Spectroscopic Investigation of Polysulfide Redox Evolution in Catholyte Solutions for Potassium–Sulfur Batteries

ABSTRACT Potassium–sulfur (K–S) batteries represent a promising class of next‐generation energy storage systems, leveraging the abundance of potassium and the high theoretical capacity of sulfur. However, their development is limited by an incomplete understanding of polysulfide chemistry. Here, we combine spectroscopic techniques with electrochemical analysis to elucidate the formation, speciation, and redox behavior of potassium polysulfides in hybrid‐electrolyte K–S cells operating at 60°C. UV–vis spectroscopy reveals that sulfur radical anions, especially S 3 • − , act as key intermediates whose evolution reflects redox equilibria and state of charge. Modulating the catholyte composition shows that incorporating a eutectic ε‐caprolactam:acetamide (CPL:Am, 1:1) mixture into tetraglyme (G4) enhances polysulfide solubility and capacity. Cell architecture plays a critical role: glass cells used to couple spectroscopy and electrochemistry exhibit rapid capacity loss and Coulombic efficiencies of ∼80%, while compact piston‐type cells boost performance. Electrolyte choice within piston cells further differentiates behavior, with tetraglyme yielding 561 mAh g − 1 and gradual fading, whereas CPL:Am–G4 electrolytes initially reach 1354 mAh g − 1 with more pronounced decay. These insights highlight the intertwined effects of solvent chemistry and cell design, offering guidance for developing stable, high‐performance K–S batteries.