How to Control the Discharge Products in Na-O Cells: Direct Evidence toward the Role of Functional Groups at the Air Electrode Surface.

Sodium-oxygen batteries have received a significant amount of research attention as a low-overpotential alternative to lithium-oxygen. However, the critical factors governing the composition and morphology of the discharge products in Na-O cells are not thoroughly understood. Here we show that oxygen containing functional groups at the air electrode surface have a substantial role in the electrochemical reaction mechanisms in Na-O cells. Our results show that the presence of functional groups at the air-electrode surface conducts the growth mechanism of discharge products toward a surface-mediated mechanism, forming a conformal film of products at the electrode surface. In addition, oxygen reduction reaction at hydrophilic surfaces more likely passes through a peroxide pathway, which results in the formation of peroxide-based discharge products. Moreover, in-line X-ray diffraction combined with solid state Na NMR results indicate the instability of discharge products against carbonaceous electrodes. The findings of this study help to explain the inconsistency among various reports on composition and morphology of the discharge products in Na-O cells and allow the precise control over the discharge products.