Study On Structure Regulation And Sodium Storage Performance Of Tin Sulfide/Nb₂CT_x Composites

At present,excellent sodium storage materials with high specific capacity and long cycle life is the bottleneck of sodium-ion batteries.Orthorhombic tin sulfide(SnS)has been considered as one of the most promising sodium storage materials because of its excellent electrochemical activity and low cost.However,SnS,as a sodium storage material of conversion type,has serious the serious volume change upon discharging/charging and poor electronic conductivity,which causes the electrode to suffer from electrode pulverization and slow reaction kinetics,thus resulting in serious capacity fading and degraded rate capability.In addition,the research on sodium storage mechanism of tin sulfide materials in sodium ion batteries is not clear,so it is worth further exploration.The purpose of this research is to optimize the microstructure and electrochemical performance of tin sulfide anode materials and explore its sodium storage reaction mechanism.We evaluated its sodium storage performance through electrochemical characterization,and assembled SnS composite with positive electrode materials into a full battery to explore the potential for practical application.This research provides an experimental basis and theoretical support for the commercial application of tin sulfide composites in sodium ion batteries.The main research of this work is as follows:1.Study on structural regulation and sodium storage of SnS/Nb2CTx MXene composite:SnS particles tend to agglomerate during the preparation process,and its electronic conductivity is poor.The electrode structure of SnS particles will inevitably be damaged during the cycling process,resulting in the attenuation of sodium storage performance.In this work,the SnS/Nb2CTx composite through simple solvothermal and calcination,in which SnS nanoparticles are uniformly distributed on the surface of layered Nb2CTx MXene.The above structural design gives SnS/Nb2CTx a threedimensional conductive network and rapid charge transfer.Due to the excellent electronic/ionic conductivity and rich active sites of SnS/Nb2CTx,the composite exhibits excellent sodium storage performance.After 100 cycles at a current density of 0.1 A g-1,the reversible specific capacity can reach 479.6 mAh g-1;After 500 cycles at a current density of 0.5 A g-1,the specific capacity remained as high as 278.9 mAh g-1,exhibiting excellent sodium storage activity.In addition,the mechanism of sodium storage of SnS/Nb2CTx was investigated by in situ XRD testing,which showed that the prepared composite electrode has good electrochemical reversibility.2.Study on structural regulation and sodium storage of SnS/Nb2CTx@NC composite:In the previous chapter,the design of SnS/Nb2CTx composite materials exhibits excellent ionic/electronic conductivity,while significantly improving electrochemical sodium storage performance,highlighting the structural advantages of combining SnS and Nb2CTx.However,it should be emphasized that there is a lack of chemical bonding between SnS and Nb2CTx,and SnS particles are prone to fall off during the cycling process,which is detrimental to long cycling and magnification performance.Therefore,we developed a polyethylene imide(PEI)assisted synthesis method to prepare nitrogen doped carbon coated SnS/Nb2CTx(SnS/Nb2CTx@NC)in this chapter.On the one hand,The PEI allows the composite to form a three-dimensional interconnected structure.On the other hand,PEI can guide the uniform growth of SnS on Nb2CTx sheets.In the subsequent carbonization process,PEI is pyrolysed into nitrogen doped carbon,which forms electronic coupling with SnS/Nb2CTx.This electronic coupling structure can not only improve charge transfer within the composite electrode,enhance ion diffusion dynamics,but also enhance the stability of the composite electrode.Therefore,the SnS/Nb2CTx@NC composite exhibits excellent sodium storage performance,showing a high specific capacity of 593.1 mAh g-1 after 100 cycles at a current density of 0.1 A g-1.After 200 cycles at a high current density of 0.5 A g-1,the electrode can still obtain a specific capacity of 403.5 mAh g-1.In addition,the study of electrochemical reaction kinetics and charge storage mechanism shows that the fast ion diffusion and high pseudo capacitance charge storage mechanism of SnS/Nb2CTx@NC play an important role in the high sodium storage performance.3.Study on structural regulation and sodium storage of SnS/Nb2CTx@CS composite:In the last chapter,PEI was carbonized to generate NC skeleton in situ to realize the coating of SnS/Nb2CTx,construct the interconnect structure,and improve the electron/ion conductivity of the complex.However,in order to further mitigate the structural changes of electrode active materials during the cycle,it is very important to construct an efficient buffer space.Therefore,in this chapter,a three-dimensional cross-linked structure of Nb2CTx wrapped SnS hollow carbon sphere was synthesized by using hydrothermal and electrostatic self-assembly methods.The design method has successfully improved the sodium storage activity of SnS carbon balls,shortened the diffusion path of sodium ions,and thus improved the specific capacity and multiplier performance.In addition,Nb2CTx MXene can effectively inhibit the SnS shedding during the cycle,and realize the long-term cycle stability of electrode materials.Therefore,SnS/Nb2CTx@CS composite has high reversible capacity,excellent cyclic stability and good rate performance.In addition,SnS/Nb2CTx@CS is assembled with Na3V2(PO4)3/C cathode material to complete the battery,which can be cycled 500 times at the current density of 1.0 A g-1 to obtain the specific capacity of 36.7 mAh g-1.The in situ carbon coating,two-dimensional Nb2CTx MXene and the hollow structure of SnS ensure the high sodium storage activity of SnS/Nb2CTx@CS.