An electrolyte-locked thermally stable composite separator for safe lithium ion batteries

The safety issue of lithium ion batteries has always received extensive attention and become the main obstacle restricting their development and applications. Commercial polyolefin separators own poor thermal stability, which cannot ensure the safe operation of lithium ion batteries after heating. Herein, an electrolyte-locked thermally stable composite separator polyimide/poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP@PI) is constructed through coating a layer of PVDF-HFP on the electrospun PI fibers membrane. In this structure, the high electrolyte retention ability and excellent thermal dimensional stability of PVDF-HFP@PI effectively inhibit the continuous reaction between electrolyte and Li anode, and prevent the battery failure after heating. In addition, the prepared PVDF-HFP@PI has good mechanical properties and high ionic conductivity of up to 1.79 mS cm⁻¹, which significantly improves the electrochemical performance of the batteries. Therefore, the pouch cell with PVDF-HFP@PI retains 77.4 % of the original capacity after 300 cycles at 0.1 C. More importantly, the cell with PVDF-HFP@PI can work normally after heated under 140 °C for 1 h, exhibiting high heat tolerance.