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Characterization of Ni–W oxide thin film electrodes
Solid State Ionics 109 (1998) 303–310
Characterization of Ni–W oxide thin film electrodes Se-Hee Lee*, Young-Sin Park, Seung-Ki Joo Department of Material Science and Engineering, Seoul National University, San 56 -1 Shillim-Dong Kwanak-Ku, Seoul 151 -742, South Korea Received 2 December 1997; accepted 9 February 1998
Abstract Ni–W oxide thin films were fabricated by co-sputtering of nickel and tungsten in oxygen ambient. It has been found that the tungsten amount is very closely related to the lithium intercalation kinetics. This new material has been found to show anodic coloration with lithium ions. Compared with NiOx thin films, Ni–W oxide thin films showed considerable improvements in charge transfer density and optical response. 1998 Elsevier Science B.V. All rights reserved. Keywords: Anodic coloration; Charge transfer density; Co-sputtering; Lithium intercalation; Optical response; Thin film
1. Introduction The intercalation process in nonstoichiometric nickel oxide, NiO x , has been studied quite consistently [1]. Following the well known proton insertion process carried out in aqueous media, many studies have been performed in the past years on the intercalation reaction of small-size cations in amorphous NiO x in aqueous, as well as in water-free systems [2,3]. Recently, it was shown by Passerini et al. [4] that nickel oxide sputtered samples, if kept in rigorously dry conditions, can undergo a reversible lithium intercalation–deintercalation process, which is accompanied by a net electrochromic effect. It has also been reported that since the NiO x is a p-type material due to the negatively charged nickel vacancies, it experiences the transformation from p-type *Corresponding author. National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401, USA. E-mail: [email protected]; fax: 11 303 384 6531.
to intrinsic and then to an n-type with electrochemical intercalation of lithium [5]. Therefore, a large charge transfer resistance occurs in the middle of lithium intercalation and deintercalation, which makes the intercalation process very slow. NiO x films which contain a sufficient amount of lithium have been used for a lithium source in electrochromic cells. The only drawback in this case is that NiO x is optically passive and does not function as a complementary counterelectrode. Hence the color contrast has to rely totally upon a cathodically coloration electrode, which produces a low electrochromic efficiency. In this work, a new electrochromic material which can be colored anodically has been synthesized by addition of tungsten oxide to NiOx thin films. The effects of tungsten oxide addition on lithium intercalation kinetics were investigated. The electrochromic behavior of Ni–W oxide thin film electrodes with lithium as an electroactive species has been investigated through electrochemical (constant current
0167-2738 / 98 / $19.00 1998 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00093-9
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charge–discharge, cyclic voltammetry, and a.c. impedance spectroscopy) and optical (transmittance spectra) measurements.
2. Experimental Thin films of Ni–W oxide were deposited by reactive RF-magnetron sputtering from composite targets comprised of pie-shaped wedges of W metal arrayed radially on circular metal Ni targets. After evacuation to about 6 3 10 26 Torr, oxygen was introduced to a pressure of 10 mTorr, and the discharge power was set at 150 W. The average thickness of the Ni–W oxide films was 120 nm. Ni–W oxide films were deposited on indium tin oxide (ITO) coated glass of 10 V / h for electrochemical measurements and on SiO 2 / Si (100) wafers for X-ray diffraction analysis. The composition of the Ni–W oxide films was evaluated by energy dispersive X-ray analysis (EDS). Thin film X-ray diffraction methods were used to confirm the crystal structure of the films. An automated Rigaku X-ray thin film diffractometer with Cu K a radiation, monochromated by a graphite crystal, was used. X-ray photoelectron spectroscopy (XPS) measurements were made using a PHI5600 ESCA system. The X-ray source was Al K a . Charge corrections were made by assigning 284.5 eV binding energy to the C 1s peak. Charge corrections were less than 0.2 eV for the data reported here. The electrochemical insertion and removal of lithium in Ni–W oxide films was carried out in a dry box, where oxygen and water-free ambient were maintained by argon. 1 M LiClO 4 –PC–DME (propylene carbonate plus 1,2-dimethoxyethane, 50 / 50 by volume) was used as the electrolyte and lithium metal as a reference and counterelectrode. The electrochemical cells were discharged and charged at a constant current between preset voltage limits using a current source (Keithley 220) and a voltmeter (Keithley 192). Optical properties were measured with a HP8452A diode array spectrophotometer and in-situ optical transmittance measurements were carried out using an optical system consisting of a He–Ne laser (633 nm) source and a silicon photodetector. Impedance measurements were made over the frequency range from 10 22 to 10 6 Hz using
a Zahner model IM5d Impedance Spectrum Analyzer. The a.c. response of the electrochemical system with either charge transfer or diffusion-limited kinetics was analyzed with the well known equivalent circuit [6]. An electrochromic display device based on combinations of Ni–W oxide film with K 0.3 WO 3.15 [7] was fabricated and its optical switching characterized. The electrochromic display devices were made by joining pre-lithiated Ni–W oxide film (25 a / o W in W/(W 1 Ni)) with K 0.3 WO 3.15 film. The polymeric solid electrolyte was laminated between two electrodes. The distance between two electrodes was about 50 m and the device area was about 9 cm 2 . The polymer was comprised of polyethylene oxide, lithium perchlorate, and propylene carbonate. The preparation and Li 1 conductivities of these polymer electrolyte films may be found in Ref. [8]. Ni–W oxide films were cycled electrochemically in a 1 M LiClO 4 –PC–DME (propylene carbonate plus 1,2dimethoxyethane, 50 / 50 by volume) electrolyte prior to lamination, and pre-reduced with about 30 mC cm 22 . K 0.3 WO 3.15 films of average thickness 400 nm were deposited from the corresponding oxide power by thermal evaporation [7].
3. Results and discussion
3.1. As-deposited films Fig. 1 shows the evolution of X-ray diffraction patterns of Ni–W–O thin films prepared on a SiO 2 / Si (100) wafer with variable tungsten content. The pure nickel oxide film exhibits a crystalline phase with a strong (200) diffraction peak, indicating the preferred orientation of the film surface. As the tungsten content increases, the intensity of the (200) peak decreases sensitively, finally reaching an amorphous phase with 25 a / o tungsten. This clearly indicates that tungsten addition induces a weakening of the long-range order in the Ni–O matrix. Tungsten ions of high oxidation state (6 1 ) would induce a large perturbation of the lattice around a tungsten cation inserted in a site left free by nickel because of the large difference between the stable valence states, 2 1 and 6 1 , respectively, and they seem to change the overall structure to a less dense state.
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Fig. 2. XPS response of Ni–W oxide thin films. Detailed nickel peaks: (a) 5 a / o W; (b) 25 a / o W. Fig. 1. X-ray diffraction patterns of as-deposited NiO x and Ni–W oxide thin films on a SiO 2 / Si (100) wafer.
Such a structurally modified phase has an essential advantage in electrochromic performance, in that it can offer a desirable diffusion pathway for lithium ions contrary to the dense cubic structure of NiO x . The XPS results of some selected samples are summarized in Table 1. The W 4f 7 / 2 peaks are found at the same position of 35.5 eV, as in WO 3 reported by Colton [9], regardless of the tungsten content. This confirms that the oxidation state of tungsten is constantly 6 1 in our samples. Two Ni 2p peaks were observed for as-deposited film samples at positions of around 854 and 857 eV, which are attributed to Ni 21 and Ni 31 states, respectively. The relative intensity ratio increased with increase of tungsten content (Fig. 2) and when the lithium ions are inserted, practically all the nickel ions are reduced to Ni 21 (Fig. 3). The decrease of the Ni 31
ion concentration with tungsten addition can be understood on the basis of defect chemistry: nonstoichiometric nickel oxide is known to be a p-type material due to nickel vacancies, giving rise to Ni 31 ions. The holes are thought to be coupled with Ni 31 ions to form polarons and are electrically compensated by the doubly negatively charged nickel vacancies in order to maintain charge neutrality. However, if W 61 ions can be assumed to replace Ni 21 , then doubly negatively charged nickel vacancies are generated for charge compensation: WO 3 ↔W 41 Ni 1 2V0 Ni 1 3Oo.
(1)
Assuming doubly ionized anion vacancies V0 Ni as predominant defects in Ni–W oxide films, the following equilibrium should be taken into account: O 2 ↔V0 Ni 1 2h 1 Oo.
(2)
Table 1 X-ray photoelectron spectroscopy binding energies for Ni–W oxide thin films Sample
Ni 2p (Ni 21 )
Ni 2p (Ni 31 )
Intensity ratio, INi 21 /INi 31
W 4f 7 / 2
5 a/o W 25 a / o W 25 a / o W (bleached)
854.9 854.2 853.0
857.5 857.4
0.94 1.05
35.5 35.5 35.5
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Fig. 4. Dependence of the electrical resistivity of Ni–W oxide thin films on tungsten content.
Fig. 3. XPS response of Ni–W oxide thin films (25 a / o W). Detailed nickel peaks: (a) as-deposited, colored state; (b) bleached with 30 mC cm 22 Li.
Considering the above two conditions, the following relationship between the hole concentration and the additive tungsten amount can be obtained: 41 21 / 2 [h] 5 P 1O/24s ]21 Kd 1 / 2 [W Ni ] ,
(3)
where PO 2 is the equilibrium oxygen partial pressure and K the reaction constant of the reaction, Eq. (2). Eq. (3) implies that the hole concentration decreases with tungsten addition in Ni–W oxide thin films. This explanation is supported by the electrical resistivity measurement (Fig. 4). The increase of electrical resistivity with tungsten content is in good agreement with Eq. (3) and XPS results (Fig. 2).
3.2. Electrochemistry of Ni–W oxide films Fig. 5 shows the effect of tungsten concentration on the cyclic voltammetry of Ni–W oxide thin films. These voltammetries were obtained after the stabilization scans. It is clear that with increasing tungsten amount added, the cathodic and anodic peaks in-
Fig. 5. Cyclic voltammetry of Ni–W oxide thin films immersed in LiClO 4 –PC–DME electrolyte. Scan rate 20 mV s 21 .
S.-H. Lee et al. / Solid State Ionics 109 (1998) 303 – 310
crease, which indicates fast kinetics for the lithium insertion and removal processes. When the amount of tungsten addition becomes .25 a / o, these peaks decrease and the anodic coloration behavior becomes ambiguous. A typical cyclic voltammogram of the as-deposited Ni–W oxide film (25 a / o W) is shown in Fig. 6. It can be seen that in the first cycle, most of the lithium is inserted at low voltage (below 2 V vs. Li), but in the following cycles, most of the lithium is inserted at around 3 V. The new cathodic peak at 3 V after the first cycle becomes deeper as the number of scans increases until no more change appears after seven or eight cycles. Most of the irreversibly retained lithium was inserted during the first cycle, which can be seen from the difference between the cathodic and anodic charge in this figure. The gradual increase of capacity with the number of cycles may reflect that lithium intercalation–deintercalation processes modify the microstructure of the Ni–W oxide film to a more favorable state for easy lithium diffusion. Even though amorphization of the film has already brought about an important macroscopic structural change compared with a NiO x film, such an additional local modification would also contribute to faster lithium diffusion. In order to understand the anodic coloration of Ni–W oxide films (25 a / o W), a series of in-situ
Fig. 6. Cyclic voltammetry of an as-deposited Ni–W oxide thin film (25 a / o W). Scan rate 20 mV s 21 .
307
optical transmittance measurements were carried out at various cut-off voltages (Fig. 7). The appearance of a bleached state with lithium insertion confirms that the overall electrochromism of our sample is dominantly related to the variation of the oxidation state of nickel ions between 21 and 31, while the tungsten ions play a minor role, as expected from the XPS data (Table 1 and Fig. 3). In all cases, dent-like small peaks are observed in the first discharge process. Their existence seems to reflect that the individual ions vary their oxidation states in the course of the first lithium insertion, changing the local structure. Actually, these small fluctuations in transmittance appear even in the fourth cycle when the cut-off voltage is 2.3 V, where the structural modification is believed to occur more slowly. When the cut-off voltage is 2.3 V (Fig. 7a), the transmittance of the bleached state (TB) is relatively low (60%) compared with the other cases (80%). This suggests that Ni 31 ions are not totally reduced by the lithium insertion process, probably due to the high cut-off voltage. As the cut-off voltage is lowered below 2.0 V, it seems that the Ni 31 concentration reducible by lithium insertion increases, and almost all the Ni 31 ions are reduced to transparent Ni 21 , leading to the maximum TB (approx. 80%). It should be mentioned that the W 61 ions probably begin to be reduced to W 51 from 3.0 V as in pure WO 3 and the inserted lithium ions can be thought to be fixed in the matrix below 2.0 V [10]. Therefore, the bleached state of Fig. 7b,c should not be interpreted simply by the reduction of nickel ions alone, but should be considered as the increase of reducible Ni 31 ions which are participating in the bleaching process due to the lowering of the cut-off voltage may compensate the simultaneously generated W 51 ions, due to the lowering of the cutoff voltage. The existence of irreversibly retained lithium ions is confirmed once again by the first discharge time being longer than the charge time. However, whether they are mainly coupled with nickel or tungsten ions depends on the cut-off voltage. The transmittance after the first charge is slightly higher than the as-deposited film at higher cut-off voltage (Fig. 7a), whereas the reverse result is observed as the voltage decreases. This strongly suggests that the retained
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Fig. 7. In-situ optical transmittance of an as-deposited Ni–W oxide thin film (25 a / o W) during constant current (100 mA cm 22 ) charge and discharge tests. (a) Lower cut-off voltage 2.3 V, (b) 2.0 V, (c) 1.5 V.
lithium ions after the first cycle with high cut-off voltage are mainly coupled with Ni 21 ions, giving rise to the slight increase of transmittance. On the other hand, the W 51 ions tend to couple with the lithium ions at cut-off voltages below 2.0 V. The initial insertion of lithium gives rise to a significant change in the charge transfer resistance of the as-deposited Ni–W oxide film (Fig. 8). The charge transfer resistance of the as-deposited Ni–W oxide film (25 a / o W) was about 10 5 V, but decreased significantly to less than 400 V after the first discharge. This may be related to the irreversibly reduced tungsten oxide phases, which are probably randomly distributed in the matrix, because such reduced sites are susceptible to act as microscopic current collecting centers
3.3. Complementary electrochromic device The electrochromic display devices were made by joining pre-lithiated NiO x and Ni–W oxide films (25 a / o W) with a K 0.3 WO 3.15 film. The optical responses of these electrochromic devices using Ni–W oxide and NiO x as a counter electrode for K 0.3 WO 3.15 are compared in Fig. 9. These current and optical responses were obtained using a potentiostat with the K 0.3 WO 3.15 acting as a working electrode and the Ni–W oxide and NiO x as a counterelectrode which were also shorted to the reference. Faster kinetics of lithium insertion in Ni– W oxide than in NiO x is obvious from this figure. In particular, a much enhanced color change can be noticed from the top figure of Fig. 9. This is due to
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Fig. 7. (continued)
Fig. 9. Comparison of the optical responses of the electrochromic devices using Ni–W oxide or NiO x as a counterelectrode for the K 0.3 WO 3.15 working electrode. (? ? ?) NiO x ; (———) Ni–W oxide. Input voltages are shown at the bottom.
Fig. 8. Dependence of the charge transfer resistance of Ni–W oxide thin films (25 a / o W) on lithium insertion.
the fact that the Ni–W oxide electrode acts as a complementary electrode compared with the optically passive NiO x electrode. The coloration efficiencies at 633 nm of Ni–W oxide, K 0.3 WO 3.15 and the ECD device composed of these two electrodes are compared in Fig. 10. As can be seen, there is a linear relationship between the change in optical density and charge transfer. The coloration efficiencies of Ni–W oxide, K 0.3 WO 3.15 and the ECD device were calculated from a least squares fit to the data in Fig. 10 giving 22, 35 and 58 cm 2 / C, respectively. It can be seen that the
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was attributed to a reduction of the charge transfer resistance and fast diffusion of lithium ions. The ECD device was fabricated from K 0.3 WO 3.15 and Ni–W oxide and showed a fast response and good contrast. The coloration efficiency of the ECD device was 58 cm 2 / C at 633 nm.
Acknowledgements This work was supported by the Korea Science and Engineering Foundation through the Research Center for Thin Film Fabrication and Crystal Growing of Advanced Materials at Seoul National University. Fig. 10. The change in optical density vs. charge transfer for Ni–W oxide, K 0.3 WO 3.15 and the ECD device composed of these two electrodes.
overall coloration efficiency of a complementary electrochromic system is the sum of the coloration efficiencies of the individual coloring films. It is clear that the Ni–W oxide functions as an anodic coloration material and the optical response of the ECD comprised of Ni–W oxide and K 0.3 WO 3.15 thin films can be very much enhanced.
4. Conclusion Ni–W oxide exhibits a faster lithium insertion– removal process than nonstoichiometric nickel oxide and can contribute to an optical modulation by coloring and bleaching synchronously with the working electrode. The NiO x phase in Ni–W oxide was amorphized by addition of tungsten oxide. The fast electrochemical response of the Ni–W oxide film
References [1] J.S.E.M. Svensson, C.G. Granqvist, Appl. Phys. Lett. 49 (1986) 1566. [2] W. Estrada, A.M. Andersson, C.G. Granqvist, J. Appl. Phys. 64 (1988) 3678. [3] F. Hahn, D. Floner, B. Beolen, C. Lamy, Electrochim. Acta 32 (1987) 1631. [4] S. Passerini, B. Scrosati, A. Gorenstein, J. Electrochem. Soc. 137 (1990) 3297. [5] S. Passerini, B. Scrosati, J. Electrochem. Soc. 141 (1994) 889. [6] C. Ho, I.D. Raistrick, R.A. Huggins, J. Electrochem. Soc. 127 (1980) 343. [7] S.-K. Joo, R.A. Huggins, Mater. Res. Bull. 20 (1985) 897. [8] S.-H. Lee, K.H. Hwang, S.K. Joo, in: K.-C. Ho, D.A. MacArthur (Eds.), Electrochromic Materials, PV 94-2, The Electrochemical Society Softbound Proceedings Series, Pennington, NJ, 1994, p. 290. [9] R.J. Colton, A.M. Gutzman, J.W. Rablais, J. Appl. Phys. 49 (1978) 409. [10] S.K. Mohapatra, S. Wagner, J. Electrochem. Soc. 125 (1978) 1603.
Characterization of Ni–W oxide thin film electrodes Se-Hee Lee*, Young-Sin Park, Seung-Ki Joo Department of Material Science and Engineering, Seoul National University, San 56 -1 Shillim-Dong Kwanak-Ku, Seoul 151 -742, South Korea Received 2 December 1997; accepted 9 February 1998
Abstract Ni–W oxide thin films were fabricated by co-sputtering of nickel and tungsten in oxygen ambient. It has been found that the tungsten amount is very closely related to the lithium intercalation kinetics. This new material has been found to show anodic coloration with lithium ions. Compared with NiOx thin films, Ni–W oxide thin films showed considerable improvements in charge transfer density and optical response. 1998 Elsevier Science B.V. All rights reserved. Keywords: Anodic coloration; Charge transfer density; Co-sputtering; Lithium intercalation; Optical response; Thin film
1. Introduction The intercalation process in nonstoichiometric nickel oxide, NiO x , has been studied quite consistently [1]. Following the well known proton insertion process carried out in aqueous media, many studies have been performed in the past years on the intercalation reaction of small-size cations in amorphous NiO x in aqueous, as well as in water-free systems [2,3]. Recently, it was shown by Passerini et al. [4] that nickel oxide sputtered samples, if kept in rigorously dry conditions, can undergo a reversible lithium intercalation–deintercalation process, which is accompanied by a net electrochromic effect. It has also been reported that since the NiO x is a p-type material due to the negatively charged nickel vacancies, it experiences the transformation from p-type *Corresponding author. National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401, USA. E-mail: [email protected]; fax: 11 303 384 6531.
to intrinsic and then to an n-type with electrochemical intercalation of lithium [5]. Therefore, a large charge transfer resistance occurs in the middle of lithium intercalation and deintercalation, which makes the intercalation process very slow. NiO x films which contain a sufficient amount of lithium have been used for a lithium source in electrochromic cells. The only drawback in this case is that NiO x is optically passive and does not function as a complementary counterelectrode. Hence the color contrast has to rely totally upon a cathodically coloration electrode, which produces a low electrochromic efficiency. In this work, a new electrochromic material which can be colored anodically has been synthesized by addition of tungsten oxide to NiOx thin films. The effects of tungsten oxide addition on lithium intercalation kinetics were investigated. The electrochromic behavior of Ni–W oxide thin film electrodes with lithium as an electroactive species has been investigated through electrochemical (constant current
0167-2738 / 98 / $19.00 1998 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00093-9
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charge–discharge, cyclic voltammetry, and a.c. impedance spectroscopy) and optical (transmittance spectra) measurements.
2. Experimental Thin films of Ni–W oxide were deposited by reactive RF-magnetron sputtering from composite targets comprised of pie-shaped wedges of W metal arrayed radially on circular metal Ni targets. After evacuation to about 6 3 10 26 Torr, oxygen was introduced to a pressure of 10 mTorr, and the discharge power was set at 150 W. The average thickness of the Ni–W oxide films was 120 nm. Ni–W oxide films were deposited on indium tin oxide (ITO) coated glass of 10 V / h for electrochemical measurements and on SiO 2 / Si (100) wafers for X-ray diffraction analysis. The composition of the Ni–W oxide films was evaluated by energy dispersive X-ray analysis (EDS). Thin film X-ray diffraction methods were used to confirm the crystal structure of the films. An automated Rigaku X-ray thin film diffractometer with Cu K a radiation, monochromated by a graphite crystal, was used. X-ray photoelectron spectroscopy (XPS) measurements were made using a PHI5600 ESCA system. The X-ray source was Al K a . Charge corrections were made by assigning 284.5 eV binding energy to the C 1s peak. Charge corrections were less than 0.2 eV for the data reported here. The electrochemical insertion and removal of lithium in Ni–W oxide films was carried out in a dry box, where oxygen and water-free ambient were maintained by argon. 1 M LiClO 4 –PC–DME (propylene carbonate plus 1,2-dimethoxyethane, 50 / 50 by volume) was used as the electrolyte and lithium metal as a reference and counterelectrode. The electrochemical cells were discharged and charged at a constant current between preset voltage limits using a current source (Keithley 220) and a voltmeter (Keithley 192). Optical properties were measured with a HP8452A diode array spectrophotometer and in-situ optical transmittance measurements were carried out using an optical system consisting of a He–Ne laser (633 nm) source and a silicon photodetector. Impedance measurements were made over the frequency range from 10 22 to 10 6 Hz using
a Zahner model IM5d Impedance Spectrum Analyzer. The a.c. response of the electrochemical system with either charge transfer or diffusion-limited kinetics was analyzed with the well known equivalent circuit [6]. An electrochromic display device based on combinations of Ni–W oxide film with K 0.3 WO 3.15 [7] was fabricated and its optical switching characterized. The electrochromic display devices were made by joining pre-lithiated Ni–W oxide film (25 a / o W in W/(W 1 Ni)) with K 0.3 WO 3.15 film. The polymeric solid electrolyte was laminated between two electrodes. The distance between two electrodes was about 50 m and the device area was about 9 cm 2 . The polymer was comprised of polyethylene oxide, lithium perchlorate, and propylene carbonate. The preparation and Li 1 conductivities of these polymer electrolyte films may be found in Ref. [8]. Ni–W oxide films were cycled electrochemically in a 1 M LiClO 4 –PC–DME (propylene carbonate plus 1,2dimethoxyethane, 50 / 50 by volume) electrolyte prior to lamination, and pre-reduced with about 30 mC cm 22 . K 0.3 WO 3.15 films of average thickness 400 nm were deposited from the corresponding oxide power by thermal evaporation [7].
3. Results and discussion
3.1. As-deposited films Fig. 1 shows the evolution of X-ray diffraction patterns of Ni–W–O thin films prepared on a SiO 2 / Si (100) wafer with variable tungsten content. The pure nickel oxide film exhibits a crystalline phase with a strong (200) diffraction peak, indicating the preferred orientation of the film surface. As the tungsten content increases, the intensity of the (200) peak decreases sensitively, finally reaching an amorphous phase with 25 a / o tungsten. This clearly indicates that tungsten addition induces a weakening of the long-range order in the Ni–O matrix. Tungsten ions of high oxidation state (6 1 ) would induce a large perturbation of the lattice around a tungsten cation inserted in a site left free by nickel because of the large difference between the stable valence states, 2 1 and 6 1 , respectively, and they seem to change the overall structure to a less dense state.
S.-H. Lee et al. / Solid State Ionics 109 (1998) 303 – 310
305
Fig. 2. XPS response of Ni–W oxide thin films. Detailed nickel peaks: (a) 5 a / o W; (b) 25 a / o W. Fig. 1. X-ray diffraction patterns of as-deposited NiO x and Ni–W oxide thin films on a SiO 2 / Si (100) wafer.
Such a structurally modified phase has an essential advantage in electrochromic performance, in that it can offer a desirable diffusion pathway for lithium ions contrary to the dense cubic structure of NiO x . The XPS results of some selected samples are summarized in Table 1. The W 4f 7 / 2 peaks are found at the same position of 35.5 eV, as in WO 3 reported by Colton [9], regardless of the tungsten content. This confirms that the oxidation state of tungsten is constantly 6 1 in our samples. Two Ni 2p peaks were observed for as-deposited film samples at positions of around 854 and 857 eV, which are attributed to Ni 21 and Ni 31 states, respectively. The relative intensity ratio increased with increase of tungsten content (Fig. 2) and when the lithium ions are inserted, practically all the nickel ions are reduced to Ni 21 (Fig. 3). The decrease of the Ni 31
ion concentration with tungsten addition can be understood on the basis of defect chemistry: nonstoichiometric nickel oxide is known to be a p-type material due to nickel vacancies, giving rise to Ni 31 ions. The holes are thought to be coupled with Ni 31 ions to form polarons and are electrically compensated by the doubly negatively charged nickel vacancies in order to maintain charge neutrality. However, if W 61 ions can be assumed to replace Ni 21 , then doubly negatively charged nickel vacancies are generated for charge compensation: WO 3 ↔W 41 Ni 1 2V0 Ni 1 3Oo.
(1)
Assuming doubly ionized anion vacancies V0 Ni as predominant defects in Ni–W oxide films, the following equilibrium should be taken into account: O 2 ↔V0 Ni 1 2h 1 Oo.
(2)
Table 1 X-ray photoelectron spectroscopy binding energies for Ni–W oxide thin films Sample
Ni 2p (Ni 21 )
Ni 2p (Ni 31 )
Intensity ratio, INi 21 /INi 31
W 4f 7 / 2
5 a/o W 25 a / o W 25 a / o W (bleached)
854.9 854.2 853.0
857.5 857.4
0.94 1.05
35.5 35.5 35.5
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Fig. 4. Dependence of the electrical resistivity of Ni–W oxide thin films on tungsten content.
Fig. 3. XPS response of Ni–W oxide thin films (25 a / o W). Detailed nickel peaks: (a) as-deposited, colored state; (b) bleached with 30 mC cm 22 Li.
Considering the above two conditions, the following relationship between the hole concentration and the additive tungsten amount can be obtained: 41 21 / 2 [h] 5 P 1O/24s ]21 Kd 1 / 2 [W Ni ] ,
(3)
where PO 2 is the equilibrium oxygen partial pressure and K the reaction constant of the reaction, Eq. (2). Eq. (3) implies that the hole concentration decreases with tungsten addition in Ni–W oxide thin films. This explanation is supported by the electrical resistivity measurement (Fig. 4). The increase of electrical resistivity with tungsten content is in good agreement with Eq. (3) and XPS results (Fig. 2).
3.2. Electrochemistry of Ni–W oxide films Fig. 5 shows the effect of tungsten concentration on the cyclic voltammetry of Ni–W oxide thin films. These voltammetries were obtained after the stabilization scans. It is clear that with increasing tungsten amount added, the cathodic and anodic peaks in-
Fig. 5. Cyclic voltammetry of Ni–W oxide thin films immersed in LiClO 4 –PC–DME electrolyte. Scan rate 20 mV s 21 .
S.-H. Lee et al. / Solid State Ionics 109 (1998) 303 – 310
crease, which indicates fast kinetics for the lithium insertion and removal processes. When the amount of tungsten addition becomes .25 a / o, these peaks decrease and the anodic coloration behavior becomes ambiguous. A typical cyclic voltammogram of the as-deposited Ni–W oxide film (25 a / o W) is shown in Fig. 6. It can be seen that in the first cycle, most of the lithium is inserted at low voltage (below 2 V vs. Li), but in the following cycles, most of the lithium is inserted at around 3 V. The new cathodic peak at 3 V after the first cycle becomes deeper as the number of scans increases until no more change appears after seven or eight cycles. Most of the irreversibly retained lithium was inserted during the first cycle, which can be seen from the difference between the cathodic and anodic charge in this figure. The gradual increase of capacity with the number of cycles may reflect that lithium intercalation–deintercalation processes modify the microstructure of the Ni–W oxide film to a more favorable state for easy lithium diffusion. Even though amorphization of the film has already brought about an important macroscopic structural change compared with a NiO x film, such an additional local modification would also contribute to faster lithium diffusion. In order to understand the anodic coloration of Ni–W oxide films (25 a / o W), a series of in-situ
Fig. 6. Cyclic voltammetry of an as-deposited Ni–W oxide thin film (25 a / o W). Scan rate 20 mV s 21 .
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optical transmittance measurements were carried out at various cut-off voltages (Fig. 7). The appearance of a bleached state with lithium insertion confirms that the overall electrochromism of our sample is dominantly related to the variation of the oxidation state of nickel ions between 21 and 31, while the tungsten ions play a minor role, as expected from the XPS data (Table 1 and Fig. 3). In all cases, dent-like small peaks are observed in the first discharge process. Their existence seems to reflect that the individual ions vary their oxidation states in the course of the first lithium insertion, changing the local structure. Actually, these small fluctuations in transmittance appear even in the fourth cycle when the cut-off voltage is 2.3 V, where the structural modification is believed to occur more slowly. When the cut-off voltage is 2.3 V (Fig. 7a), the transmittance of the bleached state (TB) is relatively low (60%) compared with the other cases (80%). This suggests that Ni 31 ions are not totally reduced by the lithium insertion process, probably due to the high cut-off voltage. As the cut-off voltage is lowered below 2.0 V, it seems that the Ni 31 concentration reducible by lithium insertion increases, and almost all the Ni 31 ions are reduced to transparent Ni 21 , leading to the maximum TB (approx. 80%). It should be mentioned that the W 61 ions probably begin to be reduced to W 51 from 3.0 V as in pure WO 3 and the inserted lithium ions can be thought to be fixed in the matrix below 2.0 V [10]. Therefore, the bleached state of Fig. 7b,c should not be interpreted simply by the reduction of nickel ions alone, but should be considered as the increase of reducible Ni 31 ions which are participating in the bleaching process due to the lowering of the cut-off voltage may compensate the simultaneously generated W 51 ions, due to the lowering of the cutoff voltage. The existence of irreversibly retained lithium ions is confirmed once again by the first discharge time being longer than the charge time. However, whether they are mainly coupled with nickel or tungsten ions depends on the cut-off voltage. The transmittance after the first charge is slightly higher than the as-deposited film at higher cut-off voltage (Fig. 7a), whereas the reverse result is observed as the voltage decreases. This strongly suggests that the retained
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Fig. 7. In-situ optical transmittance of an as-deposited Ni–W oxide thin film (25 a / o W) during constant current (100 mA cm 22 ) charge and discharge tests. (a) Lower cut-off voltage 2.3 V, (b) 2.0 V, (c) 1.5 V.
lithium ions after the first cycle with high cut-off voltage are mainly coupled with Ni 21 ions, giving rise to the slight increase of transmittance. On the other hand, the W 51 ions tend to couple with the lithium ions at cut-off voltages below 2.0 V. The initial insertion of lithium gives rise to a significant change in the charge transfer resistance of the as-deposited Ni–W oxide film (Fig. 8). The charge transfer resistance of the as-deposited Ni–W oxide film (25 a / o W) was about 10 5 V, but decreased significantly to less than 400 V after the first discharge. This may be related to the irreversibly reduced tungsten oxide phases, which are probably randomly distributed in the matrix, because such reduced sites are susceptible to act as microscopic current collecting centers
3.3. Complementary electrochromic device The electrochromic display devices were made by joining pre-lithiated NiO x and Ni–W oxide films (25 a / o W) with a K 0.3 WO 3.15 film. The optical responses of these electrochromic devices using Ni–W oxide and NiO x as a counter electrode for K 0.3 WO 3.15 are compared in Fig. 9. These current and optical responses were obtained using a potentiostat with the K 0.3 WO 3.15 acting as a working electrode and the Ni–W oxide and NiO x as a counterelectrode which were also shorted to the reference. Faster kinetics of lithium insertion in Ni– W oxide than in NiO x is obvious from this figure. In particular, a much enhanced color change can be noticed from the top figure of Fig. 9. This is due to
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Fig. 7. (continued)
Fig. 9. Comparison of the optical responses of the electrochromic devices using Ni–W oxide or NiO x as a counterelectrode for the K 0.3 WO 3.15 working electrode. (? ? ?) NiO x ; (———) Ni–W oxide. Input voltages are shown at the bottom.
Fig. 8. Dependence of the charge transfer resistance of Ni–W oxide thin films (25 a / o W) on lithium insertion.
the fact that the Ni–W oxide electrode acts as a complementary electrode compared with the optically passive NiO x electrode. The coloration efficiencies at 633 nm of Ni–W oxide, K 0.3 WO 3.15 and the ECD device composed of these two electrodes are compared in Fig. 10. As can be seen, there is a linear relationship between the change in optical density and charge transfer. The coloration efficiencies of Ni–W oxide, K 0.3 WO 3.15 and the ECD device were calculated from a least squares fit to the data in Fig. 10 giving 22, 35 and 58 cm 2 / C, respectively. It can be seen that the
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was attributed to a reduction of the charge transfer resistance and fast diffusion of lithium ions. The ECD device was fabricated from K 0.3 WO 3.15 and Ni–W oxide and showed a fast response and good contrast. The coloration efficiency of the ECD device was 58 cm 2 / C at 633 nm.
Acknowledgements This work was supported by the Korea Science and Engineering Foundation through the Research Center for Thin Film Fabrication and Crystal Growing of Advanced Materials at Seoul National University. Fig. 10. The change in optical density vs. charge transfer for Ni–W oxide, K 0.3 WO 3.15 and the ECD device composed of these two electrodes.
overall coloration efficiency of a complementary electrochromic system is the sum of the coloration efficiencies of the individual coloring films. It is clear that the Ni–W oxide functions as an anodic coloration material and the optical response of the ECD comprised of Ni–W oxide and K 0.3 WO 3.15 thin films can be very much enhanced.
4. Conclusion Ni–W oxide exhibits a faster lithium insertion– removal process than nonstoichiometric nickel oxide and can contribute to an optical modulation by coloring and bleaching synchronously with the working electrode. The NiO x phase in Ni–W oxide was amorphized by addition of tungsten oxide. The fast electrochemical response of the Ni–W oxide film
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