Study On Hydrothermal Synthesis And Performance Of LiFePO₄

Olivine-type LiFePO₄is considered as the most promising candidate for thenext-generation cathode material of Li-ion batteries due to its overwhelming advantages ofhigh theoretical specific capacity, low cost, environmental protection, excellent safety andgood cycling characteristics. However, pure LiFePO₄shows poor electrochemicalperformance, which is attributed to its low electronic conductivity and slow kinetics oflithium ion diffusion. Some measures such as reducing particle size, adding conductiveadditive, and cation ion doping are taken to improve the electrochemical performance.In this paper phospho-olivine LiFePO₄cathode material were synthesized byhydrothermal method. Carbon sources were introduced into the process of hydrothermalmethod directly for synthesizing LiFePO₄/C composite materials. Hydrothermal productswere calcined in order to improve their electrochemical performance. We use chemicaloxidative polymerization method to conduct polymer polyaniline, with polyaniline asconductive agent on LiFePO₄/C composite material to do further doping modification. Themicrostructure and morphology of products were analyzed by XRD, SEM and TEM. Theelectrochemical properties were examined by galvanostatic charge-discharge, cyclicvoltammetry （CV） and electrochemical impedance spectroscopy （EIS）.The LiFePO₄synthesized by hydrothermal method using FeSO₄, H₃PO₄and LiOH asraw materials and ascorbic acid as reducing agent. The product shows high purity, goodcrystallinity and uniform particle distribution. The effects of synthesis temperature, timeand solvent on the morphologies and electrochemical performance of LiFePO₄wereanalyzed. It was found that in the present case, LiFePO₄synthesized at180℃for6hshows better electrochemical performances using de-ionized water and ethanol mixture assolvent.Carbon sources, i. e. critic acid, polyvinyl alcohol and glucose, were introduced intothe process of hydrothermal method directly to synthesize LiFePO₄/C composite materialsand hydrothermal products were calcinated in order to improve their electrochemicalperformance. Effects of different carbon sources and the amount of best carbon source on the performance of LiFePO4have been investigated. The results demonstrate that theLiFePO4/C composite material synthesized with5%glucose shows excellent performance.The morphology of products changes from regular rectangular body to ball-like particlewith grain size200³00nm. The initial specific discharge capacity of unheated sample is104.8mAh g-1at0.1C. The first specific discharge capacity of the heat treated samplereaches151.2mAh g-1at0.1C and87.3mAh g-1at2C. And the material shows goodcycle stability. The first discharge capacity of132.2mAh g-1at0.2C and the capacityincrease to137.1mAh g-1after30cycles.We adopt homemade conductive polymer polyaniline to coating the LiFePO4/Ccomposite material which was doped with5%glucose. The results show that polyanilinecan improve the electrochemical performance of the cathode material. And the structure ofLiFePO4/C has not been affected by the polyaniline. LiFePO4/C coated with10%polyaniline shows the best electrochemical performance. Its initial discharge capacities are168.9mAh g-1,121.6mAh g-1and87.2mAh g-1at0.1C,2C and5C, respectively. Thefirst discharge capacity of167mAh g-1at0.2C and the capacity decrease to144.1mAh g-1after30cycles.