Temperature dependence of ionic association of lithium triflate in acetate solutions

Ion transport is studied in polymer and organic liquid electrolytes due to the importance of these systems in rechargeable battery applications. We have used Fourier Transform Infrared Spectroscopy (FTIR) to study the temperature dependence of ionic association in solutions of lithium trifluoromethanesulfonate (LiCF$_{3}$SO$_{3}$, LiTf) dissolved in propyl, hexyl, octyl, and decyl acetates. The IR Spectra were recorded for three salt concentrations (0.1, 0.2, 0.5 mol kg$^{ -1}$) from 0 $^{\circ}$C to 80 $^{\circ}$C. Two spectral bands were analyzed at each temperature: the CF$_{3}$ symmetric bend ($\delta_{s}$) in the 740-780 cm$^{-1}$ region and the carbonyl stretch in the 1660-1800 cm$^{-1}$ region. Relative intensities of these bands were calculated by a curve fitting procedure. Three different LiTf ionic species were observed in the $\delta_{s}$(CF$_{3}$) region. The carbonyl band due to coordination with Li ion appears at a lower frequency (1714 cm$^{-1}$) than the pure carbonyl band (1744 cm $^{-1}$).