Future of Potassium Super Oxide Batteries

Dear Colleagues,

Lithium and sodium have been significantly used in the energy storage system after the development of energy generation and storage concept. The concept of lithium-oxygen (Li–O2) and sodium-oxygen (Na–O2) batteries based on lightweight oxygen (O2) cathode evolved during the attempts for further improving the battery energy density and reducing the cathode cost without the usage of transition metal element. Though Li–O2 occupy the major space in the area of research of the alkali metal-O2 battery system, researchers are working hard within this decade to eliminate the drawbacks of the Li–O2 battery through the development of potassium (K)–O2 battery, which was first invented by Dr. Yiying Wu from the Ohio State University in 2013. The limited Li abundance on the earth’s crust and Li supply shortage due to extensive exploitation accelerate the search for its alternative. The existence of more abundant K source on the earth also added fuel in the necessity and probability of K–O2 battery.

The mechanisms of O2 reduction and evolution play a key role in understanding and controlling the alkali metal-O2 batteries. Although O2 is first reduced to superoxide anion (O2-) intermediate for all the alkali metal-O2 batteries, the unstable superoxide in the presence of Li+ ions and Na+ ions could occur disproportionation spontaneously with the formation of peroxide and singlet O2 (1O2) evolution. The peroxide requires a high overpotential to decompose and the reactive 1O2 could easily trigger the electrolyte decomposition and electrode degradation, which deteriorate the performance in terms of cycle life and energy efficiency.

Different from peroxide chemistry, the superoxide batteries involve single electron transfer between O2/O2-, which is highlighted with high reaction kinetics without any electrocatalysts. The K is the lightest element which can form the thermodynamically stable superoxide at ambient condition. The basic superoxide electrochemistry stemmed from the stabilization effect of hard-soft acid-base theory. Stabilizing the soft Lewis base of O2- with a soft Lewis acid of K+ provides a simple but elegant solution to circumvent the battery degradation due to the instability of superoxide. Thus, superoxide-based K−O2 batteries 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 alkali metal-O2 battery with the combined advantages of low costs, high energy efficiencies, abundant elements, and good energy densities.

However, due to the lower specific energy and the high reactivity of K metal, K−O2 is often underestimated and deemed unsuitable for practical applications. There is a wide room for the study of the K–O2 battery system with perspectives on the future development of the long-lasting K metal anode, the functional electrolyte, and the porous gas diffusion cathode materials for enhancement of lifespan, energy output and power capability. It also opens the door for a deeper understanding of the K–O2 chemistry and development of related materials for superoxide battery research.

Prof. Dr. Ram Krishna Hona
Dr. Lei Qin
Guest Editors

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• alkali metal
• superoxide battery
• charging cycle
• soft acid/base
• electrochemistry