Nanoengineers at the University of California San Diego have discovered that lithium metal batteries using weakly binding electrolytes perform better at ultra-low temperatures than batteries with strongly binding electrolytes.
In the study, researchers discovered it’s not necessarily how fast the electrolyte can move the ions, but how easily it lets go of them and deposits them on the anode.
Tests of the proof-of-concept battery showed it retained 84% and 76% of its capacity over 50 cycles at -40 and -60°C, respectively.
The work was collaboration between the laboratories of UC San Diego nanoengineering professors Ping Liu, Zheng Chen and Tod Pascal.
The research was published in the science journal Nature Energy.
In tests, lithium metal battery cells with the weakly binding electrolyte performed better overall at -60°C; it was still running strong after 50 cycles. In contrast, cells with the strongly binding electrolyte stopped working after just two cycles.
After cycling the cells, the researchers took the cells apart to compare the lithium metal deposits on the anodes. In the cells with the weakly binding electrolyte deposits were smooth and uniform, but chunky and needle-like in the cells with the strongly binding electrolyte.
To understand why the differences occur, the team took a detailed look at the interactions using computational simulations and spectroscopic analysis. In one of the electrolytes, called diethyl ether (or DEE), the researchers observed molecular structures consisting of lithium ions weakly bound to the surrounding electrolyte molecules. In the other electrolyte, called DOL/DME, the researchers observed structures that feature strong binding between the ions and electrolyte molecules.
The researchers said the structures and binding strengths were important because they ultimately dictated how lithium deposits on the anode surface at low temperature.
Holoubek said that in weakly bound structures, like those observed in the DEE electrolyte, lithium ions could easily leave the electrolyte’s hold, so it didn’t take much energy to get them to deposit anywhere on the anode surface. This is why deposits are smooth and uniform in DEE. But in strongly bound structures, like those in DOL/DME, more energy is needed to pull lithium ions away from the electrolyte.
As a result, lithium will prefer to deposit where the anode surface has an extremely strong electric field— anywhere there’s a sharp tip, and lithium will continue to pile up on that tip until the cell short circuits. This is why deposits are chunky and dendritic in DOL/DME.
These fundamental insights enabled the team to design a cathode that’s compatible with the electrolytes and anode for low-temperature performance. It is a sulfur-based cathode made with materials that are low-cost, abundant and environmentally benign— no expensive transition metals are used.