Design And Preparation Of Iron-based Oxides Anode For Lithium Storage Applications

Lithium-ion battery is a kind of energy conversion and storage device by means of lithiation reactions such as embedding/detaching of lithium ions back and forth.Due to its high energy density and stable cycle life,it has been broadly applied in many energy storage fields.With the fast iteration of convenient electronic products and new energy vehicles,the search for a new generation of lithium-ion batteries with higher energy density,power density and lifespan has become an inevitable trend.In conventional lithium-ion batteries,commercially available graphite carbon is usually served as the negative electrode.However,due to the low theoretical specific capacity of graphite,it is just not enough to satisfy the high demands of lithium storage performance for the new generation of energy storage devices.In recent years,iron-based oxides based on conversion reactions have become a popular research issue for new anode materials because of their extensive resources,simple preparation,low pollution and high theoretical specific capacity.However,the rapid capacity decay caused by its inherent poor electrical conductivity and the problems of particle agglomeration and volume expansion during the charging and discharging process,it has restricted the commercial application of this cathode material.Based on this,a variety of iron-based oxide anode materials with unique morphological structures have been synthesized through rational structural design.We investigate their lithium storage performance and analyze the electrochemical kinetic mechanism during the charging and discharging process.It provide ideas for the synthesis of transition metal oxide-based anode materials and high-performance energy storage applications.The main research work and innovations of this thesis are as follows.(1)CoFe2O4/rGO/C composite has been strategically synthesised via a facile two-step hydrothermal technique.The effects of graphene modification,amorphous carbon encapsulation on the morphological structure and lithium storage properties have been systematically investigated.The results show that reduced graphene oxide serves as the structural framework,amorphous carbon serves as the outer carbon encapsulation layer,and CoFe2O4 nanoparticles are well immobilized in this three-dimensional conducting framework.The graphene and amorphous carbon have excellent modification effect for lithium storage.It can exhibit a fast and reversible lithiation reaction.The CoFe2O4/rGO/C electrode exhibits a high specific capacity and excellent long span life due to the surface-induced pseudocapacitance process.When tested at 0.1 A g-1,the electrode has a reversible specific capacity of up to 945 mA h g-1.When tested at a high rate(4 Ag-1),the specific capacity is stabilized at 421 mA h g-1 and the coulombic efficiency keeps～100%after 2000 cycles.In addition,the reversible capacity shows a rapid increase after 100 cycles at 1 A g-1,and finally presents an ultra-high capacity of 1430 mA h g-1 after 500 cycles.(2)Novel α-Fe2O3@3DrGO hydrogel composite has been ingeniously synthesized by using simple hydrothermal self-assembly with freeze-drying treatment.The growth mechanism,morphological structure as well as its electrochemical properties have been further tested to analyse its lithium storage mechanism in detail.Morphological and structural characterizations have confirmed that the reduced graphene oxide sheets self-assemble to form a three-dimensional interconnected porous framework.Ultrafine α-Fe2O3 particles are uniformly anchored on the inner or outer surface of the graphene mesopores.This ideal structure provides a hierarchical porous conducting network with abundant reactive sites and large stress buffer space.The α-Fe2O3@3DrGO electrodes exhibit a dominant pseudocapacitive storage mechanism(93.9%of the total capacity),which facilitates fast electrochemical kinetics.At the current densities of 0.1,1 and 5 A g-1,α-Fe2O3@3DrGO electrode exhibits high specific capacities of 1082.3,921.6 and 812.4 mA h g-1,respectively.Even tested at a large rate(5 A g-1),the electrode still remains a good capacity(396.6 mA h g-1)after 2000 cycles,exhibiting an excellent cycling stability.(3)Porous nitrogen-doped carbon nanorod composite(FexOy@PN-CNR)anchored by hollow FexOy nanoparticles has been successfully prepared by combining electrospinning technique with multi-step gradient thermal annealing treatment.In addition,the electrochemical properties of the anode material and its kinetic mechanism have been investigated in detail.Through morphological characterization,it is clearly observed that the ultrafine hollow FexOy nanoparticles(NPs)are homogenously distributed in porous nitrogen-doped carbon nanorods.This unique structure can effectively prevent the aggregation and pulverization of iron oxide nanoparticles and maintain the integrity of the electrode structure.In addition,the FexOy@PN-CNR composite provides a hierarchical conductive network for electron transport and abundant and feasible porous channels for rapid diffusion of lithium ions.It also provides sufficient space to accommodate volume expansion,which can effectively inhibit pulverization or aggregation of active materials during the cycle process.Benefiting from these characteristics,the FexOy@PN-CNR electrode has a very high reversible specific capacity(1198.6 mA h g-1 for 100 cycles)when tested at 1 A g-1.Even in a large rate cycle test,the reversible specific capacity can still remain at 800 mA h g-1 after 200 cycles,exhibiting a stable cycle life.