Molten salt takes the heat out of lithium-metal batteries

The boom in phones, laptops and other personal devices over the past few decades has been made possible by the lithium-ion (Li-ion) battery. But as climate change demands more powerful batteries for electric vehicles and grid-scale renewable storage, lithium-ion technology might not be enough.

Lithium-metal batteries (LMBs) have theoretical capacities an order of magnitude greater than lithium-ion batteries, but a more literal boom has stymied research for decades.

“A compounding challenge that further doomed the first wave of LMB commercialization in the late 1980s was their propensity to explode,” said Chibueze Amanchukwu, a professor in the University of Chicago’s Pritzker School of Molecular Engineering.

Now, in a paper in Matter, Amanchukwu and colleagues report a way around this decades-old problem, by using solvent-free inorganic molten salts to create energy-dense, safe batteries, opening up new possibilities for electric vehicles and grid scale renewable energy storage.

“We have developed a non-flammable, non-volatile system that is safe and can actually improve energy densities by 2x (compared to Li-ion),” Amanchukwu said.

Conventional lithium-metal batteries rely on an electrolyte made by dissolving lithium salt in a solvent. Those volatile, flammable solvents – not the salt itself – caused those safety concerns.

To combat this, researchers have tried different solvents or phases and tinkered with the salt concentration. It was always a trade-off: batteries that used solid-state inorganics for their electrolytes were safer, but batteries that used liquid electrolytes were more powerful. The result was either unsafe batteries or batteries that didn’t live up to lithium-metal batteries’ massive theoretical capabilities.

Amanchukwu’s group took a novel approach to this challenge, questioning the conventional structure of the electrolyte itself. “The question was what’s the solvent doing there in the first place? Just remove it,” Amanchukwu said.

So they tried making the lithium salt a liquid not by dissolving it but by melting it. This required creating a new composition of salt that melts at low temperatures. The challenge was to hit a temperature where the lithium salt melts but the lithium metal used elsewhere in the battery doesn’t.

To give a sense of the scope of this task, pure lithium chloride melts at just over 600°C, whereas lithium metal melts at just 180°C, meaning any useful molten salt electrolyte would have to have a far lower melting point.

Amanchukwu and his colleagues have now created a salt that melts at 45°C, resulting in a powerful battery that can operate safely at 80–100°C. “That was a sweet spot to be in the middle, to still have all the safety benefits but operate at temperatures that allow it to be liquid,” Amanchukwu said.

His group is continuing to work on salt compositions that have even lower melting points, with the ultimate goal of developing a powerful lithium-metal battery that will operate safely at room temperature.

“How can you get this down to 25°C or 30°C? From a research and applied point of a view, there’s lots of excitement there. We have the opportunity to create a very impactful battery that helps to solve a key global challenge – energy storage.”

This story is adapted from material from the University of Chicago, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.