Hydrothermal Synthesis And Modification Of Lithium Ion Battery Cathode Material LiFePO₄

Lithium iron phosphate has become one of great interest as a cathode for lithium ion batteries owing to its abundant raw materials, low cost, high theoretical capacity, good cyclability, environmental benignancy and safety. However, there are some problems such as poor electronic conductivity and low ion diffusion coefficient seriously affected the rate capacity of LiFePO4and limited its particle application in high power density batteries.In this paper, we aimed at synthesizing LiFePO4cathode material with high performance. LiFePO4was prepared by several methods and carbon was coated on the surface to improve its performance. The samples were characterized by electrochemical performance test, XRD, FE-SEM, HR-TEM, FTIR and BET. The growth mechanism of LiFePO4has been discussed.1. The effect of reactant concentration, pH value and carbon source on the physicochemical properties and electrochemical properties of LiFePO4in hydrothermal reaction was investigated and the optimum reaction process was obtained. The appropriate reactant concentration was0.5M (by Fe2+concentration), the pH value was better controlled under weak alkaline environment (pH=8.2) and glucose was chosen as carbon source.2. In solvothermal reaction, nanocube-like LiFePO4/C particles were prepared through a solvothermal method in the presence of ionic liquid. Ionic liquid was used as co-solvent and surface growth directing template agent to control the size and morphology of the particles. It changed the crystal orientation in the stage of crystal nucleation and growth. A proper concentration of ionic liquid can decrease the particles size and the as-prepared sample in this concentration demonstrates excellent electrochemical performance. When1ml ionic liquid was added, the discharge capacity can get up to164.2mAh/g (close to the theoretical value of170mAh/g) at0.1C and there is small difference between the charge and discharge curves. The product delivers specific capacities as high as 98.3mAh/g at5C and78.7mAh/g at IOC respectively and1.248×10-6S·cm-1electronic conductivity was obtained. What’s more, the phase structure and morphology features of LiFePO4synthesized during different time periods have been observed to study the regulation mechanism of ironic liquid in solvothermal reaction; In addition, isopropyl alcohol was adopted as co-solvent to synthesize self-assembled layer structure LiFePO4. With the concentration of isopropyl alcohol increased, LiFePO4evolved to the hierarchical dumbbell-like structure. When the ratio of isopropyl alcohol and water (by value) was the lower, the LiFePO4product owns better electrochemical properties. The LiFePO4prepared with the value ratio5:35delivered discharge capability as high as166mAh/g, and no significant fade was observed. When the concentration of isopropyl alcohol was too high, impurities is easy to come.3. In the hydrothermal-soft template reaction, pyrogallic acid was added to serve as reducing agent, surface growth directing template and carbon source to synthesize craggy-faced LiFePO4/C composite materials. During the crystal growth process, pyrogallic acid influences the ordering manner of the crystal growth, leading to the change in shape. The discharge capacity of LiFePO4synthesized with8wt%pyrogallic acid can get up to163.9mAh/g, it also exhibits a high discharge capacity of102.1mAh/g and retains about60%capacity at5C, demonstrated excellent high rate performance and good cycling performance. What’s more, the effect of the surfactant polyacrylamide (PAM) during soft template hydrothermal synthesis of LiFeP4O4was discussed. PAM owns amphiphilic molecules structure and amido bond, which can help to synthesize nano-particles. Combining its stereo-hindrance effect, loosen and porous network structure LiFePO4was prepared. The LiFePO4particles demonstrated outstanding electrochemical properties.