A Porous Polyimide Composite Separator with an In Situ-Polymerized PDOL Gel Network for Enhanced Dendrite Suppression...

High-energy-density lithium-metal batteries (LMBs) demand separators with enhanced thermal stability, electrolyte affinity, and ionic transport capability. Herein, a composite separator composed of polyimide, poly(1,3-dioxolane), and lithium bis(trifluoromethanesulfonyl)imide (PI/PDOL/LiTFSI) is developed by preparing a porous support layer via nonsolvent-induced phase separation followed by in situ polymerization, aiming to overcome the limitations of conventional polyolefin-based separators. Density functional theory calculations confirm that the PI/PDOL system enhances Li⁺ cooperative coordination through electron-rich sites and lowers the HOMO energy of the gel polymer electrolyte, thereby improving high-voltage oxidation stability. The incorporation of LiTFSI further enhances the ionic transport capability and interfacial stability of the LMBs, enabling the composite separator to achieve an ionic conductivity (σ) of 0.61 mS cm^-1 along with a Li⁺ transference number (t_Li⁺) of 0.64. Moreover, the separator exhibits good electrolyte wettability and maintains dimensional stability without noticeable thermal shrinkage up to 200 °C. Electrochemical evaluations reveal outstanding cycling stability, as Li || LiFePO₄ cells maintain 96.8% of their capacity following 150 cycles at 0.5 C. Additionally, Li || Li symmetric cells function steadily for 600 h at 1 mA cm^-2 without dendrite-induced failure. These findings emphasize the significant promise of the PI/PDOL/LiTFSI composite separator for the upcoming generation of high-energy-density LMBs.