Porosity variation of lithium-ion battery separators under uniaxial tension

Separators in lithium-ion batteries are susceptible to uneven distributions of deformation, which may lead to inhomogeneous porosity distribution when batteries are subject to complex external loadings. In this study, uniaxial tensile tests were performed for four types of commercial separators and the in-situ 3D Digital Image Correlation (DIC) technique was used to measure the strain fields. In order to minimise the adverse effect caused by the wrinkling in strip-shaped specimens, dogbone-shaped specimens were designed to improve the measurement consistency by avoiding the occurrences of wrinkles in wet-processed separators whose deformation response has weak anisotropy. Homogeneous strain distributions were observed in both wet- and dry-processed separators orientated in the machine direction, but two deformation mechanisms of necking were found for dry-processed separators in the transverse direction. In the range of strain between 0 and 0.5, no thickness change was found in all separators, and large in-plane Poisson's ratio (up to 1.4) was found in wet-processed separators. The relationship between longitudinal and transverse strains was determined experimentally, based on which the in-plane Poisson's ratio is expressed as a polynomial function of longitudinal strain. Finally, relationship between material porosity and longitudinal strain were determined which can provide additional information in multi-physics simulations.