Fabrication And Characteristics Of LiFePO₄/C Cathode By Sol-gel And Ultrasonic Spray Pyrolysis Methods

The LiFePO4material has many advantages as cathode material for lithium-ion batteries, such as wide source and cheapness of raw material, safety and stability, good electrical performance, environment friendliness. But its low Lithium ionic and low electronic conductivities limit its performance to improve. Modified LiFePO4material can often obtain better electrical properties. The most widely used methods for modifying are carbon coating, narrowing particle size and metal ion doping. In this paper, we use sol-gel method and ultrasonic spray pyrolysis method to fabricate carbon-coated and morphology controlled LiFePO4cathode materials.A series of LiFePO4/C materials were prepared by sol-gel method by using three different material systems, namely Fe (NO3)3+LiNO3+NH4H2PO4+citric acid, Fe (NO3)3+LiH2PO4+citric acid and FePO4·2H2O+LiOH·H2O+H2C2O4·2H2O+glucose, the results obtained by using the third raw materials system are the best, with discharge capacities of156.5and139.8mAh/g at0.1C and1C, respectively.In Fe(NO3)3+LiNO3+NH4H2PO4+citric acid system, how sintering temperature and time and pH value of sol influence the composites and electrical performances of materials was researched. It was difficult to obtain pure LiFePO4, and the impurities increased with pH value became high. In Fe(NO3)3+LiH2PO4+citric acid system, the performances of LiFePO4/C became better when properly increased the amounts of citric acid.In FePO4-2H2O+LiOH·H2O+H2C2O4·2H2O+glucose system, it was found that adding the ball milling step to precursors can greatly improve the electrical properties of the material. The extent of agglomeration of the particles increased and the capacities slightly decreased with carbon content becoming higher. Higher the pH value of sol, the more unfavorable material properties. The resulting material performs better with higher tableting pressure. Among different mol rates of Fe: H2C2O4, the material’s performance is the best when the rate is1:3(the stoichiometric ratio when ferric ion’s ligancy is6).Fabrications of LiFePO4/C materials by ultrasonic spray pyrolysis method is rarely reported, mainly due to the prepared particles obtained by this method are generally at micron level, which is not suitable for high-rate performance of LiFePO4/C materials. However, this paper prepared the hollow spherical LiFePO4/C materials with nano-thickness shell, which shortened the lithium ion diffusion pathway. This material exhibited good electrical properties with high initial discharge capacity (159.6mAh/g at0.1C rate) and good high-rate performance (74.7mAh/g at as high as20C rate). Its cycle performance is also good. The study found that it could not obtain spherical LiFePO4/C precursors when the dying temperature was no more than200℃. The higher the concentration of the solution, the greater the average diameter and shell thickness of hollow spheres in the LiFePO4/C precursors and materials. The capacities decreased if the amounts of Fe2P were excessively high.Ultrasonic spray pyrolysis method has been applied in industry, the technology is mature. In this thesis LiFePO4/C was facilely fabricated by this method using cheap FePO4as raw material and as obtained material displayed good performances. It is hopeful to apply to industry production.