Stanford Li-ion Battery Quits Before Overheating

Stanford University researchers develop a lithium-ion battery that shuts down at high temperatures and restarts immediately when it cools.

Zhenan Bao, a professor of chemical engineering at the Palo Alto, CA, college says the new technology could prevent the kind of fires that have prompted recalls and bans on a wide range of battery-powered devices, from recliners and computers to navigation systems and hoverboards.

Li-ion batteries are playing a big role in automotive as developments in the technology are leading to more practical electric vehicles, such as the Chevy Bolt due later this year with a 200-mile (320-km) range and costing a relatively affordable $30,000 after U.S. government rebates.

Smaller Li-ion batteries also figure prominently in the industry’s future by way of 48V mild-hybrid vehicles, which use the technology to assist with fuel-saving stop-start systems and as alternative power for high-demand electrical components such as heating, ventilation and air-conditioning systems, turbochargers and adjustable suspensions.

Maintaining reliability will be a key component in their success in cars and trucks.

“We’ve designed the first battery that can be shut down and revived over repeated heating and cooling cycles without compromising performance,” Bao says in a statement.

To test its stability, Stanford researchers repeatedly applied heat to the battery with a hot-air gun. Each time, the battery shut down when it got too hot and resumed operating when the temperature cooled.

A typical Li-ion battery consists of two electrodes and a liquid or gel electrolyte that carries charged particles between them. Puncturing, shorting or overcharging the battery generates heat. If the temperature reaches about 300° F (150° C), the electrolyte can catch fire and trigger an explosion.

Several techniques have been tried to prevent battery fires, such as adding flame retardants to the electrolyte. Two years ago Stanford engineer Yi Cui created a smart battery that provides ample warning before it gets too hot.

“Unfortunately, these techniques are irreversible, so the battery is no longer functional after it overheats,” Cui says.

To address the problem Cui, Bao and postdoctoral scholar Zheng Chen turned to nanotechnology.

The Personal Touch

Bao created a wearable sensor to monitor human body temperature. The sensor is made of a plastic material embedded with tiny particles of nickel with nanoscale spikes protruding from their surface. Bunched together, nanoparticles of graphene-coated nickel conduct electricity. When the battery overheats, the particles separate and electric current stops flowing. During cooling, the particles reunite and the battery starts producing electricity again.

The researchers created the system by coating the spiky nickel particles with graphene, an atom-thick layer of carbon, and embedded the particles in a thin film of elastic polyethylene.

“We attached the polyethylene film to one of the battery electrodes so that an electric current could flow through it,” Chen says.

To conduct electricity, the spiky particles must physically touch one another, but during thermal expansion, the polyethylene stretches.

“This causes the particles to spread apart, making the film nonconductive so that electricity can no longer flow through the battery,” Chen says.

When the researchers heated the battery above 160° F (70° C), they report in the journal Nature Energy, the polyethylene film quickly expands like a balloon, causing the spiky particles to separate and the battery to shut down.

When the temperature dropped back down to 160° F the polyethylene shrunk, the particles came back into contact and the battery started generating electricity again.

The researchers say they can tune the temperature higher or lower depending on how many particles they put in, or what type of polymer materials they choose.

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