Have you ever wanted to cut a battery open to see how it’s made? Don’t do it. At best, the contents will irritate your skin. At worst, they’ll explode. Fortunately, we’ve got a good way to look inside batteries: a Neptune industrial CT scanner that can resolve features as small as 25 microns.
Let’s see how these things work. Want to go deeper? Join us for an interactive walkthrough on November 16.
The alkaline battery was invented at the turn of the last century. Older technologies, such as the lead-acid battery, used acid instead of an alkaline solution for electron mobility, and Alkaline batteries tended to have better charge densities. A Canadian engineer invented the modern alkaline battery design in the late 1950s (a fact our Canadian colleagues insisted we include).
An alkaline battery contains a few concentric layers. The outermost layer is called the can, typically made of steel, and is just 250 microns thick in this case. Inward one layer is the manganese dioxide that makes up the positive layer or the cathode. Then comes a rubber-like separator, followed by a layer of zinc to serve as the anode.
On this particular battery, you can see that cracks have formed in the manganese cathode. As we slide into the powdered zinc web, another body emerges: a brass pin that collects electrons from the anode and gives them a pathway out.
The pin is attached to a plate that serves as the battery's negative terminal. The battery is assembled by plunging the pin into the zinc, then sealing it in place with asphalt or epoxy. This final manufacturing step is the source of many defects; the battery will short out if the pin breaks through the rubber separator between the anode and cathode, or if the displaced zinc connects to any positively charged surfaces.
Lithium ion batteries–the foundation of modern portable electronics–are able to hold vastly more power than alkaline batteries. Although they cost more than an alkaline battery for a given form factor, they’re able to hold so much power that they wind up about 85% cheaper on a per-watt-hour basis. This example, the LG21700, powers your favorite Teslas. We used the measuring tools in our Voyager analysis software to decode the model number: it seems to mean that the battery is 21mm in diameter and 70mm tall.
This lithium-ion battery outwardly resembles a household alkaline battery, but it’s constructed very differently. Beneath the negative terminal, we find two spot-welded tabs that travel into the can. We also see concentric layers packed densely into the cylinder.
Those tabs, spot-welded together with the can, serve as the anode. Similar to the alkaline battery, they travel far into the construction to provide maximum surface area for electrons to traverse. To see what they connect to, keep scrolling.
This battery is made up of 25 layers, each of which has a negative side, a separator, and a positive side. It’s formed by fabricating a large sheet and rolling it up to fit in the battery’s cylindrical form factor. The gap in the sandwich here is where the cathode connects to the top of the battery.
This layered structure offers excellent energy density, but it’s fragile; damaging even a single layer can cause the battery to short out, leading to fires or explosions. This cylindrical battery is effectively armored, but lithium ion batteries are often packed in thinner, less-protected enclosures in phones and cars.
Engineers who design rechargeable electronics–everything from wireless earbuds to cars–need to pack as much lithium-ion power into as little space as possible. The lithium polymer battery packet has become the standard in these applications, but it’s also fragile and requires careful handling to avoid catastrophic problems. CT inspection is increasingly used in the battery industry to catch tiny defects and signs of damage before they turn into explosive failures.
Slicing in from the edge, we can get a clear view of the battery pack’s construction. Layers of lithium polymer sit between separators, the thin sections that stick up slightly higher than the rest. As the battery charges and discharges, it heats up and tends to cause the separators to delaminate from the polymer layers. Here, we see some delamination around the bend.
The positive electrode on this battery is a tab of aluminum foil. The negative electrode is copper; its yellow color in our visualization means it’s denser than the other materials in the battery. Perforations minimize battery expansion during cycling, and glue spots hold the battery’s foil wrapper together.
The polymer electrolyte can be seen in its full glory by melting away the lower-density materials in our CT visualization, revealing lithium layers that are 100 to 150 microns thick. Lithium polymer electrolytes allow for high energy densities and fast discharges, but the material is also very volatile and known to swell–which in turn can damage the battery and lead to cascading failures.