Construction And Performance Of Manganese-based Cathode Materials For Aqueous Zinc-ion Battery

The rapid development of modern industry has led to the gradual depletion of fossil energy and the gradual deterioration of the environment,so the search for clean renewable energy is imminent.Lithium-ion batteries have been widely used in portable electronic devices due to their high energy/power density.However,their poor safety and high cost limit their use in large-scale energy storage devices such as electric vehicles and energy storage power stations.Rechargeable Water System Zinc-ion batteries have shown low cost and high safety due to the use of non-flammable aqueous electrolytes,making them an ideal replacement for lithium-ion batteries.Metal zinc as a negative electrode of zinc-based battery has a high theoretical specific capacity(819 mAh g^-1and a low Zn²⁺/Zn redox potential（-0.76 V relative to a standard hydrogen electrode）,so a rechargeable water system using metal zinc as a negative electrode Zinc-ion batteries show high specific energy with low cost and good safety.In this thesis,MnO₂nanosheet array,porous MnO₂ film,Mn₂O₃ and MnCO₃ cubes were constructed by anode/cathode electrodeposition and hydrothermal method respectively.The electrochemical performance and energy storage mechanism of the positive electrode of zinc ion battery were investigated.（1）MnO₂ was prepared by anodic electrodeposition method and the electrodeposition temperature was adjusted to regulate the morphology of MnO₂.XRD showed that the samples prepared at different deposition temperatures were all MnO₂ arrays.SEM shows that as the deposition temperature increases,MnO₂ is first converted into nanofibers by nanoparticles,and finally becomes nanosheets.Electrochemical tests show that 10°C-MnO₂ has a retention of1200 times capacity at a current density of 1.8 A g^-1 close to 100%,which is the best performance compared to MnO₂ prepared at other temperatures;by comparing the EIS spectrum,10°C-MnO₂ has the fastest ion diffusion,and the internal resistance gradually decreases as the reaction progresses.（2）MnO₂ was prepared by cathodic electrodeposition method,and the deposition current and heat treatment temperature were adjusted to investigate its effect on electrochemical performance.XRD showed that the diffraction peak of h-MnO₂ was more obvious,indicating that the crystallinity of the sample after heat treatment was enhanced.The SEM and TEM showed that the prepared sample was in a film state,and the surface of the h-MnO₂ film became rougher and micropores were present,which increased the area of the electrolyte and the electrode material.Electrochemical tests show that when the deposition current is 3 mA,the prepared MnO₂ is cycled 1000 times at a current density of 1.8 A g^-1 without attenuating the capacity.The MnO₂ film prepared under this deposition condition has the best performance;The MnO₂ prepared under the conditions was heat-treated,and h-MnO₂ was circulated to a capacity of 94 mAh g^-1 at a current density of 1.8 A g^-1;when it was cycled to 3000 times at a high current density of 6.0 A g^-1,The MnO₂ capacity is 32.2 mAh g^-1,while the h-MnO₂ is as high as 45.3 mAh g^-1.（3）Preparation of micron-sized Mn₂O₃ cubes by hydrothermal method.Firstly,MnCO₃precursors were prepared.Then,the structurally stable Mn₂O₃ cubes were prepared by adjusting the heat treatment temperature.Finally,the surface was coated with C to investigate the coating modification.The impact of performance.XRD showed that the sample was a mixture of MnCO₃ and Mn₂O₃ after heat treatment at 450℃,and pure Mn₂O₃ above 480°C.The properties of the sample after coating with carbon did not change.It was preliminarily indicated that the carbon layer was amorphous.The SEM shows that the Mn₂O₃ heat-treated at 550°C is porous and firm,so it is coated with carbon.After coating with carbon,the pores become larger and the surface is smoother.The EDS characterization results show that there is no carbon before coating,and there is carbon after coating.Appearance.Electrochemical performance shows that 550℃-Mn₂O₃@C has better cycle performance and capacity retention,cycle 250times at a current density of 0.3 A g^-1,its capacity up to 250 mAh g^-1.The cycle was cycled2000 times at 1.8 A g^-1 current density,and the capacity was still maintained at 125 mAh g^-1,and the capacity retention rate was close to 100%.（4）The micron-sized MnCO₃ cube was prepared by hydrothermal method and introduced into a rechargeable water system zinc ion battery.When used as a positive electrode material for a water system zinc ion battery,the MnCO₃ cube has excellent cycle stability.After cycling5000 times at a current density of 1.8 A g^-1,the capacity retention was about 79.1%based on the maximum value(97.6 mAh g^-1.In addition,the structure and morphology evolution of the structure and its effects on the electrochemical cycle were investigated.At the end of charging,MnCO₃ partially becomes MnO₂,while the nanosheets and smaller sub-nanometers continuously appear in the MnCO₃ cube during the cycle,which is advantageous for reducing the ion transfer distance.Due to the simple synthesis strategy and superior cycleability,this study can provide new possibilities for large-scale application of zinc ion batteries in large energy storage devices.