Preparation And Electrochemical Performance Study Of Lithium Ion Battery Cathode Material LiFePO₄/C

LiFePO₄ is widely considered to be one of the most promising cathode materials for the next generation of lithium ion batteries, owing to its great advantages, such as no resources limited, cheap, environment-friendly, high theoretic specific capacity (about 170mAh/g) or specific energy (about 550Wh/Kg), moderate work potential, thermal stability and so on, especially suitable for electric vehicles. However, its low electron and lithium-ion conductivity impede its applications.On the base of reviewing the rescent research progresses of LiFePO₄ and investigating varied preparation methods and modification approaches. We mainly focus on reducing cost and seeking methods could success in mass production. Two means were used to prepare LiFePO₄/C composite, and the influence of different synthesis conditions on physical chemistry properties and electrochemical performances of materials was investigated.The main conents of this paper are as follows:1. Inexpensive iron salt FeCl₃·6H₂O, Li₂CO₃/LiOH·H₂O, NH₄H₂PO₄ and polymer X were used as iron reources, lithium resources, phosporates and carbon resources especially. The LiFePO₄/C composite materials were prepared by sol-gel method successfully, and the influence of carbon amount on material properties was studied. The crystalline phase and morphology of LiFePO₄/C were characterized by XRD, SEM and other technologies; and its electrochemical performances were tested by CV, AC impedance and charge-discharge tests, through assembled button batteries CR2032.2. The experimental results demonstrated that the physical properties and electrochemical performances of LiFePO₄/C composite is the best, when carbon amount is Fe3+/C=1/2(mol/mol) and the precursor prepared by two steps method was predecomposited at 350℃for 4⁶h and then treated at 700℃for 12h. The particle size (about 210 nm) of LiFePO₄/C composite prepared under optium conditions distribution is uniform. Its specific capacity at 0.1C rate is above 150mAh/g, and its capacity retention ratio is close to 100%. Carbon coating could restraint particle growth validly and improve conductivity of LiFePO₄. When electron conductivity increases to some value, lithium ion diffusion becomes controlling step. Thus electron conductivity and lithium ion conductivity should be improved simultaneously to ameliorate electronchemistry characteristics of LiFePO₄.3. The method to prepare precursors by drying the mixed solution containing stoichiometric lithium, iron, phosphorus resources and carbon resources directly with spray drying technology, which could mix raw materials homogeneously at molecular level, was proposed in this paper. We haven't seen similar report of this approach to prepare precursor. The precursor was predecomposited at 350℃for 4⁶ hours and then treated at 700℃for 12 hours, at last, LiFePO₄/C composite were obtained. The effect of spray condition and temperature of heat treatment on product was discussed. The optimum conditions are spray dry inport tempteture 200℃and export tempteture 118℃and precursor's heat treatment tempteture 650℃.