Hydroxylation Strategy Unlocking Multi-Redox Reaction of Manganese Hexacyanoferrate for Aqueous Zinc-Ion Battery

Abstract

Prussian blue analogues (PBAs), as one of the most promising cathodes due to their open framework and high working voltage (~1.75 V vs. Zn/Zn2+) for rechargeable aqueous zinc ion batteries (RAZIBs), commonly suffer from poor reversible capacity owing to single redox center. Here, a hydroxylation strategy is proposed for activating inactive redox pair of Mn(II)/Mn(III) in manganese hexacyanoferrate (MnHCF) to supply extra capacity delivery. As a result, OH−rich MnHCF cathode renders a high discharge capacity of 136.1 mAh g−1 (at 100 mA g−1) and a considerable energy density of 228.8 Wh kg-1 benefiting from the unlocked multi-redox reaction, which is much better than the OH−poor MnHCF (58.3 mAh g−1 and 52.6 Wh kg-1, respectively). Furthermore, the multi-redox centers in OH-rich MnHCF can be well reserved by reversible phase transition to metal oxyhydroxide during cycling, delivering long-term capacity contribution. Density functional theory (DFT) calculation reveals that abundant hydroxyl functional group favors to capture Zn2+ on the sites near Mn atoms, thereby facilitating the activation of Mn-redox reaction. This study offers new opportunities for exploiting cathodes with both high working voltage and discharge capacity for RAZIBs.

Keywords: Keywords: Zinc-ion battery, Hydroxyl functional group, MnHCF, multi-redox reaction.