Design,Preparation And Performance Regulation Of Iron-Based Anode With Confined Structure

In recent years,transition metal compound anodes for lithium storage based on the conversion reaction mechanism have received extensive attention due to their superior theoretical specific capacities.Among them,iron-based compounds such as Fe₃O₄(its theoretical specific capacity is 926 m Ah·g^-1)and Fe S(its theoretical specific capacity is 609 m Ah·g^-1)are considered to be one of the most promising candidates to replace the traditional graphite anode for lithium-ion batteries because of the superiorities of abundant resources,low cost,easy preparation and so on.However,iron-based anodes suffer from problems of low conductivity and significant volume effect during the lithiation/delithiation process,eventually leading to sluggish reaction kinetics and rapid capacity fading.Therefore,it is of great significance to perform structural optimization and compositional design to explore and develop new composite anode materials with high performance.Based on the above defects of iron-based anode materials,we prepared the Fe₃O₄@C composite with confined structure and the Fe₃O₄/Fe S@C heterogeneous composite with confined structure by the solvothermal synthesis process and gas-solid phase vulcanization technology,respectively.The evolution process of morphology and structure and the electrochemical lithium storage performance of the materials were investigated.The main research contents are as follows:（1）Firstly,the Fe₃O₄@C composite was synthesized by the solvothermal synthesis process,and the formation mechanism as well as the lithium storage properties of the composite were studied.The Fe₃O₄@C composite with pyrolytic carbon space-confined nanorod structure was constructed by sequential solvothermal reaction and high-temperature carbonization treatment with hydroxyl iron oxide（Fe OOH）which was prepared by a green corrosion reaction of metallic iron as iron source,glucose as carbon source and ethylene glycol as the solvent.And the formation process of the carbon confined nanorod structure of the composite was explored and deduced.The results show that in the solvothermal reaction process,Fe OOH nanosheet was first transformed into Fe₃O₄ nanorod,then the glucose was dehydrated and polymerized to form polyfuran（PF）and in-situ deposited on its surface to form the Fe₃O₄@PF compound,and finally the Fe₃O₄@C composite was obtained by the high-temperature calcination.Benefiting from the space-confined structure of carbon,the Fe₃O₄@C composite possesses enhanced electric conductivity and structural stability.When used as an anode material for lithium-ion batteries,the Fe₃O₄@C composite with optimized carbon content possesses an average discharge specific capacity of 390.9 m Ah·g^-1 at a current density of 6400 m A·g^-1,exhibiting a good lithium storage property.And the kinetic analysis results show that the high lithium storage capacity of the composite is mainly due to the high capacitance contribution.（2）Secondly,the Fe₃O₄/FeS@C heterogeneous composite was synthesized by a gas-solid phase vulcanization process,and the lithium storage performance and behavior of the material were studied.In addition,the mechanism of action of the heterostructure was analyzed theoretically.The Fe₃O₄/Fe S@C heterogeneous composite with pyrolytic carbon space-confined nanorod structure was constructed with the above prepared Fe₃O₄@C composite as the substrate through constructing the Fe₃O₄/Fe S heterojunctions by a gas-solid phase sulfurization reaction.The electrochemical lithium storage properties and lithium storage kinetics of the heterogeneous composite were investigated.At the same time,the heterostructure was theoretically analyzed by the first-principles calculation based on the density functional theory.The results show that the Fe₃O₄/Fe S@C heterogeneous composite possesses higher electron and lithium-ion conductivity and more stable structure due to the synergistic effect of the heterostructure and spatial confinement structure.When used as an anode material for lithium-ion batteries,the Fe₃O₄/Fe S@C heterocomposite exhibits excellent rate performance(an average discharge specific capacity of 535.79 m Ah·g^-1 at a current density of 6400 m A·g^-1)and cycle durability(an initial discharge specific capacity of 1170.5m Ah·g^-1 at 3200 m A·g^-1,and a discharge specific capacity of 439.8 m Ah·g^-1 after 1000cycles).