Investigation of the Electrical and Electrochemical Properties of

Jan 19, 2008 - the nanocomposites polypyrrole (Ppy)/V2O5 and polyaniline (PANI)/V2O5, the quadratic logistic equation. (QLE) can be used to fit the in...
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J. Phys. Chem. C 2008, 112, 2202-2209

Investigation of the Electrical and Electrochemical Properties of Nanocomposites from V2O5, Polypyrrole, and Polyaniline Fritz Huguenin*,† and Roberto M. Torresi‡ Departamento de Quı´mica, Faculdade de Filosofia, Cieˆ ncias e Letras de Ribeira˜ o Preto, UniVersidade de Sa˜ o Paulo, 14040-901 Ribeira˜ o Preto (SP), Brazil, and Instituto de Quı´mica, UniVersidade de Sa˜ o Paulo, CP 26077, 05513-970 Sa˜ o Paulo (SP), Brazil ReceiVed: July 25, 2007; In Final Form: October 29, 2007

In this work, an investigation of the electrical and electrochemical properties responsible for the energy storage capability of nanocomposites has been carried out. We demonstrate that, in the case of the V2O5 xerogel and the nanocomposites polypyrrole (Ppy)/V2O5 and polyaniline (PANI)/V2O5, the quadratic logistic equation (QLE) can be used to fit the inverse of the resistance values as a function of the injected charge in nonsteady-state conditions. This contributes to a phenomenological understanding of the lithium ion and electron transport. The departure of the experimental curve from the fitting observed for the V2O5 xerogel can be attributed to the trapping sites formed during the lithium electroinsertion, which was observed by electrochemical impedance spectroscopy. The amount of trapping sites was obtained on the basis of the QLE. Similar values used to fit the inverse of the resistance were also used to fit the absorbance changes, which is also associated with the small polaron hopping from the V(IV) to the V(V) sites. On the other hand, there was good agreement between the experimental and the theoretical data when the profile of the inverse of the resistance as a function of the amount of inserted lithium ions of the nanocomposites Ppy/V2O5 and PANI/ V2O5 was concerned. We suggest that the presence of the conducting polymers is responsible for the different electrical profile of the V2O5 xerogel compared with those of the nanocomposites. In the latter case, interactions between the lithium ions and oxygen atoms from V2O5 are shielded, thus decreasing the trapping effect of lithium ions in the V2O5 sites. The different values of the lithium ion diffusion coefficient into these intercalation materials are in agreement with this hypothesis.

Introduction Advances in the chemistry of insertion materials have been very important for the development of rechargeable lithium batteries, allowing the obtainment of electrodes with a high energy density and specific capacity.1 Some of these host matrices consist of transition-metal oxides that enable the easy access of lithium ions into their network. The V2O5 xerogel has been studied as a cathode for these rechargeable batteries due to its structural and electrochemical properties, which are suitable for lithium ion insertion/deinsertion.2-5 This electrode material has a potential higher than 3 V (vs Li/Li+) during the discharge process, an energy density close to 600 W h kg-1, and a specific capacity of about 250 A h kg-1. Lithium insertion/ deinsertion into the V2O5 xerogel can be represented by the following reaction

V2O5 + xLi+ + xe- / LixV2O5

(1)

The limiting rate step for this electrochemical reaction is lithium ion diffusion into the V2O5 matrix. The measured diffusion coefficient (D) values vary between 10-9 and 10-17 cm2 s-1, depending on the amount of inserted lithium ion, as reported in the literature.3,6 Moreover, the V2O5 xerogel conductivity (σ) is not very high, and its electrical properties * To whom correspondence should be addressed. E-mail: [email protected]. † Cie ˆ ncias e Letras de Ribeira˜o Preto, Universidade de Sa˜o Paulo. ‡ Instituto de Quı´mica, Universidade de Sa ˜ o Paulo.

have been extensively investigated due to its semiconductor nature. The conductivity values obtained for the V2O5 xerogel, which vary between 10-7 and 100 S cm-1, depend on many parameters, such as the preparation method, the nature of the precursor, the aging time of the precursor gel, and the amount of impurities.7,8 The electron hopping from the V(IV) to the V(V) sites is responsible for the conductivity of V2O5, which depends on the oxidation state of the vanadium ions.9,10 The experimental data have been analyzed by models based on smallpolaron theory, aiming at understanding the conduction mechanism in transition-metal oxides and V2O5-based glasses.11-13 Efforts have been made toward increasing the lithium ion diffusion rate and the electronic conductivity in these materials, and one way to do so is to produce the V2O5 xerogel on a synthesized nickel fiber.14 Another alternative is to obtain thin V2O5 films by either spin coating or atomic layer chemical vapor deposition.15,16 The supercritical drying of V2O5 gels,17 the addition of carbon powder in the early stages of the V2O5 gelation,18 the use of the “sticky carbon” method,19 and the synthesis of V2O5 nanotubes and V2O5 nanoribbons are some other approaches used to improve the efficiency of these materials.20 Considering the importance of interfaces for electrochemistry, novel nanocomposites have been developed. The optimization of the energy storage capacity is one of the reasons why these novel electrode materials are obtained. Therefore, conducting polymers can be intercalated into the V2O5 xerogel, offering an alternative pathway for the access of the lithium ion and

10.1021/jp0758622 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/19/2008

Nanocomposites from V2O5, Polypyrrole, and Polyaniline electron to the V2O5 sites.21-23 The polyaniline (PANI) and polypyrrole (Ppy) are some of the most commonly investigated conducting polymers because of their high conductivity values in the doped state.24,25 Moreover, the PANI specific capacity can reach values close to 160 A h kg-1 after several cycles.26 Considering the mass of the inserted anion into the Ppy, specific capacities of 108 and 122 A h kg-1 have been obtained in the Li/LiClO4-propylene carbonate (PC)/Ppy and Li/LiClO4acetonitrile (ACN)/Ppy cells.27 Therefore, the nanocomposites PANI/V2O5 and Ppy/V2O5 have been prepared and extensively studied by several research groups.28-33 Kanatzidis et al. were the first researchers to synthesize and characterize these nanocomposites.28-34 An intimate contact between the components was observed by X-ray diffraction (XRD) spectra. The electron paramagnetic resonance spectra also suggested this intimate contact, as well as a great approximation (200 Hz), associated with parallel resistive and capacitive processes. These coupled processes are related to the oxide xerogel electronic resistance (Re) and the space-charge capacitance (Csc).50 Lithium ion finite diffusion is observed at low frequencies (200 Hz) associated with the electronic resistance and the space-charge capacitance of the V2O5 xerogel at -1.2 V. Note that the Re value at this potential (750 Ω) increases compared to that obtained at -0.7 V (85 Ω), which is in agreement with the in situ resistance measurements. Lithium ion semi-infinite diffusion is observed between 7 Hz and 150 mHz (Figure 4b and c), which is characterized by the straight line with a slope of 45° in relation

to the real axis. This indicates that the lithium ion diffusion is slower at -1.2 V than that at -0.7 V. This decrease in the lithium ion diffusion rate is associated with the repulsive interactions taking place between the lithium ions within the V2O5 film, which increase as a function of the amount of inserted lithium ions. Contrary to the Nyquist diagrams shown in Figure 4a, the trapping of charge carriers into the network is observed in Figure 4b (for frequencies lower than 150 mHz), which has already been discussed in the literature for intercalation electrodes.42,43,53,55 This result corroborates the interpretation on the in situ resistance measurements mentioned above. The formation of trapping sites decreases the lithium ion mobility, which decreases the electronic delocalization and contributes to increasing the electronic resistance. Figure 5 shows absorbance changes (∆A) at 700 nm as a function of the injected charge for the V2O5 xerogel. These absorbance changes at this wavelength are also associated with the intervalence transfer from the V(IV) to the V(V) site. Just as in the case of the V2O5 conductivity, the QLE can also be used to predict the profile of these curves, according to eq 7

(

∆A ) ηq 1 -

q K - tq

)

(7)

Now, η is the electrochromic efficiency (η ) ∆A/q) for q ≈ 0. Comparing eq 7 with eq 3, one can imply that ∆A is also related to dq/dt. In fact, the magnitude of the absorbance shown in Figure 5 depends on the frequency of the charge (electron) transfer between the V(IV) and the V(V) sites. Interactions between the lithium ions and the network decrease the ionic mobility and the rate of the electron hopping. The formed trapping sites (tq) oppose the electron hopping, decreasing the electrochromic efficiency. The r, t, and K values used for the fitting in Figure 5 were 14 cm2 C-1, 1.5, and 11.5 mC cm-2, respectively. Note that the t and the K values were the same as those used in eq 6 to fit Figure 2b values. In fact, the conductivity and the absorbance changes at 700 nm are associated with the same electron hopping.36,56 This indicates that, in the phenomenological sense, the QLE is a suitable expression to represent the nonequilibrium properties associated with the electron hopping from the V(IV) to the V(V) state. Figure 6a gives the potentiodynamic profile of current and resistance, and Figure 6b shows the inverse of the resistance as a function of the injected charge for 12 µg of the [Ppy]0.3V2O5 nanocomposite. The resistance is virtually constant over the

Nanocomposites from V2O5, Polypyrrole, and Polyaniline

Figure 6. (a) Potentiodynamic profile of the current density (s), j, and in situ resistance (O), R, for [Ppy]0.3V2O5 in LiClO4 0.5 mol L-1/ PC electrolytic solution. V ) 5 mV s-1. (b) Plot of the inverse of the resistance, 1/R, as a function of the injected charge for experimental data (O) and theoretical curves 1 (b) and 2 (9).

range of -1.1 to -0.1 V, and the resistance of [Ppy]0.3V2O5 is about 1 order of magnitude lower than that of V2O5 between -0.7 and -0.1 V. This is primarily due to the fact that the Ppy component is in its doped state over this charge range. These results show the substantial enhancement in electronic conductivity that derives from embedding a conducting polymer matrix within V2O5. Figure 6b shows the experimental data and a fitting using eq 5 for the inverse of the resistance as a function of injected charge. In this case, two maxima are observed in the curve, and they can be fitted with a fitting between 0.0 and -1.0 mC cm-2 (curve 1) and another fitting between -1.0 and -5.4 mC cm-2 (curve 2), thus indicating the presence of two kinds of insertion sites. The estimated K values were 2.6 (called K1) and 6.2 mC cm-2 (called K2) in the first and second curves, respectively. In these fittings, two constant values of 60 and 40 µS were summed to 1/R (eq 5) for curve 1 and curve 2, respectively. We can suppose that the first insertion sites (curve 1) consist mainly of Ppy, once the reduction potential of Ppy is more positive than that of V2O5, as suggested above for the cyclic voltammograms in Figure 1. Moreover, the amount of Ppy in the nanocomposite is smaller than the amount of V2O5, and K1 is lower than K2. In fact, K1 and K2 are proportional to the amount of electroactive sites. The r values in curves 1 and 2 were 80 and 47 mS cm2 C-1, respectively. A high r value indicates a large change in conductivity as a function of charge.

J. Phys. Chem. C, Vol. 112, No. 6, 2008 2207

Figure 7. (a) Potentiodynamic profile of current density (s), j, and in situ resistance (O), R, for [PANI]0.3V2O5 in LiClO4 0.5 mol L-1/PC electrolytic solution. V ) 5 mV s-1. (b) Plot of the inverse of the resistance, 1/R, as a function of the injected charge for experimental data (O) and theoretical curves 1 (b) and 2 (9).

It is reported in the literature that the conductivity changes as a function of the charge when the amount of Ppy is larger than that of V2O5. The conductivity of this polymer can vary from extremely high values, 102 S cm-1 in the oxidized state, to 10-15 S cm-1 in the reduced state.57 Therefore, the r values of the first and second fittings also corroborate a larger amount of Ppy than V2O5 in the first insertion sites. Figure 7 gives (a) the potentiodynamic profile of the current and resistance and (b) the inverse of the resistance as a function of the injected charge for 12 µg of the [PANI]0.3V2O5 nanocomposite. One observes that the resistance of [PANI]0.3V2O5 as a function of the potential is similar to that of [Ppy]0.3V2O5. However, the resistance values for [PANI]0.3V2O5 are still lower than those of [Ppy]0.3V2O5. Figure 7b shows the fitting of the inverse of the resistance as a function of the injected charge using eq 5. As in the case of the [Ppy]0.3V2O5 nanocomposite, the results obtained for the [PANI]0.3V2O5 nanocomposite can also be fitted with two fitting curves. The first fitting between 0.0 and -1.0 mC cm-2 (curve 1) and the other fitting between -1.0 and -6.0 mC cm-2 (curve 2) once again indicate the occurrence of two types of insertion sites. The estimated K values were 1.5 (called K1) and 3.0 mC cm-2 (called K2) in the first and second curves, respectively. The r values for [PANI]0.3V2O5 were 110 and 38 mS cm2 C-1 for curves 1 and 2, respectively. On the basis of the same argument given above for [Ppy]0.3V2O5, we can suppose that PANI composes predominantly the first insertion sites. In fact, the conductivity

2208 J. Phys. Chem. C, Vol. 112, No. 6, 2008 values ranging from 10-10 to 10-1 S cm-1 are observed as the electron insertion level changes from 0 to 10%.58 It is also noteworthy that the symmetric (parabolic) profile of the inverse of resistance for both insertion sites of the nanocomposite (t values equal to zero) indicates a small amount (or absence) of trapping sites. This is not associated with the structural changes due to the lithium ion insertion. In fact, the VO5 units in the [PANI]0.3V2O5 nanocomposite undergo a larger local structural change than that in the V2O5 xerogel.59 Therefore, another explanation for the profiles of the inverse of the resistance in the case of the nanocomposites is the shielding of the interactions between the lithium ions and the oxygen atoms of the V2O5 due to the presence of the polymer, which should decrease the trapping effect of the lithium ions in the V2O5 sites. The hypothesis of a less shielded Li+-O interaction in the Ppy/V2O5 and PANI/V2O5 systems is consistent with several results published in previous works on PANI and poly(N-propane sulfonic acid) (PSPAN), such as follows: (1) In the UV-vis absorption spectra of the oxidized (E ) 0.4 V) and reduced (E ) -1.0 V) states of the V2O5 xerogel and the [PANI]0.3V2O5 nanocomposite,22 the band-gap energies associated with the electronic transition from O 2p to V 3d changed from 2.40 to 2.75 eV for the V2O5 xerogel and from 2.15 to 2.20 eV for the [PANI]0.3V2O5 nanocomposite. The change in the band gap energy is larger for V2O5 than that for the nanocomposite. PANI induces partial screening between the lithium ions and the oxygen atoms of the vanadium pentoxide, thus leading to a small decrease in the O 2p band compared with the V 3d band.60 (2) The distinct molecular-level interaction in the V2O5 xerogel and V2O5/PANI is also present in the Fourier transform infrared (FTIR) characterization. Huguenin et al. obtained FTIR spectra for the V2O5 xerogel, Li1.3V2O5 xerogel, [PANI]0.3V2O5, and Li1.3[PANI]0.3V2O5.59 The doublet assigned to VdO vibrations at 1022 and 1011 cm-1 for V2O5 are shifted to ∼978 cm-1 in the case of Li1.3V2O5. The wavenumber for the VdO doublet changes from 1016-1006 to 995 cm-1 when [PANI]0.3V2O5 is reduced. The change is smaller for Li1.3[PANI]0.3V2O5 than that for Li1.3V2O5 because of a partial screening promoted by PANI in the Li+‚‚‚OdV interactions, once there is an intimate contact between the organic/inorganic components.34-36 This is consistent with findings in the literature that NH-OdV hydrogen bonds and electrostatic interactions between PANI and V2O5 hinder interactions between lithium ions and vanadyl groups. (3) The molecular-level interaction between Li+-O can be better observed in LixV2O5, Lix[PANI]0.3V2O5, and Lix[PSPAN]0.3V2O5 using solid-state nuclear magnetic resonance (NMR), specifically magic angle spinning (MAS).61 A broad peak in the 7Li MAS NMR spectra for LixV2O5 that is due to lithium ions in bulk sites shifts upfield as more lithium ions are intercalated. This indicates that, as expected, more internal bulk sites are being accessed; therefore, the oxygen atoms shield more and more lithium ions from the magnetic field.61,62 For Lix[PANI]0.3V2O5, the broad peak shifted downfield as a function of the amount of lithium ions61 indicates that lithium ions are less shielded from the magnetic field, which is opposed to what is observed for the lithiated V2O5 xerogel. These results corroborate the partial screening effect of PANI on the interactions between the lithium ions and the oxygen atoms of V2O5. To understand the influence of oxygen atoms on the shielding of lithium ions from the magnetic field, the 7Li MAS NMR spectra for the poly(N-propane sulfonic acid) (PSPAN)/V2O5 nanocomposite are also analyzed.61 For Lix[PSPAN]0.3V2O5, the broad peak slightly shifts upfield as more lithium ions are

Huguenin and Torresi

Figure 8. Lithium diffusion coefficient, DLi, as a function of the molar ratio of lithium-to-vanadium ions, x, for (O) LixV2O5 xerogel, (b) Lix[Ppy]0.3V2O5, and (4) Lix[PANI]0.3V2O5.

intercalated. These spectra show chemical shifts in the same direction as those observed for lithiated V2O5 as a function of the amount of intercalated lithium ions. Because the difference between PSPAN and PANI is the sulfonic propane group, whose oxygen atoms act as sites for the intercalated lithium ions,23 we believe that this group contributes to the partial shielding effect on the interaction between the lithium ions and the oxygen atoms of V2O5. This explains the difference in the shifts between the systems containing PSPAN and PANI. In summary, upon revisiting the results with a closer inspection on the Li+-O interactions, we conclude that the UV-vis, FTIR, and NMR data are altered because of the shielding of the Li+-O interactions due to the presence of the polymer. Therefore, the trapping effect of the V2O5 sites on the lithium ions is decreased because of the lower interaction between the lithium ions and V2O5 in the nanocomposites. This helps explain the difference between the potentiodynamic profiles of the inverse of the resistance as a function of the injected charge for the nanocomposites (Figures 6 and 7) and for V2O5 (Figure 2) once electrons and the lithium ion are transported at the same time to maintain the electroneutrality within the host structure. To corroborate this explanation, the lithium ion diffusion rate must be larger in the case of the nanocomposites than that in the case of the V2O5 xerogel. The galvanostatic intermittent titration technique (GITT) was used to determine the Li+ diffusion coefficient (DLi).63 The procedure was similar to the one used to determine the diffusion coefficient for V2O5 (the experimental error was less than 2%).21 Figure 8 shows logarithmic plots of DLi versus the molar ratio of lithiumto-vanadium ions, x, for LixV2O5, Lix[PANI]0.3V2O5, and Lix[Ppy]0.3V2O5. In fact, the diffusion coefficient of the Lix[Ppy]0.3V2O5 and Lix[PANI]0.3V2O5 nanocomposites are higher compared to that of the parent oxide. Conclusions The good fitting of the inverse of the resistance as a function of the charge injected into the nanocomposites indicates that the QLE can be used for a phenomenological understanding of the electronic conductivity during the lithium ion electroinsertion. However, we included the trapping effect in the QLE to fit the inverse of the resistance as a function of the charge injected into the V2O5 xerogel. The difference between the profile of the inverse of the resistance of the V2O5 xerogel and those of the nanocomposites was attributed to the fact that the

Nanocomposites from V2O5, Polypyrrole, and Polyaniline lithium ions do not access the trapping sites in the latter cases due to the presence of the conducting polymers, which is responsible for shielding of the interaction between the lithium ion and the oxygen atoms on the V2O5 chains. Moreover, we observed the presence of two insertion sites in the case of the nanocomposites. On the basis of the K and r values of QLE, we assumed that the insertion site for high potentials consists predominantly of the conducting polymer. Therefore, the investigation done on the electrical and electrochemical properties helped to understand the electronic and ionic transport within the nanocomposites, as well as the contribution of the presence of conducting polymers in V2O5 for the energy storage properties. Acknowledgment. Financial support from FAPESP (Proc. No. 2005/00106-7) and CNPq (Proc. No. 55.0581/2005-7 and 555436/2006-3) is gratefully acknowledged. References and Notes (1) Ohzuku, T. In Lithium BatteriessNew Materials, DeVelopments and PerspectiVes, 2nd ed.; Pistoia, G., Ed.; Elsevier: Amsterdam, The Netherlands, 1995; Vol. 5, p 239. (2) Baddour, R.; Pereira-Ramos, J. P.; Messina, R.; Perichon, J. J. Electroanal. Chem. 1990, 277, 359. (3) Baddour, R.; Pereira-Ramos, J. P.; Messina, R.; Perichon, J. J. Electroanal. Chem. 1991, 314, 81. (4) Tipton, A. L.; Passerini, S.; Owens, B. B.; Smyrl, W. H. J. Electrochem. Soc. 1996, 143, 3473. (5) Livage, J. Chem. Mater. 1991, 3, 578. (6) Me`ge, S.; Levieux, Y.; Ansart, F.; Savariault, J. M.; Rousset, A. J. Appl. Electrochem. 2000, 30, 657. (7) Bullot, J.; Gallais, O.; Gauthier, M.; Livage, J. Appl. Phys. Lett. 1980, 36, 986. (8) Anaissi, F. J.; Demets, G. J.-F.; Alvarez, E. B.; Politi, M. J.; Toma, H. E. Electrochim. Acta 2001, 47, 441. (9) Barboux, P.; Morineau, H. E.; Livage, J. Solid State Ionics 1988, 27, 221. (10) Bullot, J.; Cordier, P.; Gallais, O.; Gauthier, M. J. Non-Cryst. Solids 1984, 68, 123. (11) Austin, I. G.; Mott, N. F. AdV. Phys. 1969, 18, 41. (12) Mott, N. F. AdV. Phys. 1967, 16, 49. (13) Szu, S.; Lu, S.-G. Physica B 2007, 391, 231. (14) Parent, M. J.; Passerini, S.; Owens, B. B.; Smyrl, W. H. J. Electrochem. Soc. 1999, 146, 1346. (15) Park, H. K.; Smyrl, W. H. J. Electrochem. Soc. 1994, 141, L25. (16) Li, Y. Z.; Kunitake, T.; Aoki, Y. Chem. Mater. 2007, 19, 575. (17) Le, D. B.; Passerini, S.; Tipton, A. L.; Owens, B. B.; Smyrl, W. H. J. Electrochem. Soc. 1995, 142, L102. (18) Passerini, S.; Smyrl, W. H.; Berrettoni, M.; Tossici, R.; Rosolen, M.; Marassi, R.; Decker, F. Solid State Ionics 1996, 90, 5. (19) Dong, W.; Rolison, D. R.; Dunn, B. Electrochem. Solid-State Lett. 2000, 3, 457. (20) Chan, C. K.; Peng, H. L.; Twesten, R. D.; Jarausch, K.; Zhang, X. F.; Cui, Y. Nano Lett. 2007, 7, 490. (21) Huguenin, F.; Girotto, E. M.; Torresi, R. M.; Buttry, D. A. J. Electroanal. Chem. 2002, 536, 37. (22) Huguenin, F.; Torresi, R. M.; Buttry, D. A. J. Electrochem. Soc. 2002, 149, A546. (23) Huguenin, F.; Torresi, R. M.; Buttry, D. A.; Pereira da Silva, J. E.; de Torresi, S. I. C. Electrochim. Acta 2001, 46, 3555. (24) MacDiarmid, A. G.; Chiang, J. C.; Halpern, M.; Huang, W. S.; Mu, S. L.; Somasiri, N. L. D.; Wu, W.; Yaniger, S. I. Mol. Cryst. Liq. Cryst. 1985, 121, 173. (25) Ren, X.; Pickup, P. G. Can. J. Chem. 1997, 75, 1518. (26) Desilvestro, J.; Scheifele, W.; Haas, O. J. Electrochem. Soc. 1992, 139, 2727.

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