Superoxide-based Na-O2 batteries: Background, current status and future prospects

Na-O2 batteries are considered a promising energy storage device for powering next-generation electric vehicles, due to their high theoretical energy density, high energy efficiency, and low cost. Furthermore, Na-O2 batteries that have sodium superoxide as their discharge product are highly attractive as they exhibit extremely low charge overpotentials. Though significant advances have been made on superoxide-based Na-O2 batteries, the practical application of this technology is still severely hindered by insufficient achievable capacities and limited cycling performance. This review presents a comprehensive summary of superoxide-based Na-O2 batteries, beginning with an overview of their configuration, historical development, and technical challenges, followed by their discharge/charge reaction mechanisms discussed in light of recent findings. Furthermore, the development of air electrodes, electrolytes, and the Na anode, as well as the detrimental effect of O2/O2- crossover are discussed. Finally, a general conclusion and perspective on the current limitations and recommended future research directions of Na-O2 batteries is presented. This review aims at serving as a guideline for the development of superoxide-based Na-O2 batteries with high energy density and long cycle life for next-generation applications such as electric vehicles.