Coupled Effect of Low Temperature and Electrolyte Immersion on the Tensile Properties of Separators in Lithium-Ion Batteries

The performance degradation at low temperatures and frequent safety accidents have aggravated security risks and inhibited the long-term service of lithium-ion batteries (LIBs). As a key component of LIBs, the separator has a great impact on the performance and safety of the battery. In this study, tensile tests of two commercial polyolefin separators (Celgard 2325 and Celgard PE) are performed under low-temperature and immersion conditions. Four representative temperature points and dimethyl carbonate [(DMC), the common solvent in electrolytes] are selected to investigate the coupling effect on the mechanical properties of the separators. The results show that both the separators have anisotropy, but the performance of Celgard 2325 varies more significantly than that of Celgard PE along different directions. Additionally, it is found that with a decrease in the temperature, the tensile strength of the two separators increases, while the elongation decreases. Electrolyte immersion induces a softening tendency in Celgard 2325. Due to the special effect of the residual electrolyte on polyethylene fibers, Celgard PE shows the opposite result. Furthermore, the effect of low temperature is revealed by the analysis of the crystallinity and molecular structure, which can be obtained by X-ray diffraction and Raman spectroscopy, respectively. In addition, the contact angle is measured to describe the wettability variation related to low temperature. The present work provides a theoretical basis and experimental data for the application and development of separators.