GB2216132A - Solid polyacrylamide electrolyte - Google Patents

Abstract

An ionically conducting solid polymer electrolyte comprises a non-aqueous complex of plasticized polyacrylamide or a plasticized poly(N-monosubstituted acrylamide) or a plasticized copolymerised acrylamide or plasticized copolymerised poly(N-monosubstituted acrylamide> <IMAGE> with a monovalent alkali metal salt, where R is hydrogen, or is alkyl, alkenyl, cycloalkyl, cycloalkylene or aryl (all of which may be optionally substituted). The alkali metal salts are preferably perchlorates, perfluorinated carboxylates or trifluoromethanesulphonates. In the examples, the acrylamide monomers are polymerized in the presence of the alkali metal salts. Use: as solid ionic electrolytes in galvanic cells and electrochromic displays.

Description

SOLID POLYACRYLAMIDE ELECTROLYTE This invention relates to conductive polymeric materials; more particularly, this invention relates to non-crystalline solid polymer electrolytes possessing high bulk ionic conductivity and to their use as ionic electrolytes in electrical devices such as galvanic cells and electrochromic displays.

lonically conducting polymer electrolytes generally consist of a polymer and an inorganic electrolyte; the polymer should contain electronegative atoms and be capable of dissolving and complexing the inorganic electrolyte. Many polymers can be expected to exhibit these characteristics but in practice, systems based on polyethylene oxide (PEO) are most often found both commercially and in the literature for example Duval et al, paper ECll of the European Symposium on Polymeric Materials, first meeting of the European Polymers Federation at Lyon, 14-18 September 1987. Ideally, the polymer should have a low degree of crystallinity because ionic conductivity occurs preferentially through the non-crystalline regions of the polymer, and it is desirable that it is also a solid.With linear PEO, the characteristics of solidity and a low degree of crystallinity are mutually exclusive (only very low molecular weight liquid or waxy materials are non-crystalline or exhibit a low degree of crystallinity); the crystalline content of PEO can be reduced to some extent by virtue of complexing an inorganic electrolyte, but generally, the propensity to crystallise can otherwise only be prevented by modifying the polymer.

Examples of modified PEO may be found in the prior art; typical examples include comb-polymers such as poly(alkoxy-PEO-methacrylate)s and poly(alkoxy-PE#siloxane)s in which linear PEO chains are attached as the teeth on the comb to an olefinic chain backbone or a siloxane chain backbone respectively. These materials are rubbery in the former case and are liquids in the latter.

It is desirable to have solid rather than liquid polymer electrolytes, the advantages being that the solid nature of solid polymer electrolytes prevents the solvating chains from migrating together with the ions and being co-inserted in the electrode material and inimises degradative reactions at the electrolyte-electrode interfaces. Furthermore, solid polymer electrolytes are easier to handle than liquid or viscous liquid electrolytes and are dimensionally stable.

According to the present invention, an ionically conducting solid polymer electrolyte comprises a non-aqueous complex of plasticized polyacrylamide or a plasticized poly(N-monosubstituted acrylamide) or a plasticized copolymerised acrylamide or plasticized copolymerised poly(N-monosubstituted acrylamide)

with a monovalent alkali metal salt where R is hydrogen, or is alkyl, alkenyl, cycloalkyl, cycloalkylene, aryl (all of which may be optionally substituted e.g., by methyl (non-complexing) or methoxy (complexing)). The polyacrylamide or poly(N-substituted acrylamide) referred to as "the polymer" in this disclosure, may be cross-linked by the incorporation of units derived from a polyfunctional monomer such as methylene-bis-acrylamide.The polymer may be plasticized by the presence of a low molecular weight compound such as an N-substituted or N,N-disubstituted amide or a nitrile or an oligoether or another polar or apolar organic compound. Thus secondary amide groups may be present in the form of added N-substituted acylamide such as N-methylacetamide, and tertiary amide groups may be present in the form of added N,N-disubstituted acylamide, such as N1N-dimethylacetamide, nitrile groups may be present in the form of added acetonitrile, and ether groups may be present in the form of added oligo-ethylene oxide. The polymer may also be blended with one or more other polymers such as with each other or with other members of the family of poly(N-substituted acrylamide)s, or with PEO or PEO-related materials or other polymeric materials.Thus, additional ether groups may be present in the form of added polyethylene oxide.

The monovalent alkali metal cation may be lithium or sodium or potassium and the anion may be perchlorate or trifluoromethanesulphonate or another anion that is bulky or is a weak conjugate base.

The advantages of using polyacrylamides and poly(N-substituted acrylamide)s, in contrast to PEO-based materials, are that these polymers are non-crystalline and possess superior thermal and chemical stability. Furthermore, it is possible to generate tough solids that are colourless, transparent materials.

Polyacrylamide and poly(N-substituted acrylamide)s may be prepared by polymerisation, in solution or in bulk, of acrylamide and N-substituted acrylamides respectively, referred to as "the monomers" in this disclosure, by a single or combined use of thermally activated or photo-activated radically initiated polymerisation. The polymers may be obtained by homo-polymerisation of a single monomer or by co-polymerisation of two or more monomers. Polymers may also be blended.

Copolymerisatlon of two or more monomers can lead to copolymers having properties that combine the better and desirable qualities of the parent homopolymers, and evidence of these benefits will be demonstrated later.

Blending of two or more polymers does not necessarily lead to an enhancement of individual properties, but the process nevertheless provides a simple method for bringing together polymers where copolymerisation is inappropriate due to wide differences in the relative reactivities of selected co-monomers. Blending of polymers is furthermore appropriate when it is not convenient or possible to obtain them by a single polymerisation process. An example would be the case of incorporating poly(N-substituted acrylamide)s conveniently prepared by radically initiated polymerisation with PEO prepared by anionically initiated polymerisation. The blended polymers may be miscible or they may be immiscible, but it is preferred that the several polymers are miscible and form an homogeneous blend with no visible phase separation.It is further preferred that the several polymers all dissolve and complex the inorganic electrolyte. Blending may also be used to bring together the complex with dispersed additives such as fillers and/or anti-oxidants designed to contribute to the overall properties.

Blending may be achieved by mechanically mixing the co-components, by melt extruding or solvent casting the complex with the additives.

The materials obtained from polymerisation of acrylamide in solution have been found to have markedly different physical characteristics compared with the materials obtained from polymerisation of N-monosubstituted acrylamides in solution.

Thus, compared to solution polymerisation of N-monosubstituted acrylamides dissolved in organic solvents which leads to homogeneous transparent solutions or gels, solution polymerisation of acrylamide dissolved in non-aqueous solvents leads to the formation of a two-phase product consisting of the organic solvent imbibed in swollen beads of the polymer. This is a feature of the precipitation (hereafter referred to as precipitation polymerisation) of the polymer from the monomer solution at some critical build-up of molecular weight. The product is of a stiff buttery consistency.

Preparation of a single-phase transparent polyacrylamide is, however, possible by use of a form of bulk polymerisation in which acrylamide is heated until it is molten and polymerisation is then allowed to proceed. This process has been found to yield transparent solids, but it is difficult to control and often leads to materials containing voids and other defects.

Defect-free transparent solid polyacrylamide can nevertheless be prepared by polymerising acrylamide in the presence of a few weight percent of volatile solvent (e.g., acetonitrile) and dissolved inorganic electrolyte at a temperature slightly higher than the melting temperature of the monomer and the boiling point of the solvent. In this process, polymerisation of monomer and controlled evaporation of solvent occur concurrently, and this process prevents precipitation of polymer from solvent and the formation of voids in the solid product. The product is tough and can be machined.

In photo-initiated bulk polymerisation, an Me/D high pressure mercury discharge lamp may be used to provide a source of ultra violet (UV)light of wavelengths ranging from about 240 to about 380 nanometres (nm). With such a light source polymerisation may be conducted in thin walled glass vessels. However, glass is opaque to UV light below about 350nm so it is preferable to use vessels made from polyethylene which is less opaque to UV light.

For example, with such a light source and with polyethylene vials a suitable arrangement for polymerisation is that the vials containing the monomer are placed in the plane of the light path at the circumference of a rotating disc (diameter 15cm, 6 rpm) such that they periodically pass at a distance of locum to 35cm, preferably 20cm to 35cm from the light source, or are held static at a distance of locum to 80cm, preferably 30cm to 50cm, most preferably 35cm to 45cm from the light source. Photo-Initiated polymerisation is particularly useful for in situ generation of the polymer, for example in forming a thin polymer film at the surface of an electrode.

In polymers that are cross-linked the weight percentage of cross-linking monomer (e.g., nethylene-bis-acrylamide) is preferably up to 20X on the weight of monomer in the case of polyacrylamide prepared by precipitation polymerisation, and preferably up to 30X on the weight of monomer in the case of poly(N-substituted acrylamide).

The cation used in the formation of the complex incorporating the polymers of this invention is preferably monovalent. A particularly preferred class of cation is an alkali metal cation such as lithium, sodium or potassium. Lithium is preferred by reason of its use in electrode materials. The anion (counterion) is preferably large in order to reduce gross segregation in use, but not so large as to be rendered immobile, and it is further preferred that the anion is a weak conjugate base. Examples include the monovalent anions derived from higher halogens, e.g.

Br- and I-; complex monovalent anions, preferably perfluorinated where possible, e.g. SCN-, C104-, HgI3-, BF4-; carboxylic, preferably perfluorinated, e.g. CmF2m+lco2- such as CF3CO2-, C2F5CO2-; C3FTCO2-; sulphonic, preferably perfluorinated e.g., CmF2mtlso3- such as CF3SO3-, where m is an integer from 1 to 6, preferably from 1 to 3.

Formation of the polymer-inorganic electrolyte complex, hereinafter called "the complex", may be achieved in a number of ways, for example dissolution of pre-formed polymer and inorganic electrolyte in a suitable solvent followed by evaporation of the solvent under reduced pressure. With some of the polymers and inorganic electrolytes described in this invention methanol is suitable, and the solvent is removed by evaporation under reduced pressure to yield the complex. The disadvantage of this process is that polymers that are cross-linked, either by design or because of the presence of polyfunctional monomeric impurities in the supplied monomer, and polymers that are highly hydrogen bonded as is the case for many of the polymers of this invention are at best difficult to dissolve or form only swollen gels.An alternative method which avoids this disadvantage Is to bring together the monomer and Inorganic electrolyte prior to polymerisation.

Thus, the inorganic electrolyte may be dissolved directly in the (liquid) monomer, or to assist rapid dissolution, especially as in the case of acrylamide, in the presence of a suitable co-solvent for monomer and inorganic electrolyte, for which purpose polar organic solvents such as N-methylacetamide, N,N-dimethylacetamide or acetonitrile are suitable. The solvent is removed from the monomer prior to bulk polymerisation or from the polymer subsequent to solution polymerisation.

The polymers of this invention form hydrogen bonded systems and have high glass transition temperatures (Tg) leading to materials that are rigid and tough. In order to obtain acceptable levels of ionic conductivity in the polymer/electrolyte complex it is necessary to reduce the Tg of the polymers. The Tg is reduced to some extent by virtue of the plasticizing action of residual solvent; residual solvent is extremely difficult to remove after its inclusion as an aid in the preparation of the complex. It has been found beneficial and indeed necessary to include a high proportion of plasticizing solvent in the complexes leading to a reduction in Tg without this also leading to an undue loss in the solid nature of the materials.

A plasticizer may be any non-aqueous solvent, preferably one that will complex both the inorganic electrolyte and dissolve the polymer, such as N,N-dimethylacetamide, N-methylacetamide, acetamide, formamide or acetonitrile, or it may be monomer such as N-methylacrylamide purposely added or from incomplete conversion during polymerisation, or it may be a low molecular weight polymer such as a polyether-based compound.

The plasticizer need not be present as a minor component.

Indeed, it can be the major component of a conducting system.

For example, it is known that polymerisation of acrylamide with cross-linking agent (i.e., methylene-bis-acrylamide) in water yields a transparent gel that is typically composed of between 5 and 30 weight percent of polymer the remaining weight being entrapped water. When the gel contains electrolyte, it can carry a useful current, and for this reason, polyacrylamide gels are used in gel electrophoresis. In these materials water acts as the medium through which the ions migrate.

Ionically conducting polymers exploited for use in galvanic cells cannot contain aqueous media because of the vigorous reaction of water with the electrode materials, though organic solvents that dissolve electrolytes are suitable alternatives.

Examples of suitable organic solvents include those previously noted and other polar solvents such as propylene carbonate.

In a polymer-solvent system that contains more than one phase, for example, that obtained by precipitation polymerisation of acrylamide in N,N-dimethylacetamide, the solvent is entrapped in individual polymer beads and also cohesively between them; the system must contain sufficient solvent to act as a binder to the beads, and a very high solvent content is possible leading to materials that range from being of stiff to fluid, buttery consistency. In such a phase separated system comprising a linear polymer and plasticizer, in which the polymer can be described as being both plasticized by and blended with the plasticizer, it is preferred that the proportion of plasticizer (such as methoxy-polyethyleneglycol (Mn 350)) is from 40Z to 90X, more preferably from 60% to 80X.

In a polymer-solvent system that consists of a single phase, for example that obtained with poly(N-acrylylallylamine) and N,N-dimethylacetamlde1 the solvent is entrapped in a swollen three dimensional entangled polymer network, and the system can contain very high contents of solvent leading to materials ranging from being tough, stiff solids to viscous resins. In such a single phase system comprising a linear-polymer and plasticizer, in which the polymer can be described as being plasticized, it is preferred that the proportion of plasticizer (such as N,N-dimethylacetamlde) is from 15X to 70%, more preferably from 20% to 60%.

It is useful to distinguish between external and internal plasticizers, the distinction being that the former are not covalently linked to the polymer whereas the latter are covalently linked to the polymer.

Copolymerisation may be used to generate polymers containing an "internal" plasticizer as part of the polymeric chain. For example, N-butylacrylamide may be used as an internal plasticizer in the copolymer obtained from N-methylacrylamide and N-butylacrylamide.

In complex systems that contain plasticizer, it is preferred that the plasticizer is an external plasticizer rather than an internal plasticizer.

The ideal external plasticizer has a high dielectric constant, low viscosity, is stable, and is wholly compatible with the other co-components of the complex system. Combinations of plasticizers may be used.

Combinations of plasticizers can include non-complexing plasticizers such as non-polar organic solvents (e.g. hexane).

It is preferred that the non-complexing plasticizer is miscible with the complexing plasticizer and It is further preferred that the proportion of non-complexing plasticizer incorporated in the combined plasticizers is such that the inorganic electrolyte is not precipitated.

In addition to the type and content of plasticizer, the method of preparation has been found to influence the bulk ionic conductivity of the complex. Thus, polymers prepared by thermally activated radically initiated bulk polymerisation usually exhibit lower stiffness or viscosity compared to the polymers prepared by photo-activated radically initiated bulk polymerisation. Furthermore, the complexes incorporating the polymers prepared by the former method exhibit ionic conductivities superior to those complexes incorporating polymers of nominally identical chemical composition but prepared by the latter method. It has further been observed that polymers prepared by photo-activated radically initiated polymerisation yield complexes having superior ionic conductivities if the polymer is prepared in a glass rather than a polyethylene vessel.

Thus, bulk ionic conductivities greater than 10-6 S/cm (S = ohm-1) at 200C can be obtained in the case of complexes incorporating N-monosubstituted acrylamide i.e., poly(N-acrylyl allylamine) and plasticizer i.e., N,N-dimethylacetamide (polymer/solvent ratio 80:20) prepared photochemically in polyethylene vessels. Further increase in the ambient temperature bulk ionic conductivity, at least to greater than 10-3 S/cm, is possible with higher plasticizer content (50%), but increasing the weight ratio of plasticizer to polymer above 70:30 leads to materials of inadequate rigidity unless the polymer is cross-linked.

In poly(N-monosubstituted acrylamides) that are cross-linked, the incorporated content of plasticizer can be higher than in the non-cross-linked polymer. It has been found that increasing the weight fraction of cross-linking monomer (i.e., increasing the cross-link density) allows higher levels of plasticizer to be incorporated, but too high a cross-link density leads to materials that are friable, irrespective of the content of plasticizer. In practice, it has been found that levels of up to 80Z plasticizer (on the weight of polymer) can be Incorporated in polymer containing up to 30X of cross linking monomer on the weight of monofunctional monomer.

The concentration of inorganic electrolyte in the polymer matrix is most conveniently expressed by the molar ratio of complexing atoms in the polymer and in other complexing co-components to monovalent cations derived from the inorganic electrolyte. The complexing atoms may be of identical type (e.g.

all secondary amide) or they may be different (e.g. amide and ether). Where the complexing atoms are of identical type it is convenient to express the molar ratio of combined complexing atoms to monovalent cations. For example, as in the case of poly(N-monosubstituted acrylamides), the presence of co-N-monosubstituted acylamides as internal plasticizers, or N-monosubstituted acylamides as external plasticizers provides additional secondary amide nitrogen atoms. Where the complexing atoms are different it is convenient to express the molar ratio of the different types of complexing atoms to monovalent cations.

In the polymer electrolytes according to this invention, the complexing atoms are primary and secondary amide nitrogen atoms in the respective cases of polyacrylamide and poly(N-monosubstituted acrylamides), and the preferred molar ratio of amide nitrogen atoms to monovalent cation is 10 to 30, more preferably 12 to 18. In cases where there are two or more kinds of complexing atoms, for example with the presence of primary or secondary or tertiary amides, nitriles or polyethers, the preferred molar ratio will be modified according to the nature of the complexes formed with the other kind of complexing atom or atoms.

The ionic conductivities of the complex were measured by a.c.

impedance spectroscopy. In this method, the samples were pressed between brass or other electrodes and conductivity measurements were carried out using a 1172 Solartron frequency response analyser. The complex admittance was measured as a function of frequency from 1 to 6.3 x 104 Hz. Due to the blocking nature of the electrodes, the real part of the admittance rose with increasing frequency to a frequency-independent plateau. The value at the plateau was used to calculate the bulk Sonic conductivity.

A variety of materials having a wide range of mechanical properties can be obtained with the polymers of this Invention.

Thus, It is possible to prepare polymeric materials that range from being transparent, tough, stiff solids to viscous resins, to opaque, stiff, spreadable pastes. A feature of this diversity is that the materials can be tailored to have mechanical characteristics suitable for a variety of products and end uses.

The invention will now be described by way of examples.

Example 1 Lithium perchlorate (1.5059) was dissolved in a solution of acrylamide (l.Og), methylene-bis-acrylamide (0.0279) and methoxy-poly(ethylene glycol) (MW average 350; 8.459g) and polymerisation was initiated between brass electrodes (cell dimensions lOmm x lOmm x loom) at 800C without added initiator.

The conductivity of the resultant material was 3.14 x 10-4 S/cm (S = ohm-1)(primary N:Li=l:l; secondary N:Li=0.02:l; 0:Li=12:l) at l80C.

Example 2 Lithium perchlorate (0.2329) was dissolved in a solution of acrylamide (0.229g), methylene-bis-acrylamide (0.006g) and methoxy-poly(ethylene glycol) (MW average 350; 2.5339) and polymerisation was initiated between brass electrodes (cell dimensions lOmm x lOmm x loom) at 800C without added initiator.

The conductivity of the resultant material at 800C was 2.19 x 10-3 S/cm (primary N:Li=1.47:1; secondary N:Li=0.037:1; 0:Li=24:1) at 80 C).

Example 3 The experiment described in example 2 was repeated except that the weights of reagents were lithium perchlorate (0.155g), acrylamide (0.305g), methylene-bis-acrylamlde (0.0089) and methoxy-poly(ethylene glycol) (MW average 350; l.532g).

The conductivity of the resultant material at 800C was 2.7 x 10-3 S/cm (primary N:Li=2.95:l; secondary N:Li=0.073:l; 0:Li=22:1) at 800C).

Example 4 The experiment described in example 2 was repeated except that the weights of reagents were lithium perchlorate (0.5499), acrylamide (0.9159), methylene-bis-acrylamide (0.0259) and methoxy-poly(ethylene glycol) (MN average 350; 2.069).

The conductivity of the resultant material at 800C was 4.7 x 10-3 S/cm (primary N:Li=2.5:1; secondary N:Li=0.006:l; 0:Li=8.25:l) at 800C).

Example 5 Lithium perchlorate (8.09) was dissolved in a solution of acrylamide (ll.lg), methylene-bis-acylamide (0.39) and N,N-dimethylacetamide (55.28g) and polymerisation was initiated with ammonium persulphate (0.155 w/w on monomers) and tetramethylethylenediamine (0.3% w/w on monomers) between brass electrodes at 180C. The product was a white paste.

The conductivity of the material spread onto and pressed between brass electrodes and having a diameter of 16mm and thickness of 0.45mm was 2.1 x 10-3 S/cm at l80C (primary N:Li=2:1, secondary N:Ll'i0.05:10 tertiary N:Lln9:l).

Example 6 A solution of N-'acrylylallylamine (O.718g) and lithium trifluoromethane sulphonate (0.0859) In N,N-dimethylacetamide (0.2069) in polyethylene vials was degassed by a process of freeze-thawing (x2) and was subjected to 265nm UV for 24h. Methyl benzoyl formate (0.02X on weight of monomer) was used as initiator. The sample was held at a fixed distance of 40cm from the light source. Polymerisation of the monomer produced a transparent solid product.

The conductivity of a disc cut from the plasticized polymer complex having a diameter of lOmm and thickness of 2.35mm ranged from 4.78 x 10-6 S/cm at 20AC to 9.98 x 10'5 S/cm at 1000C (monomer/solvent ratio 78%/22%; secondary N:Li=12:l, tertiary N:Li=4.4:l).

Example 7 The experiment described in example 6 was repeated except that the monomer/solvent ratio is 67Z/33Z (monomer (0.582g), inorganic electrolyte (0.069g), plasticizing solvent (0.2869) and the ratio of complexing atoms to lithium atoms is different.

The conductivity of a disc cut from the plasticized polymer complex having a diameter of lOmm and thickness of 3.06mm ranged from 1.48 x 10-4 S/cm at 200C to 8.62 x 10-4 S/cm at 1000C (secondary N:Li=12:1, tertiary N:Li=7.5:1).

Example 8 The experiment described in example 6 was repeated except that the monomer/solvent ratio is 57Z/43Z (monomer (0.5749), inorganic electrolyte (0.0689), plasticizing solvent (0.4409) and the ratio of complexing atoms to lithium atoms is different.

The conductivity of a disc cut from the plasticized polymer complex having a diameter of lOmm and thickness of 2.78mm ranged from 5.76 x 10'4 S/cm at 200C to 1.61 x 10-3 S/cm at 1000C (secondary N:Li=12:1. tertiary N:Li=11.6:1).

Example 9 The experiment described in example 6 was repeated except that the monomer/solvent ratio is 46%/54% (monomer (0.3789), inorganic electrolyte (0.0459), plasticizing solvent (0.4349) and the ratio of complexing atoms to lithium atoms is different.

The conductivity of a disc cut from the plasticized polymer complex having a diameter of lOmm and thickness of 3.2mm ranged from 1.38 x 10-3 Slcm at 200C to 3.75 x 10-3 S/cm at 1000C (secondary N:Ll=12.l, tertiary N:Li=17.3:l).

Claims (15)

1. An ionically conducting solid polymer electrolyte, comprising a non-aqueous complex of plasticized polyacrylamide or a plasticized poly(N-monosubstituted acrylamide) or a plasticized copolymerised acrylamide or plasticized copolymerised poly(N-monosubstituted acrylamide)

with a monovalent alkali metal salt where R is hydrogen, or is alkyl, alkenyl, cycloalkyl, cycloalkylene or aryl (all of which may be optionally substituted).

2. An electrolyte according to Claim 1, wherein the substituent of R is non-complexing.

3. An electrolyte according to Claim 2, wherein the substituent of R is methyl.

4. An electrolyte according to Claim 1, wherein the substituent of R is complexing.

5. An electrolyte according to Claim 4, wherein the substituent of R is methoxy.

6. An electrolyte according to any preceding claim, wherein the polyacrylamide or poly(N-substituted acrylamide) is cross-linked by the incorporation of units derived from a polyfunctional monomer.

7. An electrolyte according to Claim 6, wherein said monomer is methylene-bis-acrylamide.

8. An electrolyte according to any preceding claim, wherein the polymer is plasticized by the presence of a low molecular weight compound.

9. An electrolyte according to Claim 8, wherein the low molecular weight compound is an N-substituted or N,N-disubstituted amide or a nitrile or an oligoether or another polar or apolar organic compound.

10. An electrolyte according to any preceding claim, wherein the acrylamide is blended with one or more other polymers selected from other members of the family of poly(N-substituted or non-N-substituted acrylamide)s and polyethylene oxide.

11. An electrolyte according to any preceding claim, wherein the monovalent alkali metal salt is of lithium or sodium or potassium.

12. An electrolyte according to any preceding claim, wherein the salt comprises an anion that is bulky or is a weak conjugate base.

13. An electrolyte according to Claim 12, wherein the anion is perchlorate, perfluorinated carboxylate or trifluoromethanesulphonate.

14. An electrolyte according to any preceding claim, wherein the acrylamide is crosslinked, and wherein the weight percentage of cross-linking monomer is up to 20% of the polyacrylamide prepared by precipitation polymerisation or up to 30% of the poly(N-substituted acrylamide).

15. An electrolyte according to Claim 1, substantially as hereinbefore described with reference to any one of Examples 1 to 9.