Cu-Facet Selective Sulfur Chemistry for Ultrastable Sodium–Sulfur Batteries

Sodium-sulfur batteries face significant challenges due to the high solubility of sodium polysulfides and the resulting shuttle effect, which compromise cycling stability and efficiency. This study introduces the Cu(111) facet-selective reactivity of sulfur redox, which promotes the formation of a stable intermediate, NaCu₅S₃, enabling efficient sulfur conversion, rapid ionic transport, and a fully solid-solid reaction pathway. The system achieves exceptional performance, retaining a specific capacity of 602 mAh g^-1 over 800 cycles at 0.5 A g^-1 and delivering 463 mAh g^-1 at a high current density of 5 A g^-1 in ether-based electrolytes, representing the highest rate capability reported for cathodes with sulfur content ≥ 60 wt %. Comparative studies with Cu(100), Cu(110), and aluminum substrates highlight the unique reactivity of Cu(111). Density functional theory calculations further reveal the structural and electronic interactions between copper and sodium polysulfides, clarifying the facet-dependent mechanisms. This work establishes facet engineering as a promising approach to modulating sulfur redox pathways and improving the electrochemical reversibility in metal-sulfur batteries.