Operando Br K-Edge Dispersive X-ray Absorption Fine Structure Analysis for Br^–/Br₃^– Redox Mediator for Li–Air Batteries

The behavior of Br–/Br3– redox mediator (RM), which has been shown to suppress charging overpotential in lithium–air batteries, has been detected for the first time using operando Br K-edge dispersive X-ray absorption fine structure. Br3– ions in the electrolyte increase monotonically after ∼30% charging, which indicates that a part of Br3– formed via the oxidation of Br– during charging does not oxidize Li2O2 as RM but gets accumulated in the cell. The accumulated Br3– is estimated at ∼20% of the total amount of charging. Consistent with this result, the O2 evolution efficiency gradually decreases from the initial 100% as the charging progresses, and CO2 is emitted at the end of the charging. The integrated amount of O2 evolution during charging accounts for ∼83% of the theoretical two-electron reaction. All these results clearly show that the Br–/Br3– RM functions effectively at the beginning of charging but gradually loses its function as charging proceeds.