Multifunctional Polyimide Separator Enabling the Long-Life Operation of Sodium Ion Batteries at Elevated Temperatures

Sodium-ion batteries (SIBs) with NaPF6-carbonate electrolytes exhibit compromised high-temperature cycling stability due to NaPF6 thermal degradation and solid electrolyte interphase dissolution. Herein, a polyimide (PI) separator significantly enhances the cycling performance of HC||NaNi1/3Fe1/3Mn1/3O2 SIBs, achieving 80.9% capacity retention after 1500 cycles (30 °C), 80.0% after 1200 cycles (50 °C), and 70.3% after 500 cycles (90 °C). Mechanistic studies reveal that PI’s imide groups deactivate PF5, inhibiting corrosive HF formation and consequently alleviating the dissolution of TMs from the NFM cathode at high temperatures. The PI separator further regulates the Na+ solvation structure, promoting a stable electrode/electrolyte interphase and reducing continuous electrolyte decomposition. Accelerating rate calorimetry tests confirm improved thermal safety in 1 Ah pouch cells with PI separators compared to conventional polyolefin separators. This study highlights the critical role of multifunctional separators in enabling high-performance SIBs under extreme conditions, offering a promising approach to enhance both cycling stability and safety.