Although the lithium-ion cells you see in laptops and mobile phones pack twice as much energy per pound as the next-best kind, they haven't found their way into hybrid cars because they're worryingly prone to fires. A123, a Watertown, Mass. startup, believes it has solved the problem with a lithium-ion design using a special formulation for the battery's cathode, or positive plate.

On August 9, General Motors announced that it would use A123's batteries to turn the Chevrolet Volt, now a concept car, into what is known as a plug-in hybrid. The plug-in constitutes a kind of automotive holy grail because it would give priority to the electric part of the gasoline-electric hybrid. A plug-in would go considerable distances on battery power alone, usually gaining its charge straight from a wall socket and relying on the gasoline engine only as a range extender. Automakers around the world are hot on the trail of the energy-dense batteries such cars would require.

The safety problem that has stood in the way of lithium-ion batteries became notorious last year when laptops using such batteries were shown spouting flames in video clips that circulated on the Internet. Millions of lithium-ion batteries had to be recalled, even though no one was hurt. If masses of such batteries had been crammed into automobiles, however, the fires would likely have resulted in the deaths of the passengers.

The fires seem to begin when a small manufacturing defect, perhaps compounded by overcharging, causes oxygen to separate from the compound making up the cathode, a heat-releasing process known as oxidation. As the cell overheats, it can prime oxidation in neighboring cells, a process known as thermal runaway.

A123 overcomes the problem by making its cathodes out of iron phosphate, which bonds to the oxygen far more powerfully than does the cobalt dioxide found in conventional lithium-ion batteries. Its cells are thus far less subject to oxidation, and thus less prone to thermal runaway. The coompany has minimized iron phosphate's problem of a relatively low operating voltage with a nanopatterning design that improves conductivity in the cathode.

The company's batteries are already in use in other applications demanding a combination of power and safety. The company recently introduced them into a new line of 36-volt power tools, twice as powerful as their predecessors.