Preparation And Modifacation Research Of LiFePO₄as Lithium-ion Battery Cathode Material

Lithium iron phosphate cathode material has become a research hotspot in thebattery industry due to advantages such as abundant iron and phosphorus rawmaterials, low cost, large-capacity, good performance at high temperature, high poweroutput, long cycle life, environmentally friendly. Lithium iron phosphate battery hasthe potential to be widely used in various fields of pure electric vehicles, electricbicycles, energy storage devices. However, the disadvantages of LiFePO4such as lowlithium ion diffusion coefficient and low electronic conductivity and low tap densityweakened its large current discharge performance, which restricted its industrialapplication.In this paper, LiFePO4were prepared by microwave heating and solvothermalmethods. First, using Li2CO3H2O and FePO42H2O as raw materials, glucose as areducing agent and carbon source, Mn(CH3COO)2, TiO2, MgO respectively asmanganese source, titanium source, magnesium source, single or mixed metalcations-doped LiFePO4/C were prepared by microwave heating method. The effectsof microwave heating temperature and metal ions doing on the structure andelectrochemical properties of materials were studied. Second, using ethyleneglycol-water as co-solvent, FeSO47H2O, H3PO4, LiOH H2O as raw materials,ascorbic acid as a reducing agent, pure phase LiFePO4was prepared by solvothermalmethod. The influences of the molar ratio of reactants, solvothermal temperature,solvothermal time, ratio of solvents, surfactants on morphology and electrochemicalproperties were studied. The main results were as follows:(1) Microwave heating temperature at650℃,700℃,750℃, holding time of25min could prepare pure phase olivine structure LiFePO4/C. Sample synthesized at650℃had maximum discharge voltage, largest capacity, the best performance.(2) Whether single or mixed metal ion doping (doping amount≤1.5%) wouldnot change the crystal structure of LiFePO4. Compared to the carbon-coated LiFePO4,single or mixed Ti4+,Mn2+,Mg2+doping increased the large current discharge performance and the electrochemical reaction’s reversibility of LiFePO4. In addition,this sample showed a fine cycle performance. The LiFePO4/C doped0.5%Mn2+and1%Ti4+showed best electrochemical performance. The discharge capacity at0.2Crate was138mAh/g; discharge capacity at0.5C rate was116mAh/g; dischargecapacity at1C rate was105mAh/g; Capacity retention was96.5%at1C rate after20cycles.(3) Setting the ratio of glycol/water to1:1or adding cationic surfactant CTABwould contribute to (010) crystal orientation. The particle size of LiFePO4becamesmaller and the distribution became more uniform and the electrochemicalperformance was improved with adding cationic surfactant CTAB.(4) Under the condition of molar ratio of Li+: Fe2+: PO43-=2.5:1:1and ethyleneglycol/water=1:1and180℃and9h solvothermal process, the spindle-likemorphology of LiFePO4with the (010) facet mainly exposed was synthesized.Mixing lithium iron phosphate with12wt%of the glucose achieved LiFePO4/C, itsdischarge capacity0.1C rate was100.1mAh/g.