Dependence of lithium-ion battery separator porous structure and performance on synchronous bidirectional drawing process regulation of β-crystal polypropylene

Uneven separator porous construction inevitably raises lithium-ion migration barriers within the separator and thus limits overall lithium-ion battery (LIB) performance. In this research, serial LIB separators with various porous constructions were prepared precisely by adjusting the synchronous bidirectional drawing temperatures to confirm the bidirectional drawing mode-determined porous constructions and subsequent mechanical properties, thermal stability and electrochemical properties of separators. Crystal structure analyses, porous construction diagnoses, mechanical property characterizations and electrochemical tests reveal the competitive relationship between lamella slip and separation during the synchronous bidirectional drawing process. Lamella slip weakens at lower drawing temperatures, enhancing lamella separation and molecular chain rupture within amorphous regions. Therefore, a separator with broken fibrils presents optimized permeability and smoothed ion migration channels, but inferior mechanical properties and thermal stability. Excessive temperature highlights lamella slip, which damages the pore-forming process, worsens separator permeability and increases ion migration resistance. An appropriate drawing temperature of 100 °C maximally balances lamella slip and separation behaviors, retains intact fibrils and homogenized porous construction, which endows the separator with strengthened mechanical properties, isotropic thermal stability and optimized electrochemical performances. This study provides new insights for synchronous bidirectional drawing in practical separator fabrication and clarifies separator structure-determined LIB performances. © 2023 Society of Industrial Chemistry.