Stability of Glyme Solvate Ionic Liquid as an Electrolyte for Rechargeable Li-O₂ Batteries

A solvate ionic liquid (SIL) was compared with a conventional organic solvent for the electrolyte of the Li-O₂ battery. An equimolar mixture of triglyme (G3) and lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]), and a G3/Li[TFSA] mixture containing excess glyme were chosen as the SIL and the conventional electrolyte, respectively. Charge behavior and accompanying gas evolution of the two electrolytes was investigated by electrochemical mass spectrometry (ECMS). From the linear sweep voltammetry performed on an as-prepared cell, we demonstrate that the SIL has a higher oxidative stability than the conventional electrolyte and, furthermore, offers the advantage of lower volatility, which would benefit an open-type lithium-O₂ cell design. Moreover, CO₂ evolution during galvanostatic charge was less in the SIL, which implies less side reaction. However, O₂ evolution during charge did not reach the theoretical value in either of the two electrolytes. Several mass spectral fragments were generated during the charge process, which provided evidence for side reactions of glyme-based electrolytes. We further relate the difference in observed discharge product morphology for these electrolytes to the solubility of the superoxide intermediate, determined by rotating ring disk electrode (RRDE) measurements.