Preparation And Modification Of LiFePO₄/C Cathode Material For Lithium-ion Batteries

With the rapid development of Science and technology and the increasing prominence of energy problem and environment problem, people put forward higher request for batteries. Lithium ion battery, because of its high specific capacity, long cycle life, high working voltage, good safety performance, low self-discharge, non-memory effect, no pollution-free and so on, is recognized as "green power". Olivine-structured LiFePO4 is considered to be the ideal anode material in lithium-ion batteries, because it has many advantages such as abundant raw materials, low cost, high safety performance, good thermal stability, friendly to environment, large theoretical capacity (170mAh/g) etc. However, LiFePO4 has two obvious drawbacks:one is low electronic conductivity and low lithium ion diffusion rate, the other is low tap density. To improve its electrochemical properties effectively, lithium iron phosphate can be coated with carbon or doped some metal irons in the preparation process of materials.LiFePO4/C anode material in lithium ion batteries uses glucose, lithium dihydrogen phosphate and ferric oxide as raw materials through high temperature mechanochemistry technology. The physical and chemical properties and the electrochemical characterization of the powders are characterized by x-ray diffraction(XRD), scanning electron microscopy(SEM), charge-discharge test(CD), cyclic voltammetry(CV), electrochemical impedance spectroscopy(EIS), particle size distributions(PSD), infrared carbon sulfur analyzer(CS), specific surface area test(BET) methods and so on. The influence factors such as ball-milling temperature, ball-milling time, ball to powder weight ratio, lithium ferrum molar ratio and carbon coating amount on the properties of the powders are studied. Results show that LiFePO4/C obtained at 600℃ for 5h with ball to powder weight ratio at 8:1 and lithium ferrum molar ratio at 1:0.975 has inerratic morphology and tiny even particles. The initial discharge specific capacity is 117.2mAh/g at 0.1C, the discharge specific capacity is 112.3mAh/g over 20 cycles at 0.1C.In addition, Li1-xNbxFePO4/C is obtained by doping Nb5+ into LiFePO4/C. The effect of Nb contents on the physical and chemical properties and the electrochemical characterization is investigated. The results show when the content of Nb5+ doped is 0.02, Li0.98Nb0.02FePO4/C powder exhibits high performance and its particle size is 1μm or so. The initial discharge specific capacity is 145.6mAh/g at 0.1C, the discharge specific capacity is 142.3mAh/g over 20 cycles at 0.1C, and the attenuation rate is only 2.3%. Compared with LiFePO4/C powder, the resistance of Li0.98Nb0.02FePO4/C is smaller in the process of electric charge and lithium ion migration, and its electrochemical performance is higher.