Research On Iron-Based Oxide Composite Modification For High-Performance Lithium-Ion Battery Anode

Driven by the target of carbon peaking and neutrality,the vigorous development of new energy electric vehicles has put forward higher requirements for lithium-ion batteries(LIBs)in terms of energy density,cycling life,fast charging,and other performances.Currently,the commercial graphite anode has become a shackle for the further development of LIBs due to its low theoretical capacity(372 mAh g-1).It is urgent to develop new anodes with high performance and long cycling life for LIBs.Iron-based oxides(FexOy)based on conversion reaction are considered as one of the most promising anode materials for LIBs to replace commercial graphite due to their high theoretical capacity,rich reserves,low cost,and environmental friendliness.However,due to poor conductivity and volume expansion during cycling,FexOy is difficult to be used directly as an anode.In this thesis,FexOy is taken as the research object,the modification of compositing carbon materials and controlling morphology are performed to enhance the conductivity of and buffer the volume expansion of FexOy electrode during cycles.And thereby,a new type of FexOy/C anode with high capacity,excellent rate performance,and long cycling life can be developed.The specific research work is as follows:Constructing nitrogen-doped porous carbon-coated Fe3O4(Fe3O4@PNC)is developed to solve the problems of low conductivity and volume expansion of Fe3O4.Hexadecyl trimethyl ammonium bromide(CTAB)is used as the defect inducer while dopamine is used as a nitrogendoped carbon source.Dopamine is attracted to CTAB micelles by electrostatic interaction to form a hybrid carbon layer on the surface of Fe3O4 through polymerization.After hightemperature carbonization,CTAB decomposes and a hierarchical porous carbon layer with mesopores and macropores is formed on the surface of Fe3O4.According to the results of material characterization and electrochemical test,the construction of a carbon-coated structure improves the conductivity of the electrode and buffers the volume expansion of Fe3O4.The Li+diffusion efficiency and charge transfer of the electrode can be significantly enhanced because of the hierarchical porous structure.In addition,due to the decomposition of CTAB,the carbon layer is modified by rich surface defects,which is conducive to stimulating more active sites for lithium storage,thus improving the lithium storage capacity of the Fe3O4@PNC.Compared with the nitrogen-doped carbon-coated Fe3O4(Fe3O4@NC)electrode,Fe3O4@PNC shows excellent electrochemical performance.Fe3O4@PNC delivers a high reversible capacity of 988.2 mAh g-1 at a current density of 200 mA g-1 after 100 cycles,while the Fe3O4@NC electrode can maintain only 779 mAh g-1.This study provides a general method of constructing the nitrogen-doped porous carbon-coated structure for other materials with poor conductivity and high-volume expansion.Spent lithium battery graphite is used as the carbon source and microwave-induced thermal shock is used as special means.Microwave-puffed graphite(MPG)is prepared to load Fe2O3 as the supporting framework,and thereby constructing Fe2O3@MPG anode material with a sandwich structure.According to the material characterization and electrochemical testing,the charge transfer efficiency and cycle stability of the electrode are greatly enhanced because of the excellent conductivity and structural stability of the MPG framework.In addition,the surface chemical structure of the MPG framework can promote the infiltration of electrolytes,thus shortening the Li+diffusion path and enhancing charge transfer.Benefiting from the sandwich structure.Fe2O3@MPG can maintain a high reversible capacity of 1100 mAh g-1 after 200 cycles at 200 mA g-1,and exhibit excellent rate performance of 632 mAh g-1 at 2000 mA gl,as well as deliver excellent cycle stability for more than 500 cycles.Therefore,the construction of Fe2O3@MPG anode can not only exhibit potential in practice applications as a high-performance anode but also serves as an efficient disposal method for spent graphite,providing a reference for the closed-loop disposal of LIBs resources.A Fe2O3-microwave puffed graphite intercalation composite(Fe2O3-MPGIC)is constructed through a gas-phase intercalation coupling in-suit growth method to increase the loading volume of Fe2O3 in the interlayer of MPG framework,thereby improving the reversible capacity of the electrode at high current density.With the gasification and decomposition of FeCl3 driven by microwave,iron chloride is introduced into the interlayer of MPG framework,and Fe2O3 is grown in-suit after thermal treatment,thus constructing Fe2O3-MPGIC with a sandwich structure.an innovative strategy of constructing Fe2O3-MPGIC composite anode by gas phase intercalation is developed.Fe2O3-MPGIC anode realizes the uniform distribution of Fe2O3 among the interlayers of the MPG framework,fully leveraging the role of the MPG framework in buffering the volume expansion of Fe2O3.In addition,abundant pore structure and surface defects are provided for the electrode by the etching of Fe3+ during thermal treatment.which promotes the infiltration of electrolytes and the diffusion of Li+.Therefore,the lithium storage capacity of the composite electrode has been significantly improved at high current density,maintaining an excellent reversible capacity of 504.4 mAh g-1 after 500 cycles at 2000 mA g-1.The Fe2O3-MPGIC anode with excellent rate performance and cycling stability exhibits greater application potential and also provides a novel method for metal oxide embedding into the graphite layer as a high-performance lithium storage electrode.In summary,this thesis aims to solve the inherent problems of low conductivity and volume expansion in FexOy by means of compositing carbon materials and controlling morphology.And then,a high-performance FexOy/C anode with excellent cycle performance,rate performance,and long cycle life is developed,which shows great potential for practical application in the field of energy storage and provides a reference for composite modification methods for other materials that also have low conductivity and volume expansion issues.