Revealing the Hidden Polysulfides in Solid-State Na–S Batteries: How Pressure and Electrical Transport Control Kinetic Pathways

Room temperature operation of Na-S batteries with liquid electrolytes is plagued by fundamental challenges stemming from polysulfide solubility and their shuttle effects. Inorganic solid electrolytes offer a promising solution by acting as barriers to polysulfide migration, mitigating capacity loss. While the sequential formation of cycling products in molten-electrode and liquid electrolytes-based Na-S batteries generally aligns with the expectations from the Na-S phase diagram, their presence, stability, and transitory behavior in systems with inorganic solid electrolytes at room temperature, remain poorly understood. To address this, we employed operando scanning microbeam X-ray diffraction, operando X-ray photoelectron spectroscopy and ex-situ X-ray absorption spectroscopy to investigate the sulfur conversion mechanisms in Na-S cells with Na₃PS₄ and Na₄(B₁₀H₁₀)(B₁₂H₁₂) electrolytes. Our findings reveal the formation of crystalline and amorphous polysulfides, including those predicted by the Na-S phase diagram (e.g., Na₂S₅, Na₂S₄, Na₂S₂, Na₂S), high-order polysulfides observed in liquid-electrolyte systems (e.g., Na₂S_x, where x = 6-8), and phases like Na₂S₃ typically stable only under high-temperature or high-pressure conditions. We demonstrate that these transitions are governed by diffusion-limited kinetics and localized stress concentrations, emphasizing the critical role of pressure, which serves as both a thermodynamic variable, as well as a design parameter, for optimizing solid-state Na-S battery performance necessary for pushing these cells closer to the commercial frontier.