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Silicon nanoparticles supported on graphitic carbon paper as a hybrid anode for Li-ion batteries
Nano Energy (2013) 2, 1107–1112
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Silicon nanoparticles supported on graphitic carbon paper as a hybrid anode for Li-ion batteries Yongzhu Fu, Arumugam Manthiramn Electrochemical Energy Laboratory & Materials Science and Engineering Program, The University of Texas at Austin, Austin, TX 78712, United States Received 30 August 2013; received in revised form 12 September 2013; accepted 15 September 2013 Available online 25 September 2013
KEYWORDS
Abstract
Li-ion battery; Silicon-graphite hybrid anode; Hierarchical electrode; Structural characterization; Electrochemical performance
Graphite anode is widely used in current lithium-ion (Li-ion) batteries due its high capacity and excellent cycling stability. A conventional graphite composite anode consists of graphite powder, carbon additive, and a polymer binder. To further increase the energy density of Li-ion batteries, alternative high capacity anode materials such as silicon are needed. Here, we report a hybrid anode consisting of silicon nanoparticles supported on a woven graphitic carbon paper. The structural carbon paper itself exhibits a specific capacity of 167 mAh g 1 at a rate of C/40, while the silicon nanoparticles further increase the overall capacity of the electrode by 10% with a silicon loading of only 0.2 mg. The silicon in the hybrid electrode exhibits a specific capacity of 1300 mAh g 1 which decreases as the loading increases to 0.6 mg, but still showing good cyclability. The structural and morphological changes of graphite and silicon within the hybrid electrode during charge and discharge are also presented. & 2013 Elsevier Ltd. All rights reserved.
Introduction As the utilization of renewable energies (e.g., solar and wind) is widely implemented, electrochemical energy storage (EES) is becoming more critical than ever [1]. Also, electric vehicles, which are greener and more sustainable than internal combustion engines, are being intensively pursued. Rechargeable batteries, a crucial component in our daily life (e.g., cellphones and laptops), will play a n
Corresponding author. Tel.: +1 512 471 1791; fax: +1 512 471 7681. E-mail address: [email protected] (A. Manthiram). 2211-2855/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.nanoen.2013.09.004
significant role in these applications. Lithium-ion (Li-ion) batteries, which rely on reversible lithium-ion insertion electrodes (i.e., graphite anode and transition metal oxide cathodes), possess the highest energy density among the known battery chemistries and are the most promising technology for these applications. However, the capacities of these electrode materials have reached their limits, which prevent further increase in the energy density of Li-ion batteries [2,3]. To enhance the role of Li-ion batteries in the burgeoning EES market, alternative electrodes that can provide higher capacities are needed. Graphite has been the most successful anode material, providing a capacity of 372 mAh g 1 in Li-ion batteries.
1108 Lithium-ion insertion into the layers in graphite is highly reversible and the process has been well understood [4–6]. A conventional, commercial graphite anode is a mixture of graphite powder, activated carbon additive, and a polymer binder coated onto a copper current collector. Such electrode structure can accommodate a reasonable volume expansion during the ion insertion, but it may not be suitable for alternative anode materials that possess high capacities but exhibit huge volume change upon cycling. Silicon is a promising anode material that has a theoretical capacity of 4200 mAh g 1, but suffer from a huge volume change of 400% during charge/discharge [7]. To maintain an integrated electrode structure with efficient ion and electron transport to silicon particles upon cycling, nanoscaled silicon in a conductive but spacious matrix has been demonstrated to be more desirable [8–12]. Particularly, electrodes with hierarchical architectures containing no binders are being widely developed for silicon anodes [8,12–14]. For example, silicon nanoparticle-loaded porous carbon nanofibers as anode materials show a discharge capacity of 1100 mAh g 1 [13]. Recently, Ji et al. [12] developed a Si/graphene/ultrathin-graphite foam (UGF) composite in which Si nanoparticles encapsulated in graphene are supported by the UGF. The composite with a Si loading of 1.5 mg cm 2 shows unprecedented gravimetric and volumetric capacities, which are much higher than the graphite anode used in commercial lithium-ion cells. Carbon fiber paper, a structural component in the gas diffusion layer in electrodes of low-temperature proton exchange membrane fuel cells, is a promising platform for studying electrochemically active materials since it has a conductive but porous structure. Carbon fiber paper has been used as a structural support in EES devices. For example, Yang et al. [15] utilized carbon fiber paper as a support to accommodate cobalt oxide nanonet as a high capacity electrode for pseudocapacitors. Some of these fiber papers that contain graphitic carbon are also potential anode materials in structural batteries [16]. However, carbon fiber paper has limited capacity because of the limited content of graphitic carbon in it. Herein, we report a hybrid anode consisting of a woven carbon paper and silicon nanoparticles. Silicon nanoparticles can be readily deposited onto the microsized carbon fibers within the carbon paper, which can provide efficient electron transport. In addition, the voids between carbon fibers allow fast ion transport and accommodate the volume expansion of silicon nanoparticles. Both carbon paper and silicon are electrochemically active, but the lithiation reaction is different, i.e., ion insertion into graphite vs. lithium alloying with silicon. As a demonstration of the proof-of-concept, the capacity of a carbon paper electrode can be increased from ca. 3 mAh to 3.25 mAh with a silicon loading of only 0.2 mg, corresponding to a specific capacity of over 1300 mAh g 1 of the silicon.
Experimental Chemicals Toray carbon paper (Fuel Cell Earth, thickness: 370 mm), silicon nanopowder (Sigma Aldrich, o100 nm particle size),
Y. Fu, A. Manthiram and liquid electrolyte (Novolyte, 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1 v/v)) were purchased and used as received.
Silicon suspension preparation and cell fabrication An appropriate amount of silicon nanopowder was weighed in an argon-filled glove box and added into liquid electrolyte (1 mL) to render a silicon suspension solution with concentrations of 5, 10, and 20 g L 1. The suspension was sealed in a glass vial and taken out of the glove box for sonication for 15 min and then transferred back into the glove box for cell fabrication. Toray carbon paper was cut into circular discs with a diameter of 1.2 cm (1.13 cm2) and mass of 17.670.1 mg. For assembling the cells, 40 mL of liquid electrolyte or silicon suspension solution was added into carbon paper electrodes to render a silicon loading of 0.2, 0.4, and 0.6 mg, followed by a Celgard 2400 separator, 20 mL of additional electrolyte, and lithium metal anode. Finally, the cells were crimped for electrochemical evaluation outside the glove box.
Characterization Morphological characterizations were carried out with a FEI Quanta 650 scanning electron microscope (SEM). The elemental mapping was performed with an energy-dispersive spectrometer (EDS) attached to the FEI Quanta 650. X-ray diffraction (XRD) data were collected on a Philips X-ray diffractometer equipped with CuKα radiation in a step of 0.021. Electrode samples were collected from cells that were cycled after 1st discharge. XRD patterns of freshly made silicon-carbon paper electrodes were also collected for a comparison. These XRD samples were covered by Kaptons films.
Electrochemical evaluation The cells for electrochemical evaluation consist of a silicon loading of 0.2 mg except those indicated. In the battery cycling measurements, cells were galvanostatically discharged to 0.01 V at C/40 rate (1C =167 mA g 1) initially with an Arbin battery test station, then cycled between 0.01 and 2 V at C/40 rate. Electrochemical impedance spectroscopy (EIS) data were collected with a computer interfaced HP 4192A LF Impedance Analyzer in the frequency range of 1 MHz–0.1 Hz with an applied voltage of 5 mV and Li foil as both counter and reference electrodes.
Results and discussions Commercial carbon paper is a binder-free, woven carbon fiber matrix containing graphitic carbon, as shown in Figure 1a. The fibers with a diameter of about 10 mm provide the paper excellent mechanical stability; therefore, no extra polymer binder or current collectors are needed when used as an electrode. In addition, the fibers are also highly striated as shown in the inset in Figure 1a, which could have good interfacial adhesion with silicon nanoparticles by a mechanical interlocking mechanism. The large
Hybrid anode for Li-ion batteries
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5 µm
100 µm
20 µm
Si
5 µm
5 µm
Figure 1 SEM images of (a) carbon paper with a magnified image in the inset, (b) silicon–carbon paper, (c) magnified silicon–carbon paper, and (d) silicon mapping of the magnified silicon–carbon paper in (c).
porosity within the matrix provides ideal channels for ion transport and spaces for holding other nanoscaled active materials. Silicon nanoparticles were largely dispersed within the liquid electrolyte after the sonication. The concentration was maintained to be r 20 g L 1 to ensure good dispersity. The silicon nanoparticles were largely deposited onto the carbon matrix after the silicon suspension was added into the carbon paper electrode, as seen in Figure 1b. Carbon fibers are coated with a layer of agglomerated silicon nanoparticles, as revealed in the SEM image in Figure 1c and silicon elemental mapping in Figure 1d. The good adhesion between the carbon fiber and silicon nanoparticles seen in the SEM image could be partly due to the Van der Waals force between the oxygencontaining moieties (OH and COOH) on the surface of carbon fibers and the native oxide on the surface of silicon nanoparticles. Such good adhesion has also been observed between silicon nanowires and carbon textiles [17], which can ensure good electron transport and structural stability during cycling. The empty space on the carbon fibers has the potential to accommodate high loadings of silicon nanoparticles. The lithiation of graphite involves an ion insertion reaction, whereas silicon involves a displacement reaction to form alloys with lithium upon discharge. Figure 2a shows the XRD pattern of the pristine carbon paper (CP), silicon–carbon
paper (Si–CP) electrode, and their lithiated samples after 1st discharge. The carbon paper shows a strong peak at 27.21 and 55.21 corresponding, respectively, to the (002) and (004) planes [18]. After lithiation, the (002) plane peak shifts to a lower angle at 24.91, indicating an expansion of graphene layers within the carbon fiber from 3.28 to 3.56 Å due to the insertion of lithium ions. In addition, the discharged carbon paper becomes golden yellow which is a characteristic color of lithium intercalated graphite [19]. Similarly, the peak at 55.21 also shifts to lower angles. The peak density decreases slightly, meaning a minimum destructive interference because of the interlayer species [20]. The Si–CP electrode also undergoes a similar structural change upon lithiation. In addition to the graphite peaks, minor peaks of silicon nanoparticles can be noticed in the magnified XRD pattern in Figure 2a. Due to the low loading of silicon, no obvious peaks of silicon–lithium alloys can be observed even though the end of discharge is at 0.01 V [21]. Upon lithiation, the silicon nanoparticles on the carbon fibers form silicon–lithium alloys which cover largely the surface of the carbon fiber, while the carbon fiber still maintains its woven structure, as shown in Figure 2b. The magnified SEM image in Figure 2c clearly shows some silicon nanoparticles, maybe un-reacted, are embedded in the lithiated silicon compounds. The lithiated silicon layer on the carbon fiber is porous, which does not block the access
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Figure 2 (a) XRD pattern of carbon paper (CP), lithiated CP, silicon–carbon paper (Si–CP), and lithiated Si–CP, (b) SEM image of the lithiated Si–CP electrode, and (c) magnified SEM image of the lithiated Si–CP electrode.
Figure 3 (a) Voltage profile of the 1st discharge and following charge of the CP and Si–CP electrodes, with a silicon loading of 0.2 mg in the Si–CP electrode and the inset showing a magnified voltage profile and, and (b) Nyquist plots of half cells with the CP and Si–CP electrodes before and after 1st discharge.
of lithium ions to the carbon fibers. These results demonstrate the lithiation of carbon paper and silicon occurs simultaneously within the Si–CP electrode during discharge. The initial discharge and charge voltage profiles of the CP and Si–CP are shown in Figure 3a. Irreversible capacities at 0.5–0.8 V are observed, which are due to the solidelectrolyte interface (SEI) formation in the 1st discharge [4]. Due to the low surface area of the carbon paper, the
irreversible capacities are not significant. The major voltage profile of CP lies at a voltage below 0.1 V, showing three stages corresponding mainly to the formation of lithiated compounds LixC6 (0oxo1) similar to those of graphite anodes reported [6]. The Si–CP shows a voltage profile similar to that of the CP without any extra distinguishable voltage plateaus, but a higher capacity contributed from the silicon nanoparticles. Cyclic voltammogram is not shown here since the redox peaks of silicon are shadowed by those of graphite. Both CP and Si–CP electrodes exhibit 1st discharge and charge capacities of 43 mAh, indicating reversible lithiation and de-lithiation reactions. To understand the electrochemical behavior of the CP and Si–CP electrodes, electrochemical impedance analysis was performed. Figure 3b shows the Nyquist plots of the CP and Si–CP before and after the 1st discharge in the frequency range of 0.1 to 1 M Hz. Before cycling, the CP and Si–CP electrodes both show a depressed semicircle in the highmedium frequency region, which is mainly due to the charge-transfer resistance on the electrode surface, and a long tail in the low frequency region, which is the diffusioncontrolled impedance [22]. Obviously, the Si–CP electrode shows a much lower surface resistance (an order of magnitude lower) than the plain CP because of the increase in surface area and decrease in charge transfer resistance, both of which are due to the much smaller particle size (o100 nm) of Si nanoparticles compared to the carbon fiber (10 mm in diameter). After the 1st discharge, impedance spectra show only depressed semicircles without the diffusion impedance due to the low energy barrier for lithium-ion diffusion within the lithiated graphite and silicon. In addition, the charge transfer resistance becomes much smaller after the discharge. With silicon nanoparticles on the graphite fiber, the charge-transfer resistance is even smaller. These results indicate the presence of silicon nanoparticles on graphite can enhance charge transfer, which is beneficial for the electrochemical performance of the Si–CP electrodes. The cycling performance of the CP and Si–CP electrodes at C/40 is shown in Figure 4a. The cycle rate is limited by the lithiation/delithiation of carbon fibers because the
Hybrid anode for Li-ion batteries
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Figure 4 (a) Cycling performance and (b) specific capacities of the CP and Si–CP electrodes at a rate of C/40 with a silicon loading of 0.2 mg in the Si–CP electrode, (c) cycling performance of the Si–CP electrodes with silicon loadings of 0.2, 0.4, and 0.6 mg at a rate of C/40, and (d) specific capacities of silicon within the three Si–CP electrodes.
carbon fiber is too thick. The 1st discharge capacities of the CP and Si–CP electrodes are, respectively, 3.42 mAh and 3.61 mAh. Significant capacity losses of 0.31 mAh and 0.25 mAh occur after the 1st discharge, respectively, for the CP and Si-CP electrodes, which are mainly due to the SEI layer formation. The presence of silicon nanoparticles on the surface of carbon fibers increases the surface area, which normally increases the irreversible capacity loss due to the SEI layer formation. However, the Si–CP electrode shows a much lower capacity loss than the CP electrode. The alloying of silicon–lithium along with the significant volume expansion protects other silicon nanoparticles and carbon fiber from further SEI layer formation, which is illustrated in Figure 2c. After the first few cycles, the capacities of CP and Si–CP electrodes are maintained, respectively, at approximate 2.94 mAh and 3.22 mAh. The stabilized overall capacity of the Si–CP electrode is about 10% higher than that of the CP electrode, which is contributed from the silicon nanoparticles. The Coulombic efficiencies for both electrodes are close to 100% over 50 cycles, indicating good cycling reversibility and stable SEI layers formed on both electrodes. The specific capacities of CP and silicon in the Si–CP electrode are shown in Figure 4b. The latter were obtained by subtracting the overall capacities of the Si–CP electrode by those of the CP electrode and then dividing by the mass of Si. The CP shows an average capacity of 167 mAh g 1, which is much lower than that of graphite (i.e., 372 mAh g 1) because not all the carbon in the CP is made of graphite, maybe a disordered core surrounded by a graphitic sheath [16]. The average capacity of silicon is about 1300 mAh g 1, indicating a relatively high utilization of silicon in the lithiation reaction. It is lower than the theoretical capacity of silicon because of the un-reacted silicon nanoparticles seen in the SEM image in Figure 2c after the 1st discharge.
The electrochemical performance of Si–CP electrodes with different Si loadings (0.2, 0.4, and 0.6 mg) was evaluated, as shown in Figure 4c. After similar capacity decreases in the first cycles, the capacities of these Si–CP electrodes stabilize after 10 cycles. Obviously, the higher Si loading, the higher the overall capacity obtained. The capacity of Si–CP with a silicon loading of 0.6 mg is about 3.37 mAh, which is approximately 15% more than that of the CP electrode. The specific capacities of silicon in the 10th cycles are, respectively, 1190, 804, 650 mAh g 1 for the silicon loadings of 0.2, 0.4, and 0.6 mg, as shown in Figure 4d. The low capacities of silicon with high loadings are due to the uneven distributions of silicon nanoparticles on the carbon fibers. Apparently, the utilization of silicon nanoparticles can be improved with better dispersion of silicon suspension in the liquid electrolyte and an improved deposition process.
Conclusion In summary, we have demonstrated a simple approach to develop a hybrid anode consisting of a graphitic carbon paper and silicon nanoparticles. The carbon paper and silicon undergo lithiation and de-lithiation simultaneously upon cycling, showing specific capacities of 167 mAh g 1 and 1300 mAh g 1, respectively, with a silicon loading of 0.2 mg. The Si–CP electrode shows a smaller irreversible capacity loss in the 1st cycle than the CP electrode. The silicon loading can be increased, but the specific capacities of silicon decrease due to the uneven deposition of silicon in the Si–CP electrode. The results shown here demonstrate an efficient way to utilize an ion insertion anode and silicon in a hybrid electrode configuration to increase the overall capacity. With further improvement in the distribution of
1112 silicon nanoparticles within the silicon–graphite electrode, this method has the potential to significantly increase the capacity of graphite anode.
Acknowledgments This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award no. DE-SC0005397.
References [1] Z. Yang, J. Zhang, M.C.W. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, J. Liu, Chemical Reviews 111 (2011) 3577–3613. [2] J.B. Goodenough, Y. Kim, Chemistry of Materials 22 (2010) 587–603. [3] A. Manthiram, Journal of Physical Chemistry Letters 2 (2011) 176–184. [4] R. Fong, U. Sacken, J.R. Dahn, Journal of the Electrochemical Society 137 (1990) 2009–2013. [5] J.R. Dahn, T. Zheng, Y. Liu, J.S. Xue, Science 270 (1995) 590–593. [6] Z. Jiang, M. Alamgir, K.M. Abraham, Journal of the Electrochemical Society 142 (1995) 333–340. [7] C.K. Chan, H. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Nature Nanotechnology 3 (2008) 31–35. [8] J. Guo, A. Sun, C. Wang, Electrochemistry Communications 12 (2010) 981–984. [9] A. Magasinski, P. Dixon, B. Hertzberg, A. Kvit, J. Ayala, G. Yushin, Nature Materials 9 (2010) 353–358. [10] H. Wu, G. Zheng, N. Liu, T.J. Carney, Y. Yang, Y. Cui, Nano Letters 12 (2012) 904–909. [11] B. Liu, P. Soares, C. Checkles, Y. Zhao, G. Yu, Nano Letters 13 (2013) 3414–3419. [12] J. Ji, H. Ji, L.L. Zhang, X. Zhao, X. Bai, X. Fan, F. Zhang, R. S. Ruoff, Advanced Materials 25 (2013) 4673–4677. [13] L. Ji, X. Zhang, Electrochemistry Communications 11 (2009) 1146–1149. [14] M. Gu, Y. Li, X. Li, S. Hu, X. Zhang, W. Xu, S. Thevuthasan, D.R. Baer, J.-G. Zhang, J. Liu, C. Wang, ACS Nano 6 (2012) 8439–8447. [15] L. Yang, S. Cheng, Y. Ding, X. Zhu, Z.L. Wang, M. Liu, Nano Letters 12 (2012) 321–325. [16] J.F. Snyder, E.L. Wong, C.W. Hubbard, Journal of the Electrochemical Society 156 (2009) A215–A224.
Y. Fu, A. Manthiram [17] B. Liu, X. Wang, H. Chen, Z. Wang, D. Chen, Y.-B. Cheng, C. Zhou, G. Shen, Scientific Reports 3 (2013) 1622. [18] J.Y. Howe, C.J. Rawn, L.E. Jones, H. Ow, Powder Diffraction 18 (2003) 150–154. [19] T. Ohzuku, Y. Iwakoshi, K. Sawai, Journal of the Electrochemical Society 140 (1993) 2490–2498. [20] D.A. Stevens, J.R. Dahn, Journal of the Electrochemical Society 148 (2001) A803–A811. [21] T.D. Hatchard, J.R. Dahn, Journal of the Electrochemical Society 151 (2004) A838–A842. [22] R. Ruffo, S.S. Hong, C.K. Chan, R.A. Huggins, Y. Cui, Journal of Physical Chemistry C 113 (2009) 11390–11398. Yongzhu Fu is currently working as a Research Associate in the Texas Materials Institute at the University of Texas at Austin (UT-Austin). He obtained his B.E. (2000) and M.S. (2003) in Chemical Engineering from Tsinghua University and Dalian Institute of Chemical Physics, respectively, in China, and Ph.D. in Materials Science and Engineering from the University of Texas at Austin in 2007. He was a Chemist Postdoctoral Fellow at Lawrence Berkeley National Laboratory and Research Scientist at Lynntech, Inc. before joining UT-Austin. His current research is focused on new materials for rechargeable lithium batteries and fuel cells. Arumugam Manthiram is a Professor and holder of the Joe C. Walter Chair in Engineering in the Materials Science and Engineering Graduate Program and Department of Mechanical Engineering at The University of Texas at Austin. He is also the Director of the Texas Materials Institute and the Materials Science and Engineering Program. His research interests are in the area of materials for rechargeable batteries, fuel cells, and solar cells, including novel synthesis approaches for nanomaterials and nanocomposites. He has authored around 500 publications including more than 400 journal articles. See www.me.utexas.edu/manthiram for further details.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanoenergy
RAPID COMMUNICATION
Silicon nanoparticles supported on graphitic carbon paper as a hybrid anode for Li-ion batteries Yongzhu Fu, Arumugam Manthiramn Electrochemical Energy Laboratory & Materials Science and Engineering Program, The University of Texas at Austin, Austin, TX 78712, United States Received 30 August 2013; received in revised form 12 September 2013; accepted 15 September 2013 Available online 25 September 2013
KEYWORDS
Abstract
Li-ion battery; Silicon-graphite hybrid anode; Hierarchical electrode; Structural characterization; Electrochemical performance
Graphite anode is widely used in current lithium-ion (Li-ion) batteries due its high capacity and excellent cycling stability. A conventional graphite composite anode consists of graphite powder, carbon additive, and a polymer binder. To further increase the energy density of Li-ion batteries, alternative high capacity anode materials such as silicon are needed. Here, we report a hybrid anode consisting of silicon nanoparticles supported on a woven graphitic carbon paper. The structural carbon paper itself exhibits a specific capacity of 167 mAh g 1 at a rate of C/40, while the silicon nanoparticles further increase the overall capacity of the electrode by 10% with a silicon loading of only 0.2 mg. The silicon in the hybrid electrode exhibits a specific capacity of 1300 mAh g 1 which decreases as the loading increases to 0.6 mg, but still showing good cyclability. The structural and morphological changes of graphite and silicon within the hybrid electrode during charge and discharge are also presented. & 2013 Elsevier Ltd. All rights reserved.
Introduction As the utilization of renewable energies (e.g., solar and wind) is widely implemented, electrochemical energy storage (EES) is becoming more critical than ever [1]. Also, electric vehicles, which are greener and more sustainable than internal combustion engines, are being intensively pursued. Rechargeable batteries, a crucial component in our daily life (e.g., cellphones and laptops), will play a n
Corresponding author. Tel.: +1 512 471 1791; fax: +1 512 471 7681. E-mail address: [email protected] (A. Manthiram). 2211-2855/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.nanoen.2013.09.004
significant role in these applications. Lithium-ion (Li-ion) batteries, which rely on reversible lithium-ion insertion electrodes (i.e., graphite anode and transition metal oxide cathodes), possess the highest energy density among the known battery chemistries and are the most promising technology for these applications. However, the capacities of these electrode materials have reached their limits, which prevent further increase in the energy density of Li-ion batteries [2,3]. To enhance the role of Li-ion batteries in the burgeoning EES market, alternative electrodes that can provide higher capacities are needed. Graphite has been the most successful anode material, providing a capacity of 372 mAh g 1 in Li-ion batteries.
1108 Lithium-ion insertion into the layers in graphite is highly reversible and the process has been well understood [4–6]. A conventional, commercial graphite anode is a mixture of graphite powder, activated carbon additive, and a polymer binder coated onto a copper current collector. Such electrode structure can accommodate a reasonable volume expansion during the ion insertion, but it may not be suitable for alternative anode materials that possess high capacities but exhibit huge volume change upon cycling. Silicon is a promising anode material that has a theoretical capacity of 4200 mAh g 1, but suffer from a huge volume change of 400% during charge/discharge [7]. To maintain an integrated electrode structure with efficient ion and electron transport to silicon particles upon cycling, nanoscaled silicon in a conductive but spacious matrix has been demonstrated to be more desirable [8–12]. Particularly, electrodes with hierarchical architectures containing no binders are being widely developed for silicon anodes [8,12–14]. For example, silicon nanoparticle-loaded porous carbon nanofibers as anode materials show a discharge capacity of 1100 mAh g 1 [13]. Recently, Ji et al. [12] developed a Si/graphene/ultrathin-graphite foam (UGF) composite in which Si nanoparticles encapsulated in graphene are supported by the UGF. The composite with a Si loading of 1.5 mg cm 2 shows unprecedented gravimetric and volumetric capacities, which are much higher than the graphite anode used in commercial lithium-ion cells. Carbon fiber paper, a structural component in the gas diffusion layer in electrodes of low-temperature proton exchange membrane fuel cells, is a promising platform for studying electrochemically active materials since it has a conductive but porous structure. Carbon fiber paper has been used as a structural support in EES devices. For example, Yang et al. [15] utilized carbon fiber paper as a support to accommodate cobalt oxide nanonet as a high capacity electrode for pseudocapacitors. Some of these fiber papers that contain graphitic carbon are also potential anode materials in structural batteries [16]. However, carbon fiber paper has limited capacity because of the limited content of graphitic carbon in it. Herein, we report a hybrid anode consisting of a woven carbon paper and silicon nanoparticles. Silicon nanoparticles can be readily deposited onto the microsized carbon fibers within the carbon paper, which can provide efficient electron transport. In addition, the voids between carbon fibers allow fast ion transport and accommodate the volume expansion of silicon nanoparticles. Both carbon paper and silicon are electrochemically active, but the lithiation reaction is different, i.e., ion insertion into graphite vs. lithium alloying with silicon. As a demonstration of the proof-of-concept, the capacity of a carbon paper electrode can be increased from ca. 3 mAh to 3.25 mAh with a silicon loading of only 0.2 mg, corresponding to a specific capacity of over 1300 mAh g 1 of the silicon.
Experimental Chemicals Toray carbon paper (Fuel Cell Earth, thickness: 370 mm), silicon nanopowder (Sigma Aldrich, o100 nm particle size),
Y. Fu, A. Manthiram and liquid electrolyte (Novolyte, 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1 v/v)) were purchased and used as received.
Silicon suspension preparation and cell fabrication An appropriate amount of silicon nanopowder was weighed in an argon-filled glove box and added into liquid electrolyte (1 mL) to render a silicon suspension solution with concentrations of 5, 10, and 20 g L 1. The suspension was sealed in a glass vial and taken out of the glove box for sonication for 15 min and then transferred back into the glove box for cell fabrication. Toray carbon paper was cut into circular discs with a diameter of 1.2 cm (1.13 cm2) and mass of 17.670.1 mg. For assembling the cells, 40 mL of liquid electrolyte or silicon suspension solution was added into carbon paper electrodes to render a silicon loading of 0.2, 0.4, and 0.6 mg, followed by a Celgard 2400 separator, 20 mL of additional electrolyte, and lithium metal anode. Finally, the cells were crimped for electrochemical evaluation outside the glove box.
Characterization Morphological characterizations were carried out with a FEI Quanta 650 scanning electron microscope (SEM). The elemental mapping was performed with an energy-dispersive spectrometer (EDS) attached to the FEI Quanta 650. X-ray diffraction (XRD) data were collected on a Philips X-ray diffractometer equipped with CuKα radiation in a step of 0.021. Electrode samples were collected from cells that were cycled after 1st discharge. XRD patterns of freshly made silicon-carbon paper electrodes were also collected for a comparison. These XRD samples were covered by Kaptons films.
Electrochemical evaluation The cells for electrochemical evaluation consist of a silicon loading of 0.2 mg except those indicated. In the battery cycling measurements, cells were galvanostatically discharged to 0.01 V at C/40 rate (1C =167 mA g 1) initially with an Arbin battery test station, then cycled between 0.01 and 2 V at C/40 rate. Electrochemical impedance spectroscopy (EIS) data were collected with a computer interfaced HP 4192A LF Impedance Analyzer in the frequency range of 1 MHz–0.1 Hz with an applied voltage of 5 mV and Li foil as both counter and reference electrodes.
Results and discussions Commercial carbon paper is a binder-free, woven carbon fiber matrix containing graphitic carbon, as shown in Figure 1a. The fibers with a diameter of about 10 mm provide the paper excellent mechanical stability; therefore, no extra polymer binder or current collectors are needed when used as an electrode. In addition, the fibers are also highly striated as shown in the inset in Figure 1a, which could have good interfacial adhesion with silicon nanoparticles by a mechanical interlocking mechanism. The large
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5 µm
100 µm
20 µm
Si
5 µm
5 µm
Figure 1 SEM images of (a) carbon paper with a magnified image in the inset, (b) silicon–carbon paper, (c) magnified silicon–carbon paper, and (d) silicon mapping of the magnified silicon–carbon paper in (c).
porosity within the matrix provides ideal channels for ion transport and spaces for holding other nanoscaled active materials. Silicon nanoparticles were largely dispersed within the liquid electrolyte after the sonication. The concentration was maintained to be r 20 g L 1 to ensure good dispersity. The silicon nanoparticles were largely deposited onto the carbon matrix after the silicon suspension was added into the carbon paper electrode, as seen in Figure 1b. Carbon fibers are coated with a layer of agglomerated silicon nanoparticles, as revealed in the SEM image in Figure 1c and silicon elemental mapping in Figure 1d. The good adhesion between the carbon fiber and silicon nanoparticles seen in the SEM image could be partly due to the Van der Waals force between the oxygencontaining moieties (OH and COOH) on the surface of carbon fibers and the native oxide on the surface of silicon nanoparticles. Such good adhesion has also been observed between silicon nanowires and carbon textiles [17], which can ensure good electron transport and structural stability during cycling. The empty space on the carbon fibers has the potential to accommodate high loadings of silicon nanoparticles. The lithiation of graphite involves an ion insertion reaction, whereas silicon involves a displacement reaction to form alloys with lithium upon discharge. Figure 2a shows the XRD pattern of the pristine carbon paper (CP), silicon–carbon
paper (Si–CP) electrode, and their lithiated samples after 1st discharge. The carbon paper shows a strong peak at 27.21 and 55.21 corresponding, respectively, to the (002) and (004) planes [18]. After lithiation, the (002) plane peak shifts to a lower angle at 24.91, indicating an expansion of graphene layers within the carbon fiber from 3.28 to 3.56 Å due to the insertion of lithium ions. In addition, the discharged carbon paper becomes golden yellow which is a characteristic color of lithium intercalated graphite [19]. Similarly, the peak at 55.21 also shifts to lower angles. The peak density decreases slightly, meaning a minimum destructive interference because of the interlayer species [20]. The Si–CP electrode also undergoes a similar structural change upon lithiation. In addition to the graphite peaks, minor peaks of silicon nanoparticles can be noticed in the magnified XRD pattern in Figure 2a. Due to the low loading of silicon, no obvious peaks of silicon–lithium alloys can be observed even though the end of discharge is at 0.01 V [21]. Upon lithiation, the silicon nanoparticles on the carbon fibers form silicon–lithium alloys which cover largely the surface of the carbon fiber, while the carbon fiber still maintains its woven structure, as shown in Figure 2b. The magnified SEM image in Figure 2c clearly shows some silicon nanoparticles, maybe un-reacted, are embedded in the lithiated silicon compounds. The lithiated silicon layer on the carbon fiber is porous, which does not block the access
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Figure 2 (a) XRD pattern of carbon paper (CP), lithiated CP, silicon–carbon paper (Si–CP), and lithiated Si–CP, (b) SEM image of the lithiated Si–CP electrode, and (c) magnified SEM image of the lithiated Si–CP electrode.
Figure 3 (a) Voltage profile of the 1st discharge and following charge of the CP and Si–CP electrodes, with a silicon loading of 0.2 mg in the Si–CP electrode and the inset showing a magnified voltage profile and, and (b) Nyquist plots of half cells with the CP and Si–CP electrodes before and after 1st discharge.
of lithium ions to the carbon fibers. These results demonstrate the lithiation of carbon paper and silicon occurs simultaneously within the Si–CP electrode during discharge. The initial discharge and charge voltage profiles of the CP and Si–CP are shown in Figure 3a. Irreversible capacities at 0.5–0.8 V are observed, which are due to the solidelectrolyte interface (SEI) formation in the 1st discharge [4]. Due to the low surface area of the carbon paper, the
irreversible capacities are not significant. The major voltage profile of CP lies at a voltage below 0.1 V, showing three stages corresponding mainly to the formation of lithiated compounds LixC6 (0oxo1) similar to those of graphite anodes reported [6]. The Si–CP shows a voltage profile similar to that of the CP without any extra distinguishable voltage plateaus, but a higher capacity contributed from the silicon nanoparticles. Cyclic voltammogram is not shown here since the redox peaks of silicon are shadowed by those of graphite. Both CP and Si–CP electrodes exhibit 1st discharge and charge capacities of 43 mAh, indicating reversible lithiation and de-lithiation reactions. To understand the electrochemical behavior of the CP and Si–CP electrodes, electrochemical impedance analysis was performed. Figure 3b shows the Nyquist plots of the CP and Si–CP before and after the 1st discharge in the frequency range of 0.1 to 1 M Hz. Before cycling, the CP and Si–CP electrodes both show a depressed semicircle in the highmedium frequency region, which is mainly due to the charge-transfer resistance on the electrode surface, and a long tail in the low frequency region, which is the diffusioncontrolled impedance [22]. Obviously, the Si–CP electrode shows a much lower surface resistance (an order of magnitude lower) than the plain CP because of the increase in surface area and decrease in charge transfer resistance, both of which are due to the much smaller particle size (o100 nm) of Si nanoparticles compared to the carbon fiber (10 mm in diameter). After the 1st discharge, impedance spectra show only depressed semicircles without the diffusion impedance due to the low energy barrier for lithium-ion diffusion within the lithiated graphite and silicon. In addition, the charge transfer resistance becomes much smaller after the discharge. With silicon nanoparticles on the graphite fiber, the charge-transfer resistance is even smaller. These results indicate the presence of silicon nanoparticles on graphite can enhance charge transfer, which is beneficial for the electrochemical performance of the Si–CP electrodes. The cycling performance of the CP and Si–CP electrodes at C/40 is shown in Figure 4a. The cycle rate is limited by the lithiation/delithiation of carbon fibers because the
Hybrid anode for Li-ion batteries
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Figure 4 (a) Cycling performance and (b) specific capacities of the CP and Si–CP electrodes at a rate of C/40 with a silicon loading of 0.2 mg in the Si–CP electrode, (c) cycling performance of the Si–CP electrodes with silicon loadings of 0.2, 0.4, and 0.6 mg at a rate of C/40, and (d) specific capacities of silicon within the three Si–CP electrodes.
carbon fiber is too thick. The 1st discharge capacities of the CP and Si–CP electrodes are, respectively, 3.42 mAh and 3.61 mAh. Significant capacity losses of 0.31 mAh and 0.25 mAh occur after the 1st discharge, respectively, for the CP and Si-CP electrodes, which are mainly due to the SEI layer formation. The presence of silicon nanoparticles on the surface of carbon fibers increases the surface area, which normally increases the irreversible capacity loss due to the SEI layer formation. However, the Si–CP electrode shows a much lower capacity loss than the CP electrode. The alloying of silicon–lithium along with the significant volume expansion protects other silicon nanoparticles and carbon fiber from further SEI layer formation, which is illustrated in Figure 2c. After the first few cycles, the capacities of CP and Si–CP electrodes are maintained, respectively, at approximate 2.94 mAh and 3.22 mAh. The stabilized overall capacity of the Si–CP electrode is about 10% higher than that of the CP electrode, which is contributed from the silicon nanoparticles. The Coulombic efficiencies for both electrodes are close to 100% over 50 cycles, indicating good cycling reversibility and stable SEI layers formed on both electrodes. The specific capacities of CP and silicon in the Si–CP electrode are shown in Figure 4b. The latter were obtained by subtracting the overall capacities of the Si–CP electrode by those of the CP electrode and then dividing by the mass of Si. The CP shows an average capacity of 167 mAh g 1, which is much lower than that of graphite (i.e., 372 mAh g 1) because not all the carbon in the CP is made of graphite, maybe a disordered core surrounded by a graphitic sheath [16]. The average capacity of silicon is about 1300 mAh g 1, indicating a relatively high utilization of silicon in the lithiation reaction. It is lower than the theoretical capacity of silicon because of the un-reacted silicon nanoparticles seen in the SEM image in Figure 2c after the 1st discharge.
The electrochemical performance of Si–CP electrodes with different Si loadings (0.2, 0.4, and 0.6 mg) was evaluated, as shown in Figure 4c. After similar capacity decreases in the first cycles, the capacities of these Si–CP electrodes stabilize after 10 cycles. Obviously, the higher Si loading, the higher the overall capacity obtained. The capacity of Si–CP with a silicon loading of 0.6 mg is about 3.37 mAh, which is approximately 15% more than that of the CP electrode. The specific capacities of silicon in the 10th cycles are, respectively, 1190, 804, 650 mAh g 1 for the silicon loadings of 0.2, 0.4, and 0.6 mg, as shown in Figure 4d. The low capacities of silicon with high loadings are due to the uneven distributions of silicon nanoparticles on the carbon fibers. Apparently, the utilization of silicon nanoparticles can be improved with better dispersion of silicon suspension in the liquid electrolyte and an improved deposition process.
Conclusion In summary, we have demonstrated a simple approach to develop a hybrid anode consisting of a graphitic carbon paper and silicon nanoparticles. The carbon paper and silicon undergo lithiation and de-lithiation simultaneously upon cycling, showing specific capacities of 167 mAh g 1 and 1300 mAh g 1, respectively, with a silicon loading of 0.2 mg. The Si–CP electrode shows a smaller irreversible capacity loss in the 1st cycle than the CP electrode. The silicon loading can be increased, but the specific capacities of silicon decrease due to the uneven deposition of silicon in the Si–CP electrode. The results shown here demonstrate an efficient way to utilize an ion insertion anode and silicon in a hybrid electrode configuration to increase the overall capacity. With further improvement in the distribution of
1112 silicon nanoparticles within the silicon–graphite electrode, this method has the potential to significantly increase the capacity of graphite anode.
Acknowledgments This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award no. DE-SC0005397.
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Y. Fu, A. Manthiram [17] B. Liu, X. Wang, H. Chen, Z. Wang, D. Chen, Y.-B. Cheng, C. Zhou, G. Shen, Scientific Reports 3 (2013) 1622. [18] J.Y. Howe, C.J. Rawn, L.E. Jones, H. Ow, Powder Diffraction 18 (2003) 150–154. [19] T. Ohzuku, Y. Iwakoshi, K. Sawai, Journal of the Electrochemical Society 140 (1993) 2490–2498. [20] D.A. Stevens, J.R. Dahn, Journal of the Electrochemical Society 148 (2001) A803–A811. [21] T.D. Hatchard, J.R. Dahn, Journal of the Electrochemical Society 151 (2004) A838–A842. [22] R. Ruffo, S.S. Hong, C.K. Chan, R.A. Huggins, Y. Cui, Journal of Physical Chemistry C 113 (2009) 11390–11398. Yongzhu Fu is currently working as a Research Associate in the Texas Materials Institute at the University of Texas at Austin (UT-Austin). He obtained his B.E. (2000) and M.S. (2003) in Chemical Engineering from Tsinghua University and Dalian Institute of Chemical Physics, respectively, in China, and Ph.D. in Materials Science and Engineering from the University of Texas at Austin in 2007. He was a Chemist Postdoctoral Fellow at Lawrence Berkeley National Laboratory and Research Scientist at Lynntech, Inc. before joining UT-Austin. His current research is focused on new materials for rechargeable lithium batteries and fuel cells. Arumugam Manthiram is a Professor and holder of the Joe C. Walter Chair in Engineering in the Materials Science and Engineering Graduate Program and Department of Mechanical Engineering at The University of Texas at Austin. He is also the Director of the Texas Materials Institute and the Materials Science and Engineering Program. His research interests are in the area of materials for rechargeable batteries, fuel cells, and solar cells, including novel synthesis approaches for nanomaterials and nanocomposites. He has authored around 500 publications including more than 400 journal articles. See www.me.utexas.edu/manthiram for further details.