Polyaniline/Polyoxometalate Hybrid Nanofibers as Cathode for ...

Article pubs.acs.org/JPCC

Polyaniline/Polyoxometalate Hybrid Nanofibers as Cathode for Lithium Ion Batteries with Improved Lithium Storage Capacity Hongxun Yang,†,‡ Taeseup Song,§ Li Liu,† Anitha Devadoss,§ Fan Xia,§ Hyungkyu Han,§ Hyunjung Park,§ Wolfgang Sigmund,†,∥ Kyungjung Kwon,*,⊥ and Ungyu Paik*,†,§ †

WCU Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea Department of Materials Science Engineering, Hanyang University, Seoul 133-791, Republic of Korea ‡ School of Biology and Chemical Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China ⊥ Department of Energy and Mineral Resources Engineering, Sejong University, Seoul 143-747, Republic of Korea ∥ Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32606, United States §

S Supporting Information *

ABSTRACT: Hybrid nanofibers of polyaniline/polyoxometalate are synthesized via a facile interfacial polymerization method for the first time, and evaluated as a cathode material for lithium ion batteries. The hybrid nanofibers with 100 nm diameter consisted of phosphomolybdic acid polyanion, [PMo12O40]3−, and polyaniline matrix. Their 1D geometry improves the utilization of electrode materials and accommodates the volume change during cycling, which enables the significant improvement in lithium storage capacity and capacity retentions. The phosphomolybdic acid polyanions not only exhibit a large theoretical capacity of about 270 mAh g−1, but also reduce the charge transfer resistance of electrode leading to the enhanced reversible capacity and rate capability. The polyaniline/polyoxometalate nanofibers delivered a remarkably improved electrochemical performance in terms of lithium storage capacity (183.4 mAh g−1 at 0.1C rate), cycling stability (80.7% capacity retention after 50 cycles), and rate capability (94.2 mAh g−1 at 2C rate) compared to polyaniline nanofibers and bulk polyaniline/polyoxometalate hybrid.

1. INTRODUCTION Polyaniline (PANI) is a potential cathode material for lithium ion batteries (LIBs) because of its easy synthesis route, low cost, good electrochemical properties, and excellent environmental stability.1−6 However, low storage capacity limits its practical application.7,8 Various approaches have been explored to improve the storage capacity of PANI-based materials.9−18 One of successful strategies is to use one-dimensional (1D) nanostructured PANI with appropriate dopant materials. For the synthesis of 1D nanostructured PANI, the use of anions is essential together with suitable synthesis methods such as interfacial polymerization, template-assisted synthesis, and electro-polymerization.11−13 However, previously reported dopant anions, such as ClO4− and SO42−, are not electrochemically active,13−16,19 resulting in a significant decrease of the specific capacity. Polyoxometalate (POM), as an early transition metal anionic cluster, has gained momentum in lithium ion batteries due to its unique structure and rich reversible multielectron redox behaviors.19−24 Phosphomolybdic ([PMo12O40]3−, PMo12) acid, as a Keggin-type POM dopant showing both acidic and redox functions, not only protonates aniline during the synthesis process but also improves the electrochemical properties of organic−inorganic composites.19 It has been reported that the super-reduced state of PMo12, [PMo12O40]27−, © 2013 American Chemical Society

could store 24 electrons exhibiting a large theoretical capacity of about 270 mAh g−1, which is much higher than the current commercial cathode Li2CoO2 (140 mAh g−1).24 However, the main drawback of high solubility in solution impedes its applications as an electrode material. Doping POM anions into conducting polymer chains could effectively resolve this problem.7,19 Furthermore, a more significant improvement in electrochemical performances could be achieved by tailoring the geometry of the PANI/PMo12 composite with 1D nanostructure. To the best of our knowledge, PANI/PMo12 composite material at a nanoscale dimension has not been reported due to the difficulty of doping POM polyanions into the polymer chains of nanofibers. Herein, we report PANI/PMo12 hybrid nanofibers as a cathode material for lithium ion batteries to achieve improved electrochemical properties by combining the merits of the PMo12 polyanion and unique 1D nanofibrous morphology. The PANI/PMo12 hybrid nanofibers are synthesized via a facile interfacial polymerization method. The stable dopant PMo12 not only could increase a specific capacity, but also reduce the charge transfer resistance of electrode. Furthermore, its 1D Received: February 26, 2013 Revised: July 26, 2013 Published: July 29, 2013 17376

dx.doi.org/10.1021/jp401989j | J. Phys. Chem. C 2013, 117, 17376−17381

The Journal of Physical Chemistry C

Article

carboxyl methyl cellulose (CMC) and styrene-butadiene rubber (SBR) as binder at a weight ratio of 75:15:5:5. The coin cells were assembled in a glovebox filled with argon and their electrochemical properties were evaluated using a battery cycle tester (TOSCAT 3000, Toyo System, Tokyo, Japan) in the potential range of 4.2−1.5 versus (Li/Li +) V. Cyclic voltammetry (CV) measurements were performed using an electrochemical workstation (Parstat 2273) between 1.5 and 4.2 versus (Li/Li+) V at a sweep rate of 0.2 mV s−1. Electrochemical impedance spectroscopy (EIS) tests were also measured on the electrochemical workstation (Parstat 2273) operating in the frequency range of 0.1 to 106 Hz with an a.c. amplitude of 5 mV.

geometry of polyaniline chain would enable the effective utilization of electrode materials and accommodate the volume change of polymer during cycling. The PANI/PMo12 nanofiber hybrid exhibited a remarkably improved electrochemical performance in terms of lithium storage capacity, cycling stability, and rate capability.

2. EXPERIMENTAL SECTION Nanofibers of PANI/PMo12 were synthesized as follows:10,25 3 mL of freshly aniline and 8.12 g phosphomolybdic (PMo12) acid were dissolved in 200 mL of binary solvent of acetonitrile and dichloromethane (1:1 v/v). The solution turned yellow because of the formation of an aniline heteropolyacid salt. Simultaneously, 200 mL of 0.08 M ammonium persulphate (APS) solution containing 0.5 M H2SO4 was carefully spread over the organic solution of aniline−PMo12−acetonitrile/ dichloromethane. The reaction was carried out at room temperature undisturbed for 24 h. The products were separated by filtration and washed with copious acetonitile until the filtrate was colorless. At this stage, the hybrid polymer still contained small amount of sulfur, which was probably introduced by coinsertion of HSO4− together with phosphomolybdic acid during the polymerization. Sulfur contamination can be efficiently eliminated via ion-exchange with phosphomolybdic acid polyanions and washing with distilled water. The resulting black powders were added into phosphomolybdic acid solution, impregnated for 24 h, filtered, and washed with distilled water until the filtrate was colorless. Finally, the powders were dried under vacuum at 80 °C for 12 h to obtain the PANI/PMo12 nanofibers. For comparison, nanofiber PANI (Supporting Information, Figure S1) was synthesized following a similar condition without the presence of phosphomolydic acid. The surface morphology and the elemental composition of PANI/PMo12 composite were analyzed with scanning electron microscope (SEM; JEOL JXA-840) and element analyzer (HERAEUS CHN-O-RAPID), respectively. The composition of PANI/POM composite was also assessed by energy dispersive X-ray analysis measurements (EDX). The X-ray diffraction spectrum (XRD) data were obtained on a Rigaku DMAX 2500 diffractometer. Fourier transform infrared (FTIR) spectra were recorded in the range 400−4000 cm−1 on a Perkin-Elmer spectrum using KBr pellets. For the conductivity studies, the polymer in the form of powder was pressed. The electrical conductivities for PANI/PMo12 and PANI nanofibers were measured on the pressed pellets using a conventional four-probe technique (236 source measure unit, KEITHLEY) at room temperature under same conditions. Electrical conductivity, σ, was calculated using the expression, σ (S/cm) = L/tR, where L is distance between the electrodes in cm, t is thickness of the pellet in cm, and R is electrical resistance in Ω. Elemental analyses (C, H, and N) were carried on an Elementar Vario EL III analyzer. Mo and P were determined by a Jobin Yvon Ultima2 ICP atomic emission spectrometer. Anal. calcd. for PANI/PMo12: C, 18.25%; H, 1.27%; N, 3.55%; P, 1.31%; Mo, 48.59%. Found for (C6H5N)6.24(PMo12O40): C, 18.08%; H, 1.32%; N, 3.66%; P, 1.29%; Mo, 48.15%. Electrochemical measurements were conducted using CR2032 coin-type cell with lithium metal as counter electrode, and a solution of 1.0 M LiPF6 in ethylene carbonate (EC)/ diethyl carbonate (DEC; 1:1 v/v) as electrolyte. The working electrodes were prepared by mixing active material PANI/ PMo12 nanofibers with Super-P as conducting material, and

3. RESULTS AND DISCUSSION The PANI/PMo12 hybrid nanofibers were synthesized using a facile interfacial polymerization method (Supporting Information, Figure S2). For a typical organic/aqueous interfacial polymerization, an aqueous solution containing mineral acid (such as H2SO4) and oxidant (APS) was spread over a single hydrophobic organic solution of aniline.9,10 In our case, because directly combining PMo12 and APS results in a light yellow PMo12−APS precipitation, PMo12 was first mixed with aniline to form an aniline heteropolyacid salt (aniline−PMo12), which is easily dissolved in a binary organic solvent of hydrophilic acetonitrile and hydrophobic dichloromethane (1:1 v/v).25 APS solution as initiator was transferred over to the acetonitrile/ dichloromethane organic solution of aniline−PMo12. The polymerization takes place at the interface between the APS aqueous phase and the aniline−PMo12 acetonitrile/dichloromethane organic phase. The PANI/PMo12 nanofibers were characterized by SEM, EDX, FTIR, and XRD spectra. Figure 1a exhibits the SEM image of PANI/PMo12 hybrid nanofibers. The inset shows the magnified image of hybrid PANI/PMo12 nanofibers. The

Figure 1. (a) SEM images of PANI/PMo12 nanofibers (inset: high magnification SEM image). (b) EDX spectrometry of PANI/PMo12 nanofibers. 17377



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We subsequently examined the electrochemical properties of PANI/PMo12 nanofibers using CV studies. Figure 3a shows the

diameter of the as-prepared hybrid PANI/PMo12 nanofibers is about 100 nm. EDX spectroscopy was employed to confirm the incorporation of PMo12 into PANI polymer matrix. The peaks of molybdenum and phosphorus elements in Figure 1b reveal that the phosphomolybdate anions were successfully anchored within the PANI matrix. The FTIR spectrum of hybrid PANI/ PMo12 nanofibers was also recorded between 400 and 4000 cm−1 with KBr pellets (Figure 2a). The characteristic

Figure 2. (a) FTIR spectra of PANI/PMo12, PANI, and PMo12. (b) Xray diffraction patterns of PMo12/PANI, PANI, and PMo12. Figure 3. (a) Cyclic voltammograms of PANI/PMo12 nanofiber cathode in 1 M LiPF6 with 1:1 ethylene carbonate (EC) and diethyl carbonate (DEC) at a scan rate of 0.2 mV s−1. (b) Charge−discharge curves of PANI/PMo12 nanofibers and PANI nanofibers as cathode for lithium ion batteries at a current rate of 0.1 C. (c) Cycle performances of PANI/PMo12 nanofibers and PANI nanofibers as cathode for lithium ion batteries at a current rate of 0.1 C.

absorption peaks of Keggin anion PMo12O403− are observed in the FTIR spectrum of PANI/PMo12 nanofibers at 1064, 963, and 882 cm−1.25,26 The FTIR results show that PMo12 anions serve as counterions in the positively charged PANI matrix of hybrid PANI/PMo12 nanofibers. Figure 2b shows the XRD patterns of PANI, PMo12, and hybrid PANI/PMo12 nanofibers. The absence of crystalline peaks of PMo12 in hybrid PANI/ PMo12 nanofibers indicates that the Keggin anion units are dispersed at the molecular level and hence no phase segregation was observed.26,27 In addition, the XRD patterns suggest that hybrid PANI/PMo12 nanofibers are also amorphous, which could aid accelerating the transportation of lithium ions.28,29 An electronic conductivity of the electrode material is critical for the efficient charging and discharging rate.30−32 The electronic conductivity of hybrid PANI/PMo12 nanofibers was measured using the four-probe analysis. It shows the electronic conductivity of 1.37 S cm−1 at room temperature, which is obviously higher than those of counterpart bulk form with 0.5− 1.0 S cm−1.19 This result clearly indicates that a significant improvement in the electronic conductivity of hybrid could by achieved by engineering the electrode configuration.12 The electronic conductivity of PANI/PMo12 nanofibers is also higher than 0.85 S cm−1 conductivity of PANI nanofibers measured under similar conditions (Supporting Information, Table S1) in accordance with the previous reports.26,27 This is because PMo12 acid could more effectively protonate aniline than released sulfuric acid and PMo12 anions serve as counterions in the positively charged PANI polymer matrix, thus, leading to the formation of conducting PANI-PMo12 emeraldine salt segments.

cyclic voltammograms of PANI/PMo12 nanofiber electrode in the potential range of 4.2 to 1.5 vs (Li/Li+) V. Four redox pairs at the potential values of 2.83/2.64, 2.55/2.36, 2.26/2.05, and 1.83/1.65 versus (Li/Li+)V are ascribed to redox reactions between Mo6+ and Mo4+ ions in PMo12 polyanions,19,24 indicating that Li-ion insertion/extraction in PANI/PMo12 takes place as a multistep process. These changeless redox pairs of PANI/PMo12 at the second cycle further confirm the excellent reversible electrochemical activity of PMo12 in organic media. PMo12 is an endowed “electron sponge”, exhibiting a reversible 24-electron redox behavior during charging-discharging as an active cathode material in the potential range of 4.2 to 1.5 versus (Li/Li+) V, that is, from its super-reduced state [PMo12O40]27− to [PMo12O40]3−.24 Scheme 1 describes the charging−discharging mechanism of Li/PANI/PMo12 nanofibers. Figure 3b shows the voltage profiles of PANI/PMo12 and PANI nanofibers for the first and second cycles. There is only one voltage plateau at 2.43 versus (Li/Li+) V in the charge− discharge curves of PANI nanofiber cathode, corresponding to the doping and dedoping of anions into of PANI.12 However, it is interesting that the charge−discharge curves of PANI/PMo12 nanofiber are very different from that of PANI nanofiber. It can be seen that four potential plateaus around 2.68, 2.41, 2.12, and 1.75 versus (Li/Li+) V during charge−discharge process in the 17378



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structural stability during cycling. Comparing Figures S4 and S5, it was found that the original nanofibrous morphology of PANI/PMo12 is retained better than PANI nanofibers after cycling, demonstrating the improved cycle performance of PANI/PMo12 nanofibers. Figure 4a exhibits the rate capabilities of PANI/PMo12 nanofibers and PANI nanofibers as cathode for lithium ion

Scheme 1. Proposed Charge−Discharge Mechanism of Li/ PANI/PMo12 Battery

potential range of 4.2 to 1.5 versus (Li/Li+) V correspond well to the four redox waves in the CV curves of the PANI/PMo12 nanofiber cathode. These plateaus are still obvious after the first cycle, showing the excellent reversible capability of PANI/ PMo12 hybrid. PANI/PMo12 nanofiber delivered a superior discharge capacity of 183.7 mAh g−1 and a charge capacity of 210.3 mAh g−1 in the second cycle, which is lower than its theoretical capacity. This is mainly because the ratio of six aromatic rings to PMo12 molecule is the ideally expected value determined by the charge balance.19 During the polymerization process, a part of Mo6+ ions of PMo12 may be reduced by APS, resulting in the increase in the ratio between aromatic rings and PMo12 molecule (6.24). This indicates that PMo12 polyanions play an important role during cycling. The Keggin-type structure of PMo12 could undergo 24-electron redox behaviors during charging−discharging process in the potential range of 4.2−1.5 versus (Li/Li+) V,24 thus, leading to higher specific capacity compared to PANI nanofiber electrode. It should be noted that the specific capacity of 183.7 mAh g−1 delivered by PANI/PMo12 nanofibers is nearly three times higher than that in bulk form (53 mAh g −1).19 The present superior performance of hybrid PANI/PMo12 nanofibrous electrode is mainly attributed to 1D nanostructured geometry, which allows easy diffusion of electrolyte into the active material and shorter diffusion length of ions during the charge−discharge process.33 In addition, the nanostructure also increases the lithium ion flux at the interface between the electrode materials and electrolyte, thus, resulting in a higher utilization of electrode materials.3 The enhanced specific capacity suggests that appropriate morphological modification of the PANI/PMo12 electrode can improve its reversible storage capacity. The cycle performances for the PANI/PMo12 and PANI nanofibers are also given in Figure 3c. After 50 cycles, the PANI/PMo12 hybrid still delivers a specific capacity of 149.5 mAh g−1 corresponding to capacity retentions of 80.7% (Supporting Information, Figure S3), which is still much higher compared to that of counterpart bulk form electrode (53 mAh g−1).19 For PANI nanofiber electrode, it shows a specific capacity of 82.5 mAh g−1 corresponding to capacity retention of 61.8% after 50 cycles. This significant improvement is attributed to the facile strain relaxation via its 1D geometry and the stable structure and excellent redox property of PMo12 under the solid-state electrochemical redox changes.22,34 To demonstrate the roles of PMo12 as an active material and PANI as a polymer matrix, we explored the morphology and structural changes of the PANI/PMo12 and PANI nanofibers after 50 charge−discharge cycles. The coin cells electrochemically evaluated at 0.1 C were disassembled and characterized by SEM, XRD, and FTIR (Supporting Information, Figures S4− S7). No obvious structural changes of PANI/PMo12 and PANI were found from the XRD and FTIR patterns after cycles, indicating PANI and PANI/PMo12 show relative good

Figure 4. (a) Rate capabilities of PANI/PMo12 nanofibers and PANI nanofibers as cathode for lithium ion batteries at different charge− discharge rates. (b) Electrochemical impedance spectroscopy (EIS) spectra of PANI/PMo12 nanofibers and PANI nanofibers as cathode for lithium ion batteries after second charge/discharge cycle.

batteries in different charge−discharge rates. At high current rates, the differences between the PANI/PMo12 nanofiber and PANI nanofiber electrodes became much obvious. At the current rate of 2C, the PANI/PMo12 nanofiber and PANI nanofiber electrodes delivered specific capacities of 96.7 and 49.4 mAh g−1, corresponding to capacity retentions of 51.38 and 35.47% (Figure S8), respectively. This improved rate capability of PANI/PMo12 nanofibers is mainly due to the doped PMo12 polyanion into the PANI nanofiber, which enable significant enhancement in kinetics associated with lithium ions. To further probe the enhanced rate capability, EIS measurements were carried out using half cells consisting of PANI/ PMo12 nanofibers or PANI nanofibers as the working electrode after two discharge/charge cycles. As shown in Figure 4b, both of the Nyquist plots of the PANI/PMo12 and PANI electrodes are composed of a depressed semicircle in the high frequency region and a sloping line in the low frequency region. The impedance spectra are fitted using the equivalent circuit model (see the inset in Figure 4b), and the fitted impedance parameters are listed in Table S2. The fitting data are in good agreement with experimental data, as shown in Figure 4b. The equivalent circuit model includes Rs, a constant phase element (CPE) associated with the interfacial resistance, and the semicircle is correlated with the Li+ charge transfer resistance at the interface Rct. The linear portion is designated to Warburg impedance (W1), which is attributed to the diffusion of Li+ into the bulk of the electrode materials. Rs consists of the ionic resistance of the electrolyte, the intrinsic resistance of the active material, and the contact resistance at the interface active material/current collector. As shown in 17379



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(6) Shao, L.; Jeon, J. W.; Lutkenhaus, J. L. Polyaniline/Vanadium Pentoxide Layer-by-Layer Electrodes for Energy Storage. Chem. Mater. 2012, 24, 181−189. (7) Suppes, G. M.; Deore, B. A.; Freund, M. S. Porous Conducting Polymer/Heteropolyoxometalate Hybrid Material for Electrochemical Supercapacitor Applications. Langmuir 2008, 24, 1064−1069. (8) Egan, V.; Bernstein, R.; Hohmann, L.; Tran, T.; Kaner, R. B. Influence of Water on the Chirality of Camphorsulfonic Acid-Doped Polyaniline. Chem. Commun. 2001, 801−802. (9) Zhang, X. Y.; Manohar, S. K. Polyaniline Nanofibers: Chemical Synthesis Using Surfactants. Chem. Commun. 2004, 2360−2361. (10) Huang, J. X. Syntheses and Applications of Conducting Polymer Polyaniline Nanofibers. Pure Appl. Chem. 2006, 78, 15−27. (11) Taguchi, S.; Tanaka, T. Fibrous Polyaniline as Positive Active Material in Lithium Secondary Batteries. J. Power Sources 1987, 20, 249−252. (12) Sivakkumar, S. R.; Oh, J.; Kim, D. W. Polyaniline Nanofibres as a Cathode Material for Rechargeable Lithium-Polymer Cells Assembled with Gel Polymer Electrolyte. J. Power Sources 2006, 163, 573−577. (13) Ryu, K. S.; Kim, K. M.; Kang, S. G.; Joo, J.; Chang, S. H. Comparison of Lithium/Polyaniline Secondary Batteries with Different Dopants of HCl and Lithium Ionic Salts. J. Power Sources 2000, 88, 197−201. (14) Atanasoska, L.; Naoi, K.; Smyrl, W. H. XPS Studies on Conducting Polymers: Polypyrrole Films Doped with Perchlorate and Polymeric Anions. Chem. Mater. 1992, 4, 988−994. (15) Chen, S. A.; Hwang, G. W. Synthesis of Water-Soluble SelfAcid-Doped Polyaniline. J. Am. Chem. Soc. 1994, 16, 7939−7940. (16) Xia, Y.; Weisinger, J. M.; Macdiarmid, A. G.; Epstein, A. J. Camphorsulfonic Acid Fully Doped Polyaniline Emeraldine Salt: Conformations in Different Solvents Studied by an Ultraviolet/ Visible/Near-Infrared Spectroscopic Method. Chem. Mater. 1995, 7, 443−445. (17) Tian, S. J.; Liu, J. Y.; Zhu, T.; Knoll, W. The Effect of Ruthenium(III) Chloride on the Formation of Protonated Parent Ions in Electrospray Mass Spectrometry. Chem. Commun. 2003, 2738− 2739. (18) Suppes, G. M.; Deore, B. A.; Freund, M. S. Porous Conducting Polymer/Heteropolyoxometalate Hybrid Material for Electrochemical Supercapacitor Applications. Langmuir 2008, 24, 1064−1069. (19) Lira-cantú, M.; Gómez-Romero, P. Electrochemical and Chemical Syntheses of the Hybrid Organic-Inorganic Electroactive Material Formed by Phosphomolybdate and Polyaniline. Application as Cation-Insertion Electrodes. Chem. Mater. 1998, 10, 698−704. (20) Gómez-Romero, P.; Lira-cantú, M. Hybrid Organic−Inorganic Electrodes: The Molecular Material Formed between Polypyrrole and the Phosphomolybdate Anion. Adv. Mater. 1997, 9, 144−147. (21) Azumi, B. M.; Ishihara, T.; Nishiguchi, H.; Takita, Y. Electrochemical Intercalation of Li into Heteropoly 12 Molybdophosphoric Acid Ion-Exchanged with Cs. Electrochemistry 2002, 70, 869− 874. (22) Kawasaki, N.; Wang, H.; Nakanishi, R.; Hamanaka, S.; Kitaura, R.; Shinohara, H.; Yokoyama, T.; Yoshikawa, H.; Awaga, K. Nanohybridization of Polyoxometalate Clusters and Single-Wall Carbon Nanotubes: Applications in Molecular Cluster Batteries. Angew. Chem., Int. Ed. 2011, 50, 3471−3474. (23) Sonoyama, N.; Suganuma, Y.; Kume, T.; Quan, Z. Lithium Intercalation Reaction into the Keggin Type Polyoxomolybdates. J. Power Sources 2011, 196, 6822−6827. (24) Wang, H.; Hamanaka, S.; Nishimoto, Y.; Irle, S.; Yokoyama, T.; Yoshikawa, H.; Awaga, K. In Operando X-ray Absorption Fine Structure Studies of Polyoxometalate Molecular Cluster Batteries: Polyoxometalates as Electron Sponges. J. Am. Chem. Soc. 2012, 134, 4918−4924. (25) Hasik, M.; Turek, W.; Stochmal, E.; Lapkowski, M.; Proń, A. Conjugated Polymer-Supported Catalysts - Polyaniline Protonated with 12-Tungstophosphoric Acid. J. Catal. 1994, 147, 544−551.

Table S2, Rs is much smaller than Rct, indicating the cell impedance is mainly attributed to charge-transfer resistance. It is obvious that the Rct of PANI/PMo12 (93.6 Ω) is much smaller than that of PANI (203.8 Ω). This result is well correlated with the increased capacity and improved rate capability of PANI/PMo12 hybrid nanofiber electrode.

4. CONCLUSIONS In summary, PANI/PMo12 hybrid nanofibers as cathode material for lithium ion batteries are synthesized via a facile interfacial polymerization method. This 1D PANI/PMo12 hybrid nanofibers exhibit the high specific capacity, stable cycle performance and excellent rate capability. These improvements in the electrochemical performances are mainly attributed to the PMo12 dopant and 1D geometry. This research result reveals that the suitable engineering in a composition and electrode configuration could significantly improve the electrochemical performances of PANI-based electrode.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental materials, SEM images, conductivities and capacity retentions, XRD, and FTIR spectra for PANI/PMo12 and PANI nanofibers. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-2-2220-0502 (U.P.); +82-2-3408-3947 (K.K.). Fax: +82-2-2281-0502. E-mail: [email protected]; kjkwon@ sejong.ac.kr. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by WCU (World Class University) program through the Korea Science and Engineering Foundation (R31-2008-000-10092), Global Research Laboratory (GRL) program (K207040000037A050000310) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (Information and Communication Technologies) and Future Planning, and the International Cooperation program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government of Ministry of Trade, Industry & Energy (2011T100100369).

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