Mechanism of Oxygen Reduction in Aprotic Li–Air Batteries: The Role of Carbon Electrode Surface Structure

Electrochemical oxygen reduction in aprotic media is a key process that determines the operation of advanced metal–oxygen power sources, e.g., Li–O2 batteries. In such systems oxygen reduction on carbon-based positive electrodes proceeds through a complicated mechanism that comprises several chemical and electrochemical steps involving either dissolved or adsorbed species, and as well side reactions with carbon itself. Here, cyclic voltammetry was used to reveal the effects of imperfections in the planar sp2 surface structure of carbon on the Li oxygen reduction reaction (Li-ORR) mechanism by means of different model carbon electrodes (highly oriented pyrolytic graphite (HOPG), glassy carbon, basal, and edge planes of pyrolytic graphite), in dimethyl sulfoxide (DMSO)-based electrolyte. We show that the first electron transfer step O2 + e– ⇆ O2 – (followed by ion-coupling Li+ + O2 – ⇆ LiO2) does not involve oxygen chemisorption on carbon as evidenced by the independence of its rate on the carbon electrode surface morphology. The second electron transfer leading to Li2O2 (Li+ + LiO2 + e– ⇆ Li2O2) is strongly affected by the electrode surface even in highly solvating DMSO. Formation of Li2O2 via the electrochemical reaction could be observed only on the nearly ideal basal plane of graphite. In contrast, for more disordered electrode surfaces, (and/or bulk) the only reduction peak revealed on cyclic voltammograms corresponds to LiO2 formation, supporting that solution-mediated mechanism for Li2O2 growth is more favorable in that case. We also show that increased defect concentrations on the carbon electrode surface promote the formation of Li2CO3 during ORR, albeit relatively slower than Li2O2 formation.