Recent Advances on Sodium-Oxygen Batteries: A Chemical Perspective

Releasing greenhouse gases into the atmosphere because of widespread use of fossil fuels by humankind has resulted in raising the earth's temperature during the past few decades. Known as global warming, increasing the earth's temperature may in turn endanger civilization on the earth by starting a cycle of environmental changes including climate change and sea level rise. Therefore, replacing fossil fuels with more sustainable energy resources has been considered as one of the main strategies to tackle the global warming crisis. In this regard, energy saving devices are required to store the energy from sustainable resources like wind and solar when they are available and deliver them on demand. Moreover, developing plug-in electric vehicles (PEVs) as an alternative for internal combustion engines has been extensively pursued, since a major sector of fossil fuels is used for transportation purposes. However, currently available battery systems fail to meet the required demands for energy storage. Alkali metal-O₂ battery systems demonstrate a promising prospect as a high-energy density solution regarding the increasing demand of mankind for energy storage. Combining a metallic negative electrode with a breathing oxygen electrode, a metal-O₂ cell can be considered as a half battery/half fuel cell system. The negative electrode in the metal-O₂ cells operates a conversion reaction rather than intercalation mechanism, which eliminates the need for a host lattice. In addition, the positive electrode material (O₂) comes from the ambient air and hence is not stored in the battery. Therefore, the resultant battery systems exhibit the highest theoretical energy density, which is comparable to that of gasoline. Accordingly, an unprecedented amount of research activity was directed toward alkali metal-O₂ batteries in the past decade in response to the need for high-energy storage technology in electric transportation. This extensive research surge has resulted in a rapid expansion of our knowledge about alkali metal-O₂ batteries. The present Account summarizes the most recent findings over the underlying chemistry of all components in Na-O₂ cells as one of the most efficient members of alkali metal-O₂ family.