Advanced nano-bifunctional electrocatalysts in Li-air batteries for high coulombic efficiency

To meet the growing demand for energy storage technologies, it is crucial to develop next-generation energy storage and conversion devices with superior energy density, safety, low cost, and green sustainability. Li-air batteries (LABs) can catalyze the special redox reaction of light weight metal-oxygen (O2) couples, with superb theoretical energy density, low cost, and environmental friendliness, making them suitable for large-scale electricity storage technologies. However, an in-depth understanding of the root causes of poor battery performance and a well-defined electrochemical mechanism based on structural or material properties are still lacking. Hence, we discuss the main obstacles to the sluggish kinetics of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) at the cathode in LABs in this review. Exploration of a special redox reaction based on the design of the catalyst material is extremely instructive for battery development. Various design/manufacturing methods for nanoscale bifunctional electrocatalysts are introduced to offer general design principles. Electrocatalysts involve alloys, transition metal oxides (manganese-based, cobalt-based), other transition metal compounds (carbides/nitrides/sulfides), and carbon materials. The effects of different crystal structure designs on the bifunctional electrocatalytic mechanism of the OER/ORR are investigated at the nanoscale to guide performance improvement strategies. In addition, the root causes of poor battery performance caused by other components are discussed, as well as their possible future breakthroughs. Finally, the prospects of LABs should be highly focused on an integrated mechanism-structure-property strategy to guide catalyst synthesis and further integration with practical parameters for device development. This article focuses on the major obstacle of sluggish ORR and OER kinetics of the cathode in LABs and reviews the main advances of the major designing principles of various nanoscale bifunctional electrocatalysts, and the relation to the enhancing OER/ORR catalytic activity.