Researchers develop a CR1225 battery that can be recharged repeatedly at -60 degrees Celsius

Nanoengineers at the University of California, San Diego have discovered a new principle for lithium metal batteries that perform well at ultra-low temperatures; mainly, the weaker the electrolyte binds to lithium ions, the better. By using this weakly binding electrolyte, the researchers developed a CR1225 battery that can be recharged repeatedly at -60 degrees Celsius, a first in the field.

The researchers published their findings in the journal Nature Energy on February 25.

In tests, the proof-of-concept battery retained 84% and 76% of its capacity over 50 cycles at -40 and -60 degrees Celsius, respectively. Such performance is unprecedented, the researchers said.

Other lithium batteries designed for use in low temperatures can be discharged at low temperatures, but need to be heated when recharged. This means that additional heaters must be brought in to use these batteries in applications such as outer space and deep sea exploration. The new battery, on the other hand, can be charged and discharged at ultra-low temperatures.

The work, by UC San Diego nanoengineering professors Ping Liu, Zheng Chen and Todd Pascal, suggests a new way to improve the performance of lithium metal batteries at ultracold temperatures. Until now, many efforts have focused on selecting electrolytes that don’t freeze as easily and can keep lithium ions moving quickly between electrodes. In this study, the UC San Diego researchers found that the key is not how fast the electrolyte moves the ions, but how easily the electrolyte releases the ions and deposits them on the anode.

“We found that the binding and structure between the lithium ions and the electrolyte, and the ions in the electrolyte, means life or death for these batteries at low temperatures,” said first author John Holoubek, a nanoengineering doctoral student at the UC San Diego Jacobs School of Engineering.

The researchers made these findings by comparing the performance of batteries with two electrolytes: one that binds lithium ions weakly and one that binds lithium ions more strongly. The CR1225 battery with the weakly binding electrolyte performed better overall at -60 degrees Celsius; after 50 cycles, it was still running strong. In contrast, the cell with the strongly binding electrolyte stopped working after two cycles.

After the cells cycled, the researchers separated them and compared the lithium metal deposits on the anode. The differences were stark. Cells with weaker electrolyte binding had smooth, uniform deposits, while cells with stronger electrolyte binding had blocky, needle-like deposits.

A Matter in the Details

The differences in battery performance all come down to nanoscale interactions, the researchers say. "How the lithium ions interact with the electrolyte at the atomic level not only enables sustained cycling at very, very low temperatures, but also prevents dendrite formation," Chen said.

To understand why, the team used computational simulations and spectroscopy analysis to study these interactions in detail. In one electrolyte, called diethyl ether (DEE), the researchers observed molecular structures made up of lithium ions that were weakly bound to the surrounding electrolyte molecules. In another electrolyte, called DOL/DME, the researchers observed strongly bound structures between the ions and electrolyte molecules.

These structures and binding strengths are important, the researchers say, because they ultimately determine how lithium deposits on the anode surface at low temperatures. In weakly bound structures like those observed in DEE electrolytes, lithium ions can easily leave the electrolyte, so it doesn't take much energy to get them deposited anywhere on the anode surface, Holoubek explained. That’s why the deposits are smooth and uniform in DEE. But in strongly bound structures, like DOL/DME, it takes more energy to pull lithium ions out of the electrolyte. So the lithium will tend to deposit where there’s an extremely strong electric field on the anode surface — anywhere there’s a sharp point. The lithium will continue to accumulate at the sharp points until the battery shorts out. That’s why the deposits in DOL/DME are clumpy and dendritic.

“Finding out the different types of molecular and atomic structures that lithium forms, and how it coordinates with specific atoms — those details are important,” said Pascal, who directed the computational research. “By fundamentally understanding how these systems fit together, we can come up with all kinds of new design principles for next-generation energy storage systems. This work demonstrates the power of nanoengineering, by figuring out what’s going on at the small scale, you can design devices at the large scale.”

Compatible cathode

These fundamental insights enabled the team to design a cathode that’s compatible with the electrolyte and anode for low-temperature performance. It’s a sulfur-based cathode that’s low-cost, abundant, environmentally friendly, and doesn’t use expensive transition metals.

"The significance of this work is twofold," said Liu, whose lab designed the cathode and optimized its cycling performance under normal conditions in DEE. "Scientifically, it presents insights that are contrary to conventional wisdom. Technically, it is the first rechargeable CR1225 battery that can deliver meaningful energy density while fully operating at -60 degrees Celsius. These two aspects provide a complete solution for ultra-low temperature batteries."

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