JP2013084367A - Coating liquid for lithium ion battery separator, and lithium ion battery separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a coating liquid with high stability in manufacturing a lithium ion battery separator with a high heat resistance, and a low-resistance lithium ion battery separator arranged so that the powder falling of a layer including alumina hydrate, binder polymer, a water-soluble dispersant is reduced.SOLUTION: The coating liquid for a lithium ion battery separator comprises: alumina hydrate; aqueous fluid dispersion of binder polymer; and aqueous solution of dispersant. The alumina hydrate forms a single particle of boehmite having an average particle size of 2.0 μm or less, a specific surface area of 40 m/g or less, and an aspect ratio below 5. The lithium ion battery separator is produced by coating a nonwoven fabric with the coating liquid, followed by drying.

Description

  The present invention relates to a coating solution for a lithium ion battery separator and a lithium ion battery separator.

  A lithium ion secondary battery, which is one of the electrochemical elements, has a feature of high energy density, and is widely used as a power source for portable electric devices such as mobile phones, portable music players, and notebook personal computers. . In addition, the movement to use lithium ion secondary batteries is spreading in large equipment such as electric bicycles, hybrid cars, and electric cars. For this reason, lithium ion secondary batteries are required to have high capacity and charge / discharge characteristics at a large current. However, since lithium ion secondary batteries are non-aqueous batteries, they emit smoke compared to water-based batteries. It is known that there is a high risk of ignition, rupture, etc., and improvement in safety is also required.

  In a lithium ion secondary battery, the risk of smoking increases due to temperature rise due to external heat, overcharge, internal short circuit, external short circuit, and the like. These can be prevented to some extent by an external protection circuit. In addition, the polyolefin resin porous film used as a lithium ion battery separator melts at around 120 ° C., and the pores are blocked to block the flow of current and ions, thereby suppressing battery temperature rise. . This is called a shutdown function. However, when the temperature rises due to external heat or when a chemical reaction occurs inside the battery due to the temperature rise, the battery temperature further rises even if the shutdown function works. The porous film contracted, causing an internal short circuit, which could cause ignition.

  As described above, it is difficult to suppress the ignition of the battery by the shutdown function of the separator. In addition, with the increase in capacity of batteries, the increase in current during charging and discharging is also progressing, and in order to suppress the Joule heat generated at that time, it is also necessary to lower the resistance value of the separator impregnated with the electrolyte. It has become. For this reason, metal oxide fine particles were used for the purpose of suppressing the ignition of the battery by lowering the resistance value while making it difficult to cause an internal short circuit by raising the heat shrinkage temperature than the porous film of polyolefin resin. Separator has been developed. In this separator, the pore diameter is controlled by the metal oxide fine particles, and it is possible to suppress internal short circuit, improve heat resistance, and lower the resistance value.

  For example, a separator having a microporous pseudoboehmite layer obtained as a porous film by mixing pseudoboehmite and a binder, drying and peeling after coating on a separately prepared film has been proposed (for example, patents) Reference 1). However, although this separator has an improved heat shrink temperature, it tends to cause powder falling and cracking, so that it is difficult to take it out as a rolled-up separator alone, resulting in poor handling during battery production. It was.

  Also proposed is a separator having a porous inorganic coating on and in the nonwoven fabric, the inorganic coating having aluminum (Al), silicon (Si) and / or zirconium (Zr) oxide particles. (For example, refer to Patent Document 2). Although this separator uses non-woven fabric as a base material, handling is improved, but other metal oxides are detected by silica fine particles generated by the sol-gel method. Drops and cracks are likely to occur, which leads to the generation of pinholes and causes internal short circuits, making it difficult to be a useful separator.

  Furthermore, a coating liquid containing insulating fine particles having heat resistance, a thickener, and a dispersion medium, and a separator coated with the coating liquid on a substrate have been proposed (for example, Patent Document 3). reference). However, the coating solution has a problem that the stability of the coating solution is not sufficient, and in the separator coated with the coating solution on a substrate, primary particles are aggregated as insulating fine particles. When the secondary particles are used, the insulating fine particles may fall off from the base material because of insufficient adhesion to the base material. Further, when the flat insulating fine particles are used, the flat insulating fine particles block the pores of the separator, so that there is a problem that the internal resistance is increased by preventing the passage of lithium ions.

JP-T-2001-527274 JP 2005-536658 Gazette International Publication No. 2009/096451 Pamphlet

  The present invention provides a highly stable coating solution for producing a lithium ion battery separator having high heat resistance, and a layer containing an alumina hydrate, a binder polymer, and a water-soluble dispersant ( In the following, a lithium ion battery separator having a low resistance and a low resistance may be provided.

As a result of intensive studies to solve the above problems, the present inventors have
(1) Alumina hydrate, an aqueous dispersion of a binder polymer, and an aqueous dispersant. The alumina hydrate has an average particle size of 2.0 μm or less and a specific surface area of 40 m ² / g. Hereinafter, a coating liquid for a lithium ion battery separator, characterized by being boehmite single particles having an aspect ratio of less than 5;
(2) A lithium ion battery separator, wherein the coating liquid is applied to a nonwoven fabric and dried.
I found.

An alumina hydrate, an aqueous dispersion of a binder polymer, and an aqueous dispersant are contained. The alumina hydrate has an average particle diameter of 2.0 μm or less, a specific surface area of 40 m ² / g or less, an aspect By using boehmite single particles with a ratio of less than 5, the stability of the coating solution for a lithium ion battery separator can be enhanced.

  By applying the coating liquid to a nonwoven fabric, a lithium ion battery separator in which the coating layer is less powdered and the degree of voids of the separator is controlled can be obtained.

An example of the electron micrograph (10000 magnification) of the single particle alumina hydrate used in Example 1 of this invention. An example of an electron micrograph (10000 magnifications) of the aggregated particle alumina hydrate used in Comparative Example 1 of the present invention.

The coating liquid of the present invention comprises an alumina hydrate, an aqueous dispersion of a binder polymer, and an aqueous dispersant, and the alumina hydrate has an average particle size of 2.0 μm or less and a specific surface area. Is a boehmite single particle having an aspect ratio of less than 5 and 40 m ² / g or less.

  The particles of the alumina hydrate according to the present invention are not particles formed by agglomeration of primary particles of fine particles, but single particles. Whether it is an aggregate or a single particle can be determined from an image obtained by photographing an aqueous dispersion of alumina hydrate with a scanning electron microscope (SEM). For example, a single particle is a state in which alumina hydrate is not aggregated but dispersed as a single particle as shown in FIG. 1, and an aggregate is a fine alumina water as shown in FIG. It is a state where Japanese products are gathered and dispersed.

  The aspect ratio (ratio of particle “diameter” to particle “thickness”) of the alumina hydrate of the present invention can be determined from an image taken by SEM. In the present invention, the aspect ratio is measured for 50 alumina hydrates, and the average value is used. Here, the “diameter” of the particle represents the diameter of a circle equal to the projected area of the particle obtained by image analysis using an image obtained by observing the alumina hydrate with SEM. In the present invention, the aspect ratio is less than 5. The aspect ratio is preferably 4 or less from the viewpoint of ensuring a space between particles. More preferably, it is 3 or less. In the case where the aspect ratio is 5 or more, the voids between the particles cannot be secured due to layering in the coating layer, and the resistance may be increased.

As for the particle size of the alumina hydrate of the present invention, the average particle size when dispersed in water is 2.0 μm or less.
From the viewpoint of controlling the voids in the coating layer and the balance of powder falling, the average particle size during water dispersion is preferably 1.5 μm or less. More preferably, it is 1.0 μm or less. If the average particle size exceeds 2.0 μm, the alumina hydrate in the coating solution may precipitate and cause a problem. In addition, when the average particle size is smaller, there is no particular problem, but the viscosity of the coating solution may become too high, so 0.3 μm or more is sufficient. In addition, the average particle diameter in this invention is an average particle diameter calculated | required from the particle size distribution measurement by a laser diffraction method.

The BET specific surface area in this invention is 40 m < ² > / g or less. 30 m < ² > / g or less is preferable in the liquid property of a coating liquid and the point of powder fall-off. More preferably, it is 20 m ² / g or less. When the BET specific surface area exceeds 40 m ² / g, the stability of the coating liquid may be lowered, or the adhesiveness to the nonwoven fabric may be lowered. Further, even if the BET specific surface area is small, there is no particular problem, but 2.5 m ² / g or more is sufficient.

  The alumina hydrate used in the coating liquid of the present invention is boehmite represented by the composition formula AlOOH, APYRAL (registered trademark) AOH manufactured by Nabaltec, Germany, ACTILOX (registered trademark), and Cerasur (registered trademark) manufactured by Kawai Lime. ) BMB.

  Various latexes such as styrene-butadiene latex can be used as the aqueous dispersion of the binder polymer used in the coating liquid of the present invention. As an example of the aqueous dispersion of the binder polymer, tack load (registered by JSR) Trademark) 2001 can be exemplified.

  The addition amount of the binder polymer used in the coating liquid of the present invention is preferably 5% by mass or more and 25% by mass or less with respect to the alumina hydrate. From the viewpoint of the powder fall of the coating layer and the balance of the voids in the coating layer, 7.5% by mass or more and 20% by mass or less is more preferable.

  Examples of the aqueous dispersant used in the coating liquid of the present invention include synthetic polymers such as polyvinyl alcohol and polyvinyl pyrrolidone, cellulose derivatives such as carboxymethyl cellulose and hydroxymethyl cellulose, polycarboxylic acids, polyacrylic acids, and polycarboxylates. A polymer dispersant or the like can be used, but carboxymethyl cellulose is preferable from the viewpoint of miscibility with both the alumina hydrate and the binder polymer.

  The addition amount of the aqueous dispersant used in the coating liquid of the present invention is preferably 0.05% by mass or more and 3.0% by mass or less with respect to the alumina hydrate. From the viewpoint of the stability of the coating solution and the powder falling off of the coating layer, it is more preferably 0.1% by mass or more and 2.0% by mass or less.

  The coating liquid of the present invention is preferably applied to a nonwoven fabric. The nonwoven fabric manufactured by a conventionally known method can be used. For example, what was manufactured by methods, such as the spun bond method, the melt blow method, the dry method, the wet method, and the electrospinning method, can be used.

  In the present invention, the nonwoven fabric may be smoothed by calendaring or thermal calendaring for the purpose of controlling the surface and thickness of the nonwoven fabric.

  As the constituent material of the nonwoven fabric, polyethylene terephthalate, polybutylene terephthalate and derivatives thereof, polyester such as aromatic polyester, wholly aromatic polyester, polyolefin, acrylic, polyacetal, polycarbonate, aliphatic polyketone, aromatic polyketone, aliphatic polyamide, Aromatic polyamide, wholly aromatic polyamide, polyimide, polyamideimide, polyphenylene sulfide, polybenzimidazole, polyetheretherketone, polyethersulfone, poly (para-phenylenebenzobisthiazole), poly (para-phenylene-2,6- Benzobisoxazole), polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl alcohol, polyurethane and polyvinyl chloride Like Wei and cellulose fibers. The nonwoven fabric may contain two or more of these constituent materials.

As a nonwoven fabric used for the lithium ion battery separator of the present invention, the basis weight is preferably 5.0 to 30.0 g / m ² . If the basis weight exceeds 30.0 g / m ² , the nonwoven fabric occupies most of the separator, and the effect of the coating layer may be difficult to obtain. If it is less than 5.0 g / m ² , it may be difficult to obtain uniformity as a nonwoven fabric. As a fabric weight of a nonwoven fabric, 7.0-20.0 g / m < ² > is more preferable. The basis weight means the basis weight based on the method defined in JIS P 8124 (paper and paperboard—basis weight measurement method).

  There is no particular limitation on the method for applying the coating liquid of the present invention to the nonwoven fabric, for example, a conventionally known air doctor coater, blade coater, knife coater, rod coater, squeeze coater, impregnation coater, gravure coater, kiss roll coater, Examples include a die coater, a reverse roll coater, a transfer roll coater, and a spray coater.

In the present invention, the adhesion amount of the coating layer is preferably 1.0~20.0g / ^{m 2,} further 4.0~15.0g / ^{m 2} is more preferable. If the amount of the coating layer deposited is less than 1.0 g / m ² , the surface of the nonwoven fabric cannot be sufficiently covered, the pore diameter becomes large, and good battery characteristics such as short circuit may not be exhibited. is there. On the other hand, when the adhesion amount of the coating layer exceeds 20.0 g / m ² , it may be difficult to reduce the thickness of the separator.

  In order to produce a uniform coating layer by the above coating method, a thickener, an antifoaming agent, a wetting agent, and the like can be appropriately added to the coating solution as necessary.

  In the present invention, the method of drying after coating is not particularly limited, and a known drying method can be used. In particular, a method of drying by heating, such as a method of blowing hot air or a method of irradiating infrared rays, is productivity. Is preferably used.

In the lithium ion battery separator of the present invention, the basis weight of the separator is preferably ^{10.0~50.0g / m 2, 15.0~40.0g / m} 2 is more preferable. Moreover, 10.0-50.0 micrometers is preferable and, as for the thickness of a separator, 15.0-40.0 micrometers is more preferable. Preferably 0.4~1.2g / cm ³ as the density of the separator, 0.6~1.0g / cm ³ is more preferable.

  In the present invention, after coating and drying, the lithium ion battery separator may be smoothed by calendaring for the purpose of controlling the flatness and thickness of the coating layer surface.

  Examples of the present invention will be described below.

<Production of non-woven fabric>
45 parts by mass of oriented crystallized polyethylene terephthalate (PET) short fibers having a fineness of 0.06 dtex (average fiber diameter of 2.4 μm) and a fiber length of 3 mm, an orientation of fineness of 0.1 dtex (average fiber diameter of 3.0 μm) and a fiber length of 3 mm 15 parts by mass of crystallized PET-based short fibers, a fine component of 0.2 dtex (average fiber diameter 4.3 μm), and a fiber length of 3 mm PET-based short fibers for a single-component binder (softening point 120 ° C., melting point 230 ° C.) 40 parts by mass Were mixed together, disintegrated in water with a pulper, and a uniform papermaking slurry having a concentration of 1% by mass was prepared under stirring by an agitator. Using a circular paper machine, this papermaking slurry is made up by a wet method, and a PET short fiber for a binder is adhered by a cylinder dryer at 120 ° C. to develop a nonwoven fabric strength. The basis weight is 15.2 g / m ² . A non-woven fabric was used. Furthermore, this nonwoven fabric was subjected to heat treatment at a roll temperature of 185 ° C., a linear pressure of 740 N / cm, and a conveying speed of 20 m / min using a 1-nip thermal calendar composed of a metal roll and a metal roll, and a nonwoven fabric having a thickness of 27 μm was obtained. Produced.

Example 1
Alumina hydrate A (boehmite, single particles, average particle size 0.9 μm, specific surface area 5.5 m ² / g, aspect ratio 2.5, manufactured by Naval Tech, trade name: APYRAL (registered trademark) -AOH60) 90 parts by mass in terms of solid content, carboxymethylcellulose sodium salt (1% by weight aqueous solution B viscosity 200 mPa · s, etherification degree 0.65) 0.2 parts by mass in terms of solid content are mixed and stirred in a homogenizer, Carboxymethylcellulose sodium salt (1% by weight aqueous solution B viscosity 7000 mPa · s, etherification degree 0.7) is mixed with 0.2 parts by mass in terms of solid content, stirred, and then styrene-butadiene latex is 9 in terms of solid content. Parts were mixed and stirred, and ion exchange water was further added to prepare a coating liquid A having a solid content concentration of 25% by mass. The coating solution is uniformly applied to one side of the non-woven fabric so that the dry solid content is 10.2 g / m ² by a gravure coater on the non-woven fabric after the thermal calendar treatment, and lithium ion with a thickness of 30.2 μm A battery separator was obtained.

Example 2
In Example 1, instead of alumina hydrate A, alumina hydrate B (boehmite, single particle, average particle size 1.8 μm, specific surface area 3.6 m ² / g, aspect ratio 3.1, manufactured by Navaltech Corporation, Product name: APYRAL (registered trademark) -AOH 30) was used, and the coating liquid B was prepared and applied in the same manner as in Example 1, and the dry solid content of the coating layer was 10.5 g / m ² , thickness. A 31.0 μm lithium ion battery separator was obtained.

Example 3
In Example 1, instead of alumina hydrate A, alumina hydrate C (boehmite, single particle, average particle size 0.4 μm, specific surface area 38 m ² / g, aspect ratio 4.2, manufactured by Naval Tech Co., Ltd., trade name) : ACTILOX (registered trademark) -400SM), coating liquid C was prepared and coated in the same manner as in Example 1, and the coating layer had a dry solid content of 10.5 g / m ² and a thickness of 29. A 3 μm lithium ion battery separator was obtained.

Example 4
In Example 1, instead of alumina hydrate A, alumina hydrate D (boehmite, single particle, average particle diameter 0.6 μm, specific surface area 18 m ² / g, aspect ratio 3.4, manufactured by Naval Tech Co., Ltd., trade name) : APYRAL (registered trademark) -AOH180) was used, and the coating liquid D was prepared and applied in the same manner as in Example 1, and the dry solid content of the coating layer was 10.7 g / m ² and the thickness was 29. A 9 μm lithium ion battery separator was obtained.

Example 5
In Example 1, instead of alumina hydrate A, alumina hydrate E (boehmite, single particle, average particle size 0.5 μm, specific surface area 21 m ² / g, aspect ratio 1.7, manufactured by Kawai Lime Industry Co., Ltd., commercial product Name: Cerasur® (registered trademark) BMB) was used in the same manner as in Example 1 except that the coating liquid E was prepared and coated. The coating layer had a dry solid content of 10.5 g / m ² and a thickness of 30. A 1 μm lithium ion battery separator was obtained.

(Comparative Example 1)
In Example 1, instead of alumina hydrate A, alumina hydrate F (boehmite, aggregated particles, average particle size 1.6 μm, specific surface area 3.0 m ² / g, aspect ratio 4.7, manufactured by Daimei Chemical Co., Ltd.) Except for using the product name: C20), the coating liquid F was prepared and coated in the same manner as in Example 1, and the dry solid content of the coating layer was 10.1 g / m ² and the thickness was 29.7 μm of lithium. An ion battery separator was obtained.

(Comparative Example 2)
In Example 1, instead of alumina hydrate A, alumina hydrate G (boehmite, single particle, average particle size 6.0 μm, specific surface area 3.0 m ² / g, aspect ratio 3.5, manufactured by Navaltech Corporation, Except for using product name: APYRAL (registered trademark) -AOH20Y), a coating solution G was prepared and applied in the same manner as in Example 1, and the dry solid content of the coating layer was 10.6 g / m ² , thickness. A 32.7 μm lithium-ion battery separator was obtained.

(Comparative Example 3)
In Example 1, instead of alumina hydrate A, alumina hydrate H (boehmite, single particle, average particle size 1.9 μm, specific surface area 5.0 m ² / g, aspect ratio 6.8, manufactured by Kawai Lime Industry Co., Ltd.) , Trade name: Cerasur (registered trademark) BMT) except that the coating liquid H was prepared and applied in the same manner as in Example 1, and the dry solid content of the coating layer was 10.5 g / m ² . A lithium ion battery separator having a thickness of 31.2 μm was obtained.

(Comparative Example 4)
In Example 1, instead of alumina hydrate A, alumina hydrate I (pseudo boehmite, aggregated particles, average particle size 0.4 μm, specific surface area 71 m ² / g, aspect ratio 7.5, manufactured by Sasol Corporation, trade name : Dispersal (registered trademark) 80) was used, and coating liquid I was prepared and coated in the same manner as in Example 1, and the dry solid content of the coating layer was 10.2 g / m ² and the thickness was 28.9 μm. Lithium ion battery separator was obtained.

<Evaluation>
The coating liquid and lithium ion battery separator obtained in the examples and comparative examples were evaluated as follows, and the results are shown in Table 1.

[Stability of coating solution]
About the produced coating liquid, it was left to stand for 48 hours, respectively, the sedimentation state of the alumina hydrate at that time was confirmed visually, and it evaluated by the following degree.
A: No sedimentation is observed at all.
○: Slight sedimentation of alumina hydrate is observed at the bottom, but returns to the original state by stirring.
Δ: Some sedimentation of alumina hydrate was observed at the bottom, and some aggregates of alumina hydrate were observed even when stirred.
X: Precipitation of a large amount of alumina hydrate was observed, and a large amount of aggregate of alumina hydrate was observed even when stirred.

[Food fall]
About the produced separator, it cuts into a strip shape of 50 mm width x 200 mm, the upper end is fixed with tape, a black cloth on which a 50 g weight is placed is slid, and the black cloth and separator at that time are visually observed. It confirmed and evaluated in the following degree.
(Double-circle): A coating layer does not stick at all to the surface of a black cloth, and peeling does not look at a coating layer.
○: The coating layer is slightly attached to the surface of the black cloth, but no peeling is observed in the coating layer.
Δ: The coating layer is slightly attached to the surface of the black cloth, and peeling is seen in the coating layer.
X: A large amount of coating layer is attached to the surface of the black cloth, and peeling is seen in the coating layer.

[Separator porosity]
About the produced separator, it used as the parameter | index of the space | gap state of a separator using the Gurley permeability resistance meter, and evaluated it with the following degree.
◎: measured value is 90 sec / 100 cc or more and 150 sec / 100 cc or less ◯: measured value is 75 sec / 100 cc or more and less than 90 sec / 100 cc, or greater than 150 sec / 100 cc and 175 sec / 100 cc or less Δ: 50 sec / 100 cc or more and less than 75 sec / 100 cc, or 175 sec Greater than / 100cc and 250sec / 100cc or less x: less than 50sec / 100cc or greater than 250sec / 100cc

The separator obtained by coating and drying the coating liquid obtained in Examples and the nonwoven fabric contains alumina hydrate, an aqueous dispersion of a binder polymer, and an aqueous dispersant. Since the hydrate is boehmite single particles having an average particle size of 2.0 μm or less, a specific surface area of 40 m ² / g or less, and an aspect ratio of less than 5, the coating liquid is excellent in stability and coated. -Also in the separator produced by drying, a good result was obtained that the powder layer of the coating layer was excellently balanced with the degree of voids of the separator.

On the other hand, in Comparative Example 1, the used alumina hydrate is boehmite having an average particle size of 2.0 μm or less, a specific surface area of 40 m ² / g or less, and an aspect ratio of less than 5, but is an aggregate. The stability of the coating liquid was lower than in the Examples, and the tendency of powder falling to deteriorate was also observed in separators prepared by coating and drying this on a nonwoven fabric.

  Moreover, in Comparative Example 2, since the average particle diameter of alumina hydrate was more than 2.0 μm, the stability of the coating liquid was lower than that in Examples, and this was prepared by coating and drying the nonwoven fabric. In the separator, the degree of voids was high, and the space diameter of the nonwoven fabric tended to increase.

  In Comparative Example 3, since the aspect ratio is 5 or more, the stability of the coating liquid is lower than in Examples, and the degree of voids is low even in a separator produced by coating and drying the nonwoven fabric. There was a tendency to overfill the gaps.

  In Comparative Example 4, alumina hydrate is an aggregate pseudo-boehmite and has a large specific surface area. Therefore, the stability of the coating solution is lower than that of the example, and this was prepared by coating and drying the nonwoven fabric. Also in the separator, powder fallen worsened, the degree of voids was high, and the space diameter of the nonwoven fabric tended to increase.

  The coating liquid for lithium ion battery separator and lithium ion battery separator of the present invention are used in addition to lithium ion secondary battery applications, such as manganese dry batteries, alkaline manganese batteries, silver oxide batteries, lithium primary batteries, lead storage batteries, nickel-cadmium storage batteries. , Nickel-hydrogen storage battery, nickel-zinc storage battery, silver oxide-zinc storage battery, lithium polymer battery, various gel electrolyte batteries, zinc-air storage battery, iron-air storage battery, aluminum-air storage battery, fuel cell, solar battery, sodium sulfur It can be used for batteries, polyacene batteries, electrolytic capacitors, electric double layer capacitors, lithium ion capacitors and the like.

Claims (2)

An alumina hydrate, an aqueous dispersion of a binder polymer, and an aqueous dispersant are contained. The alumina hydrate has an average particle diameter of 2.0 μm or less, a specific surface area of 40 m ² / g or less, an aspect A coating solution for a lithium ion battery separator, wherein the coating liquid is boehmite single particles having a ratio of less than 5.   A lithium ion battery separator, wherein the coating liquid of claim 1 is applied to a nonwoven fabric and dried.