ABSTRACT
The physicochemical properties of the ionic liquid N-ethyl-2-(2-methoxyethoxy)-N,N-bis(2-(2-methoxyethoxy)ethyl)ethan-1-ammonium bis(trifluoromethylsulfonyl)imide (N2(2O2O1)3TFSI) as well as its solutions with sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) and sodium bis(fluorosulfonyl)imide (NaFSI) are compared in order to study the effects of the anion. The NaFSI solutions show weaker interactions as suggested by lower glass transition temperatures, lower densities, lower viscosities and higher conductivities as compared to their more strongly coordinating NaTFSI analogues. The transport properties follow Vogel-Tamman-Fulcher behaviour suggesting that the mixtures are fragile glass formers. The addition of a higher concentration of Na salts results in decoupling of the translational motion from the viscosity, as observed from Walden plots, and 23Na NMR data suggests that the sodium speciation is independent of the nature of the Na salt or the temperature but is affected by the salt concentration.
ABSTRACT
The ion dynamics in a novel sodium-containing room-temperature ionic liquid (IL) consisting of an ether-functionalised quaternary ammonium cation and bis(trifluoromethylsulfonyl)amide [NTf2 ] anion with various concentrations of Na[NTf2 ] have been characterised using differential scanning calorimetry, impedance spectroscopy, diffusometry and NMR relaxation measurements. The IL studied has been specifically designed to dissolve a relatively large concentration of Na[NTf2 ] salt (over 2â mol kg-1 ) as this has been shown to improve ion transport and conductivity. Consistent with other studies, the measured ionic conductivity and diffusion coefficients show that the overall ionic mobility decreases with decreasing temperature and increasing salt content. NMR relaxation measurements provide evidence for correlated dynamics between the ether-functionalised ammonium and Na cations, possibly with the latter species acting as cross-links between multiple ammonium cations. Finally, preliminary cyclic voltammetry experiments show that this IL can undergo stable electrochemical cycling and could therefore be potentially useful as an electrolyte in a Na-based device.