Much attention is being paid to high-energy density lithium batteries such as lithium-sulfur (Li-S) battery because its specific energy density is much higher than those for commercialized lithium ion batteries [1]. However, many challenges including insulating properties of active materials, sluggish electrochemical reactions, and dissolution of the sulfur species into electrolyte remain unresolved for practical application. Electrolyte properties are essentially concerned with the above serious issues of these batteries. Undesirable side-reactions between electrolytes and the reduction products (lithium polysulfides, Li₂S_m) occur in some non-aqueous electrolytes. The dissolution of the sulfur species, volatility, and flammability of electrolytes have also to be addressed for long-life operation of the Li-S cells. Therefore, development of electrolytes is a key to establishing these batteries [2].

We have determined saturated solubility of Li₂S_m and S₈ in various type of electrolytes ranging from molecular solvents to aprotic and solvate ionic liquids to study factors that govern the dissolution of S₈ and Li₂S_m species into electrolyte. In terms of solvent parameters, the solubility of Li₂S_m was well correlated with Gutmann’s donor number, whereas solubility of S₈ was explained by Hildebrand solubility parameter. Although weakly coordinating solvents having low donor number could greatly suppressed dissolution of Li₂S_m, supporting lithium salts (e.g., LiTFSA) were hardly soluble in such weakly coordinating solvents. An exception was ionic liquid-based electrolytes: ionic liquids having a weakly coordinating anion can dissolve the supporting salt while suppressing dissolution of Li₂S_m. We show how the electrolyte properties of the ionic liquids link to the cell performance such as large capacity, high Coulombic efficiency, and good reversibility.

Reference

[1] P. G. Bruce, S. A. Freunberger, L. J. Hardwick, and J-M. Tarascon, Nat. Mater., 11, 19 (2012).

[2] M. Watanabe M. L. Thomas, S. Zhang, K. Ueno, T. Yasuda, K. Dokko, Chem. Rev., ASAP (2017).