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.