Sol-gel Synthesis And Modification Research Of Nanosized Lithium Iron Phosphate

For using in lithium secondary batteries, lithium iron phosphate is an interesting active cathode material, because it has high theoretical capacity of170mAh/g and a flat discharge voltageat of3.4V. It also has high chemical and thermal stability, and because of its low cost and less toxic material, it offers economic and environmental advantages. However, the inherent poor electrical conductivity and its slow lithium ion diffusivity, are the two factors which limit its use in secondary batteries, especially at higher current densities and low temperatures. Facing the two defects, in this paper, we use the sol-gel method to synthesize the carbon coated and Rare-earth doped LiFePO4composite and the LiFePO4nanowires. Microstructure and electrochemical properties of LiFePO4were investigated using XRD, SEM and EIS measurements. The main results as follows:1、Sol-Gel method to synthesize the nanoparticle of LiFePO4An innovative sol-gel method to synthesize nanostructured LiFePO4cathode material with the low cost Fe3+as a source of iron was introduced. And the citric acid was used both as a reducing agent and a chelating agent. Through the investigation of the calcining temperature and heat preservation time, it was concluded that the optimized sintering temperature and holding time were700℃and15h, respectively.2、AAO template method for ordered LiFePO4nanowire arraysAAO template was synthesised by two-step anodization process at voltage of60V, and the pores diameter was80～100nm. The LiFePO4nanowire arrays were prepared using the sol-gel template method and hydrothermal-template method, respectively.3、Carbon coated LiFePO4nanoparticlesThe Carbon coated LiFePO4nanoparticles were prepared by the sol-gel method with the sugar and glucose respectively for carbon sources. The results of SEM, XRD showed that the particle size of the LiFePO4nanoparticles was about70～120nm. The particles are homogeneous, and the crystallinity was good. The electrochemical test showed that the doping of sugar increased the discharge capacity and improved the cycle performance. As indicated by the results, when the amounts of the sucrose was10%, the LiFePO4/C nanomaterial had good rate properties and cycle stability.At1C, the discharge capacity was138.7mAh/g. After50cycles, the capacity remained132.3mAh/g. The EIS test demonstrated that the doping of sugar could improve the cycle performance of the sample, and decrease the charge transfer resistance Rct.4、Rare-earth doped of LiFePO4nanoparticlesLiFe1-xCexPO4/C cathode materials were synthesized by the sol-gel method using Ce as the dopant source. Microstructure and electrochemical properties of LiFePO4were investigated by XRD, SEM and EIS measurements. All the samples had the crystal structure of olivine, and the particle size was decreased. The electrochemical test showed that the doping of Ce increased the discharge capacity and improved the cycle performance. The research suggested that LiFe0.99Ce0.01PO4exhibited a relatively good rate capability and cycling stability. At0.1C,the discharge capacity was153.1mAh·g-1. After50cycles, its discharge capacity remained148.0mAh·g-1. The EIS test demonstrated that the doping of Ce could improve the cycle performance of the sample, and decrease the charge transfer resistance Rct.