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 α-, β-, γ-, and δ-MnO₂. Experimental and theoretical calculation results reveal that all MnO₂ follow the H⁺ and Zn²⁺ co-intercalation mechanism during discharge, with ZnMn₂O₄, MnOOH, and Zn₄(SO₄)(OH)₆·4H₂O being the main products. ZnMn₂O₄ is formed from Zn²⁺ intercalation, while MnOOH and Zn₄(SO₄)(OH)₆·4H₂O 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 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 MnO₂, which can inform strategies for design of multiphase MnO₂ cathodes for high-performance ZIBs. © 2024 Wiley-VCH GmbH.