High‐Capacity and Stable Sodium‐Sulfur Battery Enabled by Confined Electrocatalytic Polysulfides Full Conversion

The efficient polysulfide capture and reversible sulfur recovery during reverse charging process are critical to exploiting the full potential of room temperature NaS batteries. Here, based on a core‐shell design strategy, the structural and chemical synergistic manipulation of sodium polysulfides quasi‐solid‐state reversible conversion is proposed. The sulfur is encapsulated in the multi‐pores of 3D interconnected carbon fiber as the core structure. The Fe(CN)64−‐doped polypyrrole film serves as a redox‐active polar shell to lock up polysulfides and promote complete polysulfide conversion. Importantly, the short‐chain Na2S4 polysulfides are reduced to Na2S directly leaving with a small fraction of soluble intermediates as the cation‐transfer medium at the core/shell interface, and freeing up formation of solid Na2S2 incomplete product. Further, the redox mediator with open Fe species electrocatalytically lowers the Na2S oxidation energy barrier and renders the high reversibility of electrodeposited Na2S. The tunable quasi‐solid‐state reversible sulfur conversion under versatile polymer sheath greatly enhances sulfur utilization, affording a remarkable capacity of 1071 mAh g−1 and a stable high capacity of 700 mAh g−1 at 200 mA g−1 after 200 cycles. The confined electrocatalytic effect provides a strategy for tuning electrochemical pathway of sulfur species and guarantees high‐efficiency sulfur electrochemistry. A high‐capacity NaS cell based on a freestanding flexible cathode with a core‐shell structure is investigated using electrochemical characterizations. In situ spectroscopic results confirm the facile quasi‐solid‐state conversion of polysulfide intermediates to final discharge product with fast kinetics through a confined electrocatalytic effect.