Simultaneous reversible tuning of H + and Zn 2+ coinsertion in MnO 2 cathode for high-capacity aqueous Zn-ion battery.

Protons and zinc ions are generally regarded as charge carriers for rechargeable Zn/MnO 2 batteries relying on cation coinsertion for their two-step redox energy storage. However, the irreversibility of H + insertion and especially Zn 2+ insertion unlocks the innate advantages of this scalable aqueous battery system such as high safety and low cost. Herein, an encapsulated structure with manganese hexacyanoferrate(II)-polypyrrole (MnHCF-PPy) composite thin films was in situ constructed within α-MnO 2 nanofibers to modulate the interfacial charge transfer process and direct the consequent reversible H + and Zn 2+ insertion/extraction via a synergistic action. The PPy film promotes interfacial proton transfer for the fast conversion of MnO 2 to MnOOH and suppresses cathode dissolution, whereas MnHCF with abundant interconnected open ion channels serves as a cation reservoir to facilitate continuous and reversible zinc ion transfer without the aggregation of ZnMn 2 O 4 nanocrystals, leading to protected MnO 2 cathodes with recorded high discharge capacity (263 mA h g -1 at 0.5 C), remarkable rate capability (179 mA h g -1 at 5 C), and long lifespan in ZnSO 4 electrolyte. Moreover, a flexible solid-state Zn/MnO 2 full cell was further demonstrated, which delivers a preferable energy density of 220 W h kg cathode -1 , opening a new modus operandi towards advanced flexible batteries through interfacial engineering and device optimization.