 144.0 mAh/g（3 C） ，-20 ℃时的 0.2 C 容量约为 125 mAh/g，与室温容量相比容量保持率高达 78% 河北工业大学硕士学位论文 iii 关键词：磷酸铁锂，煅烧制度，碳包覆，还原性气氛，电化学性能河北工业大学硕士学位论文 v STUDY ON CARBON COATING FOR HYDROTHERMALLY SYNTHESIZED LiFePO4 ABSTRACT Carbon coating process is a very important step for improving the conductivity of bare LiFePO4 (LFP) prepared by hydrothermal method. Carbon source and coating process have significant effect on physical and chemical performance of LiFePO4/C (LFP/C). Glucose, sucrose and PEG6000 were selected as the carbon sources. The heat behaviors were analyzed by TG-DTA for individual carbon source and the mixture of carbon sources and LiFePO4 powder. The effect of carbon source, carbon amount and calcination atmosphere on the electrochemical performance of LFP/C was characterized by X-ray diffraction, transmission electron microscope and electrochemical test. Washing treatment is needed for the LFP obtained from hydrothermal synthesis before carbon coating. The effect of water consumption and wash times on the SO42- percentage in LFP and electrochemical performance was studied. The results show that the optimal washing process conditions are mwater : mslurry = 2 : 1, and two times washing. The heat behaviors are obviously different from individual carbon source and that on the surface of LFP. The results show that the carbonization reaction is catalyzed by LFP, which can lower the starting temperature more than 8 %. The mass loss occurs mainly in the temperature less than 400 ° C. The key point of calcination temperature was determined by TG-DTA curves. The effect of calcination temperature, temperature rising rate and holding time on the electrochemical performances of LFP were investigated. The optimal calcination conditions were obtained: the mixture of carbon sources and LFP powder are heated to 350 ° C in the rising rate of 8 ° C /min then holding for two hours before it heated to 700 ° C in the same rising rate, the 水热合成磷酸铁锂的碳包覆工艺研究 vi holding time is 4 hours at 700 ° C and then cooling to ambient temperature under N2 atmosphere. The calcination temperature is the key factor that influencing the electrochemical performance of LFP/C by improving the conductivity of carbon and the crystallinity of LFP. The effect of carbon amount on the electrochemical performances of LFP was studied. The results show that the optimum amount of carbon is 2 wt.%. PEG6000 could lead to a more evenly distributed carbon on the surface of LFP and a better electrochemical performance such as discharge capacity, C-rate capability, cycling and low temperature performance. The effect of reducing atmosphere resulting from moisture in the mixture on the performance of LFP/C was studied by moisture variation in the samples and atmosphere. The results show that the reducing atmosphere has a certain effect on the electrochemical performance of LFP/C. The reducibility can be enhanced by the moisture. Enhanced reducibility can lead to less evenly distributed carbon on the surface of the LFP and lower the surface activity of the particles which resulting in the poorer conductivity and electrochemical performance of LFP/C. In this work, the optimum carbon coating condition was concluded by experimental optimizing and theoretical analysis. The LFP/C samples exhibit perfect electrochemical performance: the discharge capacities are 161.6, 156.6 and 144.0 mAh/g at 0.2, 1 and 3 C and at room temperature, respectively. The low temperature (-20 ° C) discharge capacity is 125 mAh/g at 0.2 C. The capacity retention can reach 78% compared to that at room temperature. KEY WORDS: LiFePO4, calcination system, carbon coating, reducing atmosphere, electrochemical performance河北工业大学硕士学位论文 vii 目 录 第一章 绪论 ..................................。