What’s Inside a Lithium-Ion Battery?
To understand why lithium-ion batteries sometimes fail, you need to know what’s going on under the hood. Inside every lithium-ion battery, there are two electrodes—the positively charged cathode and the negatively charged anode—separated by a thin sheet of “microperferated” plastic that keeps the two electrodes from touching. When you charge a lithium-ion battery, lithium ions are pushed by electricity from the cathode, through the microperferations in the separator and an electrically conductive fluid, and to the anode. When the battery discharges, the reverse happens with the lithium ions flowing from the anode toward the cathode. This is the reaction that powers your laptop.
Small batteries, like those found in smartphones, usually have only a single lithium-ion cell. Larger batteries, like those in laptops, normally have between 6 and 12 lithium-ion cells. The batteries in electric cars and airplanes can have hundreds of cells.
What Makes a Lithium-Ion Battery Explode?
The very thing that makes lithium-ion batteries so useful is what also gives them the capacity to catch fire or explode. Lithium is really great at storing energy. When it’s released as a trickle, it powers your phone all day. When it’s released all in one go, the battery can explode.
Most lithium-ion battery fires and explosions come down to a problem of short circuiting. This happens when the plastic separator fails and lets the anode and cathode touch. And once those two get together, the battery starts to overheat.
There are a number of reasons that the separator can fail:
- Bad Design or Manufacturing Defects: The battery is poorly designed, as with the Galaxy Note 7. In that case, there wasn’t enough space for the electrodes and separator in the battery. In some models, when the battery expanded a little as it charged, the electrodes bent and caused a short circuit. Even a well designed battery can fail if quality control isn’t kept tight enough or there’s some defect in manufacturing.
- External Factors: Extreme heat is nearly guaranteed to cause a failure. Batteries left too close to a heat source—or caught in a fire—have been known to explode. Other external factor can cause a lithium-ion battery to fail, too. If you drop your phone too hard (or too many times), there’s a chance you’ll damage the separator and cause the electrodes to touch. If you pierce the battery (either by accident or deliberately), then you’ll almost certainly cause a short circuit.