Sandwich-Structured PI-FPU/Celgard/PI-FPU Separator for Long-Life Room-Temperature Sodium–Sulfur Batteries

The shuttle effect of polysulfides and the piercing of dendrites severely limit the cycling stability and safety of room-temperature sodium–sulfur (RT Na–S) batteries. To address these challenges, a sandwich structure separator was designed and prepared by electrospinning a fluorinated polyurethane copolymerized polyimide (PI-FPU) fiber membrane and assembling it on both sides of a commercial Celgard separator (PI-FPU/Celgard/PI-FPU). The external PI-FPU fiber layers synergistically suppress the polysulfide shuttle through the physical confinement combined with chemical anchoring. Meanwhile, the toughness of the PI-FPU membrane and the high mechanical strength of the Celgard substrate create a robust and flexible structure that effectively inhibits dendrite penetration. In addition, the inherent thermal stability and heat resistance of the PI-FPU material enhance the battery's resistance to thermal runaway and fire. Due to these advanced structural features, the sodium-symmetric cell with the PI-FPU/Celgard/PI-FPU separator achieves stable cycling for over 1000 h at a current density of 1 mA cm⁻². The assembled RT Na–S battery maintains a high discharge specific capacity of 555 mAh g⁻¹ after 1500 cycles at 3 A g⁻¹. This work provides a feasible strategy for developing high-safety and long-life RT Na–S batteries.