Electrochemical behavior of LiFePO4/C cathode material for rechargeable lithium batteries

Electrochemical behavior of LiFePO4/C cathode material synthesized by ballmilling technique from a mixture of Fe powder, FePO4 center dot 4H2O, Li3PO4 center dot 1/2H(2)O, and sucrose was studied in half-cells using lithium metal as anode. The prepared LiFePO4/C cathode material exhibits excellent rate capability with initial discharge capacities of 141 mAh/g (1 C ), 127 mAh/g (2 C), 119 mAh/g (3 C), and 109 mAh/g (5 C), respectively. Good cycling performance under 2 C charge-discharge rate could be obtained at room temperature with a discharge capacity of 125-131 mAh/g for 300 cycles. Impedance spectroscopy was applied to investigate the material behavior during the 2 C rate charge-discharge cycling. It is found that the Nyquist plots of the cycled lithium cell at discharged state are comprised of one or two semicircles in high-to-medium frequency range followed by an inclined line in low-frequency range. The first semicircle at higher frequency shows close relationship with the cycling performance of the lithium cell. Thus, it may be used to predict the cycle life of the LiFePO4/C cathode. Through a combined X-ray diffraction (XRD) analysis and scanning electron microscope observation on the LiFePO4/C cathode upon the 2 C rate cycling, we correlated the observed capacity fading after 300 cycles at 2 C charge-discharge rate with the LiFePO4 particle cracking and the increase of particle contact resistance. No significant phase change of LiFePO4/C was observed even after 800 cycles according to the XRD patterns. (c) 2005 The Electrochemical Society.