The Role of the Electrode Surface in Na–Air Batteries: Insights in Electrochemical Product Formation and Chemical Growth of NaO₂

The Na–air battery, because of its high energy density and low charging overpotential, is a promising candidate for low-cost energy storage, hence leading to intensive research. However, to achieve such a battery, the role of the positive electrode material in the discharge process must be understood. This issue is herein addressed by exploring the electrochemical reduction of oxygen, as well as the chemical formation and precipitation of NaO₂ using different electrodes. Whereas a minor influence of the electrode surface is demonstrated on the electrochemical formation of NaO₂, a strong dependence of the subsequent chemical precipitation of NaO₂ is identified. In the origin, this effect stems from the surface energy and O₂/O₂⁻ affinity of the electrode. The strong interaction of Au with O₂/O₂⁻ increases the nucleation rate and leads to an altered growth process when compared to C surfaces. Consequently, thin (3 µm) flakes of NaO₂ are found on Au, whereas on C large cubes (10 µm) of NaO₂ are formed. This has significant impact on the cell performance and leads to four times higher capacity when C electrodes with low surface energy and O₂/O₂⁻ affinity are used. It is hoped that these findings will enable the design of new positive electrode materials with optimized surfaces.