Structure Design And Lithium Storage Performance Of Fe₃O₄ Anode Material Enhanced By Carbon

Lithium-ion batteries(LIBs)possess many advantages included high energy density,no memory effect and long-term life,which have been widely used in various industries.However,the commercialized LIBs cannot meet the rapidly growing energy demand due to the use of layered graphite anode with a low specific capacity(372 m Ah g^-1).Fortunately,Fe₃O₄ has a high theoretical specific capacity of 926 m Ah g^-1,abundant reserves and environmentally friendliness,which is one of the most promising alternative materials for commercial graphite anodes.However,the low conductivity and serious volume expansion of Fe₃O₄ hinder its industrialization.In this work,a series of carbon materials with different structures,morphologies,and properties were used to combine the Fe₃O₄ for improving the above-mentioned problems.Therefore,the various composite materials with controllable morphologies are successfully constructed.The main research contents and results of this work are as follows:(1)The Fe₃O₄@Fe₃O₄/Carbon with a hollow structure was successfully prepared by a solvent-induced phase separation method,and the influence of the solvent ratio on the micro-morphologies was also discussed.It can be found that when the volume ratio of tert-butanol and water is 1:1,a good cavity can be formed.The existence of the cavity provides a buffer space for the volume expansion of Fe₃O₄.In addition,the introduction of the carbonaceous layer has also effectively improved the pulverization of the Fe₃O₄ electrode structure during charging and discharging.Based on the analysis of the kinetics data measured by cyclic voltammetry,it can be seen that the electrode material of the hollow structure has a higher contribution of pseudocapacitance.Therefore,when the assembled half-cell was tested at a high rate of 5 A g^-1,it can still maintain a specific capacity of 450m Ah g^-1 after 1000 cycles.(2)The terephthalic acid was used as the main ligand to construct metal-organic framework,and the structure and morphology of the product was regulated by introducing the extra 2-methylimidazole.Then the in-situ N-doped porous carbon framework coated Fe₃O₄ was successfully prepared by combining with different carbonization tempratures.Based on the GITT test,it can be found that the introduction of nitrogen doping effectively improves the ion and electron transport during charging and discharging.At the same time,the kinetics data measured by cyclic voltammetry also shows that it has a higher pseudocapacitance contribution.Therefore,when the assembled half-cell is tested at a high rate of 5 A g^-1,it can still maintain a specific capacity of 300 m Ah g^-1 after 300 cycles.When it was assembled into the coin-type full-cell with a commercial Li Co O₂ cathode,it can still release a specific capacity of 730 m Ah g^-1 after 50 cycles at 0.5 A g^-1 current density.(3)In view of the traditional dyeing technique,Prussian blue was successfully in-situ deposited on a commercial cotton textile in large scale,and flexible self-freestanding anode materials with different morphologies were prepared by further carbonization.And the structure and morphology of the flexible self-supporting anode were adjusted by further carbonization at different temperatures.Among them,the flexible electrode carbonized at 800?has a semi-embedded coating layer and a unique mesoporous structure.The advantages of this structure are conducive to easeing volume expansion,improving electrolyte infiltration and better ion and electron transport.Therefore,when it was tested at 2m A cm^-2 in a half-cell,the areal specific capacity of 0.66 m Ah cm^-2 can still be maintained after 300 cycles.Not only that,it showed good electrochemical performance in coin-type full-cell and soft-pack full-cell.This simple,large-scale preparation technology and superior electrochemical performance show great practical prospects.(4)Polyacrylonitrile nanofibers were prepared by electrospinning,then the flexible self-supporting anode film constructed with porous nanofibers was successfully prepared via combining with in-situ vapor etching during carbonization to.Due to the introduction of sodium dihydrogen phosphate during the carbonization process,the products produced in the phase transition process can react with each other to generate steam vapor,thereby etching the carbon nanofiber.While N,P co-doping and a rich pore structure can appear on the nanofiber surface.The GITT test shows that the synergistic effect of the N,P co-doping and the porous structure give it better ion and electron transport.At the same time,cyclic voltammetry tests show that it has two methods of diffusion and capacitance of lithium storage,while the sample without sodium dihydrogen phosphate has only diffusion lithium storage.Therefore,when it was tested at 2 m A cm^-2 in a half-cell,the areal specific capacity retention rate can still reach 55%after 300 cycles.At the same time,the coin-type full-cell assembled with it also showed good electrochemical performance.(5)A hybrid anode material with multiple ionic bond interfaces was successfully prepared by means of electrostatic self-assembly.Density functional theory was used to calculate the interaction energy between ionic bonds and the adsorption energy of lithium ions embedded in the ionic bond interface.It was confirmed that the multiple ionic bond interface is beneficial to provide more capacitive lithium storage behavior.In addition,the presence of ionic bonds is beneficial to repair the pulverized Fe₃O₄ during the charging and discharging process,and to ensure the integrity of the electrode structure.Therefore,the assembled battery released a specific capacity of up to 830 m Ah g^-1 at a current density of 1 A g^-1.Even after 1000 cycles,the specific capacity of 800 m Ah g^-1 can be retained.