Industrial computer tomography with the exaCT s

Failure Analysis of a Lithium Ion Accumulator

In Order to find the cause of an electrical failure in a lithium ion accumulator, the internal structure of the component was non-destructively analyzed by a computed tomography.

The computed tomography exaCT S from Wenzel provides a view into the interior of an accumulator. As a result the error causing the failure can now be avoided during the assembly of the cell. Figure 1 shows the experimental set-up of the measurement.

The electrical failure of a lithium ion accumulator was the basis for this study.

The outer shell of the battery showed no evidence of the cause of the failure; a look into the interior was therefore necessary. In addition to the destructive testing such as metallography, non-destructive testing methods such as X-ray radiography and X-ray computed tomography were considered. The radiography provides only overlay images. An interpretation of the component is often more difficult in the X-ray image due to the superimposing of the structures. So the choice fell on the computed tomography. This method provides three-dimensional images, which make it possible to analyze non-destructively any areas inside of the component.

Reconstruction of a three-dimensional data set

To analyze the defect cause, the internal structure of the positive and negative poles was studied using X-ray computed tomography. For this purpose, the exaCT S 130kV desktop computed tomography from Wenzel Volumetrik was used. The CT has a maximum voxel resolution better than 5 microns.

Horizontal CT cutting plane of the accumulator with identification of the conductive tab.To obtain the three-dimensional data set, various two-dimensional projection images are required and to enable this, the test object rotates 360° during the measurement in an X-ray cone beam. During the rotation the detector records the projection images that are composed of the material-dependent weakening of the X-ray beam. Then a three-dimensional data set of volume elements (voxels) are reconstructed by high performance computer systems.

After the measurement, and the reconstruction of the volume, it is possible to check any layer of the component from different perspectives for errors and to display the details of the component’s interior.

The volume data can be visualized and analyzed with the analysis software.

Through the reconstructed volume of the accumulator any two-dimensional cutting planes can be placed and moved with a step size in micrometer range, so a detailed analysis inside the component is possible.

The horizontal CT cutting plane shows the wound electrode and separator layers of the accumulator into which the so-called conductive tab is inserted (see arrow).

Vertical perspective at the CT volume scan of the accumulator with marking of the crack in the conductive tab.The conductive tab is connecting the positive electrode to the cover of the cell.

Looking at the conductive tab at the positive pole from a vertical perspective at the CT volume scan, the windings can be found in the lower half, in which the conductive tab is passed to.

Above is the connection of the conductive tab to the positive pole. Above the windings in the first turn of the conductive tab an interruption can be clearly observed (see arrow).

This is the cause of the electrical failure. The movement of the cutting planes before and after the defective location shows that this extends continuously at the same location on all the other vertical cutting planes. This means that the conductive tab is completely broken.

Computed tomography offers a view into the interior of the accumulator

Continuous cracked conductive tab at the CT volume scan of the accumulator.The analysis software enables the user to cut away volume parts of the accumulator using clipping boxes.

The conductive tab could thus be selectively exposed by hiding areas so that the continuous crack in the CT volume scan is very easy to see

The direct comparison of the vertical CT-cutting planes of the electrically failed accumulator (above) with an identical electrically functional accumulator (below) shows, as expected, an intact conductive tab at the functional accumulator.

Without the comparison object, the interpretation of the defect location would be compromised, as it could be interpreted that the defect is an artifact in the CT images.

Artifacts are deviations from reality in the CT image, which are artificially created so it could also be interpreted that the crack is no real gap in the conductive tab, but caused by a shadow of the more absorbent cover of the accumulator.

Vertical CT cutting plane through the electrically failed accumulator (above) and for comparison through an intact cell (below).Due to the continuous interruption of the conductive tab there is no electrical contact between the positive electrode and the cell cover.

To avoid the error in future, care must be taken in cell production. There should be no excessive stress which could damage the 150 micron thick conductive tab during the assembly of the cell. The fact that the crack in the conductive tab does not result from an external influence, such as mechanical stress on the cell, is clear because no external force is visible in the form of a deformation. Other possible examples of applications are the visualization of internal structures as a part of design reviews, cell evaluations or the detection of foreign objects within the cells.

Authors
Miriam Rauer, Prof. Dr. Michael Kaloudis
Aschaffenburg University of Applied Sciences
Laboratory for Materials Technology
T 06021/4206-834