Salt Concentration Control of Polysulfide Dissolution, Diffusion, and Reactions in Lithium–Sulfur Battery Electrolytes

Lithium-sulfur (Li-S) batteries suffer from the dissolution of sulfur and polysulfide (PS) species in the electrolyte, leading to capacity loss, instability, and a shortened lifespan. While highly concentrated electrolytes have been explored to address this issue, the underlying mechanisms of S/PS dissolution and subsequent diffusion, particularly concerning the specific behavior of long- and short-chain PSs under varying states of charge (SOC), remain poorly understood. We here employ <i>operando</i> Raman spectroscopy to semiquantitatively monitor PS solubility and migration across a wide range of LiTFSI concentrations in DME:DOL (1:1, v/v). We find that both PS dianions (S_4-8 ^2-) and trisulfur radicals (S₃ ^•-) decrease at the lithium anode with increasing electrolyte salt concentration (0.3-7.0 m), indicating reduced solubility and slower transport. Notably, the concentration of S₃ ^•- decreases more rapidly than that of its parent PS S₆ ^2-, suggesting less favorable radical formation pathways in highly concentrated electrolytes, potentially due to Li-TFSI-PS adduct formation. These changes result from shifts in the local solvation structure at high salt concentration, thereby controlling the solubility, transport, and chemical pathways of polysulfides in the electrolyte. By providing the real-time dynamics of long- and short-chain PSs, this work advances the mechanistic understanding of PSs in order to provide valuable insight for further improvement of Li-S battery performance.