The technology is designed for conventional lithium-ion batteries now used in billions of cellphones, laptops and other electronic devices, as well as a growing number of cars and airplanes, researchers said.
    
"Our goal is to create an early-warning system that saves lives and property," said Yi Cui, an associate professor of materials science and engineering at Stanford University.
    
"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," said Cui.
    
A series of well-publicised incidents in recent years has raised concern over the safety of lithium-ion batteries.
    
"The likelihood of a bad thing like that happening is maybe one in a million," Cui said.
    
"That's still a big problem, considering that hundreds of millions of computers and cellphones are sold each year. We want to lower the odds of a battery fire to one in a billion or even to zero," said Cui.
    
A typical lithium-ion battery consists of two tightly packed electrodes – a carbon anode and a lithium metal-oxide cathode – with an ultrathin polymer separator in between. The separator keeps the electrodes apart.
    
If it's damaged, the battery could short-circuit and ignite the flammable electrolyte solution that shuttles lithium ions back and forth.
    
To address the problem, Cui and his colleagues applied a nanolayer of copper onto one side of a polymer separator, creating a novel third electrode halfway between the anode and the cathode.
    
"The copper layer acts like a sensor that allows you to measure the voltage difference between the anode and the separator," said graduate student Denys Zhuo, co-lead author of the study.
    
"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," said Zhuo.
    
The build-up of dendrites is most likely to occur during charging, not during the discharge phase when the battery is being used, researchers said.
    
"You might get a message on your phone telling you that the voltage has dropped to zero, so the battery needs to be replaced," Zhuo said.
    
"That would give you plenty of lead-time. But when you see smoke or a fire, you have to shut down immediately. You might not have time to escape," Zhuo said.