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Product Canada. ACU

PATENT NUMBER This data is not available for free
PATENT GRANT DATE April 2, 2002
PATENT TITLE Materials useful as electrolytic solutes

PATENT ABSTRACT The invention concerns novel ionic compounds with low melting point whereof the onium type cation having at least a heteroatom such as N, O, S or P bearing the positive charge and whereof the anion includes, wholly or partially, at least an ion imidide such as (FX.sup.1 O)N.sup.- (OX.sup.2 F) wherein X.sup.1 and X.sup.2 are identical or different and comprise SO or PF, and their use as solvent in electrochemical devices. Said composition comprises a salt wherein the anionic charge is delocalized, and can be used, inter alia, as electrolyte
PATENT INVENTORS This data is not available for free
PATENT ASSIGNEE This data is not available for free
PATENT FILE DATE September 7, 1999
PATENT FOREIGN APPLICATION PRIORITY DATA This data is not available for free
PATENT REFERENCES CITED Darstellung und Reaktionen des 1,2,35-Dithiadiazoliumchlorids, H. W. Roesky et al. XP-002104842 (1978, month unknown) Chem. Ber. vol. 111, No. 8.
Darstelung und Untersuchung von Fluorsulfurylverbindungen, H.W. Roesky et al. XP-002104843 (1968, month unknown) Chem. Ber. vol. 1. No. 101.
Chemistry of Imidobis(sulphyryl fluoride): Part II--Behaviour of Electroyltes in Imidobis(sulphuryl fluoride) XP-002-104844 Dhingra et al. Indian Journal of Chemistry (Jun. 1985).
Phosphor-und Schewefelhydrazin-Verbingungen, H. W. Roesky et al. XP-002104845, Zeitschrift Fur Naturforschung, Teil B, vol. 26B, No. 12, 1971 (month unknown) pp. 1232-1235.
Tetraphenylarsonium Bis(fluorosulfonyl)amide at 290 and 112K, W.Isenberg et al. XP-002104846 Acta Crystalograhica, vol. B38, No. 11, pp 2887-2889, (Oct. 1982).
Preparation and Reactions of fluorosulfonyliminosulforoxy Difluoride, H.W. Roeasky et al. XP-002104847 Inorganic Chemistry, vol. 8, No. 8, pp 1733-1735, (Aug. 1969).
PATENT PARENT CASE TEXT This data is not available for free
PATENT CLAIMS What is claimed is:

1. An ionic compound having a cation of the onium type with at least one heteroatom comprising N, O, S or P bearing the positive charge and the anion including, in whole or in part, at least one imide ion of the type (FX.sup.1 O)N.sup.- (OX.sup.2 F) wherein X.sup.1 and X.sup.2 are the same or different and comprise SO or PF,

wherein the onium type cation comprises a compound of formula: ##STR16##

a compound of formula ##STR17##

a compound of formula ##STR18##

a compound of formula ##STR19##

wherein

W is O, S or N, and wherein N isoptionally substituted with R.sup.1 when the valence allows it;

R.sup.1, R.sup.3, R.sup.4 are the same or different and represent H;

an alkyl, alkenyl, oxaalkyl, oxaalkenyl, azaalkyl, azaalkenyl, thiaalkyl, thiaalkenyl, dialkylazo, each of these can be either linear, branched or cyclic and comprising from 1 to 18 carbon atoms;

cyclic or heterocyclic aliphatic radicals of from 4 to 26 carbon atoms optionally comprising at least one lateral chain comprising one or more heteroatoms;

aryl, arylalkyl, alkylaryl and alkenyaryl of from 5 to 26 carbon atoms optionally comprising one or more heteroatoms in the aromatic nucleus;

groups comprising aromatic or heterocyclic nuclei, condensed or not, optionally comprising one or more atoms of nitrogen, oxygen, sulfur or phosphorous;

and wherein two groups R.sup.1, R.sup.3 or R.sup.4 can form a cycle or a heterocycle of from 4 to 9 carbon atoms, and wherein one or more R.sup.1, R.sup.3 or R.sup.4 groups on the same cation can be part of a polymeric chain;

R.sup.2 and R.sup.5 to R.sup.9 are the same or different and represent R.sup.1, R.sup.1 O--, (R.sup.1).sub.2 N--, R.sup.1 S--, R.sup.1 being as defined above, and

wherein R.sup.1, R.sup.3 and R.sup.4 can comprise a polymerisation active group.

2. A compound according to claim 1 wherein the polymerisation active group comprises double bonds, epoxides, or functional groups reactive in polycondensations.

3. A compound according to claim 1 wherein the cation comprises an ammonium, imidazolium, or phosphonium ion unsubstituted or substituted with an alkyl group, or an oxaalkyl group comprising from 1 to 8 carbon atoms.

4. A electrolytic composition comprising at least one ionic compound according to claim 3 in combination with at least another component comprising a metallic salt, a polar polymer and/or an aprotic co-solvent.

5. An electrolytic composition according to claim 4 wherein the cation of the metallic salt is selected from the proton, an alkaline metal, an alkaline-earth metal, a transition metal or a rare earth.

6. An electrolytic composition according to claim 4 wherein at least one metallic salt is a lithium salt.

7. An electrolytic composition according to claim 4 wherein the polar polymer comprises monomer units of ethylene oxide, propylene oxide, epichlorohydrine, epifluorohydrine, trifluoroepoxypropane, acrylonitrile, methacrylonitrile, esters and amides of acrylic and methacrylic acids, vinylidene fluoride, N-methylpyrolidone and polyelectrolytes of the polycation or the polyanion type.

8. An electrolytic composition according to claim 4 wherein the aprotic co-solvent is selected from dialkylated ethers of ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycols of weight comprised between 400 and 2000; esters, comprising those of carbonic acid, either linear or cyclic, cylic carbonylic esters nitriles, 1,2,6-tricyanohexane, amides, sulfamides and sulfonamides and mixtures thereof.

9. An electrochemical generator having at least one positive electrode and at least one negative electrode, wherein the positive electrode comprises a mixed oxide of lithium and vanadium LiV.sub.3 O.sub.8, a double oxide of cobalt and lithium optionally partially substituted of general formula Li.sub.1-a Co.sub.1-x+y Ni.sub.x Al.sub.y wherein 0
wherein said electrochemical generator uses as the electrolyte an electrolytic composition which comprises a combination of:

an ionic compound having a cation of the onium type with at least one heteroatom comprising N, O, S or P bearing the positive charge and the anion including, in whole or in part, at least one imide ion of the type (FX.sup.1 O)N.sup.- (OX.sup.2 F) wherein X.sup.1 and X.sup.2 are the same or different and comprise SO or PF; and

at least another component comprising a metallic salt, a polar polymer and/or an aprotic co-solvent.

10. An electrochemical generator according to claim 9 characterized in that the negative electrode comprises lithium or an alloy thereof, a carbon insertion compound, an oxide comprising a titanium spinel Li.sub.4x+3y Ti.sub.5-x O.sub.12 wherein 0.ltoreq.x,y.ltoreq.1, a double nitride of a transition metal and lithium comprising Li.sub.3-x Co.sub.2 N wherein 0.ltoreq.z.ltoreq.1 or having a structure of the antifluorite type comprising Li.sub.3 FeN.sub.2 or Li.sub.7 MnN.sub.4, or mixtures thereof.

11. An electrochemical generator according to claim 10, wherein the negative electrode comprises petroleum coke or graphite.

12. An electrochemical generator having at least one positive electrode and at least one negative electrode, wherein the positive electrode comprises a mixed oxide of lithium and vanadium LiV.sub.3 O.sub.8, a double oxide of cobalt and lithium optionally partially substituted of general formula Li.sub.1-a Co.sub.1-x+y Ni.sub.x Al.sub.y wherein 0
wherein said electrochemical generator uses as the electrolyte an electrolytic composition which comprises a combination of:

an ionic compound having a cation of the onium type with at least one heteroatom comprising N, O, S or P bearing the positive charge and the anion including, in whole or in part, at least one imide ion of the type (FX.sup.1 O)N.sup.- (OX.sup.2 F) wherein X.sup.1 and X.sup.2 are the same or different and comprise SO or PF; and

at least another component comprising a metallic salt, a polar polymer and/or an aprotic co-solvent, and

wherein at least one of the electrodes is mixed with the electrolytic composition to form a composite electrode.

13. A modulation system of the electrochromic type comprising at least one electrochromic material wherein the electrolyte comprises a combination of:

an ionic compound having a cation of the onium type with at least one heteroatom comprising N, O, S or P bearing the positive charge and the anion including, in whole or in part, at least one imide ion of the type (FX.sup.1 O)N.sup.- (OX.sup.2 F) wherein X.sup.1 and X.sup.2 are the same or different and comprise SO or PF; and

at least another component comprising a metallic salt, a polar polymer and/or an aprotic co-solvent, wherein the electrochromic material is dissolved in the electrolyte.

14. A modulation system according to claim 13 characterized in that the electrochromic material is deposited on a semi-conductor layer transparent in the visible, derived from tin oxide or indium oxide on a glass or polymer substrate.

15. A modulation system according to claim 14 wherein the electrochromic material is an oxide of molybdenum, tungsten, titanium, vanadium, niobium, cerium, tin or mixtures thereof.
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PATENT DESCRIPTION FIELD OF INVENTION

The present invention relates to ionic compositions having a high ionic conductivity, comprising a salt wherein the anionic charge is delocalized, and their uses, such as an electrolyte.

BACKGROUND OF THE INVENTION

Room temperature molten salts, such as triethyl ammonium nitrate, have been known for a long time. This product is of no interest except because of the presence of a leaving proton on the cation, limiting the redux or acido-basic stability domain of the compound. Methyl-ethylimidazolium or butyl-pyridinium type compounds, associated to the complex ion [Cl.sup.-, xAlCl.sub.3 ] wherein 1
The use of anions usually stables associated to imidazolium or pyridinium type cations has been proposed, but the melting points are relatively high. For example, 1-methyl-3-ethylimidazolium hexafluorophosphate melts at 60.degree. C., and 1,2-dimenthyl-3-propylimidazolium hexafluorophosphate melts at 65.degree. C. In addition, these salts, although not hygroscopic, are nevertheless soluble in water and can therefore be hardly prepared by ion exchange in water unless longer alkyl substituents are used, which results in a strong reduction in conductivity and enhanced viscosity.

U.S. Pat. No. 5,827,602 describes salts with a melting point relatively low, with a selection criteria being an anion volume higher than 100 .ANG..sup.3, thus allowing to obtain salts with high conductivity and hydrophobic character. Most representative anions are the bis-trifluoromethanesulfonimidide, that has a calculated volume of 144 .ANG..sup.3 with Hyperchem.RTM. program, or the tris-trifluoromethanesulfonylmethylide, which has a volume of 206 .ANG..sup.3.

SUMMARY OF THE INVENTION

The present invention is concerned with low melting point ionic compounds, preferably lower than room temperature, wherein the cation is of the onium type and having at least one heteroatom such as N, O, S or P bearing the positive charge and is wherein the anion comprises, in whole or in part, at least one imidide ion of the type (FX.sup.1 O)N.sup.- (OX.sup.2 F) wherein X.sup.1 and X.sup.2 are the same or different and comprise SO or PF. More specifically, the onium type cation comprises a compound of formula: ##STR1##

a compound of formula ##STR2##

a compound of formula ##STR3##

a compound of formula ##STR4##

wherein

W is O, S or N, and wherein N is optionally substituted with R.sup.1 when the valence allows it;

R.sup.1, R.sup.3, R.sup.4 are the same or different and represent

H;

an alkyl, alkenyl, oxaalkyl, oxaalkenyl, azaalkyl, azaalkenyl, thiaalkyl, thiaalkenyl, dialkylazo, each of these can be either linear, branched or cyclic and comprising from 1 to 18 carbon atoms;

cyclic or heterocyclic aliphatic radicals of from 4 to 26 carbon atoms optionally comprising at least one lateral chain comprising one or more heteroatoms;

groups comprising several aromatic or heterocyclic nuclei, condensed or not, optionally comprising one or more atoms of nitrogen, oxygen, sulfur or phosphorous; and wherein two groups R.sup.1, R.sup.3 or R.sup.4 can form a cycle or a heterocycle of from 4 to 9 carbon atoms, and wherein one or more R.sup.1, R.sup.3 or R.sup.4 groups on the same cation can be part of a polymeric chain;

R.sup.2 and R.sup.5 to R.sup.9 are the same or different and represent R.sup.1, R.sup.1 O--, (R.sup.1).sub.2 N--, R.sup.1 S--, R.sup.1 being as defined above.

The invention further comprises an electrolytic composition comprising at least one ionic compound as defined above in combination with at least another component comprising a metallic salt, a polar polymer and/or an aprotic co-solvent.

DETAILED DESCRIPTION OF THE INVENTION

It has been found that onium-type cation salts as defined above, and preferably the imidazolium, ammonium, sulfonium and phosphonium salts, associated to anions of the family represented by the general formula (FX.sup.1 O)N.sup.- (OX.sup.2 F) as defined above allow to obtain liquid salts at temperatures equal or lower than those obtained with larger ions. Further, their conductivity is, in all instances, at the same temperature, superior to that of the compounds described in U.S. Pat. No. 5,827,602. These liquid salts are hydrophobic even though the anion size is small, comprised between 85 and 92 .ANG..sup.3, and thus, easily prepared by ion exchange in water, and can be handled without any particular precaution. Unexpectedly, these salts show an oxydation stability equal to that of the bis(trifuoromethanesulfonimidide) or tris(trifluoromethanesulfonyl)methylide anions, and higher than that obtained with anions of the tetrafluoroborate or hexafluorophoborate type.

The compounds of the present invention can, in addition to the imidide anion mentioned above, comprise at least another anion selected from Cl.sup.- ; Br.sup.- ; I.sup.- ; NO.sub.3.sup.- ; M(R.sup.10).sub.4.sup.- A(R.sup.10).sub.6.sup.- ; R.sup.11 O.sub.2.sup.-, [R.sup.11 ONZ.sup.1 ].sup.-, [R.sup.11 YOCZ.sup.2 Z.sup.3 ].sup.-, 4,5-dicyano-1,2,3-triazole, 3,5-bis(R.sub.F)-1,2,4-triazole, tricyanomethane, pentacyanocyclopentadiene, pentakis-(trifluoromethyl)cyclopentadiene, barbituric acid and Meldrum acid derivatives and their substitution products;

M is B, Al, Ga or Bi;

A is P, As and Sb;

R.sup.10 is a halogen;

R.sup.11 represents H, F, alkyl, alkenyl, aryl, arylalkyl, alkylaryl, arylalkenyl, alkenylaryl, dialkylamino, alkoxy or thioalkoxy, each having from 1 to 18 carbon atoms and being unsubstituted or substituted with one or more oxa, thia, or aza substituents, and wherein one or more hydrogen atoms are optionally replaced with halogen in a ratio of 0 to 100%, and eventually being part of polymeric chain;

Y represents C, SO, S.dbd.NCN, S.dbd.C(CN).sub.2, POR.sup.11, P(NCN)R.sup.11, P(C(CN).sub.2 R.sup.11, an alkyl, alkenyl, aryl, arylalkyl, alkylaryl, arylalkenyl, alkenylaryl having from 1 to 18 carbon atoms and optionally substituted by one or more oxa, thia or aza; a dialkylamino group N(R.sup.10).sub.2 ;

Z.sup.1 to Z.sup.3 representing independently R.sup.11, R.sup.11 YO or CN, this group being optionally part of a polymeric chain.

Another advantage of the compounds of the invention is the low cost of the starting anions, their preparation not requiring perfluoroalkyl chemistry such as CF.sub.3 or C.sub.4 F.sub.9 for instance, the fluorine atoms present in the compounds of the invention being derived from inorganic chemistry products, thus easily accessible. This economical aspect is particularly important because the molten salts contain between 40 and 75% by weight of the anionic species, the remainder being the cationic species. In addition, the density of these liquids is close to 1.5, compared to about 1 for the organic solutions, which requires more important quantities of salts for all applications wherein a volume or a given thickness are necessary, such as electrolyte films, chemical reactors, etc.

Another particularly important aspect of the present invention is the possibility for these molten salts to dissolve other salts, in particular metallic salts, such as lithium salts, to give highly conductive solutions. In a similar manner, the molten salts, or their mixtures with other metallic salts, are excellent solvents or plasticizers for a great number of polymers, in particular those bearing polar or ionic functions. Liquid compounds as well as polymers plasticized by ionic mixtures behaving like solid electrolytes are applicable in electrochemistry to generators of the primary or secondary type, supercapacities, electrochromic systems, antistatic coatings, or electroluminescent diodes. The non-volatility of the molten salts of the invention, their thermal and electrochemical stability, and their enhanced conductivity are important parameters for the fabrication of devices working at low temperature and not presenting the usual flammability risks associated with the use of conventional organic solvents.

The molten salts of the invention are polar media of low volatility, and because of this, are capable of being used as solvents to perform a great number of organic chemistry reactions, such as nucleophilic an electrophilic substitutions, or anionic, cationic or radicalar polymerisations. It is also possible to dissolve catalysts in such media, in particular transition metal salts or rare earth salts eventually coordinated with ligands, to increase the catalytic properties. Examples of such catalysts include bipyridines, porphyrines, phosphines, arsines. Organometallics like metallocenes are also included as solutes that can present catalytic properties.

The non-volatility of the molten salts, their thermal stability and their non-miscibility with non-polar solvents like hydrocarbons, as well as their hydrophobic character, are particularly advantageous to separate the chemical reaction products. It is also possible to work in diphasic systems, the molten salts containing the catalyst and the reacting substrates being in solution in a hydrocarbon or non-miscible aliphatic ether. After the reaction, a simple decantation can separate the organic phase containing the reaction product and the molten salt that is purified by washing with a non-solvent such as water or hydrocarbon, and dried by simple in vacuo procedure.

The ammonium, phosphonium and sulfonium cations can have an optical isomery and the molten salts containing them are chiral solvents susceptible or favoring the formation of enantiomeric excesses in the reactions performed in these media. Preferred cations for the present invention comprise the compounds of formula: ##STR5##

that include the imidazolium, triazolium, thiazolium, and oxazolium derivatives;

the compounds of formula ##STR6##

that include the trizolium, oxadiazolium, and thadiazolium;

the compounds of formula ##STR7##

preferably pyridinium derivatives ##STR8##

the compounds of formula ##STR9##

wherein

W is O, S or N, and wherein N is optionally substituted with R.sup.1 when the valence allows it;

R.sup.1, R.sup.3, R.sup.4 are the same or different and represent

H;

an alkyl, alkenyl, oxaalkyl, oxaalkenyl, azaalkyl, azaalkenyl, thiaalkyl, thiaalkenyl, dialkylazo, each of these can be either linear branched or cyclic and comprising from I to 18 carbon atoms;

cyclic or heterocyclic aliphatic radicals of from 4 to 26 carbon atoms optionally comprising at least one lateral chain comprising one or more heteroatoms such as nitrogen, oxygen or sulfur;

aryl, arylalkyl, alkylaryl and alkenyaryl of from 5 to 26 carbon atoms optionally comprising one or more heteroatoms in the aromatic nucleus;

groups comprising several aromatic or heterocyclic nuclei, condensed or not, optionally comprising one or more atoms of nitrogen, oxygen, sulfur or phosphorous; and wherein two groups R.sup.1, R.sup.3 or R.sup.4 can form a cycle or a heterocycle from 4 to 9 carbon atoms, and wherein one or more R.sup.1, R.sup.3 or R.sup.4 groups on the same cation can be part of a polymeric chain;

R.sup.2 and R.sup.5 to R.sup.9 are the same or different and represent R.sup.1, R.sup.1 O--, (R.sup.1).sub.2 N--, R.sup.1 S--, R.sup.1 being as defined above.

R.sup.1, R.sup.3 and R.sup.4 groups can bear groups active in polymerization such as double bonds or epoxides, or reactive functions in polycondensations, such as OH, NH.sub.2 or COOH. When the cations bear double bonds, they can be homopolymerized or copolymerized, for instance with vinylidene fluoride, an acrylate, a maleimide, acrylonitrile, a vinylether, a styrene, etc. Epoxide groups can be polycondensed or copolymerized with other epoxides. These polycations are particularly useful alone or in a mixture with a solvent, including a molten salt of the present invention and/or one or more lithium salt or a mixture of lithium and potassium salts as electrolyte in lithium batteries with a lithium anode or using a cathode inserting the lithium at low potential such as titanium spinel or carbonated materials.

The invention further concerns an electrolytic composition comprising at least one ionic compound comprising at least one anion and at least one cation as defined above in combination with at least another compound comprising a metallic salt, a polar polymer and/or an aprotic co-solvent. A preferred cation of the metallic salt comprises the proton, an alkaline metal cation, an alkaline-earth metal cation, a transition metal cation, a rare earth metal cation, lithium being particularly preferred.

A preferred polar polymer comprises monomer units derived from ethylene oxide, propylene oxide, epichlorohydrine, epifluorohydrine, trifluoroepoxypropane, acrylonitrile, methacrylonitrile, esters and amides of acrylic and methacrylic acid, vinylidene fluoride, N-methylpyrrolidone and polyelectrolytes of the polycation or polyanion. Finally, examples of preferred aprotic co-solvent include di-alkylic ethers of ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycols of weight comprised between 400 and 2000; esters, in particular those of carbonic acid, linear or cyclics such as dimethylcarbonate, methyl-ethylcarbonate, diethylcarbonate, ethylene carbonate, propylene carbonate; esters like y-butyrolactone, nitrites like glutaronitrile, 1,2,6-tricyanohexane, amides such as dimethylformamide, N-methylpyrrolidinone, sulfamides and sulfonamides as well as mixtures thereof.

When the present electrolytic composition comprises more than one polymer, is at least one of those can be cross-linked.

An electrochemical generator comprising an electrolytic composition of the present invention preferably comprises; a negative electrode containing either lithium metal or an alloy thereof, or a carbon insertion compound, in particular petroleum coke or graphite, or a low potential insertion oxide such as titanium spinel Li.sub.4-x+3y Ti.sub.5-x O.sub.12 (0.ltoreq.x, y.ltoreq.1), or a double nitride of a transition metal and lithium such as Li.sub.3-x Co.sub.z N (0.ltoreq.z.ltoreq.1) or having a structure of the antifluorite type like Li.sub.3 FeN.sub.2 or Li.sub.7 MnN.sub.4, or mixtures thereof. The positive electrode of the generator preferably contains either vanadium oxide VO.sub.x (2.ltoreq.x.ltoreq.2,5), a mixed oxide of lithium and vanadium LiV.sub.3 O.sub.8, a double oxide of cobalt and lithium optionally partially substituted of general formula Li.sub.1-.alpha. Co.sub.1-x+y Ni.sub.x Al.sub.y (0.ltoreq.x+y.ltoreq.1; 0.ltoreq.y.ltoreq.0,3; 0.ltoreq..alpha..ltoreq.1), a manganese spinel optionally partially substituted of general formula Li.sub.1-.alpha. Mn.sub.2-z M.sub.z (0.ltoreq.z.ltoreq.1) wherein M=Li, Mg, Al, Cr, Ni, Co, Cu, Ni, Fe; a double phosphate of the olivine or Nasicon structure such as Li.sub.1-.alpha. Fe.sub.1-x Mn.sub.x PO.sub.4, Li.sub.1-x+2.alpha. Fe.sub.2 P.sub.1-x Si.sub.x O.sub.4 (0.ltoreq.x, .alpha..ltoreq.1), a rhodizonic acid salt, a polydisulfide derived from the oxidation of dimercaptoethane, 2,5-dimercapto-1,3,4-thiadiazole, 2,5-dimercapto-1,3,4-oxadiazole, 1,2-dimercaptocyclobutene-3,4-dione, or mixtures thereof.

Advantageously, at least one of the electrodes of the generator is mixed with the electrolytic composition to form a composite electrode.

The electrolytic composition of the invention can also be used as an electrolyte in an electrical energy storage system of the supercapacity type, optionally containing, in an electrode, carbon with high specific surface, or a conjugated polymer. Advantageously, the conjugated polymer comprises 3 degrees of oxidation, and is found in both electrodes. An example of such a polymer is a derivative of phenyl-3-thiophene.

Finally, the electrolytic composition of the invention can be used as an electrolyte in a light modulating system of the electrochromic type comprising as least one electrochromic material. In such a system, the electrochromic material is advantageously deposited on a layer of a semi-conductor transparent in the visible, preferably a tin oxide or indium oxide derivative, on a glass or polymer substrate. Examples of preferred electrochomic materials include oxides of molybdenum, tungsten, titanium, vanadium, niobium, cerium, tin, and mixtures thereof The electrochromic material can optionally be dissolved in the electrolyte.

The following examples are provided to illustrate preferred embodiments of the present invention, and should not be construed as limiting its scope.
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