Regulation On Structure Of Layered Manganese Dioxide Cathode And Zn²⁺Storage Performance By Metal Ion Pre-Intercalation Engineering

Among the cathode materials of aqueous zinc-ion batteries（ZIBs）,8-MnO₂ has attracted much attention due to its high specific capacity and discharge voltage.However,its low electronic/ionic conductivities and strong coulomb repulsion between Zn²⁺and lattice result in a slow reaction kinetics and a poor electrochemical performance.In this work,we promote the electronic/ionic conductivities of δ-MnO₂ via regulating lattice and electronic structures,which thus enhance the electrochemical reaction kinetics and H⁺intercalation activity of δ-MnO₂ cathode,eventually improving its electrochemical performance as cathode material for ZIBs.The specific research content is as follows:（1）The monovalent alkali ions Na⁺ and K⁺ possessing different radii are in-situ intercalated into the interlayer of δ-MnO₂ in the hydrothermal synthesis of δ-MnO₂.Compared with Na⁺,the pre-intercalation of K⁺ synchronously enlarges the interlayer spacing and elevates the covalency degree of Mn-O bond,which thus facilitates the electronic/ionic conductivities and results in a high rate capability.At a large current density of 3 A g^-1,the discharge capacity of K⁺pre-intercalated 8-MnO₂ cathode reaches 100.1 mAh g^-1 after 600 cycles.The diffusion-controlled and intercalation pseudocapacitive dominated charge storage process are observed at lower and higher potential sweep speeds,respectively.（2）The divalent metal ions of Cu²⁺、Mg²⁺、Ni²⁺、Co²⁺ and Zn²⁺（M-MnO₂）possessing similar radii are in-situ intercalated into the interlayer of δ-MnO₂ fabrication of M-MnO₂ in a similar hydrothermal method.It is found that the introduction of the metal ion with different types can adjust the metal-O bond type and covalency degree,then change the average charge around O atom,and thus optimize the p-band center（ε_p）of O in the δ-MnO₂.This further endows δ-MnO₂ with the equilibrium of absorption/desorption of H⁺and thus dramatically enhances the H⁺intercalation capacity.The Cu-O covalent bond induced by Cu 3d orbit endows Cu-MnO₂ with high electronic/ionic conductivities and modest H⁺adsorption energy,thus resulting in higher H⁺intercalation contribution of 73%at 3 A g^-1 compared with Mg-MnO₂ possessing the Mg-O ionic bond induced by Mg 2p orbit（62%）.The specific capacity is up to 153 mAh g^-1 after 1000 cycles.It is revealed that the ε_p of O in δ-MnO₂ cathodes could be a very important descriptor for H⁺intercalation contribution that describes a volcano curve as a function of ε_p,and the best discharge capacity and H⁺intercalation contribution are found for the Cu-MnO₂ cathode among Cu-,Ni-,Co-,Zn-MnO₂ and δ-MnO₂.