Time-Resolved Structural Insight into Zinc Metal Plating in Aqueous Batteries.

Zinc metal anodes are promising for next-generation aqueous batteries due to their low electrochemical potential, high volumetric capacity, and global abundance. However, their application is hindered by irreversibility caused by parasitic reactions on the anode, including hydrogen evolution, dendrite formation, and zinc hydroxide sulfate (ZHS) buildup. The structural origins of electrochemical irreversibility in aqueous zinc cells are related to these parasitic side reactions, plating/stripping efficiency, and the crystallographic evolution of zinc anodes during cycling. Here, we use operando X-ray diffraction to track zinc plating and stripping under varying current densities and capacities, uncovering key structural changes that govern reversibility. Zinc plates with a preferred (002) orientation during the first cycle; however, subsequent cycling disrupts this texture, especially at high capacities, shifting the orientation toward Zn(100) or producing random orientations. We show that ZHS formation is governed primarily by the total time spent at low potentials rather than current density or cycle number, implicating calendar aging as a dominant degradation pathway. Dead zinc and ZHS formation both correlate with reduced Coulombic efficiency, but dead zinc appears to be the dominant factor in early cycle reversibility loss. These findings provide structural insight into degradation mechanisms and highlight the importance of engineering interfaces and electrolytes to control crystallographic orientation and parasitic phase formation.