Poly(pyrrole) and poly(thiophene)/vanadium oxide interleaved nanocomposites: positive electrodes for lithium batteries

Transcription

1 Electrochimica Acta, Vol. 43, Nos 10±11, pp. 1307±1313, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: S (97) ±4686/98 $ Poly(pyrrole) and poly(thiophene)/vanadium oxide interleaved nanocomposites: positive electrodes for lithium batteries G. R. Goward, F. Leroux and L. F. Nazar* Department of Chemistry, University of Waterloo, Guelph-Waterloo Centre for Graduate Work in Chemistry, Waterloo, Ontario, Canada N2L 3G1 (Received 19 September 1996; accepted 10 November 1996) AbstractÐLithium insertion has been examined in a series of conductive polymer-v 2 O 5 nanocomposites that have a structure comprised of layers of polymer chains interleaved with inorganic oxide lamellae. Poly(pyrrole), [PPY]; poly(aniline) [PANI]; poly(thiophene), [PTH] and its derivatives constituted the polymer component; PTH was prepared from the monomers bithiophene, terthiophene, 3-methylthiophene and 2,5-dimethylthiophene. Compositions of the corresponding nanocomposites were [PANI] 0.4 V 2 O 5, [PPY] x V 2 O 5 (x10.4, 0.9), and [PTH] x V 2 O 5 (x10.3±0.8). We nd that for modi ed [PANI] 0.4 V 2 O 5, polymer incorporation results in better reversibility, and increased Li capacity in the nanocomposite compared to the xerogel. For PPY and PTH nanocomposites, the electrochemical response is highly dependent on the preparation method, nature of the polymer, and its location. Reversible Li insertion was maximized in the case of PTH when it was prepared from 3-methyl or terthiophene as the monomers, suggesting that chain conjugation length and polymer order area important factors. In some of these materials, the Li insertion capacity can be increased by 40% by subjecting the electrode to an initial charge step. # 1998 Published by Elsevier Science Ltd. All rights reserved Key words: conductive polymer, insertion-polymerization, vanadium oxide, nanocomposite, Li battery. INTRODUCTION Conductive polymer/transition metal oxide nanocomposites have been the focus of much synthetic research in the last 10 years. These hybrid materials comprise conductive organic polymers interleaved between the layers of an inorganic oxide lattice such as V 2 O 5 or MoO 3. Few studies have been devoted to the unique properties that may result from this combination, aside from those that have examined the enhanced electronic conductivity resulting from polymer incorporation [1, 2]. In particular, the redox active nature of both components o ers promising possibilities in electrochemical applications such as positive electrode materials in lithium batteries. Conductive polymers themselves have also been considered as possible positive electrode materials [3]. Such polymers have not realized their expected success due to numerous limitations. *Author to whom correspondence should be addressed. For example, in these systems, the electrode is composed of the oxidized (p-doped) conductive polymer, along with a charge compensating anion. During the discharge process (reduction of the polymer to its neutral state) expulsion of the anion acts as the primary charge compensation phenomenon rather than insertion of lithium cations, though the process is more complicated than originally thought [4]. To suppress anion exchange, large, immobile organic anions (surfactants) have been incorporated into the conductive polymers [5]. This suggests the use of a large, immobile, inorganic lattice as the counter anion; a component which can both act as the anion, and participate in the redox processes. On the other hand, with regard to the transition metal oxide lattice, the conducting polymer can in principle augment the electron transport properties, or enhance ionic di usion by minimizing steric and electrostatic limitations to lithium transport within the oxide host. These organic±inorganic hybrid materials (nanocomposites) thus o er the 1307

2 1308 G. R. Goward et al. appealing possibility of synergistic redox capabilities. The choice of an inorganic lattice ``anion'' is based on the selection of a host that is both amenable to polymer incorporation, and which has a high redox potential. Numerous lamellar compounds are known to intercalate large polymeric guest species. One example, sol±gel derived V 2 O 5 has been studied extensively for its promising electrochemical properties [6, 7] and previous related work has also been carried out on [PEO] x V 2 O 5 [7]. Our recent studies of [PANI]-HTaWO 6 [8], [PANI]- MoO 3 [9], [PANI]-V 2 O 5 [10] and previous work of Kanatzidis et al. [1] who rst synthesized this type of hybrid material, have prompted us to examine the electrochemical behavior of vanadium oxide nanocomposites as positive electrodes. Here, we report the e ect of the polymer on lithium insertion in poly(thiophene) (PTH) and poly(pyrrole) (PPY) polymer-v 2 O 5 nanocomposites compared to the poly(aniline)- V 2 O 5 nanocomposite and the pristine V 2 O 5 xerogel. EXPERIMENTAL V 2 O 5 xerogel was synthesized according to the method described by Lemerle et al. [11] PPY was inserted into the xerogel using two methods described by Kanatzidis et al., a room temperature aqueous reaction, and a re ux reaction in acetonitrile [1(a)]. Preparation of PTH nanocomposites were only achieved by re ux methods, as initially reported for bithiophene in V 2 O 5 [1(b)]. As thiophene possesses a redox potential greater than that of V 2 O 5, which prevents its in situ polymerization by this inorganic host, thiophene derivatives were used as starting materials. These included methylated monomers (3-methyl thiophene and 2,5- dimethyl thiophene), and bithiophene and terthiophene (TerTH) oligomers. The molar fraction of polymer per V 2 O 5 was determined by thermal analysis. Positive electrode materials were prepared from active material, carbon black (Super S, Alfa Chemicals) and KYNAR FLEX as the organic binder in the weight proportion 80, 15, and 5 respectively. These powders were mixed in cyclopentanone and then the slurry was spread onto an aluminium disk. The composite electrodes were heated at 1008C for 2±3 h prior to their introduction into the glove box. Electrodes had a surface area of 1cm 2 and contained about 2 mg of active material. The electrolyte was a 1.0 M solution of LiClO 4 in propylene carbonate (PC, Aldrich). Swagelock [12] cells were assembled in an argon atmosphere, and studied under galvanostatic conditions between constant voltage limits using a MAC-PILE2 system (BioLogic, Claix, France). RESULTS AND DISCUSSION The polymer-v 2 O 5 nanocomposites and xerogel exhibit di erences in terms of Li uptake on rst discharge, and degree of reversibility. Figure 1 shows the voltage dependence as a function of Li composition for the various nanocomposites ([Poly]V 2 O 5 where Poly = PterTH (terthiophene-derived), PPY, Fig. 1. Variation of the voltage with Li composition, at a constant current density of 20 ma/cm 2 (110 ma/g, C/20) between 1.8 and 3.8 V for (a) V 2 O 5 ; (b) [PANI] 0.4 V 2 O 5 ; (c) [PPY] 0.4 V 2 O 5 ; (d) [PTH] 0.44 V 2 O 5 *, asterisk indicates material prepared under re ux.

3 Polymer/vanadium oxide interleaved electrodes 1309 PANI), and the V 2 O 5 xerogel alone. On rst discharge, [PPY] 0.44 V 2 O 5 displays twice the Li capacity as that obtained for [PANI] 0.4 V 2 O 5 and slightly greater than the xerogel (1.23, 0.60 and 1.05 Li/formula unit respectively). In all the materials, a portion of the capacity is not recovered during charge; this irreversible capacity for the V 2 O 5 xerogel (19%) is, however, less than that of the hybrid materials (24, 42 and 43% for PPY, PANI, PterTH respectively). It therefore seems at rst, that intercalation of the conductive polymer does not improve the electrochemical response, and the Li insertion±deinsertion phenomenon appears to be hindered. This can be explained by restricted access of the Li ions to the electroactive centres due to the possible deposition of surface polymer occurring during oxidative polymerization. Based on these observations, we have implemented di erent synthesis methods, as well as post preparation oxidative treatments. The results to date are presented here. Insight into the electrochemical behavior of the nanocomposites is a orded by examining the thermal analysis, and X-ray di raction patterns of the materials. In the case of [PPY] x V 2 O 5, two methods of synthesis were used, either involving simple addition to the monomer to a thin lm of the xerogel, or reacting the monomer with the xerogel in acetonitrile under re ux conditions. This provides materials with nominal compositions of 0.44 PPY (1) and 0.87 PPY (2) per 2 vanadium atoms respectively. The TGA/DTA curve for 1 (Fig. 2) shows one discrete exothermic peak corresponding to the combustion of the polymer at 4008C. In the case of 2, two exothermic regions are observed. The rst broad exotherm at lower temperature (250±3008C) is attributed to the combustion of surface polymer. The XRD pattern of 2 shows an increased interlayer d-spacing of 15.2 A Ê compared to 13.4 A Ê for 1. The additional polymer in 2 is thus distributed on both the surface, and interlamellar regions. The surface polymer, in particular, could impede lithium mobility in the nanocomposite, speci cally by blocking the layer edge sites and/or by formation of a surface double layer due to the capacitive behavior of the quasi-free polymer. This is observed in the electrochemical response (Fig. 3). Although the initial lithium capacity is higher for 2 (1.55 Li per formula unit), the lithium is trapped by the surface polymer, and only 0.85 Li can be removed. In 1, 01.2 Li per formula unit are inserted, and subsequently 0.9 Li are removed, thereby demonstrating better reversibility than 2. The lower Li uptake in the latter case could be the result of a higher degree of partial reduction of the V centres corresponding to higher polymer content. The degree of order in the nanocomposite, as determined by X-ray di raction, also a ects the electrochemical behaviour of the material. X-ray di raction studies showed the presence of a characteristic 001 re ection, and several weaker higher Fig. 2. (a) TGA-DTA curves of [PPY] x V 2 O 5 (x = 0.44, prepared by re ux method), and (b) [PPY] y V 2 O 5 (y = 0.87, prepared at ambient temperature), obtained with a heating rate of 108C/min and air ow of 20 cm 2 / min. TGA represented by dashed lines. order re ections, indicating the lamellar nature of the xerogel was maintained. The d-spacings of the PPY and PTH (from bith monomer) were in accordance to those previously reported [1]. A comparison of the X-ray patterns for PTH composites prepared from various monomers showed a signi cant variation in the intensity and peak width of the d 001 peak [I 001 (ter)>i 001 (3meth)>I 001 (bi)> I 001 (2,5 dimethyl)] which is an indirect measure of order in the nanocomposite. This is re ected in the lithium capacity of the materials; ie the more ordered [poly(3-methylthiophene)] V 2 O 5 and [poly(terthiophene)] V 2 O 5 composites also show greater lithium uptake (0.95 and 0.94 Li per formula unit respectively, Fig. 4). This agrees with what one would intuitively predict, since lithium mobility within the lattice should be facilitated in a more highly ordered system. The higher capacity of the terthiophene composite as compared to bithiophene can be attributed to a more facile polymerization process for the trimer. The redox potential of terthiophene (lower than that of bithiophene), and presence of two additional a±a' linkages in the monomer likely results in longer chain lengths in the polymer, and/or a greater degree of polymer chain order. Poly(thiophene) and poly(pyrrole) are both known to be highly susceptible to the for-

4 1310 G. R. Goward et al. Fig. 3. E ect of synthesis method on the Li capacity for [PPY] x V 2 O 5 ; (a) aqueous room-temperature method, x = 0.44; (b) re ux method; x = mation of chain defects that result from monomer a±b' or b±b' coupling [13]. Methylation in the 3- position on thiophene encourages a±a' coupling, and therefore formation of a more highly conjugated polymer, whereas methylation in the 2 and 5 positions destroys conjugation, resulting in a less conductive polymer. This could account for the di erent lithium capacities (0.95 vs 0.58 Li per formula unit). These results indicate that nanocomposites are sensitive to both polymer nature, and overall order in the material. Our previous studies on [PANI] 0.4 V 2 O 5 indicated a large increase in Li capacity and cyclability after treatment in oxygen at 1508C [10]. These results are Fig. 4. Variation of the voltage with Li composition, at a constant current density of 20 ma/cm 2 (110 ma/g, C/20) between 1.8 and 3.8 V for [PTH] x V 2 O 5 prepared using the monomers (a) terthiophene, x = 0.44; (b) 2,5 dimethylthiophene, x = 0.76; (c) 3-methylthiophene, x = 0.31; or (d) bithiophene, x = 0.75.

5 Polymer/vanadium oxide interleaved electrodes 1311 Fig. 5. Variation of the voltage with Li composition between 1.8 and 3.8 V for (a) [PANI] 0.40 V 2 O 5, 1; O 2 - [PANI] 0.40 V 2 O 5, 2; (b) [PPY] 0.44 V 2 O 5, 3; O 2 -[PPY] 0.44 V 2 O 5 4; (at a constant current density of 20 ma/cm 2 (110 ma/g, C/20)). summarized in Fig. 5(a) which shows that the capacity has more than doubled, and the insertion± deinsertion process has become more reversible and less polarized as a result of this treatment. This was attributed in part to the re-oxidation of V 4+ (which results from the sacri cial reduction during the coupling of aniline) to V 5+. Similar treatment in the case of PPY (or PTH)- V 2 O 5 however, resulted in a degradation of the polymer, and a decrease in capacity (Fig. 5(b)). This is in accordance with the reported reactivity of these polymers towards oxygen and moisture, by comparison to poly(aniline) which is relatively stable under these conditions. We nd, however, that the capacity in these PTH (or PPY) nanocomposites can be increased by subjecting the positive electrode to an initial charge step before the discharge process. Shown in Fig. 6 are the voltage dependence curves as a function of Li composition for [poly(terthiophene)] V 2 O 5, either recorded for an initial discharge sweep or for a charge sweep rst (i.e., during an initial discharge± charge cycle. Figure 6(a) or an initial charge±discharge±charge cycle, Fig. 6(b). Signi cant modi cation of the electrochemical response occurs during the charge step that results in a substantial increase of the speci c capacity from 0.9 Li (discharge rst) to 1.3 Li (charge rst). As this takes place without signi cant electron transfer during charge, it presumably arises from an adsorption/polarization process on the electrode surface. Such a process may involve anion adsorption or swelling of surface polymer. This poorly understood phenomenon is under further investigation. The e ect of the polymer on the lithium site potential and the e ect of a charge on the following discharge has also been studied in [poly(terthiophene)] V 2 O 5 compared to pure V 2 O 5, Fig. 6. E ect of a (a) initial charge step, vs (b) initial discharge step, on the Li capacity of [PterTH] 0.44 V 2 O 5 prepared from terthiophene using a constant current of 10 ma/cm 2.

6 1312 G. R. Goward et al. Fig. 7. Incremental capacity (dx/dv vsv) curves for (a) V 2 O 5 and (b) [PTH] 0.44 V 2 O 5 in initial discharge (b,1) orin charge rst (b,2). by analyzing the incremental capacity curves (obtained by numerical di erentiation) of the xerogel and the composites. These are displayed in Fig. 7. In both cases (irrespective of charge or discharge rst), the general shapes of the (dx/dv)± V curves of [PterTH]V 2 O 5 resemble that of the pure xerogel. In the latter, sites are evident at about 3 V and 2.5 V in discharge, and 2.7 V in charge, as reported in the literature [6, 14]. By comparison, [poly(terthiophene)] V 2 O 5 also shows two potential sites on discharge, but the higher voltage site is shifted down to 2.9 V. Moreover, the relative occupancy is completely di erent; namely, these two sites are in a 2:1 ratio in pure V 2 O 5, and 1:1 in [PterTH] 0.44 V 2 O 5. Two well developed corresponding oxidation sites are also evident in [PterTH] 0.44V 2O 5, one being at 2.6 V and the other at 2.9 V. In summary, on incorporation of polythiophene, the potential sites on discharge are maintained, with a slight shift to lower voltage, while a new de ned site at 2.9 V is created during charge. The e ect of an initial charge is evident in these dx/ dv±v curves by the increase of the Li site occupation, thus contributing to an uptake of 1.3 Li per formula weight. CONCLUSIONS Both the conductive polymer and the transition metal oxide host play a role in the redox processes of the nanocomposite. We have previously found that for O 2 -treated [PANI] 0.44_V 2O 5, polymer incorporation results in better reversibility, and increased Li capacity compared to the xerogel. For PPY and PTH nanocomposites, the electrochemical response appears to be highly dependent on the preparation method, and the polymer content and location. Although the Li capacity cannot be increased by an oxidation treatment, due to the known instability of these polymers to oxidation, the Li capacity can be substantially increased by subjecting the electrode to an initial charge. Our results, though promising, are still short of theoretical expectations. Optimization of the synthesis of these materials

7 Polymer/vanadium oxide interleaved electrodes 1313 may improve electrochemical response. The speci c capacity (reversible Li insertion) can be improved by incorporation of the polymer when it is not present in excess on the surface, hence blocking access to interlayer regionsðas long as the structural integrity and V 5+ content of the host are maintained. ACKNOWLEDGEMENTS LFN gratefully acknowledges the NSERC (Canada) for funding this research through the strategic grant program. The authors thank ELF ATOCHEM North America for providing the KYNAR PVDF resin. The authors thank also Dr M. Odriemkowski for his help in purifying the electrolyte solvents. REFERENCES 1. C.-G. Wu, M. G. Kanatzidis, H. O. Marcy, D. C. DeGroot and C. R. Kannewurf, Polym. Mater. Sci. Eng., 61, 969 (1989); M. G. Kanatzidis, C.-G. Wu, H. O. Marcy, D. C. DeGroot and C. R. Kannewurf, Chem. Mater., 2, 222 (1990); M. G. Kanatzidis, C.- G. Wu, H. O. Marcy and C. R. Kannewurf, J. Am. Chem. Soc., 111, 4139 (1989); Y.-J. Liu, D. C. DeGroot, J. L. Schindler, C. R. Kannewurf and M. G. Kanatzidis, J. Chem. Soc., Chem Commun., 593 (1993). 2. L. F. Nazar, Z. Zhang, D. Zinkweg, J. Am. Chem. Soc., 114, 6239 (1992); L. F. Nazar, H. Wu and W. P. Power, J. Mater. Chem., 5, 1985, (1995); R. Bisseur, C. D. DeGroot, J. L. Schindler, C. R. Kannewurf and M. G. Kanatzidis, J. Chem. Soc., Chem Commun, 688 (1993). 3. S. Panero, E. Spila and B. Scrosati, J. Electrochem. Soc., 143, L29 (1996); B. Scrosati, in Solid State Electrochemistry, (Edited by P. G. Bruce) p. 229, Cambridge University Press, Inc., Cambridge, Great Britain (1995); and references therein. 4. K. Naoi, M. Lien and W. H. Smyrl, J. Electrochem. Soc. 138, 440 (1991). 5. M. De Paoli, S. Panero, P. Prosperi and B. Scrosati, Electrochim. Acta, 35, 1145 (1993); M. Morita, S. Miyazaki, M. Ishikawa, Y. Matsuda, H. Tajima, K. Adachi, and F. Anan, J. Power Sources, 54, 214 (1995); M. Morita, S. Miyazaki, M. Ishikawa, Y. Matsuda, H. Tajima, K. Adachi and F. Anan, J. Electrochem. Soc., 142, L3 (1995). 6. B. Araki, C. Mailhe, N. Ba er, J. Livage and J. Vedel, Solid State Ionics, 9 & 10, 439 (1983); R. Baddour, J. P. Pereira-Ramos, R. Messina, J. Perichon, J. Electroanal. Chem., 314, 81 (1991); H.- K. Park and W. H. Smyrl, J. Electrochem. Soc., 141, L25 (1994). 7. K. West, B. Zachau-Christiansen, T. Jacobsen and S. Skaarup, Electrochim. Acta, 38, 1215 (1993) and Y.-J. Liu, J. L. Schindler, D. C. DeGroot, C. R. Kannewurf, W. Hirpo and M. G. Kanatzidis, Chem. Mater., 8, 525 (1996). 8. B. E. Keone and L. F. Nazar, Solid State Ionics, 89, 147 (1996). 9. T. A. Kerr, H. Wu and L. F. Nazar, Chem. Mater. 8, 2005 (1996). 10. F. Leroux, B. E. Keone and L. F. Nazar, J. Electrochem. Soc. 143, L181 (1996), 144, 3886 (1997). 11. J. Lemerie, L. Nejem and J. Lefebvre, J. Chem. Res., 5301 (1978); J. Livage, Chem. Mater., 3, 578 (1991). 12. D. Guyomard and J. M. Tarascon, J. Electrochem. Soc. 139, 937 (1992). 13a. J. Roncali, Chem. Rev. 92, 711 (1992). 13b. G. Tourillon and F. Garnier, J. Phys. Chem. 87, 2289 (1983). 13c. R. J. Waltman, J. Bargon and A. F. Diaz, J. Phys. Chem. 87, 1459 (1983). 14. H-K. Park, W. H. Smyrl and M. D. Ward, J. Electrochem. Soc. 142, 1068 (1994).