Synthesis And Modification Of High-rate LiFePO₄Cathode Materials

Up to now, LiCoO2was generally selected as the cathode material of lithium-ionbattery, which has many advantages such as high working voltage, stable voltage atcharge-discharge process, high specific energy and so on. However, LiCoO2isexpensive and toxic. Overcharging of the battery will lead to an irreversible capacityand increasing polarization, which has potential safety hazard. The olive structurelithium iron phosphate, which has lots of advantages such as flat working voltage, longstorage and cycling life, environmental-friendliness and no memory effect, has beenconsidered as a potential cathode material of lithium-ion battery and has become one ofthe ideal material of electric vehicle power batteries.In this paper, LiFePO4powders were firstly fabricated, and then the clectricalconductivity and rate performance of the material were improved via carbon coating.Rate performance of LiFePO4/C composites was also investigated by using differentcarbon sources. The particular research contents were listed as follows:1) LiFePO4cathode materias were fabricated using LiOH、(NH4)2Fe(SO4)2andH3PO4mixed solutions, then optimized the process parameters (such as sucrose, PVAand pitch) and get the best process parameters: the concentration of precursors isn(LiFePO4)=0.015mol(hydrothermal-media is45ml deionized water), reaction time is5h, pH=7.2. Materials prepared under this condition have better crystallization, smallpartical size and homogeneous dispersity. The first discharge capacity could attain122.02mAh/g.2) Based on LiFePO4powers prepared by hydrothermal method at the optimizedexperimental conditions, the effect of carbon-coat modification on the rate performanceof LiFePO4/C composite material was investigated via using different carbon sources(sucrose, PVA and pitch), and then the experiment conditions of the optimum carbonsource were also explored. The results showed that the LiFePO4/C composite materialcoated with pitch as carbon source had best coating effect and rate performance, the firstdischarge capacity at10C can achieve99.15mAh/g. After optimization by orthogonaltests, we determined the optimum technological conditions: LiFePO4: pitch=6.5:1,coating temperature is600oC. The first discharge capacity at10C rate of LiFePO4/Ccomposite material prepared under these conditions has been improved to109.12mAh/g. 3) LiFePO4/C composite materials were also synthesized through carbothermicmethod using sucrose, PVA and pitch as carbon sources. After characterized by SEMand TEM, we found that samples prepared through pitch coating had uniform particlesize, amorphous and uniform carbon layer. The electrochemical test also indicated thatsamples prepared through pitch coating exhibited excellent rate performance, the firstdischarge capacity at10C couldreach117.72mAh/g.