Understanding the charge/discharge mechanisms and passivation reactions in Na-O2 batteries

Sodium-oxygen batteries are becoming of increasing interest in the research community as they are able to overcome some of the difficulties associated with lithium-oxygen batteries. The interpretation of the processes governing the discharge and charge of these batteries, however, has been under debate since their early development. In this work we combine different electrochemical methods to build up a model of the discharge product formation and decomposition. We initially analyze the formation and decomposition of the discharge products by means of electrochemical impedance spectroscopy. After that, and for the first time, oxygen electrode processes in Na-O2 cells are analyzed by means of electrochemical quartz crystal microbalance experiments. Based on the combination of these two techniques it is possible to evidence the stabilization of the discharge products in the electrolyte prior to their precipitation. The deposition of passivating products that cannot be stripped off during charge is also demonstrated. Cyclic voltammetry experiments at different potential limits further confirm these passivation reactions. In conclusion, this work provides an accurate picture of the mechanism of the Na-O2 cell reactions by combining different electrochemical techniques