3. Results and discussion
3.1 Eﬀect of strain on Li adsorption
To analyze the adsorption of Li atoms on silicene, we consider four typical sites with high symmetry: the site on the top of a Si atom in a higher plane (T), the site on the top of a Si atom in a lower plane (V), the site at the midpoint of a Si–Si bond (B), and the site in the center of a hexagon (H). The corresponding sites are shown in Fig. 1. After full relaxation, the H site of silicene is the most stable adsorption position, where Li resides above the center of the hexagonal silicene ring and it has three nearest neighbor Si atoms. The corresponding binding energy is �2.41 eV, which agreed with other previous works.18,19,22 The second favorable adsorption site is the valley (V) site on top of the lower silicon atoms with a binding energy of �2.02 eV. In this configuration, the silicon atom directly below the adatom undergoes a great displacement towards the outside of the plane due to the electrostatic repulsion, as shown in Fig. 2(b).
Consequently, the Li adatom also prefers to interact with the neighboring three Si atoms when it is adsorbed on the V site of the silicene. Li adsorbed on the T site of silicene with a binding energy of �1.83 eV has a similar structure to that on the V site.
Though the valley and top site adsorptions are possible, bridge site adsorption of Li is not possible on a silicene lattice in our calculations, which is in agreement with the results of Sahin et al.31 In addition, the delithiation process was also examined by computing the minimum energy path (MEP) of moving Li from the most stable adsorption site to a site 4.0 Å far away above the original one, the results are shown in Fig. 3. During delithiation, the Si atoms near the adatom are first pulled out of the plane, then after removal of the Li atom from the system, the structure of silicene returns to its initial form. Assuming no additional energy barriers to the kinetic transport of Li in the silicene, lithiation/delithiation should be spontaneous if a potential diﬀerence of B2.4 eV is applied.
For practical application of silicene as an electrode material for LIBs, the eﬀect of strain on the binding of Li atoms on silicene is examined. The in-plane biaxial tensile strains are imposed on silicene, achieved by enlarging the lattice constants. The strain was applied for every 2% set from 0 to 10% to the silicene model, which is within the aﬀordable range of material strength of silicene.32,33 Here the strain is defined as e = (a � a0)/a0 = Da/a0, where a0 and a are the lattice parameters of the unstrained and strained supercell, respectively. As strain increases, it is found that all Li atoms can stably adsorb on the H site of silicene.
Specifically, it is noted that when the strain is above 6%, lithium atoms at the V site or the T site cannot be stably adsorbed and would automatically transfer to the H site. Taking Li adsorption at the V site as an example, when tensile strain increases, there is more charge transfer from Li atoms to silicene. Meanwhile, the buckling of the sheet decreases, which results in the distance between Li and the directly interacting valley Si atom being retained. Thus the electrostatic repulsion between Li and the valley Si atom increases. On the other hand, the electrostatic attraction between Li and the neighboring three Si atoms increases because of the more charge transfer.
It is noted that the electrostatic attraction of the lithium atom to the three silicon atoms is not completely symmetrical, and this asymmetry increases with increasing tensile strain. One silicon atom is more attractive to the lithium atom than the other two atoms. Thus the combined increment of the repulsive force and asymmetric attraction makes Li move to the H site of silicene when the strain further increases to 6%. The results of binding energies with Li adsorption on sites of silicene and the related adsorption distance as a function of strain are demon- strated in Fig. 4(a). It can be observed that the binding energy of Li is reduced when the strain is applied, which indicates that the interactions between the Li atom and silicene are much more sensitive to external strains and implies that tensile strain can enhance the Li binding kinetics. Although the binding energies of Li adsorption on H or V (T) sites of silicene are all decreased with increasing strain, the adsorption distance exhib- its diﬀerent characteristics (Fig. 4(b)). The adsorption distance of Li at H sites decreases with rising strain, consistent with stronger binding at large strains. Here the adsorption distance is defined as the distance of the adsorbed Li atom to the lowest basal plane of silicene sheets. Nevertheless, the adsorption distance on the V (T) site is found to vary little with the increase in external strains from 0% to 4%. This fact is due to the in-plane relaxation of silicene monolayers induced by external strains, which results in a deformation in the perpendicular directions. Since silicene sheets are low-buckled, the strain leads to a rise of the bond length between Si–Si and makes the silicene more flat. The bond strength of Si–Si in silicene also weakens under strain. Therefore, as the strain increases, the binding distances of Li to the sheet surface are reduced due to the stronger interactions between the Li atom and the hexagon ring of silicene. While for Li adsorption on V sites, their binding distances (Li and adsorbed Si) vary little attributing to the strong interaction between Li and valley Si atom whether there is strain applied. 翻译成中文