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Study on LiFePO4/C Composite Cathode Materials Prepared by Sol-gel Route

Author: FanJie
Tutor: PanHongGe
School: Zhejiang University
Course: Materials Science
Keywords: Lithium ion battery Composite cathode material Lithium iron phosphate Carbon-coated Sol-gel method Sintering Electrochemical properties
CLC: TM912
Type: Master's thesis
Year: 2006
Downloads: 395
Quote: 1
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Abstract


The olivine LiFePO 4 because of its low-cost, non-toxic, excellent cycling performance, outstanding safety performance advantages that can replace expensive, toxic LiCoO 2 , become one of the most development and application of potential next-generation lithium ion battery cathode material. The LiFePO 4 low conductivity, charge or discharge the poor performance bottleneck restricting its commercial. In this paper, on the basis of progress at home and abroad lithium-ion battery LiFePO 4 cathode materials research, in order to improve its electrochemical performance as the main purpose, the use of sol - gel synthesis the LiFePO 4 / C composite material, the use of XRD, SEM, TEM and technical analysis of the microscopic structure and morphology of the product, and the use of constant current charge and discharge, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) technical testing its electrochemical properties. Focuses on the synthesis conditions of the different synthesis temperatures, different synthesis time, different reaction atmosphere the LiFePO 4 / C matrix composites impact of the organizational structure and electrochemical properties of ethylene glycol, citric acid, polypropylene as the carbon source precursor two steps sintering the LiFePO 4 / C performance were compared and discussed. Fe 3 compounds as iron sources and ethylene glycol as a conductive agent carbon source, sol-gel method step sintering prepared LiFePO 4 / C different synthesis temperature ( 600-850 ° C) LiFePO 4 composite structure and electrochemical performance. It was found that the high temperature conducive to the LiFePO 4 crystal growth intact, an increase in the degree of crystallinity, but the temperature is too high and lead to the LiFePO 4 particle aggregation grew up. On the the LiFePO 4 cathode, the smaller the particle size, the crystallization of the more intact, more conducive to improving its electrochemical properties. In this study within a temperature range of 700 ° C is the synthesis at the same time having a higher degree of crystallinity and smaller particle size (300 nm) the LiFePO 4 powder optimum sintering temperature, but the electrochemical performance not satisfactory, the carbon content for the 0.1 wt%, too little carbon content causes the discharge capacity of just 86.4mAh / g. Conversion with citric acid as the the conductive agent carbon source, a synthetic material having a carbon content of 2.6wt%, the carbon content increases to improve the electrochemical performance. In the Fe 2 carbon source of the compound and citric acid as a source of iron and a conductive agent, a sol-gel method the two-step sintering prepared LiFePO 4 / c, at 700 ° C. The sintering temperature to study the impact of different sintering time of 2.5h-10h the synthesis of LiFePO 4 / C structure and electrochemical properties. The results showed that the samples synthesized using the divalent iron source having good electrochemical performance. Different sintering time on structure and electrochemical properties of carbon-coated the LiFePO 4 product to some extent. The sintering time is longer, the high-rate discharge performance of the material as possible. Sintering time is too long will cause the product particles grew up affect its reversible specific discharge capacity. 10h sintered product obtained has a high carbon content of 12.4wt%, so that the LiFePO 4 / C has a good overall electrochemical properties. The increase in carbon content is effective to improve the conductivity of the material, to improve the electrochemical properties, reversible discharge capacity of 138.3 mAh / g, when the carbon content of 12.4wt%. Electrochemical impedance spectroscopy comparison shows that, after 10h the total impedance of the synthetic product was significantly less than after the 2.5h and 24h synthetic products, the total impedance. The different reaction atmosphere on the structure and electrochemical properties synthesis of LiFePO 4 product. As the hydrogen content increases, the reversible discharge capacity of the material is gradually reduced, the hydrogen content of 20%, a hetero-phase multi-synthesized product, reversible than the low capacity, poor performance of the high magnification. 5% hydrogen atmosphere for the best atmosphere.

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