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Synthesis and Modification of Spherical LiFePO4Materials

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Tutor: DuChunYu
School: Harbin Institute of Technology
Course: Chemical Engineering and Technology
Keywords: lithium ion battery,cathode material,LiFePO4,spherical structure,vanadium doping
CLC: TM912
Type: Master's thesis
Year:  2012
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Abstract:
Because of the high safety performance and good thermal stability, LiFePO4material has received extensive attention, but its electronic conductivity and lithium iondiffusion coefficient is small, resulting in its poor conductive properties and the lowvolume capacity, which restricts its practical application. This article prepared thespherical FePO4as the precursor by controlled crystallization method. SphericalLiFePO4/C and the spherical LiFePO4/V/C with the different concentration of vanadiumdoping were synthesized by carbonthermal reduction method. And then we analyzed theproperties of the materials by the characterization of physical and electrochemicalproperties of the materials.Spherical LiFePO4/C materials were prepared by a two-step synthesis method andthe structure and morphology of the materials were characterized by XRD, SEM, TG/DSC and other means. The influencing factors to FePO4precursor structure andmorphology like solution pH value and hydrothermal time were investigated, and theoptimal synthesis process was concluded. Electrochemical tests of the differentcarbon-coated materials showed that when the carbon amount of the theoretical productis10%, the electrochemical property of the material was the best. Carbon coated hadimproved the conductivity of the material, resulting in the LiFePO4/C materials withbetter electrochemical performance. The initial discharge capacity was137.6mAh/g at0.1C rate, while the discharge capacity decreased to76.7mAh/g at the10C rate. Thematerial had good cycle stability, and the capacity maintained95%after200cycles at10C rate.Spherical LiFePO4/V/C materials with different vanadium content weresuccessfully synthesized by using two-step synthesis method, XRD, EDS, SEM and ICPanalysis showed that under this experimental condition a small amount of vanadiumions could dop into the bulk phase of LiFePO4material, to make sure an ultra-valenceof metal doping. The influencing factors to the composition, structure and morphologyof the materials were investigated as the amount of doped vanadium changed. Afterinvestigated the impact of reaction temperature on the composition and properties ofmaterials, we found that the material had the highest capacity when it was synthesizedat700¡æ. When vanadium content is low, only the redox reaction peak of FePO4/LiFePO4appeared, while the doping mount increases, it appeared the reactionpeak of Li3V2£¨PO4£©3.The appropriate amount of vanadium doping could improve theelectrochemical performance of LiFePO4/C materials. When the vanadium dopingcontent was5%, the material had the best performance, the first delithiation capacitywas151.1mAh/g at0.1C rate, and at10C rate, the capacity was still able to bemaintained at about104.4mAh/g. After20cycles at10C rate, the capacity maintenancerate was97%, which showed the electrochemical performance superior to othermaterials.
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