Lithium-ion batteries, despite being one of the most important pieces of modern technology, have always had a particularly grievous flaw: they’re highly flammable. If you puncture a lithium-ion battery, or you charge and discharge them improperly, you’ll usually have a pretty bad fire on your hands — or worse, if the conditions are just right, an explosion. Most notably, the Boeing 787 Dreamliner was grounded in 2013 due to its lithium-ion battery packs causing electrical fires mid-flight, but there have also been a fair number of stories about exploding smartphone batteries recently. Now, long overdue, researchers at the University of North Carolina at Chapel Hill say they’ve built a nonflammable lithium-ion battery.
Lithium-ion batteries (LIBs) are prone to be incendiary and explosive for two reasons: because they’re pressurized, and because their electrolyte — the charge-carrying liquid that sits between the positive and negative electrodes — is flammable. The electrolyte in an LIB is nearly always a lithium salt dissolved in an organic solvent, and most of these solvents are highly flammable. Because of these factors, LIBs are generally manufactured to a very high level of quality, but that doesn’t help you if the battery is somehow pierced (as in the Tesla Model S battery pack that was impaled and burst into flames), or if thermal runaway occurs.
Now, if you mutilate an LIB or throw it haphazardly in the trash, you’re asking for trouble. (Always recycle your batteries!) Thermal runaway, however, is a much trickier problem that’s inherent to LIB tech, and which you can’t do a whole lot about. Basically, in the process of charging or discharging a lithium-ion battery, it warms up. This is just the battery chemistry in action, and is completely normal. If it discharges too quickly, though, or you overcharge it, you can rupture one of the battery’s cells. This rupture can then cascade down the line of cells, potentially causing a fire or explosion. This is why the peak power draw from LIBs is fairly low, and why there has been some discussion about whether LIBs are the right technology for high-power applications, such as electric vehicles and grid power storage.
The most common cause of LIB thermal runaway is likely faulty charging circuits. The battery itself could be suffering from a manufacturing fault, too. When the cells rupture in some cases, an LIB will just swell up rather than explode. We still don’t know what caused the Dreamliner battery fires, but it’s probably just a case of Boeing underestimating peak draw, or an inherently faulty battery design (it uses just a few big cells which are prone to cascading failure, rather than Tesla’s approach, which uses lots of smaller cells).
The University of North Carolina’s breakthrough is to replace the electrolyte’s flammable organic solvent with nonflammable perfluoropolyether (PFPE). PFPE is usually used as an industrial lubricant, and the lead researcher – Joseph DeSimone — had originally been looking at using PFPE on the bottom of ships to prevent marine life from sticking, which is a serious problem in the world of shipping. He realized that PFPE had a similar structure to another solvent used in LIBs, so he did what any sensible chemist would do: tried dissolving a lithium salt in PFPE to see what would happen. As luck would have it, it worked just fine. He created a nonflammable lithium-ion battery.
DeSimone and his team still need do more testing to see if the nonflammable electrolyte can withstand the rigors of everyday use, but the research paper puts a pretty positive spin on things: “These electrolytes not only are completely nonflammable, but they also exhibit unprecedented high transference numbers and low electrochemical polarization, indicative of longer battery life.” Moving forward, the team will now begin with the arduous task of bridging the gap between laboratory testing and industrial mass production.
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