Probing MnO ₂ Cycling Stability in Aqueous Zinc‐Ion Batteries using Chemical Strain Analysis

Abstract

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 MnO 2 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‐MnO 2 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‐MnO 2 proceeds through initial cointercalation of H + and Zn 2+ causing elastic deformation, followed by phase transformation to ZnMn 2 O 4 . During charging, this phase transformation coupled with ZnMn 3 O 7 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.