JP2021034256A - slurry - Google Patents

Abstract

To provide a slurry which enables the formation of a separator for a secondary battery superior in heat resistance.SOLUTION: A slurry according to the present invention comprises: a fine fiber of 150°C or higher in melting point (decomposition temperature if it has no melting point); a water-soluble cellulose; and water. In the slurry, a content of the fine fiber [x; wt.%] is 0.1-1.0 wt.%, and a content of the water-soluble cellulose [y; wt.%] is over 0 wt.%. The fine fiber content [x; wt.%] and the water-soluble cellulose content [y; wt.%] satisfy the following expression (1): y≤0.62x (1). It is preferred that the fine fiber content [x; wt.%] and the water-soluble cellulose content [y; wt.%] further satisfy the following expression (2): y≥0.133x+0.01 (2).SELECTED DRAWING: None

Description

The present invention relates to a slurry that can be used for forming a secondary battery separator, a separator for a secondary battery formed by using the slurry, 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 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 when lithium ions move through this separator, it is also required to ensure transparency.

Patent Document 1 describes that a non-woven fabric made of polyolefin such as polypropylene is used as a separator.

The softening point of polypropylene is 155 ° C., and when the inside of the battery exceeds the softening point and becomes an abnormally high temperature, the pores of the non-woven fabric formed of polypropylene are softened and closed. As a result, the flow of lithium ions between the electrodes can be stopped and the function of the battery can be safely stopped. This is called the "shutdown function". However, if the temperature inside the battery rises even after the flow of lithium ions is stopped, the separator shrinks (shrinks) and melts (melts down), losing the function of insulating the positive electrode and the negative electrode. As a result, the electrodes may be short-circuited and thermal runaway may occur.

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. Therefore, the voltage and capacity are further increased. 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.

Japanese Unexamined Patent Publication No. 2015-128055

Therefore, an object of the present invention is to provide a slurry capable of forming a separator for a secondary battery having excellent heat resistance.
Another object of the present invention is to provide a slurry capable of forming a separator for a secondary battery having excellent heat resistance and permeability.
Another object of the present invention is to provide a separator for a secondary battery having excellent heat resistance.
Another object of the present invention is to provide a separator for a secondary battery having excellent heat resistance and permeability.
Another object of the present invention is to provide a method for manufacturing the separator for a secondary battery.
Another object of the present invention is to provide a secondary battery provided with the separator for the secondary battery.

As a result of diligent studies to solve the above problems, the present inventors have found that they contain fine fibers having a melting point (decomposition temperature if there is no melting point) of 150 ° C. or higher in a specific ratio and water-soluble cellulose, and further contain water. If a slurry is used, a non-woven fabric that can retain its shape even in an abnormally high temperature environment can be obtained, and if the structure obtained by laminating the non-woven fabric on a porous base material is used as a separator for a secondary battery. It was found that the function of insulating the positive electrode and the negative electrode can be maintained even when the temperature inside the battery becomes abnormally high, and the short circuit of the electrodes can be prevented. The present invention has been completed based on these findings.

That is, the present invention contains microfibers having a melting point (decomposition temperature if there is no melting point) of 150 ° C. or higher, water-soluble cellulose, and water.
The content of the fine fibers [x;% by weight] is 0.1 to 1.0% by weight, the content of the water-soluble cellulose [y;% by weight] is more than 0% by weight, and the content of the fine fibers is Provided is a slurry in which the amount [x;% by weight] and the content [y;% by weight] of the water-soluble cellulose satisfy the following formula (1).
y ≦ 0.62x (1)

The present invention also provides the slurry in which the content of the fine fibers [x;% by weight] and the content of the water-soluble cellulose [y;% by weight] further satisfy the following formula (2).
y ≧ 0.133x + 0.01 (2)

The present invention also provides the slurry having a viscosity of 10 to 10000 mPa · s at a temperature of 25 ° C. and a shear rate of 100 (1 / s).

The present invention also provides the slurry in which the microfibers are aramid fibers.

The present invention also provides the slurry having an average thickness of 0.01 to 10 μm and an average length of 0.01 to 1 mm of the microfibers.

（式中、３つのＲは同一又は異なって、水素原子、又はヒドロキシル基若しく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） The present invention also provides the slurry in which the water-soluble cellulose is a cellulose derivative having a structural unit represented by the following formula (c), or a salt thereof.

(In the formula, the three Rs are the same or different and indicate a hydrogen atom or an alkyl group having a hydroxyl group or a carboxyl group and having 1 to 5 carbon atoms).

The present invention also provides the slurry having a degree of etherification of a cellulose derivative having a structural unit represented by the formula (c) of 0.6 or more.

The present invention also provides the slurry in which a 1 wt% aqueous solution of water-soluble cellulose has a viscosity of 50 mPa · s or more at 25 ° C. and 60 rpm.

The present invention also provides the slurry for forming a separator for a secondary battery.

The present invention also provides a non-woven fabric made of a solidified product of the slurry.

The present invention also provides a separator for a secondary battery, which comprises a laminate of the nonwoven fabric and a porous substrate.

The present invention also applies the slurry to the surface of the porous base material and dries the slurry to form a laminate of a non-woven fabric made of a solidified product of the slurry and the porous base material. Provided is a method for manufacturing a separator for a secondary battery, which obtains a separator for a secondary battery provided with a body.

The present invention also provides a secondary battery including the separator for the secondary battery.

The separator for a secondary battery provided with the non-woven fabric formed by using the slurry of the present invention and the porous base material has excellent heat resistance, can retain its shape even in a high temperature environment, and shrinks, melts, etc. It is possible to prevent deformation due to the above, or to make the degree of deformation extremely small. Therefore, the secondary battery provided with the separator for the secondary battery of the present invention maintains the insulating function of the separator even in a high temperature environment, so that the short circuit of the electrodes can be prevented and thermal runaway occurs due to the short circuit of the electrodes. Can be prevented. That is, it is excellent in safety.
Further, the separator for the secondary battery is lightweight.
Therefore, the separator for a secondary battery of the present invention 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 separator for a secondary battery is used in an electric vehicle, it is possible to significantly reduce the weight, thereby dramatically improving fuel efficiency.

It is a distribution map which shows the relative relationship between the fine fiber and the water-soluble cellulose content in a slurry, and the air permeability [sec / 100 mL] of the separator obtained by using the slurry is described next to the dot. ..

[slurry]
The slurry of the present invention contains microfibers, water-soluble cellulose and water. The slurry of the present invention may contain other components in addition to the above components.

<Microfibers>
The fine fibers have a melting point (decomposition temperature if there is no melting point) of 150 ° C. or higher, preferably 200 ° C. or higher, more preferably 250 ° C. or higher, and particularly preferably 300 ° C. or higher. The upper limit of the melting point is, for example, 500 ° C.

The average thickness of the fine fibers is not particularly limited, but is, for example, 0.01 to 10 μm, and among them, it has more excellent heat resistance (or shape retention in a high temperature environment), and the separator is thinned. The upper limit of the average thickness is preferably 5 μm, particularly preferably 1 μm, in terms of contributing to (or contributing to the miniaturization of the secondary battery). The lower limit of the average thickness is preferably 0.05 μm, particularly preferably 0.1 μm. As for the average thickness of the fine fibers, an electron microscope (SEM, TEM) is used to take an electron microscope image of a sufficient number (for example, 100 or more) of the fine fibers, and the thickness (diameter) of these fine fibers is taken. ) Is measured and arithmetically averaged.

The average length of the fine fibers is not particularly limited, but is, for example, 0.01 to 1 mm, and among them, it has more excellent heat resistance (for example, shape retention in a high temperature environment), and the separator is thinned. The upper limit of the average length is more preferably 0.8 mm, particularly preferably 0.6 mm, and most preferably 0.5 mm in terms of contributing to (or contributing to the miniaturization of the secondary 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). And 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 thickness) 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) can be obtained. The average aspect ratio is such that a non-woven fabric having sufficient heat resistance can be formed even if the thickness is thinner, which contributes to the thinning of the separator (or the miniaturization of the secondary battery). 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 contain 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, an aromatic or non-aromatic heterocycle may be condensed on the aromatic ring.

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, interfacial 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 commonly used method. Further, after being spun into a fibrous form, crushing treatment or the like can be performed as needed. In the present invention, a non-woven fabric having sufficient heat resistance (for example, shape retention in a high temperature environment) can be formed even if the thickness is thinner, which contributes to further thinning of the separator (or a secondary battery). It is preferable to apply a strong mechanical shearing force to the microfibrils by using an ultra-high pressure homogenizer or the like.

As the aramid fiber, for example, a commercially available product such as the trade name "Tiara" (fine fibrous aramid, manufactured by Daicel Finechem Co., Ltd.) may be used.

（式中、３つのＲは同一又は異なって、水素原子、又はヒドロキシル基若しく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） <Water-soluble cellulose>
Examples of the water-soluble cellulose include a cellulose derivative having a structural unit represented by the following formula (c), or a salt thereof.

(In the formula, the three Rs are the same or different and indicate a hydrogen atom or an alkyl group having a hydroxyl group or a carboxyl group and having 1 to 5 carbon atoms).

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. Of these, an alkyl group having 1 to 3 carbon atoms is preferable, and an alkyl group having 1 to 2 carbon atoms is particularly preferable.

Cellulose derivatives (particularly CMC) having a structural unit represented by the above formula (c) may form a salt. That is, the hydroxyl group or the carboxyl group provided in the alkyl group having 1 to 5 carbon atoms may form a salt with the alkali metal.

Examples of the salt include monovalent metal salts such as alkali metal salts (lithium salt, sodium salt, potassium salt, rubidium salt, cesium salt, etc.), alkaline earth metal salts (calcium salt, magnesium salt, etc.) and the like. Valuable metal salts, quaternary ammonium salts, amine salts, substituted amine salts or compound salts thereof and the like can be mentioned. As the salt, an alkali metal salt such as a sodium salt and a quaternary ammonium salt are preferable.

For example, when the hydroxyl group or the carboxyl group provided in the alkyl group having 1 to 5 carbon atoms forms a sodium salt, the hydroxyl group (-OH) becomes -ONa and the carboxyl group (-COOH) becomes -COONa. ..

Specific examples of the water-soluble cellulose include hydroxyethyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose (CMC), and alkali metal salts thereof (for example, sodium carboxymethyl cellulose). These water-soluble celluloses can be used alone or in combination of two or more.

The degree of etherification (or the degree of (total) substitution by an alkyl group having 1 to 5 carbon atoms having a hydroxyl group or a carboxyl group) of a cellulose derivative (particularly CMC) having a structural unit represented by the above formula (c) is , For example, it can be selected from the range of 0.1 to 3.0. In the present invention, the degree of etherification is preferably 0.6 or more, more preferably 0.8 or more, in that a non-woven fabric having low air permeability and excellent permeability to the electrolytic solution can be formed. It is particularly preferably 1.0 or more, and most preferably 1.1 or more. The upper limit of the degree of etherification is preferably 2.0, particularly preferably 1.5, and most preferably 1.4. Therefore, the degree of etherification is preferably 0.6 to 2.0, particularly preferably 0.8 to 1.5, most preferably 1.0 to 1.5, and particularly preferably 1.1 to 1.4. is there.

The degree of etherification (or the degree of (total) substitution by an alkyl group having 1 to 5 carbon atoms having a hydroxyl group or a carboxyl group) is the 2-position, 3-position, and 6-position of the glucose unit constituting the cellulose. It is an average value of the degree of substitution of a hydroxyl group with an alkyl group having a hydroxyl group or a carboxyl group and having 1 to 5 carbon atoms, or an average value of the degree of etherification.

As the water-soluble cellulose, carboxymethyl cellulose (CMC) is preferable from the viewpoint of excellent water solubility. The CMC may be a complete acid type CMC (= CMC-H), or may be a CMC (for example, CMC-Na) in which at least a part thereof forms a salt.

The average degree of polymerization (= viscosity average degree of polymerization) of the structural unit represented by the above formula (c) of the water-soluble cellulose (particularly CMC) or a salt thereof is not particularly limited, but is preferably 10 to 1000, for example. Is 50 to 900, more preferably 100 to 800.

The viscosity of a 1 wt% aqueous solution of the water-soluble cellulose (particularly CMC) or a salt thereof at 25 ° C. and 60 rpm is, for example, 50 mPa · s or more. Among them, the dispersibility of the fine fibers is improved, a non-woven fabric containing the fine fibers can be formed uniformly, and the binding property between the fine fibers is further improved to form a non-woven fabric having high strength. If a non-woven fabric having the above-mentioned characteristics is used for the separator of the secondary battery, the shrinkage of the porous substrate can be suppressed more strongly, preferably 150 mPa · s or more, more preferably 500 mPa · s. As described above, it is more preferably 500 to 5000 mPa · s, particularly preferably 1000 to 5000 mPa · s, most preferably 1500 to 4500 mPa · s, and particularly preferably 1600 to 4000 mPa · s.

[slurry]
The slurry of the present invention contains the fine fibers, the water-soluble cellulose, and water.

The content [x;% by weight] of the fine fibers in the slurry is 0.1 to 1.0% by weight, and in particular, an increase in air permeability can be suppressed and the permeability of the electrolytic solution is improved. In that respect, the lower limit of the content of the fine fibers is preferably 0.3% by weight, particularly preferably 0.4% by weight, and most preferably 0.5% by weight. The upper limit of the content of the fine fibers is preferably 0.9% by weight, particularly preferably 0.8% by weight, and most preferably 0.75% by weight.

The content [y;% by weight] of the water-soluble cellulose in the slurry is more than 0% by weight.

The content of the fine fibers [x;% by weight] and the content of the water-soluble cellulose [y;% by weight] in the slurry satisfy the following formula (1).
y ≦ 0.62x (1)

The above formula (1) is preferably the following formula (1').
y≤0.49x (1')

The above formula (1) is preferably the following formula (1 ″).
y ≦ 0.41x (1 ″)

It is preferable that the content of the fine fibers [x;% by weight] and the content of the water-soluble cellulose [y;% by weight] in the slurry further satisfy the following formula (2).
y ≧ 0.133x + 0.01 (2)

The above formula (2) is preferably the following formula (2').
y ≧ 0.133x + 0.02 (2')

The above formula (1) is preferably the following formula (2 ").
y ≧ 0.133x + 0.03 (2 ”)

Therefore, the content [y;% by weight] of the water-soluble cellulose in the slurry is, for example, 0.01 to 5% by weight, and in particular, the water-soluble cellulose is capable of uniform coating. The lower limit of the content of is preferably 0.05% by weight, particularly preferably 0.08% by weight, most preferably 0.1% by weight, and particularly preferably 0.15% by weight. Further, the upper limit of the content of the water-soluble cellulose is preferably 1.0% by weight, more preferably 1.0% by weight, in that an increase in air permeability can be suppressed and the permeability of the electrolytic solution can be improved. It is 0.6% by weight, particularly preferably 0.5% by weight, most preferably 0.3% by weight, and particularly preferably 0.25% by weight.

Excessive use of water-soluble cellulose tends to block the voids in the separator and increase electrical resistance. Further, if the amount of water-soluble cellulose used is too small, the adhesion to the porous substrate tends to be insufficient.

Since the slurry contains fine fibers and water-soluble cellulose in the above range, the binding property between the fine fibers is improved as compared with the case where only the fine fibers are used. As a result, when a separator for a secondary battery, which is a laminate of a non-woven fabric made of a solidified slurry and a porous base material, is produced using the slurry, the strength of the non-woven fabric is increased and the non-woven fabric is manufactured. Improves adhesion to porous substrates. Therefore, the obtained separator for a secondary battery is particularly excellent in heat resistance (for example, shape retention in a high temperature environment). On the other hand, when the content of the water-soluble cellulose is less than the above range, the strength of the non-woven fabric tends to decrease. In addition, the non-woven fabric and the porous base material tend to be easily peeled off, and it tends to be difficult to maintain the shape of the porous base material in a high temperature environment.

The viscosity of the slurry 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, from the viewpoint of enabling uniform coating, it is preferably 50 to 1000 mPa · s. , Particularly preferably 80 to 500 mPa · s. The viscosity of the slurry can be controlled by adjusting the water content.

The non-volatile content concentration in the slurry can be appropriately adjusted according to the thickness of the non-woven fabric to be formed, and is, for example, 0.5 to 5% by weight.

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

In addition, the slurry contains at least water as a volatile component. This is because water has a small environmental load and is excellent in safety. The slurry may contain an organic solvent other than water as a volatile component, but the content of the organic solvent is preferably, for example, 50% by weight or less of the total amount of the volatile components contained in the slurry, and is particularly preferable. Is 30% by weight or less, most preferably 10% by weight or less.

The slurry is excellent in dispersibility (particularly, dispersibility of fine fibers). For example, the slurry is placed in a screw tube bottle (capacity 6 mL, manufactured by AS ONE Corporation, No. 2) to a height of 30 mm from the bottom, and a lid is closed. When closed and allowed to stand at 25 ° C. (± 2 ° C.) for 7 days, the width of the upper layer (aqueous phase) above the separation interface of the slurry is, for example, 15 mm or less, preferably 9 mm or less, particularly preferably 5 mm or less, most preferably. It is 3 mm or less. Therefore, if the slurry is used, a non-woven fabric containing fine fibers can be formed uniformly, and the binding property between the fine fibers can be further improved to form a non-woven fabric having high strength.

Since the slurry has the above 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)).

[Separator for secondary battery]
The separator for a secondary battery of the present invention includes a laminate of a non-woven fabric and a porous base material. The non-woven fabric and the porous substrate will be described in detail later.

The separator for a secondary battery has excellent heat resistance, and the shrinkage rate at 180 ° C. (for example, the shrinkage rate in the MD direction of a porous substrate) is, for example, 50% or less, preferably 30% or less, and particularly preferably 20% or less. is there. Therefore, the shape of the separator can be maintained even in a high temperature environment, and a short circuit of the electrodes due to deformation of the separator can be prevented.

The air permeability of the secondary battery separator is, for example, 100 to 4000 sec / 100 mL. The upper limit of the air permeability is preferably 1000 sec / 100 mL, more preferably 800 sec / 100 mL, further preferably 600 sec / 100 mL, particularly preferably 500 sec / 100 mL, and most preferably 400 sec / 100 mL. The lower limit of air permeability is preferably 200 sec / 100 mL. Therefore, the rate of increase in air permeability is extremely small, and the electrolyte permeability is excellent.

Further, the secondary battery separator is very lightweight. Therefore, the weight of the secondary battery provided with the separator for the secondary battery can be reduced, which contributes to the improvement of fuel efficiency of the electric vehicle or the like using the secondary battery.

Since the separator for a secondary battery can ensure sufficient heat resistance even if the non-woven fabric is thinned, the total thickness of the separator for a secondary battery can be, for example, 10 to 50 μm, preferably 15 while maintaining the heat resistance. It can be thinned to ~ 30 μm. Therefore, the separator for a secondary battery can increase the filling density of the battery and contributes to the miniaturization of the secondary battery.

(Non-woven fabric)
The non-woven fabric is made of a solidified product of the slurry. The non-woven fabric has excellent heat resistance and can maintain the shape of the non-woven fabric (for example, a sheet-like shape) without softening or melting even in a high temperature environment. Therefore, the non-woven fabric exerts an action of adhering to the porous base material in the separator for a secondary battery and supporting the porous base material so as to maintain its shape even in a high temperature environment.

The non-woven fabric is porous, and the porosity of the non-woven fabric is, for example, 30 to 90% by volume, preferably 35 to 80% by volume. If the porosity exceeds the above range, the strength tends to decrease, and the effect of suppressing the shrinkage of the porous substrate 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 (= air permeability resistance) of the non-woven fabric is, for example, 1 to 1000 sec / 100 mL, preferably 1 to 500 sec / 100 mL, particularly preferably 1 to 300 sec / 100 mL, and most preferably 1 to 100 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 decrease, and the effect of suppressing the shrinkage of the porous substrate in a high temperature environment tends to be insufficient.

The thickness of the non-woven fabric is, for example, 0.5 to 20 μm. The lower limit of the thickness is preferably 1 μm, particularly preferably 2 μm, in that the strength of the non-woven fabric can be ensured and the effect of sufficiently suppressing the shrinkage of the porous substrate in a high temperature environment can be exhibited. .. The upper limit of the thickness is preferably 10 μm, particularly preferably 7 μm, and most preferably 5 μm in that the filling density of the electrodes can be improved and the secondary battery can be further miniaturized. ..

The non-woven fabric is extremely lightweight and has a basis weight of, for example, 10 g / m ² or less, and is preferably 9 g / m ² or less, particularly preferably 7 g / m ² or less, most preferably, in terms of contributing to weight reduction of the battery. Is 5 g / m ² or less. The lower limit of the basis weight is, for example, 0.1 g / m ² .

(Porous substrate)
In a lithium ion secondary battery, the porous substrate has a gap that allows lithium ions to move between electrodes, and when the inside of the secondary battery becomes abnormally high temperature, the porous base material softens to eliminate the gap. It is a member that exerts a "shutdown function" that stops the flow of lithium ions between electrodes by closing it.

The size of the voids in the porous substrate 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 is, for example, 20 to 70% by volume, 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 is, for example, 10 to 600 sec / 100 mL, preferably 50 to 400 sec / 100 mL, and particularly preferably 50 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.

The porous base material is a material that is insoluble in an electrolytic solution and is a base material that softens in a high temperature environment. As the material, for example, a thermoplastic polymer is preferable. Examples of the thermoplastic polymer include polyolefins such as polyethylene and polypropylene; polyesters such as polyethylene terephthalate; and aliphatic polyamides such as 2,6-nylon.

It is preferable that the material of the porous base material is appropriately selected and used from the thermoplastic polymers according to a 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 softening temperature of 140 to 150 ° C.

The thickness of the porous substrate is, for example, 5 to 40 μm, preferably 8 to 30 μm. If the thickness exceeds the above range, the electrical resistance of the battery tends to increase and the volume capacity tends to decrease. On the other hand, if the thickness is less than the above range, the strength tends to be insufficient.

The porous base material is produced, for example, by a method in which a material (for example, polyolefin) of the porous base material is heated and melted to form an extruded film, and the obtained film is stretched to make it porous. can do.

[Manufacturing method of separator for secondary battery]
The secondary battery separator is, for example, a step of applying the slurry to the surface of a porous base material and drying it to form a laminate of a non-woven fabric made of a solidified product of the slurry and the porous base material. Can be manufactured after.

The method for applying the slurry is not particularly limited, and for example, it can be applied by 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 slurry may be applied to at least one surface of the porous substrate. In the present invention, among them, the porous base material is made of a porous base material in that the total thickness of the separator for a secondary battery can be reduced, the filling density of the battery can be increased, and the size of the secondary battery can be reduced. It is preferable to apply it to only one surface.

The method for drying the slurry is not particularly limited, and examples thereof include heating, depressurizing, and blowing air. 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 has a positive electrode in which the positive electrode active material layer is arranged in the positive electrode current collector, a negative electrode in which the negative electrode active material layer is arranged in the negative electrode current collector, a separator, and a power generation element including an electrolytic solution. It is contained inside the body. The secondary battery of the present invention is characterized by using the above-mentioned separator for a secondary battery as a separator.

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

The secondary battery of the present invention 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 of the present invention is light, and the filling density of the electrodes can be improved, so that the secondary battery can be miniaturized.

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

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

The viscosity of the slurry was measured using a molten viscoelasticity measuring device (trade name "MCR-302", manufactured by Antonio Par).

Example 1 (Preparation of slurry)
To 100 parts by weight of water, 1.65 parts by weight of CMC (dry weight loss: 8.0%, manufactured by Daisel Finechem Co., Ltd., product number: 2200) shown in Table 1 below was added, and a rotating revolution agitator (Shinky Co., Ltd.) A 1.5 wt% CMC aqueous solution was prepared by stirring at 2000 rpm for 30 minutes using a product, product name: Awatori Rentaro, model number: ARE-310).
Next, aramid fine fibers (average thickness: 0.56 μm, average length: 0.43 mm, average aspect ratio: 768, decomposition temperature: 400 ° C. or higher, solid content: 20%, manufactured by Daicel Finechem Co., Ltd., trade name : Tiara, product number: KY400S) 95 parts by weight of water was added to 5 parts by weight, and the mixture was stirred at 2000 rpm for 30 minutes using a rotation / revolution stirrer to prepare a 1% by weight aramid fiber aqueous dispersion.
1.67 parts by weight of the obtained 1.5% by weight CMC aqueous solution, 7.50 parts by weight of the obtained 1% aramid fiber aqueous dispersion, and 190.83 parts by weight of water were mixed and used with a rotation / revolution stirrer. Stirring at 2000 rpm for 2 minutes gave a slurry [aramid fiber content: 0.0375% by weight, CMC content: 0.0125% by weight, water content: 99.95 parts by weight].

The dispersion stability of the obtained slurry was evaluated by the following method.
That is, put the slurry in a screw tube bottle (capacity 6 mL, manufactured by AS ONE Corporation, No. 2) at a height of 30 mm from the bottom, close the lid, and leave it at 25 ° C (± 2 ° C) for 7 days. Then, the width of the upper layer of the upper layer (aqueous phase) and the lower layer (aramid fiber phase) from the separation interface of the slurry was measured. It should be noted that the narrower the width of the upper layer, the better the dispersion stability of the fine fibers in the slurry, and the less likely it is that separation will occur.

Examples 2-7 (Preparation of slurry)
A slurry was obtained in the same manner as in Example 1 except that the CMC was changed as shown in Table 1 below, and the dispersion stability of the obtained slurry was evaluated. The product numbers in the table are the product numbers of "CMC Daicel" manufactured by Daicel Finechem Co., Ltd.

The above results are summarized in the table below.

From Table 1 above, it can be seen that in the slurry using CMC having a low degree of etherification, the dispersion stability of the fine fibers is improved by increasing the viscosity of CMC.
It can be seen that the slurry using CMC having a high degree of etherification is extremely excellent in the dispersion stability of fine fibers even if the viscosity of CMC is low.

Example 8 (Preparation of slurry)
To 100 parts by weight of water, 1.65 parts by weight of CMC (dry weight loss: 8.0%, manufactured by Daisel Finechem Co., Ltd., product number: 2200) shown in Table 2 below was added, and 2000 rpm using a rotation / revolution stirrer. A 1.5 wt% CMC aqueous solution was prepared by stirring with the mixture for 30 minutes.
Next, aramid fine fibers (average thickness: 0.56 μm, average length: 0.43 mm, average aspect ratio: 768, decomposition temperature: 400 ° C. or higher, solid content: 20%, manufactured by Daicel Finechem Co., Ltd., trade name : Tiara, product number: KY400S) 95 parts by weight of water was added to 5 parts by weight, and the mixture was stirred at 2000 rpm for 30 minutes using a rotation / revolution stirrer to prepare a 1% by weight aramid fiber aqueous dispersion.
1.67 parts by weight of the obtained 1.5% by weight CMC aqueous solution, 7.5 parts by weight of the obtained 1% aramid fiber aqueous dispersion, and 0.83 parts by weight of water were mixed and used with a rotation / revolution stirrer. Stirring at 2000 rpm for 2 minutes gave slurry (1) [aramid fiber content: 0.75% by weight, CMC content: 0.25% by weight, water content: 99 parts by weight]. The viscosity of the obtained slurry at a temperature of 25 ° C. and a shear rate of 100 (1 / s) was 306 mPa · s.

(Making a separator)
Polypropylene microporous membrane as a porous substrate (1) (manufactured by CS Tech Co., Ltd., Serion P2010, void size (average pore size): 0.03 μm, thickness: 20 μm, air permeability: 253 sec / 100 mL) The obtained slurry (1) was coated on one side at a speed of 240 mm / sec using an automatic coating device (manufactured by Tester Sangyo Co., Ltd., model number: PI-1210) (coating thickness: 200 μm). ..
Next, the coated slurry (1) was dried in a high temperature bath at 60 ° C. for 15 minutes. As a result, a non-woven fabric / PP laminate made of a solidified product of the slurry (1) (thickness of non-woven fabric: 5 μm, basis weight of non-woven fabric: 1 g / m ² , porosity of non-woven fabric: 75% by volume) was obtained, and the separator (thickness: 75% by volume) was obtained. It was set as 1).

Examples 9 to 14 (Preparation of Separator)
Slurries (2) to (7) were obtained in the same manner as in Example 8 except that the CMC was changed as shown in Table 2 below, and the obtained slurries (2) to (7) were used for non-woven fabric / PP lamination. I got a body. These were designated as separators (2) to (7).

Comparative Example 1
Only the polypropylene microporous membrane (1) was used as the separator (8) without laminating the non-woven fabric.

The air permeability and heat resistance of the separators (1) to (8) obtained in Examples and Comparative Examples were measured by the following methods.

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

(Heat resistance test)
The separators (approximately squares of 40 mm × 40 mm) obtained in Examples and Comparative Examples are placed on a stainless steel plate, and the two sides parallel to the MD direction of the polypropylene microporous membrane (1) constituting the separator are fixed with polyimide tape. did.
Subsequently, the separator fixed on a stainless steel plate was placed in a high temperature bath at 180 ° 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 part of the separator after heating in the MD direction [mm] / Length of the separator before heating in the MD direction [mm])] × 100

The above results are summarized in Table 2 below.

From Table 2 above, the separators obtained in Examples (particularly, separators containing CMC having a high degree of etherification) had low air permeability and excellent electrolytic solution permeability.
Further, since the separator obtained in the examples (particularly, the separator containing high-viscosity CMC) has excellent heat resistance, the shrinkage rate is low even under a high temperature condition of 180 ° C., and such a separator can be used as a separator for a secondary battery. It was found that if used, the insulation function can be maintained even in an abnormally high temperature environment.

On the other hand, the separator obtained in the comparative example has low air permeability and excellent electrolyte permeability, but since the non-woven fabric is not laminated on the porous substrate, the porous substrate is subjected to a high temperature condition of 180 ° C. It was found that if such a separator is used as a separator for a secondary battery, it is difficult to maintain the insulating function in an abnormally high temperature environment.

Examples 15-17
A slurry was obtained in the same manner as in Example 8 except that the CMC content and / or the aramid fiber content was changed, and the obtained slurry was used to obtain a non-woven fabric / PP laminate. This was used as a separator.

The non-woven fabric surface of the obtained separator was photographed in a range of 3 cm × 3 cm at a resolution of 300 pixels × 300 pixels with black drawing paper as a background, and the pixels on which the aramid fiber was placed were counted as follows. The aramid fiber appears white, and the lining is transparent and black in the part where the aramid fiber is not placed.
First, the gray scale is obtained by taking the average of the R value (0 to 255), G value (0 to 255), and B value (0 to 255) of the image and designating the values as the R value, G value, and B value. It became. This value is called brightness. Then, the average of the brightness of all the pixels was taken, the average of the brightness was set to -5, and when the brightness was higher than that, it was counted that the aramid fiber was on board.
Then, the coatability index was calculated from the following formula, and the coatability of the slurry was evaluated according to the following criteria.
Coatability index = number of pixels on which aramid fiber is mounted / total number of pixels
<Applicability evaluation criteria>
○ (Good): When the coatability index exceeds 0.9 △ (Yes): When the coatability index exceeds 0.85 and 0.9 or less × (No): The coatability index is 0. If it is 85 or less

Moreover, the air permeability of the obtained separator was measured.

The above results are summarized in Table 3 below.

Examples 18-23
A slurry was obtained in the same manner as in Example 11 except that the content of CMC and / or the content of aramid fiber was changed.

Further, the viscosity of the obtained slurry at a temperature of 25 ° C. and a shear rate of 100 (1 / s) was measured to evaluate the coatability of the slurry.

Further, the obtained slurry was used to obtain a non-woven fabric / PP laminate, which was used as a separator. Then, the air permeability of the obtained separator was measured.

The above results are summarized in Table 4 below.

The results of Tables 3 and 4 above are summarized in FIG. From FIG. 1, when the aramid fiber concentration in the slurry is in the range of 0.1 to 1.0% by weight and the CMC concentration [y;% by weight] is in the range of more than 0% by weight, the CMC concentration [y;% by weight] is determined. When the aramid fiber concentration [x;% by weight] was within the range represented by the following formula (1a), the obtained separator had low air permeability and excellent electrolyte permeability.
y ≦ 0.41x (1a)
Further, when the aramid fiber concentration and the CMC concentration in the slurry are in the range of 0.1 to 1.0% by weight of the aramid fiber concentration and the CMC concentration [y;% by weight] is in the range of more than 0% by weight, the above formula (1a) is used. ), And within the range represented by the following formula (2a), a non-woven fabric having excellent coatability and uniformly containing fine fibers can be formed, and further, fine fibers can be formed. It was possible to form a non-woven fabric having high strength by improving the binding property between them.
y ≧ 0.133x + 0.03 (2a)

On the other hand, when the range of the above formula (1a) is exceeded, the air permeability of the obtained separator tends to increase. Further, if it is out of the range of the above formula (2a), the coatability of the slurry tends to decrease.

Claims (13)

It contains microfibers having a melting point (decomposition temperature if there is no melting point) of 150 ° C. or higher, water-soluble cellulose, and water.
The content of the fine fibers [x;% by weight] is 0.1 to 1.0% by weight, the content of the water-soluble cellulose [y;% by weight] is more than 0% by weight, and the content of the fine fibers is A slurry in which the amount [x;% by weight] and the content [y;% by weight] of the water-soluble cellulose satisfy the following formula (1).
y ≦ 0.62x (1) The slurry according to claim 1, wherein the content of the fine fibers [x;% by weight] and the content of the water-soluble cellulose [y;% by weight] further satisfy the following formula (2).
y ≧ 0.133x + 0.01 (2) The slurry according to claim 1 or 2, which has a viscosity of 10 to 10000 mPa · s at a temperature of 25 ° C. and a shear rate of 100 (1 / s). The slurry according to any one of claims 1 to 3, wherein the fine fibers are aramid fibers. The slurry according to any one of claims 1 to 4, wherein the fine fibers have an average thickness of 0.01 to 10 μm and an average length of 0.01 to 1 mm. The slurry according to any one of claims 1 to 5, wherein the water-soluble cellulose is a cellulose derivative having a structural unit represented by the following formula (c), or a salt thereof.

(In the formula, the three Rs are the same or different and indicate a hydrogen atom or an alkyl group having a hydroxyl group or a carboxyl group and having 1 to 5 carbon atoms). The slurry according to claim 6, wherein the degree of etherification of the cellulose derivative having the structural unit represented by the formula (c) is 0.6 or more. The slurry according to any one of claims 1 to 7, wherein the viscosity of a 1 wt% aqueous solution of water-soluble cellulose at 25 ° C. and 60 rpm is 50 mPa · s or more. The slurry according to any one of claims 1 to 8, which is used for forming a separator for a secondary battery. A non-woven fabric made of a solidified slurry according to any one of claims 1 to 9. A separator for a secondary battery, comprising a laminate of the non-woven fabric and the porous base material according to claim 10. The slurry according to any one of claims 1 to 9 is applied to the surface of a porous base material and dried to form a laminate of a non-woven fabric made of a solidified product of the slurry and the porous base material. A method for manufacturing a separator for a secondary battery, which obtains a separator for a secondary battery including the laminate through the steps. A secondary battery comprising the separator for a secondary battery according to claim 12.

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