Research On Improving The Performance Of Manganese Oxides Storage Magnesium Based On Defect Engineering

Lithium-ion batteries have been widely used as energy storage systems in portable electronic devices and（hybrid）electronic vehicles.However,they still have disadvantages such as high cost,low safety,and lack of resources to limit their further development.In recent years,aqueous magnesium ion capacitors have attracted the interest of researchers due to the ability of multivalent magnesium cations to transfer two electrons,resulting in greater energy density and more excellent cycling stability than monovalent ion batteries.For these multivalent metal ion batteries,however,the large hydrated ionic radius of Mg²⁺and the strong electrostatic interaction with the host material lead to slow reaction kinetics,making reversible intercalation of Mg²⁺in the host material more difficult,and there is no suitable cathode material to achieve the desired energy density.Manganese oxides are considered to be the most suitable cathodes for energy storage devices due to their low cost,high theoretical capacity and multivalent state of Mn.However,poor ionic conductivity and dissolution problems make Mn-based materials unsatisfactory in terms of multiplicity and cycling performance.Therefore,it is important to develop a simple and reproducible strategy to further optimize Mn-based materials for Mg ion storage applications.In recent years,the strategy of using defect-induced modulation of the electronic and surface structures of materials can effectively solve the above existing problems.Therefore,in this thesis,Mn O₂precursors were studied and,firstly,aluminum substitution-dopedα-Mn O₂materials were prepared by a one-step hydrothermal method.The aluminum substitution was investigated by combining XRD,EPR,XPS characterization and density flooding theory calculations（DFT）.The role of potassium ions involved in the synthesis of manganese oxides is utilized and discussed for the first time based on CI-NEB density general function theory calculations and experiments.The presence of K⁺ions promotes the subsequent Al³⁺doping,and the successful introduction of aluminum ions shortens the oxygen distance in a unit cell,bringing about a large number of oxygen defects.after Al doping,the partial substitution of Al³⁺at the Mn⁴⁺site ofα-Mn O₂can modulate its electronic structure,favoring the electrostatic attraction of Mg²⁺cations and lowering the diffusion barrier of Mg²⁺,leading to improved reaction kinetics,adaptive volume expansion,and reduced dissolution of the active material Mn²⁺in the electrode material.As a result,the Al_xMn O_2-zcathode exhibits high capacity（197.02 m Ah/g at0.1 A/g）and stable cycling performance（82%after 2500 cycles）.In the voltage range of 0-1.9 V,the constructed Mg-ion capacitors have significantly high energy density（104.86 Wh/kg）and high-power density（736.71 W/kg）due to the advantage of their structure and the good performance of the cathode material.Next,the“anion-cation double defect”（CADD）Mn₃O₄electrode material was prepared by the reduction effect of Na BH₄and the Jahn-Teller effect of the Mn-based material.First-principles simulations based on density generalization theory（DFT）show that the presence of the double defect substantially reduces the diffusion potential barrier of magnesium ions.The reduction of the diffusion potential barrier results in a faster diffusion rate of magnesium ions in the CADDs-Mn₃O₄material.The non-in situ XRD results reveal the phase transition between spinel and Birnessite structures,and the appearance of the Mn₅O₈intermediate phase in the magnesium ion electrolyte is observed for the first time.As a result,high discharge capacity（320.93m Ah/g）and long cycle stability（2000 cycles,100%capacity retention）can be achieved based on the presence of double ionic defects.This study provides a detailed understanding of the utility of the anion-cation double defect and opens up new avenues for subsequent energy storage materials in design.