Chapter 4
Nonaqueous Polymer Electrolytes for Electrochromic Devices J. R. Stevens, W. Wieczorek, D. Raducha, and K. R. Jeffrey
Downloaded by UNIV OF LIVERPOOL on November 24, 2016 | http://pubs.acs.org Publication Date: August 19, 1999 | doi: 10.1021/bk-1999-0726.ch004
Department of Physics, University of Guelph, Guelph, Ontario N1G 2W1, Canada
Nonaqueous alkali metal salt polyether electrolytes and nonaqueous H PO proton conducting electrolyte gels have been synthesized for application in electrochromic devices. The ionic conductivity of these electrolytes has been optimized to be in the range 10 S/cm to 10 S/cm at room temperature. The most favourable, stable, polyether electrolytes are blends based upon EO/PO (= 50/50 or 60/40) copolymers with molecular weights in the range of 300-600 and LiCF SO or Li(CF SO ) N concentrations of 25:1, PMMA concentrations in the range 8 to 40 vol% and PC < 15 vol%. The most favourable, stable nonaqueous gels were those with H PO dissolved in PC (20 to 50 vol%) entrapped in a PMMA network using a crosslinking agent. These electrolytes are stablefrom-30° to 100°C. 3
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According to the Report of the World Commission on Environment and Development (7) and the research and development being carried out under the International Energy Agency (2), interest in solar energy materials, like energy efficient window coatings, is growing. Materials with optical switching (dynamic) properties were reviewed by Lampert and Granqvist (3) and Granqvist (4) and consist of photochromic, thermochromic and electrochromic materials depending on whether their optical properties are changed due to irradiation, temperature, or electric potential difference. Electrochromism is defined as a persistent but reversible optical change in absorption or reflection produced electrochemically in a medium by an applied electric field or current. An electrochromic device in which this process can be realized is schematically shown in Figure 1. Generally such a device contains transparent conductors, an ion storage layer, an ionic conductor and an optically active electrochromic layer. We will focus on properties of the ionic conductor. Since the first reports that alkali metal salts could be dissolved in poly(ethylene oxide) (PEO) (5-7) much progress has been made in the synthesis of polymer electrolytes (£). The main role of an electrolyte in electrochromic devices is to allow
© 1999 American Chemical Society Khan and Harrison; Field Responsive Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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Figure 1.
Schematic sketch of an electrochromic device.
Khan and Harrison; Field Responsive Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
53 ions to be shuttled between an electrochromic film and an ion storage film also known as a "counter electrode". It is beneficial for polymer electrolytes working in electrochromic devices to have the following properties: wide operational temperature range, high exchange current density, ionic conductivity > 10" S/cm, good to excellent adhesive properties, greater than 80% transparency in the visible region of the electromagnetic spectrum, stability against such environmental factors as moisture and temperature variation, and either be self-supporting films or have good, bubble free, coatability onto an electrochromic film substrate. A wide range of solid and liquid electrolytes has been tested in various electrochromic devices and the results of these studies have been extensively discussed (4). The main electrolyte groups are: aqueous acidic electrolytes, nonaqueous lithium electrolytes, ceramic ionic conductors (P-alumina, Nasicon etc.), polyelectrolytes, polymer solid electrolytes (complexes of polyethers or polyimines with alkali metal salts and proton donors) and plasticized polymer ionic conductors. Ambient temperature proton conducting systems studied so far are either not very stable (e.g. heteropolyacids) or their conductivity depends on the amount of water present (e.g. hydrated Nafion or Dow membranes). The presence of water is the main disadvantage of using aqueous or polyelectrolyte systems. It has been shown that most of the electrochromic oxides react with water molecules. This limits the lifetime of electrochromic devices containing electrolytes with even residual traces of water. The utilization of ceramic electrolytes leads to interfacial problems manifested by an increase in the overall resistance of the device which results in limiting current effects. Moreover most of the intercalated materials used as electrodes in electrochromic devices expand during the intercalation of ions; this often results in the cracking of the ceramic electrolyte. Because of the above mentioned limitations flexible nonaqueous polymeric electrolytes have been used as electrolyte membranes in various electrochromic devices. Polymer electrolytes with ionic conductivities up to 10' S/cm at room temperature have been synthesized and tested in "smart window" and electrochromic display configurations. Generally these electrolytes are of two types; lithium salt doped polyether copolymers (9-17) and nonaqueous, proton conducting strong acid solutions (18-20); both types entrapped in a polymeric gel. Lithium bis(triflouromethanesulfonyl)imide (Li(CF S02) and lithium triflouromethanesulfonate (LiCF SOj) are the favoured lithium salts and phosphoric, antimonic and sulphuric acids have been used as sources of protons. The gels are usually poly(methyl methacrylate) (PMMA) or a copolymer of P M M A and glycidal methacrylate (GMA), poly(vinyl acetate), poly(acrylamide) or poly(vinylidene fluoride). The highest conductivities in lithium salt doped systems have been achieved in plasticized systems (13-17). These are characterized by relatively high ionic conductivities (up to 10" S/cm at room temperature) and transparency for up to 30 mass% of P M M A where P M M A has been used (13,14). A two step polymerization of glycidyl methacrylate (GMA) in the presence of propylene carbonate (PC) and a lithium salt (75), the use of small molecule cyclotriphosphazenes in
Downloaded by UNIV OF LIVERPOOL on November 24, 2016 | http://pubs.acs.org Publication Date: August 19, 1999 | doi: 10.1021/bk-1999-0726.ch004
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Khan and Harrison; Field Responsive Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
54 poly[bis(methoxyethoxy) phosphazene] ( M E E P ) - L i C F S 0 (16), and the use of fumed-silica particulates in low molecular weight poly(ethylene oxide) (17) with lithium salts (imide and triflate anions) are examples of placticized systems. In these systems anions are the dominant mobile species. Scanning election microscope studies indicate that polyether/PMMA/Li salt blends have a microphase separated morphology with the P M M A well dispersed in sizes of 2-20 fim (11). The effect of the molecular weight of the components on the "window of compatibility" is known from the literature for polymer blends such as polyethylene oxide) (PEO)/PMMA (9-12). The blending of P M M A with the polyether was found to stabilize the electrolyte against moisture uptake and provide adhesion to a glass substrate. There has recently been widespread interest in the development of proton conducting polymeric electrolytes which can be used at ambient and moderate temperatures (18-28). The advantage of proton conducting systems in comparison with alkali metal electrolytes arises from their potentially higher conductivity. Thus we should expect a faster coloration-bleaching time in electrochromic devices compared with lithium salt electrolytes. Proton conductors are systems in which polar polymers with basic sites on a main polymer chain form compounds with strong acids such as H S 0 or H P 0 . The polymeric films formed must be chemically and mechanically stable. Properties of these systems have been reviewed by Lassegues (27). Ambient temperature conductivities obtained for some of these proton polymeric electrolytes were higher than 10" S/cm (18-21,27,28). The C - 0 bond in ethers and alcohols is broken by strong acids; such degradation is accelerated by traces of water. Under laboratory conditions all necessary precautions have been made to keep these electrolytes "dry" (22). However it is not clear what the effect of residual traces of water on conductivity will be as well as the long time stability of these electrolytes in large area applications. The main objectives of the work discussed here are to optimize the conductivity of nonaqueous electrolytes for electrochromic devices. The electrochemistry of nonaqueous alkali metal salt electrolytes is well developed because of the possible application of these systems in ambient temperature alkali metal batteries (29). However, little is known about the properties of nonaqueous concentrated solutions of strong acids which potentially have high protonic conductivity. Most of the studies of liquid proton conducting electrolytes described in the literature are devoted to aqueous electrolytes or to very dilute solutions (
