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

Abstract

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 <i>in situ</i> constructed within α-MnO₂ nanofibers to modulate the interfacial charge transfer process and direct the consequent reversible H⁺ and Zn²⁺ insertion/extraction <i>via</i> 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 kg_cathode^-1, opening a new <i>modus operandi</i> towards advanced flexible batteries through interfacial engineering and device optimization.