A Study On Synthesis And Properties Of Lithium Iron Phosphate

The crystal structure of LiFePO4 is Olivine. This type of cathode material has gradually become a new research focus in the world. Studies show that theoretical capacity of the new cathode material is 170mAh/g. The material's platform is very stable which can be compared with stable electrical source. The material has good cycle performance and the material's thermal stability is better than any other known cathode material. The material has medium voltage (3.4V) and is non-toxic and real green.In this study, we synthesized carbon-coated lithium iron phosphate using a combination of solid phase synthesis with liquid phase ones. The raw materials are lithium carbonate, phosphoric acid, ferric oxide, glucose, citric acid and proper amount of distilled water. The procedures of synthesizing carbon coated lithium iron phosphate have raw material mixing, drying, ball milling and sintering. We determined carbon-coated lithium iron phosphate crystal structure using XRD technology and measured lithium iron phosphate particle size using laser particle size analyzer. We tested its electrochemical properties using charge-discharge instrument and cyclic voltammeter.We found that lithium iron phosphate using a combination of solid phase synthesis with liquid phase ones followed by heat treatment, the addition of carbon can be effective against lithium iron phosphate particles growing up, to get a small uniform particle size material in the experiment. The role of carbon added are:①As a reducing agent to reduce trivalent iron to ferrous iron;②coated lithium iron phosphate particles in the surface to improve electrical conductivity of materials;③controlling of high temperature solid state synthesizing lithium iron phosphate material particle size. In synthesizing lithium iron phosphate material process, we added glucose, sucrose, lactose, acetylene black separately. Lithium iron phosphate materials synthesizing with adding glucose, lactose show better electrochemical properties that specific capacity are all 139.8mAh/g or more at room temperature, while lithium iron phosphate materials synthesizing with adding acetylene black has only 93.44mAh/g . Constant charge and discharge rate was 0.2C.We synthesized lithium iron phosphate with the solid state method. In the process of preparing composite materials, the sintering temperature and sintering time have great effects on the material's properties. We synthesized lithium iron phosphate at 500℃, 550℃, 600℃, 650℃, 700℃, 750℃, 800℃sintering temperature with 8 hours. Laboratory test results show: at room temperature, the constant charge and discharge rate was 0.2C, the material synthesized at 500℃, 550℃sintering temperature have no capacity. The material's specific capacity synthesized at 600℃sintering temperature is 20mAh/g. The material's specific capacity synthesized at 650℃, 700℃sintering temperature are all 140mAh/g around. The material's specific capacity synthesized at750℃, 800℃sintering temperature are 120mAh/g, 80mAh/g separately. The results show that the carbon-coated lithium iron phosphate synthesized at 700℃sintering temperature has the best chemical properties. We synthesized carbon-coated lithium iron phosphate at 700℃sintered 2, 4, 6, 8, 12, 16 hours separately. The results showed that carbon-coated lithium iron phosphate sintered 8 hours has the best electrochemical properties.We synthesized lithium iron phosphate with the solid state method. In the experiment, we studied the effects that doping 1%MgCO3, CrO3, (NH4)6Mo7O24·4H2O in synthesizing carbon-coated lithium iron phosphate on its' electrochemical properties. The experimental result showed that carbon-coated lithium iron phosphate doped 1%CrO3 has the best electrochemical properties. Constant charge and discharge rate was 0.2C.Specific capacity of carbon-coated lithium iron phosphate doped 1%CrO3 is 145mAh/g, while enhancing the material's high rate discharge specific capacity and cycle stability.