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Stanford boffins snuff out li-ion batt blaze risk

Plastic covering corrects cockups

The explosive side effects of overheated lithium-ion batteries have long been documented, and have caused everything from groin burns to the suspension of Boeing 787 flights.

Now boffins at Stanford think they have the answer.

If the electrolyte within a battery reaches 300 degrees Fahrenheit (150 degrees Celsius) then it can catch fire, and for years battery manufacturers have been trying to build in a safety system to stop this happening. While techniques have been developed to shut down a battery that overheats, it comes at a cost.

"Unfortunately, these techniques are irreversible, so the battery is no longer functional after it overheats," said Stanford engineer Yi Cui, who invented just such a system. "Clearly, in spite of the many efforts made thus far, battery safety remains an important concern and requires a new approach."

Cui and a team led by Zhenan Bao, a professor of chemical engineering at Stanford, have now built a solution – a polyethylene film studded with nickel that covers the battery's electrodes.

The nickel spikes conduct electricity, but if the temperature rises the polyethylene expands and breaks the electrical connection, then shrinks to reestablish the electrical flow as it cools.

"We can even tune the temperature higher or lower depending on how many particles we put in or what type of polymer materials we choose," said Bao. "For example, we might want the battery to shut down at 50 C or 100 C."

The technique is not only reversible, but repeatable. The team applied a heat gun to the substance to activate the tiny circuit breakers, let it cool to reestablish power, and repeated the process many times.

"Compared with previous approaches, our design provides a reliable, fast, reversible strategy that can achieve both high battery performance and improved safety," Cui said. "This strategy holds great promise for practical battery applications." ®

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