Energetics of lithium ion adsorption on defective carbon nanotubes

We apply the density-functional theory to study energetics of lithium ion adsorption on single-wall carbon nanotubes (SWNTs). We use both the local-density approximation (LDA) and the generalized-gradient approximation (GGA) for the exchange-correlation energy functional. Both the LDA and GGA calculations indicate that adsorption processes of lithium onto the pristine and defective (8,0) SWNT are exothermic with respect to the metallic lithium. On the other hand, the lithium adsorption on the (5,5) SWNT can be endothermic depending on the defect structures. As the defective ring on the surface becomes large, the energy barrier for lithium diffusion through the ring lowers and the peak position of the barrier shifts toward the center of the SWNT. Molecular dynamics simulations are also performed to study the diffusion dynamics of the lithium into the SWNT through the defective rings on the sidewall. These results suggest the possibility of accumulating lithium ions inside the SWNT with large defective rings.

Figure 1

(Color online) Fully relaxed structures of the defect-free and the defective (5,5) (upper panel) and (8,0) (lower panel) SWNTs obtained by the GGA calculations. Gray balls and rods are the carbon atoms and bonds shorter than 1.5 Å, respectively. Carbon ring defects are indicated by the thick circles. The structures obtained by the LDA calculations are indistinguishable from the GGA results on this scale.

Figure 2

Energetics of diffusion pathways for lithium to move through the center of the $n$ rings of the (5,5) and (8,0) SWNTs. The result for the defect-free (pristine) SWNT is given in the left end for each case. The energy zero is taken to be that of the most stable point. Solid (open) circles are the results of the LDA (GGA) calculations. A, B, and C represent the spatial points where lithium is on the axis, on the surface (center of the ring), and in exterior of the SWNT, respectively, as schematically shown in the inset of the upper-right graph. The scale of the horizontal axis is the distance (in angstroms) from the central axis (point A). The vertical dotted line is the SWNT surface (point B), formally defined as half of the diameter.

Figure 3

(Color) Snap shots of the lithium diffusion to the defect-free hexagonal ring (upper panel) and to the $n=7$ defective ring (lower panel) of the (5,5) SWNT. The side (left panel) and the top (right panel) views are presented. The initial structure of the SWNT (gray balls and the framework) is superimposed for the eye’s guide. The atoms making up the defective ring are indicated by the red balls. The total number of MD time steps is 125 (250 fs) and only the lithium trajectory for every five MD steps are shown (yellow balls). The initial position of the lithium is indicated by the green balls.

Figure 4

(Color) Snap shots of the lithium diffusion to the $n=8$ defective ring (upper panel) and to the $n=9$ defective ring (lower panel) of the (5,5) SWNT. The other details are the same as those in Fig. 3 except that the total number of MD time steps is 525 (1050 fs) for the $n=9$ case. The acceleration of the lithium motion on approaching the $n=9$ defective ring is clearly seen from the trajectory.