| Lithium ion battery has the characteristics of high energy density and operating voltage,low self-discharge rate and environmentally benign.Thus,it is widely used in the fields of mobile electronic products and electric vehicles.However,with the development of portable electronic devices and large-scale transportation equipment,the development of high energy density and high power density have gradually become one of research hotspots for lithium ion battery anode materials.Compared with traditional graphite anode materials,transition metal sulfide(MoS2,WS2,ReS2,etc.)has been considered as an ideal candidate of anode material due to its higher theoretical capacity,abundant sources,and easy preparation.Nevertheless,the transition metal sulfide has low electronical conductivity,large volume changes seriously restrict their practical application.Therefore,in this paper,compounding the transition metal sulfide with mesoporous carbon can obtain the composite materials with high electrical conductivity.The structure of electrode material was regulated by space-confined strategy,which is used to construct a rich buffer space to effectively suppress or alleviate the volume effect,thereby improving the electrochemical performance of lithium storage.The main contents are as follows:1.Synthesis of WS2/mesoporous carbon spheres triple-shelled composites and their applications in lithium-ion batteriesThe hollow mesoporous carbon spheres(HMCSs)were adopted as growth substrate,and the ultrathin WS2 nanosheets were vertically embedded into the inner and outer carbon shells of the HMCSs to form the unique triple-shelled hollow nano-spheres(WS2-C-WS2)by hydrothermal method,namely triple-shelled WS2/C nanocomposites.Based on the existence of the chemical bonding(C-O-W)between WS2 and HMCSs,the vertically grown WS2 nanosheets have expanded layer spacing and fewer layers,which can shorten the diffusion path of lithium ions,widen the diffusion channel of lithium ions,improve the diffusion ability of lithium ions and enhance the ion/electron transport kinetics.When testing lithium storage performance,WS2/C nanocomposites exhibit cycle stability(784 and 442 mA h g-1 at 1 A g-1 and 5 A g-1 after 1000 cycles,respectively)and superior rate capability(396 mA h g-1 at 10 A g-1).Meantime,WS2/C nanocomposites with different contents of WS2 were prepared by regulating the precursor(WCl6·2H2O),and their lithium storage performance were analyzed.To study the structural stability and morphology of WS2/C nanospheres electrode material after cycling performance,it was found that the spherical structure was still maintained without collapse.Several desirable design rationales,including low-dimensional ultrathin nanosheets,HMCSs frame structure and multi-shell hollow structure,were effectively integrated.It not only provides diffusion channels for the rapid transfer of lithium ions and electrons,improves the electrical conductivity of the electrode material,and its high specific surface area provides more lithium-embedded active sites,but also provides buffer space for WS2 nanosheets to improve the structural stability of the electrode material.2.Few-Layer ReS2/mesoporous carbon spheres with heterogeneous interface and their applications in lithium-ion batteriesBy one-step hydrothermal method,the spatial confinement strategy of the hollow mesoporous carbon spheres(HMCSs)is proposed to synthetize the few-layer ReS2 nanosheets confined in HMCSs,forming the ReS2@C nanocomposite.In this chapter,from the triple-shelled hollow sphere in the previous chapter to the filled hollow carbon sphere,the hollow carbon sphere is folly utilized to improve the volume energy density of the composite material.The ReS2 nanosheets were proved to fill the inner cavity and the inner and outer carbon walls by ultrathin sectioning technique.The ReS2 nanosheets are mainly filled in the whole cavity of hollow carbon sphere,and part of ReS2 nanosheets are intimately embedded on the outer layer of the carbon shell.For lithium-ion batteries,the ReS2@C nanocomposites exhibit superior cycle stability(capacity of 578 mA h g-1 after 1000 cycles at 1 A g-1).The density functional theory(DFT)analysis shows that the high Li adsorption energy at the heterogeneous interface improves the stability of the electrode material.By observing the structure and morphology of the ReS2@C nanospheres after cycling performance,it is clearly seen that the spherical structure remains intact,and the stable SEI film is formed on the surface of the electrode material.ReS2 nanosheets were broken into nanograins,which were confined to the inner space of the hollow carbon sphere due to the confinement effect.It is attributed to the space-confined strategy of HMCSs,which can realize the close contact between the active material and the conductive matrix,buffer the volume effect,and prevent the active material from peeling off from the conductive matrix during the pulverization process,and improve the cyclic stability.Moreover,the cyclic voltammetry curves and electrochemical impedance spectroscopy analysis show that the composite has high capacitive behavior and lithium ion diffusion coefficient.It is attributed to the multi-level heterostructure structure and porous conductive network that promote the rapid transfer of electrons and ions,improving the reversible specific capacity and cycling stability.3.Preparation of crystal spherical-like MoS2/mesoporous carbon spheres composites and their applications in lithium-ion batteriesCompared with WS2 and ReS2,MoS2 has a higher theoretical specific capacity and is a potential lithium anode material.Meantime,it further verifies the universality of transition metal sulfide growth in the confined space of hollow mesoporous carbon spheres.In this chapter,using the spatial confined growth mechanism that presented in the previous chapter,the hollow mesoporous carbon sphere as template,the thin MoS2 nanosheets were encapsulated in hollow mesoporous carbon frameworks by the confinement approach to form the crystal spherical-like MoS2/C nanocomposites.The crystal spherical-like MoS2/C nanospheres have large specific surface area and MoS2 content,provide more lithium storage sites,and improve the specific capacity.The composite material provides a buffer space,alleviates the volume expansion effect of MoS2,improves the cyclic stability Moreover,the combination of MoS2 and carbon effectively improves the conductivity of the material,which is conducive to the conduction of electrons in the material.As the anode material of lithium ion battery,the scaled MoS2/C composite nanospheres exhibit high specific capacity and excellent cycle stability.It is noted that the specific capacity of the MoS2/C electrode exhibits good cycling stability.The pseudocapacitance analysis also showed that a significant pseudocapacitance effect of MoS2/C composite nanospheres appeared when used as anode of LIBs.In addition,we have used the same method to prepare the similar morphology MoO2/C and MoSe2/C composite materials,which prove the universality of the method.4.In-situ growth preparation of bowl-like C@MoS2 composites and their applications in lithium-ion batteriesBased on the space-confined strategy of HMCSs in the first three chapters,the MS2(M=Mo,W,Re)/mesoporous carbon composite structure with multi-level heterostructure structure and rich buffer space was constructed.In this chapter,the hollow mesoporous carbon bowls(HMCBs)were prepared by adjusting the TEOS to RF feed ratio.We report the design and preparation of bowl-like composite structures whereby few layer MoS2 nanosheets are grown on the surface of MHCBs in situ by a facile hydrothermal method,forming bowl-like C@MoS2 nanocomposites.Compared with the spherical structure,the bowl-shaped structure has higher tap density.The porous carbon bowl network can effectively avoid the agglomeration of nanosheets,buffer the stress during the process of intercalation/delithium,and maintain the stability of the structure.The bowl-like C@MoS2 structure exhibits a reversible capacity and outstanding long-term stability.In addition,compared with pure MoS2 nanosheets,kinetics investigation demonstrates that the Li+storage is governed by the pseudocapacitive mechanism due to the porous bowl composites with high tap density.The high capacitive contribution is conducive to the rapid and reversibly intercalation/deintercalation of lithium ions in the electrode material,which is conducive to the improvement of the rate performance and long cycle of the material.5.Construction of yolk/shell MoS2@mesoporous carbon spheres composites through space-confined and their applications in lithium-ion batteriesIn the previous chapter,the synthesis of mesoporous carbon bowls have limited internal space,MoS2 nanosheets are distributed on the surface of the mesoporous carbon bowl,which is easy to occur pulverization and exfoliation during the cycle,leading to their poor cycling stability.In the chapter,using sodium molybdate as molybdenum source and thiourea as sulfur source,and hollow mesoporous carbon spheres(HMCSs)serve as nanoreactors can effectively control MoS2 nanosheets inside HMCSs to form the yolk-shell structured MoS2@C.Compared with core-shell C@MoS2 nanospheres and pure MoS2,the yolk-shell structured MoS2@C is proven to achieve high reversible capacity,superior rate capability,and excellent cycle performancewhen evaluated as an anode material for lithium-ion batteries.By observing the structure and morphology of the MoS2@C and C@MoS2 nanospheres after cycling performance,it is clearly shown that the size,shape,and structural integrity of the carbon skeletons are unchanged for the former.On the contrary,the C@MoS2 nanospheres suffer from severe volume expansion result in the deformation and fracture of the carbon spheres.The electrochemical properties and structural advantages of the yolk-shell MoS2@C composite are mainly attributed to the construction of a porous carbon coated layer,which promotes the contact between the electrolyte and the electrode material,and avoid loss and aggregation of active materials.Meantime,the appropriate buffer space provided by voids in yolk-shell MoS2@C nanospheres,allow free expansion/contraction of MoS2 nanosheets during the lithiumion intercalation/deintercalation process,maintain the integrity of the electrode structure,improving the electrochemical performance and cycle stability. |
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