- Email: [email protected]
A Stable Flexible Silicon Nanowire Array as Anode for High-Performance Lithium-ion Batteries
Accepted Manuscript Title: A Stable Flexible Silicon Nanowire Array as Anode for High-Performance Lithium-ion Batteries Author: Wang Jiantao Wang Hui Zhang Bingchang Wang Yao Lu Shigang Zhang Xiaohong PII: DOI: Reference:
S0013-4686(15)30069-4 http://dx.doi.org/doi:10.1016/j.electacta.2015.07.001 EA 25277
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
4-5-2015 1-7-2015 1-7-2015
Please cite this article as: Jiantao Wang, Hui Wang, Bingchang Zhang, Yao Wang, Shigang Lu, Xiaohong Zhang, A Stable Flexible Silicon Nanowire Array as Anode for High-Performance Lithium-ion Batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.07.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Stable Flexible Silicon Nanowire Array as Anode for High-Performance Lithium-ion Batteries Wang Jiantao1, 3, Wang Hui2, Zhang Bingchang2, Wang Yao 1, 3*, Lu Shigang1, 3*, Zhang Xiaohong2 1
R&D Center for Vehicle Battery and Energy Storage, General Research Institute for
Nonferrous Metals, Beijing 100088, People’s Republic of China 2
Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical
Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China 3
China Automotive Battery Research Institute Co., Ltd., Beijing 101407, People’s
Republic of China
E-mail: [email protected], [email protected]
Highlights:
A flexible SiNW array in PDMS structure is designed and fabricated as Li-ion battery anode material.
The aggregation and fracture of SiNWs are alleviated by the flexible PDMS skeleton during the process of charge and discharge.
The loose SiO2 shells coating on the SiNWs could form the protective layer in charge/discharge.
The as-obtain flexible SiNW array/PDMS composite exhibits a much better cycling stability.
Abstract: A Silicon nanowire (SiNW) array inserted into flexible poly-dimethylsiloxane (SiNW array/PDMS) composite structure as anode with high capacity and long-term cycling stability is synthesized by a cost-effective and scalable method. In this structure, the aggregation and fracture of SiNWs are alleviated by the flexible PDMS skeleton. Act as the main active component, the SiNWs are coated by loose SiO2 shells. These loose SiO2 shells not only provide space for the large volume changes of SiNW, but also react with the electrolyte and form the stable protective layer during the processes of the lithiation and delithiation. These two functions could improve the cycling stability and columbic efficiency of the SiNWs. The as-obtain flexible SiNW array/PDMS composite structure exhibits excellent long-term cycling stability with a specific capacity of 887.2 mA•h•g −1 and capacity retention of ∼83.4% over 350 cycles at 0.5 C rate compared with the fifteenth cycle. The design of this new structure provides a potential way for developing other functional composite materials. Key words: Flexible, Silicon Nanowire array, Anode materials, Lithium-ion battery, Composite
Introduction Low- or zero-emission hybrid electrical vehicles (HEVs) and electrical vehicles (EVs） have great potential to ease the Energy Crisis and environmental issues arising from the fossil fuels, such as air pollution, dust particle contamination and so on. It is necessary to develop high-energy-density and long cycle lithium (Li)-ion batteries so as to meet the requirements set for electric vehicles.1-4 Increasing the specific capacity of Li-ion battery anodes is considered as an attractive
route to attain this goal.5-8 Among all kinds of anode materials, Silicon (Si) material is one of the most promising alloy-type anode material for the next generation high-energy-density Li-ion batteries because of its highest theoretical capacity (∼4200 mA•h•g−1, ten times of conventional graphite anodes) and low discharge voltage. However, the huge volume change (＞300%) during the processes of lithiation and delithiation will induce large stress on Si structure and cause structural failure, pulverization, electrical disconnection from the current collector, and eventual rapid capacity fading, which seriously limits its commercial application.9 Great efforts had been made to overcome these problems, such as reducing the size of Si structures,10 mixing inactive material into Si structure, synthesizing composite structures,11 and so on. These methods were used to buffer electrode pulverization and capacity loss in the Si-based anode structures. Various nanostructures and composites had been synthesized, including thin films, nanowires,5,12 nanotubes,13 nanocables,14 nanospheres,15 and mixed with electronically conductive cushioning materials, such as carbon,16-18, graphene,19−21 conducting polymer,22 metal,23 and so on. These methods significantly enhance cycling stability. However, with the increase of the cycle number to several hundreds, nano-sized active Si materials would peel from the cushioning materials because of the different expansion coefficient between them, which could reduce the cycling stability of Si-based anode materials and limit the applications of Si-based materials in large scale. As an attempt to enhance cycling stability, one-dimensional (1D) Si nanowires (SiNWs) have been widely studied in recent years.24−27 To SiNWs, less stress-induced fracture would make more possible contact between the SiNWs and current collector during the
process of lithiation and delithiation, which would improve the electrochemical cycling stability.5 However, the as-fabricated disordered SiNWs were found to lack the long cycle life due to themselves congestion, existence of instable solid electrolyte interphase (SEI) layer and some other reasons, which was not good enough to satisfy the application demands in electrical vehicles or energy storage units. Meanwhile, the single component composed of ordered SiNW array without surface protection is affected by degradation during lithium cycling by SEI layer formation.28 To solve these problems, much research had been made to focus on fabricating more complex SiNW structures to retain the advantages of SiNWs including to avoid stress-induced fracture and increase possible electrical contact while reducing the amount of solid electrolyte interphase (SEI) layer. Among these approaches, surface decoration of the ordered SiNWs could increase their cycling life significantly, 29 which means a potential way to improve the electrochemical performance of Si anode during the application in Li-ion batteries. In this present work, a Si-based composite with SiNW array embedded in flexible poly (dimethylsiloxane) film (SiNWs array/PDMS) was used to overcome existing challenges. The excellent cycling stability of this structure was also demonstrated in this article. In this composite, the SiNWs were self-passivated by SiO2 shell during growth since both the Si and SiO2 came from the disproportionation reaction of SiO source. To avoid the SiNWs conglomerating and improve the cycling stability and reduce the exposed surface of SiNWs, a flexible PDMS film was introduced not only as supporting and segregating part, but also as protecting structure. 30, 31 The concept of using SiNW array/PDMS structure as anode opens a new route to explore SiNWs active component with long cycling stability and excellent capacity.
In this
structure, SiNWs are sectionally embedded in the flexible PDMS film, which not only effectively alleviates the aggregation of SiNWs via separating them from each other, but also accommodate expansion during the process of electrochemical cycle. In addition, the flexible PDMS film reduces the exposed surface between SiNWs and electrolyte, which could protect SiNWs from degradation during the lithium cycling by SEI layer formation and improve the first coulombic efficiency.
Electrochemical measurements on
lithium-ion batteries reveal that the SiNWs array/PDMS structure anode exhibits excellent long-term cycling stability and high capacity. We anticipate that our strategy would provide a way for the rational design of nanowires uniformly dispersed in functional composites. EXPERIMENTAL SECTION Synthesis of SiNW arrays: SiNW arrays were synthesized by the way of thermally evaporating SiO powders in a tube furnace. An Al2O3 boat loaded with 0.5 g SiO powders (Sigma-Aldrich, 325 mesh, 99.9%) were placed in the middle of a 1200 cm long alumina tube, and 25.00 g Sn slat loaded onto another Al 2O3 boat (10 cm long)
was placed at 16.5 cm (1048 °C)
down-stream from the SiO source (center to center). After the system pumped to 5 × 10−2 mbar, nitrogen was flown in until the pressure increased to 350 mbar. The system was heated to 1350 °C at a rate of 10 °C min−1, and then maintained at this temperature for about 90 min. After cooling to room temperature, a yellowish-brown product with the size of 2 cm× 8 cm was obtained on the Sn surface. Fabrication of SiNW array / PDMS composite structure: The PDMS base material and the curing agent (Sylgard 184, Dow Corning) were mixed
at a 10:1 w/w ratio and diluted with methylene chloride (0.25g.mL−1). Then, the mixture was dropped onto the SiNW array. With the methylene chloride evaporating slowly, the monomer was polymerized at ambient conditions for 24 h. At last, the whole SiNWs embedded in PDMS film structure was obtained. After treated by etching solution with 30 mL deionized (DI) water, 2.5 mL 18 mol L−1 H2SO4, 10 mL 44% aq. HF acid, and 2.5 mL ethanol for 5 h, the SiNW array/PDMS composite anode structure was obtained. Structural and Electrochemical Characterizations: SEM was conducted on a Hitachi S-4800 scanning electron microscope operated at 10 kV. EDX analysis was carried out with an EDAX system attached on the microscope. TEM and HRTEM were performed by using a JEOL JEM-2010F transmission electron microscope operated at 200 kV. The electronic conduction property of SiNW array in PDMS was tested by a simple device in supporting information figure s1a. In this device, the one side of the array was made to contact the ITO glass with Ag glue, the other side was made to contact Cu wire with Ag glue. The measured I-V curve with a changed slop for this prototyping was featured in supporting information figure s1b, which might be attributed to the Schottky barrier between SiNWs and Ag electrode. This experiment revealed that the SiNWs penetrated through the PDMS film had better electronic conduction property. Electrochemical experiments for these Si nanostructures as anode were performed using in coin cells. The discharge and charge performances of the batteries were performed on a Land CT2001A system in the fixed voltage window between 0.05 and 2V at room temperature. We prepared a series of electrodes, including PDMS film electrode, SiNP electrode, disordered SiNWs with HF acid treated electrode and SiNW array/PDMS structure electrode. About SiNP and disordered SiNW electrode,
we mixed the active materials, poly (vinylidenefluoride) (PVDF) and Super-P with a mass ratio of 50:20:30 into a homogeneous slurry with mortar and pestle. Then, the obtained slurries were pasted onto pure Cu foils (99.9%, Hitachi). In the PDMS and SiNW array/PDMS electrode, the PDMS and SiNW array/PDMS film were cohered on the collector by 10 μm thickness thin film，which mixed by PVDF and Super-P with a mass ratio 2:3. The surface of the electrode was also coating by the same 10 μm thickness thin film. The loading mass of SiNWs is about 0.8 mg. cm-2. The electrolyte is 1 M LiPF6 in EC/DMC (1:1 v/v) (Tianjing Jinniu Power Sources Material Co. Ltd.) plus 2 wt % vinylene carbonate (VC).
RESULTS AND DISCUSSION Figure 1 shows the schematic diagram for the synthesis process of SiNW array/PDMS composite. As demonstrated in Figure 1, the fabrication of the SiNW array/PDMS composite involves three main steps: fabricating a vertically ordered Si core with loose SiO2 shell (Si/SiO2) nanowire array, coating PDMS film all over the array, obtaining the product of SiNW array/PDMS structure with two ends of SiNWs exposed and supported by flexible PDMS film. In the first step, a vertically ordered Si core with loose SiO2 shell (Si/SiO2) nanowire array was synthesized by chemical vapor deposition (CVD) method, which had been demonstrated in our previous work.32 The whole growth process of vertically ordered Si/SiO2 core/shell nanowire array was free-floating growth in a tube furnace using SiO powders as evaporation source and using a slab of Sn (10 cm×2 cm×0.5 cm) as the growth substrate, which shows in the figure 2. The optical image of SiNW array on the
Sn substrate was shown in figure 2a. During this growth step， SiO vapor decomposed into Si and SiO2 when it reached the liquid Sn substrate and covered the liquid surface with SiO2 in high temperature and low pressure. Sn vapor was re-deposited on the surface of SiO2 layer and formed Sn droplets to catalyze the growth of Si/SiO2 nanowires. With the growing of these nanowires, the as-grown product had become a vertically ordered structure under the processes of free-floating action and self-adjustment in figure 2b. The SEM and TEM images revealed that individual SiNW was coated by SiO2 layer in figure 2c and 2d. In the second step, the Si/SiO2 core/shell nanowire array was coated by PDMS film all over. In this step, the dimethylsiloxane solution was firstly casted onto the as-prepared vertically ordered Si/SiO2 nanowires and cured as flexible film structure in ambient conditions, then the flexible film was removed away from the Sn substrate. In the last step, the product of SiNW array/PDMS structure with two ends of SiNWs exposed and supported by flexible PDMS film was fabricated. In this step, the flexible film removed from the Sn substrate was entirely etched by mixed acid solution. The length of the exposed SiNWs was controlled by growth and etching time. In this structure, SiNW array was embedded in the PDMS skeleton structure, which could not only effectively avoid the Si nanowires congestion and lodging, but also reduce the possibility of cutting off between the SiNWs during the process of charge and discharge. The optical image of SiNW array/PDMS treated by mixed acid was shown in figure 3a. The SEM images of the SiNW array in PDMS indicated that the vertically ordered SiNWs with length ranging from several tens of micrometer to hundreds of micrometer were integrated into the flexible transparent PDMS membrane in figure 3b, c.
The electrochemical properties of this SiNW array/PDMS and Si nanoparticles, disordered SiNWs and PDMS were investigated by coin cell. The electrodes of Si nanoparticles, disordered SiNWs and SiO2 nanoparticles were fabricated by assembling them on Cu collector by slurry mixed with PVDF and Super-P with a mass ratio 50:20:30. Figure s2a and s2b were shown the schematic of SiNW array/PDMS electrode before and after S-p slurry coating. In the SiNW array/PDMS electrode, to enhance the connection and electronic conduction, one side of flexible array was cohered on the Cu foil by the 10 μm thickness slurry mixed with PVDF and S-p with a mass ratio 2:3, and the other side of flexible array was coated by the same thickness slurry. Figure s2c and s2d were the optical images of the electrodes of SiNW array/PDMS before and after coated by S-p and PVDF slurry.
Figure 4a shows the discharge−charge profiles of the first cycle of the SiNW array/PDMS composite at 0.5 C (1C = 1.5 A••g−1) between the voltage limit of 0.05−2.0V vs Li+/Li. The initial discharge and charge specific capacities are respective 887.2 and 1689.7 mA•h•g −1 based on the sum mass of Si core and SiO2 shell, leading to a coulombic efficiency of 52.5%. The irreversible specific capacity loss of the SiNW array/PDMS composite can be ascribed to the formation of the SEI and the existence of SiO2 shell layer. The coulombic efficiency becomes stable after fifteen cycles and the specific capacities was 746 mA•h•g−1.
As shown in Figure 4b, the SiNW array/PDMS composite exhibits excellent cycle
performance. On one hand, the reversible specific capacity SiNW array/PDMS composite was up to 620.4 mA•h•g−1 based on the sum mass of SiNWs core and SiO2 shell even after 350 cycles. The capacity retention rate of SiNW array/PDMS composite were 70% and 83.4% compared with the first cycle and the fifteenth cycle with discharge capacity loss of only ∼0.086% and ∼0.047% per cycle respectively. On the other hand, to show the advantage of the SiNW array/PDMS composite, we compared the cycling performance of SiNPs, disordered SiNWs treated by HF acid with the SiNW array/PDMS composite under the same condition. Electrochemical test results indicate that the SiNPs electrodes exhibits a rapid capacity fading (Figure 4b), which could be attributed to the large volume change of these aggregated and irregular Si nanoparticles during the insertion and extraction of Li ion and extraction, leading to an electrical disconnection among nanoparticles. The SEM images of the Si nanoparticle electrode pad after cycling shown that the materials peeled from the collector in the supporting information (Figure s3). About the disordered SiNWs with HF treated, its initial discharge specific capacities was 1242.4 mA•h•g−1 based on the mass of SiNWs, which was higher than that of SiNW array/PDMS composite based on the sum mass of SiNWs core and SiO2 shell. However, its discharge specific capacities and capacity retention ratio were 534.5 mA•h•g−1 and 43% respectively after 200 cycles. Its discharge capacity loss was 0.285% per cycle, which was much higher than the one of SiNW array/PDMS composite. Compared with the disordered SiNWs electrodes, SiNW array/PDMS showed better cycling performance due to preferably physical and chemical properties of this specific structure. In this structure, PDMS film as buffer substrate is nearly inactive demonstrated by the electrochemical performance of PDMS film in supporting information figure s4, which ensured the
stability of SiNW array/PDMS composite. The SEM images of the SiNW array/PDMS electrode after cycling in supporting information figure s5 provide further evidence for the stability of this composite. To better understand the reason of the electrochemical stability of this structure, a schematic of possible process of lithiation and delithiation in the SiNW array/PDMS anode electrode was shown in figure 5 based on the previous theoretic and experimental study. In this schematic, we hypothesize that the S-p and PVDF coating thin layer had been not affected by this process. On the basis of this hypothesis, the proposed charge and discharge mechanism for this stable structure is shown as follows: During the process of lithiation, there were two type reactive sites in the SiNWs core and loose SiO2 shell. On one hand, with the reaction among the lithium ion, silicon and electron, Li-Si alloy was generated, which resulted in the volume expansion of the composite. On the other hand, the reaction among the lithium ion, loose silicon oxide and electron could produce compact Li2O and Li2SiO3, which resulted in the irreversible capacity and provided certain cavity for volume expansion of SiNWs core. The volume expansion of SiNWs had nearly no influence on the whole structure of electrode during this step. With the process of delithiation, the different degree of volume shrinkage for Li-Si alloy and Li2SiO3 would result in cavity around the SiNWs. The cavity could not cause the electrode collapse, since the PDMS skeleton had the favorable stability. On the contrary, the cavity could relax the subsequent volume change during the process of charge and discharge. Through these two steps, the large volume expansion, which is considered as the most serious problem standing in the way of silicon anode utilization, could be
mainly overcome and the cycling stability of the composite be ensured. In addition, the mixture of S-p and PVDF acts not only as a conductive component to improve the ion transmission performance during the process of the charge-discharge, but also as a link structure to ensure the stability of the electrode pad. In summary，this flexible SiNW array/PDMS as an anode for Li-ion battery exhibited excellent electrochemical stability. In this structure, the PDMS skeleton alleviates the aggregation and fracture of SiNWs. The loose SiO2 shell not only provides pores to accommodate large volume changes of SiNWs in the process of the lithiation and delithiation, but also react with the electrolyte and form the protective layer for the Si nanowires during the process of the lithiation and delithiation. These two functions could improve the cycling stability and columbic efficiency of the SiNWs as anode electrode. The as-obtain flexible SiNW array/PDMS composite exhibits excellent cycling stability (only 30% and 16.6% capacity loss over 350 cycles compared with the first cycle and the fifteenth cycle respectively). The approach of using flexible structure as skeleton to support the SiNWs array and subsequently form a stable SiNW array/PDMS composite can be a simple, yet very cost-effective method for extensively fabricating high performance anode materials for Li-ion batteries. Owing to its versatility, the approach reported in this work could also be extended to other stabilize functional composite materials with large volume changes during physical, chemical, or electrochemical operations.
Acknowledgments
This study was financially supported by the National Natural
Science Foundation of China (Grant No.51404030), the Beijing Natural Science
Foundation (No.3154043) and the National High Technology Research and Development Program of China (Grant No. 2013AA050903).
References: (1) M. Armand, J. Tarascon, Building better batteries, Nature 451 (2008) 652. (2) J. Maier, Nanoionics: ion transport and electrochemical storage in confined systems, Nature Mater. 4 (2005) 805. (3) B. Kang, G. Ceder, Battery materials for ultrafast charging and discharging, Nature 458 (2009) 190. (4) A. S. Arico, P. Bruce, B. Scrosati, J. M. Tarascon, W. Van Schalkwijk, Nanostructured materials for advanced energy conversion and storage devices, Nature Mater. 4 (2005) 366. (5) C. K. Chan, H. Peng, G. Liu, K. McIlwrath, X. F. Zhang, R. A. Huggins, Y. Cui, High-performance lithium battery anodes using silicon nanowires, Nature Nanotech. 3 (2008) 31. (6) P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J. M. Tarascon, Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries, Nature 407 (2000) 496. (7) L. Taberna, S. Mitra, P. Poizot, P. Simon, J. M. Tarascon, High rate capabilities Fe3O4-based Cu nano-architectured electrodes for lithium-ion battery applications, Nature Mater. 5 (2006) 567. (8) Y. Oumellal, A. Rougier, G. A. Nazri, J. M. Tarascon, L. Aymard, Metal hydrides for
lithium-ion batteries, Nature Mater. 7 (2008) 916 (9) U. Kasavajjula, C. Wang, A. J. Appleby, Nano-and bulk-silicon-based insertion anodes for lithium-ion secondary cells, J. Power Sources 163 (2007) 1003. (10) X. Zhao, D. Xia, L. Gu, J. Yue, B. Li, H. Wei, H. Yan, R. Zou, Y. Wang, X. Wang, Z. Zhang, J. Li, High-performance self-organized Si nanocomposite anode for lithium-ion batteries, Journal of Energy Chemistry 23 (2014) 291-300. (11) H. Kim, M. Seo, M. H. Park, J. Cho, A Critical Size of Silicon Nano-Anodes for Lithium Rechargeable Batteries, Angewandte Chemie International Edition 49 (2010) 2146. (12) C. Pereira-Nabais, J. Światowska, A. Chagnes, A. Gohier, S. Zanna, A. Seyeux, P. Tran-Van, C. S. Cojocaru, M. Cassir, P. Marcus , Insight into the Solid Electrolyte Interphase on Si Nanowires in Lithium-Ion Battery: Chemical and Morphological Modifications upon Cycling, J. Phys. Chem. C 118 (2014) 2919. (13) B. Hertzberg, A. Alexeev, G. Yushin, Deformations in Si− Li Anodes Upon Electrochemical Alloying in Nano-Confined Space, J. Am. Chem. Soc. 132 (2010) 8548. (14) F. F. Cao, J. W. Deng, S. Xin, H. X. Ji, O. G. Schmidt, L. J. Wan, Y. G. Guo, Cu-Si Nanocable Arrays as High-Rate Anode Materials for Lithium Ion Batteries, Adv. Mater. 23 (2011) 4415. (15) Y. Yao, M. T. McDowell, I. Ryu, H. Wu, N. Liu, L. Hu, W. D. Nix, Y. Cui, Interconnected silicon hollow nanospheres for lithium-ion battery anodes with long cycle life, Nano Lett. 11 (2011) 2949. (16) Y. S. Hu, R. Demir-Cakan, M. M. Titirici, J. O. Müller, R. Schlögl, M. Antonietti, J. Maier, Superior Storage Performance of a Si@SiOx/C Nanocomposite as Anode Material
for Lithium-Ion Batteries, Angew. Chem., Int. Ed. 47 (2008) 1645. (17) S. H. Ng, J. Wang, D. Wexler, K. Konstantinov, Z. P. Guo, H. K. Liu, Highly Reversible Lithium Storage in Spheroidal Carbon-Coated Silicon Nanocomposites as Anodes for Lithium-Ion Batteries, Angew. Chem., Int. Ed. 45 (2006) 6896. (18) J. T. Wang, Y. Wang, B. Huang, J. Y. Yang, A. Tan, S. G. Lu, Silicon Supported on Stable Si-O-C Skeleton in High-Performance Lithium-Ion Battery Anode Materials, Acta Phys. Chim. Sin. 30 (2014) 305. (19) J. K. Lee, K. B. Smith, C. M. Hayner, H. H. Kung, Silicon nanoparticles–graphene paper composites for Li ion battery anodes, Chem. Commun. 46 (2010) 2025. (20) X. Zhou, Y. X. Yin, L. J. Wan, Y. G. Guo, Facile synthesis of silicon nanoparticles inserted into graphene sheets as improved anode materials for lithium-ion batteries, Chem. Commun. 48 (2012) 2198. (21) J. T. Wang, Y. Wang, B. Huang, J. Y. Yang, A. TanS. G. , Lu, Improvement of cycle behavior of Si/Ni negative composite by the Stable Si-O-C Skeleton Supported, Journal of Nanoscience and Nanotechnology 15 (2015) 5198. (22) Y. Liu, T. Matsumura, N. Imanishi, A. Hirano, T. Ichikawa, Y. Takeda, Electrochem. Preparation and Characterization of Si/C Composite Coated with Polyaniline as Novel Anodes for Li-Ion Batteries, Solid-State Lett. 8 (2005) A599. (23) Y. Yu, L. Gu, C. Zhu, S. Tsukimoto, P. A. van Aken, J. Maier, Reversible Storage of Lithium in Silver-Coated Three-Dimensional Macroporous Silicon, Adv. Mater. 22 (2010) 2247. (24) X. H. Liu, H. Zheng, L. Zhong, S. Huang, K. Karki, L. Q. Zhang, Y. Liu, A. Kushima, W. T. Liang, J. W. Wang, J. H. Cho, E. Epstein, S. A. Dayeh, S. T. Picraux, T. Zhu, J. Li, J.
P. Sullivan, J. Cumings, C. Wang, S. X. Mao, Z. Z. Ye, S. Zhang, J. Y. Huang, Anisotropic swelling and fracture of silicon nanowires during lithiation, Nano Lett. 11 (2011) 3312. (25) J. H. Cho, X. Li, S. T. Picraux, The effect of metal silicide formation on silicon nanowire-based lithium-ion battery anode capacity, J. Power Sources 205 (2012) 467. (26) X. Li, J. H. Cho, N. Li, Y. Zhang, D. Williams, S. A. Dayeh, S. T. Picraux, Carbon Nanotube Enhanced Growth of Silicon Nanowires as an Anode for High Performance Lithium Ion Batteries, Adv. Energy Mater. 2 (2012) 87. (27) A. Vlad, A. L. M. Reddy, A. Ajayan, N. Singh, J. Gohy, S. Melinte, P. M. Ajayan, Roll up nanowire battery from silicon chips, Proc. Natl. Acad. Sci. 109 (2012) 15168. (28) J. H. Cho, S. T. Picraux, Silicon Nanowire Degradation and Stabilization during Lithium Cycling by SEI Layer Formation, Nano Lett. 14 (2014) 3088. (29) J. H. Cho, S. T. Picraux, Enhanced lithium ion battery cycling of silicon nanowire anodes by template growth to eliminate silicon underlayer islands, Nano Lett. 13 (2013) 5740. (30) H. Wang, J. T. Wang, X. M. Ou, C. S. Lee, X. H. Zhang, Si Nanowire Directly Grown on Liquid Metal Substrate--Towards Wafer Scale Transferable Nanowire Arrays with Improved Visible-Light Sterilization, Nanotechnology 25 (2014) 145601. (31) H. Wang, J. T. Wang, X. M. Ou, F. Li, X. H. Zhang, Highly Efficient and Stable Si Nanowires
Array
Embedded
into
Transparent
Polymer
for
Visible
Light
Photoelectrochemical Cell, Nanotechnology 25 (2014) 265401. (32) J. T. Wang, X. H. Zhang, H. Wang, X. M. Ou, Super-Hydrophobic Silicon/Silica Hierarchical Structure Film, Acta Phys. -Chim. Sin. 27 (2011) 2233.
Figure Captions
Figure 1. The schematic illustration for preparation of SiNW array in PDMS structure. Figure 2. The images for the surface of Si/SiO2 nanowire array (a) the optical image of SiNW array on the surface of the Sn substrate; (b) the side view SEM image of Si/SiO2 nanowire array; (c) the SEM image of as-prepared SiNWs coated by SiO2 shell; (d) TEM image of the single Si/SiO2 core/shell nanowire, inset is the relative EDX spectrum and e SAED pattern. Figure 3. The images of SiNW array in PDMS structure. (a) the optical image of the flexible SiNW array in PDMS structure; (b) the SEM image of SiNW array in PDMS structure after treated by mixed acid form side view; (c) he SEM image of SiNW array in PDMS structure after treated by mixed acid form top view; (d) TEM image of the single Si nanowire of the SiNW array/PDMS, inset is the relative EDX spectrum; (e), HRTEM image of the single Si nanowire of the SiNW array/PDMS, inset is the relative SAED pattern. Figure 4. Electrochemical performance of SiNPs, disordered SiNWs and SiNW array/PDMS with the same conditions. (a) The first discharge−charge profiles of SiNW array/PDMS composite, (b) long-term cycling stability of SiNPs, disordered SiNWs and SiNW array/PDMS composite.
Figure 5. Schematic of the process of lithiated and delithiation. (a) the process of lithiated; (b) the end of lithiated; (c) the process of delithiation.
Figure 1
Figure 2
Figure 3
Potential(V vs Li/Li+)
2.0
a
1.5
1.0
0.5
0.0 200
400
600
800
1000
1200
-1 Specific Capacity/(mAh/g )
1400
1800
b
1600
specific capacity (mAh/g)
1600
100
1400
SiNPS Scatter SiNWs SiNWs Array in PDMS
1200 1000
80
60
800 40
600 400
20 200 0 0
50
100
150
200
Cycle Number
Figure 4
250
300
0 350
coulombic efficiency (100%)
0
Figure 5
S0013-4686(15)30069-4 http://dx.doi.org/doi:10.1016/j.electacta.2015.07.001 EA 25277
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
4-5-2015 1-7-2015 1-7-2015
Please cite this article as: Jiantao Wang, Hui Wang, Bingchang Zhang, Yao Wang, Shigang Lu, Xiaohong Zhang, A Stable Flexible Silicon Nanowire Array as Anode for High-Performance Lithium-ion Batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.07.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Stable Flexible Silicon Nanowire Array as Anode for High-Performance Lithium-ion Batteries Wang Jiantao1, 3, Wang Hui2, Zhang Bingchang2, Wang Yao 1, 3*, Lu Shigang1, 3*, Zhang Xiaohong2 1
R&D Center for Vehicle Battery and Energy Storage, General Research Institute for
Nonferrous Metals, Beijing 100088, People’s Republic of China 2
Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical
Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China 3
China Automotive Battery Research Institute Co., Ltd., Beijing 101407, People’s
Republic of China
E-mail: [email protected], [email protected]
Highlights:
A flexible SiNW array in PDMS structure is designed and fabricated as Li-ion battery anode material.
The aggregation and fracture of SiNWs are alleviated by the flexible PDMS skeleton during the process of charge and discharge.
The loose SiO2 shells coating on the SiNWs could form the protective layer in charge/discharge.
The as-obtain flexible SiNW array/PDMS composite exhibits a much better cycling stability.
Abstract: A Silicon nanowire (SiNW) array inserted into flexible poly-dimethylsiloxane (SiNW array/PDMS) composite structure as anode with high capacity and long-term cycling stability is synthesized by a cost-effective and scalable method. In this structure, the aggregation and fracture of SiNWs are alleviated by the flexible PDMS skeleton. Act as the main active component, the SiNWs are coated by loose SiO2 shells. These loose SiO2 shells not only provide space for the large volume changes of SiNW, but also react with the electrolyte and form the stable protective layer during the processes of the lithiation and delithiation. These two functions could improve the cycling stability and columbic efficiency of the SiNWs. The as-obtain flexible SiNW array/PDMS composite structure exhibits excellent long-term cycling stability with a specific capacity of 887.2 mA•h•g −1 and capacity retention of ∼83.4% over 350 cycles at 0.5 C rate compared with the fifteenth cycle. The design of this new structure provides a potential way for developing other functional composite materials. Key words: Flexible, Silicon Nanowire array, Anode materials, Lithium-ion battery, Composite
Introduction Low- or zero-emission hybrid electrical vehicles (HEVs) and electrical vehicles (EVs） have great potential to ease the Energy Crisis and environmental issues arising from the fossil fuels, such as air pollution, dust particle contamination and so on. It is necessary to develop high-energy-density and long cycle lithium (Li)-ion batteries so as to meet the requirements set for electric vehicles.1-4 Increasing the specific capacity of Li-ion battery anodes is considered as an attractive
route to attain this goal.5-8 Among all kinds of anode materials, Silicon (Si) material is one of the most promising alloy-type anode material for the next generation high-energy-density Li-ion batteries because of its highest theoretical capacity (∼4200 mA•h•g−1, ten times of conventional graphite anodes) and low discharge voltage. However, the huge volume change (＞300%) during the processes of lithiation and delithiation will induce large stress on Si structure and cause structural failure, pulverization, electrical disconnection from the current collector, and eventual rapid capacity fading, which seriously limits its commercial application.9 Great efforts had been made to overcome these problems, such as reducing the size of Si structures,10 mixing inactive material into Si structure, synthesizing composite structures,11 and so on. These methods were used to buffer electrode pulverization and capacity loss in the Si-based anode structures. Various nanostructures and composites had been synthesized, including thin films, nanowires,5,12 nanotubes,13 nanocables,14 nanospheres,15 and mixed with electronically conductive cushioning materials, such as carbon,16-18, graphene,19−21 conducting polymer,22 metal,23 and so on. These methods significantly enhance cycling stability. However, with the increase of the cycle number to several hundreds, nano-sized active Si materials would peel from the cushioning materials because of the different expansion coefficient between them, which could reduce the cycling stability of Si-based anode materials and limit the applications of Si-based materials in large scale. As an attempt to enhance cycling stability, one-dimensional (1D) Si nanowires (SiNWs) have been widely studied in recent years.24−27 To SiNWs, less stress-induced fracture would make more possible contact between the SiNWs and current collector during the
process of lithiation and delithiation, which would improve the electrochemical cycling stability.5 However, the as-fabricated disordered SiNWs were found to lack the long cycle life due to themselves congestion, existence of instable solid electrolyte interphase (SEI) layer and some other reasons, which was not good enough to satisfy the application demands in electrical vehicles or energy storage units. Meanwhile, the single component composed of ordered SiNW array without surface protection is affected by degradation during lithium cycling by SEI layer formation.28 To solve these problems, much research had been made to focus on fabricating more complex SiNW structures to retain the advantages of SiNWs including to avoid stress-induced fracture and increase possible electrical contact while reducing the amount of solid electrolyte interphase (SEI) layer. Among these approaches, surface decoration of the ordered SiNWs could increase their cycling life significantly, 29 which means a potential way to improve the electrochemical performance of Si anode during the application in Li-ion batteries. In this present work, a Si-based composite with SiNW array embedded in flexible poly (dimethylsiloxane) film (SiNWs array/PDMS) was used to overcome existing challenges. The excellent cycling stability of this structure was also demonstrated in this article. In this composite, the SiNWs were self-passivated by SiO2 shell during growth since both the Si and SiO2 came from the disproportionation reaction of SiO source. To avoid the SiNWs conglomerating and improve the cycling stability and reduce the exposed surface of SiNWs, a flexible PDMS film was introduced not only as supporting and segregating part, but also as protecting structure. 30, 31 The concept of using SiNW array/PDMS structure as anode opens a new route to explore SiNWs active component with long cycling stability and excellent capacity.
In this
structure, SiNWs are sectionally embedded in the flexible PDMS film, which not only effectively alleviates the aggregation of SiNWs via separating them from each other, but also accommodate expansion during the process of electrochemical cycle. In addition, the flexible PDMS film reduces the exposed surface between SiNWs and electrolyte, which could protect SiNWs from degradation during the lithium cycling by SEI layer formation and improve the first coulombic efficiency.
Electrochemical measurements on
lithium-ion batteries reveal that the SiNWs array/PDMS structure anode exhibits excellent long-term cycling stability and high capacity. We anticipate that our strategy would provide a way for the rational design of nanowires uniformly dispersed in functional composites. EXPERIMENTAL SECTION Synthesis of SiNW arrays: SiNW arrays were synthesized by the way of thermally evaporating SiO powders in a tube furnace. An Al2O3 boat loaded with 0.5 g SiO powders (Sigma-Aldrich, 325 mesh, 99.9%) were placed in the middle of a 1200 cm long alumina tube, and 25.00 g Sn slat loaded onto another Al 2O3 boat (10 cm long)
was placed at 16.5 cm (1048 °C)
down-stream from the SiO source (center to center). After the system pumped to 5 × 10−2 mbar, nitrogen was flown in until the pressure increased to 350 mbar. The system was heated to 1350 °C at a rate of 10 °C min−1, and then maintained at this temperature for about 90 min. After cooling to room temperature, a yellowish-brown product with the size of 2 cm× 8 cm was obtained on the Sn surface. Fabrication of SiNW array / PDMS composite structure: The PDMS base material and the curing agent (Sylgard 184, Dow Corning) were mixed
at a 10:1 w/w ratio and diluted with methylene chloride (0.25g.mL−1). Then, the mixture was dropped onto the SiNW array. With the methylene chloride evaporating slowly, the monomer was polymerized at ambient conditions for 24 h. At last, the whole SiNWs embedded in PDMS film structure was obtained. After treated by etching solution with 30 mL deionized (DI) water, 2.5 mL 18 mol L−1 H2SO4, 10 mL 44% aq. HF acid, and 2.5 mL ethanol for 5 h, the SiNW array/PDMS composite anode structure was obtained. Structural and Electrochemical Characterizations: SEM was conducted on a Hitachi S-4800 scanning electron microscope operated at 10 kV. EDX analysis was carried out with an EDAX system attached on the microscope. TEM and HRTEM were performed by using a JEOL JEM-2010F transmission electron microscope operated at 200 kV. The electronic conduction property of SiNW array in PDMS was tested by a simple device in supporting information figure s1a. In this device, the one side of the array was made to contact the ITO glass with Ag glue, the other side was made to contact Cu wire with Ag glue. The measured I-V curve with a changed slop for this prototyping was featured in supporting information figure s1b, which might be attributed to the Schottky barrier between SiNWs and Ag electrode. This experiment revealed that the SiNWs penetrated through the PDMS film had better electronic conduction property. Electrochemical experiments for these Si nanostructures as anode were performed using in coin cells. The discharge and charge performances of the batteries were performed on a Land CT2001A system in the fixed voltage window between 0.05 and 2V at room temperature. We prepared a series of electrodes, including PDMS film electrode, SiNP electrode, disordered SiNWs with HF acid treated electrode and SiNW array/PDMS structure electrode. About SiNP and disordered SiNW electrode,
we mixed the active materials, poly (vinylidenefluoride) (PVDF) and Super-P with a mass ratio of 50:20:30 into a homogeneous slurry with mortar and pestle. Then, the obtained slurries were pasted onto pure Cu foils (99.9%, Hitachi). In the PDMS and SiNW array/PDMS electrode, the PDMS and SiNW array/PDMS film were cohered on the collector by 10 μm thickness thin film，which mixed by PVDF and Super-P with a mass ratio 2:3. The surface of the electrode was also coating by the same 10 μm thickness thin film. The loading mass of SiNWs is about 0.8 mg. cm-2. The electrolyte is 1 M LiPF6 in EC/DMC (1:1 v/v) (Tianjing Jinniu Power Sources Material Co. Ltd.) plus 2 wt % vinylene carbonate (VC).
RESULTS AND DISCUSSION Figure 1 shows the schematic diagram for the synthesis process of SiNW array/PDMS composite. As demonstrated in Figure 1, the fabrication of the SiNW array/PDMS composite involves three main steps: fabricating a vertically ordered Si core with loose SiO2 shell (Si/SiO2) nanowire array, coating PDMS film all over the array, obtaining the product of SiNW array/PDMS structure with two ends of SiNWs exposed and supported by flexible PDMS film. In the first step, a vertically ordered Si core with loose SiO2 shell (Si/SiO2) nanowire array was synthesized by chemical vapor deposition (CVD) method, which had been demonstrated in our previous work.32 The whole growth process of vertically ordered Si/SiO2 core/shell nanowire array was free-floating growth in a tube furnace using SiO powders as evaporation source and using a slab of Sn (10 cm×2 cm×0.5 cm) as the growth substrate, which shows in the figure 2. The optical image of SiNW array on the
Sn substrate was shown in figure 2a. During this growth step， SiO vapor decomposed into Si and SiO2 when it reached the liquid Sn substrate and covered the liquid surface with SiO2 in high temperature and low pressure. Sn vapor was re-deposited on the surface of SiO2 layer and formed Sn droplets to catalyze the growth of Si/SiO2 nanowires. With the growing of these nanowires, the as-grown product had become a vertically ordered structure under the processes of free-floating action and self-adjustment in figure 2b. The SEM and TEM images revealed that individual SiNW was coated by SiO2 layer in figure 2c and 2d. In the second step, the Si/SiO2 core/shell nanowire array was coated by PDMS film all over. In this step, the dimethylsiloxane solution was firstly casted onto the as-prepared vertically ordered Si/SiO2 nanowires and cured as flexible film structure in ambient conditions, then the flexible film was removed away from the Sn substrate. In the last step, the product of SiNW array/PDMS structure with two ends of SiNWs exposed and supported by flexible PDMS film was fabricated. In this step, the flexible film removed from the Sn substrate was entirely etched by mixed acid solution. The length of the exposed SiNWs was controlled by growth and etching time. In this structure, SiNW array was embedded in the PDMS skeleton structure, which could not only effectively avoid the Si nanowires congestion and lodging, but also reduce the possibility of cutting off between the SiNWs during the process of charge and discharge. The optical image of SiNW array/PDMS treated by mixed acid was shown in figure 3a. The SEM images of the SiNW array in PDMS indicated that the vertically ordered SiNWs with length ranging from several tens of micrometer to hundreds of micrometer were integrated into the flexible transparent PDMS membrane in figure 3b, c.
The electrochemical properties of this SiNW array/PDMS and Si nanoparticles, disordered SiNWs and PDMS were investigated by coin cell. The electrodes of Si nanoparticles, disordered SiNWs and SiO2 nanoparticles were fabricated by assembling them on Cu collector by slurry mixed with PVDF and Super-P with a mass ratio 50:20:30. Figure s2a and s2b were shown the schematic of SiNW array/PDMS electrode before and after S-p slurry coating. In the SiNW array/PDMS electrode, to enhance the connection and electronic conduction, one side of flexible array was cohered on the Cu foil by the 10 μm thickness slurry mixed with PVDF and S-p with a mass ratio 2:3, and the other side of flexible array was coated by the same thickness slurry. Figure s2c and s2d were the optical images of the electrodes of SiNW array/PDMS before and after coated by S-p and PVDF slurry.
Figure 4a shows the discharge−charge profiles of the first cycle of the SiNW array/PDMS composite at 0.5 C (1C = 1.5 A••g−1) between the voltage limit of 0.05−2.0V vs Li+/Li. The initial discharge and charge specific capacities are respective 887.2 and 1689.7 mA•h•g −1 based on the sum mass of Si core and SiO2 shell, leading to a coulombic efficiency of 52.5%. The irreversible specific capacity loss of the SiNW array/PDMS composite can be ascribed to the formation of the SEI and the existence of SiO2 shell layer. The coulombic efficiency becomes stable after fifteen cycles and the specific capacities was 746 mA•h•g−1.
As shown in Figure 4b, the SiNW array/PDMS composite exhibits excellent cycle
performance. On one hand, the reversible specific capacity SiNW array/PDMS composite was up to 620.4 mA•h•g−1 based on the sum mass of SiNWs core and SiO2 shell even after 350 cycles. The capacity retention rate of SiNW array/PDMS composite were 70% and 83.4% compared with the first cycle and the fifteenth cycle with discharge capacity loss of only ∼0.086% and ∼0.047% per cycle respectively. On the other hand, to show the advantage of the SiNW array/PDMS composite, we compared the cycling performance of SiNPs, disordered SiNWs treated by HF acid with the SiNW array/PDMS composite under the same condition. Electrochemical test results indicate that the SiNPs electrodes exhibits a rapid capacity fading (Figure 4b), which could be attributed to the large volume change of these aggregated and irregular Si nanoparticles during the insertion and extraction of Li ion and extraction, leading to an electrical disconnection among nanoparticles. The SEM images of the Si nanoparticle electrode pad after cycling shown that the materials peeled from the collector in the supporting information (Figure s3). About the disordered SiNWs with HF treated, its initial discharge specific capacities was 1242.4 mA•h•g−1 based on the mass of SiNWs, which was higher than that of SiNW array/PDMS composite based on the sum mass of SiNWs core and SiO2 shell. However, its discharge specific capacities and capacity retention ratio were 534.5 mA•h•g−1 and 43% respectively after 200 cycles. Its discharge capacity loss was 0.285% per cycle, which was much higher than the one of SiNW array/PDMS composite. Compared with the disordered SiNWs electrodes, SiNW array/PDMS showed better cycling performance due to preferably physical and chemical properties of this specific structure. In this structure, PDMS film as buffer substrate is nearly inactive demonstrated by the electrochemical performance of PDMS film in supporting information figure s4, which ensured the
stability of SiNW array/PDMS composite. The SEM images of the SiNW array/PDMS electrode after cycling in supporting information figure s5 provide further evidence for the stability of this composite. To better understand the reason of the electrochemical stability of this structure, a schematic of possible process of lithiation and delithiation in the SiNW array/PDMS anode electrode was shown in figure 5 based on the previous theoretic and experimental study. In this schematic, we hypothesize that the S-p and PVDF coating thin layer had been not affected by this process. On the basis of this hypothesis, the proposed charge and discharge mechanism for this stable structure is shown as follows: During the process of lithiation, there were two type reactive sites in the SiNWs core and loose SiO2 shell. On one hand, with the reaction among the lithium ion, silicon and electron, Li-Si alloy was generated, which resulted in the volume expansion of the composite. On the other hand, the reaction among the lithium ion, loose silicon oxide and electron could produce compact Li2O and Li2SiO3, which resulted in the irreversible capacity and provided certain cavity for volume expansion of SiNWs core. The volume expansion of SiNWs had nearly no influence on the whole structure of electrode during this step. With the process of delithiation, the different degree of volume shrinkage for Li-Si alloy and Li2SiO3 would result in cavity around the SiNWs. The cavity could not cause the electrode collapse, since the PDMS skeleton had the favorable stability. On the contrary, the cavity could relax the subsequent volume change during the process of charge and discharge. Through these two steps, the large volume expansion, which is considered as the most serious problem standing in the way of silicon anode utilization, could be
mainly overcome and the cycling stability of the composite be ensured. In addition, the mixture of S-p and PVDF acts not only as a conductive component to improve the ion transmission performance during the process of the charge-discharge, but also as a link structure to ensure the stability of the electrode pad. In summary，this flexible SiNW array/PDMS as an anode for Li-ion battery exhibited excellent electrochemical stability. In this structure, the PDMS skeleton alleviates the aggregation and fracture of SiNWs. The loose SiO2 shell not only provides pores to accommodate large volume changes of SiNWs in the process of the lithiation and delithiation, but also react with the electrolyte and form the protective layer for the Si nanowires during the process of the lithiation and delithiation. These two functions could improve the cycling stability and columbic efficiency of the SiNWs as anode electrode. The as-obtain flexible SiNW array/PDMS composite exhibits excellent cycling stability (only 30% and 16.6% capacity loss over 350 cycles compared with the first cycle and the fifteenth cycle respectively). The approach of using flexible structure as skeleton to support the SiNWs array and subsequently form a stable SiNW array/PDMS composite can be a simple, yet very cost-effective method for extensively fabricating high performance anode materials for Li-ion batteries. Owing to its versatility, the approach reported in this work could also be extended to other stabilize functional composite materials with large volume changes during physical, chemical, or electrochemical operations.
Acknowledgments
This study was financially supported by the National Natural
Science Foundation of China (Grant No.51404030), the Beijing Natural Science
Foundation (No.3154043) and the National High Technology Research and Development Program of China (Grant No. 2013AA050903).
References: (1) M. Armand, J. Tarascon, Building better batteries, Nature 451 (2008) 652. (2) J. Maier, Nanoionics: ion transport and electrochemical storage in confined systems, Nature Mater. 4 (2005) 805. (3) B. Kang, G. Ceder, Battery materials for ultrafast charging and discharging, Nature 458 (2009) 190. (4) A. S. Arico, P. Bruce, B. Scrosati, J. M. Tarascon, W. Van Schalkwijk, Nanostructured materials for advanced energy conversion and storage devices, Nature Mater. 4 (2005) 366. (5) C. K. Chan, H. Peng, G. Liu, K. McIlwrath, X. F. Zhang, R. A. Huggins, Y. Cui, High-performance lithium battery anodes using silicon nanowires, Nature Nanotech. 3 (2008) 31. (6) P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J. M. Tarascon, Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries, Nature 407 (2000) 496. (7) L. Taberna, S. Mitra, P. Poizot, P. Simon, J. M. Tarascon, High rate capabilities Fe3O4-based Cu nano-architectured electrodes for lithium-ion battery applications, Nature Mater. 5 (2006) 567. (8) Y. Oumellal, A. Rougier, G. A. Nazri, J. M. Tarascon, L. Aymard, Metal hydrides for
lithium-ion batteries, Nature Mater. 7 (2008) 916 (9) U. Kasavajjula, C. Wang, A. J. Appleby, Nano-and bulk-silicon-based insertion anodes for lithium-ion secondary cells, J. Power Sources 163 (2007) 1003. (10) X. Zhao, D. Xia, L. Gu, J. Yue, B. Li, H. Wei, H. Yan, R. Zou, Y. Wang, X. Wang, Z. Zhang, J. Li, High-performance self-organized Si nanocomposite anode for lithium-ion batteries, Journal of Energy Chemistry 23 (2014) 291-300. (11) H. Kim, M. Seo, M. H. Park, J. Cho, A Critical Size of Silicon Nano-Anodes for Lithium Rechargeable Batteries, Angewandte Chemie International Edition 49 (2010) 2146. (12) C. Pereira-Nabais, J. Światowska, A. Chagnes, A. Gohier, S. Zanna, A. Seyeux, P. Tran-Van, C. S. Cojocaru, M. Cassir, P. Marcus , Insight into the Solid Electrolyte Interphase on Si Nanowires in Lithium-Ion Battery: Chemical and Morphological Modifications upon Cycling, J. Phys. Chem. C 118 (2014) 2919. (13) B. Hertzberg, A. Alexeev, G. Yushin, Deformations in Si− Li Anodes Upon Electrochemical Alloying in Nano-Confined Space, J. Am. Chem. Soc. 132 (2010) 8548. (14) F. F. Cao, J. W. Deng, S. Xin, H. X. Ji, O. G. Schmidt, L. J. Wan, Y. G. Guo, Cu-Si Nanocable Arrays as High-Rate Anode Materials for Lithium Ion Batteries, Adv. Mater. 23 (2011) 4415. (15) Y. Yao, M. T. McDowell, I. Ryu, H. Wu, N. Liu, L. Hu, W. D. Nix, Y. Cui, Interconnected silicon hollow nanospheres for lithium-ion battery anodes with long cycle life, Nano Lett. 11 (2011) 2949. (16) Y. S. Hu, R. Demir-Cakan, M. M. Titirici, J. O. Müller, R. Schlögl, M. Antonietti, J. Maier, Superior Storage Performance of a Si@SiOx/C Nanocomposite as Anode Material
for Lithium-Ion Batteries, Angew. Chem., Int. Ed. 47 (2008) 1645. (17) S. H. Ng, J. Wang, D. Wexler, K. Konstantinov, Z. P. Guo, H. K. Liu, Highly Reversible Lithium Storage in Spheroidal Carbon-Coated Silicon Nanocomposites as Anodes for Lithium-Ion Batteries, Angew. Chem., Int. Ed. 45 (2006) 6896. (18) J. T. Wang, Y. Wang, B. Huang, J. Y. Yang, A. Tan, S. G. Lu, Silicon Supported on Stable Si-O-C Skeleton in High-Performance Lithium-Ion Battery Anode Materials, Acta Phys. Chim. Sin. 30 (2014) 305. (19) J. K. Lee, K. B. Smith, C. M. Hayner, H. H. Kung, Silicon nanoparticles–graphene paper composites for Li ion battery anodes, Chem. Commun. 46 (2010) 2025. (20) X. Zhou, Y. X. Yin, L. J. Wan, Y. G. Guo, Facile synthesis of silicon nanoparticles inserted into graphene sheets as improved anode materials for lithium-ion batteries, Chem. Commun. 48 (2012) 2198. (21) J. T. Wang, Y. Wang, B. Huang, J. Y. Yang, A. TanS. G. , Lu, Improvement of cycle behavior of Si/Ni negative composite by the Stable Si-O-C Skeleton Supported, Journal of Nanoscience and Nanotechnology 15 (2015) 5198. (22) Y. Liu, T. Matsumura, N. Imanishi, A. Hirano, T. Ichikawa, Y. Takeda, Electrochem. Preparation and Characterization of Si/C Composite Coated with Polyaniline as Novel Anodes for Li-Ion Batteries, Solid-State Lett. 8 (2005) A599. (23) Y. Yu, L. Gu, C. Zhu, S. Tsukimoto, P. A. van Aken, J. Maier, Reversible Storage of Lithium in Silver-Coated Three-Dimensional Macroporous Silicon, Adv. Mater. 22 (2010) 2247. (24) X. H. Liu, H. Zheng, L. Zhong, S. Huang, K. Karki, L. Q. Zhang, Y. Liu, A. Kushima, W. T. Liang, J. W. Wang, J. H. Cho, E. Epstein, S. A. Dayeh, S. T. Picraux, T. Zhu, J. Li, J.
P. Sullivan, J. Cumings, C. Wang, S. X. Mao, Z. Z. Ye, S. Zhang, J. Y. Huang, Anisotropic swelling and fracture of silicon nanowires during lithiation, Nano Lett. 11 (2011) 3312. (25) J. H. Cho, X. Li, S. T. Picraux, The effect of metal silicide formation on silicon nanowire-based lithium-ion battery anode capacity, J. Power Sources 205 (2012) 467. (26) X. Li, J. H. Cho, N. Li, Y. Zhang, D. Williams, S. A. Dayeh, S. T. Picraux, Carbon Nanotube Enhanced Growth of Silicon Nanowires as an Anode for High Performance Lithium Ion Batteries, Adv. Energy Mater. 2 (2012) 87. (27) A. Vlad, A. L. M. Reddy, A. Ajayan, N. Singh, J. Gohy, S. Melinte, P. M. Ajayan, Roll up nanowire battery from silicon chips, Proc. Natl. Acad. Sci. 109 (2012) 15168. (28) J. H. Cho, S. T. Picraux, Silicon Nanowire Degradation and Stabilization during Lithium Cycling by SEI Layer Formation, Nano Lett. 14 (2014) 3088. (29) J. H. Cho, S. T. Picraux, Enhanced lithium ion battery cycling of silicon nanowire anodes by template growth to eliminate silicon underlayer islands, Nano Lett. 13 (2013) 5740. (30) H. Wang, J. T. Wang, X. M. Ou, C. S. Lee, X. H. Zhang, Si Nanowire Directly Grown on Liquid Metal Substrate--Towards Wafer Scale Transferable Nanowire Arrays with Improved Visible-Light Sterilization, Nanotechnology 25 (2014) 145601. (31) H. Wang, J. T. Wang, X. M. Ou, F. Li, X. H. Zhang, Highly Efficient and Stable Si Nanowires
Array
Embedded
into
Transparent
Polymer
for
Visible
Light
Photoelectrochemical Cell, Nanotechnology 25 (2014) 265401. (32) J. T. Wang, X. H. Zhang, H. Wang, X. M. Ou, Super-Hydrophobic Silicon/Silica Hierarchical Structure Film, Acta Phys. -Chim. Sin. 27 (2011) 2233.
Figure Captions
Figure 1. The schematic illustration for preparation of SiNW array in PDMS structure. Figure 2. The images for the surface of Si/SiO2 nanowire array (a) the optical image of SiNW array on the surface of the Sn substrate; (b) the side view SEM image of Si/SiO2 nanowire array; (c) the SEM image of as-prepared SiNWs coated by SiO2 shell; (d) TEM image of the single Si/SiO2 core/shell nanowire, inset is the relative EDX spectrum and e SAED pattern. Figure 3. The images of SiNW array in PDMS structure. (a) the optical image of the flexible SiNW array in PDMS structure; (b) the SEM image of SiNW array in PDMS structure after treated by mixed acid form side view; (c) he SEM image of SiNW array in PDMS structure after treated by mixed acid form top view; (d) TEM image of the single Si nanowire of the SiNW array/PDMS, inset is the relative EDX spectrum; (e), HRTEM image of the single Si nanowire of the SiNW array/PDMS, inset is the relative SAED pattern. Figure 4. Electrochemical performance of SiNPs, disordered SiNWs and SiNW array/PDMS with the same conditions. (a) The first discharge−charge profiles of SiNW array/PDMS composite, (b) long-term cycling stability of SiNPs, disordered SiNWs and SiNW array/PDMS composite.
Figure 5. Schematic of the process of lithiated and delithiation. (a) the process of lithiated; (b) the end of lithiated; (c) the process of delithiation.
Figure 1
Figure 2
Figure 3
Potential(V vs Li/Li+)
2.0
a
1.5
1.0
0.5
0.0 200
400
600
800
1000
1200
-1 Specific Capacity/(mAh/g )
1400
1800
b
1600
specific capacity (mAh/g)
1600
100
1400
SiNPS Scatter SiNWs SiNWs Array in PDMS
1200 1000
80
60
800 40
600 400
20 200 0 0
50
100
150
200
Cycle Number
Figure 4
250
300
0 350
coulombic efficiency (100%)
0
Figure 5