Interrogation of the Reaction Mechanism in a Na-O₂ Battery Using <i>In Situ</i> Transmission Electron Microscopy.

Abstract

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 <i>in situ</i> environmental-transmission electron microscopy (TEM) experiment. The reaction mechanism and phase evolution are probed using <i>in situ</i> electron diffraction and TEM imaging. The reversible ORR and OER cycling lies upon the nanosized copper clusters that were formed <i>in situ</i> by sodiation of CuS. <i>In situ</i> 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.