Regulation Of Layered Manganese Dioxide Cathode Material And Its Study In Aqueous Zinc-ion Batteries

Aqueous zinc-ion batteries have low redox potential,high theoretical capacity,and high safety,so they have gradually emerged in the arena of electrochemistry.Among its many cathode materials,manganese dioxide has the advantages of low price,abundant crystal forms,multiple variable valence states and large electrochemical voltage window,so it has great advantages.δ-MnO₂ is a two-dimensional layered structure with large interlayer spacing,which can provide a good transport channel for ion migration.However,its electrical conductivity,rate performance and cycle stability are poor.Therefore,it is urgent to improve the rate performance and structural stability ofδ-MnO₂.Here,we adopted the compounding with the highly conductive material MXene and the doping of transition metal Cu²⁺to further improve the rate capability and structural stability ofδ-MnO₂.The details are as follows:（1）Combiningδ-MnO₂ with a single layer of highly conductive MXene improves its structural stability and rate capability.We prepared MXene@δ-MnO₂（Mx@δ-MnO₂）cathode materials for aqueous zinc-ion batteries by combining the exfoliated MXene nanosheets as monolayers withδ-MnO₂ by hydrothermal method.First,MXene has a larger specific surface area,and as a substrate material supports manganese dioxide active material,it provides more active sites,also increases the electrodeposition area,and promotes electrochemical utilization and kinetics.Second,manganese dioxide nanosheets grow vertically on MXene,shortening the ion transport path and accelerating the ion diffusion rate.In addition,the high electrical conductivity of MXene enables Mx@δ-MnO₂ to have good electrical conductivity,which accelerates its electrical conductivity and improves the rate capability and stability.The test shows that after 1000 cycles at 2 A g^-1,the retention rate of Mx@δ-MnO₂ can still reach 88%,while that ofδ-MnO₂ is only 26%.At the same time,Mx@δ-MnO₂ also has better rate performance and cycle reversibility.It is found that Mx@δ-MnO₂ has high ionic conductivity and faster ionic diffusion by EIS and GITT tests.Thermogravimetric analysis confirmed that Mx@δ-MnO₂ has better thermal stability,which is also the performance of its excellent stability.（2）Cu²⁺intercalationδ-MnO₂ is used to improve the structural stability and rate capability of aqueous zinc-ion batteries.First,we chose Cu²⁺intercalationδ-MnO₂ from the perspective of ionic radius,electronegativity and ionic valence.Cu-δ-MnO₂（CMO）cathode material was prepared by intercalating Cu²⁺,a divalent metal with high electronegativity,into the interlayer ofδ-MnO₂ by hydrothermal method.Cu²⁺and[MnO₆]octahedra form Cu-O bonds that exist in the interlayer,supporting the layered structure.Because Cu²⁺and Zn²⁺have similar diameters,the electronegativity of Cu²⁺（1.359）is slightly higher than that of Zn²⁺（1.347）.This means that Cu²⁺has stronger interaction with the MnO₂ lattice and can remain stable during intercalation/extraction cycles of Zn²⁺and H⁺.Therefore,the Cu-O bond can act as a stable"pillar of structure",which greatly improves the stability of the CMO.After 600 cycles at 2 A g^-1,the capacity of CMO remains almost unchanged,while the capacity retention ofδ-MnO₂ is only 23%,showing the excellent cycling stability of CMO.The CMO also exhibits remarkable rate capability when the current density increases from 0.2 A g^-1 to 2.0 A g^-1,with a capacity retention of 71%,which is much higher than that ofδ-MnO₂ of 32%.Meanwhile,we found that the doping of Cu²⁺also increased the oxygen vacancy content of the CMO,thereby enhancing its electronic conductivity.Through the tests of GITT and EIS,we found that the ionic conductivity of CMO is higher.Therefore,the charge transfer resistance of H⁺and Zn²⁺at the electrode/electrolyte interface is reduced,and the rate capability of the CMO cathode material is improved.Finally,we use ex-situ XRD and SEM characterization to study and discuss the energy storage mechanism of CMO//Zn batteries is the co-insertion/extraction process of H⁺and Zn²⁺.