Syntheses Of Carbon Nanofibers Composite Encapsulated By Bimetallic Alloy Nanoparticles And Electrochemical Performances For Sodium Rechargeable Batteries

With the development of science,technology and economy,people's demand for green energy is increasing.Lithium-ion batteries are widely used in various electronic equipment and energy storage devices due to their high energy density,rapid charging and discharging,and environmental friendliness.However,lithium is unevenly distributed in the earth's crust and is not a naturally abundant element.Therefore,lithium-ion batteries will face both in resources and cost difficulties in the future development.However,sodium has the advantages of low price and abundant natural resources,so sodium ion batteries have also received more and more attention.In addition,under the current research situation,it is still a serious challenge to explore the low price and excellent performance of sodium ion battery anode materials.For example,the actual application of sodium ion batteries is severely restricted by the volume changes of the anode active material during the charge and discharge process.This volume effect often leads to irreversible capacity loss and poor cycle performance.Therefore,this thesis mainly conducts research and design from the composition,structure and morphology of electrode materials.By using electrospinning technology to study the influence of different bimetallic alloy nanoparticles encapsulated in carbon nanofibers on the sodium storage performance of electrode materials.The contents of this paper as follows:(1)The modification effect of inactive metal nickel on pure metal materials.Because the pure metal undergoes huge volume changes during the Na alloying/dealloying reactiones,the electrode is severely crushed and the capacity is rapidly attenuated,which hinder its practical application in advanced SIBs.Therefore,we want to explore whether this problem can be solved by adding another metal element to prepare a bimetallic alloy.When selecting metal salts,Ni-Sn intermetallic compounds have attracted special attention due to their high charge storage and low cost.Sbhas high theoretical capacity and cut-off voltage,which also attracts more concerned.Therefore,the electrospinning technology is used to coat the metal chloride precursor salts(Sn Cl₂.2H₂O,SbCl₃,Ni Cl₂.6H₂O)in the carbon nanofibers,and then through high-temperature carbonization,hydrogen reduction reduces the metal salts to metal nanoparticles filled in the carbon nanofibers.Ni₃Sn₂@NCNFs,Ni Sb@NCNFs,Sn@NCNFs and Sb@NCNFs nanocomposites were successfully obtained.When they are used as the negative electrode materials for sodium-ion batteries,after 200 cycles at a current density of0.1A g^-1,the nickel-containing bimetallic alloy nanoparticles are encapsulated in the composite materials of carbon nanofibers(Ni₃Sn₂@NCNFs,Ni Sb@NCNFs)have a reversible specific capacity of 389 and 322m Ah^-1,respectively,and their electrochemical performance is better than that of the composite materials in which pure metal nanoparticles are encapsulated in carbon nanofibers(Sn@NCNFs reversible specific capacity of 258m Ah g^-1and Sb@NCNFs has a reversible specific capacity of 214m Ah g^-1),which is mainly due to its special structural design.Nickel,as an electrochemically inactive metal element,can provide additional mechanical strength in nanocomposites to prevent cracks and fractures.In addition,it acts as a diffusion barrier to inhibit the complete sodiumization of amorphous Na_xSn or Na_xSbto crystalline Na₁₅Sn₄or Na₃Sband can reduce the internal stress in the process of sodiumization and sodium removal,inhibit volume expansion and improve the mechanical stability of the electrode,ensuring that the Ni-Sn or Ni-Sballoy has better electrochemical performance than the pure Sn or Sbphase.(2)Electrospinning technology is also used to research the sodium storage performance of different kinds of bimetal alloy nanoparticles encapsulated in carbon nanofibers.In this chapter,we mainly research the sodium storage performance of bimetallic alloys with active/active metal composition(Sn-Sb)and the comparison with active/inactive bimetal alloy sodium storage performance.This paper proves the application prospect of nitrogen-doped carbon nanofibers coated with SbSn nanodots(SbSn@NCNFs)prepared by electrospinning as a negative electrode material for sodium ion batteries.Sn Sballoy is considered to be a promising negative electrode material due to its relatively high theoretical capacity.Both Sn and Sbhave the ability to store sodium ions during the charge and discharge processes,which helps to increase the overall capacity.Importantly,The different sodiumization/desodiumization potentials of Sn and Sbin the SbSn@NCNFs composite material help to alleviate the volume expansion during the cycling.The three-dimensionally interconnected carbon nanofibers have high specific surface area and porosity,which are beneficial to electronic and transmission of ions.The resulting SbSn@NCNFs composite exhibits impressive electrochemical performance,with large discharge capacity(capacity up to 808m Ah g^-1at a current density of 100m A g^-1)and great rate capability(current density of1.6A g^-1the capacity reaches 331m Ah g^-1)and excellent cycle stability(When the current density is 100m A g^-1,the capacity remains at 331m Ah g^-1after 500 cycles),which are better than Ni₃Sn₂@NCNFs,Ni Sb@NCNFs comparative material.At the same time,we used this material as the anode and the Na₃V₂(PO₄)₂F₃nanoflowers studied by our research group were anchored to the three-dimensional carbon sheet nanocomposite material(NVOPF@3Dc)as the cathode,then assembled a full sodium-ion battery to prove this anode material has certain potential commercial value.