Interrogation of the Reaction Mechanism in a Na-O₂ Battery Using In Situ Transmission Electron Microscopy

Critical factors that govern the composition and morphology of discharge products are largely unknown for Na-O₂ batteries. Here we report a reversible oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) process in a sodium-oxygen battery observed using in situ environmental-transmission electron microscopy (TEM) experiment. The reaction mechanism and phase evolution are probed using in situ electron diffraction and TEM imaging. The reversible ORR and OER cycling lies upon the nanosized copper clusters that were formed in situ by sodiation of CuS. In situ electron diffraction revealed the formation of NaO₂ initially, which then disproportionated into orthorhombic and hexagonal Na₂O₂ and O₂. Na₂O₂ was the major final ORR product that uniformly covered the whole wire-shape cathode. This uniform product morphology largely increased the application feasibility of Na-O₂ batteries in industry. In the following OER process, the Na₂O₂ transformed to NaO₂, which resulted in volume expansion at first, and then the NaO₂ decomposed to sodium ions and O₂ gas. Galvanostatic charge/discharge profiles of CuS in real Na-O₂ cells revealed a maximum capacity over 3 mAh cm^-2 with a discharge cutoff voltage of 1.8 V and high cycling stability. The nanosized copper catalyst plays a dominating role in controlling the morphology, chemical composition of discharge products, and reversibility of this Na-O₂ battery. Our finding shines light on the exploration of effective catalysts for the Na-O₂ battery.