R&D and Failure Analysis
As noted previously, when describing what is possible and not
possible for production inspection, battery components are,
in almost every case, very delicate, and that is the root of the
inspection challenge that requires the use of RT. Inspection
of anode/cathode alignment and surface overlap, along with
identifying flaws in connectivity, can—with the right equip-
ment—be conducted at production pace. However, a closer
and more detailed inspection cannot. These longer, more
detailed evaluations fall under the categories of R&D and
failure analysis.
The areas of interest for understanding battery efficiency
and durability require much more detailed, higher-resolution
scans and therefore require very different CT scanning equip-
ment than would be used in a production application. For
R&D, such systems need to achieve resolutions meaningful for
analyzing material structure and characteristics, such as anode
and cathode coatings, the nature of the interface between
layers, and anomalies in coatings. These systems must be
capable of reaching sub-micron resolution with excellent
image quality, and they must do so within a relatively short
scan time.
Accordingly, systems used for battery cell R&D must be
flexible enough to undertake various tasks, including exploring
sub-micron particles, evaluating design deviations, diagnosing
manufacturing issues, assessing material flaws, and exam-
ining geometric structures. This can be achieved by X-ray
CT systems, as shown in Figure 7, which use the latest focal
spot technologies, placing them on par with state-of-the-art
optical magnification scanners but with easier operation, faster
learning curves, and greater flexibility. Since users are typically
chemical and materials engineers as opposed to radiography
specialists, it is important that the systems are easy to use with
limited training.
The main areas of interest in studying cell performance
and longevity include the quality and thickness of the anode
and cathode coatings, as well as signs of particle breakdown
within the coatings, which can degrade cell performance.
Figure 8 shows cracking in particles of a Li-ion battery cathode
coating in this example, the cracks measure ~0.6 µm wide.
The cathode coating is created by applying a slurry that,
once set, looks like pebbles under magnification, while the
coating on the anode is a graphite slurry that has a more
homogeneous structure. These coatings must be very consis-
tent in thickness and free from cracks. This type of cracking is
one of the most common conditions that reduces a battery’s
ability to recharge to its original levels, thereby affecting overall
battery life.
Figure 9 shows the coating of a typical Li-ion cell anode,
where graphite flakes are visible, and Figure 10 shows
the cross section of a typical Li-ion cell cathode, both at
sub-micron discernible feature resolution.
For failure analysis, CT X-ray systems are used that can
scan larger samples, which are more flexible in terms of the
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ELECTRICVEHICLES
Figure 7. High-resolution CT scanner with sample very close to the
source.
0.04 mm
Figure 8. 2D slice through 3D CT scan of degraded cathode coating (with
red arrows pointing to cracks). Coating sample size: 3 mm voltage:
100 kV power at target: 1.8 W voxel size: 0.5 µm scan time: 11 h
FDD: 459 mm magnification: 200 × 3600 projections.
0.04 mm
Figure 9. 2D slice through 3D CT scan of anode coating (high-resolution
image). Coating sample size: 3 mm voltage: 100 kV power at
target: 1.8 W voxel size: 0.5 µm scan time: 11 h FDD: 459 mm
magnification: 200 × 3600 projections.
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resolution they can achieve. This flexibility allows scans in the
low micron range to be obtained as well as complete scans of
large cells, and even entire battery modules and small packs.
Such flexibility is not required of a production machine, which
would be custom-built and dedicated to one task.
R&D systems are typically equipped with dual X-ray
sources (tubes) to achieve the broadest possible range of
scan resolutions. These usually include both a nanofocus
tube (1 µm focal spot) and a microfocus tube (200 µm focal
spot). This dual-tube configuration (as shown in Figure 11) is
often coupled with a much larger scan envelope within the
system, allowing for much greater sample size and weight.
Such systems allow an operator to have the flexibility to
measure a range of part sizes or even perform both a low- and
high-resolution scan of the same part without dismounting it
from its fixture. However, using both sources at the same time
isn’t possible.
With larger scans, the user is typically looking for defects
such as misaligned anodes and cathodes, electrode overhang,
damaged electrodes, welds and tabs, tears in electrode
material, and inclusions. All of these defects reduce the perfor-
mance of the cells and can make the cells hazardous.
Figure 12 shows an example of inclusions of rogue mate-
rials accidentally introduced during the production process.
These inclusions are very difficult to visualize with other
inspection methods but are easily seen with the appropriate
CT machine as bright white spots.
An example of a damaged electrode in a cylindrical cell is
shown in Figure 13. Damaged electrodes can reduce the effi-
ciency of a Li-ion cell and, in the worst case, encourage the
buildup of dendrites that can breach the separator and cause
thermal runaway.
Motors
The remaining parts of the powertrain system that differ from
those in an IC vehicle are the motors. Unlike batteries, their
components are not nearly as delicate, but they are sealed
away from view and buried in solid steel casings. By their
nature, this makes them not only impossible to measure using
tactile or optical methods, but very difficult to penetrate with
X-rays except by using the most powerful MeV (over 1 million
volts) X-ray sources. The need for an MeV source precludes the
use of RT in a production environment because of the safety
implications associated with such high energies, along with the
long scan times required—often measured in hours. However,
Figure 11. CT machine with both microfocus and nanofocus sources.
0.04 mm
Figure 10. 2D slice through 3D CT scan of cathode coating (high-
resolution image). Coating sample size: 3 mm voltage: 100 kV power
at target: 1.8 W voxel size: 0.5 µm scan time: 11 h FDD: 459 mm
magnification: 200 × 3600 projections.
Figure 12. 2D slice through CT scan of inclusions, seen as bright white
spots, and broken elements (indicated by red arrows). Voltage: 450 kV
power at target: 675 W voxel size: 70 µm scan time: 3 h, 46 min FDD:
1150 mm magnification: 1.4 × 5589 projections.
Figure 13. 2D slice through CT scan of damaged electrode in a
cylindrical cell. Voltage: 200 kV power at target: 10.8 W voxel size:
5 µm scan time: 2 h, 40 min FDD: 801 mm magnification: 19.5 ×
4800 projections.
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