Study On Preparation And Properties Of Cathode Material LifePO₄for Li-ion Battery

In recent years, olivine-type LiFePO₄material has attracted much attention for its hightheoretical capacity, excellent cycle performance, good thermal stability and low raw materialcost. However, LiFePO₄has poor electronic conductivity and low diffusion coefficient oflithium, which restrict its development. To improve the electrochemical performances ofLiFePO₄material, the preparation conditions, carbon coating, metal doping and electrodekinetic performance were studied in detail in this paper. The cathode material LiFePO₄/C wassynthesized by carbothermal method using Fe2O3and LiH2PO4as raw materials.The preparation conditions were discussed. The material was synthesized at750°C for16hours and the synthesis condition was optimized to be that the ration of Li/Fe was1:1andthe amount of carbon was8%. The charge and discharge performance of LiFePO₄wassignificantly improved and the discharge capacity was127.1mAh/g at0.1C rate afterball-milling.The influence of carbon coating for LiFePO₄surface modification on elecrochemicalproperties was also studied. The results showed that the surface modificated LiFePO₄samplecould be identified as a single LiFePO₄olivine structure when using glucose as the carbonsource. With the amount of glucose increasing, the particle size becomes small and uniform.The LiFePO₄/C synthesized with20wt%glucose has the best performance. The dischargecapacity reached140.6mAh/g,130.4mAh/g and89.1mAh/g at0.2C,1C,10C rate andretained97%after50cycles at1C rate.The structure and properties of LiFePO₄doped by Mg were also investigated. The resultsshowed that the sample doped a small amount of Mg（X≤0.05） could be identified as a singleLiFePO₄olivine structure. The discharge capacity of LiFe_0.98Mg_0.02PO₄/C are130mAh/g,124.6mAh/g and93.3mAh/g at0.2C,1C,10C rate，respectively, and retained98.6%after50cycles at1C rate. The charge-discharge cycle and high rate discharge performance ofLiFe0.98Mg0.02PO4/C doped with Mg was improved compare with that of the oringinal one.Finally, the kinetics of LiFePO₄was studied by using cyclic voltammetry andelectrochemical impedance spectroscopy measurements. The results showed that only oneoxidative and reductive reaction occurred during the charge-discharge process of LiFePO₄. LiFePO₄doped with Mg have better reversibility and less electrochemical polarization in theinsertion-extraction process, and also have less charge transfer resistance of the electrodesurface, which are conducive to the improvement of the electrochemical performance.