Theoretical Study On The Lithium Polysulfide Immobilization Mechanism Of Two-dimensional Cathode Materials For Lithium-Sulfur Batteries

With the continuous development of electronic technology,people's requirements for batteries are constantly improving.In general,we hope to obtain batteries with better cycle performance,longer life and smaller volume.Lithium-sulfur batteries have a high specific capacity and high energy density,its cathode material sulfur has a low price,a wide source,is safe and non-toxic,so it has attracted widespread attention and is considered to be a powerful competitor to replace the most widely used lithium batteries.However,the lithium polysulfide generated by the sulfur anode is easily dissolved in the electrolyte to cause shuttle effect during the charging and discharging,resulting in the problems of faster capacity decay and poor cycle performance of the battery,which hinders the practical application of lithium-sulfur batteries.In order to solve these problems,researchers have conducted a lot of exploration on cathode materials in recent years.Two-dimensional materials have the characteristics of good electrical conductivity,high chemical stability,and easy to modify.They have become one of the current research hotspots and frontiers in the field of chemistry and materials.In addition,studies have shown that the combination of small sulfur species(S_3-5)and positive electrode materials can effectively prevent the formation of long-chain lithium polysulfide?LiPSs?in lithium-sulfur batteries,and inhibit the shuttle effect to a certain extent.In this paper,the first-principles calculation was used to study the lithium polysulfide immobilization mechanism of heteroatom-doped graphene and MXene two-dimensional materials combined with small sulfur molecules as cathode materials for lithium-sulfur batteries.The main contents of the two-dimensional material to suppress the shuttle effect are as follows:First,we used first-principles to study the interaction of ten heteroatom-doped graphene?h-G,h=N,O,S,P,B?with small sulfur species and their intermediate products.Through the analysis of data such as binding energy,charge density difference,and atomic partial density of states?PDOS?,it can be concluded that the key to the interaction between the h-G structure and LiPSs is that the lithium atoms in LiPSs interact with heteroatoms and benzene rings in the h-G structure separately.Further,we studied the effect of solvents on the binding properties between h-G structures and LiPSs.It is worth noting that,considering the solvent,the binding energies of LiPSs with pyridine,epoxy and boron doped graphene are still larger than that of LiPSs with solvent,and the intermediate products during charge and discharge are not easy to dissolve in the solvent,which may be to inhibit the shuttle effect and the ideal cathode material for lithium-sulfur batteries.In addition to heteroatom-doped graphene,we also studied the mechanism of anchoring Li₂S₄,a lithium polysulfide intermediate product,with one of the two-dimensional transition metal carbon/nitride?MXene?materials—transition metal nitride.The first-principles method was used to systematically study the inhibition of the shuttle effect by four oxygen functionalized transition metal nitrides of V,Cr,Ti,Mo.Through the analysis of data such as binding energy,charge density difference and PDOS,it is concluded that the interaction between Li₂S₄ and MXene mainly depends on the charge transfer between S-Li and Li-O,and the descend order of the binding performance of MXene to Li₂S₄ is V₃N₂O₂>Cr₃N₂O₂>Ti₃N₂O₂>Mo₃N₂O₂.The binding energies of these four kinds of MXene to Li₂S₄ is higher than that of common electrolytes to Li₂S₄,and at the same time,it does not cause the dissociation of Li₂S₄,so these four materials may all become ideal cathode materials for lithium-sulfur batteries.