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

• Published in: Electrochimica acta 2012-12, Vol.85, p.572-578
• Main Authors: Prabu, M., Reddy, M.V., Selvasekarapandian, S., Rao, G.V. Subba, Chowdari, B.V.R.
• Format: Article
• Language: English
• Subjects: Cathode; Diffraction; Electric potential; Electrochemical impedance spectroscopy; Electrodes; Impedance; Impedance spectroscopy; Indication; LiFePO4F; Lithium; Tavorite-structure; Voltage
• ISSN: 0013-4686; 1873-3859
• DOI: 10.1016/j.electacta.2012.08.073

[Display omitted] ► We report detailed impedance and electrochemical cycling behavior of LiFePO4F. ► The measured capacitance value is of the order of 10–30pF and thus confirmed the impedance due to the bulk of the material. ► The σ (ionic) is 0.6(±0.1)×10−7Scm−1 at 27°C which increases to 5.4(±0.1)×10−7Scm−1 at 50°C with an energy of activation (Ea)=0.75eV. ► 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. 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)×10−7Scm−1, respectively, with an energy of activation (Ea)=0.75eV. 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, 1.5–4V. LiFePO4F delivers an initial discharge capacity of 115(±3)mAhg−1 which increases to 119(±3)mAhg−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 3.3–2.6V 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.