Lithiation studies on some transition metal oxides for an all-solid thin film electrochromic system

Solid State Ionics 59 ( 1993) 47-57 North-Holland

Lithiation studies on some transition metal oxides for an all-solid thin film electrochromic system P.V. Ashrit, IL Benaissa, G. Bader, F.E. Girouard and Vo-Van Truong Departmentof Physics,Universitkde Moncton, Moncton, N.B. EIA 3E9, Canada Received 3 1 March 1992; accepted for publication 1 September 1992

A study of the lithiation behavior of some transition metal oxides has been carried out. Using a dry method of lithiation changes in optical and electrical properties of WOs, Vz05 and Nbz05 films prepared under different conditions are studied. The optimum results and techniques are used for the fabrication of an all-solid electrochromic (EC) system based on the utilization of high coloration efficiency WOs as the base electrochromic layer and the low coloration efficiency VxO, as the counter electrode. LiBO* has been used as the ion conducting layer. Such system has been operated over several hundred optical switching cycles without any degradation so far. All results indicate the suitability of the system for “smart” window application.

1. Introduction The study of thin film electrochromic (EC) systems in which an optical change can be reversibly introduced upon double injection of positive ions and electrons has become increasingly important in recent years due to their application potential as “smart” windows in automobiles and buildings [ 1,2 1. Such a system is mainly made up of solid state ionic materials playing different roles. The general configuration of such an EC system can be represented as TC 1/EC/FIC/C%/TC2 where TCs are the transparent electrodes used for the application of an uniform electric fieldto the system; EC is the base electrochromic layer which is a mixed ion-electron conductor and in which occurs the actual optical change; FIC is the fast ion conducting and electronically insulating layer and CE is the counter electrode or the ion storage layer [ 3 1. Several designs of this five layer configuration have been proposed and studied including those encompassing gel, semi-solid or polymer electrolyte in a laminate structure [ 4-6 1. However, for large area applications all-solid EC systems deposited on a single substrate are more attractive. Such five layer structures are very important for stable operation of the system since they contain ECCE layers with exactly balancing half cell reactions.

Two such CE-EC combinations are very practical: (i) complimentary electrochromic layers in which one is a cathodically coloring material and the other is an anodically coloring one ( 1) or (ii) transition metal oxide (TMO) pairs with high and low coloration efficiency in the solar spectral region employed as EC and CE layer, respectively. In this work we have examined the optical, electrical and electrochromic properties of three such metal oxides, WOJ, VzOs and Nb205 films under dry lithiation for their use as CE or EC layers in the fabrication of an all solid electrochromic system. A reversible intercalation in these oxides can be brought about by the electrical injection of an electron and a charge compensating ion or by the insertion of a metal atom resulting in the reduction of the oxide. The lithiation of WOa leads to the development of a dark absorption band due to the formation of tungsten bronze giving a very good optical modulation in the solar spectral range. Nb205 and Vz05 films for the same amount of lithiation show a very low efficiency optical change in this range. Hence, it is very convenient to employ W03 as the base EC layer and Vz05 or Nbz05 as the CE layer. Using these dry lithiation results, an all-solid-state device has been fabricated and studied using LiB02 as an ion conductor. The complete system has been tested for its electrochromic characteristics. Results indicate a good po-

0167-2738/93/S 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.

48

P. V. Ashrit et al. /Lithiation studies on transition metal oxides

tential of such a system for application as “smart window”.

thermally deposited gold layer. Optical measurements were done using a Cary double beam spectrophotometer in the wavelength range of 300 nm to 2500 nm.

2. Experimental For the study of individual materials thin films were deposited on glass substrates and lithiated. The deposition conditions were chosen from our earlier studies on these materials. W03 films were deposited by thermal evaporation at a substrate temperature of 200°C and at a rate of 0.3-0.4 rim/s.. Nb205 and VZOs films were deposited by rf sputtering in a total pressure of around 3 x 1O-’ Torr in an atmosphere of 10% O2 and 90% Ar. The film thickness in all the three cases was around 200 nm. The quartz crystal technique was used for film thickness monitoring. All the film thicknesses were measured assuming the bulk density of the material. These individual films were lithiated by an in situ and dry method using LiNb03 powder to a maximum effective mass thickness of 50 nm [ 71. We have found that when LiNb03 powder is heat treated in a tungsten boat under vacuum it gives off lithium atoms which diffuse into the bulk of the exposed electrochromic films. This behavior is similar to that of heat treated Li3N [ 8 1. The quantity of lithium thus inserted into the TMOs under study was measured by noting the frequency change of the quartz crystal as the effective mass of lithium deposited. The charge thus injected was obtained by extracting the lithium from the TMOs deposited on IT0 coated glass substrates in an electrochemical cell containing LiC104 in propylene carbonate liquid electrolyte [ 7 1. We have found that a fairly linear correlation exists between the effective mass thickness deposited and the charge extracted. LiBOz films which have been used as ion conducting layers were also deposited by thermal evaporation on glass substrates at a substrate temperature of 200” C. Using the optimum conditions from this study an all-solid-state electrochromic system has been fabricated on a single IT0 coated (Donnelly Corp., 40 n/O ) and etched glass substrate. Such a system comprises of sputter deposited V205 film as counter electrode and thermally evaporated W03 and LiBOz as electrochromic and ion conducting layers, respectively. The outer second electrode was a 15 nm thick

3. Results 3.1. Electrochromic layer

The extensive research work carried out to date on transition metal oxides has established evaporated amorphous tungsten oxide ( W03) films as excellent candidates for the EC layer [ 11. The electrochromic properties of these films are found to depend strongly on (i) the microstructure, (ii) film composition and (iii) the humidity content of the films [9-l 11. Xray diffraction studies of our W03 films deposited at substrate temperatures from room temperature to 300°C indicated that the films “as deposited” were amorphous. It has been reported that the polycrystallinity in such films onsets around deposition temperatures between 150°C and 300°C [12]. However, no peaks in the diffraction patterns in our films were recorded even at 300 ’ C. These films deposited at 300°C however, were slightly bluish in color compared to those deposited at 100°C and 200°C. Films deposited at 100’ C and 200’ C exhibited far superior transmission in the solar spectral range than those deposited at 300°C. Such slightly bluish WO, films are reported to be having y values between 2.5 and 2.7 [ 13 1. The electrical measurements were also in accordance with this behavior. The electrical resistivity of the films was found to be 1.9 X 10’ R-cm, 6.7x lo5 Q-cm and 3.2~ lo6 R-cm for the films deposited at 300” C, 200” C and lOO”C, respectively, indicating that with increasing substrate temperature the films might be tending towards polycrystallinity. Further detailed work on W03 samples was carried out only with the samples deposited at 100°C or 200°C due to their superior solar spectral transmission in the “as deposited” state. To examine the W03 film surface, we have used the replica technique [ 141 with a transmission electron microscope (Hitachi EMV-II C model). Results of this study of fnms deposited at 200°C showed that the surface roughness of the films increases as the film thickness increases from 100 nm to 300 nm.

49

P. V. Ashrit et al. / Lithiation studies on transition metal oxides

However, above 300 nm the film was found to develop smoother surface with large isolated island imperfections. From these observations it appears that films with low thickness ( c 100 nm) and with high thickness ( > 300 nm) may not present good coloration characteristics because of the densely packed structure which hinders the passage of lithium ions

1151. The sensitive method of infra-red attenuated total reflection (IR-ATR) method was used for the water content study in our W03 films. By measuring the stretching band absorbance due to water molecules centered around 3400 cm-’ the water concentration in W09 films was determined. From these studies it was found that ( 1) thin films of W03 deposited at room temperature absorb fairly high amounts of water and (2) with increasing substrate temperature there is a decrease in the water content of the films [ 15,161. From these studies it was concluded that W03 films deposited at 200 ’ C were amorphous, contained lower amounts of water and exhibited high degree of transmission in the solar wavelengths. Further, these films were also found to be stable under long term immersion in LiClO, liquid electrolyte sometimes used for lithiation studies. Hence, these films were chosen

for use as electrochromic layer in our all-solid system. Dry lithiation of these films was carried out using lithium niobate powder. In fig. 1 is shown the transmission change as a function of lithium injection in a 200 nm thick W03 films deposited at 200°C. As can be seen from this figure a significant drop in transmission occurs throughout the spectral region examined. Hence, these amorphous W09 films exhibit a fairly good optical modulation and are very suitable for EC layer application. For lithiation of an effective mass thickness of up to 40 nm there is an increasing absorption in these films. However, above this value this transmission change reverses, perhaps due to the gradual formation of a tungsten phase as reported elsewhere [ 15 1. The changes at these high density lithiation are found to be irreversible. Hence, an effective lithium mass thickness of 40 nm which corresponds to a charge density of 25 mC/cm’ was found to be optimum for our W03 samples of thickness 200 nm. These films were then checked for their cyclic ability by introducing them in the LiC104 electrolyte and applying a triangular potential to insert and extract lithium. Although, the first lo- 12 cycles were not completely reversible the system attained stable voltammograms after these initial cycles. Hence, an amorphous W03 film of thickness 200 nm

100

60 z-

500

1000

1500 WAVELENGTH

2000

2500

(NM)

Fig. 1. Transmission spectra of a 200 nm thick WOS film with different mass thicknesses of lithium: (a) 0 nm, (b) 10 nm, (c) 20 mn, (d) 30 nm, (e) 40 nm and (f) 50 nm.

50

P. V. Ashrit et al. /Lithiation studies on transition metal oxides

deposited at 200°C and injected with 40 nm effective mass thickness of lithium was chosen as the pivotal point for the tailoring of an all-solid electrochrome system. 3.2. CE layer The proposed configuration as already mentioned calls for the utilization of a low coloration efficiency transition metal oxide as the counter electrode. It is well known from earlier works that rf sputtered Nb205 and V205 films show very weak and reversible coloration in the solar spectral range under lithiation using a liquid electrolyte [ 17,181. Hence, a comparative study of this coloration behavior in NbZOs and VZOSwas undertaken using the method of dry lithiation. Reactively sputtered VZOSfilms have been shown to be very good ion storage systems due to their optical passivity under lithiation. Although, its low lithium intercalation capacity makes it unsuitable for use in lithium batteries, for the counter electrode applications in EC systems its lithium storage capacity is generally sufficient [ 191. In our case, as mentioned above, a charge capacity of 25 mC/cm’ corresponding to a lithium mass thickness of 40 nm was

500

1000

sought in such films. Vanadium oxide films were reactively sputtered using a vanadium target under different Ar/O flow rates. Brightly yellow colored films were obtained with a fairly good transmission. An example of this is shown in fig. 2 in the case of a film deposited with an Ar/O* flow ratio of 9. As can be seen from this figure the “as deposited” film exhibits a fairly high degree of transmission, in excess of 70% throughout the spectral range examined. In the same figure are also shown the transmission changes in VZOSfilm after different degrees of dry lithiation. As can be seen from the figure there is hardly any change in the transmission at higher wavelengths and only a small change manifests at lower wavelengths with lithiation. Even at 50 nm lithium mass thickness the change is not very significant. Fig. 3 in which the absorption change in a VZOSfilm on a glass substrate is shown as a function of lithiation is given shows clearly these changes occuring at lower wavelengths. With increasing lithium content the optical absorption edge below 500 nm shifts towards the ultra violet region rendering the film more transparent. Another feature to be noted is the development of an absorption peak around 500 nm in the visible region and its shift towards lower wavelengths with increasing lithiation between 0 and 40 nm. This peak, how-

1500 WAVELENGTH

2000

2500

(NM)

Fig. 2. Transmission spectra of a 200 nm thick V,O, film as a function of lithium mass thickness: (a) 0 nm, (b) 10 nm, (c) 20 nm, (d) 50 nm.

51

P. V. Ashrit et al. / Lithiation studies on transition metal oxides

500

1000

2000

1500 WAVELENGTH

2500

(NM)

Fig. 3. Absorption change in 200 nm thick V205 film with different amounts of lithium: (a) 0 nm, (b) 20 mn, (c) 30 nm, (d) 40 nm and (e) 50 nm.

ever, seems to reverse towards higher wavelengths for the maximum lithiation value of 50 nm. The electrical resistivity measurements carried out on these samples with and without lithiation also show interesting results. As shown in fig. 4 with increasing lithiation the resistivity of V20S films seems to increase rather peculiarly. Initially there is a slight drop in resistivity upon lithiation. Upon further lithiation there is a significant increase in film resistivity. A lithiation of above 40 nm again brings down the resistivity to some extent. This is an anomalous behavior in comparison with W03 and Nb205 films where there is an almost gradual decrease in resistivity with lithiation. Reactively sputtered niobium oxide films have been shown to exhibit a similar passive coloration making them candidates for use as counter electrodes in electrochromic systems. In our investigation for a suitable counter electrode material we have carried out a detailed study of rf sputtered NbzOS film. As reported elsewhere such films deposited with an Ar/Oz gas mixture of 75/25 percent and 90110 percent yield chemically stable and fairly transparent films in the spectral region of interest [ 7 1. Hence, for our comparative study the optical and electrical properties were measured in NbzOS sample sput-

l0’O I lo8

--

lo8

E

c Q

lo4

lo2

loo 0

10

20

30 Li

40

50

60

(NM)

Fig. 4. Variation of fflrn resistivity as a function of lithium mass thickness in: (a) V205, (b) Nb20S and (c) WOS films.

tered deposited with 10% oxygen. In figs. 5 and 6 are shown the transmission and absorption behavior of this film under dry lithiation. As can be seen from these figures with increasing lithiation there is hardly

52

P. V. Ashrit et al. /Lithiation studies on transition metal oxides

100

80

60 i? E.

40

20

0 500

1000

1500

WAVELENGTH

2000

2500

(NM)

Fig. 5. Transmission changes in 200 nm thick NbzOS film with lithium mass thickness: (a) 0 nm, (b) 20 nm, (c) 30 nm and (d) 50 nm.

60 s 4 d

40

20

0 500

1000

1500

WAVELENGTH

2000

2500

(NM)

Fig. 6. Absorption changes in Nb,Olr film with lithiation: (a) 0 nm, (b) 20 nm, (c) 30 nm and (d) 50 nm.

any change in film transmission at higher wavelengths and only a small drop for lithiation between 0 and 30 nm. Only for a lithiation of 50 nm does this drop become fairly significant in both these regions. The absorption peak centered around 500 nm increases monotonically with lithiation. Another in-

teresting feature of this peak is its non consistent shift first towards higher wavelengths and then to lower values. This behavior is also to some extent reflected in the resistivity measurements where unlike W03 films there is a plateau region around 20 nm of lithiation in fig. 4. These anomalous optical and elec-

53

P. K Ashrit et al. / Lithiation studies on transition metal oxides

trical behaviors in both Vz05 and NbzOs films with lithiation are currently being studied using FTIR methods. However, with a lithiation of 50 nm this peak becomes a broad absorption band throughout the visible region giving a greyish appearance to the film. Hence, the NbzOs films although more transparent in the as-deposited state than VzOs films lose their visible transmission monotonically with lithiation. Vz05 films on the other hand have a lower visible transmission in the as deposited state due to their characteristic yellow coloration. However, with increasing lithiation they become more transparent in the visible region. Also the changes in the higher wavelength transmission are less significant in the case of VzOs film than Nb205 films. Hence, these films are more suitable candidates for the application as counter electrodes. Since our aim here was to evaluate the application potentials of the three transition metal oxides as electrochromic and counter electrode layers integrated transmission values of these films with and without lithiation were obtained in the solar and visible regions. These integrated values are given by T(sol

,

vis)=

I W)Qz(soL vis)(l)

a

J @(sol, vis) (2) CLZ ’

where @(sol, vis) is the spectral solar n-radiance at an air mass of 1.5 (evaluated between 0.3 and 1.5 microns) or luminous efficiency of the eye (evaluated between 0.4 and 0.7 microns). These values are given in tables 1 and 2 below. As can be seen from these tables, the visible and solar transmission values drop very rapidly with lithiation in the case of W03 film leading to a very significant modulation. Table 1 Integrated visible transmission. Li+ mass thickness (nm)

Gi, (96) wo3

NW3

vzo,

0

87.5

10 20 30 40 50

48.8 29.2 22.0 19.9 19.7

83.6 87.5 70.2 71.3 57.1 56.7

14.4 62.4 77.6 70.0 69.8 63.7

Table 2 Integrated solar transmission. Li+ (mass thickness) (nm) 0 10 20 30 40 50

T-1 (%) wo3

NW1

vzo3

79.1 41.0 25.8 20.5 19.0 19.3

80.1 77.6 72.2 68.3 61.3 62.9

65.3 62.8 66.6 67.8 68.2 67.2

It is to be noted that such a W03 film deposited with the conditions given earlier exhibits an integrated transmission of about 80% and more in the solar and visible regions, respectively, in the as deposited state. With the insertion of 30 nm or more of lithium mass thickness this transmission can be reduced to around 200/oin both the regions of interest. Such a significant change makes amorphous W09 highly suitable for electrochromic layer. On the contrary variations in these integrated values with lithiation of V205 and NbzOs films are quite passive as required for counter electrode application. As can be seen from these tables the Vz05 film exhibits comparatively lesser changes in transmission both in visible and solar regions than NbzOs film, especially around 40 nm of lithiation. From all these results V205 films were deemed to be more suitable for application as counter electrode with W03 film as the electrochromic layer. 3.3. Fast ion conductor LiBOz films have been studied for their application as fast ion conductors in solid state batteries. Thermally deposited LiBO* films have yielded lithium ion conductivity of the order of 1O- ‘- 1O- * (Rcm) - ’ along with a very high electrical resistivity. In addition to these qualities another requirement for their application in EC systems is a high degree of transmission in the solar spectral region. Hence, with an intention of using LiBO* films as lithium ion conductors we have studied their optical properties [ 7 1. These films exhibit an optical behavior very similar to that of glass. The transmission and reflectance spectra of the LiBOz films is flat and structureless in the wavelength range between 300 nm and 2500 nm.

54

P. V. Ashrit et al. / Lithiation studies on transition metal oxides

The integrated visible and solar transmission values calculated are found to be around 94% each. The optical constants of these films have been found to be similar to those of glass with refractive index (n) being around 1.5 and absorption coefficient (k) being of the order of 10e3. Hence, such a film is extremely suitable as an ion conductor in EC systems.

4. All-solid EC system Using the results of the above studies an all-solid EC system of the following configuration has been fabricated and studied: Glass (IT0 1LiXV205 1LiB02 )W03 I Au . The outer electrode was a room temperature deposited gold film of thickness 15 nm. The electrochemically active area of the system was about 3 cm*. Tens of coloration and bleaching cycles were performed on the sample before taking any measurements both in order to know the operational parameters and to stabilize the system. Fig. 7 shows the spectral transmission of the EC system under different applied voltages. The voltage values are given with respect to ITO. As can be seen from the figure in the as de-

posited state the transmission value lies in the middle of the biased spectra in the two directions. Upon the application of a positive voltage of 1 and 2 V the spectral transmission increases and is almost the same for both the values of applied voltage. However, with increasing negative bias there is a significant drop in the transmission of the solid system. A fairly good optical modulation as can be seen between the most colored and the most bleached states is achieved with this all solid system. Bulk of the transmission in the bleached state is, however, attenuated due to strong Au absorption and interference effects. Nevertheless, it can be seen that a good optical modulation manifests in the film in the higher visible and NIR regions. Coloration and bleaching of this system has been carried out for several hundred cycles. An example of this cycling is shown in fig. 8, recorded at a wavelength of 8 15 nm. As can be seen from this figure an optical density change of about 1 can be achieved at this wavelength. At these applied voltages the system takes four minutes for each cycle. During each of these coloration and bleaching processes it was seen that the current was made up of two parts: a time dependent part corresponding to the ionic insertion and extraction and a constant electronic leak current

50.0

0.0 500

1000

1500 WAVELENGTH

2000

2500

(NM)

Fig. 7. Transmission spectra of the all-solid electrochromic system with different applied voltages: (a) 2 V, (b) 1 V, (c) 0 V, (d) - 1V and (e) -2V.

55

P. K Ashrit et al. / Lithiation studies on transition metal oxides

50.0

37.5

i? - 25.0 e-

12.5

0.0 0

15

30 TIME (MIN)

45

60

Fig. 8. Coloration and bleachig cycles of the all-solid system recorded at a wavelength of 8 15 nm with an applied voltage of k 3 V.

of the order of 9 pA at the end of each cycle. To examine quantitatively the reproducible and stable operation of the system in detail, cyclic voltammograms were carried out. In fig. 9 are shown the cyclic changes in charge injected and current as a function of the applied potential. A triangular potential at a sweep rate of 33 mV/s was applied. The charge injected or extracted into the film was obtained by integrating the time varying current in the system, that is after eliminating the electronic leak current. In fig. 10 are shown the cyclic variations of the transmission and current as a function of the applied potential which indicate clearly the cyclic ability of the system after several hundred cycles of coloration and bleaching. This variation in transmission was recorded at a wavelength of 632 nm and at an incident angle of 7’. The optical memory of the EC system was tested by coloring and bleaching the system to extreme values and recording the time variation of the transmission in open circuit. It was seen that both the extremely colored and the bleached states were retained for well over eight hours. The change in transmission in both these states was found to be less than 1% after eight hours thus indicating the excellent optical memory of such a system.

5. Conclusion The optical and electrical properties of three transition metal oxides, WOa, NbzOS and VzOShave been studied under dry lithiation for the fabrication of an all-solid electrochromic system. From these studies it has been seen that thermally evaporated amorphous WOg exhibits a good degree of optical modulation under lithiation in the solar spectral region. Hence, such a film has been employed as the electrochromic layer. A maximum lithiation of 40 nm of mass thickness of lithium corresponding to a charge density of 25 mC/cm2 has been found to give a fairly adequate solar and visible transmission change and hence is used as the pivotal point for the tailoring of other layers used in the EC system. Nb20S and V205 films for the same amount of lithiation exhibit only a small degree of coloration in the spectral region of interest and hence making these films viable candidates for counter electrode application. However, V20S film exhibiting a greater degree of transmission in the visible and NIR regions than Nb205 film under 25 mC/cm2 of lithiation is thus found to be more suitable as counter electrode. Optical properties of thermally evaporated LiB02 films have also been studied for use as lithium ion conducting layer. These films studied for application in lithium batteries are

56

P. K Ashrit et al. /Lithiation studies on transition metal oxides

4

oi- 2 3 > z.0 k $ z

(4

-2

-4

/:

(b) 20

s E

15

10

0:

-4

-2

0

APPLIED VOLTAGE

0’

16

36

2

4

(V)

Fig. 10. Cyclic voltammetry in the all-solid system after several hundred cycles.

TIME (MIN)

Fig. 9. Cyclic variation of current, charge and applied potential as a function of time.

known to exhibit ion conductivities of the order of 1O-‘- 1Om8(R-cm) - ’ and a very high electronic resistivity. Our LiBOz films exhibit an excellent degree of transmission in the solar spectral region. Hence, these qualities make these films extremely suitable for application as ion conducting layer in EC systems. Using the results of all these studies an all solid electrochromic system has been fabricated and studied. Such a system exhibits the following essential characteristics deemed useful in “smart window” applications: ( 1) The system exhibits a fairly good optical modulation in the visible and NIR regions although the over all transmission is impeded due to the presence of an absorbing Au layer. (2) The system exhibits an excellent reproducibility and operational stability. The system has been

colored and bleached for over thousand cycles so far without any change in its electrical and optical characteristics. This testing is continued at present. ( 3) The system also exhibits an excellent optical memory under open circuit conditions both in the highly colored and bleached states. These two extreme states are retained for well over eight hours with almost no change. In addition to these characteristics the system exhibits a switching speed of about 120 s for an applied voltage of 2 V. This speed is acceptable for “smart” window applications. However, efforts are underway to improve this speed by improving the ionic conductivity of the LiBOz layer. Work is also being done to replace the outer gold electrode by a more transparent IT0 layer. Acknowledgement We gratefully acknowledge the support of Energy,

P. V. Ashrit et al. /Lithiation studies on transition metal oxides

Mines and Resources (EMR), Canada and Natural Science and Engineering Research Council (NSERC) of Canada. We also thank Dr. Niall Lynam of Donnelly Corporation for generously supplying the IT0 coated glass samples.

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