Growing Intact Membrane by Tuning Carbon Down to Ultrasmall 0.37 nm Microporous Structure for Confining Dissolution of Polysulfides Toward High-Performance Sodium–Sulfur Batteries

Room temperature sodium–sulfur (Na–S) batteries are severely hampered by dissolution of polysulfides into electrolytes. Herein, a facile approach is used to tune a biomass-derived carbon down to an ultrasmall 0.37 nm microporous structure for the first time as a cathode in sodium–sulfur batteries. This produced an intact uniform Na₂S membrane to greatly confine the dissolution of polysulfides while realizing a direct solid phase conversion for complete reduction of sulfur to Na₂S, which delivers a sulfur loading of 1 mg cm⁻² (50 wt.%), an excellent rate capacity (933 mAh g⁻¹ @ 0.1 A g⁻¹ and 410 mAh g⁻¹ @ 2 A g⁻¹), long cycle performance (0.036% per cycle decay at 1 A g⁻¹ after 1500 cycles), and a high energy density for 373 Wh kg⁻¹ (0.1 A g⁻¹) based on whole electrode weight (active sulfur loading + carbon), ranking the best among all reported plain carbon cathode-based room temperature sodium–sulfur batteries in terms of the cycle life and rate capacity. It is proposed that the solid Na₂S produced in the ultrasmall pores (0.37 nm) can be squeezed out to grow an intact membrane on the electrode surface covering the outlet of the pores and greatly depressing the dissolution effect of polysulfides for the long cycle life. This work provides a green chemistry to recycle wastes for sustainable energies and sheds light on design of a unique pore structure to effectively block the dissolution of polysulfides for high-performance sodium–sulfur batteries.