Synergistic enhancement of thermal mechanical stability in polypropylene separators via carbazole functionalization and UV crosslinking

Abstract

To address the poor thermomechanical stability and strong hydrophobicity of commercial polypropylene (PP) separators in lithium-ion batteries (LIBs), this study prepared functionalized PP-based separators via melt blending and UV-induced crosslinking. Carbazole-functionalized PP (PPCZ) was incorporated into the PP matrix, and a thermally stable 3D covalent network was constructed through in-situ self-crosslinking of carbazole groups under 365 nm UV irradiation. The synthesis of carbazole monomer UDK was optimized using KOH as a deprotonating agent instead of K₂CO₃, increasing monomer yield to 92 % with a reaction rate improvement of 30 % and simplifying purification. Metallocene-catalyzed copolymerization of UDK and propylene yielded PPCZ with 1.5 mol% carbazole insertion and a number-average molecular weight of approximately 90,000, and the product showed excellent PP matrix compatibility without phase separation. Key tests demonstrated that the 10 wt% PPCZ separator delivered superior comprehensive performance. Its thermal shrinkage at 105 °C measured 12 %, in sharp contrast to 28 % for the pristine PP separator and 18 % for the commercial Al₂O₃-coated PP separator. The temperature corresponding to 5 % weight loss (T₅) hit 342.2 °C, representing a 3.7 °C increase over pristine PP. Meanwhile, electrolyte wettability was significantly enhanced, with the contact angle reduced to 75° as opposed to 98° for pure PP. Despite slight decreases in porosity from 39.5 % to 36.8 %, electrolyte absorption from 120 % to 115 %, crystallinity from 51.1 % to 43.8 %, and ionic conductivity from 0.19 to 0.14 mS·cm⁻¹, the separator achieved an optimal balance of thermomechanical stability, wettability, and mechanical properties with a tensile strength of 110 MPa, puncture strength of 850 gf and elongation at break of 128 %, thus avoiding embrittlement. The strategy is compatible with existing dry-process lines, avoids ceramic coating issues such as high cost and delamination, and shows great potential for high-safety LIBs.