Operando Br K-Edge Dispersive X-ray Absorption Fine Structure Analysis for Br – /Br 3 – Redox Mediator for Li–Air Batteries

The behavior of Br – /Br 3 – 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. Br 3 – ions in the electrolyte increase monotonically after ∼30% charging, which indicates that a part of Br 3 – formed via the oxidation of Br – during charging does not oxidize Li 2 O 2 as RM but gets accumulated in the cell. The accumulated Br 3 – is estimated at ∼20% of the total amount of charging. Consistent with this result, the O 2 evolution efficiency gradually decreases from the initial 100% as the charging progresses, and CO 2 is emitted at the end of the charging. The integrated amount of O 2 evolution during charging accounts for ∼83% of the theoretical two-electron reaction. All these results clearly show that the Br – /Br 3 – RM functions effectively at the beginning of charging but gradually loses its function as charging proceeds.