Probing the reactivity of protonated oxygen intermediate in aprotic media with in situ surface-enhanced infrared spectroscopy

A fundamental understanding of electrochemical reactions is essential to drive the practicality of batteries. The oxygen reduction reaction (ORR) that occurs on discharge in aprotic lithium-oxygen (Li-O-2) batteries, invariably encounters interference from impurities (e.g., protons) in practical environments. It has been shown that moderate proton-mediated ORR can improve discharge capacity without altering the overall pathway; however, the reactivity of protonated oxygen intermediates formed towards aprotic electrolytes during ORR, remains controversial and unexplored. Herein, we interrogate the reactivity of protonated oxygen intermediates at the model Au vertical bar propylene carbonate and Au vertical bar trimethyl phosphate interfaces containing phenol as the moderate proton source, using in situ attenuated total reflection surface-enhanced infrared spectroscopy coupled with theoretical calculations. Direct spectroscopic evidence presents that the preferential reaction between superoxide and available protons to form protonated oxygen intermediates (e.g., HO2), can significantly mitigate superoxide anion (O-2)-induced solvent degradations while not triggering additional secondary parasitic reactions. Consequently, practical Li-O-2 batteries containing phenol have also exhibited improved electrochemical performance and reversibility. We believe that the fundamental insights will provide important lessons for future practical design (e.g., protons control of electrolytes) of Li-O-2 batteries and other related electrochemical devices.