Operando
Observation of the Gold–Electrolyte
Interface in Li–O<sub>2</sub> Batteries
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Abstract
Observing
the cathode interface in Li–O<sub>2</sub> 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
(LiO<sub>2</sub>), in contrast to reports that suggest it is a mere
intermediate in the formation of lithium peroxide (Li<sub>2</sub>O<sub>2</sub>). Some LiO<sub>2</sub> is further reduced to Li<sub>2</sub>O<sub>2</sub> producing a coating of insulating discharge products
that renders the gold electrode inactive. Upon charging, a superficial
layer of these species (∼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 LiO<sub>2</sub> and
Li<sub>2</sub>O<sub>2</sub> products using frequency-dependent responses
and to correlate their reduction and oxidation potentials to the accepted
mechanism of Li<sub>2</sub>O<sub>2</sub> 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<sub>2</sub> cells