To extend range, automakers have increased the thickness of the electrodes they use in battery cells. While that has allowed them to promise longer drives between charges, it makes some of the lithium hard to access, resulting in slower charging and less power for a given battery weight.

“Charging an EV battery takes 30 to 40 minutes even for aggressive fast charging, and that time increases to over an hour in the winter,” says Neil Dasgupta, Ph.D., an associate professor of mechanical engineering and materials science at the University of Michigan. “This is the pain point we want to address.

“For the first time, we’ve shown a pathway to simultaneously achieve extreme fast charging at low temperatures, without sacrificing the energy density of the lithium-ion battery,” claims Dasgupta. “Lithium-ion EV batteries made this way can charge 500 percent faster at temperatures as low as 14 F.”

The structure and coating developed by Dasgupta and his colleagues prevented the formation of performance-hindering lithium plating on the battery’s electrodes. As a result, batteries with these modifications keep 97 percent of their capacity even after being fast-charged 100 times at very cold temperatures.

The devices were built in the U-M Battery Lab and studied at the Michigan Center for Materials Characterization.

“We envision this approach as something that EV battery manufacturers could adopt without major changes to existing factories,” says Dasgupta.

Previously, the engineers improved battery charging capability by creating pathways—roughly 40 microns in size—in the anode. Drilling through the graphite by blasting it with lasers enabled the lithium ions to find places to lodge faster, even deep within the electrode, ensuring more uniform charging.

This sped up room-temperature charging significantly, but cold charging was still inefficient. The team identified the problem: the chemical layer that forms on the surface of the electrode from reacting with the electrolyte.

Dasgupta compares this behavior to butter: you can get a knife through it whether it’s warm or cold, but it’s a lot harder when it’s cold. If you try to fast charge through that layer, lithium metal will build up on the anode like a traffic jam. That plating prevents the entire electrode from being charged, which reduces the battery’s energy capacity.

To prevent that surface layer from forming, Dasgupta and his colleagues coated the battery with a glassy material made of lithium borate-carbonate, approximately 20 nanometers thick. The addition of this coating sped up cold charging significantly, and when combined with the channels, the test cells were 500 percent faster to charge in subfreezing temperatures.