  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.
