ADVANCED MATERIALS & PROCESSES | JULY/AUGUST 2023 32 a process that involves spraying a slurry of lithium, nickel, cobalt, and other metals onto a foil made of aluminum (for the cathode) or carbon onto copper (the anode), then rolling or “calendaring” the resultant product into thin sheets, which are then made into cells—the building blocks of any lithium-ion battery. The density and distribution of individual particles is critical to efficient ion storage; but since these tiny bits of metal— the active material— are best measured in microns, nano- CT-scanning with its ability to “see” even the smallest of features is the most effective inspection method available. Advanced imaging and analysis software converts a series of two-dimensional, black-andwhite CT-scanned images into dimensionally accurate 3D models of the object. Researchers then use these to determine particle size and distribution. They can measure variation in the layer thickness, spot voids, and detect contamination. And once the coated foils are cut to size, rolled or stacked, and assembled into the cells that make up the battery pack, they can look for inclusions, thought to be the main cause of battery fires. Inclusions form when molten metal penetrates the battery pack during welding, or when small bits of foil from the cutting process make their way into the cell (Fig. 3). Regardless of how they get there, these particles can damage the thin layer of plastic that keeps the battery’s anodes and cathodes separate from one another. And even if there is no direct harm at the time of manufacturing, mechanical friction over the course of the battery’s lifetime can cause the separator to fail, possibly leading to a thermal runaway. Detecting inclusions at the time of manufacturing is therefore crucial. It is also why many used batteries are CT-scanned upon removal from a vehicle, to determine whether they might go on to a “second life” in stationary charging stations or for domestic energy storage. CT scanning and analysis finds other defects as well. Porosity in the weld lines is not likely to cause a fire, but it can lead to leakage (Fig. 2). Here, snow and rainwater can seep into the battery over time. The electrolyte within then reacts to form hydrofluoric acid, a highly corrosive gas that irritates the eyes, nose, and respiratory tract in humans. There is also layer delamination to consider, a phenomenon that reduces battery capacity. In addition, “anode overhang,” the small but necessary height differential between anode and cathode that’s critical to battery safety, is monitored (Fig. 4). Without it, lithium crystals can form on electrode edges, eventually growing large enough to puncture the separator and cause a runaway. INTERPRETING THE DATA Although CT scanning is the only metrology tool able to nondestructively inspect for these and many other defects and conditions, there’s one caveat: analysis of the resulting data is complex and requires advanced software analysis to interpret the images and identify various features. These software tools can dramatically simplify the inspection process, allowing even non-expert users to leverage CT scanning to its fullest capability. This is especially true for battery researchers, designers, and manufacturers, who have a host of application-specific tools at their disposal. For example, VGSTUDIO MAX from Volume Graphics allows operators to identify those metallic inclusions that can damage the electrode separators. Fig. 2 — Weld line porosity analysis identifies areas where leakage and water infiltration may occur, which might eventually lead to the formation of hazardous gases. Fig. 3 — The random colored bits shown here represent inclusions that could puncture the plastic separators in battery cells and potentially cause a runaway fire. Fig. 4 — The color-coded vertical sections at the bottom of the image (note mm scale) indicate anode overhang, a structure that is crucial to battery performance and safety.
RkJQdWJsaXNoZXIy MTYyMzk3NQ==