Evolution of mechanical properties of polypropylene separator in liquid electrolytes for lithium‐ion batteries

Abstract

Knowledge of the mechanical behaviors of polymeric separators immersed in liquid electrolytes is of great significance for predicting the long‐term performance of lithium batteries with high performance and safety. In terms of tensile tests, heating shrinkage, and dynamic mechanical analysis as well as the essential work of fracture method, the study reported here encompasses a systematic investigation of the mechanical properties of a typical commercial polypropylene separator in mixtures of ethylene carbonate and dimethyl carbonate and lithium hexafluorophosphate (LiPF6), comparing with the results in ionic liquid (IL) electrolyte composed of lithium tetrafluoroborate (LiBF4) and 1‐butyl‐3‐methylimidazolium tetrafluoroborate (BMIBF4) and dry condition. It has been found that liquid electrolytes have obvious negative effect on the dimensional stability at elevated temperature and mechanical properties, especially on crack resistance of the polymer separator. LiBF4‐BMIBF4 has much smaller damage on the strength, Young's modulus and fracture toughness of separator than the organic solution except the dynamic modulus at high temperature. Notably, the maximum tensile stress, Young's modulus and the reciprocal of relaxation time of the polymer separator are linearly dependent with strain rate under quasi‐static condition, and the relaxation time has clarified the coupling effect mechanism of liquid electrolyte and loading rate. Moreover, the non‐dimensional viscoelastic constitute equation could perfectly track the tensile behavior of wet and dry separators at different strain rate, and a property model could well characterize the temperature‐dependent storage modulus of polymer separators from rubbery to viscous state. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46441.

Facile Route for the Fabrication of Polypropylene Separators for Lithium-Ion Batteries with High Elongation and Strong Puncture Resistance

“…Therefore, it can be summarized here that the treatments by an oil bath can significantly improve the toughness in the TD and the puncture resistance of the separator with only slight drops of the yield strength. To understand fully, the measurement of the essential work of fraction (EWF) has been performed according to previous elegant studies. , The EWF measurement can provide detailed information concerning the fracture resistance behavior of the separators with and without the white oil treatment. The results provided in Figure S1 and Table S1 in the Supporting Information section revealed that after the white oil treatment, the load–displacement curves for the DEN-T specimens of SWO-2H are quite different from that of SWO-0, to be more exact, a longer displacement before the total failure and the lower loading force of SWO-2H can be seen compared with that of SWO-0.…”

“…One of the most important components of the LIB interior is the polymer separator, which is made of polyolefin such as polyethylene (PE) or polypropylene (PP). The manufacturing processes, to be specific, dry − and wet , processes, bring significant effects on the mechanical properties of the separators. − The PP separators manufactured via a dry process is one of the most successfully developed commercial polymer separators, featured of highly uniform and small-sized micropores, excellent permeability, high electrochemical properties and relatively low manufacturing costs . It is highly anisotropic in the machine direction (MD) and the transverse direction (TD), that is to say, it exhibits high tensile strength and medium elongation at break in the MD, while very low elongation at break in the TD (even less than 15%), which greatly restricts its resistance to the penetration of the lithium dendrites. , Its enhancement, however, can hardly be evidently elevated as far as we have observed up to date.…”

Composite Separators for Robust High Rate Lithium Ion Batteries

“…Chen et al [112] revealed the influencing factors of tensile strength for PP separators in the wetting condition. They proposed that the tensile properties of polymeric separators appear to be a function of loading rate and the relation between relaxation time and strain rate.…”

Weakening of mechanical properties of cellulose separator caused by electrolyte immersion and elevated temperature

“…Many reports have conducted investigation mechanical properties of PP separator submersed in air (or vacuum), water, dimethyl carbonate (DMC), and integrated electrolyte [1.1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) = 1/1 by volume (1.1 M LiPF6 in EC/DMC)] from experiments and molecular simulation . The effect of temperature has also studied and found to take negative influence . These studies indicate that electrolyte has a certain influence on the weakening, swelling, and softening of PP separators, causing damage on materials and structures.…”