Liquefied gas electrolytes point to safer batteries with high-energy

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Engineers in the US have demonstrated liquefied gas electrolytes that simultaneously provide lithium metal batteries with high-energy density and temperature resilience.

Pritzker Molecular Engineering Professor Y. Shirley Meng (left) and her team, including PhD student Yijie Yin (right), demonstrate how liquified gas electrolytes are not only safer but promise more sustainable operation
Pritzker Molecular Engineering Professor Y. Shirley Meng (left) and her team, including PhD student Yijie Yin (right), demonstrate how liquified gas electrolytes are not only safer but promise more sustainable operation - Baharak Sayahpour

The liquefied gas electrolyte (LGE), developed by a team led by Y. Shirley Meng, a professor at the University of Chicago’s Pritzker School of Molecular Engineering, also provides a path to sustainable, fire-extinguishing batteries that can be developed at scale. The research, involving Meng’s University of California San Diego labs, is detailed in Nature Energy.

Yijie Yin, a nanoengineering PhD student and co-first author of the paper, said: “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.” The related results were published in Science.

The LGE is claimed to greatly broaden the choice of electrolyte solvent molecules. The screened fluoromethane and difluoromethane 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 per cent), and high performance of high-voltage cathodes.

According to UChicago, the LGE electrolyte is not yet ‘perfect’ because the saturated vapour pressure of the molecules used is high and is still flammable.

Yin mentioned to Yang that in follow-up work, he wanted to replace the strong solvating power liquid co-solvents with the smallest ether molecule - dimethyl ether (Me2O).

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

“If we continue to use the current FM and DFM weakly solvated solvents, the existing high-pressure and flammability shortcomings will not be changed,” Yang said. “Instead, we should work on searching for molecules with increased fluorinated carbon bonding.”

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. With these properties in mind, the team concentrated on 1,1,1,2 tetrafluoroethane (TFE) and pentafluoroethane pentafluoroethane (PFE), which they said maintain >3 mS cm−1 ionic conductivity from −78 to +80 °C.

These two molecules are also the main components in some fire extinguishers, which means that the molecules are non-flammable and have fire-extinguishing properties.