Lithium-ion batteries became crucial to the design of Boeing Co.'s new Dreamliner jet because they offered a combination of high power and low weight. Yet the very chemistry that makes these high-tech batteries so attractive to designers may increase their risk of overheating and catching fire, a situation that has contributed to the global grounding of about 50 Dreamliners in use and a halt to new deliveries after two onboard fires.
Investigators aren't just looking at the batteries. Safety experts are combing over the jet's wiring, circuit boards and other battery-related external components as they probe the incidents.
Batteries convert stored chemical energy into usable, electrical energy. A lithium ion battery consists of a negative electrode and a positive electrode that are linked by an electrolyte, such as an organic solvent.
When the battery is being used, the electrolyte transports ions between the electrodes; electrons flow along a separate wire circuit and that electrical current is used to start an auxiliary power unit and for emergency power.
By mixing and matching different metals and chemistries, scientists have created a range of different battery types that differ in terms of voltage, service life, size and cost. Low-power devices such as those in a TV remote are usually powered by inexpensive alkaline batteries.
The challenge for battery makers has been to boost the amount of energy that can be stored in a given volume—and that is where lithium-ion technology shines. A lithium-ion power-pack can deliver more energy than a similar-size battery based on another metal, a measure known as energy density.
That is why lithium batteries were compelling to the 787's designers. The Dreamliner was crafted to allow for big fuel savings and weight reductions, some of which are enabled by the small but powerful lithium-ion batteries Boeing is using.
Lithium is the least dense of all metals and highly electropositive, which means it delivers a high voltage. Lithium-ion batteries pack twice as much energy density as nickel-metal-hydride versions, and four to six times as much energy density of the lead-acid battery found in many cars, according to Stanley Whittingham, a professor of chemistry and expert on lithium-ion batteries at Binghamton University in Binghamton, N.Y.
Lithium also is the third-smallest element after hydrogen and helium. "Because it is small, you can pull it in and out of materials easily" compared with other elements that are bigger and can't be moved so easily, said Clare Grey, professor of materials chemistry at the University of Cambridge, England.
Today, lithium batteries are the preferred power packs for an array of widely used consumer products, from laptops and tablets to cellular phones and portable drills. They are also increasingly found in cars, buses, planes, trains and satellites.
On the 787, the batteries aren't used during normal cruising flight, but they are continually charged by the plane's onboard generators. A potential problem can arise when a lithium-ion battery is plugged into an external power source for recharging. "If it is overcharged, there is electrolyte breakdown and that generates heat," said Prof. Whittingham.
Chemists call it "thermal runaway," an ever-increasing heating process that can cause the battery to ignite. It isn't clear whether this is what happened to the Dreamliner batteries. Prof. Grey speculated that a manufacturing defect could have triggered a physical short-circuit, which caused heating, released oxygen and led to a fire. Or intense heat could have melted the physical separator between anode and cathode. "Then the whole thing goes up in a puff of smoke," she said.
Prof. Whittingham noted that, when heated up, the electrolytic solvent in the battery can turn into a flammable gas, which can potentially trigger a fire. "When you get thermal runaway," he added, the battery can burn "at a temperature of 300-to-400 degrees Celsius," or between 570 degrees and 750 degrees Fahrenheit.