Probing the Reaction Interface in Li-Oxygen Batteries Using Dynamic Electrochemical Impedance Spectroscopy: Discharge-Charge Asymmetry in Reaction Sites and Electronic Conductivity.

Fundamental questions concerning the reaction interface in Li-O2 batteries, including where reactions occur and discharge-charge asymmetries come from, have stimulated a flurry of investigations; nevertheless, heated debates still prevail. Dynamic electrochemical impedance spectroscopy (EIS) is employed here to probe the reaction interface in a Li-O2 battery under potentiostatic and galvanostatic modes. Two impedance semicircles are identified during discharge with the high- and the low-frequency ones related to the Li2O2 film and the oxygen reduction reaction (ORR), respectively. However, upon triggering the oxygen evolution reaction (OER), only one semicircle is observed, implying that the reaction interface changes. Combining qualitative analysis on the EIS structure and quantitative information obtained from model fitting reveals that the ORR occurs on the Li2O2-electrolyte interface during discharge and the OER occurs on the electrode surface during charge. In addition, it is found that the electronic conductivity of Li2O2 is higher at oxidative potentials (charge) than reductive potentials (discharge). Discharge-charge differences in the reaction interface and the electronic conductivity reported here expand the scope of discharge-charge asymmetries of Li-O2 batteries.