Preparation Of Ring-shaped ?-Fe₂O₃ Based Composite Electrode Materials And Their Lithium Storage Performances

As one of the most promising energy storage technology,lithium ion battery?LIBs?has been attracted widespread attention of researchers,due to their high energy density,long cycle life,no memory effect,environmentally friendly and other outstanding features.With the rapid development of portable electronic products,high performance of lithium-ion batteries are highly desirable,however,graphite,as the main anode materials for commercialized lithium-ion batteries,and its theoretical capacity is just only 372 mA h g^-1,which seriously affects the performance of Li-ion batteries,so it has become the key to choose and prepare the anode material with high energy density and good cycle stability for the development of lithium-ion batteries.Driven by abundant sources,environmental benignity,low cost and high theoretical capacities,ect.,iron oxides have become a promising anode candidates for LIBs.However,its poor rate performance and cycling performance,caused by low intrinsic conductivity and large volume change during charge-discharge,which strongly hinder its practical commercial application.Here,for the purpose of enhancing the electrical conductivity and structural stability of iron oxide anode materials,we design and synthesize ring-shaped?-Fe₂O₃ based nanocomposites through hydrothermal methods,including?-Fe₂O₃,?-Fe₂O₃/Graphene and?-Fe₂O₃/GAs composite,the lithium storage capacities were further improved.The main works in this dissertation are divided into three parts and the specific research contents and results are listed as follows:1.The?-Fe₂O₃ nanorings were prepared via a simple hydrothermal method with the existence of FeCl₃·6H₂O as the ferric source and NaH₂PO₄·2H₂O?Na₂SO₄ as the structure-directing agents at 220? for 48 h.The composition,morphology and the nanostructure were analyzed by means of XRD,SEM,TEM and BET,and the?-Fe₂O₃nanorings as the active material was assembled to a button cell for electrochemical performance test.This results showed that the?-Fe₂O₃ nanorings have good lithium storage performance.When the current density at 100 mA·g^-1,its first discharge capacity reaches1203.6 mA h g^-1,after 100 cycles,the reversible specific capacity could maintain about 225mA h g^-1.2.Graphene oxide?GO?was prepared from natural graphite by a modified Hummers method.Subsequently,the pre-prepared?-Fe₂O₃ nanoring was combined with graphene oxide to synthesize ring-shaped?-Fe₂O₃/graphene composite using a one-step hydrothermal method.The samples were characterized by XRD,SEM,TEM,HRTEM,FT-IR,XPS,TGA and BET,and the electrochemical performance was tested after the samples were assembled into button batteries.The results show that the prepared?-Fe₂O₃/Graphene is a 2D porous network structure with ring-shaped?-Fe₂O₃ nanoparticles are distributed on the wrinkle layered graphene nanosheets.Electrochemical tests show that the?-Fe₂O₃/Graphene composite electrode exhibits superior lithium storage performance compared to the pure?-Fe₂O₃electrode,especially its cyclic stability(at a constant current density of 100 mA g^-1,the reversible capacity could maintained at 965 mA h g^-1 after 100 cycles)and the rate performance(the discharge specific capacity is still remained 450 mA h g^-1 even at a large current density of 1000 mA g^-1).3.Based on the three-dimensional network structure of graphene aerogels and the advantages of hollow nanorings,the ring-shaped?-Fe₂O₃/GAs have been successfully prepared by one-pot hydrothermal self-assembly reduction and subsequent freeze-drying method with the pre-made?-Fe₂O₃ nanorings and graphene oxide?GO?.The phase,structure,morphology and composition of composites were characterized by XRD,SEM,TEM,FT-IR,Raman,XPS,TGA,and BET,and its electrochemical performance was also observed by cyclic voltammetry and galvanostatic charge-discharge test.The characterization results show that the?-Fe₂O₃ nanorings were uniformly located on graphene nanosheets to construct a 3D porous network structure in composites.The?-Fe₂O₃/GAs nanocomposites have large specific surface area and stable structure,and show excellent electrochemical properties than bare?-Fe₂O₃ nanorings.As an anode material for lithium ion batteries,the?-Fe₂O₃/GAs exhibit a specific capacity of 1288 mA h g^-1 at a current density of 100 mA g^-1 up to 100cycles.This result indicates that the combination of graphene aerogels?GAs?and active materials will contribute to the significant improvement of the electrochemical performance,and its owned great potential in lithium ion battery applications.