Aqueous metal-air batteries: Fundamentals and applications

Aqueous metal-air batteries have gained much research interest as an emerging energy storage technology in consumer electronics, electric vehicles, and stationary power plant recently, primarily due to their high energy density derived from discarding the bulkier cathode chamber. In addition, abundant raw materials, low cost, high safety, and environmental friendliness are intrinsic advantages of metal-air batteries as well. Motivated by the desirable characteristics, the significant progress has been made in promoting the promising electrochemical performance and improving the long-term stability in aqueous metal-air batteries in recent years. This review article starts with a general description of the features and working principles of aqueous metal-air battery systems. Then, the air cathode structures are introduced and compared, as well as the electrochemical catalysts applied in the air cathode are comprehensively summarized, including noble metals, alloys, metal oxides, hydroxides or sulfides, carbon materials, as well as the composite materials such as metal (oxides)/metal oxides and (metal) oxides/carbon materials. In addition, the chemistries and major issues of metal anodes, e.g. aluminum, zinc and magnesium, are reviewed and respective strategies to alleviate these issues are offered as well. Afterwards, detailed discussions have been provided on the recent progress towards the electrolytes and separators, both of which are crucial components in the battery configuration. From the system point of view, innovative single-cell and stack system designs are also presented. Finally, the challenges and perspectives for the future development of aqueous metal-air batteries are highlighted.