| In recent years,the demand for electronics and electric vehicles from domestic and foreign markets have continuously increased,which has promoted the rapid development of lithium-ion batteries.However,the limited lithium resource and uneven distribution seriously hinder the development of the application of lithium-ion batteries in the large-scale energy storage field in the future.It is worth noting that the reserve of sodium in the earth’s crust is relatively abundant and the costs of sodiumbased chemicals are relatively low.This significant cost advantage allows sodium ion batteries to be a new type of low-cost energy storage device to supplement lithium-ion batteries.Therefore,searching suitable anode materials with higher specific capacity and good cyclic stability has become a hot research topic.In this thesis,the strategies of controllable synthesis of adjusting the material composition,carbon coating,and graphene encapsulation have been successfully applied to synthesize bimetallic sulfide/carbon composite with high electrochemical activity and explored the related electrochemical reaction mechanisms.The main contents of the research work are as follows:(1)The Sb2S3-Bi2S3@C@rGO composite material with double-protection of carbon and graphene coating was successfully synthesized via solvothermal,vapor deposition,and cation replacement methods.The advantages of Sb2S3-Bi2S3@C@rGO electrode are concentrated in the following two points:firstly,the high electrochemical activity makes Sb2S3-Bi2S3 has the higher theoretical specific capacity,and the synergic effect within the bimetallic sulfides enables the electrode material to exert the better cycle stability.More importantly,the carbon encapsulation and graphene confinement can not only improve the overall conductivity of the material,prevent the agglomeration of Sb2S3-Bi2S3 microrods,but also alleviate the volume change during the discharge/charge process at a certain extent to maintain the structural stability of the material.Moreover,the dual coating of carbonaceous material the composite provides a large surface area,which can increase the reaction active sites and wettability with the electrolyte to effectively improve the diffusion of Na+.For sodium storage,it exhibited an excellent long-term cyclic stability with a capacity of 460.5 mA h g-1 after 1100 cycles at 8 A g-1.When assembled with Na3V2(PO4)3 into a full battery,the Sb2S3-Bi2S3@C@rGO sample also showed excellent cyclic stability.At the same time,the reaction mechanism of electrode materials was explored by in-situ XRD and ex-situ HRTEM tests.During the discharge process,Sb2S3 and Bi2S3 undergo the conversion reaction with Na+to generate Na2S and the Sb and Bi firstly;then the alloying reaction of Sb,Bi with Na+produce Na3Sb and Na3Bi,respectively.Within the charging process,the above processes are reversed.(2)Based on the electrostatic adsorption between Ni2+and GO,NiS2 nanoparticles uniformly anchored on rGO(NiS2@rGO)were successfully synthesized through a simple one-step hydrothermal method.The particle size of NiS2 is about 60 nm,which can effectively shorten the diffusion path of sodium ions.The rGO layer can not only effectively improve the conductivity of the material,but also alleviate the volume change of the electrode material during the process of discharge/charge.Consequently,NiS2@rGO-160 exhibited enhanced sodium storage properties as an anode material,with a satisfactory cyclic stability(a specific capacity of 358.8 mA h g-1 was retained at 0.1 A g-1 over 100 cycles and a capacity of 206.3 mA h g-1 was achieved with at 5 A g-1).When assembled with Na3V2(PO4)3 into a full battery,the sodium storage performance of NiS2@rGO electrode was still satisfactory.In addition,the ex-situ XRD test technology was used to explore the reaction mechanism of NiS2@rGO.And the reaction kinetic of NiS2@rGO electrode was further explored by the galvanostatic intermittent titration techniques(GITT)and electrochemical impedance spectrum. |
