Binary mixtures of ionic liquids based on EMIm cation and fluorinated anions: physico-chemical characterization in view of their application as low-temperature electrolytes

Abstract

Binary mixtures of ionic liquids (ILs) consisting of 1-ethyl-3-methylimidazolium [EMIm]+ cation coupled with fluorinated anions: bis(fluorosulfonyl)imide [FSI]-, bis(trifluoromethanesulfonyl)imide [TFSI]- and tetrafluoroborate [BF4]-, were formulated to obtain ionic fluids which are liquid at low temperature and to understand their properties. The thermal behaviour of [EMIm][TFSI]x[FSI](1-x), [EMIm][FSI]x[BF4](1-x) and [EMIm][TFSI]x[BF4](1-x) mixtures with various molar ratios of IL components was investigated with differential scanning calorimetry (DSC). They exhibited reduced enthalpy of the phase transitions and lower melting temperature compared to the parent ILs, or only vitrification/devitrification occurring below −90 °C in some cases where x ranges from 0.5 to 0.8, 0.1 to 0.6 or 0.2 to 0.8 for [EMIm][TFSI]x[FSI](1-x), [EMIm][FSI]x[BF4](1-x) and [EMIm][TFSI]x[BF4](1-x), respectively. For the formulations showing solely a glass transition, considered to be prospective low-temperature electrolytes, the density, viscosity and conductivity values were determined in a wide temperature range. The density demonstrated linear temperature dependence, whilst the dynamic viscosity and ionic conductivity followed the Vogel-Tamman-Fulcher equation. The corresponding Walden plot for pure ILs and their mixtures revealed a high degree of ionicity classifying them as “good ionic liquids” even down to −25 °C. The [EMIm][FSI]x[BF4](1-x) mixtures with x ranging from 0.5 to 0.6 displayed the lowest viscosity and highest conductivity, indicating them as promising electrolytes for IL-based electrochemical capacitors operating at low temperature, down to at least −40 °C.