First Principles Study On Several Kinds Of Low Dimensional Structures As Anode Materials For Metal Ion Batteries

In recent years,with the rapid development of portable electronic equipment and electric vehicle industry,metal ion batteries with high energy density and high charge-discharge rate have attracted extensive attention,and promoted the development of anode materials for high-capacity metal ion batteries.Two dimensional materials are considered as potential candidates for the anode materials of the next generation metal ion batteries because of their high specific surface area,high flexibility and high carrier mobility.In this paper,the first principles calculation based on density functional theory is used to systematically evaluate the properties of four kinds of low dimensional nano structures such as borophene/graphene bilayer structure,antimonene and its derivatives and borophenes as anode materials for metal ion batteries.The adsorption characteristics and diffusion process of metal atoms on the material surface are mainly analyzed and studied,and the maximum theoretical specific capacity and average open circuit voltage of metal ion battery anode materials are calculated.The main results are as follows:(1)It is found that Li,Na and K can be stably adsorbed on the surface of borophene and graphene.On the side of borophene,the most stable adsorption site is directly above the B atom forming B-C bond,and on the side of graphene,it is above the center of C hexagonal ring,and the adsorption energy is larger than the corresponding cohesive energy.The analysis of charge-density difference shows that there is obvious charge transfer between the adsorbed metal atoms and the surface,which further shows the bilayer structure has strong adsorption for metal atoms.The study on the diffusion process shows that the diffusion barriers of Li,Na and K on the surface of borophene/graphene bilayer structure decrease in turn,and the diffusion barrier of K atom is the lowest(0.09 e V),which has a higher diffusion rate,indicating a higher charge-discharge rate.The adsorption of multiple metal atoms by borophene/graphene bilayer structure was further studied.The maximum adsorption concentrations of Li,Na and K are 2.0,1.56 and0.5,respectively.Among them,the theoretical specific capacity of Li is the highest,which is about 1880.7 m A·h/g.(2)The study of antimonene and its derivatives shows that monolayer antimonene,bilayer antimonene and antimonene/graphene heterostructure have good adsorption characteristics for Mg atoms.The most stable adsorption site is directly above the center of Sb hexagonal ring.From the perspective of adsorption energy,the adsorption process belongs to spontaneous reaction process.The study of diffusion process shows that the diffusion barrier of Mg along the 4-4'path on the surface of antimonene is the lowest,which is only 73 me V.Further study found that 4×4 antimonene supercell can adsorb up to 24 Mg atoms,and the maximum theoretical specific capacity is 735.5 m A·h/g,which is much higher than the maximum theoretical specific capacity of antimonene for storing Li/Na ions.Compared with monolayer antimonene,the diffusion barrier of Mg on the surface of antimonene/graphene heterostructure is further reduced,and it has better stability in the battery cycle.(3)The Mg storage properties of borophenes with?₁₂,?₃ and pmmn structures were studied.It is found that the most stable adsorption site of a single Mg atom is just above the center of the B hexagonal ring for?₁₂ and?₃ structure borophenes,while Mg is more inclined to adsorb above the B-B bond in the'valley'of pmmn borophene,and its adsorption energy is about-2.24e V.Analysis of the diffusion process shows that Mg has a lower diffusion barrier when diffusing in the'valley'of pmmn borophene,indicating higher charge-discharge rates.With the increase of Mg atoms concentration on the surface of borophene,Mg atoms are adsorbed on both sides of borophene uniformly.On the premise of no B-B bonds fracture,the theoretical specific capacity is obtained by theoretical calculation,in which the theoretical specific capacity of?₁₂structure borophene can be as high as 3970 m A·h/g.The results show that borophene is a potential candidate for anode materials of Mg ion batteries.