1 Description ELECTROLYTE COMPRISING EUTECTIC MIXTURE AND ELECTROCHEMICAL DEVICE USING THE SAME Technical Field [1] The present invention relates to a eutectic mixture-containing electrolyte having high thermal and chemical stability, high electric conductivity and a broad elec- trochemical window. Also, the present invention relates to an electrochemical device having improved safety and quality by using the same electrolyte. Background Art [2] In general, various kinds of electrolytes have been used in electrochemical devices widely used in recent years, such as lithium secondary batteries, electrolytic condensers, electric double layer capacitors or electrochromic display devices, and dye-sensitized solar cells that have been studied intensively for their practical use in the future. Under these circumstances, importance of such electrolytes has increased more and more. [3] Electrolytes that have been used the most widely in recent years include non- aqueous solutions containing electrolyte salts dissolved in organic solvents such as ethylene carbonate, propylene carbonate, dimethoxyethane, gamma-butyrolactone (GBL), N,N-dimethyl formamide, tetrahydrofuran or acetonitrile. Such non-aqueous electrolytes have to serve as media performing conduction of ions and electrons between both electrodes, and should be stable in the drive voltage range of a device to which they are applied and show sufficiently high ion conductivity. However, the organic solvents used in such electrolytes have a low viscosity, leak out easily, and have high volatility to cause evaporation. Additionally, such organic solvents are ignitable, and thus are problematic in terms of long-term reliability, durability and stability. Therefore, recently, many studies have been conducted to utilize an ionic liquid as an electrolyte for an electrochemical device. However, conventional ionic liquids are expensive, and are obtained via a complicated preparation and purification process. Moreover, a liquid electrolyte is disadvantageous because it has possibility of leakage and it is not applicable to devices designed to have a large size or a small thickness. [41 Meanwhile, G. Berchiesi and coworkers have prepared a eutectic mixture by using acetamide, LiSCN (lithium thiocyanate), CH COOLi (lithium acetate), or the like ( Thermochimica Acta, 1983, 70, pp 317-324). However, there is no disclosure of the use of the above eutectic mixture as an electrolyte for an electrochemical device. Disclosure of Invention 2 Technical Problem [5] Therefore, the present invention has been made in view of the above-mentioned problems. We have found that when a cost-efficient eutectic mixture having excellent thermal and chemical stability is used in an electrolyte for electrochemical devices, it is possible to solve the problems of evaporation, exhaustion and flammability of electrolytes caused when using a conventional organic solvent as electrolyte, thereby improving the safety. We have also found that it is possible to improve the quality of an electrochemical device by virtue of excellent conductivity and a broad elec- trochemical window of the eutectic mixture. [6] It is an object of the present invention to provide an electrolyte, which comprises a eutectic mixture, and an electrochemical device using the same electrolyte. Technical Solution [7] According to an aspect of the present invention, there is provided an electrolyte comprising a eutectic mixture formed of: (a) an amide group-containing compound; and (b) a lithium-free ionizable salt. There is also provided an electrochemical device, preferably an electrochromic device, comprising the above electrolyte. [8] Hereinafter, the present invention will be explained in more detail. [9] The present invention is characterized by using a eutectic mixture to form an electrolyte for an electrochemical device, wherein the eutectic mixture is free from lithium. [10] Generally, a eutectic mixture is referred to as a mixture containing two or more substances and having a decreased melting point. Particularly, such eutectic mixtures include mixed salts present in a liquid phase at room temperature. Herein, room temperature means a temperature of up to 100°C, or a temperature of up to 60°C in some cases. [11] According to the present invention, the eutectic mixture comprises at least two materials, one of which includes any metal salt having higher conductivity when compared to a conventional lithium salt. Thus, it is possible to improve the quality of an electrochemical device by virtue of increased ion conductivity of an electrolyte caused by such excellent conductivity of the metal component. For example, when a sodium (Na)-containing salt is used as a metal salt, it is possible to obtain a con- ductivity improved by about at least three times even to ten times or more when compared to a lithium salt Additionally, when an electrolyte contains such a metal salt, it shows a significantly decreased viscosity when compared to an electrolyte using a lithium salt, and thus ion movement and/or transfer via the electrolyte is facilitated, resulting in improvement of the quality of an electrochemical device. It is thought that such decreased viscosity results from weakening of binding force in the salt due to the 3 metal larger than lithium. Further, the electrolyte according to the present invention utilizes an organic salt to form a eutectic mixture, while a conventional electrolyte utilizes a metal salt to form a eutectic mixture. Therefore, it is possible to overcome the limitation in composition of a eutectic mixture according to the present invention. [12] In addition, the electrolyte comprising the aforementioned eutectic mixture shows a broader electrochemical window when compared to conventional organic solvents and ionic liquids due to the physical stability of the eutectic mixture itself, so that an elec- trochemical device using the above electrolyte can have an extended range of drive voltage. In fact, conventional electrolytes using ionic liquids and organic solvents show an upper limit of electrochemical window of approximately 4-4.5V, while the eutectic mixture according to the present invention shows an upper limit of elec- trochemical window of 4.5-5.5V, which is significantly extended when compared to the conventional electrolytes based on ionic liquids and organic solvents. [13] Further, the eutectic mixture contained in the electrolyte according to the present invention has no vapor pressure contrary to conventional solvents, and thus shows no problem of evaporation and exhaustion of the electrolyte. Also, the eutectic mixture has flame resistance, thereby improving the safety of an electrochemical device. Moreover, the eutectic mixture itself is very stable, and thus can inhibit side reactions in the electrochemical device. In fact, a eutectic mixture has a broad temperature range where it exists as liquid, high solvation capability, non-coordinate character, or the like. Thus, it is known that a eutectic mixture has physicochemical properties as an eco-friendly solvent that can substitute for a conventional harmful organic solvent. Ad- ditionally, a eutectic mixture is prepared via a relatively simple process when compared to conventional ionic liquids and has a high ion concentration. Therefore, it is expected that such eutectic mixtures have a broad spectrum of applications. [14] According to a preferred embodiment of the present invention, one of the con- stitutional elements forming the eutectic mixture is an amide group-containing compound having two different polar functional groups, i.e. a carbonyl group and an amine group, in its molecule. However, any compound having at least two polar functional groups (e.g. an acidic group and a basic group) in the molecule may be used with no particular limitation. [15] The polar functional groups different from each other serve as complexing agents that weaken the bond between the cation and the anion of the ionizable salt, thereby forming a eutectic mixture, resulting in a drop in melting temperature. In addition to the above functional groups, compounds comprising polar functional groups that can weaken the bond between a cation and anion of an ionizable salt and thus capable of forming a eutectic mixture are also included in the scope of the present invention. [16] The amide group-containing compound may be an amide group-containing 4 compound having a linear structure, a cyclic structure or a combination thereof. Non- limiting examples of the amine group-containing compound include C1-C10 alkyl amide, alkenyl amide, aryl amide or alkylaryl amide compounds. Also, Primary, secondary or tertiary amide compounds may be used. It is more preferable to use a cyclic amide compound showing a broader electrochemical window, because such cyclic amide compounds have a smaller number of hydrogen atoms and are stable under a high voltage so as to prevent decomposition thereof. Non-limiting examples of the amide compound that may be used in the present invention include acetamide, urea, methyl urea, caprolactam, valerlactam, carbamate, formamide and mixtures thereof. [17] The other constitutional element forming the eutectic mixture according to the present invention includes any lithium-free ionizable salt. Non-limiting examples of such salts include metal salts, organic salts or mixed salts thereof containing at least one metal selected from the group consisting of alkali metals except lithium, alkaline earth metals, transition metals, metalloids, lanthanides and actinides. [18] The eutectic mixture according to the present invention may be represented by the following Formula 1, but is not limited thereto: [19] [Formula 1] [20] [21] wherein R is a hydrogen atom, a C1-C20 alkyl group, a C1-C20 alkylamine group, alkenyl group, aryl group or alkylaryl group; [22] R is a hydrogen atom, a halogen atom, a C1-C20 alkyl group, alkenyl group, aryl group or alkylaryl group; [23] A is selected from the group consisting of carbon, oxygen, hydrogen, nitrogen and sulfur, with the proviso that when A is hydrogen, R is null; [24] X represents at least one metal cation or organic cation selected from the group consisting of alkali metals except lithium, alkaline earth metals, transition metals, metalloids, lanthanides and actinides; [25] Y represents an anion capable of forming a salt with X; and [26] n represents an integer of 0-10, and when n is 1 or more, A is selected from the group consisting of carbon, oxygen, nitrogen and sulfur, excluding hydrogen. 5 [27] Preferably, in the compound represented by the above Formula 1, cation X represents secondary, tertiary and quaternary ammonium, phosphonium, magnesium, potassium or calcium, while anion Y is thiocyanate, formate, acetate, nitrate, perchlorate, sulfate, hydroxide, alkoxide, halogenide, carbonate, oxalate or tetraflu- oroborate. [28] As described above, the constitutional elements of the eutectic mixture, i.e. the amide group-containing compound and the lithium-free ionizable salt (XY) cause the formation of a coordination bond between the carbonyl group present in the amide group-containing compound and the metal or organic cation (X4) of the hthium-free ionizable salt, as well as the formation of a hydrogen bond between the anion (Y~) of the salt and the amine group present in the amide group-containing compound, as shown in the following Reaction Scheme 1. In this manner, chemical bonds in each compound are weakened. As a result, the amide group-containing compound and the lithium-free ionizable salt, which was originally present in a sold state, show a decreased melting point, while they form a eutectic mixture present in a liquid state at room temperature. [29] [Reaction Scheme 1] [30] [31] Although there is no particular limitation in melting point of the eutectic mixture according to the present invention, it is preferable that the eutectic mixture is in a liquid state at a temperature of up to 100°C, more preferably at room temperature. Also, although there is no particular limitation in viscosity of the eutectic mixture according to the present invention, the eutectic mixture preferably has a viscosity of 100 cp or less. [32] The eutectic mixture may be prepared by a conventional process known to one skilled in the art. For example, a compound having an amide group is mixed with an ionizable lithium-free salt at room temperature and then the mixture was reacted by 6 heating it at a suitable temperature of 70°C or less, followed by purification. Herein, the molar ratio (%) of the compound having an acidic functional group and basic functional group to lithium salt suitably ranges from 1:1 to 8:1, more preferably from 2:1 to 6:1. [33] [34] The eutectic mixture according to the present invention may be applied to any electrolytes regardless of the forms of electrolytes. Preferably, the eutectic mixture may be applied to two types of electrolytes, i.e. liquid electrolytes and gel polymer electrolytes. [35] Herein, the electrolyte may further comprise at least one salt, preferably an ionizable lithium salt. Non-limiting examples of the anion of lithium salt that may be used in the present invention include F, CI", Br", V, NO ", BF ", PF ', N(CN) ", SCN, CIO ", (CF ) PF ', (CF ) PF \ (CF ) PF, (CF ) P", (CF CF SO ') N, (CF SO ) N", CF 4 3 3 3 3 4 2 3 5 3 6 3 2 2 2 3 2 2 3 SO ", CF CF (CF ) CO", (CF SO ) CH", (CF SO ) C, CF (CF ) SO ", CF CO ", CH 3 3 2 3 2 3 2X2 3 2 3 3 2 7 3 3 2 3 CO " or the like. If possible, the anion is preferably the same as the anion forming the eutectic mixture. This is because when the anion contained in the lithium salt is different from the anion of the eutectic mixture, solubility of the lithium salt to the eutectic nnxtare-containing electrolyte may be degraded. The lithium salt is used preferably in a concentration of 0-1 mole/L, but is not limited thereto. [36] (1) According to an embodiment of the present invention, the electrolyte is a liquid type eutectic mixture-containing electrolyte, which may be obtained by using the eutectic mixture formed of the above amide group-containing compound and the lithium-free ionizable salt alone, or in combination with at least one salt. [37] (2) According to another embodiment of the present invention, the electrolyte is a gel polymer type eutectic mixture-containing electrolyte. The gel polymer serves to support the eutectic mixture. Thus, in this case, it is possible to solve the problem of electrolyte leakage, and to form an electrochemical device in the form of a thin film or other films. [38] The gel polymer electrolyte may be prepared by using a method generally known to those skilled in the art. The method may be performed according to the following three types of embodiments. It is matter of course that the eutectic mixture may further comprises at least one salt as described above. [39] (D According to a preferred embodiment of the method, polymerization of monomers is performed in the presence of the eutectic mixture to form a gel polymer electrolyte. The method of forming a gel polymer electrolyte via polymerization of monomers may be performed by in-situ polymerization inside an electrochemical device. Otherwise, a gel polymer electrolyte may be introduced into an electrochemical device, after the gel polymer electrolyte is formed. 7 [40] The gel polymer electrolyte can be formed by polymerizing an electrolyte pre-gel solution (electrolyte precursor solution) containing: (i) a eutectic mixture comprising an amide group-containing compound and a lithium-free ionizable salt; and (ii) monomers capable of forming a gel polymer via polymerization. [41] There is no limitation in the kind of monomer as long as it is capable of forming a gel polymer by polymerization and particular examples of such monomers include vinyl monomers, etc. Vinyl monomers have advantages in that they can provide transparent polymerization products when mixed with a eutectic mixture and they are amenable to simple polymerization conditions. [42] Non-limiting examples of the vinyl monomer that may be used according to the present invention include acrylonitrile, methyl methacrylate, methyl acrylate, methacrylonitrile, methyl styrene, vinyl esters, vinyl chloride, vinylidene chloride, acrylamide, tetrafluoroethylene, vinyl acetate, methyl vinyl ketone, ethylene, styrene, para-methoxystyrene, para-cyanostyrene, etc. [43] Preferably, the monomer capable of forming a gel polymer by polymerization provides low volumetric shrinkage upon polymerization and permits in-situ poly- merization inside of an electrochemical device. [44] The polymerization of the monomers is generally performed under heat or UV ir- radiation, and thus the electrolyte pre-gel solution may further comprise a poly- merization initiator or a photoinitiator. [45] Initiators are decomposed by heat or UV rays to form radicals, and then react with a monomer through free radical polymerization to form a gel polymer electrolyte. It is also possible to carry out polymerization of monomers without using any initiator. Generally, free radical polymerization includes an initiation step in which transient molecules or active points having strong reactivity are formed; a propagation step in which a monomer is added to the end of an active chain to form another active point at the end of the chain; a chain transfer step in which active points are transferred to other molecules; and a termination step in which the center of an active chain is broken. [46] Thermal initiators that may be used in the polymerization include organic peroxides or hydroperoxides such as benzoyl peroxide, acetyl peroxide, dilauryl peroxide, di- tert-butyl peroxide, cumyl hydroperoxide, hydrogen peroxide, etc., and azo compounds such as 2,2-azobis(2-cyanobutane), 2,2-azobis(methylbutyronitrile), AIBN(azobis(iso-butyronitrile), AMVN (azobisdimethyl-valeronitrile), organometallic compounds such as alkylated silver compounds, etc. Additionally, non-limiting examples of the photoinitiator that permits formation of radicals by the light such as UV rays include chloroacetophenone, diethoxy acetophenone(DEAP), l-phenyl-2-hydroxy-2-methyl propaneone(HMPP), 1-hydroxy cyclrohexyl phenyl ketone, a-aminoacetophenone, benzoin ether, benzyl dimethyl ketal, benzophenone, 8 thioxanthone, 2-ethylanthraquinone(2-ETAQ), etc. [47] Additionally, the mixing ratio in the electrolyte precursor solution according to the present invention on the weight basis, i.e., the weight ratio of (eutectic mixture) x : (monomer capable of forming a gel polymer by polymerization) y : (polymerization initiator) z, is 0.5-0.95 : 0.05-0.5 : 0.00-0.05, with the proviso that x+y+z=l. More preferably, x is 0.7-0.95, y is 0.05-0.3 and z is 0.00-0.01. [48] In addition to the above-described materials, the precursor solution of gel polymer electrolyte according to the present invention optionally further comprises other additives known to one skilled in the art. [49] As described above, the in-situ polymerization is initiated by irradiation of heat or UV rays so as to form a gel polymer electrolyte. Herein, polymerization degree of the gel polymer depends on reaction conditions, i.e. polymerization time and temperature in the case of heat polymerization, or light irradiation dose in the case of UV poly- merization. Therefore, it is possible to control the polymerization degree of a gel polymer as desired by controlling the reaction conditions including polymerization time, polymerization temperature or light irradiation dose. Additionally, poly- merization time depends on the kind of the initiator used for the polymerization and polymerization temperature. It is preferable that polymerization is performed for a period of time during which leakage of the gel polymer electrolyte cannot occur and the electrolyte cannot be over-polymerized to such a degree that it causes volumetric shrinkage. For example, polymerization is generally performed for about 20-60 minutes at a temperature of about 40~80°C. [50] ® According to another preferred embodiment of the present invention, the eutectic mixture is injected to a preformed polymer or gel polymer so that the polymer or gel polymer is impregnated with the eutectic mixture. [51 ] Non-limiting examples of the polymer that may be used in the present invention include polymethyl methacrylate, polyvinylidene difluoride, polyvinyl chloride, polyethylene oxide, polyhydroxyethyl methacrylate, etc. Any gel polymers known to one skilled in the art may also be used. In this case, it is possible to simplify processing steps compared to the above in-situ polymerization method. [52] (D According to still another preferred embodiment of the present invention, a polymer and the eutectic mixture are dissolved in a solvent and then the solvent is removed to form a gel polymer electrolyte. Herein, the eutectic mixture is included in the polymer matrix. [53] Although there is no particular limitation in selecting the solvent, non-limiting examples of the solvent include toluene, acetone, acetonitrile, THF, etc. Additionally, there is no particular limitation in the method for removing the solvent and any con- ventional heating methods may be used. However, the third method has a disadvantage 9 in that there is a need of a post-treatment step for removing a solvent in order to form the gel polymer electrolyte. [54]