CNT Complex for Potential Li-Ion

Jul 8, 2009 - Institute of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis, Singapore 13862 Science Park II, Singapore 117528, ...
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J. Phys. Chem. C 2009, 113, 14015–14019

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DFT Study on Nano Structures of Sn/CNT Complex for Potential Li-Ion Battery Application J. W. Zheng,† S. M. L. Nai,‡ M.-F. Ng,† P. Wu,*,† J. Wei,§ and M. Gupta‡ Institute of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis, Singapore 13862 Science Park II, Singapore 117528, Department of Mechanical Engineering, National UniVersity of Singapore, 9 Engineering DriVe 1, Singapore 117576, and Singapore Institute of Manufacturing Technology, 71 Nanyang DriVe, Singapore 638075 ReceiVed: October 14, 2008; ReVised Manuscript ReceiVed: March 9, 2009

From first principles calculations, we investigate the adsorption of a number of Sn structures (a single atom, a cluster, and an atomic wire) on the inner and outer surfaces of single-walled carbon nanotubes (CNTs); with an interest in the potential applications of these systems to Li-ion batteries. We find that Sn clusters have a much weaker interaction with the CNTs than single Sn atoms and that the interaction of Sn with the outer surface of the CNT is about 2-3 times greater than with the inner surface. The Sn atomic wire is stable inside zigzag (n, 0) CNTs, when n is greater than 10; moreover, we find that the adsorption energy reaches a maximum (-0.19 eV) at n ) 14 or 15. Our simulation results explain well experimental observations and suggest that CNT-encapsulated Sn is a potential anode material for Li-ion batteries, with the ability to withstand the huge volume changes that occur upon Sn alloying with Li. Introduction Commercial lithium ion batteries use graphite as the anode material due to safety considerations, low cost, and long cyclability.1-4 Its low theoretical capacity (372 mAh/g), however, limits the application of lithium ion batteries. Tin (Sn) metal with a theoretical capacity of 994 mAh/g is one of the top potential graphite substitutes, but it suffers from the colossal change of volume upon alloying with Li,5,6 which will eventually lead to the pulverization of Sn anode. Encapsualtion of Sn inside carbon nanotubes (CNT) seems a promising way to resolve the problem. Oh et al.7 demonstrated that encapsulation of Sn inside spherical hollow carbon can suppress the aggregation and pulverization of nanosized Sn metal particles. Tin nanoparticles encapsulated in elastic carbon spheres8 and nanostructured Sn-C composites9 were also reported to be good anode materials for Li-ion batteries. Kumar et al.10 found that Sn-filled multiwalled CNT gave a first-cycle insertion and deinsertion capacities of 2474 and 889 mAh/g, respectively, which were much higher than those of parent CNT or Sn. The reversible capacities of Sn-filled CNT maintained within the 720-800 mAh/g region over the first 20 cycles. However, relatively low encapsulation of Sn in CNT (only less than 7 wt % of Sn) is reported. The role of the encapsulated Sn and the influence of its concentration are not understood yet. Attempts to encapsulate metal Sn into the hollow space of CNT date back to the early 1990s when Guerret-Ple´court et al.11 failed to introduce Sn inside CNT by using the arc-discharge method. It is believed that the high surface tension of liquid Sn inhibits the direct encapsulation.12 Recently, CNT encapsulated Sn nanowires have been successfully synthesized by two separate methods: by first introducing a Sn salt or oxide into the CNT then managing to recover the Sn12,13 or by using Sn * E-mail: [email protected]. † Singapore Science Park II. ‡ National University of Singapore. § Singapore Institute of Manufacturing Technology.

as a catalyst to grow the CNT on the surface of Sn nanowire.14 The encapsulation mechanism, however, is still not fully understood. In this paper, we report our study on the interactions of Sn with the inner and outer surfaces of single-walled CNT by using first principles methods based on density functional theory (DFT). We correlate our simulation results with experimental findings and demonstrate theoretically that CNT encapsulated Sn can withstand the colossal volume change upon Sn alloying with Li. Simulation Details First principles calculations were performed within the DFT framework using the generalized gradient approximation (GGA) exchange-correlation functional of Perdew and Wang (PW91). The total energies were calculated by means of the projectoraugmented-wave (PAW) method,15,16 as implemented by the VASP code.17,18 In all of the calculations, the plane-wave cutoff energy was set to 520 eV; at this point, the electronic energies were seen to be well-converged. A Monkhorst-Pack mesh with 1 × 1 × 3 k-point sampling was tested and found to provide adequate accuracy for geometry optimizations, and in the case of density of states (DOS) calculations, 1 × 1 × 10 sampling was performed. The atomic positions in all of the computational models were fully relaxed until reaching a convergence energy of 10-4 eV. The adsorption energies were calculated using ∆E ) E(Snn + CNT) - E(Snn) - E(CNT). By definition, ∆E < 0 indicates an attractive interaction between Sn and the CNT surface. Results and Discussion We examined three situations: a single Sn atom, a Sn3 cluster, and a Sn nanowire adsorbed on the outer and inner surfaces of the CNT. Cells of (5, 5) and (8, 0) CNTs with 80 and 96 carbon atoms, respectively, were used for the cases of the single Sn atom and the Sn cluster. The lateral separation between the tube

10.1021/jp809266n CCC: $40.75  2009 American Chemical Society Published on Web 07/08/2009

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Figure 2. Structures of Sn3 clusters: (a) Linear (b) V-shaped. Figure 1. High-symmetry adsorption sites on (a) semiconducting (8, 0) CNT and (b) metallic (5, 5) CNT.

TABLE 1: Adsorption Energies E (eV) and the Minimum Distance (dmin, Å) between Single Sn and (5, 5) or (8, 0) CNTs single Sn CNT

adsorption sites

Eouter

dmin

Einner

dmin

5, 5

HT* B1 B2 HT* B1 B2

-1.10 -1.07 -0.95 -1.51 -1.35 -1.11

2.38 2.41 2.43 2.45 2.37 2.46

-0.35 -0.38 -0.31 -0.99 -0.98 -0.99

2.72 2.64 2.63 2.65 2.62 2.83

8, 0

* Sn atom initially put at T or H sites would move to a site between H and T. The new site is denoted HT.

and its image is 22 Å for a single Sn atom and 30 Å for the Sn3 cluster, respectively. For the single Sn atom, we studied four high-symmetry adsorption sites on the surface of metallic (5, 5) and semiconducting (8, 0) single-walled CNTs. As shown in Figure 1, they are as follows: (i) top (T), (ii) hollow (H), (iii) bridge 1 (B1), and (iv) bridge 2 (B2), respectively. It is interesting to note that the single Sn atom initially put at either T or H sites shifts to the center between T and H after geometry optimization (please see Figure 1). We denote the new site as HT. The adsorption energies and minimum Sn-C distances of the single Sn atom at various sites of outer and inner surfaces of CNTs are presented in Table 1. For single Sn atoms adsorbed on the outer CNT surface, the most stable site is found to be HT, with adsorption energies of -1.10 and -1.51 eV for (5, 5) and (8, 0) CNTs, respectively. The minimum distances between Sn and the nearest C atoms of (5, 5) and (8, 0) CNTs are 2.38 and 2.45 Å, respectively. The nearest C atoms are seen to move up, resulting in an incremental increase of 0.03-0.07 Å in the C-C bond lengths of the C6 rings underneath the Sn atom. Mulliken charge analysis showed that there are more electrons transferred from the single Sn atom to (8, 0) than (5, 5) CNT. The partial charges of the single Sn atoms are 0.73 and 0.55 on HT sites of (8, 0) and (5, 5) CNTs, respectively. Interestingly, the adsorption energy was found to decrease when the single Sn atom was adsorbed on the inner CNT surface. Here, the adsorption energy was calculated to be -0.38 eV for the (5, 5) CNT at the B1 site, and -0.99 to -0.98 eV for the (8, 0) CNT at any of the four adsorption sites, as shown in Table 1. Moreover, the minimum distances between Sn and the nearest C atoms of the CNTs are seen to increase to 2.62 and 2.64 Å for the (5, 5) and (8, 0) CNTs, respectively, which is about 0.2-0.3 Å longer than those of the single Sn atom on the outer surface of CNTs. Also, the C-C bond lengths in the C6 rings underneath the Sn atom elongate by only 0.01-0.02

Å, which is much smaller than those of the single Sn atom on the outer surfaces of the CNTs. Mulliken charge analysis revealed that there are more electrons transferred to the nearby C atoms from single Sn atoms on the inner surface of CNTs than on the outer surface of CNTs. The partial charges of a single Sn atom on the inner surface were found to be 1.13 and 1.16 for the T site on (8, 0) and B1 site on (5, 5) CNTs, respectively. Here, the electrons transferred from the Sn atom distribute almost evenly on the surrounding C atoms. The maximum charge on a C atom is found to be only -0.08, regardless of whether it was (8, 0) or (5, 5) CNT. Most are, in fact, smaller than -0.02. In contrast, when Sn is adsorbed on the outer surface of CNTs, the electrons transferred from Sn are seen to be localized on only a few C atoms directly underneath the Sn atom. The maximum charges on the C atoms are -0.18, and most of them are larger than -0.10, resulting in the stronger adsorption of the single Sn atoms on the outer surface of CNTs. Both linear and V-shaped forms of the Sn3 cluster were used in the simulation. Their structures are shown in Figure 2. The V-shaped Sn3 cluster is 0.48 eV more stable than the linear form. The Sn-Sn bond length in the linear form is 2.72 Å, and the Sn1-Sn2-Sn3 angle is 179.6°. The Sn-Sn bond length in the V-shaped form is a bit shorter (2.70 Å), and the Sn1-Sn2-Sn3 angle is 79.8°. For the linear Sn3 cluster, we examined an orientation parallel to the surface of the CNT. The first Sn atom of the linear Sn3 cluster was used as a reference atom to generate a number of structures at different adsorption sites. With regard to the Vshaped form, we have considered four orientations: (i) The Sn-Sn-Sn face is perpendicular to the CNT surface, with the second Sn atom (the middle Sn atom) pointing to the CNT surface. Here, we take the middle Sn atom as the reference atom, and denote this structure perp 1. (ii) The Sn-Sn-Sn face is again perpendicular to CNT surface, but this time, the reference atom points away from the CNT surface; this conformation is denoted perp 2. (iii) The Sn-Sn-Sn face is again perpendicular to the CNT surface, but this time, one of the Sn-Sn bonds lies parallel to CNT surface. (iv) The Sn-Sn-Sn face is parallel to the CNT surface. We denote the latter two orientations as Par and Par3, respectively. The adsorption energies and minimum distances between Sn and the nearest C atoms are listed in Table 2. When the linear Sn3 cluster is adsorbed on the outer surfaces of (5, 5) CNT, the most stable adsorption site is B2 with an adsorption energy of -0.19 eV; this value is much smaller than those for single Sn atoms on the outer surface of the CNTs. Before optimization, only the first Sn atom was put at the B2 site, though the other two atoms were close to the B2 site. After optimization, each Sn atom sits at B2 sites of adjoining C6 rings, resulting in the bending of the linear Sn3 cluster. The Sn1-Sn2-Sn3 angle decreases from 179.6° to 153.8°, and the Sn-Sn bonds shorten from 2.72 Å to 2.67 Å. The second stable

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TABLE 2: Adsorption Energies (eV) and Minimum Distances of Sn-CNT for Sn3 Clusters on the Outer and Inner Surfaces of (5, 5) and (8, 0) CNTs V-shaped Sn3c

linear Sn3 CNT 5, 5

8, 0

adsorption sites

Eouter

dmin

Einner

dmin

B2 H

-0.19 -0.17

3.62 3.70

1.34 1.34

3.43 3.44

B1 B2 H T

-0.51 -0.47 -0.15 -0.15

2.50 3.39 3.54 3.35

2.41 2.23 2.39 2.41

2.93 3.18 3.12 3.20

adsorption sites Perp2-HT Perp2-B1 Perp2-B2 Perp2-H Perp2-T Perp2-HT Perp2-B1 Perp2-B2 Perp2-H Perp2-T Par3-Tb Par3-B1 Par3-B2

a

Eouter

dmin

Einner

dmin

-0.11 -0.14 -0.17 -0.17 -0.16

3.40 3.00 3.25 3.22 3.35

1.79 1.80 1.80 1.80

3.43 3.43 3.43 3.43

-0.24 -0.23 -0.25 -0.25 -0.34 -0.33 -0.33

2.96 2.97 3.31 2.92 2.48 2.48 2.46

2.68 2.82 2.82 2.68

3.17 3.13 3.07 3.15

a Perp2 denote the second orientation where Sn-Sn-Sn face is perpendicular to CNT surface and the second Sn atom (middle Sn) points out of CNT surface. b Par3 denote the fourth orientation where Sn-Sn-Sn face is parallel to CNT surface. c V-shaped Sn3 cluster becomes linear and moves to the center of CNT when it is put inside CNT after optimization.

adsorption site is HT, and after optimization, each Sn atom sits at an HT site; here, a somewhat smaller adsorption energy of -0.17 eV is observed. Similarly, the Sn1-Sn2-Sn3 angle decreases to 153.1° and the Sn-Sn bonds shorten to 2.68 Å. The minimum distances between Sn and C atoms are 3.6 Å at B2 sites and 3.7 Å at HT sites, respectively, which are much larger than those observed for single Sn atoms adsorbed on the outer surface of CNTs. Thus, it can be seen that the structure of (5, 5) CNT is not disturbed much by the adsorption of the Sn. The C-C bond lengths of the C6 rings directly underneath the linear Sn3 clusters remain almost unchanged. When we modeled the adsorption of the linear Sn3 cluster on the outer surface of (8, 0) CNT, clusters that retained a linear conformation after geometry optimization were found to exhibit quite small adsorption energies of around -0.1 eV. At the most stable adsorption sites, the linear Sn3 cluster was found to bend significantly to form V-shaped conformations after geometry optimization. The most stable adsorption site was found to be a top site (T), with an adsorption energy of -0.54 eV; here, the linear Sn3 cluster bends to form a V-shaped conformation with an Sn1-Sn2-Sn3 angle of 70.2° and where the Sn-Sn-Sn face is almost parallel to the CNT surface. In this conformation, the first and third Sn atoms are at T sites, while the second Sn atom is at a B2 site. Also, the Sn-Sn bonds elongate from 2.72 Å to 2.98 Å. The minimum distances between these three Sn atoms and the nearest C atoms are almost the same: 2.50, 2.52, and 2.56 Å, respectively. As the energies of both the Sn3 cluster and the (8, 0) CNT change only a small degree after adsorption, it is reasonable to deduce that the attractive interaction between Sn3 and CNT is the main contributor to the most stable adsorption. Interestingly, at the second most stable adsorption site, the linear Sn3 cluster bends to the most stable V-shaped structure with the Sn1-Sn2-Sn3 angle at 80.3° and the Sn-Sn bond lengths at 2.70 Å. Here, the Sn-Sn-Sn face is perpendicular to the (8, 0) CNT surface, and the second Sn atom points out of the CNT surface. The first Sn atom is at a T site, and the third Sn atom sits at the B2 site. The minimum distance between Sn and the nearest C atoms is 3.39 Å, and the C-C bond lengths of the C6 rings, underneath Sn3 cluster, remain almost unchanged. The adsorption energy is found to be -0.47 eV. Importantly, this energy value corresponds to a linear to V-shaped structural change in the Sn3 cluster, revealing that the stabilization of the Sn3 cluster is the main contribution to the second most stable adsorption energy.

In comparison to the linear Sn3 cluster, in the V-shaped conformation the cluster adheres weakly on the outer surface of the (8, 0) CNT. The most stable adsorption configuration is par3 T with an adsorption energy of -0.34 eV; here, the Sn-Sn-Sn face is parallel to CNT surface, and each Sn atom sits alternatively on the top of C atoms in the same C6 ring. Moreover, the Sn3 clusters initially in either a par3 B1 or par3 B2 configuration changed to par3 T site after geometry optimization (see Table 2). The second stable adsorption situation is where the Sn-Sn-Sn face is perpendicular to CNT surface and the middle Sn atom points out of CNT surface. In the par3 T configuration, the structure of the V-shaped Sn3 cluster changes to a quasi-equilateral triangle after geometry optimization; this structure is about 0.45 eV higher in energy than the most stable V-shaped arrangement. The lengths of the three sides (Sn-Sn bond) are 2.99, 3.01, and 3.19 Å, and the three Sn-Sn-Sn angles are 57.6°, 58.2°, and 64.2°, respectively. The minimum distances between each Sn atom and the nearest C atoms are 2.48, 2.48, and 2.57 Å, respectively, which are comparable to those of single Sn atoms on the surfaces of CNT. Considering that the energy of the V-shaped Sn3 cluster in par3 T configuration after optimization is 0.45 eV higher than that before optimization and the energy of (8, 0) CNT changes only a small degree after adsorption, it is reasonable to deduce that the attractive interaction between Sn3 and (8, 0) CNT is the main contributor to the most stable adsorption. With regard to the adsorption of the V-shaped Sn3 cluster on the outer surface of the (5, 5) CNT, it was found that the second orientation, as mentioned above, is the most stable, namely, where the Sn-Sn-Sn face is perpendicular to the CNT surface and the middle Sn atom points out of the CNT surface; moreover, two stable adsorption configurations have been identified. In the first conformation, the first and third Sn atoms are observed to sit at HT sites. In the second conformation, the first Sn atom sits at a T site and the third Sn atom at a HT site. Their adsorption energies are almost identical, each around -0.17 eV, which is comparable to that of a linear Sn3 cluster on the outer surface of a (5, 5) CNT, though smaller than that of a V-shaped Sn3 cluster on the outer surface of a (8, 0) CNT. The minimum distance between the Sn atoms and the nearest C atoms is around 3.2 Å. It is interesting to note that the most stable configuration on the (8, 0) CNT, namely, where the Sn-Sn-Sn face is parallel to the CNT surface, is not stable here. For example, when each Sn atom sits at a T site, the adsorption energy is 0.29 eV, although the structure of the Sn3

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TABLE 3: Adsorption Energies (eV) and the Minimum Distance between Sn and CNT (dmin, Å) of Sn Nanowire inside Zigzag (n, 0) CNTs Sn nano wire at the center Sn nano wire at 3.5 Å zigzag diameter CNT (Å) 8, 0 9, 0 10, 0 11, 0 12, 0 13, 0 14, 0 15, 0 16, 0 25, 0 30, 0

6.42 7.17 7.93 8.71 9.48 10.26 11.04 11.82 12.59 19.75 23.58

E

dmin

E

dmin

2.28 0.86 0.18 -0.078 -0.17 -0.18 -0.19 -0.19 -0.16 -0.048 -0.049

3.21 3.58 3.96 4.35 4.74 5.13 5.52 5.91 6.30 9.88 11.79

2.28 0.86 0.18 -0.079 -0.16 -0.17 -0.16 -0.16 -0.14 -0.10 -0.10

3.20 3.58 3.95 4.33 4.41 4.65 4.50 4.59 4.59 4.28 4.33

cluster after optimization is similar to that of a Sn3 cluster in the par3 T adsorption mode on the (8, 0) CNT; moreover, the minimum distances between the Sn atoms and the nearest C atoms are around 2.6 Å, which highlights the fact that the interaction between the V-shaped Sn cluster and the outer surface of (5, 5) CNT is quite weak. When either the linear or V-shaped Sn3 cluster was placed on the inner surface of the (5, 5) or (8, 0) CNTs, the absorption energies are largely positive, indicating that the Sn3 cluster is unstable inside (5, 5) and (8, 0) CNTs. The V-shaped Sn3 cluster stretches to a linear chain and moves to the center of the tube after optimization, regardless of its initial placement. Similarly, the linear Sn3 cluster also moves to the center of the tube after optimization. In order to examine whether the Sn cluster is stable inside the CNT, we put a linear atomic Sn wire inside a series of zigzag (n, 0) CNTs (n varies from 8 to 30, and their diameters vary from 6 to 23 Å). We used the linear atomic Sn wire to represent our Sn nanowire. In order to match the lattice constant of the CNT, the Sn-Sn bond was elongated by about 1%. The lateral separation between the tube and its image was set to exceed 10 Å. The Sn nanowire was initially placed along the axis of the tube, either in the center of the tube or at a position where the Sn atoms are about 3.5 Å off the sidewall of the CNT. As shown in Table 3, the adsorption energy becomes negative when n is greater than 10, no matter where the initial position is, indicating that the Sn wire is becoming stable inside the CNT. Interestingly, the strongest adsorption energy is found to be only -0.19 eV (for the (14, 0) and (15, 0) CNTs), showing that the interaction between Sn and the inner surface of the CNT is very weak. A Mulliken charge analysis showed that there is almost no charge transfer between the Sn nano wire and the (n, 0) CNT when n is greater than 10. Due to this very weak interaction between the Sn atoms and the inner surface of the CNT, the position of the Sn nanowire (which initially was placed in the center of the CNT) remains virtually unchanged after geometry optimization. Moreover, the Sn nanowire, which was initially placed at 3.5 Å off the CNT wall, is seen to move closer to the center of the tube after optimization. In both of the above cases, the Sn nanowire remains linear during the geometry optimization. For the latter case, the adsorption energy reaches a maximum when n ) 13 and subsequently decreases with increasing diameter of the CNT due to the gradual flattening of the inner CNT surface. Figure 3 shows the typical density of states (DOS) of a single Sn atom and the linear Sn cluster on the outer surface of the (5, 5) and (8, 0) CNTs, respectively. Its is important to note that

Figure 3. Density of states (DOS) of (a) single Sn atom at HT site of the outer surface of (5, 5) CNT; (b) linear Sn3 cluster at B2 site of the outer surface of (5, 5) CNT; (c) single Sn atom at HT site of the outer surface of (8, 0) CNT; (d) linear Sn3 cluster at B1 site of the outer surface of (8, 0) CNT.

the adsorption of Sn does not change the conducting nature of the (5, 5) CNT. Conversely, when the linear Sn cluster is adsorbed on the outer surface of the (8, 0) CNT, the system becomes metallic; this is clearly important for any materials under consideration as potential anodes for Li-ion batteries. Our simulation results have shown that Sn clusters have a much smaller adsorption energy than a single Sn atom on the outer surfaces of CNTs; this indicates that bulk Sn would adhere only weakly to the outer surfaces of CNTs, which results in noncontinuous bead-like Sn particles on the CNTs (Figure 4).20 In the case of other metals, such as Au, Pd, Fe, Al, and Pb, the same discrete particles were observed on the surfaces of CNTs; here, the weak metal-CNT interaction was proposed to be the main cause.21 Because the adsorption energy of Sn on the outer surface of the CNT is about 2-3 times greater than that on the inner surface, it is not surprising that the arc-discharge method failed to encapsulate Sn into CNT. On the other hand, the weak interaction between Sn and the CNTs is a distinct advantage for the potential application of these systems as anodic materials in Li-ion batteries. First, the CNT backbone is not perturbed by the alloying of Sn and Li; this is because there is no direct bonding between Sn and the CNT. Second, our calculations have shown that (i) the Sn nanowire is stable inside the CNT when the average Sn-C distance is about 4.4 Å, and (ii) it can stay in the center of (30, 0) CNTs, even though it is about 12 Å off the inner sidewall.. Indeed, if we consider as a forbidden void the region from the inner side wall to 4.4 Å out (which is much larger than the sum of covalent radii of C and Sn (or Li) atoms

DFT Study on Nano Structures

J. Phys. Chem. C, Vol. 113, No. 31, 2009 14019 Conclusion In summary, this work shows that Sn clusters have a much weaker interaction with CNTs than single Sn atoms. The interaction of Sn with the outer surface of a CNT is about two to three times stronger than that with the inner surface; this has two consequences: (i) the formation of discrete metal particles on the outer surface of the CNT and (ii) the failure to encapsulate Sn inside the CNT by conventional methods. Importantly, from the above calculations it can be seen that the (n, 0) CNT/ encapsulated Sn nanowire system can easily cope with the huge volume expansion during Li+ insertion when n g 21. Insights into the interaction of Sn with CNTs provided here should aid in the future design of Sn/CNT complexes for potential applications in Li-ion batteries. Acknowledgment. The authors gratefully acknowledge the support received from the Institute of High performance Computing of A*STAR of Singapore. References and Notes

Figure 4. TEM image of CNT coated with Sn metal.

Figure 5. Void in CNT: (a) forbidden void from inner sidewall to 4.4 Å out, which is the average minimum distance between Sn and the CNT when Sn is stabilized inside the CNT; (b) remaining void for encapsulating Sn and alloying of Sn with Li.

(∼2.2 Å), the remaining void can be used for Sn alloying with Li (Figure 5). The remaining volume of void for one Sn atom in a (21,0) CNT (which corresponds to a cylinder with a radius of 3.84 Å and a height of 1.4 Å), is 1.1 times the volume occupied by one SnLi4 unit, indicating that a (n, 0) CNT encapsulated Sn nanowire can theoretically withstand the colossal volume change of alloying when n g 21.

(1) Zheng, T.; Dahn, R. Application of carbon in lithium-ion batteries. In Carbon Materials of AdVanced Technology, Burchell, T. D., Ed.;Pergamon Press: Oxford, UK, 1999. (2) Flandrois, S.; Simon, B. Carbon 1999, 37, 165. (3) Yazami, R. In Materials for Li-ion Batteries, Julien, C., Stoyhov, Z., Eds.; Kluwer Academic Publishers: London, 2000. (4) Noel, M.; Suryanarayanan, V. J. Power Sources 2002, 111, 193. (5) Tirado, J. L. Mater. Sci. Eng. 2003, R40, 103. (6) Larcher, D.; Beattie, S.; Morcrette, M.; Edstro¨m, K.; Jumas, J. C.; Tarascon, J. M. J. Mater. Chem. 2007, 17, 3759. (7) Lee, K. T.; Jung, Y. S.; Oh, S. M. J. Am. Chem. Soc. 2003, 125, 5652. (8) Zhang, W. M.; Hu, J. S.; Guo, Y. G.; Zheng, S. F.; Zhong, L. S.; Song, W. G.; Wan, L. J. AdV. Mater. 2008, 20, 1160. (9) Hassoun, J.; Derrien, G.; Panero, S.; Scrosati, B. AdV. Mater. 2008, 20, 3169. (10) Kumar, T. P.; Ramesh, R.; Lin, Y. Y.; Ting-Kuo Fey, G. Electrochem. Commun. 2004, 6, 520. (11) Pleˇcourt, C. G.; Bouar, Y. L.; Lolseau, A.; Pasxard, H. Nature 1994, 372, 761. (12) Dujardin, E.; Ebbesen, T. W.; Hiura, H.; Tanigaki, K. Science 1994, 265, 1850. (13) Jankovicˇ, L.; Gournis, D.; Trikalitis, P. N.; Arfaoui, I.; Cren, T.; Rudolf, P.; Sage, M. H.; Palstra, T. T. M.; Kooi, B.; Hosson, J. D.; Karakassides, M. A.; Dimos, K.; Moukarika, A.; Bakas, T. Nano Lett. 2006, 6, 1131. (14) Li, R. Y.; Sun, X. C.; Zhou, X. R.; Cai, M.; Sun, X. L. J. Phys. Chem. C. 2007, 111, 9130. (15) Blochl, P. E. Phys. ReV. B. 1994, 50, 17953. (16) Kresse, G.; Joubert, D. Phys. ReV. B. 1999, 59, 1758. (17) Kresse, G.; Hafner, J. Phys. ReV. B. 1993, 48, 13115. (18) Kresse, G.; Hafner, J. Phys. ReV. B. 1994, 49, 14251. (19) Perdew, J. P.; Wang, Y. Phys. ReV. B. 1992, 45, 13244. (20) Nai, S. M. L. Ph. D. thesis, National University of Singapore, 2008. (21) Zhang, Y.; Dai, H. J. Appl. Phys. Lett. 2000, 77, 3015.

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