Publication date: 15th December 2025
Volumetric expansion of sulfur during electrochemical conversion is widely regarded as one of the major challenges limiting the practical deployment of metal-sulfur batteries. The drastic theoretical volume increase associated with the S₈ to M₂S (M = Li, Na, K) reaction is thought to generate mechanical stress, disrupt electronic pathways, and accelerate electrode degradation. Yet despite its central role in performance loss, the actual magnitude, origin, and spatial distribution of sulfur expansion remains poorly understood, particularly in room-temperature Na-S batteries, where most sulfur cathodes originally developed for Li-S, suffer from severe polysulfide shuttling. In commonly employed electrolytes for Na-S batteries, long-chain polysulfides exhibit higher solubility and mobility, causing extensive sulfur loss and preventing meaningful quantification of electrode-level dimensional changes.
To address this long-standing limitation, we employ a sulfur-carbon hybrid in which sulfur is highly confined suppressing polysulfide dissolution, retaining nearly all sulfur within the cathode, and enabling near-quantitative sulfur utilization (~99%).[1] This uniquely stable system provides, for the first time, a reliable platform for correlating molecular-scale sulfur conversion with electrode-level volume evolution in room-temperature Na-S batteries.
Operando electrochemical dilatometry measurements reveal that the effective electrode expansion is only ~3%, far below the theoretically predicted 171% for S₈ to Na₂S conversion. Multiscale characterization explains this discrepancy: FIB-SEM 3D reconstruction resolves dimensional evolution and material redistribution at the micrometer scale, clarifying how the electrode accommodates reaction-driven changes. Depth-resolved XPS and ToF-SIMS track sodium incorporation and sulfur speciation through the electrode depth, offering insight into interfacial and compositional processes linked to volumetric behavior. At the nanoscale, ex-situ cryo-TEM visualizes structural evolution of individual sulfur–carbon domains, revealing how local confinement shapes particle-level deformation. Together, these operando and ex-situ insights establish the first quantitative and mechanistic picture of volumetric expansion in room-temperature Na-S cathodes.
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