Eli5: Why do batteries have the potential to explode?
In: Technology
It has to do with heat. Movement in real life creates friction which developed heat. In the world of electricity, a similar thing happens because every material will absorb some electrons. When you absorb energy you create heat. So as the materials in the battery have more and more current pushed through then the hotter they get. If too much current is pulled through the battery then the matria will get too hot.
In the case of modern lithium ion batteries, that heat will begin to melt the lithium metal. Hot lithium metal bonds with oxygen in the air in a rapid and explosive release of energy.
There’s a lot of energy in there.
If you cause a short through the whole thing, instead of nicely providing 5v for a long duration, it’s all the volts all at once. That flow of electricity heats up everything acting as a resistor along the path, just like a space-heater but uncontrolled. Whatever the electricity is flowing through can heat up so much that it vaporizes. Gases forming inside of solids can build up pressure and explode really quickly.
I tend to think of it as three things. Energy, size and time.
The sun provides a load of energy. But it’s right across the surface of the earth. We can deal with that. Use a lens to focus just a bit of it, though, and you can set stuff on fire. You’ve concentrated spread out energy into a small area.
Now for time. Fill your car with petrol. That’s a lot of energy. It can power a car to go at 80mph for hours. Set fire to that can of petrol and you’ll blow up a house. The difference is time: releasing it all at once or releasing it slowly.
A lithium ion battery does all of these things. You need enough energy to power a computer, screen, radios, audio etc etc for a day. You squeeze that into a really, really small space. That’s all fine provided that you release it slowly over that day. But if the battery gets damaged or short circuited so that there’s no control over its release, it all comes out at once.
Batteries have electrical charge stored inside of them, but that electricity is only released when you give it somewhere to go, like when you connect a piece is metal to the positive and the negative sides so that it can run your car or your cell phone.
If you have ever touched 2 hot wires together, or seen someone stick a piece of aluminum foil into a household socket, then you’ll know that electricity really wants to escape, and when you touch those 2 wires together you get a big bad spark.
The same thing happens in a battery when the liquid inside starts to have problems that basically create little “wires” inside of the battery that connect both sides.
The explosion is actually a whole bunch of sparks that are the electricity trying to find it’s way out. Combine that with materials that may be flammable, and you have the potential to explode.
The little”Wires” I referred to earlier are called dendrites. They look pretty cool under a microscope.
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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