Design Of Transition Metal Sulfide Heterostructures And Study On Sodium Storage Performance

The electrochemical performance of sodium ion battery depends on the choice of electrode material to a large extent.On the cathode side,polyanionic compounds,layered metal oxides and Prussian blue analogues have been successfully used as positive terminals of sodium ion batteries,providing high specific capacity and stable electrochemical performance.However,due to the large ionic radius of sodium ions,the graphite electrode used in commercial lithium-ion anode materials is difficult to accommodate sodium ions effectively.Therefore,it is urgent to develop advanced anode materials with excellent rate performance,large specific capacity and long cycle life.Transition metal sulfide have been proved to be promising anode material for sodium ion batteries due to their rich variety,large theoretical capacity,low cost and environmental friendliness.However,the poor ionic/electronic conductivity as well as large volume changes of transition metal sulfides during cycling lead to poor sodium storage properties.To solve these problems,this paper uses the combination of transition metal sulfide and conductive matrix to accelerate the transmission of electrons and reduce the volume expansion of active materials.The heterogeneous interfaces of different transition metal sulfides are designed to create a large number of lattice defects and provide abundant active sites.The mechanism of sodium storage and the structure-activity relationship of the materials were revealed by the analysis of microstructure,the characterization of nonin-situ technique,and the electrochemical performance test,which provided theoretical support for the commercial application of transition metal sulfide as anode of sodium ion batteries.The main research contents of this thesis include:1.Design of NiS/FeS carbon nanofibers and study on their sodium storage properties:The heterostructure of NiS/FeS carbon nanofibers were prepared by introducing evenly distributed NiS and FeS nanocrystals into the conductive carbon nanofibers network by combining electrospinning with high-temperature carbonization and vulcanization.On the one hand,carbon nanofibers solve the problem of low electron conductivity of transition metal sulfide;On the other hand,the NiS/FeS interface has abundant lattice defects and active sites,which greatly improve the electrochemical performance of the materials.The NiS/FeS carbon nanofiber composites obtained 359.5 mAh g-1 after 200 cycles at 1.0 A g-1 and 298.4 mAh g-1 at 5 A g-1.In addition,the charge diffusion kinetics was investigated by means of cyclic voltammetry,alternating current impedance,and intermittent current titration.2.Design of NiS/FeS/NC hollow nanocube and study on its sodium storage performance:In chapter 1,the synthesized NiS/FeS carbon nanofiber composites exhibited high ionic/electronic conductivity.However,metal ions easily spilled onto the surface of the carbon nanofibers during the sintering process at high temperature,and the prepared carbon fibers do not have enough internal space to relieve the volume expansion,which leads to poor electrochemical properties of the materials.The Prussian blue analogues(PBAs)synthesized by metal ions and different cyanides have the advantages of controllable size,uniform morphology,and diverse composition.Therefore,in this chapter,we proposed the research idea of taking Ni-Fe PBA nanocubes as precursor,coating a layer of nitrogen-doped carbon layer on the shell of NiS/FeS hollow nanocube through carbonization and vulcanization.C-N decomposition produces a large amount of internal space.At the same time,the dopamine breakdown of the outer layer can also be converted into a layer of N-doped carbon coating on the surface of the material,providing high electrical conductivity while also acting as a good protective shell.The electrochemical performance of the composites obtained in this chapter has been significantly improved compared with that in the previous chapter.The capacity of the composites is almost no decay after 180 cycles at 0.1 A g-1,and the specific capacity of 322.2 mAh g-1 can still be maintained at 5 A g-1.In addition,in this chapter,the charge diffusion kinetics is investigated by pseudocapacitance effect analysis,and the mechanism of sodium storage is discussed by in-situ XRD test.3.Design of carbon nanotubes coated Fe7S8/FeS2 composites and research on their sodium storage properties:Due to the poor interfacial miscibility of different metal sulfides,the rate performance of bimetallic sulfides is not ideal.The interfacial compatibility of the same metal compounds is better and has more excellent rate performance.Based on this,we choose iron sulfide with lower cost and higher theoretical capacity as the research object.By controlling the vulcanization conditions,Fe7S8/FeS2 nanoparticles encapsulated by N-doped carbon nanotubes were grown in situ.On the one hand,because Fe7S8/FeS2 is encapsulated in N-doped carbon nanotubes,the conversion reaction takes place inside the carbon nanotubes,which effectively alleviates the volume expansion of the composites.On the other hand,the interfacial compatibility between Fe7S8 and FeS2 ensures rapid electron transfer.Compared with the previous two chapters,the electrochemical properties of the composites obtained in this chapter have been significantly improved,and the specific capacity of 359.7 mAh g-1 can be maintained at 5 A g-1,while the specific capacity of 273.4 mAh g-1 can be maintained at 20 A g-1.At the same time,we prepared carbon-doped sodium vanadium phosphate cathode material by ball milling and high temperature sintering,and assembled the full battery by carbon nanotubes coated Fe7S8/FeS2 composite material,and explored the feasibility of its practical application.The experimental results show that the assembled battery can stably cycle 500 times at the current density of 1 A g-1 indicating that carbon nanotubes coated Fe7S8/FeS2 composites have great application prospects.