Study On The Synthesis And Electrochemical Performance Of Nitrogen-Doped Reduced Graphene Oxide And Their Composites Materials

Lithium-ion battery is considered as the best secondary battery because of its high energy density,long service life,safety and other advantages.Lithium-ion battery plays an important role in the field of energy storage.The electrode material is the most important part of the lithium-ion battery,and the synthetises of high performance electrode materials are the main research work of the lithium-ion battery.Graphene has high conductivity and large specific surface area,transition metal oxides have high specific capacity.There has been a lot of researchs on them as the anode material of lithium-ion batteries.But graphene and transition metal oxides have very obvious shortcomings when they are used as anode materials for lithium-ion batteries respectively.The combination of different materials by using physical or chemical methods is a common scientific research methods.The composite materials can obtain their advantages,overcome their shortcomings,and play a complementary role,thereby improving the electrochamical performance of the material.This study used urea and ammonia as the nitrogen source respectively for preparing nitrogen-doped graphene?NG?and nitrogen-doped graphene/transition metal compound composite electrode materials.X-ray diffraction?XRD?,Raman spectroscopy?Raman?,transmission electron microscopy?TEM?,scanning electron microscopy?SEM?and other characterization methods were used to analyze the the structure and morphology of the materials.These products were used as anode materials for lithium-ion batteries,and their electrochemical properties were tested by cyclic voltammetry,galvanostatic charge/ discharge and electrochemical impedance spectroscopy?EIS?.The morphology and properties of nitrogen-doped materials were analyzed,and the electrochemical performances of the products were compared.Graphene oxide?GO?was prepared by a modified Hummers' method,and then NG1 and NG2 were prepared by hydrothermal method using urea and ammonia as the nitrogen source respectively.By means of Raman spectroscopy and TEM,it was found that the prepared NG1 and NG2 were in the shape of gauze and had obvious folds.Because of the doping of nitrogen,the confusion degree of graphene becomes higher,the defects are more,the lithium-ion storage capacity of the material is enhanced,the capacity loss is reduced,and the cycle performance is better.The coulombic effciency of them in the recycling process are nearly 100%,about 75% of the specific capacity is maintained.In contrast,the confusion degree of NG2 is higher,and its electrochemical performance is better than NG1,which indicates that the ammonia is a better nitrogen source for preparing nitrogen-doped graphene.Fe₂O₃/NG1 and Fe₂O₃/NG2 were prepared by hydrothermal method using urea and ammonia as the nitrogen source respectively.The formation mechanism of composite material was explored.Under hydrothermal conditions,the reduction and nitrogen doping of graphene oxide and the formation of Fe₂O₃ were achieved simultaneously.The introduction of N can provide more active sites,also limit the growth of Fe₂O₃.It was found that average particle size of Fe₂O₃ nanoparticles in the Fe₂O₃/NG nanocomposite is about 50 nm and Fe₂O₃ nanoparticles are dispersed at the surface,between the lamellae and the edge of the NG.Fe₂O₃/NG1 and Fe₂O₃/NG2 showed a high discharge capacity of 1110.7 and 1297.2 m Ah/g respectively in the first cycle process.After 50 cycles the discharge capacity still remianed 707.4 and 766.8 m Ah/g respectively.The size of Fe₂O₃/NG2 nanoparticles is smaller,the dispersion is better,the confusion degree is higher,and the electrochemical performance is better.It proved that the ammonia's reduction effect is better.Mn₃O₄/NG1 was prepared by one step hydrothermal method.The Mn₃O₄/NG2 was prepared by two step method.Firstly,NG2 was prepared by hydrothermal method using ammonia as the nitrogen source,then Mn₃O₄/NG2 was prepared by stirring and ultrasonic method at room temperature.The average particle size of Mn₃O₄ in Mn₃O₄/NG1 and Mn₃O₄/NG2 is about 90 nm and 50 nm respectively.Mn₃O₄/NG1 and Mn₃O₄/NG2 showed a high discharge capacity of 1204 and 1279.5 m Ah/g respectively in the first cycle process which is much higher than bare Mn₃O₄.After 50 cycles,the electrochemical performance of Mn₃O₄/NG2 was slightly higher than that of Mn₃O₄/NG1,and both of them are higher than bare Mn₃O₄.The results showed that cycle performance of the nitrogen-doped composite materials was improved,and the two step method is better because of its smaller size,less accumulation,more active sites,and higher electrochemical activity.Fe₃O₄/NG was prepared by two step method.Firstly,NG2 was prepared,then Fe₃O₄/NG was prepared by solvothermal method.And Fe₃O₄ was prepared by the same method without the addition of NG2.After the recombination of Fe₃O₄ and NG,the size of Fe₃O₄ nanoparticles is about 90 nm.Fe₃O₄/NG showed a high discharge capacity of 1424.8 m Ah/g in the first cycle process,far more than the discharge capacity of the Fe₃O₄.After 50 cycles the discharge capacity still remianed 763.8 m Ah/g,the capacity retention rate is very high.It is found that the interfacial resistance of Fe₃O₄/NG is very small and the electrical conductivity of the material has been greatly improved.This is because NG can improve the conductivity of the metal oxide,alleviate the stress of the metal oxide volume expansion,and the metal oxide can also prevent nitrogen-doped graphene accumulating again.