Understanding the microstructure behaviour of the Li-ion battery separator under compression using 3D image-based modelling

A 3D image-based modelling method is developed to predict compressive behaviour of Li-ion battery (LIB) separators. The separator sample of LIB, consisting of polypropylene (PP) matrix and pores, is imaged using nanoscale X-ray computed tomography (XCT). The microstructure of the solid PP phase is obtained by the segmentation of the image. The segmented microstructures are used to characterise the geometrical properties of LIB separators, including porosity and tortuosity, which are important parameters for characterizing the performance of separator related to the transport of Li-ion through the separator. A 3D finite element model based on the microstructure of separator is established for the study of the compressive behaviour of separator from micro-scale. Due to the limitation of relatively low resolution of nanoscale XCT (∼100 nm), the fibrils (< 100 nm) of polymer separator cannot be captured in the image-based model. A method is proposed to add fibrils into the image-based model, which is validated using experimental stress-strain curves and the images obtained from scanning electronic microscope (SEM). Finally, the porosity and tortuosity of LIB separator under compression are determined analytically to obtain a relationship between tortuosity and porosity of the separator, which can be used to analyse the performance of LIB with deformed separators.