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.