In-situ imaging Co3O4 catalyzed oxygen reduction and evolution reactions in a solid state Na-O2 battery

Co3O4 nano materials have attracted tremendous attention as effective catalysts for sodium oxygen batteries (SOBs). However, their electrochemical processes and fundamental catalytic mechanism remain unclear till now. Herein, in-situ environmental transmission electron microscopy technique was used to study the catalysis mechanism of the Co3O4 nanocubes in SOBs during discharge and charge processes. It is found that during the 1st discharge and charge processes, Na2O2 formed and decomposed, respectively, around the Co3O4 nanocubes, but the following discharge and charge processes were very difficult. In order to promote the charge kinetics, we increased the charging temperature up to 500 °C, when the decomposition of Na2O2 became facile. Aberration corrected high-angle annular dark field imaging indicated that a thin layer of CoO grew epitaxially on the surface of Co3O4 nanocubes after the first discharge. Density functional theory calculations indicate that the CoO surface is energetically more favorable than Co3O4 for the nucleation of Na2O2. This study provides not only new fundamental understandings to the electrochemical reaction mechanisms of SOBs, but also strategies to improve the cycling performance of solid state SOBs.