Preparation And Electrochemical Properties Of LiFePO₄/C Composite Electrode Material

Olivine-LiFePO₄ is acknowledged to be the most promising cathode material in HEV for its high theoretical capacity,brilliant cycle stability, cost effective raw materials and non-toxicity.But the low electrical conductivity which will affect the high rate performance of this material obstructs its' process of commercialization.To cope with this problem, researchers try to improve the electrochemical properties of LiFePO₄ by the way of decreasing the particle size,adding the conductive agent and doping metal ions which must be realized by proper preparation methods. In this work,a novel synthesis route(dual step inputting carbon source) was proposed for preparing LiFePO₄/C composites.We synthesized the LiFePO₄/C aggregations with carbon coated and uniform particle size by inputting different carbon sources in different phase of reaction. Technological conditions of this synthesis method were investigated systematically and optimized.The electrochemical characteristics and reaction mechanism of prepared LiFePO₄/C samples were investigated.LiFePO₄ was synthesized by solid state reaction method using Fe₂O₃, LiH₂PO₄,Super-P and sucrose as starting materials.The reaction can be described as:0.5Fe₂O₃+LiH₂PO₄+C—LiFePO₄+xCO+yCO₂+H₂O+0.5CFirstly the precursor was synthesized by mixing the Fe₂O₃,LiH₂PO₄ and Super-P;Then the obtained material was preheated at 350℃for 4 h under the N₂ atmosphere;The sucrose was mixed with the preheated materials and the mixture was calcined at 650℃for 6 h under N₂ atmosphere;Finally the LiFePO₄/C aggregation with carbon coated and uniform particle size was prepared utilizing the different functions of various carbon sources.The XRD data showed that the obtained material was almost coherent with the standard diagram.The SEM date showed that the particle size of aggregation is about 1μm.The EDS analysis proved that there are carbon layer on the surface of LiFePO₄/C aggregation.Just because of this kind of special structure,the synthesized LiFePO₄/C shows the excellent electrochemical properties.The synthesized LiFePO₄ showed the good high rate properties and cycle stability in the electrochemical test.The discharge special capacity was 168 mAh/g,146 mAh/g,134 mAh/g,124 mAh/g and 92 mAh/g respectively under the rates of 0.1C,1C,2C,3C and 5C.Especially after the high rate discharging,the LiFePO₄/C still showed the good recovery properties.TG-MS was used to analyze the mechanism of reaction.We found that the sucrose had the higher temperature in which the Fe³⁺ was reduced than Super-P as the result in the calcinations the inner Super-P was responsible to reduce the Fe³⁺ and outer sucrose just carbonized as the coating layer.