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

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. (c) 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46441.