We
report real time imaging of the oxygen reduction reactions (ORRs)
in all solid state sodium oxygen batteries (SOBs) with CuO nanowires
(NWs) as the air cathode in an aberration-corrected environmental
transmission electron microscope under an oxygen environment. The
ORR occurred in a distinct two-step reaction, namely, a first conversion
reaction followed by a second multiple ORR. In the former, CuO was
first converted to Cu<sub>2</sub>O and then to Cu; in the latter,
NaO<sub>2</sub> formed first, followed by its disproportionation to
Na<sub>2</sub>O<sub>2</sub> and O<sub>2</sub>. Concurrent with the
two distinct electrochemical reactions, the CuO NWs experienced multiple
consecutive large volume expansions. It is evident that the freshly
formed ultrafine-grained Cu in the conversion reaction catalyzed the
latter one-electron-transfer ORR, leading to the formation of NaO<sub>2</sub>. Remarkably, no carbonate formation was detected in the oxygen
cathode after cycling due to the absence of carbon source in the whole
battery setup. These results provide fundamental understanding into
the oxygen chemistry in the carbonless air cathode in all solid state
Na–O<sub>2</sub> batteries