Isothermal transient ionic current study of laminated electrochromic devices for smart window applications

The isothermal transient ionic current (ITIC) in three different kinds of laminated electrochromic devices has been determined. The devices consisted of one 300 nm thick layer of nickel oxide and one 300 nm thick layer of WO₃ deposited onto separate In₂O₃:Sn (ITO) covered glass substrates by DC magnetron sputtering at room temperature. They were then laminated with a polymeric ion conductor, acting as electrolyte, in symmetric and asymmetric configurations, i.e. WO₃/WO₃, NiO_xV_yH_z/NiO_xV_yH_z and WO₃/NiO_xH_y. The electrolyte was prepared by mixing polyethylene glycol of average molecular weight 400 (PEG 400) and lithium triflouromethanesulfonate (LiSO₃CF₃) for 12 h at 70°C and with a ratio oxygen/lithium (O/Li)=10. The samples were first polarized, i.e. the ions are transported to one of the electrodes, which in the asymmetric devices is the nickel oxide electrode. At the first applied potential step, the ions move through the electrolyte towards the opposite electrode. The potential is then switched and the ions move back to the first electrode. The ITIC curves are found to depend on the electrode material and in the asymmetric case also the direction of the ion current.     

The temperature behavior of smart windows under direct solar radiation

The purpose of this paper was to investigate the variation in temperature of electrochromic devices under direct solar radiation and to compare the results with double-glazed glass. The devices consisted of a V₂O₅ layer as an ion storage film and a WO₃ layer as an electrochromic layer. The V₂O₅ and WO₃ films were prepared by thermal and electron beam evaporation, respectively. The optical properties and structures of these films were investigated. Both the ion storage film and the electrochromic layer were amorphous. The optical absorption was caused by a direct-forbidden transition in V₂O₅ and by an indirect-allowed transition in WO₃. The maximum temperatures under solar radiation were measured for colored and bleached devices, double glass and air, they were found to be approximately 63, 63, 53 and 36 °C, respectively. The rates of increasing temperature to the incident power density for colored, bleached devices and double glass were 0.051, 0.049 and 0.041 °C/(W/m²), respectively.     

Electrochromism in amorphous lithium tungsten oxide films

A small ac-signal impedance analysis was utilized to characterize the surface property of an ITO electrode and Li transport in an Li_xWO₃ electrode. The as-deposited ITO film has a surface layer with extremely high carrier density, which could be easily removed by mechanical polishing. Based on the kinetic model of Li_xWO₃, the diffusion coefficient of Li transport was obtained. It depends on the various deposition conditions of WO₃ films and the composition of the electrolyte used. The response of Li_xWO₃ film showed a gradual decrease and reached a certain equilibrium after repeated cycling (more than 10⁴ times). The examination of such a degradation phenomenon leads to the conclusion that there are two active sites in the WO₃ structure, which are available for the Li ion; one for coloration (xLi⁺ + WO₃ + xe⁻ ↔ Li_xWO₃), and the other for an ion exchange reaction expressed by WO-H+Li⁺ ↔ WO-Li+H⁺. The mechanism of these phenomena are further discussed. Impedance analysis has been proved to be very sensitive and applicable enough to the quantitative characterization of ITO and Li_xWO₃ electrodes involved in the electrochromic device.     

Effect of modified ITO substrate on electrochromic properties of polyaniline films

In this work, we report the morphological and electrochromic properties of electrochemically synthesized polyaniline (PANI) thin films on bare and modified indium–tin oxide (ITO) glass substrates. In the last case, the surface of ITO glass was covered by a self-assembled monolayer of N-phenyl-γ-aminopropyl-trimethoxysilane (PAPTS). Atomic force microscopy images and perfilometry show that smoother and thinner PANI films were grown on PAPTS-modified ITO substrates. PANI-based electrochromic devices (ECDs) were assembled by using a viscous polymeric electrolyte (PE) of LiClO₄ and polymethyl methacrylate (PMMA) co-dissolved in a mixture of propylene and ethylene carbonate. The architectural design of the devices was glass/ITO/PANI/PE/ITO/glass. A dual ECD was also prepared by collocating a poly(3-methylthiophene) (P3MT) thin film as a complementary electrochromic element. The effect of the PAPTS-modified ITO substrate is reflected in a higher optical transmittance at bleach state and a little less color change at 550 nm of PANI-based ECDs.     

An all-solid-state electrochromic device based on NiO/WO3 complementary structure and solid hybrid polyelectrolyte

An all-solid-state electrochromic (EC) device based on NiO/WO₃ complementary structure and solid polyelectrolyte was manufactured for modulating the optical transmittance. The device consists of WO₃ film as the main electrochromic layer, single-phase hybrid polyelectrolyte as the Li⁺ ion conductor layer, and NiO film as the counter electrochromic layer. Indium tin oxide- (ITO) coated glass was used as substrate and ITO films act as the transparent conductive electrodes. The effective area of the device is 5×5 cm². The device showed an optical modulation of 55% at 550 nm and achieved a coloration efficiency of 87 cm² C⁻¹. The response time of the device is found to be about 10 s for coloring step and 20 s for bleaching step. The electrochromic mechanism in the NiO/WO₃ complementary structure with Li⁺ ion insertion and extraction was investigated by means of cyclic voltammograms (CV) and X-ray photoelectron spectroscopy (XPS).     

Electrochromic properties of nanocomposite WO3 films

A new nanocomposite WO₃ (NWO) film-based electrochromic layer was fabricated by a spray and electroplating technique in sequence. An indium–tin oxide (ITO) nanoparticle layer was employed as a permanent template to generate the particular nanostructure. The structure and morphology of the NWO film were characterized. The optical and electrochromic properties of the NWO films under lithium intercalation are described and compared to the regular WO₃ film. The NWO films showed an improved cycling life and an improved contrast with compatible bleach-coloration transition time, owing to the larger reactive surface area. The nanocomposite WO₃ film-based electrochromic device (NWO-ECD) was also successfully fabricated. Most importantly, the NWO film can be prepared on a large scale directly onto a transparent conductive substrate, which demonstrates its potential for many electrochromic applications, especially, smart windows, sunroof and displays.     

Electrochromic devices employing methacrylate-based polymer electrolytes

Poly(ethyl methacrylate) (PEMA)- and poly(2-ethoxyethyl methacrylate) (PEOEMA)-based polymer gel electrolytes with entrapped solutions of lithium perchlorate in propylene carbonate (PC) were prepared by direct, UV-initiated polymerization. The electrolytes were studied using electrochemical methods and they exhibit good ionic conductivity (up to 0.7 mS cm⁻¹ at 20 °C) as well as electrochemical stability up to 2.5 V vs. Cd/Cd²⁺ (5.1 V vs. Li/Li⁺) on gold electrode. The electrolytes have thermal stability up to 125 °C. The electrolytes were successfully tested as ionic conductors in the electrochromic device FTO/WO₃/Li⁺-electrolyte/V₂O₅/FTO using coupled optoelectrochemical methods to discuss the relationship between the electrolyte composition and parameters such as change of transmittance, response time and stability. The transmittance change Δτ was found to be 30-45% at 634 nm.     

Performance of Li-ion secondary batteries in low power,hybrid power supplies

Small, portable electronic devices need power supplies that have long life, high energy efficiency, high energy density, and can deliver short power bursts. Hybrid power sources that combine a high energy density fuel cell, or an energy scavenging device, with a high power secondary battery are of interest in sensors and wireless devices. However, fuel cells with low self-discharge have low power density and have a poor response to transient loads. A low capacity secondary lithium ion cell can provide short burst power needed in a hybrid fuel cell–battery power supply. This paper describes the polarization, cycling, and self-discharge of commercial lithium ion batteries as they would be used in the small, hybrid power source. The performance of 10 Li-ion variations, including organic electrolytes with Li_xV₂O₅ and Li_xMn₂O₄ cathodes and LiPON electrolyte with a LiCoO₂ cathode was evaluated. Electrochemical characterization shows that the vanadium oxide cathode cells perform better than their manganese oxide counterparts in every category. The vanadium oxide cells also show better cycling performance under shallow discharge conditions than LiPON cells at a given current. However, the LiPON cells show significantly lower energy loss due to polarization and self-discharge losses than the vanadium and manganese cells with organic electrolytes.     

Stabilizing lithium plating-stripping reaction between a lithium phosphorus oxynitride glass electrolyte and copper thin film by platinum insertion

Lithium (Li) plating-stripping reaction properties at the lithium phosphorus oxynitride glass electrolyte (LiPON)/copper thin film (Cu) interface is improved by the insertion of nano-thickness platinum (Pt) layer at the interface. The LiPON films are formed on mirror-polished lithium-ion conductive solid electrolyte sheets, and current collector thin films of Li, Cu-Pt multi layer, and Cu are formed on the LiPON films. The plating-stripping reactions at the LiPON/current collector films interface are carried out by galvanostatic and potential sweep measurements. Galvanostatic measurements reveal that Pt layer insertion reduces the overvoltage of the reaction and improves its coulomb efficiency. Also, cyclic voltammetry measurement suggests formation of Li-Pt alloys at higher voltages than 0 V (vs. Li/Li⁺) during the lithium plating process. Scanning electron microscopy observation clarifies that platinum insertion moderate non-uniform lithium plating reaction. Most probably, Li-Pt alloys increase the reaction sites, resulting in both the stabilization of current collector and the reduction of the overvoltage of the lithium plating-stripping reaction upon cycling. The results shown here will be useful in improving the anode reaction of the “Li-free” all-solid-state lithium batteries.     

Cycling performance and thermal stability of lithium polymer cells assembled with ionic liquid-containing gel polymer electrolytes

Gel polymer electrolytes containing 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide and a small amount of additive (vinylene carbonate, fluoroethylene carbonate, and ethylene carbonate) are prepared, and their electrochemical properties are investigated. The cathodic limit of the gel polymer electrolytes can be extended to 0 V vs. Li by the formation of a protective solid electrolyte interphase on the electrode surface. Using these gel polymer electrolytes, lithium metal polymer cells composed of a lithium anode and a LiNi_1/3Co_1/3Mn_1/3O₂ cathode are assembled, and their cycling performances are evaluated at room temperature. The cells show good cycling performance, comparable to that of a cell assembled with gel polymer electrolyte containing standard liquid electrolyte (1.0 M LiPF₆ in ethylene carbonate/diethylene carbonate). Flammability tests and differential scanning calorimetry studies show that the presence of the ionic liquid in the gel polymer electrolyte considerably improves the safety and thermal stability of the cells.     

Integrated low-emittance–electrochromic devices incorporating ZnS/Ag/ZnS coatings as transparent conductors

We have prepared and tested integrated low-emittance–electrochromic devices using ZnS/Ag/ZnS coatings as transparent electrodes and WO₃ films as electrochromic layers. These devices exhibit adequate coloration and can withstand more than 1000 bleaching-coloration cycles, provided that the metal layer is protected from the liquid electrolyte by a combination of thick dielectric films (ZnS/WO₃). We have also predicted the optimum configuration of the WO₃/ZnS/Ag/ZnS/Glass stack that maximizes transmission in the visible. Integration of low emittance and electrochromic films into one device could improve the performance and reduce the cost of electrochromic windows.     

Performance of an electrochromic window based on polyaniline, prussian blue and tungsten oxide

In our laboratory various electrochromic windows (ECWs) have been investigated using mainly tungsten oxide (WO₃), polyaniline (PANI) and prussian blue (PB) as electrochromic materials in combination with poly(2-acrylamido-2-methyl-propane-sulphonic acid) (PAMPS) as a solid proton-conducting electrolyte. The ECWs have been characterized by AC-impedance, linear sweep voltammetry and spectroelectrochemical studies in the 290–3300 nm spectral region. The ECWs have the following general multilayered structure: Glass/ITO/EC1/IC/EC2/ITO/Glass, where ITO=indium oxide doped with tin, IC=ionic conductor, EC1 is either PANI or PANI including PB, and EC2 is WO₃. The best of these ECWs has been able to regulate up to 56% (typical 50%) of the transmission of the total solar energy in the 290–3300 nm spectral range. The combination of the two electrochromic materials PANI and PB has been shown to be mutually beneficial in such a way that the colouration of the window is enhanced by the addition of a layer of PB onto PANI, while the adhesion of PB is improved by the presence of PANI. The energy consumption of the ECW is about 0.01 Wh/m² for one complete cycle (−1.8 V/1.2 V). The switching time for 90% colouring/bleaching is typically 10–30 s. A PANI/PB//WO₃ window has been operated for about 50 days (3700 complete cycles) without substantial loss of transmission regulation, though with an increase in switching time (10 min.). Spectra from individual layers in the ECWs have been recorded by making holes in one or two of the electrochromic layers. In this way (the hole method), it has been possible to study the transmission regulation properties for each electrochromic material separately in complete solid state windows. In addition, spectra for complete windows have been simulated by adding contributions from individual electrochromic layers.     

Comparative studies of “all sol–gel” electrochromic devices with optically passive counter-electrode films, ormolyte Li ion-conductor and WO3 or Nb2O5 electrochromic films

Laminated electrochromic (EC) devices are becoming increasingly important for making “smart” windows and switchable displays. Mostly, polymeric Li⁺ ionic conductors in combination with vacuum deposited active electrochromic and counter-electrode films are used. In this paper we report on the development of all sol–gel EC devices, that is, those where all three internal layers are prepared via the sol–gel route, including the ionically conductive inorganic–organic hybrid (ormolyte). The electrochemical and optical properties of EC devices are presented and the cycling stability and reversibility of their optical modulation assessed. The results show that WO₃/ormolyte/SnO₂ : Mo, WO₃/ormolyte/SnO₂ : Sb, WO₃/ormolyte/SnO₂ : Sb : Mo, Nb₂O₅/ormolyte/SnO₂ : Sb : Mo and WO₃/ormolyte/LiCo-oxide exhibit a transmission modulation dependent on the thickness of the active electrochromic and counter-electrode films and the thickness of the ormolyte layer. Electrochemical and optical properties of individual films are described and correlated with the stability of the all sol–gel EC devices.     

All solid state electrochromic devices on glass and polymeric foils

Tungsten oxide (WO₃), vanadium and nickel-hydroxide (VO_xH_y and NiO_xH_y)-films were evaporated on glass and polymeric substrates covered with indium-tin oxide (ITO). Films of nickel-oxide (NiO_x) were reactively sputtered from a nickel target. In order to obtain electrochromic devices the WO₃ film was used as one electrode and with a polymeric solid state electrolyte (PSSE) glued to each of the other films which served as different counter electrodes. The films for themselves and the complete devices were investigated by optoelectrochemical and other methods. The most stable device was the WO₃–VO_xH_y system which even improved the electrochromic properties after 3×10⁴ cycles.     

Electrochromic devices operating with electrolytes based on boronate ester compounds and various alkali metal salts

Various polymer electrolytes based on boronate esters and different lithium and sodium salts have been evaluated in electrochromic (EC) devices based on WO₃ films. The results showed that the ionic conductivity of the electrolytes was not the most important parameter for the colouration performance of these devices. The use of solid electrolytes containing LiClO₄ resulted in a higher colouration performance than the corresponding liquid electrolytes, even though the latter exhibited a significantly higher conductivity. The results also showed that the transfer process at the interface between the WO₃ layer and the electrolyte played a major role for the colouration process. The presence of Lewis acidic boronate ester groups in the electrolytes significantly improved the performance of the EC devices. The incorporation of boron in the composition of the electrolytes allowed the use of solid electrolytes instead of liquid ones, thus avoiding leakage problems. Furthermore, the highest colouration performance was found in EC devices operating with inexpensive salts. Because of their poor stability, the electrolytes based on LiCl and LiBr were not suitable, while those incorporating LiClO₄ salt exhibited excellent overall characteristics.     

Galvanostatic cycling of vanadium oxide (V6O13) in a nonaqueous secondary lithium cell

The galvanostatic cycling of Li(Al)-V₆O₁₃ cells in 1M LiAsF₆/propylene carbonate (PC)-acetonitrile (AN) and 1M LiAsF₆/PC electrolytes is reported. The discharge capacity and voltage of the Li(Al)-V₆O₁₃ cell were shown to be consistently higher than those of the corresponding Li(Al)-TiS₂ cell. Discharge rates of 1.2, 2.5, 5.0 and 10.0 mA cm^?2 with active material utilisations of 20 – 60% were obtained from the Li(Al)-V₆O₁₃ cell. The addition of fresh PC-AN electrolyte resulted in an improvement in cell capacity, but did not stop capacity decline on cycling. Cycle life of Li(Al)-V₆O₁₃ cells was normally less than 50 cycles. It was also found that exposure of the V₆O₁₃ electrode to moist air reduced the OCV and the discharge capacity of the cell. Short circuiting and complete discharge of the cell resulted in lower capacity and this may have been due to some degree of irreversible reduction of the V₆O₁₃.     

Imidazolium-based ionic liquid derivatives for application in electrochromic devices

Novel proton-conducting electrolytes were prepared from the sol–gel precursor 1-[3-(trimethoxy-λ⁴-silyl)propyl]imidazole with the addition of either trifluoroacetic or acetic acid. The presence of trimethoxysilyl groups enabled the solvolysis and condensation reactions of silsesquioxane species. IR spectroscopy revealed that more cube-like species formed in the electrolyte prepared from trifluoroacetic acid, while cube- and ladder-like silsesquioxanes were present in the electrolyte with acetic acid. This assignation was independently confirmed by ²⁹Si NMR analyses revealing the T³ signals of trisiloxane bonding. IR spectroscopy also pointed to the formation of hydrogen bonding in the latter electrolyte, since the frequencies of the observed bands at 1710, 1409, and 1272 cm⁻¹ approached those of acetic acid. In contrast, the IR bands at 1662, 1204, and 1130 cm⁻¹ confirmed the existence of trifluoroacetate anions in the case when the electrolyte was prepared from trifluoroacetic acid. The presence of free trifluoroacetate anions contributed to the moderately higher specific conductivity of this electrolyte (4.6×10⁻⁵ S/cm) compared to that of acetic acid (1.6×10⁻⁵ S/cm). The specific conductivity of the electrolytes could be further increased by the addition of a lithium salt. All electrolytes were employed in electrochromic devices with optically active WO₃ and various inorganic counter-electrodes (CeVO₄, V₂O₅, Ti/V-oxide). Photopic transmittance changes from 30% to 40% were achieved.     

WO3 pillar-type and helical-type thin film structures to be used in microbatteries

In the present study WO₃ thin films were deposited by sputtering onto ITO glass, W/ITO and Si substrates by using the glancing angle deposition (GLAD) technique, with the objective of applying these materials in electrochemical intercalation devices. The thin films microstructure and electrochemical behavior were determined through scanning electron microscopy (SEM) and cycling at constant current with potential limitation. By mainly adjusting the substrate holder speed rotation, pillar-type and helical-type structures were obtained under high and low speed rotation levels, respectively. The electrochemical results showed that the best charge capacity performance was obtained for the WO₃/W/ITO films with pillar-type structures, which are more porous.     

Electrochemical performance and thermal stability of LiCoO2 cathodes surface-modified with a sputtered thin film of lithium phosphorus oxynitride

A lithium phosphorus oxynitride (LiPON) glass-electrolyte thin film is coated on a lithium cobalt oxide (LiCoO₂) composite cathode by means of a radio frequency (RF) magnetron sputtering method. The effect of the LiPON coating layer on the electrochemical performance and thermal stability of the LiCoO₂ cathode is investigated. The thermal stability of the delithiated LiCoO₂ cathode in the presence of liquid electrolyte is examined by differential scanning calorimetry (DSC). It is found that the LiPON coating, improves the rate capability and the thermal stability of the charged LiCoO₂ cathode. The LiPON film appears to suppress impedance growth during cycling and inhibits side-reactions between delithiated LiCoO₂ and the electrolyte.     

Irradiation effect in WO3 thin films

Tungsten oxide (WO₃) films were prepared on indium tin oxide coated glass substrates and Corning glass substrates by sol–gel deposition. The samples coated on the glass substrates have been irradiated to approximately 0.93–21.1 kGy dose using Co-60 gamma radioisotope. Co-60 radioisotope changed the color of the WO₃ films on samples after the irradiation. Their color turned to brownish color tones depending on the applied dose. Optical and structural properties of the samples are examined for both gamma irradiated and unirradiated coated samples. To compare the effect of the irradiation on the electrochromic properties, additional measurements were done with WO₃ coated on ITO substrates irradiated by gamma rays, separately. The coated films were characterized by atomic-force microscopy, NKD-analyzer and cyclic voltammograms. The influence of irradiation on the spectra of transmittance and on the surface structure has been investigated. These showed that the surface texture was changed dramatically by the irradiation. The electrochemical insertion and removal of lithium and proton ions was carried-out using 1 M LiClO₄ propylene carbonate (PC) electrolyte and 1 M KCl in aqueous solutions,respectively.