Rate‐Dependent Capacity Evolution and Time‐Controlled Degradation of MnO 2 Cathodes in Aqueous Zinc‐Ion Batteries

Abstract

The excellent cycling stability of MnO 2 cathodes in aqueous zinc‐ion batteries (AZIBs) at high current densities often deteriorates at lower current densities, yet the origin of this rate‐dependent behavior remains unclear. Here, we uncover the interplay between current density and cumulative cycling time by tracking the electrochemical and structural evolution of α ‐MnO 2 cathodes in Mn 2+ ‐containing electrolytes. Low current densities promote more extensive Mn 2+ ‐mediated dissolution–deposition, leading to greater accumulation of a porous, metastable Zn–Mn–O intermediate and pronounced capacity enhancement. Notably, while the magnitude of the capacity enhancement strongly depends on current density, the capacity maximum occurs within a relatively narrow cumulative cycling time of ~350–450 h across the tested current densities in 2 M ZnSO 4 + 0.2 M MnSO 4 electrolyte. Upon prolonged cycling, the metastable Zn–Mn–O intermediate gradually diminishes, accompanied by the emergence of crystalline ZnMn 2 O 4 , increased electrode polarization, and rapid capacity fading. These findings reveal that current density affects the extent of capacity enhancement, whereas cumulative cycling time provides a more consistent measure of the subsequent capacity evolution than cycle number. This time‐domain understanding explains the contrasting cycling behavior of MnO 2 cathodes at different current densities and provides mechanistic guidance for evaluating and improving their long‐term durability in AZIBs.