Preparation Of Graphene-based Cathode Materials And Their Application In Aqueous Zinc Ion Batteries

With the dramatic increase in the consumption of non-renewable energy,energy crisis and environmental pollution are the two major problems facing the society,the development and utilization of new energy and the concept of“Green mountains and clear water are equal to mountains of gold and silver”have pointed out the direction to optimize the energy structure and promote energy saving and emission reduction.Lithium ion batteries（LIBs）have the advantages of high energy density,discharge capacity,and cycling stability,making them the mainstream devices in energy storage.However,due to the shortcomings of high cost and flammability,it has hindered the further development of lithium ion batteries.Aqueous zinc ion battery（ZIBs）benefits from low cost,safe operation,environmental protection and other advantages to become one of the suitable replacement of lithium ion battery.As an important part of ZIBs,the capacity of the positive electrode is low,unable to match the high capacity of the metal zinc negative electrode is one of the serious problems restricting the performance of ZIBs.Manganese based oxides and vanadium based oxides as positive electrode materials of ZIBs generally have the disadvantage of poor electrical conductivity,while reduced graphene oxide（r GO）has the advantages of high electrical conductivity and high specific surface area,so it can be a candidate for composite with metal oxides,metal-organic frames and other materials.Therefore,this paper attempts to prepare suitable graphene-based composites in order to improve the electrochemical performance of ZIBs.Therefore,in this paper,the following research work is done:（1）Flexible self-supporting composite films of manganese oxide and graphene（H₂-Mn₃O₄/r GO）were prepared by hydrogen reduction method at high temperature,and used as positive electrode material for aqueous zinc ion battery.By adding r GO,a three-dimensional conductive network with abundant pores is formed,which improves the conductivity of traditional manganese oxide cathode materials and promotes the electron transmission rate in the charging and discharging process.Meanwhile,the existence of graphene can effectively slow the structural collapse of manganese oxide in the charging and discharging process,and improve the cycle stability performance.Based on this unique structure,H₂-Mn₃O₄/r GO electrode achieves good electrochemical performance:The H₂-Mn₃O₄/rGO electrode exhibited higher discharge capacity(up to 338.1 m Ahg^-1 at 0.1 Ag^-1 and good rate performance（current density increased by 10 times and capacity retention rate was 52.3%）compared to the electrode without graphene.And cycle stability(90%of initial capacity is maintained after 2000 cycles at 4 A g^-1 current density).In addition,the ZIBs based on the H₂-Mn₃O₄/r GO flexible electrode can successfully light the LED indicator,and the LED brightness does not decrease in the bending test,showing good flexibility.（2）In order to further explore the electrochemical properties of ZIBs cathode material.A composite material（MIL-88B（V）@r GO）was synthesized by a simple one-step hydrothermal method with V-MOF nanorods（MIL-88B（V））uniformly anchored to the surface of r GO.The presence of graphene restricts the degree of freedom of MOF growth,resulting in the formation of smaller MOF structures,which help to improve the interface between electrode and electrolyte,and improve the interface charge transport of composite electrodes.In electrochemical research,we found that V-MOF transforms into amorphous V₂O₅ electrochemically in situ during the first cycle charging and discharging process,which can be considered as the true redox center in the subsequent charging and discharging process.Thanks to this unique multistage composite structure and amorphous V₂O₅characteristics,the electrode material exhibits excellent electrochemical performance:zinc storage capacity up to 479.6 m Ah g^-1 at 50 m A g^-1 current density;The current density increases by 100 times,and the capacity retention rate is 54.9%,which has excellent magnification performance;After 400 cycles of 2 A g^-1,the initial capacity of 80.3%can still be maintained,and the cycle stability is relatively ideal.The charge and discharge mechanism of the electrode material was further studied by various non-in-situ characterization methods,and the co-embedding behavior of Zn²⁺and H₂O in the electrode material was found,as well as the reversible deposition and dissolution process of Zn₃V₃O₈.This work provides experimental and theoretical basis for preparing and broadening the application of graphene and MOFs materials in ZIBs.