(29d) Poly(ethylene glycol) Containing Ionic Liquids and Lithium Salt Blends: An Experimental and Molecular Dynamics Simulation Study

PEGylated imidazolium ionic liquids are of interest because of their non-volatility, non-flammability, and unique ion transport properties attributed to poly(ethylene glycol) (PEG). The ether oxygen atoms in the PEG side groups interact with electron deficient centers in the imidazolium ring, leading to increased separation of cation–anion pairs in the fluid, higher ionic conductivity, and higher than expected fluidity for these relatively high molar mass compounds [1]. However, the addition of lithium salts to these ionic liquids is found to result in a significant increase in viscosity, an increase in the glass transition temperature, and a decrease in ionic conductivity [2].

In this work, we used molecular dynamics (MD) simulations to study the interactions of Li⁺ in a blend of 1-(monomethoxy PEG)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (bistriflamide) ([PEGMeIm][Tf₂N]) ionic liquid and lithium bistriflamide (LiTf₂N) salt. Simulations show that Li⁺ coordinates not only with the ether oxygen atoms of the PEG side group, but also with the oxygen and nitrogen atoms of the [Tf₂N]^– anion, which can adversely affect Li⁺ mobility. This finding is corroborated by the following trend of self-diffusion coefficients obtained from the mean squared displacement data: [PEGMeIm]⁺ > [Tf₂N]^– > Li⁺.

Toward improving the lithium ion mobility, we explored solvating it with tetraethylene glycol dimethyl ether (tetraglyme, G4). The 1-(2-methoxyethyl)-3-methylimidazolium bistriflamide ([mEtMeIm][Tf₂N]) ionic liquid, was blended with 1:1 complex of LiTf₂N and G4 (a solvate ionic liquid, denoted herein by [Li(G4)][Tf₂N]). The addition of [mEtMeIm][Tf₂N] to [Li(G4)][Tf₂N] lowers the viscosity and increases the ionic conductivity of the blends, compared with the ionic conductivity of neat [Li(G4)][Tf₂N] [3]. The blends also exhibit improved thermal stability in thermogravimetry experiments.

The dissolution of unchelated LiTf₂N in [mEtMeIm][Tf₂N] results in a decrease in the ionic conductivity at all temperatures of measurement. However, when LiTf₂N is added as a 1:1 complex with G4, the conductivity is higher (at the same molar concentration of LiTf₂N). We also find that the complex mixture, comprising of three different ionic species (the lithium ion, and the ionic liquid cation and anion) and one nonionic compound (tetraglyme), of significantly different chemical structures and polarities, show nearly linear mixing rules for density, viscosity and conductivity data.

In this talk, the experimentally determined diffusion coefficients and other transport properties will be analyzed in the context of the MD derived radial distribution functions and coordination numbers.

References

[1] Ganapatibhotla LV, Zheng J, Roy D, Krishnan S. PEGylated imidazolium ionic liquid electrolytes: thermophysical and electrochemical properties. Chemistry of Materials. 2010; 22(23): 6347-6360.

[2] Wu L, Venkatanarayananan RI, Shi X, Roy D, Krishnan S. Glass transition, viscosity, and conductivity correlations in solutions of lithium salts in PEGylated imidazolium ionic liquids. Journal of Molecular Liquids. 2014; 198: 398-408.

[3] Wang Y, Turk MC, Sankarasubramanian M, Srivatsa A, Roy D, Krishnan S. Thermophysical and transport properties of blends of an ether-derivatized imidazolium ionic liquid and a Li⁺-based solvate ionic liquid. Journal of Materials Science. 2017; 52(7): 3719-3740.