Abstract: F-doped lithium iron phosphates were synthesized and in~situ carbon coated by two-step solid-state reaction using Fe(Ⅲ) compound as the iron source and polypropylene as the reductive agent and carbon source. The results show that the synthesized powders are near-spherical particles of 50--200nm with the olivine type LiFePO₄ structure. Two-step solid-state reaction inhibits the growth of LiFePO₄ particles more efficient compared with one-step solid-state reaction. The results of electrochemical measurement show that fluorin doping improves the rate capability of LiFePO₄ and decreases the polarization of LiFePO₄ electrode effectively. The discharge capacities of LiFePO_3.98F_0.02/C cathode material are 146mAh/g, 137mAh/g, 122mAh/g at charge and discharge rates of 1C, 2C, 3C, respectively. Cycling efficiency of 99.3% can be obtained after 55 cycles at 1C rate.

Key words: lithium iron phosphate, lithium ion batteries, cathode materials, fluorin doping