Reversible sulfate-anion involved charge storage in layered MnO2 for aqueous symmetric batteries

Aqueous rechargeable batteries are promising for large-scale energy storage, but MnO2 electrodes suffer from manganese dissolution and Jahn-Teller-related instability. Here, we investigate sulfate-anion-involved charge storage in layered δ-MnO2 symmetric cells. Ex situ spectroscopy, thermal analysis, and elemental mapping show state-dependent sulfur signals accompanied by Mn valence evolution and modest, partially reversible local coordination changes. First-principles calculations indicate that sulfate accommodation and migration within a modeled interlayer are energetically plausible. These results support sulfate-coupled charge compensation and suggest possible interlayer accommodation, while surface adsorption, near-surface storage, and electrolyte trapping cannot be excluded under the present ex situ conditions. The MnO2 electrode delivers 154.2 mAh g-1 at 0.2 A g-1, and the symmetric cell achieves 107.94 Wh kg-1 at 180 W kg-1 with 93.7% capacity retention after 3,000 cycles. This work provides evidence for sulfate-coupled charge compensation in layered MnO2.