Synthesis Of Iron(Cobalt)-Based/C Hollow Mesoporous Carbon Nanospheres Composites And Their Lithium Storage Performances

The excessive use of fossil fuels has increasingly made problems prominent such as the greenhouse effect and smog.It also causes a serious challenge to our living environment.Fossil fuels are non-renewable resources.There are not inexhaustible.So it is not sustainable to depend on fossil fuels exceedingly and its prospects are worrying.Compared with batteries made of other materials,lithium ion batteries have the superiority of lightweight materials,more cycle times,little environmental pollution,no memory effect and low self-discharge rate which have got more and more favor.For the time being,nanoscale transition metals,oxides,sulfides and phosphates have got more and more concern on account of their small particle size,low cost,ecological friendliness and high theoretical specific capacity.However,their practical application in lithium ion batteries have limitted due to their poor cycling stability,large irreversible capacity loss,low specific capacity,large volume expansion and low conductivity.Hollow mesoporous carbon nanospheres(HMCNs)have aroused the great attention,owing to their good electrical conductivity,porous shells and large specific surface area.In addition,HMCNs use a porous shell structure to provide large holes to accommodate guest compounds,reducing the aggregation of nanoparticles.With hollow mesoporous carbon nanospheres as a growth substrate,iron and cobalt nanoparticles were grown in situ on the inner and outer walls of the hollow mesoporous carbon nanosphere by thermal decomposition to form a new type of iron-based/C,cobalt-based/C hollow ball based on interface anchoring and used in electrochemical energy storage.The synthesis of anode materials for lithium ion batteries provides a new way.The research content of this dissertation is divided into three parts:(1)The synthesis of sesame balls-like Fe3O4/C hollow nanospheres based on interfacial anchoring effect to enhance lithium storage performanceHollow mesoporous carbon nanospheres(HMCNs)are used as the growth skeleton.Fe3O4 nanoparticles(Fe3O4NPs,a diameter of 10 nm)grow in situ on the shell of HMCNs to form a new type of sesame balls-like Fe3O4/C hollow nanospheres(HNSs)by thermal decomposition reaction.The Fe3O4/C HNSs display great cycling stability.At a current density of 1 A g-1,the reversible specific capacity of Fe3O4/C HNSs is as high as 946 mA h g-1 after 250 cycles.Compared with Fe3O4 NPs without composite structure,the electrochemical properties of Fe3O4/C HNSs are significantly improved.The anchoring of Fe3O4 NPs on the shell of HMCNs can avoid the aggregation of Fe3O4 NPs,increase the lithiation sites and accelerate the rapid migration of Li ions.In addition,the framework of Fe3O4/C HNSs,i.e.HMCNs,can improve the conductivity of materials and boost the migration of electron.This notion and expedient method of construction can be broadened to synthesize other hollow nanostructured material with preferable electrochemistry performance.(2)In situ derivatization and synthesis of Fe7S8/C hollow nanospheres and their electrochemical lithium storage propertiesOn the basis of the previous chapter,Fe3O4/C hollow nanospheres(Fe3O4/C HNSs)were used as precursors.After the in situ vulcanization and phosphating,Fe3O4/C HNSs are transformed into Fe7S8/C HNSs and Fe(PO3)2/Fe2O3/C HNSs,respectively.The influence of derived temperature and phosphorus source on morphology and crystal structure were researched.When the current density is 1 A g-1,the reversible specific capacity of Fe7S8/C HNSs is 1006 mA h g-1 after 250 cycles.Compared with Fe3O4/C HNSs and Fe(PO3)2/Fe2O3/C HNSs,the electrochemical properties of Fe7S8/C HNSs are significantly improved.The remarkable improvement in electrochemical performance can put down to the rapid design of nanostructures,the reduction of the polarization effect and the acceleration of dynamic diffusion of Li ions.The anchoring of Fe7S8 NPs on the shell of HMCNs avoids the aggregation of Fe7S8 NPs,promotes the conductivity of composite and boosts the delivery of electron.It is a prospective material with great electrochemical properties in the field of energy storage and conversion.(3)Synthesis of ultrafine Co/C hollow nanospheres and their lithium storage propertiesIn order to prove the universality of composite materials prepared by thermal decomposition with hollow mesoporous carbon spheres as the substrate,cobalt acetylacetonate was used as cobalt source in this chapter.Co nanoparticles(NPs)with an average diameter of 6 nm are grown in situ on the shell wall of HMCNs to form ultrafine Co particle composite hollow carbon nanospheres(Co/C HNSs).Due to the interface effect of tiny nanoparticles,well-designed Co/C hollow nanospheres can provide more lithium ion storage sites and accelerate the rapid migration of lithium ions.The in-situ growth of Co NPs on the HMCNs shell has a good constraint effect on Co NPs,which can reduce the aggregation,loss and accumulation of nanoparticles to maintain the complete structure of the composite.At the current density of 10 A g-1,the specific capacity of Co/C composites after 1000 cycles can still reach approximately 264 mA h g-1.The electrochemical properties of Co/C HNSs are significantly improved compared with those of the ultrafine Co nanoparticles without composite structure.