Article pubs.acs.org/cm
Freestanding Macroporous Silicon and Pyrolyzed Polyacrylonitrile As a Composite Anode for Lithium Ion Batteries Madhuri Thakur,† Roderick B. Pernites,† Naoki Nitta,† Mark Isaacson,‡ Steven L. Sinsabaugh,§ Michael S. Wong,†, ∥ and Sibani Lisa Biswal*,† †
Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas 77005, United States Lockheed Martin Space Systems, Palo Alto, California 94304, United States § Lockheed Martin MS2, Akron, Ohio 44315, United States ∥ Department of Chemistry, Rice University, Houston, Texas 77005, United States ‡
S Supporting Information *
ABSTRACT: Silicon continues to draw great interest as an anode material for lithium ion batteries due to its large specific capacity for lithium. Macroporous silicon produced by electrochemical etching is one of several anode materials of interest, but its energy density is oftentimes limited due to its attachment to an unreactive silicon substrate. Here, we present a novel “lift-off” method by which a freestanding macroporous silicon film (MPSF) is electrochemically detached from the underlying bulk silicon and combined with pyrolyzed polyacrylonitrile (PAN), a conductive polymer that forms a conjugated-chain chemical structure. We report the performance of these silicon thin films with and without pyrolyzed PAN. KEYWORDS: anode materials, lithium-ion battery, multistep lift-off process, freestanding macroporous silicon film (MPSF)
1. INTRODUCTION The development of high energy density betteries continues to be of interest due to their numerous applications in information technology, consumer electronic devices, electric vehicles, implantable devices, and the telecommunication industry.1,2 Developing a low-cost electrode material with high energy capacity can lead to a significant improvement in the performance and lifetimes of products that use rechargeable batteries. In this regard, considerable effort have been made to improve the performance of lithium ion batteries (LIBs). Silicon has become an attractive material because it can react with lithium to form binary alloys with a maximum uptake of 4.4 lithium atoms per silicon atom, Li22Si5.3 At room temperature, the highest achievable specific capacity for silicon is 3579 mAh g−1, far greater than the theoretical capacity of 372 mAh g−1 for graphite,4 which is the most commonly used anode material. However, lithium alloying with silicon results in a large volume change (∼280%), which results in severe cracking in the silicon and eventual electrode failure. Several strategies have been developed to accommodate this severe volume expansion, including silicon sub micrometer pillars,5 silicon nanowires,6 silicon carbon composite,7 crystalline- amorphous silicon nanowires,8 porous thin films,9,10 and silicon nanowire arrays attached to silicon substrates,11−13 which have all been considered as promising candidates. Among these designs, the film structures remain one of the most cost-effective, since it is compatible with common microfabrication techniques for easy scale up. The challenge with silicon films is that oftentimes, the adherence of these films © 2012 American Chemical Society
to the current collector, one of the key factors in electrochemical performance, is poor. Many groups have included additional surface treatments, such as metal coatings,10,14 but this adds costly processing steps. Another limitation with silicon films has been that they oftentimes include a bulk silicon substrate that does not contribute to the specific capacity,10,15 leading to an increase in the weight of the anode. Some groups have removed this bulk silicon substrate through backside chemical etching processes,16 but at the expense of possibly useful silicon material. In this paper, we report a facile method to fabricate thin films of macroporous silicon (MPSF) to be used as an anode material for LIB. Unlike structured porous silicon9,10 or silicon nanowire arrays attached to a substrate,11,12,16 our thin films are electrochemically removed from the bulk silicon substrate. This freestanding MPSF is fabricated by electrochemical etching of silicon wafer in hydrofluoric acid (HF) solution using a multistep lift-off procedure. This leads to materials that are significantly lighter. Another benefit is that multiple porous silicon liftoff procedures can be performed using the same wafer, leading to little silicon waste. To these porous silicon thin films, we infiltrate a polymer binder, polyacrylonitrile (PAN). When PAN is pyrolyzed, it forms a conjugated-chain chemical structure with a specific capacity for lithium.17,18 Our experimental results indicate that a composite formed from Received: May 4, 2012 Revised: June 29, 2012 Published: July 6, 2012 2998
dx.doi.org/10.1021/cm301376t | Chem. Mater. 2012, 24, 2998−3003
Chemistry of Materials
Article
freestanding MPSF and pyrolyzed PAN can deliver a specific capacity of 1200 mAh g−1 and a better cycle life and Coulombic efficiency compared to bare MPSF.
2. EXPERIMENTAL SECTION Freestanding macroporous silicon can be fabricated by etching crystalline silicon wafers with an HF/solvent solution at a constant current density. The porosity and depth of the porous silicon can be controlled by varying the current density or concentration of HF acid.19 Solanki and co-workers showed that the porous silicon layer can be removed from the bulk silicon using either a one step process or a two step process.20 In the one step anodization liftoff procedure, the in situ change in fluoride ion concentration results in the formation of a high porosity layer beneath a low porosity layer under the same etching conditions. The pores in the high porosity layer then eventually expand in width and overlap with one another until the porous silicon breaks away from the substrate. In the two step procedure, a silicon wafer is first etched at a constant current density to create isotropic pores. Then the current density is suddenly increased causing the base of the pores to rapidly expand and overlap, thus lifting the porous silicon from the bulk substrate. To fabricate MPSF, we have modified the two-step liftoff procedure described above. Prime grade, boron doped, p-type (100) silicon wafers with thickness of 275 ± 25 μm and resistivity between 14 and 22 Ω cm (Siltronix Corp) were used. First, pores were etched into the silicon wafers at a constant current density of 2 mA cm−2, delivered by an Agilent power supply (E3612A) at room temperature. The etch time determined the depth of the pores. For an etch time of 1 h, the pore depth was typically 12 μm. The etching solution was composed of 30 mL of dimethylformamide (DMF, Sigma Aldrich) and 4 mL of 49% HF (Fisher Scientific) solution and was previously optimized10 to obtain pores with an average diameter of 1 μm. In the second step, we deviated from the two step procedure described by Solanki et al. Instead of using a sudden change, the current density was increased in small increments of 1 mA cm−2 and held for 10 min at each current up to 15 mA cm−2. This initial etching condition generates the macroporous silicon film, as illustrated in Figure 1a and shown in a corresponding scanning electron microscope (SEM) image in Figure 1b. The MPSF layer can be fabricated with an average pore diameter between 500 nm and 2 μm and thicknesses between 10 and 50 μm by adjusting the current density or HF concentration. A silicon wafer was etched for 3 h at 2 mA cm−2 to create a 36 μm thick macroporous silicon film. The current density was then increased in a stepwise manner. With increasing pore depth, the availability of fluoride ions at the pore tip decreases, which leads to isotropic etching at the tip of the pores, resulting in a layer of silicon that is more porous at the point of contact with the bulk silicon, as shown in Figure 1c,d. At some point, the pore walls near the substrate are thin enough so that substantial gaps between the substrate and MPSF appear. This leads to separation of a 50 μm thick MPSF from the bulk silicon substrate as illustrated and shown in a corresponding SEM image in Figure 1e,f. After fabrication, the MPSF was removed from the etching cell and washed with methanol and DI water. The MPSF was then dried in the vacuum for 1 h. Once dry, the freestanding film can be physically lifted off with a razor blade and weighed to determine the silicon mass. This multistep liftoff procedure is reliable and can separate the MPSF off the bulk substrate in its entirety. The use of incremental increases in current density was a significant improvement over the two-step process which frequently resulted in pieces of MPSF being removed at various times from the wafer. The process can then be repeated on the bulk silicon subtrate to generate multiple MPSFs from a single wafer, resulting in a cost-effective process with little Si wasted. To form the MPSF with the pyrolyzed PAN composite anode, the MPSF was treated with a PAN solution. A polyacryonitrile (PAN, MW150 000, Sigma Aldrich) solution was made by dissolving 1 g of PAN into 100 mL of DMF and stirring at 60 °C for 4 h. The freestanding MPSF is placed into the PAN solution in an argon atmosphere for 24 h. After PAN infiltration, the MPSF/PAN film was
Figure 1. Schematic of macroporous silicon film (MPSF) formation: (a) initial stage of MPSF formation at current density of 2 mA cm−2 for 1 h, (c) branching of the pores as the current density is increased in step 1 mA cm−2 up to 15 mA cm−2 for several minutes, (e) macroporous layer separated from bulk silicon. Parts b, d, and f are the corresponding SEM images of the process.
heated to 550 °C for 1 h to pyrolyze the PAN. Figure 2 illustrates the infiltration and pyrolyzation of PAN into MPSF. For our anodes, we typically fabricate freestanding MPSFs that have an average pore diameter of 1.5 μm and a thickness of 50 μm. Scanning electron microscopy (SEM, FEI Quanta 400) images of a typical MPSF film are shown in Figure 3. Figure 3a shows the lift-off of the porous silicon film. Because of the increase in current density during the electrochemical etching process, the pores on the backside of the lift-off film are much larger than the front side, as shown in Figure 3b,c. Figure 3d shows the side view of the MPSF with PAN. Figure 3e,f shows the SEM top and side-view of the free-standing MPSF with pyrolyzed PAN. The back-side of the MPSF film was coated with 100 nm of titanium and 200 nm of copper and placed in contact with a copper current collector. For electrochemical testing, a half-cell was assembled in an argon environment, in which the MPSF film or the MPSF/ pyrolyzed PAN composite is used as the working electrode and lithium foil as a counter electrode. The separator used was a trilayer polypropylene membrane (Celgard 2320) wetted with an electrolyte. The electrolyte used was 1.0 M LiPF6 in 1:1 (w/w) ethylene 2999
dx.doi.org/10.1021/cm301376t | Chem. Mater. 2012, 24, 2998−3003
Chemistry of Materials
Article
Figure 2. Schematic of the fabrication process for creating freestanding MPSF/pyrolyzed PAN composite.
Figure 3. SEM images of the surface morphology of (a) freestanding, (b) top-view, (c) back-side view of MPSF, (d) side-view of MPSF with PAN, and (e,f) side-view and back-view with pyrolyzed PAN.
Figure 4. (a) Comparison of discharge capacity and Coulombic efficiency vs cycle number for the freestanding MPSF and MPSF with pyrolyzed PAN composite during galvanostatic charge/discharge tested between 0.07 and 1.5 V at 200 μA cm−2. carbonate/diethyl carbonate (Ferro Corporation). Our anode material was cycled between 0.07 and 1.5 V at 200 μA cm−2. To confirm the PAN infiltration, chemical analysis was performed using energy dispersive spectroscopy (EDAX, FEI Quanta 400) and Xray diffraction (XRD, Rigaku D/Max Ultima II Powder). X-ray photoelectron spectroscopy (XPS, PHI Quantera Scanning X-ray Microprobe) was performed to probe the surface composition. To characterize the pyrolyzed PAN, attenuated total reflection Fourier
transform infrared (ATR FTIR) spectroscopy measurements were performed on the sample before and after pyrolysis at 550 °C. The ATR FTIR spectra were obtained on a Digilab FTS 7000 equipped with a HgCdTe detector from 4000 to 600 (cm−1) wavenumbers. All spectra were taken with a nominal spectral resolution of 4 cm−1 in absorbance mode. All films were measured under ambient and dry conditions for several trials at different areas of the sample surface. The UV−vis spectrum was recorded within 300−700 nm range using a HP3000
dx.doi.org/10.1021/cm301376t | Chem. Mater. 2012, 24, 2998−3003
Chemistry of Materials
Article
8453 UV−vis spectrometer. The UV−vis measurement of the pyrolyzed PAN films (adsorbed onto glass substrate) was done at ambient and dry conditions at different areas of the sample and an average curve is reported. Finally, the resistivity measurements of the sample were determined with a four point probe technique using the Keithley 2700 Multimeter Integra series. All films were measured at least 7−10 times on different areas of the sample under ambient and dry conditions. The conductivity was calculated using the resistivity value and the measured thickness of the sample.
battery, infrared spectroscopy measurement was performed before and after pyrolysis at 550 °C (Figure 5). The infrared
3. RESULTS AND DISCUSSION The cycling performance of bare freestanding MPSF and MPSF with pyrolyzed PAN is shown in Figure 4. The size of the MPSF with pyrolyzed PAN is 1 cm2 with a mass of 4 mg, which is measured using a microbalance (A&D, HR-202i, resolution 0.1 μg). In Figure 4, the discharge capacity for the first cycle of MPSF/pyrolyzed PAN is 850 mAh g−1, whereas for the bare MPSF is 757 mAh g−1. The bare MPSF rapidly drops in capacity after the second cycle and is 200 mAh g−1 by cycle 10 and completely fails by cycle 15. For MPSF with pyrolyzed PAN, the discharge capacity increases for the first four cycles to a discharge capacity of 1260 mAh g−1 and remains constant through cycle 20. This initial increase in capacity is typical for porous silicon films.10 The ratio of PAN to Si is 1:1 by mass. To calculate the contribution of the pyrolyzed PAN to the total capacity of the MPSF/pyrolyzed PAN composite, testing of pyrolyzed PAN was performed. The pyrolyzed PAN has a capacity of 55 mAh g−1 after the first cycle (130/35 mAh g−1) (Figure S1 in the Supporting Information), which indicates that the PAN contribution to the capacity is small. When analyzing galvanostatic cycling data, it is important to note the difference between the charge (lithiation) and discharge (delithiation) capacities, as this is an indication of reversibility (Coulombic efficiency). The average Coulombic efficiency for MPSF with pyrolyzed PAN is 95% after the first cycle while the bare MPSF reaches a maximum of 83% after 4 cycles and then drops to 70%. These results indicate that the pyrolyzed PAN can improve the cycleability and Coulombic efficiency. After 17 cycles, the Coulombic efficiency drops to 90% for the MPSF with pyrolyzed PAN. It is apparent that there is an irreversibe capacity fade that correlates with the Coulombic efficiency, which is most likely related to the electrolyte decomposition and an increase in thickness of the solid electrolyte interface (SEI) layer. To validate the infiltration of PAN into the MPSF, EDAX was performed for various depth profiles (Figure S2 in the Supporting Information). For our 50 μm thick MPSF, EDAX illustrates that the PAN infiltrates into the macroporous layer to an average depth of 12 μm from the top surface and 10 μm from the bottom surface. The wide scan XPS analysis (Figure S3a in the Supporting Information) shows a higher concentration of PAN compared to silicon at the surface of the sample, which supports the EDAX results that PAN is not able to infiltrate fully into the pores but rather it is more abundant closer to the film surfaces. Additionally, XRD was performed to identify undesired elements in our MPSF/pyrolyzed PAN composite. XRD pattern for PAN infiltrated MPSF (Figure S4 in the Supporting Information) shows carbon and silicon peaks without the formation of other alloys such as silicon carbide during the heat treatment. Further work is underway to determine if direct polymerization of PAN into MPSF instead of solution infiltration would further improve the electrochemical performance of the composite material. To analyze the PAN and understand its role in improving the performance of the MPSF as an anode material for the Li-ion
Figure 5. Infrared spectrum of PAN (red curve) before and (black curve) after pyrolysis at 550 °C with the chemical structures of the proposed products (inset). (Left inset) Focused IR region after pyrolysis: (1) CH and CH2 aliphatic stretch, (2) −CN stretch, and (3) −CC and/or −CN stretch.
spectrum of the native PAN (red curve with structure (I) on inset) divulges its signature IR peaks:22 −CH and CH2 aliphatic stretch (2800−3000 cm−1) due to the hydrocarbon chain polymer backbone, −CN stretch (∼2240 cm−1) due to the nitrile group side chain, −CH2 (∼1458 cm−1), and −CH (∼1365 cm−1) bending due to the polymer backbone. After pyrolysis of the PAN, the broad aliphatic −CH and −CH2 stretching vibrational peak (peak 1) and the distinct nitrile peak (peak 2) have apparently decreased in intensity while the peak due to −CH bending has increased (black curve). Interestingly, new IR bands have appeared in the spectrum such as the broad peak between 1500 and 1650 cm−1 (peak 3) and the sharp peak at 1265 cm−1 (peak 4), which are assigned to the −CC and/ or −CN and −C−N functional groups, respectively. The appearance of a triplet in the former peak (peak 3, magnified right inset 2) implies the formation of a vinyl group in a conjugated ring. The same IR spectrum was obtained for the pyrolysis of the composite sample of PAN and MPSF. Therefore, the IR analysis of the pyrolyzed sample evidence the presence of conjugated −CC and −CN sequences. Our results are supported by previous literature23 that suggested the formation of conjugated chain like structures after pyrolysis of PAN and their proposed reaction pathway and chemical structures of the products are shown on the inset of Figure 5. The proposed structure III, which consists of a cyclic polymer chain with a delocalized π-electron system, has been reported to exhibit outstanding mechanical and relatively high electrical conductivity properties.23,24 Furthermore, much earlier collaborative studies by MacDiarmid and Heeger24 also suggested the formation of singly (inset structure II) and doubly (inset structure III) conjugated structures of PAN after pyrolysis. It is noteworthy to mention that their IR data resembles that of Figure 5, which revealed the emergence of −CC and/or −CN and −C−N peaks, proving the resulting conjugated cyclic structures of pyrolyzed PAN. From the results, we insinuate that the products of the pyrolysis of PAN are composed of a heterogeneous mixture of II and III (mainly). It may also contain a trace amount of the native PAN (I) since there are still the appearance of minute nitrile peak at ∼2240 cm−1 and CH2 bending at ∼1458 cm−1. 3001
dx.doi.org/10.1021/cm301376t | Chem. Mater. 2012, 24, 2998−3003
Chemistry of Materials
Article
Figure 6. SEM images of the surface morphology of (a) bare MPSF after 3 cycles (b) MPSF with pyrolyzed PAN after 3 cycles, and (c) MPSF with pyrolyzed PAN after 20 cycles.
increased electrical conductivity and functioning as a good binder. This structure can also be fabricated using processing steps that are compatible with modern thin film processing techniques for easy scale up.
This is feasible since a relatively shorter amount of time was utilized to pyrolyze the sample. Moreover, since the sample was heated to 550 °C, it is also possible that the pyrolyzed products may contain few amounts of the more conducting graphitic carbon, which is reported earlier.25 The formation of a conjugated ring structure after pyrolysis of native PAN was also evidenced in the UV−vis analysis (Figure S5 in the Supporting Information), which shows a broad absorption peak between 350 and 650 cm−1 that is a signature peak for cyclized PAN.26 The absorption peak in this region has been explained to be due to the formation of a delocalized π-electron structure along the conjugated −CC and −CN bonds.26 Highresolution XPS scans for carbon (C 1s) and nitrogen (N 1s) (Figure S3b,c in the Supporting Information) further confirm the structural changes in PAN17,18,21 after heating. To investigate the electrical conductivity of the pyrolyzed PAN, standard four point probe measurements were performed on the PAN before and after pyrolysis at 550 °C. As a summary, the conductivity of the innate PAN was determined to be 9.08 × 10−1 S/m, which augmented to 2.36 S/m after pyrolysis at 550 °C. This value is comparable to the previous conductivity of pyrolyzed PAN that is equal to 5 S/cm.27 For our LIBs, we do not intend to completely carbonize the PAN but increase the electric conductivity of the PAN upon heating, which improves the electrochemical performance of the MPSF with pyrolyzed PAN. We have observed that our pyrolyzed PAN porous silicon composite material is better able to handle stress build-up during lithiation. As shown in Figure 6, the surface morphology of bare MPSF (Figure 6a) and MPSF with pyrolyzed PAN (Figure 6b) is compared after three cycles. The bare MPSF contains cracks as a result of the large volume expansion in silicon during lithiation. The cracks form to relieve the stress build-up in the material and cause loss of contact with the current collector, resulting in a decrease in capacity and cycle life. The MPSF with pyrolyzed PAN contains no observable fractures after 3 cycles or even after 20 cycles (Figure 6c). The pyrolyzed PAN appears to provide a percolation network where it acts as a stress reliever during the expansion and contraction of the silicon. This prevents the cracking and eventual pulverization of the MPSF, thus improving the capacity and lifetime.
■
ASSOCIATED CONTENT
S Supporting Information *
Plot of capacity and coulombic efficiency vs cycle number for PAN pyrolyzed at 550 °C under an argon atmosphere, EDAX spectra, XPS analysis of MPSF-PAN, X-ray diffraction (XRD) spectrum of MPSF with and without pyrolyzed PAN, and UV− vis spectrum of cyclized PAN after heating at 550 °C (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*Phone: +1 713-348-6055. Fax: +1 713-348-4902. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work is supported by the Lockheed Martin Advanced Nanotechnology Center of Excellence at Rice University (LANCER).
■
REFERENCES
(1) Simon, P.; Gogotsi, Y. Nat. Mater. 2008, 7, 845. (2) Kang, B.; Ceder, G. Nature 2009, 458, 190. (3) Hatchard, T. D.; Dahn, J. R. J. Electrochem. Soc. 2004, 151, A838. (4) Li, J.; Dahn, J. R. J. Electrochem. Soc. 2007, 154, A156. (5) Green, M.; Fielder, E.; Scrosati, B.; Wachtler, M.; Moreno, J. S. Electrochem. Solid State Lett. 2003, 6, A75. (6) Chan, C. K.; Peng, H.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. Nat. Nanotechnol. 2008, 3, 31. (7) Zhang, T.; Fu, L.; Gao, J.; Yang, L.; Wu, Y.; Wu, H. Pure Appl. Chem. 2006, 78, 1889. (8) Cui, L. F.; Ruffo, R.; Chan, C. K.; Peng, H. L.; Cui, Y. Nano Lett. 2009, 9, 491. (9) Shin, H. C.; Corno, J. A.; Gole, J. L.; Liu, M. L. J. Power Sources 2005, 139, 314. (10) Thakur, M.; Isaacson, M.; Sinsabaugh, S. L.; Wong, M. S.; Biswal, S. L. J. Power Sources, 2012, DOI:10.1016/j/jpowsour.2012.01.058. (11) Föll, H.; Carstensen, J.; Ossei-Wusu, E.; Cojocaru, A.; QuirogaGonzalez, E.; Neumann, G. J. Electrochem. Soc. 2011, 158, A580. (12) Leisner, M.; Cojocaru, A.; Ossei-Wusu, E.; Carstensen, J.; Foell, H. Nanoscale Res. Lett. 2011, 5, 1502. (13) Ossei-Wusu, E.; Cojocaru, A.; Hartz, H.; Carstensen, J.; Foell, H. Phys. Status Solidi, A: Appl .Mater. Sci. 2011, 208, 1417.
■
CONCLUSION We have demonstrated an inexpensive and facile process for generating MPSFs with pyrolyzed PAN as an anode material for lithium ion batteries. Our results show that MPSF with pyrolyzed PAN has a better cycle life compared to bare MPSF. Pyrolyzed PAN provides a number of benefits, including an 3002
dx.doi.org/10.1021/cm301376t | Chem. Mater. 2012, 24, 2998−3003
Chemistry of Materials
Article
(14) Takamura, T.; Sumiya, K.; Suzuki, J.; Yamada, C.; Sekine, K. J. Power Sources 1999, 81, 368. (15) Kang, D. K.; Corno, J. A.; Gole, J. L.; Shin, H. C. J. Electrochem. Soc. 2008, 155, A276. (16) Astrova, E. V.; Fedulova, G. V.; Smirnova, I. A.; Remenyuk, A. D.; Kulova, T. L.; Skundin, A. M. Tech. Phys. Lett. 2011, 37, 731. (17) Xue, J. S.; Dahn, J. R. J. Electrochem. Soc. 1995, 142, 3668. (18) Kim, C.; Yang, K. S.; Kim, Y. J.; Endo, M. J. Mater. Sci. 2003, 38, 2987. (19) Turner, D. R. J. Electrochem. Soc. 1958, 105, 402. (20) Solanki, C. S.; Bilyalov, R. R.; Poortmans, J.; Celis, J. P.; Nijs, J.; Mertens, R. J. Electrochem. Soc. 2004, 151, C307. (21) Janus, R.; Natkanski, P.; Wach, A.; Drozdek, M.; Piwowarska, Z.; Cool, P.; Kuśtrowski, P. J. Therm Anal Calorim 2012, DOI: 10.1007/ s10973-011-2157-6. (22) Zhao, Q.; Cao, T. Ind. Eng. Chem. Res. 2012, 51, 4952. (23) Xue, G.; Dong, J.; Zhang, J. Macromolecules 1992, 25, 5855. (24) Chung, T. C.; Schlesinger, Y.; Etemad, S.; Macdiarmid, A. G.; Heeger, A. J. J. Polym. Sci. Polym. Phys. 1984, 22, 1239. (25) Wang, L.; Yu, Y.; Chen, P. C.; Zhang, D. W.; Chen, C. H. J. Power Sources 2008, 183, 717. (26) Pospisil, J.; Samoc, M.; Zieba, J. Eur. Polym. J. 1998, 34, 899. (27) Teoh, H.; Metz, P. D.; Wilhelm, W. G. Mol. Cryst. Liq. Cryst. 1982, 83, 1329.
3003
dx.doi.org/10.1021/cm301376t | Chem. Mater. 2012, 24, 2998−3003
