Ionic Charge Transport and Molecular Reorientation in Deep Eutectic Solvents studied by Dielectric Spectroscopy and Nuclear Magnetic Resonance

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Electrolytes are essential components of energy-storage and -conversion devices such as batteries, fuel cells or supercapacitors. The non-continuous availability of solar and wind energy and a future success of electromobility require to advance these technologies significantly. Finding better electrolytes, thus, is key to ensuring tomorrow's sustainable energy supply. A promising new class of electrolytes are deep eutectic solvents (DESs), which are superior compared to other materials concerning ease of preparation, low cost, sustainability and biocompatibility. DESs are commonly composed of a salt and a molecular hydrogen-bond donor. The mixing of the two components generates the typical eutectic melting-point reduction, rendering DESs liquid at room temperature. This can transform salts, that otherwise are crystalline at room temperature, to liquid electrolytes. As often found for eutectic mixtures, DESs tend to exhibit a glass transition when cooled sufficiently fast. To understand the ionic transport mechanism in DESs is an important goal to facilitate their optimization with respect to electrochemical applications.Within the present project, the ionic charge-transport and glass-forming properties of DESs shall be investigated using dielectric spectroscopy (DS) and nuclear magnetic resonance (NMR), supplemented by rheological and calorimetric measurements. DS is sensitive to the translational displacements of the ions as well as to the reorientational motions of the dipolar molecules, where the two dynamics can be coupled. NMR is able to detect both dynamics in a mixing-partner-specific manner by employing suitable nuclear probes and isotopomers. Thus, NMR ideally complements DS, which covers an exceptionally broad dynamic range but does not provide such selective information. Moreover, the combination of these methods is also ideal to investigate the glassy freezing of the ionic and molecular dynamics as they can provide detailed information concerning the continuous slowing down of these dynamics in the course of the glass transition.It is the goal of the present project to achieve a better understanding of the microscopic mechanisms that govern the ionic charge transport in DESs. Especially, we wish to explore the so-far often neglected reorientational motion in these materials and its relevance for the translational ion dynamics. In other classes of ionic conductors, the molecular reorientation was, suggested to open pathways for the ionic charge transport via a revolving-door-like mechanism. We aim at clarifying whether this may also play a role for DESs. Moreover, we plan to study the only sparsely investigated glass transition and the glassy properties of DESs in detail. Especially, it needs to be clarified how the glass temperature and the typical non-Arrhenian glassy dynamics affect the ionic charge transport in DESs.