Ideal design of air electrode-A step closer toward robust rechargeable Zn-air battery

To develop a structural design that could provide accessible active sites to oxygen, electrolyte, and electron, it is necessary to modify the overall structure of an air electrode, which is considered as the most significant and complicated part of Zn-air batteries (ZABs). This review highlights the structural features essential to satisfy the design of the cathode compartment of ZABs and presents the associated factors that drive the oxygen reactions in the air electrode based on the relationship between the intrinsic activities of bifunctional O-2 catalysts and the collective strategies employed to modify the electronic structure of such electrocatalysts. The first part describes the fundamentals of an ideal air electrode with its corresponding oxygen electrochemical reactions and typical bifunctional O-2 catalysts. In-depth discussion of O-2 catalysts for air electrodes and progress of binder-free air electrodes for ZABs are presented in the following based on three major modification strategies: defect engineering, cation/anion regulation in multi-components transition metal compounds, and single or multi-heteroatom doping in carbon materials (metal-free and metal-based material). The final part summarizes the properties of air electrodes needed to fulfill the requirements of electrically rechargeable ZABs and provides ideas for the future designs of air electrodes.