Cu–Sn Core–Shell Nanowire Arrays as Three-Dimensional Electrodes

ARTICLE pubs.acs.org/JPCC

Cu
Sn Core
Shell Nanowire Arrays as Three-Dimensional Electrodes for Lithium-Ion Batteries Jiazheng Wang, Ning Du, Hui Zhang, Jingxue Yu, and Deren Yang* State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China ABSTRACT: Cu
Sn core
shell three-dimensional electrodes have been successfully synthesized by directly depositing metallic Sn on the surface of presynthesized Cu nanowire arrays. When used as an anode for lithium-ion batteries, the Cu
Sn three-dimensional electrode exhibits a high reversible capacity, a high initial Coulombic efficiency, a good rate capability, and an improved capacity retention. The effect of the thickness of the Sn layer on the Li-ion battery performance has also been investigated. The efficient buffering of the volume change, fast transport of electrons, and good contact to the current collector of the array structure may be responsible for the good cycling performance.

’ INTRODUCTION Currently, as one of the most important energy storage and conversion devices, lithium-ion batteries are widely used in portable electronic devices, such as mobile phones, laptops, and so forth.1 However, lithium-ion batteries still cannot satisfy the ever-growing needs for high-power applications due to the low energy density of the commercial graphite anode material.2 Therefore, large research efforts are devoted to developing alternative anode materials with large capacities, low operating potentials, an excellent high-rate performance, and a good cyclic life.3 Tin-based materials have been regarded as one of the most promising alternative anodes for lithium-ion batteries due to the high theoretical capacity, low potential of lithium-ion intercalation, and low price.4 Among them, SnO2 is the most widely studied anode material. However, the high initial irreversibility related to the formation of Li2O in the first cycle limits its practical application.5 In the past years, tin-based intermetallic compounds, such as SnxMy (M = inactive metal), have attracted great attention because these materials exhibited longer cycle ability. Nevertheless, the intermetallic compound anodes prolong the cycling life at the price of decreasing capacity.6 In addition, they have the same problem of low initial Coulombic efficiency. Compared with SnO2 and tin-based intermetallic compounds, pure Sn exhibits a higher theoretical capacity (990 mA h g
1) and a higher first Coulombic efficiency, which can be expected to lead to a comparable capacity. Despite the above-mentioned merits, the key problem of a pure Sn anode is the large volume change during the alloying/dealloying process of lithiation.7 To overcome this issue, several strategies have been proposed to improve the cyclability of Sn materials, which includes decreasing the particle sizes and using porous Sn thin films.7,8 These strategies can improve the electrochemical performance of Sn anodes, but only to a limited extent. More recently, Cu nanowire r 2011 American Chemical Society

array current collectors grown onto a planar copper surface covered with an active material using an electrodeposition method have been reported.9 These nanoarchitectured current collectors showed an improved performance and a high rate capability due to many advantages of the novel electrode envisioned, such as good electrical contact of the active materials with current collectors, fast electron transport, and good strain accommodation.9 Herein, it can be imagined that a Cu nanowire array current collector supported metallic Sn electrode could show an improved cycling stability. In this study, a pure Sn layer has been deposited directly on a Cu nanowire array current collector by a sputtering technique. The structure and electrochemical properties of as-prepared Cu
Sn three-dimensional electrodes have been investigated. We believe that the performance of pure Sn anodes can be significantly improved by employing the Cu nanowire array current collector.

’ EXPERIMENTAL SECTION Synthesis of Cu Nanowire Arrays on Cu Substrates. Cu nanowire arrays on copper substrates were fabricated by cathodic electrodeposition with anodized aluminum oxide (AAO) nanotemplates by an LK2006A electrochemical workstation. The AAO membranes were prepared by a two-step anodization process as described everywhere.10 The pore diameter of the as-prepared templates is estimated to be about 40
50 nm with a density of 1010
1011 pores cm
2. The cathode Cu substrate was first mechanically polished with 1.0 μm α-alumina and 0.25 μm γ-alumina polishing slurries. The cathodes were then further Received: July 3, 2011 Revised: October 18, 2011 Published: October 18, 2011 23620

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The Journal of Physical Chemistry C ultrasonically cleaned in ethanol and diluted HCl solution (10 vol %) and rinsed with DI water. The polished cathode Cu foil, AAO, separator (filter paper soaked with electrolyte, Whatman), and the anode Cu disk were packed in sequence and assembled using two stainless steel clamps. The outer parts of the copper anode and cathode were protected from dissolution or deposition by isolating adhesive films, which is similar to the method previously reported.9a The electrolyte systems consisted of CuSO4 3 5H2O, 100 g L
1; (NH4)2SO4, 10 g L
1; and diethylenetriamine (DETA), 40 mL L
1. All the reagents were used without further purification. The Cu nanowires were electrochemically deposited under cyclic voltammetry conditions by sweeping the potential from
0.8 to
1.0 V at room temperature. After deposition, the as-prepared samples were taken out from the electrolyte and immersed in 2 M NaOH solution for 1 h to remove AAO templates. Current collectors were then rinsed several times with deionized water and stored into a glovebox filled with an argon atmosphere. Synthesis of Cu
Sn Three-Dimensional Electrodes on Cu Substrates. Pure Sn layers were deposited on Cu nanowire arrays by the direct current sputtering of a 99.99% pure Tin target at a working pressure of 10 Pa. The working gas for deposition was 99.99% pure Ar, and the gas flow was 20 sccm. The sputtering power was 60 W, and the substrate was kept at 180 °C. Four types of thickness-controlled Sn thin films were prepared for comparison by depositing for 200 s (sample A), 400 s (sample B), 600 s (sample C), and 800 s (sample D). Characterization and Electrochemical Measurement. The morphology and structure of the obtained samples were examined by scanning electron microscopy (SEM HITACH S4800) with an energy-dispersive X-ray spectrometer (EDX) and transmission electron microscopy (TEM, PHILIPS CM200). Electrochemical measurements were performed by coin-type cells (CR2025) that were assembled in a glovebox (Mbraun, labstar, Germany) under an argon atmosphere by directly using the as-synthesized Cu
Sn three-dimensional electrodes as the anodes. The counter and reference electrodes were lithium metal foils (15 mm diameter), and the electrolyte solution was a 1 M solution of LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume). Finally, the cells were then aged for 12 h before measurements. A galvanostatic cycling test of the assembled cells was carried out on a Land CT2001A system in the potential range of 1 mV to 2.0 V at a discharge/charge current density of 500 mA g
1. Cyclic voltammetry (CV) was recorded on a Arbin BT 2000 system at a scan rate of 0.1 mV s
1. The accurate mass of the active materials on the Cu nanoarray current collector was examined using a microbalance. We measured the masses of the bare Cu substrate, the substrate with Cu nanowire arrays, and the substrate after sputtering. Thus, the total masses of the active material could be obtained.

’ RESULTS AND DISCUSSION The fabrication of vertical arrays of Cu
Sn core
shell nanowires on a Cu substrate is displayed in Figure 1. It can be seen that the whole process involves two steps: (1) growth of a Cu nanowire array current collector by cathodic electrodeposition via a template-assisted method and (2) coating of a Sn layer onto the surface of Cu nanowires by a sputtering technique. Scanning electron microscopy (SEM) was used to investigate the morphology of as-obtained samples. It can be seen from Figure 1b that the as-prepared Cu nanowires are smooth and

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Figure 1. (a) Schematic illustration for the synthesis of a Cu
Sn threedimensional electrode. (b) SEM image of a Cu nanowire array current collector. (c) SEM image of a Cu
Sn three-dimensional electrode.

perpendicular to the Cu substrate. The diameters of the Cu nanowires are estimated to be about 40
50 nm, while the lengths are 1
2 μm, which is much smaller than that of previously reported Cu pillar nanoarrays (200
300 nm in diameter) by Taberna’s group.9a We emphasize here that the smaller-diameter nanowires are meaningful for Li-ion batteries: First, a smaller-diameter nanowire current collector could support more active material; Second, the smaller size suggests a more efficient electron transport path. Third, a smaller-diameter nanowire current collector allows a faster electrolyte diffusion and better strain accommodation. Figure 1c is an SEM image of Cu nanowire arrays after sputtering, from which we can notice that the surface of Cu nanowires becomes rough, indicating the successful deposition of Sn layer. Compared to the electrochemical growth method reported by Bazin et al.,9d the coating process used here is very clean, easily handled, and reproducible and can be extended to the preparation of other material-coated Cu nanostructured electrodes. Figure 2a,b shows the SEM images of cross-sectional views of Cu
Sn core
shell nanowire arrays. It can be seen that a covering layer of Sn nanoparticles was on the surface of Cu nanowires. Moreover, there is much space available among the parallel orientation of the nanowire arrays, suggesting a good volume variation accommodation of the as-prepared product when used as anodes for lithium-ion batteries. To further verify the Cu
Sn core
shell nanowire structure, TEM and energy-dispersive X-ray (EDX) analyses were employed. Panels c
e in Figure 2 are the TEM images of Cu
Sn core
shell nanowires that were scraped out of the substrate. As observed, most of the nanowires were well-coated and the thickness of the rough coating is about 15 nm. Figure 2d is the EDX spectrum of the as-synthesized product. Elements Cu and Sn are detected, which come from the Sn layer and the Cu nanoarray current collector, respectively. This further reveals that the metallic Sn is coated on the surface of the Cu nanowire arrays. The above-mentioned characterizations confirm the successful synthesis of Cu
Sn core
shell nanowire arrays on a copper substrate. Motivated by the unique structure of the core
shell nanostructure array electrodes, we performed the lithium storage test as anodes. Figure 3a displays its cyclic performance. When cycled 23621

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The Journal of Physical Chemistry C

Figure 2. (a, b) Cross-sectional view and top-view SEM images of a Cu
Sn core
shell three-dimensional electrode. (c
e) TEM images of Cu
Sn core
shell nanowires. (f) EDX spectrum of the Cu
Sn core
shell three-dimensional electrode.

Figure 3. (a) Discharge and charge capacities versus cycle number for the Cu
Sn core
shell three-dimensional electrode at the rate of 500 mA g
1. (b) Cycling performance at various C rates of the Cu
Sn core
shell nanowire three-dimensional electrode. (c) The first three CV curves of the Cu
Sn three-dimensional electrode at a scan rate of 0.1 mV s
1 and a temperature of 20 °C.

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between 0.01 and 2 V at a constant current density of 500 mA g
1, the anodes deliver an initial discharge capacity of 1230 mA h g
1 and a Coulombic efficiency (the ratio between charge and discharge capacities) of 80.1%. The array structure and the formation of a solid electrolyte film (SEI) may be responsible for the high capacity in the first cycle.4b,11 The Coulombic efficiency reached 94% at the second cycle, while the others keep steady at more than 95% thereafter. To the best of our knowledge, the Coulombic efficiency for the first cycle in this study is much higher than that for most tin-based materials previously reported,4c,6d,8b indicating a good cycle reversibility. In the following cycles, we have observed a reversible capacity higher than 720 mA h g
1 after 30 cycles with the retention of about 82.7%. The discharge capacities of the electrode in the 50th, 55th, 60th, and 62th cycles are 587, 556, 552, and 551 mA h g
1, respectively. Recently, Ke et al. fabricated a macroporous Sn
Cu alloy through a colloidal crystal template method.6d When tested under a current density of 160 mA g
1, the Cu
Sn alloy anode showed a discharge capacity of 430 mA h g
1. More recently, Li et al. reported the synthesis of a three-dimensional porous Sn thin-film electrode by electroless deposition on a copper foam,8 which exhibited a discharge capacity of about 400 mA h g
1 after 30 cycles at a current of 50 mA g
1. Considering that our current density (500 mA h g
1) is much larger and the reversible capacity after 30 cycles is much higher, we claim here that the Cu
Sn core
shell three-dimensional electrode shows a significantly improved electrochemical performance. We believe that the unique three-dimensional structure, which favors fast electron transport and effectively buffers the volume expansion during cycling, may enable the high performance of the as-prepared electrode. The rate capability of the three-dimensional electrode was further studied. Figure 3b shows the variation of the discharge and charge capacities as a function of cycle number. It can be seen that the cell reveals good cyclability at 2C (1980 mA g
1) with a reversible capacity around 570 mA h g
1 and a Coulombic efficiency of 95.8%. Even at a rate as high as 8C (7920 mA g
1), the electrode is capable of delivering a stable capacity of about 460 mA h g
1. With the increase in the current rates of 2C, 4C, and 8C, the measured charge capacities are 584, 527, and 474 mA h g
1 and the capacity retentions are 91.6, 82.8, and 74.5%, respectively. Upon decreasing the rate from 8C to 4C, a 93.8% retention of the capacity at 4C can be recovered, indicating the superior rate cyclability of the as-prepared Cu
Sn core
shell nanowire array electrode. To further understand electrochemical reactions during the charging and discharging process, a cyclic voltammetry (CV) measurement was carried out. Figure 3c shows the first three CV curves of the Cu
Sn three-dimensional electrode at a scan rate of 0.1 mV s
1 and a temperature of 20 °C. During the first discharge process, the CV profiles show two apparent reduction peaks around 0.53 and 0.33 V derived from different lithium intercalation processes of LixSn (x < 2.33) and LiySn (3.5 < y < 4.4), as described in eqs 1 and 2, which is in agreement with previous reports.12 xLiþ þ Sn þ xe
f Lix Sn

ð1Þ

Lix Sn þ ðy
xÞLiþ þ ðy
xÞe
f Liy Sn

ð2Þ

Upon charging, three anodic peaks located at 0.56, 0.68, and 0.78 V were observed, which can be ascribed to the dealloying of 23622

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Figure 5. Discharge and charge capacity versus cycle number of four types of products: sample A, 200 s; sample B, 400 s; sample C, 600 s; and sample D, 800 s. Figure 4. SEM images of different thickness-controlled Sn thin films on Cu nanoarray current collectors after depositing pure Sn for (a) 200, (b) 400, (c) 600, and (d) 800 s.

the Li
Sn alloy to Li and Sn. In the second and third cycles, the CV curves show almost no change, suggesting high reversibility of the Cu
Sn three-dimensional electrode in the subsequent cycles. We also investigated the effects of sputtering time on the morphology and electrochemical performance of Cu
Sn threedimensional electrodes. When the reaction time was set at 200 s, it can be seen from the SEM image (Figure 4a) that homogeneous layers cannot be formed except a few Sn nanoparticles on the surface of the Cu nanowire arrays. Figure 4b shows the sample of the 400 s reaction. The surface of the product seems slightly rough, which means that a very thin Sn layer is deposited. When the reaction time is increased to 600 s, the deposited Sn layer becomes denser and thicker, suggesting the formation of a core
shell structure. If the deposition time is further prolonged to 800 s, excessive Sn nanoparticles will cover the top of the nanowire arrays, as observed in Figure 4d. The cycling performance of different thickness-controlled Sn thin films on Cu nanoarray current collectors were tested under the same conditions. Figure 5 exhibits the discharge capacity versus the cycle number for four Sn electrodes with a deposition time ranging from 200 to 800 s. For the sample A (200 s), a high initial discharge capacity of 1704 mA h g
1 is obtained, while the reversible capacity of the electrode continuously degrades after 30 cycles with a capacity of only 392 mA h g
1 since the randomly deposited Sn nanoparticles may suffer severely from the aggregation and irreversible surface reaction. A significant difference is observed from the sample B (400 s), which delivers a reversible capacity of 662 mA h g
1 after 30 cycles. It can be noticed that the optimal deposition time is found to be around 600 s (sample C). The electrochemical performance of sample C is still stable for 30 cycles at the C/2 rate. This phenomenon could be explained by the good advantage of the Cu
Sn core
shell three-dimensional electrode, which allows a good charge transport and strain accommodation. When the sputtering time of Sn is increased to 800 s (sample D), a lot of excessive Sn nanoparticles will cover the top of the Cu nanoarrays, indicating that there is insufficient free space available for buffering the volume expansion of the electrode during cycling.

Figure 6. (a) Cycling performance for the Cu
Sn core
shell nanowire array collector and the Sn layer on a plane Cu current collector at the current density of 500 mA g
1. (b) SEM image of the Cu
Sn threedimensional electrode after 20 cycles. (c) SEM image of the Sn film on a Cu substrate after 20 cycles.

As a result, the capacity fading is unavailable because sample D cannot take advantage of the merits of the superior array structure efficiently. To further illustrate the advantages of three-dimensional current collectors, the electrochemical performance of Sn films deposited (600 s) directly on a plane Cu substrate was tested under the same condition for comparison. It can be seen in Figure 6a that the performance of the Sn layer on a plane Cu current collector decays rapidly and gives a poor capacity of 342 mA h g
1 after 30 cycles. By contrast, the Cu
Sn threedimensional electrode exhibits a larger capacity and better cycling performance. Figure 6b is an SEM image of a Cu
Sn three-dimensional electrode after 20 cycles of lithium alloying and dealloying. As observed, the Cu
Sn three-dimensional 23623

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The Journal of Physical Chemistry C electrode maintains the array architecture, which indicates the good stability of the product after the cycling. For contrast, the Sn layer on a plane Cu substrate after 20 cycles no longer shows a clear structure (Figure 6b). The cracking of the anode caused by the huge volume change may be responsible for the capacity fading during the lithium alloying and dealloying process. These results clearly demonstrated that the unique current collector plays an important role in improving the electrochemical performance. The high reversible capacity and cycling stability can be attributed to the array architecture electrode that could accommodate the volume change and enhance the contact to the current collector.

’ CONCLUSION In summary, Cu
Sn core
shell three-dimensional electrodes have been successfully synthesized by directly depositing a pure Sn layer on the surface of presynthesized Cu nanowire arrays. When used as an anode for lithium-ion batteries, the threedimensional nanowire array electrode exhibits a high reversible capacity, a high initial Coulombic efficiency, and an improved capacity retention. The electrochemical performance of the Cu
Sn nanowire array anode depends on the deposition parameters of the Sn layer. We believe that the efficient buffering of the volume change, fast transport of electrons, and good contact to the current collector of the array structure improved the cycling performance of the Cu
Sn core
shell three-dimensional electrode.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: 86 571 87952322. Tel: 86 571 87951667.

’ ACKNOWLEDGMENT The authors appreciate the financial support from the 973 Project (No. 2007CB613403) and NSFC (Nos. 50802086 and 51002133). ’ REFERENCES (1) (a) Scrosati, B. Nature 1995, 373, 557. (b) Hosono, E. J.; Kudo, T.; Honma, I.; Matsuda, H.; Zhou, H. S. Nano Lett. 2009, 9, 1045. (2) (a) Sun, X. M.; Liu, J. F.; Li, Y. D. Chem. Mater. 2006, 18, 3486. (b) Wen, Z. H.; Wang, Q.; Zhang, Q.; Li, J. H. Adv. Funct. Mater. 2007, 17, 2772. (c) Chen, G.; Wang, Z. Y.; Xia, D. G. Chem. Mater. 2008, 20, 6951. (d) Chen, J. S.; Cheah, Y. L.; Chen, Y. T.; Jayaprakash, N.; Madhavi, S.; Yang, Y. H.; Lou, X. W. J. Phys. Chem. C 2009, 113, 20504. (3) (a) Du, N.; Zhang, H.; Chen, B. D.; Wu, J. B.; Ma, X. Y.; Liu, Z. H.; Zhang, Y. Q.; Yang, D. R.; Huang, X. H.; Tu, J. P. Adv. Mater. 2007, 19, 4505. (b) Park, M. S.; Needham, S. A.; Wang, G. X.; Kang, Y. M.; Park, J. S.; Dou, S. X.; Liu, H. K. Chem. Mater. 2007, 19, 2407. (c) Park, J. P.; Kim, J. H.; Kwon, H. S.; Song, H. J. Adv. Mater. 2009, 21, 803. (d) Cui, L. F.; Yang, Y.; Hsu, C. M.; Cui, Y. Nano Lett. 2009, 9, 3370. (4) (a) Idota, Y.; Kubota, T.; Matsufuji, A.; Maekawa, Y.; Miyasaka, T. Science 1997, 276, 1359. (b) Liu, J. P.; Li, Y. Y.; Huang, X. T.; Ding, R. M.; Hu, Y. Y.; Jiang, J.; Liao, L. J. Mater. Chem. 2009, 19, 1859. (c) Park, M. S.; Wang, G. X.; Kang, Y. M.; Wexler, D.; Dou, S. X.; Liu, H. K. Angew. Chem., Int. Ed. 2007, 46, 750. (d) Ortiz, G. F.; Hanzu, I.; Lavela, P.; Knauth, P.; Tirado, J. L.; Djenizian, T. Chem. Mater. 2010, 22, 1926. (5) (a) Yang, J.; Takeda, Y.; Imanishi, N.; Yamamoto, O. J. Electrochem. Soc. 1999, 146, 4009. (b) Liu, Y.; Xie, J. Y.; Takeda, Y.; Yang, J. J. Appl. Electrochem. 2002, 32, 687. 23624

dx.doi.org/10.1021/jp206277a |J. Phys. Chem. C 2011, 115, 23620–23624