Invention and Development of Liquefied Gas Electrolytes for Lithium Ion Batteries and Beyond
Lithium (Li) ion batteries have been successfully commercialized under decades of development and made a great impact as energy storage devices for electronics industries. The demand keeps increasing and expands to larger areas like grid storage and electric vehicles, which calls for higher energy density, longer cycle life, and wider operation conditions. However, there have been little changes in electrolyte compositions since its commercialization and the balanced-properties of the conventional electrolytes can’t meet the requirements for next-generation electrodes, such as Li-metal anode, high-voltage cathodes.
Herein, we design and develop a novel electrolyte system from a unique direction. The use of molecules that are gases under standard conditions is liquefied under moderate pressure to form liquefied gas electrolyte for the first time. Combing the superior physical and chemical properties, the liquefied gas electrolytes show a wide potential window of stability and impressive performance in extended temperature ranges. Electrolytes using difluoromethane (DFM) and fluoromethane (FM) demonstrate excellent performance in a wide temperature range for electrochemical capacitor and 4-volt lithium-ion battery, respectively.
Comprehensive approaches are applied to have a deeper understanding of liquefied gas electrolytes and further improve the overall performance. Tetrahydrofuran is first introduced as a cosolvent to improve the salt solubility and conductivity. The unique solvation structure and stable solid electrolyte interface enable improved Li-metal coulombic efficiency and rate capability in a wide temperature range down to -60°C. Besides, there are demands to further expand the upper-end boundary of both voltage and temperature window of liquefied gas electrolytes. Therefore, in a separate study, we formulate liquefied gas electrolytes using acetonitrile as a cosolvent and a higher salt concentration of 1.2 M for 4.5 V Li-metal batteries in a wider temperature range (-78~+75°C). Possibilities of using other liquefied gas solvents for Li-ion batteries are also been explored. A mixture of liquefied solvents is selected to improve safety. The electrolyte shows unique properties on fire extinguishing and one step recycling with excellent electrochemical performance.