Structure And Properties Of La-doped LiF-SEI On The Surface Of Lithium Metal Anode:Ab Initio Study

Lithium metal has attracted people's attention as an electrode material,and lithium-sulfur battery systems with relatively high energy density have become a research hotspot.Studies have shown that the performance of the solid electrolyte interface(SEI)directly affects the performance of Li metal anodes.This paper uses a first-principles calculation method based on density functional theory to study the major inorganic components LiF,LiCl,and LiBr in SEI,It is expected that a feasible method for improving the SEI performance will be found through a calculation method.By optimizing the surface and bulk structure,the adsorption energy,vacancy energy,charge differential density,Bader and mechanical strength were calculated.The elastic band(NEB)method was used to simulate the diffusion transport behavior of Li atoms in the structure.This paper contains the following sections: Adsorption and Diffusion of Li Atoms on LiHa(F,Cl,Br)(001)(111)Surface;Plating and stripping within the Ha2-Li2 island and evolution behavior of Ha2-Li3 island structure;Calculation of mechanical strength of LiF-SEI and La-doped LiF / La-SEI,calculation of transport of Li atoms in LiF / La-SEI,and influence of tensile stress on the transport process.The results of the analysis and calculation are as follows:(1)The adsorption energy of the Li atom at the Ha top site(OTHa)on the surface of LiHa(001)is greater than that of the remaining sites(OTLi,BR,HL).We calculated the Bader discovery of different pairs of base heights and the larger the Bader value as the electronegativity of F,Cl,and Br atoms decreased(F = 4.0,Cl = 3.0,Br = 2.8).The migration energy curve is basically similar,with the increase of the ionicity of the system(LiF <LiCl <LiBr),the migration activation energy decreases.The LiHa(111)plane is divided into Li termination plane and Ha termination plane.The adsorption energy of Li atoms on the Ha termination plane is much larger than that of the Li termination plane.The activation activation energy of Li on the Li termination surface is in a small range(0-0.4eV),while the migration process of Li atoms on the Ha termination surface needs to repeatedly break the Li-Ha bond and then re-form a new Li-Ha bond.Different from LiHa(001),as the electronegativity of F,Cl and Br decreases,the Bader value decreases,and the migration energy increases.(2)In order to study the de-embedding behavior of Li atoms on the surface of LiHa(001),an island configuration of Ha2-Li3 that evolved from Li atoms around the base island Ha2-Li2 was established.It is found through calculation that the adsorption energy of the island configuration is much larger than that of a single Li atom.With the decrease of Ha electronegativity,the adsorption energy of the island configuration also weakens.By comparison the adsorption and migration behavior of Li atoms on the surface of three of LiHa-SEI(LiF,LiCl,LiBr),shows that LiF has better superiority as SEI.(3)Due to the difference in the lattice constants of LiF and Li crystal structures,LiF will be regionally distributed when LiF is placed on the Li substrate.Two types of SEI models are constructed,LiF-SEI and the boundary Li is replaced by La.LiF / La-SEI can be found through mechanical strength calculations.After joining La,the slope of the stress-strain curve of LiF-SEI decreases,and the ideal shear strength and ideal tensile strength of the configuration are significantly reduced.(4)Using LiF / La-SEI as the research object,calculated the transport of Li atom at unit cell boundary and unit cell body.It is found that unit cell boundary is more likely to form vacancies due to the large number of dangling bonds in La,and its migration activation energy is lower.Has promoted the deposition of Li atoms.By calculating the activation energy of the vacancy level migration at the interface between LiF and Li substrate before and after applying tensile stress,it is found that the application of tensile stress reduces the vacancy formation energy at the interface,reduces the activation energy barrier for same layer migration,Increasethe layer migration activation energy.