(133e) Investigating the Diffusivity of Ionic Liquids in Solvent-in-Salt System Using Molecular Dynamics Simulation

Highly concentrated salt electrolytes, known as solvent-in-salt (SIS) systems, have attracted significant attention recently as electrolytes for energy storage devices.^1–3 Due to the high cation transference number (the ratio of the diffusion coefficient of cation to the total diffusion coefficient of ions), wide range of electrochemical stability, suppression of lithium dendrite formation, low flammability, low volatility, and high charge density, SIS systems are now considered as promising electrolyte systems for the next generation batteries and supercapacitors. Compared to diluted electrolytes and pristine ionic liquids, it has been shown that the small amount of solvent makes dynamics and structure of SIS systems significantly different.^4,5. However, the microscopic transport properties of ions in SIS systems remain poorly studied. Thus, understanding the factors that influence the charge transport in SIS systems is crucial for their design and applications.

In this presentation, we discuss our recent work⁶ on SIS systems consisting of highly concentrated LiTFSI-acetonitrile, and LiTFSI-water, in which the ratio of ion to solvent is 1:3. Using a combination of broadband dielectric spectroscopy (BDS) and molecular simulation, we analyzed the role of ion-ion correlation, ion-solvent correlation, and solvent-solvent correlation on the ion transport properties. By implementing the autocorrelation function of electric current through the Green-Kubo relation, the conductivity of the solvent-in-salt system can be directly generated, and the different contributions from the correlations between different ions and solvent can be analyzed explicitly. Moreover, the effect of concentration and temperature on the conductivity were also examined. The results show that a decrease in anion-cation correlations leads to a higher transference number of lightest ion, and that the cation-anion correlations can be tuned by changing solvent concentration.

References

1. Li, M., Wang, C., Chen, Z., Xu, K. & Lu, J. New Concepts in Electrolytes. Chem. Rev. 120, 6783–6819 (2020).
2. Suo, L., Hu, Y.-S., Li, H., Armand, M. & Chen, L. A new class of Solvent-in-Salt electrolyte for high-energy rechargeable metallic lithium batteries. Nat. Commun. 4, 1481 (2013).
3. Azov, V. A., Egorova, K. S., Seitkalieva, M. M., Kashin, A. S. & Ananikov, V. P. “Solvent-in-salt” systems for design of new materials in chemistry, biology and energy research. Chem. Soc. Rev. 47, 1250–1284 (2018).
4. Seo, D. M., Borodin, O., Balogh, D., O’Connell, M., Ly, Q., Han, S.-D., Passerini, S. & Henderson, W. A. Electrolyte Solvation and Ionic Association III. Acetonitrile-Lithium Salt Mixtures–Transport Properties. J. Electrochem. Soc. 160, A1061–A1070 (2013).
5. Popov, I., Sacci, R. L., Sanders, N. C., Matsumoto, R. A., Thompson, M. W., Osti, N. C., Kobayashi, T., Tyagi, M., Mamontov, E., Pruski, M., Cummings, P. T. & Sokolov, A. P. Critical Role of Anion–Solvent Interactions for Dynamics of Solvent-in-Salt Solutions. J. Phys. Chem. C 124, 8457–8466 (2020).
6. Popov, I., Khamzin, A., Matsumoto, R. A., Zhao, W., Lin X., Cummings, P. T., Sokolov, A. P. Controlling ion transport number in Solvent-in-Salt Solutions. (Submitted)