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Enhancement of the electrochemical performance of a Li-intercalated V2O5 xerogel doped with Eu
Solid State Ionics 160 (2003) 61 – 67 www.elsevier.com/locate/ssi
Enhancement of the electrochemical performance of a Li-intercalated V2O5 xerogel doped with Eu E.C. Almeida a, M. Abbate b, J.M. Rosolen a,* b
a Departamento de Quı´mica, FFCLRP-USP, 14040-901 Ribeira˜o Preto SP, Brazil Departamento de Fı´sica, UFPR, Caixa Postal 19091, 81531-990 Curitiba PR, Brazil
Received 26 June 2002; accepted 20 February 2003
Abstract We studied the electrochemical response of a Li-intercalated V2O5 xerogel doped with Eu. We found that the electrochemical performance of the rare-earth doped xerogel is improved. This is attributed to bonding of the rare-earth ions to the residual H2O in the xerogel structure. The electronic structure of the original V2O5 xerogel is not drastically affected by the rare-earth doping. D 2003 Elsevier Science B.V. All rights reserved. PACS: 81.20.Fw; 82.30Nr; 78.70.Dm Keywords: V2O5 xerogel; Li intercalation; X-ray absorption
1. Introduction V2O5 xerogel (xrg) is a useful electrode material in Li ion batteries and electrochromic devices [1– 5]. The electrochemical characteristic of V2O5 xerogel depends mostly on structural factors such as pillaring [1– 5]. Residual H2O in the xerogel provokes alterations in the V2O5 pillaring, affecting the electrochemical response and decreasing the electrical conductivity. In addition, the H2O molecules tend to bind to unreacted species in the structure, decreasing the mobility of the Li ions. Coustier et al. [6] doped V2O5 xerogel with diverse metal ions in an effort to neutralize this tendency. * Corresponding author. Tel.: +55-16-602-8787; fax: +55-16633-8151. E-mail address: [email protected] (J.M. Rosolen).
Here, we have studied the electrochemical response of a crystalline Li intercalated V2O5 xerogel doped with Eu. The principal characterization techniques were Fourier transform infra-red (FTIR), X-ray diffraction (XRD), cyclic voltammetry, galvanostatic charge– discharge and X-ray absorption spectroscopy (XAS). It was found that the reversible specific capacity of the rare-earth doped V2O5 xerogel composite electrode is improved. The improvement is attributed to bonding of the rare-earth ions to the residual H2O in the xerogel structure. However, the electronic structure of the original V2O5 xerogel is not drastically affected by the rare-earth doping. XAS is a useful tool in the study of the electronic structure. This technique provides selected information concerning the site and symmetry of unoccupied electronic states. The O 1s X-ray absorption edge is particularly useful when vanadium oxides are involved
0167-2738/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-2738(03)00126-7
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[7– 9]. The O 1s spectra are related to the O 2p unoccupied electronic states in the conduction band [10]. The diagnostic capability of XAS in this area has been demonstrated in studies of various Li-intercalated compounds [11 – 15].
2. Experimental details The V2O5 xerogel was prepared using a solution of 0.5 M sodium metavanadate (Alfa) and a proton exchange resin (Bayer, Lewatit S 100). For the preparation of the Eu0.11V2O5 xerogel, a Eu(OH)CO3 hydrated solution was added to the yellow HVO3. The high solubility of Eu(OH)CO3 in acid pH warrants a complete ionic exchange reaction between Eu3 + and HVO3 [16]. Both the undoped and Eudoped hydrogels were aged for 8 days and then heated at 50 jC for 12 h in air. The xerogels were then heated further at 300 jC for 4 h in air (treatment T1) or at 400 jC for 1 h in a 0.9 bar partial vacuum (treatment T2) to remove the residual water. The resulting powders were then milled in a mortar, sieved (300 mesh) in air, and stored in a dry-box. The concentrations of H2O in the xerogels submitted to thermal treatment T1 were approximately 0.2 mol in the Eu0.11V2O5 and 0.1 mol in the V2O5. These data were obtained from thermogravimetric/differential thermal analysis (heating rate 10 jC min 1, N2 atmosphere). Membrane composite electrodes were prepared with 10% polyvinylidene fluoride (PVDF) binder (Solvay), 10% carbon (ketjen black, MMB) and 80% xerogel (V2O5 or Eu0.11V2O5). The xerogel and carbon were mixed in a mortar and afterward added into the binder solution (PVDF dissolved in acetone). The slurry obtained was spread on a glass surface using a Doctor Blade. The membrane composite was cut in the form of disks (8-mm diameter). The electrochemical characterizations were performed in a coin cell with stainless-steel current collectors where lithium was used as the auxiliary and reference electrodes. Two layers of separator (Celgard) were used in the electrochemical cells. The electrolyte was a 1 M solution of LiClO4 (Aldrich, dried at 120 jC under vacuum) in propylene carbonate (Aldrich). All membrane electrodes were dried in a vacuum (0.9 bar) at 70 jC for 4 h before the cells were assembled (inside the dry-box). Galvanostatic charge/discharge
tests were performed using a Mac-Pile cycler, while the linear sweep voltammetry experiments were carried out with a PAR-362 and LabView-PC system. The powder XRD patterns were obtained using a Phillips 2001 diffractometer (Cu Ka radiation, graphite monochromator, 10– 90j range for 2h, 0.02j steps, and 3 s/step). The FTIR spectra were obtained using a Nicolet 5ZDX spectrometer (KBr beam splitter, 64 acquisitions, 2 cm 1 resolution, and transmission mode). The V 2p and O 1s XAS spectra were taken in the SGM beam line at the Laborato´rio Nacional de Luz Sincrontron (LNLS, Brazil). The spectra were collected in the total electron yield mode measuring the sample current. In this method of X-ray absorption, ˚ [15]. The the probing depth is approximately 50 A energy resolution at the V 2p and O 1s absorption edges was approximately 0.5 eV. The energy scale was calibrated using the known peak positions in the V2O5 spectrum. The base pressure in the experimental chamber was in the low 10 9 mbar range. The spectra were normalized to the maximum intensity after a constant background subtraction. For the XAS characterizations, the samples were intercalated by a chemical route. The details of the procedure are given elsewhere [17].
3. Results and discussion 3.1. Structural characterization The left side of Fig. 1 shows the XRD patterns for the V2O5 and Eu0.11V2O5 xerogels after both thermal treatments. All patterns show the features of polycrystalline orthorhombic V2O5 indicating interstitial Eu. However, the XRD patterns for Eu0.11V2O5 after thermal treatments T1 and T2 (Fig. 1(A) and (B), respectively) indicate less crystallinity than those for V2O5 (Fig. 1(C) and (D), respectively). The Bragg reflections in the Eu0.11V2O5 patterns are less intense and less sharp than those in the V2O5 patterns. Another difference observed in the patterns of Fig. 1 is the displacement of the Eu0.11V2O5 peaks with thermal ˚ treatment. While the lattice parameter d001 c 4.45 A for V2O5 after both thermal treatments, and also ˚ for Eu0.11V2O5 after treatment T2; d001 c 4.45 A
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Fig. 1. XRD patterns (left) and FTIR spectra (right) for (A) Eu0.11V2O5 xerogel after thermal treatment T1, (B) Eu0.11V2O5 after T2, (C) V2O5 after T1 and (D) V2O5 after T2.
˚ for Eu0.11V2O5 after d001 expands to about 4.75 A treatment T1. The right side of Fig. 1 shows the FTIR spectra for the xerogels after the two thermal treatments. The FTIR spectrum for V2O5 has been calculated by Sruca et al. [19]. In short, the bands in the range from 1020 to 950 cm 1 correspond to the V –O vanadyl stretching modes, while the band between 900 and 700 cm 1 can be ascribed to the bridging V – O – V stretching mode. For wave numbers smaller than 700 cm 1 the vibrations could be due to edge-sharing 3V – O stretching or bridging V – O –V deformations. The shift to lower wave numbers in the crystalline V2O5 spectra indicates either the presence of V4 + –O group modes or the lowering of the V2O5 crystal symmetry. The enhancement of the shoulder at about 980 cm 1 is an indication of the increased crystallinity of V2O5 [19,20]. Clearly, the FTIR spectra confirm that the V2O5 xerogel after thermal treatment T2 (see spectrum (D)) is the more crystalline of the four studied and that the Eu was inserted in the V2O5 layers. The Eu0.11V2O5 xerogel spectra (A) and (B) for thermal treatments T1 and T2, respectively, present the same vibrations mode of V2O5 but the bands associated with the V – O vanadyl stretching
and bridging V –O – V stretching modes are shifted to higher wave numbers. In summary, the FTIR results are in agreement with the XRD, showing that the Eu is in the V 2O5 interlayers and bonded to H2O molecules. In fact, not one of the samples with Eu gives any luminescence signal. All the xerogels studied here are single phase and crystalline, and, where applicable, the Eu insertions are responsible for the dehydration [18]. 3.2. Electrochemical characterization Fig. 2(A) and (B) reports the cyclic voltammetric curves for composite electrodes prepared with the V2O5 and Eu0.11V2O5 xerogels submitted to thermal treatments T1 and T2, respectively. During the first cathodic scan for both of the (T1) samples (see Fig. 2(A)), the peaks appear at the same potentials. In the anodic direction, the voltammograms for these electrodes show two superposed peaks that also occur at equivalent potentials. Subsequently, the cathodic/anodic peaks observed in the first discharge/charge cycle are either reduced or they disappear. After the first cycle, the voltammogram for V2O5 (T1) has only two broad cathodic and anodic peaks, while the voltam-
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Fig. 2. Voltammograms (0.1 mV. s 1 scan rate) for composite electrodes prepared with the Eu0.11V2O5 and V2O5 xerogels submitted to thermal treatments (A) T1 and (B) T2 (notice that the voltammogram of V2O5 was shifted up 1.0 mA).
mogram for Eu0.11V2O5 (T1) has two superposed peaks above 3 V and superposed broad peaks above 2 V. Thus, the voltammograms of Fig. 2(A) reveal similar features: irreversibility in the first cycle and stabilization of the anodic and cathodic charge ratio thereafter. Furthermore, the peak currents for the Eu0.11V2O5 (T1) electrode are more intense than those for the V2O5 (T1) electrode. In the case of the (T2) samples, the role of Eu – H2O interaction becomes more evident. The voltammogram for V2O5 (T2) in Fig. 2(B), in its main features, resembles the voltammogram for V2O5 (T1) in Fig. 2(A). It contains clear cathodic peaks in the first scan, with a similar peak distribution, and the same irreversibility. For the Eu0.11V2O5 (T2) electrode, the electrochemical response is quite different. The voltammogram for Eu0.11V2O5 (T2) shows only two cathodic and anodic peaks, independent of the cycle number. For both (T2) electrodes, the anodic and cathodic charge ratio does not become stable with cycling. The voltammograms of Fig. 2(B) also suggest that the electrodes prepared with (T2) xerogels are less efficient for lithium intercalation (i.e., the capacitive and ohmic effects increase) than those prepared with (T1) xerogels. It is well known that the peaks in
voltammograms involving lithium intercalation are associated with different occupation sites (oxi-reduction sites in the host material). Alterations in the distribution of the peaks are thus correlated with structural modifications that affect the intercalation. The results shown in Fig. 2 suggest that the presence of Eu in V2O5 improves its electrochemical performance, depending on the thermal treatment used in the crystallization of the xerogel. The response of Eu0.11V2O5 (T1) is better for lithium intercalation and its structure appears to be more stable to lithium intercalation cycles. The peak currents in the Eu0.11V2O5 (T1) voltammogram have the larger values and the superposition of the peaks is less than in the other voltammograms. However, the Eu3 + does not seem to participate of the oxi-reduction process associated with lithium site occupation in the range of potentials between 1.5 and 3.9 V. Fig. 2A shows that the Eu0.11V2O5 (T1) oxireduction peaks for the first insertion are practically at the same potential as those in the voltammograms for the pristine V2O5 xerogel (this hypothesis will be confirmed by the XAS characterization). Finally, the results of Fig. 2 show that an irreversible structural modification occurs during the first insertion in the crystalline xerogels. In fact, this is expected for polycrystalline vanadium pentoxide, where the irreversible
E.C. Almeida et al. / Solid State Ionics 160 (2003) 61–67
structural alterations are associated with phase transition and/or phase segregation [21]. Fig. 3 shows the initial discharge voltage as a function of the lithium concentration x for composite electrodes prepared with V2O5 (circles) and Eu0.11V2O5 (line) xerogels submitted to thermal treatment T1. These curves have several plateaus that occur at the same potentials as the cathodic peaks during the voltammogram first cycles (see Fig. 2(A)). An initial discharge voltage curve is sensitive to the chemical potential of the ions and electrons, and, for an amorphous xerogel, it does not contain any plateaus [1]. The curves in Fig. 3 reveal that the structural effect provoked by the presence of Eu is evident for x > 0.5. The plateaus in the LixEu0.11V2O5 curve (line) are less well defined than those in the LixV2O5 curve (circles). This confirms what was detected by XRD, FTIR and the voltammograms, ie, that a sample doped with Eu is more amorphous than an undoped sample for the same thermal treatment. Finally, Table 1 presents values of the specific capacities for the less crystalline (T1) xerogel composite electrodes during given discharge/charge cycles, measured at different currents between the cutoff voltages of 2.0 and 3.7 V. The reversible specific capacity of Eu0.11V2O5 (T1) was 215 mA h g 1
Fig. 3. Variation of voltage as a function of lithium concentration x for composite electrodes prepared with the vanadium pentoxide xerogels LixEu0.11V2O5 (line) and LixV2O5 (circles). Both xerogels were submitted to thermal treatment T1 before preparation of the electrodes, and the data were taken at a discharge current of 0.010 mA cm 2.
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Table 1 Specific capacity of the xerogel composite electrodes during given discharge/charge cycles Electrode
Eu0.11V2O5 (T1)
V2O5 (T1)
1
Current density (AA g 1)
Specific capacity (mA h g 1st discharge
2nd discharge
10th cycle
(A) 15
269
215
215
(B) 21 (C) 26 (A) 22 (B) 23 (C) 22
260 255 225 255 270
215 215 216 246 216
215 215 181 146 119
)
The currents used in the cycles were (A) 100 AA, (B) 150 AA and (C) 120 AA.
(26 AA g 1) for the 10th discharge/charge cycle, the same value was found for the 2nd cycle. The specific capacity of V2O5 (T1) did not become reversible, and its value was only 119 mA h g 1 (22 AA g 1) for the 10th cycle. Therefore, the Eu0.11V2O5 (T1) composite electrode has better lithium insertion kinetics and structural stability than the V2O5 (T1) electrode. If the H2O concentration is very small, as is expected in the case of V2O5 (T2), electrochemical improvement upon Eu doping is not anticipated. Indeed, the Eu0.11V2O5 (T2) composite electrode was found to operate irreversibly with cutoff voltages of 1.5 and 3.9 V. The interaction between Eu and H2O molecules provokes alterations which enhance the electrochemical performance of a crystalline V2O5 xerogel. The Eu is hydrolyzed in the structure (for example, Eu(H2O)n3 + or Eu(H2O)n2 + 1). The typical layered structure of crystalline V2O5 is preserved by Eu insertion, but the active sites for lithium and the structural stability are affected. Still, the intercalation charge remains largely associated with the vanadium cation and/or oxygen as was observed in the pure V2O5 xerogel [17]. The Eu3 + is a polyvalent cation that does not participate in the oxi-reduction process of lithium intercalation. Further improvement would be expected from increasing the polarization of the hydration sphere; by decreasing the size of the lanthanide, for example. 3.3. XAS characterization Fig. 4(a) shows the O 1s XAS spectra of V2O5 and Li-intercalated LixV2O5 xerogels after thermal treat-
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Fig. 4(b) shows the O 1s XAS spectra of Eu0.11V2O5 and Li-intercalated LixEu0.11V2O5 xerogels after thermal treatment T1. First, we note that the spectra of Eu0.11V2O5 and V2O5 are strikingly similar. This shows that the electronic structure of the original material is not greatly affected by Eu doping. Further, we note that the spectra of LixV2O5 and LixEu0.11V2O5 are also rather similar. Again, this indicates that Eu doping does not affect the electronic structure of the material. The main effect of the Eu doping is for Eu to bind to unreacted H2O within the structure.
4. Conclusions
Fig. 4. O 1s X-ray absorption spectra for Li-free and chemically Liintercalated (a) V2O5 and (b) Eu0.11V2O5 xerogels after thermal treatment T1.
ment T1. The spectrum of V2O5 presents two peaks at threshold and broader bumps at higher energies. The first two peaks correspond to O 2p character mixed in the V 3d band, whereas the bumps correspond to O 2p character mixed in the V 4sp bands. The so-called V 3d band region at threshold is further split by crystalfield effects. These assignments are supported by band structure calculations of the related VO2 compound [10]. The spectra of Li-intercalated LixV2O5 present changes in both the peak and bumps regions. The V 3d mixed bands are broader and the bumps grow larger. All these effects signal an increase in the degree of covalency with the O 2p states. The most relevant change appears around 532 eV and is signaled by the arrow. This emerging spectral weight coincides with the main absorption peak in the Li2O spectrum [16]. This suggests that Li intercalation affects O 2p – V 3d mixed states and leads to Li2O formation.
The present work has shown that Eu insertion into a V2O5 xerogel provokes a meaningful improvement of its electrochemical performance. We obtained crystalline Eu-doped V2O5 composite electrodes which were tested in Li ion coin cells, yielding specific capacity and capacity fading results superior to those of crystalline V 2 O 5 composite electrodes also obtained from the sol – gel route. The rare earth does not participate in the oxi-reduction process. The charge transfer involved in the lithium insertion occurs only with V and/or O ions. The beneficial effect of Eu doping appears to be significant only for samples that have residual H2O.
Acknowledgements This work was supported by CNPq-PADCT (620238/97-6, 301493/95-2), LNLS and Fundacßa˜o Araucaria.
References [1] B.B. Owens, S. Passerini, W.H. Smyrl, Electrochim. Acta 45 (1999) 215. [2] K. West, B. Zachauchristiansen, T. Jacobsen, S. Skaarup, Electrochim. Acta 38 (1993) 1215. [3] H.K. Park, W.H. Smyrl, J. Electrochem. Soc. 141 (1994) L25. [4] H.K. Park, W.H. Smyrl, M.D. Ward, J. Electrochem. Soc. 142 (1995) 1068. [5] A.L. Tipton, S. Passerini, B.B. Owens, W.H. Smyrl, J. Electrochem. Soc. 143 (1996) 3473. [6] F. Coustier, G. Jarero, S. Passerini, W.H. Smyrl, J. Power Sources 83 (1999) 9.
E.C. Almeida et al. / Solid State Ionics 160 (2003) 61–67 [7] M. Abbate, F.M.F. de Groot, J.C. Fuggle, Y.J. Ma, C.T. Chen, F. Sette, A. Fujimori, Y. Ueda, K. Kosuge, Phys. Rev., B 43 (1991) 7263. [8] M. Abbate, F.M.F. de Groot, J.C. Fuggle, A. Fujimori, Y. Tokura, Y. Fujishima, O. Strebel, M. Domke, G. Kaindl, J. van Elp, B.T. Thole, G.A. Sawatzky, M. Sacchi, N. Tsuda, Phys. Rev., B 44 (1991) 5419. [9] H.F. Pen, M. Abbate, A. Fujimori, Y. Tokura, H. Eisaki, S. Uchida, G.A. Sawatzky, Phys. Rev., B 59 (1999) 7422. [10] M. Abbate, H. Pen, M.T. Czyzyk, F.M.F. de Groot, J.C. Fuggle, Y.J. Ma, C.T. Chen, F. Sette, A. Fujimori, Y. Ueda, K. Kosuge, J. Electron Spectrosc. 62 (1993) 185. [11] L.A. Montoro, M. Abbate, E.C. Almeida, J.M. Rosolen, Chem. Phys. Lett. 309 (1999) 14. [12] L.A. Montoro, M. Abbate, J.M. Rosolen, J. Electrochem. Soc. 147 (2000) 1651. [13] L.A. Montoro, M. Abbate, J.M. Rosolen, Electrochem. SolidState Lett. 3 (2000) 410.
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[14] J.M. Rosolen, M. Abbate, Solid State Ionics 139 (2001) 83. [15] S. Passerini, W.H. Smyrl, M. Berretoni, R. Tossici, J.M. Rosolen, R. Marassi, F. Decker, Solid State Ionics 90 (1996) 4. [16] L. Znaidi, D. Lemordant, N. Baffier, Solid State Ionics 28 – 30 (1988) 1750. [17] E.C. Almeida, M. Abbate, J.M. Rosolen, Solid State Ionics 140 (2001) 241. [18] H.P. Oliveira, J.M. Rosolen, P.S. Calefi, C.A. Brunello, C.F.O. Graeff, in: M.E. Hawley, D.H. Blank, C.-B. Eom, S.K. Streitter, D.G. Schlom (Eds.), Multicomponent Oxide Films for Electronics, vol. 574, MRS, Pittsburgh, 1999, pp. 137 – 143. [19] A. Sruca, B. Orel, G. Drazie, B. Pihlar, J. Electrochem. Soc. 146 (1999) 232. [20] L. Abello, E. Husson, Y. Repelin, G. Lucazeau, Spectrochim. Acta 39A (1983) 641. [21] B. Pecquenard, D. Gourier, N. Baffier, Solid State Ionics 78 (1995) 287.
Enhancement of the electrochemical performance of a Li-intercalated V2O5 xerogel doped with Eu E.C. Almeida a, M. Abbate b, J.M. Rosolen a,* b
a Departamento de Quı´mica, FFCLRP-USP, 14040-901 Ribeira˜o Preto SP, Brazil Departamento de Fı´sica, UFPR, Caixa Postal 19091, 81531-990 Curitiba PR, Brazil
Received 26 June 2002; accepted 20 February 2003
Abstract We studied the electrochemical response of a Li-intercalated V2O5 xerogel doped with Eu. We found that the electrochemical performance of the rare-earth doped xerogel is improved. This is attributed to bonding of the rare-earth ions to the residual H2O in the xerogel structure. The electronic structure of the original V2O5 xerogel is not drastically affected by the rare-earth doping. D 2003 Elsevier Science B.V. All rights reserved. PACS: 81.20.Fw; 82.30Nr; 78.70.Dm Keywords: V2O5 xerogel; Li intercalation; X-ray absorption
1. Introduction V2O5 xerogel (xrg) is a useful electrode material in Li ion batteries and electrochromic devices [1– 5]. The electrochemical characteristic of V2O5 xerogel depends mostly on structural factors such as pillaring [1– 5]. Residual H2O in the xerogel provokes alterations in the V2O5 pillaring, affecting the electrochemical response and decreasing the electrical conductivity. In addition, the H2O molecules tend to bind to unreacted species in the structure, decreasing the mobility of the Li ions. Coustier et al. [6] doped V2O5 xerogel with diverse metal ions in an effort to neutralize this tendency. * Corresponding author. Tel.: +55-16-602-8787; fax: +55-16633-8151. E-mail address: [email protected] (J.M. Rosolen).
Here, we have studied the electrochemical response of a crystalline Li intercalated V2O5 xerogel doped with Eu. The principal characterization techniques were Fourier transform infra-red (FTIR), X-ray diffraction (XRD), cyclic voltammetry, galvanostatic charge– discharge and X-ray absorption spectroscopy (XAS). It was found that the reversible specific capacity of the rare-earth doped V2O5 xerogel composite electrode is improved. The improvement is attributed to bonding of the rare-earth ions to the residual H2O in the xerogel structure. However, the electronic structure of the original V2O5 xerogel is not drastically affected by the rare-earth doping. XAS is a useful tool in the study of the electronic structure. This technique provides selected information concerning the site and symmetry of unoccupied electronic states. The O 1s X-ray absorption edge is particularly useful when vanadium oxides are involved
0167-2738/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-2738(03)00126-7
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E.C. Almeida et al. / Solid State Ionics 160 (2003) 61–67
[7– 9]. The O 1s spectra are related to the O 2p unoccupied electronic states in the conduction band [10]. The diagnostic capability of XAS in this area has been demonstrated in studies of various Li-intercalated compounds [11 – 15].
2. Experimental details The V2O5 xerogel was prepared using a solution of 0.5 M sodium metavanadate (Alfa) and a proton exchange resin (Bayer, Lewatit S 100). For the preparation of the Eu0.11V2O5 xerogel, a Eu(OH)CO3 hydrated solution was added to the yellow HVO3. The high solubility of Eu(OH)CO3 in acid pH warrants a complete ionic exchange reaction between Eu3 + and HVO3 [16]. Both the undoped and Eudoped hydrogels were aged for 8 days and then heated at 50 jC for 12 h in air. The xerogels were then heated further at 300 jC for 4 h in air (treatment T1) or at 400 jC for 1 h in a 0.9 bar partial vacuum (treatment T2) to remove the residual water. The resulting powders were then milled in a mortar, sieved (300 mesh) in air, and stored in a dry-box. The concentrations of H2O in the xerogels submitted to thermal treatment T1 were approximately 0.2 mol in the Eu0.11V2O5 and 0.1 mol in the V2O5. These data were obtained from thermogravimetric/differential thermal analysis (heating rate 10 jC min 1, N2 atmosphere). Membrane composite electrodes were prepared with 10% polyvinylidene fluoride (PVDF) binder (Solvay), 10% carbon (ketjen black, MMB) and 80% xerogel (V2O5 or Eu0.11V2O5). The xerogel and carbon were mixed in a mortar and afterward added into the binder solution (PVDF dissolved in acetone). The slurry obtained was spread on a glass surface using a Doctor Blade. The membrane composite was cut in the form of disks (8-mm diameter). The electrochemical characterizations were performed in a coin cell with stainless-steel current collectors where lithium was used as the auxiliary and reference electrodes. Two layers of separator (Celgard) were used in the electrochemical cells. The electrolyte was a 1 M solution of LiClO4 (Aldrich, dried at 120 jC under vacuum) in propylene carbonate (Aldrich). All membrane electrodes were dried in a vacuum (0.9 bar) at 70 jC for 4 h before the cells were assembled (inside the dry-box). Galvanostatic charge/discharge
tests were performed using a Mac-Pile cycler, while the linear sweep voltammetry experiments were carried out with a PAR-362 and LabView-PC system. The powder XRD patterns were obtained using a Phillips 2001 diffractometer (Cu Ka radiation, graphite monochromator, 10– 90j range for 2h, 0.02j steps, and 3 s/step). The FTIR spectra were obtained using a Nicolet 5ZDX spectrometer (KBr beam splitter, 64 acquisitions, 2 cm 1 resolution, and transmission mode). The V 2p and O 1s XAS spectra were taken in the SGM beam line at the Laborato´rio Nacional de Luz Sincrontron (LNLS, Brazil). The spectra were collected in the total electron yield mode measuring the sample current. In this method of X-ray absorption, ˚ [15]. The the probing depth is approximately 50 A energy resolution at the V 2p and O 1s absorption edges was approximately 0.5 eV. The energy scale was calibrated using the known peak positions in the V2O5 spectrum. The base pressure in the experimental chamber was in the low 10 9 mbar range. The spectra were normalized to the maximum intensity after a constant background subtraction. For the XAS characterizations, the samples were intercalated by a chemical route. The details of the procedure are given elsewhere [17].
3. Results and discussion 3.1. Structural characterization The left side of Fig. 1 shows the XRD patterns for the V2O5 and Eu0.11V2O5 xerogels after both thermal treatments. All patterns show the features of polycrystalline orthorhombic V2O5 indicating interstitial Eu. However, the XRD patterns for Eu0.11V2O5 after thermal treatments T1 and T2 (Fig. 1(A) and (B), respectively) indicate less crystallinity than those for V2O5 (Fig. 1(C) and (D), respectively). The Bragg reflections in the Eu0.11V2O5 patterns are less intense and less sharp than those in the V2O5 patterns. Another difference observed in the patterns of Fig. 1 is the displacement of the Eu0.11V2O5 peaks with thermal ˚ treatment. While the lattice parameter d001 c 4.45 A for V2O5 after both thermal treatments, and also ˚ for Eu0.11V2O5 after treatment T2; d001 c 4.45 A
E.C. Almeida et al. / Solid State Ionics 160 (2003) 61–67
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Fig. 1. XRD patterns (left) and FTIR spectra (right) for (A) Eu0.11V2O5 xerogel after thermal treatment T1, (B) Eu0.11V2O5 after T2, (C) V2O5 after T1 and (D) V2O5 after T2.
˚ for Eu0.11V2O5 after d001 expands to about 4.75 A treatment T1. The right side of Fig. 1 shows the FTIR spectra for the xerogels after the two thermal treatments. The FTIR spectrum for V2O5 has been calculated by Sruca et al. [19]. In short, the bands in the range from 1020 to 950 cm 1 correspond to the V –O vanadyl stretching modes, while the band between 900 and 700 cm 1 can be ascribed to the bridging V – O – V stretching mode. For wave numbers smaller than 700 cm 1 the vibrations could be due to edge-sharing 3V – O stretching or bridging V – O –V deformations. The shift to lower wave numbers in the crystalline V2O5 spectra indicates either the presence of V4 + –O group modes or the lowering of the V2O5 crystal symmetry. The enhancement of the shoulder at about 980 cm 1 is an indication of the increased crystallinity of V2O5 [19,20]. Clearly, the FTIR spectra confirm that the V2O5 xerogel after thermal treatment T2 (see spectrum (D)) is the more crystalline of the four studied and that the Eu was inserted in the V2O5 layers. The Eu0.11V2O5 xerogel spectra (A) and (B) for thermal treatments T1 and T2, respectively, present the same vibrations mode of V2O5 but the bands associated with the V – O vanadyl stretching
and bridging V –O – V stretching modes are shifted to higher wave numbers. In summary, the FTIR results are in agreement with the XRD, showing that the Eu is in the V 2O5 interlayers and bonded to H2O molecules. In fact, not one of the samples with Eu gives any luminescence signal. All the xerogels studied here are single phase and crystalline, and, where applicable, the Eu insertions are responsible for the dehydration [18]. 3.2. Electrochemical characterization Fig. 2(A) and (B) reports the cyclic voltammetric curves for composite electrodes prepared with the V2O5 and Eu0.11V2O5 xerogels submitted to thermal treatments T1 and T2, respectively. During the first cathodic scan for both of the (T1) samples (see Fig. 2(A)), the peaks appear at the same potentials. In the anodic direction, the voltammograms for these electrodes show two superposed peaks that also occur at equivalent potentials. Subsequently, the cathodic/anodic peaks observed in the first discharge/charge cycle are either reduced or they disappear. After the first cycle, the voltammogram for V2O5 (T1) has only two broad cathodic and anodic peaks, while the voltam-
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Fig. 2. Voltammograms (0.1 mV. s 1 scan rate) for composite electrodes prepared with the Eu0.11V2O5 and V2O5 xerogels submitted to thermal treatments (A) T1 and (B) T2 (notice that the voltammogram of V2O5 was shifted up 1.0 mA).
mogram for Eu0.11V2O5 (T1) has two superposed peaks above 3 V and superposed broad peaks above 2 V. Thus, the voltammograms of Fig. 2(A) reveal similar features: irreversibility in the first cycle and stabilization of the anodic and cathodic charge ratio thereafter. Furthermore, the peak currents for the Eu0.11V2O5 (T1) electrode are more intense than those for the V2O5 (T1) electrode. In the case of the (T2) samples, the role of Eu – H2O interaction becomes more evident. The voltammogram for V2O5 (T2) in Fig. 2(B), in its main features, resembles the voltammogram for V2O5 (T1) in Fig. 2(A). It contains clear cathodic peaks in the first scan, with a similar peak distribution, and the same irreversibility. For the Eu0.11V2O5 (T2) electrode, the electrochemical response is quite different. The voltammogram for Eu0.11V2O5 (T2) shows only two cathodic and anodic peaks, independent of the cycle number. For both (T2) electrodes, the anodic and cathodic charge ratio does not become stable with cycling. The voltammograms of Fig. 2(B) also suggest that the electrodes prepared with (T2) xerogels are less efficient for lithium intercalation (i.e., the capacitive and ohmic effects increase) than those prepared with (T1) xerogels. It is well known that the peaks in
voltammograms involving lithium intercalation are associated with different occupation sites (oxi-reduction sites in the host material). Alterations in the distribution of the peaks are thus correlated with structural modifications that affect the intercalation. The results shown in Fig. 2 suggest that the presence of Eu in V2O5 improves its electrochemical performance, depending on the thermal treatment used in the crystallization of the xerogel. The response of Eu0.11V2O5 (T1) is better for lithium intercalation and its structure appears to be more stable to lithium intercalation cycles. The peak currents in the Eu0.11V2O5 (T1) voltammogram have the larger values and the superposition of the peaks is less than in the other voltammograms. However, the Eu3 + does not seem to participate of the oxi-reduction process associated with lithium site occupation in the range of potentials between 1.5 and 3.9 V. Fig. 2A shows that the Eu0.11V2O5 (T1) oxireduction peaks for the first insertion are practically at the same potential as those in the voltammograms for the pristine V2O5 xerogel (this hypothesis will be confirmed by the XAS characterization). Finally, the results of Fig. 2 show that an irreversible structural modification occurs during the first insertion in the crystalline xerogels. In fact, this is expected for polycrystalline vanadium pentoxide, where the irreversible
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structural alterations are associated with phase transition and/or phase segregation [21]. Fig. 3 shows the initial discharge voltage as a function of the lithium concentration x for composite electrodes prepared with V2O5 (circles) and Eu0.11V2O5 (line) xerogels submitted to thermal treatment T1. These curves have several plateaus that occur at the same potentials as the cathodic peaks during the voltammogram first cycles (see Fig. 2(A)). An initial discharge voltage curve is sensitive to the chemical potential of the ions and electrons, and, for an amorphous xerogel, it does not contain any plateaus [1]. The curves in Fig. 3 reveal that the structural effect provoked by the presence of Eu is evident for x > 0.5. The plateaus in the LixEu0.11V2O5 curve (line) are less well defined than those in the LixV2O5 curve (circles). This confirms what was detected by XRD, FTIR and the voltammograms, ie, that a sample doped with Eu is more amorphous than an undoped sample for the same thermal treatment. Finally, Table 1 presents values of the specific capacities for the less crystalline (T1) xerogel composite electrodes during given discharge/charge cycles, measured at different currents between the cutoff voltages of 2.0 and 3.7 V. The reversible specific capacity of Eu0.11V2O5 (T1) was 215 mA h g 1
Fig. 3. Variation of voltage as a function of lithium concentration x for composite electrodes prepared with the vanadium pentoxide xerogels LixEu0.11V2O5 (line) and LixV2O5 (circles). Both xerogels were submitted to thermal treatment T1 before preparation of the electrodes, and the data were taken at a discharge current of 0.010 mA cm 2.
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Table 1 Specific capacity of the xerogel composite electrodes during given discharge/charge cycles Electrode
Eu0.11V2O5 (T1)
V2O5 (T1)
1
Current density (AA g 1)
Specific capacity (mA h g 1st discharge
2nd discharge
10th cycle
(A) 15
269
215
215
(B) 21 (C) 26 (A) 22 (B) 23 (C) 22
260 255 225 255 270
215 215 216 246 216
215 215 181 146 119
)
The currents used in the cycles were (A) 100 AA, (B) 150 AA and (C) 120 AA.
(26 AA g 1) for the 10th discharge/charge cycle, the same value was found for the 2nd cycle. The specific capacity of V2O5 (T1) did not become reversible, and its value was only 119 mA h g 1 (22 AA g 1) for the 10th cycle. Therefore, the Eu0.11V2O5 (T1) composite electrode has better lithium insertion kinetics and structural stability than the V2O5 (T1) electrode. If the H2O concentration is very small, as is expected in the case of V2O5 (T2), electrochemical improvement upon Eu doping is not anticipated. Indeed, the Eu0.11V2O5 (T2) composite electrode was found to operate irreversibly with cutoff voltages of 1.5 and 3.9 V. The interaction between Eu and H2O molecules provokes alterations which enhance the electrochemical performance of a crystalline V2O5 xerogel. The Eu is hydrolyzed in the structure (for example, Eu(H2O)n3 + or Eu(H2O)n2 + 1). The typical layered structure of crystalline V2O5 is preserved by Eu insertion, but the active sites for lithium and the structural stability are affected. Still, the intercalation charge remains largely associated with the vanadium cation and/or oxygen as was observed in the pure V2O5 xerogel [17]. The Eu3 + is a polyvalent cation that does not participate in the oxi-reduction process of lithium intercalation. Further improvement would be expected from increasing the polarization of the hydration sphere; by decreasing the size of the lanthanide, for example. 3.3. XAS characterization Fig. 4(a) shows the O 1s XAS spectra of V2O5 and Li-intercalated LixV2O5 xerogels after thermal treat-
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Fig. 4(b) shows the O 1s XAS spectra of Eu0.11V2O5 and Li-intercalated LixEu0.11V2O5 xerogels after thermal treatment T1. First, we note that the spectra of Eu0.11V2O5 and V2O5 are strikingly similar. This shows that the electronic structure of the original material is not greatly affected by Eu doping. Further, we note that the spectra of LixV2O5 and LixEu0.11V2O5 are also rather similar. Again, this indicates that Eu doping does not affect the electronic structure of the material. The main effect of the Eu doping is for Eu to bind to unreacted H2O within the structure.
4. Conclusions
Fig. 4. O 1s X-ray absorption spectra for Li-free and chemically Liintercalated (a) V2O5 and (b) Eu0.11V2O5 xerogels after thermal treatment T1.
ment T1. The spectrum of V2O5 presents two peaks at threshold and broader bumps at higher energies. The first two peaks correspond to O 2p character mixed in the V 3d band, whereas the bumps correspond to O 2p character mixed in the V 4sp bands. The so-called V 3d band region at threshold is further split by crystalfield effects. These assignments are supported by band structure calculations of the related VO2 compound [10]. The spectra of Li-intercalated LixV2O5 present changes in both the peak and bumps regions. The V 3d mixed bands are broader and the bumps grow larger. All these effects signal an increase in the degree of covalency with the O 2p states. The most relevant change appears around 532 eV and is signaled by the arrow. This emerging spectral weight coincides with the main absorption peak in the Li2O spectrum [16]. This suggests that Li intercalation affects O 2p – V 3d mixed states and leads to Li2O formation.
The present work has shown that Eu insertion into a V2O5 xerogel provokes a meaningful improvement of its electrochemical performance. We obtained crystalline Eu-doped V2O5 composite electrodes which were tested in Li ion coin cells, yielding specific capacity and capacity fading results superior to those of crystalline V 2 O 5 composite electrodes also obtained from the sol – gel route. The rare earth does not participate in the oxi-reduction process. The charge transfer involved in the lithium insertion occurs only with V and/or O ions. The beneficial effect of Eu doping appears to be significant only for samples that have residual H2O.
Acknowledgements This work was supported by CNPq-PADCT (620238/97-6, 301493/95-2), LNLS and Fundacßa˜o Araucaria.
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