Preparation,Characterization And Electrochemical Performance Of Ti₃C₂T_x MXene Based Anode Materials For Lithium Ion Batteries

As a kind of promising energy storage equipment,lithium-ion batteries have been widely concerned for possessing many advantages,such as high energy density,long cycle life,and environment friendly.However,with the development of large-scale energy storage applications such as electric vehicles,power grids,and battery banks,the storage performance of lithium-ion batteries needs to be further improved to meet the need for large-scale energy storage systems.Electrode materials,as an important part of lithium-ion batteries,play a decisive role in battery performance.So that it's most urgent to develop new materials with excellent performance.As a new type of two-dimensional(2D)material,graphene-like 2D transition metal carbides/nitrides,MXenes,have been proved to be a kind of competitive high-performance electrode material.Among them,due to the low cost,high conductivity(?6500 S cm^-1),and low lithium-ion transport barrier(?0.07 e V for pure Ti₃C₂),Ti₃C₂T_xMXene has become a new research hotspot in the MXenes family.However,the further development of Ti₃C₂T_xis still limited by its drawbacks such as low theoretical capacity and aggregation.Therefore,a series of high-performance anode materials were prepared by optimizing the structure of Ti₃C₂T_x-based materials.The relationship between the structure and properties of the electrode materials was analyzed in detail,and the lithium storage mechanism was studied in depth.The main contents of this thesis are as follows:(1)The preparation and electrochemical performance of Sn O_xnanosheets(Sn O_xNSs)modified multilayer Ti₃C₂T_x(m-Ti₃C₂T_x)composite:ultrathin Sn O_xNSs with a thickness of?5nm were uniformly grown on the interlayer/surface of m-Ti₃C₂T_xsubstrate by hydrothermal method,forming a unique accordion-like Sn O_x@m-Ti₃C₂T_xcomposite.In the composite,the ultra-thin Sn O_xNSs not only play the role of capacity contribution but also increase the layer spacing of m-Ti₃C₂T_x,which increases the capacity significantly.The large atomic interface contact between Sn O_xNSs and m-Ti₃C₂T_xcan not only enhance the conductivity of the entire electrode and avoid the loss of electroactive surface caused by material aggregation but also buffer the volume change during Li⁺insertion/desertion which inhibits the capacity decline during cycling.So that the Sn O_x@m-Ti₃C₂T_xanodes show the high reversible capacity,excellent rate performance,and long-term cycle stability for lithium-ion batteries.It still exhibits a reversible capacity of?540 m Ah g^-1after 1000 cycles at a current density of 500 m A g^-1.(2)Preparation and electrochemical performance of one-dimensional(1D)Ti₃C₂T_xMXene hollow tube(MX-T):due to the limitation of interlayer spacing,there will be a large number of active sites in m-Ti₃C₂T_xthat can't be fully utilized.Therefore,the single-layer Ti₃C₂T_x(Ti₃C₂T_xNSs)was further prepared.However,the serious aggregation of Ti₃C₂T_xNSs during the collection and electrode preparation will also lead to the loss of active sites and affect the wetting of electrolytes.Besides,the stacking of Ti₃C₂T_xNSs increases the transport path of Li⁺during the charging/discharging process.Therefore,2D Ti₃C₂T_xNSs were designed to be transformed into a 1D MX-T structure by electrospinning to inhibit the stacking and aggregation of Ti₃C₂T_xNSs.The preparation conditions,including the concentration and the type of spinning solution,and the concentration of Ti₃C₂T_xsuspension,were explored.The capacity of the MX-T-0.4 anode,which exhibits excellent morphology and properties,is three times higher than that of 2D Ti₃C₂T_xNSs.(3)Preparation and electrochemical performance of large-scale 2D composite with Co₃O₄nanocrystalline(Co₃O₄NCs)anchored on single layer Ti₃C₂T_x(s-Ti₃C₂T_x):to inhibit the aggregation and improve the capacity of s-Ti₃C₂T_x,s-Ti₃C₂T_xbased composite was prepared,in which transition metal oxide with high theoretical capacity served as the main capacity contribution component and s-Ti₃C₂T_xserved as the conductive/flexible substrate.Co₃O₄NCs@s-Ti₃C₂T_x2D composite was prepared by simple freeze-drying then annealing method,in which Co₃O₄NCs with controllable size were uniformly loaded on the surface of s-Ti₃C₂T_x.The in-situ growth of Co₃O₄NCs reduced the accumulation tendency of s-Ti₃C₂T_x.Due to the synergistic effect of composition and structure,the Co₃O₄NCs@s-Ti₃C₂T_xanode shows excellent electrochemical performance.The reversible capacity of?223 m Ah g^-1can be obtained at a current density of 10 A g^-1,and the reversible capacity of?550 m Ah g^-1can be maintained after 700 cycles at a current density of 1 A g^-1.(4)Spray drying prepared and electrochemical performance of Ti₃C₂T_xNSs loaded carbon-coated Si(Si@C-MX)three-dimensional(3D)composite:3D Si@CS-MX precursor was prepared through one-step spray drying method,which used Si nanoparticles(Si NPs)with ultra-high theoretical capacity as the load material,chitosan(CS)as carbon(C)source and Ti₃C₂T_xNSs as the flexible substrate.Then Si@C-MX was obtained by subsequent annealing.Benefiting from the strong confinement of the C-shell,the buffer effect of the flexible Ti₃C₂T_xsubstrate,and the tight contact of the C-Ti₃C₂T_x,the Si@C-MX composite can fully suppress/buffer the volume change of Si and enhance the stability of the entire electrode during cycling.Also,the tight continuous network in the composite can accelerate the ion/electron transport during the cycling process,which makes the anode structure more stable.The Si@C-MX anode exhibits excellent lithium storage capacity and significantly improved cycle stability.The reversible capacity of?803.8 m Ah g^-1can be maintained after1000 cycles at 500 m A g^-1.When Si@C-MX was assembled to the full battery with Li Ni_0.8Co_0.1Al_0.1O₂,the reversible capacity of 169.4 m Ah g^-1can be obtained at the current density of 50 m A g^-1(based on the cathode electrode).