Identifying Reactive Sites and Transport Limitations of Oxygen Reactions in Aprotic Lithium-O2 Batteries at the Stage of Sudden Death

Discharging of the aprotic Li‐O2 battery relies on the O2 reduction reaction (ORR) forming solid Li2O2 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‐O2 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|Li2O2|Li+ electrolyte, in which the Au electrode was passivated with a thin Li2O2 film by discharging to the state of sudden death. Direct conductivity measurement of the Li2O2 film and in situ spectroscopic study of ORR using 18O2 for passivation and 16O2 for further discharging provide compelling evidence that ORR (and O2 evolution reaction as well) occurs at the buried interface of Au|Li2O2 and is limited by electron instead of Li+ and O2 transport. Direct conductivity measurement of an electrodeposited Li2O2 film and an in situ SERS study on the O2 reduction reaction (ORR) using 18O2 for passivation and 16O2 for further discharging unveiled the origin of sudden death of aprotic Li‐O2 batteries. Both ORR and O2 evolution (OER) occur at the buried interface of cathode|Li2O2, and thus the Li‐O2 electrochemistry is limited by electron transport instead of Li+ and O2 transport.