Study On The Preparation And Performance Of Fe₃O₄@rGO/C Anode Materials For Lithium Ion Batteries

With the development of electrification,the demand for energy is getting higher and higher.As a kind of clean and environment-friendly energy storage,lithium-ion battery has been enthusiastically sought.However,the current commercial lithium-ion battery anode material graphite has a relatively small theoretical specific capacity and cannot meet the requirements of high-power electromechanical equipment such as electric vehicles,crane trucks,and mining machinery.Therefore,looking for the development of high specific capacity lithium-ion battery anode materials is the core to solve this problem.Compared with the intercalation and deintercalation mechanism of graphite-based materials and titanium-based materials,Fe₃O₄,which uses redox as the charge and discharge mechanism,has a higher theoretical specific capacity and better safety,and is considered by researchers to be a very ideal lithium Anode material for ion battery.However,due to the inherent defects of Fe₃O₄,such as the expansion of Fe₃O₄ when lithium ions are inserted and extracted during the oxidation-reduction process,the volume becomes larger,which easily causes the active material to fall off the electrode and pulverize,resulting in rapid battery capacity degradation and extremely poor cycle stability.Such problems seriously hinder the application range of lithium-ion batteries.At present,the single-layer carbon coating of Fe₃O₄ is one of the main means to solve this problem,but such modification still has a certain space in improving the electrochemical performance of Fe₃O₄.This paper aims at the shortcomings of Fe₃O₄ mainly by constructing a single-layer,multi-layer coating structure,combining coating and encapsulation technology,and constructing a reasonable micro-nano structure.The coating structure and the encapsulation structure can construct a continuous inert protective layer on the periphery of the material to limit the volume deformation of Fe₃O₄ during the inserted and extracted of lithium ions,thereby limiting the shedding and pulverization of active materials,to improving electrochemical performance.In addition,a reasonable micro-nano structure can also limit the volume expansion of the active material in the full electric process.At the same time,it can shorten the diffusion path of lithium ions in the electrode material,and improve the rate performance of the active material.By modifying Fe₃O₄,the electrochemical performance of Fe₃O₄ has been greatly improved,The main research work of this paper is as follows:(1)Preparation and electrochemical performance of carbon-coated Fe₃O₄.The volume expansion of carbon materials during charge-discharge is almost negligible and has high conductivity.Therefore,the carbon material was selected as the coating material to coating the Fe₃O₄.The precursor porous Fe₂O₃ was obtained by a simple hydrothermal method and high temperature calcination method,and then a carbon-coated porous Fe₃O₄@C material was obtained by carbothermal reduction,which was named CP-Fe₃O₄@C.The presence of porosity helps the electrolyte to infiltrate the electrode material and provide a larger contact area between the electrode and the electrolyte,thereby increasing the electron transfer rate.At the same time,the influence of the added amount of pore former and the coating hydrothermal time on the morphology of the product was discussed.The charge-discharge results show that the specific capacity of CP-Fe₃O₄@C with core-shell structure after 200cycles at a current density of 0.3 A g^-1 is 785 m Ah g^-1,and the specific capacity can reach 130 m Ah g^-1 at a high current density of 6 A g^-1.According to the calculation of the reaction kinetics of CP-Fe₃O₄@C rate performance,the pseudocapacitance accounts for 91%of the total capacity when the scan rate is 5 m V s^-1,indicating that carbon materials are a good coating material.(2)Preparation and electrochemical performance of core-shell Fe₃O₄encapsulated by defect reduced graphene oxide(Dr GO).Graphene is a two-dimensional layered carbon-based material with excellent electrical conductivity,porosity,high specific surface area and good mechanical properties.It is very suitable for encapsulation engineering and building micro-nano materials with three-dimensional structures.In this paper,r GO with carbon defects was prepared by the alkaline etching method.The oxygen-containing functional groups in the carbon vacancy defects are closely combined with Fe₃O₄ nanospheres through chemical bonds to form a stable structure.In the process of synthesis,the Dr GO encapsulated core-shell Fe₃O₄@C composite material(Dr GO-CP-Fe₃O₄@C)was synthesized by hydrothermal and carbothermic reduction methods.The influence between the graphene etching time and the electrochemical performance of the composite material is explored,and the mechanism of graphene etching was briefly described.The charge-discharge results show that the specific capacity of the three-dimensional Dr GO-CP-Fe₃O₄@C after 200 cycles at a current density of 0.3 A g^-1 is 820 m Ah g^-1.It is worth noting that the specific capacity of Dr GO-CP-Fe₃O₄@C at a high current density of 6 A g^-1 is 251 m Ah g^-1.Compared with CP-Fe₃O₄@C,the capacity of Dr GO-CP-Fe₃O₄@C has been increased by 93%,proving that the introduction of Dr GO can effectively improve the charge-discharge performance under high current conditions.(3)Preparation and electrochemical performance of carbon-encapsulated Fe₃O₄/r GO composite materials.The oxygen concentration group of r GO is used to directly self-assemble with Fe₃O₄ to form a stable"sandwich"structure of r GO/Fe₃O₄composites.Under the encapsulation of layered r GO,the volume expansion of Fe₃O₄in the charge-discharge process is greatly suppressed.On this basis,the introduction of carbon mesh structure to encapsulate the surface of r GO can not only inhibit the volume expansion of Fe₃O₄ on the surface of r GO,but also further inhibit the overall expansion of the r GO/Fe₃O₄ composite.At the same time,the introduction of carbon network can build three-dimensional network electronic transmission structure;shorten the transmission distance of electrons.In this paper,a simple and environmentally friendly one-pot solvothermal method is used to directly grow cauliflower-like Fe₃O₄ between and on the surface of r GO(r GO/Fe₃O₄).Then in the process of calcining r GO/Fe₃O₄ into phase formation,a layer of carbon mesh was prepared on its surface to obtain a"sandwich"structure of r GO/Fe₃O₄/C composite material.The charge-discharge results show that the specific capacity of r GO/Fe₃O₄/C after 300 cycles at a current density of 0.3 A g^-1 is 844 m Ah g^-1.It is worth noting that the specific capacity of r GO/Fe₃O₄/C after 300 cycles at a high current density of 4 A g^-1 is 363 m Ah g^-1.The charge-discharge results fully prove that the composite material with this"sandwich"structure has a certain improvement in electrochemical cycling performance.The changes of the elements contained in the electrode material before and after the cycle was analyzed by infrared and XRD,and the mechanism of the electrode in the fully electric process is briefly described.(4)Study on the modification of lithium battery performance of carbon-coated Fe₃O₄@r GO by hydrogenated oxygen vacancy defect titanium dioxide coating.The stability of titanium-based materials in lithium-ion battery anode materials is much higher than that of transition metal oxides,especially Ti O₂.The volume expansion coefficient of Ti O₂ during the deintercalation of lithium ions is about 4%,which can maintain the stability of the structure.Its poor electrical conductivity(?1*10^-12S/m)can be improved by hydrogenation engineering.Through hydrogenation reduction treatment,part of Ti⁴⁺in Ti O₂ can be reduced to Ti³⁺,generating a certain number of single electrons and improving its electronic conductivity.In this paper,a multilayer with Fe₃O₄@r GO as the core,carbon layer as the middle coating layer,and hydrogenated Ti O₂(H-Ti O₂)as the outermost coating layer was synthesized by simple solvothermal method,carbothermal reduction and hydrogenation methods.The charge-discharge results show that the specific capacity of H-Ti O₂/C/Fe₃O₄@r GO after 200 cycles at a current density of 0.3 A g^-1 is 867 m Ah g^-1.It is particularly important to note that the specific capacity of H-Ti O₂/C/Fe₃O₄@r GO after 700 cycles at a high current density of 1 A g^-1 is 505 m Ah g^-1.It can be seen that the introduction of the H-Ti O₂ coating layer and the double coating layer structure is greatly helpful to improve the cycle stability of electrode materials.