If you're a regular smartphone user, you've probably felt how warm such devices can get -- and, although cases of lithium ion batteries overheating and destroying devices are in the minority, they certainly do happen. The hazard isn't just for smartphones, either -- any machine powered by lithium ion battery is at potential risk, including laptops, e-readers, even cars and planes -- such as the Boeing 787 fleet that was grounded after a battery fire in 2013.
A team of researchers at Stanford University has developed a system that warns users when this might be about to occur.
"Our goal is to create an early-warning system that saves lives and property," said Yi Cui, associate professor of materials science and engineering at Stanford. "The system can detect problems that occur during the normal operation of a battery, but it does not apply to batteries damaged in a collision or other accident."
A lithium ion battery consists of two electrodes, packed tightly side-by-side: carbon anode and a lithium metal-oxide cathode. A thin layer of polymer separates the two -- but if this separator gets damaged, the electrodes could short and explode.
The polymer layer, which is the same material as plastic bottles, is also porous enough to allow the flow of lithium ions between the electrons via a flammable electrolyte solution. This is by design, but any flaw in the construction could be hazardous.
Another hazard is overcharging, which leads to a buildup of lithium ions, forming dendrites that could breach the separator, reaching the cathode and causing the battery to short.
The system created by Cui and his colleagues involves applying a thin layer of copper to one side of the polymer separator, creating an additional barrier -- what the researchers describe as a third electrode.
"The copper layer acts like a sensor that allows you to measure the voltage difference between the anode and the separator," said co-lead author and graduate student Denys Zhuo. "When the dendrites grow long enough to reach the copper coating, the voltage drops to zero. That lets you know that the dendrites have grown halfway across the battery. It's a warning that the battery should be removed before the dendrites reach the cathode and cause a short circuit."
The copper layer also allowed the team to locate precisely where the dendrites had penetrated by measuring the electrical resistance between the separator and the cathode.
"The coated separator is quite flexible and porous, like a conventional polymer separator, so it has negligible effect on the flow of lithium ions between the cathode and the anode," said Hui Wu, co-lead author and postdoctoral fellow in the Cui group. "Adding this thin conducting layer doesn't change the battery's performance, but it can make a huge difference as far as safety.
"And it can be used in any battery that could require you to detect a short before it explodes -- a very useful feature as more and more large machines -- particularly vehicles -- make use of lithium ion batteries.
"The bigger the battery pack, the more important this becomes," Cui said. "Some electric cars today are equipped with thousands of lithium-ion battery cells. If one battery explodes, the whole pack can potentially explode."
The full study can be found online in the latest issue of journal Nature Communications.