Anomalous behaviour of the ionic conductivity of nanoconfined IL -lithium salt mixtures

Abstract

The effect of salt addition on the protic ionic liquid (IL) ethylimidazolium nitrate is analysed in this work in terms of structural changes measured by NMR, thermal behaviour and ionic conductivity. Additionally, the effect of the immobilization of these mixtures in nano-silica structures on the same properties is also studied. Results show that the influence of salt on the chemical shifts of the signals associated to hydrogen atoms in the apolar regions of the ionic liquid is almost negligible, both in liquid and gel states. The salt accommodates in the polar region of the IL, according to the nanostructured solvation paradigm. The liquid range increases with salt concentration in both states, being this effect especially interesting for high and low temperature applications. The ionic conductivity takes appropriate values to use these compounds as electrolytes in electrochemical devices; pure IL presents 6.9 mS·cm−1 at 298 K rising to 17.38 mS·cm−1 at 323 K and this property decreases with salt concentration in the liquid state, as can be expected. Although gel samples present lower ionic conductivity than liquid samples, an anomalous behaviour can be observed in the ionogel samples upon the salt concentration, characterized by an absolute maximum, 13.26 mS·cm−1 at 323 K, at the lowest salt concentration (0.5 mol·kg−1 of Li+ salt addition), which represents the 12 % higher than the corresponding values of pure ionogel at this temperature.