Understanding the Microscopic Behavior of the Mixture of Ionic Liquid/Ethylene Glycol/Lithium Salt through Time-Resolved Fluorescence, Nuclear Magnetic Resonance (NMR), and Electron Paramagnetic Resonance (EPR) Studies

Abstract

The present study has been undertaken with an aim to find out the suitability of a binary solvent system, comprising an anionic liquid, 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([OHEMIM][NTf]) and ethylene glycol (EG), toward lithium ion battery applications. For this purpose, the behavior in terms of structure, intermolecular interaction, and dynamics of several solvent systems, [OHEMIM][NTf], [OHEMIM][NTf]-LiNTf(lithium bis(trifluorimethylsulfonyl)imide), [OHEMIM][NTf]-EG, and [OHEMIM][NTf]-EG-LiNTf, is investigated by carrying out steady state and time-resolved fluorescence, nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR) measurements. Both steady-state fluorescence and EPR studies have pointed out that the micropolarities of [OHEMIM][NTf]-EG-LiNTf are close to those of neat RTIL. However, studies on rotational dynamics have revealed that the structural organization of [OHEMIM][NTf]-LiNTf is significantly influenced upon addition of EG. Interestingly, the average solvation time is observed to be relatively faster in [OHEMIM][NTf]-EG-LiNTf than those in other solvent systems. Since average solvation time and conductivity are inversely related to each other, the present observation indicates that the introduction of EG is helpful in increasing the electrical conductivity of [OHEMIM][NTf]-EG-LiNTf. Translational diffusion coefficient measurements in [OHEMIM][NTf]-EG-LiNTf and [OHEMIM][NTf]-LiNTf through NMR spectroscopy have also indicated the suitability of [OHEMIM][NTf]-EG-LiNTf as a potential electrolytic medium for battery applications.

Effect of Lithium-Ion on the Structural Organization of Monocationic and Dicationic Ionic Liquids

“…From the temperature-dependent viscosities plots that are shown in Figure , one can also see that the viscosity of IL media increases with the adding of Li + ion concentration. Such increase in viscosity has also been obtained by other researchers while working on ionic liquid-electrolyte systems. ,,, We also note here that the viscosities of the monocationic as well as dicationic ILs may follow nonexponential behavior which can be expressed by the Vogel–Tammann– Fulcher (VTF) equation for a wide range of temperatures . The observation of exponential behavior in the present study might have happened since the data are collected within a small temperature range.…”

“…Pulse field gradient NMR (PFG-NMR) technique used for measuring the translational diffusion of ions in a given media is expected to be helpful in understanding the microscopic structural organization of the fluid system. ,,, In view of this, we have investigated the translational diffusion of the ILs, specifically the cationic part of the ILs, in the absence and presence of lithium salt by monitoring the 1 H NMR. Table and Figure illustrates the D value of both MILs and DIL in the absence and presence of lithium salt in a temperature range of 298–318 K. From Table ; we can see that at any particular temperature, the value of D is found to be smallest for [C 6 (mim) 2 ]­[NTf 2 ] 2 .…”

“…While the majority of the research work in this regard is carried out by employing monocation-based ILs, such studies on another type of ILs, for example, geminal dicationic ILs (GDILs), are very limited. Moreover, since recent studies − have demonstrated that anions of ILs aggregates in the presence of lithium-ion leading to an increase in viscosity of the medium, proper understanding on the structure and dynamics of ILs in the absence and the presence of lithium-ion becomes a worthwhile objective.…”

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

“…But the average solvation times (t solv ) in these media, as reported earlier (e.g. t solv = 160 ps for ethaline and 560 ps for OHEMIM), 31,68 are found to be much higher than the t avg of ANF. Therefore, it is possible to observe fluorescence from the excited species which are not fully solvent equilibrated.…”

“…A more detailed understanding about the interaction of C153 with both the DES and ILs can be obtained by considering the friction coefficient, C obs value (ratio between experimentally measured t r and theoretically calculated t r values). 26,68 Since the C obs value is obtained by the aforementioned ratio, the value of C obs usually depicts the extent of solute-solvent interaction. 26,69 Here, we would like to mention that an increase in temperature can also cause a change in the hydrogen bonding and thus change the structure of the medium which would also be expected to cause a change in the C obs value.…”