The novel liquefied gas electrolytes can function both at room temperature and at extremely low temperatures, down to -60°C. Photo: UC San Diego Jacobs School of Engineering.
The novel liquefied gas electrolytes can function both at room temperature and at extremely low temperatures, down to -60°C. Photo: UC San Diego Jacobs School of Engineering.

Improvements to a class of battery electrolyte first introduced in 2017 – liquefied gas electrolytes – could pave the way for a high-impact and long-sought advance in rechargeable batteries: replacing the graphite anode with a lithium-metal anode.

The research, reported in paper in Joule, builds on innovations first reported in a paper in Science in 2017 by the same research group at the University of California (UC), San Diego and the university spinout South 8 Technologies.

Finding cost-effective ways to replace the graphite anode in commercial lithium-ion batteries is of great interest because it could lead to lighter batteries capable of storing more charge, via a 50% increase in energy density at the cell level. This increased energy density comes from a combination of factors, including the lithium-metal anode's high specific capacity, low electrochemical potential and light weight (low density).

As a result, switching to lithium-metal anodes would significantly extend the range of electric vehicles and lower the cost of batteries used for grid storage, explained UC San Diego nanoengineering professor Shirley Meng, a corresponding author on the paper.

However, making the switch comes with technical challenges. The main hurdle is that lithium-metal anodes are not compatible with conventional liquid electrolytes. Two long-standing problems arise when these anodes are paired with conventional electrolytes: low cycling efficiency and dendrite growth. Meng and her colleagues' solution to these problems was to switch to a more compatible electrolyte, called a liquefied gas electrolyte.

One of the tantalizing aspects of these liquefied gas electrolytes is that they function both at room temperature and at extremely low temperatures, down to -60°C. They are made from liquefied gas solvents – gases that are liquefied under moderate pressures – which are far more resistant to freezing than standard liquid electrolytes.

In the new Joule paper, the researchers report how, through both experimental and computational studies, they managed to improve their understanding of some of the shortcomings of liquefied gas electrolyte chemistry. With this knowledge, they were able to tailor their liquefied gas electrolytes for improved performance in key metrics for lithium-metal anodes, both at room temperature and -60°C.

In lithium-metal half-cell tests, the team reports that the anode's cycling efficiency (Coulombic efficiency) was 99.6% for 500 charge cycles at room temperature. This is up from the 97.5% cycling efficiency reported in the 2017 Science paper, and an 85% cycling efficiency for lithium-metal anodes with a conventional liquid electrolyte.

At -60°C, the team demonstrated a lithium-metal anode cycling efficiency of 98.4%. In contrast, most conventional electrolytes fail to work at temperatures below -20°C.

The UC San Diego team's simulation and characterization tools, many developed in the Laboratory for Energy Storage and Conversion, which is led by Meng, allowed the researchers to explain why lithium-metal anodes perform better with liquefied gas electrolytes. At least part of the answer has to do with how the lithium particles deposit on the metal anode surface.

The researchers report the smooth and compact deposition of lithium particles on lithium-metal anodes when liquefied gas electrolytes are used. In contrast, when conventional electrolytes are used, needle-like dendrites form on the lithium-metal anode. These dendrites can degrade the battery efficiency, cause short circuits and lead to serious safety threats.

One measure for how densely lithium particles deposit on anode surfaces is porosity: the lower the porosity, the better. The research team reports that the porosity of lithium particle deposition on a metal anode is 0.9% at room temperature with liquefied gas electrolytes. In the presence of conventional electrolytes, the porosity jumps to 16.8%.

There is currently a big push to find or improve electrolytes that are compatible with lithium-metal anodes and competitive in terms of cost, safety and temperature range. Research groups have mainly been looking at highly concentrated solvents (liquid) or solid-state electrolytes, but there is currently no silver bullet.

"As part of the battery research community, I am confident that we are going to develop the electrolytes that we need for lithium-metal anodes. I hope that this research inspires more research groups to take a serious look at liquefied gas electrolytes," said Meng.

This story is adapted from material from the University of California, San Diego, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.