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–O2) batteries, the oxygen reduction reaction (ORR) has been
confronted with the “sudden-death” phenomenon caused
by the cathode passivation from discharge product Li2O2. 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–O2 batteries performance remains hypothetical. Here, we present in situ surface-enhanced Raman spectroscopy and obtain direct
spectroscopic evidence (i.e., LiAQ and LiAQO2) 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–O2 batteries.