JP2004127595A - Lithium ion conductive gel electrolyte - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion conductive gel electrolyte with high lithium ion conductivity at low temperatures. <P>SOLUTION: A lithiated monomer having a polymerizable functional group is used. The concentration of lithium ions in a system is increased without increasing the viscosity of the system and high ionic conductivity can be realized. Moreover, by the simultaneous existence of salt monomers having ionic interaction and lithiated monomers having the polymerizable functional groups in the system, since the dissociation of the lithium ions from the lithiated monomers is accelerated, ionic conductivity is effectively increased. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a lithium ion conductive gel electrolyte.
[0002]
[Prior art]
2. Description of the Related Art As electronic devices have become smaller and lighter and more portable, lithium secondary batteries having characteristics of high voltage and high energy density have attracted attention in recent years and have been actively researched and developed. It is also a very promising field in the development of electric vehicles and the like. Against this background, it is expected that a lithium secondary battery capable of realizing higher voltage and higher energy density will be required in the future.
[0003]
As a conventional electrolyte for an electric device, a liquid electrolyte, particularly one obtained by dissolving an ionic compound in an organic electrolyte has been used, but the liquid electrolyte leaks out of the electrolyte, volatilizes, Since the elution of the electrode material easily occurs, long-term reliability has been a problem. In order to solve these problems, a strong material such as a steel can is required as a package. However, the package is heavy and is not suitable for making electronic devices lighter and more portable. The use of an aluminum can as a package material is very effective in reducing the weight of a battery, but the strength is inferior to steel cans. It is necessary also from the countermeasures such as leakage, and various studies have been made.
[0004]
Various methods have been studied as a method for making the electrolyte non-liquefied, but for convenience, they are often classified into two types. One method is to use a gel polymer electrolyte in which the electrolyte solution is gelled with a polymer compound and the fluidity of the electrolyte solution is lost as the electrolyte.In this case, reliability such as leakage resistance and storage stability are improved. There is an advantage that an excellent battery can be formed. Another method is to use an electrolyte that does not use an organic solvent at all, or use a low-boiling organic solvent during electrolyte synthesis, but then remove the low-boiling organic solvent by heating or the like, and then remove the low-boiling organic solvent. There is.
[0005]
As the gel-like polymer electrolyte, for example, a gel-like polymer alloy electrolyte comprising a polymer alloy film obtained by mixing or compatibilizing a polymer and a polyanion polymer which are hardly soluble in an organic solvent and a gel-like polymer electrolyte has been proposed. Has a high transport number of 0.99, but a low conductivity of 1.4 × 10 ⁻⁴ S / cm, and is limited to use at low current to avoid voltage loss due to electrolyte resistance (for example, see Patent Reference 1). In addition, a semi-solid polymer electrolyte membrane has been proposed, and an ion conductivity of 10 ⁻⁴ S / cm has been achieved (for example, see Patent Document 2). Furthermore, a lithium ion conductive gel electrolyte comprising a lithium salt and a polymer has been proposed, and an ion conductivity of 10 ⁻⁴ S / cm has been achieved (for example, see Patent Document 3). However, these values are not sufficient when compared to 10 ⁻³ to 10 ⁻² S / cm of the organic electrolyte.
[0006]
On the other hand, as the polymer solid electrolyte, for example, a polymer solid electrolyte containing a solid solvent and an inorganic salt has been proposed, and an ion conductivity of 10 ⁻⁴ S / cm has been achieved at room temperature. Is expected to further decrease at low temperatures (for example, see Patent Document 4). Further, in application development of a solid electrolyte to a lithium battery or the like, electronic devices require low-temperature characteristics.
In general, a polymer all-solid electrolyte may be a better material than a gel electrolyte in terms of reliability, but at present, materials that sufficiently satisfy battery requirements such as ionic conductivity are not available. I can't find it. Further, polyethylene oxide, polypropylene oxide, and the like are well known as base polymers of the polymer all-solid electrolyte, but materials having an ether bond generally have a low glass transition temperature, and have a high flexibility at a high temperature. However, there is a problem that the mechanical strength is reduced.
[0007]
[Patent Document 1]
JP-A-10-50345 (pages 4-5)
[Patent Document 2]
Japanese Patent No. 2715309 (page 3-4)
[Patent Document 3]
Japanese Patent Publication No. 7-3022 (page 4-5, Table 1)
[Patent Document 4]
JP-A-6-187822 (page 4-5, Example 1)
[0008]
[Problems to be solved by the invention]
When an all-solid polymer electrolyte is used as the electrolyte, the polymer chain plays the role of propagating ions because no organic solvent is used. In general, the main chain of the polymer has lower mobility than the side chains, and therefore, by using a polymer solid electrolyte having highly branched side chains, the ion propagation capacity is increased, and the high ionic conductivity is improved. Aiming methods have been proposed. However, it is inevitable that the movement of the molecular chains at low temperatures is hindered, and so high ionic conductivity has not been achieved. In addition, lowering the glass transition temperature of the polymer in order to increase the mobility at low temperatures can improve the ionic conductivity, but in that case, the mechanical strength is greatly reduced, and the stability at high temperatures is reduced. Cause problems.
On the other hand, it can be said that the gel electrolyte achieves better ionic conductivity than the all solid-type polymer electrolyte, but the ionic conductivity at low temperature is not sufficient.
The present invention has been made in view of the above problems, and has as its object to provide a lithium ion conductive gel electrolyte having high ionic conductivity at a low temperature.
[0009]
[Means for Solving the Problems]
The present inventor has made intensive efforts to solve such problems, and as a result, by using a lithium salt monomer having a polymerizable functional group, without increasing the viscosity of the gel, the lithium ion concentration in the gel was increased. A lithium salt is added to further increase the lithium ion concentration, and a monomer having an acid component and a monomer having a base component to promote the dissociation of lithium ions from the lithium-containing monomer. It has been found that by simultaneously adding the polyfunctional salt monomer, good ionic conductivity can be obtained, and further studies have led to the completion of the present invention.
[0010]
That is, the present invention relates to a lithium salt comprising a lithium salt monomer having a polymerizable functional group and a copolymer comprising a polyfunctional salt monomer comprising a monomer having an acid component and a monomer having a base component, a lithium salt and an organic solvent. It is intended to provide an ion-conductive gel electrolyte.
In the lithium ion conductive gel electrolyte, preferred copolymers are those comprising a lithium salt monomer having a polymerizable functional group, a salt monomer having an ionic interaction, and a component that forms a chemical crosslink, and are also preferable. The lithium salt monomer having a polymerizable functional group is selected from lithium carboxylate monomer having a polymerizable functional group, lithium sulfonic acid salt monomer having a polymerizable functional group, and phenol lithium salt monomer having a polymerizable functional group. It is what is done.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
The lithium ion conductive gel electrolyte of the present invention uses a lithium salt monomer having a polymerizable functional group, increases the lithium ion concentration in the system without increasing the viscosity of the system, and further has a salt having an ionic interaction. The main point is to promote the dissociation of lithium ions from a lithium salt monomer having a polymerizable functional group by using a monomer in combination, and to develop good ion conductivity.
[0012]
As the lithium salt monomer having a polymerizable functional group used in the present invention, a lithium carboxylate having a polymerizable functional group, a lithium sulfonic acid salt having a polymerizable functional group, and a lithium phenol salt having a polymerizable functional group are preferable. Examples of the polymerizable functional group include an acryl group, a methacryl group, and a vinyl group. Specific examples of the lithium salt having these functional groups include lithium carboxylate such as lithium acrylate and lithium methacrylate; Lithium vinylbenzene sulfonate, lithium 3-vinylbenzenesulfonate, lithium 4-vinylbenzenesulfonate, lithium 2-methyl-1-pentene-1-sulfonate, lithium 1-octene-1-sulfonate, 4-vinylbenzene Lithium sulfonic acids such as lithium methanesulfonate and lithium 2-acrylamido-2-methyl-1-propanesulfonate; lithium phenols such as lithium 2-vinylphenol, lithium 3-vinylphenol and lithium 4-vinylphenol; And the like.
[0013]
As an example of the synthesis of a lithium salt monomer having a polymerizable functional group used in the present invention, when a carboxylic acid having a double bond is used, a methanol solution thereof is prepared, and lithium hydroxide is added thereto to form a carboxylic acid. There is a method in which a carboxyl group is lithiated and acetone is added to precipitate a product.
[0014]
In the present invention, the purpose of the lithium salt monomer having a polymerizable functional group is to increase the concentration of lithium ions in the system without increasing the viscosity of the system. For example, when the concentration of the lithium salt is increased in the lithium electrolyte, the viscosity of the solution increases, so that the conductivity decreases above a certain concentration. However, when a lithium salt monomer is introduced into the monomer component, an increase in the viscosity of the solution is suppressed, and therefore, a decrease in conductivity is suppressed. Furthermore, since the anionic monomer portion, which is the counter ion of lithium ions, is polymerized and fixed in the polymer matrix, it prevents the charged anionic monomer from moving to the electrode during repeated charging and discharging. This can suppress a decrease in the capacity of the battery or a significant decrease in the cycle characteristics.
[0015]
The polyfunctional salt monomer comprising a monomer having an acid component and a monomer having a base component used in the present invention is preferably a salt monomer synthesized from an amine component having a double bond and an acid component having a double bond. . When a multifunctional salt monomer composed of a monomer having an acid component and a monomer having a base component and a lithium salt monomer having a polymerizable functional group are simultaneously present in the system, dissociation of lithium ions from the lithium salt monomer is promoted. Therefore, it acts effectively on the ionic conductivity.
[0016]
As the amine component having a double bond, primary, secondary, tertiary, and quaternary ammonium salts having a double bond can be used, but from the viewpoint of the stability of the resulting salt monomer, a quaternary ammonium salt is preferable. Ammonium salts, among others, the chlorides thereof. For example, 2-ethyl methacrylate trimethyl ammonium chloride, 3-acrylamido propyl trimethyl ammonium chloride, 2-ethyl acrylate trimethyl ammonium chloride, 3- meth amide propyl trimethyl ammonium chloride, dimethyl amino ethyl benzyl methacrylate, etc. are mentioned.
[0017]
As the acid component having a double bond, preferably, a carboxylic acid or a sulfonic acid having a double bond may be mentioned, and if the acid component can form an ionic bond with the amine component having a double bond, particularly, It is not limited. Among them, a sulfonic acid having a double bond with high acidity is preferred from the viewpoint of the magnitude of polarization when introduced into a gel and the ease of synthesis of a salt monomer. Examples include 2-vinylbenzenesulfonic acid, 3-vinylbenzenesulfonic acid, 4-vinylbenzenesulfonic acid, 2-methyl-1-pentene-1-sulfonic acid, 1-octene-1-sulfonic acid, and 4-vinylbenzenesulfonic acid. It is possible to use vinylbenzene methanesulfonic acid, 2-acrylamido-2-methyl-1-propanesulfonic acid and the like.
[0018]
The chemical crosslinking component used in the present invention includes N, N'-methylenebisacrylamide, ethylene glycol diacrylate, ethylene glycol dimethacrylate, polyethylene glycol diacrylate, 1,6-hexanediol diacrylate, Pentyl glycol diacrylate, tripropylene glycol diacrylate, polypropylene glycol diacrylate, trimethylol propane trimethacrylate, trimethylol propane triacrylate, tetramethylol methane tetraacrylate, triethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, 1, 3-butylene glycol dimethacrylate, 1,6-hexanediol dimethacrylate, neopentyl glycol Rudimethacrylate and the like.
[0019]
In the present invention, the thermal stability of the gel electrolyte is improved by adding a chemical crosslinking component. Gel electrolytes such as polyacrylonitrile and polyvinylidene fluoride, which do not contain a chemically crosslinked portion and enable gel formation with intermolecular force, are inferior in stability at high temperatures compared to gel electrolytes containing a chemically crosslinked component. In these, it is preferable to add a chemical crosslinking component.
[0020]
In the present invention, the addition of a monomer having a polymerizable functional group and not reacting with lithium ions as a component other than the above components can improve the strength of the generated gel electrolyte or a lithium salt monomer having a polymerizable functional group. In order to improve the compatibility of the polymer with the lithium electrolyte and further increase the affinity between the produced polymer and the solvent and to be able to hold a large amount of the solvent used for batteries and the like, it is considered that such a component is used. It is also possible to add. Specific examples of the monomer having a polymerizable functional group and not reacting with lithium ion include methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, and methacryl. Acrylates such as butyl acrylate, and methacrylates. In addition to these, one of acrylonitrile, vinylene carbonate, acrylamide, methacrylamide, and styrene, or a combination of a plurality of them may be used.
[0021]
Examples of the lithium salt used in the present invention include LiPF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiBF ₄ and LiAsF ₆ .
[0022]
The organic solvent used in the present invention includes propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone, γ-butyrolactone, and the like. The lithium salt is dissolved to form an electrolyte. These can be used alone or as a mixture of two or more.
[0023]
For forming a polymer network in the gel electrolyte of the present invention, various methods such as a method of applying heat, a method of irradiating light in a visible or ultraviolet region, and a method of irradiating radiation such as an electron beam can be applied. In the polymerization by heat, a thermal polymerization initiator can be added as needed. Examples of the thermal polymerization initiator include an azo-based polymerization initiator such as azobisisobutyronitrile, and a peroxide-based polymerization initiator such as benzoyl peroxide.
[0024]
Examples of the method for obtaining the lithium ion conductive gel electrolyte of the present invention include a lithium salt monomer having a polymerizable functional group and a polyfunctional salt monomer comprising a monomer having an acid component and a monomer having a base component, If necessary, a chemical cross-linking component is added to a lithium electrolyte solution obtained by dissolving a lithium salt in an organic solvent, and the mixture is stirred at room temperature until the mixture becomes uniform to obtain a solution 1. In parallel with the preparation of the solution 1, an azo-based polymerization initiator such as azobisisobutyronitrile or a peroxide-based polymerization initiator such as benzoyl peroxide is dissolved in another lithium electrolyte solution to form a solution 2. Get. Solution 2 obtained is added to solution 1 and stirred to obtain solution 3. The solution 3 thus obtained is placed in an oven at 60 to 80 ° C. for about 10 minutes to 2 hours to have a lithium salt monomer having a polymerizable functional group, a monomer having a chemical crosslinking component, an acid component, and a base component. A copolymer comprising a polyfunctional salt monomer comprising a monomer is obtained, a polymer is formed, and a lithium ion conductive gel electrolyte obtained by gelling the polymer with an organic solvent is obtained.
[0025]
In the lithium ion conductive gel electrolyte of the present invention, as an example of the mixing ratio of each component, a lithium salt monomer having a polymerizable functional group is preferably 10 mmol / L to 2 mol / L, more preferably 100 mmol / L. -1 mol / L, and the chemical crosslinking component is preferably 5 mmol / L-500 mmol, more preferably 10 mmol / -200 mmol / L, and a polyfunctional salt monomer composed of a monomer having an acid component and a monomer having a base component is used. , Preferably 10 mmol / L to 2 mol / L, more preferably 50 mmol / L to 1 mol / L. The polymerization initiator preferably has a concentration of 5 to 200 mmol / L, and the concentration of the lithium salt dissolved in the organic solvent is preferably 0.1 to 2 mol / L.
[0026]
The lithium ion conductive gel electrolyte of the present invention is useful as a battery, an electric double layer capacitor, a material for electrochromic and other electrochemical devices, and the like.
[0027]
【Example】
Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited thereto.
[0028]
[Example 1]
<Synthesis of lithium salt monomer having polymerizable functional group>
3.61 g (50 mmol) of acrylic acid and 1.0 g (42 mmol) of lithium hydroxide were dissolved in 25 ml of methanol and reacted at room temperature for 2 hours. The obtained solution was added dropwise to 500 ml of acetone to obtain a white precipitate. . The obtained white precipitate was washed several times with acetone, and then collected by filtration. It was dried at room temperature using a vacuum drier to obtain 2.8 g of lithium acrylate (white powder). The yield was 85.5%.
[0029]
<Synthesis of a polyfunctional salt monomer composed of a monomer having an acid component and a monomer having a base component (hereinafter, referred to as a polyfunctional salt monomer)>
10.36 g (50 mmol) of 2-acrylamido-2-methyl-1-propanesulfonic acid was dissolved in 500 ml of water, 8.27 g (30 mmol) of silver carbonate was added thereto, and the mixture was stirred for 8 hours. Was obtained. An aqueous solution of 3-amidopropyltrimethylammonium chloride 3-methacrylate was prepared so as to have a concentration of 100 mmol / L, and the obtained liquid was dropped and reacted. As the reaction proceeded, silver chloride white solid precipitated. The reaction was carried out while measuring the conductivity with a conductivity meter. When 492.0 ml of an aqueous solution of 3-amidopropyltrimethylammonium chloride 3-methacrylate was dropped, the conductivity showed the minimum value, and that point was regarded as the end point. The precipitated silver chloride was removed by filtration to obtain a colorless and transparent aqueous solution. The filtrate was concentrated by an evaporator to obtain a slightly viscous aqueous solution.
The resulting solution was diluted with ethanol and added dropwise to a large amount of tetrahydrofuran to obtain a white crystal. The white crystals obtained by filtration were dried under vacuum, and the melting point of the product was confirmed by differential scanning calorimetry (DSC). The melting point was 152 ° C., and it was confirmed that the obtained compound was a single salt monomer.
[0030]
<Synthesis of lithium ion conductive gel electrolyte and evaluation of ionic conductivity>
0.140 g (0.36 mmol) of the polyfunctional salt monomer obtained above and 0.117 g (1.5 mmol) of lithium acrylate are placed in a test tube, and 2.5 ml of an organic electrolyte solution is added thereto, followed by stirring to dissolve. Let it. As the organic electrolyte solution, a solution prepared by using N-methyl-2-pyrrolidone as a solvent and adjusting LiClO ₄ to 1 mol / L was used. The resulting solution was degassed, and 0.5 ml of a solution obtained by dissolving 0.029 g (0.12 mmol) of benzoyl peroxide in 1.0 ml of the organic electrolyte solution was added. After stirring, the obtained uniform transparent solution was heated to 80 ° C., and after 15 minutes, a milky white gel electrolyte was obtained. The production of the gel electrolyte was performed under a nitrogen atmosphere. The ionic conductivity of the lithium ion conductive gel electrolyte obtained above was measured at −20 ° C. by the AC impedance method. The measured frequency range was 50 Hz to 30 MHz, and the voltage was 0.5 V. The measurement result was 1.9 × 10 ⁻³ S / cm.
[0031]
[Example 2]
<Synthesis of lithium ion conductive gel electrolyte and evaluation of ionic conductivity>
0.117 g (0.30 mmol) of the polyfunctional salt monomer obtained in the same manner as in Example 1, 0.117 g (1.5 mmol) of lithium acrylate, and 0.023 g (0.15 mmol) of methylenebisacrylamide were placed in a test tube. Then, 2.5 ml of an organic electrolyte solution was added thereto and stirred to dissolve. As the organic electrolyte solution, a solution prepared by using N-methyl-2-pyrrolidone as a solvent and adjusting LiClO ₄ to 1 mol / L was used. The resulting solution was degassed, and 0.5 ml of a solution obtained by dissolving 0.029 g (0.12 mmol) of benzoyl peroxide in 1.0 ml of the organic electrolyte solution was added. After stirring, the obtained uniform transparent solution was heated to 80 ° C., and after 15 minutes, a milky white gel electrolyte was obtained. The production of the gel electrolyte was performed under a nitrogen atmosphere.
The ionic conductivity of the lithium ion conductive gel electrolyte obtained above was measured at −20 ° C. by the AC impedance method. The measured frequency range was 50 Hz to 30 MHz, and the voltage was 0.5 V. The measurement result was 2.0 × 10 ⁻³ S / cm.
[0032]
[Example 3]
<Synthesis of lithium ion conductive gel electrolyte and evaluation of ionic conductivity>
A gel electrolyte was prepared in the same manner as in Example 2 except that 0.285 g (1.5 mmol) of lithium 4-vinylbenzenesulfonate was used instead of lithium acrylate in the preparation of the gel electrolyte in Example 2. Then, the ionic conductivity was measured. The result was 2.0 × 10 ⁻³ S / cm at −20 ° C.
[0033]
[Example 4]
<Synthesis of lithium ion conductive gel electrolyte and evaluation of ionic conductivity>
A gel electrolyte was obtained in the same manner as in Example 2 except that 0.189 g (1.5 mmol) of 4-vinylphenol lithium salt was used instead of lithium acrylate in the preparation of the gel electrolyte in Example 2. Then, the ionic conductivity was measured. The result was 1.8 × 10 ⁻³ S / cm at −20 ° C.
[0034]
[Comparative Example 1]
<Synthesis of lithium ion conductive gel electrolyte and evaluation of ionic conductivity>
A gel electrolyte was synthesized under the same conditions as in Example 2 except that lithium acrylate was not added in the preparation of the gel electrolyte in Example 2. The ionic conductivity of the obtained gel electrolyte was 1.3 × 10 ⁻³ S / cm at −20 ° C.
[0035]
[Comparative Example 2]
A gel electrolyte was synthesized in the same manner as in Comparative Example 1 except that the concentration of LiClO ₄ was set to 1.5 mol / L in Comparative Example 1 in order to match the lithium ion concentration in the system with the concentration of Example. The conductivity of the obtained gel electrolyte was 0.7 × 10 ⁻³ S / cm at −20 ° C., and the conductivity decreased as the LiClO ₄ concentration was increased.
[0036]
【The invention's effect】
According to the present invention, a lithium ion conductive gel electrolyte having a high ionic conductivity at a low temperature can be provided, and the gel electrolyte is useful as a material for batteries, electric double layer capacitors, and other electrochemical devices. .

Claims (3)

Lithium ion conductive gel comprising a copolymer comprising a polyfunctional salt monomer comprising a lithium salt monomer having at least a polymerizable functional group and a monomer having an acid component and a monomer having a base component, a lithium salt and an organic solvent Electrolytes. The lithium ion conductive gel electrolyte according to claim 1, wherein the copolymer comprises a lithium salt monomer having a polymerizable functional group, a salt monomer having an ionic interaction, and a component that forms a chemical crosslink. The lithium salt monomer having a polymerizable functional group is selected from a lithium carboxylate monomer having a polymerizable functional group, a lithium sulfonic acid monomer having a polymerizable functional group, and a phenol lithium salt monomer having a polymerizable functional group. The lithium ion conductive gel electrolyte according to claim 1, wherein