B/Cu Doping β-MnO₂ Cathode Materials And Their Zinc Storage Mechanism

In order to meet the huge energy demand of the people in the future,it is necessary to make the necessary efforts to develop a global resource storage strategy in the future.There is an urgent need to find an effective strategy to store intermittent energy such as tidal energy.Aqueous zinc ion batteries are certainly one of the best choices for these purposes.Manganese-based cathode materials have obvious advantages,such as low environmental hazard,easy availability,relatively low cost,and high storage capacity in nature,so they have been the most studied by researchers.However,the commercialization of manganese-based materials has been hindered by the poor recyclability of the materials themselves.In this thesis,β-MnO₂cathode materials are firstly prepared by one-step hydrothermal method to study the failure mechanism ofβ-MnO₂cathode materials;then the performance ofβ-MnO₂cathode materials is improved by（B/Cu）element doping and the role of energy storage mechanism after element doping is studied.The main studies are as follows.1.β-MnO₂cathode materials were prepared by a one-step hydrothermal method.The synthesizedβ-MnO₂cathode material exists in the form of nanorods with a diameter of about100 nm and a length of about 2-3μm.The starting specific capacity of thisβ-MnO₂cathode material is 176.77 mAh/g under constant current charge/discharge at a current density of 1A/g.and the capacity retention is only 20.57%after 1000 turns,and the capacity retention is27.72%after 1500 turns at a current density of 2 A/g.The capacity retention rate was 27.72%after 1500 turns at a current density of 2 A/g.It was found that the pure phaseβ-MnO₂underwent a partially irreversible process of Zn²⁺and H⁺embedding and stripping during the first charge/discharge,and its structure gradually collapsed and its morphology and physical phase changed after a long cycle of charge/discharge,transforming into a new substance containing Zn Mn₂O₄,Zn MnO₃,and Mn OOH,and its morphological structure was reconstructed.2.Doping of metallic Cu elements inβ-MnO₂by one-step hydrothermal method.The morphology and crystal structure ofβ-MnO₂with different doping ratios were compared and analyzed,and the doping of Cu element led to the increase of nanorod diameter.It was found that Cu doping improved the kinetic properties of the electrode reaction,enhanced its reaction rate and electrode conductivity,enhanced the reversible embedding and deembedding process of zinc ions,promoted the electron transport and ion diffusion rate,and also inhibited the dissolution of manganese,thus improving the energy density,power density and cycle life of the battery.Among them,the electrochemical performance of Cu:Mn=1:10 Cu-Mn doping ratio is the best,which greatly enhances the cycle stability and specific capacity ofβ-MnO₂.Under the constant current charge/discharge of current density 1 A/g,the starting current is190.9 mAh/g,and the specific capacity of 190.35 mAh/g is still available after 1000 cycles,and its retention rate is close to 100%.3.In this chapter,non-metallic B elements are doped inβ-MnO₂by one-step hydrothermal method.The doping of B elements makes B ions replace some Mn ions in the MnO₂lattice,reduces the average oxidation state of Mn inβ-MnO₂,generates a large number of active defects,improves the crystal structure of MnO₂,inhibits the dissolution of Mn ions,enhances the reversible embedding and detachment process of hydrogen ions and zinc ions,and improves theβ-MnO₂chemical stability and electrical conductivity.The results show that doping with a ratio of B:Mn=1:20 has the best performance,with a capacity retention of67.36%for 1000 cycles at a current density of 1.0 A/g,compared with only 20.53%for the undoped sample.69.9%for 2000 cycles at 2.0 A/g.In summary,both metallic Cu elements and non-metallic B elements effectively improve the specific capacity and cycling stability of pureβ-MnO₂,extend the cycle life of zinc ion batteries to a certain extent,and provide an experimental basis for the development of high-performance energy storage devices.