High-Energy-Density Metal–Oxygen Batteries: Lithium–Oxygen Batteries vs Sodium–Oxygen Batteries

The development of next-generation energy-storage devices with high power, high energy density, and safety is critical for the success of large-scale energy-storage systems (ESSs), such as electric vehicles. Rechargeable sodium–oxygen (Na–O₂) batteries offer a new and promising opportunity for low-cost, high-energy-density, and relatively efficient electrochemical systems. Although the specific energy density of the Na–O₂ battery is lower than that of the lithium–oxygen (Li–O₂) battery, the abundance and low cost of sodium resources offer major advantages for its practical application in the near future. However, little has so far been reported regarding the cell chemistry, to explain the rate-limiting parameters and the corresponding low round-trip efficiency and cycle degradation. Consequently, an elucidation of the reaction mechanism is needed for both lithium–oxygen and sodium–oxygen cells. An in-depth understanding of the differences and similarities between Li–O₂ and Na–O₂ battery systems, in terms of thermodynamics and a structural viewpoint, will be meaningful to promote the development of advanced metal–oxygen batteries. State-of-the-art battery design principles for high-energy-density lithium–oxygen and sodium–oxygen batteries are thus reviewed in depth here. Major drawbacks, reaction mechanisms, and recent strategies to improve performance are also summarized.