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., S₄^2-/S₃^2-) 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 S₄^2-/S₃^2- owing to its weaker solvation energy. In contrast, Li⁺ is much more strongly solvated by solvent and thus exhibits a weaker electrostatic interaction with S₄^2-/S₃^2-. 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.