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

Abstract Rechargeable Zn/MnO 2 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‐MnO 2 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 + /Zn 2+ coinsertion mechanism of α‐MnO 2 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 Zn 2+ ‐insertion is found to be less reversible than H + ‐insertion in view of the dramatic capacity fading occurring in the Zn 2+ ‐insertion step, which is further evidenced by the discovery of an irreversible ZnMn 2 O 4 layer at the surface of α‐MnO 2 . Hence, the H + ‐insertion process actually plays a crucial role in maintaining the cycling performance of the aqueous Zn/α‐MnO 2 battery. This work is believed to provide an insight into the charge‐storage mechanism of α‐MnO 2 in aqueous systems and paves the way for designing aqueous ZIBs with high energy density and long‐term cycling ability.