Preparation Of Carbon-Based Substrate With Manganese Dioxide Cathode Material And Its Performance In Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries（AZIBs）have attracted increasing attention because of their low cost,excellent intrinsic safety and good electrochemical performance.However,some challenges still hinder the development of AZIBs,such as low operating voltage,solubility of the cathode material,and dendrites in the zinc cathode.Among the many cathode materials,manganese oxide can usually provide relatively high discharge voltages and more desirable discharge capacities due to the multivalent state of manganese.In addition,the various crystal structures of manganese oxides can achieve different degrees of cycling performance.abundant reserves and various structures.Among them,layeredδ-MnO₂can provide abundant active sites as well as ion diffusion channels,but the structural collapse during cycling and low electrical conductivity greatly limit its practical application.In this thesis,Ni-MOF-derived hollow microspheres,three-dimensional porous nitrogen-doped carbon derived from salt templates and hollow porous carbon fibers derived from natural biomass lucerne flesh were used as substrates for the growth ofδ-MnO₂,thus modifying the electrical conductivity and ion diffusion rate ofδ-MnO₂to achieve improved rate performance,cycling performance,energy density and power density,etc.electrochemical properties,and the main contents are as follows:（1）Honeycomb-likeδ-MnO₂/Ni-MOF（NBM）integrated cathode for advanced AZIBs.It is made ofδ-MnO₂nanosheets grown in situ on hollow Ni@C microspheres.Among them,the structure of hollow microspheres is formed by pyrolyzing Ni-MOF to produce graphene carbon layers wrapped with nickel nanoparticles.The enhanced NBM electrode material provides an ultra-high specific capacity of 352 m Ah g^-1at 0.1 A g^-1.After 2000 cycles at 2 A g^-1,the NBM electrode exhibits excellent cycling stability with 81%capacity retention and99.4%high average coulombic efficiency.Experimental and first-principles calculations demonstrate that the excellent electrochemical properties of the NBM electrode can be attributed to the high specific surface area,enhanced electrical conductivity,suitable adsorption and low diffusion potential of the honeycomb framework.In addition,the hollow microsphere structure promotes rapid ion/electron diffusion and alleviates the structural collapse ofδ-MnO₂.（2）Few-layerδ-MnO₂nanosheets grown on three-dimensional N-doped hierarchically porous carbon networks for long-life AZIBs.In situ growth of few-layerδ-MnO₂nanosheets on three-dimensional N-doped hierarchically porous carbon networks（NCM）by using the green salt template method and simple hydrothermal process.The synthesized NCM possess the characteristics,such as excellent electric conductivity and abundant active sites,which can enhance the electron/ion transfer kinetics and buffer the volume changes ofδ-MnO₂nanosheets.Owing to the structural properties,the specific capacity can deliver 335 m Ah g^-1at a current density of 0.1 A g^-1and exhibits superior cycle stability(92.5%capacity retention after 1000 cycles at 1 A g^-1and 88%capacity retention after 5000 cycles at 2 A g^-1).First-principles calculations show that the introduction of NC improves the number of electrons near the Fermi level ofδ-MnO₂and reduces the diffusion barriers of Zn²⁺ions.（3）Biomass-derived nitrogen-doped carbon fiber drivenδ-MnO₂for AZIBs with high energy and power density.Theδ-MnO₂nanorods are grown in situ on the surface of hollow carbon fibers derived from the loofah sponge（HCM）.Multiple types of nitrogen-doped carbon combined withδ-MnO₂improve the intrinsic conductivity of HCM composites.Moreover,the HCF with a hollow porous structure can shorten the charge transfer path and accelerate ion diffusion between the interfaces,thereby improving the overall reaction kinetics.Notably,the abundance of crystal water inδ-MnO₂nanorods can effectively mitigate the electrostatic interaction between Zn²⁺ions and the crystal structure ofδ-MnO₂.Electrochemical performance tests show that HCM electrode has excellent electrochemical performance,with ultra-high specific capacity(354 m Ah g^-1at 0.1 A g^-1),long cycling stability(100%capacity retention after 1000 cycles at 1 A g^-1,87%capacity retention after3500 cycles at 2 A g^-1),and extremely high energy and power density(557 Wh kg^-1and 3203W kg^-1)based on the mass of cathode.In this thesis,three structural carbon-based substrates were designed by Ni-MOF,salt templates and natural biomass loofah,andδ-MnO₂with different morphologies（nanosheets,few-layer nanosheets and nanorods）were grown in situ on their surfaces.This composite not only improves the conductivity and reaction kinetics ofδ-MnO₂,but also enhances the specific capacity,rate performance,high current density of cycling performance,energy density and power density ofδ-MnO₂.In addition,the results of first-principles calculations exhibit that the composites have appropriate ion-host interactions and low diffusion energy barriers.Therefore,our design idea is feasible and has some practical prospects for application in commerce.