Preparation Of LiFePO₄ Cathode Materials And Study Of LiFePO₄/Graphene

Since the LiFePO4 has high specific capacity which is 170mAh/g, it attracted wide attention and research by a lot of researchers. However, lithium iron phosphate has some shortcomings such as low electronic conductivity, low lithium ion diffusion coefficient and low tap density, which hinder its large-scale commercial applications. By using different methods for preparing lithium iron phosphate cathode material, and doped graphene, hoping to find the right conditions to improve the electrochemical properties of lithium iron phosphate.First, using hydrothermal method to synthesis lithium iron phosphate cathode material. There are a lot of factors which affect the hydrothermal method such as different pH values、different reaction time, so we prepared four different pH values samples which pH value is 4、8、9、10 then compared their properties. We find that the sample synthesized with pH value of 10 has the highest discharge capacity and best cycle performance than the other three samples, indicating that under alkaline conditions will produce a good electrochemical properties of lithium iron phosphate. Then we study the influence of different reaction time on lithium iron phosphate. By contrast four samples synthesized in 5h,1 Oh, 15h and 20h we found that the samples prepared in 15h and 20h have the best properties, suggesting the best time of hydrothermal method was 15h-20h.Then using solid state method and hydrothermal method to synthesis lithium iron phosphate cathode material and contrast their property. XRD patterns and SEM photographs show that lithium iron phosphate cathode material prepared by solid state method has a higher degree of crystallinity, and no impurity peaks were observed. The particles of lithium iron phosphate cathode material prepared by hydrothermal method is uniform and the shape is regular. Lithium iron phosphate cathode material prepared by two methods both exist agglomeration. Lithium iron phosphate prepared by the solid state method has a higher charge-discharge capacity and cycle performance than the sample synthesized by hydrothermal method. The sample prepared by solid state method also has better property than the hydrothermal method at high magnification. The discharge capacity of lithium iron phosphate prepared by solid state is 136mAh/g, and the discharge capacity of lithium iron phosphate prepared by hydrothermal method is 126mAh/g.Finally, we study the influence by doping graphene on lithium iron phosphate cathode material. XRD patterns and SEM photographs show that doping 5% of grapheme would not affect the generation of lithium iron phosphate. When the amount of grapheme is 10%, we can observe the graphite peak on the pattern because the graphene can easily pile to become graphite. The sample doped 5% grapheme its discharge capacity can reach 150mAh/g at 0.2C and 110mAh/g at 3C which is much better than the undoped sample. When graphene is added in excess, due to the relative reduction of the active substance, resulted in decreased charge-discharge capacity. Doping graphene can improve the conductivity of the sample, reduce the polarization, reduce the impedance of the charge transfer, increase the charge-discharge capacity at large current and improve the electrochemical performance of the samples.