Electrochemical Performance Of Nb₄C₃T_x And Its Composite Material In Li-ion Batteries

Energy is the basis of human survival,but the excessive use of fossil fuels and oil has caused a heavy burden on the earth,causing a series of environmental problems such as Greenhouse Effect and air quality decline,which directly threaten the health of human beings.Therefore,it is an inevitable choice for the world’s energy development to improve the energy efficiency and reduce the consumption of coal and oil.With the increasing awareness of environmental protection and the concern about the utilization of resources,rechargeable battery has gradually become an important field of research,in particular lithium ion battery.The anode material at the negative electrode of the lithium ion battery is an key part of a lithium ion battery.The electrochemical performance of lithium ion battery greatly depends on the composition and structure of its negative electrode material.The development of anode materials has promoted the development of lithium ion batteries into commercial stage.Graphite has the advantages of high electronic conductivity,large lithium ion diffusion coefficient,small volume change of layered structure before and after lithium intercalation,high lithium intercalation capacity and low lithium intercalation potential.It has become the mainstream anode materials in commercial lithium ion battery.However,due to the intrinsic restriction of graphite properties,the development of graphite anode materials has reach its limit,in particular the specific capacity has reached its theoretical limit,which is not sufficient to power large batteries at a large discharge current.In recent years,graphene and other two-dimensional（2D）materials have shown very important theoretical and engineering value to replace graphite,thanks to their excellent properties such as high conductivities,high thermal conductivities,high specific surface areas and so on.Among the many two-dimensional materials,a series of 2D transition metal carbides（such as Ti₂C）or nitrides（such as Ti₄N₃）called MXene have become the focus in the field of energy.Considering that MXene is prepared by liquid etching method,the more accurate expression should be M_n+1X_n T_x,where M is an early transition metal,X is carbon and/or nitrogen,n is equal to 1,2 or 3,and T_x represents surface functional groups such as O or F.The multilayered powders obtained by etching can be separated into monolayers by intercalation and ultrasonic method.MXene exhibits excellent properties in supercapacitors due to its high metal-like conductivity,excellent electrochemical reactivity and hydrophilicity.At the same time,MXene is one of the most promising anode materials due to its excellent conductivity and cationic intercalation.This paper focuses on the synthesis and preparation of MXene phase Nb₄C₃T_x materials,its electrochemical performance tests,the change of the solid electrolyte interphase（SEI）in the reaction process,and the performance of the composite in lithium batteries.Using commercial niobium powder,aluminum powder and carbon black as raw materials,the MAX phase Nb₄AlC₃ was prepared by adjusting the atomic ratio,sintering temperature and holding time by solid-state sintering method.On the basis of this,Nb₄AlC₃ was mixed with hydrofluoric acid at different mass ratios.The crystallinity,morphology and chemical composition of the samples were studied by XRD,SEM,TEM,FTIR and XPS,and then combined with cyclic voltammetry.The properties of the materials in lithium batteries were analyzed by constant current charge-discharge and AC impedance analysis.The results show that the interlayer spacing of Nb₄C₃T_x can be gradually opened during the intercalation of Li ions,which increases the capacity gradually.For example,at the current density of 100 m A g^-1,the capacity increases from 310 mAh g^-11 to 380 mAh g^-11 after 100 cycles.In the reaction process,due to the increase of volume,the surface SEI film is gradually broken down.Finally,with the reaction,the capacity and the layered structure tend to become stable,and the SEI film is stable.Ti₃C₂T_x/Nb₄C₃T_x composites were synthesized by using Nb₄C₃T_x and Ti₃C₂T_x as raw materials.From SEM and TEM,we found that Ti₃C₂T_x was separated in single layer because of its softer texture.After drying by oven,the layered structure of Ti₃C₂T_x was restacked,while Nb₄C₃T_x was a bulk material,and the volume can increase gradually in the process of intercalation and release of lithium ion.Adding Nb₄C₃T_x in between Ti₃C₂T_x layers can not only support the layered frame,reduce the stacking phenomenon,but also make more lithium ions enter the interlayer space.The results show that Ti₃C₂T_x/Nb₄C₃T_x composite is superior to the original material in rate performance and cyclic performance test.