Thermophysical and transport properties of blends of an ether-derivatized imidazolium ionic liquid and a Li+-based solvate ionic liquid

Abstract

The ‘solvate’ ionic liquid (IL), comprising of a 1:1 complex of lithium bis(trifluoromethanesulfonyl)amide (LiTf2N) and tetraglyme (G4), denoted herein by [Li(G4)][Tf2N], is promising as a relatively nonvolatile, nonflammable, and safer electrolyte for lithium-ion batteries. It, however, suffers from the drawback of low ionic conductivity compared with the conventional organic carbonate electrolytes. We report herein the enhancement in the thermal and transport properties of [Li(G4)][Tf2N] by blending it with an ether-derivatized imidazolium IL, namely 1-(2-methoxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, [mEtMeIm][Tf2N]. The volumetric and transport properties of [mEtMeIm][Tf2N], and its blends with [Li(G4)][Tf2N], were investigated at temperatures in the range of 10–85 °C using oscillating U-tube densitometry, cone and plate viscometry, and electrochemical impedance spectroscopy. The addition of [mEtMeIm][Tf2N] to [Li(G4)][Tf2N] lowered the viscosity and increased the ionic conductivity of the blends. The blends also exhibited improved thermal stability in thermogravimetry experiments. Notwithstanding the complex intermolecular interactions existing in the mixture of the [mEtMeIm][Tf2N], LiTf2N, and G4, the density, viscosity, and conductivity data could be modeled assuming the blend to be a simple binary mixture of [Li(G4)][Tf2N] and [mEtMeIm][Tf2N], instead of a ternary mixture of [mEtMeIm][Tf2N], LiTf2N, and G4. The addition of unchelated LiTf2N to [mEtMeIm][Tf2N] resulted in a decrease in ionic conductivity at all temperatures of measurement. However, when LiTf2N was added as a 1:1 complex with G4, the conductivity was higher, at the same molar concentration of LiTf2N. The implications of these results are discussed in view of developing thermally and chemically stable IL-based electrolytes for Li ion batteries, especially for operation at elevated temperatures.