Non-contact multimodal ultrasonic testing of cell batteries and components

This work reviews the use of air-coupled ultrasound for testing cell batteries and components (separator membranes and electrodes). High-efficient and broad band piezoelectric air-coupled ultrasonic transducers, frequency range 0.1–3.0 MHz, in through transmission or pitch catch configuration have been used. Two transducers designs have been tested: i) flat and ii) focused. The former permits an easier interpretation of the results, while the later allows for an increased spatial resolution, convenient for location and identification of defects, testing of small cells and testing of non-planar cells. Focused transducers with two different field geometries have been used: spherical and cylindrical focusing, where the later one has been used for cylindrical cells. In addition, two different focusing techniques have been employed: i) using parabolic off-axis mirrors (for spherical focusing) preserving the bandwidth and ii) insertion of lenses (for cylindrical focusing). Multimodality refers to the fact that different modes of operation or analysis are integrated. This includes both propagation and resonant techniques, time domain and frequency domain analysis, testing along different directions, and use of different types of propagation modes: longitudinal, shear and Lamb waves.

Tested separators have been PP, PE and cellulose membranes from three main manufacturers: Celgard, TMAX and Nippon Kodoshi. These membranes are tested in the frequency domain, in through transmission and normal incidence. The extraction of information about thickness, density, porosity and pore size is explained. Tested anode materials correspond to metallic thin films (current collector, either aluminium or copper) coated with graphite or hard carbon (both single or double sided coated). For the cathode, the metallic film is aluminium and two different coatings have been studied; lithium manganese and lithium iron phosphate (both single or double sided coated). Electrodes have been mainly tested in the frequency domain, in through transmission configuration and normal incidence. The extraction of information about the coating thickness, density, porosity elastic moduli are shown. The possibility to use Lamb waves is also analysed and some results reported.

Concerning batteries, results for both pouch and cylindrical cells are shown. In this case propagation in all directions have been tested for both longitudinal and shear waves analysing both propagation in the time domain and resonances (thickness resonances) in the frequency domain, for these two modes (longitudinal and shear). This permits to fully characterize the anisotropy of the cells. The existence of forbidden propagation directions for both longitudinal and shear waves is presented and discussed. The role of the cell microstructure (finely layered material) in these features is discussed and analysed. In this case, both pouch cells and cylindrical cells have been studied. Results for cylindrical cells are reported here for the first time.

Main uses of this technique include: characterization of new materials and cells during the research and development stage, quality control during fabrication, as the technique can be used to test the 100% of the production (both components and cells) in real time, after-life sorting of cells and components for re-use or recycling.