Influence of Ion Diffusivity and Gas Solubility in Ionic Liquids on Li-Air Battery Performance

Abstract

Owing to its extreme high theoretical energy density1, the non-aqueous Li-air battery has received remarkable attention in recent years as a promising candidate for electric vehicle applications. Ionic Liquids (ILs), molten salts that are liquid at room temperature, have been investigated as potential electrolytes in Li-air batteries due to their negligible vapor pressure, good thermal stability, and wide electrochemical windows.However, comparing to conventional organic electrolytes, for example, ethylene carbonate (EC), ILs generally have higher viscosity and consequently lower ionic conductivity and ion diffusivity. Therefore, two low viscosity ILs, [Pyrr12O1][TFSI], N-methoxyethyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide, and [PP12O1][TFSI], N-methoxyethyl-N-methyl piperidinium bis(trifluoromethanesulfonyl)imide, and their mixture with Li[TFSI], were investigated.In this work, the diffusivity of the cation and anion of the IL, as well as the diffusivity of the Li+ cation, were measured with pulsed-field-gradient spin-echo (PFG-SE) NMR Spectroscopy. In addition, the I-V performances of the two ILs were characterized in a Li-O2 battery and compared with a non-ether-functionalized IL, [PP13][TFSI]2. Physical properties of the ILs and IL-Li[TFSI] mixtures, including densities, viscosities, ionic conductivities, and electrochemical windows are reported. An additional property that can affect Li-O2 battery performance, the solubility of O2 in the ILs and IL-Li[TFSI] mixtures, was also measured, using a gravimetric microbalance.