H⁺ -Insertion Boosted α-MnO₂ for an Aqueous Zn-Ion Battery

Rechargeable Zn/MnO₂ batteries using mild aqueous electrolytes are attracting extensive attention due to their low cost, high safety, and environmental friendliness. However, the charge-storage mechanism involved remains a topic of controversy so far. Also, the practical energy density and cycling stability are still major issues for their applications. Herein, a free-standing α-MnO₂ cathode for aqueous zinc-ion batteries (ZIBs) is directly constructed with ultralong nanowires, leading to a rather high energy density of 384 mWh g^-1 for the entire electrode. Greatly, the H⁺ /Zn²⁺ coinsertion mechanism of α-MnO₂ cathode for aqueous ZIBs is confirmed by a combined analysis of in situ X-ray diffractometry, ex situ transmission electron microscopy, and electrochemical methods. More interestingly, the Zn²⁺ -insertion is found to be less reversible than H⁺ -insertion in view of the dramatic capacity fading occurring in the Zn²⁺ -insertion step, which is further evidenced by the discovery of an irreversible ZnMn₂ O₄ layer at the surface of α-MnO₂ . Hence, the H⁺ -insertion process actually plays a crucial role in maintaining the cycling performance of the aqueous Zn/α-MnO₂ battery. This work is believed to provide an insight into the charge-storage mechanism of α-MnO₂ in aqueous systems and paves the way for designing aqueous ZIBs with high energy density and long-term cycling ability.