Mechanical Properties Of Two Dimensional B₂P₂ And Its Application In Anode Materials For Lithium Ion Batteries

New energy technologies are essential for achieving sustainable energy development in the future.Rechargeable lithium-ion batteries have been widely used in portable electronic devices and power energy storage systems due to their advantages of high energy density,long cycle life,and no pollution to the environment.Since the anode material has always been regarded as the core part of the lithium-ion battery,and its performance is directly related to the overall performance of the battery,the development of higher performance anode materials is of great significance.Due to its large surface area,two-dimensional materials are beneficial to the diffusion of ions and increase the storage capacity,and have become the key research direction of anode material for lithium-ion battery.In this paper,a first-principles system based on density functional theory is used to study the mechanical properties of the new two-dimensional B₂P₂and its application in anode material for lithium-ion battery.The main results are as follows:（1）Mechanical properties of monolayer B₂P₂.Firstly,the stability of monolayer B₂P₂was verified by using phonon spectrum and ab initio molecular dynamics.Then the mechanical parameters of the monolayer B₂P₂were fitted by the energy method,including Young’s modulus,shear modulus and Poisson’s ratio.The results show that the Young’s modulus of the monolayer B₂P₂along the x and y directions are 421.5 and374.1 GPa,respectively,and the Poisson’s ratio is 0.255 and 0.227,respectively,showing anisotropy,and the shear modulus is 51.2 GPa.Compared with the superhard material graphene,monolayer B₂P₂is a soft material.In addition,the stress-strain curve of the monolayer B₂P₂is also calculated.The results show that the monolayer B₂P₂can withstand 21%and 15%of the maximum tensile strain in the x and y directions,respectively,and the corresponding ultimate strength is 44 and 35.6 GPa.（2）Application of two dimensional B₂P₂ in anode materials for lithium-ion batteries.Firstly,the adsorption and theoretical storage capacity of monolayer B₂P₂for lithium ions are calculated.The results show that there are two stable adsorption sites on the surface of monolayer B₂P₂.The adsorption energies for lithium ions are-1.06 and-0.81 e V,respectively,and the theoretical capacity is predicted to be high,up to 1282.7 m Ah/g.Secondly,the diffusion properties of lithium ions on monolayer B₂P₂are calculated.The results show that there are three diffusion paths on the surface of monolayer B₂P₂,and the three diffusion pathways in monolayer B₂P₂share similar energy barriers of around 0.5 e V,which indicates the isotropic transportation of Li ions.In addition,the structure of monolayer B₂P₂before and after lithiation has also changed,from the previous graphene-like phase to phosphorene-like phase.Therefore,the diffusion of lithium ions on the monolayer B₂P₂after the structural transition is also calculated.The results show that the structural transition reduces the diffusion barrier of lithium ions in the phosphorus atom region from 0.5 e V to 0.12～0.14 e V,resulting in improved transportation of Li ions.Four kinds of bilayer B₂P₂models were constructed by changing the stacking mode between layers,and the adsorption and diffusion properties of lithium ions between the layers of the most stable model were studied.