Fig 4. The optimized structures of a) lithium-decorated pristine silicene b) lithium-decorated single vacancy silicene c) lithium-decorated Stone-Wales silicene
As observed in Fig. 4, in all the three cases the Li atom binds with three Si atoms in the structure. While the Li atom binds symmetrically to all the three Si atoms in the pristine silicene system with a bond length of 2.79 Å, in the case of SV silicene two different Li-Si bond lengths can be observed. The Li-Si bond length when bonded to the in-plane Si atoms (atom 25 and atom 27) was measured to be 2.53 Å with the third Li-Si bond length being longer at 2.70 Å. Similar behaviour can also be seen in the SW silicene system, wherein two of the three Si-Li bonds (atom 23 and atom 33) measure to be 2.70 Å with the last bond being a bit shorter at 2.66 Å. The Li binding energy calculated via Equation (2) was found to be -2.36 eV for pristine silicene which is higher than the reported lithium cohesive energy i.e., -1.97 eV [46]. This prevents the clustering of lithium atoms over the surface of the substrate which could hinder hydrogen storage. The binding energy of lithium onto the silicene substrate was shown to be further improved in the defective silicene systems with the SV silicene showing the highest Li binding energy of -3.44 eV. The Li binding energies in the case of SW silicene was measured to be -2.73 eV. The average Li-Si bond length in the case of SV silicene was much lower than in the other two systems measured as 2.58 Å which further relates to the higher Li binding energy found in this system compared to the other two systems. In fact, a direct correlation between the average Si-Li bond length and the corresponding Li binding energies for the three systems can be seen in Table 1.