Remedies of capacity fading in room-temperature sodium-sulfur batteries

Liquid-electrolyte sodium-sulfur battery operated at room temperature is encountering challenges brought by the complex sulfur redox reactions, including (i) the dissolved polysulfide intermediates trigger serious side reactions on Na anode surface; (ii) the short-chain sulfide precipitation exhibits sluggish kinetics and the sulfur utilization is generally below 50% with unclear reasons. In this work, employing an ion selective polybenzimidazole-based separator we successfully suppress the polysulfide corrosion on the Na anode, which allows the investigation of the precipitation reaction. Combining DFT calculation and characterization techniques, we determine Na₂S₂ particles as the final discharge product and reveal that Na₂S₂ passivation is the predominant attributes of large polarization and capacity fading. To address these issues, we present the use of a bifunctional NaI-P₂S₅ based electrolyte additive, which (i) improves the Na₂S₂ precipitation kinetics by forming soluble Na₂S₂-P₂S₅ complex and (ii) promotes the dissolution of Na₂S₂ by the chemical mediation of I⁻/ I₃⁻. As a result, a much improved capacity retention (92.9% for 50 cycles at 0.2C) is attained, which sheds light on enabling the stable operation of sodium-sulfur batteries via combining advanced separator and electrolyte engineering strategies.