The Forgotten Allotrope: γ-Sulfur Stabilization in Carbon Matrices for Energy Storage Applications.

Lithium-sulfur (Li-S) batteries are widely regarded as one of the most promising candidates for next-generation electrochemical energy storage, owing to their very high theoretical energy density and reliance on abundant, low-cost elements. However, the practical deployment of Li-S technology remains severely constrained by the polysulfide shuttle effect, originating from the dissolution, migration and parasitic redox cycling of lithium polysulfide intermediates, which leads to rapid capacity fading, low coulombic efficiency and incompatibility with industrial carbonate-based electrolytes. Recent reports on the formation and stabilization of γ-sulfur, a rare monoclinic allotrope, within porous carbon matrices have identified a prospective direction in sulfur electrochemistry, theoretically enabling polysulfide-free cycling and improved stability, also in conventional carbonate electrolytes. These findings challenge the long-held assumption that polysulfide formation is unavoidable in sulfur cathodes and suggest that control over sulfur allotropy and nanoconfinement, as well as carbon-sulfur chemistry, may unlock previously inaccessible performance and integration windows for metal-sulfur batteries, including most technologically advanced Li-S batteries. Based on recent studies, this Perspective article critically evaluates the evidence for γ-sulfur stabilization in carbon hosts, discusses the interplay between pore geometry, carbon surface chemistry and sulfur speciation, and finally identifies key knowledge gaps.