Cathode material design of static aqueous ZnI2 batteries

Aqueous zinc-iodine (ZnI2) batteries are one kind of appealing battery systems due to their high energy density (310 W h kg−1), intrinsic safety, low cost, long lifetime, and environmental-friendliness. Nevertheless, ZnI2 batteries still suffer from severe problems such as polyiodide shuttle, fast self-discharge, slow iodine conversion kinetics, and low I2 loading mass, besides the common hurdles of Zn metal anodes including inhomogeneous Zn electrodeposition and water-related passivation/corrosion. Over the past decades, enormous research efforts have been dedicated to tackling the above obstacles. In this review, we first introduce the electrochemistry of iodine conversion and the underlying working mechanism in aqueous rechargeable ZnI2 batteries. Then, we in-depth discuss the fundamental challenges of iodine conversion reactions and the component/structural design of I2 host materials. Finally, we share our thoughts on the prospects of ZnI2 batteries. This review is anticipated to provide valuable guidance for the further development of high-performance ZnI2 batteries.