Rate-Dependent Capacity Evolution and Time-Controlled Degradation of MnO₂ Cathodes in Aqueous Zinc-Ion Batteries

The excellent cycling stability of MnO₂ 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-MnO₂ cathodes in Mn²⁺-containing electrolytes. Low current densities promote more extensive Mn²⁺-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 ZnSO₄ + 0.2 M MnSO₄ electrolyte. Upon prolonged cycling, the metastable Zn-Mn-O intermediate gradually diminishes, accompanied by the emergence of crystalline ZnMn₂O₄, 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 MnO₂ cathodes at different current densities and provides mechanistic guidance for evaluating and improving their long-term durability in AZIBs.