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Preparation And Electrochemical Properties Of Na-doped LiFePO4and Graphene Modified LiFePO4Composites

Posted on:2015-05-10Degree:MasterType:Thesis
Country:ChinaCandidate:Y J JinFull Text:PDF
GTID:2181330422491228Subject:Materials science
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Lithium-ion batteries include cathode material, anode material, separator andelectrolyte. The slow development of cathode material is an important reason forrestricting the rapid development of lithium-ion batteries. Olivine lithium ironphosphate LiFePO4is a promising cathode material for lithium ion batteries because ofits low cost, high theoretical capacity, good safety, no pollution, good cycle performance,which is valued by the people. However, its poor performance has limited itsapplications. The major reason for its poor performance is its low electronicconductivity and poor charge-discharge performance under large rate. In order toimprove its performance, several strategies have been used over the years, such asoptimized synthesis procedures, carbon coating, particle-size minimization, metalpowder composites and doping with alien cations.In this study, the method of hydrothermal method has been used. Throughadjustment of the synthesis process and the regulation of the influence factors, such asthe sequential addition of raw materials, additives type, additive content, the volume ofthe charge, the reaction temperature and reaction time, influence of these factors hasbeen analyzed. The hydrothermal products were analyzed and characterized by X-raydiffraction, scanning electron microscopy. According to the analysis results reveal theinfluence of these factors and arrive at a relatively reasonable optimization of theprocess parameters. Good dispersion, morphology integrity, fine particles and uniformnanoparticles has been prepared and its mechanism was analyzed.The optimized process is as follows: first, lithium salt and phosphate mixeduniformly. Then an iron salt was added and mixed homogeneously. The mixed solutionwas placed in a Teflon-lined stainless steel autoclave,200oC, and5h. Polyethyleneglycol400was used as dispersant, and its amount of32mL, loading volume80mL,additive optimum volume ratio of0.4.The hydrothermal products of optimized process were analyzed and characterizedby X-ray diffraction, scanning electron microscopy and Fourier transform infraredspectroscopy. The analysis results that it is a pure phase LiFePO4, does not containimpurities. After electrochemical performance test, found that the cycle performanceand rate performance is good, and the polarization phenomenon is smaller.Na doped with the optimized process parameters, and its tproducts were analyzedand characterized by X-ray diffraction, scanning electron microscopy and Fouriertransform infrared spectroscopy. The analysis results that the product all containLiFePO4characteristic peak, does not contain impurities. Calculating the crystal cell parameters and lattice is in slight distortion. The dopant material is formed, in which Nasuccessfully to solid solution to the crystal structure of olivine lithium iron phosphateLiFePO4. Through its electrochemical performance test, found that after doping the Na,discharge specific capacity is slightly lower, but the polarization phenomenon decreases.In various doped samples, when the doping amount of3%, the electrochemicalperformance is best.Graphene composite about pure phase LiFePO4doped phase Li1xNaxFePO4wereprepared. Through the electrochemical properties of the test found that electrochemicalperformance was not significantly increased, but the polarization was decreased.Combination of doping and compounding simultaneously, coupling will occur. Thedischarge capacity, rate properties of materials will be improved substantially, andmaterial polarization and impedance will substantially reduce, so that acceleratedelectron transfer. When the doping amount of Na is3%, the electrochemicalperformance is best.
Keywords/Search Tags:Lithium-ion battery, Cathode material, LiFePO4, Na-doping, Compositemodified
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