| Lithium-sulfur batteries are considered as one of the most promising high-capacity storage systems with the advantages of wide raw material sources,long cycle life,good safety performance and high energy density.However,the Li2S is deposited on the surface of electrode materials during charging and discharging,which reduces the utilization of active material,leads to poor kinetic and multiplicative performance and makes the electrode expand in volume,which leads to battery failure and other problems.The two-dimensional transition metal sulfide as the cathode material of lithium-sulfur batteries has certain conductive properties of its own and weak layer-layer interactions,which makes Li+diffuse easily.It is well known that Coulomb interaction accounts for about 90%of the atom-to-atom interaction,and the essence of the battery is to change the electron flow in the chemical reaction from undirected to directed,thus generating electric field energy.Hence,the charge distribution has an essential influence on the performance of the electrode material for lithium-sulfur batteries.To this end,in this thesis,the electrical properties of 2D-Fe S2 and lithium polysulfide are theoretically analyzed after a more detailed basic study of the atomic charge distribution in three-dimensional simple substances using density generalized theory ab initio calculations,and then the adsorption properties of 2D-XS（X=Co,Cu,Zn,and Ni）on lithium polysulfide are investigated,as well as the atomic charge changes of lithium polysulfide before and after the reaction of each system.We investigated the Hirshfeld charges of atoms in 1772 different configurations of 90 elemental substances using the First-principles calculation before analyzing the charge distribution of the lithium polysulfide two-dimensional transition metal sulfide adsorption system.Charge inhomogeneities were found in 136 crystalline configurations of 44 elemental substances,manifested by three types of charge rise and fall present in different local lattice environments of the simple substances:atomic charge ordering,cluster charge ordering,and atomic interstitial charge ordering.The atomic charge distribution of each atom in the material is inhomogeneous due to the different atomic environments,which in turn affects the properties of the material.On the basis of calculating the charge of a simple substance,the changes of atomic charge and the amount of transfer before and after adsorption were analyzed based on the interaction between lithium polysulfide and the 2D substrate material Fe S2,and it was found that the adsorption energy of Li2S molecules adsorbed on the2D-Fe S2 surface was-4.28 e V,where the total charge of the adsorbed Li2S molecules increased by 1.32 e.The adsorption energy of Li2S2 molecules adsorbed on the2D-Fe S2 was-1.73 e V,and the total charge of Li2S2 molecules increased by 0.86 e.The charge increments were transferred from the 2D-Fe S2 surface,mainly the d orbital electrons of Fe atoms were transferred.The results show that 2D-Fe S2 has a moderate intensity of adsorption on lithium polysulfide,has a good anchoring effect,and can maintain the integrity of the polysulfide molecular structure during the adsorption process.In addition,we also selected two-dimensional sulfide materials of other transition metals of the fourth cycle as substrate materials for adsorption.Among them,the adsorption energy of Co S on Li2S,the end product of lithium-sulfur batteries,is-1.69 e V,where the charge is transferred by 0.76 e.The adsorption energy of Cu S on Li2S is-2.42 e V,which is transferred by 0.98 e.The adsorption energy of Zn S on Li2S is-2.26 e V,which is transferred by 0.84 e.The adsorption energy of Ni S on Li2S is-1.63 e V,which is transferred by comparing the adsorption energy of lithium polysulfide on the surface of the two-dimensional transition metal sulfide XS.It is found that the adsorption effect of XS on lithium polysulfide is not satisfactory and needs to be improved if it is used in lithium-sulfur battery cathode materials.By comparing the adsorption energy and charge transfer of each adsorption system,the two-point-eighth power of the change in atomic charge of the 2D-Fe S2adsorption polysulfide system is linearly related to the value of the corresponding adsorption energy.The one-point-fifth power of the change in atomic charge of the2D-transition metal sulfide XS adsorption polysulfide system is linearly related to the value of the corresponding adsorption energy.The larger the adsorption energy of the system,the more charge transfer is.The larger the adsorption energy,the more significant the charge transfer.There is a direct relationship between charge transfer and material performance.This study of material charge will provide theoretical help to improve the performance of lithium-sulfur batteries in the future. |
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