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Evaluation of Sn Nanowire Encapsulated Carbon Nanotube for a Li-Ion Battery Anode by DFT Calculations Man-Fai Ng, Jianwei Zheng, and Ping Wu* Institute of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632, Singapore ReceiVed: January 18, 2010; ReVised Manuscript ReceiVed: March 26, 2010
Hybrid nanomaterial of Sn nanowire encapsulated carbon nanotube (SnNW@CNT) for a 1-D nanostructure Li-ion battery anode is evaluated by using DFT electronic structure and molecular dynamics calculations. We reveal that ultrathin SnNW is thermodynamically stable within CNT and it is prone to have a disorder structure. In addition, SnNW@CNT can enhance Li uptake as compared with pure CNT due to the energetically favorable Li-SnNW alloying process. More importantly, we reveal that an optimized SnNW@CNT has a void space thickness of ∼5 Å between the nanostructures, indicating that the radial void spaces are only enough for accommodating the fully lithiated SnNW within CNT(n,0) when n < 27. The results suggest that such nanowire encapsulated nanotube materials have to be made ultrathin in order to become an effective hybrid nanomaterial for a Li-ion battery anode. Introduction Since its first commercialization, the Li-ion battery has become the most widely used secondary battery for portable electronic devices nowadays.1 The breakthrough of the Li-ion battery makes use of carbon material as the anode (negative electrode). Graphite is one of the best forms of carbon to intercalate with Li reversibly for such a purpose. However, due to its relatively low theoretical charge capacity (372 mAh/g for LiC6),2 intensive research pursuing new materials with higher energy capacity and a longer life cycle for a Li-ion battery remains active. In recent years, nanomaterials such as carbon nanotubes (CNTs) and nanowires (NWs) have been widely investigated both experimentally3-9 and theoretically10-16 for the possibility to replace graphite as the anode materials for a Li-ion battery. In particular, CNTs have been proven to have higher Li storage capacity, e.g. Li2.7C6 and LiC3, as compared to that of graphite.3,4 Recently, group IV nanowires such as silicon nanowire (SiNW) and germanium nanowire (GeNW) have been utilized as high performance anodes for their high charge capacity and columbic efficiency: >1000 mAh/g capacity and 84-96% efficiency for GeNW;7 >3000 mAh/g capacity and 90% efficiency for SiNW.8 In addition, tin nanowire (SnNW) has also been used as anode material.9 Both CNTs and NWs show promising potential as anode materials due to their ability to capture large amounts of Li ions. In addition, since CNT captures Li ions by intercalation while group IV nanowire by alloy formation, anode materials possessing these two Li uptake mechanisms simultaneously should be able to enhance the charge storage performance of a Li-ion battery. Recently, Sn nanowire encapsulated carbon nanotube (SnNW@CNT) has been synthesized successfully.17,18 Crystalline SnNW within CNT has a preferred growth orientation of 〈001〉 with a diameter distribution of 200-400 nm17 and 15-35 nm.18 On the other hand, Sn-filled CNT has shown remarkable performance as the anode material for a Li-ion battery;19 and Snencapsulated spherical hollow carbon is another promising anode * To whom correspondence should be addressed. E-mail: wuping@ ihpc.a-star.edu.sg.
material.20 This is because Sn itself has a high theoretical capacity of 994 mAh/g.21 All these experimental works demonstrate that growing SnNW within CNT and utilizing such a hybrid nanostructure for a Li-ion battery anode are feasible. We have investigated the potential of using CNT/Sn atom or Sn clusters for a Li-ion battery anode in our recent study.22 Here we focus on the hybrid nanomaterial of SnNW@CNT. In fact, 1-D hybrid nanomaterials such as SnO2/In2O3,23 SnO2/Sn nanoclusters24 for anode, and MnO2/CNT25 for cathode very recently have been proven to improve the efficiency of a Liion battery. However, theoretical work on 1-D hybrid nanomaterials for such a purpose has not yet been reported. In this work, we use density-functional-theory (DFT) electronic structure and molecular dynamics (MD) calculations to examine SnNW@CNT for its application as a 1-D nanostructure Li-ion battery anode. We first evaluate thermodynamically and energetically stable models for SnNW@CNT. We then examine its conducting properties as an anode by calculating its electronic structure. More importantly, the issue of volume expansion of SnNW within CNT is explored. Also, we study the process of Li uptake by this hybrid nanostructure in terms of the energy barrier for Li to penetrate through the CNT side wall. Methodology Geometry optimization and MD calculations are performed by using a generalized gradient approximation (GGA)26 scheme within DFT implemented in the Vienne Ab initio Simulation Package (VASP).27,28 The frozen-core projector augmented wave (PAW) method is used to describe the interaction between ions and electrons.29 The cutoff energy for the plane wave expansion is set at 450 eV. Monkhorst-Pack sampling with a 1×1×6 k-point grid is used. More than 10 Å of vacuum spaces in the lateral directions are used to avoid interactions between neighboring ions. For geometry optimization, both the ion and unit cell are fully relaxed until the absolute value of force acting on each atom is less than 0.01 eV/Å. For MD calculations, the algorithm of Nose´ is adopted to control the temperature of the system.30 We use 5000 ionic steps and the time step is set at 2.0 fs for all MD calculations. Temperature is set at 300 K.
10.1021/jp100495y 2010 American Chemical Society Published on Web 04/20/2010
Evaluation of SnNW@CNT for Li-Ion Battery Anode
Figure 1. (a) 1×1×1 and 1×1×2 unit cells of bulk R-Sn and β-Sn, respectively. Gray balls represent Sn atoms. Red and blue balls highlight the zigzag and linear chains of Sn atoms, respectively. (b) Geometries of zigzag SnNW@CNT, standalone zigzag SnNW, linear SnNW@CNT, and standalone linear SnNW before (left) and after (right) MD calculations at 300 K. T stands for time. T′ ) 2000 fs for the two SnNW@CNT systems while T′ ) 10000 fs for the two standalone SnNWs. The coordinates of the CNT are held fixed during the calculations.
The initial velocities of the systems are scaled at 300 K by 300 ionic steps. The lattices of SnNWs are slightly compressed along the axial direction so that it can be commensurate with the lattices of their respective CNTs as the starting models. Results and Discussion Thermal Stability of Ultrathin SnNW. Bulk gray (R) and white (β) Sn are shown in Figure 1a. The former adopts a diamond-liked tetragonal crystal structure while the latter adopts a body-centered tetragonal structure. We note that the thinnest wire components in these structures are linear and zigzag SnNWs. To establish a simple model for SnNW@CNT, we determine the thermodynamic stability of these two types of nanowire by MD calculations. The MD results show that the linear SnNW, both encapsulated by CNT and standalone, turns into a Sn cluster, indicating that the linear SnNW is not a thermodynamically stable model. In contrast, the zigzag SnNW is more stable as it remains unbroken but it is prone to become a disorder structure (Figure 1b). We have also run MD calculations to test the standalone SnNWs with thicker diameters of 5 Å and 9 Å (insets in Figure 2a). These thicker SnNWs also show the same behavior, indicating that such ultrathin SnNWs would adopt disorder structures. This is attributed to
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Figure 2. (a) Interaction energies as functions of the chiralities of CNT for zigzag SnNW@CNT, 5-Å-diam SnNW@CNT and 9-Å-diam SnNW@CNT. Arrows indicate the most stable SnNW@CNT for each class. (b) Band structures of CNT(15,0), zigzag SnNW@CNT(15,0), and zigzag SnNW. The Fermi level is set at 0 eV.
the high surface energy caused by the dangling bonds on thin and unpassivated SnNWs that causes the distortion of the structure. Thermodynamically, it can be attributed to phonon instability of 1-D crystalline nanomaterial. Nevertheless, we use zigzag SnNW as our thinnest nanowire model. Interaction Energy between SnNW and CNT. The interaction energies are evaluated by using the following equation: EINT ) ESnNW@CNT - ECNT - ESnNW, where EINT, ESnNW@CNT, ECNT, and ESnNW represent the interaction energy, the total energies of SnNW@CNT, standalone CNT, and standalone SnNW, respectively. Negative energy indicates an attractive interaction. The interaction energies as functions of the chiralities of CNT for zigzag SnNW@CNT, 5-Å-diam SnNW@CNT, and 9-Ådiam SnNW@CNT are shown in Figure 2a. The negative interaction energies indicate that the SnNW is energetically stable to stay within the CNT. As such, the CNT can provide a matrix effect to prevent the SnNW from segregating, which is important for enhancing the life cycle of the anode. Moreover, our calculations reveal that the zigzag, 5-Å-diam, and 9-Å-diam SnNWs fit best with CNT(15,0), CNT(20,0), and CNT(25,0) with coupling energies of -0.18, -0.45, and -0.75 eV, respectively, indicating that the interaction energy between SnNW and CNT increases with the system size. This is attributed to the increased interacting surface areas between the SnNW and CNT. We also investigate the symmetry effect of the SnNW by displacing it away from the center of the CNT (Figure S1 in the Supporting Information). We conclude that the SnNW is the most stable when it positions itself at the center of the CNT.
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Figure 3. Ratio of cross section of void of CNT to SnNW for zigzag SnNW@CNT(15,0), 5-Å-diam SnNW@CNT(20,0), and 9-Å-diam SnNW@CNT(25,0). The dotted circle indicates the mean diameters of the SnNW.
Electronic Properties of SnNW@CNT. The band structures of zigzag SnNW@CNT(15,0) and its counterparts are shown in Figure 2b. From the band structures, SnNW@CNT(15,0) is found to be metallic mainly due to the SnNW, because the numbers of bands around the Fermi level are increased. The standalone CNT(15,0) has a negligible band gap that is regarded as metallic. The region around the Fermi level shows a moderate band mixing between the CNT and SnNW, indicating that the two nanostructures are hybridized moderately to each other. Moreover, comparing with the standalone CNT(15,0), SnNW@ CNT(15,0) is more metallic and the metallicity is important for an anode to conduct electricity. In addition, we have also calculated their density-of-states (DOS). The DOS of the standalone CNT shows insignificant states around the Fermi level while SnNW@CNT(15,0) shows more states (Figure S2 in the Supporting Information). These results indicate that SnNW@CNT should have higher conductivity than its standalone counterparts for anode application. Also, the 1-D nature of SnNW@CNT can enhance effective electron transport as well. Volume Expansion of SnNW within CNT. Another issue is the volume expansion of SnNW upon lithiation. Previous study shows a volume change of ∼259% when Sn is fully lithiated to Li22Sn5 in the bulk phase;31 the void of the CNT should thus provide such a buffer for the volume expansion of SnNW. To illustrate this, we compare the free void spaces inside SnNW@CNT with different diameters. Assuming the volume expansion is completely accommodated by facile strain relaxation, we estimate the volume expansion issue from the ratio of cross section of the CNT void to SnNW as shown in Figure 3. Since the calculated optimal distance (dopt) between the SnNW and CNT is ∼5 Å on average (dopt ) mean radius of optimized CNT - mean radius of optimized SnNW, as shown in the insets of Figure 3), it is found that the radial free void spaces are not enough for accommodating the lithiated SnNW, which could undergo 2.59 times volume expansion, within CNT(n,0) when n > 27. The larger the hybrid system, the relatively fewer free void spaces along the radial directions. In addition, previous studies have shown that the tensile stress (σ) generated due to the volume expansion of Sn as a result of Li charging is 210 GPa32,33 while the radial Young modulus of multiwalled CNT is found to be 30 ( 10 GPa.34 From the mechanical point of view, the CNT would likely be pulverized when SnNW is lithiated within if the free void spaces along
Ng et al.
Figure 4. (a) Top views of the optimized Li-CNT and Li-SnNW. Purple balls represent Li. (b) Upper: Topological ring defects at the CNT side wall with Li at binding site and defect centroid (inset on the left). Orange balls indicate the ring defect. Lower: Energy barriers of the topological ring defects and binding energies between SnNW@CNT and Li at the binding sites and defect centroids. (c) Schematic drawing of a SnNW@CNT anode.
the radial directions are not enough. As a result, SnNW@CNT has to be made ultrathin in order to become an effective hybrid nanomaterial for a Li-ion battery anode. Li Uptake in SnNW@CNT. We calculate the binding energies between Li and the standalone SnNW as well as the perfect CNT (Figure 4a). The calculated binding energy for LiSnNW is -2.34 eV, while it is -1.78 eV (-1.74 eV) for LiCNT when Li is located outside (inside) the CNT, respectively. The outside binding site is slightly preferred. These results indicate that Li would bind stronger to the SnNW (alloy formation) than to the CNT (intercalation). Hence, in the presence of SnNW, Li could gain an extra attractive force to stay in the interior part of the CNT. As indicated by previous theoretical studies in which cluster CNT models are used, the energy barrier for Li to penetrate through the hexagonal ring of the CNT is very high (∼11-15 eV).11,13 Hence it is unlikely for Li to penetrate through the perfect CNT side wall under normal battery conditions. As such, we consider CNT with topological defects on its side wall: heptagonal, octagonal, and nonagonal ring defects. The topological defects at the CNT side wall are shown in Figure 4b. The energy barrier is defined as the energy required for Li to go from its outside binding site to the defect centroid on SnNW@CNT. The energy barriers at different topological defects and binding energies between SnNW@CNT and Li at the binding site, as well as at the defect centroid, are summarized in Figure 4b. It is shown that the energy barrier decreases sharply with the size of the defects. The energy barrier drops from 9.49 eV (hexagonal ring) to 0.38 eV (nonagonal ring). For comparison, we also calculate the energy barriers of the defects when the SnNW is taken out of the systems. It is found that the presence of SnNW reduces the energy barriers slightly by ∼0.01 eV (Table S1 in the Supporting Information), indicating that the SnNW does not have a significant effect to lower the energy barriers. For the binding energies, it is found that Li binds stronger with increasing size of the defects at both sites. Our results indicate that although the energy barrier of the nonagonal ring
Evaluation of SnNW@CNT for Li-Ion Battery Anode defect is as small as 0.39 eV, the binding energy (-2.76 eV) is so large that Li can bind strongly to the defect site and, consequently, can block the diffusion channel for Li. On the basis of both kinetic and energetic considerations, defects must be larger than the nonagonal ring for Li to penetrate through the CNT side wall. Nevertheless, using the ring shape topological models is for ideal consideration, and defects are likely to be more irregular in shape in real situations. Here we demonstrate that the energy barrier and binding energy are both important parameters to be considered for investigating the interaction between the defects and Li. Although our results indicate that large topological defects are essential for the initial stage of Li uptake, the SnNW plays a significant role for Li uptake once Li has entered the CNT subsequently. We do not consider Li uptake from the open ends of the CNT for two reasons: First, the total surface area of the CNT side wall is much larger than that of the open ends, and in the presence of large topological defects, Li should have higher probability to penetrate through the CNT side wall. Second, experiments have already shown that SnNW@CNT usually has closed tips.17,18 In addition, as an anode, one end of it would attach to the current collector (Figure 4c). However, if the tip end can be opened, Li should also be able to enter through it. Conclusions In conclusion, we have used DFT electronic structure and MD calculations to examine SnNW@CNT for its potential application as a 1-D nanostructure Li-ion battery anode. We highlight that (1) ultrathin SnNW is thermodynamically stable within CNT and it is prone to have a disorder structure. Also, (2) the role of the SnNW is for enhancing Li uptake, hence the charge capacity. The driving force is attributed to the more favorable Li-SnNW alloying process than the Li-CNT intercalating process, taking into account that Li can go reversibly through the large topological defects. Meanwhile, the role of the CNT is for providing a matrix effect to prevent the SnNW from segregating, which can enhance the life cycle of the anode. More importantly, we reveal that (3) SnNW@CNT have to be made ultrathin in order to become an effective hybrid nanomaterial for a Li-ion battery anode because the radial void spaces are only enough for accommodating the fully lithiated SnNW within CNT(n,0) when n < 27. Finally, (4) large topological defects (>nonagonal ring) are essential for not blocking the diffusion path for Li and the SnNW takes over the main role for Li uptake once it has reached the interior part of the CNT. The proposed modeling scheme should be able to be applied to evaluate other 1-D nanowire encapsulated nanotube hybrid nanomaterials for a Li-ion battery anode. Acknowledgment. The authors thank Dr. William YIM WaiLeung and Dr. WONG Chiong Teck for helpful discussion. Supporting Information Available: Figures giving the interaction energy as a function of chirality of the CNT for two SnNW@CNT systems with the SnNW located at different initial positions (Figure S1), plots of density-of-states for zigzag SnNW@CNT(15,0) and its counterparts (Figure S2), and a
J. Phys. Chem. C, Vol. 114, No. 18, 2010 8545 comparison of energy barriers of the defects in the presence and absence of the SnNW (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Nagaura, T; Tozawa, K. Prog. Batteries Sol. Cells 1990, 9, 209. (2) Lee, K.-L.; Jung, J.-Y.; Lee, S.-W.; Moon, H.-S.; Park, J.-W. J. Power Sources 2004, 270, 129. (3) Gao, B.; Bower, C.; Lorentzen, J. D.; Fleming, L.; Kleinhammes, A.; Tang, X. P.; McNeil, L. E.; Wu, Y.; Zhou, O. Chem. Phys. Lett. 2000, 327, 69. (4) Shimoda, H.; Gao, B.; Tang, X. P.; Kleinhammes, A.; Fleming, L.; Wu, Y.; Zhou, O. Phys. ReV. Lett. 2002, 88, 015502. (5) Wu, G. T.; Wang, C. S.; Zhang, X. B.; Yang, H. S.; Qi, Z. F.; He, P. M.; Li, W. Z. J. Electrochem. Soc. 1999, 146, 1696. (6) Gao, B.; Kleinhammes, A.; Tang, X. P.; Bower, C.; Fleming, L.; Wu, Y.; Zhou, O. Chem. Phys. Lett. 1999, 307, 153. (7) Chan, C. K.; Zhang, X. F.; Cui, Y. Nano Lett. 2008, 8, 307. (8) Chan, C. K.; Peng, H.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. Nat. Nanotechnol. 2008, 3, 31. (9) Kim, J.-H.; Khanal, S.; Islam, M.; Khatri, A.; Choi, D. Electrochem. Commun. 2008, 10, 1688. (10) Zhao, J.; Buldum, A.; Han, J.; Lu, J. P. Phys. ReV. Lett. 2000, 85, 1706. (11) Udomvech, A.; Kerdcharoen, T.; Osotchan, T. Chem. Phys. Lett. 2005, 406, 161. (12) Lemos, V.; Veloso, M. V. D.; Fagan, S. B.; Mendes-Filho, J. Phys. Status Solidi C 2004, 1, S219. (13) Kar, T.; Pattanayak, J.; Scheiner, S. J. Phys. Chem. A 2001, 105, 10397. (14) Garau, C.; Frontera, A.; Quin˜onero, D.; Costa, A.; Ballester, P.; Deya`, P. M. Chem. Phys. Lett. 2003, 374, 548. (15) Meunier, V.; Kephart, J.; Roland, C.; Bernholc, J. Phys. ReV. Lett. 2002, 88, 075596. (16) Mpourmpakis, G.; Froudakis, G. E.; Tylianakis, E. Appl. Phys. Lett. 2006, 89, 233125. (17) Li, R.; Sun, X.; Zhou, X.; Cai, M.; Sun, X. J. Phys. Chem. C 2007, 111, 9130. (18) Jankovieˇ, L.; Gournis, D.; Trikalitis, P. N.; Arfaoui, I.; Cren, T.; Rudolf, P.; Sage, M.-H.; Palstra, T. T. M.; Kooi, B.; De Hosson, J.; Karakassides, M. A.; Dimos, K.; Moukarika, A.; Bakas, T. Nano Lett. 2006, 6, 1131. (19) Prem Kumar, T.; Ramesh, R.; Lin, Y. Y.; Fey, G. T.-K. Electrochem. Commun. 2004, 6, 520. (20) Lee, K. T.; Jung, Y. S.; Oh, S. M. J. Am. Chem. Soc. 2003, 125, 5652. (21) Winter, M.; Besenhard, J. O. Electrochim. Acta 1999, 45, 31. (22) Zheng, J. W.; Nai, S. M. L.; Ng, M.-F.; Wu, P.; Wei, J.; Gupta, M. J. Phys. Chem. C 2009, 113, 14015. (23) Kim, D.-W.; Hwang, I.-S.; Kwon, S. J.; Kang, H.-Y.; Park, K.-S.; Choi, Y.-J.; Choi, K.-J.; Park, J.-G. Nano Lett. 2007, 7, 3041. (24) Meduri, P.; Pendyala, C.; Kumar, V.; Sumanasekera, G. U.; Sunkara, M. K. Nano Lett. 2009, 9, 612. (25) Reddy, A. L. M.; Shaijumon, M. M.; Gowda, S. R.; Ajayan, P. M. Nano Lett. 2009, 9, 1002. (26) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671. (27) Kresse, G.; Furthmu¨ller, J. Comput. Mater. Sci. 1996, 6, 15. (28) Kresse, G.; Furthmu¨ller, J. Phys. ReV. B 1996, 54, 11169. (29) Blochl, P. E. Phys. ReV. B 1994, 50, 17953. (30) Bylander, D. M.; Kleinman, L. Phys. ReV. B 1992, 46, 13756. (31) Sivashanmugam, A.; Premkumar, T.; Gopukumar, S.; Renganathan, N. G.; Wohlfahrt-Mehrens, M.; Garche, J. J. Appl. Electrochem. 2005, 35, 1045. (32) Wolfenstine, J. J. Power Sources 1999, 79, 111. (33) Wolfenstine, J.; Foster, D.; Read, J.; Behl, W. K.; Luecke, W. J. Power Sources 2000, 87, 1. (34) Palaci, I.; Fedrigo, S.; Brune, H.; Klinke, C.; Chen, M.; Riedo, E. Phys. ReV. Lett. 2005, 94, 175502.
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