  Lithium-salt-based deep eutectic solvents, where the only cation is Li+, are
promising candidates as electrolytes in electrochemical energy-storage devices
like batteries. We have performed broadband dielectric spectroscopy on three
such systems, covering a broad temperature and dynamic range that extends from
the low-viscosity liquid around room temperature down to the glassy state
approaching the glass-transition temperature. We detect a relaxational process
that can be ascribed to dipolar reorientational dynamics and exhibits the clear
signatures of glassy freezing. We find that the temperature dependence of the
ionic dc conductivity and its room-temperature value also are governed by the
glassy dynamics of these systems, depending, e.g., on the glass-transition
temperature and fragility. Compared to the previously investigated
corresponding systems, containing choline chloride instead of a lithium salt,
both the reorientational and ionic dynamics are significantly reduced due to
variations of the glass-transition temperature and the higher ionic potential
of the lithium ions. These lithium-based deep eutectic solvents partly exhibit
significant decoupling of the dipolar reorientational and the ionic
translational dynamics and approximately follow a fractional
Debye-Stokes-Einstein relation, leading to an enhancement of the dc
conductivity, especially at low temperatures. The presented results clearly
reveal the importance of decoupling effects and of the typical glass-forming
properties of these systems for the technically relevant room-temperature
conductivity.
