ELECTROCHROMIC DEVICE WITH IMPROVED CYCLING STABILITY

Abstract

An electrochromic device incorporating a high-energy absorbing polymer is presented to have an improved cycling stability. Various types of stable electrochromic devices are presented, including a highly stable yellow electrochromic device. Methods to improve the device cycling stability are also presented.

Claims

1. An electrochromic device, comprising an electrochromic layer, wherein: the electrochromic layer comprises an electrochromic polymer having an absorbance peak within 350-500 nm or having a maximal absorbance within 350-500 nm that is at least 50% of a maximal absorbance within 400-750 nm; and the electrochromic layer is configured to work under an effective overpotential of less than 1V; such that the electrochromic device maintains more than 95% optical contrast after more than 500 cycles.

2. The electrochromic device in claim 1, wherein electrochromic polymer is a high-energy absorbing electrochromic polymer comprising a propylenedioxythiophene structure.

3. The electrochromic device in claim 2, wherein the electrochromic layer is configured to work under an effective overpotential of less than 0.3V such that the electrochromic device maintains more than 95% optical contrast after more than 1000 cycles.

4. The electrochromic device in claim 1, wherein the electrochromic layer displays one of yellow, orange, cyan, green, or black color or a combination thereof.

5. The electrochromic device in claim 1, wherein when the electrochromic layer comprises an electrochromic polymer with a yellow color, and the electrochromic layer is configured to work under an effective overpotential of less than 0.3V, the electrochromic device maintains more than 95% optical contrast after more than 3000 cycles.

6. The electrochromic device in claim 1, wherein when the electrochromic layer comprises an electrochromic polymer with an orange color, and the electrochromic layer is configured to work under an effective overpotential of less than 0.2V, the electrochromic device maintains more than 95% optical contrast after more than 3000 cycles.

7. An electrochromic device, comprising an electrochromic layer, wherein: the electrochromic layer comprises an electrochromic polymer having a polyfluorene structure; and the electrochromic layer is configured to work under an effective overpotential of less than 1V such that the electrochromic device maintains at least 50% optical contrast after more than 100 cycles.

8. A method for improving cycling stability of an electrochromic device that comprises an electrochromic layer, the method comprises limiting an effective overpotential for the electrochromic layer to work under 1V.

9. The method of claim 8, wherein factors to limiting the effective overpotential include an electrolyte material, an electrolyte concentration, and a device temperature of the electrochromic device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Certain features of various embodiments of the present technology are set forth with particularity in the appended claims. A better understanding of the features and advantages of the technology will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings below. For the purpose of illustrating the invention, the drawings show aspects of one or more embodiments of the invention. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings. For example, in all the figures and the embodiments, the higher ?.sub.eff means 0.5V or about 0.5V for 0.2M of LiTFSI/PC, and the lower ?.sub.eff means 0.2V or about 0.2V for 0.4M of LiPF.sub.6/PC. PC stands for propylene carbonate.

[0013] FIG. 1 is an equation illustrating the relationship between effective overpotential ?.sub.eff, oxidation onset potential E.sub.ox, and open circuit potential E.sub.oc.

[0014] FIGS. 2(A)-(B) are diagrams showing the charge-discharge curves for an ECP-yellow thin film under different effective overpotentials (gay). FIG. 2(A) is under the higher ?.sub.eff, and FIG. 2(B) is under the lower ?.sub.eff.

[0015] FIG. 3 is a diagram showing the transmittance changes of a bleached ECP-yellow thin film during self-coloration under open circuit conditions for 30 minutes at 445 nm, under the higher ?.sub.eff and the lower ?.sub.eff, respectively.

[0016] FIGS. 4(A)-(B) are cyclic voltammogram data of an ECP-yellow thin film in a three-electrodes system under different ?.sub.eff with the scan rates changing from 5 mV/s to 100 mV/s. FIG. 4(A) is under the higher ?.sub.eff. FIG. 4(B) is under the lower ?.sub.eff.

[0017] FIG. 5 is a diagram showing the impedance of an ECP-yellow thin film in a bleached state at 1.2V vs. Ag/AgCl, under the higher ?.sub.eff and the lower ?.sub.eff, respectively.

[0018] FIG. 6 is a diagram showing the charge density changes of an ECP-yellow thin film during bleaching (solid lines) and coloration (dashed lines) processes, under the higher ?.sub.eff and the lower ?.sub.eff, respectively.

[0019] FIGS. 7(A)-(B) are diagrams showing the frequency (f.sub.3) responses of the third overtone from electrochemical quartz crystal microbalance (EQCM)-D of an ECP-yellow thin film at alternating oxidative and reduction step voltages, under the higher ?.sub.eff (shown in FIG. 7(A)) and the lower ?.sub.eff (shown in FIG. 7(B)).

[0020] FIGS. 8(A)-(B) are diagrams showing in-situ transmittance responses of an ECP-yellow thin film at alternating oxidative and reduction step voltages at 445 nm, under the higher ?.sub.eff (shown in FIG. 8(A)) and the lower ?.sub.eff (shown in FIG. 8(B)).

[0021] FIGS. 9(A)-(D) contain electrochemical and spectroelectrochemical data of an example solid-state electrochromic device comprising ECP-yellow before (solid lines) and after (dashed lines) 20K cycles. FIG. 9(A) is cyclic voltammogram data of the device. FIG. 9(B) is absorbance spectra of the device at bleached and colored states. FIG. 9(C) is a diagram showing a* and b* coordinate values of the device illustrating the coloration of the device at bleached and colored states. FIG. 9(D) is a diagram showing switching kinetics of the device taken at 445 nm.

[0022] FIGS. 10(A)-(D) contain electrochemical and spectroelectrochemical data of an example solid-state electrochromic device comprising ECP-orange before (solid lines) and after (dashed lines) 20K cycles. FIG. 10(A) is cyclic voltammogram data of the device. FIG. 10(B) is absorbance spectra of the device at bleached and colored states. FIG. 10(C) is a* and b* coordinate values of the device showing the coloration of the device at bleached and colored states. FIG. 10(D) is a diagram showing switching kinetics of the device taken at 490 nm.

[0023] FIGS. 11(A)-(B) contain optical and electrochemical data of a three-electrode system with an ECP-orange layer before (solid lines) and after (dash lines) 3K cycles. FIG. 11(A) is cyclic voltammogram data of the thin film. FIG. 11(B) is a diagram showing switching kinetics of the thin film taken at 490 nm.

[0024] FIGS. 12(A)-(B) contain optical and electrochemical data of a three-electrode system with an ECP-black layer before (solid lines) and after (dash lines) 1K cycles. FIG. 12(A) is cyclic voltammogram data of the thin film. FIG. 12(B) is a diagram showing switching kinetics of the thin film taken at 550 nm.

[0025] FIGS. 13(A)-(B) contain optical and electrochemical data of a three-electrode system with a polyfluorene layer before (solid lines) and after (dash lines) 100 cycles. FIG. 13(A) is cyclic voltammogram data of the thin film. FIG. 13(B) is a diagram showing switching kinetics of the thin film taken at 390 nm.

[0026] FIG. 14 is a diagram showing the transmittance changes at 445 nm after 3K full-contrast cycles at different ?.sub.eff under different electrolyte conditions for a three-electrode system with an ECP-yellow thin film.

DETAILED DESCRIPTION OF EMBODIMENTS

[0027] In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details. Moreover, while various embodiments of the invention are disclosed herein, many adaptations and modifications may be made within the scope of the invention in accordance with the common general knowledge of those skilled in this art. Such modifications include the substitution of known equivalents for any aspect of the invention in order to achieve the same result in substantially the same way.

[0028] Unless the context requires otherwise, throughout the present specification and claims, the word comprise and variations thereof, such as comprises and comprising are to be construed in an open, inclusive sense, that is as including, but not limited to. Recitation of numeric ranges of values throughout the specification is intended to serve as a shorthand notation of referring individually to each separate value falling within the range inclusive of the values defining the range, and each separate value is incorporated in the specification as it was individually recited herein. Additionally, the singular forms a an, and the include plural referents unless the context clearly dictates otherwise.

[0029] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases in one embodiment or in an embodiment in various places throughout this specification are not necessarily all referring to the same embodiment, but maybe in some instances. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0030] Three specific electrochromic polymers are used throughout this specification for example purposes. It is to be understood that the general inventive concept can be extracted from these examples, and that the specific examples do not limit the scope of this disclosure. ECP-yellow is a yellow-colored ECP with Formula 1. ECP-orange is an orange-colored ECP with Formula 2. ECP-black is a black-colored ECP with Formula 3. Polyfluorene is a fluorescent electrochromic (EC) material with Formula 4. All the formulas are listed below.

##STR00001##

[0031] The present disclosure is related to an electrochromic device (ECD) comprising an electrochromic layer. The electrochromic layer comprises an electrochromic polymer (ECP) either having an absorbance peak within 350-500 nm or having a maximal absorbance within 350-500 nm that is at least 50% of a maximal absorbance within 400-750 nm. The electrochromic layer in configured to work under an effective overpotential of less than 1V. The disclosed electrochromic device maintains more than 95% optical contrast after more than 500 cycles.

[0032] The lack of cycling durability for high-energy absorbing ECPs is typically ascribed to a high oxidation potential required for a bleaching process (i.e., oxidation process). Conventional approaches to improve cycling stability of a high-energy absorbing ECP focus on lowering its oxidation potential via a molecular design. The present disclosure, for the first time, demonstrates that in some embodiments, a high-energy absorbing ECP can be electrochemically stabilized by simply limiting the effective overpotential with or without any molecular modification.

[0033] An effective overpotential (?.sub.eff) describes an oxidative bias away from equilibrium. It represents the activation energy barrier required to oxidize a polymer and may better explain the thermodynamics and kinetics of an ECP's oxidation process than the conventional factor of ECP's oxidation onset potential (E.sub.ox) alone. ?.sub.eff is determined as the difference between ECP's oxidation onset potential (E.sub.ox) and open circuit potential (E.sub.oc), as shown in FIG. 1.

[0034] E.sub.ox is an oxidation onset potential to start ECP's oxidation reaction which can be experimentally observed and is highly related to ECP's structure and can be extracted from electrochemical voltammograms. Open circuit potential E.sub.oc is a result of potential difference built up at the polymer-electrolyte surface when no external bias is applied to the system, and it can be measured between the working and reference electrodes when no current passes through the system.

[0035] A stable low-energy absorbing ECP typically has a low E.sub.ox. Thus, its resulting ?.sub.eff is usually small. However, a high-energy absorbing ECP usually has a high E.sub.ox, therefore, when E.sub.oc is low, ?.sub.eff is high, and it becomes challenging to stabilize the ECP. As rationalized through the equation in FIG. 1, for an ECP, E.sub.ox is more or less fixed when the polymer structure is fixed, but by increasing E.sub.oc, ?.sub.eff can be decreased. The decreased ?.sub.eff theoretically makes the oxidation of ECPs more feasible and highly reversible, also speeds up the oxidation reaction kinetics, and reduces the deterioration of electrochemical and electrochromic properties of ECPs during cycling. Thus, the decreased ?.sub.eff improves cycling stability.

[0036] To demonstrates that lowering ?.sub.eff indeed improves a polymer's cycling stability, various characterizations are performed, including electrochemical, mass change and electrochromic characterizations for two different electrolyte conditions, i.e., 0.2M of LiTFSI in PC (LiTFSI/PC) and 0.4M of LiPF.sub.6 in PC (LiPF.sub.6/PC). An ECP-yellow thin film with a thickness of about 140 nm is deposited on ITO/glass substrate and tested in a three-electrode system with Pt wire as the counter electrode, and Ag/AgCl as the reference electrode. In these three-electrode systems, ?.sub.eff is 0.5V or about 0.5V for 0.2M of LiTFSI/PC (referred to as the higher ?.sub.eff in the figures and the embodiments) while it is 0.2V or about 0.2V for 0.4M of LiPF.sub.6/PC (referred to as the lower ?.sub.eff in the figures and the embodiments). The specific effective overpotential values here are representative values to illustrate a higher and a lower effective overpotentials for the example ECP-yellow thin film, the absolute values may not be applicable to other ECPs.

[0037] Higher ?.sub.eff means higher activation energy barrier and thus creates more challenges to oxidize a polymer. This can be indicated by charge-discharge kinetics and self-coloration kinetics under different ?.sub.eff conditions. As shown in FIGS. 2(A)-(B), during the charging and discharging experiments, constant currents of 10 ?A, 20 ?A, and 50 ?A are applied. The working electrode potential increases slower under the higher ?.sub.eff than that under the lower ?.sub.eff. Therefore, it leads to asymmetric charge-discharge kinetics that is particularly prominent at a smaller current of 10 ?A. The asymmetry in charge and discharge profile indicates that oxidation is more challenging at the higher ?.sub.eff condition. In addition, the self-coloration of bleached films at open circuit conditions also indicates the electrochemical instability of the thin film at a bleached state (i.e., oxidized state). After the polymer is oxidized, if its oxidized form is not stable, the polymer intends to return back to its neutral state and self colored. As shown in FIG. 3, the transmittance of bleached thin films under both ?.sub.eff conditions are monitored under open circuit conditions for 30 minutes. Under the higher ?.sub.eff condition, the transmittance drops significantly from 70% to 20%, while it drops insignificantly (less than 10%) under the lower ?.sub.eff condition. This indicates that the oxidized thin film is less stable when working under the higher ?.sub.eff. So when ?.sub.eff is higher, it is more challenging to oxidize the polymer, and the oxidized form is also less stable and more prone to get back to the neutral state. On the contrary, lowering ?.sub.eff makes the polymer oxidation easier and the oxidized form is more stable.

[0038] More challenging oxidation processes under higher ?.sub.eff conditions lead to obviously more asymmetric electrochemical responses. The more reversible of the redox process is, the higher symmetry we can get on the voltammogram. As shown in FIG. 4(A), under the higher ?.sub.eff, the cathodic and the anodic portion of the cyclic voltammogram is more asymmetric, with higher hysteresis between the oxidation and reduction voltages. As the scan rate increases, the asymmetry in the voltammogram becomes more prominent. On the contrary, under the lower ?.sub.eff in FIG. 4(B), the voltammogram is more symmetric across the x-axis, and symmetry persists better at increasing scan rates. Thus, a lower ?.sub.eff leads to a more symmetric voltammogram and better reversibility of the oxidation process.

[0039] The greater loss of voltammogram symmetry under a higher ?.sub.eff condition is a result of a more sluggish oxidation reaction and slower charge transfer kinetics. The charge transfer kinetics is studied by AC impedance measurement. As shown in FIG. 5, the charge transfer resistance R.sub.CT under the higher ?.sub.eff is larger than that under the lower ?.sub.eff condition. This can be also reflected on slower charge transfer kinetics (as shown in FIG. 6), which leads to slower speeds of charge density accumulation under step voltages. On the contrary, under a lower ?.sub.eff conditions, faster charge transfer kinetics are observed during both bleaching and coloration processes.

[0040] The faster switching kinetics at a lower ?.sub.eff is also demonstrated by in-situ mass change kinetics. During the electrochemical bleaching process, ions and solvents enter the polymer matrix and induce an increase in total polymer mass and thus a decrease in resonant frequency. In the reverse process (i.e., coloration process), ions and solvent exit the polymer matrix. The associated mass changes, represented by changes in resonant frequency, are measured in situ by electrochemical quartz crystal microbalance (EQCM). As shown in FIGS. 7(A)-(B), although the changes in resonant frequency f.sub.3 are in a same magnitude, within the first few seconds in the bleaching process from ?0.2 V to 1.2 V, the change in f.sub.3 under the lower ?.sub.eff condition is faster than that in the higher ?.sub.eff condition. Thus, faster mass change kinetics, representing better reversibility can be achieved by lowering ?.sub.eff condition.

[0041] The improved cycling stability with a lower ?.sub.eff is demonstrated by electrochromic characterization. The optical transition is monitored as step voltages are applied between 1.2 V and ?0.2V. As shown in FIGS. 8(A)-(B), the optical transmittance at ?.sub.max=445 nm is monitored in situ. The optical contrasts under both ?.sub.eff conditions are similar before 3K cycles, at a value of 55%. However, after 3K cycles, the optical contrasts under the higher ?.sub.eff condition decrease to 45% while that under the lower ?.sub.eff condition remains the same. Thus, lowering ?.sub.eff, optical contrast is better reversed, indicating a better cycling stability. In addition, as shown in FIGS. 8(A)-(B), a faster switching kinetics for a bleaching process under the lower ?.sub.eff condition is also observed.

[0042] By establishing a low ?.sub.eff for a high-energy absorbing ECP, an ECD incorporating the high-energy absorbing ECP can be set up to have a significantly improved cycling stability. In some embodiments, the high-energy absorbing ECP is a single-band absorbing ECP with a maximal absorbance peak within 350-500 nm, for example, yellow-colored ECPs, orange-colored ECPs. In some embodiments, the high-energy absorbing ECP is a multiple-band absorbing ECP having at least one absorbing band within 350-500 nm, for example, green-colored ECPs, cyan-colored ECPs. In some embodiments, the high-energy absorbing ECP has a broad absorption with a maximal absorbance within 350-500 nm at least 50% of the maximal absorbance within 400-750 nm, for example, black-colored ECPs, grey-colored ECPs. In some embodiments, the high-energy absorbing ECP is a blending of ECPs with a maximal absorbance within 350-500 nm at least 50% of the maximal absorbance within 400-750 nm, for example, a polymer mixture blended from a cyan-colored ECP, a yellow-colored ECP, and a magenta-colored ECP.

[0043] In some embodiments, the electrochromic polymer is a high-energy absorbing electrochromic polymer comprising a propylenedioxythiophene (ProDOT) structure unit.

[0044] In some embodiments, the electrochromic layer in configured to work under an effective overpotential of less than 0.3 V, and the disclosed electrochromic device maintains more than 95% optical contrast after more than 1K cycles.

[0045] In some embodiments, the high-energy absorbing electrochromic polymer in the disclosed electrochromic layer displays one of the following colors: yellow, orange, cyan, green or black, or a combination of some of those colors. In some embodiments, the high-energy absorbing electrochromic polymer in the disclosed electrochromic layer includes one or more of ECP-yellow, ECP-orange, ECP-cyan, ECP-green, and ECP-black.

[0046] In some embodiments, the electrochromic layer in the disclosed device comprises an electrochromic polymer with a yellow color. The yellow electrochromic layer in configured to work under an effective overpotential of less than 0.3V. The electrochromic device maintains more than 95% optical contrast after more than 3K cycles. In one embodiment, a highly stable solid-state yellow ECD is configured with ECP-yellow used in the electrochromic layer, 1M of LiPF.sub.6 in PEGMEA as the electrolyte and a Li.sup.+ doped vanadium oxide thin film as the ion storage layer. The ECP-yellow layer is configured to work under an ?.sub.eff of 0.2V, and the optical contrast of the ECP-yellow layer at 445 nm slightly dropped from ?42% to ?40% even after 20K full-contrast cycles. FIGS. 9(A)-(D) show the electrochemical and optical performances of the example device. By switching between 1.4V and ?0.2V, the device color switches between vibrant yellow color and transparent. The cycling stability of this yellow device is unprecedented for ECDs displaying yellow color to the best of our knowledge.

[0047] In some embodiments, the electrochromic layer in the disclosed device comprises an electrochromic polymer with an orange color. The electrochromic layer in configured to work under an effective overpotential of less than 0.2V, and the electrochromic device maintains more than 95% optical contrast after more than 3K cycles. In one embodiment, a stable solid-state orange ECD is configured with ECP-orange used in the electrochromic layer, 1M of LiPF.sub.6 in PEGMEA as the electrolyte and Li.sup.+ doped vanadium oxide thin film as the ion storage layer. The electrochromic layer in configured to work under an ?.sub.eff of 0V, and the optical contrast of the electrochromic layer at 490 nm only slightly dropped from 40% to ?36% even after 20K full-contrast cycles. FIGS. 10(A)-(D) show the electrochemical and optical performances of the example device. By switching between 1.0V and ?0.2V, the device color switches between orange color and transparent. The cycling stability of this device is unprecedented for ECDs displaying orange colors to the best of our knowledge. In another embodiment, a three-electrode system with an electrochromic layer comprising ECP-orange is constructed. 1M LiPF.sub.6/PC is used as electrolyte, Pt wire is used as counter electrode and Ag/AgCl is used as reference electrode. The electrochromic layer in configured to work under a ?.sub.eff of 0 V, and ECP-orange layer maintains more than 95% optical contrast after 3K cycles in the three-electrode system, as shown in FIGS. 11(A)-(B). In the embodiments above, the cycling stability in the three-electrode system is not as good as the one from the corresponding solid state device. That is because of the contact with water and oxygen, concentrated electrolyte in the three-electrode system goes through degradation, thus limiting the cycling stability of an electrochromic layer in a three-electrode system.

[0048] The same working principle can be extended to other conventionally unstable high-energy absorbing ECPs, including ECP-black and a blue OLED material polyfluorene, to improve their cycling stability.

[0049] In one embodiment, a three-electrode system with an electrochromic layer comprising ECP-black is constructed. 1M LiPF.sub.6/PC is used as electrolyte, Pt wire is used as counter electrode and Ag/AgCl is used as reference electrode. FIGS. 12(A)-(B) show the electrochemical and optical performances of the example device. The ECP-black layer is configured to work under an ?.sub.eff of 0V. By switching between 0.8V and ?0.2V, the color of the ECP-black layer switches between black color and transparent. The optical contrast of the ECP-black layer at 550 nm only slightly dropped from 49% to ?47% even after 1K full-contrast cycles in the three-electrode system. Black color is a broad absorbing color, but only 550 nm is selected for monitoring because it is the most sensible wavelength to human eyes.

[0050] In some embodiments, a three-electrode system with an electrochromic layer comprising polyfluorene is constructed. Polyfluorene is a commonly used emission-based conjugated polymer, but well-known to be electrochemically unstable. By the same strategy as disclosed herein, a stable electrochromic device comprising an electrochromic layer having a polyfluorene structure is constructed. The electrochromic layer in configured to work under an ?.sub.eff of less than 1V. The disclosed electrochromic device maintains at least 50% optical contrast after more than 100 cycles. In some embodiments, the electrochromic device comprises an electrochromic layer with a polyfluorene structure. The electrochromic layer in configured to work under an ?.sub.eff of 0.7V. The electrochromic device maintains more than 70% optical contrast after 100 cycles in a three-electrode system. In one embodiment, a three-electrode system is configured with polyfluorene layer as the working electrode with 1M of LiPF.sub.6 in PC as the electrolyte, Pt as the counter electrode and Ag/AgCl as the reference electrode. FIGS. 13(A)-(B) show the electrochemical and optical performance of the example three-electrode system. The polyfluorene layer in configured to work under an ?.sub.eff of 0.7 V. By switching between 1.6V and 0.8V, the transmittance at 390 nm changes. The optical contrast of the polyfluorene layer at 390 nm changed from ?53% to ?41% after 100 full-contrast cycles.

[0051] In another aspect, a method for improving the cycling stability of an electrochromic device is provided. And the method comprises minimizing the ?.sub.eff of the electrochromic layer in the electrochromic device. The factors to control the ?.sub.eff include an electrolyte material, an electrolyte concentration, and a device temperature.

[0052] To improve the cycling stability of an electrochromic device, an electrolyte material may be selected. Due to different conductivities and ion sizes, different electrolyte salt materials can lead to different open circuit potentials. As shown in FIG. 14, the relationship between cycling stability and ?.sub.eff under different electrolyte conditions is demonstrated. The colored and bleached transmittance values in the initial state are represented by solid dots, and those after 3K full-contrast switching cycles are represented by open circles. Smaller LiPF.sub.6 and LiBF.sub.4 lead to lower ?.sub.eff than larger LiTFSI does and yield relatively constant transmittance after 3K switching cycles. Although LiPF.sub.6 is larger in size than LiBF.sub.4, its better conductivity leads to a lower ?.sub.eff when used at the same electrolyte concentration. In some embodiments, LiPF.sub.6 in PC is used to achieve an improved cycling stability for the disclosed ECD.

[0053] To improve the cycling stability of an electrochromic device, an electrolyte concentration may be selected. Typically, a higher electrolyte concentration will lead to a lower ?.sub.eff and a better cycling stability, as shown for each electrolyte in FIG. 14. At a lower concentration of LiTFSI (e.g., 0.2M) or LiBF.sub.4 (e.g., 0.4M), ?.sub.eff is relatively higher, transmittances at a colored state and a bleached state both shift to greater values, while the shift in a bleached state is smaller than that in a colored state, leading to a decreased optical contrast, indicating a worse cycling stability. However, in carbonate electrolyte systems, a concentrated electrolyte may experience various problems, including salt precipitation, generation of HF at ambient conditions, and changes in solvation configuration and ion-ion interactions, the electrolyte concentration used in the device need to guarantee the presence of free electrolyte ions. In some embodiments, 0.4M-1M LiPF.sub.6 in PC, or 0.6M-1M LiBF.sub.4 in PC, or 0.8M-1M LiTFSI in PC is used as electrolyte to achieve improved cycling stability for the disclosed ECD.

[0054] To improve the cycling stability of an electrochromic device, a device temperature may be selected. Within the temperature range in which each of the components of ECD is chemically and electrochemically stable, a higher temperature (i.e., above room temperature), such as 40-80? C. or up to 100? C. as long as the device can be operated safely will lead to a lower ?.sub.eff and better cycling stability.