Preparation And Electrochemical Properties Of Lithium Iron Phosphates As Cathode Material

With the rapid consuming of fossil fuel and the increasingly seriousenvironmental problems, the development and utilization of green and cleanresources become more and more meaningful. As a next generation of green energystorage with high capacity, lithium ion batteries (LIBs) are widely used in portableelectronic products and new resources for transportation vehicles, such as EV, HEV.LiFePO4(LFP) was reported as a positive electrode material for LIBs with excellentproperties. Compared to traditional cobalt-based materials, LFP shows theadvantages of inherent merits including low toxicity, potential for low cost andabundant raw materials, excellent thermal stability, high safety, long cycle abilityand high theoretical capacity, which makes it the most promising candidate forpower batteries.In this thesis, we performed a solvothermal reaction to synthesis the positivematerial LiFePO4sucessfully, using ferric (Ⅲ) salt as iron source, EG as solvent andthe molar ratio of starting materials was Fe: Li: P=1:1:1. The structure,morphology and electrochemical properties of our synthesized LiFePO4werecharacterized by XRD, SEM, IR and electrochemical testing system. The results ofIR test revealed that the solvent, EG, also played a role as reductant to reduce Fe3+toFe2+in this solvothermal strategy. Products with different morphology were obtainedby changing the condition of our solvothermal reaction, such as time, temperatureand concentration. We proposed a new sphere-forming mechanism when weconducted a study on the forming mechanism of the Sphere-like and Dumbbell-likeproducts. Because of the different growth rates in different part of the rod-shapedor pancake-like shapes, the sphere-shaped morphology was formed by enclosing theends toward the middle. And we called this mechanism “orientation as in a magneticfield”, vividly. We also found phenomena of gelation in the solvothermal reaction.According to result of electrochemical test, products with different morphologyshowed different electrochemical properties. The nest-like, sphere-like, dumbbell-like, and belt-like LiFePO4exhibited initial discharge capacities of113.5,101.7,81.8, and50.5mAh/g at a current density of20mA/g, respectively.Precursors NH4FePO4·H2O with different nature of preferred orientation wereprepared by hydrothermal and solution processes. Then LiFePO4products weresynthesized through high temperature solid state reaction by calcining the mixture ofprecursors, lithium source and carbon source. The crystal structure, morphology andelectrochemical properties of our synthesized LiFePO4were characterized by XRD,SEM, and electrochemical testing system. According to SEM images, precursors andLiFePO4presented plate-like morphologies. And XRD data revealed that productLFP-1synthesized from the hydrothermal treated precursor showed a preferredorientation along (100), while LFP-2synthesized from precursor without anyaddition of NH4HCO3seemed normal. LFP-1, LFP-2and LFP-3synthesized fromprecursor through a solution process exhibited initial discharge capacities of91.5、120.2、138.7mAh/g at the current density of20mA/g, respectively.