Abstract
Lithium-salt-based deep eutectic solvents, where the only cation is Li+, are promising candidates as electrolytes in electrochemical energy-storage devices, such as 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 in 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.
Highlights
Deep eutectic solvents (DESs) have come into the focus of recent research as their properties make them promising candidates for numerous applications, e.g., in material synthesis or electrochemical devices.[1,2,3,4,5,6,7,8,9,10,11] Most DESs are easy to produce, sustainable, and biocompatible, partly being even composed of constituents found in nature
We provide the real part of the conductivity σ′, which is proportional to ε′′ν and enables the direct identification of the dc conductivity
We have performed broadband dielectric spectroscopy measurements on three DESs where the hydrogen-bond acceptor is a lithium salt. These measurements cover a broad dynamic range extending from the low-viscosity liquid around room temperature well into the supercooled-liquid state and approaching the glass state close to Tg
Summary
Deep eutectic solvents (DESs) have come into the focus of recent research as their properties make them promising candidates for numerous applications, e.g., in material synthesis or electrochemical devices.[1,2,3,4,5,6,7,8,9,10,11] Most DESs are easy to produce, sustainable, and biocompatible, partly being even composed of constituents found in nature. In the most common class of DESs, these components are molecular hydrogen-bond donors (HBDs), such as glycerol or urea mixed with a salt acting as a hydrogen-bond acceptor The latter commonly is a quaternary ammonium salt, like in the often-investigated DESs (glyceline, ethaline, and reline), which are mixtures of choline chloride with glycerol, ethylene glycol, or urea, respectively, all with 1:2 molar ratio. Several previous works have revealed that the reorientational molecular motions, present in certain ionic conductors, including ionic liquids and superionic and plastic crystals, seem to be highly relevant for ionic mobility.[19–22] Consistent with this notion, for the DESs glyceline and ethaline, we recently found a close correlation of their ionic dc conductivity with the reorientational dynamics as detected by dielectric spectroscopy.[23]. It should be noted that so far there are only a few earlier dielectric investigations of DESs16,23,24,26–30 often not addressing the reorientational dynamics, and to our knowledge, none of them has tackled the technically relevant purely Li-saltbased DESs
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
