The Synthesis And Research On LiFePO₄/C As Cathode Materials

Pure LiFePO₄have the low reversible capacity and poor charge-dischargeperformance at high current density because of its extremely low electronic conductivity.In view of this situation, many researchers tried to improve the performance of LiFePO₄with the method such as synthesizing nanocrystals of the material particle, carbon-coating,metal ions doping and so on. This dissertation consist of modification research onas-prepared hydrothermal synthesized LiFePO₄as cathode materials, the industrialproduction and its modification research on LiFePO₄/C with solid phase method.This dissertation is to propose a new kind of carbon-coated method combined withorganic and inorganic carbon. The polyvinyl alcohol （PVA） and ultra-fine conductivecarbon （SP）are homogeneously coated on the surface of the hydrothermally synthesizedLiFePO₄due to the different solubility of PVA in water and alcohol. After sintering at600℃for5h in N2,LiFePO₄/C cathode material is obtained. SEM and TEM photosshowed that the PVA formed a three-dimensional network structure among the LiFePO₄particles and the LiFePO₄particles were homogeneously coated by a layer of amorphouscarbon. The study shown that: after carbon coating with PVA and SP, the first dischargespecific capacity of LiFePO₄/C at0.2C increased from120mAh/g to140mAh/g, and thespecific capacity at5C rate increased from10mAh/g to100mAh/g. The capacity retentionafter80times at1C rate current density increased from53.85%to97.96%.In view of the solid phase method, firstly, we optimized the synthesis conditions oninlet temperature at spray drying, the solid content of slurry, the sintering temperature,then, in order to improve the processing performance, we must reduce the particle size andbreak the hollow spheres structure. The study shown that: when the inlet temperature atspray drying is350°C, the solid content of slurry is30%, the sintering temperature is720°C, the production of LiFePO₄/C as cathode materials have the best electrochemicalperformance. Next, we make the second carbon-coated by liquid stirring mill, addedwt.1%of sucrose, after drying, sinter it at650℃for10h in N2, the capacity of theobtained LiFePO₄/C is150mAh/g at0.1C,122mAh/g at2C rate,96mAh/g at5C rate, the capacity retention rate after150cycle numbers at2C is still98.04%, the balance of chargeand discharge platform is only0.061V, the BET surface area is17.69m2/g, and theprocessing performance is excellent.