Friday, December 9, 2022

New electrolyte paves the way for fire-safe, recyclable lithium-metal batteries

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Energy storage technology must evolve and adapt to meet the growing demand. Lithium-ion batteries will require dramatic improvements in high energy density, safety, temperature resilience, and environmental sustainability in order to provide the type of emission-free future that so many envision.

Now, inspired by the compositions of clean fire-extinguishing agents, a team of engineers from the University of California San Diego has demonstrated the ‘Liquefied Gas Electrolyte’ (LGE) that can simultaneously provide all four essential properties. The research provides a path to sustainable, fire-extinguishing, state-of-the-art batteries that can be developed at scale.

The liquefied gas electrolyte greatly broadens the choice of electrolyte solvent molecules. The screened fluoromethane and difluoromethane small molecules have a low melting point, fast kinetics, and wide voltage window. These characteristics, in combination with co-solvents, make these liquefied gas electrolytes exhibit excellent low-temperature performance (−60 to +55 degrees C), Li metal Coulombic efficiency (more than 99.8%) for over 200 cycles, and high performance of high-voltage cathodes.

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.

In follow-up work, researchers 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 Yijie Yin, a nanoengineering Ph.D. student and co-first author of the paper. “It may only work under the pressurized 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,” said Yangyuchen Yang, a nanoengineering Ph.D. student at UC San Diego. “Instead, we should work on searching for molecules with increased fluorinated carbon bonding.”

They both 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 requirements, researchers decided to use 1,1,1,2-tetrafluoroethane and pentafluoroethane, which are the main components in some fire extinguishers. This means the molecules are not only non-flammable but also have excellent fire-extinguishing properties.

The research was performed between the University of California San Diego and UChicago labs and was published in Nature Energy.

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