The development of a new generation of electrolytes for energy storage, which simultaneously enable substantial improvements in energy and power density, safety and, reductions of environmental impact and cost, is a big challenge at the present time. Deep eutectic solvents (DES), which sometimes are considered a sub-class of ionic liquids due to their similar properties, have shown to be promising as electrolytes. These solvents are defined as a mixture of two or more species, typically a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA) which may be solid or liquid, and that at a particular composition present a melting point depression usually resulting in being a liquid at room temperature. Among their most relevant properties are low volatility, low flammability and wide electrochemical and thermal stability windows. Unlike typical ionic liquids, DESs can be inexpensive, readily synthesized, and can often be prepared from biodegradable, and nontoxic constituents. This makes them attractive candidates, when suitable electrochemically active reagents are added, for redox flow battery applications. On the other hand addition of lithium salts to DESs has recently shown promise toward inexpensive and safe electrolytes in lithium based batteries. For the work described in this presentation, the molecular rotational and translational dynamics of several different DESs based on choline chloride (ChCl) were studied using the fast field-cycling nuclear magnetic resonance relaxometry (FFC-NMR) and pulsed-field gradient (PFG) NMR techniques. Specifically, ChCl was chosen as the HBA at concentrations between 5 and 33mol% and glycerol, ethylene glycol (EG) or phenol were used as the HBDs. It was observed that the translational and rotational motions of all species became slower with increasing ChCl concentration for EG and phenol as the HBDs, but the opposite trend was observed for glycerol. Finally, transport properties of electrolytes prepared by addition of lithium salts commonly used in electrolyte formulations (i.e. LiTFSI, LiFSI) to EG/ChCl DES were investigated by PFG NMR.

Deep eutectic solvents based on choline chloride as electrolyte components for redox flow batteries or lithium batteries: NMR studies of molecular and ionic dynamics

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