Boeing’s 787 Dreamliner has not had a charmed birth. While some of its problems—a windshield crack, minor fuel leaks—can easily be dealt with, the issues that have arisen with the plane’s batteries are much more serious.
In two incidents in recent weeks, lithium-ion batteries malfunctioned—the battery in a 787 on the tarmac in Boston caught fire, and a second 787 was forced to make an emergency landing Wednesday morning in Japan, when smoke from the battery made its way into the cockpit. That led to a worldwide grounding of Dreamliners while authorities investigate the batteries and other 787 systems.
It isn’t the first time this type of battery has shown itself to be combustible. Recent years have seen reports of lithium-ion batteries catching fire or exploding in smartphones and laptop computers. What is it about these batteries that makes them so prone to blowing up?
The first thing to know about lithium-ion batteries (li-ion batteries, for short) is that lithium is extremely flammable. The other thing to know is that li-ion batteries carry much more energy per weight than any other battery; in technical parlance, they have a higher energy density. That’s why they’re the battery of choice in everything from iPhones to laptops to electric cars, whose designers want to get the greatest potential power out of the smallest, lightest power source. In the Dreamliner, the use of li-ion batteries was part of what made the plane so much lighter—and therefore more fuel-efficient than its predecessors.
The problem is that these qualities make li-ion batteries much more sensitive to heat than traditional lead acid batteries (the sort that start your car). And any time you run an electric current through something, whether it’s a light bulb filament or a battery, you produce heat. If a battery is being used to start the engine of a jetliner, as one of the Dreamliner’s li-ion batteries is, that requires a considerable pulse of energy and so it creates a correspondingly large amount of heat (the battery doesn’t directly start the engine; it starts a device called the auxiliary power unit, which in turn starts the engine). It’s too soon to know exactly what happened in the 787s, but according to Donald Sadoway, a battery researcher and MIT professor, if a li-ion battery is generating that much energy and isn’t being properly cooled, it’s very possible that it could heat itself to the point at which it would burst into flames. “You’ve got to draw very, very high amperage for a very short time, so you’re generating a boatload of heat,” he says.
Another option is that the problem plaguing the 787 batteries has plagued li-ion laptop batteries in the past: an internal short-circuit. A byproduct of the battery manufacturing process is that tiny bits of metal float in the lithium liquid in the electrolyte, the portion of the battery that separates its positive and negative ends. The metal dust serves no purpose, but so far manufacturers have been unable to figure out how to eliminate it. Normally those metal bits are harmless, but once in a very long while they can cause short-circuits (PDF) in the battery cell; if enough of the particles cause shorts at the same instant, this can trigger a “thermal runaway.” In essence, the battery releases all its stored energy at once, and since li-ion batteries can carry so much energy, their thermal runaways are particularly dramatic. The resulting spike in temperature can then ignite the lithium in the battery, causing the battery to catch fire, melt down, spew flames, explode, or some unpleasant combination of all four.
Battery makers try to limit the likelihood of shorts and to protect against the consequences of them. Li-ion batteries have what are, essentially, different sorts of internal circuit-breakers, plus vents to allow for pressure relief. Many are built using lithium compounds that have been synthesized to maximize stability at the expense of energy density. Sadoway’s own work at MIT is on developing a solid polymer electrolyte that would avoid the volatility and flammability of today’s li-ion batteries.
Still, it’s a testament to the cost of jet fuel that airplane makers (Airbus’s forthcoming A350 also relies heavily on li-ion batteries) are putting them in planes. And it’s a testament to our unquenchable desire for smaller, more powerful electronics that we’re willing to place something so potentially combustible on our laps and in our pockets—and even put them next to our ears.