Analysis of the ionic conductivity in lithium salt-containing ionic liquids based on the bond strength–coordination number fluctuation model

Abstract

The temperature dependence of the ionic conductivity of ionic liquids (ILs) and lithium salt mixtures (Li-ILs) that consist of N-methyl-N-propyl-pyrrolidinium (P13+) and 1-ethyl-3-methylimidazolium (emim+) as cations, and bis(trifluoromethanesulfonyl)amide (TFSA−) and bis(fluorosulfonyl)amide (FSA−) as anions has been analyzed in terms of the fractional Stokes–Einstein (FSE) law and the bond strength–coordination number fluctuation (BSCNF) model of viscosity. In the present paper a new expression for the ionic conductivity is introduced to describe the temperature dependence of the ionic conductivity. It is shown that the derived relation of the ionic conductivity is in good agreement with the experimental values for the TFSA−-based ILs and Li-ILs. Meanwhile, a noticeable change in the FSE behavior is confirmed for the case of FSA−-based system. The relation between the molar conductivity Λ and the viscosity η is also discussed based on the fractional form of these quantities, ΛT ∝ (T/η)p, which is in turn, in good agreement with the experimental value for both systems. The present analysis also shows that according to the strong/fragile classification of liquids, the FSA−-based ILs and Li-ILs are stronger than the TFSA−-based ones, implying that the component anions influence qualitatively the difference in electrical and structural relaxations.