The Modification Research Of Cathode Material LiFePO₄by Carbon Coating And Doping For Lithium Ion Battery

Lithium Ion Battery(LIB), which plays an important role in alleviating the energy crisis and improving the utilization rate of materials, is a highly efficient media of energy conversion. As a cathode material of LIB, LiFePO4which has a promising development owns some advantages, such as high theoretical specific capacity, good cycle performance, non-polution for environment, low price and so on. However, due to its low tap density, electronic conductivity and Li+diffusion coefficient, LiFePO4has low volumetric specific energy and bad ability of large charge/discharge current which severely hamper the application in commercial battery. But it can improve the performance of LiFePO4by modifying the preperation technology, carbon coating and doping metal ion.LiFePO4is synthesized by and solid reaction method. The experiments are conducted from three aspects:the choice of preperation material, the choice and optimization of carbon source, the transition metal ion doping and optimization. The prepared products are characterized by X-ray diffraction, Scanning electron microscope, Thermogravimetric analysis, Constant current charge/discharge, cyclic voltammetry and Electrochemical impedence spectroscopy.The influence of performance of LiFePO4/C which synthesized by different Lithium and iron sources is studied. The lithium sources were LiOH·H2O and Li2CO3, the iron sources are FePO4and FeC2O4·2H2O, respectively. The result indicated:all the samples which synthesized by different lithium and iron sources were pure olivine-type LiFePo4, the LiFePO4/C sample which synthesized by Li2CO3and FeC2O4·2H2O had highest specific discharge capacity, lowest charge transfer impedance, best dynamic performance and the most stable cyclic performance. At0.2C,0.5C,1C and5C discharge rate, the discharge specific capacities were142mAh/g,134mAh/g,110mAh/g and42mAh/g, respectively. The capacity retention rate at1C was100%after50cycles.By adopting Li2CO3, FeC2O4-2H2O, NH4H2PO4as raw materials, the performance influence of LiFePO4/C was studied by adding different carbon sources including glucose, citric acid, sucrose. The results indicated:the carbon coating LiFePO4/C particles had lower extent of agglomeration. The specific discharge capacities of carbon coated LiFePO4/C at different discharge rate were higher than which uncoated. The LiFePO4/C which adopted glucose as caobon source had highest discharge specific capacity. At the same time, the cycling stability was enhanced. The LiFePO4/C when the molar ratio of glucose and lithium was4:15had highest discharge specific capacities which were142mAh/g,133mAh/g,28mAh/g at0.2C,0.5C,1C, respectively. The LiFePO4/C when the molar ratio of glucose and lithium was6:15had the highest discharge specific capacity which was81mAh/g at5C.By adopting Li2CO3, FeC2O4-2H2O, NH4H2PO4, C6H12O6(glucose) as raw materials, the performance influence of LiFePO4/C was studied by adding different transition metal acetates(manganese acetate, cobaltous acetate, nickel acetate, zinc acetate) and different ion content. The results indicated: LiFe0.9M0.iPO4/C(M=Mn, Co, Ni, Zn) and LiFe1-xNixPO4/C(x=0,0.05,0.1,0.15) had the same crystal structure of olivine-style with LiFePO4The doped transition metal ion existed in LiFePO4by means of solid solution. LiFe0.9M0.1PO4/C (M=Mn、Co、Ni、Zn) had lower discharge specific capacity than LiFePO4/C at low discharge rate, and the ability of cycling stability was impaired. But the doped samples have a better discharge performance at5C, especially LiFe0.9Ni0.1PO4/C which had the highest discharge specific capacity. The discharge specific capacity of LiFe1-xNixPO4/C(x=0、0.05、0.1、0.15) decreased with the increasing content of Ni. LiFe0.95Ni0.05PO4/C had scarcely capacity loss compared with LiFePO/C. The discharge specific capacity of LiFe0.95Ni0.05PO4C were143mAh/g,134mAh/g,127mAh/g,85mAh/g at0.2C,0.5C,1C,5C, respectively. But, the discharge specific capacity of LiFe1-xNixPO4/C(x=0.05、0.1、0.15) surpass that of LiFePO4/C, LiFe0.9Ni0.1PO4.also had the highest discharge specific capacity which is89mAh/g.