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Preparation of High Performance Lithium Iron Phosphate by a Novel Precipitation Approach

Author: LuoChuanXi
Tutor: LiaoShiJun
School: South China University of Technology
Course: Applied Chemistry
Keywords: Lithium-ion battery Lithium iron phosphate Precipitation method Carbon coated Electrochemical performance
CLC: TM912.9
Type: Master's thesis
Year: 2011
Downloads: 371
Quote: 1
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Abstract


The olivine LiFePO4 is viewed widely as a promising cathode material of lithium ion battery due to its advantages such as high capacity, low cost, good thermal stability and environmental benignancy. Well, solid-state method which is still adopted to prepare LiFePO4 in industrial production has some defects such as high energy consumption, impure and unsteady product. Wet-chemical approach to prepare LiFePO4 has attracted more and more attention because of low consumption, uniform particle size of product and being prone to modification.A novel wet-chemical method, mixing precipitation method is proposed and designed to prepare LiFePO4 in this thesis. Using （NH4）2Fe（SO4）2·6H2O、LiOH·H2O、H3PO4 and （NH4）3PO4·3H2O as raw material , Li3PO4 and Fe3（PO4）2 precipitation were prepared respectively at the first step. After aged and removed part of mother liquor, the two precipitations were fully mixed. Then the mixture was aged again, followed by filtering and drying and then the precursor of LiFePO4 was prepared. This approach effectively solves some problems of traditional precipitation and co-precipitation method such as the difficulty of simultaneous precipitation of Li and Fe, low yield coefficient, poor homogeneity of product. Phase -pure LiFePO4/C powders with good performance, which is better than co- precipitation product and industrial product, were prepared by this method. Effects of the preparation conditions on electrochemical performance were studied by using galvanostatic charge-discharge tests, cyclic voltammetry and electrochemical impedance spectroscopy. The structure and morphology of product were investigated by using XRD and SEM.The effects of the molar ratio of Li/Fe, aging temperature, calcination temperature and the amount of ethanol in liquid phase on the property of LiFePO4 material are investigated. The result of study shows that pure LiFePO4 could be prepared when the ratio of Li/Fe is 1.0 or 1.1 and impurity would be exist when excess or insufficient Li is added in raw material. The specific capacity of sample that Li/Fe is 1.1 is 151.7 mAh·g-1 at 0.1 C, which is the best in the all of four samples. However, the capacity of this sample is only 109.2 mAh·g-1 at 2 C, which is less than 119.0 mAh·g-1 of sample that Li/Fe is 1.0. Sample that Li/Fe is 1.0 shows good electrochemical performance at high rate discharge condition. The sample that aging htemperature is 50℃shows high capacity at various discharge rate, and the specific capacity is 141.0 mAh·g-1 and 125.7 mAh·g-1 at 0.1 C and 2 C respectively, so 50℃is a adequate aging temperature. Calcination temperature has a great effect on the electrochemical performance of the product. In the range of investigation, the capacity of product increases with the calcination temperature because of increasing degree of crystallinity. The sample of 750℃calcintion temperature shows best performance, and its specific capacity is 155.9 mAh·g-1 and 118.9 mAh·g-1 at 0.1 C and 2 C respectively.The effect of kind and amount of carbon source coated on LiFePO4 property are investigated. Sample coated by PVA as carbon source has good modification result whose specific capacity is 154.3 mAh·g-1 and 134.5 mAh·g-1 at 0.1 C and 2 C respectively when carbon content is 8 wt.%. The result of study show that sample coated by glucose also possess good electrochemical performance and its capacity achieve 148.1 mAh·g-1 at 0.1 C when carbon content is 10 wt.%.We find that sample with low carbon content shows as high capacity as samples with relative high carbon content. For example, the capacity of sample with 3 wt.% carbon is only 72.4 mAh·g-1 at 2 C, which is much lower than 125 mAh·g-1 of sample with 12 wt.% carbon.

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CLC: > Industrial Technology > Electrotechnical > Independent power supply technology (direct power) > Battery > A variety of materials, battery
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