Tailoring Electrolytes by Decoupling the Roles of Li ⁺ and Lithium Polysulfides in Li–S Batteries

Understanding the distinct roles of lithium ions (Li+) and lithium polysulfide intermediates, Li2Sx (LiPS) is critical for designing electrolytes that can extend the practical cycle life of lithium–sulfur (Li–S) batteries. In this work, we decouple the solvation and solubility effects of Li+ and LiPS and correlate them with electrochemical performance through a cosolvent strategy. Li+ solubility and solvation primarily dictate the electrolyte’s ionic conductivity and the reversibility of lithium anode stripping/plating. In contrast, LiPS solvation governs the thermodynamics of sulfur (S8), LiPS, and lithium sulfide (Li2S) interconversion, while the LiPS solubility determines their redox kinetics. By employing a fluorinated–glyme (F-glyme) cosolvent, specifically 1,2-bis(2,2-difluoroethoxy)ethane (F4DEE), that exhibits low LiPS solubility yet moderate Li+ solvation, we designed an electrolyte that enhances lithium anode stability while maintaining sufficient sulfur cathode kinetics, thereby prolonging Li–S cell cycle life. This study provides mechanistic insights into the interplay between Li+ and LiPS in Li–S electrochemistry and offers design principles for next-generation electrolytes for Li–S batteries.