Preparation And Study Of Sodium Storage Properties Of Iron-based Chalcogenide Compounds

Lithium-ion batteries（LIBs）,as a rechargeable energy storage device,has been widely used in various electronic devices and products due to its unique advantages.But the cost of lithium-ion batteries limits its development.Sodium-ion batteries（SIBs）are attracting attention as the next-generation energy storage devices because sodium crust is abundant and cheap compared to lithium.As an important component of SIBs,anode material directly affects the electrochemical performance of batteries.Therefore,the development of alternative anode materials with a high specific capacity and long cycle life has become the focus of current research.In addition to carbon materials,iron-based chalcogenide compounds with conversion mechanism have gradually attracted great attention from researchers due to their high theoretical specific capacities.However,such materials will produce huge volume expansion during repeated sodiation/desodiation,resulting in serious mechanical fracture/crushing of electrode materials,and failure of electronic contact with conductive substrate,resulting in instability of solid electrolyte interface（SEI）and increased interface impedance.In order to solve the above problems,this paper takes iron-base chalcogenide compounds as the research object,uses ether-like electrolyte to construct a stable electrode interface,and conducts in-depth and systematic research on the sodium storage performance with the help of composition and morphology regulation.（1）Preparation of Fe S_0.5Se_0.5@NC nanorods and their sodium storage properties.The synergistic effect between the heterogeneous structure and the carbon layer induced by the dual-anion improves the electrical conductivity of the iron-based chalcogenide compounds,alleviates the volume expansion during cycling,effectively improves the cycle stability and increases the capacity retention rate.In this chapter,Fe OOH@PDA precursor was first prepared by a hydrothermal method,and then the mixture of sulfur powder and selenium powder was annealed to obtain nitrogen doped carbon coated iron-based bianionic compound(Fe S_0.5Se_0.5@NC).Using it as a sodium storage anode for electrochemical performance.The experimental results show that at a test temperature of25°C,Fe S_0.5Se_0.5@NC composites can still maintain a significantly reversible capacity of450 m Ah g^-1 after 220 cycles of charge and discharge when the current density is 5 A g^-1.And it’s also exhibit good electrochemical performance at 0°C.In addition,the full battery composed of Na₃V₂（PO₄）₃@r GO//Fe S_0.5Se_0.5@NC demonstrates excellent long-term cycle stability and high sodium storage capacity under a wide range of test temperatures.The defects introduced in the process of sulfurization and selenization provide a large number of active sites for sodium ion adsorption,which can improve the storage capacity of sodium.The effective coating of nitrogen doped carbon layer can maintain the stable electrode structure and improve the conductivity of the whole electrode.（2）Preparation of bulk phase Fe₇Se₈ and its sodium storage propertiesIn order to realize the large-scale application of iron-base chalcogenide compounds,the bulk phase Fe₇Se₈（bulk Fe₇Se₈）was obtained by one-step calcination with reduced iron powder and selenium powder.Its advantages are as follows:first,the cost of raw materials is low,the preparation method is simple,easy to realize commercial production;Secondly,high specific capacity can be maintained without carbon modification due to the high conductivity of iron selenide itself.Thirdly,the bulk phase Fe₇Se₈ will decompose into three-dimensional interconnected nanoparticle-like structure during cycling,which saves the complicated structural design process,reduces the diffusion distance of sodium ions,and improves the diffusion kinetics.The electrochemical test results show that the bulk Fe₇Se₈ used as a SIBs anode material,even at a current density of 40 A g^-1,the specific capacity can still maintain 168 m Ah g^-1at 25°C.When the test temperature was 0°C,the bulk phase Fe₇Se₈ also shows good electrochemical performance.In addition,the full battery composed of Na₃V₂（PO₄）₃@r GO//bulk Fe₇Se₈ shows good cycle and rate performance at test temperatures of 25°C and 0°C.The simple method of preparing bulk Fe₇Se₈ by one step calcination can not only improve the performance of sodium storage but also save cost and facilitate the large-scale commercial production.