Ionic liquids (ILs) are salts that are liquid close to room temperature. Their possible applications are numerous, e.g., as solvents for green chemistry, in various electrochemical devices, and even for such “exotic” purposes as spinning-liquid mirrors for lunar telescopes. Here we concentrate on their use for new advancements in energy-storage and -conversion devices: Batteries, supercapacitors or fuel cells using ILs as electrolytes could be important building blocks for the sustainable energy supply of tomorrow. Interestingly, ILs show glassy freezing and the universal, but until now only poorly understood dynamic properties of glassy matter, dominate many of their physical properties. We show that the conductivity of ILs, an essential figure of merit for any electrochemical application, depends in a systematic way not only on their glass temperature but also on the so-called fragility, characterizing the non-canonical super-Arrhenius temperature dependence of their ionic mobility.

Ionic liquids are promising candidates for electrolytes in energy-storage systems. We demonstrate that mixing two ionic liquids allows to precisely tune their physical properties, like the dc conductivity. Moreover, these mixtures enable the gradual modification of the fragility parameter, which is believed to be a measure of the complexity of the energy landscape in supercooled liquids. The physical origin of this index is still under debate; therefore, mixing ionic liquids can provide further insights. From the chemical point of view, tuning ionic liquids via mixing is an easy and thus an economic way. For this study, we performed detailed investigations by broadband dielectric spectroscopy and differential scanning calorimetry on two mixing series of ionic liquids. One series combines an imidazole based with a pyridine based ionic liquid and the other two different anions in an imidazole based ionic liquid. The analysis of the glass-transition temperatures and the thorough evaluations of the measured dielectric permittivity and conductivity spectra reveal that the dynamics in mixtures of ionic liquids are well defined by the fractions of their parent compounds.

Modern ionic liquids (ILs) are considered green solvents for the future applications due to their inherited advantages and remarkable transport properties. One of the ubiquitous properties of ILs is their intrinsic ionic conductivity. However, understanding of the super-Arrhenius behavior of the ionic conductivity process at elevated pressure still remains elusive and crucial in glass science. In this work, we investigate the ion transport properties of 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide: [Cmim][NTf], 1-butylimidazolium bis[(trifluoromethyl)-sulfonyl]imide: [CHim][NTf] and 1-butylimidazolium hydrogen sulfate: [CHim][HSO] ILs in the supercooled liquid state using dielectric spectroscopy at ambient and high pressure. We present the experimental data in the dynamic window of the conductivity formalism to examine the charge transport properties. The frequency-dependent ionic conductivity data have been analyzed using the time-temperature superposition principle. In the Arrhenius diagram, the thermal evolution of the dc-conductivity reveals similar temperature dependence for both protic and aprotic ILs thus making it difficult to distinguish the ion dynamics. However, our results demonstrate the key role of high pressure that unambiguously separates the charge transport properties of protic ILs from aprotic ones through the apparent activation volume parameter. We also highlight that the activation volume can be employed to assess the information connecting the ability of ionic systems to form H-bond networks and the impact of proton transfer involved in the conduction process.