Reunderstanding the uneven deposition in aqueous zinc-based batteries

Secondary zinc-based batteries, as the new generation of energy storage devices, boast advantages such as safety, reliability, abundant resources, environmental friendliness, and low cost. However, phenomena such as dendritic growth and hydrogen evolution significantly impact the battery's cycle life, hindering commercialization. Among them, the phenomenon of electrode shape changes caused by Zn uneven deposition is overlooked, especially the deposition at the bottom of the battery under alkaline conditions. Previous research attributes this phenomenon to excessive stress at the base of nucleation sites on the electrode in the early deposition stage, leading to fracture and detachment, resulting in bottom accumulation. Besides, the hydrogen evolution causes an uneven distribution of ion concentration, exacerbating this issue. This study challenges the existing mechanisms and presents a novel explanation using particle tracing experiments and external gravity field simulations. Throughout charge and discharge processes, the ion concentration on the electrode surface changes due to the dissolution and deposition of zinc, causing alterations in the density of the electrolyte solution on the electrode surface. Driven by gravity, the electrolyte experiences natural convection, leading to a higher concentration of zinc ions at the bottom of the battery than at its top. Eventually, different electrolyte concentrations result in varied deposition morphologies, leading to uneven deposition. Furthermore, the upper deposition phenomenon in the weakly acidic electrolytes is also observed, validating the proposed mechanism.