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.