Superoxide-Based K-O2 Batteries: Highly Reversible Oxygen Redox Solves Challenges in Air Electrodes.

In the past 20 years, research in metal-O2 batteries has been one of the most exciting interdisciplinary fields of electrochemistry, energy storage, materials chemistry, and surface science. The mechanisms of oxygen reduction and evolution play a key role in understanding and controlling these batteries. With intensive efforts from many prominent research groups, it becomes clear that the instability of superoxide in the presence of Li ions (Li+) and Na ions (Na+) is the fundamental root cause for the poor stability, reversibility, and energy efficiency in aprotic Li-O2 and Na-O2 batteries. Stabilizing superoxide with large K ions (K+) provides a simple but elegant solution. Superoxide-based K-O2 batteries, invented in 2013, adopt the one-electron redox process of O2/potassium superoxide (KO2). Despite being the youngest metal-O2 technology, K-O2 is the most promising rechargeable metal-air battery with the combined advantages of low costs, high energy efficiencies, abundant elements, and good energy densities. However, the development of the K-O2 battery has been overshadowed by Li-O2 and Na-O2 batteries because one might think K-O2 is just an analogous extension. Moreover, due to the lower specific energy and the high reactivity of K metal, K-O2 is often underestimated and deemed unsuitable for practical applications. The objective of this Perspective is to highlight the unique advantages of K-O2 chemistry and to clarify the misconceptions prompted by the name "superoxide" and the judgment bias based on the claimed theoretical specific energies. We will also discuss the current challenges and our perspectives on how to overcome them.