JP2021180097A - Separator for secondary battery, manufacturing method of separator, and secondary battery equipped with separator - Google Patents

Abstract

To provide a separator for a secondary battery with excellent heat resistance and air permeability, a manufacturing method of the separator, and a secondary battery equipped with the separator.SOLUTION: A separator for a secondary battery according to the present disclosure includes a porous base material layer (A), and a heat-resistant layer (B) composed of a non-woven fabric containing fine fibers having a melting point or decomposition temperature of 200°C or higher, a binder, and a filler having polarity, and the weight ratio (filler/binder) of the filler to the binder in the heat-resistant layer (B) is 0.17 or more and less than 4. Further, in a manufacturing method of a separator according to the present disclosure, a dispersion medium solution of fine fibers, a binder, and a filler having polarity is applied to the surface of the porous base material layer (A), and the dispersion medium is volatilized to obtain a separator. Further, a secondary battery according to the present disclosure is provided with the separator for the secondary battery.SELECTED DRAWING: None

Description

The invention according to the present disclosure relates to a separator for a secondary battery, a method for manufacturing the separator, and a secondary battery provided with the separator.

In a power storage device such as a lithium ion secondary battery, a porous separator that insulates a positive electrode and a negative electrode while holding an electrolytic solution is used. Such a separator is required to ensure electrical insulation. In addition, since the battery functions, it is also required to ensure the permeability that lithium ions can move through the voids (holes) of the separator.

In recent years, lithium-ion secondary batteries have been considered to be installed in hybrid vehicles and electric vehicles in addition to information-related devices such as smartphones and laptop computers. Is required. For this reason, the separator is required to have heat resistance that can maintain the insulating function even in an abnormally high temperature environment.

In order to achieve both permeability and heat resistance, for example, Patent Document 1 describes a non-aqueous electrolyte battery separator provided with a microporous membrane and a heat-resistant porous layer containing a specific polymer and a specific inorganic filler. It has been disclosed. However, the separator of Patent Document 1 is not sufficiently satisfactory in terms of both permeability and heat resistance.

Further, Patent Document 2 describes a separator for a secondary battery including a porous base material and a heat-resistant layer made of a non-woven fabric containing fine fibers having a melting point (decomposition temperature of those having no melting point) of 200 ° C. or higher and a binder. Is disclosed. The separator of Patent Document 2 exhibits excellent transparency and heat resistance, but in recent years, in lithium ion secondary batteries, which are required to have higher voltage and larger capacity, the separator is more suitable. Further excellent permeability is required.

Japanese Unexamined Patent Publication No. 2019-149361 Japanese Unexamined Patent Publication No. 2009-231281

An object of the present invention is to provide a separator for a secondary battery having excellent heat resistance and permeability.
Another object of the invention according to the present disclosure is to provide a method for manufacturing the above-mentioned separator for a secondary battery.
Another object of the invention according to the present disclosure is to provide a secondary battery provided with the above-mentioned separator for a secondary battery.

As a result of diligent studies to solve the above problems, the present inventor has found that a non-woven fabric containing fine fibers, a binder and a filler having a melting point or a decomposition temperature of 200 ° C. or higher and having a specific weight ratio with respect to the binder and the filler has a specific weight ratio. If the structure obtained by laminating the above-mentioned non-woven fabric as a heat-resistant layer on a porous substrate, which has excellent heat resistance and permeability, is used as a separator for a secondary battery, even when the inside of the battery becomes abnormally high temperature. It was found that the function of insulating the positive electrode and the negative electrode can be maintained and the short circuit of the electrodes can be prevented. The invention according to the present disclosure has been completed based on these findings.

That is, the invention according to the present disclosure comprises a porous substrate layer (A) and a heat-resistant layer (B) made of a non-woven fabric containing microfibers, binders and fillers having a melting point or decomposition temperature of 200 ° C. or higher. Provided is a separator for a secondary battery, which comprises, and has a weight ratio (filler / binder) of the filler to the binder in the heat-resistant layer (B) of 0.17 or more and less than 4.

The invention according to the present disclosure also provides a separator for a secondary battery in which the microfibers are aramid fibers.

The invention according to the present disclosure also provides a separator for a secondary battery in which the binder is an aqueous binder.

（式中、Ｒは水酸基、カルボキシル基、フェニル基、Ｎ−置換又は無置換カルバモイル基、又は２−オキソ−１−ピロリジニル基を示す。）

（式中、ｎは２以上の整数を示し、Ｌはエーテル結合又は（−ＮＨ−）基を示す。） In the invention according to the present disclosure, the binder also has a polysaccharide derivative (1), a compound having a structural unit represented by the following formula (2), and a compound having a structural unit represented by the following formula (3). Provided is a separator for a secondary battery, which is at least one selected from the above.

(In the formula, R represents a hydroxyl group, a carboxyl group, a phenyl group, an N-substituted or unsubstituted carbamoyl group, or a 2-oxo-1-pyrrolidinyl group.)

(In the formula, n represents an integer of 2 or more, and L represents an ether bond or a (-NH-) group.)

The invention according to the present disclosure also provides a separator for a secondary battery, wherein the binder is at least one selected from cellulose, starch, glycogen, and derivatives thereof.

The invention according to the present disclosure also provides a separator for a secondary battery having an average fiber diameter of 0.01 to 1 μm and an average length of 0.01 to 2 mm.

The invention according to the present disclosure also provides a separator for a secondary battery in which the filler is an inorganic oxide filler.

The invention according to the present disclosure also provides a separator for a secondary battery in which the total thickness of the separator for a secondary battery is 10 to 50 μm and the thickness of the heat-resistant layer (B) is 0.5 to 20 μm.

The invention according to the present disclosure also comprises a step of applying a dispersion medium solution of the fine fibers, the binder, and the filler on the surface of the porous substrate (A) to volatilize the dispersion medium, and then the above two. Provided is a method for manufacturing a separator for a secondary battery, which obtains a separator for a secondary battery.

The invention according to the present disclosure also provides a secondary battery provided with the above-mentioned separator for a secondary battery.

The separator for a secondary battery of the present disclosure includes a porous base material layer (A) and a heat-resistant layer (B) made of a non-woven fabric containing microfibers, binders, and fillers having a specific melting point or decomposition temperature. Since the binder and the filler have a specific weight ratio, excellent air permeability and heat resistance are exhibited. The secondary battery separator can maintain the same heat resistance as the conventional one even if the thickness is reduced, and the degree of deformation due to shrinkage or melting can be extremely reduced even in an abnormally high temperature environment. Since it can be formed, the insulating function can be maintained. Therefore, the secondary battery provided with the separator can prevent a short circuit of the electrodes even in a high temperature environment, and can prevent the occurrence of thermal runaway due to the short circuit of the electrodes. That is, it is excellent in safety. Therefore, the separator for a secondary battery of the present disclosure can be suitably used for information-related devices such as smartphones and notebook computers, hybrid vehicles, electric vehicles, and the like. In particular, when a secondary battery provided with the above-mentioned separator for a secondary battery is used in an electric vehicle, it is possible to significantly reduce the weight, thereby dramatically improving fuel efficiency.

[Separator for secondary battery]
The separator for a secondary battery of the present disclosure comprises a porous base material layer (A) and a heat-resistant layer (B) made of a non-woven fabric containing microfibers having a specific melting point or decomposition temperature, a binder, and a filler having polarity. include.

The air permeability of the separator for a secondary battery of the present disclosure is preferably 100 to 800 sec / 100 mL, more preferably 100 to 600 sec / 100 mL, still more preferably 100 to 400 sec / 100 mL, and particularly preferably 100 to 350 sec / 100 mL. .. If the air permeability is lower than the above range, the insulating property is lowered and a short circuit tends to occur easily. On the other hand, when the air permeability exceeds the above range, the electric resistance tends to increase.

Further, the separator for a secondary battery of the present disclosure is very lightweight because the heat resistant layer (B) is formed of a non-woven fabric. Therefore, the weight of the secondary battery provided with the separator for the secondary battery of the present invention can be reduced, and the weight of the electric vehicle or the like using the secondary battery can be significantly reduced.

Since the separator for a secondary battery of the present disclosure can exhibit sufficient heat resistance even if the heat-resistant layer (B) is thin, the separator for a secondary battery can be thinned, and its total thickness is , 10 to 50 μm, more preferably 10 to 30 μm. Therefore, the separator for a secondary battery of the present invention can increase the filling density of the battery and contributes to the miniaturization of the secondary battery.

<Porous substrate layer (A)>
The porous substrate layer (A) has voids, and the size of the voids is, for example, 0.01 to 1 μm, preferably 0.02 to 0.06 μm. If the size of the void exceeds the above range, the insulating property tends to decrease and a short circuit tends to occur easily. On the other hand, when the size of the void is less than the above range, the electric resistance tends to increase.

The porosity of the porous substrate layer (A) is preferably 20 to 70% by volume, more preferably 30 to 60% by volume. If the porosity exceeds the above range, the strength tends to be insufficient. On the other hand, when the porosity is lower than the above range, the permeability of lithium ions tends to deteriorate and the electrical resistance tends to increase.

The air permeability of the porous substrate layer (A) is preferably 100 to 600 sec / 100 mL, more preferably 100 to 400 sec / 100 mL, and even more preferably 100 to 350 sec / 100 mL. When the air permeability exceeds the above range, the permeability of lithium ions tends to deteriorate and the electrical resistance tends to increase. On the other hand, when the air permeability is lower than the above range, the strength tends to be insufficient.

It is preferable that the porous base material layer (A) is insoluble in an electrolytic solution and has a function of softening in a high temperature environment to close voids, that is, a shutdown function. Therefore, a thermoplastic polymer is preferable as the material of the porous base material layer (A). Examples of the thermoplastic polymer include polyolefins such as polyethylene and polypropylene; polyester resins such as polyethylene terephthalate; and aliphatic polyamides such as 2,6-nylon. Then, it is preferable to appropriately select and use the thermoplastic polymer according to the desired shutdown temperature. For example, when the shutdown temperature is set to 150 ° C., it is preferable to use a thermoplastic polymer having a melting point or a softening temperature of 140 to 150 ° C.

The thickness of the porous substrate layer (A) is preferably 10 to 40 μm, more preferably 10 to 30 μm. If the thickness exceeds the above range, the electric resistance of the battery tends to increase and the volume capacity tends to decrease. On the other hand, when the thickness is lower than the above range, the strength tends to be insufficient.

The porous base material layer (A) is made porous by, for example, heating and melting the material of the porous base material (for example, polyolefin) to form an extruded film, and stretching the obtained coating film. It can be manufactured by a method or the like.

The surface of the porous substrate layer (A) is roughened, easily adhered, antistatic, sandblasted (sandmat), corona discharge, plasma, chemical etching, and water mat. One or more selected from treatment, flame treatment, acid treatment, alkali treatment, ultraviolet irradiation treatment, silane coupling agent treatment and the like can be applied.

For example, the surface of the porous substrate layer (A) is subjected to a treatment selected from corona discharge treatment, plasma treatment, flame treatment, acid treatment, alkali treatment, and ultraviolet irradiation treatment, and then treated with a silane coupling agent. If applied, the effect of improving the adhesion between the porous substrate layer (A) and the heat-resistant layer (B) can be obtained as compared with the case where the silane coupling agent treatment is applied alone.

In the present disclosure, in particular, a polar group (for example, a hydroxyl group, a carbonyl group, a carboxyl group, etc.) is imparted to the surface of the porous substrate layer (A) such as a corona discharge treatment and a plasma treatment to improve wettability. It is preferable to perform a treatment for improving the air permeability in that the effect of lowering the air permeability, improving the permeability of lithium ions and lowering the electric resistance can be obtained.

Therefore, for the porous base material layer (A), it is preferable to use a porous base material made of a thermoplastic polymer and having a polar group imparted to the surface thereof, and in particular, a polar group is imparted to the surface thereof. It is preferable to use a polyolefin-based porous substrate.

<Heat-resistant layer (B)>
The heat-resistant layer (B) is made of a non-woven fabric containing microfibers, binders, and fillers having a specific melting point or decomposition temperature.

The heat-resistant layer (B) is formed by applying a dispersion medium solution containing at least microfibers, binders, and fillers to at least one surface of the porous substrate layer (A) and volatilizing the dispersion medium. be able to.

The porosity of the heat-resistant layer (B) is preferably 30 to 60% by volume, more preferably 35 to 55% by volume. If the porosity exceeds the above range, heat resistance (for example, shape retention in a high temperature environment) tends to be insufficient. On the other hand, when the porosity is lower than the above range, the permeability of lithium ions tends to deteriorate and the electrical resistance tends to increase.

The air permeability of the heat-resistant layer (B) is preferably 1 to 500 sec / 100 mL, more preferably 1 to 400 sec / 100 mL, still more preferably 1 to 30 sec / 100 mL, and particularly preferably 1 to 300 sec / 100 mL. When the air permeability exceeds the above range, the permeability of lithium ions tends to deteriorate and the electrical resistance tends to increase. On the other hand, when the air permeability is lower than the above range, the heat resistance (for example, the shape retention in a high temperature environment) tends to be insufficient.

The thickness of the heat-resistant layer (B) is preferably 0.5 to 20 μm, more preferably 1 to 15 μm, still more preferably 2 to 12 μm, particularly preferably 3 to 10 μm, and particularly preferably 3 to 8 μm. When the thickness is less than the above range, the heat resistance of the separator (for example, the shrinkage suppressing effect and the shape retention in a high temperature environment) tends to be insufficient. On the other hand, when the thickness exceeds the above range, the packing density of the electrodes tends to be insufficient.

The basis weight of the heat-resistant layer (B) ^{is preferably 0.3 to 10 g / m 2} , more preferably 0.5 to 9 g / m ² , still more preferably 0.8 to 7 g / m ² , and particularly preferably 1. It is ~ 6 g / m ² , most preferably 1.8 ~ 2.5 g / m ² . If the basis weight is below the above range, it tends to be insufficient to contribute to the weight reduction of the battery. On the other hand, if the thickness exceeds the above range, the shape retention of the separator tends to be insufficient.

As the dispersion medium of the dispersion medium solution, it is preferable to use water because it has a small environmental load and is excellent in safety. The dispersion medium may contain an organic solvent in addition to water.

(Fine fiber)
The fine fibers have a melting point or decomposition temperature, that is, those having no melting point or melting point have a decomposition temperature of 200 ° C. or higher, preferably 250 ° C. or higher, and more preferably 300 ° C. or higher. The upper limit of the melting point is, for example, 500 ° C.

The average fiber diameter of the fine fibers is not particularly limited, but is, for example, 0.01 to 1 μm, and has more excellent heat resistance (or shape retention in a high temperature environment), and the separator is thinned. The upper limit of the average fiber diameter is preferably 0.85 μm, particularly preferably 0.7 μm, in terms of contributing to the miniaturization of the secondary battery (or contributing to the miniaturization of the secondary battery). The lower limit of the average fiber diameter is preferably 0.05 μm, particularly preferably 0.1 μm. For the average fiber diameter of the fine fibers, an electron microscope image was taken for a sufficient number (for example, 100 or more) of fine fibers using an electron microscope (SEM, TEM), and the diameter (diameter) of these fine fibers was taken. Is obtained by measuring and arithmetically averaging.

The average length of the fine fibers is preferably 0.01 to 2 mm, and among them, it has more excellent heat resistance (for example, shape retention in a high temperature environment) and contributes to the thinning of the separator (or two). The upper limit of the average length is more preferably 1.5 mm, particularly preferably 1.0 mm, and most preferably 0.8 mm in terms of (contributing to miniaturization of the next battery). The lower limit of the average length is more preferably 0.03 mm, particularly preferably 0.07 mm, and most preferably 0.1 mm. The average length of the fine fibers is measured by taking an electron microscope image of a sufficient number (for example, 100 or more) of the fine fibers using an electron microscope (SEM, TEM). However, it is calculated by arithmetic averaging. The length of the microfibers should be measured in a linearly stretched state, but in reality, many of them are bent, so the projected diameter and projection of the microfibers from the electron microscope image using an image analyzer. The area shall be calculated, and it shall be calculated from the following formula assuming a cylindrical body.
Length = projected area / projected diameter

The average aspect ratio (average length / average fiber diameter) of the fine fibers is not particularly limited, but is, for example, 10 to 2000, and among them, more excellent heat resistance (for example, shape retention in a high temperature environment) is obtained. Even if it has a thinner thickness, it can form a non-woven fabric having sufficient heat resistance, which contributes to the thinning of the separator (or to the miniaturization of the secondary battery), and has an average aspect ratio. The upper limit is more preferably 1500, particularly preferably 1000, and most preferably 900. The lower limit of the average aspect ratio is more preferably 50, particularly preferably 100, most preferably 500, and particularly preferably 600.

Examples of the fine fibers include cellulose fibers, aramid fibers, polyphenylene sulfide fibers, polyimide fibers, fluorine fibers, glass fibers, carbon fibers, polyp-phenylene benzoxazole fibers, polyether ether ketone fibers, liquid crystal polymer fibers and the like. Be done. These can be used alone or in combination of two or more.

The fine fibers preferably include at least aramid fibers. This is because the aramid fiber has excellent heat resistance and can be easily made into fine fibers.

The aramid fiber is a fiber composed of a polymer having a structure in which two or more aromatic rings are bonded via an amide bond (that is, a total aromatic polyamide), and the total aromatic polyamide has a meta type and a para type. included. The total aromatic polyamide is, for example, a polymer having a structural unit represented by the following formula (a).

In the above formula, Ar ¹ and Ar ² indicate the same or different aromatic rings, or groups in which two or more aromatic rings are bonded via a single bond or a linking group.

Examples of the aromatic ring include an aromatic hydrocarbon ring having 6 to 10 carbon atoms such as a benzene ring and a naphthalene ring. The aromatic ring has various substituents [eg, halogen atom, alkyl group (eg, C _1-4 alkyl group), oxo group, hydroxyl group, substituted oxy group (eg, C _1-4 alkoxy group, C _1-4). (Acyloxy group, etc.), carboxyl group, substituted oxycarbonyl group (eg, C _1-4 alkoxycarbonyl group), cyano group, nitro group, substituted or unsubstituted amino group (eg, mono or di-C _1-4 alkylamino group). , Sulf group, etc.]. Further, the aromatic ring may be condensed with a heterocyclic ring having an aromatic or non-aromatic attribute.

Examples of the linking group include a divalent hydrocarbon group (for example, a linear or branched alkylene group having 1 to 18 carbon atoms, a divalent alicyclic hydrocarbon group having 3 to 18 carbon atoms, and the like. ), A carbonyl group (-CO-), an ether bond (-O-), an ester bond (-COO-), -NH-, -SO _2-, and the like.

The aramid fiber can be produced, for example, by reacting a halide of at least one aromatic dicarboxylic acid with at least one aromatic diamine (for example, solution polymerization, surface polymerization, etc.).

Examples of the aromatic dicarboxylic acid include isophthalic acid, terephthalic acid, 1,4-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 4,4'-biphenyldicarboxylic acid, and 3,3'-biphenyldicarboxylic acid. Examples thereof include 4,4'-diphenyl ether dicarboxylic acid.

Examples of the aromatic diamine acid include p-phenylenediamine, m-phenylenediamine, 4,4'-diaminobiphenyl, 2,4-diaminodiphenylamine, 4,4'-diaminobenzophenone, and 4,4'-diaminodiphenyl ether. , 4,4'-Diaminodiphenylmethane, 4,4'-diaminodiphenylamine, 4,4'-diaminodiphenylsulfone, 2,4-diaminotoluene, 2,6-naphthalenediamine, 1,5-naphthalenediamine and the like. ..

The aramid fiber can be produced by spinning the all-aromatic polyamide into a fibrous form (for example, through steps such as spinning, washing, and drying) by a well-known and conventional method. Further, after being spun into a fibrous form, a crushing treatment or the like can be performed as needed. In the invention according to the present disclosure, a nonwoven fabric having sufficient heat resistance (for example, shape retention in a high temperature environment) can be formed even if the thickness is thinner, which further contributes to the thinning of the separator (or). In terms of contributing to the miniaturization of the secondary battery), it is preferable to apply a strong mechanical shearing force to the microfibril with an ultra-high pressure homogenizer or the like.

The content of the fine fibers in the heat-resistant layer (B) is, for example, 30 to 70% by weight, and the strength, flexibility, and porosity of the heat-resistant layer (B), which is a non-woven fabric, are particularly suitable. From this point of view, the lower limit of the content of the fine fibers is preferably 35% by weight, particularly preferably 40% by weight. The upper limit of the content of the filler is preferably 66% by weight, particularly preferably 62% by weight.

(Binder)
The binder of the present disclosure exerts an effect of imparting an appropriate viscosity to the dispersion medium solution to improve coatability, and further imparts adhesiveness to fix the fine fibers to the surface of the porous base material layer. It is a compound that exerts an action to make it.

It is not essential to use a binder having excellent heat resistance in terms of developing the heat resistance of the heat-resistant layer (B), but it is preferable, and the melting point or decomposition temperature of the binder is, for example, 160 ° C. or higher (preferably 180 ° C. or higher). It is particularly preferable to use a binder having a temperature of 200 ° C. or higher. The upper limit of the melting point or decomposition temperature of the binder is, for example, 400 ° C.

As the binder, the viscosity of the 1 wt% aqueous solution (at 25 ° C. and 60 rpm) is preferably, for example, 100 to 5000 mPa · s, particularly preferably 500 to 3000 mPa · s, and most preferably 1000 to 2500 mPa · s. Is.

Examples of the binder include non-aqueous binders such as fluorine-based binders (for example, polyvinylidene fluoride, etc.), polyester-based binders, epoxy-based binders, acrylic-based binders, vinyl ether-based binders, and water-based binders. As the binder, it is preferable to use a water-based binder because it has a small environmental load and is excellent in safety.

（式中、Ｒは水酸基、カルボキシル基、フェニル基、Ｎ−置換又は無置換カルバモイル基、又は２−オキソ−１−ピロリジニル基を示す。）

（式中、ｎは２以上の整数を示し、Ｌはエーテル結合又は（−ＮＨ−）基を示す。） Examples of the aqueous binder include a polysaccharide derivative (1), a compound having a structural unit represented by the following formula (2), a compound having a structural unit represented by the following formula (3), and the like. These can be used alone or in combination of two or more.

(In the formula, R represents a hydroxyl group, a carboxyl group, a phenyl group, an N-substituted or unsubstituted carbamoyl group, or a 2-oxo-1-pyrrolidinyl group.)

(In the formula, n represents an integer of 2 or more, and L represents an ether bond or a (-NH-) group.)

As the N- substituted carbamoyl _{group, -CONHCH (CH 3) 2,} -CON (CH 3) such as ₂ groups include N-C _1-4 alkyl-substituted carbamoyl group.

The carboxyl group may form a salt with an alkali metal.

The above n is an integer of 2 or more, for example, an integer of 2 to 5, preferably an integer of 2 to 3. _{Therefore, the [C n} H _2n ] group in the formula (3) is an alkylene group having 2 or more carbon atoms, and examples thereof include a dimethylene group, a methylmethylene group, a dimethylmethylene group, and a trimethylene group.

The compound having the structural unit represented by the above formula (2) and the compound having the structural unit represented by the following formula (3) are each represented by the structural unit represented by the formula (2) or the formula (3). It may have a structural unit other than the represented structural unit.

Examples of the compound having a structural unit represented by the above formula (2) include styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), methyl methacrylate butadiene rubber (MBR), and butadiene rubber (BR). Diene-type rubbers such as; acrylic polymers such as polyacrylic acid, sodium polyacrylate, acrylic acid / maleic acid copolymer / sodium salt, acrylic acid / sulfonic acid copolymer / sodium salt; polyacrylamide, poly-N. Acrylamide-based polymers such as -isopropylacrylamide, poly-N, N-dimethylacrylamide; polyvinylpyrrolidone and the like can be mentioned.

Examples of the compound having a structural unit represented by the above formula (3) include polyalkylene glycols such as polyethylene glycol and polypropylene glycol; polyethyleneimine and the like.

The polysaccharide derivative (1) is a compound obtained by polymerizing two or more monosaccharides by glycosidic bonds. The polysaccharide derivative (1) is preferably a compound obtained by polymerizing glucose (for example, α-glucose or β-glucose) by a glycosidic bond, or a derivative thereof, and in particular, cellulose, starch, glycogen, or At least one selected from these derivatives is preferred.

As the polysaccharide derivative (1), it is particularly excellent in heat resistance to use cellulose or a derivative thereof, and by adding a small amount, excellent adhesive strength and viscosity can be imparted to the dispersion medium solution. Is preferable.

（式中、Ｒ¹〜Ｒ³は、同一又は異なって、水素原子、ヒドロキシル基又はカルボキシル基を有する炭素数１〜５のアルキル基を示す。尚、上記ヒドロキシル基及びカルボキシル基はアルカリ金属と塩を形成していてもよい。） Examples of the cellulose or its derivatives include compounds having a structural unit represented by the following formula (1-1).

(In the formula, R ^{1 to} R ³ represent the same or different alkyl groups having hydrogen atoms, hydroxyl groups or carboxyl groups and having 1 to 5 carbon atoms. The hydroxyl groups and carboxyl groups are alkali metals and salts. May be formed.)

Examples of the alkyl group having 1 to 5 carbon atoms include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, an s-butyl group, a t-butyl group, a pentyl group and the like.

The hydroxyl group and the carboxyl group may form a salt with an alkali metal, for example, the -OH group may form a salt with sodium to form a -ONa group, and the -COOH group may form a sodium. It may form a salt to form a -COONa group.

Specific examples of the cellulose derivative include hydroxyethyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, and alkali metal salts thereof (for example, sodium carboxymethyl cellulose).

Based on the above, aqueous binders are preferable as the binders of the present disclosure, and among them, the effect of imparting viscosity is excellent, the coatability of the dispersion medium solution can be improved by adding a small amount, and the fine fibers are used in the porous substrate layer. The polysaccharide derivative (1) is preferable, and cellulose or a derivative thereof is particularly preferable, because it can exert an action of adhering to the surface and is excellent in heat resistance.

The content of the binder in the heat-resistant layer (B) is, for example, 10 to 60% by weight, and in particular, the binding property between the fine fibers and the filler is improved and the strength of the heat-resistant layer (B) is increased. In addition, the lower limit of the binder content is preferably 13% by weight, particularly preferably 15% by weight, from the viewpoint of improving the adhesion of the heat-resistant layer (B) to the porous base material layer (A). Is. Further, from the viewpoint of suppressing the blockage of the voids of the porous base material layer (A) and the porous base material layer (A), the upper limit of the binder content is preferably 40% by weight, particularly preferably 20% by weight. %.

The weight ratio (microfibers / binder) of the fine fibers to the binder in the heat-resistant layer (B) is determined from the viewpoint of the strength of the heat-resistant layer (B) and the adhesion to the porous base material layer (A). , 0.5 to 7, more preferably 0.9 to 5, still more preferably 2 to 4.

(Filler)
The filler is not particularly limited, and known fillers can be used. The filler may be an organic filler or an inorganic filler, but an inorganic filler is preferable because it is excellent in adsorbing binder.

Examples of the organic filler include polyurethane fine particles, epoxy resin fine particles, nylon fine particles (nylon 6 fine particles, nylon 6-6 fine particles, nylon 6-10 fine particles, nylon 6T fine particles, nylon 9T fine particles, etc.), polyphenylene sulfide fine particles, and polyphenylene sulfone. Fine particles, polyether sulfone fine particles, polyethylene terephthalate fine particles, polyamideimide fine particles, polyimide fine particles, polyetherimide fine particles, polycarbonate fine particles, polyether ketone fine particles, polyether ether ketone fine particles, silicone fine particles, polylactic acid fine particles, PMMA fine particles, polystyrene fine particles, Examples thereof include aramid fine particles (meth-based aramid fine particles, para-based aramid fine particles, etc.), cellulose nanocrystals, and the like.

The inorganic filler has polarity and is more excellent in binder adsorption. Therefore, the inorganic oxide filler (silica (SiO ₂ ), alumina (Al ₂ O ₃ ), titania (TIO ₂ ), zinc oxide (Zn O)) ), Calcium oxide (CaO), Zirconium oxide (ZrO ₂ ), Magnesia (MgO), Barium oxide, Boron oxide, Mica, Tarku, Wallastnite, Attapulsite, Boehmite (Al ₂ O ₃ · H ₂ O), etc.) Preferred, more preferably silica, alumina, titania, magnesia, still more preferably silica, alumina.

The above fillers can be used alone or in combination of two or more.

The shape of the filler is not particularly limited, and may be any shape such as granular, plate-shaped, and needle-shaped.

When the filler is a granular filler or a plate-shaped filler, the volume average particle size is preferably 0.005 to 50 μm, more preferably 0.01 to 40 μm, and further preferably 0.02 to 10 μm. When the volume average particle size is 0.005 μm or more, the heat-resistant layer (B) tends to form a porous structure, and the filler does not easily close the voids of the porous substrate layer (A). When the volume average particle size is 50 μm or less, the heat-resistant layer (B) tends to be thinned, and the heat resistance tends to be improved. The average particle size of the granular filler and the plate-shaped filler is the median diameter (D50) by the laser diffraction / scattering method.

When the filler is a needle-shaped filler, the average length is preferably 0.02 to 100 μm, more preferably 0.05 to 60 μm, and further preferably 0.1 to 40 μm. The average diameter is preferably 0.002 to 10 μm, more preferably 0.005 to 6 μm, and even more preferably 0.01 to 4 μm. The aspect ratio (average length / average diameter) is preferably 1.1 or more and less than 10, more preferably 1.2 to 8, and even more preferably 1.5 to 6. When the average length, average diameter and aspect ratio are within the above ranges, the heat-resistant layer (B) tends to form a porous structure, and the filler closes the voids of the porous substrate layer (A). Hateful. Further, the heat-resistant layer (B) tends to be thinned easily and the heat resistance tends to be improved. As for the average length and average diameter of the filler, an electron microscope image of a sufficient number (for example, 100 or more) of needle-shaped fillers can be obtained by using an electron microscope (SEM, TEM) as in the case of the above-mentioned fine fibers. It is obtained by taking a picture, measuring the length and diameter of the filler, and arithmetically averaging them.

The reason why the separator of the present disclosure is excellent in permeability is that the filler present in the dispersion medium dissolving forming the heat-resistant layer adsorbs an excess binder regardless of the shape and particle size of the filler. It is considered that the blockage of the voids (pores) of the porous substrate due to the above-mentioned is suppressed, and the decrease in permeability at the time of forming the heat-resistant layer is suppressed.

The content of the filler in the heat-resistant layer (B) is, for example, 10 to 70% by weight, and in particular, the permeability of the separator can be improved by suppressing the blockage of the voids of the porous substrate. From the possible points, the lower limit of the content of the filler is preferably 15% by weight, particularly preferably 20% by weight. The upper limit of the content of the filler is preferably 60% by weight, particularly preferably 50% by weight, from the viewpoint of not impairing the flexibility of the heat-resistant layer (B).

The weight ratio (filler / microfiber) of the filler to the microfibers in the dispersion medium is preferably 0.15 to 2.3, more preferably 0.23 to 1.7, and even more preferably 0.35. ~ 1.2. When the weight ratio is within the above range, the heat-resistant layer (B) is likely to be stably formed, so that the flexibility and permeability of the heat-resistant layer (B) can be easily obtained.

The weight ratio (filler / binder) of the filler to the binder in the dispersion medium is 0.17 or more and less than 4.0, preferably 0.35 to 3.8, and more preferably 1.1 to 3. It is .5. When the weight ratio is within the above range, the binder is easily adsorbed by the filler, and the voids of the porous base material layer are less likely to be blocked by the binder.

(Dispersion medium solution)
The dispersion medium solution according to the present disclosure contains at least the fine fibers, the binder, the filler, and the dispersion medium.

In the dispersion medium solution, for example, fine fibers are mixed with a binder and water, and a mechanical shearing force having a processing pressure of 30 to 300 MPa is applied using an ultra-high pressure homogenizer or the like to microfibrillate the fine fibers. It can be prepared by adding a filler and further mixing.

The content of the fine fibers in the dispersion medium solution is, for example, 0.15 to 5.0% by weight, and above all, an increase in air permeability can be suppressed and the permeability of the electrolytic solution can be improved. The lower limit of the content of the fine fibers is preferably 0.18% by weight, particularly preferably 0.2% by weight. The upper limit of the content of the fine fibers is preferably 4.7% by weight, particularly preferably 4% by weight.

The content of the binder in the dispersion medium solution is, for example, 0.003 to 5% by weight, and the lower limit of the content of the binder is preferable from the viewpoint of enabling uniform coating. Is 0.015% by weight, particularly preferably 0.03% by weight. Further, the upper limit of the content of the binder is preferably 3% by weight, more preferably 1.5% by weight, in that the increase in air permeability can be suppressed and the permeability of the electrolytic solution can be improved. %.

The content of the filler in the dispersion medium solution is, for example, 0.03 to 7% by weight, and in particular, the strength and heat resistance of the heat-resistant layer (B), which is a nonwoven fabric, can be improved. The lower limit of the filler content is preferably 0.05% by weight, particularly preferably 0.06% by weight. The upper limit of the content of the filler is preferably 4.8% by weight, particularly preferably 3.5% by weight.

Water is preferable as the dispersion medium, but a mixed solution of water and an organic solvent may be used. The content of the organic solvent in the mixed solution is preferably 50% by weight or less, particularly preferably 30% by weight or less, and most preferably 10% by weight or less, based on the total amount of the dispersion medium.

The dispersion medium solution may contain other components (for example, a dispersant, a surfactant, etc.) in addition to the fine fibers, the binder, and the filler as non-volatile components, but the dispersion medium solution may contain. The ratio of the total content of the fine fibers, the binder, and the filler to the total amount of the non-volatile content contained is, for example, 50% by weight or more, preferably 60% by weight or more, more preferably 70% by weight or more, and particularly preferably 70% by weight or more. It is 80% by weight or more, most preferably 90% by weight or more, and particularly preferably 95% by weight or more.

The concentration of the non-volatile content in the dispersion medium solution can be appropriately adjusted according to the thickness of the nonwoven fabric to be formed, and can be, for example, 0.5 to 10% by weight.

The viscosity of the dispersion medium solution at a temperature of 25 ° C. and a shear rate of 100 (1 / s) is, for example, 10 to 10000 mPa · s, and in particular, 100 to 7000 mPa is preferable in terms of enabling uniform coating. -S, particularly preferably 300 to 5000 mPa · s.

Since the dispersion medium solution has the above-mentioned characteristics, it can be suitably used for forming a separator for a secondary battery (particularly, a separator for a secondary battery having excellent heat resistance (= heat-resistant separator for a secondary battery)).

[Manufacturing method of separator for secondary battery]
The separator for a secondary battery is, for example, a heat-resistant layer (B) which is a non-woven fabric made of a solidified product of the dispersion medium solution by applying the dispersion medium solution to the surface of the porous substrate (A) and drying the mixture. ) And the porous substrate (A) can be manufactured through a step of forming a laminate.

The method for applying the dispersion medium solution is not particularly limited, and may be, for example, a printing method, a coating method, or the like. Specific examples thereof include screen printing method, mask printing method, offset printing method, inkjet printing method, flexographic printing method, gravure printing method, silk screen printing method, stamping, dispense, squeegee method, spraying, and brush coating. Further, the coating may be performed by a film applicator, a bar coater, a die coater, a comma coater, a gravure coater or the like.

The dispersion medium solution may be applied to at least one surface of the porous substrate (A). In the invention according to the present disclosure, among them, the total thickness of the separator for a secondary battery can be reduced, the filling density of the battery can be increased, and the porosity can be contributed to the miniaturization of the secondary battery. It is preferable to apply it only to one surface of the base material (A).

The method for drying the dispersion medium solution is not particularly limited, and examples thereof include heating, depressurization, and ventilation. The heating temperature and heating time, decompression degree and decompression time, air volume, air velocity, air temperature, type and dryness of gas to be blown, area to be blown, direction of air blow, etc. can be arbitrarily selected. ..

[Secondary battery]
The secondary battery of the present disclosure is a power generation element including a positive electrode in which a positive electrode active material layer is arranged in a positive electrode current collector, a negative electrode in which a negative electrode active material layer is arranged in a negative electrode current collector, a separator, and an electrolytic solution. Is included inside the exterior body. The secondary battery is characterized in that the above-mentioned separator for a secondary battery is used as the separator.

The secondary battery is a wound battery in which a positive electrode, a negative electrode, and a separator are laminated and wound, and the battery is sealed in a container such as a can together with an electrolytic solution. A laminated battery in which a sheet-like material is enclosed together with an electrolytic solution inside a relatively flexible exterior body may be used.

The secondary battery is excellent in safety because it includes the above-mentioned separator for a secondary battery having excellent heat resistance (for example, shape retention in a high temperature environment). Further, the separator for the secondary battery is lightweight and thin. Therefore, the secondary battery is light, and the filling density of the electrodes can be improved, so that it is possible to cope with the miniaturization of the secondary battery.

Therefore, by using the above-mentioned secondary battery, it is possible to reduce the weight of information-related devices such as smartphones and notebook computers, hybrid vehicles, electric vehicles, and the like while maintaining high safety. For example, an electric vehicle using the secondary battery can be significantly reduced in weight, thereby dramatically improving fuel efficiency.

As described above, each configuration of the invention according to the present disclosure and combinations thereof are examples, and the configurations can be added, omitted, replaced, and changed as appropriate within the range not deviating from the gist of the invention according to the present disclosure. .. Further, the invention according to the present disclosure is not limited to the embodiments, but is limited only by the description of the scope of claims.

Also, each aspect disclosed herein can be combined with any other feature disclosed herein.

Hereinafter, the invention according to the present disclosure will be described more specifically by way of examples, but the invention according to the present disclosure is not limited to these examples.

[Example 1]
Add 15.89 parts by weight of sodium carboxymethyl cellulose (CMC) (dry weight loss: 5.6%, manufactured by Daisel Finechem Co., Ltd., product number: 2200) to 1484 g of water, and add a rotating revolution agitator (manufactured by Shinky Co., Ltd., product name: Awatori). A 1.0 wt% CMC aqueous solution was prepared by stirring at 3000 rpm for 30 minutes using Neritaro, model number: ARE-310).

Next, aramid fine fibers (average diameter D: 0.56 μm, average length L: 0.43 mm, average aspect ratio: 768, decomposition temperature: 400 ° C. or higher, solid content: 20%, manufactured by Daisel Finechem Co., Ltd., product Name: Tiara, product number: KY400S) 19 parts by weight of water was added to 1 part by weight, and the mixture was stirred at 2000 rpm for 5 minutes using a rotating revolution stirrer. Further, the treatment was performed using a high-pressure homogenizer (50 MPa, 30 passes) to obtain a 1.0 wt% aramid fiber aqueous dispersion.

100 parts by weight of 1.0 wt% CMC aqueous solution, 300 parts by weight of 1.0 wt% aramid fiber aqueous dispersion (1), 100 parts by weight of water, and silica (granular, volume average particle diameter 28.1 nm, Nippon Aerodil Co., Ltd.) ), Aerodil 50) 1.05 parts by weight was blended and stirred at 3000 rpm for 30 minutes using a rotating revolution stirrer to obtain a dispersion medium solution for forming a heat-resistant layer.

An automatic coating device (thickness 20 μm, air permeability 235 sec / 100 mL, manufactured by Double Scope Co., Ltd.) is applied to one side of a polyethylene microporous film (thickness 20 μm, air permeability 235 sec / 100 mL), which is a porous substrate subjected to corona discharge treatment. Using Tester Sangyo Co., Ltd., model number: PI-1210), the coating was applied at a rate of 240 mm / sec, and then dried at 60 ° C. for 20 minutes to form a heat-resistant layer, which is a non-woven fabric, to form a laminate (a laminate (). A separator which is a porous base material layer / heat-resistant layer) was obtained. The total thickness was 26 μm, and the thickness of the heat-resistant layer was 6 μm.

The obtained separator was evaluated for air permeability and heat resistance.

[Example 2]
The procedure was the same as in Example 1 except that the amount of silica added was 3.3 parts by weight. The total thickness of the separator was 27 μm, and the thickness of the heat-resistant layer was 7 μm.

[Example 3]
The same procedure as in Example 1 was carried out except that alumina (granular, volume average particle diameter 0.7 μm, manufactured by Sumitomo Chemical Co., Ltd., AKP-3000) was used instead of silica. The total thickness of the separator was 26 μm, and the thickness of the heat-resistant layer was 6 μm.

[Example 4]
Alumina (volume average particle diameter 0.7 μm, manufactured by Sumitomo Chemical Co., Ltd., AKP-3000) was used instead of silica, and the same as in Example 1 except that the amount of alumina added was 3.3 parts by weight. The total thickness of the separator was 27 μm, and the thickness of the heat-resistant layer was 7 μm.

[Comparative Example 1]
The procedure was the same as in Example 1 except that no filler was added. The total thickness of the separator was 25 μm, and the thickness of the heat-resistant layer was 5 μm.
[Comparative Example 2]
The air permeability and heat resistance of the polyethylene microporous membrane that did not form a heat-resistant layer were evaluated.

(Air permeability test)
The air permeability was measured by a method according to JIS P8117 using a Garley type densometer B type manufactured by Tester Sangyo Co., Ltd. The number of seconds was measured with a digital auto counter. The smaller the value of air permeability (Garley value), the higher the air permeability.

(Heat resistance test)
The separators (40 mm × 40 mm square) obtained in Examples and Comparative Examples were placed on an aluminum plate. Subsequently, the separator placed on an aluminum plate was placed in a high temperature bath at 130 ° C. and heated for 30 minutes. Then, the shrinkage rate of the separator after heating was calculated from the following formula, and this was used as an index of heat resistance. The smaller the shrinkage rate, the better the heat resistance.
Shrinkage rate (%) = [1- (Length of the most shrunk portion of the separator after heating in the MD direction [mm] / Length of the separator before heating in the MD direction [mm])] × 100

The results of Examples 1 to 4 and Comparative Examples 1 and 2 are summarized in Table 1 below.

From Table 1, the separators obtained in Examples 1 to 4 are more excellent in air permeability and heat resistance than Comparative Example 1 in which the heat-resistant layer does not contain a filler, and are more excellent in heat resistance than Comparative Example 2. I understood.

Claims (10)

The heat-resistant layer (A) includes a heat-resistant layer (A) and a heat-resistant layer (B) made of a non-woven fabric containing microfibers having a melting point or decomposition temperature of 200 ° C. or higher, a binder, and a filler having polarity. A separator for a secondary battery, wherein the weight ratio (filler / binder) of the filler to the binder in B) is 0.17 or more and less than 4. The separator for a secondary battery according to claim 1, wherein the fine fibers are aramid fibers. The separator for a secondary battery according to claim 1 or 2, wherein the binder is a water-based binder. The binder is at least one selected from a polysaccharide derivative (1), a compound having a structural unit represented by the following formula (2), and a compound having a structural unit represented by the following formula (3). , The separator for a secondary battery according to any one of claims 1 to 3.

(In the formula, R represents a hydroxyl group, a carboxyl group, a phenyl group, an N-substituted or unsubstituted carbamoyl group, or a 2-oxo-1-pyrrolidinyl group).

(In the formula, n indicates an integer of 2 or more, and L indicates an ether bond or a (-NH-) group). The separator for a secondary battery according to any one of claims 1 to 3, wherein the binder is at least one selected from cellulose, starch, glycogen, and derivatives thereof. The separator for a secondary battery according to any one of claims 1 to 5, wherein the fine fibers have an average fiber diameter of 0.01 to 1 μm and an average length of 0.01 to 2 mm. The separator for a secondary battery according to any one of claims 1 to 6, wherein the filler is an inorganic oxide filler. The separator for a secondary battery according to any one of claims 1 to 7, wherein the total thickness of the separator for a secondary battery is 10 to 50 μm, and the thickness of the heat-resistant layer (B) is 0.5 to 20 μm. .. The surface of the porous substrate (A) is coated with a dispersion medium solution of the fine fibers, the binder, and the filler, and the dispersion medium is volatilized. A method for manufacturing a separator for a secondary battery, which obtains the separator for the secondary battery described above. A secondary battery provided with the separator for a secondary battery according to any one of claims 1 to 8.