Testing of polymeric battery separators using air-coupled, high-frequency and wideband ultrasonic pulses

Separator membranes are one of the components in batteries and, although being a passive component, they may strongly affect both battery performance and security. Hence, effective control of both its main properties and their variability are critical issues for the battery industry. This work proposes testing these membranes by the measurement of the spectra of the ultrasonic transmission coefficient (magnitude and phase) using high-frequency and wideband air-coupled ultrasonic pulses. This non-conventional ultrasonic technique for materials testing offers several advantages: it is completely contactless and non-invasive, it is very fast, and it has the potential to be deployed in the fabrication line to test the 100% of the production in real time. This work shows that it is possible: a) to propagate high-frequency, wideband air-coupled ultrasonic pulses in the frequency range 0.15-2.4 MHz in battery separators, b) to obtain the transmission coefficient spectra, both magnitude and phase; b) to theoretically model this propagation and the transmission coefficient spectra in thin films and then c) to solve the inverse problem to extract some separator properties of interest. The obtained properties depend on the theoretical model use to describe the propagation. For separators with porosity < 60% and pore size big enough (approximately < 1 μm for the separators studied here), effective medium approach can be successfully used, while for the other cases, the porous nature of the separator has to be taken into account and the Biot’s theory is used. • Air-coupled ultrasonic pulses are transmitted through polymeric separator membranes with good signal to noise ratio, showing that it is possible to measure transmission coefficient spectra (0.2-2.4 MHz). • In separators with porosity < 60% and pore size < 1 μm, transmission coefficient can be modelled by the effective medium approach. Separator thickness and porosity are obtained • Otherwise, transmission coefficient can be modelled by Biot’s theory. Pore aperture at the surface and ultrasonic velocity and attenuation in the pores are obtained. • The technique is fully non-invasive, contactless, fast, cheap and robust. Given the relevance of the data obtained, this makes possible to use it in production line to test 100% of the material in real time. • This can contribute to more consistent material production, and safer batteries.