JP7794593B2 - Cured material for lithium ion secondary battery, negative electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Description

This disclosure relates to a cured material for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

In recent years, secondary batteries have come to be used as power sources for electronic devices such as laptops and mobile phones, and the development of hybrid and electric vehicles that use secondary batteries as their power source is progressing with the aim of reducing environmental impact. These vehicles require secondary batteries with high energy density, high voltage, and high durability. Lithium-ion secondary batteries are attracting attention as a secondary battery that can achieve high voltage and high energy density.

However, it is believed that the capacity of currently used lithium-ion secondary batteries, primarily composed of lithium cobalt oxide and carbon, is approaching its limit. For this reason, new active materials for positive and negative electrodes are being developed as alternative approaches to increasing capacity. In particular, for negative electrode active materials, the use of metal materials, such as silicon and tin, that alloy with lithium as an alternative to carbon, such as graphite, has been investigated. This is because the theoretical charge/discharge capacity of graphite is 372 mAh/g, while that of silicon (Li _4.4 Si) is 4199 mAh/g, and that of tin (Li _4.4 Sn) is 993 mAh/g, which are approximately 3 to 10 times larger than that of graphite. However, when metal materials alloy with lithium, the volume of the negative electrode can expand several times as much as the volume before alloying due to the alloying reaction. This causes the negative electrode active material layer to expand and contract violently during charging and discharging, resulting in significant stress on the negative electrode. This leads to problems such as cracks in the negative electrode active material layer formed on the current collector and peeling between the negative electrode active material layer and the current collector, resulting in deterioration of charge-discharge cycle characteristics. Therefore, in order to solve the above problems, cured materials for lithium ion secondary batteries have been proposed that have improved binding properties.

For example, Patent Document 1 proposes a binder composition for lithium secondary batteries that contains a crosslinked compound of polyvinyl acrylic acid substituted with an alkali cation and polyvinyl alcohol, and discloses that the crosslinking rate of the binder composition is approximately 0.1% to approximately 70%.

JP 2012-64574 A

However, lithium ion secondary batteries using the binder composition of Patent Document 1 had the problem of not being able to achieve sufficient charge-discharge cycle characteristics. As a result of our extensive research, we found that the binder composition is easily soluble in aqueous solutions and therefore does not have sufficient elasticity and binding properties to be used as a binder for lithium ion secondary batteries.

In view of the above-mentioned problems, one aspect of the present disclosure aims to provide a cured material for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

In order to solve the above problems, one aspect of the present disclosure provides a cured product for secondary batteries, which is obtained by curing a curable composition, and which has urethane bonds, a water solubility of less than 10% when the cured product is immersed in water, and a liquid absorption rate of -10% or more and less than 5% when the cured product is immersed in an electrolyte solution.

From this perspective, if a cured material for a secondary battery has urethane bonds, and the water solubility when immersed in water is less than 10%, and the liquid absorption when immersed in an electrolyte is between -10% and 5%, it can have excellent elasticity, which can mitigate volumetric changes in the electrode and suppress cracking during charge and discharge. Furthermore, by combining sufficient curability and electrolyte resistance, it is possible to achieve a cured material for a secondary battery with excellent mechanical strength.

Also provided is a curable composition for a lithium ion secondary battery, which comprises a water-soluble polymer (A) having a hydroxyl group, a crosslinking agent (B) having a functional group reactive with the hydroxyl group, and water, and contains 35 parts by weight or more but less than 400 parts by weight of the crosslinking agent (B) per 100 parts by weight of the water-soluble polymer (A).

Also provided is a cured material for a lithium-ion secondary battery, in which the water-soluble polymer (A) includes one of polyvinyl alcohol, carboxymethyl cellulose, and methyl cellulose, and the crosslinking agent (B) is an active methylene-blocked diisocyanate.

Also provided is a cured material for a lithium-ion secondary battery, in which the water-soluble polymer (A) includes one of polyvinyl alcohol, carboxymethyl cellulose, and methyl cellulose, and the crosslinking agent (B) is a polyisocyanate having three or more functional groups.

Also provided is a cured material for lithium-ion secondary batteries, in which the weight-average molecular weight of the polyvinyl alcohol is 500 to 200,000.

From this perspective, the number of hydroxyl groups is sufficient to allow the molecular weight to react with active methylene-blocked diisocyanates and polyisocyanates with three or more functional groups, and the reaction can be easily carried out.

Also provided is a negative electrode for a lithium ion secondary battery, comprising a negative electrode active material, a negative electrode active material layer containing at least the cured material for a lithium ion secondary battery of the present disclosure, and a negative electrode current collector.

From this perspective, it is possible to create a negative electrode with excellent charge-discharge cycle characteristics.

Also provided is a lithium ion secondary battery comprising the above-mentioned negative electrode for lithium ion secondary batteries, a positive electrode, an electrolyte, and a separator.

From this perspective, it is possible to realize a lithium-ion secondary battery with excellent charge-discharge cycle characteristics.

As described above, the present disclosure makes it possible to provide a cured material for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery that exhibit excellent charge-discharge cycle characteristics.

FIG. 1 is a cross-sectional view showing a secondary battery according to an embodiment of the present disclosure.

Preferred embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings.
In this specification and drawings, components having substantially the same functional configurations are denoted by the same reference numerals, and redundant explanations will be omitted.

[Configuration of lithium ion secondary battery]
First, the configuration of a lithium ion secondary battery 100 according to this embodiment will be described with reference to FIG.

The lithium-ion secondary battery 100 comprises a positive electrode 20, a negative electrode 30, and a separator 10. The shape of the lithium-ion secondary battery 100 is not particularly limited. That is, the lithium-ion secondary battery 100 may be cylindrical, rectangular, laminated, button-shaped, or the like.

The negative electrode 30 has a negative electrode current collector 32 and a negative electrode active material layer 34 provided on one surface thereof.

The negative electrode current collector 32 can be any conductive plate material, such as a thin metal plate made of copper or nickel foil.

The negative electrode active material used in the negative electrode active material layer 34 may be silicon (Si) or a silicon compound. Two or more of these may be used in combination.

Examples of silicon compounds include compounds containing silicon and oxygen as constituent elements, represented by SiO _x , where x is preferably in the range of 0.5≦x≦1.5 (hereinafter referred to as "SiO _x ").

The SiO _x may contain a microcrystalline or amorphous phase of Si, in which case the atomic ratio of Si to O includes the Si in the microcrystalline or amorphous phase of Si. That is, SiO _x includes a structure in which Si (e.g., microcrystalline Si) is dispersed in an amorphous SiO ₂ matrix, and the atomic ratio x of this amorphous SiO ₂ and the Si dispersed therein together satisfies 0.5≦x≦1.5. For example, in the case of a compound in which Si is dispersed in an amorphous SiO ₂ matrix and the molar ratio of SiO ₂ to Si is 1:1, x = 1, so the structural formula is represented as SiO.

Furthermore, when SiO _x is used, it is preferable that its surface is coated with carbon. Because SiO _x has poor electrical conductivity, when used as a negative electrode active material, in order to ensure good battery characteristics, it is necessary to use a negative electrode conductive additive and improve the mixing and dispersion of SiO _x and the negative electrode conductive additive in the negative electrode active material layer to form an excellent conductive network. Therefore, if the surface of SiO _x is coated with carbon, a better conductive network is formed in the negative electrode than, for example, when SiO _x and the negative electrode conductive additive are simply mixed and used.

The negative electrode active material layer 34 may also contain a negative electrode conductive additive. Examples of negative electrode conductive additives include carbon powders such as carbon black, carbon nanotubes, carbon materials, metal fine powders such as copper, nickel, stainless steel, and iron, mixtures of carbon materials and metal fine powders, and conductive oxides such as indium tin oxide (ITO). If sufficient conductivity can be ensured with the negative electrode active material alone, the negative electrode active material layer 34 does not need to contain a negative electrode conductive additive.

The negative electrode active material layer 34 also contains a negative electrode cured material. The negative electrode cured material bonds the negative electrode active material together and also bonds the negative electrode active material to the negative electrode current collector 32. The negative electrode cured material is a cured material in which urethane bonds are formed by curing a curable composition.

Furthermore, the cured product in which urethane bonds have been formed has a water solubility of less than 10% when immersed in water, and a liquid absorption rate of -10% or more and less than 5% when immersed in an electrolyte solution.

The water solubility rate when immersed in water can be calculated using the following formula by measuring the weight of the cured product before and after immersion in water.

Water solubility (%) = 100 - [weight of cured product after immersion in water / weight of cured product before immersion in water x 100]

The weight of the cured product was measured before immersion in water, and then it was immersed in water. The immersion in water was continued at 25°C for one week. The cured product was then removed from the water and vacuum dried at 85°C for three hours, and the weight of the cured product after immersion was measured.

In a cured product in which urethane bonds have formed, if the water solubility is less than 10%, the crosslinking reaction will have progressed sufficiently, resulting in sufficient urethane bond formation and making it possible to suppress the occurrence of cracks that occur with repeated charging and discharging. On the other hand, if the temperature is within the typical range of 20 to 95°C, where the base reacts, and the P water solubility is 10% or more, the above effect will not be achieved.

The liquid absorption rate when immersed in an electrolyte solution can be calculated using the following formula by measuring the weight of the cured product before and after immersion in the electrolyte solution.

Liquid absorption rate (%) = [(weight of cured material after immersion in electrolyte - weight of cured material before immersion in electrolyte) / weight of cured material before immersion in electrolyte] x 100

As the electrolyte for determining the liquid absorption rate, for example, an electrolyte solution is used in which _LiPF6 is dissolved as an electrolyte to a concentration of 1 mol/L in a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of EC:DEC = 3:7.

When calculating the liquid absorption rate, the material is immersed in the electrolyte at 25°C for one week. After one week of immersion at 25°C, the cured material will reach a saturated state of liquid absorption. Note that saturated state of liquid absorption refers to the state in which the weight of the cured material does not increase even if it is further immersed in the electrolyte.

When the cured product in which urethane bonds have been formed has a liquid absorption rate of -10% or more but less than 5%, the cured product does not dissolve or swell in the electrolyte, and can therefore function as a cured product. However, if the liquid absorption rate is less than -10%, the cured product dissolves in the electrolyte, and the above-mentioned effect is not achieved. Furthermore, if the liquid absorption rate is 5% or more, the cured product will have absorbed the electrolyte, and will not be resistant to the electrolyte, will swell, and the above-mentioned effect will not be achieved.

The curable composition according to this embodiment contains a water-soluble polymer (A) having a hydroxyl group, a crosslinking agent (B) having a functional group that reacts with the hydroxyl group, and water. The curable composition contains 35 parts by weight or more and less than 400 parts by weight of the crosslinking agent (B) per 100 parts by weight of the water-soluble polymer (A). Each component will be explained below using examples.

<Water-soluble polymer (A)>
Examples of the water-soluble polymer (A) having a hydroxyl group (hereinafter sometimes abbreviated as "water-soluble polymer (A)") include polyvinyl alcohol, carboxymethyl cellulose, methyl cellulose, etc. Among these, polyvinyl alcohol is preferred.
These water-soluble polymers (A) may be used singly or in combination of two or more.
In this specification, a polymer being "water-soluble" means that when a mixture obtained by adding 1 part by weight of polymer (equivalent to the solid content) to 100 parts by weight of ion-exchanged water and stirring is adjusted to at least one of the following conditions: a temperature in the range of 20 to 95°C and a pH in the range of 3 to 12 (pH adjustment is performed using an aqueous NaOH solution and/or an aqueous HCl solution), and the mixture is passed through a 250-mesh screen, the weight of the solid content of the residue that does not pass through the screen and remains on the screen does not exceed 50% by weight of the solid content of the polymer added. Note that even if the mixture of the polymer and water is in an emulsion state that separates into two phases when left to stand, the polymer is considered to be water-soluble if it satisfies the above definition.

When polyvinyl alcohol is used as the water-soluble polymer (A), there is no particular limitation, but for example, polyvinyl alcohol obtained by saponifying polyvinyl acetate is preferred.
Examples of polyvinyl alcohol include vinyl alcohol-vinyl acetate copolymer, vinyl alcohol-vinyl butyral copolymer, and ethylene-vinyl alcohol copolymer, with vinyl alcohol-vinyl acetate copolymer being preferred. The weight-average molecular weight of polyvinyl alcohol is preferably 500 to 200,000. If the weight-average molecular weight is less than 500, the number of hydroxyl groups is small, resulting in a small number of urethane bonds formed by reaction with the crosslinking agent (B) described below. Therefore, when used as a cured product for lithium ion secondary batteries, excellent charge-discharge cycle characteristics cannot be obtained. On the other hand, if the weight-average molecular weight exceeds 200,000, the viscosity increases, resulting in gelation. Therefore, when the crosslinking agent (B) described below is added, it becomes difficult to achieve a uniform reaction. Furthermore, the viscosity of the cured product for lithium ion secondary batteries is too high, making it difficult to handle as a cured product.

The copolymerization ratio of polyvinyl alcohol is expressed by the degree of saponification. The degree of saponification of polyvinyl alcohol according to this embodiment is preferably 60 mol% or more and 99 mol% or less. When the degree of saponification of polyvinyl alcohol is 60 mol% or more, it can more effectively form a crosslinked structure with the crosslinking agent (B) described below. The degree of saponification of polyvinyl alcohol can be determined by the amount of alkali consumption required for hydrolysis of copolymerization units such as vinyl acetate or by composition analysis using NMR.

<Crosslinking agent (B)>
The crosslinking agent (B) having a functional group reactive with the water-soluble polymer (A) having a hydroxyl group (hereinafter sometimes abbreviated as "crosslinking agent (B)") may be an active methylene-blocked diisocyanate or a polyisocyanate having three or more functional groups, in which the isocyanate group is protected with a blocking agent. Representative compounds of the blocking agent and the isocyanate are listed separately here.

Blocking agents include phenols, alcohols, oximes, lactams, etc.

Diisocyanate compounds include aromatic diisocyanates having 6 to 20 carbon atoms (excluding carbon atoms in NCO groups, the same applies below), aliphatic diisocyanates having 2 to 18 carbon atoms, alicyclic diisocyanates having 4 to 15 carbon atoms, araliphatic diisocyanates having 8 to 15 carbon atoms, modified products of these diisocyanates (carbodiimide-modified products, urethane-modified products, uretdione-modified products, etc.), and mixtures of two or more of these.

Specific examples of the aromatic diisocyanates include 1,3- and/or 1,4-phenylene diisocyanate, 2,4- and/or 2,6-tolylene diisocyanate, m-xylylene diisocyanate, 2,4'- and/or 4,4'-diphenylmethane diisocyanate (hereinafter, diphenylmethane diisocyanate will be abbreviated as MDI), 4,4'-diisocyanatobiphenyl, 3,3'-dimethyl-4,4'-diisocyanatobiphenyl, 3,3'-dimethyl-4,4'-diisocyanatodiphenylmethane, and 1,5-naphthylene diisocyanate.

Specific examples of the aliphatic diisocyanates include ethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, dodecamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, lysine diisocyanate, 2,6-diisocyanatomethyl caproate, bis(2-isocyanatoethyl)carbonate, and 2-isocyanatoethyl-2,6-diisocyanatohexanoate.

Specific examples of the above-mentioned alicyclic diisocyanates include isophorone diisocyanate, dicyclohexylmethane-4,4'-diisocyanate, cyclohexylene diisocyanate, 1,3-bis(isocyanatomethyl)cyclohexane, methylcyclohexylene diisocyanate, bis(2-isocyanatoethyl)-4-cyclohexylene-1,2-dicarboxylate, and 2,5- and/or 2,6-norbornane diisocyanate.

Specific examples of the above-mentioned araliphatic diisocyanates include m- and/or p-xylylene diisocyanate, α,α,α',α'-tetramethylene xylylene diisocyanate, etc.

The tri- or higher functional polyisocyanate compound is not particularly limited as long as it has 3 to 8 isocyanate groups, but examples include compounds containing the chemical structures of triisocyanate, tetraisocyanate, isocyanurate, and biuret.

Specific examples of triisocyanate compounds include compounds represented by the following chemical formula (1):

Specific examples of tetraisocyanate compounds include compounds represented by the following chemical formula (2):

[In the formula, R1 represents an alkylene group.]

Examples of compounds having an isocyanurate structure include an isocyanurate trimer and an isocyanurate pentamer, and also include isocyanurate heptamer, nonamer, and higher polymers.
The isocyanurate trimer is a polyisocyanate consisting of three diisocyanate monomer molecules and having an isocyanurate group, and examples thereof include a compound represented by the following chemical formula (3).

[Chemical 3]

[In the formula, R represents the residue obtained by removing one isocyanate group from a diisocyanate monomer.]

An isocyanurate pentamer is a polyisocyanate having an isocyanurate structure and consisting of six diisocyanate monomer molecules, and examples include compounds represented by the following chemical formula (4):

[In the formula, R represents the residue obtained by removing one isocyanate group from a diisocyanate monomer.]

Compounds with a biuret structure are formed from urea and isocyanate groups, and examples include compounds represented by the following chemical formula (5):

[In the formula, R represents the residue obtained by removing one isocyanate group from a diisocyanate monomer.]

Among these, aromatic diisocyanates, alicyclic diisocyanates, and triisocyanates whose parent structure is a biuret structure are preferred, and aromatic diisocyanates and triisocyanates whose parent structure is a biuret structure are even more preferred, with MDI and HDI biuret forms being particularly preferred.

The curable composition according to this embodiment preferably contains 35 to 400 parts by weight of crosslinking agent (B) per 100 parts by weight of water-soluble polymer (A), more preferably 38 to 360 parts by weight. By maintaining the crosslinking agent content within this range, a favorable crosslinked structure is formed and excellent elasticity and binding properties of the cured product are ensured. When the crosslinking agent (B) content is less than 35 parts by weight, the number of crosslinking points that react with the hydroxyl groups in the water-soluble polymer (A) is small, resulting in reduced elasticity. Therefore, the composition is unable to withstand volume changes during charge and discharge, and cracking cannot be suppressed. On the other hand, when the crosslinking agent (B) content is 400 parts by weight or more, the number of crosslinking points that react with the hydroxyl groups in the water-soluble polymer (A) is large, resulting in excellent elasticity. However, the number of free hydroxyl groups not involved in crosslinking is very small, resulting in poor binding properties with the negative electrode active material and current collector. Furthermore, the composition may become less dispersible in water or become thicker, potentially resulting in gelation.

The curing temperature of the cured product obtained by curing the curable composition cannot be generally specified because it varies depending on the type of blocking agent used to protect the isocyanate groups. However, it is typically performed at a temperature of 90 to 180°C for 1 to 3 hours. To reduce the effects of moisture and oxygen in the air, the curing is performed under an inert atmosphere such as nitrogen, resulting in the formation of a crosslinked structure containing urethane bonds. As a result, crosslinking between the water-soluble polymers (A) results in a crosslinked structure with excellent elasticity and binding properties, as well as low solubility in water and low liquid absorption when immersed in an electrolyte solution. Therefore, using the curable composition according to this embodiment in a cured product for a lithium-ion secondary battery can effectively bind components in the electrode member (e.g., negative electrode active material) while improving the charge-discharge cycle characteristics of the lithium-ion secondary battery. Specifically, the crosslinked structure containing urethane bonds provides excellent elasticity, thereby mitigating volumetric changes in the negative electrode active material layer and suppressing cracking associated with repeated charge-discharge cycles. In addition, free hydroxyl groups that are not involved in crosslinking interact with the negative electrode active material, improving binding properties and ensuring strong adhesion between the negative electrode active material layer and the negative electrode current collector.

The composition ratio of the negative electrode cured material in the negative electrode active material layer 34 is preferably in the range of 0.5 to 20% by weight, more preferably 5 to 20% by weight.

The negative electrode active material layer 34 is formed, for example, by dispersing the negative electrode active material, negative electrode conductive additive, and negative electrode cured material in water as a medium to prepare a coating liquid, applying this coating liquid to the negative electrode current collector 32, drying, curing at 150°C in a nitrogen atmosphere for 1 to 3 hours, and rolling as necessary.

The positive electrode 20 has a positive electrode current collector 22 and a positive electrode active material layer 24 provided on one surface of the positive electrode current collector 22. The positive electrode current collector 22 may be made of any conductive material, such as a thin metal plate made of aluminum or nickel foil.

As the positive electrode active material, a material capable of absorbing and desorbing lithium ions, such as a lithium-containing compound such as lithium oxide, lithium sulfide, or an intercalation compound containing lithium, is suitable, and two or more of these may be mixed and used. In particular, to increase the energy density, a lithium composite oxide represented by the general formula Li _x MO ₂ or an intercalation compound containing lithium is preferred. Note that M is preferably one or more transition metals, specifically, at least one of cobalt (Co), nickel (Ni), manganese (Mn), iron (Fe), aluminum (Al), vanadium (V), and titanium (Ti) is preferred. x varies depending on the charge/discharge state of the battery and is typically a value within the range of 0.05≦x≦1.10. Other materials that can achieve high energy density include manganese spinel (LiMn ₂ O ₄ ) having a spinel-type crystal structure and lithium iron phosphate (LiFePO ₄ ) having an olivine-type crystal structure, and are therefore also preferred.

Specific examples include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), LiNi _x Co _y Mn _z M _a O ₂ (x+y+z+a=1, 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦a≦1, and M represents one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), lithium vanadium compound (LiV ₂ O ₅ ), olivine-type LiMPO ₄ (wherein M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr, or VO), lithium titanate (Li ₄ Ti ₅ O ₁₂ ), and LiNi _x Co _y Al _z O ₂ (0.9<x+y+z<1.1), etc. Furthermore, the present invention is not limited to these materials, and there are no particular limitations on the positive electrode active material as long as it is a material that electrochemically inserts and extracts lithium ions.

Suitable positive electrode cured materials for the positive electrode 20 include solvent-based cured materials such as polyvinylidene fluoride (PVDF), polyimide, polyurethane, ethylene vinyl alcohol, and polyacrylate, as well as water-based cured materials such as carboxymethyl cellulose (CMC) and styrene-butadiene copolymer.

A positive electrode conductive additive may be added to improve conductivity. Examples of positive electrode conductive additives include carbon powders such as carbon black, carbon nanotubes, carbon materials, metal fine powders such as copper, nickel, stainless steel, and iron, mixtures of carbon materials and metal fine powders, and conductive oxides such as ITO. If sufficient conductivity can be ensured with the positive electrode active material alone, the positive electrode active material layer 24 does not need to contain a positive electrode conductive additive.

The positive electrode active material layer 24 is formed, for example, by preparing a coating liquid by dispersing the positive electrode active material, positive electrode conductive additive, and positive electrode cured material in an appropriate organic solvent (e.g., N-methyl-2-pyrrolidone), applying this coating liquid to the positive electrode current collector 22, drying it, and rolling it as necessary.

The separator 10 may be formed from an electrically insulating porous structure, such as a single layer or laminate of a film made of polyethylene, polypropylene, or polyolefin, or a stretched film made of a mixture of the above resins, or a fibrous nonwoven fabric made of at least one constituent material selected from the group consisting of cellulose, polyester, and polypropylene.

The separator 10 may also be coated with inorganic particles or polymer components. Examples of inorganic particles include oxides such as alumina, silica, zirconium oxide, titanium oxide, and magnesium oxide, as well as dielectric materials such as barium titanate. Examples of polymer components include cured materials for negative electrodes, cured materials for positive electrodes, polymer electrolytes (composites of lithium salts with polymeric materials such as polyethylene glycol and polyethylene carbonate), ion exchange resins (polydimethyldiallylammonium salt, polystyrene sulfonate, etc.), and others such as polyvinyl alcohol, CMC, polybutyral, and polyacrylic acid.

The electrolyte solution contains a lithium salt as an electrolyte and a solvent. Examples of the lithium salt include _LiPF6 , _LiBF4 , LiClO4, _LiSO3CF3 , LiN( _SO2F ) _{2 ,} LiN( _SO2CF3 ) ₂ , LiN( _SO2F )( _SO2CF3 ), LiN _{( SO2CF2CF3} ) ₂ , _{LiC (SO2CF2CF3)3 , LiC} ( _{SO2CF3 ) 3 ,} LiI, LiCl, LiF _{, LiPF5} ( _{SO2CF3 )} , _{and LiPF4} ( _{SO2CF3 ) 2 .}

The concentration of the lithium salt is preferably approximately 0.8 to 5.0 mol/L.

Examples of solvents include dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, di-n-propyl carbonate, methyl-n-propyl carbonate, methyl isopropyl carbonate, ethyl-n-propyl carbonate, ethyl isopropyl carbonate, diisopropyl carbonate, 3-fluoropropyl methyl carbonate, propylene carbonate, ethylene carbonate, butylene carbonate, 4-chloro-1,3-dioxolan-2-one, 4-fluoro-1,3-dioxolan-2-one, 4-trifluoromethyl-1, Carbonate esters such as 3-dioxolan-2-one, vinylene carbonate, dimethylvinylene carbonate, vinylene carbonate, and fluoroethylene carbonate; carboxylic acid esters such as methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, methyl butyrate, methyl isobutyrate, methyl trimethylacetate, and ethyl trimethylacetate; cyclic esters such as γ-butyrolactone and γ-valerolactone; chain sulfonate esters such as dimethyl sulfoxide and dimethyl sulfite; cyclic sulfonate esters such as sulfolane and propane sultone; acetonitrile, glutaroni nitrile compounds such as tolyl, adiponitrile, methoxyacetonitrile, 3-methoxypropionitrile, and succinonitrile; chain ethers such as 1,2-dimethoxyethane, dimethyl ether, methyl ethyl ether, diethyl ether, butyl methyl ether, dipropyl ether, cyclopentyl methyl ether, dibutyl ether, diisopentyl ether, triglyme, and tetraglyme; cyclic ethers such as oxetane, tetrahydrofuran, tetrahydropyran, 1,3-dioxolane, and 1,4-dioxane; Examples of suitable solvents include hydrofluoroethers such as 2,3,3-tetrafluoropropyl ether, phosphate esters such as triethyl phosphate and trimethyl phosphate, phosphonate esters such as dimethyl methylphosphonate, and fluorinated ethers such as 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, 2,2,2-trifluoroethyl ether, difluoromethyl 2,2,3,3-tetrafluoropropyl ether, and 1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-(trifluoromethyl)pentane. These may be used alone or in combination. From the viewpoint of solubility of lithium salts, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, triglyme, tetraglyme, and acetonitrile are preferred.

The solvent may also contain an ionic liquid. The ionic liquid may be a compound containing a cation species and an anion species that is liquid at temperatures between -30°C and 120°C.

Cation species that can be used include nitrogen-based cations containing nitrogen, phosphorus-based cations containing phosphorus, and sulfur-based cations containing sulfur. These cation components may be used alone or in combination. Examples of nitrogen-based cations include linear or cyclic ammonium cations such as imidazolium cation, pyrrolidinium cation, piperidinium cation, pyridinium cation, and azoniaspiro cation. Examples of phosphorus-based cations include linear or cyclic phosphonium cations. Examples of sulfur-based cations include linear or cyclic sulfonium cations.

アニオン種としては、ＡｌＣｌ _４ ^－ 、ＮＯ _２ ^－ 、ＮＯ _３ ^－ 、Ｉ ^－ 、ＢＦ _４ ^－ 、ＰＦ _６ ^－ 、ＡｓＦ _６ ^－ 、ＳｂＦ _６ ^－ 、ＮｂＦ _６ ^－ 、ＴａＦ _６ ^－ 、Ｆ（ＨＦ） _２．３ ^－ 、ＣＨ _３ ＣＯ _２ ^－ 、ＣＦ _３ ＣＯ _２ ^－ 、ＣＨ _３ ＳＯ _３ ^－ 、ＣＦ _３ ＳＯ _３ ^－ 、（ＣＦ _３ ＳＯ _２ ） _３ Ｃ ^－ 、Ｃ _３ Ｆ _７ ＣＯ _２ ^－ 、Ｃ _４ Ｆ _９ ＳＯ _３ ^－ 、（ＣＦ _３ ＳＯ _２ ）（ＣＦ _３ ＣＯ）Ｎ ^－ 、（ＣＮ） _２ Ｎ ^－ 、次式で表されるイミドアニオン（（ＳＯ _２ （ＣＦ _２ ) _x F)(SO ₂ (CF ₂ ) _y F)N ⁻ (wherein x and y are each independently an integer of 0 to 5.) These anion species may be used alone or in combination of two or more.

From the viewpoint of the solubility of the lithium salt, the cationic component is preferably a pyrrolidinium cation or a piperidinium cation, and the anionic component is preferably an imide anion, PF ₆ ⁻ , or BF ₄ ⁻ anion, with (SO ₂ F) ₂ N ⁻ , (SO ₂ CF ₃ ) ₂ N ⁻ , or (SO ₂ CF ₃ )(SO ₂ F) N ⁻ being more preferred.

The electrolyte may contain various additives (e.g., SEI (Solid Electrolyte Interface) forming agents, surfactants, etc.). Examples of such additives include vinylene carbonate, vinylethylene carbonate, phenylethylene carbonate, succinic anhydride, lithium bisoxalate, lithium tetrafluoroborate, dinitrile compounds, propane sultone, butane sultone, propene sultone, 3-sulfolene, fluorinated allyl ethers, and fluorinated acrylates.

The exterior body 50 seals the laminate 40 and the electrolyte solution inside. There are no particular limitations on the exterior body 50, as long as it can prevent the electrolyte solution from leaking to the outside and the intrusion of moisture and other foreign matter into the secondary battery 100.

For example, as shown in Figure 1, the exterior body 50 can be a metal laminate film in which a metal foil 52 is coated on both sides with a polymer film 54. The metal foil 52 can be, for example, aluminum foil, and the polymer film 54 can be a film such as polypropylene. For example, the outer polymer film 54 is preferably made of a polymer with a high melting point, such as polyethylene terephthalate (PET) or polyamide, while the inner polymer film 54 is preferably made of polyethylene (PE), polypropylene (PP), or the like.

The leads 60, 62 are made of a conductive additive such as aluminum. Then, using a known method, the leads 60, 62 are welded to the positive electrode current collector 22 and the negative electrode current collector 32, respectively. The separator 10 is sandwiched between the positive electrode active material layer 24 of the positive electrode 20 and the negative electrode active material layer 30 and 34. The separator 10 is then inserted into the exterior body 50 together with the electrolyte, and the entrance of the exterior body 50 is sealed to produce the secondary battery 100.

The present disclosure will be explained in more detail below using examples. For Examples 1 to 81, curable compositions were prepared, the water solubility and liquid absorption rate of the cured products were measured, and negative electrodes and lithium ion secondary batteries were fabricated using the compositions, and the charge/discharge cycle characteristics of the lithium ion secondary batteries were evaluated, as described below. For comparison, Comparative Examples 1 to 15 were also fabricated and evaluated in the same manner. The results are shown in Tables 1 and 2.

Example 1
[Preparation of curable composition]
100 g of carboxymethyl cellulose (water-soluble polymer A) was added to water, the temperature was raised from room temperature to 95°C, and the mixture was stirred until completely dissolved. The carboxymethyl cellulose aqueous solution was then cooled to room temperature, and 33.0 g of blocked diphenylmethane diisocyanate (crosslinking agent B) was added and stirred to obtain a curable composition (the water was adjusted to a solids concentration of 30 wt%). The curable composition was cured at 150°C for 1 hour under a nitrogen atmosphere to obtain a cured product with urethane bonds. The water solubility of the cured product according to Example 1 was 9%, and the liquid absorption rate was -8.8%.

[Fabrication of negative electrode active material layer and negative electrode]
A coating solution was prepared by dispersing 25 g of _SiOx (a negative electrode active material that had been subjected to a disproportionation reaction by heat treatment at 1000 °C under reduced pressure), 1.4 g of Super-P (registered trademark) as a negative electrode conductive additive, and 4.5 g of the curable composition of the negative electrode cured product (solid content concentration 30%) in water. The total solid content concentration in the coating solution was 35 wt%. The coating solution was then applied to a copper foil negative electrode current collector using a doctor blade, vacuum dried at 100 °C for 2 hours, cured at 150 °C for 1 hour in a nitrogen atmosphere, and rolled to form a negative electrode active material layer 34. A 22 × 32 mm electrode size was punched out using a mold to prepare a negative electrode.

[Preparation of Positive Electrode]
A coating liquid was prepared by mixing and dispersing 96 wt% of lithium cobalt oxide (LiCoO ₂ ) as the positive electrode active material, 2 wt% of Ketjen black as a conductive additive, 2 wt% of PVDF as a cured material, and N-methyl-2-pyrrolidone as a solvent. The coating liquid was then applied to an aluminum foil positive electrode current collector using a doctor blade, and the resulting mixture was vacuum dried at 100°C for 2 hours and rolled to form a positive electrode active material layer 24. A mold was used to punch out an electrode size of 20 x 30 mm, to prepare a positive electrode.

[Fabrication of Lithium-ion Secondary Battery]
The prepared negative electrode and positive electrode were laminated via a 16 μm-thick polyethylene separator measuring 24 × 35 mm to produce an electrode assembly. Furthermore, in the negative electrode of the electrode assembly, a nickel negative electrode lead was attached to the protruding end of the copper foil where the negative electrode active material layer was not provided, while in the positive electrode, an aluminum positive electrode lead was attached to the protruding end of the aluminum foil where the positive electrode active material layer was not provided using an ultrasonic fusion machine. The electrode assembly was then fused to an aluminum laminate film for the exterior body, and the laminate film was folded to insert the electrode assembly into the exterior body. A closed opening was formed by heat-sealing the exterior body except for one side, and an electrolyte solution prepared to a concentration of 1 mol/L, containing LiPF ₆ , in a mixed solvent of EC:DEC in a ratio of 3:7, was injected through this opening. The opening of the exterior body was then heat-sealed while reducing the pressure using a vacuum sealer, to produce a lithium-ion secondary battery.

[Charge/discharge cycle characteristics]
The lithium ion secondary battery was repeatedly charged and discharged under the charge and discharge test conditions shown below to evaluate its charge and discharge cycle characteristics. The charge and discharge cycle test conditions were as follows: at a temperature of 25°C, the battery was charged at a constant current of 0.5 C to 4.2 V (CC-CV charging), and the charge capacity was measured. Thereafter, the battery was discharged at a constant current of 1.0 C (CC discharge) until the battery voltage reached 2.5 V, and the discharge capacity was measured. This constituted one cycle, and the charge and discharge cycle characteristics were evaluated based on the discharge capacity retention rate after 100 cycles.

Note that 0.5C refers to the current value at which charging and discharging are completed in two hours when a battery cell with a nominal capacity is charged or discharged at a constant current. In the case of 1C, this means the current value at which charging and discharging are completed in one hour.

The discharge capacity retention rate after 100 cycles is calculated using the following formula: Discharge capacity retention rate after 100 cycles (%) = (discharge capacity after 100 cycles / discharge capacity after 1 cycle) x 100

Example 2
The same procedure was carried out as in Example 1, except that in compounding the curable composition, blocked diphenylmethane diisocyanate was replaced with blocked 1,3-bis(isocyanatomethyl)cyclohexane as the crosslinking agent (B). The water solubility of the cured product in Example 2 was 7%, and the liquid absorption rate was -9%.

Example 3
The curable composition was prepared in the same manner as in Example 1, except that the water-soluble polymer (A) was changed from carboxymethyl cellulose to methyl cellulose, and the crosslinking agent (B) was changed from blocked diphenylmethane diisocyanate to blocked xylylene diisocyanate. The water solubility of the cured product in Example 3 was 8%, and the liquid absorption rate was 4%.

Example 4
The same procedure as in Example 1 was carried out except that in compounding the curable composition, carboxymethyl cellulose was replaced with methyl cellulose as the water-soluble polymer (A). The water solubility of the cured product in Example 4 was 8% and the liquid absorption rate was −9.2%.

Example 5
The same procedure as in Example 3 was carried out to prepare the curable composition, except that methyl cellulose was replaced with polyvinyl alcohol having a weight-average molecular weight of 500 as the water-soluble polymer (A). The water solubility of the cured product in Example 5 was 7%, and the liquid absorption rate was −4.7%.

Example 6
The same procedure was carried out as in Example 5, except that in the preparation of the curable composition, the water-soluble polymer (A) was changed from polyvinyl alcohol having a weight-average molecular weight of 500 to polyvinyl alcohol having a weight-average molecular weight of 2,000, and the crosslinking agent (B) was changed from blocked xylylene diisocyanate to blocked diphenylmethane diisocyanate. The water solubility of the cured product in Example 6 was 6%, and the liquid absorption rate was -3.9%.

Example 7
The same procedure as in Example 6 was carried out to prepare a curable composition, except that as the water-soluble polymer (A), polyvinyl alcohol having a weight-average molecular weight of 2,000 was used instead of polyvinyl alcohol having a weight-average molecular weight of 30,000. The cured product of Example 7 had a water solubility of 6% and a liquid absorption rate of -5%.

(Examples 8 to 11)
The same procedure as in Example 2 was repeated to prepare the curable compositions, except that the amount of crosslinking agent (B) added was changed from 33.0 g to 44.0 g, 88.0 g, 176.0 g, and 352.0 g. The water solubility of the cured product of Example 8 was 7%, and the liquid absorption rate was -7.7%. The water solubility of the cured product of Example 9 was 6%, and the liquid absorption rate was -6%. The water solubility of the cured product of Example 10 was 6%, and the liquid absorption rate was 4%. The water solubility of the cured product of Example 11 was 5%, and the liquid absorption rate was 4.8%.

Example 12
The same procedure was carried out as in Example 4, except that the amount of crosslinking agent (B) added in compounding the curable composition was changed from 33.0 g to 380 g. The water solubility of the cured product in Example 12 was 1%, and the liquid absorption rate was 3%.

Example 13
The curable composition was prepared in the same manner as in Example 5, except that the crosslinking agent (B) was changed from blocked xylylene diisocyanate to blocked diphenylmethane diisocyanate and the amount of crosslinking agent (B) added was changed from 33.0 g to 44.0 g. The cured product of Example 13 had a water solubility of 7% and a liquid absorption rate of 3%.

(Examples 14 to 15)
The same procedure as in Example 6 was repeated to prepare the curable compositions, except that the amount of crosslinking agent (B) added was changed from 33.0 g to 44.0 g and 88.0 g, respectively. The cured products of Example 14 and Example 15 had water solubility of 5% and liquid absorption of 2.5%, respectively.

(Examples 16 to 20)
The same procedure as in Example 7 was repeated to prepare the curable compositions, except that the amount of crosslinking agent (B) added was changed from 33.0 g to 44.0 g, 88.0 g, 176.0 g, 352.0 g, and 400.0 g. The cured product of Example 16 had a water solubility of 3% and a liquid absorption rate of -8.5%. The cured product of Example 17 had a water solubility of 6% and a liquid absorption rate of -3.8%. The cured product of Example 18 had a water solubility of 4% and a liquid absorption rate of 3.6%. The cured product of Example 19 had a water solubility of 1% and a liquid absorption rate of -0.7%. The cured product of Example 20 had a water solubility of 3% and a liquid absorption rate of 0.5%.

(Example 21)
The same procedure was carried out as in Example 13, except that in compounding the curable composition, blocked diphenylmethane diisocyanate was replaced with blocked 1,3-bis(isocyanatomethyl)cyclohexane as the crosslinking agent (B). The water solubility of the cured product in Example 21 was 1%, and the liquid absorption rate was 1%.

Example 22
The same procedure was carried out as in Example 14, except that in compounding the curable composition, blocked diphenylmethane diisocyanate was replaced with blocked 1,3-bis(isocyanatomethyl)cyclohexane as the crosslinking agent (B). The water solubility of the cured product in Example 22 was 5%, and the liquid absorption rate was 2.2%.

(Example 23)
The same procedure was carried out as in Example 15, except that in compounding the curable composition, blocked diphenylmethane diisocyanate was replaced with blocked 1,3-bis(isocyanatomethyl)cyclohexane as the crosslinking agent (B). The water solubility of the cured product in Example 23 was 4%, and the liquid absorption rate was 1.2%.

(Example 24)
The same procedure was carried out as in Example 16, except that in compounding the curable composition, blocked diphenylmethane diisocyanate was replaced with blocked 1,3-bis(isocyanatomethyl)cyclohexane as the crosslinking agent (B). The water solubility of the cured product in Example 24 was 3%, and the liquid absorption rate was -8%.

(Examples 25 to 27)
The same procedure as in Example 24 was repeated to prepare the curable compositions, except that the amount of crosslinking agent (B) added was changed from 44.0 g to 88.0 g, 176.0 g, and 352.0 g. The cured product of Example 25 had a water solubility of 4% and a liquid absorption rate of -4%. The cured product of Example 26 had a water solubility of 3% and a liquid absorption rate of 3.2%. The cured product of Example 27 had a water solubility of 1% and a liquid absorption rate of 0.3%.

(Example 28)
The curable composition was prepared in the same manner as in Example 5, except that the water-soluble polymer (A) was changed from polyvinyl alcohol having a weight-average molecular weight of 500 to polyvinyl alcohol having a weight-average molecular weight of 70,000, and the amount of crosslinking agent (B) added was changed from 33.0 g to 352.0 g. The cured product of Example 28 had a water solubility of 2% and a liquid absorption rate of -1.3%.

Example 29
The curable composition was prepared in the same manner as in Example 21, except that the water-soluble polymer (A) was changed from polyvinyl alcohol having a weight-average molecular weight of 500 to polyvinyl alcohol having a weight-average molecular weight of 160,000, and the amount of crosslinking agent (B) added was changed from 44.0 g to 400.0 g. The cured product of Example 29 had a water solubility of 3% and a liquid absorption rate of 0.8%.

(Example 30)
The same procedure as in Example 20 was carried out to prepare a curable composition, except that the water-soluble polymer (A) was changed from polyvinyl alcohol having a weight-average molecular weight of 30,000 to polyvinyl alcohol having a weight-average molecular weight of 200,000. The cured product of Example 30 had a water solubility of 3% and a liquid absorption rate of 1.2%.

(Example 31)
The same procedure as in Example 7 was repeated to prepare a curable composition, except that the crosslinking agent (B) was changed from blocked diphenylmethane diisocyanate to blocked xylylene diisocyanate and the amount of crosslinking agent (B) added was changed from 33.0 g to 38.0 g. The water solubility of the cured product in Example 31 was 9% and the liquid absorption rate was −0.2%.

(Example 32)
The same procedure was carried out as in Example 5, except that the amount of crosslinking agent added in compounding the curable composition was changed from 33.0 g to 35.0 g. The water solubility of the cured product in Example 32 was 2%, and the liquid absorption rate was 3.5%.

(Example 33)
The same procedure was carried out as in Example 15, except that in compounding the curable composition, blocked diphenylmethane diisocyanate was used as the crosslinking agent (B) instead of blocked xylylene diisocyanate. The water solubility of the cured product in Example 33 was 1%, and the liquid absorption rate was 3%.

(Example 34)
The same procedure as in Example 31 was carried out to prepare a curable composition, except that the amount of crosslinking agent (B) added was changed from 38.0 g to 152.0 g. The water solubility of the cured product in Example 34 was 2% and the liquid absorption rate was −4.0%.

(Example 35)
The same procedure as in Example 34 was repeated except that the amount of crosslinking agent (B) added was changed from 152.0 g to 305.0 g in the preparation of the curable composition. The water solubility of the cured product in Example 35 was 7% and the liquid absorption rate was 0.4%.

(Example 36)
The same procedure was carried out as in Example 1, except that in compounding the curable composition, blocked diphenylmethane diisocyanate was replaced with blocked 1,3,5-tris(6-isocyanatohexyl) biuret as the crosslinking agent (B). The water solubility of the cured product in Example 36 was 8%, and the liquid absorption rate was -7.9%.

(Example 37)
The same procedure was carried out as in Example 36, except that in compounding the curable composition, a block 1,3,5-tris(6-isocyanatohexyl) biuret compound was used as the crosslinking agent (B) instead of a block 1,2,4-triisocyanatobenzene compound. The water solubility of the cured product in Example 37 was 9%, and the liquid absorption rate was -9%.

(Example 38)
The same procedure was carried out as in Example 36, except that in compounding the curable composition, carboxymethyl cellulose was changed to methyl cellulose as the water-soluble polymer (A) and block 1,3,5-tris(6-isocyanatohexyl) biuret was changed to block 1,3,5-tris(isocyanatomethyl)benzene as the crosslinking agent (B). The water solubility of the cured product in Example 38 was 9%, and the liquid absorption rate was -8.7%.

(Example 39)
The same procedure as in Example 36 was carried out to prepare a curable composition, except that carboxymethyl cellulose was used as the water-soluble polymer (A). The water solubility of the cured product of Example 39 was 7% and the liquid absorption rate was −8%.

(Example 40)
The same procedure as in Example 38 was carried out to prepare a curable composition, except that methyl cellulose was replaced with polyvinyl alcohol having a weight-average molecular weight of 500 as the water-soluble polymer (A). The cured product of Example 40 had a water solubility of 5% and a liquid absorption rate of -6.4%.

(Examples 41 to 42)
The same procedure as in Example 36 was repeated to prepare the curable cured product, except that the water-soluble polymer (A) was changed from carboxymethyl cellulose to polyvinyl alcohol having a weight-average molecular weight of 2,000 and polyvinyl alcohol having a weight-average molecular weight of 30,000. The cured product of Example 41 had a water solubility of 5% and a liquid absorption of 3%. The cured product of Example 42 had a water solubility of 5% and a liquid absorption of 2.8%.

(Examples 43 to 46)
The same procedure as in Example 37 was repeated to prepare the curable compositions, except that the amount of crosslinking agent (B) added was changed from 33.0 g to 39.0 g, 79.0 g, 158.0 g, and 316.0 g. The cured product of Example 43 had a water solubility of 8% and a liquid absorption rate of 3%. The cured product of Example 44 had a water solubility of 6% and a liquid absorption rate of 4%. The cured product of Example 45 had a water solubility of 3% and a liquid absorption rate of -2%. The cured product of Example 46 had a water solubility of 5% and a liquid absorption rate of 4%.

(Example 47)
The same procedure as in Example 39 was repeated except that the amount of crosslinking agent (B) added was changed from 33.0 g to 380.0 g in the preparation of the curable composition. The water solubility of the cured product of Example 47 was 4% and the liquid absorption rate was 3%.

(Example 48)
The same procedure was carried out as in Example 40 to prepare a curable composition, except that the crosslinking agent (B) was changed from blocked 1,3,5-tris(isocyanatomethyl)benzene to blocked 1,3,5-tris(6-isocyanatohexyl) biuret, and the amount of crosslinking agent (B) added was changed from 33.0 g to 39.0 g. The cured product of Example 48 had a water solubility of 6% and a liquid absorption rate of 2.8%.

(Examples 49 to 50)
The same procedure as in Example 41 was repeated to prepare the curable compositions, except that the amount of crosslinking agent (B) added was changed from 33.0 g to 39.0 g and 79.0 g, respectively. The cured products of Example 49 and Example 50 had water solubility of 5% and liquid absorption of 3%.

(Examples 51 to 55)
The same procedure as in Example 42 was repeated to prepare the curable compositions, except that the amount of crosslinking agent (B) added was changed from 33.0 g to 39.0 g, 79.0 g, 158.0 g, 316.0 g, and 400.0 g. The water solubility of the cured product of Example 51 was 6%, and the liquid absorption rate was 1.6%. The water solubility of the cured product of Example 52 was 6%, and the liquid absorption rate was 2%. The water solubility of the cured product of Example 53 was 9%, and the liquid absorption rate was 1%. The water solubility of the cured product of Example 54 was 7%, and the liquid absorption rate was 1%. The water solubility of the cured product of Example 55 was 5%, and the liquid absorption rate was 1.5%.

(Example 56)
The same procedure as in Example 48 was carried out to prepare a curable composition, except that as the water-soluble polymer (A), polyvinyl alcohol having a weight-average molecular weight of 70,000 was used instead of polyvinyl alcohol having a weight-average molecular weight of 500. The cured product of Example 56 had a water solubility of 5% and a liquid absorption rate of 0.6%.

(Examples 57 to 58)
The same procedure as in Example 56 was repeated to prepare the curable compositions, except that the amount of crosslinking agent (B) added was changed from 39.0 g to 79.0 g and 158.0 g, respectively. The cured product of Example 57 had a water solubility of 8% and a liquid absorption of 1%. The cured product of Example 58 had a water solubility of 6% and a liquid absorption of 1%.

(Example 59)
The same procedure as in Example 48 was carried out to prepare a curable composition, except that as the water-soluble polymer (A), polyvinyl alcohol having a weight-average molecular weight of 500 was used instead of polyvinyl alcohol having a weight-average molecular weight of 160,000. The cured product of Example 59 had a water solubility of 2% and a liquid absorption rate of 2.5%.

(Examples 60 to 62)
The same procedure as in Example 59 was repeated to prepare the curable compositions, except that the amount of crosslinking agent (B) added was changed from 39.0 g to 79.0 g, 158.0 g, and 316.0 g. The cured product of Example 60 had a water solubility of 4% and a liquid absorption of 2%. The cured product of Example 61 had a water solubility of 7% and a liquid absorption of 1.6%. The cured product of Example 62 had a water solubility of 5% and a liquid absorption of 1%.

(Example 63)
The same procedure as in Example 54 was carried out to prepare a curable composition, except that as the water-soluble polymer (A), polyvinyl alcohol having a weight-average molecular weight of 30,000 was used instead of polyvinyl alcohol having a weight-average molecular weight of 200,000. The cured product of Example 63 had a water solubility of 2% and a liquid absorption rate of 2.8%.

(Example 64)
The same procedure was carried out as in Example 48, except that in compounding the curable composition, a block 1,3,5-tris(6-isocyanatohexyl) biuret compound was used as the crosslinking agent (B) instead of a block 1,2,4-triisocyanatobenzene compound. The water solubility of the cured product of Example 64 was 4%, and the liquid absorption rate was 6%.

(Example 65)
The same procedure as in Example 64 was carried out to prepare a curable composition, except that as the water-soluble polymer (A), polyvinyl alcohol having a weight-average molecular weight of 500 was used instead of polyvinyl alcohol having a weight-average molecular weight of 2,000. The water-solubility of the cured product in Example 65 was 4%, and the liquid absorption rate was −3%.

(Example 66)
The same procedure as in Example 65 was repeated except that the amount of crosslinking agent (B) added in compounding the curable composition was changed from 39.0 g to 79.0 g. The water solubility of the cured product in Example 66 was 5% and the liquid absorption rate was 2%.

(Example 67)
The same procedure as in Example 64 was carried out to prepare a curable composition, except that as the water-soluble polymer (A), polyvinyl alcohol having a weight-average molecular weight of 30,000 was used instead of polyvinyl alcohol having a weight-average molecular weight of 500. The cured product of Example 67 had a water solubility of 2% and a liquid absorption rate of 3%.

(Examples 68 to 70)
The same procedure as in Example 67 was repeated to prepare the curable compositions, except that the amount of crosslinking agent (B) added was changed from 39.0 g to 79.0 g, 158.0 g, and 316.0 g. The cured product of Example 68 had a water solubility of 2% and a liquid absorption of 3.5%. The cured product of Example 69 had a water solubility of 3% and a liquid absorption of -1%. The cured product of Example 70 had a water solubility of 4% and a liquid absorption of 2.5%.

(Example 71)
The same procedure as in Example 70 was carried out to prepare a curable composition, except that as the water-soluble polymer (A), polyvinyl alcohol having a weight-average molecular weight of 70,000 was used instead of polyvinyl alcohol having a weight-average molecular weight of 30,000. The cured product of Example 71 had a water solubility of 4% and a liquid absorption rate of 2%.

(Example 72)
The same procedure as in Example 64 was repeated to prepare the curable composition, except that the water-soluble polymer (A) was changed from polyvinyl alcohol having a weight-average molecular weight of 500 to polyvinyl alcohol having a weight-average molecular weight of 160,000, and the amount of crosslinking agent (B) added was changed from 39.0 g to 400.0 g. The cured product of Example 72 had a water solubility of 3% and a liquid absorption rate of 2.5%.

(Example 73)
The same procedure as in Example 72 was repeated to prepare a curable composition, except that the water-soluble polymer (A) was changed from polyvinyl alcohol having a weight-average molecular weight of 160,000 to polyvinyl alcohol having a weight-average molecular weight of 200,000. The cured product of Example 73 had a water solubility of 2% and a liquid absorption rate of -3%.

(Example 74)
The same procedure was carried out as in Example 51, except that in compounding the curable composition, a block 1,3,5-tris(6-isocyanatohexyl) biuret compound was used as the crosslinking agent (B) instead of a block 1,3,5-tris(isocyanatomethyl)benzene. The water solubility of the cured product of Example 74 was 7%, and the liquid absorption rate was 1%.

(Example 75)
The same procedure as in Example 40 was repeated except that the amount of crosslinking agent (B) added was changed from 33.0 g to 35.0 g in the preparation of the curable composition. The water solubility of the cured product in Example 75 was 9% and the liquid absorption rate was −0.5%.

(Example 76)
The same procedure was carried out as in Example 50, except that in compounding the curable composition, a block 1,3,5-tris(6-isocyanatohexyl) biuret compound was used as the crosslinking agent (B) instead of a block 1,3,5-tris(isocyanatomethyl)benzene. The water solubility and liquid absorption rate of the cured product of Example 76 were 3% and 3%, respectively.

(Example 77)
The same procedure was carried out as in Example 53, except that in compounding the curable composition, a block 1,3,5-tris(6-isocyanatohexyl) biuret compound was used as the crosslinking agent (B) instead of a block 1,3,5-tris(isocyanatomethyl)benzene. The water solubility of the cured product of Example 77 was 4%, and the liquid absorption rate was -1.5%.

(Example 78)
The same procedure as in Example 77 was repeated except that the amount of crosslinking agent (B) added was changed from 158.0 g to 305.0 g in the preparation of the curable composition. The water solubility of the cured product in Example 78 was 2% and the liquid absorption rate was 1%.

(Example 79)
The same procedure was carried out as in Example 54, except that in compounding the curable composition, a blocked 1,3,5-tris(6-isocyanatohexyl) biuret was used as the crosslinking agent (B) instead of a blocked polymethylene polyphenyl polyisocyanate. The water solubility and liquid absorption rate of the cured product of Example 79 were 3% and 3%, respectively.

(Example 80)
The same procedure was carried out as in Example 56, except that in compounding the curable composition, a blocked 1,3,5-tris(6-isocyanatohexyl) biuret was used as the crosslinking agent (B) instead of a blocked polymethylene polyphenyl polyisocyanate. The water solubility of the cured product in Example 80 was 2%, and the liquid absorption rate was 2.5%.

(Example 81)
The same procedure as in Example 80 was repeated to prepare a curable composition, except that the water-soluble polymer (A) was changed from polyvinyl alcohol having a weight-average molecular weight of 70,000 to polyvinyl alcohol having a weight-average molecular weight of 160,000. The cured product of Example 81 had a water solubility of 4% and a liquid absorption rate of -1%.

(Comparative Example 1)
The same procedure as in Example 7 was carried out except that the crosslinking agent (B) was not added in preparing the curable composition. The water solubility of the cured product in Comparative Example 1 was 100%, and the liquid absorption rate was 7.4%.

(Comparative Example 2)
The same procedure as in Example 7 was carried out except that the amount of crosslinking agent (B) added was changed from 33.0 g to 25.0 g in the preparation of the curable composition. The water solubility of the cured product in Comparative Example 2 was 70% and the liquid absorption rate was 7%.

(Comparative Example 3)
The same procedure as in Example 7 was carried out except that the amount of crosslinking agent (B) added was changed from 33.0 g to 605.0 g in the preparation of the curable composition. However, the amount of crosslinking agent (B) added was too large, causing an increase in viscosity and making it impossible to evaluate.

(Comparative Examples 4 to 6)
The same procedure as in Example 7 was repeated to prepare the curable compositions, except that the amount of crosslinking agent (B) added was changed from 33.0 g to 10.0 g, 15.0 g, and 18.0 g. The cured product of Comparative Example 4 had a water solubility of 50% and a liquid absorption rate of 50%. The cured product of Comparative Example 5 had a water solubility of 52% and a liquid absorption rate of -15%. The cured product of Comparative Example 6 had a water solubility of 40% and a liquid absorption rate of 10%.

(Comparative Example 7)
The curable composition was prepared in the same manner as in Example 13, except that the water-soluble polymer (A) was changed from polyvinyl alcohol having a weight-average molecular weight of 500 to polyvinyl alcohol having a weight-average molecular weight of 300, and the amount of crosslinking agent (B) added was changed from 44.0 g to 35.0 g. The cured product of Comparative Example 7 had a water solubility of 93% and a liquid absorption rate of 7.2%.

(Comparative Example 8)
In preparing the curable composition, 100 g of polyvinyl alcohol having a weight-average molecular weight of 250,000 as the water-soluble polymer (A) was added to water, and the temperature was raised from room temperature to 95°C, followed by stirring until complete dissolution. At this stage, the viscosity was too high, causing gelation, and it was not possible to add the crosslinking agent (B).

(Comparative Example 9)
The curable composition was prepared in the same manner as in Example 5, except that the water-soluble polymer (A) was changed from polyvinyl alcohol having a weight-average molecular weight of 500 to polyvinyl alcohol having a weight-average molecular weight of 45,000, the crosslinking agent (B) was changed from blocked xylylene diisocyanate to polyacrylic acid, and the amount of crosslinking agent (B) added was changed from 33.0 g to 380.0 g. The cured product of Comparative Example 9 had a water solubility of 72% and a liquid absorption rate of -30%.

(Comparative Example 10)
The same procedure as in Example 42 was repeated except that the amount of crosslinking agent (B) added was changed from 33.0 g to 25.0 g in the preparation of the curable composition. The water solubility of the cured product in Comparative Example 10 was 75% and the liquid absorption rate was 7.1%.

(Comparative Example 11)
The same procedure as in Example 74 was carried out except that the amount of crosslinking agent (B) added was changed from 39.0 g to 605.0 g in the preparation of the curable composition. However, the amount of crosslinking agent (B) added was too large, causing an increase in viscosity and making it impossible to evaluate.

(Comparative Example 12)
The same procedure as in Example 74 was repeated except that the amount of crosslinking agent (B) added was changed from 39.0 g to 10.0 g in the preparation of the curable composition. The water solubility and liquid absorption rate of the cured product of Comparative Example 12 were 50% and 50%, respectively.

(Comparative Examples 13 to 14)
The same procedure as in Example 51 was repeated to prepare the curable compositions, except that the amount of crosslinking agent (B) added was changed from 39.0 g to 15.0 g and 18.0 g, respectively. The cured product of Comparative Example 13 had a water solubility of 47% and a liquid absorption rate of -15%. The cured product of Comparative Example 14 had a water solubility of 42% and a liquid absorption rate of 10%.

(Comparative Example 15)
The curable composition was prepared in the same manner as in Example 48, except that the water-soluble polymer (A) was changed from polyvinyl alcohol having a weight-average molecular weight of 500 to polyvinyl alcohol having a weight-average molecular weight of 300, and the amount of crosslinking agent (B) added was changed from 39.0 g to 35.0 g. The cured product of Comparative Example 15 had a water solubility of 95% and a liquid absorption rate of 7%.

These results demonstrate that lithium ion secondary batteries using the cured material according to the presently disclosed embodiment as a cured material for lithium ion secondary batteries exhibit favorable charge-discharge cycle characteristics.

10...Separator, 20...Positive electrode, 22...Positive electrode current collector, 24...Positive electrode active material layer, 30...Anode, 32...Anode current collector, 34...Anode active material layer, 40...Laminate, 50...Outer case, 52...Metal foil, 54...Polymer film, 60, 62...Leads, 100...Lithium ion secondary battery

Claims (6)

A cured product obtained by curing a curable composition,
The curable composition includes a water-soluble polymer (A) having a hydroxyl group, a crosslinking agent (B) having a functional group reactive with the hydroxyl group, and water;
The water-soluble polymer (A) includes one or more of polyvinyl alcohol, carboxymethyl cellulose, and methyl cellulose,
The weight average molecular weight of the polyvinyl alcohol is 2,000 to 200,000,
The cured product has a urethane bond,
the water solubility of the cured product when immersed in water is less than 10%, and the liquid absorption rate when immersed in an electrolyte is −10% or more and less than 5%;
Hardened material for lithium-ion secondary batteries. The curable composition comprises
Per 100 parts by weight of the water-soluble polymer (A),
The crosslinking agent (B) is contained in an amount of 35 parts by weight or more and less than 400 parts by weight.
The cured product for a lithium ion secondary battery according to claim 1 . The crosslinking agent (B) is an active methylene diisocyanate.
The cured product for a lithium ion secondary battery according to claim 1 or 2. The crosslinking agent (B) is a polyisocyanate having three or more functional groups.
The cured product for a lithium ion secondary battery according to claim 1 or 2. a negative electrode active material;
A negative electrode active material layer comprising at least the cured material for a lithium ion secondary battery according to any one of claims 1 to 4 , a negative electrode current collector,
1. A negative electrode for a lithium-ion secondary battery comprising: The negative electrode for a lithium ion secondary battery according to claim 5 ,
A positive electrode and
An electrolyte;
A lithium ion secondary battery comprising: