Preparation And Characterization Of FePO₄and LiFePO₄Nanosheets

With the development of society, people pay more attention to the field of environmental protection, new energy development and IT. Humans begin to pursue efficient, convenient, fine environmental, safe and reliable energy. Therefore, developing rechargeable lithium-ion batteries which is low-cost, high energy, has an extremely scientific significance and considerable potential commercial value. Iron-based cathode materials has stood out for its advantages of low prices, abundant raw materials, environment-friendly, etc. The battery researchers have excited great interest in them, especially for lithium iron phosphate and iron phosphate material. Iron phosphate material can be used as a cathode material, it also can be used as a precursor to the preparation of lithium iron phosphate cathode material. The lithium iron phosphate has been considered as a promising lithium-ion battery beeause of its rich raw materials, stable structure, high capacity, safety et al, which is expected to replace the lithium cobalt oxide (LiCoO2), lithium nickel (LiNiO2) and lithium manganese oxide (LiMn2O4) as electrode materials. The main weak point of LiFePO4is its low electronie and low tap density.The iron phosphate nanosheets were synthesized with the precipitation method, using the surfactant CTAB as particle dispersant. The iron phosphate lithium was prepared by carbothermal reduction, using homemade iron phosphate nanosheets, ithium carbonate and glucose as raw material. The two products were characterized by BET surface area, tap density, SEM, TG-DSC, DTA, XRD and IR. Lithium battery test system and the electrochemical workstation were used to test the electrochemical properties of the sample of lithium iron phosphate. The main experiments and results were as follows:The FePO4nanosheets was synthesized with the precipitation method, using hexahydrate ferric chloride, phosphoric acid as raw materials, and CTAB as particle dispersant. The effect of surfactant, reaction temperature, the molar ratio of P and Fe, feeding way of the raw material, drying way and calcination temperature on the mean size of iron phosphate powder were studied. SEM and particle size distribution test showed that the thickness of the iron phosphate particles were70-100nm and the average particle size were1000nm. The appearance of the morphology of FePO4particles was lamellar, with uniform particle size distribution. TG-DSC/DTA test showed the dihydrate iron phosphate water lost at120-200℃. The XRD results showed that the iron phosphate was crystallized to orthorhombic system of space group Pcab (61), with cell parameters a=11.724A, b=10.024A, c=7.518A and cell volume v=883.52A3. IR test showed that FePO4.2H2O had characteristic peak of the PO4groups, the water vibration and Fe characteristic absorption peak. There was also the bending vibration of the O-P-O or phosphate lattice vibrations in the lower wave range.Lithium iron phosphate/carbon composite was prepared by carbothermal reduction, which under the protection of the N2atmosphere, using homemade iron phosphate nanosheets, ithium carbonate and glucose as raw materials. XRD analysis showed that the Lithium iron phosphate was crystallized to orthorhombic system of space group Pmnb (62), with cell parameters a=6.051A, b=10.324A, c=4.688A and volume v=292.86A3. Particle size distribution test and SEM analysis showed the average particle size was920nm. The appearance of the morphology of LiFePO4particles was lamellar, with uniform particle size distribution. The BET surface area was46.82m2/g and tap density was1.2073g/cm3. IR test showed the sample of lithium iron phosphate was ideal pure phase LiFePO4. Electrochemical performance test showed that the lithium iron phosphate/carbon composite had a high discharge capacity and stable cycling performance. The discharge capacity were140.65mAh.g-1,132.97mAh.g-1,122.19mAh.g-1respectively under discharge current density of0.1C,0.5C,1.0C. The charge and discharge efficiency for the first time were82.74%,85.81%,83.70%. After100cycles respectively under0.1C,0.5C,1.0C, the discharge capacity retention of samples were still95.57%,96.22%and91.41%.