Testing of battery separators: transducers and methods for air-coupled ultrasonic sensing

Battery separator me3mbranes are thin polymeric microporous films placed between battery electrodes. They are a key component that strongly affects both battery performance and security and controlling its main properties is critical for the industry. In particular, the manufacturing process demands tests that allow checking both the mechanical integrity (porosity, thickness, pore size, etc.) of the materials and the consistency of these properties during the construction of the batteries. The use of an air-coupled ultrasonic system is proposed in this work. It is expected that this will make possible the non-destructive and quick testing of these materials, in order to evaluate thickness, stiffness, pore distribution and pore size of the separator. By means of three pairs of transducers, the modulus and phase of the transmission coefficient was measured from 0.35 to 1.4 MHz in different materials. The measurements show a clear modification of the film response produced by the presence of porosity. This suggests that the transmission of ultrasonic waves in battery separators using air-coupled ultrasound may have two contributions: one corresponds to the propagation though the solid part, the other corresponds to propagation in the pores. The former allows to determine properties like film thickness and elastic constants, while the latter allows to determine porosity and pore size and pore tortuosity. This work set the basis for the development of a quality test for the in-line inspection of these materials during their fabrication. The observed ultrasonic response in microporous separators suggest the possibility to use low frequency air-coupled transducers with limited sensitivity, where the use of MEM transducers at industrial scale can be advantageousm project Listen2Future. Under grant 101096884, Listen2Future is co-funded by the European Union and by the Project Ref. PCI2022-135060-2 funded by CIN/AEI/10.13039/501100011033 and by EU “NextGenerationEU”/PRTR.Peer reviewe

Relating the ultrasonic and aerosol filtration properties of filters

Abstract Non-contact methods are useful to improve the quality control of particle filtration media. The purpose of this paper is to investigate the correlation between the filtration efficiency of a porous sheet and its ultrasonic properties obtained using a non-contact technique. An air-coupled ultrasonic technique is used to obtain rapid measurements without affecting the integrity of the material. High frequencies (from 0.1 to 2.5 MHz) are used to improve technique sensitivity, and transmitted waves are measured to probe the internal properties of the material. Measurements of transmission coefficient spectra (amplitude and phase) and the corresponding ultrasound velocity and attenuation coefficient at different frequencies are obtained for a set of filtration media with well-characterized properties. Results show that the ultrasonic properties of filtration media vary as a function of basis weight, and therefore filtration efficiency, for a given charge state. However, the effect of electrostatic charge on ultrasonic propagation is almost negligible, as expected. We conclude that ultrasonic transmission may provide a valuable tool for the continuous online monitoring of material quality during fabrication and as a method to tease apart mechanical and electrostatic contributions to particle filtration