K-O 2 electrochemistry at the Au/DMSO interface probed by in situ spectroscopy and theoretical calculations.

The reaction mechanism underpinning the operation of K-O 2 batteries, particularly the O 2 reactions at the positive electrode, is still not completely understood. In this work, by combining in situ Raman spectroelectrochemistry and density functional theory calculations, we report on a fundamental study of K-O 2 electrochemistry at a model interface of Au electrode/DMSO electrolyte. The key products and intermediates (O 2 - , KO 2 and K 2 O 2 ) are identified and their dependency on the electrode potential is revealed. At high potentials, the first reduction intermediate of O 2 - * radical anions (* denotes the adsorbed state) can desorb from the Au electrode surface and combine with K + cations in the electrolyte producing KO 2 via a solution-mediated pathway. At low potentials, O 2 can be directly reduced to on the Au electrode surface, which can be further reduced to at extremely low potentials. The fact that K 2 O 2 has only been detected in the very high overpotential regime indicates a lack of KO 2 disproportionation reaction both on the Au electrode surface and in the electrolyte solution. This work addresses the fundamental mechanism and origin of the high reversibility of the aprotic K-O 2 batteries.