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Porous Carbon Nanofibers Derived from Conducting Polymer: Synthesis and Application in Lithium-Ion Batteries with High-Rate Capability Chengchao Li, Xiaoming Yin, Libao Chen, Qiuhong Li, and Taihong Wang* Key Laboratory for Micro-Nano Optoelectronic DeVices of Ministry of Education and State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan UniVersity, Changsha, 410082, P. R. China ReceiVed: March 4, 2009; ReVised Manuscript ReceiVed: May 11, 2009

Nanoscale porous carbon fibers (CNFs) were synthesized successfully by pyrolysis of conducting polymer in an argon atmosphere, which were synthesized by the self-degradation template method. The diameter and length of the fibers are 100 nm and 2-3 µm, respectively. Owing to the high porosity and conducting onedimension network, these porous carbon nanofibers show enhanced lithium-ion storage properties when used as anode material in lithium-ion battery. The reversible specific capacity of the CNFs at a 0.5C rate is about 400 mAhg-1. Moreover, they exhibit considerably specific capacity even at a high charge-discharge current; i.e., the reversible capacities are around 250 and 194 mAhg-1 at a rate of 10 and 20C, respectively. 1. Introduction Lithium-ion batteries are widely regarded as the choice of power source for the upcoming century due to the growing concern about the energy crisis and environmental protection. The main advantages of the lithium-ion batteries are its lightweight and very high energy density, whereas its disadvantage is lower power which limits its application in some situations especially for electric vehicles (EVs). It is highly desired to develop better cathode and anode materials leading to improved electrochemical properties and economical lithiumion batteries.1–5 Currently, most of the commercial lithium-ion batteries have graphite as an anode material, however, the lithium-storage capacity of graphite (theoretical maximum capacity of 372 mAhg) is not enough to meet the demand of electric devices. Nanostructured materials are attractive for lithium-ion batteries with their unique features that arise from their nanoscale structures. Due to the small size, the optimum transport of both electrons from the back contact to the front of the electrode and ions from the electrolyte to the electrode particles can lead to a rapid discharging and charging rate. Up to now, various nanomaterials such as metal oxides, carbonaceous materials, phosphate, and sulfides, etc., have been widely used as anode material for lithium-ion battery.6–11 Among the families of materials studied, nanostructured carbon materials have a lot of advantages such as availability, chemical stability, good cyclability, and low cost as anode materials for lithium-ion batteries. However, in order to further improve the performance of carbon nanomaterials as anodes for lithium-ion batteries, much effort has been devoted to the exploration of amorphous carbon nanomaterials. Recent studies demonstrated that an effective porous structure in a controllable fashion is needed to provide desirable surface area and open structure, which can achieve higher rate capability and better cycle performance.12–14 The nanoscale porous carbon fibers demonstrated here exhibit a porous structure with a specific surface area of 74.5 m2 g-1, Hence, they can be expected to perform as high-rate and highcapacity anode materials for lithium-ion battery applications. * Corresponding author. Phone: +86-0731-8823407. Fax: +86-07318823407. E-mail: [email protected].

In this work, we synthesized carbon nanofibers by pyrolysis of conducting polymer in an argon atmosphere and demonstrated their potential application in lithium-ion batteries. Their microstructure and configuration were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and nitrogen adsorption/desorption analysis. The fibers have an average diameter and length of 100 nm and 2-3 µm, respectively. The electrochemical property of the sample was evaluated as anode material in lithium-ion battery. Due to the large surface area and short transport length, the CNFs preserve a high capacity even at high charge/discharge current density. Constant current charge and discharge tests showed stable Coulombic efficiency and desirable cyclability. The reversible discharging capacity is as high as 254 mAh g-1 after 100 cycles at 10C rate. 2. Experimental Section 2.1. Synthesis of Carbon Nanofibers. The methyl orange, pyrrole, and ferric chloride (analytical grade) were used as raw materials without further purification. Methyl orange (5 mmol) was dissolved in 30 mL of distilled water completely under vigorous stirring; then, 1.5 mmol of ferric chloride was added into it. About 5-8 min later, the color of the mixture turns to brown, and 105 µL of pyrrole was added to the mixture. A reaction time of 24 h was used to prepare the polymeric nonofibers.15 The as-prepared polymeric nanofibers were washed with distilled water and ethanol by vacuum filtration before being kept at 60 °C for 12 h. Finally, the porous carbon was annealed in an argon atmosphere at 900 °C for 3 h and cooled to room temperature naturally. 2.2. Characterizations. The morphology and size of the synthesized carbon fibers were characterized by scanning electron microscopy (SEM) [JEOL-JSM-6700F] and transmission electron microscopy (TEM) [JEOL-3010F]. The operating voltage of SEM and TEM is 5 and 200 kV, respectively. The pore diameter distribution and surface area were tested by nitrogen adsorption/desorption analysis [BECKMAN COULTER SA3100]. The crystal structure of the sample was determined by X-ray diffraction (XRD) [D/max 2550 V, Cu KR radiation].

10.1021/jp901968v CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009

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Figure 1. (a) SEM image of as-grown carbon nanofibers. (b and c) TEM images of as-grown carbon nanofibers. (d) XRD pattern of carbon nanofibers.

2.3. Preparation of CNF-CHIT Composite Film. The glass carbon (GC) electrode was polished by 0.3 and 0.05 µm aluminum slurries, and then was cleaned by dipping into 1:1 (v/v) aqueous solution of HNO3, deionized water, and ethanol with the assistance of ultrasonication prior to the experiment. The as-prepared CNFs were ultrasonically dispersed in 0.5 wt % chitosan to give a suspension with different CNF concentrations. An 8 µL portion of the CNF suspension was cast on the surface of the glass carbon electrode. The electrode was dried at 4 °C overnight in a refrigerator. The electrode was stored in phosphate butter solution (PBS) and kept at 4 °C in a refrigerator when not in use. 2.4. Lithium-Ion Battery Measurements. Electrochemical studies were characterized in a CR2016-type coin cell with a multichannel current static system Arbin (Arbin Instruments BT 2000, USA). The prepared material was dispersed in ethanol with ultrasonic for 30 min and dried at 60 °C totally before the slurry process. The electrode materials were prepared by mixing the active material with 10 wt % carbon black and 20 wt % binder (LA133 and CMC) in distilled water to form a homogeneous slurry. The well-mixed slurry was then spread onto a copper foil and dried at 105 °C in a vacuum oven for 12 h. Circular disk electrodes were punched from the foil and used as the cathode. 1 M LiPF6 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (EC:DC:DMC ) 1:1:1) was used as the electrolyte. The assembly of the test cells was performed in an argon-filled glovebox using Li foil as the counter electrode and polypropylene (PP) film (Celgard 2400) as the separator; the cells were discharged and charged galvanostatically between 0 and 2.0 V at room temperature.

3. Results and Discussion By pyrolysis of polymer nanofibers, the porous carbon nanofibers were obtained, which have similar fiber structures to the polymer precursor. Figure 1a is the scanning electron micrograph of the carbon nanofibers, on which one can observe that almost all of the nanofibers or nanofiber bundles in the sample have uniform size and smooth surface. The diameter and length of the fibers are about 100 nm and 2-3 µm, respectively. Figure 1b displays the transmission electron micrograph of the fibers. The HRTEM image (Figure 1c) illustrates that the fibers consist of poorly packed, highly crimpled, underdeveloped graphene layers with many edges. Figure 1d shows the X-ray diffraction (XRD) of a carbon nanofiber sample. The peaks located at 24.3° can be assigned to the (002) reflection of the CNFs. Besides, the peak of (100) is unconspicuous, which indicates that the crystallization of the fibers is poor and the structure of the sample is little graphitic. The result is accordant with the high-resolution transmission electron microscopy (Figure 1c) of as-prepared sample. In the pyrolysis process, the loss of both nitrogen atoms and dopant ions possibly resulted in the porous structure. The nitrogen-sorption data of the synthesized carbon fiber can be noted in Figure 2a, and reveal an average pore diameter of 27.98 nm (Barrett Joyner and Halenda algorithm) and a specific Brunauer-Emmett-Teller (BET) surface area of about 74.5 m2 g-1. Figure 2b shows the cumulative pore volume of the synthesized material using an NLDFT model; the dotted line is the demarcation between micropore and mesopore. The micropores have a pore diameter of less than 2 nm, while the mesopores are those with diameters between 2 and 80 nm. For the given sample, the volume ratio of mesopores to micropores is 8.0:1, revealing a very low micropore content. It is rather

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Figure 2. (a) Nitrogen adsorption/desorption isotherms of the carbon nanofibers. (b) The cumulative pore volume of the carbon nanofibers.

Figure 4. (a) Voltammetric response of 2 mg/mL CNF-CHIT composite modified electrode to 1 mM hydroquinone in 1/15 M PBS (pH 6.98) at different scan rates. (b) Plot of the peak current versus the square root of the scan rate in the range 20-200 mV s-1.

Figure 3. Cyclic voltammetric response of 5 mM hydroquinone in 1/15 M PBS (pH 6.98) for CNF-CHIT composites at different loading of CNF concentrations.

uncommon for thermally treated nongraphitic carbons to have such a small ratio of micropores. A higher micropore volume would lead to an enhanced irreversible Li storage and thus results in a bad cycling performance.16–18 The carbon nanofiber/chitosan nanocomposite film was characterized by cyclic voltammetry for electrochemical evaluation of the transducers. The cyclic voltammogram obtained at the nanofiber/chitosan nanocomposite film modified electrode immersed in aqueous 1/15 M PBS containing 5 mM hydroquinone is shown in Figure 3. A drastic decrease in the peak current was observed if the glassy carbon electrode was only coated with 0.5 wt % chitosan. This result can easily be understood because chitosan film acts as a barrier to electron transfer. The addition of carbon nanofibers greatly enhanced the redox peak currents due to their particular electrical property.

However, when the carbon nanofiber content increases to 2 mg/ mL, the redox peak currents do not increase; this is probably related to the limitation of the ability of chitosan to solubilize carbon nanofibers. The carbon nanofibers could not effectively increase the electroactive surface area due to their aggregation at a high content. On the basis of these results, 2 mg/mL CNFs were employed to modified glass carbon electrode for investigation of electrochemical properties. Figure 4 shows the cyclic voltammograms of 2 mg/mL CNF-CHIT composite film at different scan rates in a range from 20 to 200 mV-1. The welldefined peaks show that the carbon nanofiber/chitosan nanocomposite film is highly homogeneous. Peak-to-peak separation increased with increasing scan rate, suggesting a quasi-reversible behavior. Moreover, it is found from Figure 4b that the peak currents are proportional to the square roots of the scan rates, which suggest a typical semi-infinite, linear, diffusion-controlled electrochemical behavior feature of the redox process. Every nanofiber behaves as a very small electrode. On the basis of the Randles-Sevcik equation,19 the electroactive surface area can be estimated:

Ip ) 2.69 × 105AD1/2n3/2r1/2C where n is the number of electrons participating in the redox reaction, A is the area of the electrode, D is the diffusion coefficient of the molecule, C is the concentration of the target molecule in the solution, and r is the scan rate. The effective

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Figure 5. (a) Discharge/charge curve of carbon nanofibers for the first and second cycles at a rate of 0.5C. (b) The cyclic voltammogram profiles of the electrode from the 1st to the 10th cycles.

electroactive surface area for 2 mg/mL carbon nanofiber/chitosan composite film obtained using the equation above is about 0.258 cm2. The results imply that the CNFs are good electrochemical transducers and have a high electroactive surface area, which are advantageous for lithium-ion battery applications. Figure 5a shows the discharge (Li insertion)/charge (Li extraction) curve of our carbon nanofibers for the first and second cycles at a rate of 0.5C. At the current density, the first discharge and charge specific capacity is 631.9 and 358.9 mAhg-1, respectively. For graphitic materials, we can often observe a three-staging lithium insertion mechanism.20 However, no staging mechanism was observed in the discharge/charge curves of the first cycle, which is characteristic of the carbon nanomaterials with low graphitization degree. In the discharge process, the slope of the curve starts approximately at 1.8 V and has large specific capacity below 0.5 V. The capacity of the potential region lower than 0.5 V can be related to lithium intercalation into the graphene layers, and the capacity above 0.5 V vs Li/Li+ may be ascribed to the faradic capacitance on the surface of CNFs.21,22 Similar Li-storage behavior can also be observed in hard carbon.23,24 The pseudoplateau at 0.8 V indicated the surface film formation over the CNFs because of the reaction with the electrolyte. In the second cycle, none such behavior was observed, since the film formation reaction is limited to the first cycle. To better understand the scheme of the electrochemical process, the cycle voltammogram (CV) profiles of the electrode from the 1st to the 10th cycles are depicted in Figure 5b. The shape of the CV curves matches well with the charge/discharge profiles; during the first cycle, the apparent irreversible areas from 0 to 0.5 V seem to be formed by reduction between Li+ and the defects of CNFs. These reactions definitely involve an irreversible process, because the extrusive area disappeared in subsequent cycles. The main intercalation of lithium into carbon occurred below 0.4 V, and the extraction part is at 0.1-0.5 V with a broader shoulder. Figure 6a displays the cycle performance of synthesized material

Figure 6. (a) Cyclic performance of synthesized material at 0.5C. (b) The rate performance of synthesized material at different current densities.

at 0.5C. The reversible capacity at the 45th cycle is about 400 mAhg-1, which is comparable with some new materials reported recently.25,26 It is found that the Coulombic efficiency is a little larger than 100% since the 15th cycle. We speculate that the water-soluble binder (LA133) is hard to soak and absorb the electrolyte; the carbon materials are not fully activated during the beginning cycles, but the lithium-ion transport channel is extended as the charge and discharge proceed, activating the rest of the carbon. The performance of our sample in high current density is notable, as can be clearly seen in Figure 6b. The cell was first cycled at 5C for 100 cycles. Then, it was tested at a rate of 10, 20, and 30C for 100 cycles, respectively. After 100 cycles at 5C, the material maintained a specific capacity of 301.7 mAhg-1. At 10C, a capacity of 250.3 mAhg-1 is retained, 194.2 mAhg-1 at 20C, and 155.4 mAhg-1 at 30C. To the best of our knowledge, these results are extremely high when compared with what was reported in the literature previously.27,28 For example, the reversible capacity of hierarchical carbon derived from rice straw is only 254 mAhg-1 at a cycling rate of 2C and 168 mAhg-1 for vapor-grown carbon nanofibers at 10C.29,30 The high-rate capability can be ascribed to the porous and one-dimensional nanostructure of CNFs. The porous channels facilitate electrolyte transportation and lithiumion diffusion,31 and the conducting one-dimensional network ensures good electron transfer between the electrode and CNFs, which can be confirmed by the characterization of cyclic voltammograms above. All of these factors lead to high-rate performance and high reversible capacity of CNFs.

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4. Conclusion A novel kind of porous carbon nanofibers has been synthesized by an easy pyrolysis of conducting polymer in an argon atmosphere, with an average pore diameter of 27.98 nm and a specific surface area of about 74.5 m2 g-1. As anode material in lithium-ion battery, the CNFs show a large reversible capacity and good high-rate performance. Our study offers a promising material to overcome hurdles of lithium-ion batteries for highpower applications. Acknowledgment. This work was partly supported by “973” National Key Basic Research Program of China (Grant No. 2007CB310500), Chinese Ministry of Education (Grant No. 705040), and National Natural Science Foundation of China (Grant No. 90606009). References and Notes (1) Reddy, A. L. M.; Shaijumon, M. M.; Gowda, S. R.; Ajayan, P. M. Nano Lett. 2009, 9, 1002–1006. (2) Ma, H.; Zhang, S. Y.; Ji, W. Q.; Tao, Z. L.; Chen, J. J. Am. Chem. Soc. 2008, 130, 5361–5367. (3) Park, M. S.; Needham, S. A.; Wang, G. X.; Kang, Y. M.; Park, J. S.; Dou, S. X.; Liu, H. K. Chem. Mater. 2007, 19, 2406–2410. (4) Wang, Y.; Cao, G. Z. AdV. Mater. 2008, 20, 2251–2269. (5) Guo, Y. G.; Hu, J. S.; Wan, L. J. Electrochem. Commun. 2007, 9, 1867–1872. (6) Singhal, A.; Skandan, G.; Amatucci, G.; Badway, F.; Ye, N.; Manthiram, A.; Ye, H.; Xu, J. J. J. Power Sources 2004, 129, 38–34. (7) Ahn, H. J.; Choi, H. C.; Park, K. W.; Kim, S. B.; Sung, Y. E. J. Phys. Chem. B 2004, 108, 9815–9820. (8) Yu, Y.; Chen, C. H.; Shi, Y. AdV. Mater. 2007, 19, 993–997. (9) Shaju, K. M.; Gruce, P. G. Chem. Mater. 2008, 20, 5557–5562.

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