Volumetric and viscosity properties of water-in-salt lithium electrolytes: A comparison with ionic liquids and hydrated molten salts

Abstract

The density and viscosity of LiTf, LiTFSI and LiTFSI + LiTf (mole ratio 3:1) aqueous solutions have been measured at temperatures between 25 °C and 55 °C, over a wide range of concentrations covering the Water-in-Salt (WiS) region, where no free water is present in the system. As it was observed in mixtures of ionic liquids with water and mixtures of melted salt hydrates, the molar volumes of these WiS electrolytes are linear functions of the salt mole fraction. We propose a new procedure to calculate the intrinsic volume of the salts in the WiS solutions, corresponding to the volume of the hypothetical supercooled pure salts. The contributions of electrostriction and conformational changes of the anion to the partial molar volume of the WiS are discussed. The presence of Li+ ions in the salts free of water (supercooled salts) produces a large contraction of the Tf− and TFSI− volumes as compared with ionic liquids containing the same anions in contact with bulky cations. In terms of the apparent partial molar volume of water we could identify a dilute regime (x ≤ 0.1) where the volumetric properties are dominated by the water electrostriction, and a WiS regime (x > 0.1), without free-water, where the molar volume is determined by the volumes of the hydrated Li+ ion and the corresponding dehydrated anion, compatible with a proposed WiS structure formed by a percolating network of anions embedded by Li(H2O)n+ cations. The excess volume of the ternary WiS (LiTf + LiTFSI) is very small at all temperatures and concentrations, while the excess viscosity is positive and small, but increases near the solubility limit. The viscosity of the WiS electrolytes exhibit a normal Arrhenius dependence, but simple extrapolation to the glass transition temperature indicates that the WiS electrolytes behave as fragile fluids.