Preparation And Performance Of Mn-based Nanostructured Cathode Materials For Aqueous Zinc Ion Batteries

Because Zn has many outstanding characteristics such as low oxidation reduction potential,high safety,low cost,abundant reserves,and environmental friendliness,aqueous zinc ion batteries?AZIBs?have great advantages of safety and low cost in large-scale energy storage applications.However,compared with Li⁺ion intercalation chemistry,due to the strong electrostatic interaction between the divalent Zn²⁺ions and the lattice of the cathode material,the cathode material tends to exhibit a slow Zn²⁺diffusion rate and poor reversibility.Therefore,the biggest challenge for the development of AZIBs is to design and prepare cathode materials with high Zn²⁺storage performance.Manganese-based oxides are considered to be one of the most promising cathode materials for AZIBs because of their large theoretical capacity and high operating voltage.Among them,Mn O₂has attracted wide attention because of its tunnel or layered structure,which is conducive to the reversible insertion/extraction of Zn²⁺ions.However,the disadvantages of unstable structure and poor conductivity lead to poor electrochemical performance of Mn O₂in AZIBs.Therefore,this paper has carried out a modification study on the defects of Mn O₂nanowire cathode materials,hoping to further improve its electrochemical performance through micro-nano structure design,polypyrrole coating,carbon combination,doping and other methods.The main research results are as follows:?1?A facile and scalable method to fabricate a free-standing and flexible film composed of carbon nanotubes?CNTs?and polypyrrole?PPy?coated Mn O₂nanowires?NWs?.The free-standing flexible film exhibits hierarchically porous 3D architecture composed of PPy coated Mn O₂NWs and CNTs networks.Such unique architecture not only improves the reaction kinetics and structural stability of the electrodes during repeated cycling,but also provides abundant active sites for Zn²⁺and H⁺adsorption,giving rise to additional pseudocapacitive charge storage.In addition,the freestanding and binder-free design with hierarchical porous structure facilitates transport of ions and electrons,enabling efficient utilization of the active materials for performance enhancement of the electrode.Therefore,when the material is directly used as a cathode of AZIBs,it has excellent rate performance(It has a specific capacity of 222.7 m A h g^-1at a current density of 0.3 A g^-1,and still has specific capacity of 84.2 m A h g^-1at a large current density of 2.0 A g^-1),and stable cycling performance(The capacity retention ratio is 87.4%at 1.0 A g^-1current density after 1000 cycles).?2?Se ions doped Mn O₂nanowires with oxygen vacancies were synthesized by hydrothermal method and vacuum suction filtration.In this nanowire,Se ions are doped in the Mn O₂nanowire,which changes the chemical properties of the nanowire.Se ions doped Mn O₂nanowires not only effectively increase the diffusion rate of ions,but also accelerate the migration rate of electrons.At the same time,the Se ions combined with the manganese ions on the surface of the nanowire not only play a coating role,but also inhibit the occurrence of disproportionation reaction,thereby alleviating the dissolution of manganese in the Mn O₂nanowire.When the material is used as a cathode of AZIBs,it has excellent rate performance(It can achieve a high reversible capacity of 284.9 m A h g^-1at a current density of 0.1 A g^-1,and can also Maintain a high specific capacity of 81.0 m A h g^-1when the current density increases to 5.0 A g^-1),and stable cycle performance(The capacity retention ratio is 96.7%at1.0 A g^-1current density after 500 cycles).