Research On Lithium Iron Phosphate As Cathode Material By Hydrothermal Synthesis

Lithium ion battery is a type of battery using lithium (Li) containing materials as an electrode; it stands for the modern battery of high performance. The Li-ion battery has many advantages, such as high voltage, large specific capacity, good circulation performances, safety, rapid charging and so on. So it is widely used in the field of cameras, laptops, mobile phones, satellites, weapons and electrical vehicles. LiFePO₄ with Olivine structure is one new type cathode material for use in a Li-ion battery, and its theoretical specific capacity can reach to 170mAh/g, it is considered as one material with high capacity, non-toxic, non-pollution, safety, cheap and long circle life. Because of wide material sources, it is a new ideal cathode material.However, there is some intrinsic limitation of LiFePO₄. In the process of charge/discharge of LiFePO₄, the presence of PO₄^3- limits the space of movement of Li⁺, which makes Li⁺ move only in two dimensionedly in the extraction/reinsertion of Li⁺. Therefore, LiFePO₄ has a low electrical conductivity and ion diffuse coefficient which causes the loss of capacity at the high Capacity-rates and it becomes a bottleneck of limiting the development in the application of high power battery. In order to solve this problem, in this project we improved electrochemical performance of LiFePO₄ by adjusting the synthesis process and adding surfactant. We prepared the powder material of LiFePO₄ by hydrothermal method using FeSO₄·7H₂O, H₃PO₄ and LiOH·H₂O for raw materials, and then carbon coated LiFePO₄. In addition an in-situ carbon coating method was devised using cetyl trimethytammoium bromide (CTAB) as carbon source and this was successful in obtaining nano-powder material of LiFePO₄/C. We reduced the particle size of LiFePO₄, which can promote the practical application of LiFePO₄ material in powder battery. There are some results and conclusions, as follows:1. By hydrothermal synthesis and adding proper reluctant ascorbic acid to prevent the oxidation of Fe²⁺, we got LiFePO₄ with good performance. In addition, by changing synthesis process we studied the influence of synthesis temperature and holding time to microstructure and morphology of LiFePO₄. The results showed that the sample prepared at 190℃for 5 hours is the best of all. Then chose some better samples and carbon coated the samples by glucose (mixed in a proper ratio (w/w) and fired) and compared the structure and morphology of LiFePO₄/C material. At last we obtain fine LiFePO₄/C material with smaller and homogenous particles.2. Using CTAB as the surfactant, an in-situ carbon coating of the powders of lithium iron phosphate was done. Researches on the influence of different qualities of surfactant to the particle size showed that the sample adding 6g CTAB gave good dispersion, low content of impurities nano-particles of LiFePO₄ with good dispersion and few impurities had synthesized by increasing the concentration of CTAB. The use of CTAB acted both as surfactant to prevent grain growth and as carbon source. In this way we could obtain nano-powders of LiFePO₄/C material with better electrochemical performance and the sample added 6g CTAB is the best one.