JP2021057236A - Separator for lithium ion secondary battery - Google Patents

Abstract

To provide a separator for a lithium ion secondary battery having high heat resistance and high productivity.SOLUTION: In a separator for a lithium ion secondary battery, a coating layer containing inorganic particles, an organic polymer binder, and fine fibers is provided on at least one surface of a porous substrate, and the fine fibers are total aromatic polyamide fibers.SELECTED DRAWING: None

Description

The present invention relates to a separator for a lithium ion secondary battery.

In recent years, with the increase in capacity and energy density of lithium ion secondary batteries (hereinafter, "lithium ion secondary batteries" may be referred to as "batteries"), safety has become a serious problem. In order to improve the safety of the battery, measures have been taken such as containing a flame retardant in the electrolytic solution and improving the thermal stability of the positive electrode and the negative electrode. In a separator for a lithium ion secondary battery (hereinafter, "separator for a lithium ion secondary battery" may be referred to as a "separator"), it is said that the higher the heat resistance, the higher the safety of the battery (for example). Patent Document 1). Therefore, a highly heat-resistant separator in which inorganic particles are coated on a porous base material such as a polyolefin-based microporous membrane or a polyester non-woven fabric has been proposed (see, for example, Patent Documents 2 and 3).

However, although the heat resistance of the separator is increased to some extent by coating with inorganic particles, the effect is limited, and there is a problem that the heat resistance of the porous base material is largely dependent on it.

Nonwoven fabric base materials such as polyester non-woven fabrics have higher heat resistance of the porous base materials themselves than polyolefin-based microporous membranes, and can be expected to be safe. However, in the method for manufacturing a separator in which a coating liquid is applied to a non-woven fabric base material, the coating liquid is easily pushed into the inside of the porous base material due to the strong dynamic pressure in the coating device, and the coating liquid is dried at high speed. Even with the wind pressure coming from the large amount of hot air, the coating liquid is likely to be pushed into the inside of the porous substrate. This tendency is particularly remarkable when the separator is to be manufactured at high speed. Therefore, a phenomenon in which the pushed coating liquid exudes from the surface opposite to the one to which the coating liquid is applied (hereinafter, may be referred to as “striking through the coating liquid”) occurs, and the roll of the coating device is soiled. In addition, dirt adhering to the roll may be retransferred to the separator and the surface thereof may become non-uniform, and it has been extremely difficult to produce the separator at high speed by this production method.

As a method of preventing strike-through of the coating liquid, a method of adding fine cellulose to the coating liquid has been proposed (for example, Patent Document 4). However, although this method improves the strike-through of the coating liquid, the heat resistance of the separator is not significantly improved by the fine cellulose.

International Publication No. 2006/061936 Pamphlet Japanese Unexamined Patent Publication No. 2008-4439 Japanese Patent Application Laid-Open No. 2011-505663 JP-A-2015-84318

An object of the present invention is to provide a separator for a lithium ion secondary battery having high heat resistance and high productivity.

As a result of diligent research to solve the above problems, the following inventions have been found.

(1) In a separator for a lithium ion secondary battery in which a coating layer containing inorganic particles, an organic polymer binder and fine fibers is provided on at least one surface of a porous substrate, the fine fibers are all aromatic polyamide fibers. A separator for a lithium-ion secondary battery.

(2) The separator for a lithium ion secondary battery according to (1), wherein the porous base material is a non-woven fabric base material.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a separator for a lithium ion secondary battery having high heat resistance and high productivity.

The separator for a lithium ion secondary battery of the present invention is provided with a coating layer containing inorganic particles, an organic polymer binder and fine fibers on at least one surface of a porous base material, and the fine fibers are all aromatic polyamide fibers. It is characterized by being.

In the present invention, examples of the porous substrate include a microporous membrane and a non-woven fabric substrate. The microporous membrane is generally obtained by uniaxially stretching or biaxially stretching a polyolefin such as polypropylene or polyethylene. In addition, a porous base material made of polyamide or polyimide is also mentioned as an example of a microporous membrane. In the present invention, it is considered that the fine fibers of the total aromatic polyamide contained in the coating layer are mixed with the porous base material together with the inorganic particles to exhibit heat resistance. The more the coating layer penetrates into the porous substrate, the more the coating layer and the porous substrate are mixed. Therefore, it is preferable that the porous substrate is a non-woven fabric substrate.

Fibers contained in the non-woven substrate include polyolefins such as polypropylene and polyethylene, polyesters such as polyethylene terephthalate (PET), polyethylene isophthalate and polyethylene naphthalate, acrylics such as polyacrylonitrile, and polyamides such as 6,6 nylon and 6 nylon. Examples thereof include various synthetic fibers such as wood pulp, hemp pulp, and various cellulose pulps such as cotton pulp, and cellulose-based regenerated fibers such as rayon and lyocell. Among these, a non-woven fabric base material containing polyester or polypropylene is preferable because of heat resistance, low hygroscopicity, and the like.

The method for producing the non-woven fabric base material is not particularly limited. Examples of the method for forming the fiber into a sheet include a spunbond method, a melt blow method, an electrostatic spinning method, a wet method and the like. Among these, the wet method is preferable because a non-woven fabric base material having a thin and dense structure can be obtained.

The basis weight and thickness of the porous substrate of the present invention are not particularly limited, but the porous substrate of the separator for a lithium ion secondary battery preferably has a basis weight of 5 to 30 g / m ² and a thickness of 8 to 30 μm. , Basis weight 6 to 15 g / m ² , thickness 10 to 20 μm are more preferable. The basis weight is a value measured based on the method specified in JIS P8124: 2011 (paper and paperboard-basis weight measurement method), and the thickness is the outer micrometer specified in JIS B 7502: 2016 (micrometer). In use, it means the value measured under a 7N load.

The inorganic particles are not particularly limited as long as they are suitable for use in the coating layer of the separator. Examples include alumina such as α-alumina, β-alumina and γ-alumina, alumina hydrate such as boehmite, magnesium compounds such as magnesium oxide and magnesium hydroxide, and calcium oxide. Among these, α-alumina, alumina hydrate, magnesium oxide, and magnesium hydroxide are preferably used because of their high stability to the electrolytic solution used in the lithium ion secondary battery.

In the present invention, the coating layer contains an organic polymer binder in order to increase the strength of the coating layer. The organic polymer binder is not particularly limited as long as it is suitable for use in the coating layer of the separator. Examples thereof include acrylic acid ester polymers, methacrylic acid ester polymers, styrene-butadiene polymers, polyvinylidene fluoride and the like.

The content of the organic polymer binder is preferably 0.5% by mass to 20.0% by mass, more preferably 1.0% by mass to 10.0% by mass, based on the inorganic particles. When the content of the organic polymer binder is less than 0.5% by mass with respect to the inorganic particles, the adhesion to the porous substrate and the coating layer strength are weakened, and the inorganic particles are likely to fall off. On the other hand, when the content of the organic polymer binder exceeds 20.0% by mass with respect to the inorganic particles, the organic polymer binder may be coated and the internal resistance of the separator may be increased.

Total aromatic polyamide fibers are used as the fine fibers. The fine fibers in the present invention are preferably fibers having a modified drainage degree of 400 ml or less. The modified drainage degree was measured in accordance with JIS P8121: 2012 except that an 80-mesh wire mesh with a wire diameter of 0.14 mm and a mesh size of 0.18 mm was used as a sieving plate and the sample concentration was 0.1%. The value.

Fine fibers are obtained by treating all aromatic polyamide fibers using a wet ball mill, a wet vibration mill, a wet paint shaker, a refiner, a beater, a grinder, a high pressure homogenizer, a high speed homogenizer, an ultrasonic crusher and the like. Since the fine fibers are all aromatic polyamide fibers, the effect of improving the heat resistance of the separator is higher than when other materials such as cellulose are used. It is considered that this is because the total aromatic polyamide fibers are mixed when the porous substrate is hot-melted. As a result, the viscosity of the entire separator is increased, so that the separation function is maintained even in the molten state, and the heat resistance is improved. The finer the fiber, the higher the effect of increasing the viscosity of the porous substrate during hot melting, and the higher the heat resistance, the higher the strength can be maintained even at high temperatures. The modified drainage degree of the fine fibers in the present invention is preferably 0 to 400 ml, preferably 0 to 350 ml, and more preferably 0 to 250 ml. When the modified water flow rate exceeds 400 ml, the viscosity when the porous base material and the fine fibers are mixed is low, and it becomes difficult to improve the heat resistance.

The content of the fine fibers is preferably 0.5% by mass or more and 4.0% by mass or less, and more preferably 1.0% by mass or more and 2.0% by mass or less with respect to the porous substrate. When the content of the fine fibers is less than 0.5% by mass with respect to the porous substrate, it becomes difficult to develop heat resistance. When the content of fine fibers exceeds 4.0% by mass with respect to the porous substrate, the internal resistance of the separator may increase.

In addition to the inorganic particles, organic polymer binder and fine fibers, various dispersants such as polyacrylic acid and sodium carboxymethyl cellulose, various thickeners such as hydroxyethyl cellulose, sodium carboxymethyl cellulose and polyethylene oxide, and various thickeners are used in the coating layer. Various additives such as a wetting agent, an antiseptic agent, and an antifoaming agent can be added as needed.

In the present invention, the content of the coating layer (coating amount after drying of the coating liquid) is preferably 5 to 30 g / m ² , and more preferably 8 to 20 g / m ² . If the amount of coating is too small, pinholes may occur. If the amount of coating is too large, the internal resistance may become too high. In addition, the thickness of the separator becomes thicker. In the present invention, the thickness of the separator is preferably 10 to 40 μm, more preferably 15 to 25 μm. If the separator is too thick, it becomes difficult to increase the energy density, and if the separator is too thin, safety may not be ensured. In producing the separator of the present invention, in the present invention, the coating layer may be applied to only one side of the non-woven fabric base material, or may be applied to both sides. Moreover, you may apply it to each surface twice or more.

In the production of the separator of the present invention, various coating devices can be used as a method of coating the coating liquid on the porous substrate. As the coating device, various coaters such as a gravure coater, a die coater, a blade coater, a rod coater, and a roll coater can be used. When the porous base material is a non-woven fabric base material, the coating liquid strikes through, so compared to the conventional coating liquid, which is difficult to use with a coating device that applies dynamic pressure in the depth direction during coating. It is also an excellent feature of the coating liquid used in the present invention that a wider variety of coating devices can be selected from the viewpoint of productivity and the like.

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

<Preparation of non-woven fabric base material>
A single fiber with a fineness of 0.06 dtex (average fiber diameter of 2.4 μm) and a fiber length of 3 mm, 60 parts by mass of oriented crystallized polyethylene terephthalate (PET) short fibers and a fineness of 0.2 dtex (average fiber diameter of 4.3 μm) with a fiber length of 3 mm. 40 parts by mass of PET short fibers for component type binders (softening point 120 ° C., melting point 230 ° C.) were dispersed in water with a pulper to prepare a uniform papermaking slurry having a concentration of 1% by mass. This slurry for making is made by a wet method with a tilting paper machine, and PET-based short fibers for binders, and PET-based short fibers for binders and oriented crystallized PET-based short fibers are used by a cylinder dryer at 135 ° C. The intersections of the above were fused to develop tensile strength, and a non-woven fabric ^{having a grain size of 10 g / m 2 was obtained.} Further, this non-woven fabric is subjected to the conditions of a thermal roll temperature of 200 ° C., a linear pressure of 100 kN / m, and a processing speed of 30 m / min using a 1-nip thermal calendar composed of a dielectric heating jacket roll (metal thermal roll) and an elastic roll. A non-woven fabric base material having a thickness of 15 μm was prepared by thermal calendar treatment.

<Microporous membrane>
A polyethylene microporous membrane having a thickness of 15 μm was used.

<Preparation of fine fiber suspension A>
Tiara (registered trademark) KY400S (manufactured by Daicel FineChem), which is an all-aromatic polyamide fiber, is dispersed in ion-exchanged water so as to have a concentration of 1% by mass, and treated with a VF pump (manufactured by Shinhama Pump Mfg. Co., Ltd.) for 5 minutes. As a result, a fine fiber suspension A having a modified drainage degree of 50 ml and a solid content concentration of 1% by mass was prepared.

<Preparation of fine fiber suspension B>
Tiara KY400S (manufactured by Daicel FineChem), which is an all-aromatic polyamide fiber, is dispersed in ion-exchanged water so as to have a concentration of 2% by mass, and treated with a VF pump (manufactured by Shinhama Pump Mfg. Co., Ltd.) for 5 minutes to change the concentration. A fine fiber suspension B having a legal drainage degree of 50 ml and a solid content concentration of 2% by mass was prepared.

<Preparation of fine fiber suspension C>
Towaron (registered trademark) 1094 (manufactured by Teijin Limited), which is a total aromatic polyamide fiber, is dispersed in ion-exchanged water so as to have a concentration of 1% by mass, and treated with a VF pump (manufactured by Shinhama Pump Mfg. Co., Ltd.) for 5 minutes. Then, a fine fiber suspension C having a modified drainage degree of 340 ml and a solid content concentration of 1% by mass was prepared.

<Preparation of fine fiber suspension D>
Cellulose fiber Serish (registered trademark) KY-100G (manufactured by Daicel FineChem) is dispersed in ion-exchanged water so as to have a concentration of 1% by mass, and treated with a VF pump (manufactured by Shinhama Pump Mfg. Co., Ltd.) for 5 minutes. A fine fiber suspension D having a modified drainage content of 220 ml and a solid content concentration of 1% by mass was prepared.

(Example 1)
To 100 parts by mass of the fine fiber suspension A, 100 parts by mass of magnesium hydroxide having an average particle size of 3.0 μm was added, and the mixture was stirred and mixed. Next, 75 parts by mass of a 2% by mass aqueous solution of carboxymethyl cellulose sodium salt having a viscosity of 200 mPa · s at 25 ° C. of the 1% by mass aqueous solution was added and stirred and mixed. Subsequently, 5 parts by mass of an emulsion (solid content concentration: 50% by mass) of a carboxy-modified styrene-butadiene polymer having a glass transition point of −18 ° C. and an average particle size of 0.2 μm was added and stirred and mixed. Finally, a coating solution was prepared by adding prepared water so that the solid content concentration became 25% by mass. The prepared coating liquid is applied onto a non-woven fabric substrate placed on black drawing paper with a ^{hand coating bar so that the solid content coating amount is 10 g / m 2} , dried with a hot air dryer at 80 ° C., and separated. Got

(Example 2)
A separator was obtained by preparing a coating liquid and coating the non-woven fabric substrate in the same manner as in Example 1 except that the fine fiber suspension A was changed to the fine fiber suspension B.

(Example 3)
A separator was obtained by preparing a coating liquid and coating the non-woven fabric substrate in the same manner as in Example 1 except that the fine fiber suspension A was changed to the fine fiber suspension C.

(Example 4)
A coating solution was prepared in the same manner as in Example 2. The prepared coating liquid is applied onto a microporous film placed on black drawing paper with a ^{hand coating bar so that the solid content coating amount is 10 g / m 2} , dried with a hot air dryer at 80 ° C., and separated. Got

(Comparative Example 1)
A separator was obtained by preparing a coating liquid and coating the non-woven fabric substrate in the same manner as in Example 1 except that the fine fiber suspension A was changed to the fine fiber suspension D.

(Comparative Example 2)
A separator was obtained by preparing a coating liquid and coating the non-woven fabric substrate in the same manner as in Example 1 except that the fine fiber suspension A was changed to ion-exchanged water.

(Comparative Example 3)
A coating solution was prepared in the same manner as in Comparative Example 2. The prepared coating liquid is applied onto a microporous film placed on black drawing paper with a ^{hand coating bar so that the solid content coating amount is 10 g / m 2} , dried with a hot air dryer at 80 ° C., and separated. Got

[Evaluation of strike-through of coating liquid]
The separators prepared in Examples 1 to 4 and Comparative Examples 1 to 3 were peeled off from the black drawing paper after drying, and the strike-through of the coating liquid was evaluated according to the following criteria.

◯: No adhesion of the coating liquid to the black drawing paper is observed.
Δ: The coating liquid is attached to the black drawing paper, but the black drawing paper and the separator do not stick to each other after drying.
X: The coating liquid is attached to the black drawing paper, and the black drawing paper or the separator is partially damaged when the separator is peeled off from the black drawing paper after drying.

[Heat-resistant temperature evaluation]
The separators, non-woven fabric base material, and microporous film of Examples 1 to 4 and Comparative Examples 1 to 3 cut into 60 mm square were attached to a metal frame having a 50 mm square hole, and the center was a needle (French embroidery needle No. 7). , Thickness 0.69 mm). Hot air was blown to the portion where the needle hole was made with a hot air gun for 10 seconds, and the maximum temperature at which the hole did not expand (the diameter did not become 2.0 mm or more) was set as the heat resistant temperature. At this time, the distance between the outlet of the hot air gun and the metal frame was fixed at 5 mm. The results are shown in Table 1.

◯: Difference in heat resistant temperature between the porous base material and the separator is 100 ° C. or more Δ: Difference in heat resistant temperature between the porous base material and the separator is 50 ° C. or more and less than 100 ° C. ×: Difference in heat resistant temperature between the porous base material and the separator Is less than 50 ° C

As shown in Table 1, the separators for lithium ion secondary batteries of Examples 1 to 4 contain inorganic particles, an organic polymer binder, and total aromatic polyamide fibers (fine fibers) in the coating layer, and therefore, therefore, the coating liquid. No strike-through occurred and it showed high heat resistance.

On the other hand, in the separator of Comparative Example 1, although the coating layer contained fine cellulose, it did not contain all aromatic polyamide fibers, so that the increase in heat resistant temperature due to the coating layer was small. Further, in the separator of Comparative Example 2, since the coating layer did not contain fine fibers and the porous base material was a non-woven fabric base material, strike-through of the coating liquid occurred and the increase in heat resistant temperature was small. In the separator of Comparative Example 3, since the porous base material was a microporous film, strike-through of the coating liquid did not occur, but since the coating layer did not contain all aromatic polyamide fibers, the heat resistant temperature due to the coating layer was high. The rise was small.

From the comparison between Examples 2 and 4, the increase in heat resistant temperature was larger in the non-woven fabric base material in the case where the porous base material was the non-woven fabric base material and the case where the microporous membrane was used. This is considered to be the difference in the contact area between the porous substrate and the coating layer. In the non-woven fabric base material, since the coating layer penetrates inside the base material, the contact area between the porous base material and the coating layer is larger than that of the microporous film. It is considered that this caused the hot-melted porous substrate to quickly mix with the coating layer and increase the viscosity, thereby facilitating the development of heat resistance.

The present invention can be used in a lithium ion secondary battery.

Claims (2)

A separator for a lithium ion secondary battery in which a coating layer containing inorganic particles, an organic polymer binder and fine fibers is provided on at least one surface of a porous base material, wherein the fine fibers are all aromatic polyamide fibers. Separator for lithium-ion secondary batteries. The separator for a lithium ion secondary battery according to claim 1, wherein the porous base material is a non-woven fabric base material.