Lithium coordination and diffusion coefficients of PEGylated ionic liquid and lithium salt blends: A molecular dynamics simulation study

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PEGylated imidazolium ionic liquids are of interest because of their non-volatility, nonflammability, and unique ion transport properties attributed to poly(ethylene glycol) (PEG). In this work, we used molecular dynamics (MD) simulations to study the interactions of Li+ in an equimolar blend of 1-(monomethoxy PEG)-2-methylimidazolium bistriflamide ([PEGmim][Tf2N]) and lithium bistriflamide (LiTf2N) salt. Analysis of the radial distribution function for Li+ showed that the oxygen atoms from the bistriflamide anion are replaced by, on average, 3.1 oxygen atoms from the PEG side groups, indicating improved solvation of the lithium salt in the PEGylated ionic liquid than in conventional alkylated ionic liquids. Li+ coordination number showed an increase in the affinity for PEG oxygen atoms when the temperature increased from 300 to 350 K. The mean square displacement (MSD) versus time data were analyzed using a combination of a power-law model, representative of the subdiffusive regime, and a linear model for the Brownian diffusion. A transition from subdiffusive to normal diffusion behavior was observed at around 10 ns for all ionic species present in the system. Contrary to the expected trend based on ion sizes, the diffusion coefficients in the linear MSD regime followed the trend [PEGmim]+ > [Tf2N]− > Li+, with the larger [PEGmim]+ exhibiting the highest diffusion coefficient. Interestingly, the mean square displacement profile, normalized by the square of the ion size represented by a characteristic radius, was similar for the cation and anion, indicating equivalent ionic displacements in the absence of an electric field. In the asymptotic limit, the ratio of the MSD of [PEGmim]+ to the MSD of [Tf2N]− was about 1.3. Diffusion coefficients of [PEGmim]+, measured using pulsed field gradient stimulated echo nuclear magnetic resonance spectroscopy, showed excellent correlation with the liquid viscosity, based on the Sutherland–Einstein equation, for both the neat ionic liquid and its blend with the lithium salt. Values of diffusion coefficients increased with temperature, consistent with an Arrhenius diffusion behavior with an activation energy of about 49 kJ mol−1 for [PEGmim][Tf2N] and 55 kJ mol−1 for [PEGmim][Tf2N] + LiTf2N.