Direct In Situ Spectroscopic Evidence for Solution-Mediated Oxygen Reduction Reaction Intermediates in Aprotic Lithium-Oxygen Batteries.

A fundamental understanding of the reaction process is essential to predict and enhance the performance of electrochemical devices. As a central reaction in aprotic lithium-oxygen (Li-O 2 ) batteries, the oxygen reduction reaction (ORR) has been confronted with the "sudden-death" phenomenon caused by the cathode passivation from discharge product Li 2 O 2 . The soluble catalyst (e.g., reduction mediator) promoted solution-mediated ORR represents an elegant solution. However, no direct molecular evidence is available so far, and its link to Li-O 2 batteries performance remains hypothetical. Here, we present in situ surface-enhanced Raman spectroscopy and obtain direct spectroscopic evidence (i.e., LiAQ and LiAQO 2 ) of the solution-mediated ORR on a model anthraquinone (AQ, a typical reduction mediator)-immobilized Au electrode. With the assistance of density functional theory calculations and differential electrochemical mass spectrometry, the related elementary reaction steps of the solution-mediated ORR are proposed. This work provides intuitive insights into the AQ-catalyzed solution-mediated ORR mechanism that is helpful in the optimization and tailor-design of soluble catalysts for excellent next-generation Li-O 2 batteries.