How Does Addition of Lithium Salt Influence the Structure and Dynamics of Choline Chloride-Based Deep Eutectic Solvents?

In recent times, deep eutectic solvents (DESs) have emerged as an environment-friendly alternative to both common organic solvents and ionic liquids (ILs). The present study has been undertaken with an objective to understand the intermolecular interaction, structural organization, and dynamics of two DES systems in the absence and presence of lithium salt so that the potential of these mixtures in electrochemical application is realized. For this purpose, the steady-state, time-resolved fluorescence, electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) behavior of two DESs (ethaline and glyceline) and their mixture with lithium bis(trifluoromethylsulfonyl) imide (LiNTf2) has been investigated. Measurements of polarity through EPR technique have revealed that the polarities of DESs are close to aliphatic polyhydroxy alcohol and the polarities of the medium increase with the increase in lithium salt concentration. Studies on solvation dynamics have indicated that there is an increase in average solvation time with the increase in lithium salt concentration. Investigation of rotational dynamics of some selected fluorophore in these media has shown that addition of lithium salt significantly alters the nano/microstructural organization of both DESs. Further, measurements of the self-diffusion coefficient through NMR have also supported the perturbation of the nanostructural organization of the solvent systems by addition of lithium salts. Essentially, all of these investigations have suggested that addition of lithium salt significantly alters the microscopic behavior of DESs. The outcome of this study is expected to be helpful in realizing the potential of these media for various electrochemical applications including application in lithium-ion battery.