Finding economical ways to replace the graphite anodes in commercial lithium-ion batteries could lead to lighter batteries that can store more charge. The improved energy density would result from a combination of factors including the lithium-metal anode's high specific capacity, light weight (low density), and low electrochemical potential.
Switching to lithium-metal anodes would greatly extend the range of electric vehicles and lower the cost of batteries used for grid storage, according to UC San Diego nanoengineering professor Shirley Meng, an author of a new paper in Joule.
However, the switch to lithium metal anodes comes with technical challenges. The main difficulty is that lithium metal anodes are not compatible with conventional electrolytes. Two long-standing problems emerge when lithium metal anodes are coupled with conventional electrolytes: dendrite growth, and low cycling efficiency.
So Meng and colleagues' at UC San Diego switched to a more compatible electrolyte, called liquefied gas electrolytes.
Liquefied gas electrolytes
One of the attractive aspects of these liquefied gas electrolytes is that they function both at room temperature and can continue functioning even at extremely-low temperatures, down to minus 60°C. These electrolytes are made of gases that are liquefied under moderate pressures (liquified gas solvents). They are known to be far more resistant to freezing than standard liquid electrolytes.
In the 2019 paper in Joule, the researchers reported about how, through a combination of both experimental and computational studies, they increased their understanding of some of the deficiencies of the liquefied gas electrolyte chemistry.
With this knowledge, they were able to customize their liquefied gas electrolytes for enhanced performance in critical metrics for lithium-metal anodes, both at room temperature and -60°C.
In lithium-metal half-cell tests at room temperature, the team reported that the anode's cycling efficiency (Coulombic efficiency) was 99.6% for 500 charge cycles. This is improved from the 97.5% cycling efficiency that they 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 observed lithium-metal anode cycling efficiency of 98.4%. In contrast, most conventional electrolytes fail to work below -20°C.
The UC San Diego team used simulation and characterization tools, many of which were developed in the Laboratory for Energy Storage and Conversion led by Shirley Meng. These tools allow the researchers to explain why lithium metal anodes perform better with liquefied gas electrolytes. Part of the reason has to do with how the lithium is deposited on the metal anode surface.
When liquefied gas electrolytes are used, the researchers found the smooth and compact deposition of lithium particles on lithium-metal anodes.
However, when conventional electrolytes were used, needle-like dendrites form on the lithium metal anode. Such dendrites can degrade the efficiency, cause short circuits, and result in serious safety threats.
Porosity is one measure of how densely lithium particles deposit on anode surfaces. The lower the porosity the better. They reported that at room temperature the porosity of lithium particle deposition on a metal anode is 0.90%, but the porosity in the presence of conventional electrolytes is boosted to 16.8%.
Pursuing the right electrolyte
Many are currently looking to find or improve electrolytes that are compatible with the lithium metal anodes and are competitive in cost, safety, and temperature range. Research groups have primarily been examining 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.
Meng is also an author of a related article in the May 2019 issue of Trends in Chemistry titled, "Key Issues in Hindering a Practical Lithium-Metal Anode."
Reference Material
Yang, Y., Davies, D. M., Yin, Y., Borodin, O., Lee, J., Fang, C., Olguin, M., Zhang, Y., Sablina, E. S., Wang, X., Rustomji, C. S., Meng, Y. S. High Efficiency Lithium Metal Anode Enabled by Liquefied Gas Electrolytes. Joule, July 1, 2019. DOI:https://doi.org/10.1016/j.joule.2019.06.008
