JP2000082328A - Gel-form electrolyte and lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gel-form electrolyte for lithium secondary battery, which meets the requirements of high ionic conductivity and excellent mechanical strength compatibly. SOLUTION: This gel-form electrolyte is characterized, in an electrolytic solution included in a polymer, by 1) the polymer consists of multi-functional monomers of polyalkylene glycol base or polyalkylene sulfide base having a plurality of unsaturated double bonds of polymerization and monofunctional monomers having one unsaturated double bond of polymerization, by 2) the polyalkylene glycol unit number or polyalkylene sulfide unit number of multi- functional monomer is no less than 7 in the mean, and by 3) the ratio by weight of the polymer to the gel-form electrolyte ranges form 1 to 18%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gel polymer solid electrolyte. More specifically, the present invention relates to a gel electrolyte capable of constructing a lithium secondary battery having excellent discharge characteristics and the like.

[0002]

2. Description of the Related Art Heretofore, studies have been made on using a gel-like solid electrolyte which can completely suppress liquid leakage in principle as an electrolyte used in a lithium secondary battery or the like. Polyethers represented by polyethylene oxide (PEO) have long been considered as electrolytes since they were found to have ionic conductivity, but cannot provide a practical battery due to low ionic conductivity. Was. Therefore, various solid electrolytes containing a plasticizer have recently been proposed in order to improve ionic conductivity. They are roughly classified into a type in which a polymer such as polyvinylidene fluoride (PVdF) or polyacrylonitrile (PAN) is impregnated with an electrolytic solution, and a type in which a monomer having an unsaturated double bond is polymerized in the presence of the electrolytic solution. Is done. The former polymer-impregnated electrolyte has a heavy load in the production process, for example, it needs to be heated to a high temperature to once dissolve the polymer during the impregnation.

On the other hand, the latter monomer-crosslinked electrolyte has a high degree of freedom in design, for example, various kinds of monomers can be combined. However, when the amount of the monomer is reduced to increase the ionic conductivity, there is a problem that the strength of the gel electrolyte is reduced and the solid electrolyte may not be formed. In general, a gel electrolyte adopts a structure in which an electrolyte solution obtained by dissolving a supporting electrolyte such as a lithium salt in a solvent is held in a polymer. Therefore, in order to increase the mechanical strength, the proportion of the polymer may be increased, but in that case, the ionic conductivity is deteriorated by the decrease in the proportion of the electrolytic solution. Conversely, if the proportion of the electrolytic solution is increased, the ionic conductivity is improved, but the mechanical strength of the gel is deteriorated. For example, in the case of a battery using a gel electrolyte, the ionic conductivity of the gel electrolyte is an important factor directly affecting the performance of the battery, and the mechanical strength is an important factor directly affecting the safety of the battery. Therefore, the emergence of a gel electrolyte realizing these simultaneously has been desired.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a gel electrolyte which has both high ionic conductivity and strength.

[0005]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, in a region where the polymer concentration in the gel electrolyte is relatively low, the polymer is polymerized as a polymer monomer component. And a polyalkylene glycol-based or polyalkylene sulfide-based polyfunctional monomer having one and a plurality of polyalkylene glycol units or polyalkylene sulfide units (hereinafter referred to as the number of polyalkylene glycol units or polyalkylene sulfide units in the polyfunctional monomer). It has been found that by setting the “unit number” to 7 or more, high ionic conductivity and excellent mechanical strength can be obtained. In general, when a monofunctional monomer and a polyfunctional monomer are used in combination, the polyfunctional monomer is considered to have a function as a cross-linking agent that binds the chain polymers of the monofunctional monomer. The higher the number of units, the lower the crosslinking point density per unit weight, but nevertheless, in regions where the polymer concentration in the gel electrolyte is low, the mechanical strength of the gel is improved. That is a truly surprising finding.

[0006] That is, the gist of the present invention is to provide a gel electrolyte in which an electrolytic solution is held in a polymer, wherein 1) the polymer is a polyalkylene glycol or a polyalkylene having a plurality of polymerizable unsaturated double bonds. A polymer obtained by polymerizing a monomer containing a sulfide polyfunctional monomer and a monofunctional monomer having one polymerizable unsaturated double bond, and 2) the number of polyalkylene glycol units of the polyfunctional monomer or A gel electrolyte characterized in that the number of polyalkylene sulfide units is 7 or more on average, and 3) a weight ratio of the polymer to the gel electrolyte is 1 to 18%, and lithium secondary using the same. Exists in batteries.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
The gel electrolyte of the present invention comprises an electrolyte held in a polymer. The polymer comprises a monofunctional monomer having one polymerizable unsaturated double bond and a polyalkylene glycol-based or polyalkylene sulfide-based polyfunctional monomer having a plurality of polymerizable unsaturated double bonds. It is a polymer obtained by polymerizing the containing monomer.

As monofunctional monomers, acrylates,
Examples include unsaturated esters such as methacrylate, simple olefins such as ethylene and styrene, and vinyl compounds such as vinyl chloride, vinyl acetate and acrylonitrile. Among them, unsaturated esters such as acrylate and methacrylate are preferable because radical polymerization is easy. In particular, acrylate is preferable because the reaction speed is high and the productivity of the electrolyte is improved. As an example of such a preferable monofunctional monomer, an acryloyl group or a methacryloyl group is bonded to one end of an alkylene oxide such as ethylene oxide or propylene oxide or an alkylene sulfide monomer or multimer such as ethylene sulfide or propylene sulfide. Compounds can be mentioned. More specifically, a compound represented by the following general formula (I) can be mentioned.

[0009]

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(Where m is 1 to 4; X is an oxygen atom or a sulfur atom; R ¹ is a hydrogen atom or a lower alkyl group; R ² is a lower alkyl group or an aryl group; R ³ and R ⁴ are each a hydrogen atom Or a lower alkyl group, and a plurality of R ³ or R ⁴ may be the same or different.) Among them, X is preferably an oxygen atom, and m is preferably 1 or 2. R ¹ is preferably hydrogen or an alkyl group having 1 to 4 carbon atoms, particularly preferably a hydrogen atom or a methyl group. The alkyl group for R ² is preferably an alkyl group having 1 to 4 carbon atoms, and more preferably a methyl group or an ethyl group. Examples of the aryl group for R ² include a phenyl group and a naphthyl group.
Preferably it is a phenyl group. Furthermore, R ³ and R
⁴ is preferably a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, particularly preferably a hydrogen atom or a methyl group, and most preferably a hydrogen atom.

Specific examples of preferably used monomers include 2-alkoxyethyl acrylates such as 2-methoxyethyl acrylate and 2-ethoxyethyl acrylate, and diethylene glycol alkyl ether acrylates such as diethylene glycol methyl ether acrylate and diethylene glycol ethyl ether acrylate. And polyethylene glycol aryl ether acrylates such as polyethylene glycol phenyl ether acrylate and polyethylene glycol nonyl phenyl ether acrylate, and sulfur-containing acrylates such as 2- (methylthio) ethyl acrylate. Particularly, 2-alkoxyethyl acrylates and diethylene glycol alkyl ether acrylates having a small molecular weight and having an ether unit are preferably used. When the molecular weight is small, the ratio of a portion having a polymerizable unsaturated double bond such as an acryloyl group or a methacryloyl group per unit volume increases, which leads to an improvement in strength. The ether unit is effective because it has a high affinity for metal ions of a supporting electrolyte in an electrolyte such as lithium and contributes to an improvement in ionic conductivity. In any of the above monomers, a part of the hydrogen atoms may be replaced by a halogen atom or the like.

The polyfunctional monomer in the present invention may be an acryloyl group, a methacryloyl group or a vinyl group at both ends of an alkylene glycol compound such as polyethylene glycol or polypropylene glycol or an alkylene sulfide compound such as polyethylene sulfide or polypropylene sulfide. , Two or more groups having an unsaturated double bond such as an allyl group. Preferably, an acrylic group such as an acryloyl group and / or a methacryloyl group is bonded to both ends. Further, the number of the units is 7 or more, and specific examples include a compound represented by the following general formula (II).

[0013]

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(Where n is 7 or more; R ⁵ is a hydrogen atom or a lower alkyl group; a plurality of R ⁵ may be the same or different; R ⁶ and R ⁷ are each a hydrogen atom or a lower alkyl group) A plurality of R ⁶ or R ⁷ may be the same or different; X is an oxygen atom or a sulfur atom) R ⁵ is preferably a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, Particularly preferred is a hydrogen atom or a methyl group. R ⁶ and R ⁷ are preferably a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, particularly preferably a hydrogen atom or a methyl group, and most preferably a hydrogen atom. X is preferably an oxygen atom. Furthermore, n is 7 or more, usually 7 to 100,
Preferably it is 7-50.

If the number n of units is smaller than 7, the strength of the electrolyte becomes weak, which is not preferable. This is because while the ratio of unsaturated double bonds per unit volume increases with a decrease in molecular weight, mass transfer tends to occur, so the reaction between polyfunctional monomers easily progresses, and the crosslinking point becomes heterogeneous. It is presumed that it becomes. That is, the balance between the reactivity of the monofunctional monomer as the main skeleton and the reactivity of the polyfunctional monomer as the cross-linking agent is not balanced, and a heterogeneous and weak solid electrolyte is provided. On the other hand, if the number of units n is too large, the number of cross-linking points per unit weight becomes too small as the molecular weight increases, so that curing becomes difficult and the solubility in a solvent may be deteriorated. Some of the commercially available compounds are mixtures of compounds having several units n. Therefore, in this case, the number n of units is a value as an average value.

More specific polyfunctional monomers include polyalkylene glycol diesters such as polyethylene glycol diacrylate, polyethylene glycol dimethacrylate, polypropylene glycol diacrylate, polypropylene glycol dimethacrylate, polybutylene glycol diacrylate, and polybutylene glycol dimethacrylate. And polyalkylene glycol divinyl ethers such as polyethylene glycol divinyl ether, polypropylene glycol divinyl ether, and polybutylene glycol divinyl ether. In particular, polyethylene glycol diacrylate and polyethylene glycol dimethacrylate are preferably used because they have high reactivity, are inexpensive and easily available.

As the polyfunctional monomer, besides the above-mentioned bifunctional monomer, a trifunctional monomer or a polyfunctional monomer having a higher functionality can be used. Examples of the trifunctional monomer include trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, trimethylolpropane ethoxylate triacrylate, trimethylolpropane ethoxylate trimethacrylate, trimethylolpropanepropoxylate triacrylate, trimethylolpropanepropanepropoxylate trimethacrylate, Pentaerythritol triacrylate, pentaerythritol trimethacrylate and the like can be mentioned. Also, 4
Functional or higher monomers include pentaerythritol tetraacrylate, pentaerythritol tetramethacrylate, ditrimethylolpropane tetraacrylate, ditrimethylolpropane tetramethacrylate,
Examples include dipentaerythritol pentaacrylate, dipentaerythritol pentamethacrylate, and the like.

The polymer in the gel electrolyte of the present invention can be obtained by polymerizing the above monofunctional monomer and polyfunctional monomer. The molar ratio between the monofunctional monomer and the polyfunctional monomer in the polymer is usually 1/99 or more, preferably 50
/ 50 or more, more preferably 55/45 or more,
It is usually at most 99/1, preferably at most 90/10. If the ratio is too large or too small, the effect of the present invention tends to be insufficient. As the monomer component, a monomer other than the above may be used, but usually, 90% by mole or more of the monomer component is the above monofunctional monomer or polyfunctional monomer.

The weight average molecular weight of the polymer is usually 10,000 or more,
Preferably it is 100,000 or more, usually 5,000,000 or less, preferably 1,000,000 or less. If it is too large, the viscosity is too high and handling becomes difficult, and if it is too small, gel tends to be difficult to form. The ratio of the polymer in the gel electrolyte of the present invention to the entire gel electrolyte is 1 to 18% by weight. If this ratio is too small, the mechanical strength of the gel tends to be insufficient, so it is preferably at least 4% by weight,
On the other hand, if it is too large, ionic conductivity tends to be insufficient, and the specific effect of the present invention tends to be remarkably hard to appear. Therefore, the content is preferably 15% by weight or less. The ratio of the polymer to the electrolyte is usually 1% by weight or more, preferably 5% by weight.
As described above, it is usually at most 25% by weight, preferably at most 20% by weight. When the ratio is too large or too small, the same tendency as above is observed.

The gel electrolyte of the present invention contains an electrolytic solution. As the electrolytic solution, a solution in which a supporting electrolyte is dissolved in a non-aqueous solvent can be used. As the supporting electrolyte, lithium salt,
Various alkali metal salts such as sodium salt and potassium salt and alkaline earth metal salts can be used, and lithium salts are preferred.

The lithium salt is not particularly limited,
LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSb
F ₆ , LiAlCl ₄ , LiClO ₄ , CF ₃ SO ₃ L
i, C ₄ F ₉ SO ₃ Li, CF ₃ COOLi, (CF ₃
(CO) ₂ NLi, (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F
₅ SO ₂ ) NLi and the like. Particularly, LiPF ₆ and LiC which are easily soluble in a solvent and exhibit a high degree of dissociation
IO ₄ and CF ₃ SO ₃ Li are preferably used. A plurality of these may be used simultaneously.

The amount of the supporting electrolyte is usually at least 1% by weight, preferably at least 5% by weight, and usually at most 30% by weight, preferably at most 20% by weight, based on the gel electrolyte. Also, usually 0.5 to 2.5 mol with respect to the electrolytic solution
/ L. If the amount of the supporting electrolyte is too small or too large, the ionic conductivity tends to decrease.

The solvent used in the electrolytic solution is not particularly limited as long as it dissolves the supporting electrolyte. However, in order to obtain high ionic conductivity, dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate and the like are usually used. (DEC), propylene carbonate (P
C), alkyl carbonates such as butylene carbonate (BC) and methyl ethyl carbonate (MEC);
esters such as γ-butyrolactone and methyl formate,
Ethers such as 1,2-dimethoxyethane and tetrahydrofuran, and sulfur-containing compounds such as sulfolane and dimethylsulfoxide are used. Further, a mixture of these solvents may be used. In addition, those in which a predetermined amount of a lithium salt is previously dissolved in these solvents are commercially available, and these may be used. Preferably, dimethyl carbonate,
It is at least one selected from the group consisting of ethylene carbonate, propylene carbonate, diethyl carbonate, and methyl ethyl carbonate.

The amount of the solvent is usually at least 50% by weight, preferably at least 70% by weight, and usually at most 95% by weight, preferably at most 90% by weight, based on the gel electrolyte.
If the amount of the solvent is too small, the ionic conductivity tends to decrease, and if the amount is too large, the strength of the solid electrolyte tends to decrease. The gel electrolyte may contain various additives, if necessary. For example, trifluoropropylene carbonate, vinylene carbonate, catechol carbonate,
1,6-Dioxaspiro [4,4] nonane
-2,7-done, 12-crown-4-ether and the like can be added.

The gel electrolyte of the present invention can be produced by separately preparing a polymer and an electrolytic solution, bringing them into contact with each other, and subjecting them to a gelling treatment if necessary. Examples of the treatment for gelation include a method of cooling after heating, and a method of irradiating energy rays such as ultraviolet rays. The gel electrolyte of the present invention can also be obtained by producing a composition in which a polymer monomer component and an electrolytic solution are mixed and then polymerizing the monomer component, which is a preferable method.

As a method of polymerizing the monomer, a method of directly irradiating radiation such as an electron beam or γ-ray without adding a polymerization initiator, a method of irradiating ultraviolet rays in the presence of an ultraviolet polymerization initiator, an organic peroxide, For example, a method of heating in the presence of a thermal polymerization initiator, a redox-based room-temperature curing method using an oxidation-reduction initiator, and the like. In particular, the method of irradiating with ultraviolet rays is preferable in that polymerization at a low temperature is possible and the time required for curing is short, and the method of polymerization by heat is preferable because it does not require a special device and is simple.

In preparing the gel electrolyte, care must be taken to prevent the incorporation of moisture. Usually, it is usually to dehydrate each component in advance and to minimize the incorporation of moisture during preparation. It is. The water content in the gel electrolyte is usually 50 ppm or less, preferably 30 ppm or less. Too much water tends to be unsuitable for use in non-aqueous batteries such as lithium secondary batteries.

The gel electrolyte in the present invention can be used as an electrolyte layer of a secondary battery such as a lithium secondary battery. Further, it can also be used as an ion mobile phase in the active material layer of the electrode. Hereinafter, a lithium secondary battery to which the gel electrolyte of the present invention can be applied will be described. The basic configuration of a lithium secondary battery to which the gel electrolyte of the present invention can be applied is the same as that of a conventionally known battery, and a positive electrode and a negative electrode are stacked via an electrolyte layer and housed in a case.

First, the positive electrode and the negative electrode will be described. The positive electrode and the negative electrode can be composed of an active material-containing layer capable of inserting and extracting lithium ions, usually provided on a current collector, and an ion mobile phase formed in the layer. The ion mobile phase is composed of the same electrolyte as that constituting the electrolyte layer, and the electrolyte constituting the ion mobile phase of the positive electrode and the negative electrode is, for example, a collector for an electrode containing an active material, a binder and a solvent. After forming on the active material layer having a void by applying and drying on, after applying a gel precursor such as the composition of the present invention for the electrolyte formation on the active material layer and impregnating in the voids It is formed by a gelling method.

As the current collector, a metal foil such as an aluminum foil or a copper foil is usually used, and its thickness is appropriately selected.
Usually, it is 1 to 50 μm, preferably 1 to 30 μm. If the thickness of the current collector is too small, the mechanical strength becomes weak, which causes a problem in production. If the thickness is too large, the capacity of the battery as a whole decreases. The current collector is preferably used after being subjected to a roughening treatment in advance in order to increase the adhesive strength of the active material-containing layer.
Examples of the surface roughening method include a mechanical polishing method, an electrolytic polishing method, and a chemical polishing method. In the mechanical polishing method, a polishing cloth paper having abrasive particles fixed thereon, a grindstone, an emery buff, a wire brush provided with a steel wire, or the like is used. Further, an intermediate layer may be formed on the surface of the current collector in order to increase the adhesive strength and the conductivity of the active material-containing layer. Positive electrode active materials capable of inserting and extracting lithium ions are roughly classified into inorganic compounds and organic compounds.

As the positive electrode active material composed of an inorganic compound,
Transition metal oxides, composite oxides of lithium and transition metals,
Transition metal sulfide and the like. As the above transition metal, Fe, Co, Ni, Mn and the like are used. Specific examples of the inorganic compound used for the positive electrode active material include Mn
Transition metal oxides such as O, V ₂ O ₅ , V ₆ O ₁₃ and TiO ₂ ; composite oxides of lithium and transition metals such as lithium nickelate, lithium cobaltate and lithium manganate;
Transition metal sulfides such as TiS ₂ , FeS, and MoS ₂ are mentioned. These compounds may be partially elementally substituted in order to improve their properties.

As a positive electrode active material composed of an organic compound,
For example, polyaniline, polypyrrole, polyacene, disulfide-based compounds, polysulfide-based compounds, N-fluoropyridinium salts and the like can be mentioned. The positive electrode active material may be a mixture of the above-mentioned inorganic compound and organic compound. The particle size of the positive electrode active material is appropriately selected in consideration of other components of the battery. From the viewpoint of improving battery characteristics such as rate characteristics and cycle characteristics, the particle size is usually 1 to 30 μm.
Preferably it is 1 to 10 μm.

Examples of the negative electrode active material capable of inserting and extracting lithium ions include carbon-based active materials such as graphite and coke. Such carbon-based active materials include metals, metal salts,
It can also be used in the form of a mixture with an oxide or a coating. Examples of the negative electrode active material include oxides and sulfates such as silicon, tin, zinc, manganese, iron, and nickel, lithium metal, Li-Al, Li-Bi-Cd, and Li-Sn.
A lithium alloy such as -Cd, a lithium transition metal nitride, silicon or the like can be used. The particle size of the negative electrode active material is appropriately selected in consideration of other components of the battery. However, from the viewpoint of improving battery characteristics such as initial efficiency, rate characteristics, and cycle characteristics, it is usually 1 to 50 μm, preferably 15 μm. ~ 30μ
m.

[0034] The electrode may optionally contain additives exhibiting various functions such as a conductive material and a reinforcing material. The conductive material is not particularly limited as long as it is capable of imparting conductivity by being mixed in an appropriate amount with the active material, and usually includes acetylene black, carbon black, carbon powder such as graphite, and various metal fibers and foils. . In addition, to increase the stability and life of the battery, trifluoropropylene carbonate, vinylene carbonate,
Catechol carbonate, 1,6-Dioxaspir
o [4,4] nonane-2,7-dione, 12
-Crown-4-ether and the like can be used. Furthermore, various inorganic and organic spherical, plate-like, rod-like or fibrous fillers can be used as the reinforcing material. Examples of the binder constituting the electrode paint include inorganic compounds such as silicate and glass, and various resins.

Examples of the binder resin include alkane-based polymers such as polyethylene, polypropylene and poly-1,1-dimethylethylene; unsaturated polymers such as polybutadiene and polyisoprene; polystyrene, polymethylstyrene and polyvinylpyridine. , Poly-
Polymers having a ring such as N-vinylpyrrolidone, acrylics such as polymethyl methacrylate, polyethyl methacrylate, polybutyl methacrylate, polymethyl acrylate, polyethyl acrylate, polyacrylic acid, polymethacrylic acid, polyacrylamide, etc. Polymers, fluorine resins such as polyvinyl fluoride, polyvinylidene fluoride, and polytetrafluoroethylene; CN group-containing polymers such as polyacrylonitrile and polyvinylidene cyanide; polyvinyl alcohol polymers such as polyvinyl acetate and polyvinyl alcohol; and polyvinyl chloride And halogen-containing polymers such as polyvinylidene chloride, and conductive polymers such as polyaniline. Further, a mixture, a modified product, a derivative, a random copolymer, an alternating copolymer, a graft copolymer, a block copolymer or the like of the above-mentioned polymers can be used. The molecular weight of these resins is usually 10,000 to 300
0000, preferably 100,000 to 1,000,000. If the molecular weight is too low, the strength of the coating decreases,
If it is too high, the viscosity becomes high and it becomes difficult to form an electrode.

In preparing the electrode coating material, the active material 100
The compounding amount of the binder with respect to parts by weight is usually 0.1 to 3
0 parts by weight, preferably 1 to 15 parts by weight. If the amount of the binder is too small, the strength of the electrode decreases,
If the amount is too large, the amount of voids in the active material-containing layer will decrease, making it difficult to impregnate the electrolyte for forming a gel electrolyte. The type of solvent used for preparing the electrode coating material is not particularly limited as long as it is inert to the active material and can dissolve the binder, and may be any of inorganic and organic solvents. One example of a suitable solvent is N-methylpyrrolidone. In addition, a ball mill, a sand mill, a twin-screw kneader, or the like is used for preparing the electrode coating material.

The active material layer having voids can also be formed by a method in which a mixture of the active material and a binder is softened by heating and then pressed or sprayed onto a current collector. Also in this case, the blending amount of the binder with respect to 100 parts by weight of the active material is in the above range. Further, a method of baking only the active material on the current collector can be adopted.

The electrolyte of the ion mobile phase of the positive electrode and the negative electrode can be preferably formed by a method in which a composition for forming an electrolyte is applied to the active material layer, impregnated into voids, and then gelled. Needless to say, a method of polymerizing and gelling after applying a mixture of the components forming the active material layer and the above-described composition, or forming a gel electrolyte by polymerizing the above-described composition and the like, A method of impregnating the active material layer in a state where the viscosity is reduced by heating and then cooling after that can be adopted. When the gel electrolyte of the present invention is used as the electrolyte of the ion mobile phase, a composition containing a monomer component serving as a raw material of a polymer and an electrolytic solution can be used as the composition. In the preferred method, since the viscosity of the electrolytic solution is low, it is easy to impregnate the voids in the active material layer with the electrolytic solution. Since the thickness of the active material layer is usually 1 mm or less, impregnation with the electrolytic solution is completed quickly. In any case, by applying a calender treatment to the coating film, the coating film can be compacted and the filling amount of the active material can be increased.

Next, the electrolyte layer will be described. The electrolyte layer can be composed of the gel electrolyte of the present invention.
At this time, the electrolyte can be impregnated and held in a structure such as a porous membrane. The above-mentioned structure for an electrolyte layer has an initial elastic modulus to compression of 1 in order to further enhance the liquid retaining property of the electrolyte layer.
It is preferably at least × 10 ⁵ (N / m ² ). Such an initial elastic modulus is obtained from a slope of a straight line in a linear response region near zero stress in a stress-strain curve. And
A high value of the initial elastic modulus means that the deformation of the structure is small from the stage where the load is small. Therefore, by using a structure having a high initial elastic modulus, it is possible to effectively support the pressure applied to the gel electrolyte.

The voids in the structure for an electrolyte layer are as follows:
In order to increase the ion mobility, it is preferable that most of the structure has a structure extending in the vertical direction (electrode direction). The porosity of the structure is preferably in the range of 60 to 90% by volume, particularly preferably in the range of 70 to 90% by volume. If the porosity is smaller than 60%, the ion conductivity may be reduced, and the amount of the electrolyte bleeding may be reduced, making it difficult to form a gel electrolyte. If the porosity exceeds 90%, the structure becomes too soft, which causes difficulty in the manufacturing process of the electrolyte layer. The porosity can be calculated by calculating the apparent specific gravity from the volume and the weight, and comparing it with the true specific gravity of the material of the structure.

The material of the above-mentioned structure for the electrolyte layer is not particularly limited as long as it satisfies the above-mentioned physical properties. Examples thereof include papers such as paper and Japanese paper, cloths made from various natural and synthetic fibers, and separation and purification. Commercially available filters used for such purposes. In particular, filters are preferably used because their structures are dense and homogeneous. The thickness of the structure is usually 5
２００200 μm, preferably 10-60 μm. When the thickness is less than 5 μm, the strength is reduced and a safety problem is caused. In addition, it becomes too soft, and it becomes difficult to perform positioning and the like during battery production. When the thickness exceeds 200 μm, the resistance of the electrolyte portion increases and the capacity of the battery as a whole decreases. The portion of the structure for the electrolyte layer which comes into contact with the electrolyte may be subjected to a surface treatment in order to control the affinity with the electrolyte.

As a method for causing the electrolyte to be contained in the structure for the electrolyte layer, the same method as that for forming the electrolyte for the ion transfer layer of the positive electrode and the negative electrode can be adopted. Not all of the electrolyte in the electrolyte layer may be incorporated into the electrolyte layer structure. The existence of the electrolyte single layer of a certain thickness on the surface of the electrolyte layer composed of the structure-electrolyte composite can contribute to the reduction of the resistance at the lamination interface between the electrolyte layer and the electrode. Therefore, all of the electrolyte does not need to be taken into the electrolyte layer structure as long as the liquid retention of the entire electrolyte layer is not hindered.

Next, the lithium secondary battery will be described. In a lithium secondary battery, the positive electrode and the negative electrode configured as described above are stacked via an electrolyte layer and housed in a case. As the case, a shape-changeable case having flexibility, flexibility, flexibility, and the like is suitably used.
Examples of the material include a plastic, a polymer film, a metal film, rubber, and a thin metal plate. Specific examples of the case include a bag made of a polymer film such as a plastic bag, a vacuum packaging bag or a vacuum pack made of a polymer film, a vacuum packaging bag or a vacuum pack made of a laminated material of metal foil and a polymer film. , A can formed of plastic, or a case in which the periphery is fixed by welding, bonding, fitting, or the like sandwiched between plastic plates.

In the above, a vacuum packaging bag or a vacuum pack made of a polymer film in terms of airtightness and shape changeability, or a vacuum packaging bag or a vacuum pack made of a laminated material of a metal foil and a polymer film Is preferred. These cases do not have the weight and rigidity of a metal can and are flexible, bendable, and flexible, so they have the advantage of being able to bend after storage of the battery and having the shape freedom and lightening. . The lithium secondary battery of the present invention,
Various shapes such as a cylinder, a box, a paper, and a card can be realized with a light weight. Of course, for convenience such as mounting the battery on the device, it is also possible to enclose the battery in a shape-variable case, deform it into a preferable shape, and then store it in a rigid outer case. As a preferred embodiment, there is a lithium secondary battery in which a positive electrode, an electrolyte layer, and a negative electrode are laminated in a flat plate and housed in a shape-variable case by vacuum sealing.

[0045]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples.
Although the present invention will be described, the present invention is not limited to these.
It is not limited by the embodiment. Example 1 Diethylene glycol ethyl ether acrylate (A
1.80 g, polyethylene glycol
Dimethacrylate (Aldrich, average ethylene glycol
Recall unit number 13.3) 0.60 g, Solulite
(Mitsubishi Chemical's propylene carbonate and ethylene carbonate
LiClO in an equal weight mixture of carbonate _FourTo 1 mol /
17.40 g, dissolved to a concentration of L)
And 0.2 g of benzoyl peroxide (manufactured by Kishida Chemical Co.)
The mixture was weighed into a square flask and mixed under an inert atmosphere. this
After weighing 18 g of the composition in a beaker and sealing, the oven
And heat-treated at 90 ° C. for 20 minutes. The resulting gel-like
Rheometer for compression rupture test of solid (10mm thickness)
(Manufactured by Leotech, RT-2020D / D, diameter 1mm
(Using an elastic rod), the breaking strength was 9
170 g / cm^TwoMet. In addition, the above solid is a platinum electrode
And heat-treated at 90 ° C for 20 minutes.
Conductivity was measured by the impedance method (25 ° C)
At this time, it was 3.7 mS / cm.

Example 2 1.80 g of diethylene glycol ethyl ether acrylate (manufactured by Aldrich), 0.60 g of polyethylene glycol dimethacrylate (manufactured by Aldrich, average number of ethylene glycol units: 8.7), sollite (manufactured by Mitsubishi Chemical Corporation, propylene carbonate) LiClO ₄ in a mixture of equal weights of
17.40 g, dissolved to a concentration of L)
And 0.2 g of benzoyl peroxide (manufactured by Kishida Chemical Co.)
The mixture was weighed into a square flask and mixed under an inert atmosphere. After 18 g of this composition was weighed and sealed in a beaker, it was placed in an oven and heat-treated at 90 ° C. for 20 minutes. When a compression rupture test of the obtained gel-like solid was performed in the same manner as in Example 1,
The breaking strength was 2040 g / cm ² . The conductivity was 3.5 mS / cm.

Comparative Example 1 1.80 g of diethylene glycol ethyl ether acrylate (manufactured by Aldrich), 0.60 g of tetraethylene glycol dimethacrylate (manufactured by Kishida Chemical Co., Ltd., number of ethylene glycol units: 4), sollite (manufactured by Mitsubishi Chemical Corporation, propylene carbonate) 17.40 g of a solution obtained by dissolving LiClO ₄ at a concentration of 1 mol / L in an equal weight mixture of ethylene carbonate and 0.2 g of benzoyl peroxide (manufactured by Kishida Chemical Co., Ltd.) were weighed into a triangular flask. Mix under an inert atmosphere. This composition 18
g in a beaker, sealed and placed in an oven.
Heat treated at 20 ° C. for 20 minutes. When a compression rupture test was performed on the obtained gel-like solid in the same manner as in Example 1, the rupture strength was below the detection limit (200 g / cm ² or less). The conductivity was 3.5 mS / cm.

Example 3 1.20 g of diethylene glycol ethyl ether acrylate (manufactured by Aldrich), 0.60 g of polyethylene glycol diacrylate (manufactured by Polyscience, average number of ethylene glycol units: 9.1), sollite (manufactured by Mitsubishi Chemical Corporation, propylene) LiClO ₄ was added to an equal weight mixture of carbonate and ethylene carbonate at 1 mol / mol.
17.40 g, dissolved to a concentration of L)
And 0.2 g of benzoyl peroxide (manufactured by Kishida Chemical Co.)
The mixture was weighed into a square flask and mixed under an inert atmosphere. After 18 g of this composition was weighed and sealed in a beaker, it was placed in an oven and heat-treated at 90 ° C. for 20 minutes. When a compression rupture test of the obtained gel-like solid was performed in the same manner as in Example 1,
The breaking strength was 1300 g / cm ² . The conductivity was 4.2 mS / cm.

Comparative Example 2 1.20 g of diethylene glycol ethyl ether acrylate (manufactured by Aldrich), 0.60 g of tetraethylene glycol diacrylate (manufactured by Aldrich, number of ethylene glycol units: 4), sollite (manufactured by Mitsubishi Chemical Corporation, propylene carbonate and ethylene) 17.40 g of a solution obtained by dissolving LiClO ₄ at a concentration of 1 mol / L in a mixed solution of an equal weight of carbonate) and 0.2 g of benzoyl peroxide (manufactured by Kishida Chemical Co., Ltd.) were weighed into a triangular flask and unweighed. Mix under an active atmosphere. This composition 18
g in a beaker, sealed and placed in an oven.
Heat treatment at 20 ° C. for 20 minutes did not cure and remained a viscous liquid.

Comparative Example 3 Polyethylene glycol dimethacrylate (manufactured by Aldrich, average number of ethylene glycol units: 13.3)
1.20 g, Sollite (manufactured by Mitsubishi Chemical Corporation, mixed with an equal weight mixture of propylene carbonate and ethylene carbonate)
18.60 g of iClO ₄ dissolved at a concentration of 1 mol / L) and 0.2 g of benzoyl peroxide (manufactured by Kishida Chemical Co., Ltd.) were weighed into a triangular flask and mixed under an inert atmosphere. . After 18 g of this composition was weighed and sealed in a beaker, the composition was placed in an oven and heat-treated at 90 ° C. for 20 minutes. However, the composition was not cured and remained a viscous liquid. Thus, it can be seen that when the bifunctional monomer is used alone, the effect of using the monomer having 7 or more alkylene glycol units is not recognized. Table 1 summarizes the above results.

[0051]

[Table 1]

Reference Examples 1-2 A gel electrolyte was produced and the breaking strength was measured in the same manner as in Example 1 except that the materials shown in Table-2 were used in the amounts shown in Table-2. Table 2 shows the results.

[0053]

[Table 2] * Figures other than breaking strength indicate the amount of orientation by weight.

As is apparent from Table 1, when the polyfunctional monomer is used alone (Comparative Example 3), or when the number of units of the polyfunctional monomer is smaller than 7, even when used together with the monofunctional monomer. It can be seen that (Comparative Examples 1 and 2) cannot obtain a good gel electrolyte. Reference Example 1 in Table-2
Comparing and 2, the polymer concentration was 20 (= 15 +
5) When it is as high as% by weight, the same level of breaking strength is exhibited when the number of units of the polyfunctional monomer is 7 or more or less than 7; It can be seen that the effect is unique when the amount is small.

In the reference example, the reason why the breaking strength is higher than that of the example is simply that the amount of the polymer used is large. Further, it is apparent that the reference example has extremely low ionic conductivity as compared with the example because the concentration of the electrolytic solution is low.

Claims (7)

[Claims] 1. A gel electrolyte in which an electrolyte is held in a polymer. 1) The polymer is a polyalkylene glycol or a polyalkylene sulfide-based polyfunctional monomer having a plurality of polymerizable unsaturated double bonds. And a polymer obtained by polymerizing a monomer containing a monofunctional monomer having one polymerizable unsaturated double bond, and 2) the number of polyalkylene glycol units or polyalkylene sulfide units of the polyfunctional monomer is 3) The weight ratio of the polymer to the gel electrolyte is 1 to 18% on average.
A gel electrolyte, characterized in that: 2. The gel electrolyte according to claim 1, wherein the monofunctional monomer is represented by the following general formula (I). Embedded image (Where m is 1-4; X is an oxygen atom or a sulfur atom;
R ¹ is a hydrogen atom or a lower alkyl group; R ² is a lower alkyl group or an aryl group; a R ³ and R ⁴ are each a hydrogen atom or a lower alkyl group, even more R ³ or R ⁴ is the same respectively May be different) 3. The gel electrolyte according to claim 1, wherein the polyfunctional monomer is represented by the following general formula (II). Embedded image (Provided that R ⁵ is a hydrogen atom or a lower alkyl group, a plurality of R ⁵ may be the same or different from each other; X is an oxygen atom or a sulfur atom; R ⁶ and R ⁷ are each a hydrogen atom or a lower alkyl group) And a plurality of R ⁶ or R ⁷ may be the same or different; n is 7 or more) 4. The gel electrolyte according to claim 1, wherein the electrolyte contains a lithium salt and a solvent. 5. The lithium salt is LiPF ₆ , LiClO.
₄ and gel electrolyte according to claim 4 is at least one selected from the group consisting of CF ₃ SO ₂ Li. 6. The gel electrolyte according to claim 4, wherein the solvent contains at least one selected from the group consisting of dimethyl carbonate, ethylene carbonate, propylene carbonate, diethyl carbonate, and methyl ethyl carbonate. 7. A lithium secondary battery using the gel electrolyte according to any one of claims 4 to 6.