TW201810773A - Separator for battery-use - Google Patents

Abstract

The present invention addresses the problem of providing a separator for battery-use having excellent pin-release properties and remaining safe even if thinning of the separator thickness advances. In the invention, alumina particles include particles having: two mode diameters which exhibit maxima in a primary particle size distribution; and an absorption peak near 3475 cm-1 by Fourier transform infrared spectroscopy (FT-IR), the peak disappearing at 300 DEG C or higher. This makes it possible to obtain a slurry having extremely excellent dispersibility and dispersion stability, and a separator for battery-use having excellent resistance to short-circuits, resistance to voltage, and pin-release properties.

Description

   Battery separator   

The invention relates to a battery separator.

Microporous membranes made of thermoplastic resins are widely used as substance separation membranes, selective osmosis membranes, and separation membranes. Examples include lithium ion secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, battery separators for polymer batteries, or separators for electric double-layer capacitors, reverse osmosis filtration membranes, ultrafiltration membranes, and precision filtration membranes. Various filters, moisture-permeable waterproof clothing, medical materials and so on.

In particular, as a separator for lithium ion secondary batteries, it is impregnated with electrolytic solution to have ion permeability and good electrical insulation. It can be suitably used at temperatures around 120-150 ° C when the battery is abnormally heated up. It is a polyolefin microporous membrane with pore sealing function that cuts off the current and suppresses excessive temperature rise. However, after the pores are closed for some reason, the polyolefin microporous membrane will shrink and rupture when the temperature inside the battery continues to rise. This phenomenon is not limited to the phenomenon of polyolefin microporous membranes. When microporous membranes of other thermoplastic resins are used, the melting point of the resin or higher cannot be avoided.

The separator for a lithium ion secondary battery is deeply related to battery characteristics, battery productivity, and battery safety, and requires heat resistance, electrode adhesion, penetration, and resistance to melt film cracking. Hitherto, for example, a battery separator using a polyolefin microporous film as a substrate and a porous layer provided on at least one side of the substrate has been known. As the resin used in the porous layer, a resin that imparts heat resistance to a separator called polyimide resin, polyimide resin, and polyimide resin, or a separator called fluororesin can be suitably used. Resin that imparts adhesion to electrodes. In recent years, a water-soluble or water-dispersible resin in which a porous layer can be laminated can be used in relatively simple steps. From the viewpoint of improving heat resistance, it is also considered to include inorganic particles in the porous layer.

The separator is laminated between a positive electrode and a negative electrode, and is applied to a battery having a wound electrode body wound. When the wound electrode body is manufactured, the pin is in direct contact with the separator, and therefore the separator requires good pin extraction. When the plug is poorly pulled out, the following problems occur: the separator that is in contact with the plug is pulled by the plug when the plug is pulled out, causing the inner circumference of the electrode wound body to protrude into a bamboo or telescope shape and lose the electrode The insulation structure between the positive and negative electrodes of the wound body. In order to improve the pull-out property of the spacer, it is proposed to set the static friction coefficient or the dynamic friction coefficient of the spacer to a specific value or less.

Patent Document 1 describes a three-layer separator including a positive electrode, a negative electrode, polypropylene, polyethylene, and polypropylene, and a bonding agent including polydifluoroethylene and alumina powder disposed between the electrodes and the separator. Electrode body of a flexible resin layer.

Test Example 2 of Patent Document 2 uses a jet mill to perform dry pulverization (wind pressure: 0.2 MPa, 5 minutes), and uses an air-flow type powder classification device to classify magnesium carbonate powder to 4 μm or less, and an acrylic adhesive aqueous solution. And the thickener mixture, pre-knead with a high-speed stirring disperser (15000 rpm, 5 minutes), and then formally knead (20,000 rpm, 15 minutes) to prepare a slurry for forming a porous layer and apply it to a polymer On the olefin microporous membrane, a battery separator was obtained.

In Example 1 of Patent Document 3, a mixed wet pulverized slurry 1 (a suspension of alumina-water suspension and wet pulverization using DYNO-MILL), carboxymethyl cellulose (CMC), and a solvent was obtained, and A battery separator in which a porous layer is laminated on a polyolefin-based porous film by applying a slurry treated under high-pressure dispersion conditions (20 MPa × 1 pass, 60 MPa × 3 pass) using Gaulinhomogenizer.

Example 3 of Patent Document 4 is obtained by stirring at room temperature until the emulsion and water of the self-crosslinking acrylic resin are uniformly dispersed. The boehmite powder is added to the dispersion in 4 times, and the dispersion is applied using a disperser. The obtained slurry was stirred (2800 rpm, 5 hours), and a porous separator was laminated with a porous layer on a polyethylene microporous membrane.

Patent Document 5 discloses reducing the static friction coefficient of the surface layer of the separator in order to improve the sliding property between the pin and the separator, and Patent Document 6 discloses reducing the dynamic friction coefficient of the surface layer of the separator.

On the other hand, if the porous layer contains inorganic particles, coarse protrusions are generated on the surface of the porous layer, and if the separator is a roll or the like, the separator in contact with the coarse protrusions may be indented. After that, it is predicted that the film thickness of the battery separator becomes thinner, and the thinner the thickness of the separator, the more obvious the effect of the indentation, and the film of the separator may be broken.

Prior art literature Patent literature

Patent Document 1 Japanese Re-examined Publication No. 1999-036981

Patent Document 2 Japanese Re-examined Publication No. 2011-158335

Patent Document 3 Japanese Patent Application Publication No. 2014-040580

Patent Document 4 Japanese Patent Laid-Open No. 2008-123996

Patent Document 5 Japanese Patent Application Laid-Open No. 2011-126275

Patent Document 6 Japanese Patent Application Publication No. 2014-012857

The subject of the present invention is to obtain a battery separator that is excellent in short-circuit resistance, voltage resistance, and excellent pin extraction.

As a result of careful discussions on the above-mentioned subject by the inventors of the present case, attention is focused on controlling the dispersibility and dispersion stability of the inorganic particles in the slurry, which is extremely important in battery separators. When a porous layer having inorganic particles is provided on the surface of a substrate, a method of adjusting a slurry containing inorganic particles, a binder, and a solvent, applying the slurry to the substrate, and drying the porous layer is generally used. The inorganic particles used in this case are liable to aggregate in the slurry if the size is small, and have the problem of dispersibility which is liable to precipitate if the size is large. Further, even if the particles can be sufficiently dispersed, if the dispersion stability of the inorganic particles in the slurry is low, there is a possibility that the particles may re-aggregate during the period from the preparation of the slurry to the application. When aggregates of inorganic particles are present in the slurry, the aggregates are mixed in the porous layer, and coarse protrusions are generated on the surface of the porous layer. When the separator obtained as described above is used as a roll, there is a case where the separator in contact with the large protrusions is indented.

As a result of careful research on the inorganic particles and their dispersion technology, the inventors of the present case found that the first-order particle size distribution has two types of mold diameters that are extremely large, and at least a part of the alumina particles have specific surface characteristics to obtain dispersion It has excellent dispersion and dispersion stability, and can provide battery separators with excellent short-circuit resistance, voltage resistance, and pin extraction.

In order to solve the above problems, the battery separator of the present invention has the following configuration.

That is,

(1) A battery separator comprising a polyolefin microporous film and a battery separator having a porous layer on at least one side of the polyolefin microporous film, characterized in that the porous layer contains alumina Particles and binders. When the total of the alumina particles and the binder is set to 100% by volume, the volume ratio of the alumina particles is 50% by volume or more. The alumina particles are included in a Fourier transform infrared spectroscopic method ( FT-IR) is a particle with an absorption peak near 3475cm ^-1 and the peak disappears above 300 ° C. The alumina particles have at least two kinds of maximums satisfying the following 1 and 2 in the first-order particle size distribution. When the alumina particles A having a particle diameter of 1.0 μm or more in the first-order particle diameter and the alumina particles B having a particle diameter of less than 1.0 μm, the total volume ratio of each of them satisfies the following formula 3; A (r1) ≦ 3.0 (μm) ... Equation 1

0.3 (μm) ≦ B (r1) <1.0 (μm) ... Equation 2

0.5 ≦ A (vol) / B (vol) ≦ 2.0 ... Eq. 3

Here, A (r1) and B (r1) are the largest die diameters in the first-order particle size distribution of alumina particles, and A (vol) / B (vol) is the total of alumina particles A and alumina particles B. Volume ratio.

(2) The battery separator of the present invention preferably has an absorption peak near 3475 cm ^-1 using a Fourier transform infrared spectroscopy (FT-IR), and the particles whose peaks disappear above 300 ° C are the alumina. Particle A.

(3) The battery separator of the present invention preferably has a moisture content of alumina particles measured by heating generated gas mass analysis (TPD-MS) when the temperature rises from room temperature to 1000 ° C, which is 2000 mass ppm or less.

(4) The battery separator of the present invention is preferably 0.7 ≦ A (vol) / B (vol) ≦ 1.5.

(5) The battery separator of the present invention is preferably 0.8 ≦ A (vol) / B (vol) ≦ 1.3.

(6) The battery separator of the present invention, preferably, the binder contains a fluororesin.

(7) The separator for a battery of the present invention, preferably, the fluororesin contains a difluoroethylene-hexafluoropropylene copolymer.

(8) The battery separator of the present invention, preferably, the thickness of the polyolefin microporous membrane is less than 10 μm.

(9) In the battery separator of the present invention, the thickness of the polyolefin microporous film is preferably 7 μm or less.

According to the present invention, by using alumina particles having two kinds of mold diameters having extremely large first-order particle size distributions and having specific surface characteristics, a slurry having excellent dispersibility and dispersion stability can be obtained. The porous layer formed by the body can suppress coarse protrusions to a high degree, and can provide a battery separator that is safe and thin for the development of separators. In particular, when the thickness of the separator is less than 10 m, the present invention can exert a greater effect. In addition, the battery separator of the present invention is excellent in the pin pullability, so that the productivity can be improved in the battery assembly step, and the effect of forcibly reducing the manufacturing cost can be exerted.

1‧‧‧ resin insulator

2‧‧‧ Negative electrode for lithium ion battery

3‧‧‧ Insulation

4‧‧‧ aluminum foil

5‧‧‧ metal ball

6‧‧‧ Compression fixture (lower side)

6’‧‧‧ compression clamp (upper side)

7‧‧‧ sample laminate containing metal balls

FIG. 1 is a diagram showing an infrared spectroscopic spectrum of the alumina particles 1 of Example 1 by a diffuse reflection method.

FIG. 2 is a diagram showing an infrared spectroscopic spectrum of the alumina particles 1 of Comparative Example 2 by a diffuse reflection method.

Fig. 3 is a graph showing a TPD-MS spectrum of alumina.

FIG. 4 is a diagram showing a laminated body of a sample used in a short-circuit resistance test.

FIG. 5 is a diagram showing a method for measuring a short-circuit resistance test.

Implementation of the invention     [1] Polyolefin microporous membrane    

The polyolefin resin constituting the polyolefin microporous film is preferably polyethylene or polypropylene. In addition, it may be a single substance or a mixture of two or more different polyolefin resins, for example, a mixture of polyethylene and polypropylene, or a copolymer of different olefins. In addition to the basic properties such as electrical insulation and ion permeability, the polyolefin microporous membrane formed from the resin described above also has the effect of blocking the pores by blocking the current during abnormal temperature rise of the battery and suppressing excessive temperature rise.

The polyethylene microporous membrane can be freely selected according to the purpose. As a method for manufacturing a microporous membrane, there are a foaming method, a phase separation method, a dissolution recrystallization method, an extended pore opening method, and a powder sintering method. Among these, from the viewpoint of homogenization of micropores and cost, Is preferably a phase separation method.

As a manufacturing method using a phase separation method, for example, a molten mixture obtained by heating and kneading polyethylene and a molding solvent, extruding from a mold, and cooling to form a gel-like molded article may be mentioned. A method of obtaining a microporous film by extending the gel-like molded product in at least a uniaxial direction and removing the aforementioned molding solvent.

The polyolefin microporous film may be a single-layer film or a layer composed of two or more layers having different polyolefin resin types, molecular weights, or average pore diameters. The layer structure can be a laminate of different polyolefins such as polypropylene / polyethylene / polypropylene or polyethylene / polypropylene / polyethylene, or it can be used by mixing polyolefin resins in any layer or all layers. .

As a method for producing a multilayer film including two or more layers, for example, each of the polyethylene constituting the a layer and the b layer is melt-kneaded with a molding solvent, and the obtained molten mixture is supplied to 1 by respective extrusion machines. Each mold is a method of integrating and extruding gel sheets constituting each component, and a method of superimposing gel sheets constituting each layer and performing heat fusion. The co-extrusion method is easy to obtain high-layer indirect strength, and it is easy to form communication holes between layers. Therefore, it is easy to maintain high penetration and excellent productivity, so it is preferable.

The upper limit of the thickness of the polyolefin microporous membrane of the present invention is preferably 25 μm, more preferably 9 μm, and even more preferably 7 μm. The lower limit is preferably 3 μm, and more preferably 5 μm. If the thickness of the polyolefin microporous membrane is in the above-mentioned preferred range, the practical membrane strength and pore sealing function can be maintained, and the area per unit volume of the battery case will not be limited. After that, it is also suitable for development. Increased battery capacity.

The upper limit of the air resistance of the polyolefin microporous membrane is preferably 500sec / 100ml Air, more preferably 400sec / 100ml Air, and the lower limit, preferably 50sec / 100ml Air, more preferably 70sec / 100ml Air, and even more preferably 100sec. / 100ml Air.

Regarding the porosity of the polyolefin microporous membrane, the upper limit is preferably 70%, more preferably 60%, even more preferably 55%. The lower limit is preferably 30%, more preferably 35%, and even more preferably 40%. If the polyolefin microporous membrane has air resistance and porosity in the above-mentioned preferred ranges, when it is used as a battery separator, in the charge and discharge characteristics of the battery, especially in ion permeability (charge and discharge operating voltage) And the life of the battery (closely related to the retention of the electrolyte), the battery can be fully utilized. In addition, since the polyolefin microporous membrane can obtain sufficient mechanical strength and insulation, the possibility of causing a short circuit during charging and discharging using the battery using the same is low.

The polyolefin microporous membrane needs to have a function of closing pores when the charge-discharge reaction is abnormal. Therefore, the melting point (softening point) of the resin formed is preferably 70 to 150 ° C, more preferably 80 to 140 ° C, and still more preferably 100 to 130 ° C. If the melting point of the resin is in the above-mentioned preferred range, the battery can be prevented from becoming unusable by exhibiting the hole-sealing function during normal use, and the safety can be ensured by exhibiting the hole-sealing function during abnormal reactions. .

    [2] Porous layer    

The porous layer of the present invention is formed by applying a slurry containing alumina particles, a binder, and a solvent to a polyolefin microporous membrane, immersing the slurry in a coagulation solution, and drying the porous layer. Alumina particles are responsible for improving pin pullout, voltage resistance, and short-circuit resistance.

The alumina particles used in the present invention include particles having an absorption peak near 3475 cm ^-1 by Fourier transform infrared spectroscopy (FT-IR), and the peak disappears at 300 ° C or higher. Further, in the present specification, refers to the range of 3475 ± 5cm ^-1 in the vicinity of 3475cm ^-1.

In the present invention, by using alumina particles having an absorption peak in the vicinity of 3475 cm ^-1 using FT-IR, the alumina particles can be uniformly dispersed without adding a dispersant such as a surfactant. When a dispersant such as a surfactant is used, the electrode active material may adhere to the electrode, and the battery performance may be deteriorated. Although the mechanism for improving the dispersibility by using alumina particles having an absorption peak as described above has not been determined, the inventors of the present invention consider the following. General alumina particles have a broad-spectrum absorption peak around 3400 cm ^-1 . This absorption peak suggests the presence of hydroxyl groups present on the surface of the alumina particles and the presence of adsorbed water that is hydrogen-bonded to the hydroxyl groups. On the other hand, the alumina particles used in the present invention include those having relatively sharp peaks around 3475 cm ^-1 . This absorption peak suggests the presence of crystal water adsorbed on a specific side of the surface of the alumina particles. Accordingly, it is estimated that the dispersibility is improved with respect to water or a water-soluble solvent system.

From the viewpoint of the durability of the non-aqueous secondary battery, the moisture content of the inorganic particles used in the separator for a non-aqueous electrolyte secondary battery is preferably as small as possible. If moisture is present in the non-aqueous electrolyte secondary battery, the oxidative decomposition of the moisture or the reaction between the moisture and the electrolyte results in significant gas generation, and the cycle characteristics are deteriorated due to battery expansion or electrolyte consumption. The absorption peak near 3475 cm ^-1 of the specific alumina particles used in the present invention disappears at 300 ° C or higher. That is, it is estimated that the crystal water adsorbed on a specific side of the surface of a specific alumina particle does not detach even if the temperature is raised to about 200 ° C, and does not adversely affect the non-aqueous secondary battery.

The content of a specific alumina particle having an absorption peak in the vicinity of 3475 cm ^-1 as the alumina particle used in the present invention is preferably when FT-IR measurement is performed on the entire alumina particle used in the present invention. The degree of the peak as described above can be confirmed.

The moisture content of the alumina particles generated when heated from room temperature to 1000 ° C. as measured by TPD-MS is preferably 2000 mass ppm or less, and more preferably 1900 mass ppm or less. If the amount of water generated from the entire alumina particles exceeds 2000 mass ppm, there is a possibility that battery characteristics may be deteriorated due to the influence of moisture. When the moisture content of the alumina particles is within the above-mentioned preferred range, deterioration of the battery characteristics can be suppressed when the alumina particles are used in a battery separator.

Then, in order to improve the pull-out property of the plug, it is effective to add a particle having a relatively large particle diameter to the porous layer to reduce the coefficient of friction between the separator and the plug. However, when only relatively large particles having a particle diameter of 1 μm or more are used, large particles are aggregated, and when larger coarse protrusions are formed on the surface of the porous layer, as described above, the separator that comes into contact with the coarse protrusions may be indented. Or the film may rupture. In addition, the gap between the alumina particles on the surface of the separator becomes large, and the resistance to a short circuit due to foreign matter mixed into the battery may decrease. Therefore, it is necessary to use a relatively small particle having a particle size of 1 μm or less to mix the separator. The surface is coated with alumina.

The alumina particles used in the present invention have at least two kinds of maximum in the first-order particle size distribution, and can be distinguished as alumina particles A having a particle diameter of 1.0 μm or more in the particle diameter distribution, and particles having a diameter of less than 1.0 μm Diameter of alumina particles B.

The alumina particle A represents an upper limit value of A (r1), which is an extremely large mode diameter (hereinafter also referred to as a “first-stage mode diameter”) in the first-order particle size distribution, preferably 3 μm, more preferably 2 μm, and a lower limit. It is preferably 1 μm, and more preferably 1.2 μm. When the first-order mold diameter A (r1) is less than 1.0 μm, sufficient plug extraction properties may not be obtained. If it exceeds 3 μm, in addition to the case where the withstand voltage is reduced, it may easily fall off the porous layer. .

The alumina particle B represents the lower limit value of the extremely large mode diameter B (r1) in the first-order particle size distribution, and is preferably 0.3 μm, and more preferably 0.4 μm. When the first-order mold diameter B (r1) is less than 0.3 μm, protrusions due to aggregates tend to be generated, and if it exceeds 1.0 μm, short-circuit resistance may be reduced. The upper limit value does not exceed 1 μm, and is preferably 0.8 μm.

Particles having an absorption peak near 3475 cm ^-1 using Fourier transform infrared spectroscopy (FT-IR) and the peak disappears above 300 ° C are preferably alumina particles A, and more preferably alumina particles A and B. Both sides.

The Mohs hardness of the alumina particles A and B is preferably 9. In the dispersing step, since the alumina particles are not easily shaved, it is difficult to generate fine powder, and it is difficult to generate coarse aggregates due to the fine powder. By using alumina particles having a high Mohs hardness, a slurry having excellent dispersibility and dispersion stability can be obtained, and the formation of coarse protrusions can be suppressed when it becomes a porous layer.

The alumina particles A, alumina particles B, and the binder are set to 100% by volume, and the total content of the alumina particles A and B of the porous layer is 50% by volume or more and 90% by volume or less, preferably 60% by volume. Above 80% by volume. When the total volume ratio of the alumina particles A and B is within the above-mentioned preferable range, the strength of the porous layer can be maintained, and a decrease in coatability can be suppressed.

The volume ratio (A (vol) / B (vol)) of the alumina particles A and B is preferably 0.5 to 2.0, more preferably 0.7 to 1.5, and still more preferably 0.8 to 1.3. If it is in the said range, favorable short-circuit resistance and latching-out property can be obtained. The volume V (cm ³ ) of each particle is determined as the true specific gravity d (g / cm ³ ) of the particle and the weight w (g) is determined by the following Equation 4.

V = w / d ... Equation 4.

The alumina particles used in the present invention are obtained, for example, by appropriately adjusting firing conditions and pulverization conditions. As long as it has the characteristics of the present invention, commercially available alumina particles may be used.

    (Binder)    

The binder used in the present invention is not particularly limited as long as it is a resin that binds alumina particles or a substrate and a porous layer. Examples thereof include a fluororesin, a polyamide-imide resin, an acrylic resin, polyvinyl alcohol, and carboxymethyl cellulose. From the viewpoints of heat resistance and electrolyte permeability, polyamidoamine imine resins or aromatic polyamidoamine resins are suitable. From the viewpoint of electrode adhesion, a fluororesin is suitable.

The following uses fluororesin as an example for detailed description. As the fluororesin, it is preferable to use one selected from the group consisting of a difluoroethylene vinylene homopolymer, a difluoroethylene vinylene-fluorinated olefin copolymer, a fluoroethylene homopolymer, and a fluoroethylene-fluorinated olefin copolymer. More than that. Further, the fluororesin may be graft-polymerized with maleic acid or the like. These polymers have excellent adhesion to electrodes, high affinity with non-aqueous electrolytes, and high chemical and physical stability relative to non-aqueous electrolytes, so they can be fully maintained at high temperatures. Affinity of electrolyte.

    (Manufacturing method of slurry)    

The slurry used in the present invention can be obtained by the following production method. The slurry used in the present invention can be obtained by mixing and dispersing alumina particles 1 corresponding to alumina particles A, alumina particles 2 corresponding to alumina particles B, and a binder and a solvent in order to obtain a more uniform slurry. The ground dispersion slurry can also include (1) the step of dissolving the binder in a solvent to obtain a binder solution, and (2) adding alumina particles 1 and alumina particles 2 to the binder solution, and pre-dispersing Then, it is manufactured by a method of further dispersing to obtain a slurry.

    (1) the step of dissolving the binder in a solvent to obtain a binder solution    

The solvent is not particularly limited as long as it can dissolve the binder and can be mixed with water, and can be freely selected according to the solubility of the binder. Examples include N-methyl-2-pyrrolidone (NMP), acetone, and the like.

    (2) a step of adding alumina particles 1 and alumina particles 2 to the binder solution, and pre-dispersing, and further dispersing to obtain a slurry    

Next, alumina particles 1 and alumina particles 2 were sequentially added while agitating the binder solution obtained in the foregoing step, and pre-dispersed once. Here, it is preferable to gradually add the alumina particles 1 and the alumina particles 2 to the binder solution.

Gradually add, for example, setting the addition rate of the binder solution per 10 L to 10 to 50 g / min. By doing so, the generation of fine powder can be suppressed. In addition, the pre-dispersion is stirred for a certain period of time (for example, about 1 hour), a mechanical stirrer, or the like to reduce agglomerated alumina particles. Without pre-dispersion, agglomerates of alumina particles included in the slurry may precipitate, and a part of the slurry may become a paste. In this case, sufficient dispersion becomes difficult, and not only the coarse protrusions are liable to be formed in the porous layer, but also the transportation pump may be blocked.

After pre-dispersion, dispersion is further performed using a bead mill or the like. Such as a bead mill, the dispersion method of applying high shear force to the slurry can further disperse alumina particles and reduce aggregates. Dispersion. Generally, if sufficient dispersion is desired, a high shear force is applied and the number of times (hereinafter sometimes referred to as the number of passes) needs to be 4 to 5 times. However, a part of the slurry becomes excessively dispersed and re-agglomerates. Happening. In the present invention, by using the aforementioned alumina particles, not only can the number of passes be shortened to 1 to 3 times, but also reaggregation can be suppressed even when a high shear force is applied.

The thickness of the porous layer is preferably 0.5 to 3 μm per side, more preferably 1 to 2.5 μm, and even more preferably 1 to 2 μm. If the film thickness of each surface is 0.5 μm or more, functions such as adhesion to the electrode and heat resistance can be ensured.

If the thickness of each surface is 3 μm or less, the volume of the roll can be suppressed, and thereafter, it is suitable for increasing the capacity of the battery if it is developed.

The porosity of the porous layer is preferably 30 to 90%, and more preferably 40 to 70%. When the porosity of the porous layer is set to the above-mentioned preferable range, an increase in the resistance of the separator can be prevented, a large current can flow, and the film strength can be maintained.

    [3] Battery separator    

The method for manufacturing a battery separator of the present invention includes the following steps (1) to (3) in this order.

(1) the step of dissolving the binder in a solvent to obtain a binder solution, (2) the step of adding alumina particles 1 and 2 to the binder solution, and further dispersing to obtain a slurry after pre-dispersing, (3) The slurry is applied to a polyolefin microporous membrane, immersed in a coagulation solution, and then washed and dried.

The method for applying the obtained slurry to a polyolefin microporous membrane may be a known method, and examples thereof include a dipping and coating method, a reverse roll coating method, a gravure coating method, an anastomotic coating method, a roll brush method, and a spray coating method. Method, air knife coating method, Meyer bar coating method, pipe doctor method, doctor blade coating method, die coating method, etc., and these methods may be performed individually or in combination.

The following uses fluororesin as an example for detailed description. The microporous membrane of the coating slurry is immersed in a coagulation liquid to coagulate the fluororesin to form a void between the fluororesin and the alumina particles. As the coagulation liquid, an aqueous solution containing 1 to 20% by weight of a good solvent relative to a fluororesin can be used, and more preferably 5 to 15% by weight.

Examples of good solvents include N-methyl-2-pyrrolidone, N, N-dimethylformamide, and N, N-dimethylacetamide. The coagulation liquid may also be added with a phase separation aid if necessary.

From the viewpoint of coagulation of the fluororesin, the immersion time in the coagulation liquid is preferably set to 2 seconds or more, and the upper limit is not limited, and 10 seconds is sufficient. Thereafter, a battery separator can be obtained through a washing step of removing a solvent by immersion in pure water and a drying step using hot air at 100 ° C or lower.

The air resistance of the battery separator is one of the most important characteristics, preferably 50 to 600 sec / 100 ml Air, more preferably 100 to 500 sec / 100 ml Air, and even more preferably 100 to 400 sec / 100 ml Air. The desired air resistance is obtained by adjusting the porosity of the porous layer and adjusting the degree of penetration of the adhesive into the polyolefin microporous membrane. If the air resistance of the battery separator is in the above-mentioned preferred range, charge-discharge characteristics and life characteristics in a range suitable for practical use can be obtained.

The upper limit of the overall film thickness of the battery separator obtained by laminating the porous layer is preferably 30 μm, and more preferably 25 μm. The lower limit is preferably 5 μm, and more preferably 7 μm. By setting it to be above the lower limit of the above-mentioned preferred range, sufficient mechanical strength and insulation can be ensured. By setting it to be below the upper limit of the above-mentioned preferable range, the electrode area that can be filled in the container can be ensured, and thus capacity reduction can be avoided.

Examples

Examples will be described below to specifically explain them, but the present invention is not limited to these examples. In addition, the measured value in an Example is a value measured by the following method.

    1.Fourier transform infrared spectroscopy (FT-IR)    

Alumina particles were added to a ceramic container and installed in a heating diffusion reflection device chamber provided with a heater. Inert gas (N ₂ ) was circulated in the chamber at a flow rate of 50 ml / min at about 20 ° C / min. The temperature was raised from room temperature to 700 ° C, and an infrared spectrum was obtained by a diffuse reflection method at 1 minute intervals. The measurement was performed under the following conditions using a commercially available FT-IR device (FTS-7000 manufactured by Varian) and a heated diffuse reflection device (made by PIKE Technologies). The alumina particles to be subjected to the measurement method may be alumina particles used as a raw material, or alumina particles from which a binder is removed from a porous layer using a solvent.

Detector: Deuterium Tri-Glycine Sulfate (DTGS)

Reference: Au

Wavelength range: 400 ~ 4000cm ^-1

Resolution: 4cm ^-1

Accumulated times: 16 times

The obtained diffuse reflectance spectrum was subjected to Kubelka-Munk conversion.

    2. Heated Gas Generation Mass Analysis (TPD-MS)    

Approximately 100 mg of alumina particles were added to a container, and a helium gas flow of 50 ml / min was used, and the temperature was raised from room temperature to 1000 ° C. at a temperature increase rate of 20 ° C./min. Amount of water generated per unit weight of alumina particles. For the measurement, a GS-MS device (QP2010 Ultra manufactured by Shimadzu Corporation) was used.

    3. Dispersion    

The dispersibility is the ratio of the difference between the mold diameter of the alumina particles and the diameter of the first-order mold diameter of the alumina particles in the slurry immediately after the slurry has been prepared with respect to the first-order mold diameter of the alumina particles (hereinafter expressed by the formula (5) The calculated value) is used as an index.

{(Die diameter of alumina particles in slurry-first-order die diameter of alumina particles) ÷ first-order die diameter of alumina particles} × 100 ... Eq. 5

The first-order die diameter of alumina particles, after adding alumina particles in water to 0.05% by weight, the laser diffraction / scattering type particle diameter distribution measuring device LA-960V2 (manufactured by Horiba, Ltd.) will be fully ultrasonic For the cracked sample, the particle size distribution was measured under the following conditions, and the maximum value was detected from the obtained frequency distribution chart. The maximum value in the range of 1.0 μm or more and 3.0 or less was defined as A (r1), and it was 0.3 or more and less than 1.0. The maximum value in the range of μm is defined as B (r1).

The die diameter of the alumina particles in the slurry immediately after being made into the slurry is the same as that of the primary mold except that the slurry is diluted with a solvent that can dissolve the binder so that the concentration of alumina particles becomes 0.05 wt%, and it is not cracked by ultrasound. The diameter was measured in the same manner, and A (r2) and B (r2) were obtained.

When the value of Formula 5 is within 30%, uniform dispersion can be achieved, agglomeration of alumina particles is small, formation of coarse protrusions of the obtained separator can be suppressed, and dispersibility is good.

    4. Decentralized stability    

The dispersion stability refers to the rate of change of the die diameter of the alumina particles in the slurry from the die diameters A (r2) and B (r2) of the alumina particles immediately after the slurry is prepared (the following formula 6). The mold diameter of the alumina particles in the slurry after 1 week was reduced to half of the capacity by using a container made of polypropylene that was prepared from the slurry produced until 1 week later. For the sample after hours, the mold diameter was determined in the same manner as in 3. Dispersibility.

{(The diameter of the alumina particles in the slurry from the time of making the slurry to one week-the diameter of the alumina particles in the slurry immediately after making the slurry) ÷ Die diameter of alumina particles) × 100 ... Equation 6

For the dispersion stability of the alumina particles 1 and 2, if the value of the formula 6 is 10% or less, the alumina particles in the slurry are stably present, the formation of coarse protrusions of the obtained separator can be suppressed, and it is good.

    5. Latch extraction    

The method of using a cylinder-shaped plug with a diameter of 4.0 mm was used to evaluate the plug pullability. First, a spacer material A to D with a width of 40 mm was applied with a tensile load of 200 g (each spacer material was 5 g / mm in width), and five turns were placed around a cylindrical plug having a diameter of 4.0 mm. The cylindrical plug was pulled out from the wound spacers A to D, and the plug pullability was evaluated as described below.

Good: The amount of bamboo shoot protrusion is less than 1mm

Defective: The amount of bamboo-like protrusion is 1mm or more

    6. Withstand voltage test method    

The wound body of the battery separator obtained from the examples and comparative examples was released from the core by about 10 m, and was used as a sample. Using a withstand voltage tester TOS5051A (manufactured by Kikusui Electronics Co., Ltd.), the withstand voltage test of the separator was performed in the following procedure. A separator (50 mm × 50 mm size) was placed on a sample stand on which aluminum foil was laid, and then an aluminum foil of φ15 mm was placed on the separator, and a conductive rubber of 13 mm was placed on the aluminum foil of φ 15 mm. Next, a metal hammer (φ50mm × height 32mm, weight about 500g) was placed on the conductive rubber, and the metal hammer and the aluminum foil on the sample stand were connected to the withstand voltage tester by a cable. The voltage between the sample table and the metal hammer is stepped up to 2kV (voltage step-up speed: 0.1kV / sec) with a withstand voltage tester. When converted to a film thickness of 20μm and an electrical short circuit occurs below 2kV, it is determined as a withstand voltage. The voltage is poor, and when the electrical short circuit does not occur, the withstand voltage is determined to be good.

    7. Short-circuit resistance test    

The short-circuit resistance was evaluated using a desktop precision universal testing machine Autograph AGS-X (manufactured by Shimadzu Corporation). As shown in FIGS. 4 and 5, a laminated polypropylene insulator 1 (thickness 0.2 mm), a negative electrode 2 for lithium ion batteries (total thickness: about 140 μm, base material: copper foil (thickness about 9 μm), active material: artificial A sample laminate of graphite (particle diameter of about 30 μm), double-sided coating), separator 3, and aluminum foil 4 (thickness of about 0.1 mm) was fixed to a compression jig (lower side) 6 of a universal testing machine with a double-sided tape. Next, the aluminum foil, the negative electrode of the sample laminate, and the circuit including the capacitor and the sheath resistor were connected with a cable. The capacitor was charged to about 1.5 V, and a metal ball 5 (material: chromium (SUJ-2)) with a diameter of about 500 μm was placed between the separator and the aluminum foil in the sample laminate. A compression jig is installed in the universal testing machine. As shown in FIG. 5, a sample laminate including the metal ball 5 is placed between the two compression jigs, and the compression is performed at a speed of 0.3 mm / min. The test is ended when the load reaches 100N. At this time, in the change in the compressive load, the part where the inflection point appears is determined as the film rupture point of the separator, and the moment when the above circuit is formed through the metal ball and the current is detected is determined as the short-circuit generation point. Measure the compressive displacement A (t) when the separator film is broken by compression and the compressive stress generates a buckling point, and the compressive displacement B (t) at the moment when the circuit is flowing a current, and calculate the value using the following Equation 7. When it is 1.1 or more, it means that even if the separator film is broken due to foreign matter mixed in the battery, insulation can be maintained by adhering the coating layer composition on the surface of the foreign matter, and therefore, short-circuit resistance is good. On the other hand, when the value obtained by Equation 7 is greater than 1.0 and less than 1.1, the separator film will not be broken and short-circuited at the same time, but in order to prevent the tension or charge / discharge associated with the winding applied to the battery structure. Even when the internal pressure of the battery is increased during the expansion of the electrode, a short circuit does not occur, and a certain level of resistance is required. Therefore, the short circuit resistance is slightly inferior. When the value obtained by Equation 7 is 1.0, film breakage and short-circuiting of the separator occur simultaneously, and no improvement in short-circuit resistance by the coating layer is observed, so the short-circuit resistance is poor.

B (t) ÷ A (t) ... Equation 7.

    Examples 1 to 11         (Adjustment of slurry)    

A difluoroethylene vinylene-hexafluoropropylene copolymer (VdF-HFP copolymer (weight average molecular weight: 1 million)) as a fluororesin was blended at a weight ratio of about 5 wt% based on N-methyl-2-pyrrolidone. Then, a fluororesin solution in which the VdF-HFP copolymer was completely dissolved was obtained. Next, the total volume of the alumina particles 1 and alumina particles 2 and the volume of the VdF-HFP copolymer were set to 50:50, and the alumina particles 1 were added to the fluororesin solution while stirring at 350 rpm using a mechanical stirrer. With 2. The characteristics and volume ratios of the alumina particles used in the examples are shown in Table 1. In addition, the first-order die diameters of the alumina particles 1 and 2 are based on each sample in which each alumina particle is adjusted to 0.05 wt% in water, and the particle size distribution is measured using the same measurement conditions as in the above-mentioned 3. Dispersibility, and is determined as the self-frequency The maximum value obtained from the distribution map.

The addition rate of alumina particles was set to 30 g / min per 10 L of the fluororesin solution. After the alumina particles were added, the process was continued, and the mixture was stirred for 1 hour with a mechanical stirrer to perform pre-dispersion. Next, DYNO-MILL (manufactured by Shinmaru Enterprises Co., Ltd., DYNO-MILL MULTI LAB (1.46L container, filling rate 80%, φ0.5mm alumina beads)) was used at a flow rate of 11 kg / hr and a peripheral velocity of 10 m / s Under the conditions shown in Table 1, the dispersion was carried out to prepare a slurry.

At this time, the temperature of the slurry is adjusted to a range of 20 to 45 ° C. In addition, the slurry should be kept sealed until it is coated without contacting fresh air.

    (Laminated porous layer)    

The slurry was applied to both sides of a polyethylene microporous membrane (thickness: 7 μm, air resistance: 100 sec / 100 ml Air) by dip coating, immersed in water (coagulation liquid), washed with pure water, and then passed through hot air at 70 ° C. Drying was performed in a drying furnace to obtain a battery separator having a final thickness of 11 μm.

    Example 12    

Except that the polyethylene microporous membrane was replaced with a thickness of 12 μm and air resistance of 120 sec / 100 ml of Air, it was carried out in the same manner as in Example 1 to obtain a battery separator.

    Example 13    

Except that the polyethylene microporous membrane was replaced with a thickness of 9 μm and air resistance of 180 sec / 100 ml of Air, it was carried out in the same manner as in Example 1 to obtain a battery separator.

    Example 14    

Except that the polyethylene microporous membrane was replaced with a thickness of 7 μm and air resistance of 180 sec / 100 ml of Air, it was carried out in the same manner as in Example 1 to obtain a battery separator.

    Example 15    

Except having replaced the polyethylene microporous membrane with 5 micrometers in thickness and 110 sec / 100 ml of air resistance, it carried out similarly to Example 1, and obtained the battery separator.

    Comparative Example 1    

Except that alumina particles 1 were not used, and only alumina particles 2 were used as alumina particles, the same procedure as in Example 1 was performed to obtain a battery separator.

    Comparative Examples 2 to 7    

Except that the alumina particles 1 and alumina particles 2 were the ratios shown in Table 1, and the dispersion conditions were the number of passes shown in Table 1, the same procedures as in Example 1 were performed to obtain a battery separator.

Table 1 shows the characteristics of the polyolefin microporous membranes and alumina particles 1 and 2 used in the examples and comparative examples.

Table 2 shows the number of passes of the examples and comparative examples, the dispersibility and dispersion stability of the alumina particles, and the characteristics of the obtained separator.

Claims (9)

A battery separator comprising a polyolefin microporous film and a battery separator having a porous layer on at least one side of the polyolefin microporous film, characterized in that the porous layer includes alumina particles and a bond. When the total amount of the alumina particles and the binder is 100% by volume, the volume ratio of the alumina particles is 50% by volume or more. The alumina particles are included in a FT-IR ), Which has an absorption peak near 3475 cm ^-1 , and the peak disappears above 300 ° C. The alumina particles have at least two kinds of maximums satisfying the following formulae 1 and 2 in the first-order particle size distribution. When the alumina particles A having a particle diameter of 1.0 μm or more and the alumina particles B having a particle diameter of less than 1.0 μm in the first-order particle size, the respective total volume ratios satisfy the following formula 3; 1.0 (μm) ≦ A ( r1) ≦ 3.0 (μm) ... Equation 1 0.3 (μm) ≦ B (r1) <1.0 (μm) ... Equation 2 0.5 ≦ A (vol) / B (vol) ≦ 2.0 ... Equation 3 is in Here, A (r1) and B (r1) are the largest die diameters in the first-order particle size distribution of alumina particles, and A (vol) / B (vol) are alumina particles A in the porous layer. The total volume of particles B alumina. For example, the battery separator according to claim 1, wherein the particle having an absorption peak near 3475 cm ^-1 using a Fourier transform infrared spectroscopy (FT-IR) and the peak disappearing at 300 ° C or higher is the alumina particle A.     For example, the battery separator of claim 1 or 2, wherein the alumina particles have a generated water content measured by heating generated gas mass analysis (TPD-MS) of 2000 mass ppm or less when the room temperature rises to 1000 ° C.         For example, the battery separator according to any one of claims 1 to 3, wherein 0.7 ≦ A (vol) / B (vol) ≦ 1.5.         For example, the battery separator according to any one of claims 1 to 3, wherein 0.8 ≦ A (vol) / B (vol) ≦ 1.3.         The battery separator according to any one of claims 1 to 5, wherein the binder contains a fluororesin.         The battery separator according to claim 6, wherein the fluororesin comprises a difluoroethylene-hexafluoropropylene copolymer.         The battery separator according to any one of claims 1 to 7, wherein the thickness of the polyolefin microporous membrane is less than 10 μm.         The battery separator according to any one of claims 1 to 8, wherein the thickness of the polyolefin microporous membrane is 7 μm or less.