Competitive Oxygen Reduction Pathways to Superoxide and Peroxide during Sodium-Oxygen Battery Discharge

The sodium-air battery offers a sustainable, high-energy alternative to lithium-ion batteries. Discharge in the cell containing glyme-based electrolytes can lead to formation of large cubic NaO₂ particles via a solution-precipitation mechanism. While promising, high rates result in sudden death. The exact nature of the discharge product has been a matter of contention, and Na₂O₂ has never been directly detected in a dry glyme Na−O₂ cell. If Na₂O₂ were to form during discharge in the Na−O₂ cell it would have a detrimental impact on cell performance. Here we show that Na₂O₂ forms during discharge at high overpotential in the glyme-based Na−O₂ batteries. Na₂O₂ formation is confirmed by spectroscopic and electrochemical analysis and electron microscopy demonstrates that it results in thin insulating films at the electrode surface. The formation of these thin films results in rapid cell death during discharge, introducing an inherent chemical limitation to the Na−O₂ battery.