The Potassium–Air Battery: Far from a Practical Reality?

ConspectusAn energy storage system is the key bottleneck toward the widespread use of renewable energy and the development of electric vehicles (EVs). Alkali metal–oxygen batteries, which have higher gravimetric energy densities (3500–935 Wh kg–1) than conventional lithium–ion batteries (100–265 Wh kg–1), are considered to be one of the promising next-generation energy storage systems. Over the past decade, Li–O2 batteries have been the center of the research effort owing to their highest energy density. However, the poor reversibility, low round-trip efficiency, and limited cycle life originating from sluggish kinetic and serious parasitic chemistry induced by singlet oxygen hamper the development of Li–O2 batteries. Both the sluggish kinetics and severe parasitic reactions are closely related to the discharge product Li2O2. Unlike Li–O2 batteries, K–O2 batteries based on potassium superoxide offer an attractive theoretical energy density (935 Wh kg–1) with a significantly improved energy efficiency and lifetime compared to other alkali metal–O2 batteries. The fast and reversible O2/KO2 single–electron reaction exhibits higher redox kinetics compared to the Li–O2 redox chemistries and removes the needs of catalysts or redox mediators. In addition, the earth abundant K greatly alleviates the global shortage and uneven regional distribution of Li. These unique advantages of the K–O2 system make it a promising candidate for low-cost and large-scale energy storage. However, the development of a K–O2 battery is still in its early stages and its round-trip efficiency is still lower than that of lithium–ion batteries. Further improvement in energy efficiency and cycle life of the K–O2 batteries is crucial prior to practical applications. The present Account combines our efforts and other representative works on fundamental understandings and design strategies toward next-generation K–O2 batteries. Insights are offered on oxygen electrode reversibility and stability, anode stabilization and alternative anodes, and the closed system based on KO2–K2O2 conversion. Five physicochemical factors that affect the oxygen electrode reversibility and stability are discussed in light of recent findings, including electrolyte design, growth mechanism, operation environment, degradation mechanism, and electrode–electrolyte design. Furthermore, the alternative anode materials development to solve the long-standing potassium anode issue are discussed and the pros and cons of alternative anodes are compared. In addition, due to oxygen crossover to the anode and the electrolyte evaporation problem in open K–air battery systems, the feasibility and strategies to develop closed systems are briefly discussed. At the end of the Account, future directions in deepening understanding of K–O2 reaction and battery design to realize practical applications of K–O2 systems are highlighted.