Development For Materials Of Lifepo4Cathode By Microwave Hydrothermal Process

Olivine LiFePO₄has been the most potential Li-ion battery cathode materialand has attracted extensive interest due to its high safety, environmentalbenignancy, fine cycle performance and high theoretical specific capacity(170mAh·g^-1). Meanwhile, the rich of raw materials and low production costs isalso the significant advantages of LiFePO₄. Therefore, LiFePO₄ion lithiumbattery has become the main direction of powering electric vehicles （EVs）,hybrid electric vehicles （HEVs）, and other power sources for variousapplications.Microwave hydrothermal process has been used in preparing manyimportant materials for its characteristics of innovative, rapid and economic.Microwave hydrothermal process takes both advantages of the microwavemethod and the hydrothermal method, absorpt the electromagnetic wavesthrough the material itself, which made the material heated uniform and rapid ina short time as to obtain good crystallization product. LiFePO₄crystallites wereprepared by a microwave hydrothermal method using ferrous oxalate（FeC₂O₄·2H₂O）, ammonium dihydrogen phosphate （NH₄H₂PO₄） and lithiumhydroxide （LiOH·H₂O） as source materials. The crystalline structure,microstructure, elementary composition and electrochemical performances of theas-prepared crystallites were investigated by X-ray diffraction （XRD）, scanningelectron microscopy （SEM）, energy dispersive spectrometer （EDS） andcharge-discharge test. We compared the impact of high temperature solid methodand microwave hydrothermal method on the synthesis of LiFePO₄powders. Theinfluence of microwave hydrothermal temperature, microwave hydrothermalreaction time, Fe²⁺concentration, Li/Fe/PO₄molar ratio on the phasecompositions, morphologies and electrochemical performances of the LiFePO₄powders were investigated. The influence of citric acid as a complexing agent onthe synthesis of LiFePO₄powders was also investigated. First, adding acticarbonwhich react in the microwave hydrothermal reaction system in the precursor solution. The effect of acticarbon on the electrochemical characteristics ofLiFePO₄battery powder was dicussed. Then, under the optimal syntheticconditions, carbon coated LiFePO₄/C composite materials use glucose, sucroseor citric acid as the carbon source were prepared as to obtain highelectrochemical performance.The results show that the product synthesized by high temperature solidmethod contains the impurity Fe2O3, while single phase LiFePO₄crystalliteswith regular sharp, rules of morphology, and excellent electrochemicalperformance can be successfully prepared by microwave hydrothermal process.The optimal synthetic conditions to obtain LiFePO₄crystallites are microwavehydrothermal temperature with170°C, microwave hydrothermal reaction timewith40min, Fe²⁺concentration with0.4mol·L-1, molar ratio Li/Fe/PO₄with3:1:1.The grain size of LiFePO₄crystallites synthesized at the optimal conditions isabout1μm. At0.1C rate, the initial discharge capacity of LiFePO₄/C cell was136.1mAh·g^-1at the first cycle and133.9mAh·g^-1after10cycles, respectively.The product synthesized without citric acid contains the impurity Li3PO₄, whilesingle phase LiFePO₄crystallites can be successfully prepared with citric acid.Because of the effect of citric acid, the smaller particle was obtained comparedwith the sample without citrc acid. The introduction of acticarbon will notchange the LiFePO₄crystal structure, but achieve more uniform and finecrystallites and result in better electrochemical performance. The sample coatedby citric acid has the better electrochemical properties than the sample coated bysucrose or glucose. At0.1C rate, the initial discharge capacity of LiFePO₄/C cellcoated by citric acid was160.0mAh·g^-1at the first cycle and159.6mAh·g^-1after10cycles, respectively.