Researchers at Rice University have made test cells for lithium metal batteries with a coat of red phosphorus on the separator that keeps the anode and cathode electrodes apart. The phosphorus acts as a spy for management systems used to charge and monitor batteries, by detecting the formation of dendrites — protrusions of lithium that can cause them to fail.
Lithium metal anodes charge much faster and hold about 10 times more energy by volume than the common lithium-ion anodes used in just about every electronic device on the market. But charging lithium-infused anodes forms dendrites that, if they reach the cathode, cause a short circuit and possibly a fire or explosion. When a dendrite reaches a red phosphorus-coated separator, however, the battery’s charging voltage changes. That tells the battery management system to stop charging. Unlike other proposed dendrite detectors, the Rice strategy doesn’t require a third electrode, which is very hard to manufacture.
The red phosphorus layer had no significant effect on normal performance in experiments with test batteries. The researchers built a transparent test cell with an electrolyte known to accelerate aging of the cathode and encourage dendrite growth. That let them monitor the voltage while they watched dendrites grow. With an ordinary separator, they saw the dendrites contact and penetrate the separator with no change in voltage, a situation that would lead a normal battery to fail. But with the red phosphorus layer, they observed a sharp drop in voltage when the dendrites contacted the separator. “As soon as a growing dendrite touches the red phosphorus, it gives a signal in the charging voltage,” said chemist James Tour. “When the battery management system senses that, it can say, ‘Stop charging, don’t use.'”
“Literally, when you make a new battery, you’re making over a billion of them,” Tour said. “Might a couple of those fail? It only takes a few fires for people to get really antsy. Our work provides a further guarantee for battery safety. We’re proposing another layer of protection that should be simple to implement.”