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 (σ ionic) of LiFePO4F estimated using impedance analysis at 27 °C and at 50 °C are 0.6(±0.1) × 10−7 and 5.4(±0.1) × 10−7 S cm−1, respectively, with an energy of activation (Ea) = 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 0.1C rate in the voltage range, 1.5–4 V. LiFePO4F delivers an initial discharge capacity of 115(±3) mAh g−1 which increases to 119(±3) 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.

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► We report detailed impedance and electrochemical cycling behavior of LiFePO4F.
► The measured capacitance value is of the order of 10–30 pF and thus confirmed the impedance due to the bulk of the material.
► The σ (ionic) is 0.6(±0.1) × 10−7 S cm−1 at 27 °C which increases to 5.4(±0.1) × 10−7 S cm−1 at 50 °C with an energy of activation (Ea) = 0.75 eV.
► Reversible capacity shows increasing trend up to 20 cycles, then after degrades over 100 cycles, with a capacity loss of 19% due to electrode polarization.