As an energy storage system, Li-ion batteries are not safe because they use flammable organic electrolytes. A safer alternative is rechargeable Zn-based batteries with aqueous electrolytes. Among them, Zn-air batteries have high theoretical volumetric energy density (4400 Wh/L), which can even compete with lithium-sulfur batteries. Alkaline electrolyte is preferable for Zn-air batteries. However, the performance of Zn anodes in alkaline electrolyte is limited by passivation, dissolution and hydrogen evolution. Through SEM investigation, critical passivation size was found to be ~ 2 µm. Sub-micron-sized Zn anodes won’t have passivation problem. As a result, we focus our research on nanoscale. However, Zn dissolution of nanosized anodes will be accelerated because of large electrode-electrolyte surface area.

Thus, anode modification and protection are needed to alleviate the dissolution. We designed a (1) Zn mesh@GO anode (Fig. 1a): graphene oxide (GO) layers on the Zn mesh surface deliver electrons across insulating ZnO and can slow down the Zn dissolution; (2) lasagna-inspired ZnO@GO anode (Fig. 1b): ZnO nanoparticles encapsulated by GO can solve simultaneously the passivation and dissolution problems; (3) core-shell ZnO@TiN nanorod anode (Fig. 1c): thin and conformal TiN coating mitigates Zn dissolution, mechanically maintains the nanostructure, and delivers electron to nanorods. These coatings are ion selective, which allow permeation of OH^- and water, and prevents loss of Zn active material through blocking bigger Zn(OH)₄^2-.

Hydrogen evolution is a competitive side reaction on the zinc anodes, which causes low efficiency of Zn based batteries. Two approaches are investigated to suppress hydrogen evolution, including (1) modify the anode with a hydrogen suppressive material: core-shell ZnO@TiO₂ nanorod anode was made with hydrogen suppressive TiO₂ coating, which solves hydrogen evolution, passivation and dissolution problems at the same time; (2) design a hydrogen evolution suppressive nanostructured alloy anode: Zn-Ag alloy with increased standard reduction potential was used, which is more competitive compared with hydrogen evolution and more stable in aqueous electrolyte.

All of these anodes show superior performance compared with unmodified anodes. These anodes can be paired with air cathodes to make high energy Zn-air batteries. The nanoscale design principles here can potentially be applied to overcome intrinsic limitations of other battery materials.