Study On The Structure And Interface Regulation Of Carbon Modified Metal Sulfides/Chlorides-based Anode Materials And Their Enhancement On Alkali Metal Ions Storage Properties

To develop alkali metal(Li,Na,K)ion batteries for electric vehicles and large-scale energy storage technologies,it is essential to fulfill the requirements of electrode materials with high energy density,fast charge and discharge capability and long cycle stability.In general,the interface state greatly affects the electrochemical performance,especially the solid-liquid interface of electrolyte/electrode and the internal phase interface of electrode material.How to controllably design reasonable interface characteristics for improved electrochemical performance of alkali metal ion batteries according to the physical and chemical properties of active materials,remains an enormous challenge.Considering the many merits of conversion-typed anode materials(such as metal sulfides and chlorides),herein,my research works have focused on designing and synthesizing a series of sulfide-based and chloride-based anode materials under the guidance of interface optimization and control,aiming to enhance the alkali metal ions transport and storage performance in the anode materials.On this basis,the electrochemical reaction mechanism of electrode materials was studied in depth by means of various test and characterization methods and first principle analysis.Meanwhile,the dependence of the electrochemical properties of the electrode materials on the interface characteristics is clarified.The specific research content includes the following six parts:(1)Aiming to the issues that the metal sulfide is easy to react with the electrolyte and form the unstable solid-electrolyte interphase(SEI).MoS₂@C and Fe₇S₈@C are taken as the research objects.The ether solvent is used to replace the traditional ester solvent to optimize the SEI layer between the metal sulfide and the electrolyte,so as to reduce the side reaction between the metal sulfide and the electrolyte,improve the stability of SEI and enhance Na/K-ions storage.Through electrochemical test,SEM,ex-situ XPS and DFT calculation,it is found that ether solvent can optimize the SEI layer composition,form thin and dense SEI layer,thus promoting the electron transport,reducing the energy barrier of Na/K-ions diffusion,and improving the transport and storage performance of Na/K-ions in MoS₂and Fe₇S₈.(2)To further improve the rate performance and long cycle stability of Fe₇S₈.In addition to the optimal design of solid-liquid interface,we have also optimized the multi-nanometer scale interface reaction(Fe₇S₈ultrafine nanoparticles encapsulated in hollow carbon nanofibers)and introduced the strategy of adjusting Fe:S atomic ratio.Excellent sodium storage performance could be obtained:in ether-based electrolyte,the NHCFs/Fe₇S₈electrode holds capacity of 444 m Ah g^-1at 20 A g^-1.The excellent electrochemical performance of NHCFs/Fe₇S₈is attributed to its improved intrinsic electronic conductivity and faster Na-ions diffusion coefficient as well as lower diffusion energy barriers.(3)To improve the ion transport property of metal sulfide crystal in essence,the principle of effective heterostructures formation(the two components need to have energy gap difference and reversibility difference at the same time)should be studied in-depth.Zn S with wide energy gap(3.60 e V)and poor reversibility(3.55 e V)was introduced into Fe₉S₁₀with narrow energy gap(0 e V)and good reversibility(1.49 e V),constructing Zn S/Fe₉S₁₀@C heterostructures interface(hetero-interface)with strong internal electric field(E-field).Because the hetero-interface can induce the formation of internal E-field,the improvement of its ion transport performance will be an order of magnitude.Synthesized Zn S/Fe₉S₁₀@C heterostructures material with strong internal E-field has excellent sodium storage performance:the electrode holds reversible capacity of 235 m Ah g^-1at 50 A g^-1.The excellent electrochemical performance is attributed to enhanced E-field in the constructed heterostructure between Zn S and Fe₉S₁₀,which can reduce the ion diffusion resistance,accelerating the electron transfer rate and offer more active sites at the hetero-interface.In addition,the corresponding relationship between the internal E-field intensity induced by the hetero-interface and the energy gap difference is clarified,which provides a theoretical basis for other materials to reasonably design the hetero-interface.(4)To further precisely control the direction of the internal E-field induced by the hetero-interface,the two-dimensional material MoS₂was used as the research object,and the compact structure with Fe₉S₁₀as the core,MoS₂as the shell and carbon coated was designed Fe₉S₁₀@MoS₂@C heterostructure materials.In the process of discharge,the E-field(driven by hetero-interface)from the particle surface MoS₂to the internal Fe₉S₁₀,while in the process of discharge,it is the opposite(driven by reversible difference),further strengthening the hetero-interface effect.Introducing Fe₉S₁₀cores could significantly increase electronic conductivity,facilitate the densifying of MoS₂component and decrease ion diffusion energy barrier driven by a strong E-field at the hetero-interface.Excellent sodium storage performance could be obtained:a high volumetric capacity of 662 m Ah cm^-3at 200 m A g^-1;a stable reversible capacity of132 m Ah g^-1at a high current density of 50 A g^-1.In ether-based electrolyte,the stable cycle performance is also achieved with a capacity retention of 93.4%after 1000cycles at a high current density of 2.0 A g^-1.potassium storage performance,the electrode also delivers a high initial volumetric capacity of 408 m Ah cm^-3.(5)Considering the many merits of metal chloride intercalated graphite intercalation compounds(GICs)in the use of electrode materials,such as high electronic conductivity and high vibration density,they are one type of most promising candidates for electrode materials.Aiming to solve the dissolution problem of FeCl₃and Li Cl in the electrolyte,rich epoxy functional groups are introduced into the graphite layers,and the strong chemical anchoring effect on chloride is used to limit the dissolution of chloride.We fabricated stable FeCl₃-intercalated GICs as anode materials for high-performance lithium-ion storage by optimizing the interface between graphite layer and ferric chloride.Furthermore,the loss of electronic coupling between adjacent graphite layers after intercalating FeCl₃monolayers can effectively promote the amounts of Li-ion storage in graphite accompanying with a stable discharge product of CLi compound.This designed FeCl₃-intercalated GICs with a high epoxy group content acting as anodes for LIBs exhibits an ultra-high reversible capacity(1371 m Ah g^-1)and improved cycle stability(98%capacity retention after 50 cycles).(6)To develop the new application of GIC in anode materials of sodium ion battery,AlCl₃-microcrystalline graphite intercalation compound(AlCl₃-MGIC)was taken as the research object.The solid-liquid interface between AlCl₃-MGIC and electrolyte was optimized by adjusting electrolyte solvent(three ether solvents and one ester solvent were selected),to improve the sodium ion transport and storage performance of AlCl₃-MGIC.When GIC was used as anode material of sodium ion battery,excellent storage performance has been achieved:in diethylene glycol dimethyl ether(DEGDME)electrolyte,AlCl₃-MGIC exhibits an ultra-high reversible capacity of 198 m Ah g^-1after 900 cycles at 0.5 A g^-1.DFT calculation and analysis show that the structural integrity of GIC can be maintained in ether electrolyte,especially in DEGDME solvent,which has suitable desolvation energy and the best structural integrity.