Synthesis Of Nitrogen Doped Graphene/Transition Metal Oxide Composite And Their Lithium Storage Properties

With the development of society,people rely on high-power portable devices increasingly.And as the power source for portable devices,lithium-ion batteries have attracted more and more' attentions.In order to meet people's needs for high-performance lithium-ion batteries,many researchers have been exploring new electrode material,especially the anode material for lithium-ion batteries.Because of the high theoretical specific capacitance of Fe₃O₄ and Co₃O₄?926mAh/g and 890mAh/g,respectively?,they have absorbed more and more attention.Although synthesizing nanoscale material or forming composite with some other high conductivity material can solve the problems effectively,which mainly include their own poor electrical conductivity and the structure collapse of the electrode materials,the damage of electrode,poor cycling stability and so onduring the charging and discharging course of the Fe₃O₄ and Co₃O₄ electrode materials,there are still many other difficulties in restricting their lithium storage performance,such as the poor dispersion of active material in the composite,the unsatisfactory conductivity,the low content of the doped nitrogen in carbon-based material and so on.Novel N-G/Fe₃O₄ and N-G/Co₃O₄ composite materials with many excellent characteristics,such as good dispersion,high nitrogen content,good dispersion of uniform particles,good conductivity,excellent lithium storage performance and so on have been synthesized by the hydrothermal process and subsequent heat treatment.The graphene oxide,as conductive precursor,obtained by simple Hummers method,FeCl2·4H2O,FeCl3·6H2O and CoCl2·6H2O as active substance precursor,dicyandiamide as nitrogen source and thiourea as dispersant,have been used to synthesize the N-G/Fe₃O₄ and N-G/Co₃O₄,respectively,and subsequently,the morphology and lithium storage performance of the composites have been obtained.The detailed research results are as follows:1)Graphene have been obtained from grapheme oxide by hydrothermal reducing method and subsequently the influence of the doped nitrogen content and the hydrothermal temperatureon the performance of the nitrogen-doped graphene has been investigated in detail.The result shows that when the dicyandiamide isused as nitrogen source and the hydrothermal temperature is 170?,the as-prepared N-doped graphene delivered the specific capacity of 405.8mAh/g even after 100 cycles at the current density of 100mA/g,which is higher than the theoretical specific capacity of graphene.2)rGO/Fe₃O₄ and rGO/Co₃O₄ have been prepared by thermostatic stirring,hydrothermal process and subsequent heat treatment.It is shown that graphene-based composition can significantly improve the conductivity of the composite,relieve the volume variation and further improve lithium storage performance.When the concentration of precursor,graphene oxide,is 0.25mg/ml,the hydrothermal temperature and heat-treat temperature is 150? and 300?,respectively,the excellent electrochemical performance of the composite has been obtained.The rGO/Fe₃O₄ and rGO/Co₃O₄ delivered the specific capacity of 741.2 mAh/g and 688.6mAh/g after 70 cycles,respectively,which is much higher than those of pure Fe₃O₄ and Co₃O₄ nanomaterials.And it's cycle coulombic efficiency as high as above 98%.3)The N-G/Fe₃O₄ and N-G/Co₃O₄ have been prepared by the hydrothermal method and subsequent by heat-treatment process.It is shown that the thiourea addition can effectively enhance the dispersion of the material.The dicyandiamide used as nitrogen sourcecan effectively increased nitrogen-doped content?the atomic ratio:8%?to further enhance cycle specific capacity of composite materials significantly.When the hydrothermal time is 12h and the heat treatment temperature is 500?,the material has obtained excellent performance.After 100 cycles,the N-G/Fe₃O₄ and N-G/Co₃O₄ delivered the specific capacities of 1011mAh/g and 794.1mAh/g,respectively,which are much higher than those of rGO/Fe₃O₄ and rGO/Co₃O₄.Their charge and discharge cycle curve are almost overlap,and they have stable cycling performance,high coulombic effciency.N-G/Fe₃O₄ and N-G/Co₃O₄ composites with high-content of doped nitrogen and uniformly dispersed nanoparticles have been formed by simple method.This effectively solves the current problem of low lithium storage capacity and poor cycle performance owing to poor dispersion and low content of nitrogen in the nitrogen-doped graphene-related carbon materials especially in graphene/transition metal oxide composition and further effectively improves the lithium storage performance of transition metal oxide.Therefore,it lays the foundation for the preparation and further practical application of transition metal oxide based anode materials.