The importance of transport property studies for battery electrolytes: revisiting the transport properties of lithium–N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide mixtures

Abstract

Transport properties are examined in some detail for samples of the low temperature molten salt N-propyl-N-methyl pyrrolidinium bis(fluorosulfonyl)imide [Pyr₁₃][FSI] from two different commercial suppliers. A similar set of data is presented for two different concentrations of binary lithium–[Pyr₁₃][FSI] salt mixtures from one supplier. A new and significantly different production process is used for the synthesis of Li[FSI] as well as the [Pyr₁₃]⁺ salt used in the mixtures. Results for the viscosity, conductivity, and self-diffusion coefficients, together with the density and expansivity and apparent molar volume, are reported over the temperature range of (0 to 80) °C. The data for neat [Pyr₁₃][FSI] are discussed in the context of velocity cross correlation (VCC or f_ij) and Laity resistance (r_ij) coefficients. Unusually, f₊₋ ∼ f₊₊ < f₋₋. The three resistance coefficients are of similar magnitude indicating all three ion–ion interactions contribute to the transport properties, not just the cation–anion interaction. The composition dependence of the transport properties is compared to previously reported data for the same and related compounds: in contrast to high-temperature molten salt mixtures, this is an exponential dependence. The Nernst–Einstein parameter Δ, which contains information on the correlations of the ionic velocities and is determined by differences in the VCC for the various ion–ion combinations, was calculated for both the neat ionic liquid and its binary mixture. It increases with increasing lithium concentration. The new data set also allows some conclusions with regards to the lithium–[FSI]⁻ coordination environment.