β-MnOOH Converted δ-MnO₂ with New Layer-to-Layer Charge Storage as Cathode for Zinc-Ion Batteries

Layer-structured birnessite δ-MnO₂ is viewed as a promising cathode material for harnessing the copious potential of aqueous zinc-ion batteries (ZIBs). However, discharging δ-MnO₂ has resulted in diverse non-layer-structured products being reported, rendering it unfavorable for reversible charge storage compared to intercalation electrochemistry in conventional layered materials. Here, feitknechtite β-MnOOH and its isostructural δ-MnO₂ obtained from an anodization method are first applied as cathode materials for ZIBs. Relative to the β-MnOOH, the anodized δ-MnO₂ with larger interlayer spacing manifested boosted charge transfer kinetic and reversibility of Mn-based redox electrochemistry, showing significant enhancements in cyclicity and rate capability when coupled with Zn metal anodes in aqueous electrolytes. Importantly, experimental results combined with theoretical simulations revealed that the minor presence of β-MnOOH in the anodized δ-MnO₂ (due to incomplete deprotonation) is linked to a new layer-to-layer charge storage mechanism, where anodized δ-MnO₂ transformed into β-MnOOH during the reversible charge and discharge process. This work uncovers a new phase transformation in Mn-based cathodes and can pave the way for the development of more efficient layered materials for ZIBs.