Synthesis, impedance and electrochemical studies of lithium iron fluorophosphate, LiFePO4F cathode

Tavorite-structured LiFePO4F is synthesized by solid-state reaction and characterized by Rietveld refined X-ray diffraction, scanning electron microscopy, BET surface area, IR and Raman spectroscopy techniques. The ionic conductivity (a ionic) of LiFePO4F estimated using impedance analysis at 27 degrees C and at 50 degrees C are 0.6(+/- 0.1) x 10(-7) and 5.4(+/- 0.1) x 10(-7) S cm(-1), respectively, with an energy of activation (E-a) = 0.75 eV. Its electrochemical behavior were examined by galvanostatic charge-discharge cycling up to 100 cycles, cyclic voltammetry (CV) and electrochemical impedance spectroscopy using Li-metal as the counter and reference electrode, at 0.1 degrees C rate in the voltage range, 1.5-4V. LiFePO4F delivers an initial discharge capacity of 115(+/- 3)mAh g(-1) which increases to 119(+/- 13) mAh g(-1) at the 20th cycle. The capacity degrades slowly thereafter over 100 cycles, with a capacity loss of 19%. CV data show a clear indication of the Fe3+/2+ redox couple at 3.3-2.6 V that involves a two-phase reaction. Electrochemical impedance spectra measured at various voltages at selected cycles were fitted to an equivalent circuit and the variation of impedance parameters interpreted. C) 2012 Elsevier Ltd. All rights reserved.