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

## TL;DR

This study explains how sulfurized polyacrylonitrile (SPAN) cathodes work better in lithium-sulfur batteries due to stable chemical groups and unique charge storage behavior.

## Contribution

The paper identifies stable S–O functionalities and a broad redox density of states as key to SPAN's pseudocapacitive behavior.

## Key findings

- Higher sulfur content in SPAN leads to sharper redox peaks and increased capacity.
- S–O functionalities like sulfone and sulfonate groups are newly identified in SPAN and evolve during cycling.
- SPAN's charge storage is pseudocapacitive, not diffusion-limited, and increases with sulfur content.

## Abstract

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 sp
2 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.

We report a fundamental mechanism underlying charge storage in sulfurized polyacrylonitrile cathodes. Stable sulfuroxygen groups and a broad redox density of states enable pseudocapacitive behavior distinct from conventional sulfur cathodes. Through combined spectroscopic and electrochemical analysis, this study reveals key structurefunction relationships that advance the design of high‐performance polymer‐based cathodes for lithiumsulfur energy storage systems.