Operando Observation of the Gold-Electrolyte Interface in Li-O2 Batteries

Observing the cathode interface in Li-O-2 batteries during cycling is necessary to improve our understanding of discharge product formation and evolution in practical cells. In this work a gold electrode surface is monitored by operando surface-enhanced Raman spectroscopy during typical discharge and charge cycling. During discharge, we observe the precipitation of stable and reversible lithium superoxide (LiO2), in contrast to reports that suggest it is a mere intermediate in the formation of lithium peroxide (Li2O2). Some LiO2 is further reduced to Li2O2 producing a coating of insulating discharge products that renders the gold electrode inactive. Upon charging, a superficial layer of these species (similar to 1 nm) are preferentially oxidized at low overpotentials (<0.6 V), leaving residual products in poor contact with the electrode surface. In situ electrochemical impedance spectroscopy is also used to distinguish between LiO2 and Li2O2 products using frequency-dependent responses and to correlate their reduction and oxidation potentials to the accepted mechanism of Li2O2 formation. These operando and in situ studies of the oxygen electrode interface, coupled with ex situ characterization, illustrate that the composition of discharge products and their proximity to the catalytic surface are important factors in the reversibility of Li-O-2 cells.