Preparation Of Hereoatom-doped Metal Sulfide-Based Composite Material And Its Application In Lithium-Sulfur Batteries

Lithium-Sulfur batteries have attracted the attention of researchers due to the advantages of high energy density,abundant sulfur sources,low cost and environmental friendliness.However,lithium-sulfur batteries face many problems,such as the low electronic conductivity of sulfur and lithium sulfide and the shuttle effect of lithium polysulfide.These problems have seriously hindered the commercialization of lithiumsulfur batteries.In view of the above problems,the electrochemical performance of lithium-sulfur batteries have been improved in this paper from two aspects:firstly,the transition metal sulfide is uniformly loaded on the surface of reduced graphene oxide to obtain the transition metal sulfides/reduced graphene oxide composite.It can improve the conductivity of the cathode when the composite is used as the sulfur host material.Secondly,heteroatom doping on the surface of transition metal sulfide can control the surface morphology and structural defects of the material and improve the adsorption ability and electrocatalytic activity of material to the intermediate lithium polysulfide.These composites can effectively reduce the shuttle effect of lithium polysulfide,so as to obtain excellent electrochemical performance.The main work of this paper includes:(1)Nickel-doped molybdenum disulfide/reduced graphene oxide(Ni-MoS2/rGO)composite with sandwich structure is synthesized by hydrothermal method and used as sulfur host material.The 3%Ni-MoS2/rGO composite with sandwich structure is synthesized by using GO as the matrix,ammonium molybdate tetrahydrate((NH4)6Mo7O24·4H2O)as Mo source,nickel acetate tetrahydrate(Ni(Ac)2·4H2O)as Ni source and thiourea(CH4N2S)as S source.The S@3%Ni-MoS2/rGO composite can be obtained by melt diffusion method.The cathode can obtain the initial discharge specific capacity of 1343.6 mA h g-1 at 0.2 C.In addition,the initial capacity of S@3%NiMoS2/rGO cathode is as high as 937.8 mA h g-1 at 0.5 C.It is calculated that the average attenuation rate per cycle is only 0.077%after 140 cycles.Furthermore,when the actual sulfur loading of the cathode reaches 4.21 mg cm-2,the high discharge specific capacity of 553.5 mA h g-1 can be maintained after 150 cycles.(2)Cobalt-doped vanadium tetrasulfide/reduced graphene oxide(Co-VS4/rGO)composite with three-dimensional porous structure is prepared by hydrothermal method and used as sulfur host material.This host material is synthesized by using GO as the matrix,sodium orthovanadate(Na3VO4)as V source,cobalt chloride hexahydrate(CoCl2·6H2O)as Co source and thioacetamide(CH3CSNH2)as S source.The sulfur-loaded 3%cobalt-doped vanadium tetrasulfide/reduced graphene oxide(S@3%Co-VS4/rGO)shows exhibit excellent electrochemical performance.Among them,the discharge specific capacity of 311.7 mA h g-1 can be maintained after 1000 cycles at a high current of 3 C and the capacity decay rate per cycle is only 0.05%.When the sulfur loading of the cathode is 4.48 mg cm-2,the discharge specific capacity of 756.2 mA h g-1 and the capacity retention rate of 89.5%can be obtained after 100 cycles.(3)It is found that the above two materials have similar energy storage mechanism through the characterization of structural morphology,catalytic and adsorption ability and electrochemical performance.First,GO with a large specific surface area not only improves the conductivity of materials,but also provides a support substrate for hydrothermal deposition of transition metal sulfides.Further,the transition metal sulfide/reduced graphene oxide composites(cylindrical appearance)with aerogel structure constructed from sandwich nanosheets are obtained through freeze-drying technology.This three-dimensional aerogel structure can not only increase the sulfur loading of the composite,but also restrict the dissolution of lithium polysulfide through physical confinement action.In addition,Heteroatom(Co/Ni)doping can disrupt the normal arrangement of local lattices in sulfides(MoS2/VS4).These defects can increase the active sites of the materials,the active sites of the materials can enhance the material's affinity to lithium polysulfide and accelerate the conversion of lithium polysulfide to lithium sulfide.Owing to the synergistic effect of physical limitation,chemical adsorption and electrocatalysis,these cathodes can finally achieve good electrochemical performance.