Study On Synthesis Of LiFePO 4 Cathode Material For Lithium Ion Batteries By Molten Salt Method

Olivine-type LiFePO4 is considerd as one of the most promising cathode intercalation materials for lithium-based secondary batteries because it shows many advantages, such as environmental friendiness,abundant raw materials, low cost, excellent structural stability and high theoretical capacity (170mAh·g-1), especially for for the large-size lithium battery. However, the commercial application of this material is impeded due to its low intrinsic electronic conductivity and poor lithium ion diffusion. To overcome the disadvantage, a new kind of preparation-MS method is explored in this paper to control the morphology of lithium iron phosphate, reduce the synthesis temperature and the grain size, also to shorten the lithium ion diffusion distance. On the basis on the method, electrochemical of the material is improved by enhanceing the inherent conductivity and improving ion diffusion rate through a small amount of magnesium-doped lithium-site and Iron- site.1. The iron phosphate Lithium precursor is performed by thermogravimetric (TG) and Differntial scanning calorimetry. And the best condition for synthesing iron phosphate Lithium Technology is at 600℃for 12h.2. The prepared LiFePO4 composites are characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM) and Transition electron microscope (TEM). The phase of material are olives structure with a high degree of crystallinity; the particles of material is spherical, and the grain sizes is about 200nm. Amorphous C, distributes uniformly between lithium iron phosphate particles.3. The Charge-discharge test shows that the materials prepared at 600℃with the lithium-sodium ratio of 1:4 for 12 hours have good electrochemical performance. At the rate of 0.2C(34 mA·g-1) and 0.5C, the discharge capacities were 144.6 and 122.3 mAh·g-1 respectively, together with good cycle performances. The CV and EIS method is used to characterize electrochemical properties of the materials, The results show that the electrochemical reaction, which occurring reversible redox reaction in iron phosphate Li-ion battery materials, is controlled by lithium ion diffusion and the charge transfer resistance Rct is only 400?.4. Through the study of magnesium content, sintering time, and sintering temperature on the LiFePO4 physical and electrochemical properties, LiFePO4 prepared in 600℃for 8h have a good electrochemical and cycle performances. When magnesium-doped lithium-site is 6%, at the rate of 0.1C, 0.2C, 1C, the discharge capacity is 131.0mAhg^-1, 120.4mAhg^-1and 109.3mAhg^-1 respectively. After 20 charge-discharge tests, the discharge capacity is 108.3 mAhg^-1 at 1C. And when magnesium-doped Iron- site is 3%, at the rate of 0.1C, 0.2C, 1C, the discharge capacity is 150.3 mAhg^-1, 141.1 mAhg^-1, 115.1 mAhg^-1 respectively, with a good cycle performance. The cyclic voltammetry tests showed that Mg doping improves the reversibility of the material and reduces the polarization of the material. And the AC impedance tests show that the charge transfer resistance Rct of Mg doping on lithium-site and Iron- site is 450? and 500? respectively, and the lithium-ion diffusion coefficient is 8.5748 * 10^-17m²s^-1 and 9.0275*10^-17m²s^-1 respectively.