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In this study, effects of heat setting treatment (pre-set heat setting ratio λ and heat setting temperature Tsetting) on the crystalline structures, orientation status, microporous structures, gas permeability, and electrochemical performance of the separators were studied in detail by means of differential scanning calorimetry (DSC), infrared dichroism, scanning electron microscope (SEM), ultrasmall angle X-ray scattering (U-SAXS), Gurley value test, and electrochemical measurement. It was found that with the decrease of λ or the increase of Tsetting, the melting temperature and degree of crystallinity change regularly and the orientation degree decreases gradually, resulting in small-sized micropores and lower porosity and S/Vratio of the separator. Finally, the gas permeability and ionic conductivity of the separator decreased while the dimensional stability increased. By tuning λ and Tsetting, the microstructure, morphology, and performance of the separators can be efficiently controlled.

Compression stress on thickness direction of separators exists in assembling and operation of all lithium ion batteries (LIBs). In this study, influences of compression on microporous morphologies, electrolyte uptake behavior, and electrochemical properties of three types of commercial separators (PP-U, PP-B, and PE-B) for LIBs manufactured via different processes, were investigated. Results of scanning electronic microscope (SEM) and X-ray measurements revealed special stacked-multilayer structures of PP-B and PE-B, while PP-U was featured of micropores arranged in lines separated by lamellae in both surface view and section view. Compression was found to be key factor influencing the microporous structures, electrolyte uptake, and electrochemical properties of separators. Different microporous structures of separators were important factors determining the structural stability (tolerance to compression) of the separator and therefore influencing their performances before and after compression. A novel delamination experiment was designed to explain different structural stabilities of separators under compression. A related mechanism was proposed.