Stanford researchers are using spiky nanoparticles of graphene-coated nickel to create a lithium-ion battery that shuts down when it’s too hot, then quickly restarts when it cools (1µ =1 micrometer). Credit: Zheng Chen, Stanford University
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. “People have tried different strategies to solve the problem of accidental fires in lithium-ion batteries,” said Prof. Zhenan Bao, chemical engineering, Stanford. It “can be shut down and revived over repeated heating and cooling cycles without compromising performance.”
A typical Li-ion battery consists of 2 electrodes and a liquid or gel electrolyte that carries charged particles between them. Puncturing, shorting or overcharging the battery generates heat. If the temp reaches ~300F the electrolyte could catch fire and trigger an explosion. Several techniques have been used to prevent battery fires, such as adding flame retardants to the electrolyte. In 2014, 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,” said A/Prof. Cui.
To address the problem Cui, Bao and Chen turned to nanotechnology. Bao recently invented 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. For the battery experiment, the researchers coated the spiky nickel particles with graphene, 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,” said Chen, lead author of the study. “To conduct electricity, the spiky particles have to physically touch one another. But during thermal expansion, polyethylene stretches. That causes the particles to spread apart, making the film nonconductive so that electricity can no longer flow through the battery.” When the researchers heated the battery >160 F, the polyethylene film quickly expanded like a balloon, causing the spiky particles to separate and the battery to shut down. But when the temperature dropped back to <160 F, the polyethylene shrunk, the particles came back into contact, and the battery started generating electricity again.
“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 Prof. Bao. “For example, we might want the battery to shut down at 50 C or 100 C.” “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.” http://www.eurekalert.org/pub_releases/2016-01/su-nsb010716.php
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