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

Abstract

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.

Deep eutectics and analogues as electrolytes in batteries

“…Overall, the Li salt causes an increase in viscosity (with LiPF6 < LiTFSI, Table 3), decrease in conductivity (Table 3), and decrease in 1 H diffusion values (for instance from 2.2 x 10 -11 m 2 s -1 in neat ChCl:EG to 0.65 x 10 -11 m 2 s -1 after addition of LiTFSI at 0.3 mole fraction). 175,176 In line with previous observations, EG-based systems exhibited lower viscosities, higher diffusivities and higher conductivity than systems based on G or LA, due to the presence of a less extensive hydrogen bonding network between ChCl and EG as well as within EG molecules. 175,176 ChCl:EG containing Li salts has then the potential to act as a discrete medium for electrochemical applications.…”

“…175,176 In line with previous observations, EG-based systems exhibited lower viscosities, higher diffusivities and higher conductivity than systems based on G or LA, due to the presence of a less extensive hydrogen bonding network between ChCl and EG as well as within EG molecules. 175,176 ChCl:EG containing Li salts has then the potential to act as a discrete medium for electrochemical applications. In particular ChCl:EG + LiPF6 0.5 M displayed a ionic conductivity of 7.95 mS cm −1 and viscosity of 88.4 mPa s at room temperature, together with a reasonable electrochemical stability against glassy carbon as the working electrode (3.5 V).…”

“…173 Mixtures of ChCl with different HBDs (EG, G, LA) were also tested upon addition of a Li salt (LiTFSI, LiPF6). 175,176 The addition of the salt significantly perturbs the nano/microstructural organization of the pristine DESs.…”

Structural properties of supercooled deep eutectic solvents: choline chloride–thiourea compared to reline

“…To provide pathways for broadening the DES applications, elucidation of the molecular level interactions, structural organization and structure–property relationship in DESs is certainly desirable. Different experimental and computational techniques have evidenced a nanoscopic heterogeneous environment in DESs: X-ray structural analysis, neutron scattering, vibrational spectroscopy, 9–11 time-resolved fluorescence, 12 nuclear magnetic resonance (NMR), 12 spin-probe electron paramagnetic resonance (EPR), 12–14 pulse-field gradient (PFG) nuclear magnetic resonance (NMR), 15–17 molecular dynamic (MD) simulations, 9,18 and others.…”

Choline chloride and ethylene glycol based deep eutectic solvent (DES) versus hydroxyl functionalized room temperature ionic liquids (RTILs): assessing the differences in microscopic behaviour between the DES and RTILs

“…[7][8][9] Since the starting materials for the preparation of DESs are relatively cheaper as compared to ILs, large scale preparation of DESs is possible for their use in various fields such as electrochemistry, 19,20 synthesis, 21 catalysis, 18,22 nanotechnology, 23,24 and gas adsorption. 25 Some notable experimental techniques like fluorescence, [26][27][28][29][30][31][32] small angle neutron scattering, 33,34 NMR diffusion [35][36][37][38][39] and theoretical studies based on molecular dynamics (MD) simulation [40][41][42][43][44][45] have been used to explore the microscopic structural organisation and dynamics of DESs and RTILs independently. Atkins and co-workers 17 suggested that ILs have huge structural diversity depending on the nature of ions as well as their selforganisation into ion pairs, ions' crystal lattice, H-bonding network and micellar domains.…”