Preparation And Lithium Storage Properties Of Molybdenum Dioxide/Carbon Composites

With the rapid development of economy,people’s demand for portable electronic devices,smart phones and pure electric vehicles is increasing day by day.It is urgent to develop a lithium-ion battery with high energy density,long cycle life and stable performance to meet people’s daily needs.At present,most commercial lithium-ion batteries use silicon-based oxides and carbon-based materials as negative electrodes,but there is still much room for improvement in reversible specific capacity,cycle and rate performance.It is very important to develop new anode substitute materials with high energy density,power density and capacity retention.MoO₂ has a broad application prospect in the field of energy storage because of its high theoretical specific capacity,stable electrochemical activity and rich natural reserves.At present,MoO₂ is mainly used in the field of supercapacitors,but it is restricted in lithium-ion batteries because of its poor conductivity,serious volume expansion during charging and discharging and few reactive sites.In view of the above-mentioned problems that need to be solved urgently,this thesis introduces various forms of highly conductive carbon materials to compound them,studies the lithium storage performance and analyzes the modification mechanism.The main research content is divided into the following three parts:（1）Using cheap glucose as carbon source,MoO₂@C composites were successfully prepared by carbon coating modification of MoO₂ by hydrothermal method combined with calcination process.When the hydrothermal temperature is adjusted to 190℃and the glucose concentration is 0.4 mol L^-1,the electrochemical performance of the prepared material is the best.The first coulombic efficiency reaches65.4%at 0.1 C current density,and the specific capacity can be maintained at 350.8m Ah g^-1 after 100 cycles,and at 1.0 C high current density,the specific capacity can be maintained at 226.0 m Ah g^-1.The carbon nano-particle layer is evenly coated on the surface of MoO₂ sheet,which acts as a good conductive medium to promote charge transfer,and can alleviate the mechanical stress caused by the volume effect of MoO₂material during charging and discharging,and maintain the stability of the microstructure of the material.Compared with pure MoO₂,it shows better cycle stability and rate performance.（2）In order to further improve the electrochemical performance of MoO₂@C composites,carbon nanoparticles were doped with nitrogen by using urea as nitrogen source based on hydrothermal method combined with calcination process,and MoO₂@NC composites were successfully prepared.When the urea concentration is0.08 mol L^-1 and the glucose concentration is 0.4 mol L^-1,the lithium storage performance of the material is obviously improved.At the current density of 0.1 C,the first coulombic efficiency is 77.2%,and the specific capacity can be maintained at527.6 m Ah g^-1 after 100 cycles,and at the high current density of 1.0 C,the specific capacity can be maintained at 336.8 m Ah g^-1.By adding a proper amount of nitrogen into carbon nanoparticles,a micro-porous structure can be formed,which makes the MoO₂@NC composite contact with electrolyte more fully,and shortens the Li⁺diffusion and electron transfer path.With the modification of carbon nanoparticles by nitrogen,the specific surface area of the composite material is increased,more reaction sites are provided,the contact with electrolyte is increased,and the electrochemical reaction activity is enhanced.At the same time,the micro-porous structure plays a role of skeleton support,alleviating the mechanical stress caused by volume effect and preventing the aggregation of MoO₂.（3）On the basis of MoO₂@NC composite,graphene with excellent physical and chemical properties was further selected to modify its surface.Firstly,reduced graphene oxide（r GO）with oxygen-containing functional groups and many defects was prepared by Hummers method,and then it was compounded with MoO₂@NC through hydrothermal process,and the MoO₂@NC/r GO composite was successfully prepared.When the dosage of r GO is 12 mg,the lithium storage performance is the best.At the current density of 0.1 C,the first coulombic efficiency reaches 79.6%.After 100 cycles,the reversible specific capacity is 572.4 m Ah g^-1,and the capacity retention rate is64.5%.At the high current density of 1.0 C,the specific capacity can be maintained at364.9 m Ah g^-1.r GO is uniformly loaded on the surface of MoO₂ coated with nitrogen-doped carbon nanospheres,which provides buffer space for the charging and discharging volume expansion of MoO₂ with its own lamellar gap,and provides low-resistance channels for lithium ions and charge transfer.The introduction of more defects and oxygen-containing functional groups can increase the active sites of the composites,and the contribution rate of pseudocapacitance is improved,thus improving the power density of the materials.By compounding various carbon components with appropriate contents and forms,the cycle and rate performance of MoO₂ anode materials are significantly improved by effective synergy.