GB2325236A - Polymer electrolyte for an electrochromic device - Google Patents

Abstract

A conducting polymer electrolyte for an electrochromic device comprising a cross-linked polysaccharide derivative, such as cellulose cinnamate or allyl cellulose, a solvent, such as N,N-dimethylacetamide (DMAC) or N-methyl-2-pyrrolidinone (NMP), and a salt, such as lithium chloride.

Description

Electrochromic Devices The invention relates to electrochromic devices, and in particular to polymer electrolytes for such devices.

A number of the world's suppliers of glass products are currently developing so-called "intelligent glazings". Unlike conventional glazings, whose properties, such as their solar heat or light transmission, are static, intelligent glazings may have their properties dynamically varied in response to sensed ambient conditions.

Electrochromic devices offer one source of intelligent glazings. Electrochromic devices are known which comprise a conducting polymer electrolyte, such as polymethylmethacrylate incorporating a salt and plasticiser, sandwiched between a pair of glass panes. The inner faces of the glass panes may be coated, one with an electrochromic layer on top of a transparent electrically conductive layer and the other with a counter-electrode layer also on top of a transparent electrically conductive layer. The electrolyte contains mobile ions, such as lithium cations, and the colour of the device may be altered by reversibly transferring cations between the electrochromic and counter-electrode layers. The direction and size of the transfer is determined by the polarity and magnitude of the voltage applied across the electrically conducting layers.

A device with a tungsten trioxide electrochromic layer and a cerium titanium oxide counter-electrode layer, each over an electrically conducting layer of indium tin oxide, can be varied between a blue, coloured state and a clear, bleached state. The solar heat absorption of the device increases with the blue colouration. Thus, with such a device as an intelligent glazing, solar heat transmission, for example, can be modulated automatically by continually adjusting the polarity and magnitude of the applied voltage in response to, say, the sensed sunlight intensity.

Electrochromic devices of the structure described above have been made using the well known cast-in-place laminating technique. The cast-in-place technique involves holding the two glass panes spaced apart by means of a peripheral seal, for example a double-sided adhesive tape, and filling the space between the sheets, the cell, with a mixture which is subsequently polymerised to form a solid polymer electrolyte. It is, therefore, a requirement of a cast-in-place electrochromic device that the polymer electrolyte material not only has the necessary conductivity characteristics but is also easily castable. There is an upper limit on the acceptable viscosity of a castable electrolyte material, and this limit decreases with the width of the cell being cast.

It has been suggested by J M G Cowie, Macromolecular Symposium 98 843-853 (1995), that cellulose gels could be used as polymer electrolytes in a variety of systems including electrochromic devices. Cowie found that a transparent, cross-linked gel-like material could be prepared by dissolving cellulose in a solvent/salt mixture, such as N,Ndimethylacetamide (DMAC) and lithium chloride or N-methyl-2-pyrrolidinone (NMP) and lithium chloride, and then by adding isophorone di-isocyanate as a cross-linking agent.

However, more recently it has been discovered that a di-isocyanate cross-linked cellulose gel is impractical for electrochromic devices. The difficulty is that it is not possible completely to dry the cellulose, even under the most stringent of drying procedures, and the cellulose/solvent/salt solution will always contain some water. This causes a problem because isophorone di-isocyanate is susceptible to a side reaction with water resulting in the formation of carbamic acid which is highly unstable and decomposes to an amine derivative with the evolution of carbon dioxide. The amine and the carbon dioxide complicate the electrochemistry of the electrolyte and affect its performance, but, more importantly in the context of glazings, bubbles of carbon dioxide degrade the appearance of the electrolyte.

Furthermore, unreacted isocyanate groups, which will always be present, are electrochemically unstable under the influence of even low applied voltages.

The invention provides a conducting polymer electrolyte for an electrochromic device comprising a cross-linked polysaccharide derivative, a solvent and a salt.

Polysaccharide derivatives are not susceptible to any side reactions with water which could ultimately result in the evolution of carbon dioxide. In addition, in order to be crosslinked, polysaccharide derivatives do not require the addition of a cross-linking agent, which involves a separate processing stage in the formation of the electrolyte. Also, polysaccharide derivatives exhibit better mechanical properties than other electrolyte materials such as polymethylmethacrylate and offer a significant cost advantage.

The polysaccharide derivative may contain sidegroup substituents which carry polymerisable functionalities including cellulose cinnamate or allyl cellulose. Cellulose cinnamate may be produced by mixing regenerated cellulose with cinnamoyl chloride in the presence of a suitable base. Allyl cellulose may be produced by mixing regenerated cellulose with allyl bromide.

Cross-linking is preferably effected after the derivative/solvent/salt mixture has been poured into the cell formed by the spaced apart panes of the electrochromic device. Crosslinking may be effected by any suitable thermal, photochemical or radiation process. The mixture may also include a cross-linking initiator, such as those commercially available under the designation Irgacure. Although a cross-linking agent is not necessary, one may be included to assist the process.

The salt may be lithium chloride (LiCl). It is known that LiCI is required in order to assist in the dissolving of polysaccharides in a solvent, but from the point of view of electrochromics, the presence of LiCl has the added advantage that it provides a source of lithium cations to take part in the electrochromic ion transfer process. LiCl is also cheaper than lithium perchlorate which is conventionally used as a source of lithium cations. Lithium perchlorate has also been found to be a suitable lithium cation source with cellulose cinnamate.

The solvent may be of the polar aprotic type such as DMAC or NMP.

The invention further provides an electrochromic device comprising a conducting polymer electrolyte as described above.

The invention is illustrated but not limited by the following examples: Example 1 Cellulose Cinnamate Regenerated cellulose was prepared by the following method: 30-SOg of cellulose was stirred in 500 ml of deionised water for 16 hours, after which the resulting mixture was filtered and the solids collected. The cellulose was then washed 5 times with 400 ml aliquots of dry methanol, and each time the cellulose/methanol mixture was stirred for 30 minutes and then filtered. This was repeated with DMAC in place of methanol and finally the last filtered solid was dried under a flow of dry nitrogen for 16 hours. The final regenerated cellulose obtained in this way contains approximately 33% dry cellulose and 67% DMAC by weight.

Cellulose cinnamate was prepared as follows: 4g of regenerated cellulose was mixed into as s tion of 9g of LiCl in 91g of DMAC under a nitrogen atmosphere. 1.78g of triethss e (a base catalyst) in lOml DMAC was then added, followed by 2.94g cinnamoyl chlc 'Oml of DMAC. The latter was added dropwise. The resulting solution was stirred at re lperature for 48 hours. The solution was then poured into hot water to precipitate the t product which was washed with hot water and finally soxhlet extracted with methanol for 3 hours. The resultant cellulose cinnamate was then dried at 50 C under vacuum overnight.

A gel was prepared from 57.6% substituted cellulose cinnamate (functionality of 1.73 with respect to hydroxyl groups) as follows: 0.5g of cellulose cinnamate and 0.5g LiCI were dissolved in 5.19 g NMP by standing at 1300 C for five hours and then slowly cooling the solution. After a gel had formed, 0.006 g of Irgacure 1700 photoinitiator was added and mixed by agitation. A gelled film was produced by pouring out the gel to a thickness of O."m and exposing it for 15 minutes to UV radiation, wavelength 360nm, so as to cross ae cellulose cinnamate. The conductivity of the film was measured by AC impedance sis and found to be 8.9 x 104 S at 200 C and 7 2 x 103 S cm~l at 800 C.

le 2: Allvl Cellulose Allyl cellulose was prepared as follows: 4g of regenerated cellulose (made as in example 1) was mixed into a solution of 9g of LiCI in 91 g of DMAC under a nitrogen atmosphere. 1.78g triethylamine in lOml DMAC was then added, followed by 2.33g allyl bromide in 20ml DMAC. The latter was added dropwise. The resulting solution was stirred at room temperature for 48 hours. The solution was then poured into hot water to precipitate the crude product which was washed with hot water and finally soxhlet extracted with methanol for 3 hours. The allyl cellulose was then dried at 500C under vacuum overnight.

A gel was prepared from 49.3% substituted allyl cellulose (functionality equal to 1.48) as follows: 0.5g allyl cellulose, 0.62g tetraethylene glycol diacrylate (a cross-linking agent) and 0.5g LiCl were dissolved in 5.19g NMP by standing at 1300C for 5 hours and then slowly cooling the solution. After a gel had formed, 0.007 g of Irgacure 1700 photoinitiator was added and mixed by agitation. A gelled film was produced by pouring out the solution to a thickness of 0.5mm and exposing it for 15 minutes to UV radiation, wavelength 360nm, so as to cross-link the allyl cellulose. The conductivity of the film was measured by AC impedance analysis and found to be 1.5x 10 Scum' at 20"C and 2.15x10-2 Scm' at 800C.

Claims (12)

Claims

1. A conducting polymer electrolyte for an electrochromic device comprising a cross-linked polysaccharide derivative, a solvent and a salt.

2. A conducting polymer electrolyte according to claim 1 wherein the polysaccharide derivative contains sidegroup substituents which carry polymerisable functionalities

3. A conducting polymer electrolyte according to claim 2 wherein the polysaccharide derivative is cellulose cinnamate or allyl cellulose.

4. A conducting polymer electrolyte according to any of claims 1 to 3 wherein the electrochromic device comprises spaced apart panes and the cross-linking is effected with the derivative/solvent/salt mixture between the panes.

5. A conducting polymer electrolyte according to any of claims 1 to 4 wherein cross-linking is effected by thermal, photochemical or radiation processes.

6. A conducting polymer electrolyte according to claim 5 further comprising a cross-linking initiator.

7. A conducting polymer electrolyte according to any preceding claim wherein the salt is lithium chloride or lithium perchlorate.

8. A conducting polymer electrolyte according to any preceding claim wherein the solvent is of the polar aprotic type.

9. A conducting polymer electrolyte according to claim 8 wherein the solvent is DMAC or NMP.

10. A method of making a conducting polymer electrolyte according to any preceding claim.

11. An electrochromic device comprising a conducting polymer electrolyte according to any of claims 1 to 9.

12. A conducting polymer electrolyte substantially as herein described in the examples.