Preparation And Electrochemical Properties Of Transition Metal Ion-doped Manganese Oxides

Aqueous zinc ion batteries?AZIBs?have been developed and researched gradually because of the advantages of high safety,low cost and non-pollution to the environment,which are expected to become a new generation of safe and reliable green energy storage device.Manganese oxide is often considered as the most promising cathode material for AZIBs because of its characteristics of medium open circuit voltage?OCV?,high discharge capacity and abundant resources.However,due to its poor electrochemical activity and cycling stability,manganese oxide is limited by further development and application.In this dissertation,the electrochemical performance of manganese oxide in AZIBs has been studied and discussed to increase the electrochemical active sites of manganese oxide.The strategy of the experimental scheme mainly adopts from inside to outside,and the approaches includes two aspects,doping engineering and surface engineering.In addition,the influence of structural stability and electrical conductivity on materials is the focus of this thesis as well.Firstly,the electrochemical activity and cyclic stability of Cu-Mn₃O₄/PVP composites were investigated.Cu-Mn₃O₄/PVP composites with high electrochemical activity were prepared by chemical precipitation.The defect structure was introduced by copper ion doping to improve metal ion adsorption capacity and ionic conductivity.One of the reasons for the poor stability of the positive electrode cycle of manganese tetroxide is that the volume changes easily during charging and discharging.Adding polyvinylpyrrolidone with good affinity into the system,on the one hand,the volume expansion of manganous tetroxide can be inhibited,and the dissolution of manganous can be alleviated.On the other hand,it can improve the dispersion of materials,increase the specific surface area,and produce a large number of active sites.The Cu-Mn₃O₄/PVP composite shows higher electrochemical activity and cycle stability than those of pure Mn₃O₄ depending on the behavior of charge-discharge with constant current.Secondly,the cyclic stability of Fe/?-Mn O₂@PPy composites was studied.We successfully prepared Fe/?-Mn O₂@PPy composites with mesoporous structure.The doping of iron ions is beneficial to increase the layer spacing of manganese dioxide and widen the ion migration channel.On the one hand,the surface coating of polypyrrole can alleviate the corrosion of electrolyte to the active material and reduce the collapse of manganese dioxide.On the other hand,it can be used as a conductive network of materials to accelerate the electron migration and effectively improve the conductivity of composites.The constant current charge-discharge test showed that Fe/?-Mn O₂@PPy composites had a cyclic capacity of nearly 240 m Ah g^-1 at the current density of 100 m A g^-1,and the capacity retention rate reached 96.6%after 100 cycles.In addition,the apparent diffusion coefficient calculation shows that Fe/?-Mn O₂@PPy composites has good diffusion kinetics.Thirdly,the study is focused on the initial cycle capacity of Zn-rich Mn₃O₄/GO composites.The Zn-rich Mn₃O₄/GO composites was prepared by chemical precipitation from the perspective of zinc ion pre-intercalated.The pre-intercalated of zinc ions effectively shortens the transport path between ions and active substances.The addition of graphene oxide improves the material's electrical conductivity and facilitates the rapid migration of electrons.In addition,GO contains a large number of oxygen-containing functional groups,which can react with polyethylenimine in a nucleophilic manner to form a PEI-GO network.The network acts as an elastic protective layer of active substances to inhibit large volume expansion.The Zn-rich Mn₃O₄/GO composites can reach the initial cycle capacity of 170 m Ah g^-1 at the current density of 200 m A g^-1,and it only needs about 10 cycles of activation to reach the stable charge and discharge capacity,which indicates that the composites have excellent initial cycle performance.Finally,we summarized the kinetic properties of Cu-Mn₃O₄/PVP,Zn-rich Mn₃O₄/GO and MCM4@Mn₃O₄ composites.The results show that Zn-rich Mn₃O₄/GO composite has excellent diffusion kinetics,which is mainly due to the fact that the pre-embedded zinc ions in the composite effectively shorten the diffusion channel between ions and active substances,thus facilitating the rapid migration of zinc ions.This provides a reference for improving the dynamic performance of manganese oxides.In summary,the transition metal ion doped manganese oxide composites have the advantages of low cost,simple preparation and non-toxic and non-pollution.It has good research value in improving ion conductivity,increasing reactive sites,especially increasing ion diffusion rate.