Cation-Directed Selective Polysulfide Stabilization in Alkali Metal-Sulfur Batteries

Alkali metal sulfur redox chemistry offers promising potential for high-energy-density energy storage. Fundamental understanding of alkali metal sulfur redox reactions is the prerequisite for rational designs of electrode and electrolyte. Here, we revealed a strong impact of alkali metal cation (Li ⁺ , Na ⁺ , K ⁺ , and Rb ⁺ ) on polysulfide (PS) stability, redox reversibility, and solid product passivation. We employed operando UV-vis spectroscopy to show that strongly negatively charged short-chain PS (e.g., S ₄ ^2- /S ₃ ^2- ) is more stabilized in the electrolyte with larger cation (e.g., Rb ⁺ ) than that with the smaller cation (e.g., Li ⁺ ), which is attributed to a stronger cation-anion electrostatic interaction between Rb ⁺ and S ₄ ^2- /S ₃ ^2- owing to its weaker solvation energy. In contrast, Li ⁺ is much more strongly solvated by solvent and thus exhibits a weaker electrostatic interaction with S ₄ ^2- /S ₃ ^2- . The stabilization of short-chain PS in K ⁺ -, Rb ⁺ -sulfur cells promotes the reduction of long-chain PS to short-chain PS, leading to high discharge potential. However, it discourages the oxidation of short-chain PS to long-chain PS, leading to poor charge reversibility. Our work directly probes alkali metal-sulfur redox chemistry in operando and provides critical insights into alkali metal sulfur reaction mechanism.