Quaternary Ammonium Ionic Liquid Electrolyte for a Silicon Nanowire

Sep 30, 2011 - the silicon. However, cycle life greater than a few hundred cycles has yet to ... anodes in these electrolytes is good, their long-term...
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Quaternary Ammonium Ionic Liquid Electrolyte for a Silicon Nanowire-Based Lithium Ion Battery Vidhya Chakrapani, Florencia Rusli, Micheal A. Filler, and Paul A. Kohl* School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ABSTRACT: The performance of a silicon nanowire battery anode was investigated in electrolytes consisting of butyl-trimethyl ammonium bis(trifluoromethylsulfonyl)imide (QATFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The lithiation capacity of silicon nanowires in 1 M LiTFSI/QATFSI was low due to the lack of a stable solid electrolyte interface (SEI) layer. The addition of 10 wt % propylene carbonate to the ionic liquid electrolyte promoted the formation of a stable SEI layer and resulted in substantially higher lithiation capacity. Galvanostatic cycling at a rate of C/20 for silicon halfcells and Si/LiCoO2 full cells showed good performance with a discharge capacity of ∼2000 mAh/g and Coulombic efficiency of 97% after 50 cycles. The effect of electrolyte additives, including ethylene carbonate and vinylene carbonate, was investigated by chronopotentiometry and electrochemical impedance spectroscopy. Results are discussed in terms of the SEI layer and its influence on battery capacity as well as cycle life.

’ INTRODUCTION Nanostructured silicon shows promise as an anode for high density Li ion batteries due to its demonstrated lithiation capacity of ∼3500 mAh/g.1 Although the lithium silicon system has a high capacity, there are several issues that limit its utilization as a battery anode. In particular, structural degradation caused by the large volume expansion of the silicon (>400%) during lithiation leads to low cycle life. The use of silicon nanowires allows for radial strain relaxation, thus preventing mechanical destruction of the silicon. However, cycle life greater than a few hundred cycles has yet to be demonstrated due, in part, to the continual formation and degradation of the solid electrolyte interface (SEI) during cycling. Improvements in the battery life require optimization of the electrode structure and an appropriate choice of electrolyte. Commonly used electrolytes are based on mixtures of carbonates such as ethylene carbonate (EC) or dimethyl carbonate (DMC) with LiPF6. Although the performance of carbon anodes in these electrolytes is good, their long-term stability and compatibility with a silicon anode has yet to be demonstrated. Furthermore, the use of an organic electrolyte in a battery poses a flammability concern. The organic electrolyte also limits the operating temperature range. Ionic liquids (ILs) are of interest due to their wide electrochemical potential window, low volatility, low flammability, and high thermal stability. However, the ionic conductivity and viscosity of ILs are modest compared to organic solvents, which can result in low capacity and cycle life. There have been several investigations of IL electrolytes with carbon and lithium metal anodes.2 4 A review by Lewandowski and Swiderska-Mocek provides a comprehensive summary of various ILs as well as the additives that have been investigated.5 In particular, a number of reports have studied the behavior of ILs with imidazolium and pyrrolidinium cations with bis-fluorosulfonylimide (FSI) or bis-trifluoromethanesulfonyl imide (TFSI) anions for carbon anodes.6 9 r 2011 American Chemical Society

Relative to other ILs, these exhibit high ionic conductivity and low viscosity. However, imidazolium-based ILs have limited electrochemical stability at high negative potentials, where severe electrolyte degradation occurs.4 The TFSI anion, on the other hand, intercalates into graphene layers and renders the electrode surface inactive for lithium intercalation.10 In both cases, the addition of an SEI-forming additive, such as vinylene carbonate (VC) or chloroethylene carbonate (Cl-EC), to the electrolyte suppresses electrolyte decomposition and allows reversible lithium ion intercalation into graphite.11,12 This has resulted in limited success with carbon electrode capacities approaching theoretical values. However, optimization of the electrolyte composition to extend the battery life beyond ∼100 cycles remains challenging. There have been very few reports on the use of silicon anodes in IL electrolytes.13 16 Baranchugov et al.14 reported a capacity of 3000 mAh/g and cycle life of 35 cycles with a piperidiniumbased IL for amorphous thin film Si anodes. Importantly, the irreversible capacity loss for the first cycle was 17%. Nguyen et al.15 showed a capacity of 1500 mAh/g for 200 cycles with a Si Cu amorphous thin films in a pyrrolidium-based IL with 29.5% irreversible capacity loss for the first cycle. Although these capacities are promising, SEI formation and degradation leads to significant electrolyte consumption during the initial cycles. In this study, we report the lithiation performance of silicon nanowire anodes in a quaternary ammonium TFSI IL electrolyte. Nanowires exhibit a stable capacity of 2000 mAh/g at 50 cycles as well as a high first cycle efficiency of 91.2%. The results are discussed in terms of the nature of the SEI layer and its impact on the long-term stability in the IL. Received: August 8, 2011 Revised: September 19, 2011 Published: September 30, 2011 22048

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Figure 1. (A) Top-down SEM image of Si nanowires grown on stainless substrates. (B) Cross sectional SEM image of the anode showing randomly oriented nanowire of several 100 μm length.

Figure 2. Representative galvanostatic charge discharge profiles of silicon nanowire anodes at a rate of C/20 in (A) 1 M LiTFSI/QATFSI IL electrolyte and (B) 1 M LiPF6 in EC/DMC organic electrolyte.

’ EXPERIMENTAL SECTION Nominally undoped silicon nanowires were grown by chemical vapor deposition using 5 nm Au catalyst dots on stainless steel (type 304, McMaster Carr) current collectors at 550 °C and a total pressure of 15 Torr pressure for 20 min. The partial pressure of SiH4 and H2 was 1.4 Torr and 13.6 Torr, respectively. Before growth, stainless steel substrates were cleaned using a UHV cleaning procedure, which begins with successive rinsing in soapy and hot distilled water. Substrates were then soaked in trichloroethylene for 30 min, sonicated in methanol for 10 min, and subsequently dried in an oven at 100 °C overnight. Au catalyst was deposited on the substrates via e-beam evaporation at 10 7 Torr. As-grown nanowires were immediately transferred to an Arfilled glovebox to minimize exposure to air. No conductive agents or binders were used for electrode fabrication. The electrode mass was measured using a Metler microbalance before testing. Electrochemical half-cell testing was performed in a split, flat-cell (MTI Corp.) using metallic lithium foil as the counter and reference electrode and Celgard as the separator. One molar LiPF6 in a 1:1 mixture of EC and DMC was used as the organic electrolyte for control experiments. The IL electrolyte was prepared by mixing 1.0 M lithium trifluoromethanesulfonyl imide (LiTFSI) in butyl-trimethyl ammonium trifluoromethanesulfonyl imide (QA-TFSI). Propylene carbonate (PC), VC, and EC were investigated as the SEI promoters. All chemicals were purchased from Aldrich. Full cell battery testing of the silicon nanowires in the IL electrolyte was performed in a 2032 coin cell with LiCoO2 (MTI Corp.) as the cathode material. To ensure the anode was the capacity-limiting electrode for coin cell studies, the weight of cathode was about 60 times the weight of the silicon anode. Electrochemical impedance spectroscopy (EIS) was performed using a PARSTAT 2263 (Princeton Applied Research)

potentiostat in the frequency range from 25 mHz to 500 kHz at an AC amplitude of 5 mV. Galvanostatic cycling of the cells was performed using an Arbin, Inc. battery tester. All electrochemical measurements were done in an Ar filled glovebox, whose H2O content was maintained below 0.1 ppm. After electrochemical testing, silicon anodes were soaked overnight in neat DMC to remove excess salt and high boiling solvent. Scanning electron microscopy (SEM), using a Zeiss Ultra60 field emission microscope, was used to examine silicon nanowire morphology.

’ RESULTS AND DISCUSSION Silicon nanowires grown on stainless steel substrates consisted of a mat of randomly oriented nanowires. Figure 1A,B shows the top-down and cross-sectional SEM images of the electrode, respectively. Nanowires exhibit diameters between 80 and 100 nm and lengths of several hundred micrometers. This spaghetti-like nanoarchitecture is perhaps better suited for battery applications than vertically oriented, epitaxial nanowires arrays. Randomly coiled nanowires create an interconnected network for electron transport and mitigate the severe capacity loss that would otherwise result from loss of electrical contact with the substrate during lithiation. Furthermore, the nanowires are less likely to punch through the thin separator. Figure 2A shows the representative chronopotentiometric lithiation of the silicon nanowires at a C/20 rate in 1 M LiTFSI in quaternary ammonium TFSI (QATFSI) IL. The first charge (lithiation) and discharge (delithiation) capacities were 172 mAh/g and 156 mAh/g, respectively. Despite this low capacity, a 90.7% Coulombic efficiency indicates that both silicon and lithium were stable in the QATFSI electrolyte. We attribute the low first cycle capacity to the initial absence of an SEI layer. Ammonium-based ILs are known for their wide electrochemical potential window (>5 V). When used as electrolytes in Li-ion 22049

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Figure 3. (A) Galvanostatic charge discharge profile of silicon nanowire anodes at C/20 rate in 1 M LiTFSI/QATFSI containing 10 wt % PC. Inset shows the cyclic voltammogram at a scan rate of 1 mV/s measured after the 20th cycle. (B) Comparison of the Nyquist plot of Si nanowire anode at 1 mV vs Li/Li+ after the 1st charge in IL electrolyte with and without PC.

batteries, they reduce at potentials near 0.5 V vs Li+/Li,17 which is more negative than the lithiation potential of the silicon anode. Hence, ILs are poor SEI forming electrolytes. The poor conductivity of the IL also contributed to the low capacity when the electrode was charged to a specific voltage due to the potential drop in the IL. When the C-rate for the charging was lowered, the apparent capacity was slightly higher because the potential drop across the electrolyte was lower. For reference, Figure 2B shows similar data collected for silicon nanowires in organic electrolyte. Charge and discharge capacities were 3029 and 2641 mAh/g, respectively, which yields a columbic efficiency of 87%. To promote the formation of a stable SEI layer in IL electrolyte, 10 wt % PC was used as an additive. PC is known to decompose at 0.75 V versus Li+/Li on graphite surfaces.18 This reduction helps form an SEI layer during the initial cycle. Alternatively, an SEI forming agent can be chemically grafted into the IL cation or anion chain that would preclude the need for an additive, but is beyond the scope of this work. The inset in Figure 3A shows a typical cyclic voltammogram of the silicon electrode after the 20th lithiathion/delithiation cycle measured at a scan rate of 1 mV/s. The rapid increase in the cathodic current at potentials negative of ∼0.35 V is attributed to the progressive formation of various amorphous LixSi phases.19 An anodic peak was observed near 0.7 V during delithiation, which corresponds to dealloying of the lithium rich phase and formation of amorphous silicon. As shown in Figure 3A, silicon nanowire electrodes exhibit far better lithiation and delithiation performance when PC is added. Galvanostatic cycling at a current density of 80 μA/cm2 yields first cycle charge and discharge capacities of 2014 mAh/g and 1836 mAh/g, respectively. The first cycle columbic efficiency was 91.2% and subsequently increased to 97%, indicating that only a

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small degree of irreversible capacity loss occurs. This suggests that consumption of the electrolyte or PC during SEI formation or intercalation of TFSI ions into silicon, as seen with graphite,11 is not prevalent in the silicon nanowire system. Irreversible capacity loss with silicon anodes is reported to occur through other reactions such as the reaction of lithium ions with the surface oxygen dangling bonds,20 surface degradation and subsequent activation of the silicon surface during lithiation,19 and possible reaction of the electrolyte with the native SiO2 layer.21 The first cycle efficiency for the IL electrolyte is better than that recorded with organic electrolyte (87.2%, see Figure 2B) as well as literature values for planar silicon anodes.1,14 EIS was used to study the changes in the electrode/electrolyte interface during lithiation and delithiation. Figure 3B shows the Nyquist plot for the silicon nanowire electrode after the first charge in the IL electrolyte, with and without 10 wt % PC. Both impedance spectra consist of a depressed semicircle, and a straight line at low frequencies. The interface was modeled using as a simple equivalent circuit consisting of bulk resistance, Rs, in series with the parallel charge transfer resistance, Rct, and capacitance. The high frequency x-intercept includes the series solution resistance Rs, electrical contact resistance, and ohmic resistance along the length of the nanowires.22 The difference between the extrapolated x-axis values of the semicircle (diameter of the semicircle) corresponds to the charge transfer resistance, Rct, which includes the impedance associated with the formation of a passivating SEI layer. The straight line at low frequency represents lithium ion diffusion resistance in the electrode. The negative Zim value seen at very high frequency is likely due to the inductive behavior of the battery and arises from the porous nature of the electrode.22 Detailed modeling of the interface is difficult due to the complex surface area and pore volume of the silicon nanowire matrix, and is beyond the scope of this paper. Comparison between the two spectra in Figure 3B reveal that the total impedance of the cell decreases dramatically with the addition of an SEI promoter (i.e., PC) to IL electrolyte. Both the series resistance (Rs) and the charge transfer resistance (Rct) decreased with the addition of PC. The decrease in the series resistance, Rs, likely results from the decrease in solution resistance.23 The addition of a low boiling solvent such as PC not only decreases the dynamic viscosity of the IL electrolyte but also increases the ionic conductivity. The decrease in electrolyte viscosity improves diffusion of ions through the porous nanowire mat and also promotes electrolyte wetting. Formation of an electrically nonconducting SEI layer during charging raises the cell impedance slightly. However, its formation promotes better lithiation of the silicon, which ultimately raises the electrical conductivity and electrode capacity. The net effect is a decrease of the cell impedance with the addition of PC. In addition to PC, the SEI forming properties of EC and VC were also tested. Figure 4A summarizes the measured discharge capacity for the first few cycles in each case. The first cycle discharge capacities of 10 wt % each of EC, PC, and VC in LiTFSI/QATFSI electrolyte were 1713, 1836, and 1884 mAh/g, respectively. This corresponds to Coulombic efficiencies of 85.2%, 91.2%, and 91.6%, respectively. These similar capacity values suggest that all three additives are capable of forming an SEI layer; however, PC and VC were more effective than EC for the formation of an SEI film on the silicon surface. The cycling performance dramatically diverged during subsequent cycling. During the third cycle, the discharge capacities and Coulombic efficiencies were 1231 mAh/g and 92.5% for EC, 1750 mAh/g 22050

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Figure 4. (A) Specific capacity versus cycle number of a Si nanowire anode in 1 M LiTFSI/QATFSI IL electrolytes containing various organic additives. (B) Nyquist plot of the anode at 1 mV vs Li+/Li in 1 M LiTFSI/QATFSI containing 10 wt % PC or VC.

and 96.6% for PC and 1008 mAh/g and 94.6% for VC, respectively. Both EC and VC showed capacity fades in excess of 30% after just three cycles. On the other hand, PC discharge capacities remained steady with a 2% capacity fade after six cycles. These data indicate that the reduction of PC leads to a more compact and stable SEI film relative to EC or VC in this IL. To evaluate the properties of the SEI layer and its influence on long-term battery capacity, EIS was performed after the cell was charged to 1 mV vs Li+/Li during the first cycle. The corresponding Nyquist plot is shown in Figure 4(B) for electrolytes containing 10 wt % PC or VC. Little change was observed in the bulk electrolyte resistance, as shown by the value of Rs (∼30 Ω cm2 in both cases). However, Rct for the electrolyte containing VC was nearly double that of the PC-containing electrolyte (∼250 Ω cm2). The higher impedance observed for the electrode with VC likely arises from the increased polarization resistance common for thicker SEI layers. This is desirable for nanostructured silicon because thicker and more compact SEI layers can better accommodate the nearly 400% volumetric expansion that occurs during complete lithiation. However, if the SEI layer is too thick, it can also block the Li+ charge transfer process, which can lead to capacity loss. This conclusion is consistent with the Coulombic efficiencies and capacity values shown in Figure 4A. It is not clear why PC forms the best SEI layer among the additives that were tested. EC and VC are well-known SEI promoting additives used in commercial organic electrolytes. Furthermore, all the additives share nearly similar dielectric and material properties. These results are consistent with the observations of Zheng et al.18 who compared the performance of EC and PC in trimethylhexyl ammonium TFSI IL at a natural graphite anode. Although the exact mechanism of SEI formation is not known, the studies show the importance of the correct additive choice for achieving long and stable cycle life.

Figure 5. (A) Cycling performance of Si nanowire anode in 1 M LiTFSI/QATFSI IL electrolyte containing 10 wt % PC. The electrode was cycled at a C/20 rate between 0.001 and 2.0 V vs Li+/Li. (B) Charge transfer resistance, Rct, estimated from the electrochemical impedance measurement of Si NW anode at 1 mV vs Li+/Li after various charge discharge cycles.

The long-term galvanostatic cycling of the silicon nanowire in a half-cell configuration was performed at C/20 in the potential range of 0.001 2.0 V. The results are shown in Figure 5A. The silicon nanowires exhibited stable discharge capacity even up to 50 cycles. The Coulombic efficiency at the end of 50 cycles was 93%, indicating repeatable lithiation and delithiation in the IL. It was observed that both charge and discharge capacity of the battery first increases until the 12th cycle, after which the capacity fades with the number of cycles. By contrast, silicon nanowire anodes immersed in organic electrolyte show a steady drop in both the capacity and charge transfer resistance with cycling.24 To understand these trends, the impedance of the half cell was measured at the end of each charge cycle at a potential of 1 mV vs Li+/Li in the IL electrolyte. The estimated charge transfer resistance of the silicon anode as a function of cycle number is shown in Figure 5B. The Rct for the silicon nanowires first increased with cycle number until the 12th cycle, after which it decreased with cycling. The increase in Rct in the charged state is commonly associated with a thicker SEI layer. Lee et al.25 showed that Rct for silicon in organic electrolyte reaches a minimum value at an intermediate state-of-charge followed by an increase as the SEI layer gets thicker at more negative potentials when the stateof-charge increased. In the IL electrolyte, the changes in Rct suggest that the SEI layer is continuously formed or gets compacted in the initial cycles. The Coulombic efficiency steadily increased from 91% seen for the first cycle to 98% in the 10th cycle. Although the resistance increased, a more stable and compact SEI layer is better able to facilitate Li+ insertion 22051

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Figure 6. (A) SEM image of the nanowire electrode after 50 cycles in IL electrolyte. (B) Comparison of cycling performance of Si nanowire anode in 1 M LiPF6 in 1:1 EC/DMC organic electrolyte and 1 M LiTFSI/QATFSI IL electrolyte containing 10 wt % PC. Shown here is the discharge capacity normalized to the value observed for the first cycle.

into the silicon lattice as measured by higher capacity with cycle number. To examine the structural integrity of silicon nanowires during cycling, the electrode was analyzed using SEM after 50 cycles. As shown in Figure 6A, the nanowires clearly retain their morphology, but appear more coiled than the as-grown wires. This may be due to the irreversible crystalline to amorphous phase transition that occurs during lithiation. This was confirmed using X-ray diffraction (not shown) that showed that the nanowires are mostly amorphous after the first charge cycle. Further, this result is consistent with the observation of Chan et al.26 For comparison, a silicon nanowire anode was cycled in the LiPF6 organic electrolyte. The extent of capacity fade in the two electrolytes is compared in Figure 6B. The graph shows the normalized discharge capacity of the battery with increasing cycle number. As can be seen, the capacity fade of silicon nanowires in the organic electrolyte after 20 cycles was 19.7%. In comparison, silicon nanowires showed only 5.5% capacity fade in the IL electrolyte. Note that the actual capacity of the Si nanowires in organic electrolyte are generally much higher (see Figure 2B) than that observed in IL. Finally, the compatibility of QATFSI IL electrolyte with a LiCoO2 cathode was tested by constructing a Si NW/LiTFSIQATFSI/LiCoO2 full cell battery. The cell was cycled in the potential range of 2.5 4.3 V at a rate of C/20 with respect to the capacity of the silicon nanowire anode. The results are shown in Figure 7(A). The cell showed a first cycle charge and discharge capacity of 3120 and 2111 mAh/g, respectively, which corresponds to a Coulombic efficiency of 68%. The high discharge capacity indicates that LiCoO2 is stable in the IL. The high surface area of the silicon nanowire anode is potentially valuable in situations where short, high-power discharges are required, such as in wireless sensor devices. To explore this possibility, Si NW/LiTFSI-QATFSI/LiCoO2 full cell battery was cycled in a high current, short pulse discharge mode. The battery was

Figure 7. (A) Galvanostatic first cycle charge discharge profile of Si/ LiCoO2 full cell in 1 M LiTFSI/QATFSI + 10 wt % PC at a current density of 120 μA/cm2 corresponding to a C/20 rate. (B) Performance of Si/LiCoO2 full cell in IL electrolyte as a function of test time at high current pulse discharge. The battery was discharged at 15 mA for 15 ms and charged at constant potential. (C) Si/LiCoO2 full cell battery life at the end of 118 586 cycles.

initially galvanostatically charged at a C/20 rate to a potential of 4.3 V, and subsequently discharged at a constant current density of 10 A/g (∼3C rating) for 15 ms followed by charging at constant 4.3 V for 10 s. The results for the first 30 000 cycles are shown in Figure 7B. As can be seen from the voltage profile, very little drop in the battery voltage and discharge capacity was seen even after 30 000 cycles. Figure 7C shows the battery performance of the last 17 000 cycles before it degraded. It is important to emphasize that the silicon nanowire/IL battery was successfully cycled 118 586 times.

’ CONCLUSIONS We report here the first use of a quaternary ammonium-based IL as an electrolyte for silicon nanowire-based Li ion batteries. The nanowire electrode was stable in the IL and exhibited discharge capacities as large as 2000 mAh/g, which is 5 times more than that of graphite anodes, with a Coulombic efficiency of 97% after 50 cycles. The full cell batteries were successfully cycled 118 586 times at a 3C rate in short discharge pulses. The high discharge capacity, first-cycle Coulombic efficiency (higher than 22052

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(26) Chan, C. K.; Ruffo, R.; Hong, S. S.; Huggins, R. A.; Cui, Y. J. Power Sources 2009, 189, 34.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors would like to thank Prof. Jud Ready for his assistance with the coin cell set up. This work was funded by the Semiconductor Research Corporation though the Interconnect Focus Center, one of six Focus Center Research Programs. ’ REFERENCES (1) Chan, C. K.; Peng, H.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. Nat. Nanotechnol. 2008, 3, 31. (2) Appetecchi, G. B.; Scaccia, S.; Tizzani, C.; Alessandrini, F.; Passerini, S. J. Electrochem. Soc. 2006, 153, A1685. (3) Diaw, M.; Chagnes, A.; Carre, B.; Willmann, P.; Lemordant, D. J. Power Sources 2005, 146, 682. (4) Ishikawa, M.; Sugimoto, T.; Kikuta, M.; Ishiko, E.; Kono, M. J. Power Sources 2006, 162, 658. (5) Lewandowski, A.; Swiderska-Mocek, A. J. Power Sources 2009, 194, 601. (6) Hassoun, J.; Fernicola, A.; Navarra, M. A.; Panero, S.; Scrosati, B. J. Power Sources 2010, 195, 574. (7) Nadherna, M.; Reiter, J.; Moskon, J.; Dominko, R. J. Power Sources 2011, 196, 7700. (8) Seki, S.; Ohno, Y.; Kobayashi, Y.; Miyashiro, H.; Usami, A.; Mita, Y.; Tokuda, H.; Watanabe, M.; Hayamizu, K.; Tsuzuki, S.; Hattori, M.; Terada, N. J. Electrochem. Soc. 2007, 154, A173. (9) Ui, K.; Minami, T.; Ishikawa, K.; Idemoto, Y.; Koura, N. J. Power Sources 2005, 146, 698. (10) Sutto, T. E.; Duncan, T. T.; Wong, T. C. Electrochim. Acta 2009, 54, 5648. (11) Zheng, H.; Jiang, K.; Abe, T.; Ogumi, Z. Carbon 2006, 44, 203. (12) Katayama, Y.; Yukumoto, M.; Miura, T. Electrochem. Solid-State Lett. 2003, 6, A96. (13) Sugimoto, T.; Atsumi, Y.; Kono, M.; Kikuta, M.; Ishiko, E.; Yamagata, M.; Ishikawa, M. J. Power Sources 2010, 195, 6153. (14) Baranchugov, V.; Markevich, E.; Pollak, E.; Salitra, G.; Aurbach, D. Electrochem. Commun. 2007, 9, 796. (15) Nguyen, C. C.; Song, S.-W. Electrochem. Commun. 2010, 12, 1593. (16) Usui, H.; Yamamoto, Y.; Yoshiyama, K.; Itoh, T.; Sakaguchi, H. J. Power Sources 2011, 196, 3911. (17) Egashira, M.; Okada, S.; Yamaki, J.-i.; Dri, D. A.; Bonadies, F.; Scrosati, B. J. Power Sources 2004, 138, 240. (18) Zheng, H.; Liu, G.; Battaglia, V. J. Phys. Chem. C 2010, 114, 6182. (19) Green, M.; Fielder, E.; Scrosati, B.; Wachtler, M.; Moreno, J. S. Electrochem. Solid-State Lett. 2003, 6, A75. (20) Lee, K.-L.; Jung, J.-Y.; Lee, S.-W.; Moon, H.-S.; Park, J.-W. J. Power Sources 2004, 130, 241. (21) Graetz, J.; Ahn, C. C.; Yazami, R.; Fultz, B. Electrochem. SolidState Lett. 2003, 6, A194. (22) Laman, F. C.; Matsen, M. W.; Stiles, J. A. R. J. Electrochem. Soc. 1986, 133, 2441. (23) Seki, S.; Ohno, Y.; Kobayashi, Y.; Miyashiro, H.; Usami, A.; Mita, Y.; Tokuda, H.; Watanabe, M.; Hayamizu, K.; Tsuzuki, S.; Hattori, M.; Terada, N. J. Electrochem. Soc. 2007, 154, A173. (24) Ruffo, R.; Hong, S. S.; Chan, C. K.; Huggins, R. A.; Cui, Y. J. Phys. Chem. C 2009, 113, 11390. (25) Lee, Y. M.; Lee, J. Y.; Shim, H.-T.; Lee, J. K.; Park, J.-K. J. Electrochem. Soc. 2007, 154, A515. 22053

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