Nanoengineers from the University of California, San Diego (UC San Diego) have gained a new fundamental insight for designing lithium metal batteries that work well at very low temperatures; specifically, the weaker the electrolyte binds to lithium ions, the better.
The researchers have now used this weakly binding electrolyte and have successfully designed a lithium metal battery that can be continuously recharged at temperatures down to −60°C—the first-of-its-kind in the field. The team has reported the study results in an article published in the Nature Energy journal on February 25th, 2021.
Tests were performed in which the proof-of-concept battery was found to retain 84% and 76% of its capacity across 50 cycles at −40°C and −60°C, in that order. According to the researchers, such performance is truly unparalleled.
While other lithium batteries have been designed for use in sub-freezing temperatures and can discharge in cold surroundings, they still require warmth when charging.
This fact indicates that an additional heater should be brought on board to utilize these batteries in applications, like deep-sea exploration and outer space. By contrast, the new battery can be both charged and discharged at very low temperatures.
This latest study, which is a joint association between the laboratories of UC San Diego nanoengineering professors Ping Liu, Zheng Chen, and Tod Pascal—offers a new method for enhancing the performance of lithium metal batteries at very low temperatures.
To date, several attempts have focused on selecting electrolytes that do not freeze up so easily and can allow lithium ions to move rapidly between the electrodes.
In the new analysis, the UC San Diego team found that it is not actually how fast the electrolyte can shift the ions, but rather how easily it releases the ions and eventually deposits them on the anode.
We found that the binding between the lithium ions and the electrolyte, and the structures that the ions take in the electrolyte, mean either life or death for these batteries at low temperature.
John Holoubek, Study First Author and Nanoengineering PhD Student, Jacobs School of Engineering, University of California, San Diego
The team made these findings by comparing the performance of batteries with two kinds of electrolytes—one that attaches strongly to lithium ions, and one that attaches weakly to lithium ions.
It was observed that lithium metal battery cells containing the weakly-binding electrolyte performed better overall at −60°C; and even after 50 cycles, the cells were still running strong. On the other hand, cells with the strongly binding electrolyte ceased working after only a couple of cycles.
After the cells were cycled, the investigators took them apart to compare the deposits of the lithium metal on the anodes and found a distinct difference.
The cells with the weakly binding electrolyte contained smooth and uniform deposits, while the cells with the strongly binding electrolyte contained chunky and needle-like deposits.
The variations in the performance of the battery all boil down to nanoscale interactions, added the researchers.
How lithium ions interact with the electrolyte at the atomic level not only enables sustainable cycling at very, very low temperature, but also prevents dendrite formation.
Zheng Chen, Nanoengineering Professor, University of California, San Diego
To find out why, the researchers used spectroscopic analysis and computational simulations to closely look at these interactions. In one such electrolyte, known as diethyl ether (DEE), the team detected molecular structures in which the lithium ions were weakly attached to the surrounding electrolyte molecules.
In the other electrolyte, known as DOL/DME, they observed molecular structures that exhibit powerful binding between the electrolyte molecules and ions.
According to the researchers, these molecular structures and binding strengths are extremely significant, because they eventually govern how lithium accumulates on the surface of the anode at very low temperatures.
Holoubek explained that in the case of weakly bound structures, such as those seen in the DEE electrolyte, lithium ions can effortlessly detach from the electrolyte, so that it does not take a considerable amount of energy to get them to deposit anywhere on the surface of the anode.
This is the reason why deposits are even and smooth in the DEE electrolyte. However, in the case of robustly bound structures, such as those found in DOL/DME electrolyte, additional energy is required to tug the lithium ions away from the electrolyte.
Consequently, lithium will choose to deposit where the surface of the anode has an extremely powerful electric field—that is, wherever there is a sharp tip. Meanwhile, lithium will continue to deposit on that tip until a short circuit occurs in the cell. This is the reason why deposits are dendritic and chunky in the DOL/DME electrolyte.
Figuring out the different types of molecular and atomic structures that lithium forms, how lithium coordinates with certain atoms—these details matter. By understanding fundamentally how these systems come together, we can come up with all kinds of new design principles for the next generation of energy storage systems. This work demonstrates the power of nanoengineering, where figuring out what happens at the small scale enables the design of devices at the large scale.
Tod Pascal, Nanoengineering Professor, University of California, San Diego
Pascal had also directed the computational analyses.
Such a fundamental understanding allowed the researchers to develop a cathode that can be used with the anode and electrolytes for performance even at low temperatures. The cathode is based on sulfur and made with abundant, low-cost, and environmentally benign materials, without using any costly transition metals.
“The significance of this work is really two-fold,” stated Liu, whose laboratory had developed the cathode and has been improving the cycling performance of this cathode in the DEE electrolyte for regular conditions.
“Scientifically, it presents insights that are contrary to conventional wisdom. Technologically, it is the first rechargeable lithium metal battery that can deliver meaningful energy density while being fully operated at -60 C. Both aspects present a complete solution for ultra-low temperature batteries,” Liu concluded.
Holoubek, J., et al. (2021) Tailoring electrolyte solvation for Li metal batteries cycled at ultra-low temperature. Nature Energy. doi.org/10.1038/s41560-021-00783-z.