METHOD FOR PRODUCING MEMBRANE OF GEL COMPOSITE ELECTROLYTE

Abstract: A method is provided for the production of a membrane of gel composite electrolyte having a uniform membrane thickness. The method has the steps of: mixing a lithium salt, an electrolyte solvent having a boiling point at one atmospheric pressure of not less than 240 DEG C and a diluting solvent having a boiling point at one atmospheric pressure of from 40 DEG to 210 DEG C to obtain an electrolyte solution; mixing the electrolyte solution with a gelling agent to obtain a gel composite electrolyte; shaping the gel composite electrolyte into a membrane configuration; and removing the diluting solvent from the gel composite electrolyte. The method is characterized by the use of a diluting solvent. The diluting solvent facilitates the shaping step, thereby producing an electrolyte membrane having a uniform thickness. The gelling agent may be fumed silica.

Description: DESCRIPTION METHOD FOR PRODUCING MEMBRANE OF GEL COMPOSITE ELECTROLYTE TECHNICAL FIELD This invention relates to a method for producing a membrane of a gel composite electrolyte. BACKGROUND ART An electrochemical cell consists of a pair of electrodes and an electrolyte provided between the electrodes . The electrolyte may be a liquid or solid electrolyte. A liquid electrolyte, for example, employs an electrolyte solution formed by dissolving a lithium salt in an organic solvent. In recent years , attention has been directed to the use of gel composite electrolytes in such batteries as lithium ones which are produced by adding silica or the like to liquid electrolytes to form gels. For example, in J. Electrochem. Soc, Vol. 144, No. 2, 1997, pp 399-408; Composite Electrolytes Prepared from Fumed Silica, Ethylene Oxide Oligomers and Lithium Salts , there are described gel composite electrolytes comprising lithium salts, ethylene glycol esters and fumed silica particles. However, because of their limited fluidity gel composite electrolytes suffered from various problems in practical application. For example, where a gel composite electrolyte is to be used in a cell, it needs to be shaped into a membrane configuration. However, it is difficult to shape the electrolyte into a membrane of uniform thickness. 1 Also, while the fumed silica particles are mixed in the electrolyte solution comprising the lithium salt and a solvent therefor, the viscosity of the solution rises gradually, which makes it difficult to uniformly disperse the silica particles . SUMMARY OF THE INVENTION Accordingly, the aim of this invention is to solve the above-mentioned problems arising during production of membranes of gel composite electrolyte. Specifically, in the invention, the gel composite electrolytes that are diluted with diluting solvents are used to produce membranes , and then the diluting solvents are removed following membrane production. This enables the formation of membranes of gel composite electrolyte having a uniform thickness, as well as the uniform dispersion of gelling agents such as silica particles in the membranes. . According to this invention, there is provided: a method for producing a membrane of gel composite electrolyte, the method comprising the steps of: mixing a lithium salt , an electrolyte solvent having a boiling point at one atmospheric pressure of not less than 240°C and a diluting solvent having a boiling point at one atmospheric pressure of from 40° to 210°C to obtain an electrolyte solution; wherein the electrolyte solvent is an organic solvent capable of dissolving the lithium salt and is liquid at 25°C, and the diluting solvent is an aprotic polar organic solvent and is liquid at 25°C; 2 mixing the electrolyte solution with a gelling agent to obtain a gel composite electrolyte; shaping the gel composite electrolyte into a membrane configuration; and removing the diluting solvent from the gel composite electrolyte. In the invention, it is preferred that the mixing step comprise the steps of mixing the lithium salt with the electrolyte solvent to obtain a mixture and mixing the mixture with the diluting solvent . Preferably, the diluting solvent has a boiling point at one atmospheric pressure of from 50° to 180°C. Furthermore, the diluting solvent preferably contains a carbonic ester, a cyclic ester, a polyether, a cyclic ether, an amide, a nitrile, a ketone, a cyclic ketone, or a sulfur compound. Still further, it is preferred that the weight ratio of the total of the lithium salt, the electrolyte solvent and the gelling agent to the diluting solvent be in the range of 75 : 25 - 25 : 75. Preferably, the gelling agent is an inorganic filler. Further, the electrolyte solvent preferably contains an ethylene glycol derivative having a molecular weight of from 200 to 1,000. Still further, it is preferred that the step of removing a diluting solvent include a step of sending air to the gel composite electrolyte in a membrane configuration. DETAILED DESCRIPTION OF THE INVENTION According to this invention, a lithium salt, a prescribed electrolyte solvent, and a prescribed diluting solvent are first mixed to obtain an electrolyte solution. Lithium salts known in the art can preferably be used as the lithium salt to be suited for lithium ion secondary batteries. For example, mentioned are LiC10 4 , LiPF 6 , LiBF 4 , LiAsF 6 , LiCF 3 S0 3 , LiClF 4 , LiAlCl 4 , LiSbF 6 , LiA10 4 , LiCl, Lil , LiN(CF 3 S0 2 ) , etc. The electrolyte solvent employs a solvent that is liquid at 25°C. Its boiling point at one atmospheric pressure is not less than 240°C, and preferably not less than 245°C. It is also required that the electrolyte solvent can dissolve the lithium salts: specifically aprotic polar organic solvents are preferable. The electrolyte solvent preferably contains an. ethylene glycol derivative having a molecular weight of from 200 to 1,000, more preferably an ethylene glycol derivative having a molecular weight of from 200 to 800. As used herein, "the ethylene glycol derivative" contains a polyethylene glycol ether or a polyethylene glycol ester. The polyethylene glycol ethers include polyethylene glycol monolower alkyl ethers represented by the formula: R-0(C 2 H 4 0) n H (wherein R is lower alkyl of C-^Cg, preferably methyl or ethyl, and more preferably ethyl; and n is an integer); and polyethylene glycol dilower alkyl ethers represented by the formula: R- 0(C 2 H 4 0) n R (wherein R and n are as previously defined) . The polyethylene glycol dilower alkyl ethers are particularly preferable as the electrolyte solvent, for example. The electrolyte solvent further employs a propylene carbonate (with a boiling point of 240°C), a vinylene carbonate or the like. The above-mentioned solvents may be used individually or in admixture as the electrolyte solvent . The electrolyte solvent preferably dissolves 0.1 - 3 moles of the lithium salt per liter, more preferably 0.7 - 1.5 moles . The diluting solvent employs a solvent that is liquid at 25°C. Its boiling point at one atmospheric pressure is from 40° to 210°C, preferably from 50° to 180°C, and more preferably from 75° to 180°C. If the boiling point of the diluting solvent is higher than 210°C, its vapor pressure is low and its removal will be time-consuming. The boiling point is also proximate to that of the electrolyte solvent and thus, a sufficient difference can not necessarily be found. On the other hand, if the boiling point of the diluting solvent is lower than 50°C, then its volatility is too high. Also, where the solvents are put to use in targets like plants , special safety precautions need to be taken, because they are liable to catch fire. The diluting solvent employs an aprotic polar organic solvent that can preferably dissolve the lithium salts. It is also preferred that the diluting solvent be mutually soluble with an electrolyte solvent . Furthermore, the diluting solvent preferably contains a carbonic ester, a cyclic ester, a polyether, a cyclic ether, 5 an amide, a nitrile, a ketone, a cyclic ketone, or a sulfur compound; the diluting solvent may be any single one of the carbonic ester, the cyclic ester, the polyether, the cyclic ether, the amide, the nitrile, the ketone, the cyclic ketone, and the sulfur compound. By "the polyethers" are meant compounds having two or more ether linkages and they include, for example, ethylene glycol derivatives , 1,2-diethoxyethane , ethoxymethoxyethane , etc. The ethylene glycol derivatives may employ ones that are mentioned as the electrolyte solvent and are liquid at 25°C. The solvents that are used as the diluting solvent are shown in Tables 1 and 2. Table 1 illustrates ethylene glycol ethers as examples of the polyethers . In Tables 1 and 2, the boiling points represent values at one atmospheric pressure. Table 1 carbonic esters boiling point (°C) dimethylcarbonate 90 diethylcarbonate 126-128 propylenecarbonate 240 cyclic ester γ-butyrolactone 204-205 ethylene glycol derivatives ethylene glycol dimethyl ether 85 ethylene glycol diethyl ether 121 diethylene glycol dimethyl ether 162 diethylene glycol diethyl ether 180-190 cyclic ethers tetrahydrofuran 65-67 2-methyltetrahydrofuran 78-80 ; 4-methyl-l,3-dioxolane 82-83 1,3-dioxolane 74-75 Table 2 amides boiling point (°C) N, N-dimethylformamide 153 , -dimethylacetamide 164.5-166 nitrile acetonitrile 81-82 ketones acetone 56 2-butanone (methyl ethyl ketone) 80 2-pentanone (methyl propyl ketone) 100-110 3-pentanone (diethyl ketone) 102 3-methyl-2-butanone (methyl isopropyl ketone) 94-95 cyclic ketones cyclohexanone 155 cyclopentanone 130-131 sulfur compound dimethyl sulfoxide 189 The sulfur compound employs, for example, a thioether, a thioester, a sulfone, or a sulfoxide; and the sulfoxide is preferable. The hydrogen atom bound to a carbon atom of the compound that is used as a diluting solvent may be substituted by a fluorine atom. In this invention, it is preferred that the weight ratio of the total of the lithium salt, the electrolyte solvent and the gelling agent to the diluting solvent be in the range of 75 : 25 - 25 : 75; and the ratio is more preferably in the range of 50 : 50 - 25 : 75. If the amount of the diluting solvent is excessive, its removal after the production of membranes would be time-consuming. Also, it will be difficult to maintain the porosity of the membranes within a predetermined range after removal of the diluting solvents. On the other hand, if the amount of the diluting solvent is too small, the viscosity can not be adequately lowered. According to this invention, the lithium salt and the electrolyte solvent are mixed and then this mixture may be mixed with the diluting solvent. In this case, it is preferred that after the lithium salt has been sufficiently dissolved in the electrolyte solvent, the diluting solvent be added. Alternatively, the electrolyte solvent and the diluting solvent may be mixed in advance and then added to the lithium salt to effect mixing. Subsequently, an electrolyte solution formed by dissolving the lithium salt in the electrolyte solvent and the diluting solvent is mixed with a gelling agent to obtain a gel composite electrolyte. The gelling agent is preferably an inorganic filler, and more preferably fumed silica particles . When the fumed silica particles are' used as the gelling agent, two or more fumed silica particles form network structure with each other; and the lithium salt and the electrolyte solvent are entrapped inside the network structure. This is believed to decrease the fluidity and 9 cause gelation. On the surfaces of the fumed silica particles , hydroxyl groups bound to the silicon atoms of the silica particles are present; and these hydroxyl groups are believed to participate in the reaction forming the network structure . The fumed silica particles are characterized in that their particle diameters are small, and they are typically obtained by the hydrolysis of silicon tetrachloride in a hydrogen flame at a temperature of not less than 1,000°C. 2H, + O, 2H 2 0 SiCl 4 + 2H 2 0 → Si0 2 + 4HC1 The fumed silica particles can preferably employ particles containing not less than 99.8% by weight of Si0 2 . Alternatively, particles containing less than 20% by weight of A1 2 0 3 and not less than 80% by weight of Si0 2 may be used. The mean of the primary particle diameters of the fumed silica particles is, for example, 5 - 60 nm, and it is preferably 5 - 30 nm. Therefore, fumed silica particles the surface area of which is large and which, for example, have specific surface area of 50 - 380 m 2 /g may be chosen as desired. Typically they are not porous, nor do they have an inner surface area. On the surfaces of the fumed silica particles, hydroxyl groups (Si-OH) bound to the silicon atoms of the silica particles are present. Preferred for use are the fumed silica particles wherein 20 - 80% of these surface hydroxyl 10 groups is modified with hydrophobic groups; and more preferred are those wherein 30 - 70% of the surface hydroxyl groups is modified with hydrophobic groups . The hydrophobic groups include C 4 -C 30 alkyls such as octyl. This means, for example, that octyl silane groups (Si-C 8 H 17 ) in place of the silanol groups (Si-OH) are present on the silica surface. Modification with such hydrophobic groups improves the mechanical stability of the gel composite electrolytes . The hydrophobic group may also be provided with polymerizable functional groups such as acryloyl and methacryloyl . In the case of such hydrophobic groups, mechanical stability can be further improved by bridge formation of the functional groups . It is preferred that based on the total amount of the lithium salt, the electrolyte solvent, and the gelling agent 2.5 - 30% by weight of the gelling agent be used; and the use of 5 - 25% by weight of the gelling agent is more preferable . Subsequently, the gel composite electrolyte is shaped into a membrane configuration and the diluting solvent is removed from the gel composite electrolyte. This shaping step can employ shaping methods and devices that are standard in the production of membranes. For example, a coater may be used to form a membrane, and while the coater surface is covered with the gel composite electrolyte in a membrane configuration, air may be sent to remove the diluting solvent. The doctor-blade method may be used in the formation of a membrane. Namely, according to the 11 method the membrane thickness can be altered by varying the gap between the blade and the base material. The base material may have a roll shape such as that of polyethylene terephthalate, or a plate shape such as a glass plate. This invention will be illustrated by way of examples hereinbelow; however, it should not be limited to the following examples . EXAMPLES AEROSIL® R805 was used as the fumed silica particles in Example 1 and Comparative Example : it is manufactured by Degussa Inc. and imported and marketed by Nippon Aerogel Co. Ltd. The surfaces of these particles are modified with octyl groups; and the CAS registration number therefor is 92797-60-9. Their specific surface area as measured by BET method is 150±25 m 2 /g; the mean particle diameter of their primary particles is about 7 nm; the apparent specific gravity is 50 g/L; and the Si0 2 content is not less than 99.8%. EXAMPLE 1 A diluted gel composite electrolyte was first prepared. Lithium trifluoromethanesulfonate (75.66 g) was dissolved in 524.34 g of polyethylene glycol dimethyl ether (MW 250), which served as an electrolyte solvent, under a nitrogen atmosphere to give an electrolyte solution. Next, the resulting electrolyte solution and 1250 g of 2- methoxyethyl ether, which served as a diluting solvent, were mixed. To this diluted electrolyte solution (1850 g) was added, with stirring, 150 g of the fumed silica particles 12 the surfaces of which were modified with octyl groups . Then vigorous stirring was effected using a blender to give the diluted gel composite electrolyte. Next, a membrane was produced from the diluted gel composite electrolyte. The diluted gel composite electrolyte as prepared above was degassed by repeating the cycle that consists of reducing pressure and purging nitrogen at room temperature. The diluted gel composite electrolyte was coated on a roll made of polyethylene terephthalate 100 μm in thickness, with varying the wet membrane thickness by using a multicoater M- 200L (available from Hirano Techseed Co. Ltd.): a comma- coater head was used as the coater head. After the diluted gel composite electrolyte was coated to form a membrane, a dryer was applied to the membrane and 2-methoxyethyl ether contained therein was allowed to evaporate. A membrane of the gel composite electrolyte containing 20 weight parts of silica could thus be obtained. By varying the roll speed, temperature of warm wind, velocity of the wind, and gap between the blade and the base material, it was possible to alter the dried membrane thickness within the range of 10 - 100 μm; and membranes having smooth surfaces resulted. These conditions are illustrated in Table 3. 13 Table 3 roll speed temperature wind velocity membrane thickness gap (m/min) ( β C) (m/s) (μm) (μm) -1 1.1 63 12.1 10.0 130 -2 2.0 70 12.1 90.0 380 -3 1.0 60 12.1 44.0 250 Comparative Example In contrast with Example 1 , no diluting solvent was employed in the comparative example. Lithium trifluoromethanesulfonate (28.86 g) was dissolved in 200 g of polyethylene glycol dimethyl ether under a nitrogen atmosphere to give an electrolyte solution. The resulting electrolyte solution (75 g) and 15 g of the above-mentioned fumed silica particles (Product #: R805) were mixed under strong shear to give a gel composite electrolyte containing 16.7 weight parts of silica. The resulting gel composite electrolyte had hardly any fluidity. Using a doctor blade with a 250-μm gap, this gel composite electrolyte was applied on a glass substrate. In this case, because of its low fluidity the gel composite electrolyte did not spread uniformly, thus affording no good membrane. Accordingly, good membranes comprising gel composite electrolytes with uniform membrane thicknesses could be obtained by diluting the electrolyte solutions with diluting 14 solvents having boiling points lower than that of polyethylene glycol dimethyl ether, which served as the electrolyte solvent. Use of the diluting solvents can yield good membranes comprising the gel composite electrolytes with uniform membrane thicknesses. 15

Claims: CLAIMS 1. A method for producing a membrane of gel composite electrolyte, the method comprising the steps of: mixing a lithium salt, an electrolyte solvent having a boiling point at one atmospheric pressure of not less than 240°C and a diluting solvent having a boiling point at one atmospheric pressure of from 40° to 210°C to obtain an electrolyte solution; wherein the electrolyte solvent is an organic solvent capable of dissolving the lithium salt and is liquid at 25°C, and the diluting solvent is an aprotic polar organic solvent and is liquid at 25°C; mixing the electrolyte solution with a gelling agent to obtain a gel composite electrolyte; shaping the gel composite electrolyte into a membrane configuration; and removing the diluting solvent from the gel composite electrolyte. 2. The method according to claim 1 , wherein the mixing step comprises the steps of mixing the lithium salt with the electrolyte solvent to obtain a mixture and mixing the mixture with the diluting solvent . 3. The method according to claim 1 or claim 2 , wherein the diluting solvent has a boiling point at one atmospheric pressure of from 50° to 180°C. 4. The method according to claim 1 , claim 2 or claim 3 , wherein the diluting solvent contains a carbonic ester, a 16 cyclic ester, a polyether, a cyclic ether, an amide, a nitrile, a ketone, a cyclic ketone, or a sulfur compound. 5. The method according to any of claims 1 - 4 , wherein the weight ratio of the total of the lithium salt, the electrolyte solvent and the gelling agent to the diluting solvent is in the range of 75 : 25 - 25 : 75. 6. The method according to any of claims 1 - 5 , wherein the gelling agent is an inorganic filler. 7. The method according to any of claims 1 - 6 , wherein the electrolyte solvent contains an ethylene glycol derivative having a molecular weight of from 200 to 1,000. 8. The method according to any of claims 1 - 7, wherein the step of removing a diluting solvent includes a step of sending air to the gel composite electrolyte in a membrane configuration. 17

Inventors: MURATA MAKOTO, YAMAMOTO TETSU, MURATA, MAKOTO, YAMAMOTO, TETSU
Applicants: AVENTIS RES & TECH GMBH & CO, MURATA MAKOTO, YAMAMOTO TETSU, AVENTIS RESEARCH & TECHNOLOGIES GMBH & CO KG, MURATA, MAKOTO, YAMAMOTO, TETSU
Application year: 1999
Main classification: H01M6
IPC codes: H01B13, H01M6, H01M10