Potential-Dependent Generation of O₂– and LiO₂ and Their Critical Roles in O₂ Reduction to Li₂O₂ in Aprotic Li–O₂ Batteries

Discharging of the aprotic Li–O 2 battery relies on the oxygen reduction reaction (ORR) producing Li 2 O 2 in the positive electrode, which remains incompletely understood. Here, we report a mechanistic study of the Li-ORR on a model system, i.e., an Au electrode in a Li + dimethyl sulfoxide (DMSO) electrolyte. By spectroscopic identification of the reaction intermediates coupled with density functional theory calculations, we conclude that the formation of O 2 – and LiO 2 in the Li-ORR critically depends on electrode potentials and determines the Li 2 O 2 formation mechanism. At low overpotentials (> 2.0 V vs Li/Li + ) O 2 – is identified to be the first surface intermediate, which diffuses into the bulk electrolyte and forms Li 2 O 2 therein via a solution-mediated disproportionation mechanism. At high overpotentials (ca. 2.0–1.6 V vs Li/Li + ) LiO 2 has been observed, which can rapidly transform to Li 2 O 2 by further electro-reduction, suggesting a surface-mediated mechanism. The solution-mediated Li 2 O 2 formation that can account for the widely observed toroid-shaped discharged Li 2 O 2 particles has also been thoroughly examined. Thus, O 2 – formation controls the overall reaction onset potential, and LiO 2 formation demarcates the change from a solution- to surface-mediated reaction mechanism. The new findings and improved understandings of the Li-ORR in DMSO will contribute to the further development of aprotic Li–O 2 batteries.