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

Document Type

Article

Publication Date

2-25-2016

Abstract

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.

Publication Source (Journal or Book title)

Journal of Physical Chemistry C

First Page

3690

Last Page

3698

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