Identifying Reactive Sites and Transport Limitations of Oxygen Reactions in Aprotic Lithium‐O 2 Batteries at the Stage of Sudden Death

Discharging of the aprotic Li‐O 2 battery relies on the O 2 reduction reaction (ORR) forming solid Li 2 O 2 in the positive electrode, which is often characterized by a sharp voltage drop (that is, sudden death) at the end of discharge, delivering a capacity far below its theoretical promise. Toward unlocking the energy capabilities of Li‐O 2 batteries, it is crucial to have a fundamental understanding of the origin of sudden death in terms of reactive sites and transport limitations. Herein, a mechanistic study is presented on a model system of Au|Li 2 O 2 |Li + electrolyte, in which the Au electrode was passivated with a thin Li 2 O 2 film by discharging to the state of sudden death. Direct conductivity measurement of the Li 2 O 2 film and in situ spectroscopic study of ORR using 18 O 2 for passivation and 16 O 2 for further discharging provide compelling evidence that ORR (and O 2 evolution reaction as well) occurs at the buried interface of Au|Li 2 O 2 and is limited by electron instead of Li + and O 2 transport.