Stopping batteries from starting fires.

A new flame-retardant substance can prevent rechargeable lithium-ion batteries from overheating and perhaps starting a fire, researchers say. With such an additive, the light-but-powerful batteries now used in small consumer electronics could be safely scaled up to power cars and other large, energy-hungry machines.

Jai Prakash and his colleagues at the Illinois Institute of Technology in Chicago added small amounts of the organic substance hexamethoxycyclotriphosphazene to laboratory samples of the conductive liquid, or electrolyte, typically used in lithium-ion batteries. They found that just a small concentration--1.68 percent by weight--effectively reduces the flammability of the electrolyte without breaking down performance.

They note that they chose the substance, in part, because it is considered safe for the environment. Prakash, Chang Woo Lee, and Rajeev Venkatachalapathy report the work in the February ELECTROCHEMICAL AND SOLID-STATE LETTERS.

Lithium-ion batteries are popular for portable gadgets such as cell phones, camcorders, and laptop computers because they cram a lot of energy inside a small package. "You can have a very tiny battery and get the same performance as other [larger] ones," Prakash says.

That high energy density, however, makes lithium-ion batteries susceptible to fire and explosion. "If you connect the leads together, you get a huge flow of heat," explains Kim Kinoshita of the Lawrence Berkeley (Calif.) National Laboratory.

"The liquid in the battery is an organic, so it vaporizes," he says. "You can have a fire just like with gasoline." Ordinary lead-acid car batteries instead contain water-based electrolytes, which don't burn.

Still, the lithium-ion batteries inside consumer electronics pose little fire hazard. They're small, and manufacturers install safety switches in the batteries, ensuring that firing up a laptop computer means nothing more than turning it on. These switches "add to the safety but also add to the cost," Kinoshita notes.

Researchers are hoping to use much larger lithium-ion batteries to power electric vehicles (SN: 10/7/95, p. 232). Such batteries would have "so much energy in there, it would be like a bomb)," says Prakash.

The flame-retardant additive could therefore lessen the chance of an explosion, as well as make lithium-ion batteries cheaper, says Prakash.

Although other groups have recently patented novel electrolytes that resist flames, manufacturers can simply add the new compound to the organic liquids now used in commercial electrolyte systems, says Prakash. That approach minimizes the need for changes in the battery-manufacturing process.

Although the Illinois researchers have had success in laboratory studies, Kinoshita cautions, "that's only the first step."

Prakash and his group plan to test the additive in larger electrochemical cells that more closely resemble commercial batteries. If their results live up to the promise of the preliminary work, research on lithium-ion batteries is sure to heat up.