Triethylsulfonium-based ionic liquids enforce lithium salt electrolytes.

Abstract

The demand for cheap production of energy and its efficient storage is huge nowadays. Sulfonium-based ionic liquids have exhibited a useful set of physical-chemical and electrochemical properties, which make them good prospective electrolytes for electrochemical double-layer capacitors and rechargeable lithium batteries. The ability of the researchers to correctly describe local ionic structural patterns in the electrochemical systems is a cornerstone of achieving sustainable progress in this field. Herein, we report an in silico investigation of a few lithium-triethylsulfonium electrolytes and correlate our results with the recently published electrochemical study. All chosen organic and inorganic anions have been recently used in the supercapacitor and lithium-battery electrolyte systems: bis(trifluoromethylsulfonyl)imide, perchlorate, hexafluorophosphate, and trifluoromethanesulfonate. Analyzing potential energy surfaces, ion-ionic coordination, electron density distributions, and structure properties, we identified that the best-performing electrolyte system is lithium bis(trifluoromethylsulfonyl)imide dissolved in triethylsulfonium bis(trifluoromethylsulfonyl)imide. In the mentioned system, we found the weakest cation-anion binding that resulted in the fastest ionic transport. The lithium-ion plays a paramount role in the coordination of all investigated anions, whereas the impact of the triethylsulfonium cation is relatively insignificant. The lithium-induced structural changes in the local order of the electrolyte are reflected by computed vibrational spectra. In the lithium-free systems, the anions strongly bind the triethylsulfonium cation via its electron-deficient α-methyl groups. Some of these electrostatically driven interactions may be classified as medium-strength hydrogen bonds. The computed cohesion energies explain the conductivity and viscosity trends obtained for similar electrolyte compositions in the recent experiments. The reported results will be interesting for researchers who develop Li-based energy storage devices that use room-temperature ionic liquids as non-volatile and electrochemically stable media.