Preparation And Modification Research Of Lithium Iron Phosphate Cathode Material

The Olivine LiFePO₄, with high theoretical capacity, flat dischargecurve,excellent cycle stability, good thermal and structural stability, low cost andenvironmental friendly, becomes a promising new cathode material forlithium-ion batteries. However, the faults of its low electronic conductivity andlittle Li+diffusion coefficient limit its practical process. Therefore addingconductive materials and metal ion doping are supplied to improve theelectrochemical performance.Carbonthermal reduction method is used to synthesize the carbon coatedLiFePO₄in inert atmosphere. Using X-ray diffraction, scanning electronmicroscopy and electrochemical test methods to analyze the products. The resultshows that the samples all have olivine structure and the added carbon resourcecan effectively descrease the particle size and the best sintering temperature is650℃. In order to improve the electrochemical properties, the LiFePO₄materialcomposite-coated with carbon and iron is composed by carbon-thermal reductionmethod. The experimental results show that the LiFePO₄/（C+Fe） material hasbetter electrochemical performance, the first discharge capacity reached154mAh/g at0.2C, capacity loss is4%after30cycles, the addition of Fe caneffectively improve the problem of rapid reduction of the capacity at big rates.Using solid state method to prepare Na⁺doped LiFePO₄/C material, thepaper analyzes how the sintering temperature and doping content influence thestructure and morphology. The sample has good crystal structure andmorphology, which is sintered at650℃、doping with3%Na⁺and using glucoseas carbon source and reducing agent. a small amount of sodium ions to joinwould not affect the structure, but excess Na⁺may cause changes in the structure,thus influence electrochemical performance. The Mn²⁺doped LiFePO₄/Cmaterial is also prepared by solid state method. How manganese ions of dopinginfluence the crystal structure is also analyzed. The added Mn2+can join to the olivine structure and form the mischcrystal Li （Mn, Fe） PO₄and the dirrractionpeak moves to low angle with the content of Mn²⁺increasing.The LiFePO₄/C and LiFePO₄/C doping with Li⁺and Al³⁺materials aresynthesized by ultrasound auxiliary sol-gel method combined with microwavemethod, the influence of the heating temperature and effects of doping quantityare discussed. The LiFePO₄/C sample prepared with the microwave heating timeof30min gets the perfect crystal structure and size distribution. The Li⁺andAl³⁺doped LiFePO₄/C sample with good crystal structure and morphology isproducted with the following conditions: using glucose as carbon source andreducing agent, Li+and Al3+respectly doping2%, heating30min withmicrowave.