Study On The Construction And Electrochemical Properties Of Molybdenum Dioxide And Carbon Composite Microspheres

As a new energy storage device,lithium ion batteries?LIBs?in the new generation are currently attracting extensive attentions by the researchers due to the advantages of high energy and output voltage.And as the main part of lithium storage,the active materials play an important role to improve the properties of LIBs.Molybdenum dioxide?MoO₂?has been regarded as one of the ideal anode materials because of its high theoretical capacity and good chemical stability.However,bulk MoO₂ has a serious capacity decay during the charging/discharging process,which restricts their application in energy storage.Therefore,in order to develop high performance anode materials,this thesis studies the synthesis of MoO₂@C microspheres electrode by the template method and co-polymerization method respectively,explored the effect of the unique three dimensional?3D?microspheres on the electrochemical lithium storage properties of MoO₂ electrode.The main research contents of this thesis could be seen as follows:MoO₂@PS microspheres were successfully synthesized via a hydrothermal method with the template of PS microspheres,the precursors of ammonium molybdate and reducing agent of ethanol,then composite them with dopamine?DA?solution under the condition of room temperature.Finally,the precipitates are annealed to preparing internal and external carbon coating MoO₂@C sandwich microspheres.On the one hand the design of the composite microspheres with internal and outernal carbon coating can alleviate the agglomeration of MoO₂ nanoparticles,on the other hand the introduction of carbon materials can reduce the charge transfer resistance of MoO₂material.As a result,the MoO₂@C sandwich microspheres electrode shows excellent electrochemical performances:the electrode retains reversible capacity of 717 mAh g^-1?about 80%of the original high capacity?after 100 cycles at a current density of 0.1C;it is worth mentioning that the coulombic efficiencies are more than 98%during the charging/discharging process,suggesting high reversible capacity and outstanding cycling stability of MoO₂@C sandwich microspheres.MoO₂ nanocrystallines?NCs?were prepared by a hydrothermal method with the precursors of ammonium molybdate and the reducing agent of ethanol,then combined them with the DA solution under continuously stirring in room temperature,in which MoO₂ NCs induce the dopamine?DA?monomer to form the polydopamine?PDA?nanosheets,and the composite nanosheets are self-assembling the MoO₂@C 3D flower-like microspheres?FLMs?in the process of further annealing.It can be concluded that MoO₂ NCs with the size of 5 nm are uniformly loaded on the interpenetrated carbon nanosheets,which are connected to form the 3D network.More importantly,this special3D flower-like network framework can not only improve electronic conductivity of composites,but also increase the contact area between active materials and electrolyte,which can short pathway for lithium ion diffusion.When compared with traditional MoO2 NCs electrode,the MoO2@C FLMs electrode exhibits excellent electrochemical performances:The MoO₂@C FLMs electrode retains reversible capacity of 983 mAh g^-1 after 200 cycles at a current density of 0.1 C,which indicates the high reversible capacity and cycling stability of the composite electrode;And it delivers the discharge capacities of 890,826,787 and 606 mAh g^-1 at the different current densities of 0.1,0.2,0.5 and 1.0 mA cm^-1,when the current density backs to 0.1 mA cm^-1,the capacity still retains to 816 mA h g^-1,indicating the optimal rate capability of MoO₂@C 3D FLMs,and providing a basis study for the study of anode materials for lithium-ion batteries.