| In this paper, based on the extensive literature, how to simple prepare LiFePO4 cathode material modified by graphene was studied. LiFePO4 cathode material was synthesized for high current charge and discharge, high capacity and good cycle performance, and the following systematic study was made:Graphene oxide (GO), which was the intermediate of graphene, was synthesised by the modified Hummers method. The best synthesis condition were synthesis temperature of 0℃(2h),35℃(3h) and 95℃(0.5h), and auxiliary ultrasonic processing when adding H2O2. By XRD, FTIR, SEM and TEM, the results showed that GO appeared characteristics of diffraction peak(001), which showed that there are richly in hydroxyl, carboxyl and epoxy functional groups, the morphology observation found that graphene oxide dispersed evenly in the space, which has a typical two-dimensional crystal structure, with few layer number and the high quality.LiFePO4/graphene composites are synthesized by high heat organic carbon thermal reduction method using GO and LiFePO4 precursor materials under dry gel certain conditions. The effect on performance of LiFePO4/graphene composites are studied by the different mass ratio of GO doped (0,1%,3%,6%,3%). LiFePO4/graphene composites with the 6wt% adding GO had the best electrochemical performance at the discharge current rate of 0.2 C,0.3 C,1 C and 2 C, and that has the lowest of residual carbon which is only 1.48wt%. By XRD, SEM and physical test, the results showed that LiFePO4/graphene composites were simple pure phase olivine-type crystal structure with particle size of around 200 nm. Polarization, Lithium diffusion rate and electrical conductivity of the composite were improved. LiFePO4/graphene composite exhibited high specific capacity and superior rate performance with the first discharge capacities of 163.81mAh·g-1 at 0.1 C and 101.57 mAh·g-1 at 5C. The products also showed an excellent cycling stability of about 97.46% of capacity retention after 60 cycles at 0.5 CLiCoxFe1-xPO4/C composites materials are synthesized by high heat sintering with N2 atmosphere using different molecular weight ratio of Co2+ doped which mixing LiFePO4 raw materials for precursor formation. XRD analysis showed that LiCoo.o3Feo.97P04/C composite material corresponds to the olivine structure, no Co2+ or other diffraction peaks of the impurities. SEM analysis showed that the crystal growth was complete, particles size was reduced. Constant current charging and discharging test showed that the voltage difference between charge and discharge platform minimum for the first time which is only 0.11 V. First discharge specific capacity was the maximum of 153.34 mAh·g-1 at 0.1C with coulomb efficiency of 96.95%. Discharge specific capacity was 123.83 mAh·g-1 and cycling stability of 90.66% of capacity retention after 70 cycles at 0.5 C.Lithium ion battery cathode material of LiCo0.03Fe0.97/PO4/graphene was synthesized by a sol-gel facile carbothermal treatment. Physical testing results showed that the synthetic LiCo0.03Fe0.97PO4/graphene composites had standard olivine structure, crystalline became better, the particles were homogeneous growth and nearly spherical which were in graphene sheets with the diameters of about 200 nm, and the graphitization degree increase, doped Co2+ ions completely replaced the part of Fe2+, which narrowed the cell volume of LiFePO4 composite. The test of CV and EIS showed that polarization, Lithium diffusion rate and electrical conductivity of LiCoo.o3Feo 97PO4/graphene composites were improved. The charge-discharge test showed that LiCoo.o3Feo.97PO4/graphene composites had the discharge capacities of 159 mAh·g-1 at 0.1 C and capacities of 74 mAh·g-1 at 10C, and had the excellent cycling stability of about 99.70% capacity retention after 100 cycles at 0.5 C. |
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