Influence of microstructure on the chemical diffusion of lithium ions in amorphous lithiated tungsten oxide films

Size: px

Start display at page:

Download "Influence of microstructure on the chemical diffusion of lithium ions in amorphous lithiated tungsten oxide films"

Clifton Cox
6 years ago
Views:

Electrochimica Acta 46 (2001) 3415 3419 www.elsevier.

1 Electrochimica Acta 46 (2001) Influence of microstructure on the chemical diffusion of lithium ions in amorphous lithiated tungsten oxide films Se-Hee Lee a, *, Hyeonsik M. Cheong a,b, C. Edwin Tracy a, Angelo Mascarenhas a, Roland Pitts a, Gary Jorgensen a, Satyen K. Deb a a National Renewable Energy Laboratory, Center for Basic Sciences, 1617 Cole Boule ard, Golden, CO 80401, USA b Sogang Uni ersity, Shinsoo-Dong, Seoul , South Korea Received 2 February 2001; received in revised form 26 April 2001 Abstract We describe how the chemical diffusion of lithium ions is affected by microstructural changes in amorphous lithiated tungsten oxide (a-li x WO 3 ) films as a function of lithium ion concentration. The chemical diffusion of lithium ions in a-li x WO 3 films is investigated using ac impedance spectroscopy and Raman scattering measurements. Two different a-wo 3 films deposited by thermal evaporation at different partial pressures of nitrogen are used for these measurements. Based on the results of ac impedance spectroscopy and Raman scattering measurements, we conclude that the mechanism of these diffusion phenomena is attributed to an increase or decrease of the W 6+ O/O W 6+ O ratio with lithium ion insertion Elsevier Science Ltd. All rights reserved. Keywords: Chemical diffusion; Tungsten oxide; ac Impedance; Raman; Lithium insertion 1. Introduction * Corresponding author. Tel.: ; fax: address: se hee lee@nrel.gov (S.-H. Lee). Electrochromism in amorphous tungsten oxide films has been studied extensively since it was discovered in 1969 [1]. Transmittance modulation is obtained by electrically controlling the oxidation states of an electrochromic electrode and a counter electrode by inserting or extracting small alkali metal ions. WO 3, MoO 3 and Nb 2 O 5 are well-known electrochromic materials that show cathodic coloration with H + or Li + ion insertion [2]. Understanding the mechanism of ion insertion into a-wo 3 films is one of the key issues that remains to be resolved to better understand the coloring/bleaching process in EC devices because this mechanism determines the response time and the durability of the device. It has been reported that the chemical diffusion coefficients of lithium ions in a-wo 3 thin films are influenced by lithium concentration in the films and are related closely to the microstructure [3 5]. However, the exact mechanism that can explain the relationship between the chemical diffusion coefficients of lithium ions and the microstructural changes of the a-wo 3 films with lithium insertion is not clear as yet. Therefore, finding a correlation between the chemical diffusion of lithium ions and the microstructure of the a-wo 3 films can provide important insights into the mechanism of ion insertion and eventually result in improved device performance. In this work, we have studied the effect of the microstructural changes in amorphous lithiated tungsten oxide (a-li x WO 3 ) films on the chemical diffusion of lithium ions as a function of lithium concentration and have correlated these kinetic properties with changes in the Raman spectra resulting from electrochemical ion insertion or extraction /01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S (01)

2 3416 S.-H. Lee et al. / Electrochimica Acta 46 (2001) Table 1 WO 3 deposition conditions and film characteristics Sample Nitrogen pressure (10 5 Torr) Density (g cm 3 ) Coloration efficiency (cm 2 C 1 ) A 1 a B a No nitrogen was introduced intentionally (base pressure). 2. Experimental The a-wo 3 films used in this study were prepared on thin polished stainless steel substrates (type 430) by evaporation of WO 3 powder of purity 99.99%. Different partial pressures of nitrogen were introduced during thermal evaporation to adjust densities of a-wo 3 films. A thin aluminum foil ( 0.01 mm thick) was placed alongside the stainless steel substrates during film deposition and carefully weighed immediately before and after deposition to determine film density. The a-wo 3 film preparation conditions and characterization parameters are listed in Table 1. The thickness of the WO 3 layer is about 250 nm. In order to prevent water absorption into the lithiated samples, we deposited nm of LiAlF 4 solid electrolyte on top of the a-wo 3 films. LiAlF 4 is known to have a high ionic conductivity and stability in ambient atmosphere [6] and is deposited only on the samples that would be used for the Raman measurements. The electrochemical insertion and removal of lithium in a-wo 3 films was carried out in a dry box using 1 M LiClO 4 in propylene carbonate (PC) as an electrolyte with lithium metal as both a reference and a counter electrode. The lithium ion diffusion in the a-li x WO 3 films was studied with alternating current (ac) impedance spectroscopy, using a Solartron model 1260/1265. An ac impedance spectrum is obtained by measuring the complex impedance of the WO 3 working electrode as a function of the applied frequency, f, which we varied between 50 khz and Hz. The electrical behavior of an ion-insertion electrode in a liquid electrolyte is well known to be equivalent to the so-called Randles circuit (illustrated in Fig. 1a). In this circuit, R e is the electronic resistance of the system. R ct is the chargetransfer resistance (interfacial reaction resistance), C dl the double layer capacitance of the electrode electrolyte interface, Z w the so-called Warburg diffusion impedance (ionic diffusion in the electrode), C L the limiting capacitance, and R L the limiting resistance. The complex impedance spectrum (Nyquist plot) of the circuit in Fig. 1a exhibits three regions corresponding to high, low and intermediate frequencies (Fig. 1b). From the Warburg impedance data, the chemical diffusion coefficient, D, of the inserted species into WO 3 can be calculated by the following formula: Z = V M(dE/dx) 1/2 zfd 1/2 a where F is the Faraday constant, V M the molar volume of WO 3,(dE/dx) the slope of the coulometric titration curve, =2 f, a the surface area of WO 3, and z the number of electrons involved in the oxidation reduction process. Under the condition of a finite thickness limiting diffusion, the diffusion coefficient can be calculated by the following equations without knowledge of de/dx of WO 3 electrode: X= Z = V M(dE/dx) zf la R=R L = V M(dE/dx)l 3zFaD (1) 1 = (2) C L (3) where l is the film thickness. From these equations, the following equation can be easily derived: C L R L = l 2 (4) 3D R L and C L can be calculated from a straight line at very low frequencies. More detail about ac impedance analysis of the insertion oxide films is available in the literature [7 9]. Fig. 1. (a) Equivalent electrical model of a WO 3 thin film electrode on a transparent electronic conductor. (b) Typical complex ac impedance spectrum (Nyquist plot) of the circuit shown in (a).

3 S.-H. Lee et al. / Electrochimica Acta 46 (2001) the accuracy in the Raman shift are estimated to be 2 cm Results and discussion Fig. 2. Typical impedance diagram of a Li 0.09 WO 3 thin film electrode (sample A) in LiClO 4 PC electrolyte. The Raman spectra were taken in the quasi-backscattering geometry using 100 mw of the nm line of an Ar ion laser, focused to a line of 5 mm 100 m, as the excitation source. The signal was dispersed by a Spex 0.6 m triple spectrometer and detected with a liquid-nitrogen cooled high-resolution charge-coupled device detector array. Both the spectral resolution and The typical impedance diagram of a Li 0.09 WO 3 thin film electrode (sample A) in LiClO 4 PC electrolyte is shown in Fig. 2. At very low frequencies the phase angle begins to increase due to the onset of finite length effects. This Nyquist plot of the Li 0.09 WO 3 thin film electrode suggests a low-resistance electrode interface process with the spectrum mostly dominated by a diffusion-limited process. The diffusion coefficients of lithium ions were obtained from the limiting resistance and the limiting capacitance. Although the limiting capacitance is not a pure capacitance due to the surface roughness of the film, we could assume it to be pure because the degree of aberration from a pure capacitance is negligible. Tables 2 and 3 give the electrochemical parameters of two different a-wo 3 films (samples A and B in Table 1) at various levels of lithium insertion. The limiting resistance and the limiting capacitance were measured using ac impedance spectroscopy. The diffusion coefficients increase with increasing x in a-li x WO 3 showing a maximum at x= and for samples A and B, respectively, and then decrease. Table 2 Results from the ac impedance spectroscopy measurements of sample A Level of lithium insertion (x) inli x WO 3 R L ( ) C D ( cm 2 s 1 L (mf) ) Table 3 Results from the ac impedance spectroscopy measurements of sample B Level of lithium insertion (x) inli x WO 3 R L ( ) C L (mf) D ( cm 2 s 1 )

4 3418 S.-H. Lee et al. / Electrochimica Acta 46 (2001) Fig. 3. Raman spectra of a-wo 3 films (sample A) as a function of lithium concentration. The value of x in a-li x WO 3 is: (a) 0.0 (as-deposited); (b) 0.018; (c) 0.054; (d) 0.090; (e) 0.180; and (f) Fig. 3 shows the Raman spectra for an evaporated a-wo 3 film (sample A) with various amounts of lithium insertion. The spectrum of an as-deposited film (a) shows a broad peak at 770 cm 1, due to the W 6+ O bonds [10]. There is also a relatively sharp peak at 950 Fig. 4. (a) The diffusion coefficient (D ) and (b) the Raman intensity ratio of W 6+ O/O W 6+ O as a function of lithium concentration (x) in a-li x WO 3 thin film. cm 1, which has been assigned to the W 6+ O stretching mode of terminal oxygen atoms on the surfaces of the cluster and microvoid structures in the film [11]. With lithium insertion, the overall intensity of the two peaks decreases due to the reduction of the (W 6+ and oxygen) bonds to the (W 5+ and oxygen) bonds. In our previous work [12 14], it has been established that the inserted lithium ions and electrons reduce W 6+ ions to W 5+ ions thus creating O W 5+ O and W 5+ O bonds. It has also been confirmed that the inserted lithium ions do not form any type of bonds but stay near the W 5+ O and O W 5+ O [12,13]. For a more detailed microstructural analysis, we calculated the ratio of the integrated intensity of the W 6+ O bond peak at 950 cm 1 to that of the O W 6+ O peak at 770 cm 1. Fig. 4 shows the W 6+ O/ O W 6+ O ratio (b) and the diffusion coefficients (a) of the lithium ions in a-li x WO 3 film as function of lithium concentration. The W 6+ O/O W 6+ O ratio and diffusion constant increase and decrease in similar fashion with increasing x in Li x WO 3. The data show that with initial lithium insertion, the W 6+ O/O W 6+ O ratio increases up to x=0.05 in a-li x WO 3 and then decreases. The initial increase of the W 6+ O/O W 6+ O ratio in the early stages of lithium insertion indicates that, when lithium ions and electrons are inserted into a-wo 3 films, the inserted lithium ions and electrons preferentially distribute to the O W 6+ O bonds rather than to the W 6+ O bonds. After a certain amount of lithium insertion, the decrease of the W 6+ O/O W 6+ O ratio indicates that the inserted lithium ions and electrons preferentially distribute to W 6+ O bonds rather than to the O W 6+ O bonds. Historically, many groups have reported separately on Raman spectroscopic results and diffusion kinetic results. However, all these investigations have been done independently resulting in little correlation between the chemical diffusion of lithium ions and the microstructure of the a-wo 3 films. Analyses of this literature indicate that there are no obvious general trends in the diffusion constant and Raman measurements. Delichere et al. [15] and Paul et al. [16] observed that upon H + /Li + insertion, an increase of the W 6+ O peak at 950 cm 1 occurs, while Ohtsuka et al. [17] showed that a depression of the W 6+ O peak occurs with lithium insertion. With respect to diffusion constants, Zhang et al. [4] reported a decrease of diffusion coefficients of lithium ions at low insertion levels, while Pyun et al. [5] reported an increase in diffusion coefficients of lithium ions at low insertion levels. Based on our experimental results, we propose a new diffusion mechanism in a-li x WO 3 films that can explain these seemingly contradictory results reported by other groups so far. We assume that there are two diffusion paths in a-wo 3 films. One is the surface of the

5 S.-H. Lee et al. / Electrochimica Acta 46 (2001) WO 3 clusters where the W 6+ O bonds are located. The other is the inside of the WO 3 clusters where the O W 6+ O bonds are located. In our previous work [12,13], it has been confirmed that when lithium ions and electrons are inserted, the electrons reduce W 6+ ionstow 5+ ions thus creating O W 5+ O and W 5+ O bonds. The inserted lithium ions also remain in close proximity to the (W 5+ and oxygen) bonds without forming any type of Li bonding. As a result, the O W 5+ OandW 5+ O bonds are no longer able to serve as a diffusion path because the lithium ions are combined with W 5+ and oxygen bonds. It also has been established that the surfaces of WO 3 clusters exhibit a faster lithium ion diffusion than that for the inside of the clusters and more porous a-wo 3 films show a faster lithium ion diffusion [4,18]. Therefore, it follows logically that the films having a higher W 6+ O/O W 6+ O ratio show a faster diffusion than the films having a lower W 6+ O/O W 6+ O ratio. 4. Conclusions The chemical diffusion of lithium ions in a-li x WO 3 films has been studied with respect to results of ac impedance spectroscopy and Raman scattering measurements. The diffusion coefficients increase with increasing x in a-li x WO 3 showing a maximum at x=0.072 and for samples A (5.9 g cm 3 ) and B (5.2 g cm 3 ), respectively, and then decrease. The W 6+ O/O W 6+ O ratio and diffusion constant increase and decrease in similar fashion with increasing x in Li x WO 3. From Raman measurements, the mechanism of these kinetic phenomena are attributed to an increase or decrease of the W 6+ O/O W 6+ O ratio with lithium ion insertion. We suggest and infer that a similar diffusion mechanism may occur for all amorphous intercalation materials having two diffusion paths; one that is on the surface area of a metal oxide cluster and another that is on the inside of the cluster. Acknowledgements This work was supported by the DOE Office of Building Technology, State and Local Programs, Sam Taylor, Program manager under Contract No. DE- AC36-99GO The work at Sogang was supported by Korea Research Foundation Grant No. KRF DP0135. References [1] (a) S.K. Deb, Appl. Optics Suppl. 3 (1969) 192; (b) S.K. Deb, Philos. Mag. 27 (1973) 801. [2] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, New York, 1995, p. 2. [3] G. Xu, L. Chen, Solid State Ionics (1998) [4] J.-G. Zhang, C.E. Tracy, D.K. Benson, S.K. Deb, J. Mater. Res. 8 (1996) [5] S.-I. Pyun, J.-S. Bae, J. Alloys Compd. 245 (1996) L1. [6] T. Oi, K. Miyauchi, K. Uehara, J. Appl. Phys. 53 (1982) [7] C. Ho, I.D. Raistrick, R.A. Huggins, J. Electrochem. Soc. 127 (1980) 343. [8] C. Bohnke, O. Bohnke, Solid State Ionics 39 (1990) 195. [9] J. Wang, J.M. Bell, Sol. Energy Mater. Sol. Cells 58 (1999) 411. [10] Y. Shigesato, A. Murayama, T. Kamimori, K. Matsuhiro, Appl. Surf. Sci (1988) 804. [11] J.V. Gabrusenoks, P.D. Chikmach, A.R. Lusis, J.J. Kleperis, G.M. Ramans, Solid State Ionics 14 (1984) 25. [12] S.-H. Lee, H.M. Cheong, J.-G. Zhang, A. Mascarenhas, D.K. Benson, S.K. Deb, Appl. Phys. Lett. 74 (1999) 242. [13] S.-H. Lee, H.M. Cheong, J.-G. Zhang, C. Edwin Tracy, A. Mascarenhas, D.K. Benson, S.K. Deb, Electrochim. Acta 44 (1999) [14] S.-H. Lee, H.M. Cheong, C. Edwin Tracy, A. Mascarenhas, A.W. Czanderna, S.K. Deb, Appl. Phys. Lett. 75 (1999) [15] P. Delichere, P. Falaras, M. Froment, A. Hugot-Le Goff, B. Aguis, Thin Solid Films 161 (1988) 35. [16] J.L. Paul, J.C. Lassegues, J. Solid State Chem. 106 (1993) 357. [17] T. Ohtsuka, N. Goto, N. Sato, J. Electroanal. Chem. 287 (1990) 249. [18] J. Nagai, T. Kamimori, M. Mizuhashi, Sol. Energy Mater. Sol. Cells 13 (1986) 279..

Thermodynamic study of WO 3 and WO 3 :Li + thin films

Materials Science and Engineering B 138 (2007) 123 127 Thermodynamic study of WO 3 and WO 3 :Li + thin films César O. Avellaneda Laboratório Interdisciplinar de Eletroquímica e Cerâmica-LIEC, Departamento