Silicon Oxide Core–Shell Anodes Grown onto the

LETTER pubs.acs.org/NanoLett

Helical Silicon/Silicon Oxide Core Shell Anodes Grown onto the Surface of Bulk Silicon Hana Yoo,† Jung-In Lee,† Hyunjung Kim, Jung-Pil Lee, Jaephil Cho,* and Soojin Park* Interdisciplinary School of Green Energy, Converging Research Center for Innovative Battery Technologies, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Korea

bS Supporting Information ABSTRACT: We demonstrate a simple route for preparing Si/SiOx urchin-like structures in which Si/SiOx core shell nanocoils protruded out from the surface of bulk Si, via high-temperature annealing of Pt-decorated Si powders. The carbon-coated urchin-like anodes with micro- and nanostructured composite exhibit a significantly improved electrochemical performance with a high specific capacity of 1600 mAh/g and a superior cycling performance of 70 cycles at a rate of 0.2 C due to the nanocoil conformation and SiOx buffer layer. More importantly, the composite results in a significantly enhanced the volumetric capacity with ∼3780 mAh/cc, compared to bulk Si (∼2720 mAh/cc) after fully lithiation to 0 V. KEYWORDS: Lithium ion batteries, block copolymer, nanocoils, urchin-like structure, anode materials

L

ithium ion batteries (LIBs) are becoming a great technology for a number of applications, ranging from portable electronics to electric, or hybrid electric vehicles.1 Despite the significant advances in carbonaceous anode materials that are still used in most conventional LIBs, considerable attention has been paid to the development of new anode materials that exhibit superior performance, including energy density, rate capability, and cost. Since the 1970s, a rich variety of lithium (Li) alloying materials (i.e., Si, Ge, Al, Sn, and Sb) that can exhibit a high theoretical specific capacity by forming the alloy, like Li4.4Si (4200 mAhg 1), Li4.4Ge (1600 mAhg 1), LiAl (993 mAhg 1), Li4.4Sn (992 mAhg 1), and Li3Sb (660 mAhg 1), have been developed as alternative anode materials.2,3 These materials can provide higher energy density than conventional graphite (theoretical capacity of 372 mAh/g), however, they are suffering from significant volume changes during Li insertion/extraction cycle, leading to cracking of the electrode materials and the consequent loss of electrical contacts between particles, resulting in a rapid capacity decay.4 8 In addition, due to a large volume change during Li lithiation, LixSi leads to low volumetric capacity, which is a very critical factor for developing high capacity Li-ion cells. One strategy to overcome these problems is to use nanostructured materials, like zero-dimensional (0D) nanoparticles, 1D nanowires and nanotubes, and 2D nanosheets and nanoflakes, which can play an important role in improving the performance of LIBs. Nanomaterials have several advantages as follows: (i) the diffusion length of electron and Li+ ion is significantly decreased to allow operation at higher power; (ii) electrode/electrolyte contact area increased by high surface areas of nanomaterials leads to higher charge/discharge rates; (iii) large strain of nanomaterials caused by Li insertion/extraction is alleviated. Highly improved performances have been found to benefit from nanostructured materials. However, there are some disadvantages that cannot be ignored, such as low r 2011 American Chemical Society

volumetric capacity due to poor packing, low thermodynamic stability, undesirable reaction due to high surface area, and high cost by complicated synthetic process.9 11 The combination of nano- and microstructured materials that can use advantages of both materials is another strategy for not only enhancing electrochemical performance, but also improving volumetric capacity (mAh/cc). The disadvantages of nanostructured materials may be improved with advantages of microsized materials, such as good stability and easy fabrication. Several approaches have been developed to combine nanomaterials with micromaterials, such as nanomaterials encapsulated by a microshell,12,13 microsized active materials with a nanosized coating layer,14,15 active nanomaterials confined in an inactive microscale matrix,16,17 and conducting networks with 1D nanostructured materials on the surface of microsized active materials.18,19 Among these strategies, the use of urchin-like structures with numerous nanowires protruding out from the surface of a microsized inner core have the following several functions: (i) forming a continuous conductive network from 1D nanowires to improve cyclic performance; (ii) improving the electrode reaction kinetics due to increase of surface areas caused by the networks of nanowires; (iii) acting as a buffer layer of networks consisting of numerous nanowires due to excellent resiliency of 1D nanostructured materials, thus alleviating large volume change of electrode materials; and (iv) leading to easy fabrication of electrodes due to microsized inner core.20 22 For instance, Zhang et al. demonstrated a fabrication of urchinlike carbon-based material, where carbon nanofibers (CNFs) were grown from a microsized graphite inner core using catalytic chemical vapor deposition process.20 Park et al. prepared CuO Received: July 14, 2011 Revised: August 29, 2011 Published: September 14, 2011 4324

dx.doi.org/10.1021/nl202417c | Nano Lett. 2011, 11, 4324–4328

Nano Letters urchin-like particles through a sequential dissolution-precitation process from uniform-sized Cu2O nanocubes.22 Moreover, CuO/graphene composite electrodes with urchin-like structures are other examples that showed excellent electrochemical performance, compared to individual nano- and microstructured particles.23 Herein, we report a simple, direct synthetic route for the formation of Si/SiOx urchin-like composite, in which Si/SiOx core shell nanocoils protruded out from the surface of bulk Si, via thermal annealing of platinum (Pt) decorated Si powders at high temperature. Subsequent carbon coating on the surface of Si/SiOx nanowires that have an average diameter of ∼60 nm and length of tens of micrometers, play an important role in acting as a buffer layer to alleviate the volume change of bulk Si and as a conducting layer to improve electronic conductivity of active materials. Accordingly, the composite shows an excellent cycling performance of 70 cycles and very high volumetric capacity of ∼3780 mAh/cc. One of the key factors in the growth of uniform-sized 1D Si/SiOx nanostructures is to control the size of the Pt nanoparticle that acts as a catalyst. A rich variety of 1D Si or SiOx nanowires have been prepared on the 2D surfaces, like Si wafer and quartz substrates, onto which metal catalysts were deposited by thermal evaporator or sputter. The subsequent thermal annealing process leads to the formation of a metal nanocluster due to an effect of surface tension.24,25 With these approaches, it is not easy to control the size and spatial location of the metal catalyst and it cannot be extended to 3D surfaces, like bulk Si powder. In this study, we employed block copolymer micelles to make uniform-sized Pt nanoparticles. Polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) forms spherical micelles in toluene, a selective solvent for PS, in which the core is composed of the P4VP insoluble block and the corona by the PS soluble block. Since the core with vinylpyridyl group can be loaded with metal precursor salts, like Au, Ag, Pt, and Ni, the adsorption of these micelles onto the surface of solid objects provides an efficient way to control the deposition of metal nanoparticles.26 A 0.7 wt % PS-b-P4VP copolymers were dissolved in toluene and chloroplatinic acid (H2PtCl6) as a precursor of Pt nanoparticles was added in the copolymer solution (molar ratio of H2PtCl6/vinyl pyridine = 0.8) to bind Pt salts with P4VP cores. Subsequent loading of bulk Si powder to Pt-incorporated polymer solutions and an addition of n-hexane, nonsolvent for both blocks, led to the formation of bulk Si uniformly coated with micellar films. The copolymers used as templates are removed with an oxygen plasma etching (Scheme in Figure 1). The size and periodicity of copolymer micelles can be tuned easily by controlling the molecular weight and chemical composition of copolymer with tens of nanometers, which, in turn, can change the dimension of metal nanoparticles. Figure 1 shows scanning electron microscope (SEM) images of bare Si powder, bulk Si coated with Pt-incorporated micelles, Pt-decorated Si, and Si/SiOx urchin-like structures. Pt nanoparticles having an average diameter of 30 nm were uniformly decorated onto the surface of 3D bulk Si particle (Figure 1c). When the Pt-coated Si is annealed at 1050 °C under Ar stream for 2 h, 1D Si/SiOx core shell nanowires protrudes out from the surface of Si particles with an average diameter of 60 nm and tens of micrometers in length. Individual nanowires grown by Pt nanoparticles are interconnected together to make a 3D network (Figure 1d). Surprisingly, a large quantity of 1D nanocoils that was obtained by thermal annealing was observed in the magnified

LETTER

Figure 1. Upper: schematic view of Si/SiOx nanocoils grown onto the surface of bulk Si by Pt catalyst. Lower: SEM images of (a) bare Si, (b) coating of Pt-incorporated copolymer, (c) Pt-decorated Si, and (d) 1D Si/SiOx nanostructures protruded out from the surface of bulk Si. In the inset of panel d, helical Si/SiOx structures were seen. (e) Crosssectional SEM image showing Si core and Si/SiOx nanowire shell, and the corresponding mapping images of (f) silicon and (g) oxygen.

SEM image (inset of Figure 1d). In large areas, the nanocoils (∼70%) and straight nanowires (∼30%) were simultaneously shown at this annealing condition. It should be noted that the growth temperature of 1D Si/SiOx nanostructures should be above the Pt Si eutectic temperature, 847 °C,27 and nanocoils with helical structures were not observed at annealing temperature of 1100 °C. These results indicate that annealing temperature is a critical value to determine the formation of nanocoils in this system. As the annealing temperature is close to the eutectic temperature of Pt Si, the population of nanocoils increases, but the growth rate is very slow. By taking account of growth rate of 1D nanostructures and existences of nanocoils and straight nanowires, we chose the annealing temperature as 1050 °C. X-ray photoelectron spectroscopy results suggest that 1D nanostructures grown from the surface of bulk Si consist of Si, SiO, and SiOx (1 < x < 2). The powdery X-ray diffraction pattern shows the presence of Si, SiOx, and PtSi which act as a seed catalyzing the nanowire growth (Supporting Information, Figure S1). The cross-section of Si/SiOx urchin-like structures was obtained from a focused ion beam using Ga-ions to etch Si and SiOx with a high spatial precision. A few micrometer Si core and tens of micrometer Si/SiOx nanowires were clearly seen (Figure 1e). The corresponding mapping images show the spatial locations of silicon and oxygen, respectively (Figure 1f and 1g). In order to characterize 1D nanostructures further, the Si/SiOx core shell nanowires were dispersed in ethanol by ultrasonication, and collected on Fomvar-coated copper grids. Surprisingly, a large quantity of nanocoils with helical shape consisting of core shell structure was observed in high-resolution transmission electron micrograph (HR-TEM) (Figure 2a). In the magnified TEM image of the inset in the Figure 2a, crystalline Si core was seen as a darker phase due to a high density than amorphous SiOx shell. TEM image of the nanocoils terminated with a Pt Si 4325

dx.doi.org/10.1021/nl202417c |Nano Lett. 2011, 11, 4324–4328

Nano Letters

Figure 2. TEM images of (a) helical Si/SiOx structures, (b) EDXS line profiles showing the spatial locations of Si and oxygen, (c) HR-TEM image showing crystalline Si core and amorphous SiOx, and (d) EDXS profile showing the amount of Si and oxygen in the SiOx.

particle at its tip (seen with the dark contrast), which indicates that the nanowire growth mechanism follows vapor liquid solid (VLS) model.28 When the nanocoils were characterized with dark-field detector in the scanning TEM (STEM) mode, the helical Si core with a diameter of ∼15 nm is uniformly wrapped with a helical SiOx shell with a diameter of ∼20 nm (Figure 2b). The energy dispersive X-ray spectroscopy (EDXS) line profiles also confirmed that the Si core is located in the middle of the coaxial core shell structures, while the SiOx is seen in the outer regions (Inset of Figure 2b). The Si core was proven as a singlecrystalline phase in the HR-TEM image (Figure 2c), and the elemental composition analysis of SiOx phase indicated that the value of x in the SiOx is between 1 and 2 (Figure 2d). Previously, Zhang et al. reported that helical nanowires consisting of a crystalline SiC core and an amorphous SiO2 shell were formed with a screw-dislocation mechanism that is common in crystalline materials.29,30 The helical SiC structures were formed via a growth mechanism that derives from screw dislocations present in the initial crystalline SiC nanoparticles and the SiO2 outer layer was grown simultaneously. Si/SiO x core shell nanostructures prepared in this study may be formed with a similar growth mechanism as follows: Si (in bulk Si) + xSiO (environment) f Si (core) + SiOx (shell), where the source of SiO comes from the surface of the Si powder and/or the residual oxygen in Ar gas. Commercially available Ar gas usually contains a small amount of oxygen that is enough to oxidize Si material at high temperatures. Therefore, during the annealing at elevated temperature, both the nanocoils and the Si powder would be oxidized, accompanied with the growth of 1D nanostructures.31 We may conclude that helical crystalline Si nanocoils are formed as an epitaxial growth following the initial screw dislocation sites on the crystalline Si nanoparticles, and amorphous SiOx layers are simultaneously grown from the surface of Si nanocoils to stabilize the helical Si core. In addition to the

LETTER

Figure 3. Characterization of carbon-coated Si/SiOx urchin-like structures. (a) SEM images of carbon-coated Si/SiOx structures (inset: magnified SEM image), (b) HR-TEM image showing crystalline Si core, amorphous SiOx, and ∼65 nm thick carbon layer, (c) EDXS line profiles showing the spatial locations of Si and oxygen, and (d) Raman spectrum showing D-band and G-band of the carbon layer.

helical structures, straight Si/SiOx nanowires consisting of crystalline Si core and amorphous SiOx shell are clearly seen from the high temperature annealing process (Supporting Information, Figure S2). Carbon coating on the surface of Si-based electrodes, which have low intrinsic electronic conductivity and large volume changes during the battery operation, is well-known to be a very promising strategy to improve the electrochemical performance.11 The carbon layer with a thickness of ∼65 nm was coated on the surface of Si/SiOx urchin-like electrodes by decomposition of C2H2 gas at 850 °C for 30 min under an Ar atmosphere. Figure 3a shows a SEM image of carbon-coated Si/SiOx active materials, and the morphology of the 1D nanostructure was sustained after the carbon coating as seen in the inset. The bright-field TEM image and the corresponding EDXS line profiles confirmed that crystalline Si core and amorphous SiOx shell remained unchanged after the carbon coating, and the carbon layer was uniformly coated on the outer surface of SiOx layer (Figure 3b,c). Raman scattering of the carbon-coated electrodes shows two peaks at ∼1360 and ∼1580 cm 1 corresponding to the disordered (D) and the graphene (G) band, respectively. The dimensional ratio of the D band to the G band for the carbon layer was estimated to be 2.55, indicating the amorphous carbon structure (Figure 3d).32 The electrochemical properties of carbon-coated bulk Si and Si/SiOx urchin-like electrodes were investigated. The voltage profiles of the bulk Si shows the discharge of 3018 mAh/g and the charge capacity of 2630 mAh/g in the first cycle in the range of 0.01 V and 1.2 V versus Li/Li+ at a rate of 0.1 C, while the Si/ SiOx urchin-like electrodes exhibit the discharge of 2270 mAh/g and the charge capacity of 1940 mAh/g at the same condition (Figure 4a). The voltage profiles of Si/SiOx electrodes are quite similar as those of bulk Si anodes, indicating that Si materials mainly contribute to the lithium storage capacity, while SiOx (1 < x < 2) acts a buffer layer. 4326

dx.doi.org/10.1021/nl202417c |Nano Lett. 2011, 11, 4324–4328

Nano Letters

LETTER

Figure 4. (a) Voltage profiles of carbon-coated bulk Si (top) and Si/SiOx urchin-like electrodes (bottom) after 1, 2, 5, 10, and 20 cycles at a rate of 0.1 C between 1.2 and 0.01 V in coin-type half cells at room temperature. (b) Charge capacity and Coulombic efficiency of the cell (a) versus cycle number was plotted (solid and open circles, bulk Si; solid and open squares, urchin-like Si/SiOx). Schematic view illustrates morphologies of Si/SiOx urchin-like electrodes before and after cycling.

The bulk Si exhibits a high storage capacity at the first cycle, however, a significant capacity fading is seen after a few cycles at a rate of 0.1 C due to the huge volume change associated with the insertion and extraction of Li (Figure 4b). Whereas, the Si/SiOx urchin-like electrodes show a relative lower storage capacity than bulk Si due to an existence of SiOx shell, however a markedly reduced capacity fading is seen even after 20 cycles at a rate of 0.1 C and further 50 cycles at a 0.2 C rate with a high Coulombic efficiency of >98% (Figure 4b). A specific Li storage capacity of Si/SiOx electrodes may be estimated from the specific capacity of each bulk Si (2600 mAh/g, seen in Figure 4a) and SiO/carbon composite (700 mAh/g) electrode.33 From this calculation, the Si of 65 wt % and SiO of 35 wt % may contribute to exhibit the specific capacity of 1940 mAh/g in Si/SiOx urchin-like electrodes. And also EDXS and elemental analysis results confirm that the oxygen contents in the Si/SiOx materials are ∼28 wt %. From the combination of oxygen content and composition of active materials, the compositions of Si, SiO, and SiO2 may be estimated as 65, 30.5, and 4.5%, respectively. Of course, it is hard to distinguish amorphous SiO from amorphous SiO2. From here, SiOx in the as-synthesized Si/SiOx urchin-like materials represents the combination of SiO and SiO2. Compared with the irregular bulk Si electrodes, the protruding spines, Si/SiOx core shell 1D nanostructures, of urchin-like structures may connect with one another to make a 3D network and thus improve the conductivity of the electrodes (schematic of Figure 4). Moreover, the empty space formed between the spines and the inner Si core may facilitate the contact of active materials with electrolyte and enhance the transport of Li+ ion. Also the existence of a SiOx layer acts as a buffer layer to alleviate the large volume changes during battery operation. The HRTEM image showing the 1D Si/SiOx nanostructures in the urchin-like electrode materials confirms that the 1D nanostructure is clearly seen without a significant change in size and shape after 70 cycles (Supporting Information, Figure S3). More importantly, the composite results in a markedly enhanced the

Table 1. Volumetric Capacities of Bulk Si and Si/SiOx Urchin-like Composite Electrodes

samplea

thickness

mass of

energy

of active

of active layer

active

density per

layer

after lithiation

materials

volume ∼2720 mAh/cc

∼20 μm

∼50 μm

∼9 mg

Si/SiOx composite ∼20 μm

∼30 μm

∼10 mg ∼3780 mAh/cc

bulk Si a

thickness

Area of electrode: 2 cm2.

volumetric capacity of ∼3780 mAh/cc compared to bulk Si (∼2720 mAh/cc). Thickness of carbon-coated bulk Si and Si/SiOx electrodes was investigated by SEM before and after lithiation (Supporting Information, Figure S4). These results are summarized in Table 1. We believe such a drastic difference is due to that the nanocoils are effectively filled in the empty voids of the bulk Si and acts as the buffer layer for the volume change of the bulk Si simultaneously. In summary, we described a facile synthesis of helical Si/SiOx core shell structures protruding out from the surface of bulk Si particles using Pt catalyst via a screw-dislocation mechanism. The resulting Si/SiOx urchin-like electrodes showed a significantly improved electrochemical performance including a high initial Coulombic efficiency, highly reversible Li storage capacity, excellent cycling performance, and increased volumetric capacity. Although the effect of pitch and diameter of nanocoils on the electrochemical properties should be further studied, the concept of nanocoils grown onto the surface of inner active materials can be considered as promising candidates for anode materials.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental details, XRD pattern of Si/SiOx urchin-like structure, electrode thickness,

4327

dx.doi.org/10.1021/nl202417c |Nano Lett. 2011, 11, 4324–4328

Nano Letters and TEM image of active material after cycling. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Authors

LETTER

(31) Wang, C.-Y.; Chang, L.-H.; Xiao, D.-Q.; Lin, T.-C.; Shih, H. C. J. Vac. Sci. Technol., B 2006, 24, 613. (32) Lee, H.; Cho, J. Nano Lett. 2007, 7, 2638. (33) Doh, C.-H.; Park, C.-W.; Shin, H.-M.; Kim, D.-H.; Chung, Y.-D.; Moon, S.-I.; Jin, B.-S.; Kim, H.-S.; Veluchamy, A. J. Power Sources 2008, 179, 367.

*E-mail: (J.C.) [email protected]; (S.P.) [email protected]. Author Contributions †

These authors are equally contributed.

’ ACKNOWLEDGMENT This work was supported by the Converging Research Center Program (2011K000637) and IT R&D program of MKE/KEIT (KI001784). ’ REFERENCES (1) Tollefson, J. Nature 2008, 456, 436. (2) Yamaura, J.; Ozaki, Y.; Morita, A.; Ohta, A. J. Power Sources 1993, 43, 233. (3) Tirado, J. L. Mater. Sci. Eng. R 2003, 40, 103. (4) Cho, J. J. Mater. Chem. 2010, 20, 4009. (5) Lee, W. J.; Park, M. H.; Wang, Y.; Lee, J. Y.; Cho, J. Chem. Commun. 2010, 46, 622. (6) Zhou, G. W.; Li, H.; Sun, H. P.; Yu, D. P.; Wang, Y. Q.; Huang, X. J.; Chen, L. Q.; Zhang, Z. Appl. Phys. Lett. 1999, 75, 2447. (7) Li, H.; Huang, X. J.; Chen, L. Q. Electrochem. Solid State Lett. 1998, 1, 241. (8) Kasavajjula, U.; Wang, C.; Appleby, A. J. J. Power Sources 2007, 163, 1003. (9) Lee, K. T.; Cho, J. Nano Today 2011, 6, 28. (10) Guo, Y. G.; Hu, J. S.; Wan, L. J. Adv. Mater. 2008, 20, 2878. (11) Liu, C.; Li, F.; Ma, L. P.; Cheng, H.-M. Adv. Mater. 2010, 22, E28. (12) Deng, D.; Lee, J. Y. Chem. Mater. 2008, 20, 1841. (13) Guo, Y.-G.; Hu, J.-S.; Wan, L.-J. Adv. Mater. 2008, 20, 2878. (14) Hu, Y. S.; Guo, Y. G.; Dominko, R.; Gaberscek, M.; Jamnik, J.; Maier, J. Adv. Mater. 2007, 19, 1963. (15) Obrvac, M. N.; Krause, L. J. J. Electrochem. Soc. 2007, 154, A103. (16) Cui, L. F.; Ruffo, R.; Chan, C. K.; Peng, H. L.; Cui, Y. Nano Lett. 2009, 9, 491. (17) Cui, L. F.; Hu, L. B.; Choi, J. W.; Cui, Y. ACS Nano 2010, 4, 3671. (18) Zhang, Y.; Zhang, X. G.; Zhang, H. L.; Zhao, Z. G.; Li, F.; Liu, C.; Cheng, H. M. Electrochim. Acta 2006, 51, 4994. (19) Chen, J.; Minett, A. I.; Liu, Y.; Lynam, C.; Sherrell, P.; Wang, C.; Wallce, G. G. Adv. Mater. 2008, 20, 566. (20) Zhang, H. L.; Zhang, Y.; Zhang, X. G.; Li, F.; Liu, C.; Tan, J.; Cheng, H. M. Carbon 2006, 44, 2778. (21) Shu, J. Electrochem. Solid State Lett. 2008, 11, A219. (22) Park, J. C.; Kim, J.; Kwon, H.; Song, H. Adv. Mater. 2009, 21, 803. (23) Wang, B.; Wu, X.-L.; Shu, C.-Y.; Guo, Y.-G.; Wang, C.-R. J. Mater. Chem. 2010, 20, 10661. (24) Sunkara, M. K.; Sharma, S.; Chandrasekaran, H.; Talbott, M.; Krogman, K.; Bhimarasetti, G. J. Mater. Chem. 2004, 14, 590. (25) Zhang, H.-F.; Wang, C.-M.; Buck, E. C.; Wang, L.-S. Nano Lett. 2003, 3, 577. (26) F€orster, S.; Plantenberg, T. Angew. Chem., Int. Ed. 2002, 41, 688. (27) Crystallographic and Thermodynamic Data of Binary Alloys; Madelung, O., Ed.; Springer: Berlin, 1991. (28) Garnett, E. C.; Liang, W.; Yang, P. Adv. Mater. 2007, 19, 2946. (29) Blakey, F. A. Nature 1952, 170, 1119. (30) Zhang, H.-F.; Wang, C.-M.; Wang, L.-S. Nano Lett. 2002, 2, 941. 4328

dx.doi.org/10.1021/nl202417c |Nano Lett. 2011, 11, 4324–4328