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

The energy storage mechanism of MnO2 in aqueous zinc ion batteries (ZIBs) is investigated using four types of MnO2 with crystal phases corresponding to alpha-, beta-, gamma-, and delta-MnO2. Experimental and theoretical calculation results reveal that all MnO2 follow the H+ and Zn2+ co-intercalation mechanism during discharge, with ZnMn2O4, MnOOH, and Zn-4(SO4)(OH)(6)<middle dot>4H(2)O being the main products. ZnMn2O4 is formed from Zn2+ intercalation, while MnOOH and Zn-4(SO4)(OH)(6)<middle dot>4H(2)O are formed from H+ intercalation. Charging generates MnO2 through the extraction of Zn2+ and H+ from ZnMn2O4 and MnOOH, indicating reversible reactions. The study also reveals that H+ intercalation exhibits better kinetics than Zn2+ 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 Zn2+ diffusion in MnO2, with the layered delta phase exhibiting the lowest diffusion energy barrier and highest apparent diffusion coefficient, while the alpha 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 MnO2, which can inform strategies for design of multiphase MnO2 cathodes for high-performance ZIBs.