Hierarchical Porous Polyimide Separator Prepared by Sodium Chloride Salt for High-Performance Lithium Ion Batteries

Lithium-ion batteries have been widely used as energy storage and power batteries due to their unique advantages. However, with increasing demands for battery performance and application scenarios, battery safety has become a significant obstacle to their application. To address this issue, this paper proposes and fabricates an advanced polyimide (PI) separator material with high porosity and excellent thermal stability. By introducing sodium chloride (NaCl) as a pore-forming template into a polyamic acid (PAA) precursor, a PI-based separator with a uniformly interpenetrating sponge-like pore structure was successfully constructed. The obtained PI-NaCl separator exhibits outstanding thermal structural stability, maintaining dimensional integrity without significant thermal shrinkage even when tested at temperatures as high as 250 °C. Furthermore, the porous structure of the PI-NaCl separator demonstrates excellent electrolyte wettability, as the electrolyte rapidly spreads upon contact (contact angle approaching 0°), which is significantly superior to commercial separators. In lithium symmetric cell tests, this separator achieves long-term stable stripping/plating cycling by virtue of its outstanding ionic conductivity, effectively mitigating interfacial side reactions with lithium metal. In LiFePO4||C full-cell applications, the PI-NaCl-based battery exhibits good rate capability and cycling stability. Additionally, in an open-circuit voltage (OCV) monitoring experiment at a high temperature of 80 °C, the voltage of the PI-NaCl-based battery remained stable continuously for 8 h in comparison to that of the commercial separator-based battery.