Synthesies And Electrochemical Investigation Of Cathode Material LiFePO₄ For Lithium-ion Battery

Olivine-sturctured LiFePO₄ is considerd as a promissing cathode material of lithium ion battery due to its advantages such as environmental friendliness, abundant raw materials, inexpensive price and high safety. However, it suffers from low conductivity and poor rateperformance, which prevents it from commercial use. To improve the electrochemical performance of LiFePO₄ material, the synthesis condition, carbon coating, metal doping of LiFePO₄ were studied in detail.The cathode material LiFePO₄ was synthesized through solid-state reaction method, using LiOH·H₂O, NH₄H₂PO₄, FeC₂O₄·2H₂O as raw materials. The optimum processing were obtained from the effect of different preparing conditions. The effects of synthesizing temperature and the rate of Li/Fe on the electrochemical performances of LiFePO₄ material were discussed. The optimum conditions obtained from this work to be synthesized at 750℃for 18h under a flowing inert gas with the rate of Li/Fe was 1.10.LiFePO₄/C cathode material was synthesized through solid-state reaction method with glucose, sucrose, carbon black as carbon source. The crystalline sturcture, morphology, elelectrochemical performance were invetisgated by X-ray diffraction, scanning electronic microscope, hydrochloric acid analyzer and electrochemical tests. XRD results proved that the addition of carbon had not evident effect on the structure of LiFePO₄. The particles of LiFePO₄/C are smaller than that of LiFePO₄. Charge and discharge tests of simulated battery using LiFePO₄/C as cathode material showed that the initial discharge specific capacity of sample p3# is 157.5mAh·g^-1 in the current denisity of 0.1C and the highest is 164.9mAh·g^-1 at the 3rd cycle. After 10 cycles it could remain 161.7mAh·g^-1. Cyclic voltammetry results indicated that LiFePO₄/C has the same charge-discharge mechanism as LiFePO₄. The wrap of carbon could improve the electrochemical performance by increasing its conductance.The structure and properties of LiFePO₄ doped by Mg²⁺, Nb⁵⁺ and Cr³⁺were invetisgated. Mg²⁺ and Nb⁵⁺ dpoed LiFePO₄ exhibited initial discharge capcity Of 135.4mAh·g^-1and 148.4 mAh·g^-1. These indicate the doped mateiral has high dischcarge capacity and good cycling property...