Preparation Of Transition Metal Sulfide/Ti₃C₂ MXene Composites And Their Application In Lithium-sulfur Batteries

As the shortage of non-renewable resources in earth,the human resource crisis is facing the increasingly prominent.Solar energy and wind energy,etc are very important for the use of renewable resources,and energy storage devices play a more and more important role in today’s daily life.Lithium-Sulfur batteries（Li SBs）with high energy density,low cost,and environmentally friendly features,become the focus on the next-generation energy storage devices.However,the further development of Li SBs is seriously hindered by the challenges of“shuttle effect”,poor conductivity,large volume expansion,and short cycle life of sulfur cathode.Metal disulfide is used as the active center for the adsorption and catalytic conversion of polysulfides due to its simple preparation process,good electrical conductivity at room temperature,strong adsorption and catalytic conversion ability for lithium polysulfides,and effective improvement of chemical reaction kinetics.At present,disulfides such as VS₂,Ti S₂,and SnS₂ have been widely studied as the sulfur host materials of Li SBs.In recent years,the newly developed graphene-like Ti₃C₂ MXene nanosheets have good mechanical flexibility,electrical conductivity,and abundant surface functional groups（-OH,-O,-F,etc.）,which are the ideal materials for sulfur host materials in Li SBs.Based on the characteristics of the two kinds of materials,in this work,we adopt a simple hydrothermal method to form the in-situ growth of transition metal sulfide nanomaterials,such as SnS₂ nanoparticles or VS₂ nanosheets on the surface Ti₃C₂ MXene matrix with high conductivity and good flexibility.Such design not only can effectively avoid the stacking or agglomeration of pure phase material,but also expose more active sites for catalytic reaction.In addition,the combination of the two kinds of materials can also produce a synergistic effect on each other,thus effectively improving the electrochemical performance of Li SBs.Then,to solve the problem of low volumetric energy density of sulfur cathode materials,a low-content graphene（～15 wt%）was used as a“self-assembly agent”to densify the as-obtained sulfur hybrid composites,and finally obtaining high-density monolith sulfur cathode materials after hydrothermal reaction and followed by air drying.The detailed contents mainly include the following aspects:（1）Preparation and electrochemical properties of SnS₂-MXene nanocomposites.SnS₂-MXene composites with SnS₂ nanoparticles uniformly anchored on MXene surface were prepared by uisng hydrothermal reaction,and then the S/SnS₂-MXene cathode materials were obtained by using melting-diffusion method with the mixture of sulfur and SnS₂-MXene.In the direct adsorption experiment of polysulfides,SnS₂-MXene composite material can quickly adsorb the claybank polysulfides solution and make the solution become clear and transparent.At the same time,the conversion of polysulfides shows excellent characteristics,that is,it can give the rapid conversion of lithium polysulfides to form Li₂S deposition in a short time,thus achieving the Li₂S deposition capacity of 132 m Ah/g.After assembling into Li SBs,the initial specific discharge capacity can achieve 1210 m Ah/g at 0.2 C.After 100 cycles,the capacity can remain at 976 m Ah/g,and the capacity retention reaches up to 81%.In addition,at the higher current density of 1.0 C,the initial capacity can realize 810 m Ah/g,which maintain at600 m Ah/g after 500 cycles with the capacity retention of 74.1%.SnS₂ owns layered CDI2-type structure,this unique structure can intermix each layer of Tin atoms with sulfur atoms in the hexagonal double-layer structure.The van der Waals force interaction between adjacent sulfur atom layers is conducive to the rapid transfer of lithium ions,which is the reason that the battery performance can be improved.In addition,SnS₂nanoparticles in SnS₂-MXene composite material,as sulfur host in Li SBs,act as the adsorption center of polysulfides,which can confine the polysulfides and inhibit the shuttle effect of polysulfides,enrich the valid sites on the surface of MXene,and quickly catalyze the transformation of polysulfides into Li₂S₂/Li₂S.Therefore,Lithium-Sulfur batteries with excellent electrochemical performance can be obtained.（2）Preparation of VS₂-MXene heterostructures materials and their electrochemical properties in Li SBs.The VS₂ nanosheets were in-situ grown on the surface of MXene by one-step hydrothermal method to form VS₂-MXene heterostructures with close interface.At the same time,the VS₂-MXene heterostructures showed strong adsorption ability in the adsorption experiment of lithium polysulfides,and the solution became clear after the adsorption of lithium polysulfides in a short time.In the catalytic conversion test of lithium polysulfides,VS₂-MXene showed sharp redox peak and strong peak current response in the CV results of symmetric batteries.In addition,in the deposition test of Li₂S,the heterostructures could achieve a deposition capacity of 156.4 m Ah/g.However,pure VS₂ and MXene can only reach91.67 and 112 m Ah/g,respectively,indicating the excellent conversion characteristics of heterostructures to lithium polysulfides.The DFT calculation results prove the excellent electrical conductivity of VS₂ and VS₂-MXene heterostructure and the adsorption ability for lithium polysulfides.This result indicats that the heterostructure can well regulate the deposition of Li₂S,thus improving the utilization of sulfur.As a result,S/VS₂-MXene cathode can realize a specific capacity of 1211.7 m Ah/g at 0.2 C and maintain at 965.2 m Ah/g after100 cycles,with a capacity retention of 80%and a nearly 100%coulomb efficiency.The initial discharge capacities of S/MXene and S/VS₂ were 1120.1 and 965.2 m Ah/g,respectively.And their capacity retention was close to 71%and 64%after 100 cycles.Even with a higher rate of 1 and 2 C,S/VS₂-MXene still remain the capacity retention of 61%and 62%after 500cycles.（3）A high-density sulfur cathode material was prepared by using a simple high-temperature hydrothermal and natural shrinkage method.The as-prepared S/VS₂-MXene powder was combined with a low content graphene suspension（～15 wt%）to form a hydrogel by hydrothermal self-assembly,and then after natural drying and shrinking to obtain the high density sulfur cathode material--HD-S/VS₂-MXene（high density S/VS₂-MXene monolith material（～1.93 mg/cm³））.Graphene oxide as a“self-assembling agent”is reduced into reduced graphene oxide（r GO）via hydrothermal method,thus forming a conductive three-dimensional network structure that greatly promotes the electronic conduction,alleviates the volume expansion during cycling,and improves the wettability of the electrolyte.The VS₂-MXene heterostructure gives the high-efficiency catalytic conversion of lithium polysulfides,smartly confine the“shuttle effect”,and improve the performance of Li SBs.The electrochemical performance of the compact HD-S/VS₂-MXene cathode was tested.the CV curves of HD-S/VS₂-MXene cathode at different scanning cycles showed ability excellent overlap and strong redox peak current response,and the results showed a good redox reversibility.In addition,the initial discharge capacity of dense sulfur cathode with the high sulfur loading of 9 mg/cm² can reach up to 7.4 m Ah/cm² under a lean electrolyte dosage（E/S=5μL）,which shows an ideal electrochemical performance and potential practical application for Li SBs.