Solid-State Sodium-Sulfur Batteries: Opportunities, Challenges, and Emerging Strategies

Sodium-sulfur (Na-S) batteries are promising candidates for large-scale energy storage but are fundamentally limited by polysulfide shuttling and parasitic reactions in liquid electrolytes. Transitioning Na-S batteries to the solid state intrinsically suppresses polysulfide dissolution and, more importantly, reconstructs sulfur electrochemistry from solution-mediated reactions to interface-governed solid-solid conversion pathways. This transformation fundamentally alters sulfur redox behavior, creating new opportunities to regulate reaction pathways and interfacial evolution that are inaccessible in liquid systems. Despite these advantages, solid-state sodium-sulfur batteries (SSNSBs) face new challenges arising from sluggish solid-solid reaction kinetics, discontinuous ion and electron transport, and coupled chemo-mechanical instabilities at electrolyte-electrode interfaces. This review clarifies the mechanistic distinctions between liquid and solid-state Na-S chemistries, and systematically analyzes the key challenges limiting SSNSB performance. Recent advances in cathode architecture, solid-state electrolyte engineering, and Na anode/interface modification aimed at enabling reversible solid-state sulfur conversion are summarized. Finally, critical research directions required to advance SSNSBs toward practical implementation are discussed.