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][NTf2]) 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][NTf2], [OHEMIM][NTf2]-LiNTf2(lithium bis(trifluorimethylsulfonyl)imide), [OHEMIM][NTf2]-EG, and [OHEMIM][NTf2]-EG-LiNTf2, 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][NTf2]-EG-LiNTf2 are close to those of neat RTIL. However, studies on rotational dynamics have revealed that the structural organization of [OHEMIM][NTf2]-LiNTf2 is significantly influenced upon addition of EG. Interestingly, the average solvation time is observed to be relatively faster in [OHEMIM][NTf2]-EG-LiNTf2 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][NTf2]-EG-LiNTf2. Translational diffusion coefficient measurements in [OHEMIM][NTf2]-EG-LiNTf2 and [OHEMIM][NTf2]-LiNTf2 through NMR spectroscopy have also indicated the suitability of [OHEMIM][NTf2]-EG-LiNTf2 as a potential electrolytic medium for battery applications.