New battery electrolytes are a gas

To power our increasingly electrified society, energy storage technologies must evolve and adapt to meet the growing demand. Lithium-ion batteries, already essential to myriad technologies, will require dramatic improvements in energy density, safety, temperature resilience and environmental sustainability to provide the type of emission-free future that is envisioned.

Now, a team of engineers led by Shirley Meng, professor in the Pritzker School of Molecular Engineering at the University of Chicago, has demonstrated liquefied gas electrolytes that can simultaneously provide all four essential properties. This research, performed by Meng’s labs at the University of Chicago and the University of California (UC) San Diego, provides a path to sustainable, fire-extinguishing, state-of-the-art batteries that can be developed at scale. The engineers report their work in a paper in Nature Energy.

“In 2017, a team of UC San Diego nanoengineers discovered hydrofluorocarbon molecules that are gases at room temperature and will liquefy under a certain pressure,” said Yijie Yin, a nanoengineering PhD student at UC San Diego and co-first author of the paper. “They then invented a new type of electrolyte, which is called 'Liquefied Gas Electrolyte' (LGE).” The nanoenginers reported this advance in a paper in Science.

The LGE greatly broadens the choice of electrolyte solvent molecules. The screened fluoromethane (FM) and difluoromethane (DFM) small molecules have a low melting point, fast kinetics and wide voltage window. When combined with co-solvents, these characteristics allowed these LGEs to exhibit excellent low-temperature performance (< -60°C) and lithium metal Coulombic efficiency (>99.8%), and to work with high-performance, high-voltage cathodes.

The LGE electrolyte was not yet ‘perfect’, however, because the saturated vapor pressure of the molecules used is high and, like most electrolytes, it is still flammable, which creates safety and environmental issues.

The idea for this work came from a chat between Yin and Yangyuchen Yang, a fellow nanoengineering PhD student at UC San Diego. Yin mentioned that in follow-up work he wanted to try 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,” said Yin. “It may only work under the pressurized system, and it may provide better lithium-metal interface and stability while maintaining fast kinetics.”

Yang agreed and hoped the system could be further improved. “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.”

Using the structure of fluoromethane as a guide, they searched for fluorinated molecules with longer carbon chains that retained the inherent advantages of liquefied gases, 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.

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