Importance of rotational dynamics for dc-conductivity in ionic liquids
With the upcoming changes in our daily energy supply new classes of materials have come into the focus of academic research. One of them are room-temperature molten salts, so called ionic liquids, which are of importance due to their wide range of possible applications [1]. Particularly for the improvement of short-term energy storage in supercapacitors ionic liquids are already commercialized, but the large-scale application of these materials is still hampered by their limited ionic conductivity [2]. Therefor much of the research on ionic liquids is directed towards the discovery of higher dc-conductivities at room temperature. Due to this fact, a deeper understanding of the dynamical aspects and underlying mechanisms of the dc-conductivity in ionic liquids is necessary to find the optimized compound from the vast number of possible ion combinations. Moreover, it was already shown that dynamical properties in the glass transition region strongly influence the physical properties of ionic liquids [3]. Consequently the dielectric spectroscopy, well known as a method to study important properties of glass forming systems, and rate-dependent differential-scanning calorimetry measurements were used for precise determination of the relaxation dynamics of ionic liquids in the glass transition region. Measurements at the system BPyr dicyanamid revealed decoupling tendencies of the structural dynamics from the dc-conductivity while, in contrast, rotational and translational motions were strongly coupled. Those results indicate that the dc-conductivity in ionic liquids is directly related to the rotational motions of the molecules.
[1] M. Armand et al., Nat. Mater. 8, 621 (2009).
[2] D. R. MacFarlane et al., Energy Environ. Sci. 7, 232 (2014).
[3] P. Sippel et al., arXiv:1502.06851.