Lighter, less expensive and more energy dense than other secondary battery technologies, lithium-ion has become the dominant form of rechargeable battery today. It powers everything from mobile phones and laptops to cars and commercial jets. But there is one little problem—these batteries don't just short circuit, they explode into flames. Here's what goes on inside a lithium cell before it goes boom.
The lithium-ion battery's explosive tendencies are the result of a process known as thermal runaway. It's essentially an energetic positive feedback loop whereby increasing temperature causes the system to get hotter, which increases the temperature, which causes the system to get even hotter, and so on. Thermal runaway is surprisingly common and can be found in a wide array of physical and chemical processes, from curing concrete (which releases heat) to exploding stars (supernovae are the result of TR on a cosmic scale).
Li-ion batteries can suffer thermal runaway for a variety of reasons. Much like bridging a 9V battery's terminals with a coin, short circuits caused by a tear in the membrane that separates the negative and positive poles of a Li-ion battery will often cause a thermal meltdown. Ambient temperatures exceeding 60 degrees C, repeated overcharging, or unauthorized modifications to the case have all also been reported as the source of battery fires.
Regardless of the reason, when conditions are right, it's the battery's cobalt oxide chemistry that actually undergoes the reaction. "When you heat this material up," Joe Lamoreux, vice president of research and development at Valence Technology, explained to Infoworld. "It [can] reach an onset temperature that begins to self-heat and progresses into fire and explosion." In some cases, the organic electrolyte will vent, causing the battery to rupture. It may also burn openly if exposed to high temperatures or a spark.
While this uncontrollable reaction doesn't happen particularly often, the sheer number of Li-ion cells in the world today amplifies the perceived effect. In 2006, for example, high-profile recalls came from virtually every major laptop manufacturer—Apple, HP, Toshiba, Lenovo, Dell, and others—after a small number of batteries overheated. Apple alone yanked 1.8 million iBook and PowerBook battery packs from market after just nine reports of overheating according to the CPSC. What's more, the size of the battery, its configuration, and the number of cells all affect the intensity of the runaway. Small batteries, such as those used in DSLRs, only contain a few cells so the chances of the runaway spreading from the failed cell to the rest is relatively low. Large, power-dense battery packs like those aboard the troubled Boeing 787 Dreamliner on the other hand, seem practically purpose-built to instigate thermal runaway. These batteries were packed into sealed metal boxes with no means of venting waste heat. As a result, when a single cell became hot enough to ignite its electrolyte, the rest quickly followed suit.
There is no need to fear your laptop as a ticking time bomb, however. With a bit of regular maintenance, Li-ion batteries will perform safely and reliably throughout their operational service lives. Li-ions are powerful but short lived, lasting two to three years at most (whether or not you use it or leave it on a shelf). As such, all Li-ion packs should be replaced every 36 months or so to avoid pushing a worn cell too hard. Also, these batteries should be recharged once their capacity hits 50 percent. Li-ions don't suffer from memory effects but are severely damaged if the voltage drops too low.