Material characterization at different length-scales is key to understand physical device properties and improve manufacturing parameters. In this work we will illustrate this on the example of Lithium-ion battery-cathode foils.
Lithium-ion batteries are widely used as rechargeable power sources. A main field of interest for researchers is increasing their capacity. One way to achieve this is compressing the active material coating so that a battery of same size simply holds more of it. But this can have an influence on the performance of the material cancelling the benefits of its compression. To find the perfect tradeoff the material has to be investigated at micrometer length scale to identify e.g. voids between particles but also at nanometer scale to characterize e.g. the integrity of single particles.
The samples investigated in this work were Aluminum foils coated with an active mass layer containing LiNi0,33Co0,33Mn0,33O2, binder and carbon black. The foils were compressed with different pressures up to 1000 MPa. To get a first impression about the effect of compression two samples of the pristine and most calendered foil were imaged destruction free in a ZEISS Xradia 520 Versa x-ray microscope. Figure 1 shows a 3D reconstruction of this data. The as coated layer on the left shows a larger thickness than the calendered one on the right. The scanned volume covers several hundred micrometers in each direction with a voxel size of 400 nm. The reconstructed 3D data provides insight about particle integrity, sizes and voids on a larger scale.
Transferring the samples then into a ZEISS Crossbeam 540 allows to investigate chosen particles at a smaller scale and higher resolution. The x-ray data was imported into the microscope software and aligned with the sample. Dedicated particles of interest could be chosen and localized. Using the focused ion beam it was possible to mill trenches into the layer material and obtain cross-section images of single particles with the SEM. By an iterative automated workflow of milling and imaging it was possible to acquire image stacks with 10 nm voxel size. The 3D reconstruction of such an image stack is shown in Figure 2. In this data cracks inside the particles or their structural deformation when they are pressed into each other can be analyzed.
This example shows the combination of 3D x-ray microscopy with FIB-SEM tomography to gain maximum insight at high resolution over multiple length scales.