Ionic Liquid Electrolytes for Lithium–Sulfur Batteries

A variety of binary mixtures of aprotic ionic liquids (ILs) and lithium salts were thoroughly studied as electrolytes for rechargeable lithium–sulfur (Li–S) batteries. The saturation solubility of sulfur and lithium polysulfides (Li2Sm), the active materials in the Li–S battery, in the electrolytes was quantitatively determined, and the performance of the Li–S battery using the electrolytes was also investigated. Although the solubility of nonionic sulfur was low in all of the electrolytes evaluated, the solubility of Li2Sm in the IL-based electrolyte was strongly dependent on the anionic structure, and the difference in the solubility could be rationalized in terms of the donor ability of the IL solvent. Dissolution of Li2Sm in the ILs with strong donor ability was comparable to that achieved with typical organic electrolytes; the strongly donating IL electrolyte did not prevent redox shuttle reaction in the Li–S cells. The battery performance was also influenced by unfavorable side reactions of the anions (such as tetrafluoroborate (BF4–) and bis(fluorosulfonylamide) (FSA−)) with polysulfides and by slow mass transport in viscous ILs, even though the dissolution of Li2Sm into the IL electrolyte was greatly suppressed. Among the IL-based electrolytes, the low-viscosity TFSA-based ILs facilitated stable charge/discharge of the Li–S batteries with high capacity and high Coulombic efficiency. The unique solvent effect of the ILs can thus be exploited in the Li–S battery by judicious selection of ILs that exhibit high lithium-ion-transport ability and electrochemical stability in the presence of Li2Sm.