KR101981079B1 - Battery separator - Google Patents

Abstract

A problem to be solved by the present invention is to provide a separator for a battery which is excellent in the removability of the fin and is safe even when the thickness of the separator is reduced. The alumina particles have two mode diameters representing the maximum in the primary particle diameter distribution and have absorption peaks near 3475 cm ^-1 by Fourier transform infrared spectroscopy (FT-IR) and disappear at the above peak at 300 ° C or higher By including the particles, it is possible to obtain a slurry having an excellent dispersibility and dispersion stability, and to obtain a separator for a battery which is excellent in short-circuit resistance, withstand voltage and pin removability.

Description

Battery separator

The present invention relates to a battery separator.

BACKGROUND ART [0002] Microporous membranes made of a thermoplastic resin are widely used as separation membranes for materials, selective permeable membranes, and separator membranes. For example, various filters such as a lithium ion secondary battery, a nickel-hydrogen battery, a nickel-cadmium battery, a battery separator used for a polymer battery, a separator for an electric double layer capacitor, a reverse osmosis filtration membrane, an ultrafiltration membrane, Breathable waterproof medicine, medical materials and so on.

Particularly, as a separator for a lithium ion secondary battery, a separator having ion permeability by impregnation with an electrolyte and excellent in electrical insulation, and having a capacity for blocking an excessive temperature rise by interrupting the current at a temperature of about 120 to 150 deg. A polyolefin microporous membrane having a closing function is preferably used. However, if the internal temperature of the battery continues to increase after closing of the hole for any reason, the polyolefin microporous membrane may generate a shrinkage / peeling film. This phenomenon is not a phenomenon confined to the polyolefin microporous membrane, and even in the case of the microporous membrane using other thermoplastic resin, it can not be avoided at a temperature above the melting point of the resin.

The separator for a lithium ion secondary battery is deeply involved in battery characteristics, battery productivity, and battery safety, and is required to have heat resistance, electrode adhesiveness, permeability, and content of rust-preventive film. Heretofore, for example, a separator for a battery has been known in which a porous layer is formed on at least one surface of the above-described substrate with a polyolefin microporous membrane as a base. As the resin used for the porous layer, a resin that imparts heat resistance to a separator such as a polyamideimide resin, a polyimide resin, and a polyamide resin, or a resin that imparts adhesiveness to an electrode to a separator called a fluororesin is preferably used. In recent years, a water-soluble or water-dispersible resin capable of laminating a porous layer with a relatively simple process has also been used. In addition, from the viewpoint of improving the heat resistance, it has been studied 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 wound electrode body. Since the fins and the separator come in direct contact when producing the wound electrode body, good separability of the separator is required. When the pin removability is poor, the separator in contact with the pin is attracted to the fin at the time of removing the pin, so that the inner peripheral portion of the electrode winding protrudes in a bamboo shape or a telescope shape, There arises a problem that the insulation structure between the electrodes is lost. In order to improve the removability of the separator, it has been proposed to set the static friction coefficient or the dynamic friction coefficient of the separator to a specific value or less.

Patent Document 1 describes an electrode body including a positive electrode, a negative electrode, a three-layer separator made of polypropylene, polyethylene, and polypropylene, and an adhesive resin layer made of polyvinylidene fluoride and alumina powder disposed between the electrodes and the separator have.

In Test Example 2 of Patent Document 2, dry milling (wind pressure: 0.2 MPa, 5 minutes) was carried out using a jet mill, and a mixture of magnesium carbonate powder, acrylic binder aqueous solution and thickener (15,000 rpm, 5 minutes) with a high-speed agitating disperser, followed by kneading (20,000 rpm, 15 minutes) to prepare a slurry for forming a porous layer and applying this slurry on a polyolefin microporous membrane to obtain a battery separator.

In Example 1 of Patent Document 3, the wet grinding slurry 1 (suspension in which the alumina-water suspension was wet pulverized using Dyno mill), carboxymethyl cellulose (CMC) and the solvent were mixed and subjected to a high-pressure dispersion condition using a coline homogenizer (20 MPa x 1 pass, 60 MPa x 3 passes) was applied to obtain a battery separator in which a porous layer was laminated on a polyolefin based porous film.

In Example 3 of Patent Document 4, the emulsion of the self-crosslinking acrylic resin and water were stirred at room temperature until uniformly dispersed, boehmite powder was added in four divided portions to this dispersion, Followed by stirring (2800 rpm, 5 hours) to obtain a separator for a battery in which a porous layer was laminated on a polyethylene-made microporous membrane.

Patent Document 5 discloses lowering the static friction coefficient of the surface layer of the separator in order to improve the slipperiness of the fin and the separator, and Patent Document 6 discloses lowering the coefficient of dynamic friction of the surface layer of the separator.

On the other hand, if inorganic particles are contained in the porous layer, coarse protrusions may be generated on the surface of the porous layer. If the separator is made of a winding material or the like, indentations may occur in the separator contacting the coarse protrusions. In the battery separator, it is predicted that the thickness of the separator becomes thinner, and as the thickness of the separator becomes thinner, the influence of the indentation becomes more present and may lead to the breakage of the separator.

Japanese Patent Publication No. 1999-036981 Japanese Patent Publication No. 2011-158335 Japanese Patent Application Laid-Open No. 2014-040580 Japanese Patent Application Laid-Open No. 2008-123996 Japanese Patent Application Laid-Open No. 2011-126275 Japanese Patent Application Laid-Open No. 2014-012857

An object of the present invention is to obtain a separator for a battery which is excellent in short-circuit resistance and withstanding voltage resistance and which is excellent in removing ability of a pin.

As a result of intensive investigation into the above problems, the present inventors have noted that controlling the dispersibility and dispersion stability of the inorganic particles in the slurry is extremely important for a battery separator. When a porous layer having inorganic particles is formed on the surface of a substrate, a method is generally used in which a slurry containing inorganic particles, a binder and a solvent is prepared, applied to a substrate, and dried to form a porous layer. If the size of the inorganic particles used is small, aggregation tends to occur in the slurry. If the size of the inorganic particles is large, the inorganic particles tend to precipitate. In addition, even if the particles can be sufficiently dispersed, if the dispersion stability of the inorganic particles in the slurry is low, there is a fear that the particles are re-aggregated until the slurry is prepared and then coated. When aggregates of inorganic particles are present in the slurry, aggregates are mixed into the porous layer, and coarse protrusions are generated on the surface of the porous layer. If the thus obtained separator is made of a cinder, indentations may be generated in the separator in contact with the coarse protrusion.

As a result of intensive research on inorganic particles and their dispersion techniques, the present inventors have found that the two mode diameters showing the maximum in the primary particle diameter distribution and at least a part of the alumina particles have specific surface properties, It is possible to obtain a very excellent slurry and to obtain a separator for a battery excellent in short-circuit resistance, withstanding voltage and pin removing property.

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

In other words,

(1) A separator for a battery comprising a polyolefin microporous membrane and a porous layer on at least one side of the polyolefin microporous membrane, wherein the porous layer comprises alumina particles and a binder, and the total amount of the alumina particles and the binder is 100 vol% Wherein the volume ratio of the alumina particles is 50 vol% or more, the alumina particles have an absorption peak at about 3475 cm < ^-1 > by Fourier transform infrared spectroscopy (FT-IR) Wherein the alumina particles have at least two maximum peaks satisfying the following formulas 1 and 2 in the primary particle diameter distribution, and the alumina particles A having a particle diameter of 1.0 mu m or more in the primary particle diameter distribution and 1.0 mu m By mass of alumina particles B having a particle diameter of less than < RTI ID = 0.0 > Cell separator.

1.0 (占 퐉)? A (r1)? 3.0 (占 퐉) ... Equation 1

0.3 (占 퐉)? B (r1) <1.0 (占 퐉) Equation 2

0.5? A (vol) / B (vol)? 2.0 ... Equation 3

Here, A (r1) and B (r1) are mode diameters showing the maximum in the primary particle diameter distribution of alumina particles, and A (vol) / B (vol) are total volume ratios of alumina particle A and alumina particle B, respectively.

(2) It is preferable that the separator for a battery of the present invention has an absorption peak at about 3475 cm ^-1 by Fourier transform infrared spectroscopy (FT-IR), and the particle disappearing at a peak of 300 ° C or higher is the alumina particle A.

(3) It is preferable that the battery-produced separator of the present invention has an amount of generated alumina particles of not more than 2000 ppm by mass as measured by heating gas mass analysis (TPD-MS) when the temperature is raised from room temperature to 1000 캜.

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

(5) The separator for a battery of the present invention preferably has 0.8? A (vol) / B (vol)? 1.3.

(6) In the battery separator of the present invention, it is preferable that the binder contains a fluororesin.

(7) In the battery separator of the present invention, it is preferable that the fluororesin includes a vinylidene fluoride-hexafluoropropylene copolymer.

(8) In the battery separator of the present invention, it is preferable that the thickness of the polyolefin microporous membrane is less than 10 mu m.

(9) In the battery separator of the present invention, it is preferable that the thickness of the polyolefin microporous membrane is 7 탆 or less.

According to the present invention, it is possible to obtain a slurry having excellent dispersibility and dispersion stability by using alumina particles having two mode diameters showing the maximum in the primary particle diameter distribution and having specific surface characteristics, The porous layer can highly suppress the coarse protrusion and can provide a battery separator which is safe even when the separator is made thinner. Particularly, the present invention exerts a greater effect when the film thickness of the separator is less than 10 mu m. Further, since the separator for a battery of the present invention is excellent in the removability of the fin, the productivity can be improved in the cell assembling process, and further, the manufacturing cost can be reduced.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram showing an infrared spectroscopic spectrum of alumina particle 1 of Example 1 by a diffuse reflection method. FIG.
2 is a view showing an infrared spectroscopic spectrum of the alumina particle 1 of Comparative Example 2 by the diffuse reflection method.
3 is a view showing a TPD-MS spectrum of alumina.
Fig. 4 is a view showing a laminate of samples used in the short-circuit immunity test. Fig.
5 is a diagram showing a method for measuring a short-circuit immunity test.

[1] Polyolefin microporous membrane

The polyolefin resin constituting the polyolefin microporous membrane is preferably polyethylene or polypropylene. Further, 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 other olefins. The polyolefin microporous membrane formed by the resin has a hole closing effect that cuts off the current at the time of battery abnormality increase and suppresses an excessive increase in temperature, in addition to basic characteristics such as electrical insulation and ion permeability.

The polyethylene microporous membrane can be freely selected according to the purpose. Examples of the method for producing the microporous membrane include a foaming method, a phase separation method, a dissolution recrystallization method, a stretching method, a powder sintering method, and the like. Among these methods, a phase separation method is preferable in terms of uniformity of fine holes and cost.

As a production method by the phase separation method, for example, there can be used a production method in which, for example, polyethylene and a molding solvent are heated and melted and kneaded, the obtained molten mixture is extruded from a die and cooled to form a gel molding, And a method in which a microporous membrane is obtained by removing the molding solvent, and the like.

The polyolefin microporous membrane may be a single layer film or a layer composed of two or more layers of different types of polyolefin resin, molecular weight or average pore diameter. The layer constitution may be a laminate of other polyolefins such as polypropylene / polyethylene / polypropylene, polyethylene / polypropylene / polyethylene or the like, and these polyolefin resins may be blended in any one layer or all layers.

As a method for producing a multilayer film composed of two or more layers, for example, each of polyethylene constituting the a layer and the b layer is melted and kneaded with a molding solvent, and the resulting molten mixture is supplied to one die from each extruder, A method of pneumatic delivery by integrating a gel sheet constituting the respective layers, and a method of superposing a gel sheet constituting each layer to heat fusion. The co-extrusion method is more preferable because it is easy to obtain a high interlaminar bond strength, easily forms a communication hole between the layers, easily maintains high permeability, and is excellent in productivity.

The upper limit of the thickness of the polyolefin microporous membrane of the present invention is preferably 25 占 퐉, more preferably 9 占 퐉, and still more preferably 7 占 퐉. The lower limit is preferably 3 占 퐉, more preferably 5 占 퐉. When the thickness of the polyolefin microporous membrane is within the above-mentioned preferable range, it is possible to have a practical film strength and a hole closing function, so that the area per unit volume of the battery case is not restricted, and is suitable for high capacity of the battery to be advanced in the future.

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

With respect to the porosity of the polyolefin microporous membrane, the upper limit is preferably 70%, more preferably 60%, still more preferably 55%. The lower limit is preferably 30%, more preferably 35%, still more preferably 40%. When the polyolefin microporous membrane is used as a separator for a battery, it is preferable that the porosity and the porosity of the polyolefin microporous membrane are in the above-described preferable range, and the charging / discharging characteristics of the battery, particularly the ion permeability (charge / discharge operation voltage) The function of the battery can be sufficiently exhibited. In addition, since the polyolefin microporous membrane has sufficient mechanical strength and insulation property, the use of the polyolefin microporous membrane reduces the possibility of a short circuit occurring during charging and discharging.

It is necessary that the polyolefin microporous membrane has a function of closing the hole at the time of abnormality of the charge-discharge reaction. Therefore, the melting point (softening point) of the constituent resin is preferably 70 to 150 占 폚, more preferably 80 to 140 占 폚, and still more preferably 100 to 130 占 폚. When the melting point of the resin to be constituted is in the above-mentioned preferable range, it is possible to prevent the battery from becoming unusable due to the manifestation of the hole closing function at the time of normal use, and to secure safety by manifesting the hole closing function at the time of abnormal reaction.

[2] Porous layer

The porous layer in 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 coagulating solution, and drying the slurry. The alumina particles are responsible for improving the pin removability, withstand voltage and short-circuit resistance.

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

In the present invention, by using alumina particles having an absorption peak at about 3475 cm ^-1 by FT-IR, uniform alumina particles can be dispersed without adding a dispersant such as a surfactant. If a dispersant such as a surfactant is used, it may be adhered to an electrode active material to deteriorate battery performance. The mechanism by which the dispersibility is improved by using the alumina particles having such an absorption peak is not certain, but the inventors of the present invention consider the following. Typical alumina particles have a broad absorption band in the vicinity of 3400cm ^-1. This absorption peak suggests the presence of adsorbed water which is hydrogen bonded to the hydroxyl groups and hydroxyl groups present on the alumina particle surface. On the other hand, the alumina particles used in the present invention include those having relatively sharp peaks near 3475 cm ^-1 . This absorption peak suggests the presence of crystal water adsorbed on a specific site on the alumina particle surface. It is thus presumed that the dispersibility is improved with respect to a solvent soluble in water or water.

From the viewpoint of the durability of the nonaqueous secondary battery, it has been desired that the water content of the inorganic particles used in the separator for the nonaqueous electrolyte secondary battery is very small. When moisture is present in the non-aqueous electrolyte secondary battery, generation of gas by oxidative decomposition of water and reaction of moisture and electrolyte becomes significant, and cycle characteristics are deteriorated due to expansion of the battery and consumption of electrolyte. 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 speculated that the number of crystals adsorbed to a specific site on the surface of a specific alumina particle does not deteriorate even by raising the temperature to about 200 ° C, and does not adversely affect the nonaqueous secondary battery.

As the degree of incorporation of the specific alumina particle having an absorption peak near 3475 cm ^-1 in the alumina particle used in the present invention, when the measurement is carried out by FT-IR on the whole alumina particles used in the present invention, Is preferable.

The alumina particles preferably have a water content of 2000 ppm by mass or less, more preferably 1900 ppm by mass or less, which is generated when heated from room temperature to 1000 占 폚, which is measured by heating gas mass analysis (TPD-MS). If the amount of water generated from the whole alumina particles exceeds 2000 ppm by mass, the deterioration of the battery characteristics may be caused by the influence of moisture. When the moisture content of the alumina particles is within the above preferable range, deterioration of the battery characteristics can be suppressed when the alumina particles are used for the battery separator.

However, in order to improve the pin removability, it is effective to add particles having a relatively large particle size to the porous layer and reduce the coefficient of friction between the separator and the pin. However, in the case where only relatively large particles having a particle size of 1 占 퐉 or more are used, large particles aggregate to form larger coarse protrusions on the surface of the porous layer. As described above, . In addition, since the gap of the alumina particles on the surface of the separator becomes large and the resistance to short-circuiting caused by the foreign matter mixed in the battery may decrease, relatively small particles having a particle diameter of 1 탆 or less are used in combination to form the separator surface It is necessary to coat with alumina.

The alumina particles used in the present invention have alumina particles A having at least two maximum peaks in the primary particle diameter distribution and having a particle diameter of 1.0 mu m or more in the particle diameter distribution and alumina particles B having a particle diameter of less than 1.0 mu m .

The upper limit value of the mode diameter A (r1), which represents the maximum value in the primary particle diameter distribution (hereinafter also referred to as &quot; primary mode diameter &quot;) A is preferably 3 m, more preferably 2 m, Is preferably 1 占 퐉, and more preferably 1.2 占 퐉. If the primary mode diameter A (r1) is less than 1.0 占 퐉, sufficient pin removability may not be obtained. If the primary mode diameter A (r1) is more than 3 占 퐉, the withstand voltage may be lowered.

The lower limit value of the mode diameter B (r1) representing the maximum in the primary particle diameter distribution of the alumina particles B is preferably 0.3 mu m, more preferably 0.4 mu m. If the primary mode diameter B (r1) is less than 0.3 탆, protrusions due to agglomerates are likely to be formed. If the primary mode diameter B (r1) is more than 1.0 탆, short-circuit resistance may decrease. The upper limit value does not exceed 1 탆, and is preferably 0.8 탆.

Particles having an absorption peak at about 3475 cm ^-1 by Fourier transform infrared spectroscopy (FT-IR) and disappearing at a peak above 300 ° C. are preferably alumina particles A, more preferably alumina particles A and B .

The Mohs hardness of the alumina particles A and B is preferably 9. The alumina particles are hardly cut in the dispersion step, so that fine particles are hardly generated and coarse aggregates due to the fine particles are hardly produced. By using the alumina particles having high Mohs hardness, it is possible to obtain a slurry excellent in dispersibility and dispersion stability, and the formation of coarse projections can be suppressed when the porous layer is used.

The total content of the alumina particles A and B in the porous layer is not less than 50% by volume and not more than 90% by volume, preferably not less than 60% by volume and not more than 80% by volume based on 100% by volume of the alumina particle A, the alumina particle B and the binder. Volume% or less. 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 deterioration of the coating property 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. Within the above range, good short-circuit resistance and pin removability can be obtained. In addition, each particle volume V (cm ³⁾ is obtained by the equation (4) below when the specific gravity d (g / cm ^3), the weight w (g) of the true particles.

V = w / d ... Equation 4

The alumina particles used in the present invention are obtained, for example, by appropriately adjusting calcination conditions and crushing conditions. If it has the characteristics of the present invention, the alumina particles may be used.

(bookbinder)

The binder used in the present invention plays a role of binding alumina particles to each other, but is not particularly limited as long as it is a resin that bonds the substrate and the porous layer. For example, fluorine resin, polyamideimide resin, acrylic resin, polyvinyl alcohol, carboxymethyl cellulose and the like are listed. From the viewpoints of heat resistance and electrolyte permeability, polyamideimide resin or aromatic polyamide resin is preferable. From the viewpoint of the electrode adhesion, fluororesin is preferable.

Hereinafter, the fluororesin will be described as an example. As the fluororesin, it is preferable to use at least one selected from the group consisting of vinylidene fluoride homopolymer, vinylidene fluoride-fluoroolefin copolymer, vinyl fluoride homopolymer and vinyl fluoride-fluoroolefin copolymer. Further, maleic acid or the like may be graft-polymerized to the fluororesin. These polymers are excellent in adhesion with electrodes, have high affinity with nonaqueous electrolytes, and have high chemical and physical stability against nonaqueous electrolytes. Therefore, they can sufficiently maintain affinity with electrolytes even at high temperatures.

(Method for producing slurry)

The slurry used in the present invention can be obtained by the following production method. The slurry used in the present invention is obtained by mixing and dispersing alumina particles 1 corresponding to alumina particles A, alumina particles 2 corresponding to alumina particles B, a binder and a solvent, but in order to obtain a more uniformly dispersed slurry, (2) adding alumina particles 1 and alumina particles 2 to the binder solution, preliminary dispersion, and further dispersing the dispersion to obtain a slurry; and You can.

(1) a 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 in accordance with the solubility of the binder. For example, N-methyl-2-pyrrolidone (NMP), acetone and the like are listed.

(2) adding alumina particles 1 and alumina particles 2 to the binder solution, preliminarily dispersing them, and further dispersing them to obtain a slurry

Next, the alumina particles 1 and the alumina particles 2 are sequentially added while stirring the binder solution obtained in the above step, and preliminary dispersion is performed once. Here, the alumina particles 1 and the alumina particles 2 as the binder solution are preferably added gradually. The slow addition means that, for example, the addition rate per 10 L of the binder solution is 10 to 50 g / min, whereby generation of fine powder can be suppressed. Further, in the preliminary dispersion, the particles are agitated with a mechanical stirrer or the like for a predetermined period of time (for example, about 1 hour) to reduce aggregated alumina particles. If the preliminary dispersion is not carried out, aggregates of the alumina particles contained in the slurry precipitate, and a part of the slurry may be in a paste form. In this case, sufficient dispersion becomes difficult, and coarse projections are easily formed on the porous layer, and the transport pump may be clogged.

After preliminary dispersion, further dispersion is carried out using a bead mill or the like. The alumina particles can be further dispersed by a dispersion method in which a high shear force is applied to the slurry, such as a bead mill, thereby reducing aggregates. The dispersion is usually carried out four to five times (hereinafter sometimes referred to as the number of passes) by applying a high shearing force to achieve sufficient dispersion, but a part of the slurry may become hyperbranched and may re-aggregate . In the present invention, by using the above alumina particles, not only the number of passes can be shortened by one to three, but also the re-aggregation can be suppressed even if a high shear force is applied.

The thickness of the porous layer is preferably 0.5 to 3 占 퐉, more preferably 1 to 2.5 占 퐉, and even more preferably 1 to 2 占 퐉 per one side. If the film thickness per one surface is 0.5 mu m or more, functions such as adhesion with electrodes and heat resistance can be ensured. When the film thickness per one surface is 3 m or less, the winding volume can be suppressed, which is suitable for increasing the capacity of the battery to be advanced in the future.

The porosity of the porous layer is preferably 30 to 90%, more preferably 40 to 70%. By setting the porosity of the porous layer within the above-described preferable range, the electrical resistance of the separator can be prevented from increasing, a large current can be passed, and the film strength can be maintained.

[3] separator for batteries

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

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

(2) adding alumina particles 1 and 2 to the binder solution, preliminarily dispersing and then further dispersing to obtain a slurry,

(3) A step of applying a slurry to a polyolefin microporous membrane, immersing it in a coagulating solution, washing and drying.

The slurry may be applied to the polyolefin microporous membrane by a known method. Examples of the method include a dip coating method, a reverse roll coating method, a gravure coating method, a kiss coating method, a roll brush method, a spray coating method, An air knife coating method, a Mayer Bar coating method, a pipe doctor method, a blade coating method and a die coating method, and these methods can be used alone or in combination.

Hereinafter, the fluororesin will be described as an example. The microporous membrane coated with the slurry is immersed in the coagulating solution to coagulate the fluororesin, thereby forming a gap between the fluororesin and the alumina particles. The coagulating solution may be an aqueous solution containing 1 to 20% by weight, more preferably 5 to 15% by weight, of a good solvent for the fluororesin. Examples of the good solvent include N-methyl-2-pyrrolidone, N, N-dimethylformamide and N, N-dimethylacetamide. A phase-separation auxiliary may be added to the coagulating solution as necessary.

The immersion time in the coagulating liquid is preferably 2 seconds or more from the viewpoint of solidification of the fluororesin, and the upper limit is not limited, but 10 seconds is sufficient. Thereafter, the separator for a battery can be obtained through a washing step of removing the solvent by dipping in pure water and a drying step of hot air at 100 ° C or less.

The specular resistance of the battery separator is one of the most important characteristics, and is preferably 50 to 600 sec / 100 ml Air, more preferably 100 to 500 sec / 100 ml Air, further preferably 100 to 400 sec / 100 ml Air. In order to achieve desired durability, the porosity of the porous layer is adjusted and the degree of penetration of the binder into the polyolefin microporous membrane is adjusted. When the specular resistance of the battery separator is within the above-mentioned preferable range, charge / discharge characteristics and life characteristics in a range preferable for practical use can be obtained.

The upper limit of the total thickness of the separator for a battery obtained by laminating the porous layers is preferably 30 占 퐉, more preferably 25 占 퐉. The lower limit is preferably 5 占 퐉, more preferably 7 占 퐉. When it is at least the lower limit of the above-mentioned preferable range, sufficient mechanical strength and insulating property can be ensured. By setting the upper limit value to the above-mentioned preferable range, it is possible to secure an electrode area which can be filled in the container, so that the capacity can be prevented from being lowered.

Example

EXAMPLES Hereinafter, examples will be specifically described, but the present invention is not limited by these examples. The measurement value in the examples is a value measured by the following method.

1. Fourier Transform Infrared Spectroscopy (FT-IR)

The alumina particles were placed in a container made of ceramics and placed in a heating and diffusing reflector chamber provided with a heater. The inert gas (N ₂ ) was circulated in the chamber at a flow rate of 50 ml / min. The temperature was raised, and an infrared spectrum was obtained by a diffuse reflection method at intervals of one minute. The measurement was performed under the following conditions using a commercially available FT-IR apparatus (FTS-7000 manufactured by Varian) and a heat diffuse reflection apparatus (manufactured by PIKE Technologies). The alumina particles to be subjected to this measurement method may be alumina particles used as a raw material or alumina particles obtained by removing a binder by using a solvent from a porous layer.

Detector: Deuterium Tri-Glycine Sulfate (DTGS)

Reference: Gold (Au)

Wavelength range: 400 to 4000 cm ^-1

Resolution: 4 cm ^-1

Accumulated count: 16

The obtained diffuse reflection spectrum was subjected to Kubelka-Munk transformation.

2. Mass production of heating gas (TPD-MS)

About 100 mg of alumina particles were placed in a vessel and the temperature was raised from room temperature to 1000 占 폚 at a heating rate of 20 占 폚 / min while flowing helium gas at a flow rate of 50 ml / min. The amount of generated water was quantified by a mass spectrometer, And the amount of water generated was calculated. A GS-MS apparatus equipped with a heating device (QP2010Ultra manufactured by Shimadzu Corporation) was used for the measurement.

3. Dispersibility

The dispersibility is determined by the ratio of the difference between the mode diameter of the alumina particles in the slurry immediately after the preparation of the slurry to the primary mode diameter of the alumina particles and the difference in the primary mode diameter of the alumina particles (hereinafter referred to as the value obtained by the equation (5)) .

{(Mode diameter of alumina particles in slurry - primary mode diameter of alumina particles) / primary mode diameter of alumina particles} x 100 ... Equation 5

Alumina particles were added so that the primary mode diameter of the alumina particles was 0.05 wt% in water, and the sample was thoroughly ultrasonicated and then subjected to laser diffraction / scattering particle size distribution measurement apparatus LA-960V2 (manufactured by HORIBA, Ltd.) A maximum value within a range of 1.0 μm or more and 3.0 or less is defined as A (r1), a maximum value within a range of 0.3 or more and 1.0 μm or less is defined as B ( r1).

The mode diameter of the alumina particles in the slurry immediately after preparation of the slurry was measured in the same manner as the primary mode diameter except that the slurry was diluted with a solvent in which the binder was soluble so that the alumina particle concentration was 0.05 wt% A (r2) and B (r2) were obtained.

Number of data injections 5000

Intensity of ultrasonic dispersion 7

Ultrasonic Dispersion Time 1 min

Circulation Speed 5

Stirring speed 3

When the value of the formula (5) is 30% or less, it is possible to uniformly disperse the alumina particles so that aggregation of the alumina particles is less and the generation of coarse projections of the obtained separator can be suppressed and the dispersibility is good.

4. Dispersion stability

The dispersion stability is determined by the rate of change of the mode diameter of the alumina particles in the slurry after one week from the mode diameters A (r2) and B (r2) of the alumina particles immediately after the slurry preparation (hereinafter referred to as the value obtained by the equation (6)). The mode diameter of the alumina particles in the slurry after one week was measured by putting the slurry after one week from the production in a container made of polypropylene so as to have a half capacity and vibrating the sample up and down ten times by hand, . The mode diameter was obtained as a function of dispersion.

{Mode diameter of alumina particles in slurry after 1 week from slurry production - mode diameter of alumina particles in slurry immediately after slurry preparation) / mode diameter of alumina particles in slurry immediately after slurry production} x 100 ... Equation 6

When the value of the formula 6 is 10% or less, the alumina particles 1 and 2 can be stably present in the slurry, and the generation of coarse projections of the resulting separator can be suppressed.

5. Pin Removability

The pin removability was evaluated by a method using a cylindrical pin having a diameter of 4.0 mm. First, Separators A to D having a width of 40 mm were wound five times around a circumferential pin having a diameter of 4.0 mm by applying a tensile load of 200 g (5 g / mm 2 per separator width). The circumferential fins were removed from the wound separators A to D, and the pin removability was evaluated as follows.

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

Bad: The amount of bamboo protrusion is more than 1mm

6. Withstand voltage test

About 10 m from the wound core of the battery separator obtained in Examples and Comparative Examples was taken out and supplied to the sample. A withstand voltage tester TOS5051A (manufactured by KIKUSUI ELECTRONICS CORPORATION) was used, and the withstand voltage test of the separator was carried out in the following order. A separator (50 mm x 50 mm in size) was placed on the aluminum foil sample bed, and then an aluminum foil of? 15 mm and an aluminum foil of? 15 mm were placed on the separator. Next, a metal weight (φ50 mm × height 32 mm, weight about 500 g) was placed on the conductive rubber, and the metal weight and the aluminum foil on the sample table were each hung with an electric strength tester. When a voltage between the sample stand and the metal weight is increased to 2 kV (boosting rate: 0.1 kV / sec) by an electric strength tester and an electrical short circuit occurs at a rate of 2 kV or less in terms of a film thickness of 20 탆, The withstand voltage is set to be good.

7. Short immunity test

Evaluation of short-circuit resistance was carried out using a bench-top precision universal testing machine, Autograph AGS-X (Shimadzu Corporation). (Total thickness: about 140 占 퐉, base material: copper foil (thickness: about 9 占 퐉), active material: polytetrafluoroethylene (polytetrafluoroethylene) (Lower side) 6 of a universal testing machine was laminated on both sides of a double-sided tape (hereinafter referred to as &quot; double-sided tape &quot;) of a sample stacked body obtained by laminating an artificial graphite . Next, the aluminum foil and the negative electrode of the sample laminate were connected to a circuit comprising a capacitor and a clad resistor by a cable. The capacitor was charged to about 1.5 V, and a metal hole 5 (material: chrome (SUJ-2)) having a diameter of about 500 mu m was placed between the separator and the aluminum foil in the sample stack. A compression jig was attached to the universal testing machine. As shown in Fig. 5, a sample laminate including the metal balls 5 was sandwiched between the two compression jigs at a speed of 0.3 mm / min. When the load reached 100 N To end the test. At this time, the point where the inflection point appeared in the compression load change was regarded as the wave film point of the separator, and the moment when the circuit was formed through the metal ball and the current was detected was regarded as a short occurrence point. The compressive displacement A (t) at the time when the separator is broken due to compression and the inflection point at the compressive stress is generated and the compressive displacement B (t) at the moment when the current flows through the circuit are measured. When the numerical value obtained by the following formula 7 is 1.1 or more , Meaning that even if the separator is broken due to the foreign matter mixed in the battery, the coating layer composition adheres to the surface of the foreign matter, thereby maintaining the insulation. On the other hand, when the numerical value obtained by the equation (7) is larger than 1.0 and less than 1.1, the breakage of the separator and the short circuit do not occur at the same time, but even in the case of the increase in the internal pressure of the battery due to the tension applied to the winding of the battery member, In order not to cause a short circuit, the resistance to short-circuit is somewhat poor because a certain level of tolerance is required. When the numerical value obtained by the equation (7) is 1.0, short-circuiting occurs simultaneously with the breakage of the separator, and improvement in short-circuit resistance by the application layer is not seen.

B (t)? A (t) ... Equation 7

Examples 1 to 11

(Adjustment of slurry)

A vinylidene fluoride-hexafluoropropylene copolymer (VdF-HFP copolymer (weight average molecular weight: 1,000,000)) as a fluororesin was blended at a weight ratio of about 5 wt% based on N-methyl-2-pyrrolidone, VdF -HFP copolymer was completely dissolved to obtain a fluororesin solution. Then, alumina particles 1 and 2 were added to the fluororesin solution under stirring at 350 rpm with a mechanical stirrer so that the total volume of the alumina particles 1 and the alumina particles 2 and the volume of the VdF-HFP copolymer became 50:50. The characteristics and the volume ratio of the alumina particles used in the respective Examples are shown in Table 1. The primary mode diameters of the alumina particles 1 and 2 were determined by measuring the particle size distribution under the same measurement conditions as in the above 3. dispersion using each of the alumina particles adjusted to be 0.05 wt% The maximum value was obtained.

The addition rate of the alumina particles was 30 g / min per 10 L of the fluororesin solution. Alumina particles were added, and then the mixture was stirred with a mechanical stirrer for 1 hour to carry out preliminary dispersion. Next, using a dyno mill (manufactured by SHINMARU ENTERPRISES CORPORATION, Dyno Mill Multilabo (1.46L vessel, filling rate 80%, 0.5 mm alumina beads)) was used and the results are shown in Table 1 under the conditions of a flow rate of 11 kg / hr and a peripheral speed of 10 m / And the slurry was dispersed by the number of pass shown. At this time, the temperature of the slurry was adjusted to be in the range of 20 to 45 캜. In addition, the slurry was kept in a sealed state so as not to contact with the outside atmosphere until the application.

(Lamination of the porous layer)

The slurry was applied to both surfaces of a polyethylene microporous membrane (thickness 7 占 퐉, air resistance 100 sec / 100 ml Air) by dip coating method, immersed in water (coagulation liquid), washed with pure water, And dried to obtain a battery separator having a final thickness of 11 mu m.

Example 12

A separator for a battery was obtained in the same manner as in Example 1 except that the polyethylene microporous membrane had a thickness of 12 탆 and a durability of 120 sec / 100 ml Air.

Example 13

A separator for a battery was obtained in the same manner as in Example 1 except that the polyethylene microporous membrane had a thickness of 9 탆 and a durability of 180 sec / 100 ml Air.

Example 14

A separator for a battery was obtained in the same manner as in Example 1 except that the polyethylene microporous membrane had a thickness of 7 탆 and a durability of 180 sec / 100 ml Air.

Example 15

A separator for a battery was obtained in the same manner as in Example 1, except that the polyethylene microporous membrane had a thickness of 5 占 퐉 and a durability of 110 sec / 100 ml Air.

Comparative Example 1

A battery separator was obtained in the same manner as in Example 1 except that alumina particles 1 were not used as alumina particles but only alumina particles 2 were used.

Comparative Examples 2 to 7

A separator for a battery was obtained in the same manner as in Example 1 except that alumina particles 1 and alumina particles 2 were used in the ratios shown in Table 1 and the dispersion conditions were set to the number of passes shown in Table 1. [

Table 1 shows the characteristics of the polyolefin microporous membrane and alumina particles 1 and 2 used in Examples and Comparative Examples.

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

1: Resin insulator
2: Negative electrode for lithium ion battery
3: Separator
4: Aluminum foil
5: Metal ball
6: Compression jig (lower side)
6 ': Compression jig (upper side)
7: Sample laminate including metal balls

Claims (9)

A separator for a battery comprising a polyolefin microporous membrane and a porous layer on at least one side of the polyolefin microporous membrane, wherein the porous layer comprises alumina particles and a binder, and when the total amount of the alumina particles and the binder is 100 vol% Wherein the volume ratio of alumina particles is 50 vol% or more, the alumina particles include particles having an absorption peak at about 3475 cm < ^-1 &gt; by Fourier transform infrared spectroscopy (FT-IR) , The alumina particles having at least two maximum peaks satisfying the following formulas 1 and 2 in the primary particle diameter distribution and having an average particle diameter of 1.0 mu m or more in the primary particle diameter and a particle diameter And the alumina particles B having the total amount of the alumina particles The facilitator.
1.0 (占 퐉)? A (r1)? 3.0 (占 퐉) ... Equation 1
0.3 (占 퐉)? B (r1) <1.0 (占 퐉) Equation 2
0.5? A (vol) / B (vol)? 2.0 ... Equation 3
A (vol) / B (vol) is a mode diameter indicating a maximum in a primary particle diameter distribution of alumina particles, and A (vol) / B (vol) B is the total ratio] The method according to claim 1,
Characterized in that said alumina particle (A) has an absorption peak at about 3475 cm ^-1 by Fourier transform infrared spectroscopy (FT-IR), and a particle disappearing at a peak above 300 캜. 3. The method according to claim 1 or 2,
Wherein the alumina particles have a water content of not more than 2000 ppm by mass as measured by heating gas mass analysis (TPD-MS) when the temperature of the alumina particles is raised from room temperature to 1000 占 폚. 3. The method according to claim 1 or 2,
(Vol) / B (vol) &lt; / = 1.5. 3. The method according to claim 1 or 2,
(Vol) / B (vol) &lt; / = 1.3. 3. The method according to claim 1 or 2,
Wherein the binder comprises a fluororesin. The method according to claim 6,
Wherein the fluororesin comprises a vinylidene fluoride-hexafluoropropylene copolymer. 3. The method according to claim 1 or 2,
Wherein the thickness of the polyolefin microporous membrane is less than 10 占 퐉. 3. The method according to claim 1 or 2,
Wherein the thickness of the polyolefin microporous membrane is 7 占 퐉 or less.

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