Preparation And Properties Of LiFePO₄/C As A Cathode Material For Lithium-ion Batteries

LiFePO₄ as a cathode material for lithium-ion battery has the advantages of high specific capacity, stable discharge plateau, long cycle life, high safety, low cost, no pollution, and so on. However, the low electronic conductivity and moderate lithium-ion diffusion coefficient of LiFePO₄ limit its applications.In this thesis, LiFePO₄ coated carbon was synthesized by the solid state reaction method and the co-precipitation method, respectively. Optimization of synthesis conditions was used to improve the properties of samples. TG-DTA, XRD, and SEM were used to study the sinter temperatures, microstructure, and superficial morphology. The electrochemical properties were investigated by galvanostatic charge/discharge measurement, cyclic voltammegram, and electrochemical impedanc spectroscopy. The results are showed as follow.LiFePO₄/C cathode material was prepared by solid state reaction method under nitrogen atmosphere. The preparation process of solid state reaction is simple, therefor suitable for large scale production. The quality of coated carbon was optimized when different iron sources were used. And effect of different iron sources on material properties have been deeply investigated. The synthesis process was performed at 400 oC for 5 h and 700 oC for 10 h. As the results shown, using FeC2O₄·2H₂O as iron source,whenβ-cyclodextrin coated reaches up to 45 wt%, the sample has a simple pure olivine-type phase structure with small and uniform particles, and presents the best electrochemical performance. Its discharge specific capacity reaches 125.32, 108.95, and 97.69 mAh/g at 0.1 C, 0.5 C, and 1 C rates, respectively. It also shows good cycle performance. Cheap raw material Fe(NO₃)₃·9H₂O as iron source was used to synthesize LiFePO₄/C cathode material by solid state reaction under nitrogen atmosphere. The sample with a simple pure olivine-type phase and the best electrochemical performance was prepared by coated 60 wt%β-cyclodextrin. The results indicate that its discharge specific capacity reaches 130.23, 115.55, and 108.99 mAh/g at 0.1 C, 0.5 C and 1 C rates, respectively, with good cycle performance. Compared with the sample using FeC2O₄·2H₂O as a iron source, the sample using Fe(NO3)3·9H₂O as a iron source has better discharge specific capacity and rate capacity. In addition, Fe(NO3)3·9H₂O as a iron source may largely reduce the cost of production.To further improve the material properties, pure phase olivine-type LiFePO₄/C cathale material was successfully synthesized by co-precipitation method. To optimize the preparation conditions of co-precipitation method, the value of pH and the quality of carbon coated were investigated. The synthesis process was carried out at 400 oC for 5 h and 700 oC for 10 h. The results show that the LiFePO₄/C composite prepared at pH value of 2.0 andβ-cyclodextrin coated being 60wt% obtains small particles and uniformly distribution of sizes, and exhibits the best electrochemical performance. Its discharge specific capacity is 150.77 mAh/g at 0.1C rate. It also shows good rate capability with 141.38, 136.81 and 118.77 mAh/g at high rate of 0.5 C,1 C and 5 C, respectively. In addition, its retention capacity is higher than 99% after 100 cycles. Compared with solid state reaction method, the co-precipitation method may achieve molecular level hybrid, resulting in that the sample synthesized by co-precipitation method retains the smaller and more uniform particles, and exhibits better electrochemical performance.