Ionic liquid derived novel deep eutectic solvents as low viscous electrolytes for energy storage

Abstract

This work investigates a newly emerging class of solvents to be used as alternatives to or complementarily with ionic liquid (IL) electrolytes. Three novel deep eutectic solvents (DESs) comprised of the hydrogen bond donors (HBDs), namely N-Methylacetamide (NMAc) and formamide, were formulated with three variants of the methyl imidazolium-based ionic liquids (ILs) as hydrogen bond acceptors (HBAs). ILs consisting of 1-butyl-3-methyl imidazolium ([BMIM]) cation with anions namely methanesulphonate ([MeSO3]), bis(trifluoromethylsulfonyl)imide ([Tf2N]), and hexafluorophosphate ([PF6]) were used as hydrogen bond acceptors. Initially, the eutectic points of the DESs were estimated using a quantum chemical-based thermodynamic model, namely Conductor like Screening Model-Segment Activity Coefficient (COSMO-SAC). The calculated melting points of the DESs were determined to be markedly lower than those of both the constituent components. Further, physical and chemical characterization was done on the synthesized DESs. The viscosities (<20 cp) were found to be significantly lower than those of conventionally used pure room temperature ILs, and ionic conductivities (>5 mS/cm) were found to be appreciably higher than those of the corresponding constituent ILs. Thermogravimetric analysis revealed negligible mass loss up to 80–100 °C. Linear scan voltammetry (LSV) was employed to determine the electrochemical stable potential window (ESPW) of the solvents with a glassy carbon (GC) electrode. LSV provided electrochemical stabilities in the range of 3.8–4 V for all three studied deep eutectic solvents, comparable to pure ILs and far higher than the potential windows of mixtures of ILs and organic solvents, a blend commonly used as electrolytes.