Impact of Die Exit Temperature on the Crystalline Orientation and Performance of Polypropylene Battery Separators

Lithium-ion battery separators play a crucial role in ensuring the efficiency and safety of modern energy storage systems. The melt-stretching method is commonly used for polypropylene battery separator fabrication, with research extensively exploring how extrusion parameters influence the final product's structure and properties. However, the specific impact of die exit temperature on separator quality remains largely unexamined. This study investigates the effect of die exit temperature during the dry process on the performance of polypropylene microporous membranes used as battery separators. Separators were produced using a co-rotational twin-screw extruder at various die exit temperatures (215–245 °C) and characterized for their crystalline orientation, porosity, and battery performance. Polarized FTIR and 2D-WAXS analyses revealed that lower die exit temperatures improve crystalline orientation, resulting in more uniform pore structures. At 215 °C, the separators exhibited superior electrolyte uptake (109.4%) and better pore morphology. The coin cell tests revealed that separators fabricated at 215 °C achieved higher charge storage capacity (181.45 mAh) and greater efficiency compared to those produced at elevated temperatures. These findings underscore the critical role of optimizing die exit temperature in the production of high-performance battery separators.