Simultaneous reversible tuning of H+ and Zn2+ coinsertion in MnO2 cathode for high-capacity aqueous Zn-ion battery

Protons and zinc ions are generally regarded as charge carriers for rechargeable Zn/MnO2 batteries relying on cation coinsertion for their two-step redox energy storage. However, the irreversibility of H+ insertion and especially Zn2+ 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-MnO2 nanofibers to modulate the interfacial charge transfer process and direct the consequent reversible H+ and Zn2+ insertion/extraction via a synergistic action. The PPy film promotes interfacial proton transfer for the fast conversion of MnO2 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 ZnMn2O4 nanocrystals, leading to protected MnO2 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 ZnSO4 electrolyte. Moreover, a flexible solid-state Zn/MnO2 full cell was further demonstrated, which delivers a preferable energy density of 220 W h kgcathode-1, opening a new modus operandi towards advanced flexible batteries through interfacial engineering and device optimization.