Synthesis of Spherical Lithium Iron Phosphate and Electrochemical Properties
|Keywords||lithium rechargeable battery cathode materials LiFePO4 synthesis mechanism electrochemical properties|
Lithium rechargeable batteries have been used in mobile communications and portable electronic products for many years, and are potentially applied for hybrid electric vehicles and electric vehicles, because of its high capacity, high voltage, wide temperature range, long cycle life, and low self-discharge. As the cathode materials determine in some degree the properties of lithium ion cells, it is necessary to develop alternative cathode materials. In present, lithium cobalt oxide and spinel lithium manganese oxide are the dominating cathode materials, but lithium cobalt oxide is costly and poisonous, and lithium manganese oxide is limited by its capacity attenuation and bad property in high temeparatures. Therefor, researchers focus on a new cathode material LiFePO4 with the high capacity, broad temperature range, and innocuity. LiFePO4 is obtained mainly by chemical methods, for example solid reaction method and wet method, each method has its advantage and disadvantage. So it is essential to explore a relationship between the synthesis process and electrochemical properties.The present paper studied the preparation of spherical olivine lithium iron phosphate by ferrous sulfate, phosphoric acid, ammonia, and lithium hydroxide and characterized its electrochemical properties. The preparative process of lithium iron phosphate was as follow, first synthesized the spherical ferrous phosphate precursor, then the precursor of ferrous phosphate coated by the lithium phosphate, at last the lithium iron phosphate by sintering at high temperature in a protected atmosphere.The morphology of particles, functional group, heat treatment process, and crystal structure of spherical ferrous phosphate were investigated via scanning electron microscope（SEM）, fourier transform infrared（FTIR） spectrometer, thermogravimetric and differential scanning calorimetry（TG-DSC） and X-ray powder diffraction（XRD）. The morphologies and crystal structures of the precursor and lithium iron phosphate were examined by SEM and XRD. The formation mechanism of lithium iron phosphate was researched by XRD, TG-DSC, FTIR, and energy dispersive X-ray （EDS）. The experimental results show that the optimal synthesis process of spherical ferrous phosphate was 0.42mol/L ferrous sulfate, 0.28mol/L ammonium phosphate, and pH 6.0-6.5, the synthesized ferrous phosphate was amorphous and combined eight hydration; the precursor consisted of lithium phosphate and ferrous phosphate, and the later coated by the former; lithium iron phosphate and the composite lithium iron phosphate/carbon were heat treated at 500～800℃in protected atmosphere for 5～10h, the carbon content 2%（wt%） and 10%（wt%） was determined by EDS.The cathode powder lithium iron phosphate, lithium iron phosphate/carbon and their working electrode were tested via cyclic voltammograms（CV） and electrochemical impedance spectroscopy（EIS） using a VMP2 multi-potentiostat system. The performances of charge/discharge, capacity, discharge voltage, etc. were investigated by PCBT-100 test system. The electrochemical results indicated that the discharge voltage of lithium iron phosphate, lithium iron phosphate/carbon was approximate 3.4V, and lithium iron phosphate/carbon was more excellent than lithium iron phosphate. Their properties were affected deeply by heat treatment, and the high capacity, good capacity self-containing were obtained at 700℃for 10h.