Broad Redox Density of States and S–O Functionalities Drive Stable Pseudocapacitive Behavior in Sulfurized Polyacrylonitrile (SPAN) Cathodes

Sulfurized polyacrylonitrile (SPAN) cathodes offer a promising route for improving Li-S batteries by eliminating polysulfide shuttling and enabling stable, high-rate performance. Here, a comprehensive mechanistic study of SPAN cathodes with varying sulfur content (0-35 wt.%), revealing how structural and electronic factors that govern charge storage is presented. Cyclic voltammetry shows that SPAN exhibits distinct redox features without soluble polysulfides, and that higher sulfur content leads to sharper redox peaks and increased capacity. In situ Raman spectroscopy reveals that electrochemical cycling induces the formation of nanocrystalline sp² carbon domains and a decline in φ-S_x species. X-ray photoelectron spectroscopy shows the presence of stable S- O functionalities, including sulfone and sulfonate groups, which are previously unreported in SPAN. These S-O motifs evolve with cycling and are correlated with SPAN's redox activity. Trasatti analysis demonstrates that SPAN's charge storage is dominated by surface-controlled (pseudocapacitive) processes, unlike the diffusion-limited (redox) behavior of elemental sulfur. The pseudocapacitive contribution to the total capacity is found to increase with increasing S content. The redox density of states, g_r(μ), is further quantified using electrochemical capacitance spectroscopy through a density functional theory (DFT) inspired approach. The broad and stable g_r(μ), enabled by diverse S-O redox sites and the active participation of the carbon backbone, underpins SPAN's pseudocapacitive behavior and superior cycling stability.