  Glass formation and reorientational motions are widespread, but
often-neglected features of deep eutectic solvents, although both can be
relevant for the technically important ionic conductivity at room temperature.
Here we investigate these properties for two mixtures of ethylene glycol and
ZnCl2, which were recently considered as superior electrolyte materials for
application in zinc-ion batteries. For this purpose, we employed dielectric
spectroscopy performed in a broad temperature range, extending from the
supercooled state at low temperatures up to the liquid phase around room
temperature and beyond. We find evidence for a relaxation process arising from
dipolar reorientation dynamics, which reveals the clear signatures of glassy
freezing. This freezing also governs the temperature dependence of the ionic dc
conductivity. We compare the obtained results with those for deep eutectic
solvents that are formed by the same hydrogen-bond donor, ethylene glycol, but
by two different salts, choline chloride and lithium triflate. The four
materials reveal significantly different ionic and reorientational dynamics.
Moreover, we find varying degrees of decoupling of rotational dipolar and
translational ionic motions, which partly can be described by a fractional
Debye-Stokes-Einstein relation. The typical glass-forming properties of these
solvents strongly affect their room-temperature conductivity.
