Electrochromic Properties of LixNiyO Films Deposited

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Journal of The Electrochemical Society, 156 共8兲 H629-H633 共2009兲

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0013-4651/2009/156共8兲/H629/5/$25.00 © The Electrochemical Society

Electrochromic Properties of LixNiyO Films Deposited by RF Magnetron Sputtering Takaya Kubo,a,z Yoshinori Nishikitani,b,z Yuko Sawai,c Hirosuke Iwanaga,c Yasushi Sato,c and Yuzo Shigesatod,z a

Research Center for Advanced Science and Technology, University of Tokyo, Tokyo 153-8904, Japan Nippon Oil Corporation, Kanagawa 231-0815, Japan Department of Chemistry and Biological Science and dGraduate School of Science and Engineering, Aoyama Gakuin University, Kanagawa 229-8558, Japan b c

Electrochromic properties of a series of LixNiyO films were studied to obtain the films that give a good reversible electrochromic reaction in a nonaqueous electrolyte solution. We paid attention to the lithium content in the films. The films were deposited by radio-frequency 共rf兲 magnetron sputtering with LixNiyO targets under different conditions. The lithium contents were selected by using three different LixNiyO targets. NiO pellets were also used for cosputtering with the targets to finely adjust the lithium content in LixNiyO films. We found that a LixNiyO film gave a high electrochromic coloration efficiency when the lithium content in the film was adjusted to about 0.2. The X-ray diffraction 共XRD兲 study showed that the crystalline Li2Ni8O10 film yielded good electrochromic performance, and that the 共102兲 XRD peak was dominant in the films. © 2009 The Electrochemical Society. 关DOI: 10.1149/1.3141704兴 All rights reserved. Manuscript submitted October 15, 2008; revised manuscript received February 24, 2009. Published June 10, 2009.

Our society has been encountering worldwide problems such as global warming and diminishing natural resources. Therefore, efficient utilization of renewable energy, such as solar energy and hydropower, are becoming increasingly important. Solar cells, which convert sunlight to electricity, have already commercialized, and the production volume of the solar cell has steadily increased. The conservation of the conventional energy form of fossil fuels is another important issue. So-called smart windows1 such as electrochromic 共EC兲 windows have great potential for contributing to the conservation of fossil fuels. The phenomenon known as electrochromism is found in various materials that change their colors in response to the application of an electric field.2 These materials include, for example, metal oxides,3 conjugated polymers, and monomers,4 and are good candidate materials for displays and switchable glazing such as EC mirrors and EC windows.5-7 If we install EC windows in a building and control them in the right way, the energy-saving efficiency of the building can be improved.8 There are, however, many things to be done, such as improving durability, before putting these EC windows on the market. EC devices are in essence composed of three parts 共Fig. 1a兲: an EC electrode, a counter electrode 共ion reservoir兲, and an electrolyte layer. The EC electrodes are formed by several different deposition methods. The vacuum deposition methods, such as sputtering and vacuum evaporation, are usually employed. Compared to several cathodic EC materials such as WO3, IrO, MoO, and so forth, anodic materials are limited to a few metal oxide films such as CeO2, CuO, and NiO. The tungsten oxide films have been most widely studied and used a cathodic EC material.9-12 This is not only because the tungsten oxide film shows the high coloration efficiency 共60 cm2 /C at 550 nm兲, but also because the film has been proven to repeat a coloring/beaching EC cycle more than 105 times. The EC devices are colored by applying negative 共positive兲 potential to a cathodic EC electrode wherein the EC material is reduced 共oxidized兲 to color, and positive 共negative兲 charge is simultaneously stored in the counter electrode to balance the charge. Switching the polarity of an applied driving potential to the device causes the EC reaction to occur in reverse. To improve the coloration efficiency of EC devices even further, a complementary-type EC device structure has been proposed 共Fig. 1b兲. An anodic EC material is employed as a counter electrode in the device.2 In fabricating complementary EC devices, the anodic EC material is required to have as high a coloration efficiency as that of the

cathodic EC material. Among various anodic EC metal oxide materials so far explored, nickel oxide films give higher coloration efficiency. Nickel oxide is oxidized to color according to the following reaction: NiO 共transparent兲 + OH− → NiOOH共brown兲 + e−. On applying a negative potential to the NiOOH film, the film is bleached back to the original color. The electrochromic performance of the nickel oxide film is usually studied with a KOH aqueous solution.13-16 Recently, Lee et al. reported on the EC properties of NiO/Ta2O5 with pH neutral KCl aqueous solutions,17 and Abe et al. examined the EC performance of NiO using various acidic solutions.18 Considering the applications for EC windows, long-term durability is required. Therefore, nonaqueous solvents such as propylene carbonate and ␥-butyrolactone 共GBL兲 are more suitable for an electrolyte solution than for an aqueous electrolyte solution because the nonaqueous electrolyte solution has a wider potential window for electrochemical reactions. Compared with substantial studies on the EC properties of nickel oxide films using aqueous electrolyte solutions, few papers have been published on the electrochromism of NixO films studied with nonaqueous electrolyte solutions. Nickel oxide-based ternary oxide films such as LixNiyO films and NiMO 共M = V, Mg, Al, etc.兲 films have been relatively well studied.19-25 Campet et al. once reported that LixNiyO films deposited by sputtering with a Li0.3Ni0.7O target showed good reversible EC performance.26 As shown in the latter part of this paper, however, the lithium content in the films was lower than that in the target employed. The films then must have a lithium content lower than

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E-mail: [email protected]; [email protected]

[email protected];

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Figure 1. 共Color online兲 EC device structures: 共a兲 typical EC device and 共b兲 complementary-type EC device.

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Journal of The Electrochemical Society, 156 共8兲 H629-H633 共2009兲

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2θ degs. Figure 2. XRD of NiO thin films deposited with six different oxygen flow ratios. 共a兲 XRD patterns of the films deposited with six different oxygen flow ratios. 共b兲 Full width at half-maximum and diffraction angle of the 43° XRD peak. Sputtering condition: the working pressure is 5.0 Pa and the substrate temperature is 423 K. The dotted line given in Fig. 4b is the 共400兲-peak position 共43.291°兲 of the film obtained from JCPDS.

0.3. Michalak et al. also deposited LixNiyO films by radio frequency 共rf兲 magnetron sputtering from a LiNiO2 target. The LixNiyO films exhibited an excellent reversibility of EC reaction.24 Although the authors gave no data about the lithium content of the films, the lithium content is also assumed to be lower than the lithium content of 0.5 because the sputtering efficiency of lithium atom is lower than that of nickel atom. The purpose of this study is to obtain LixNiyO films, giving a good EC reaction in a ␥-butyrolactone electrolyte solution containing LiClO4 as a supporting salt, where LixNiyO films are colored in the following reaction: LixNiyO → Lix−␣NiyO + ␣Li+ + ␣e− and are reduced to the transparent state back again. A series of LixNiyO films was deposited by rf magnetron sputtering. EC properties of the films were studied by centering our focus on the lithium content of the film. We found out that a LixNiyO film gave the high EC coloration efficiency when the lithium content in the film was adjusted to about 0.2. The X-ray diffraction 共XRD兲 study showed that a crystalline Li2Ni8O10 film yielded a good EC performance, and that the 共102兲 XRD peak was dominant in the films. This is partly because the larger lattice constant of 共102兲 gives lithium ions an efficient path to diffuse in and/or out of the films. Experimental LixNiyO films were formed by rf magnetron sputtering either on indium tin oxide 共ITO兲 共Sn-doped In2O3, 10 ⍀/䊐兲 glass substrates or on glass substrates 共10 mm wide and 50 mm long兲. Three different targets 共Li2NiO2, LiNiO2, and LiNi2Ox兲 with 3 in. diameters were used for the sputtering. To finely adjust the lithium content in the films, cosputtering was also carried out using the three targets along with different numbers of NiOx pellets 共5 mm diameter and 1 mm thickness兲; the pellets were placed on the erosion area of each

target. Sputtering conditions were, unless otherwise stated, as follows: An oxygen flow ratio, a working pressure, a power density, and a substrate temperature were 30%, 5.0 Pa, 4.9 W/cm2, and 423 K, respectively. The base pressure obtained before deposition was 6 ⫻ 10−4 Pa. A target–substrate distance was 55 mm. The XRD patterns of the LixNiyO films formed on ITO substrates were taken with the ␪-2␪ method 共XRD-6000 Shimadzu兲. Film thickness was measured using a surface stylus profiler 共Dektak3 Sloan Tech.兲. Cyclic voltammetry of the films was measured in the nonaqueous electrolyte solution of LiClO4 共1 M兲 in ␥-butyrolactone by a standard three-electrode method using a potentiostat 共HA-301, HOKUTO DENKO兲 and a function generator 共HA-104, HOKUTO DENKO兲. A counter electrode and a reference electrode were a platinum wire and Ag/AgCl, respectively. All the electrochemical measurements were performed in a nitrogen-filled glove box. The H2O content in the electrolyte solution was checked after each electrochemical measurement to ensure that the H2O content was less than 200 ppm. In situ EC measurements were carried out on the films placed in an electrochemical cell with two optical windows. The transmittance spectra of the films placed in the electrochemical cell were obtained in the visible to near-IR region with a spectrophotometer 共Shimadzu UV-3100兲. The coloration efficiency ␩: cm2 /C of the films was obtained from the slope of the differential optical density, ⳵ ⌬OD/⳵ Q 兩Q=0, by the least-squares method, where Q 共C/cm2兲 is the injected electrical charge and ⌬OD is the differential optical density defined by the logarithm of the ratio of a colored transmittance to a bleached one. In the present case, the degradation of electrolyte was not taken into account because of the following two reasons: 共i兲 A nonaqueous solvent, GBL, was assumed to be stable in the voltage region that we employed, and 共ii兲 if the degradation of solvent ever happened, it occurred at higher doping levels, which might decrease apparent coloration efficiency. The coloration efficiency was obtained at lower doping levels with the equation of ⳵ ⌬OD/⳵ Q 兩Q=0. Inductively coupled plasma 共ICP兲 emission spectroscopy analysis was employed to determine the content of Li and Ni atoms in the LixNiyO films. Films formed on glass substrates were immersed in a nitric acid solution to dissolve the film, and the solution was used to measure the Li and Ni content using an ICP Auger electron spectroscope 共SPS7800 Seiko Instruments兲. Results and Discussion The XRD patterns of NiO films deposited with six different oxygen flow ratios are plotted in Fig. 2. There exist four diffraction peaks at about 37, 43, 50, and 60°. The two dominant peaks at around 37 and 43° are attributed, respectively, to the 共222兲 and 共400兲 peaks of NiO. The diffraction peaks at about 30 and 35° are assigned to the peaks of the ITO film underneath the NiO film. The XRD signals of ITO were marked by an asterisk. The XRD patterns obtained on the LixNiyO films formed with three different targets, and with different oxygen flow ratios are also shown in Fig. 3a-c. All the films given in Fig. 3 were deposited by cosputtering with eight pieces of NiO pellets because our preliminary results implied that cosputtering with eight pieces of NiO pellets gave the higher coloration efficiency.27 Unlike NiO films, all the XRD patterns are composed of four peaks at around 60, 50, 43, and 37°. These peaks appear with varying degrees of intensity and are broader than those of NiO. The XRD patterns of the three LixNiyO films show that as the working pressure increases, the 43° XRD peak becomes blurred and the 37° peak gains its intensity. These peaks are located closely to the peaks of Li2Ni8O10 共104兲 and Li2Ni8O10 共102兲, respectively. Figure 4a-c shows transmittance spectra of NixLiyO films deposited, respectively, from the NiO, LiNi2Ox, and LiNiO2 targets. The transmittance spectra of the films formed using the Li2NiO2 target are not shown due to a reason to be mentioned later. Each figure gives the transmittance spectra of the films of three different states 关共a兲 as deposited, 共b兲 bleached state, and 共c兲 colored state兴. The bleached/colored films were obtained by repeating a coloring/ bleaching reaction by applying constant potentials of 1.45 and

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Journal of The Electrochemical Society, 156 共8兲 H629-H633 共2009兲

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⫺1.95 V, respectively. Applied voltages employed in the present manuscript were determined by considering practical applications, where the driving voltage for EC devices was kept as small as possible. NiO and LiNi2Ox films became pale brown with an increasing number of cycles, while LiNiO2 films remained darker brown. This is because additional Li atoms work as acceptor states and induce Ni+3 states causing the coloration of the films. All films sputtered with the Li2NiO2 target peeled off from the ITO substrates during the EC reactions. This is probably caused by residual large stress in the Li2NiO2 films. The XRD peaks of the four different films observed at around 43°, together with the 共104兲 peak 共43.291°兲 obtained from JCPDS, are plotted in Fig. 5. The 43° XRD peak of the Li2NiO2 film is located at an angle larger than 43.291°, while those of the three other peaks appear below 43.291°. This indicates that the lattice constant of the Li2NiO2 film is smaller than those of the others, and that a tensile stress is induced in the Li2NiO2 film. Thus, this large lattice distortion in the films explains the reason why all the films deposited with the Li2NiO2 target peeled off from the substrates during the EC reactions. Therefore, no data are available to discuss the EC properties of the Li2NiO2 films. Figure 6a-c gives electrochemically induced absorption spectra of the films deposited, respectively, from NiO, LiNiO2, and LiNi2Ox targets. All the spectra peak at around 370 nm and tail off toward the near-IR region. Differential optical density values at 550 nm are plotted with injected charges in Fig. 7a-c. The coloration efficiency, ␩ 共550 nm兲, of NiO, LiNiO2, and LiNi2Ox are 39.5, 40.6, and 45.8 cm2 /C, respectively. The LiNi2Ox film achieves the highest coloration efficiency value. NiO has a rock-salt structure, whereas LiNiO2 and LiNi2Ox have a layered rock-salt structure. This layered structure is expected to give efficient ion paths that are favorable for lithium ions to diffuse in and out of the films. Similar observations were reported on lithium ion batteries. Li1−xNi1+xO2 is considered to be the same class of materials as Li2Ni8O10, and is a candidate material for an anodic electrode of lithium ion batteries. Because of its layered rock-salt structure, lithium ion diffusion in Li1−xNi1+xO2 occurred efficiently. Arai et al. prepared Li1−xNi1+xO2 materials with different lithium contents to clarify the relationship between the

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Figure 4. Transmittance spectra of NixLiyO thin films. The films were formed from 共a兲 NiO, 共b兲 LiNi2Ox, and 共c兲 LiNiO2 targets.

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Figure 5. XRD peaks of the four different films observed at around 43°. The dotted line given in Fig. 4b is the 共400兲-peak position 共43.291°兲 of the film obtained from JCPDS.

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Journal of The Electrochemical Society, 156 共8兲 H629-H633 共2009兲

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span of transmittance change in the visible region, as shown in Fig. 4. We then employed the LiNi2Ox target for the following experiments and tried to understand the relationship between lithium contents in the films and their EC performance. Figure 8 shows the variations of the lithium content in the films deposited from the LiNi2Ox target. Although error bars are not shown in Fig. 8, the lithium content values obtained from our ICP measurement contain about a 10% error, which was confirmed from separate measurements. The lithium content values decrease in the order of increasing pieces. The lithium content value of the film always remains lower than the value expected from the LiNi2O target compositions of 0.33关=1/共1 + 2兲兴. This is chiefly due to the difference in the sputtering efficiency of lithium and nickel atoms. Figure 9a-c shows the transmittance spectra of NixLiyO films formed by cosputtering with different pieces of NiO pellets: a working pressure is 5.0 Pa, an oxygen flow ratio is 30%, and a substrate temperature is 423 K. The coloration efficiencies of the films are 32.2, 47.3, and 50.6 cm2 /C, respectively. The film formed by cosputtering using eight pieces of NiO pellets yields the highest coloration efficiency. In practice, however, the most important factor is the lithium content in the films rather than the number of NiO pellets used and the target composition employed. The lithium contents of the films given in Fig. 9a-c are 0.36, 0.27, and 0.22, respectively. The results show that the film with a lithium content of 0.22 exhibits the highest coloration efficiency. In Fig. 10a-c, 550 nm transmittance changes of three different NixLiyO films are plotted with the number of EC cycles. The transmittance changes of the film deposited with four and eight pieces of NiO pellets are shown, respectively, in Fig. 10a

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charge and/or discharge properties and the lithium contents. The charge and/or discharge capacity is found to increase with decreasing lithium content in the films.28 In addition to the high EC efficiency, the films deposited using the LiNi2Ox target give a wider

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and b. These transmittance changes gradually increase with the number of cycles and need several EC cycles to become almost constant, whereas the reversible EC reactions are observed in the film yielding the higher coloration efficiency value of 50.6 cm2 /C, as shown in Fig. 10c. The transmittance at the colored state obtained at the first EC cycle is lower than the transmittances of the colored state after the second cycle. This observation indicates a possibility that the amount of lithium ions in the film giving a stable transmittance change becomes slightly lower than the initial value of 0.22. The coloration efficiencies of LixNiyO films that were deposited under different conditions are plotted in Fig. 11, where we focused solely on lithium contents in the films. Although the data points are scattered, the coloration efficiency increases with decreasing lithium content in the films and reaches the highest value when the lithium content is about 0.2. Further increase beyond this point in turn decreases the EC performance probably because the larger the amount of lithium atoms incorporated, the larger the lattice distortion induced. There is then a trade-off between the two factors relating to the introduction of lithium atoms in the films. The XRD study also clearly showed that the crystalline Li2Ni8O10 film yielded a good EC performance and that the 共102兲 orientation became more dominant than did the 共104兲 orientation. This is because the lattice constant of 共102兲 is larger than that of 共104兲, which allows lithium ions to diffuse in the films more efficiently. Conclusion

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LixNiyO films with varying degrees of lithium content were deposited by rf magnetron sputtering. The EC properties of the films were studied by focusing on the lithium content in the films. The

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Figure 11. Coloration efficiency values of LiNi2Ox thin films formed under several different conditions. Each data point in parentheses gives the sputtering condition: The value on the left is the number of NiO pellets used, and the oxygen flow ratio is given on the right. The dotted line is drawn as a guide for the eye.

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Figure 10. Transmittance changes at 550 nm of three different NixLiyO films formed by cosputtering with different numbers of NiO pellets: 共a兲 4, 共b兲 8, and 共c兲 12 pieces. The transmittance spectra of the films 关共a兲–共c兲兴 correspond, respectively, to those of 关共a兲–共c兲兴 given in Fig. 9.

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following findings were obtained: 共i兲 the lithium content in the films was always smaller than in the target used to deposit the films; 共ii兲 the XRD study clearly showed that the crystalline Li2Ni8O10 film exhibited good EC performance; 共iii兲 the films were confirmed to yield the highest coloration efficiency when the lithium content was about 0.2. We are optimizing the deposition conditions to achieve an even higher coloration efficiency.

Acknowledgments This work was partially supported by a High-Tech Research Center project for private universities with matching fund subsidy from the Ministry of Education, Culture, Sports, Science and Technology 共MEXT兲 of the Japanese Government to Aoyama Gakuin University. University of Tokyo assisted in meeting the publication costs of this article.

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