Hydroxylation strategy unlocking multi-redox reaction of manganese hexacyanoferrate for aqueous zinc-ion battery

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. As a result, OH − rich MnHCF cathode renders a high discharge capacity of 136.1 mAh/g (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 and 52.6 Wh kg−1, respectively). Furthermore, the abundant hydroxyl in OH-rich MnHCF can continuously activate the multi-redox centers to provide high capacity in the subsequent cycles. 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.