JP7144794B2 - Binder for manufacturing lithium ion secondary battery and lithium ion secondary battery using the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a binder used for manufacturing a lithium ion secondary battery, an electrode and a separator using the same, and a lithium ion secondary battery using the same.

Among secondary batteries, lithium-ion secondary batteries are lighter and have higher capacity than nickel-cadmium batteries and nickel-metal hydride batteries, so they are widely used as power sources for mobile electronic devices such as smart phones. Moreover, in recent years, it is becoming popular as a power source for hybrid vehicles and electric vehicles. As a power source for these, lithium-ion secondary batteries are expected to have higher capacity and longer life.

As for the negative electrode of a lithium ion secondary battery, as a negative electrode active material capable of increasing the capacity, many alloy-based negative electrode active materials such as silicon and silicon oxide having a large discharge capacity are being studied. However, since the silicon-based negative electrode active material expands and contracts significantly during charging and discharging, repeated charging and discharging may cause disconnection of the conductive paths between the negative electrode active material particles in the negative electrode mixture layer, and There is concern that peeling or the like will occur between the layers, and that the life characteristics will deteriorate. On the other hand, an electrode using polyimide, which has high elasticity and strong adhesion, as a binder for the negative electrode has been reported (Patent Document 1).

In addition, as for the positive electrode, there are increasing examples of using a ternary composite oxide (hereinafter also referred to as "NCM positive electrode material") composed of nickel, cobalt, and manganese with a large amount of nickel, which is excellent in capacity expression, as a positive electrode active material. . However, if the nickel content of the NCM positive electrode active material is increased in order to increase the capacity, the hydrofluoric acid removal reaction proceeds when PVDF is used as a binder, and there is a problem that a stable positive electrode slurry cannot be produced. rice field. In addition, PVDF swells due to the electrolyte solution when the temperature of the battery becomes high, causing disconnection of the conductive path between the positive electrode active materials and peeling of the positive electrode mixture layer and the current collector, etc., resulting in deterioration of life characteristics. There's a problem. In order to solve such problems of PVDF, an electrode using polyimide as a binder for a positive electrode has been reported (Patent Document 2).

Binders for lithium ion secondary batteries containing polyamic acid have also been reported (Patent Documents 1 and 3). However, polyamic acid is prone to hydrolysis with water and easily changes at room temperature, so there is a problem that the properties tend to vary. Furthermore, when a binder containing polyamic acid is used, a high temperature treatment such as 350° C. is required for complete imidization. As a result, the copper foil is oxidized when the electrode is manufactured, and the resistance increases, so there is a problem that it is difficult to put it into practical use unless it is in a special atmosphere such as an inert gas atmosphere.

Furthermore, as an electrode binder, a lithium ion secondary battery using a solvent-soluble block copolymer polyimide that has completed imidation has been reported (Patent Documents 4, 5, and 6), and as an electrode binder, polyimide water A lithium ion secondary battery using a dispersion has also been reported (Patent Document 7).

JP 2014-067592 JP 2002-124264 JP 2012-209219 Japanese Patent Laid-Open No. 11-097028 JP 2009-283284 International Publication No. 2013/115219 JP 2017-190409

Originally, polyimide has excellent solvent resistance, but by controlling the imide group concentration in polyimide, refraction, and bending bonding, polyimide soluble in solvents such as N-methylpyrrolidone, dimethylacetamide, dimethylformamide, and dimethylsulfoxide can be synthesized.

In order to use such a solvent-soluble polyimide as a binder for manufacturing an electrode of a lithium ion battery, it is necessary to maintain the solubility of the polyimide in the solvent and prevent the polyimide from dissolving in the electrolyte of the lithium ion secondary battery. It is necessary that the degree of swelling of the polyimide with respect to the electrolytic solution is small.

However, when the present inventors try to increase the solubility of the polyimide in the solvent, the solvent resistance of the polyimide to the electrolyte used in the lithium ion secondary battery becomes insufficient, and the degree of swelling in the electrolyte increases. I found out. Further, the inventors have found that the solubility of polyimide in a solvent decreases when the degree of swelling of polyimide in the electrolytic solution is reduced. That is, it has been difficult for conventional solvent-soluble polyimides synthesized from a combination of a tetracarboxylic dianhydride and a diamine to achieve both high solubility in solvents and low swelling in electrolytic solutions.

As a result of intensive studies in view of the above problems, the present inventors have found that by controlling the molecular weights of the tetracarboxylic dianhydride and diamine used in polyimide synthesis, high solvent solubility in solvents and low swelling in electrolyte solutions can be achieved. The inventors have found that it is possible for the first time to synthesize a solvent-soluble polyimide suitable as a binder for lithium-ion secondary batteries, and have completed the present invention.

That is, the present invention provides a binder for producing a lithium ion secondary battery containing a solvent-soluble polyimide, wherein the solvent-soluble polyimide is a block copolymer having an imide group concentration of 20 to 32%, and N-methyl It is soluble in at least one solvent selected from the group consisting of pyrrolidone, dimethylacetamide, dimethylformamide and dimethylsulfoxide, and all tetracarboxylic dianhydride components constituting the polyimide have a molecular weight of 360 or less, and , Among all the diamine components constituting the polyimide, the ratio of the diamine component having a molecular weight of 220 or less is at least 30 mol%, and the film produced from the polyimide is immersed in an electrolyte for lithium ion secondary batteries, and heated at 80 ° C. The degree of swelling when treated for 72 hours is 50% by weight or less, and the electrolytic solution for a lithium ion secondary battery is a mixed solvent consisting of ethylene carbonate: ethyl methyl carbonate: dimethyl carbonate (volume ratio 3: 3: 4) Provided is a binder for manufacturing a lithium ion secondary battery, which is an electrolytic solution obtained by dissolving 1 mol/L of lithium hexafluorophosphate (LiPF ₆ ) in

The present invention also provides a binder for manufacturing a lithium ion secondary battery containing a solvent-soluble polyimide synthesized from a tetracarboxylic dianhydride and a diamine, wherein the polyimide is a block copolymer, and the imide group of the polyimide A diamine having a concentration of 20 to 32%, a molecular weight of 360 or less for all tetracarboxylic dianhydride components constituting the polyimide, and a molecular weight of 220 or less among all the diamine components constituting the polyimide. Provided is a binder for manufacturing a lithium ion secondary battery, wherein the proportion of the component is at least 30 mol %.

Furthermore, the present invention is a method for producing a solvent-soluble polyimide for use in a binder for producing a lithium ion secondary battery by reacting a tetracarboxylic dianhydride and a diamine, wherein all the The molecular weight of the tetracarboxylic dianhydride is 360 or less, and the ratio of diamines with a molecular weight of 220 or less is at least 30 mol% among all the diamines used in the reaction, and the imide group concentration of the polyimide is 20. A method for producing a binder for producing a lithium ion secondary battery, which is ∼32% .

By synthesizing a specific solvent-soluble polyimide according to the present invention in which the molecular weights of the tetracarboxylic dianhydride component and the diamine component that constitute the polyimide are controlled, a solvent-soluble polyimide having an electrolytic solution swelling degree of 50% by weight or less is synthesized for the first time. became possible. In addition, by using the binder for producing a lithium ion secondary battery of the present invention containing a polyimide capable of achieving both high solvent solubility in such a solvent and low swelling degree in an electrolytic solution, adhesion between electrode active materials It is possible to produce an electrode having excellent properties and adhesion between the electrode mixture layer and the current collector. Moreover, by using such an electrode, it is possible to produce a lithium-ion secondary battery having excellent life characteristics (cycle characteristics).

The binder for lithium ion secondary batteries of the present invention contains a solvent-soluble polyimide. The features of the solvent-soluble polyimide in the present invention and the method for synthesizing the polyimide are specifically described below.

<Solvent-soluble polyimide>
The solvent-soluble polyimide in the present invention is a polyimide having a degree of swelling of 50% by weight or less when a film made from polyimide is immersed in an electrolytic solution for lithium ion secondary batteries and treated at 80° C. for 72 hours.

A specific method for measuring the degree of swelling in the present invention is as follows. That is, a test piece of 3 cm×3 cm is prepared from a film having a thickness of about 50 μm obtained by pouring a solvent-soluble polyimide into a dish-shaped container and drying it at 120° C. for 15 hours. Next, the test piece was immersed in an electrolytic solution prepared by dissolving 1 mol/L of lithium hexafluorophosphate (LiPF ₆ ) in an electrolytic solution solvent consisting of ethylene carbonate:ethylmethylcarbonate:dimethylcarbonate=3:3:4, After heating the electrolyte to 80° C., the specimen is immersed at 80° C. for 72 hours. The weight increase rate (%) of the film obtained by measuring the weight of the polyimide film before and after the immersion treatment in the electrolytic solution is defined as the degree of swelling (also referred to as "swelling degree of electrolytic solution") in the present invention. Here, the film preparation method does not affect the numerical value of the swelling degree of the electrolyte, and the weight of the film after the immersion treatment is determined by removing the test piece from the electrolyte and quickly wiping the surface with a tissue so as not to dry. It was measured after wiping off the electrolyte adhering to the

In the present invention, the degree of swelling of the polyimide film measured by the above method is preferably 50% by weight or less, more preferably 40% by weight or less, and particularly preferably 30% by weight or less. When the degree of swelling in the electrolyte is large, the adhesion between the active materials tends to be insufficient, and the adhesion between the electrode mixture layer and the current collector tends to be insufficient. life characteristics tend to deteriorate. The lower the degree of swelling of the polyimide film, the better, but the lower limit of the degree of swelling is 15% by weight or more, further 17% by weight or more, particularly 19% by weight or more, and sufficiently good battery life characteristics can be obtained. can be achieved

The term "solvent-soluble" in the present invention is a term used for the organic polar solvent used in the synthesis of polyimide and the solvent used for the mixture described later, and polyimide that dissolves 5 g or more in 100 g of solvent. It means that there is As the solvent, at least one organic solvent selected from the group consisting of N-methylpyrrolidone (NMP), dimethylacetamide, dimethylformamide and dimethylsulfoxide is preferred. The synthesized polyimide can be used as a binder for lithium ion secondary batteries in the form of a solution in which the solid content is 10 to 30% by weight in the above solvent.

(Method for synthesizing solvent-soluble polyimide)
The solvent-soluble polyimide in the present invention can be synthesized by subjecting a tetracarboxylic dianhydride and a diamine to a dehydration and ring closure reaction with a catalyst in a solvent.
Examples of the catalyst include acid catalysts such as sulfuric acid, phosphoric acid, and toluenesulfonic acid, and imidization can be promoted by heating tetracarboxylic dianhydride and diamine to 160 to 200° C. and using an acid catalyst. .

If the acid catalyst remains in the polyimide solution produced, it can cause discoloration and deterioration of polymer products. , the separation of the polyimide and the catalyst is effected by redissolving (WH Miller, USP 4,927,736).

A more preferred method for synthesizing the solvent-soluble polyimide in the present invention is a method of promoting the dehydration imidization reaction using a two-component acid-base catalyst utilizing the equilibrium reaction of lactone (USP5502143). This method uses a binary catalyst of γ-valerolactone and pyridine or N-methylmorpholine. As the imidization progresses, water is produced, and the produced water participates in the equilibrium of the lactone and becomes an acid/base catalyst, exhibiting catalytic action.

Water produced by the imidization reaction is removed from the system by azeotropy with toluene or xylene coexisting in the polar solvent. When the imidization reaction is completed, the water in the solution is removed, and the acid/base catalyst becomes γ-valerolactone and pyridine or N-methylmorpholine and is removed from the system. Thus, a highly pure polyimide solution can be obtained.

Other binary catalysts are oxalic acid or malonic acid and pyridine or N-methylmorpholine. In the reaction solution at 160-200°C, oxalate or malonate promotes the imidization reaction as an acid catalyst. A catalytic amount of oxalic acid or malonic acid remains in the resulting polyimide solution. When this polyimide solution is applied and heated to 200° C. or higher to remove the solvent and form a film, oxalic acid or malonic acid remaining in the polyimide is thermally decomposed and removed as a gas from the system. Thus, a high-purity polyimide product can be obtained. Oxalic acid-pyridine-based catalysts are more active than valerolactone-pyridine-based catalysts, and can produce high-molecular-weight polyimides in a short period of time.

(tetracarboxylic dianhydride)
As the tetracarboxylic dianhydride used in the present invention, pyromellitic dianhydride (PMDA), 1,2,5,6- naphthalenetetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 2,2',3,3'- biphenyltetracarboxylic dianhydride, 2,3,3′,4′-biphenyltetracarboxylic dianhydride, 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 2,2 ',3,3'-benzophenonetetracarboxylic dianhydride, 2,3,3',4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA), bis(3,4-dicarboxyphenyl)sulfone dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, 1 , 1-bis(2,3-dicarboxyphenyl)ethane dianhydride, 1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride, 2,2-bis[3,4-(dicarboxy Phenoxy)phenyl]propane dianhydride (BPADA), 4,4′-(hexafluoroisopropylidene)diphthalic anhydride, 4,4′-oxydiphthalic anhydride (ODPA), bis(3,4-dicarboxyphenyl ) sulfone dianhydride, bis(3,4-dicarboxyphenyl) sulfoxide dianhydride, thiodiphthalic dianhydride, 3,4,9,10-perylenetetracarboxylic dianhydride, 2,3,6,7 -anthracenetetracarboxylic dianhydride, 1,2,7,8-phenanthrenetetracarboxylic dianhydride, 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride, 9,9-bis[ Aromatic tetracarboxylic dianhydrides such as 4-(3,4'-dicarboxyphenoxy)phenyl]fluorene dianhydride, cyclobutanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic acid dianhydride, 2,3,4,5-tetrahydrofurantetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, 3,4-dicarboxy-1-cyclohexylsuccinic dianhydride 3,4-dicarboxy-1,2,3,4-tetrahydro-1-naphthalenesuccinic dianhydride etc. can be exemplified, but are not limited to these. These tetracarboxylic dianhydrides may be used alone or in combination of two or more.

In the present invention, it is preferable to use a tetracarboxylic dianhydride having a molecular weight of 360 or less, more preferably 350 or less, particularly 330 or less. Tetracarboxylic dianhydrides having a molecular weight of 360 or less include pyromellitic dianhydride (PMDA; molecular weight 218.1), 4,4′-oxydiphthalic anhydride (ODPA; molecular weight 310.2), 3,3 ',4,4'-biphenyltetracarboxylic dianhydride (BPDA; 294.2), 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA; molecular weight 322.2) and the like. be done.

When the molecular weight of the tetracarboxylic dianhydride is greater than 360, the solubility of the obtained polyimide in N-methylpyrrolidone and dimethylacetamide is improved, but the degree of swelling in the electrolytic solution of the lithium ion secondary battery is more than 50%. When it is used as an electrode binder for a lithium ion secondary battery, the cycle characteristics tend to deteriorate.

In the present invention, by reacting a tetracarboxylic dianhydride having a molecular weight of 360 or less with a diamine, a polyimide having a structure in which all the tetracarboxylic dianhydride components constituting the polyimide have a molecular weight of 360 or less. can be synthesized, and it is possible to provide a polyimide that can achieve both high solvent solubility and low swelling degree in electrolyte solution.

(diamine)
Diamines used in the present invention include paraphenylenediamine (PPD), metaphenylenediamine (MPDA), 2,5-diaminotoluene, 2,6-diaminotoluene, 4,4'-diaminobiphenyl, 2,5-dimethyl-1 ,4-phenylenediamine (DMPDA), 4,4′-diaminobiphenyl, 3,3′-dimethyl-4,4′-diaminobiphenyl, 3,3′-dimethoxy-4,4′-diaminobiphenyl, 2,2 -bis(trifluoromethyl)-4,4'-diaminobiphenyl, 3,3'-diaminodiphenylmethane, 4,4'-diaminodiphenylmethane (MDA), 2,2-bis-(4-aminophenyl)propane, 3 , 3′-diaminodiphenyl sulfone (33DDS), 4,4′-diaminodiphenyl sulfone (44DDS), 3,3′-diaminodiphenyl sulfide, 4,4′-diaminodiphenyl sulfide, 3,4-diaminodiphenyl ether (m- DADE), 4,4'-diaminodiphenyl ether (p-DADE), 1,5-diaminonaphthalene, 4,4'-diaminodiphenyldiethylsilane, 4,4'-diaminodiphenylsilane, 4,4'-diaminodiphenylethyl phosphine oxide, 1,3-bis(3-aminophenoxy)benzene (APB), 1,3-bis(4-aminophenoxy)benzene (TPE-R), 1,4-bis(4-aminophenoxy)benzene, bis[4-(3-aminophenoxy)phenyl]sulfone (BAPSM), bis[4-(4-aminophenoxy)phenyl]sulfone (BAPS), 2,2-bis[4-(4-aminophenoxy)phenyl] Propane (BAPP), 3,3-diamino-4,4-dihydroxydiphenylsulfone (ABPS), hydroxybenzidine (HAB), 2-bis(3-aminophenyl)1,1,1,3,3,3-hexa Examples include fluoropropane, 2,2-bis(4-aminophenyl)1,1,1,3,3,3-hexafluoropropane, 9,9-bis(4-aminophenyl)fluorene, and the like. Not limited. These diamines may be used alone or in combination of two or more.

In the present invention, as the diamine, the ratio of diamine components having a molecular weight of 220 or less is at least 30 mol% or more, further 40 mol% or more, and particularly 50 mol% or more of all diamine components used in polyimide synthesis. It is preferable to become

Examples of diamines having a molecular weight of 220 or less include 4,4′-diaminobiphenyl, 2,5-dimethyl-1,4-phenylenediamine (DMPDA; molecular weight 136.19), diaminobenzoic acid (molecular weight 152.15), isophoronediamine (molecular weight 170.3), 4,4-diaminodiphenylmethane (MDA; molecular weight 198.26), 3,4-diaminodiphenyl ether (3,4-DADE; molecular weight 200.24), 4,4′-diaminodiphenyl ether (4,4 '-DADE; molecular weight 200.24), 3,3'-dihydroxybenzidine (HAB; molecular weight 216.24), bis(4-aminophenyl) sulfide (ASD; molecular weight 216.3) and the like.

When the ratio of the diamine component having a molecular weight of 220 or less is less than 30 mol%, the solubility of the obtained polyimide in N-methylpyrrolidone or dimethylacetamide is improved, but the degree of swelling in the electrolytic solution of the lithium ion secondary battery is low. It tends to be greater than 50%, and when used as an electrode binder for a lithium ion secondary battery, the cycle characteristics tend to deteriorate.

(Preferred embodiment of solvent-soluble polyimide)
In the present invention, it is preferable to react a tetracarboxylic dianhydride having a molecular weight of 360 or less as described above with a diamine having a molecular weight of 220 or less in a proportion of at least 30 mol % of the total diamine. As a result, a polyimide having a structure in which all the tetracarboxylic dianhydride components constituting the polyimide have a molecular weight of 360 or less, and the ratio of the diamine component having a molecular weight of 220 or less is at least 30 mol% or more of the total diamine components. It is possible to provide a polyimide that can be synthesized, has even better solubility in a solvent, and has a smaller degree of swelling in the electrolyte solution of a lithium ion secondary battery.

(Imide group concentration)
In the present invention, setting the molecular weight of the carboxylic acid dianhydride or diamine used in synthesizing the polyimide to a specific range means controlling the imide group concentration of the polyimide to be synthesized.
In the present invention, the imide group concentration in the polyimide resin is a value calculated by the following formula (1).
Imide group concentration = [(molecular weight of imide group portion 140) / (molecular weight of polyimide)] x 100 (1)

When an imide ring is produced by the reaction of a carboxylic acid dianhydride and a diamine, 1 mol of water can be removed, so two imide rings are produced by the reaction of a pair of carboxylic acid dianhydride and a diamine, so the above formula (1) can be represented by the following formula (2).
[(molecular weight of imide group portion 140)/(molecular weight of tetracarboxylic dianhydride+molecular weight of diamine-molecular weight of 2 moles of water)]×100 (2)

When multiple tetracarboxylic dianhydrides and diamines are used, the average molecular weight calculated from the amount of each component used is used as the molecular weight of the tetracarboxylic dianhydrides and diamines in the above formula (2). Here, the average molecular weight of the tetracarboxylic dianhydride is obtained by multiplying the usage ratio of each component by the molecular weight of the component when the total amount of the tetracarboxylic dianhydride used is 1. be able to. Similarly, the average molecular weight of diamine can be obtained by multiplying the ratio of each component used by the molecular weight of the component, when the total amount of diamine used is 1.

As can be seen from the above formula (2), the use of a low-molecular-weight tetracarboxylic dianhydride or a low-molecular-weight diamine increases the imide group concentration of the resulting polyimide. As described above, by synthesizing a polyimide varnish by containing a specific amount of a tetracarboxylic dianhydride having a specific molecular weight or less and a diamine having a specific molecular weight or less, a polyimide that dissolves in the solvent and has a low degree of electrolyte swelling can be obtained. Therefore, the lower limit of the imide group concentration of the polyimide is 20% or more, further 25% or more, particularly 25% or more, in terms of being able to obtain a polyimide having good solubility in a solvent and a low degree of swelling of the electrolyte. 28% or more is preferred.

A polyimide having a high imide group concentration has a large irreversible capacity when used in a lithium ion secondary battery. Therefore, considering the irreversible capacity, the imide group concentration of the polyimide used as the electrode binder for the lithium ion secondary battery is preferably 33% or less, more preferably 32.5% or less, particularly 32% or less. When the imide group concentration of the polyimide exceeds 33%, the irreversible capacity of the lithium ion secondary battery tends to increase.
As described above, the imide group concentration of the solvent-soluble polyimide can be set to an optimum range in consideration of the degree of swelling of the electrolyte and the irreversible capacity.

In addition, in order to obtain a solvent-soluble varnish in which the imide group concentration is in the optimum range, it is possible to introduce a tetracarboxylic dianhydride with a small molecular weight and a diamine with a small molecular weight into different block structures. It is preferred to use a polymerization method. By using the block copolymerization method, it is possible to obtain a polyimide that is easily dissolved in a solvent and has a low degree of swelling in an electrolytic solution.

On the other hand, if synthesis is carried out by random polymerization without introducing a tetracarboxylic dianhydride with a small molecular weight and a diamine with a small molecular weight into different block structures, the reaction solution becomes cloudy or gels during synthesis, resulting in a solvent-soluble polyimide. It is difficult.

(Block copolymer)
Furthermore, the solvent-soluble polyimide of the present invention is preferably a block copolymer. In the case of block copolymers, it is possible to expand the range of polyimides that achieve both high solvent solubility in the above solvents and a degree of swelling of electrolyte solution of 50% by weight or less.

Polyimide is inherently difficult to dissolve in solvents such as N-methylpyrrolidone, dimethylacetamide, dimethylformamide, and dimethylsulfoxide, and it is difficult to ensure solubility in these solvents in random polymerization. However, if it is a block copolymer, it is possible to introduce a combination of tetracarboxylic dianhydride and diamine, which is difficult to ensure solubility, into separate block structures, so it is possible to ensure solubility in solvents. Conceivable.

Further, when the solvent-soluble polyimide of the present invention is a block copolymer, the intermolecular interaction between the tetracarboxylic dianhydride and the diamine introduced into separate block structures in the film obtained by drying the polyimide It is considered that a small degree of swelling of the electrolyte can be achieved because the influence of
The method described in WO2006/057036 can be referred to for the method for synthesizing the block copolymer polyimide.

In the present invention, the weight average molecular weight of the soluble polyimide is preferably 100,000 or more, more preferably 130,000 or more. If the weight average molecular weight is less than 100,000, the stability of the electrode slurry composition produced using polyimide tends to be poor. On the other hand, the upper limit of the weight average molecular weight is not particularly limited as long as it can be produced, but if the molecular weight is too large, the resistance of the lithium ion battery tends to increase, so it is necessary to reduce the amount used as a binder.

The solvent-soluble polyimide in the present invention preferably has a B-type viscosity of 2000 cps or more in an 8% NMP solution. If the B-type viscosity is lower than 2000 cps, the stability of the electrode slurry tends to deteriorate. The B-type viscosity of the 8% NMP solution is more preferably 3000 cps or more, particularly preferably 5000 cps or more.

<Binder for Lithium Ion Secondary Battery>
The solvent-soluble polyimide in the present invention can be used as both a positive electrode binder and a negative electrode binder for lithium ion batteries. In addition, since the degree of swelling with respect to the electrolytic solution is also important in the binder for ceramic coating, the solvent-soluble polyimide of the present invention can be suitably used as a binder for ceramic coating.

(Amount of binder used)
When the solvent-soluble polyimide of the present invention is used as a positive electrode binder for a lithium ion secondary battery, the solvent-soluble polyimide is 0.5 parts by mass to 10 parts by mass, preferably 1 mass, per 100 parts by mass of the electrode active material. parts to 5 parts by weight, more preferably 1.5 parts to 3 parts by weight. When the amount of the solvent-soluble polyimide of the present invention used is less than 0.5 parts by mass, the adhesiveness between active materials and the adhesiveness between the electrode layer and the current collector tend to decrease, resulting in a decrease in peel strength.

When the solvent-soluble polyimide of the present invention is used as a negative electrode binder for a lithium ion secondary battery and a carbon material is used as an electrode active material, the solvent-soluble polyimide is 0.5 mass parts per 100 parts by mass of the electrode active material. parts to 10 parts by weight, preferably 1 part to 5 parts by weight, more preferably 1.5 parts to 3 parts by weight. When the amount of the solvent-soluble polyimide of the present invention used is less than 0.5 parts by mass, the adhesiveness between active materials and the adhesiveness between the electrode layer and the current collector tend to decrease, resulting in a decrease in peel strength.

When a silicon material is used alone or in combination with a carbon material as a negative electrode active material, the solvent-soluble polyimide is 2 parts by mass to 20 parts by mass, preferably 1 part by mass, per 100 parts by mass of the electrode active material. ~10 parts by weight can be used. The amount of the solvent-soluble polyimide used as the binder varies depending on the ratio of the silicon material used, and is appropriately determined.

<Electrode for lithium ion secondary battery and method for producing the same>
Materials used for lithium ion secondary battery electrodes will be described.
(Positive electrode active material)
Positive electrode active materials for lithium ion secondary batteries are not particularly limited, but include lithium-containing cobalt oxide (LiCoO ₂ ), lithium manganate (LiMn ₂ O ₄ ), lithium-containing nickel oxide (LiNiO ₂ ), Co— Ni—Mn lithium-containing composite oxide (Li(CoMnNi)O ₂ ), Ni—Mn—Al lithium-containing composite oxide, Ni—Co—Al lithium-containing composite oxide, olivine-type lithium iron phosphate (LiFePO ₄ ), olivine-type lithium manganese phosphate (LiMnPO ₄ ), Li ₂ MnO ₃ —LiNiO ₂ solid solution, lithium-excess spinel compound represented by Li _1+x Mn _2-x O ₄ (0<X<2), Li Known positive electrode active materials such as [Ni _0.17 Li _0.2 Co _0.07 Mn _0.56 ]O ₂ and LiNi _0.5 Mn _1.5 O ₄ can be used. The compounding amount and particle size of the positive electrode active material are not particularly limited, and those within a known range can be used.

(Negative electrode active material)
The active material for lithium ion secondary batteries is not particularly limited, but coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fiber, pyrolytic vapor growth carbon fiber, phenolic resin sintered body, polyacrylonitrile-based Carbon-based negative electrode active materials such as carbon fiber, pseudoisotropic carbon, furfuryl alcohol resin calcined body (PFA), hard carbon, natural graphite, artificial graphite; silicon (Si), alloys containing silicon, SiO, SiOx, Si A non-carbon negative electrode active material such as a composite of a Si-containing material and conductive carbon, which is obtained by covering or combining a containing material with conductive carbon, and the like. The amount and particle size of the negative electrode active material are not particularly limited, and those within a known range can be used.

(Conductive material)
The electrode slurry composition obtained by dispersing the electrode active material using a binder may contain a conductive material. The conductive material is for ensuring electrical contact between the electrode active materials. Conductive materials include carbon black (e.g., acetylene black, Ketjenblack (registered trademark), furnace black, etc.), graphite, carbon fibers, carbon flakes, and ultrashort carbon fibers (e.g., carbon nanotubes, vapor-grown carbon fibers, etc.). ), and other conductive carbon materials; fibers and foils of various metals can be used. Among them, as the conductive material, carbon black is preferable, and acetylene black is more preferable. These can be used singly or in combination of two or more.

The amount of the conductive material to be blended in the slurry composition is not particularly limited, but is preferably 1 part by mass or more and preferably 10 parts by mass or less per 100 parts by mass of the electrode active material. When the slurry composition contains the conductive material in an amount within the above range, a good conductive path is formed in the composite material layer of the electrode produced using the slurry composition, and in particular the slurry composition is used as a positive electrode. When used, the battery characteristics such as the rate characteristics of the lithium ion secondary battery can be improved.

In the case of a slurry composition for slurry ceramic coating, the conductive material is used in an amount of 2-10 parts by weight with respect to 100 parts by weight of the inorganic filler such as alumina. If the amount of the conductive material used is less than 2 parts by mass, the heat resistance of the separator tends to decrease. Further, when the amount of the conductive material used is more than 10 parts by mass, the air permeability of the heat-resistant separator having a heat-resistant layer coated with a ceramic slurry decreases, and when used in a battery, the resistance of the battery tends to increase. .

(Slurry composition for electrode)
The electrode slurry composition (electrode resin composition) can be prepared by dispersing each of the above components together with the solvent-soluble polyimide binder of the present invention in a dispersion medium such as N-methylpyrrolidone (NMP). Specifically, using mixers such as ball mills, sand mills, bead mills, pigment dispersers, crushing machines, ultrasonic dispersers, homogenizers, planetary mixers, and Filmix, each of the above components, a solvent-soluble polyimide binder and a dispersion medium are mixed. A slurry composition can be prepared by mixing.

The solid content concentration of the obtained electrode slurry composition is preferably 40% by mass or more and 80% by mass or less. If the solid content concentration of the electrode slurry composition is within the above range, the drying efficiency can be increased when the electrode mixture layer is produced by removing the dispersion medium from the electrode slurry composition. In addition, since the electrode slurry composition contains solvent-soluble polyimide, it exhibits excellent coatability even when the solid content concentration is relatively high.

The viscosity of the electrode slurry composition is preferably 3000 mPa·s or more and 8000 mPa·s or less. If the viscosity of the electrode slurry composition is within the above range, the coatability will be good, holes formed by air entrainment during preparation of the slurry composition and uneven coating will be sufficiently suppressed, and the slurry composition will have a uniform thickness. An electrode mixture layer can be formed. The "viscosity" of the slurry composition for the electrode of the lithium ion secondary battery is the viscosity measured using a single cylindrical rotational viscometer (B-type viscometer). It can be measured using the method described.

(Electrodes for lithium-ion secondary batteries)
A lithium ion secondary battery electrode of the present invention includes a current collector and an electrode mixture layer formed on the current collector. The electrode mixture layer is formed using the slurry composition for the electrode of the lithium ion secondary battery, and specifically, it can be formed by applying the slurry composition for the electrode onto a current collector and drying it. can. That is, the electrode mixture layer is made of the dried electrode slurry composition, and usually contains the solvent-soluble polyimide of the present invention and the electrode active material, and optionally the conductive material. In addition to the solvent-soluble polyimide of the present invention, the electrode mixture layer may optionally contain other components such as polymers such as PVDF, epoxy resins, and synthetic rubbers as binders, antifoaming agents, leveling agents, and flame retardants. may be

A suitable abundance ratio of each component contained in the electrode mixture layer is the same as a suitable abundance ratio of each component in the slurry composition.
The lithium-ion secondary battery electrode of the present invention is produced using the slurry composition of the present invention, and therefore has excellent peel strength. By using the electrode of the present invention, a lithium ion secondary battery having excellent battery characteristics such as cycle characteristics can be obtained.

(Method for producing electrode for lithium ion secondary battery)
The electrode for a lithium ion secondary battery of the present invention includes, for example, a step of applying the above-described slurry composition on a current collector (application step), and drying and collecting the slurry composition applied on the current collector. It is manufactured through a process (drying process) of forming an electrode mixture layer on an electric body.

[Coating process]
The method for applying the electrode slurry composition onto the current collector is not particularly limited, and a known method can be used. Specifically, as a coating method, a doctor blade method, a dipping method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, or the like can be used. At this time, the slurry composition may be applied to only one side of the current collector, or may be applied to both sides. The thickness of the slurry film on the current collector after application of the slurry composition and before drying can be appropriately set according to the thickness of the electrode mixture layer obtained by drying.

Here, as the current collector to which the slurry composition is applied, a material having electrical conductivity and electrochemical durability is used. Specifically, as the current collector, for example, a current collector selected from iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, platinum, and the like can be used. In addition, one type of the above materials may be used alone, or two or more types may be used in combination at an arbitrary ratio.

[Drying process]
The method for drying the slurry composition on the current collector is not particularly limited, and known methods can be used. drying method. By drying the slurry composition on the current collector in this way, an electrode mixture layer is formed on the current collector to obtain an electrode for a lithium ion secondary battery comprising the current collector and the electrode mixture layer. be able to.
After the drying step, the electrode mixture layer may be pressurized using a mold press, a roll press, or the like. The pressure treatment can improve the adhesion between the electrode mixture layer and the current collector, and can increase the density of the electrode mixture layer.

<Lithium ion secondary battery and its manufacturing method>
The lithium ion secondary battery of the present invention includes a positive electrode, a negative electrode, an electrolytic solution, and a separator, and uses the lithium ion secondary battery electrode of the present invention for at least one of the positive electrode and the negative electrode. And since the lithium ion secondary battery of this invention is equipped with the electrode for lithium ion secondary batteries of this invention, it is excellent in battery characteristics, such as cycling characteristics.

(electrode)
Electrodes other than the lithium ion secondary battery electrode of the present invention described above can also be used in the lithium ion secondary battery of the present invention. Electrodes other than the electrode of the present invention are not particularly limited, but known electrodes used in the production of lithium ion secondary batteries can be used. Specifically, an electrode obtained by forming an electrode mixture layer on a current collector using a known manufacturing method can be used.

(Electrolyte)
As the electrolytic solution, an organic electrolytic solution in which a supporting electrolyte is dissolved in an organic solvent is usually used. Lithium salts, for example, are used as supporting electrolytes for lithium ion secondary batteries. Examples of lithium salts include LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAlCl ₄ , LiClO ₄ , CF ₃ SO ₃ Li, 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. Among these, LiPF ₆ , LiClO ₄ and CF ₃ SO ₃ Li are preferred, and LiPF ₆ is particularly preferred, from the viewpoint of being easily soluble in a solvent and exhibiting a high degree of dissociation. One type of electrolyte may be used alone, or two or more types may be used in combination at an arbitrary ratio. Generally, lithium ion conductivity tends to increase as a supporting electrolyte with a higher degree of dissociation is used, so the lithium ion conductivity can be adjusted depending on the type of supporting electrolyte.

The organic solvent used in the electrolytic solution is not particularly limited as long as it can dissolve the supporting electrolyte. Examples include dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), carbonates such as butylene carbonate (BC) and ethyl methyl carbonate (EMC); esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; sulfur-containing compounds such as sulfolane and dimethylsulfoxide and the like are preferably used. A mixture of these solvents may also be used. Among them, it is preferable to use carbonates from the viewpoint of high dielectric constant and wide stable potential region.

The concentration of the electrolyte in the electrolytic solution can be adjusted as appropriate, and is preferably 0.5 to 15% by mass, more preferably 2 to 13% by mass, and more preferably 5 to 10% by mass. More preferred. In addition, known additives such as fluoroethylene carbonate and ethyl methyl sulfone can be added to the electrolytic solution.

(separator)
The separator is not particularly limited, but a microporous film made of resin such as polyolefin (polyethylene, polypropylene), polyamide, polyimide, or the like is preferable. A heat-resistant separator having a ceramic coating layer provided on one or both sides of the microporous membrane to increase the heat resistance of the separator can also be used. The solvent-soluble polyimide of the present invention can also be suitably used as a binder for this ceramic coating layer.

(Manufacturing method of lithium ion secondary battery)
In the lithium ion secondary battery of the present invention, for example, a positive electrode and a negative electrode are superimposed with a separator interposed therebetween, and if necessary, wound according to the shape of the battery, or the positive electrode and the negative electrode are It can be manufactured by laminating a plurality of sheets with a separator sandwiched therebetween, putting the battery container into a battery container, injecting an electrolytic solution into the battery container, and sealing the battery container. In order to prevent the internal pressure rise of the lithium ion secondary battery and the occurrence of overcharge/discharge, etc., a fuse, an overcurrent prevention element such as a PTC element, an expanded metal, a lead plate, etc. may be provided as necessary. . The shape of the lithium-ion secondary battery may be, for example, coin-shaped, button-shaped, sheet-shaped, cylindrical, rectangular, or flat.

EXAMPLES The present invention will be specifically described below based on examples, but the present invention is not limited to these examples. In the following description, "%" and "parts" representing amounts are based on mass unless otherwise specified.

The abbreviations and compound names of the starting compounds used in the Synthesis Examples and Comparative Synthesis Examples below are described below.
s-BPDA: 3,3',4,4'-biphenyltetracarboxylic dianhydride (molecular weight: 294.2)
ODPA: 4,4'-oxydiphthalic dianhydride (molecular weight: 310.2)
PMDA: pyromellitic anhydride (molecular weight: 218.1)
BTDA: benzophenonetetracarboxylic dianhydride (molecular weight: 322.2)
DMPDA: 2,5-dimethyl-1,4-phenylenediamine (molecular weight: 136.19)
DETDA: diethylmethylbenzenediamine (molecular weight: 178.28)
m-DADE: 3,4-diaminodiphenyl ether (molecular weight: 200.24)
TPE-R: 1,3-bis(4-aminophenoxy)benzene (molecular weight: 292.33)
ABPS: 3,3-diamino-4,4-dihydroxydiphenylsulfone (molecular weight: 280.33)
p-DADE: diaminodiphenyl ether (molecular weight: 200.24)
APB: 1,3-bis(3-aminophenoxy)benzene (molecular weight: 292.33)
BAPS: bis[4-(4-aminophenoxy)phenyl]sulfone (molecular weight: 432.49)

<Synthesis of polyimide>
Synthesis Example 1
A separable three-necked glass flask fitted with a stainless steel anchor stirrer was fitted with a ball condenser with a moisture separation trap. 4,4′-oxydiphthalic dianhydride (ODPA, molecular weight: 310.2) 10.86 g (35 mmol), 2,5-dimethyl-1,4-phenylenediamine (DMPDA, molecular weight: 136.19) 9.53 g (70 mmol), γ-valerolactone 1.40 g, pyridine 2.22 g, N-methylpyrrolidone (NMP) 109 g, and toluene 45 g were added to a three-necked flask. In a nitrogen atmosphere, the mixture was heated and stirred in a silicon bath at 180° C. and 180 rpm for 1.5 hours. Further, the mixture was air-cooled to room temperature and stirred for 15 minutes. Then, 7.01 g (35 mmol) of 3,4-diaminodiphenyl ether (m-DADE, molecular weight: 200.24) and 1,3-bis(4-aminophenoxy)benzene (TPE-R, molecular weight: 292.33) 10 0.23 g (35 mmol) was added in portions to a three-necked flask with good mixing. Furthermore, 30.99 g (105 mmol) of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA, molecular weight: 294.2), NMP 146 g, and toluene 10 g were added, and the mixture was heated to 180° C. under a nitrogen atmosphere. , heated and stirred at 180 rpm. After 6 hours, the reaction was stopped to obtain a 20% by weight polyimide solution. Further, 106 g of NMP was added and stirred for 30 minutes to obtain a 15% by weight polyimide solution.

The molar ratios of each component used in polyimide synthesis were ODPA:s-BPDA=1:3 and DMPDA:m-DADE:TPE-R=2:1:1. All the tetracarboxylic dianhydride components used in polyimide synthesis had a molecular weight of 360 or less, and the ratio of diamine components having a molecular weight of 220 or less was 75 mol % of the total diamine components.

The imide group concentration of the polyimide was 30.9 according to the following formula.
(Calculation formula for imide group concentration)
- Average molecular weight of tetracarboxylic dianhydride:
0.25 x 310.2 + 0.75 x 294.2 = 298.2
・Average molecular weight of diamine:
0.5 x 136.19 + 0.25 x 200.24 + 0.25 x 292.33
= 191.18
・Imide group concentration:
[140/(298.2+191.18-2×18)]×100=30.9

The measurement results of the molecular weight and glass transition temperature (Tg) of the obtained polyimide are as follows.
Molecular weight Mw: 144,478
Tg: 296°C

Synthetic Example 2
An apparatus similar to that of Synthesis Example 1 was used. 10.86 g (35 mmol) of ODPA (molecular weight: 310.2), 9.53 g (70 mmol) of DMPDA (molecular weight: 136.19), 1.40 g of γ-valerolactone, 2.22 g of pyridine, 109 g of NMP, and 45 g of toluene. added to the mouth flask. In a nitrogen atmosphere, the mixture was heated and stirred in a silicon bath at 180° C. and 180 rpm for 1.5 hours. Further, the mixture was air-cooled to room temperature and stirred for 15 minutes. Then, 10.23 g (35 mmol) of TPE-R (molecular weight: 292.33) and 9.81 g (35 mmol) of 3,3-diamino-4,4-dihydroxydiphenylsulfone (ABPS, molecular weight: 280.33) were thoroughly mixed. The resulting mixture was added little by little to a three-necked flask. Furthermore, 30.99 g (105 mmol) of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA, molecular weight: 294.2), NMP 146 g, and toluene 10 g were added, and the mixture was heated to 180° C. under a nitrogen atmosphere. , heated and stirred at 180 rpm. After 6 hours and 30 minutes, the reaction was terminated to obtain a 20% by weight polyimide solution. Further, 106 g of NMP was added and stirred for 30 minutes to obtain a 15% by weight polyimide solution.

The molar ratio of each component used in synthesizing the polyimide was ODPA:DMPDA=1:2 and s-BPDA:TPE-R:ABPS=3:1:1. Further, the molecular weight of all the tetracarboxylic dianhydride components used in the synthesis of the polyimide is 360 or less, the ratio of the diamine component having a molecular weight of 220 or less is 50 mol% of the total diamine component, and the imide group concentration of the polyimide is It was 29.6%.
The measurement results of the molecular weight and glass transition temperature (Tg) of the obtained polyimide are as follows.
Molecular weight Mw: 172,691
Tg: 328.4°C

Synthesis Example 3
An apparatus similar to that of Synthesis Example 1 was used. 15.27 g (70 mmol) of PMDA (molecular weight: 218.1), 18.72 g (105 mmol) of DETDA (molecular weight: 178.28), 0.9 g of oxalic anhydride, 1.9 g of pyridine, 130 g of NMP and 30 g of toluene Added a 1-necked flask. The reaction was carried out at 180° C. and 175 rpm for 1 hour in a nitrogen atmosphere. Furthermore, the reaction was carried out at 180° C. and 165 rpm for 4 hours and 20 minutes. Then, 22.55 g (70 mmol) of BTDA (molecular weight: 322.2), 7.01 g (35 mmol) of p-DADE (molecular weight: 200.24) and 102 g of NMP were added, and the mixture was heated at 180°C and 165 rpm for 6 hours and 20 minutes. It reacted and obtained the polyimide solution of 20 weight%.

The molar ratios of the respective components used in synthesizing the polyimide were PMDA:DETDA=2:3 and BTDA:p-DADE=2:1. Further, the molecular weight of all the tetracarboxylic dianhydride components used in the synthesis of the polyimide is 360 or less, the ratio of the diamine component having a molecular weight of 220 or less is 100 mol% of the total diamine component, and the imide group concentration of the polyimide is It was 33.5%.
The measurement results of the molecular weight of the obtained polyimide are as follows.
Molecular weight Mw: 199,610
Tg: 319.5°C

Synthesis Comparative Example 1 (random polymerization example)
Using the same apparatus as in Synthesis Example 1, Synthesis Example 3 was carried out by random polymerization. Three mouths of DMPDA (molecular weight: 136.19) 21.79 g (160 mmol), TPE-R (molecular weight: 292.33) 23.39 g (80 mmol), ABPS (molecular weight: 280.33) 22.42 g (80 mmol) added to the flask. It was dissolved in NMP by stirring at 180 rpm for 10 minutes under a nitrogen atmosphere. Then, ODPA (molecular weight: 310.2) 24.82 g (80 mmol), s-BPDA (molecular weight: 294.2) 70.61 g (240 mmol), γ-VL 3.20 g, pyridine 5.06 g, NMP, toluene 45 g In addition, the mixture was stirred and heated at 180° C. and 180 rpm in a silicon bath in a nitrogen atmosphere. The total amount of NMP charged at this stage was 858 g. The reaction was terminated after 4 hours due to cloudiness of the solution.

Synthetic Comparative Example 2
Using the same device as in Synthesis Example 1, 34.60 g (80 mmol) of BAPS (molecular weight: 432.49) was added to a three-necked flask. It was dissolved in NMP by stirring at 180 rpm for 10 minutes under a nitrogen atmosphere. Then, s-BPDA (molecular weight: 294.2) 47.1 g (160 mmol), ODPA (molecular weight: 310.2) 24.82 g (80 mmol), γ-VL 3.20 g, pyridine 5.06 g, NMP, toluene 45 g In addition, the mixture was stirred and heated at 180° C. and 180 rpm in a silicon bath in a nitrogen atmosphere. The total amount of NMP charged at this stage was 390 g. After reacting for 5 hours and 30 minutes, a 20% by weight polyimide solution was obtained.

The molar ratio of each component used in synthesizing the polyimide was ODPA:BPDA=1:2, and the diamine was 100% BAPS. Further, the molecular weight of all the tetracarboxylic dianhydride components used in the synthesis of the polyimide is 360 or less, the ratio of the diamine component having a molecular weight of 220 or less is 0 mol% of the total diamine component, and the imide group concentration of the polyimide is It was 20.1%.
The measurement results of the molecular weight and glass transition temperature (Tg) of the obtained polyimide are as follows.
Molecular weight Mw: 123680
Tg: 272°C

Synthetic Comparative Example 3
An apparatus similar to that of Synthesis Example 1 was used. s-BPDA (molecular weight: 294.2) 58.84 g (200 mmol), 1,3-diaminobenzoic acid (molecular weight: 151.15) 15.21 g (100 mmol), γ-valerolactone 1.50 g, pyridine 2.4 g , 200 g of NMP and 30 g of toluene were added to a three-necked flask. In a nitrogen atmosphere, the mixture was heated and stirred in a silicon bath at 180° C. and 180 rpm for 1.0 hour. Furthermore, it was air-cooled to room temperature and stirred for 15 minutes. Then, 46.53 g (150 mmol) of ODPA (molecular weight: 310.2), 73.08 g (250 mmol) of TPE-R (molecular weight: 292.33), 360 g of NMP and 90 g of toluene were added, and the mixture was heated at 180°C and 180 rpm under a nitrogen atmosphere. Heat and stir. After 2 hours and 30 minutes, the reaction was stopped, NMP was added, and the mixture was stirred for 30 minutes to obtain a 20% by weight polyimide solution.

The molar ratios of the components used in polyimide synthesis were s-BPDA:ODPA=57.1:42.9 and diaminobenzoic acid:TPE-R=28.6:71.4.
Further, the molecular weight of the tetracarboxylic dianhydride component used in the synthesis of the polyimide is 360 or less, the ratio of the diamine component having a molecular weight of 220 or less is 28.6 mol% of the total diamine component, and the imide group concentration of the polyimide is It was 27.1%.
The measurement results of the molecular weight and glass transition temperature (Tg) of the obtained polyimide are as follows.
Molecular weight Mw: 80,000
Tg: 230°C

Examples 1-3, Comparative Examples 1-3
Using the polyimides synthesized in Synthesis Examples 1 to 3 and Synthesis Comparative Examples 1 to 3, a positive electrode slurry composition, a positive electrode for a lithium ion secondary battery, a separator and a lithium ion secondary battery are prepared by the following method. made.

<Preparation of positive electrode slurry composition>
Made in China NCM (nickel-cobalt-manganese composite oxide, N: C: M = 5: 3: 2) 1000 parts by mass, conductive agent Denka Black 20 parts by mass, the solvent-soluble polyimide of the present invention (8 mass% N- 250 parts by mass of methylpyrrolidone solution) were mixed with a planetary mixer, N-methylpyrrolidone (NMP) was added in several portions until the target viscosity was reached, and stirring was repeated to prepare a slurry composition.

<Preparation of positive electrode for lithium ion secondary battery>
An aluminum foil having a thickness of 18 μm was prepared as a current collector. The slurry composition prepared above was applied to one side of an aluminum foil with a comma coater and dried to obtain a positive electrode. This drying was carried out by transporting the aluminum foil with the coating film at a speed of 0.5 m / min in an oven at 120 ° C. for 2 minutes, and then further heat-treating it in an oven at 120 ° C. for 2 minutes. . The obtained positive electrode raw material was rolled by a roll press to obtain an electrode for a lithium ion secondary battery having a positive electrode mixture layer with a thickness of 60 μm.

<Preparation of separator for lithium ion secondary battery>
A single-layer polypropylene separator ("CELGARD (registered trademark) 2500" manufactured by Celgard) was punched out and used as a separator.

<Production of lithium-ion secondary battery>
As the positive electrode, the positive electrode produced above was heat-treated in a vacuum oven at 150° C. for 5 hours, and a 2032-type coin cell was produced by a normal method. Li foil was used for the negative electrode, and a 20 μm thick porous polyolefin film was used for the separator. As the electrolytic solution, an electrolytic solution obtained by dissolving 1 mol/L of LiPF ₆ in an electrolytic solution solvent consisting of ethylene carbonate:ethyl methyl carbonate:dimethyl carbonate=3:3:4 was used.

The molecular weight of the solvent-soluble polyimides produced in Synthesis Examples 1-3 and Synthesis Comparative Examples 1-3, the glass transition temperature (Tg), the viscosity of the 8% solution, the electrolyte swelling degree of the film obtained by drying, and , the viscosity of the slurry composition, the peel strength of the electrode, the cycle characteristics of the lithium ion secondary battery, and the irreversible capacity were measured and evaluated by the following methods.

<Molecular weight of solvent-soluble polyimide>
The solvent-soluble polyimide was diluted with NMP, and the weight average molecular weight was measured using a high-performance liquid chromatograph (manufactured by Tosoh).

<Glass transition temperature (Tg) of solvent-soluble polyimide>
Using Shimadzu Corporation DSG-50, after measuring the temperature up to 400 ° C. at a temperature increase rate of 10 ° C./min, the sample was air-cooled, and the temperature was raised to 420 ° C. at a temperature increase rate of 10 ° C./min. A glass transition temperature (Tg) was determined.

<Electrolytic solution swelling degree of solvent-soluble polyimide>
The solvent-soluble polyimide was poured into a petri dish made of PTX resin and dried at 120° C. for 15 hours to obtain a film with a thickness of about 50 μm. A test piece of 3 cm x 3 cm was prepared from the film, and the test piece was placed in an electrolytic solution obtained by dissolving 1 mol/L of LiPF ₆ in an electrolytic solution solvent consisting of ethylene carbonate: ethylmethyl carbonate: dimethyl carbonate = 3:3:4. Soaked. After heating the electrolytic solution to 80° C., the test pieces were immersed at 80° C. for 72 hours. The change in weight of the test piece before and after the immersion treatment was measured to determine the weight increase rate (%), which was defined as the swelling degree of the electrolyte. Here, the film preparation method does not affect the numerical value of the swelling degree of the electrolyte, and the weight of the film after the immersion treatment is determined by removing the test piece from the electrolyte and quickly wiping the surface with a tissue so as not to dry. It was measured after wiping off the electrolyte adhering to the

<Viscosity of 8% solvent-soluble polyimide solution)
According to JIS Z8803:1991, it was measured with a B-type viscometer (25° C., rotation speed=60 rpm, spindle shape: 4), and the value 60 seconds after the start of measurement was taken as the B-type viscosity of the 8% polyimide solution.

<Viscosity of slurry composition>
According to JISZ8803:1991, it was measured with a B-type viscometer (25°C, rotation speed = 60 rpm, spindle shape: 4), and the value 60 seconds after the start of measurement was taken as the B-type viscosity of the slurry composition.

<Electrode peel strength>
The prepared lithium ion secondary battery electrode was cut into a rectangle with a length of 100 mm and a width of 10 mm to form a test piece, and the surface having the electrode mixture layer was placed down on the surface of the electrode mixture layer with cellophane tape (JIS Z1522). ) was attached, and the stress when one end of the current collector was pulled vertically upward at a speed of 50 mm/min and peeled off was measured (the cellophane tape was fixed to the test stand). The measurement was performed 5 times, and the average value was obtained as the peel strength. The larger the peel strength value, the better the adhesion between the electrode mixture layer and the current collector.

<Cycle characteristics of lithium ion secondary battery>
The produced lithium ion secondary battery half cell was allowed to stand in an environment of 25° C. for 24 hours. After that, in an environment of 25° C., charge and discharge operations were performed by charging to 4.2 V and discharging to 3.0 V by a low current method at 0.5 C, and the initial capacity Cs was measured. Furthermore, this lithium ion secondary battery was repeatedly charged and discharged 200 times (200 cycles) in an environment of 25° C., and the capacity Cf after 200 cycles was measured.
Cycle characteristics were calculated by ΔC=(Cf/Cs)×100(%) and taken as a capacity retention rate. A higher value of this capacity retention ratio indicates better cycle characteristics.

<Presence or absence of irreversible capacity>
The produced lithium ion secondary battery was allowed to stand in an environment of 25° C. for 24 hours. After that, in an environment of 25 ° C., charging to 4.2 V by a low current method of 0.1 C and discharging to 3.0 V were performed. In comparison with the capacity of the lithium ion secondary battery produced with the electrode using the amount used, it was determined that the irreversible capacity was present when the capacity was small. The case where the irreversible capacity is not exhibited is indicated by ◯, and the case where the irreversible capacity is exhibited is indicated by x.

Table 1 below shows the physical properties of the solvent-soluble polyimides produced in Synthesis Examples 1 to 3 and Comparative Synthesis Examples 1 to 3, the evaluation results of the properties of the slurry compositions, electrodes, and lithium ion secondary batteries.

From the results in Table 1 above, block copolymerization in which the molecular weight of all the tetracarboxylic dianhydride components constituting the solvent-soluble polyimide is 360 or less and the ratio of the diamine component having a molecular weight of 220 or less is 30 mol% or more. It can be seen that when the coalesced polyimide is used (Examples 1 to 3), a low electrolyte swelling degree of 50% by weight or less can be achieved. Moreover, in the case of Examples 1 to 3, it can be seen that a lithium ion secondary battery having excellent cycle characteristics can be produced.

The solvent-soluble polyimide in the present invention is a polyimide capable of achieving both high solvent solubility in solvents such as N-methylpyrrolidone, dimethylacetamide, dimethylformamide, and dimethylsulfoxide, and low swelling in electrolyte solutions used in lithium ion secondary batteries. be. By using such a solvent-soluble polyimide as a binder for a lithium ion secondary battery, it is possible to produce an electrode having good adhesion between electrode active materials and adhesion between an electrode mixture layer and a current collector. It becomes possible. Moreover, by using such an electrode, it is possible to produce a lithium-ion secondary battery having excellent life characteristics (cycle characteristics).

Claims (14)

A binder for manufacturing a lithium ion secondary battery containing a solvent-soluble polyimide,
The solvent-soluble polyimide is a block copolymer having an imide group concentration of 20 to 32%, and is compatible with at least one solvent selected from the group consisting of N-methylpyrrolidone, dimethylacetamide, dimethylformamide and dimethylsulfoxide. is soluble,
All the tetracarboxylic dianhydride components constituting the polyimide have a molecular weight of 360 or less, and among all the diamine components constituting the polyimide, the ratio of diamine components having a molecular weight of 220 or less is at least 30 mol%. can be,
The degree of swelling when the film made from the polyimide is immersed in the electrolyte for lithium ion secondary batteries and treated at 80 ° C. for 72 hours is 50% by weight or less,
The electrolytic solution for a lithium ion secondary battery is prepared by dissolving 1 mol/L of lithium hexafluorophosphate (LiPF ₆ ) in a mixed solvent of ethylene carbonate:ethylmethylcarbonate:dimethylcarbonate (volume ratio 3:3:4). A binder for manufacturing a lithium ion secondary battery, which is an electrolytic solution consisting of 2. The binder for producing a lithium ion secondary battery according to claim 1 , wherein said polyimide has a weight average molecular weight of 100,000 or more. 3. The binder for producing a lithium ion secondary battery according to claim 1, wherein an 8% NMP solution of said polyimide has a B-type viscosity of 2000 cps or more. A binder for manufacturing a lithium ion secondary battery containing a solvent-soluble polyimide synthesized from a tetracarboxylic dianhydride and a diamine, wherein the polyimide is a block copolymer and the polyimide has an imide group concentration of 20 to 32%. and all the tetracarboxylic dianhydride components constituting the polyimide have a molecular weight of 360 or less, and among all the diamine components constituting the polyimide, the proportion of diamine components having a molecular weight of 220 or less is at least 30. A binder for manufacturing a lithium ion secondary battery, which is mol%. A resin composition for a lithium ion secondary battery, comprising an active material and the binder for producing a lithium ion secondary battery according to any one of claims 1 to 4 . An electrode for a lithium ion secondary battery, comprising a current collector coated with a resin composition containing an active material and the binder for producing a lithium ion secondary battery according to any one of claims 1 to 4 . 7. The electrode for a lithium ion secondary battery in accordance with claim 6 , wherein said active material is a nickel-cobalt-manganese ternary positive electrode active material, and said electrode is a positive electrode. 7. The electrode for a lithium ion secondary battery in accordance with claim 6 , wherein said active material is a negative electrode active material containing a Si material, and said electrode is a negative electrode. A lithium ion secondary battery comprising the electrode according to any one of claims 6 to 8 . A separator for a lithium ion secondary battery using the binder for manufacturing a lithium ion secondary battery according to any one of claims 1 to 4 as a binder for ceramic coating. A lithium ion secondary battery using the separator according to claim 10 . A method for producing a solvent-soluble polyimide for use in a lithium ion secondary battery manufacturing binder by reacting a tetracarboxylic dianhydride and a diamine, wherein all tetracarboxylic dianhydrides used in the reaction has a molecular weight of 360 or less, and the ratio of diamines with a molecular weight of 220 or less among all the diamines used in the reaction is at least 30 mol%, and the imide group concentration of the polyimide is 20 to 32%. A method for producing a binder for producing an ion secondary battery. Manufacture of an electrode for a lithium ion secondary battery comprising coating a current collector with a resin composition containing an active material and a binder for manufacturing a lithium ion secondary battery manufactured by the method according to claim 12 and drying the composition. Method. A method for producing a lithium ion secondary battery using the electrode produced by the method according to claim 13 .