(643d) Investigation of Ion Correlations in Ionic Liquid and Organic Solvent Mixtures through Scalable Screening

Ray A. Matsumoto^1,2, Matthew W. Thompson^1,2, and Peter T. Cummings^1,2

¹Multiscale Modeling and Simulation Center, Vanderbilt University, Nashville, TN, 37235-1604, USA

²Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, TN, 37235-1604, USA

ABSTRACT

Considerable efforts have been made to improve the performance of supercapacitors, electrochemical devices that exhibit high power density but low energy density. As part of these efforts, room temperature ionic liquids have shown to improve the energy density of supercapacitors due to their wide operational voltage windows in comparison to other electrolytes¹. Ionic liquids however exhibit high viscosities and slow dynamic properties which can negatively affect the charging rates of such devices. To mitigate slow dynamics, ionic liquids can be solvated in organic solvents without a substantial loss in operational voltage window².

We have previously used a screening approach to encompass a larger chemical space and expand our knowledge of interactions between ionic liquids and organic solvents. As a result, we implemented the necessary algorithms and force fields within MoSDeF³ to conduct screening of [BMIM⁺][Tf2N^-] in 22 distinct solvents over a range of 18 different concentrations. Self-diffusivities of ions in these solutions were computed via mean squared displacement (MSD) calculations, while conductivity was estimated via the Nernst-Einstein (NE) equation. From the ability to sample a larger portion of the solvent-ionic liquid design space, we were able to identify several key trends related to ionic liquid diffusivity and conductivity.

Using the previous screening parameter space, we now focus our efforts towards understanding the structure of these ionic liquid and organic solvent mixtures. Ion correlations have been well-characterized for bulk or so-called “neat” ionic liquids, which have shown to impact conductivity through the concept of “ionicity” ⁴. On the other hand, ion correlations in ionic liquid and solvent mixtures are still relatively unknown and therefore, we characterize ion correlations within these ionic liquid and organic solvent mixtures by analyzing ion pairing and caging. Furthermore, we compare the Einstein-Helfand (EH) approach⁵ to the NE equation for estimating the ionic conductivity of these solutions. Unlike the NE equation, the EH formalism considers ion correlations, which enables us to investigate ionicity through a ratio of the two conductivities. A greater understanding of the relationship between ion correlations and dynamics presented can further guide the selection of solvents and concentration for electrochemical applications of ionic liquids.

References

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[2] R. Lin, P. Huang, J. Segalini, C. Largeot, P.L. Taberna, J. Chmiola, Y. Gogotsi, P. Simon, “Solvent Effect on the Ion Adsorption from Ionic Liquid Electrolyte Into Sub-nanometer Carbon Pores”, Electrochimica Acta,2009, 54, 7025-7032.

[3] “MoSDeF” [Online]. Available: https://github.com/mosdef-­hub.

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