Ordered Mesoporous Carbon (OMC) Nanocomposites for

Cobalt-doped pyrochlore-structured iron fluoride as a highly stable cathode material for lithium-ion batteries. Ghulam Ali , Gul Rahman , Kyung Yoon C...
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FeF3/Ordered Mesoporous Carbon (OMC) Nanocomposites for Lithium Ion Batteries with Enhanced Electrochemical Performance Hyeyun Jung,† Jihyun Shin,† Changju Chae,‡ Jung Kyoo Lee,‡ and Jongsik Kim*,† †

Department of Chemistry and ‡Department of Chemical Engineering, Dong-A University, Busan 604-714, South Korea S Supporting Information *

ABSTRACT: FeF3 is of great interest as a potential candidate cathode material because of its low cost, abundance, environmental friendliness, and high theoretical capacity of about 237 mAh·g−1 in the voltage range of 2.0−4.5 V. However, FeF3 has drawbacks of poor cycling stability and rate performance because of its low intrinsic electrical conductivity and slow diffusion of lithium ions. These issues should be improved for the practical application of FeF3 in lithium-ion battery systems. In this study, FeF3/ordered mesoporous carbon (OMC) nanocomposites were synthesized by an incipient-wetness impregnation technique in a facile and scalable method. The tubular shaped OMC was utilized as both a conductive agent and a hard template for the formation of nanosized FeF3 particles. The FeF3/OMC nanocomposites showed enhanced capacity, cycling stability, and rate performance compared to bulk FeF3 in the voltage range of 2.0−4.5 V at room temperature. 1.5−4.5 V and 237 mAh·g−1 in the range of 2.0−4.5 V), high operating voltage, environmental friendliness, and low cost.8−14 Despite these merits, the practical application of this material in commercial rechargeable LIBs has been hindered by its poor capacity retention and rate capability. These drawbacks are caused by its low intrinsic electrical conductivity, which is attributed to the high ionic character of the Fe−F bond.15,16 Several approaches have been reported to overcome these issues through nanostructure fabrication, metal doping, and ball-milling with conductive agents such as carbon, MoS2, and V2O5.13,16−26 In particular, Badway et al. synthesized carbon metal fluoride nanocomposites (CMFNCs) by high-energy mechanical ball-milling, achieving a capacity of 216 mAh·g−1 in the voltage range of 2.8−3.5 V, but at a very low current density of 7.58 mA·g−1.13 The electrochemical properties of FeF3 were further improved through the formation of carbon nanotube (CNT)/FeF3 nanocomposites by ball-milling, resulting in a capacity of about 210 mAh·g−1 with an excellent cycling stability at a current density of 20 mA·g−1 as reported by Kim et al.21 However, the content of the active material in the

1. INTRODUCTION Lithium-ion batteries (LIBs) are effective energy storage systems for portable electronic devices, such as cell phones, laptop computers, and microchips.1−3 However, the demand for LIBs with enhanced reversible lithium-storage capacities is still growing for their widespread application in electric vehicles (EVs) and energy storage systems for renewable energy sources. The positive electrode material is a crucial factor in the improvement of the electrochemical performance of LIBs, including energy storage capacity, cycling stability, rate performance, and so on. Since LIBs were commercialized by Sony in 1991, LiCoO2 has been the most commonly used as a cathode material.4 However, because of the toxicity, high cost, and safety issues of LiCoO2, alternative cathode materials such as LiNi1/3Mn1/3Co1/3O2, LiMn2O4, and LiFePO4, among others, have been extensively examined for the past decade.5−7 Despite significant improvements in the cycling stabilities and rate capabilities of these materials, their insufficient theoretical capacities have limited the enhancement of reversible lithiumstorage capacities. Therefore, new cathode materials with higher theoretical capacities still need to be developed. Recently, iron trifluoride (FeF3) emerged as one of the most promising cathode materials because of its high theoretical specific capacity (about 712 mAh·g−1 in the voltage range of © XXXX American Chemical Society

Received: March 6, 2013 Revised: June 28, 2013

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Scheme 1. Schematic Illustration of the Preparation of FeF3/OMC Nanocomposite Samples (FeOMC11 and FeOMC13)

heated at a rate of 10 °C/min. The obtained yellow powder is designated as bulk FeF3. FeF3/OMC Nanocomposite. Ordered mesoporous carbon (OMC) with a tubular morphology was provided by Unicam Ltd., Kunsan, Korea. The OMC was prepared by replicating a hexagonally ordered mesoporous silica (OMS). Phenanthrene was used as a carbon source to increase the electrical conductivity of the OMC for use in LIBs.20,21 The detailed synthetic method can be found in the previous reports.29,30 FeF3/OMC nanocomposite samples were synthesized by a pore-filling wet-impregnation method. A proper amount of the freshly prepared FeF3 precursor (FeCl3/HF) solution (21.6 M) was added dropwise onto OMC, and the sample was then dried at 80 °C in an oven. This process was repeated several times until the desired amount of precursor solution had been impregnated into the pores of the OMC. The total volume of the impregnated FeF3 precursor was adjusted to be about 3.25 mL, which is equivalent to the pore volume of the OMC. The pore volume was estimated by Brunauer−Emmett−Teller (BET) measurements. The FeF3-impregnated OMC was dried at 80 °C for 12 h in a vacuum oven and then calcined at 200 °C for 36 h under an Ar flow (50 mL/min). The obtained sample was designated as FeOMC11. For comparison, another sample, FeOMC13, was prepared with different synthetic conditions. The amount of impregnated precursor solution was increased to 3 times that used for FeOMC11. In this case, the obtained FeF3-impregnated OMC was dried under a vacuum at 130 °C for 12 h and subsequently calcined at 300 °C for 2 h under an Ar flow (50 mL/min). 2.2. Characterization. X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Field-Emission Scanning Electron Microscopy (FESEM), BET (Brunauer−Emmett− Teller) Surface Area, Electrical Conductivity, and Elemental Analysis (EA). XRD data were collected for all samples on a Rigaku Ultima IV D/MAX X-ray diffractometer (Cu Kα radiation, λ = 1.5418 Å). Low-angle XRD patterns were obtained on Panalytical X’Pert PRO materials research diffractometer (MRD) using Cu Kα radiation (λ = 1.5418 Å). All the diffraction patterns were compared with those in the Joint Committee on Powder Diffraction Standards (JCPDS). TEM and SEM images were obtained on JEOL JEM-2010 and JEOL JSM-6700F microscopes, respectively. The BET surface areas of the samples were measured on a Micromeritics ASAP 2010 gas sorption analyzer using N2. The pore size distributions of the samples were estimated by the Barrett−Joyner−Halenda (BJH) method. The contents of carbon in the samples were determined using an element analyzer (Flash EA 1112). The electrical conductivities were measured by the four-point-probe method (Keithely 2400).

composites needs to be increased for practical applications of this material. In addition, the high-energy ball-milling used in these reports often induces significant structural defects and difficulty in controlling the particle size of FeF3. Liu et al. reported spherical FeF3/activated carbon microbead (ACMB) composites with a specific capacity of 179 mAh·g−1 at a rate of 0.1C, but the cycling stability was not satisfactory, with 1.1% fading rate per cycle.17 Li et al. recently reported that nanostructured single-walled carbon nanotube (SWNT)/ FeF3·0.33H2O synthesized in ionic liquids at a low temperature delivered a high discharge capacity of 220 mAh·g−1 at 0.1C in the wider range of 1.7−4.5 V.27 However, their synthetic method is not appropriate for industrial production. (Note that the charge−discharge process of FeF3·0.33H2O is attributed to solid-solution behavior,24 in contrast to that of ReO3-type FeF3, for which the charge−discharge process is based on multiple reaction stages and phase transformations, thus insertion and conversion reactions.10) In this study, we report a facile and scalable method to synthesize FeF3/ordered mesoporous carbon (OMC) nanocomposites using OMC as a hard template. The capacities, cycling stabilities, and rate performances of the FeF3/OMC nanocomposites were thoroughly compared with those of bulk FeF3. The FeF3/OMC nanocomposites were prepared by the incipient-wetness impregnation of an aqueous FeCl3/HF solution into the pores of the OMC used as a hard template, followed by drying and calcination under argon (Ar) (Scheme 1). During calcination, FeF3·3H2O was dehydrated to form FeF3. To the best of our knowledge, this is the first report to use the pore-filling wetness-impregnation technique to prepare FeF3/carbon matrix composite materials. To compare the electrochemical properties of the as-synthesized FeF3/OMC nanocomposites, bulk FeF3 was prepared following a previously reported method using FeCl3 and an excess amount of aqueous HF.21 The formation of crystalline FeF3 was monitored by measuring X-ray diffraction (XRD) patterns.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Bulk FeF3. For the preparation of bulk FeF3, 0.4 mL of 4.5 M FeCl3 solution (Aldrich) was added dropwise to 1.6 mL of HF aqueous solution (48%, Aldrich) in Teflon containers under stirring. (Warning: HF is highly corrosive. Avoid skin contact or inhalation.28) The resulting solution was stirred for 2 h to complete the reaction of FeCl3 with HF and heated to 80 °C until the water and unreacted HF had evaporated. The dried sample was further dried at 130 °C under a vacuum for 12 h and calcined at 300 °C for 2 h in a tube furnace under an Ar flow (50 mL/min). The furnace was B

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Electrochemical Measurements. The working electrodes were prepared by mixing bulk FeF3 or FeF3/OMC nanocomposites, carbon black (Super P Li carbon, Timcal) as a conductive additive, and poly(vinyl difluoride) (PVDF) as a binder at a weight ratio of 85:10:5 in N-methylpyrrolidone (NMP, Aldrich). The resulting slurry was applied to aluminum foil using a doctor blade and dried at 80 °C for 1 h. After this predrying, the electrodes were dried in a vacuum oven at 110 °C for 12 h. The electrodes were punched into disks with a diameter of 14 mm. Coin cells were assembled in an argonfilled glovebox. Lithium metal and Celgard 2400 polypropylene membrane were used as the counter electrode and separator, respectively. The electrolyte was 1.0 M LiPF6 in an ethylene carbonate (EC)/dimethyl carbonate (DMC) mixture (3:7 v/v) (Ukseung Ltd.). Galvanostatic charge/discharge measurements were carried out at different current densities in the voltage ranges of 4.5−2.0 V at 25 °C on a WonATech WBCS3000 battery test system. (Herein, 1C refers to one lithium per formula unit of FeF3 discharged/charged in 1 h.) The cells were discharged using constant-current (CC) mode to 2.0 V and then charged in constant-current/constant-voltage (CCCV) mode to 4.5 V. After the cells had been charged to 4.5 V, an additional voltage holding step was performed at 4.5 V with a 10% cutoff of the original current value. The specific capacities of the samples were calculated on the basis of the total mass of FeF3/OMC nanocomposite. The Coulombic efficiency was calculated from the first charge capacity to the 30th charge capacity as the ratio between charge capacity of cycle N and the discharge capacity of cycle N + 1.14 Cyclic voltammetry (CV) experiments were performed using a WonATech WBCS3000 battery test system at a scan rate of 0.5 mV/s.

process of calcination in Figure 1b. The average particle sizes of bulk FeF3 and samples FeOMC13 and FeOMC11 were calculated with the (012) peak to be about 25.7, 22.6, and 12.1 nm, respectively, by the Debye−Scherrer equation using the data in Figure 1a−c. These values are consistent with those determined from TEM images (see Figure 4b−f later in this section). (Note that, when FeOMC11 precursor was annealed at 300 °C to obtain more of the crystallized FeF3 phase, only the FeF2 phase was formed, probably through the reduction of Fe3+ to Fe2+ because of the increased amount of carbon matrix.) The BET analysis of the bare OMC showed a IV-type N2 adsorption−desorption isotherm with an H1-type hysteresis loop, as is typical of mesoporous materials (Figure 2a).29−31 The surface area and pore volume were estimated by the BET method to be about 1255 m2·g−1 and 1.43 cm3·g−1, respectively, which are higher than the values reported previously (Table 1).29−31 The pore diameter distribution calculated from the isotherm indicated that pores with a diameter of approximately 3.8 nm were predominant (Figure 2b). As FeF3 was loaded into the OMC, the BET surface areas and pore volumes of the FeF3/OMC nanocomposites decreased to about 283 m2·g−1 and 0.42 cm3·g−1, respectively, for FeOMC11 and about 142 m2·g−1 and 0.26 cm3·g−1, respectively, for FeOMC13 (Figure 2b and Table 1). These decreases indicate that FeF3 was successfully impregnated into the pores of the OMC. According to the SEM and TEM images (Figures 3a and 4a), the OMC had a tubular-shaped morphology with a pore diameter of about 3.8 nm. This value is consistent with that from the BET measurements. The as-synthesized bulk FeF3 had a morphology of rectangular shapes with lengths of about 2−6 μm and diameters of about 300−400 nm (see Figures 3b and 4b).17,18,32 However, the higher-magnification TEM images in Figure 4c show that the bulk FeF3 was composed of nanoparticles with sizes of about 25−30 nm. The SEM images of FeOMC11 and FeOMC13 (Figure 3c,d) indicate that the morphology of the bare OMC was mostly retained during the synthetic procedure of FeF3. For FeOMC13, small FeF3 particles anchored on the surface of the OMC were observed. The TEM images in Figure 4d clearly show that the FeF3 precursor was mainly crystallized to FeF3 on the surface of the OMC with its characteristic lattice fringes of (012). However, by adjusting the amount of FeF3 precursor solution to be equivalent to the pore volume of the OMC, the formation of FeF3 particles on the surface of the OMC was decreased significantly (Figure 4e). The high-magnification TEM image in Figure 4f shows the coexistence of FeF3 nanoparticles with sizes of about 4 and 10 nm, indicating that strong capillary forces mainly drove the absorption of the initial portion of the precursor solution into the channels of the mesoporous carbon, followed by the crystallization of FeF3. However, FeF3 particles also partially formed on the external surface of the OMC. The decreased particle size of FeOMC11, as observed in the XRD patterns and TEM images, can be attributed to the lower synthesis temperature and the confinement of the FeF3 precursor solution into the nanosized channels by capillary forces and the subsequent formation of FeF3 nanoparticles inside the mesopores. The low-angle XRD pattern of the OMC shows an intense (100) reflection and weaker (110) and (200) reflections, characteristic of the two-dimensional hexagonal mesostructure in Figure 5.29,30 The mesoscopic order of the OMC in FeOMC11 was fairly preserved, as shown in the low-angle XRD pattern. For FeOMC13, the intensities of the XRD peaks

3. RESULTS AND DISCUSSION 3.1. XRD, SEM, TEM, BET Surface Area, and EA. The synthesized FeF3 samples were characterized by XRD measurements. The powder XRD patterns of bulk FeF3 and FeOMC13 were consistent with those found in JCPDS file 33-0647 for FeF3 (Figure 1a,c). FeF2 was observed to be a minor impurity phase in both samples. On the other hand, sample FeOMC11 had FeF3 as a major phase and FeF3·0.33H2O as a minor phase because of the lower calcination temperature of 200 °C in the

Figure 1. XRD patterns of (a) bulk FeF3, (b) FeOMC11, and (c) FeOMC13. C

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Figure 2. (a) N2 adsorption/desorption isotherms and (b) BJH pore size distribution plots for bare OMC, FeOMC11, and FeOMC13.

ances of bulk FeF3 and the FeF3/OMC nanocomposites were estimated by galvanostatic charge and discharge measurements (Figure 6). Figure 6a shows the initial discharge and charge profiles of bulk FeF3, FeOMC13, and FeOMC11 at a current density of 0.1C in the voltage range of 2.0−4.5 V. Whereas the bulk FeF3 delivered a low initial discharge capacity of about 117 mAh·g−1, FeOMC13 and FeOMC11 had enhanced initial discharge capacities of 162−164 mAh·g−1. It is worth noting that FeOMC11 delivered a reversible capacity as high as about 178 mAh·g−1 during the second cycle. As shown in Figure 6b, the bulk sample exhibited a capacity of only 68 mAh·g−1 at the 30th cycle, with a poor capacity fading of about 42% (specific capacity fading rate of 1.4% per cycle) because of its poor insulating behavior, which are similar to the previously reported data.17−19 On the other hand, the FeF3/OMC nanocomposites showed better cycling stabilities than bulk FeF3. For example, FeOMC13 delivered a capacity of about 134 mAh·g−1 during the 30th cycle with a capacity fading of about 17.3% (specific capacity fading rate of 0.6% per cycle). FeOMC11 still discharged a capacity of about 150 mAh·g−1 at the 30th cycle, achieving a further improved capacity fading of 8.8%

Table 1. BET Surface Areas, Total Pore Volumes, and Pore Sizes of Bare OMC, FeOMC11, and FeOMC13 sample

SBET (m2·g−1)

VTOT (cm3·g−1)

pore size (nm)

bare OMC FeOMC11 FeOMC13

1255.33 283.36 141.95

1.43 0.42 0.26

3.62 3.55 3.58

decreased significantly, indicating that the mesostructure of the OMC was barely maintained during the impregnation process. These results suggest that the growth of FeF3 particles in a large quantity inside the channels of the OMC can induce the breakdown of the ordered mesostructure.33 The carbon contents of FeOMC13 and FeOMC11 were estimated to be about 10 and 20 wt %, respectively, by elemental analysis (EA) experiments. The electrical conductivities of bulk FeF3, FeOMC13, and FeOMC11 were measured to be about 2.53 × 10−7, 0.19, and 0.347 S/m, respectively, by four-point-probe measurements. 3.2. Electrochemical Performance of Bulk FeF3 and FeF3/OMC Nanocomposites. The electrochemical perform-

Figure 3. FESEM images of (a) OMC (ordered mesoporous carbon), (b) bulk FeF3, (c) FeOMC11, and (d) FeOMC13. D

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Figure 4. TEM images of (a) OMC (ordered mesoporous carbon), (b) bulk FeF3, (c) magnification of image b, (d) FeOMC13 (inset: magnification of image d), (e) FeOMC11, and (f) magnification of image e.

cycles for FeF3/V2O5 composites at 0.1C19). This indicates that FeOMC11 can deliver a higher reversible specific capacity during prolonged cycles. The Coulombic efficiency of bulk FeF3 was higher than about 96% during 30 cycles. However, FeOMC11 and FeOMC13 showed Coulombic efficiencies of about 86% and 93%, respectively, at the first cycle, which increased in the following cycles above about 96%. As shown in Figure 6a, FeOMC11 and FeOMC13 were overcharged in the first cycle. FeOMC11 showed a higher overcharge capacity than FeOMC13, mainly because of its larger content of OMC. Bare OMC showed a large overcharge capacity in the first cycle, probably because of undesirable side reactions such as electrolyte oxidation on the OMC (see Figure S1 of the Supporting Information). However, the overcharge capacity decreased significantly during the following cycles. The CV curves of bulk FeF3 and FeOMC11 were obtained in the voltage range of 2.0−4.5 V and are shown in Figure 7. The two curves exhibit similar shapes with a pair of reduction/ oxidation reversible peaks corresponding to the insertion/ extraction of lithium ions. These curves indicate that these samples have similar electrochemical reaction mechanisms. However, FeOMC11 had a significantly smaller difference between its cathodic and anodic peaks, ΔE, of 0.38 V, compared to that of bulk FeF3 (0.58 V). Thus, the polarization was decreased in FeOMC11 sample. In addition, FeOMC11 showed a higher current and larger area in the CV curve, which are directly related to the achieved capacity. These results show that the kinetics of lithium insertion/extraction was improved

Figure 5. Low-angle X-ray diffraction patterns of bare OMC, FeOMC11, and FeOMC13.

(0.3% fading per cycle). Although the initial capacity of FeOMC11 was significantly enhanced compared to that of bulk FeF3, it was still lower than or similar to those of ball-milled FeF3 with a carbon matrix or V2O5.17−19,21,32 However, the capacity retention of FeOMC11 (∼0.3% fading per cycle) was found to be superior to those in previous reports (e.g., capacity fading rates of 0.33% at 10 mA·g−1 after 20 cycles for annealed FeF3/acetylene black composite,32 0.7% at 0.1C after 50 cycles for FeF3·0.33H2O/CNT composites,17 and 0.41% after 30 E

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Figure 6. (a) Initial discharge/charging curves of bulk FeF3 (black solid line), FeOMC13 (red dashed line), and FeOMC11 (blue dashed line) and (b) cycling performances of bulk FeF3 (black solid squares), FeOMC13 (red solid circles), and FeOMC11 (blue solid triangles) in a voltage range of 2.0−4.5 V at 0.1C rate (herein, 1C = 237 mA·g−1). The Coulombic efficiencies of the samples are plotted with open symbols corresponding to those used for the discharge capacities.

Figure 7. CV curves of bulk FeF3 and FeOMC11.

Figure 8. Discharging rate performances of bulk FeF3 (solid squares), FeOMC13 (solid circles), and FeOMC11 (solid triangles) in the voltage range of 2.0−4.5 V as a function of current densities. 1C = 237 mA·g−1. Note that the discharge/charge experiments were performed at the same rate for each cycle.

through the formation of a nanocomposite with the OMC. They are well matched by the improvements in the cycling and rate performances of sample FeOMC11 shown in Figures 6 and 8, respectively. The rate performances of bulk FeF3 and FeF3/OMC nanocomposites at different current densities in the range of 0.1−10C are compared in Figure 8. As the current density increased, the discharge capacities of these samples decreased. However, the FeF3/OMC nanocomposites showed a significantly enhanced rate capability compared to bulk FeF3. The discharged capacity of bulk FeF3 was about 114 mAh·g−1 at 0.1C, 60 mAh·g−1 at 0.25C, 30 mAh·g−1 at 0.5C, and 12 mAh·g−1 at 1C for the first cycle. However, FeOMC13 exhibited values of 160, 131, 109, 87, 65, and 35 mAh·g−1 at rates of 0.1C, 0.25C, 0.5C, 1C, 2C, and 5C, respectively. FeOMC11 showed a further improved rate capability, delivering 165, 156, 143, 131, 117, 90, and 69 mAh·g−1 at rates of 0.1C, 0.25C, 0.5C, 1C, 2C, 5C, and 10C, respectively. Notably, at current rates of 0.5C and above, the discharge capacities of the FeF3/OMC nanocomposites were more than 3 times larger than the corresponding values for bulk FeF3. As the current rate increased to 1C, FeOMC11 still delivered a discharged capacity of about 131 mAh·g−1 (vs 87 mAh·g−1 for FeOMC13 and 12 mAh·g−1 for bulk FeF3). Furthermore, when the current density was decreased back to 0.1C, bulk FeF3, FeOMC13, and FeOMC11 regained 48%, 76%, and 89% of their original capacities, respectively. This indicates that

FeOMC11 had a higher structural stability and reversibility than the other samples. The improved electrochemical performances of FeOMC11 were first ascribed to the increased electrical conductivity due to the higher content of carbon matrix, compared to those of bulk FeF3 and FeOMC13. FeOMC11 had a higher ratio of FeF3 particles inside the mesopores to those on the surface compared to FeOMC13. Confinement of the FeF3 particles in nanosized spaces ensured intimate contact between FeF3 and the conducting carbon matrix (OMC), resulting in improved electron transport in FeOMC11. The smaller particle size of the FeF3 in sample FeOMC11 provided shorter diffusion paths for the lithium ions during insertion/extraction cycles. The carbon matrix can also play the role of a protective buffer layer to mitigate the local volume changes in the FeF3 structure and the undesirable surface reactions of FeF3 with electrolytes during repeated charge/discharge cycling. In addition, bulk FeF3 and FeOMC13 contain the FeF2 phase as a minor impurity, which is less active above 2 V.32 However, FeOMC11 was synthesized at a relatively lower temperature, forming FeF3·0.33H2O instead of FeF2. Several previous reports have already shown that the FeF3·0.33H2O phase is electrochemically active in the F

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voltage range of 2.0−4.5 V.17−20,24,25,27 This indicates that the formation of FeF3·0.33H2O increased the ratio of electrochemically reactive portions in sample FeOMC11 and consequently contributed to the improvement of the electrochemical performance.

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4. CONCLUSIONS A facile and scalable method was developed to synthesize FeF3/ OMC composites by the wetness-impregnation technique. These composite materials exhibited improved electrochemical properties, such as reversible specific capacity, cycling stability, and rate capability, compared to bulk FeF3. When the formation of FeF3 particles was well controlled inside the mesopores, the synergistic benefits were further increased. It will be interesting to investigate whether a similar approach can be extended to enhance the electrochemical performance of FeF3 in the wide voltage window of 1.5−4.5 V, delivering a high theoretical capacity of about 712 mAh·g−1 through intercalation and conversion reactions. Furthermore, the applications of this work to other alternative cathode materials (e.g., metal fluorides, polyanion materials) with low intrinsic electrical conductivity can be envisaged.

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ASSOCIATED CONTENT

S Supporting Information *

Initial discharge/charge profiles of the OMC. Complete author list of ref 8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +82 51 200 7259. Tel.: +82 51 200 7253. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was financially supported by the Ministry of Education, Science, and Technology (MEST), the National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (2012H1B8A2025809), and the Basic Science Research Program (2011-0023512).

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REFERENCES

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