Electrochemical Properties Of Manganese Oxides As Electrode Materials For Aqueous Multivalent Metal-ion Batteries

Aqueous rechargeable multivalent metal-ion batteries（ARMMIBs）,as a powerful alternative battery system to Li-ion batteries,which has the advantages of low cost,high safety,abundant raw materials,environmental friendliness,and multiple electron transfer.There have promising applications in wearable devices,large-scale energy storage systems and electric vehicles.Compared with lithium,multivalent metals（Mg,Zn,etc.）have high natural abundance,compatibility with clean and non-flammable aqueous electrolytes,high safety,and multi-electron redox capabilities.Manganese-based materials are of great interest because of their low cost,non-toxicity and high voltage.Among them,manganese oxides possess abundant valence states and crystal structures,and exhibit different electrochemical properties when used as electrode materials for ARMMIBs.However,the inherent poor electrical conductivity and structural instability of manganese oxides affect their cyclic performance,and conductive materials are introduced to improve their electrochemical performance in order to ameliorate this problem.The main content of this thesis are:（1）Different crystal types of MnO₂ materials（α,β,γ,δ）were prepared by hydrothermal method,and the successful preparation of MnO₂ was confirmed by XRD and FTIR test methods.It was further applied in aqueous magnesium ion batteries（AMIBs）.The results of CV and charge-discharge tests showed thatδ-MnO₂ had better electrochemical performance than other crystal forms,and the current of 100 mA g^-1 the first discharge capacity at the density can reach 231 mAh g^-1.By calculating the apparent diffusion coefficient D_Mg,the theoretical conclusion is consistent with the electrochemical test.It is fully proved that the crystal structure of the material determines the electrochemical performance of the material.Theδ-MnO₂ with a larger interlayer spacing is superior to theα-,β-,andγ-MnO₂ materials with a tunnel structure,indicating that the relatively open structure is beneficial to the insertion or extraction of ions.（2）The MnOOH precursor was prepared by the hydrothermal method,and the layered Mn₅O₈ material was obtained by high-temperature calcination,which was used in AMIBs for the first time.The three-electrode system was used to explore the optimal system composition in different electrolytes（MgSO₄,Mg（NO₃）₂,Mg（CH₃COO）₂）and different concentrations(0.5,1,3 mol L^-1).The Mn₅O₈ electrode in 1 mol L^-1 MgSO₄electrolyte can exhibit better discharge capacity(204.16 mAh g^-1)and capacity retention（50%）by electrochemical tests and ion diffusion coefficient D_Mg calculation.The mechanism of Mg²⁺in the charging and discharging process was further explored by XPS and EDS tests.（3）Using the typical manganese oxide Mn₂O₃ as a comparison term,it was hydrothermally composited with nitrogen-doped rGO.Applied in aqueous zinc-ion batteries（AZIBs）.Through physical characterization methods such as Raman and XPS,the successful preparation of the composite material is proved.In the electrochemical performance test,the discharge capacity of Mn₂O₃/N-rGO is 222.31 mAh g^-1 after 100cycles,which is significantly higher than that of Mn₂O₃(118.1 mAh g^-1).The co-intercalation mechanism of H⁺and Zn²⁺was explored by XPS,XRD and other tests.Based on the outstanding performance of Mn₅O₈ in AMIBs,rGO was compounded with it to explore the electrochemical performance of Mn₅O₈ in AZIBs.The highest discharge capacity of Mn₅O₈/rGO electrode can reach 266.2 mAh g^-1,which is higher than that of the Mn₅O₈ electrode(140.9 mAh g^-1).The above two experiments demonstrate that rGO has a large positive effect on the capacity and cycling stability of manganese oxides.