Design Of Anode Host Material For Lithium-sulfur Batteries Based On Two-dimensional Sulfide Materials

With the increasingly serious environmental problems,the society has a higher demand for energy storage and conversion.In energy storage systems,rechargeable secondary batteries have attracted much attention due to their advantages of good reversible capacity,high energy density and environmental friendliness.The performance of a secondary battery depends largely on the performance of its electrode material.Therefore,the development of new high-performance electrode materials to meet the needs of the next generation of rechargeable secondary batteries has never stopped.The low capacity of conventional commercial lithium-ion batteries has greatly hindered their large-scale commercialization.Therefore,it is necessary to develop other potential high-performance alternative batteries.Lithium-sulfur battery has several times the theoretical capacity of lithium-ion battery,and the sulfur anode used has the characteristics of low cost,low toxicity and abundant reserves.It is the key development system of the next generation secondary battery.However,the development of lithium-sulfur batteries still face many challenges,such as poor conductivity of cathode materials and discharge products,“shuttle effect”and volume expansion of sulfur anode.“Shuttle effect”is the main factor leading to the degradation of lithium-sulfur batteries’performance.In order to suppress shuttle effect,sulfur and host materials can be used to construct the positive electrode of lithium-sulfur batteries to anchor polysulfide,so as to overcome the above problems.Two-dimensional materials have large specific surface area and strong mechanical stability,so they are suitable for anchoring materials for lithium-sulfur batteries.Among them,graphene has excellent mechanical strength and electrical conductivity,making it an ideal electrode for lithium-sulfur batteries.However,due to the non-polar nature of graphene in the vertical direction,polysulfide shedding can occur after multiple charge-discharge cycles.One way to change the polarity of graphene is to build a composite with a two-dimensional polar material.Among them,two-dimensional transition metal sulfides are a kind of polar materials with excellent characteristics.They have moderate affinity for active polysulfides and good electronic conductivity.In addition,they have the ability to adjust the kinetics of REDOX reaction and contribute high active capacity.Therefore,van-der-Waals polar heterostructure composed of graphene and two-dimensional transition metal sulfide has high research value.In addition,as the allotrope of graphene,the graphyne has larger in-plane space holes,allowing Li⁺to diffuse and shuttle freely in the three dimensions,which can be used as an important regulatory means to optimize the heterostructure.In this paper,we first constructed a monolayer Ti S₂as a two-dimensional transition metal sulfide representative,and calculated and studied the performance of monolayer Ti S₂as a anode host material for lithium-sulfur batteries.The results show that monolayer Ti S₂is an ideal anode host material for lithium-sulfur batteries.However,although the monolayer Ti S₂has electron state distribution at Fermi level,its state density is low.So,it can be inferred that its own electrical conductivity is not particularly strong,which has limited ability to improve the problem of poor electrical conductivity of cathode materials for lithium-sulfur batteries.Therefore,we then designed Ti S₂/graphene van-der-Waals polar heterostructure as the anchoring material for the anode of lithium-sulfur batteries.Monolayer Ti S₂mainly contributes to enhance the anchoring ability of polysulfide with polar characteristics,and graphene mainly contributes to improve the poor electrical conductivity of the sulfur anode.The results show that Ti S₂/graphene heterostructure,as an anode material for lithium-sulfur batteries,exhibits good electrical conductivity,suitable adsorption effect for polysulfide and low Li⁺migration barrier,which fully proves that Ti S₂/graphene heterostructure is an ideal anode material for lithium-sulfur batteries.Finally,in order to explore the performance of graphyne as electrode material,we designed a VS₂/graphyne heterostructure as anode host material for lithium-sulfur batteries.The results show that graphyne is also a promising electrode material for lithium-sulfur batteries.To sum up,this paper discusses the theoretical characteristics of two-dimensional sulfide as the host material of lithium-sulfur batteries through first-principles calculation.Two-dimensional sulfide is polar in the vertical direction,which is conducive to the stable interaction between it and polar polysulfide.Even after several charge and discharge cycles,polysulfide will not fall off,which inhibits the shuttle effect and ensures the long-term cycle stability of lithium-sulfur batteries.The important role of polarity in enhancing the anchoring ability of polysulfide was elucidated from atomic scale,which provided theoretical reference for the design of composite materials as host materials for lithium-sulfur batteries.