Potential-Dependent Generation of O2– and LiO2 and Their Critical Roles in O2 Reduction to Li2O2 in Aprotic Li–O2 Batteries
2016
Zhang, Yelong | Zhang, Xinmin | Wang, Jiawei | McKee, William C. | Xu, Ye | Peng, Zhangquan
Discharging of the aprotic Li–O₂ battery relies on the oxygen reduction reaction (ORR) producing Li₂O₂ 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 O₂– and LiO₂ in the Li-ORR critically depends on electrode potentials and determines the Li₂O₂ formation mechanism. At low overpotentials (> 2.0 V vs Li/Li⁺) O₂– is identified to be the first surface intermediate, which diffuses into the bulk electrolyte and forms Li₂O₂ therein via a solution-mediated disproportionation mechanism. At high overpotentials (ca. 2.0–1.6 V vs Li/Li⁺) LiO₂ has been observed, which can rapidly transform to Li₂O₂ by further electro-reduction, suggesting a surface-mediated mechanism. The solution-mediated Li₂O₂ formation that can account for the widely observed toroid-shaped discharged Li₂O₂ particles has also been thoroughly examined. Thus, O₂– formation controls the overall reaction onset potential, and LiO₂ 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–O₂ batteries.
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