Novel ionic liquid based electrolytes for non-aqueous lithium-oxygen batteries

Abstract

Superoxide (O2•-) is the key discharge intermediary driving many non-aqueous metal-oxygen (M-O2) battery chemistries.1–4 Characteristic dioxygen (O2x, where x = -2, -1, 0, +1) stretching vibrations (νO-O) have been well documented spectroscopically in numerous chemically unique systems.5–8 Therefore, the O2•- intermediary can be used as a diagnostic molecule to spectroscopically probe electrolyte effects on O2 reduction-evolution reaction (OR/ER) processes at the electrode/electrolyte interface, providing fundamental insight into non-aqueous M-O2 reaction mechanisms. However, it is first important to understand the vibrational spectra of O2x, though a concise overview of the large amount of empirical data in different chemical environments is lacking. Herein, reviewing the spectroscopy of O2•- and other O2x species gave a good spectroscopic grounding that supported in situ spectroscopic studies of OR/ERs in novel ionic liquid (IL) based electrolytes. Fundamental studies of OR/ERs at the roughened gold (rAu) model electrode interface in a variety of IL electrolytes using in situ surface enhanced Raman spectroscopy (SERS) were performed to study IL cation and anion effects on the chemical nature of O2•-. Analysis of νO-O and IL vibrational peak intensities and Stark shifts provided valuable information about electrolyte interactions. Furthermore, IL:solvent (IL:sol) blended electrolytes were studied to determine the effect of the solvent and salt additives on the properties of the IL electrolyte. Overall, four key parameters were shown to affect the chemical nature of O2•- at the electrode/electrolyte interface, the: (1) IL cation, (2) IL anion, (3) solvent additive Gutmann acceptor/donor number (AN/DN) and (4) the electrode potential. To optimise various physicochemical properties of the electrolyte, A general heuristic methodology for “tailoring”, screening and optimising the electrolyte was developed and a series of novel electrolytes were formulated. These novel electrolytes showed improved O2 reduction reaction (ORR) electrochemistry and exceptional stability in contact with lithium metal (Li-metal) with good physicochemical properties. Our study provides valuable information for the design and tailoring of novel IL based electrolytes for Li and other M-O2 batteries.