Unveiling the Energy Storage Mechanism of MnO₂ Polymorphs for Zinc‐Manganese Dioxide Batteries

The energy storage mechanism of MnO 2 in aqueous zinc ion batteries (ZIBs) is investigated using four types of MnO 2 with crystal phases corresponding to α‐, β‐, γ‐, and δ‐MnO 2 . Experimental and theoretical calculation results reveal that all MnO 2 follow the H + and Zn 2+ co‐intercalation mechanism during discharge, with ZnMn 2 O 4 , MnOOH, and Zn 4 (SO 4 )(OH) 6 ·4H 2 O being the main products. ZnMn 2 O 4 is formed from Zn 2+ intercalation, while MnOOH and Zn 4 (SO 4 )(OH) 6 ·4H 2 O are formed from H + intercalation. Charging generates MnO 2 through the extraction of Zn 2+ and H + from ZnMn 2 O 4 and MnOOH, indicating reversible reactions. The study also reveals that H + intercalation exhibits better kinetics than Zn 2+ intercalation due to its higher diffusion ability and easier charge transfer. The crystal structures and diffusion energy barriers differences are responsible for the differences of H + and Zn 2+ diffusion in MnO 2 , with the layered δ phase exhibiting the lowest diffusion energy barrier and highest apparent diffusion coefficient, while the α phase exhibits the highest diffusion energy barrier and lowest apparent diffusion coefficient. These findings highlight the importance of crystal phases in determining the diffusion ability and energy storage of MnO 2 , which can inform strategies for design of multiphase MnO 2 cathodes for high‐performance ZIBs.