Study On Preparation Of LiFePO₄/C Composite Materials With Different Crystal Of Fe₂O₃

Since having been proposed for lithium ion battery in 1997, Lithium iron phosphate（LiFePO₄） has been widely used in power and energy storage type lithium ion battery, because of advantages of safety, environmental protection, long cycle life, rich resources. What’s more, its intrinsic low electronic conductivity and poor diffusion rate of lithium ion have been conquered by the nano particles, coated carbon on the surface, and metal ion doping in recent years.The iron source of Lithium iron phosphate can be divided into bivalent iron（such as: ferrous oxalate, acetic acid, ferric citrate, etc.） and ferric iron（such as: ferric nitrate, iron phosphate, ferric oxide, citric acid iron, etc.）. Iron oxide red used as raw material of Lithium iron phosphate has been reported in some literature. But with a variety of isomer, the relationship between crystal of Fe₂O₃ and the electrochemical performance of lithium iron phosphate prepared is still not clear. The question was discussed in this paper, and the results showed as follows:Respectively by chemical precipitation method, thermal decomposition method and acoustic degradation method, rectangular α-Fe₂O₃（length about 500 nm, width about50 nm）, short rod like β-Fe₂O₃（length about 400 nm, width about 100 nm） and acicular γ-Fe₂O₃（360⁷20 nm） were prepared successfully. And use these three Fe₂O₃ for iron source, Li OH, NH4H2PO4, sucrose as raw materials, through ball mill mixing and high temperature solid reduction process to prepare LiFePO₄/C composites. And the crystal structure, morphology and the electrochemical properties of LiFePO₄/C were characterized and analyzed by XRD, SEM, and cell electrochemical performance test.Through the selection for temperature and reaction time of the high temperature solid reaction process, with α-Fe₂O₃, β-Fe₂O₃ and γ-Fe₂O₃ for iron source to prepare LiFePO₄/C composites, the suitable reaction temperature was 650 oC, and the suitable reaction time was 10 h.A battery charging and discharging experiments showed that with carbon content of（1-3wt.%）, the discharge specific capacity of LiFePO₄/C prepared with γ-Fe₂O₃, were better than another two LiFePO₄/C at the rate of 0.1C-10.0C. With more carbon content（4wt.%, 5wt.%）, the discharge specific capacity of LiFePO₄/C prepared with α-Fe₂O₃ or β-Fe₂O₃ had a substantial improvement, even more than the that of LiFePO₄/C by γ-Fe₂O₃ at the rate of 0.1C-10.0C.Using α-Fe₂O₃ to prepare LiFePO₄/C, the appropriate carbon content was 5wt.%; using β-Fe₂O₃ or γ-Fe₂O₃ to prepare LiFePO₄/C, the appropriate carbon content was 3wt.%. With α-Fe₂O₃, β-Fe₂O₃ and γ-Fe₂O₃ for iron source of LiFePO₄/C composites, charge and discharge specific capacity and cycle performance under different rate had little relationship with Fe₂O₃ crystal, but closer relevance with particle size of Fe₂O₃.On the basis of the optimization of carbon content, screening for amount of Mg²⁺ doping, the results showed that: α-Fe₂O₃ and β-Fe₂O₃ for iron source to prepare LiFePO₄/C, when the mole ratio of Mg²⁺ doping was 0.04, the discharge specific capacity of the sample was higher. But compared with the undoped samples, its discharge specific capacity changed little. With γ-Fe₂O₃ for the iron source, when the mole ratio of Mg²⁺ doping was 0.01, the electrochemical performance of LiFePO₄/C sample was better. And compared with that of undoped samples, its discharge specific capacity significantly increased.