Understanding sulfur confinement and chemical transformation in hybrid sulfur-carbon materials is critical for advancing metal-sulfur batteries. Here, we investigate the structural evolution of a sulfur-rich polymer into a hybrid sulfur-carbon via inverse vulcanization and thermal condensation. Multiscale analyses reveal a stepwise transformation, beginning with the emergence of sulfur radicals at ∼175°C, followed by the progressive development of a carbon matrix above 300°C that stabilizes the radical species. Around 450°C, a transitional phase forms, consisting of conjugated carbon clusters covalently bonded to sulfur chains. This hybrid structure confines sulfur within pseudo-graphitic nanodomains, effectively suppressing polysulfide dissolution and enhancing redox stability. DFT simulations show how sulfur confinement modulates Na-S reaction energetics, while electrochemical testing confirms high sulfur utilization, delivering ∼1000 mAh (Formula presented.) and 1200 Wh (Formula presented.), setting a new performance benchmark for room-temperature Na─S batteries. These findings provide critical insights into the correlation between structural evolution and electrochemical performance, offering design principles for next-generation sulfur-based electrodes.

Nanoconfinement is a promising strategy in chemistry enabling increased reaction rates, enhanced selectivity, and stabilized reactive species. Sulfur's abundance and highly reversible two-electron transfer mechanism have fueled research on sulfur-based electrochemical energy storage. However, the formation of soluble polysulfides, poor reaction kinetics, and low sulfur utilization are current bottlenecks for broader practical application. Herein, a novel strategy is proposed to confine sulfur species in a nanostructured hybrid sulfur-carbon material. A microporous sulfur-rich carbon is produced from sustainable natural precursors via inverse vulcanization and condensation. The material exhibits a unique structure with sulfur anchored to the conductive carbon matrix and physically confined in ultra-micropores. The structure promotes Na+ ion transport through micropores and electron transport through the carbon matrix, while effectively immobilizing sulfur species in the nanoconfined environment, fostering a quasi-solid-state redox reaction with sodium. This translates to ≈99% utilization of the 2e− reduction of sulfur and the highest reported capacity for a room temperature Na−S electrochemical system, with high rate capability, coulombic efficiency, and long-term stability. This study offers an innovative approach toward understanding the key physicochemical properties of sulfurcarbon nanohybrid materials, enabling the development of high-performance cathode materials for room-temperature Na-S batteries with efficient sulfur utilization.