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Research finds that weakening electrolytes' ability to bind lithium ions could help develop button battery cr1620 with low-temperature performance

Nanoengineers at the University of California San Diego have come up with new insights into lithium metal batteries that perform well at ultra-low temperatures; mainly, the weaker the electrolyte's binding to lithium ions, the better. By using this weakly bound electrolyte, the researchers developed a lithium metal battery that can be repeatedly charged at temperatures as low as -60 degrees Celsius, a first in the field.

The researchers reported their work in a paper published February 25 in Nature Energy.

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

Other button battery cr1620 have been developed for use in subfreezing temperatures and are able to discharge in the cold, but require warmth to charge. This means that in order to use these batteries in applications such as outer space and deep-sea exploration, an additional heater must be carried. The new battery, on the other hand, can be charged and discharged at ultra-low temperatures.

The work, a collaboration between the labs of UC San Diego nanoengineering professors Ping Liu, Zheng Chen and Todd Pascal, offers a new way to improve the ultra-cold performance of lithium metal batteries. Until now, much research has focused on selecting electrolytes that are less prone to freezing and can keep lithium ions moving quickly between electrodes. In this study, the UC San Diego researchers found that it’s not necessarily how fast the electrolyte moves ions, but how easily the electrolyte releases the ions and deposits them on the anode.

“We found that the binding between the lithium ions and the electrolyte, and the structure of the ions in the electrolyte, is what determines whether these batteries live or die at low temperatures,” said first author John Holobeck, 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 strongly. The lithium metal battery with the weakly binding electrolyte performed better overall at -60 degrees Celsius and was still running strong after 50 cycles. In contrast, the battery with the strongly binding electrolyte stopped working after just two cycles.

After cycling the batteries, the researchers separated them and compared the lithium metal deposits on the anode. The difference was also clear, with smooth, uniform deposits in cells with weakly bound electrolytes and clumpy and needle-like deposits in cells with strongly bound electrolytes.

Details matter

"The differences in battery performance all come down to interactions at the nanoscale," the researchers said. "How lithium ions interact with the electrolyte at the atomic level not only allows for sustainable cycling at very, very low temperatures, but also prevents the formation of dendrites."

To understand why, the team used computational simulations and spectroscopic analysis to study these interactions in detail. In one of the electrolytes, called diethyl ether (DEE), the researchers observed molecular structures where lithium ions were weakly bound to surrounding electrolyte molecules. In the other electrolyte, called DOL/DME, the researchers observed structures with strong bonds between the ions and electrolyte molecules.

These structures and bond strengths are important, the researchers say, because they ultimately determine how lithium deposits on the anode surface at low temperatures. Holoubek explains that in the weakly bound structures observed in the DEE electrolyte, lithium ions can easily leave the electrolyte, so it doesn't take much energy to get them to deposit anywhere on the anode surface, which is why the deposits in DEE are smooth and uniform. But in strongly bound structures, like those in DOL/DME, more energy is needed to pull lithium ions out of the electrolyte. As a result, lithium prefers to deposit where there are extremely strong electric fields on the anode surface, anywhere there are sharp points. The lithium will continue to accumulate on the sharp points until the battery shorts out, which is why the deposits in DOL/DME are clumpy and dendritic.

"It's important to figure out the different types of molecular and atomic structures that lithium forms, how lithium coordinates with certain atoms," 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." The work demonstrates the power of nanoengineering, where figuring out what's happening at a small scale allows you to design devices at a large scale.

Compatible cathode

These fundamental insights enabled the team to design a cathode that is compatible with both the electrolyte and the anode, with low-temperature performance. It is a sulfur-based cathode made of low-cost, abundant and environmentally friendly materials, without the use of expensive transition metals.

The significance of this work is twofold, said Liu, whose lab designed the cathode and has been optimizing the cycling performance of this cathode under normal conditions of DEE. Scientifically speaking, it offers insights contrary to conventional wisdom. Technically, it is the first rechargeable lithium metal battery that can provide meaningful energy density while fully operating at -60°C. Both aspects provide a complete solution for ultra-low temperature batteries.

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