Diffusion and condensation of lithium atoms in single-walled carbon nanotubes

We have carried out theoretical investigations to explore the diffusion of lithium on the exteriors and the interiors of single-walled carbon nanotubes (SWNTs). We found that lithium can adsorb on both interior and exterior surfaces with adsorption energy around $-2.44\mathrm{eV}$. Lithium has a higher mobility with the diffusion barriers less than $0.2\mathrm{eV}$ on both the exteriors and interiors of SWNTs. The diffusion barriers (exo- and endohedral) of lithium depend on the radius and chirality of SWNTs. The lithium capacity trapped inside SWNTs increases with the increasing tube diameter and can be as high as $\mathrm{Li}{\mathrm{C}}_{2.6}$ in a (10,10) SWNT. These lithium atoms tend to form single- or multishelled coaxis nanotubes, together with a linear atomic chain in the axis at low temperature depending on the diameter of SWNTs, suggesting a potential way to synthesize small metal nanotubes and metal nanowires.

Figure 1

The relative energy of a (5,5) SWNT with an adsorbed lithium varies as the endo- and exohedral approaches of the lithium toward the center of a carbon hexagon in the tube wall. The horizontal axis is the distance of the lithium to the tube axis. The structure was relaxed during the approaches. The equilibrium configurations of endo- and exohedral adsorption of a lithium on the tube wall are shown as the insets of this figure, where the black balls and white circles denote lithium and carbon atoms, respectively.

Figure 2

Variation of the total energies of (a) (5,5), (b) (9,0), and (c) (10,10) SWNTs with a lithium adsorbed on the interior (endo-) and exterior (exo-) surface as the lithium is diffusing from one stable adsorption site to the adjacent adsorption sites along different paths shown as (d) and (e): (1) $\mathrm{A}\to \mathrm{B}$ (exo); (2) $\mathrm{A}\to \mathrm{C}$ (exo); (3) $\mathrm{A}\to \mathrm{C}$ (endo); and (4) $\mathrm{A}\to \mathrm{B}$ (endo). For each path, the total energy of the initial structure is set to zero. The solid lines are the fitting results.

Figure 3

The variation of diffusion barrier along different pathways as a function of tube diameter.

Figure 4

The pair distribution functions in the radial direction $g\left(r\right)$ and axial direction $g\left(z\right)$, and the equilibrium configurations of lithium trapped inside (a), (b) (6,0) SWNT, (c) (5,5) SWNT, (d) (7,7) SWNT, (e) (9,9) SWNT, and (f) (10,10) SWNTs. White and gray circles in this figure represent carbon and lithium atoms, respectively.

Figure 5

(a) The equilibrium configuration of lithium trapped inside a (7,7) SWNT (side view). Carbon atoms are denoted by white circles, while lithium atoms are represented by gray and black circles. The dashed lines indicate the wall of a (7,7) SWNT. (b) The pair distribution function in the axial direction $g\left(z\right)$ of the linear atomic chain in the center (denoted by black circles). (c) The pair distribution function in the axial direction $g\left(z\right)$ of the lithium tube (denoted by gray circles).

Figure 6

The variation of $\mathrm{Li}∕\mathrm{C}$ ratio as a function of the diameter of SWNTs.