Sodium-oxygen batteries currently stimulate extensive research due to their high theoretical energy density and improved operational stability when compared to lithium-oxygen batteries. Cell stability, however, needs to be demonstrated also under resting conditions before future implementation of these batteries. In this work we analyze the effect of resting periods on the stability of the sodium superoxide (NaO2) discharge product. The instability of NaO2 in the cell environment is demonstrated leading to the evolution of oxygen during the resting period and the decrease of the cell efficiency. In addition, migration of the superoxide anion (O2(-)) in the electrolyte is observed and demonstrated to be an important factor affecting Coulombic efficiency.

Rechargeable metal-oxygen batteries are receiving significant interest as a possible alternative to current state of the art lithium ion batteries due to their potential to provide higher gravimetric energies, giving significantly lighter or longer-lasting batteries. Recent advances suggest that the Na-O2 battery, in many ways analogous to Li-O2 yet based on the reversible formation of sodium superoxide (NaO2), has many advantages such as a low charge overpotential (∼100 mV) resulting in improved efficiency. In this Perspective, we discuss the current state of knowledge in Na-O2 battery technology, with an emphasis on the latest experimental studies, as well as theoretical models. We offer special focus on the principle outstanding challenges and issues and address the advantages/disadvantages of the technology when compared with Li-O2 batteries as well as other state-of-the-art battery technologies. We finish by detailing the direction required to make Na-O2 batteries both commercially and technologically viable.

In the search for a new energy strategy, metal–O2 batteries stand out as a potential alternative. To reduce losses due to high overpotential and to improve their performance, redox mediators emerge as a powerful strategy for the commercialization of these promising devices.

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