Preparation And Its Sodium Storage Performance Of Layered Transition Metal Dichalcogenide Composites

In the past few decades,rechargeable lithium-ion batteries have been successfully developed as a major mobile power source.However,due to the global shortage and uneven distribution of lithium resources,the further development of lithium-ion batteries to large-scale energy storage systems(ESS)is restricted.Compared with lithium,sodium has higher natural reserves and lower prices.Therefore,sodium storage devices that replace lithium with sodium have a wider application prospect.Sodium storage devices mainly include sodium ion batteries and sodium ion capacitors.Both sodium ion batteries and sodium ion capacitors require their electrode materials to have high rate capacity and long cycle life.Whether the sodium storage material can exert excellent performance directly affects the performance of the entire device.Therefore,the development of high-performance sodium storage electrode materials is very important for the development of sodium ion batteries or sodium ion capacitors.Among them,layered transition metal chalcogenides(LTMDs)have large interlayer spacing(6-7(?))and weak interlayer van der Waals forces,which enable Na⁺to have a fast and stable insertion/extraction rate;in addition,LTMDs also have a higher theoretical capacity(600-1000 m A h g^-1).Therefore,LTMDs materials are currently one of the most promising anode materials.However,after long-term cycling,the crystal structure of LTMDs will undergo serious volume expansion,which will eventually lead to a rapid decrease in stability and rate capacity.Based on the above problems,this thesis modified the sodium storage performance of the three LTMDs materials of VSe₂,MoS₂ and WS₂ through surface growth,nanocrystallization and interlayer engineering,and explored their structure,charge and discharge mechanism,and sodium storage performance.The main research contents and results are as follows:(1)Preparation and Sodium Storage Properties of Rough Endoplasmic Reticular-like VSe₂/rGOThe slow kinetic process of Na⁺in VSe₂ and the volume expansion caused by continuous ion insertion/extraction limit the sodium ion capacitor to exert high rate capacity and cycle stability.In this work,inspired by the structure and function of the rough endoplasmic reticulum in the cell,the layered vanadium diselenide(VSe₂)is confined in the form of nanoflowers on both sides of the redox graphene oxide(rGO)sheet,and then,it is further assembled into a porous three-dimensional(3D)VSe₂/rGO aerogel.The 3D rGO framework provides a stable substrate for fixing VSe₂,can promote the diffusion and adsorption of electrolyte in the electrode material,and buffer the volume expansion of VSe₂ during the cycle.As a sodium storage anode material,the prepared 3D VSe₂/rGO aerogel has a higher reversible capacity and stability than pure VSe₂.Finally,3D VSe₂/rGO is used as the negative electrode and the positive electrode activated carbon(AC)is paired to assemble a sodium ion full capacitor.The assembled device shows both high power density and high energy density(providing energy densities of 106 W h kg^-1and 68 W h kg^-1 at 125 W kg^-1 and 5000 W kg^-1,respectively).(2)Flexible electrode constructed by encapsulating ultrafine VSe₂ in carbon fiber for quasi-solid-state sodium ion batteriesThe 3D VSe₂/rGO aerogel prepared in the first work is a powder material,which needs to be ground into a slurry with a binder and a conductive agent during the preparation of the electrode and then scraped onto the current collector.After a long cycle of charging and discharging,the electrode material is easily pulverized and falls off from the current collector,resulting in a significant decrease in capacity.And the additional binder is an Na⁺/electronic insulator,which will block the transmission of Na⁺and electrons during charging and discharging.On the other hand,the liquid organic electrolyte used in the first job is prone to leaks,causing safety hazards.Therefore,in this work,in order to improve the above two problems.First,a flexible binder-free film composed of N-doped carbon nanofibers(VSe₂/NCNFs)coated with ultrafine VSe₂nanodots is directly used as the anode of a quasi-solid sodium ion battery.The ultra-small VSe₂ nano-dots greatly shorten the diffusion path of Na⁺,and the highly graphitized carbon fiber provides a fast path for electron transmission and stabilizes the structure of VSe₂ during cycling.Secondly,we introduced a quasi-solid gel polymer electrolyte containing only a small amount of electrolyte into the battery system.The elastic coordination between VSe₂/NCNF and gel electrolyte can further buffer the overall volume expansion of VSe₂/NCNF.Moreover,the sodium ion half-cell combined with flexible VSe₂/NCNFs and gel polymer electrolyte can provide high reversible capacity of420.8 m A h g^-1 at the current density of 0.05 A g^-1,and at 5.0 A g^-1 after 10,000 cycles,it can still provide a high specific capacity of 278.1 m A h g^-1.(3)Flexible MXene-Ti₃C₂T_x bond few-layers transition metal dichalcogenides(MoS₂,WS₂)/C spheres for Fast and Stable Sodium StorageIn the second work,we downsized the layered transition metal dichalcogenide(VSe₂)into nano-dots and encapsulated them in carbon fibers to shorten the diffusion paths of Na⁺.However,this method only solves the problem of the length of the diffusion path of Na⁺in LTMDs,and does not solve the problem of difficulty in the diffusion of sodium ions between layers of LTMDs.In this work,MoS₂ was prepared into monolayer or interlayer expanded few-layer structure and uniformly confined in N-doped carbon matrix,and further assembled into a hierarchical porous hollow sphere(H-MoS₂@NC).Monolayer/few-layer MoS₂ can realize fast Na⁺migration,which is beneficial to reduce ion diffusion energy barrier and accelerate the electrochemical reaction kinetics.The protective carbon layer between MoS₂ sheets can not only stabilize the layered structure of MoS₂,but also further improve the electrical conductivity of the composite material.Moreover,the hierarchical porous hollow structure is beneficial to the rapid infiltration of the electrolyte and alleviates the volume expansion of MoS₂ during cycling process.In addition,MXene-Ti₃C₂T_x sheets were adapted to instead of polymer binder as a multifunctional binder to prepare flexible,binder-free electrodes(MX-H-MoS₂@NC)where electron and ion transport faster due to the absence of polymer binders.In addition,the flexible MX-H-MoS₂@NC electrode can also endure the overall volume expansion of the H-MoS₂@NC spheres.Therefore,the electrochemical performance of MX-H-MoS₂@NC in half-cell and full battery has been significantly improved.In addition,we also synthesized H-WS₂@NC with the same structure by the same method,which shows that the method is versatile and can produce more H-LTMDs@NC with monolayer LTMDs.