Controlling Solution-Mediated Reaction Mechanisms of Oxygen Reduction Using Potential and Solvent for Aprotic Lithium–Oxygen Batteries

Fundamental understanding of growth mechanisms of Li₂O₂ in Li–O₂ cells is critical for implementing batteries with high gravimetric energies. Li₂O₂ growth can occur first by 1e^– transfer to O₂, forming Li⁺–O₂^– and then either chemical disproportionation of Li⁺–O₂^–, or a second electron transfer to Li⁺–O₂^–. We demonstrate that Li₂O₂ growth is governed primarily by disproportionation of Li⁺–O₂^– at low overpotential, and surface-mediated electron transfer at high overpotential. We obtain evidence supporting this trend using the rotating ring disk electrode (RRDE) technique, which shows that the fraction of oxygen reduction reaction charge attributable to soluble Li⁺–O₂^–-based intermediates increases as the discharge overpotential reduces. Electrochemical quartz crystal microbalance (EQCM) measurements of oxygen reduction support this picture, and show that the dependence of the reaction mechanism on the applied potential explains the difference in Li₂O₂ morphologies observed at different discharge overpotentials: formation of large (∼250 nm–1 μm) toroids, and conformal coatings (<50 nm) at higher overpotentials. These results highlight that RRDE and EQCM can be used as complementary tools to gain new insights into the role of soluble and solid reaction intermediates in the growth of reaction products in metal–O₂ batteries.