Controlled Synthesis Of MnO₂-based Nanocomposites And Their Application In Zinc-ion Batteries

In recent years,the excellent performance of water-based zinc-ion batteries has attracted great attention.Among them,manganese-based materials are one of the main cathodes of zinc-ion battery,due to the advantages of low cost and high energy density.However,its shortcomings can not be ignored in the applications of zinc-ion battery.It is prone to phase change during the charging and discharging process,which leads to the collapse of the mechanism and the low electrical conductivity.Therefore,to improve its stability has become the main method.In this thesis,we prepareα-MnO₂ andδ-MnO₂through a facile effective synthesis method.Additionally,by kinds of modification,the improvements ofα-MnO₂ andδ-MnO₂ as cathodes in ZIBs can be achieved.The experimental details are as follows:（1）Nitrogen-doped carbon（NC）is synthesized by calcination method using urea and glucose,and then needle-shapedα-MnO₂ is loaded on the layered nitrogen-doped carbon by a simple stirring method to prepareα-MnO₂@NC composite material.The morphology characterization results show that nitrogen-doped carbon nanosheets can effectively suppress the volume expansion and contraction ofα-MnO₂ during charging and discharging,and its porous structure facilitates ion diffusion and promotes electron transport,thereby increasing the specific capacity of the battery.The electrochemical test results show that theα-MnO₂@NC composite material has a specific capacity of 120 m Ah g^-1 at a current density of 1 A g^-1,whileα-MnO₂ under the same conditions has a specific capacity of only 50 m Ah g^-1 specific capacity.At the same time,at a low current density of 0.1 A g^-1,theα-MnO₂@NC composite material provides a discharge specific capacity of 195 m Ah g^-1,and the capacity retention rate is 92.26%after 500 cycles,which shows thatα-MnO₂@NC composite material has excellent stability and good long cycle life.（2）In this chapter,theδ-MnO₂/graphene quantum dot（GQD）composite electrode material is prepared by the hydrothermal method,and a series of characterization tests are performed on the material,and finally it is used as the anode of the zinc ion battery.The modification of graphene quantum dots can improve the shortcomings of low conductivity when pureδ-MnO₂ is used as a positive electrode,and at the same time increase the reaction area in the electrochemical process.It is precisely because of these advantages that through electrochemical analysis,it is not difficult to see that under the modification of graphene quantum dots,δ-MnO₂/GQD has a specific capacity of 178 m Ah g^-1 at a current density of 1A g^-1.Theδ-MnO₂ is only 70 m Ah g^-1,which shows that the specific capacity of graphene quantum dots has been greatly improved after adding graphene quantum dots.At the same time,theδ-MnO₂/GQD composite electrode material has a capacitance retention rate of92.23%after 500 cycles.Compared with the case where the capacity of pureδ-MnO₂ begins to decrease at 100 cycles,after the modification of graphene quantum dots The cycle performance of MnO₂/GQD has been greatly improved.（3）In this chapter,theδ-MnO₂/GQD composite material is pre-doped with zinc ions by the hydrothermal method to obtain theδ-MnO₂/GQD composite electrode material（ZMC）pre-doped with zinc ions.The material not only has the advantages of increased reaction area and improved conductivity caused by the modification of graphene quantum dots,but also has the characteristics of shortening the diffusion distance between the ion and the active material after the zinc ion is pre-intercalated,resulting in improving the dynamic performance of the material.By comparing the electrochemical performance of pureδ-MnO₂,δ-MnO₂/GQD composite material andδ-MnO₂ with only zinc ions pre-intercalated,it is not difficult to see that the doping of zinc ions and the modification of graphene quantum dots can improveδ-MnO₂ material performance has a synergistic effect.Electrochemical tests show that its specific capacity is as high as 260 m Ah g^-1after 500cycles at a current density of 3 A g^-1.In summary,compared with the performance of pureδ-MnO₂,ZMC has a significant improvement in both cycle stability and specific capacity.It also indirectly proves that the doping of zinc ions and graphene quantum dots have an effect onδ-MnO₂.The study of chemical properties has a synergistic effect.