Research On Preparation Process Of Lithium Iron Phosphate/multilayer Graphene Composite

Lithium iron phosphate(LFP)is one of the most important cathode materials in the world for its high specific capacity,low cost,long cycle life,high safety and environmental friendliness.However,low conductivity of lithium iron phosphate and low lithium ion diffusion coefficient limit the power density of lithium iron phosphate.Reducing the size of lithium iron phosphate and compounding with carbon materials are effective strategies.Different from the existing GO /LFP composites,LFP was prepared by chemical method on the surface of multilayer graphene without activation in this thesis.The main researches are as follows:(1)The thesis studied the preparation process of preparing nanometer iron oxide particle films on the surface of multi-layer graphene,then depositing lithium dihydrogen phosphate,and then converting it into lithium iron phosphate after heating.The preparation of iron oxide on the surface of multilayer graphene was studied,and the influence of the amount of ethanol added in the reaction solution on the uniformity of iron oxide films were studied.Uniform iron oxide nanoparticle films were obtained on the surface of the multilayer graphene.The effect of calcination process on lithium iron phosphate/graphene was investigated.The phase composition and microstructure of the composite were studied by XRD and SEM,and its electrochemical properties were tested.The experimental data show that the best calcination condition is calcination at 650 °C for 10 hours in pure nitrogen atmosphere.The lithium iron phosphate prepared by this process is uniformly distributed on the surface of the multi-layer graphene.At current density of 17 mA/g,the specific discharge capacity of the sample is 49.5mAh/g,and at current density of 85 mA/g,the specific discharge capacity of the sample is 16.74 mAh/g.(2)The influence of the mixed process of lithium dihydrogen phosphate solvent and iron oxide films/multi-layer graphene on the morphology of lithium iron phosphate films was studied.The effects of different proportion of ethanol and water as solvent and mixing process on the morphology of lithium iron phosphate/graphene were studied.The experimental data show that the proper proportion of water and anhydrous ethanol is used as solvent,anhydrous ethanol is added several times and the process of evaporation is grinding.Uniform lithium iron phosphatefilms can be obtained on the surface of graphene.The results of electrochemical tests show that the first discharge specific capacity of the composite is 76.48 mAh/g at a current density of 17 mA/g,and the capacity retention is 90% after 100 cycles,discharge specific capacity is still 68.87 mAh/g.The discharge specific capacity is 31.82 mAh/g at 85 mA/g current density.