Three-Dimensional Nanostructured Air Electrode for Sodium-Oxygen Batteries: A Mechanism Study toward the Cyclability of the Cell

A binder-free three-dimensional (3D) nanostructured air electrode Composed of vertically grown nitrogen doped carbon nanotubes on carbon paper (NCNT-CP) is developed and applied to Na-O-2 cells. The 3D architecture of the air electrode results in increased discharge capacity by optimizing the utilized area of the electrode material. The chemical and electrochemical reaction mechanisms of the cell are also explored with the use of synchrotron-based X-ray absorption spectroscopy (XAS). Investigation of the discharge product of Na-O-2 cells during discharge and charge cycles using X-ray absorption near-edge structure (XANES) indicates that both,sodium superoxide and peroxide are produced under various physicochemical conditions and can be subsequently decomposed with different overpotentials. Furthermore, formation of carbonate-based parasitic products is also shown to restrict the cyclability of the cell.