Probing MnO₂ Cycling Stability in Aqueous Zinc-Ion Batteries using Chemical Strain Analysis

The mechanical degradation of cathodes during charge–discharge cycling poses a critical limitation to the cycle life of aqueous zinc-ion batteries (AZIBs). Although the degradation of MnO₂ cathodes has been extensively investigated, the underlying reaction mechanisms have long remained a subject of debate, and the associated mechanical evolution during cycling is still poorly understood. In this work, a comprehensive investigation of electrochemical phase transitions and chemical strain evolution in δ-MnO₂ cathode is presented using a custom-built in situ strain testing system based on digital image correlation. The results reveal that the discharge–charge mechanism of δ-MnO₂ proceeds through initial cointercalation of H⁺ and Zn²⁺ causing elastic deformation, followed by phase transformation to ZnMn₂O₄. During charging, this phase transformation coupled with ZnMn₃O₇ formation induces irreversible plastic deformation, generating substantial residual strain and cathode volume expansion. Increasing current density can effectively reduce residual strain by suppressing phase transformation, thereby enhancing electrode cycling stability.