Toward Better Lithium-Sulfur Batteries: Functional Non-aqueous Liquid Electrolytes

Having been extensively studied in the past five decades, lithium–sulfur (Li–S) batteries possess a high theoretical energy density (~ 2600 Wh kg−1), offering the potential to power advanced twenty-first-century technologies such as electric vehicles and drones. However, a surprisingly complex engineering challenge remains in the application of these batteries: the identification of appropriate electrolytes that are compatible with both sulfur cathodes and lithium metal anodes. Non-aqueous liquid electrolytes, typically consisting of a lithium salt dissolved in an organic solvent, cannot themselves demonstrate effective electrochemical performances. Researchers have found that functional electrolytes offer unique possibilities to engineer the surface chemistries of sulfur cathodes and lithium anodes to enable long-term cycling. In this article, recent progresses in the development of functional non-aqueous liquid electrolytes in Li–S batteries are reviewed, including novel co-solvent solutions, lithium salts, additives, redox mediators, and ionic liquids. Characterization techniques and interpretations are cited to elucidate the effects of these components on the kinetics of sulfur redox reactions, lithium passivation, and cell performance. The information presented and the studies highlighted in this review will provide guidance for future optimized electrolyte designs.