Minimizing Long-Chain Polysulfide Formation In Li-S Batteries By Using Localized Low Concentration Highly Fluorinated Electrolytes

Electrolytes that suppress the formation of long-chain polysulfides and limit the sulfur (S) cathode dissolution and polysulfide shuttle effect, while maintaining high active material utilization and long cycle life, have long been sought for advancing Li-S battery applications. Here we demonstrate that localized electrolytes based on fluorinated ethers with the total salt concentration of only 0.1 M show remarkable performance characteristics. By systematically reducing the fraction of sulfolane (SL) as a cosolvent with 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropylether (HFE), we reveal a dramatic change in the cell discharge behavior. The higher voltage similar to 2.4 V plateau corresponding to the formation of long-chain polysulfide shrank, while the medium voltage similar to 2.1 V plateau corresponding to the transformation to short-chain polysulfides almost disappeared. Additionally, a third low voltage plateau (similar to 1.9 V) emerges as S transform directly to a short chain polysulfides and Li2S, which have significantly lower solubilities. This plateau extended with increasing HFE and became a dominant process. When compared with the traditional dimethoxyethane/1,3-dioxolane (DME/DIOX) electrolyte solvent mixture, the reported electrolytes offer substantially higher capacity and coulombic efficiency due to decreased solubility for Li+ in the HFE, causing a shift to favoring short chain length polysulfides in the discharge reaction.