Potassium Superoxide: A Unique Alternative for Metal-Air Batteries.

Lithium-oxygen (Li-O2) batteries have been envisaged and pursued as the long-term successor to Li-ion batteries, due to the highest theoretical energy density among all known battery chemistries. However, their practical application is hindered by low energy efficiency, sluggish kinetics, and a reliance on catalysts for the oxygen reduction and evolution reactions (ORR/OER). In a superoxide battery, oxygen is also used as the cathodic active medium but is reduced only to superoxide (O2•-), the anion formed by adding an electron to a diatomic oxygen molecule. Therefore, O2/O2•- is a unique single-electron ORR/OER process. Since the introduction of K-O2 batteries by our group in 2013, superoxide batteries based on potassium superoxide (KO2) have attracted increasing interest as promising energy storage devices due to their significantly lower overpotentials and costs. We have selected potassium for building the superoxide battery because it is the lightest alkali metal cation to form the thermodynamically stable superoxide (KO2) product. This allows the battery to operate through the proposed facile one-electron redox process of O2/KO2. This strategy provides an elegant solution to the long-lasting kinetic challenge of ORR/OER in metal-oxygen batteries without using any electrocatalysts. Over the past five years, we have been focused on understanding the electrolyte chemistry, especially at the electrode/electrolyte interphase, and the electrolyte's stability in the presence of potassium metal and superoxide. In this Account, we examine our advances and understanding of the chemistry in superoxide batteries, with an emphasis on our systematic investigation of K-O2 batteries. We first introduce the K metal anode electrochemistry and its corrosion induced by electrolyte decomposition and oxygen crossover. Tuning the electrolyte composition to form a stable solid electrolyte interphase (SEI) is demonstrated to alleviate electrolyte decomposition and O2 cross-talk. We also analyze the nucleation and growth of KO2 in the oxygen electrode, as well its long-term stability. The electrochemical growth of KO2 on the cathode is correlated with the rate performance and capacity. Increasing the surface area and reducing the O2 diffusion pathway are identified as critical strategies to improve the rate performance and capacity. Li-O2 and Na-O2 batteries are further compared with the K-O2 chemistry regarding their pros and cons. Because only KO2 is thermodynamically stable at room temperature, K-O2 batteries offer reversible cathode reactions over the long-term while the counterparts undergo disproportionation. The parasitic reactions due to the reactivity of superoxide are discussed. With the trace side products quantified, the overall superoxide electrochemistry is highly reversible with an extended shelf life. Lastly, potential anode substitutes for K-O2 batteries are reviewed, including the K3Sb alloy and liquid Na-K alloy. We conclude with perspectives on the future development of the K metal anode interface, as well as the electrolyte and cathode materials to enable improved reversibility and maximized power capability. We hope this Account promotes further endeavors into the development of the K-O2 chemistry and related material technologies for superoxide battery research.