JP6939569B2 - Battery separator - Google Patents

Description

The present invention relates to a battery separator.

Microporous membranes made of thermoplastic resins are widely used as substance separation membranes, selective permeation membranes, isolation membranes and the like. For example, various types of battery separators used for lithium ion secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, polymer batteries, separators for electric double layer capacitors, reverse osmosis filtration membranes, ultrafiltration membranes, precision filtration membranes, etc. Filters, breathable waterproof clothing, medical materials, etc.

In particular, as a separator for a lithium ion secondary battery, it has ion permeability due to impregnation with an electrolytic solution, has excellent electrical insulation, and cuts off current at a temperature of about 120 to 150 ° C. when the temperature inside the battery rises abnormally, resulting in an excessive amount. A polyolefin microporous film having a pore closing function that suppresses temperature rise is preferably used. However, if the temperature inside the battery continues to rise even after the pores are closed for some reason, the microporous polyolefin membrane may shrink or rupture. This phenomenon is not limited to the polyolefin microporous membrane, and even in the case of a microporous membrane using another thermoplastic resin, it cannot be avoided above the melting point of the resin.

The separator for a lithium ion secondary battery is deeply related to battery characteristics, battery productivity and battery safety, and is required to have heat resistance, electrode adhesion, permeability, melt rupture resistance and the like. So far, for example, a battery separator in which a polyolefin microporous membrane is used as a base material and a porous layer is provided on at least one surface of the base material is known. As the resin used for the porous layer, a resin that imparts heat resistance to the separator such as a polyamide-imide resin, a polyimide resin, and a polyamide resin, or a resin that imparts adhesiveness to an electrode to a separator such as a fluororesin is preferably used. In recent years, water-soluble or water-dispersible resins capable of laminating porous layers in a relatively simple process have also been used. Further, from the viewpoint of improving heat resistance, it is also considered to include inorganic particles in the porous layer.

The separator is applied to a battery having a wound electrode body laminated and wound between a positive electrode and a negative electrode. Since the pin and the separator come into direct contact with each other when the wound electrode body is manufactured, the separator is required to have good pin pull-out property. If the pin pull-out property is poor, the separator in contact with the pin will be dragged by the pin when the pin is pulled out, causing the inner peripheral portion of the electrode winding body to protrude like a bamboo shoot or a telescope, resulting in an electrode winding. Problems such as loss of the insulating structure between the positive and negative electrodes of the body occur. In order to improve the pin removal property of the separator, it has been proposed to set the coefficient of static friction or the coefficient of dynamic friction of the separator to a specific value or less.

Patent Document 1 describes an electrode provided with a three-layer separator made of a positive electrode, a negative electrode, polypropylene, polyethylene, and polypropylene, and an adhesive resin layer made of polyvinylidene fluoride and alumina powder arranged between these electrodes and the separator. The body is listed.

In Test Example 2 of Patent Document 2, a magnesium carbonate powder, an acrylic binder aqueous solution, which was dry pulverized using a jet mill (wind pressure 0.2 MPa, 5 minutes) and classified to 4 μm or less using an air flow type powder classifier. The mixture of the thickener and the thickener is pre-kneaded (15000 rpm, 5 minutes) with a high-speed stirring / dispersing machine, and then the main kneading (20,000 rpm, 15 minutes) is performed to prepare a slurry for forming a porous layer, and the mixture is prepared on a polyolefin microporous film. This is applied to obtain a battery separator.

In Example 1 of Patent Document 3, a wet pulverized slurry 1 (a suspension obtained by wet pulverizing an alumina-aqueous suspension using a dyno mill), carboxymethyl cellulose (CMC) and a solvent were mixed, and a gorin homogenizer was used. A slurry treated under high-pressure dispersion conditions (20 MPa × 1 pass, 60 MPa × 3 passes) is applied to obtain a battery separator in which a porous layer is laminated on a polyolefin-based porous film.

In Example 3 of Patent Document 4, the emulsion of the self-crosslinking acrylic resin and water are stirred at room temperature until uniformly dispersed, boehmite powder is added to the dispersion in 4 portions, and the mixture is stirred by a disper (2800 rpm, 5). The slurry obtained after (time) is applied to obtain a separator for a battery in which a porous layer is laminated on a polyethylene microporous film.

Patent Document 5 discloses that the static friction coefficient of the separator surface layer is lowered in order to improve the slipperiness between the pin and the separator, and Patent Document 6 discloses that the dynamic friction coefficient of the separator surface layer is lowered. ..

On the other hand, if the porous layer contains inorganic particles, coarse protrusions may be formed on the surface of the porous layer, and if the separator is a wound product or the like, indentations may be generated on the separator in contact with the coarse protrusions. In the future, it is predicted that the film thickness of the battery separator will be reduced, and as the thickness of the separator becomes thinner, the effect of indentation will become apparent, which may lead to film rupture of the separator.

Re-table 1999-036981 Re-table 2011-158335 Japanese Unexamined Patent Publication No. 2014-040580 Japanese Unexamined Patent Publication No. 2008-123996 Japanese Unexamined Patent Publication No. 2011-126275 Japanese Unexamined Patent Publication No. 2014-012857

An object of the present invention is to obtain a battery separator having excellent short-circuit resistance, withstand voltage resistance, and excellent pin pull-out property.

As a result of diligent studies on the above problems, the present inventors have focused on the fact that controlling the dispersibility and dispersion stability of inorganic particles in a slurry is extremely important for a battery separator. When a porous layer having inorganic particles is provided on the surface of a base material, a method of preparing a slurry containing the inorganic particles, a binder and a solvent, applying the slurry to the base material, and drying to form the porous layer is generally used. Used. If the size of the inorganic particles used at this time is small, they tend to aggregate in the slurry, and if the size is large, they tend to settle, which is a problem of dispersibility. Further, even if the particles can be sufficiently dispersed, if the dispersion stability of the inorganic particles in the slurry is low, the particles may reaggregate between the preparation of the slurry and the coating. If agglomerates of inorganic particles are present in the slurry, the agglomerates will be mixed in the porous layer, and coarse protrusions will be generated on the surface of the porous layer. If the separator thus obtained is a wound product, indentations may be generated on the separator in contact with the coarse protrusions.

As a result of diligent research on inorganic particles and their dispersion technology, the present inventors have two mode diameters showing maximum in the primary particle size distribution, and at least some alumina particles have specific surface characteristics. It was found that a slurry having extremely excellent dispersibility and dispersion stability can be obtained, and a battery separator having excellent short-circuit resistance, withstand voltage resistance and pin pull-out property can be obtained.

In order to solve the above problems, the battery separator of the present invention has the following configuration.
That is,
(1) A battery separator having a polyolefin microporous film and a porous layer on at least one side of the polyolefin microporous film, the porous layer containing alumina particles and a binder, and the alumina particles and the binder. When the total is 100% by volume, the volume ratio of the alumina particles is 50% by volume or more, and the alumina particles have an absorption peak in the vicinity of ^{3475 cm-1 by Fourier transform infrared spectroscopy (FT-IR).} The alumina particles include particles whose peaks disappear at 300 ° C. or higher, and the alumina particles have at least two maximums satisfying the following formulas 1 and 2 in the primary particle size distribution, and particles having a primary particle size distribution of 1.0 μm or more. A battery separator, characterized in that the ratio of the total volume of each of the alumina particles A having a diameter and the alumina particles B having a particle diameter of less than 1.0 μm satisfies 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
Here, A (r1) and B (r1) are mode diameters showing the maximum in the primary particle size distribution of the alumina particles, and A (vol) / B (vol) is the total volume ratio of the alumina particles A and the alumina particles B. be.
(2) The battery separator of the present invention ^{has an absorption peak in the vicinity of 3475 cm-1} by Fourier transform infrared spectroscopy (FT-IR), and the particles whose peak disappears at 300 ° C. or higher are the alumina particles A. It is preferable to have.
(3) In the battery separator of the present invention, the amount of water generated by the alumina particles measured by mass spectrometry of heated generated gas (TPD-MS) when the temperature is raised from room temperature to 1000 ° C. is preferably 2000 mass ppm or less.
(4) The battery separator of the present invention preferably has 0.7 ≦ A (vol) / B (vol) ≦ 1.5.
(5) The battery separator 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 contains a vinylidene fluoride-hexafluoropropylene copolymer.
(8) In the battery separator of the present invention, the thickness of the polyolefin microporous film is preferably 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, it is possible to obtain a slurry having extremely excellent dispersibility and dispersion stability by using alumina particles having two mode diameters showing maximums in the primary particle size distribution and having specific surface characteristics. The porous layer formed by the slurry can highly suppress coarse protrusions, and can provide a battery separator that is safe even if the separator is thinned. In particular, the present invention exerts a greater effect when the film thickness of the separator is less than 10 μm. Further, since the battery separator of the present invention is excellent in pin pull-out property, it is possible to improve productivity in the battery assembly process, and it is possible to reduce the manufacturing cost.

It is a figure which shows the infrared spectroscopic spectrum of the alumina particle 1 of Example 1 by the diffuse reflection method. It is a figure which shows the infrared spectroscopic spectrum of the alumina particle 1 of the comparative example 2 by the diffuse reflection method. It is a figure which shows the TPD-MS spectrum of alumina. It is a figure which shows the laminated body of the sample used for a short circuit resistance test. It is a figure which shows the method of measuring a short circuit resistance test.

[1] Polyolefin Microporous Film As the polyolefin resin constituting the polyolefin microporous film, polyethylene or polypropylene is preferable. 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 different olefins. In addition to basic characteristics such as electrical insulation and ion permeability, the polyolefin microporous film formed of the resin has a pore-blocking effect of blocking current when the battery abnormally rises and suppressing excessive temperature rise.

The polyethylene microporous membrane can be freely selected by a production method according to the purpose. As a method for producing a microporous membrane, there are a foaming method, a phase separation method, a dissolution recrystallization method, a stretch opening method, a powder sintering method, etc. Among these, phase separation in terms of homogenization of fine pores and cost. The method is preferred.

As a production method by the phase separation method, for example, polyethylene and a molding solvent are heat-melted and kneaded, and the obtained melt mixture is extruded from a die and cooled to form a gel-like molded product, and the obtained gel-like molding is performed. Examples thereof include a method of obtaining a microporous film by stretching an object in at least one axial direction and removing the molding solvent.

The microporous polyolefin membrane may be a single-layer membrane, or may have a layer structure consisting of two or more layers having different types, molecular weights, or average pore diameters of the polyolefin resin. The layer structure may be a laminate of different polyolefins such as polypropylene / polyethylene / polypropylene or polyethylene / polypropylene / polyethylene, or any layer or all layers may be blended with these polyolefin resins. You may.

As a method for producing a multilayer film composed of two or more layers, for example, polyethylene constituting the a layer and the b layer are melt-kneaded with a molding solvent, and the obtained melt mixture is supplied from each extruder to one die. It can be produced by either a method of integrating the gel sheets constituting each component and coextruding them, or a method of superimposing the gel sheets constituting each layer and heat-sealing them. The coextrusion method is more preferable because it is easy to obtain high interlayer adhesion strength, it is easy to form communication holes between layers, it is easy to maintain high permeability, and it is also excellent in productivity.

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, more preferably 5 μm. When the thickness of the polyolefin microporous membrane is in the above-mentioned preferable range, it is possible to have a practical membrane strength and a pore closing function, and the area per unit volume of the battery case is not restricted, which will be advanced in the future. Suitable for high battery capacity.

The upper limit of the air permeation resistance 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, still more preferably 100 sec. / 100 ml Air.

The upper limit of the porosity of the polyolefin microporous membrane is preferably 70%, more preferably 60%, and even more preferably 55%. The lower limit is preferably 30%, more preferably 35%, and even more preferably 40%. When the polyolefin microporous membrane is used as a battery separator when the air permeation resistance and the porosity are in the above preferable ranges, the charge / discharge characteristics of the battery, particularly the ion permeability (charge / discharge operating voltage) and the life of the battery ( The function of the battery can be fully exerted in (closely related to the holding amount of the electrolytic solution). Further, since the polyolefin microporous film can obtain sufficient mechanical strength and insulating property, a battery using the polyolefin microporous film is less likely to cause a short circuit during charging / discharging.

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 constituent resin is preferably 70 to 150 ° C., more preferably 80 to 140 ° C., and even more preferably 100 to 130 ° C. When the melting point of the constituent resin is within the above-mentioned preferable range, it is possible to prevent the battery from becoming unusable due to the appearance of the pore closing function during normal use, and to ensure safety by exhibiting the pore closing function during an abnormal reaction. can.

[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 film, immersing it in a coagulating liquid, and drying it. Alumina particles are responsible for improving pin pull-out resistance, withstand voltage resistance, and short-circuit resistance.

^{The alumina particles used in the present invention include particles having an absorption peak in the vicinity of 3475 cm-1} by Fourier transform infrared spectroscopy (FT-IR) and the peak disappearing at 300 ° C. or higher. Note that the 3475Cm ^-1 near herein refers to a range of 3475 ± ^{5 cm -1.}

In the present invention, ^{by using alumina particles having an absorption peak in the vicinity of 3475 cm-1} by FT-IR, uniform dispersion of alumina particles is possible without adding a dispersant such as a surfactant. If a dispersant such as a surfactant is used, it may adhere to the electrode active material and deteriorate the battery performance. The mechanism by which the dispersibility is improved by using alumina particles having such an absorption peak is not clear, but the inventors think as follows. Common alumina particles have a broad absorption peak near ^{3400 cm-1.} This absorption peak suggests the presence of hydroxyl groups present on the surface of the alumina particles and the adsorbed water hydrogen-bonded to the hydroxyl groups. On the other hand, the alumina particles used in the present invention include those having a relatively sharp peak near ^{3475 cm -1.} This absorption peak suggests the presence of water of crystallization adsorbed on specific sites on the surface of the alumina particles. It is presumed that this will improve the dispersibility in water or a solvent soluble in water.

From the viewpoint of the durability of the non-aqueous secondary battery, it has been said that the water content of the inorganic particles used in the separator for the non-aqueous electrolyte secondary battery should be as small as possible. When water is present in the non-aqueous electrolyte secondary battery, oxidative decomposition of water and gas generation due to the reaction between the water and the electrolyte become remarkable, and the cycle characteristics deteriorate due to the expansion of the battery and the consumption of the electrolyte. The absorption peak of the specific alumina particles used in the present invention in the ^{vicinity of 3475 cm-1} disappears at 300 ° C. or higher. That is, it is presumed that the water of crystallization adsorbed on the specific site on the surface of the specific alumina particles does not desorb even when the temperature is raised to about 200 ° C. and does not adversely affect the non-aqueous secondary battery.

As for the degree of content of specific alumina particles having an absorption peak in the vicinity of 3475 cm- ^{1 in} the alumina particles used in the present invention, the above peaks are obtained when the entire alumina particles used in the present invention are measured by FT-IR. It is preferable that it can be confirmed.

The amount of water generated when the alumina particles are heated from room temperature to 1000 ° C. as measured by heat generation gas mass spectrometry (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 concern that the battery characteristics may deteriorate due to the influence of the water. When the water 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 in the battery separator.

By the way, in order to improve the pin pull-out property, it is effective to add particles having a relatively large particle size to the porous layer to reduce the coefficient of friction between the separator and the pin. However, when only relatively large particles having a particle size of 1 μm or more are used, when the large particles aggregate to form larger coarse protrusions on the surface of the porous layer, indentations are made on the separator in contact with the coarse protrusions as described above. And may cause film rupture. In addition, since the gaps between the alumina particles on the surface of the separator become large and the resistance to short circuits generated by foreign matter mixed in the battery may decrease, relatively small particles having a particle size of 1 μm or less are used together to prepare the surface of the separator. Must be coated with alumina.

The alumina particles used in the present invention have at least two maximums in the primary particle size distribution, and the alumina particles A having a particle size of 1.0 μm or more and the alumina particles B having a particle size of less than 1.0 μm in the particle size distribution. Can be distinguished from.

The upper limit of the mode diameter (hereinafter, also referred to as “primary mode diameter”) A (r1) showing the maximum in the primary particle size distribution of the alumina particles A is preferably 3 μm, more preferably 2 μm, and the lower limit is 1 μm. It is preferably, more preferably 1.2 μm. If the primary mode diameter A (r1) is less than 1.0 μm, sufficient pin pull-out property may not be obtained, and if it exceeds 3 μm, the withstand voltage resistance may decrease and the primary mode diameter A (r1) may easily fall off from the porous layer.

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

^{The particles having an absorption peak in the vicinity of 3475 cm-1} by Fourier transform infrared spectroscopy (FT-IR) and disappearing at 300 ° C. or higher are preferably alumina particles A, and more preferably alumina particles A. And B.

The Mohs hardness of the alumina particles A and B is preferably 9. Since the alumina particles are hard to be scraped in the dispersion step, fine powder is hard to be generated, and coarse agglomerates due to the fine powder are hard to be generated. 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 the porous layer is formed.

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

The volume ratio of alumina particles A to B (A (vol) / B (vol)) is preferably 0.5 to 2.0, more preferably 0.7 to 1.5, and even more preferably 0.8 to 1. It is 3. Within the above range, good short-circuit resistance and pin pull-out property can be obtained. The volume V (cm ³ ) of each particle is calculated by the following equation 4 when the true specific density d (g / cm ³ ) and the weight w (g) of the particles are taken.
V = w / d ... Equation 4.

The alumina particles used in the present invention can be obtained, for example, by appropriately adjusting the firing conditions and the pulverizing 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 has a role of binding alumina particles to each other and a resin that binds a base material and a porous layer. For example, fluororesin, polyamide-imide resin, acrylic resin, polyvinyl alcohol, carboxymethyl cellulose and the like can be mentioned. Polyamide-imide resin and aromatic polyamide resin are suitable from the viewpoint of heat resistance and electrolyte permeability. Further, from the viewpoint of electrode adhesion, fluororesin is preferable.

Hereinafter, a fluororesin will be described in detail as an example. As the fluororesin, one or more selected from the group consisting of vinylidene fluoride homopolymers, vinylidene fluoride-olefin fluorinated copolymers, vinyl fluoride homopolymers and vinyl fluoride-olefin fluorinated copolymers can be used. It is preferable to use it. Further, maleic acid or the like may be graft-polymerized on the fluororesin. These polymers have excellent adhesion to electrodes, have high affinity with non-aqueous electrolytes, and have high chemical and physical stability with respect to non-aqueous electrolytes, so they can be used with electrolytes even at high temperatures. Affinity can be sufficiently maintained.

(Slurry manufacturing method)
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, and a slurry in which the slurry is more uniformly dispersed is obtained. In order to obtain, (1) a 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, pre-dispersing them, and then further dispersing them to obtain a slurry. It may be manufactured by a method including sequential steps.

(1) Step of Dissolving Binder in Solvent to Obtain Binder Solution The solvent is not particularly limited as long as it can dissolve the binder and can be miscible with water, and can be freely selected according to the solubility of the binder. For example, N-methyl-2-pyrrolidone (NMP), acetone and the like can be mentioned.

(2) A step of adding alumina particles 1 and alumina particles 2 to a binder solution, pre-dispersing them, and then further dispersing them to obtain a slurry. Next, the alumina particles 1 and alumina are stirred while stirring the binder solution obtained in the above step. Particles 2 are added sequentially and pre-dispersion is performed once. Here, it is preferable to gradually add the alumina particles 1 and the alumina particles 2 to the binder solution. Gradually adding means, for example, setting the addition rate per 10 L of the binder solution to 10 to 50 g / min, whereby the generation of fine powder can be suppressed. Further, in the pre-dispersion, the agglomerated alumina particles are reduced by stirring with a mechanical stirrer or the like for a certain period of time (for example, about 1 hour). If pre-dispersion is not performed, agglomerates of alumina particles contained in the slurry may settle and a part of the slurry may become a paste. In this case, sufficient dispersion becomes difficult, and not only is it easy to generate coarse protrusions in the porous layer, but also the transportation pump may be clogged.

After pre-dispersion, it is further dispersed using a bead mill or the like. Alumina particles can be further dispersed and aggregates can be reduced by a dispersion method in which a high shearing force is applied to the slurry such as a bead mill. Generally, if sufficient dispersion is to be performed, a high shearing force is applied and the number of times (hereinafter, may be referred to as the number of passes) needs to be performed 4 to 5 times, but a part of the slurry becomes overdispersed and re-dispersed. May agglomerate. In the present invention, by using the above alumina particles, not only the number of passes can be shortened to 1 to 3 times, but also reaggregation can be suppressed even if a high shearing force is applied.

The film 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. When the film thickness per side is 0.5 μm or more, functions such as adhesiveness to electrodes and heat resistance can be ensured. If the film thickness per side is 3 μm or less, the winding volume can be suppressed, which is suitable for increasing the capacity of batteries, which will 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 preferable range, it is possible to prevent an increase in the electrical resistance of the separator, allow a large current to flow, and maintain the film strength.

[3] Battery Separator The method for manufacturing a battery separator of the present invention sequentially includes the following steps (1) to (3).
(1) A step of dissolving the binder in a solvent to obtain a binder solution, and
(2) A step of adding alumina particles 1 and 2 to a binder solution, pre-dispersing them, and then further dispersing them to obtain a slurry.
(3) A step of applying a slurry to a microporous polyolefin membrane, immersing it in a coagulating liquid, washing and drying it.

The method of applying the obtained slurry to the microporous polyolefin membrane may be a known method, for example, a dip coating method, a reverse roll coating method, a gravure coating method, a kiss coating method, a roll brush method, or a spray coating method. Methods, air knife coating method, Meyer bar coating method, pipe doctor method, blade coating method, die coating method and the like can be mentioned, and these methods can be used alone or in combination.

Hereinafter, a fluororesin will be described in detail as an example. By immersing the microporous membrane coated with the slurry in the coagulating liquid to solidify the fluororesin, voids are formed between the fluororesin and the alumina particles. As the coagulating liquid, an aqueous solution containing 1 to 20% by weight, more preferably 5 to 15% by weight of a good solvent with respect to the fluororesin can be used. Examples of good solvents include N-methyl-2-pyrrolidone, N, N-dimethylformamide, and N, N-dimethylacetamide. A phase separation aid may be added to the coagulation liquid if necessary.

The immersion time in the coagulating liquid is preferably 2 seconds or more from the viewpoint of coagulation of the fluororesin, and the upper limit is not limited, but 10 seconds is sufficient. After that, a battery separator can be obtained through a cleaning step of removing the solvent by immersing in pure water and a drying step of hot air at 100 ° C. or lower.

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

The upper limit of the overall film thickness of the battery separator obtained by laminating the porous layers is preferably 30 μm, more preferably 25 μm. The lower limit is preferably 5 μm, more preferably 7 μm. Sufficient mechanical strength and insulating properties can be ensured by setting the value to be equal to or higher than the lower limit of the above preferable range. By setting the value to or less than the upper limit of the above preferable range, it is possible to secure an electrode area that can be filled in the container, and thus it is possible to avoid a decrease in capacity.

Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited to these Examples. The measured values in the examples are values measured by the following methods.

1. 1. Fourier Transform Infrared Spectroscopy (FT-IR)
Alumina particles are placed in a ceramic container, installed in a heat diffusion reflector chamber equipped with a heater, and an inert gas (N ₂ ) is circulated in the chamber at a flow velocity of 50 ml / min from room temperature to 700 ° C. The temperature was raised at about 20 ° C./min, and infrared spectra were acquired by the diffuse reflection method at 1-minute intervals. The measurement was carried out under the following conditions using a commercially available FT-IR device (FTS-7000 manufactured by Varian) and a heat diffusion reflector (manufactured by PIKE Technologies). The alumina particles that are the target of this measurement method may be alumina particles used as a raw material, or may be alumina particles in which the binder is removed from the porous layer using a solvent.

Detector: Deuterium Tri-Glycine Sulfate (DTGS)
Reference: Gold (Au)
Wavelength range: 400-4000 cm ^-1
Resolution: 4 cm ^-1
Number of integrations: 16 times The obtained diffuse reflection spectrum was subjected to Kubelka-Munk conversion.

2. Heating gas generation mass spectrometry (TPD-MS)
Approximately 100 mg of alumina particles were placed in a container, and with helium gas flowing at 50 ml / min, the temperature was raised from room temperature to 1000 ° C at a temperature rise rate of 20 ° C./min, and the amount of water generated was quantified with a mass spectrometer. , The amount of water generated per unit weight of alumina particles was calculated. For the measurement, a GS-MS apparatus equipped with a heating apparatus (QP2010 Ultra manufactured by Shimadzu Corporation) was used.

3. 3. Dispersibility Dispersibility is the ratio of the difference between the mode diameter of the alumina particles in the slurry immediately after the slurry is made and the primary mode diameter of the alumina particles (hereinafter, the value obtained by the formula (5)) with respect to the primary mode diameter of the alumina particles. Use as an index.
{(Mode diameter of alumina particles in slurry-Primary mode diameter of alumina particles) ÷ Primary mode diameter of alumina particles} × 100 ... Equation 5
The primary mode diameter of the alumina particles is LA-960V2 (stock), which is a laser diffraction / scattering type particle size distribution measuring device for a sample obtained by adding alumina particles so as to be 0.05 wt% in water and then sufficiently crushing the sample with ultrasonic waves. Using the company Horiba Seisakusho), measure the particle size distribution under the following conditions, detect the maximum value from the obtained frequency distribution map, and set the maximum value within the range of 1.0 μm or more and 3.0 or less to A (r1). ), The maximum value in the range of 0.3 or more and less than 1.0 μm was defined as B (r1).

The mode diameter of the alumina particles in the slurry immediately after the slurry is prepared is the primary mode except that the slurry is diluted with a solvent capable of dissolving the binder and not crushed by ultrasonic waves so that the alumina particle concentration is 0.05 wt%. A (r2) and B (r2) were determined by measuring in the same manner as the diameter.

Number of data acquisitions 5000 times Ultrasonic dispersion strength 7
Ultrasonic dispersion time 1 minute Circulation speed 5
Stirring speed 3
When the value of the formula 5 is 30% or less, the particles can be uniformly dispersed, the aggregation of alumina particles is small, the formation of coarse protrusions of the obtained separator can be suppressed, and the dispersibility is good.

4. Dispersion stability Dispersion stability is determined by the rate of change in the mode diameter of the alumina particles in the slurry one week after the mode diameters A (r2) and B (r2) of the alumina particles immediately after the slurry is prepared (hereinafter, formula 6). Value) is used as an index. For the mode diameter of the alumina particles in the slurry after 1 week, put the slurry 1 week after preparation into a polypropylene container so that the volume is halved, and shake it up and down 10 times by hand for 1 hour. Using the sample after standing, the mode diameter was determined in the same manner as in 3. Dispersibility above.
{(Mode diameter of alumina particles in the slurry one week after the slurry preparation-Mode diameter of the alumina particles in the slurry immediately after the slurry preparation) ÷ Mode diameter of the alumina particles in the slurry immediately after the slurry preparation} × 100 ... Equation 6
The dispersion stability of the alumina particles 1 and 2 is good because when the value of Equation 6 is 10% or less, the alumina particles in the slurry are stably present and the formation of coarse protrusions of the obtained separator can be suppressed. ..

5. Pin pull-out property The pin pull-out property 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 around a cylindrical pin having a diameter of 4.0 mm five times by applying a tensile load of 200 g (5 g / mm per separator width). Cylindrical pins were pulled out from the wound separators A to D, and the pin pull-out property was evaluated as follows.
Good: Bamboo shoot-shaped protrusion less than 1 mm Defective: Bamboo shoot-shaped protrusion 1 mm or more 6. Withstanding voltage test method A portion of about 10 m from the winding core was unwound from the wound body of the battery separator obtained in Examples and Comparative Examples, and used as a sample. Using the withstand voltage tester TOS5051A (manufactured by Kikusui Electronics Co., Ltd.), the withstand voltage test of the separator was performed by the following procedure. A separator (50 mm × 50 mm size) was placed on a sample table covered with aluminum foil, then a φ15 mm aluminum foil was placed on the separator, and a φ13 mm conductive rubber was placed on the φ15 mm aluminum foil. 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 connected to the withstand voltage tester with a cable. Withstand voltage tester boosts the voltage between the sample table and the metal weight to 2 kV (boost rate: 0.1 kV / sec), and when converted to a film thickness of 20 μm and an electrical short circuit occurs at 2 kV or less, the withstand voltage When a defect or an electrical short circuit did not occur, the withstand voltage was considered 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, polypropylene insulator 1 (thickness 0.2 mm), negative electrode 2 for lithium ion battery (total thickness: about 140 μm, base material: copper foil (thickness about 9 μm), active material: A sample laminate in which artificial graphite (particle size: about 30 μm), double-sided coating), separator 3, and aluminum foil 4 (thickness: about 0.1 mm) are laminated is attached to the compression jig (lower side) 6 of the universal testing machine with double-sided tape. Fixed. Next, the aluminum foil and negative electrode of the sample laminate were connected to a circuit consisting of a capacitor and a clad resistor with a cable. The capacitor was charged to about 1.5 V, and a metal ball 5 (material: chromium (SUJ-2)) having a diameter of about 500 μm was placed between the separator and the aluminum foil in the sample laminate. A compression jig was attached to the universal testing machine, and a sample laminate containing the metal balls 5 was placed between the compression jigs as shown in FIG. 5, and the speed was 0.3 mm / min. The test was terminated when the load reached 100 N. At this time, the portion where the inflection point appeared due to the change in the compressive load was defined as the film rupture point of the separator, and the moment when the circuit was formed via the metal ball and the current was detected was defined as the short circuit generation point. The compressive displacement A (t) when the separator breaks due to compression and an inflection point occurs in the compressive stress, and the compressive displacement B (t) at the moment when a current flows through the circuit are measured and calculated by the following equation 7. When the value is 1.1 or more, it means that even if the separator breaks due to the foreign matter mixed in the battery, the coating layer composition adheres to the foreign matter surface to maintain the insulation, so that it is short-circuit resistant. Was good. On the other hand, when the value obtained by Equation 7 is larger than 1.0 and less than 1.1, the separator film breakage and short circuit do not occur at the same time, but the tension applied to the winding of the battery member and the expansion of the electrode during charging and discharging occur. Since a certain level of resistance is required to prevent a short circuit from occurring even when the internal pressure of the battery rises, the short circuit resistance is considered to be slightly poor. When the value obtained by the formula 7 is 1.0, a short circuit occurs at the same time as the film rupture of the separator, and the short circuit resistance is not improved by the coating layer. Therefore, the short circuit resistance is considered to be poor.
B (t) ÷ A (t) ... Equation 7.

Examples 1-11
(Adjustment of slurry)
As a fluororesin, vinylidene fluoride-hexafluoropropylene copolymer (VdF-HFP copolymer (weight average molecular weight 1 million)) is blended in a weight ratio of about 5 wt% with respect to N-methyl-2-pyrrolidone, and VdF- A fluororesin solution in which the HFP copolymer was completely dissolved was obtained. Next, the alumina particles 1 and 2 were added to the fluororesin solution while stirring with a mechanical stirrer at 350 rpm so that the total volume of the alumina particles 1 and the alumina particles 2 and the volume of the VdF-HFP copolymer were 50:50. bottom. The characteristics and volume ratio of the alumina particles used in each example are shown in Table 1. The primary mode diameters of the alumina particles 1 and 2 were set to the above 3. using each sample adjusted so that each alumina particle was 0.05 wt% in water. The particle size distribution was measured under the same measurement conditions as the dispersibility, and the maximum value obtained from the frequency distribution map was used.

The addition rate of the alumina particles was 30 g / min per 10 L of the fluororesin solution. After adding the alumina particles, the mixture was continuously stirred with a mechanical stirrer for 1 hour to perform pre-dispersion. Next, using a Dyno Mill (Dyno Mill Multilab (1.46 L container, filling rate 80%, φ0.5 mm alumina beads) manufactured by Simmal Enterprises Co., Ltd.), the flow rate is 11 kg / hr and the peripheral speed is 10 m / s. The slurry was prepared by dispersing in the number of passes shown in Table 1 under the conditions of. At this time, the temperature of the slurry was adjusted so as to be in the range of 20 to 45 ° C. The slurry was stored in a hermetically sealed manner so as not to come into contact with the outside air as much as possible until the time of coating.

(Lamination of porous layers)
Slurry is applied to both sides of a polyethylene microporous membrane (thickness 7 μm, air permeability resistance 100 sec / 100 ml Air) by the dip coating method, immersed in water (coagulant), washed with pure water, and then at 70 ° C. It was passed through a hot air drying oven and dried to obtain a battery separator having a final thickness of 11 μm.

Example 12
A battery separator was obtained in the same manner as in Example 1 except that the polyethylene microporous membrane was replaced with one having a thickness of 12 μm and an air permeation resistance of 120 sec / 100 ml Air.

Example 13
A battery separator was obtained in the same manner as in Example 1 except that the polyethylene microporous membrane was replaced with one having a thickness of 9 μm and an air permeation resistance of 180 sec / 100 ml Air.

Example 14
A battery separator was obtained in the same manner as in Example 1 except that the polyethylene microporous membrane was replaced with one having a thickness of 7 μm and an air permeation resistance of 180 sec / 100 ml Air.

Example 15
A battery separator was obtained in the same manner as in Example 1 except that the polyethylene microporous membrane was replaced with one having a thickness of 5 μm and an air permeation resistance of 110 sec / 100 ml Air.
Comparative Example 1
A battery separator was obtained in the same manner as in Example 1 except that the alumina particles 1 were not used and only the alumina particles 2 were used.

Comparative Examples 2-7
A battery separator was obtained in the same manner as in Example 1 except that the ratio of the alumina particles 1 and the alumina particles 2 was set as shown in Table 1 and the dispersion condition was set to the number of passes shown in Table 1.

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

Table 2 shows the number of passes in Examples and Comparative Examples, the dispersibility and 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 containing metal spheres

Claims (9)

A battery separator having a polyolefin microporous film and a porous layer on at least one side of the polyolefin microporous film, the porous layer containing alumina particles and a binder, and the total of the alumina particles and the binder is 100. When the volume% is taken, the volume ratio of the alumina particles is 50% by volume or more, and the alumina particles have an absorption peak in the vicinity of ^{3475 cm-1 by Fourier transform infrared spectroscopy (FT-IR) and the peak.} The alumina particles have at least two maximums satisfying the following formulas 1 and 2 in the primary particle size distribution, and the alumina has a particle size of 1.0 μm or more in the primary particle size. A battery separator, characterized in that the ratio of the total volume of each of the particles A and the alumina particles B having a particle size of less than 1.0 μm satisfies 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
Here, A (r1) and B (r1) are mode diameters showing the maximum in the primary particle size distribution of the alumina particles, and A (vol) / B (vol) are the alumina particles A and the alumina particles B in the porous layer. Is the total volume ratio of. Claim 1 is characterized in that the alumina particles A have an absorption peak in the vicinity of ^{3475 cm-1} by Fourier transform infrared spectroscopy (FT-IR), and the peak disappears at 300 ° C. or higher. Battery separator described in. The present invention according to claim 1 or 2, wherein the alumina particles have a water content of 2000 mass ppm or less as measured by mass spectrometry of heated generated gas (TPD-MS) when the temperature is raised from room temperature to 1000 ° C. Battery separator. The battery separator according to any one of claims 1 to 3, wherein 0.7 ≦ A (vol) / B (vol) ≦ 1.5. 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 contains a vinylidene fluoride-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.

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