Thursday, June 16, 2022

One step closer to fire safe, recyclable lithium-metal batteries

High-energy density, improved safety, temperature resilience and sustainability are desirable properties for lithium-battery electrolytes, yet these metrics are rarely achieved simultaneously. Inspired by the compositions of clean fire-extinguishing agents, a team of nanoengineers from UC San Diego demonstrated inherently fire safe liquefied gas electrolytes, as well as a one-step solvent-recycling process which promises sustainable operation at scale, in a Nature Energy paper published on June 16.

Yijie Yin and Professor Shirley Meng working in
the lab to develop this lithium-battery electrolyte

This work provides a route to sustainable, temperature-resilient lithium-metal batteries with fire-extinguishing properties that maintain state-of-the-art electrochemical performance.

Yijie Yin, a nanoengineering PhD student and co-first author of the paper, shares how this work came about in the following paragraphs. Yin and co-first author Yangyuchen Yang are both graduate students in adjunct professor Shirley Meng’s Laboratory for Energy Storage and Conversation. Read the Nature Energy paper here.

“In 2017, a team of UC San Diego nanoengineers discovered hydrofluorocarbon molecules that are gasses at room temperature and will liquefy under a certain pressure. They then invented a new type of electrolyte, which is called "Liquefied Gas Electrolyte"(LGE). The related results were published in Science1.

Yijie Yin

The liquefied gas electrolyte greatly broadens the choice of electrolyte solvent molecules. The screened fluoromethane and difluoromethane 2 small molecules have a low melting point, fast kinetics, and wide voltage window. With the combination of co-solvents, these characteristics make these liquefied gas electrolytes exhibit excellent low temperature performance (< -60°C), Li metal Coulombic efficiency (>99.8%)3 and high performance of high-voltage cathodes4.

However, the LGE electrolyte is not yet "perfect", because the saturated vapor pressure of the molecules used is high, and like most electrolytes, it is still flammable, which makes the safety and environmental protection of the system irrational.

The idea of this work came from a chat between Yin and Yang, also a nanoengineering PhD student at UC San Diego. Yin mentioned that he wanted to try to replace the strong solvating power liquid co-solvents with the smallest ether molecule - methyl ether (Me2O) in follow-up work.

Yangyuchen Yang

‘"As a gas molecule, Me2O can only be used in liquefied gas,” said Yin. “It may only work under the pressurized system, and it may provide better lithium metal interface and stability while maintaining fast kinetics."’

During the discussion, Yang also agreed with this idea and hoped that this system could be further improved. He said, "If we continue to use the current FM and DFM weakly solvated solvents, the existing high-pressure and flammability shortcomings will not be changed, instead we should work on the searching for molecules with increased fluorinated carbon bonding".

Next, the two referred to the structure of fluoromethane to search for fluorinated molecules with longer carbon chains, while maintaining the inherent advantages of liquefied gasses, such as low melting point, low viscosity, and maintaining a certain polarity. Considering all the above requirements, 1,1,1,2 tetrafluoroethane (TFE) and pentafluoroethane pentafluoroethane(PFE) came to mind.

What's even more surprising is that these two molecules are the main components in some fire extinguishers, which means that the molecules are not only non-flammable, but also have excellent fire-extinguishing properties.

1             Rustomji, C. S. et al. Liquefied gas electrolytes for electrochemical energy storage devices. Science, doi:10.1126/science.aal4263 (2017). 

2             Davies, D. M. et al. A Safer, Wide-Temperature Liquefied Gas Electrolyte Based on Difluoromethane. Journal of Power Sources 493, 229668 (2021). 

3             Yang, Y. et al. High-Efficiency Lithium-Metal Anode Enabled by Liquefied Gas Electrolytes. Joule 3, 1986-2000, doi: (2019). 

4             Yang, Y. et al. Liquefied gas electrolytes for wide-temperature lithium metal batteries. Energy & Environmental Science 13, 2209-2219, doi:10.1039/D0EE01446J (2020).

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