| In recent years, Olivine-type LiFePO4as cathode material of lithium-ion secondary battery has been paid more attention due to the advantages of high theoretical capacity, good cycling performance, environmental compatibility, abundant resources, cheap price cost and so on. But the charge and discharge rate and specific capacity are restricted by its low electronic conductivity and low internal Li+diffusion coefficient and the bad tap density.In order to improve the electronic conductivity and Li+diffusion rate of LiFePO4materials, to improve the macro electrochemical performance, preparation of nanometer particles and carbon coated are effective methods. And realizing take off/embedded Li+energy storage and static electric double layer capacitance energy storage in the same electrode, it can improve the charging and discharging rate material performance of LiFePO4. This paper carried out the work based on previous studies:1. FePO4/PANI are prepared using in situ limit polymerization method with FeCl3.6H2O,NH4H2PO4, aniline as raw materials, and using FePO4/PANI as the precursor, CH3COOLi, ascorbic acid as raw materials, LiFePO4coated carbon in nano size are prepared using chemical embedded Li+method. The result shows that the crystalstructure of LiFePO4is indexed as olivinic structure by XRD diffraction analysis. HRTEM electron microscopy shows that the particles size ranged in18.2-54.5nm, carbon coated on the particles surface with thickness in2-10nm. The particles size and carbon layer density are different along with the different synthesis temperature. The electrochemical test shows that the LiFePO4/C powder prepared in700℃has142mAh/g capacity and132mAh/g capacity after40times cycle in0.1C.2. Thermal stability and reaction mechanism of the LiFePO4/C materials is studied by non-isothermal analysis and model-free analysis with the TG curves of LiFePO4/C. The result showed that:the right mechanism function is the Al, the mechanism of decomposition is random nuclear. The thermal dynamic decomposition equation is da/dT=(1-α)(A/β)exp(-Eα/RT). Kinetic parameters:lgA=10.386min-1Ea=138.849kJ·mol-1. Activation energy Eα has three processes with the process of thermal decomposition in the process of thermal decomposition. When conversion α<30%, reaction activation energy increases with the rise of conversion; Ea has no significant increase or decrease when α is in30-70%; When conversion a>70%, reaction activation energy increases gradually with the rise of conversion。3. It is studied about the specific surface area and pore size distribution of porous carbon prepared by activated by ZnCl2with different impregnation ratio, different activation temperature. The result shows that:BET and Vt of porous carbon are increasing with rise of impregnation ratio and activation temperature.When the activation temperature is450℃and impregnation ratio is7, the BET of porous carbon is771.6m2/g; When the impregnation ratio is5and activation temperature is900℃, the BET of porous carbon is951.34m2/g.4. Using the porous carbon with different surface area load LiFePO4/C, the influence of different surface area loading LiFePO4/C is studied. The result shows that:The bigger of BET of porous carbon, the easier loading LiFePO4The composite material of porous carbon loading LiFePO4/C with different porous carbon content are prepared, as batteries positive materials, charge and discharge capacity reduce gradually with the increase of the content of the composite material in charge-discharge curve, rate charge and discharge performance is improved some.Circulation current-voltage curve shows that electric double layer capacitance energy storage is gradually increasing with the increase of the content of porous carbon in composite material. Electrochemical impedance spectroscopy (EIS) shows that, with the increase of the content of porous carbon, the resistance of charge transfer and electrode reaction decreased and charge transfered more easily. |
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