5 0 ( 2 0 1 2 ) 1 4 2 2 –1 4 4 4
Carbon nanowalls decorated with silicon for lithium-ion batteries Victor A. Krivchenko a, Daniil M. Itkis b,c,*, Stanislav A. Evlashin a, Dmitry A. Semenenko b, Eugene A. Goodilin b,c, Alexander T. Rakhimov a, Anton S. Stepanov a, Nikolay V. Suetin a, Andrey A. Pilevsky a, Pavel V. Voronin a b c
Institute of Nuclear Physics, Moscow State University, Moscow 119991, Russia Department of Materials Science, Moscow State University, Moscow 119991, Russia Department of Chemistry, Moscow State University, Moscow 119991, Russia
A R T I C L E I N F O
A B S T R A C T
Carbon nanowalls (CNWs) are suggested as a promising nanostructured substrate for 3D
Received 25 June 2011
anodes of lithium-ion batteries. Silicon sputtering onto CNWs results in improved electro-
Accepted 24 October 2011 Available online 29 October2011 2011 6 October
chemical performance due to either a large surface area of free-standing CNWs or easy adhesion of deposited silicon clusters via the SiC interface layer formation. The 3D silicon-decorated nanowall framework (SDNF) could give the possibility to minimize the lithium diffusion length and make charge collection more effective yielding better cycling performance at high rates exceeding 2000 mA h per 1 g of silicon in the range of 0.05– 2.00 V at 1.5 C rate. 2011 Elsevier Ltd. All rights reserved.
Lithium-ion batteries  are known to play already the most important role in power tools, hybrid and electric vehicles and other high-power and high-energy applications. This trend faces a problem of further development of new effective and environmentally friendly electrode materials accommodating large amount of lithium in their structure. Nanocrystalline silicon is one of the most attractive anode materials for lithium-ion batteries due to its extraordinary capacity exceeding 4000 mA h/g [2–7]. Unfortunately significant lattice expansion/contraction in the process of alloying/de-alloying leads to fast pulverization of the active material, loss of an electrical contact with a current collector and, finally, to the anode capacity descent. Thus it seems important to design a material combining easy strain relaxation, a short diffusion length for lithium transport and a sustainable contact with a current collector. A variety of silicon crystalline nanopowders [2–4] and nanowires [5–7] have been studied to enhance cycleability of anodes. Amorphous Si (a-Si) thin films demonstrate a superior cycling performance as compared to crystalline Si (c-Si) while specific capacities of both a- and c-Si are quite high . Different approaches were used to provide a stable electrical contact and better mechanical stability of a-Si films such as metallic cone-like nanostructures or c-Si nanowires as an effective
support for amorphous silicon films as well as reinforcement of silicon with a CNT 3D conductive network [9–11]. Here we report the properties of a new Si/C anode material composed of nanostructured silicon film deposited on carbon nanowall arrays grown by plasma enhanced chemical vapor deposition in DC glow discharge (DC PECVD).
SDNF was prepared by a two-step process involving DC PECVD growth of carbon nanowall arrays as reported elsewhere . A centimeter-sized piece of titanium foil was exposed to hydrogen plasma for 10 min to remove organic and oxide contamination, and then used as a substrate for carbon nanowall growth. Silicon was deposited onto carbon nanowall coated substrates held at 700 C using a standard procedure of magnetron sputtering with a deposition rate of 1 nm/s that gave 1.1 · 10 7 g of silicon per second. After 100 s of deposition, the silicon film becomes continuous while a longer exposure leads to an increase of silicon film thickness. SEM pictures were taken using LEO Supra 40 SEM with a field emission cathode at 10 kV. JEOL JEM 2100 setup was used for imaging the separated SDNF samples. Adsorption isotherms for specific surface area estimation were registered with Quantachrome Nova 4200 analyzer. Raman scattering spectra were collected using Renishaw inVia Raman micro-
* Corresponding author at: Department of Materials Science, Moscow State University, Moscow 119991, Russia. Fax: +7 499 709 7087. E-mail address: [email protected]
(D.M. Itkis). 0008-6223/$ - see front matter 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.10.042
scope with a laser operating at 514 nm wavelength and 10 mW power output. 50· objective was used to focus the laser beamspot of ca. 10 lm in diameter to analyze the samples locally. XPS analysis was performed with a Kratos AXIS Ultra DLD spectrometer. An Al Ka monochromatized radiation (hm = 1486.6 eV) was employed as an X-ray source. Binding energies were calibrated vs the carbon signal at 285.0 eV. Electrochemical testing of SDNF was carried out using three-electrode cells with both a metallic lithium counter and reference electrodes. One molar solution of LiClO4 (Aldrich, battery grade) in a propylene carbonate (Aldrich, anhydrous): dimethoxyethane (Aldrich, anhydrous) mixture (7:3 by volume) was used as the electrolyte. The cells were assembled and sealed in an argon-filled glovebox. Galvanostatic charge– discharge cycles were performed using the Ecochemie Autolab PGSTAT 302 system in the range of 0.05–2.00 V at 1.5, 3 and 6 C rates (2.4, 4.8 and 9.6 A/g, respectively).
5 0 ( 20 1 2 ) 1 4 2 2–14 4 4
Results and discussion
CNWs in the SDNF architecture (Fig. 1) provide a mechanically stable and highly conductive support for mixed crystalline/ amorphous silicon island films. CNWs themselves demonstrate a modest lithium intercalation capacity of up to 200 mA h/g with electrochemical behavior similar to graphite . Such a carbon film offers high electronic conductance
Fig. 1 – 3D architecture of SDNF.
and a high specific surface area becoming attractive as a three-dimensional framework for silicon films deposition. In this work CNWs were grown on titanium foils by a catalyst-free DC PECVD process in a H2/CH4 gas mixture . A SEM image of the as-prepared carbon films (Fig. 2(a)) reveals that they were mainly composed of 1 lm high, 0.5–1 lm wide and 10–15 nm thin nanowalls oriented perpendicular to the metallic substrate. According to the BET data, the samples demonstrated a specific surface area of about 1000 m2/g. An analysis of TEM micrographs (Fig. 2(b)) proves that CNWs are composed of periodic carbon layers with a typical 0.34 nm‘‘graphite’’ distance. An image of 60–70 nm thick silicon film is presented in Fig. 2(c). A TEM study (Fig. 2(d)) shows that silicon crystallites reach 15 nm in size and are covered with an amorphous sili-
Fig. 2 – Micrographs of as prepared CNWs (a, b) and SDNF (c, d); a-Si denotes amorphous silicon while c-Si stands for its crystalline phase.
5 0 ( 2 0 1 2 ) 1 4 2 2 –1 4 4 4
Fig. 3 – (a) Raman spectra of silicon and CNWs in SDNF; C 1s (b) and Si 2p photoemission spectra of as-prepared SDNF.
con shell. This finding is further confirmed by the Raman spectroscopy (Fig. 3(a)) since the spectrum contains several wide maxima in the range of 300–600 cm 1 characteristic of these types of silicon. Well-defined TO phonon peak of crystalline silicon (c-Si) located at 520 cm 1 for a bulk crystal is shifted to 515 cm 1 and broadened as expected for a nanostructured silicon films due to an increased percent of surface states in nanocrystals. A wide maximum at 485 cm 1 originates of an amorphous phase (a-Si); a broad low-frequency shoulder around 350 cm 1 might be the result of amorphization of sputtered silicon film as well . Raman spectrum deconvolution allowed to estimate c-Si volume fraction to be about 15% . CNWs Raman spectrum in the range of 1200–1700 cm 1 (Fig. 3(a)) has two dominant spectral features at 1360 cm 1 (D-mode) and 1585 cm 1 (G-mode). The relative intensity of the maxima was used to determine the in-plane domain size estimated as 10 nm [15,16]. The presence of a certain amount of defects and disordering in CNWs are believed to be crucial for the silicon film adhesion as e.g. carbide interlayer between silicon and carbon nanowall surface is much easily formed at
the defect sites. To shed the light on the interface composition, photoemission spectra were registered. C 1s spectrum shown in Fig. 3(b) has a complex structure. The most intense components correspond to sp2 (284.4 eV) and sp3 (284.9 eV) carbon . Another component is observed at 283.1 eV and can be ascribed to SiC enhancing silicon film adhesion to CNWs. Si 2p photoemission spectrum (Fig. 3(c)) has also a component typical for SiC (100.8 eV) . Another doublets in Si 2p are related to crystalline Si (99.35 eV) and silicon dioxide (102.72 eV) formed on the silicon surface due to a reaction with environmental oxygen. So both C 1s and Si 2p spectra evidence for the formation of a carbide layer between CNWs and the silicon film that occurs at a high temperature during amorphous silicon film deposition. At the same time crystallization heat of silicon released inside the amorphous matrix provides a complex microstructure shown in Fig. 2(d). In order to study electrochemical performance, SDNF anodes were charged/discharged in a galvanostatic mode vs a metallic lithium foil. Large irreversible capacity loss occurs during the first cycle. A short plateau observed at 0.6–0.8 V (Fig. 4(a)) could be ascribed to a SEI formation process on
5 0 ( 20 1 2 ) 1 4 2 2–14 4 4
No plateau is observed during further cycling evidencing that no crystalline phase is formed . High specific capacity (about 2100 mA h/g at 1.5 C rate) obtained at the 1st discharge cycle remains at the level of 2000 mA h/g after more than 40 cycles (Fig. 4(b)). The initial Coulombic efficiency is rather low (<50%) that is connected with SEI formation and other irreversible processes as discussed above. However, efficiency is gradually rising during cycling getting close to 100% at the 5th cycle. In order to demonstrate the ability of synthesized material to be charged and discharged at high current densities it was cycled at 3 and 6 C rates (Fig. 4(c)). These results make us believe that silicon sputtering onto CNWs leads to a promising anode material with an improved electrochemical performance. The reason of strong adhesion of the silicon film and better stability of the electrode seems to be an interface layer between CNWs surface and silicon. The 3D architecture of SDNF as a whole could give the possibility to minimize the lithium diffusion length and make charge collection more effective providing better cycling performance at high rates.
Acknowledgements Authors sincerely acknowledge Natalia Tabachkova for help with TEM investigations. This work was financially supported by Scientific School grant # 3322.2010.2, G.C. No 0055 and by Federal program for scientific personnel support (contract #P649).
R E F E R E N C E S
Fig. 4 – (a) Voltage profiles for the first, 10th, 20th, 30th and 40th galvanostatic charge and discharge cycles at the 1.5 C rate. Cycling was performed in the 0.05–2.00 V range. (b) Charge (closed circles) and discharge (open circles) capacities and Coulombic efficiency dependencies upon cycle number. (c) Discharge capacities of the SDNF anode for different current rates.
well-developed SDNF surface . The process occurring at 1.2–1.3 V might be connected with the reduction of a small amount of SiO2 detected on the surface by XPS but it is still unclear and requires further investigation.
 Armand M, Tarascon J. Building better batteries. Nature 2008;451:652–7.  Xiao J, Xu W, Wang D, Choi D, Wang W, Li W, et al. Stabilization of silicon anode for Li-ion batteries. J Electrochem Soc 2010;157:A1047–51.  Kasavajjula U, Wang C, Appleby A. Nano- and bulk-siliconbased insertion anodes for lithium-ion secondary cells. J Power Sources 2007;163:1003–39.  Wilson A, Dahn J. Lithium insertion in carbons containing nanodispersed silicon. J Electrochem Soc 1995;142: 326–332.  Chan CK, Peng H, Liu G, McIlwrath K, Zhang XF, Huggins RA, et al. High-performance lithium battery anodes using silicon nanowires. Nature Nanotechnol 2008;3:31–5.  Laik B, Eude L, Pereira-Ramos J, Cojocaru C, Pribat D, Rouviere E. Silicon nanowires as negative electrode for lithium-ion microbatteries. Electrochim Acta 2008;53:5528–32.  Peng K, Yan Y, Gao S, Zhu J. Synthesis of large-area silicon nanowire arrays via self-assembling nanoelectrochemistry. Adv Mater 2002;14:1164–7.  Yin J, Wada M, Yamamoto K, Kitano Y, Tanase S, Sakai T, et al. Micrometer-scale amorphous Si thin-film electrodes fabricated by electron-beam deposition for Li-ion batteries. J Electrochem Soc 2006;153:A472–7.  Zhang S, Du Z, Lin R, Jiang T, Liu G, Wu X, et al. Nickel nanocone-array supported silicon anode for highperformance lithium-ion batteries. Adv Mater 2010;22:5378–82.  Cui L, Ruffo R, Chan CK, Peng H, Cui Y. Crystallineamorphous core-shell silicon nanowires for high capacity and high current battery electrodes. Nano Lett 2009;9:491–5.
5 0 ( 2 0 1 2 ) 1 4 2 2 –1 4 4 4
 Cui L, Hu L, Choi JW, Cui Y. Light-weight free-standing carbon nanotube-silicon films for anodes of lithium ion batteries. ACS Nano 2010;4:3671–8.  Krivchenko VA, Pilevsky AA, Rakhimov AT, Seleznev BV, Suetin NV, Timofeyev MA, et al. Nanocrystalline graphite: promising material for high current field emission cathodes. J Appl Phys 2010;107:014315.  Tanaike O, Kitada N, Yoshimura H, Hatori H, Kojima K, Tachibana M. Lithium insertion behavior of carbon nanowalls by dc plasma CVD and its heat-treatment effect. Solid State Ionics 2009;180:381–5.  Gajovic´ A, Gracin D, Juraic´ K, Sancho-Parramon J, Cˇeh M, et al. Correlating Raman-spectroscopy and high-resolution transmission-electron-microscopy studies of amorphous/ nanocrystalline multilayered silicon thin films. Thin Solid Films 2009;517:5453–8.
 Ferrari A, Robertson J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys Rev B 2000;61:14095–107.  Sato K, Saito R, Oyama Y, Jiang J, Cancado LG, Pimenta MA, et al. D-band Raman intensity of graphitic materials as a function of laser energy and crystallite size. Chem Phys Lett 2006;427:117–21.  Swain B. The analysis of carbon bonding environment in HWCVD deposited a-SiC:H films by XPS and Raman spectroscopy. Surf Coat Technol 2006;201:1589–93.  Obrovac M, Christensen L. Structural changes in silicon anodes during lithium insertion/extraction. Electrochem Solid State Lett 2004;7:A93–6.
Residual acetone produces explosives during the production of graphite oxide Seungjun Lee a, Junghoon Oh a, Rodney S. Ruoff b, Sungjin Park
Department of Chemistry, Inha University, Incheon 402-751, Republic of Korea Department of Mechanical Engineering and the Materials Science and Engineering Program, The University of Texas at Austin, One University Station C2200, Austin, TX 78712-0292, USA
A R T I C L E I N F O
A B S T R A C T
Chemically modified graphenes (CMGs) are promising candidates for a wide range of appli-
Received 31 August 2011
cations. Graphite oxide (GO) is most commonly used to produce CMGs. We note that resid-
Accepted 27 October 2011 Available Available online online 4 6 October October 2011 2011
ual acetone can produce dangerous explosives during the synthesis of GO. Addition of acetone produced acetone peroxides via a reaction with H2O2 and H2SO4, which are used in the Hummers/modified Hummers methods of making GO. The use of Na2S2O3 instead of H2O2 yielded GO without making explosives. 2011 Elsevier Ltd. All rights reserved.
Graphene has excellent mechanical, electrical, thermal, and optical properties, and a high specific surface area [1,2]. Chemically modified graphenes (CMGs) and their colloidal suspensions allow for use of CMG materials in polymer composites, in ultracapacitors, for hydrogen storage materials, in rechargeable batteries, as conducting inks, for thermal management, and in chemical/bio sensors [1–4]. Graphite oxide (GO), produced via oxidation of graphite, has been most frequently used as a starting material to generate various CMGs . We have identified a safety concern in the synthesis of GO and present it here. We accidentally formed white powder during the production of GO using the modified Hummers method and separately, the Hummers method, each a common method to produce GO (Fig. 1) [5–7]. The isolated powders were a strong explosive at high temperature (200–300 C) and also when ‘hammered’ on.
We then learned that it was acetone that generated such explosives during the synthesis of GO using the Hummers/ modified Hummers methods. Acetone peroxide (dimer, or trimer), a strong explosive , has caused lab accidents  and is produced by the reaction between H2O2 and acetone in an acid-catalyzed nucleophilic addition . H2O2 and acid (H2SO4) are commonly used in the synthesis of GO using the Hummers/modified Hummers methods. Nuclear magnetic resonance spectroscopy of the isolated powder confirmed the generation of acetone peroxide from the GO preparation (see Supporting Information (SI)). We found that the addition of H2O2 at different processing steps (Fig. 1) to reaction vessels containing residual acetone produced mixtures of graphite/graphite oxide with such explosives, and also caused explosions in the liquid-phase mixtures (see SI for experimental details). We have not attempted to further investigate these explosions because of the obvious dangers.
* Corresponding author: Fax: +82 32 860 5604. E-mail address: [email protected]
(S. Park). 0008-6223/$ - see front matter 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.10.045