Samsung announced last week that it’s recalling millions of its new Galaxy Note 7 smartphones amid reports the devices were overheating and, in some cases, catching fire.
There haven’t been any known cases of people being physically hurt by the phones. But the devices are being blamed for fires that destroyed a Jeep and did serious damage to a garage. Meanwhile, U.S. aviation officials are now telling fliers to avoid turning the phones on in flight.
The South Korean electronics manufacturer has yet to explain exactly what’s causing the fires. What we do know, however, is that the problem stems from their battery.
Like almost all modern smartphones and lots of other consumer electronics, the Note 7 uses a rechargeable lithium-ion battery. Electronics companies favor these kinds of batteries because they’re cheap, they pack plenty of power, and they don’t lose a lot of their charge over time when they’re sitting idle. Yet they have also been involved in several high-profile fire incidents, including episodes that more or less ended the hoverboard craze and caused problems with Boeing’s 787 jetliner.
How do lithium-ion batteries work, and why do they seem to be prone to catching fire? We asked Dr. Donald R. Sadoway, the John F. Elliott Professor of Materials Chemistry at the Massachusetts Institute of Technology (and onetime TIME Most Influential Person), to explain. First, he offered a quick primer on battery chemistry:
[Lithium-ion batteries] are a classical battery in the sense that they have two electrodes separated by an electrolyte. In this case, the negative electrode is typically some kind of a carbon, like a graphite, and the positive electrode is a metal oxide, something like lithium cobalt oxide or lithium manganese oxide. The electrolyte, because it’s shuttling lithium, it has to be non-aqueous. They can’t use an acid or an alkaline solution, they have to use something that’s not water, and that’s part of the problem. It’s an organic liquid and so therefore it’s volatile and flammable. But it does dissolve lithium salt and it allows lithium to shuttle back and forth between the negative electrode and the positive electrode.
Let’s [talk about] the battery at full state of charge. At full state of charge, all of the lithium is up inside the negative electrode. It’s sitting inside the graphite. On discharge, the lithium wants to go from the graphite over to the cobalt oxide. So it takes a swim through the electrolyte and enters the cobalt oxide, and electrons go through the external circuit and power your devices. That’s what generates the current and so on.
Sadoway then brought up the battery fires that plagued Sony laptops nearly a decade ago:
What happened there was, somehow during the manufacturing process, very tiny particles of metal had ended up in the electrolyte . . . under the action of electric current, these metal fragments aligned, and they eventually formed an unbroken chain of metal from one electrode to another. So then you’ve got the current shorting through this filamentary wire, if you will, so you’ve turned the inner electrode gap into a toaster oven. That causes all of the current to short, and generates a lot of heat, and that causes the electrolyte to bloat, and then the next thing you know, it explodes.
If the temperature gets high enough . . . at some point, if you get up to about 400-500 degrees Centigrade, the metal oxide in the negative electrode actually starts liberating oxygen. And that’s really dangerous, because now, instead of having a fire . . . getting its oxygen from the air surrounding it, it’s getting its oxygen from inside the battery itself. The term of art is, this has now become a bomb. You’ve got fuel and oxygen in the same place at the same time.
Right now, it’s hard to tell exactly what’s causing Samsung’s problems. But Sadoway has some theories:
It’s either a statistical fluke in the manufacturing process where there are some local hotspots or there are perhaps some metal shavings, some kind of a shorting. But it seems as though, from what I’ve been able to read, the fires occurred when the owners are charging the phone. That means that while they’re forcing current through it, somehow there is a side reaction that is very different from just recharging the battery. And that starts charging the battery, and then the thing goes into thermal runaway.
So does Sadoway think lithium-ion batteries are still safe to use in smartphones and other electronics?
They can be manufactured safely. The vast majority of them operate without incident. But this is a cautionary tale for us, saying that if you want to put a lot of energy into a small volume, you want to make sure that energy doesn’t let go all at the same time. So it means that you’ve got to be attentive to detail in the manufacturing process.
I don’t suspect anybody does this out of spite. I’m sure Samsung wants to get this thing corrected as quickly as possible. Boeing of course didn’t want to sink its own 787. But it shows you that batteries are complex devices.
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