New coating revitalises lithium metal batteries

Lithium metal batteries could make a dent in the $30bn lithium-ion battery market – and break into EVs - with a new coating that makes them safe and long lasting.

Lithium metal batteries
Lead authors and Ph.D. students David Mackanic, left, and Zhiao Yu with their battery tester at right. Yu holds a dish of already tested cells that they call "the battery graveyard" (Credit: Mark Golden)

Key to the breakthrough from researchers at Stanford University and SLAC National Accelerator Laboratory is the prevention of dendrites building up in the battery. Their results are published in Joule.

In laboratory tests, the coating is said to have significantly extended the battery's life. It also dealt with the combustion issue by greatly limiting the dendrites that pierce the separator between the battery's positive and negative sides. In addition to ruining the battery, dendrites can create a short circuit within the battery's flammable liquid.

"We're addressing the holy grail of lithium metal batteries," said Zhenan Bao, a professor of chemical engineering, who is senior author of the paper along with Yi Cui, professor of materials science and engineering and of photon science at SLAC, a US Department of Energy Office of Science laboratory operated by Stanford. Bao added that dendrites had prevented lithium metal batteries from being used in what may be the next generation of electric vehicles.

Lithium metal batteries can hold at least a third more power per pound as lithium-ion batteries and are lighter because they use lightweight lithium for the positively charged end rather than heavier graphite. If they were more reliable, these batteries could benefit a range of consumer electronic products but the real breakthrough would be for cars, Cui said, as biggest drawback on electric vehicles is that their batteries spend about a fourth of their energy carrying themselves around.

"The capacity of conventional lithium-ion batteries has been developed almost as far as it can go," said Stanford PhD student David Mackanic, co-lead author of the study. "So, it's crucial to develop new kinds of batteries to fulfil the aggressive energy density requirements of modern electronic devices."

The team from Stanford and SLAC tested their coating on the anode of a standard lithium metal battery, which is where dendrites typically form. They combined their specially coated anodes with other commercially available components to create a fully operational battery. After 160 cycles, their lithium metal cells still delivered 85 per cent of the power that they did in their first cycle. Regular lithium metal cells deliver about 30 per cent after that many cycles, rendering them nearly useless even if they don't explode.

The new coating prevents dendrites from forming by creating a network of molecules that deliver charged lithium ions to the electrode uniformly. It prevents unwanted chemical reactions typical for these batteries and also reduces a chemical build-up on the anode, which hinders the battery's ability to deliver power.

"Our new coating design makes lithium metal batteries stable and promising for further development," said the other co-lead author, Stanford PhD student Zhiao Yu.

The group is now refining their coating design to increase capacity retention and testing cells over more cycles.

"While use in electric vehicles may be the ultimate goal, commercialiation would likely start with consumer electronics to demonstrate the battery's safety first," Cui said in a statement.