Eli5: Why do batteries have the potential to explode?
In: Technology
In addition to the heat of electrical resistance, it helps to understand how many batteries work. Not all batteries will explode for the same reason.
Lithium batteries are notorious for being explosive. Lithium batteries have a barrier inside that prevents electrons from flowing from the anode to the cathode, but lithium ions can cross freely. The positive lithium ions flow through the electrolyte across the barrier, and the electrons needed for the ions to bind to the anode have to flow *around* through the wires and whatever device you’re powering.
The process, as others have said, creates heat. If the electrons are allowed to flow too quickly, too many ions will go through the barrier and create heat within the battery. This internal heat degrades the barrier, meaning it can’t stop electrons as well. If electrons are allowed to flow through the barrier they aren’t limited by the wire and devices anymore, and can flow freely to the other side. This is a short circuit, inside the battery. This means *more* ions will flow, creating more heat, which degrades the barrier even more, which allows even more electrons to flow, so more ions flow, and so on as the battery fails catastrophically.
An explosion is, essentially, the rapid release of heat: the battery releases all of its stored energy at once instead of over time, and you get an explosion. Plus, as others have said, lithium is very reactive to oxygen, water, and even nitrogen in the air. Once the battery casing is breached by the internal heat and fire the lithium will react with the air, too. That reactivity is lithium is used in batteries, though. Because it’s so reactive, it can store a lot of energy as long as it’s done safely.
Lead/acid batteries, like the ones in cars, can explode for a different reason. The reaction that creates electricity also creates hydrogen gas. Hydrogen is [also very reactive to oxygen](https://upload.wikimedia.org/wikipedia/commons/1/1c/Hindenburg_disaster.jpg), but it won’t spontaneously ignite in the air like lithium can. It’s actually pretty hard to get lead/acid batteries to explode. However, *if* there is a build-up of hydrogen gas inside the battery, and if the battery is damaged and enough heat is generated, or a spark, the battery can explode. The acid in the batteries (sulfuric acid) is not itself flammable, but it *is* acid so you don’t want to get that on yourself. And if it spills onto metal it will react and create more heat and more hydrogen gas, so the initial explosion from the battery can cause more fire as the acid is flung around the battery.
Other battery chemistries typically don’t have enough stored energy or strong enough reactions to *explode*, but they can still create a lot of heat and/or leak corrosive chemicals out when damaged or short circuited.
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