| Since the successful commercialization of lithium-ion batteries(LIBs)in the 1990s,LIBs have been widely used in portable electronic products.However,energy density and long-term cyclic life of lithium-ion batteries are not enough to meet the current increasing demand.The concept of sodium-ion batteries(SIBs)started in the 1980s almost at the same time as LIBs.Compared with lithium,sodium has the advantage of being rich in sodium resources.The reserve of sodium in the earth’s crust is about 2.64%,about 400 times that of lithium.It is also very simple to obtain sodium and has lower cost.Therefore,it is very important to explore anode materials with long cycle life and excellent rate performance for LIBs and SIBs.Transition metal sulfides(TMDs)(MoS2,WS2,FeS2,etc.)with semiconductor characteristics and high theoretical specific capacity are considered as ideal anode materials.However,the following problems still exist when TMDs are used as electrode materials,which restrict the improvement of their electrochemical properties:① The prepared products have a serious volume effect in the charging/discharging process,which leads to the powder of the electrode material.② The low intrinsic conductivity severely restricts the conduction and electrochemical performance of electrons in electrode materials.In order to solve the above problems,this paper designs multi-morphologic necklace-like structure composite nanofibers by using multi-morphologic nanotemplate,electro spinning and hydrothermal synthesis techniques.The necklace-like structured nanofiber can provide multidimensional transport path for electrons and ions,increasing the conductivity of materials,so as to solve the problem of low intrinsic conductivity of TMDs.The necklace-like structure fiber with confined effect can provide sufficient buffer space for volume expansion during charge and discharge process,thereby improving the cyclic stability of electrode materials.The bimetallic sulfide heterostructure enhances the electrical conductivity of the active electrode material and alleviates the volume effect from the micro-macro scale.The main research contents in this dissertation are divided into the following five parts:1.Synthesis of spherical necklace-like MoS2-CNFs composite nanofibers for enhancing lithium/sodium storage performanceThe hollow mesoporous carbon spheres are spun with PAN through electro spinning,and then calcination and alkali etching are used to obtain the hollow necklace-like nanoreactor with confined space.Then MoS2 nanosheets are confined growth by hydrothermal method to achieve the amber necklace-like MoS2-CNFs.The one-dimensional nanofibers,two-dimensional nanosheets and three-dimensional cross-linking network structure constructed by fibers not only improve the structural stability and conductivity of the anode materials from a multidimensional perspective,but also alleviate the volume expansion and powderization of MoS2 nanosheets during the process of Li+/Na+deembedding.In the lithium storage properties,it shows excellent rate performance(352.2 mA h g-1 specific capacity at 2 A g-1 current density)and cycle stability(413.9 mA h g-1 specific capacity at 500 cycles at 1 A g-1).At the same time,the assembled SIBs still have good rate performance and good cycle performance(315 mA h g-1 at 0.5 A g-1 after 500 cycles).2.The synthesis of boxed necklace-like Fe3O4/MoS2-CNFs composite nanofibers to study Lithium/Sodium storage performanceA nanoboxed necklace-like fiber is designed in this chapter,other transition metal introduced to compound multiple components effectively compensates for the intrinsic defects of electrode materials.α-Fe2O3 nano cubes as the template mix with PAN to get α-Fe2O3/PAN.And then the αFe2O3 nano cubes calcine to form Fe3O4 with more valence state under high temperature,which get the box-shape necklace-like structure Fe3O4-CNFs.The confined space has formed by high concentrated HCl etching,the Fe3O4/MoS2-CNFs anode material is obtained by hydrothermal nethod of confining growth of MoS2 nanosheets.The mixed valence state of Fe3O4(Fe3+/Fe2+)can effectively improve the conductivity of MoS2 nanosheets.The box-shape necklace structure constructed by α-Fe2O3 nanocube has mesoporous structure and cnfined space,which can alleviate the volume effect of electrode material.When the electrode material is applied to LIBs,the reversible capacity is 704.7 mA h g-1 after 500 cycles at 0.5 A g-1.When the current density is 8 A g-1,the reversible capacity can still be 337.6 mA h g-1.Through pseudocapacitance analysis,the electrochemical kinetic process is dominated by capacity control.For the SIBs,the reversible capacity is 354 mA h g-1 after 500 cycles at 0.5 A g-1,and the specific capacity of 131.5 mA h g-1 can be maintained at 8 A g-1.On the one hand,the heterogeneous structure in the confined space alleviates the volume effect of the material,thereby improving the long cycle performance,on the other hand,it effectively enhances the electrical conductivity and increases more lithium/sodium storage sites.3.Boxed necklace-like Fe7S8/MoS-CNFs composite nanofibers with dual confined effect for synergistically improving the performance of Li+/Na+storageBased on the research in the previous chapter,oxygen is replaced sulfur with low electronegativity that can further improve the electrochemical performance.In this chapter,in the process of calcination for fabricating the necklace-like nanofibers,the sulfuration process is carried out,and MoS2 is confined growth into the cavity of composite nano fibers by hydrothermal method,and the box-shape necklace-like structured Fe7S8/MoS2 heterostructure composite nanofibers are obtained.The free space and mesoporous channels provide by the box-shape cavity constitute the first spatial confinement,and the heterojunction interface of the bimetallic sulfide constructs the second interfacial confiment.A variety of ex-situ detection techniques have shown that the phase transition products of the polyvalent iron sulfur compounds are more diverse,increasing the generation of more lithium/Sodium storage sites.In the test of LIBs,the coulombic efficiency of the first charge/discharge cycle is 80.8%,and the specific capacity is 393 mA h g-1 after 1000 cycles at the current density of 8 A g-1.Even applied in the SIBs,it showed excellent cycling performance(368 mA h g-1 after 500 cycles at 0.5 A g-1)and rate performance(276.1 mA h g-1 at 8 A g-1).The adsorption energy and transition state of the composite material and the single component material are calculated by density functional theory,revealing the electrochemical storage mechanism and electrochemical kinetic process.The electrochemical kinetics of various alkaline metal ion batteries are extended and compared.Finally,using LiFePO4 as the cathode material and Fe7S8/MoS2-CNFs as the anode material,the lithium-ion full cell is assembled,showing excellent rate performance and cycle stability,which further proves that Fe7S8/MoS2CNFs is an extremely superior electrode material and has broad application prospects.4.Anchoring effect of spherical necklace-like WS2-CNFs composite nanofibers for enhancing lithium/sodium storage performanceThe necklace-like fiber structure with hollow mesoporous carbon spheres as the nanoreactor in the previous chapter is used to confined growth WS2 nanosheet,which can obtain necklace-like structured WS2-CNFs.Part of WS2 nanosheets are cnfined growth inside the hollow mesoporous spheres,and another part of WS2 nanosheets are anchored on the surface of the necklace structure.This composite nanofiber based on anchoring and confinement effect can greatly improve the conductivity,and further boost the utilization rate of electrode material.As an anode material of LIBs,the WS2-CNFs maintain a specific capacity of 446 mA h g-1 after 100 cycles at 1 A g-1,and also show excellent specific capacity of 351 mA h g-1 at 1 A g-1.The experimental results of SIBs show that the specific capacity of the composite nanofiber is 188 mA h g-1 at 2 A g-1,and 337 mA h g-1 after 500 cycles at 0.5 A g-1.In the process of repeated deembedding of Li+/Na+,the outer anchored WS2 nanosheets can protect the necklace-like carbon fiber,and the hollow carbon spheres can inhibit the volume effect of the inner WS2 nanosheets,which effectively improve the cycling stability and rate performance of the battery.5.Boxed necklace-like FeS2/WS2-CNFs composite nanofibers to study mechanism and structural evolution of lithium/sodium storageIn this chapter,the box-shape necklace-like structural α-Fe2O3/PAN composite fiber is used as the precursor,the α-Fe2O3 nanocube is reduced to iron nanoparticles at high temperature.While ensuring the integrity of the box structure,the confined space is generated by means of high temperature self-contraction,and box-shape necklace-like Fe-CNFs composite fiber is obtained.The iron sulfide is formed by vulcanization at high temperature.Finally,WS2 nanosheets are confined growth by hydrothermal reaction,which obtain the box-shape necklace-like FeS2/WS2CNFs heterojunction composite nanofibers.The structure of the material not only has dual confined effect,but also has anchoring effect.The FeS2/WS2 heterojunction grows within the cavity of hollow mesoporous nanobox and WS2 nanosheets are anchored on the surface of carbon nanofibers,providing more migration channels and storage sites for Li+/Na+,which further improve the storage performance of the material.The initial coulombic efficiency is 84.3%,and the reversible specific capacity of 789 mA h g-1 can be maintained after 500 cycles at 1A g-1.When the anode material is applied in SIBs,the first cycle coulombic efficiency is 87.0%at 0.5A g-1,and the specific capacity remains about 423 mA h g-1 after 500 cycles.At 4 A g-1,the specific capacity is at～307.6 mA h g-1 after 1000 cycles.In addition,the kinetic process of SIBs is analyzed by pseudocapacitance method.The results show that the battery process is mainly controlled by capacitance,indicating that the battery has excellent rate performance and high theoretical specific capacity.The structure evolution and electrochemical reaction mechanism of the electrode material during Na+ deembedding are characterized by ex-situ detection,which further verified its structural advantages and excellent electrochemical performance. |
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