X-ray Microscopic Comparison of Rechargeable AA Batteries

Animated High-Resolution Views of Rechargeable Batteries

Figure 1: USB rechargeable lithium-ion battery in AA size format

While more and more of our lives require electricity to keep things moving, the need to reduce waste streams leads to the increased adoption of rechargeable batteries in place of standard disposable alkaline cells. While rechargeable batteries are not a new technology, advances on this front are ongoing.

Micro-CT is an established method for quality control in product design and industrial production and is useful for analysis of traditional batteries and for viewing the internal details of new designs, non-destructively. We utilized the SkyScan 1273 desktop micro-CT to visualize and compare differences between two rechargeable battery formats because it provides a high level of detail and versatility for product development or failure analysis.

X-Ray Microscopic Imaging of Rechargeable Batteries.

Two commercially available rechargeable battery products were obtained and mounted in a low-density plastic tube for imaging. One product is a traditionally assembled nickel metal hydride (NiMH) battery, which externally shows unremarkable differences compared to a conventional battery. The second product contains on-board microelectronics to convert USB power directly into recharging a small lithium-ion battery inside the device. In this way, this alternative battery style can be recharged by any USB connection rather than requiring a dedicated device devoted to recharging batteries.

We examined both batteries using our high-power SkyScan 1273 micro-CT with an isotropic voxel size of 10 µm. With a 130kV X-ray source capable of operating at up to 39W of power, the SkyScan 1273 is a great match for battery samples where density can be a concern on lower-powered instruments. Due to the aspect ratio of the AA battery design, we chose to maximize our resolution for these datasets by basing our resolution on the diameter of the sample and using the oversize imaging mode to digitally combine four connected acquisitions along the length of the battery into a single, unified dataset during the reconstruction process.

Figure 2: Planar 2D slices through the NiMH battery cell (top) and lithium-ion battery cell (bottom)

As shown in Figure 2, DataViewer provides us with a linked set of 2D images to navigate through the dataset. These views allow us to interactively move through each dataset to boost our comparative ability. For the traditional NiMH rechargeable battery, we observe the positive electrode material (light gray) and the negative electrode material (bright white), which is comprised of a metal mesh physically adhered to the sides of the can and wrapped in alternating layers with the positive electrode material to form the spiral pattern.

Alternatively, the lithium-ion battery shows that about 40% of the volume within the AA can is occupied by micro-electronics and the physical micro-USB receptacle. The remaining volume within the can is occupied by the lithium-ion cell with a similar rolled design holding the anode and cathode materials physically isolated by a separator.

Comparing the stated capacities for both batteries, the NiMH batteries promise a 2800 mAh capacity at 1.2V while the lithium-ion batteries only promise 1560 mAh capacity at 1.5V. While the capacity of the lithium-ion batteries does not match the NiMH, the lithium-ion cell life would be expected to far exceed that of NiMH with the NiMH cells often only rated for ~1000 recharge cycles. Likewise, if charged and stored, the NiMH battery would be expected to lose up to 50% of its power after 12 months whereas the lithium-ion battery would be expected to lose less than 10%.1 Thus, even though the lithium-ion batteries will need to be recharged more often than the NiMH to provide the same amount of power, long terms effects of increased charging and overall durability should be higher for the lithium-ion cells.

Figure 3: CTVox rendered volumetric representation of the NiMH (top) and lithium-ion (bottom) datasets

Using CTVox, we digitally sliced into each dataset to explore differences on their internal construction as shown in Figure 3. While the negative electrode mesh dominates the signal for the NiMH battery, the lithium-ion battery signal is highest in the solder joints present in the connections between the battery cell collectors and the micro-electronics.

Figure 4: CTVox rendered representation of the micro-electronics and micro-USB receptacle

Likewise, if we want to isolate a portion of a dataset to explore in greater detail, CTVox allows us to set our clipping planes or use clipping shapes to highlight only our region of interest, as shown in Figure 4.

Conclusion

Among the SkyScan product line, the SkyScan 1273 is a workhorse instrument with the most flexibility for different sample types within our portfolio. As the highest-powered desktop model available within the SkyScan portfolio, the SkyScan 1273 is a great match for imaging battery cells for inspection or comparison.

We hope you found this Image of the Month informative and encourage you to subscribe to our newsletter and social media channels in preparation for the continuation of our Image of the Month series next month.

Scan Specifications

Sample Rechargeable AA Batteries
Voltage (kV) 130
Current (µA) 107
Filter 2 mm Copper
Voxel Size (nm) 10
Rotation Step 0.25
Exposure Time (ms) 981
Rotation Extent (deg.) 360
Scan Time (HH:MM:SS) 16:38:08

These scans were completed on our SkyScan 1273 micro-CT system at the Micro Photonics Imaging Laboratory in Allentown, PA. Reconstructions were completed using NRecon 2.0 while visualization and volumetric inspection of the 2D and 3D results were completed using DataViewer and CTVox.

Would you like your work to be featured in our monthly newsletter? If so, please contact us by calling Seth Hogg at 610-366-7103 or emailing seth.hogg@microphotonics.com.

References

  1. https://data.energizer.com/wp-content/uploads/2020/11/nimhhandbook_ver2-2.pdf

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