JP6969143B2 - Porous Membrane and Laminated Porous Membrane - Google Patents

Description

The present invention relates to a porous film and a laminated porous film, and more particularly to a porous film that can be suitably used as a separator for a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery.

Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries (LIBs) are widely used mainly for mobile device applications, and generally, as their separators, a porous film made of a polyolefin resin having a shutdown function is used. It is used. The shutdown function is a kind of safety function that closes the hole when the temperature of the battery rises and blocks ion permeation to prevent the battery from running out of control.

One of the development issues of LIB is to further increase the energy density. Especially in the case of in-vehicle LIB, there is a demand for heat resistance to the separator, especially improvement of dimensional stability at high temperature, due to the increase in capacity due to the increase in size. It is getting higher and higher.

Against this background, a configuration in which a heat-resistant porous membrane is laminated on a polyolefin porous membrane has been proposed. For example, Patent Documents 1 and 2 describe heat-resistant resin and inorganic material on one or both sides of the polyolefin porous membrane. A separator made of a laminated porous film having a heat-resistant porous film made of a composite with particles has been proposed.

Japanese Unexamined Patent Publication No. 2000-30686 Japanese Unexamined Patent Publication No. 2006-322466

However, in the prior art, when the content of the inorganic particles in the heat-resistant porous membrane is set to a certain amount or more, there is a problem that powder drops due to aggregation of the inorganic particles and insufficient binding property. On the other hand, when the content of the heat-resistant resin in the heat-resistant porous film is increased, the heat resistance (particularly dimensional stability at high temperature) is lowered, the film resistance is increased, and the mechanical properties (especially compression resistance) at the time of thinning are lowered. There are issues such as.

In view of the above circumstances, it is an object of the present invention to provide a porous membrane having excellent dispersibility and supportability of inorganic particles, high heat resistance, low resistance, and high strength, and a laminated porous membrane having this porous membrane. do.

The present invention for achieving the above object is characterized by the following configuration.

Porous characterized by containing an aromatic heat-resistant resin and inorganic particles, having an inorganic particle content of 60 to 99% by mass, and having an electron-attracting group on the aromatic ring of the aromatic heat-resistant resin. film.

The porous film of the present invention (hereinafter, may be referred to as a heat-resistant porous film) contains an aromatic heat-resistant resin and inorganic particles, has an inorganic particle content of 60 to 99% by mass, and has an aromatic heat resistance. It is characterized by having an electron-attracting group on the aromatic ring of the resin. Since this heat-resistant porous film has a large content of inorganic particles and is excellent in dispersibility and supportability thereof, it is possible to reduce the film resistance. Therefore, good output characteristics and cycle characteristics can be obtained when used as a separator for a secondary battery. Further, since it is excellent in compression resistance and heat resistance, it is possible to maintain the insulation between the positive and negative electrodes even if pressure is applied to the inside of the battery for some reason or the battery is continuously exposed to high temperature.

The aromatic heat-resistant resin used in the present invention means a resin having an aromatic ring on the main chain and having a glass transition temperature of 150 ° C. or higher measured by a method according to ASTM E1640-13. A resin having a glass transition temperature of 200 ° C. or higher is preferable, and a resin having a glass transition temperature of 250 ° C. or higher is more preferable. Further, at least a part of the aromatic ring on the main chain is an aromatic ring substituted with an electron-attracting group. Preferably, 30-100 mol% of the total of all aromatic rings is an electron-withdrawing group substituted aromatic ring, more preferably 50-100 mol%, still more preferably 70-100 mol%. be. Here, the electron-attracting group in the present invention means a group having an electronegativity of 2.5 or more. Examples of such an electron-attracting group include a halogen group such as a fluoro group, a chloro group and a bromo group, an alkyl halide group such as a trifluoromethyl group, a nitro group, a cyano group, a cyanate group and a phenyl group. Be done. Of these, a fluoro group, a chloro group and a trifluoromethyl group are particularly preferable.

As the polymer skeleton, for example, an aromatic polyamide (aramid) having a repeating unit represented by the following chemical formula (1) and / or a chemical formula (2), and an aromatic polyimide having a repeating unit represented by the chemical formula (3). , Aromatic polyamide-imide having a repeating unit represented by the chemical formula (4) and the like.
Chemical formula (1):

Chemical formula (2):

Chemical formula (3):

Chemical formula (4):

Here _{, examples of the skeletons of Ar 1 to} Ar ₇ include the following chemical formulas (5) to (9). For each of Ar _{1 to} Ar _7, the copolymerization may be carried out by a plurality of groups selected from the chemical formulas (5) to (9).
Chemical formulas (5) to (9):

The chemical formula (8), X, Y are in _{(9), -O -, -} CO -, - SO 2 -, - CH 2 -, - S -, - C (CH 3) 2 - is selected from such It can, but is not limited to.

Generally, an aromatic heat-resistant resin has a rigid molecular structure such that the aromatic ring has a para-orientation or the intramolecular rotation of the bond is restricted, and the larger the molecular weight, the stronger the strength and heat resistance. Excellent in characteristics. On the other hand, since the rigid and high molecular weight aromatic heat-resistant resin has a strong cohesive force of molecular chains, its solubility in a solvent is low, it is difficult to polymerize or prepare a solution, and it is difficult to make it porous, and the film resistance increases. It can be easy. On the other hand, the aromatic heat-resistant resin used in the present invention has an electron-attracting group on the aromatic ring, so that a repulsive force acts, and even if it is rigid and has a high molecular weight, intramolecular and intermolecular aggregation in a solvent. Is suppressed, and high solvent solubility and pore-forming ability can be obtained. Further, the fact that the molecular chains do not aggregate and spread easily in the solvent leads to excellent dispersibility and supportability even if the content of the inorganic particles is increased as in the present invention.

Further, the _{binding hands in Ar 1 to} Ar ₇ may be any of ortho-orientation, meta-orientation, and para-orientation, but those having para-orientation are 50 to 100 of all aromatic rings. It is preferably mol%, more preferably 80 to 100 mol%. The term "para-orientation" as used herein means a state in which the bonds constituting the main chain on the aromatic ring are coaxial or parallel to each other. If this para-orientation is less than 50 mol%, the strength and heat resistance of the obtained heat-resistant porous film may be insufficient.

Further, it is more preferable to have a skeleton having an oxygen atom O as X and Y in the chemical formulas (8) and (9) in a part of _{Ar 1 to} Ar _{7 because higher solvent solubility and pore-forming ability can be obtained.} ..

If it is necessary to identify the chemical structure of the aromatic heat-resistant resin, perform structural analysis by combining nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FT-IR), and mass spectrometry (MS). Can be done. The chemical structure of the aromatic heat-resistant resin is identified from the laminated porous membrane formed on at least one of the porous membrane base materials containing the polyolefin resin (hereinafter, may be simply referred to as the polyolefin porous membrane). In that case, the analysis can be performed by the following method. The polyolefin porous membrane is separated from the sample by immersing it in 100 parts by mass of concentrated sulfuric acid for 24 hours under heating at 60 ° C. with respect to 100 parts by mass of the laminated porous membrane sample. Then, insoluble matter (for example, inorganic particles) is removed from the recovered concentrated hydrochloric acid solution by centrifugation or the like, and the obtained heat-resistant resin component can be identified by performing the above structural analysis.

Examples of the inorganic particles used in the present invention include metal oxides, metal hydroxides, metal carbonates, metal phosphates, metal silicon salts, metal sulfates, clay minerals and the like. More specifically, silicon dioxide (wet and dry silica, colloidal silica), titanium oxide (titania), aluminum oxide (alumina), zinc oxide (zinc oxide), antimony oxide, cerium oxide, zirconium oxide, tin oxide, oxidation. Lantern, magnesium oxide, aluminum hydroxide, magnesium hydroxide, calcium carbonate, magnesium carbonate, zinc carbonate, barium carbonate, basic lead carbonate (lead white), calcium phosphate, aluminum silicate, barium sulfate, calcium sulfate, lead sulfate, zinc sulfide , Mica, mica titanium, talc, clay, kaolin, lithium fluoride, calcium fluoride and the like. These particles may be used alone or in admixture of two or more. Among these, silicon dioxide, titanium oxide, aluminum oxide, magnesium oxide, aluminum hydroxide and magnesium hydroxide are more preferable for use as a separator for a battery, and aluminum oxide is particularly preferable. The average primary particle size of the inorganic particles is preferably 0.01 to 5.0 μm. Examples of the shape of the inorganic particles include a spherical shape, a plate shape, a needle shape, a rod shape, an elliptical shape, and the like, and any shape may be used, but the shape is preferably spherical from the viewpoint of dispersibility and coatability.

The content of the inorganic particles in the heat-resistant porous membrane of the present invention is 60 to 99% by mass, preferably 70 to 99% by mass, and more preferably 80 to 99% by mass. If the content of the inorganic particles is less than 60% by mass, the heat resistance, particularly the dimensional stability at a high temperature may decrease. In addition, the Gale air permeability and membrane resistance may increase. In order to keep the content of the inorganic particles within the range of the present invention, it is preferable that the structure of the aromatic heat-resistant resin is as described above and the inorganic particles are dispersed in the membrane-forming stock solution under the conditions described below. Here, the content of the inorganic particles in the heat-resistant porous membrane can be determined by high-frequency inductively coupled plasma (ICP) emission spectroscopic analysis for a solution in which the porous membrane sample is incinerated and then heated and dissolved in hydrochloric acid.

The thickness of the heat-resistant porous membrane of the present invention is preferably 0.5 to 30 μm, more preferably 1.0 to 20 μm. If the thickness is less than 0.5 μm, sufficient heat resistance and strength may not be obtained. If the thickness exceeds 30 μm, the film resistance is high, the output may decrease when used as a battery separator, or the thickness of the active material layer that can be incorporated into the battery may become thin, and the energy density per volume may decrease.

The heat-resistant porous membrane of the present invention preferably has a Gale air permeability of 1 to 200 seconds / 100 ml. More preferably, it is 1 to 150 seconds / 100 ml. If the Gale air permeability is less than 1 second / 100 ml, the strength may decrease, the film may break during processing, or a short circuit between the electrodes may easily occur when used as a battery separator. If the Gale air permeability is larger than 200 seconds / 100 ml, the film resistance is high, and when used as a battery separator, the output characteristics may be deteriorated, or the capacity may be significantly deteriorated when used repeatedly. In order to keep the Gale air permeability within the above range, the structure of the aromatic heat-resistant resin and the content of the inorganic particles are as described above, and the dispersion of the inorganic particles in the membrane-forming stock solution and the membrane-forming conditions of the porous membrane will be described later. It is preferably within the range of.

When measuring the Gale air permeability of the heat-resistant porous membrane from the laminated porous membrane formed on at least one of the polyolefin porous membranes, the polyolefin porous is measured from the Gale air permeability of the laminated porous membrane sample. Calculated by reducing the Gale air permeability of the quality film substrate. When the galley air permeability of the polyolefin porous membrane substrate is unknown, the polyolefin porous membrane substrate can be separated and measured by the following method. The heat-resistant porous membrane is removed from the laminated porous membrane sample by immersing the laminated porous membrane sample in 100 parts by mass of concentrated sulfuric acid for 24 hours under heating at 60 ° C. Then, it is washed with running water and dried in a vacuum dryer at 80 ° C. for 12 hours to separate the polyolefin porous membrane substrate.

The heat-resistant porous membrane of the present invention preferably has a heat shrinkage rate of −0.5 to 5.0% at 200 ° C. in at least one direction in the in-plane direction, preferably −0.5 to 3.0%. Is more preferable. If the heat shrinkage rate exceeds 5.0%, a short circuit may occur at the end of the battery due to the shrinkage of the separator when the battery generates abnormal heat. In order to keep the heat shrinkage within the above range, the structure of the aromatic heat-resistant resin and the content of the inorganic particles are as described above, and the conditions for dispersing the inorganic particles in the membrane-forming stock solution and forming the porous membrane are described later. It is preferably within the range.

The heat-resistant porous membrane of the present invention preferably has a heat shrinkage rate of −0.5 to 15.0% at 500 ° C. in the thickness direction, more preferably −0.5 to 10.0%. When the heat shrinkage rate exceeds 15.0%, a short circuit may occur between the positive and negative electrodes due to the decrease in the thickness of the separator during abnormal heat generation of the battery. Since the content of the inorganic particles in the heat-resistant porous membrane is high and the dispersibility is excellent, the heat shrinkage rate in the thickness direction can be suppressed to a low level. In order to keep the heat shrinkage rate at 500 ° C. in the thickness direction within the above range, the structure of the aromatic heat-resistant resin and the content of the inorganic particles are set as described above, and the dispersion of the inorganic particles in the membrane-forming stock solution and the porous membrane are made. It is preferable that the film forming conditions are within the range described later.

The heat-resistant porous membrane of the present invention may be used alone as a separator for a secondary battery, or at least one of a porous membrane base material containing a polyolefin resin (hereinafter, may be simply referred to as a polyolefin porous membrane). It may be formed on the surface of the above and used as a laminated porous film. The heat-resistant porous film is preferably formed on both surfaces of the polyolefin porous film from the viewpoint of suppressing curling of the laminated porous film and obtaining high heat resistance.

Examples of the polyolefin resin used in the present invention include polyethylene, polypropylene, ethylene-propylene copolymer, polybutene-1, poly4-methylpentene-1, and cyclic polyolefin resins obtained by ring-opening metathesis polymerization of norbornene-based derivatives. , A cyclic polyolefin copolymer resin obtained by copolymerizing a norbornene-based derivative with α-olefins such as ethylene, propylene, butene, and penten is suitable. Further, a plurality of types of these polyolefin resins may be mixed or laminated for use.

The polyolefin porous membrane used in the present invention means a porous membrane in which the content of the above-mentioned polyolefin resin in the polyolefin porous membrane is 55 to 100% by mass. If the content of the polyolefin resin is less than 55% by mass, a sufficient shutdown function may not be obtained.

The thickness of the porous polyolefin membrane used in the present invention is preferably 2 to 30 μm, more preferably 3 to 20 μm. If the thickness is less than 2 μm, the film may be broken during processing or a sufficient shutdown function may not be obtained. If the thickness exceeds 30 μm, the film resistance may increase, the output may decrease when used as a battery separator, or the thickness of the active material layer that can be incorporated into the battery may become thin, resulting in a decrease in energy density per volume. ..

The laminated porous membrane of the present invention preferably has a membrane resistance of 1.6 to 9.6 Ω · cm ² at 25 ° C. More preferably, it is 1.6 to 7.2 Ω · cm ² . By setting the membrane resistance within the above range, when used as a battery separator, ion permeability is high, and excellent output characteristics and cycle characteristics can be obtained. If the membrane resistance ^{exceeds 9.6 Ω · cm 2} , the ion permeability may be low when used as a battery separator, the output characteristics may be deteriorated, or the capacity may be significantly deteriorated when used repeatedly. In order to keep the film resistance within the above range, it is preferable to form a heat-resistant porous film formed on at least one of the polyolefin porous films under the conditions described in the present invention.

The laminated porous membrane of the present invention preferably has a heat shrinkage rate of −0.5 to 20.0% at 150 ° C. in at least one direction in the in-plane direction, preferably −0.5 to 10.0%. Is more preferable. When the heat shrinkage rate exceeds 20.0%, a short circuit may occur at the end of the battery due to the shrinkage of the separator when the battery generates abnormal heat. In order to keep the heat shrinkage within the above range, it is preferable to form a heat-resistant porous film formed on at least one of the polyolefin porous films under the conditions described in the present invention.

Next, the method for producing the heat-resistant porous membrane and the laminated porous membrane of the present invention will be described below.

First, a method for obtaining an aromatic heat-resistant resin that can be used in the present invention will be described by taking aromatic polyamide and aromatic polyimide as examples. Not limited.

Various methods can be used to obtain aromatic polyamides. For example, when a low-temperature solution polymerization method using acid dichloride and diamine as raw materials is used, N-methylpyrrolidone, N, N-dimethylacetamide, and dimethylformamide are used. , A method of synthesizing by solution polymerization in an aprotic organic polar solvent such as dimethylsulfoxide, a method of synthesizing by surface polymerization using an aqueous medium, or the like can be taken. Solution polymerization in an aprotic organic polar solvent is preferred because it is easy to control the molecular weight of the polymer. When acid dichloride and diamine are used as raw materials, hydrogen chloride is by-produced as the polymerization reaction progresses, but when neutralizing this, an inorganic neutralizer such as lithium carbonate, calcium carbonate, or calcium hydroxide, Alternatively, an organic neutralizing agent such as ethylene oxide, propylene oxide, ammonia, triethylamine, triethanolamine, or diethanolamine may be used.

When polyamic acid, which is an aromatic polyimide or a precursor thereof, is polymerized using, for example, tetracarboxylic acid anhydride and aromatic diamine as raw materials, N-methyl-2-pyrrolidone, N, N-dimethylacetamide, dimethylformamide , A method of synthesizing by solution polymerization in an aprotic organic polar solvent such as dimethylsulfoxide can be taken. As a method for imidizing the synthesized aromatic polyamic acid to obtain an aromatic polyimide, heat treatment, chemical treatment, and a combination thereof are used. The heat treatment method is generally a method of imidizing a polyamic acid by heat-treating it at about 100 to 500 ° C. On the other hand, the chemical treatment includes a method using a tertiary amine such as triethylamine as a catalyst and a dehydrating agent such as aliphatic acid anhydride and aromatic acid anhydride, and a method using an imidizing agent such as pyridine.

_{The logarithmic viscosity (η inh} ) of the aromatic polyamide, the aromatic polyimide or its precursor, polyamic acid, is preferably 1.5 to 7.0 dl / g, preferably 2.0 to 5.0 dl / g. Is more preferable. If the logarithmic viscosity is less than 1.5 dl / g, the degree of polymerization is low, sufficient strength may not be obtained, the supportability of the inorganic particles may be lowered, and powder dropping may occur. In order to maintain the solubility of the resin and keep the logarithmic viscosity within the above range, it is preferable that the structure of the aromatic heat-resistant resin is as described above.

Next, a film-forming stock solution containing an aromatic heat-resistant resin and inorganic particles used in the process of producing a heat-resistant porous film (hereinafter, may be simply referred to as a film-forming stock solution) will be described. The aromatic heat-resistant resin solution after polymerization may be used as it is as the film-forming stock solution, or the aromatic heat-resistant resin may be isolated once and then redissolved in a solvent before use. The method for isolating the aromatic heat-resistant resin is not particularly limited, but the solvent and the neutralizing salt are extracted into the water by putting the polymerized aromatic heat-resistant resin solution into a large amount of water, and the precipitated aromatic resin is extracted. Examples thereof include a method of separating only the heat-resistant resin and then drying it. Further, a metal salt or the like may be added as a dissolution aid at the time of re-dissolution. The metal salt is preferably a halide of an alkali metal or an alkaline earth metal, and examples thereof include lithium chloride, lithium bromide, sodium chloride, sodium bromide, potassium chloride, and potassium bromide.

As a method for obtaining a slurry-like membrane-forming stock solution by dispersing inorganic particles in an aromatic heat-resistant resin solution, a known method such as a bead mill, a ball mill, a sand mill, a roll mill, a homogenizer, a high-pressure homogenizer, or an ultrasonic homogenizer can be used. .. Here, it is preferable that the liquid property of the aromatic heat-resistant resin solution for dispersing the inorganic particles is basic in terms of the dispersibility of the inorganic particles, and the pH value is more preferably 8 to 12. This is considered to be the effect of the repulsive force between the inorganic particles. That is, by making the liquid basic, the protons on the surface of the inorganic particles are dissociated, and the inorganic particles are negatively charged. It is considered that this causes the inorganic particles to repel each other and suppress aggregation. Here, the liquid property of the aromatic heat-resistant resin solution is measured for the extract after collecting 10 g of the aromatic heat-resistant resin solution in a beaker having a capacity of 50 ml, adding 40 g of pure water, and stirring at 25 ° C. for 10 minutes. Can be done. The liquidity of the aromatic heat-resistant resin solution is that basic substances such as ethylene oxide, propylene oxide, ammonia, triethylamine, triethanolamine, and diethanolamine are added to the aromatic heat-resistant resin solution after polymerization or redissolution if isolated. It can be adjusted by adding it.

A hydrophilic polymer may be mixed with the film-forming stock solution for the purpose of improving the pore-forming ability. Examples of the hydrophilic polymer include polyvinylpyrrolidone, polyethylene glycol, polyvinyl alcohol, polyacrylamide, polyacrylic acid, polyethyleneimine and the like.

In addition to the above, the undiluted film-forming solution includes a fluororesin or acrylic resin that imparts adhesiveness to electrodes, a dispersant, a thickener, a stabilizer, a defoaming agent, and a leveling agent, if necessary. Etc. may be added.

The total content of the aromatic heat-resistant resin and the inorganic particles in the film-forming stock solution is preferably 5 to 30% by mass, more preferably 7 to 20% by mass.

A heat-resistant porous membrane is produced by a so-called solution membrane-forming method using the membrane-forming stock solution prepared as described above. Typical examples of the solution film forming method include a wet method, a hygroscopic method, or a combination thereof, but the method is not particularly limited. In each method, the membrane-forming stock solution is first cast (cast) on a support (or a polyolefin porous film when formed on at least one of the polyolefin porous films to form a laminated porous film), and then wet. In the case of the method, it is introduced into a coagulation bath composed of water or a poor solvent, and in the case of the moisture absorption method, it is gradually absorbed under a temperature-controlled and humidity-controlled atmosphere to precipitate an aromatic heat-resistant resin to obtain a porous film. In addition, these combinations may be introduced into the coagulation bath after absorbing moisture in a temperature-controlled and humidity-controlled atmosphere. The bath composition of the coagulation bath in the wet method is not particularly limited, but it is preferable to use water or an organic solvent / water mixed system. In addition, the wet bath may contain an inorganic salt. The temperature and humidity control atmosphere conditions in the moisture absorption method ^{are preferably 10 to 500 g / m 3 in} volume absolute humidity, and the temperature of the atmosphere is 20 to 100 ° C. and the relative humidity is 60 within a range satisfying this volume absolute humidity. It is more preferable to set it to ~ 100% RH. The treatment time is preferably 0.1 to 5 minutes.

After forming a porous film in this way, heat treatment is performed using a tenter or the like. The heat treatment temperature is preferably 40 to 300 ° C, more preferably 150 to 280 ° C. However, when it is formed on at least one of the polyolefin porous membranes to form a laminated porous membrane, the heat treatment temperature is preferably 40 to 120 ° C.

Next, a method for forming a porous polyolefin membrane used in the present invention will be described. The raw material polyolefin solution is prepared by heating and dissolving the above-mentioned polyolefin in a solvent. The solvent is not particularly limited as long as it can sufficiently dissolve the polyolefin, and examples thereof include aliphatic or cyclic hydrocarbons such as nonane, decane, undecane, dodecane, and paraffin oil. The heat dissolution is carried out with stirring at a temperature at which the polyolefin is completely dissolved in the solvent. The temperature varies depending on the polymer and solvent used, but in the case of polyethylene, for example, the temperature is preferably in the range of 140 to 250 ° C. The concentration of the polyolefin solution is preferably 10 to 50% by mass. Next, the heated solution of this polyolefin is extruded from the die and then cooled to form a gel. The solution extruded from the die is preferably taken up at a take-up ratio of 1 to 10 before or during cooling. Next, this gel-like molded product is heated with a tenter or the like, and biaxial stretching is performed at a predetermined magnification. The stretching temperature is the melting point of the polyolefin used + 10 ° C. or lower, preferably in the range from the crystal dispersion temperature to less than the crystal melting point. For example, in the case of multistage polymerized polyethylene, the temperature is in the range of 90 to 140 ° C. The draw ratio varies depending on the thickness of the original fabric, but the surface ratio is 10 to 400 times. The obtained stretched molded product is washed with a solvent to remove the residual solvent. Examples of the cleaning solvent include hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, fluorinated hydrocarbons such as ethane trifluoride, and ethers such as diethyl ether and dioxane. Volatile ones can be used. After that, the cleaning solvent is dried, and the cleaning solvent can be dried by heating or air-drying. The dried stretched molded product is preferably heat-fixed in the temperature range from the crystal dispersion temperature to the melting point.

The heat-resistant porous film of the present invention contains an aromatic heat-resistant resin and inorganic particles, has an inorganic particle content of 60 to 99% by mass, and has an electron-attracting group on the aromatic ring of the aromatic heat-resistant resin. It is characterized by having. Since this heat-resistant porous film has a large content of inorganic particles and is excellent in dispersibility and supportability thereof, it is possible to reduce the film resistance. Therefore, good output characteristics and cycle characteristics can be obtained when used as a separator for a secondary battery. Further, since it has excellent compression resistance and heat resistance, it is possible to maintain the insulation between the positive and negative electrodes even if pressure is applied to the inside of the battery for some reason or the battery is continuously exposed to high temperature. Further, the laminated porous membrane in which the heat-resistant porous membrane of the present invention is formed on at least one of the porous membrane base materials containing a polyolefin resin has the above-mentioned characteristics of the heat-resistant porous membrane in addition to the shut-down function of the base material. Since it is a laminated porous membrane having excellent low resistance, compression resistance, and heat resistance, it can be suitably used as a separator for a secondary battery for a wide range of applications.

[Measurement method of physical properties and evaluation method of effect]
The method for measuring the physical properties in the examples was as follows.

(1) Thickness The thickness (μm) of the sample was measured using a constant pressure thickness measuring device FFA-1 (manufactured by Ozaki Seisakusho Co., Ltd.). The stylus diameter is 5 mm and the measured load is 1.25 N. Ten points were measured in the width direction of the sample, and the average value was calculated.

(2) Gale air permeability A B-type Gale Densometer (manufactured by Yasuda Seiki Seisakusho Co., Ltd.) is used to measure the Gale air permeability (seconds / 100 ml) of the sample according to the method specified in JIS-P8117 (1998). went. The sample is tightened in a circular hole with a diameter of 28.6 mm and an area of 642 mm ² , and the air inside the cylinder is passed through the test circular hole to the outside of the cylinder by the inner cylinder (inner cylinder mass 567 g), and the time for 100 ml of air to pass is measured. By doing so, the air permeability of Gale was set.

(3) As an electrode 1 for measuring film resistance at 25 ° C., an aluminum sheet having a thickness of 20 μm was cut out into a long side of 50 mm and a short side of 40 mm. Of these, the short side 40 mm × the long side end 10 mm is a margin for connecting the tabs, and the effective measurement area is 40 mm × 40 mm (1,600 mm ² = 16 cm ² ). After ultrasonically welding an aluminum tab with a width of 5 mm, a length of 30 mm, and a thickness of 100 μm to an arbitrary position of the margin of the cut aluminum sheet, the entire margin including the weld is covered with Capton (registered trademark) tape. Insulation treatment was performed. As the measurement electrode 2, a similar aluminum sheet was cut out into a long side of 55 mm and a short side of 45 mm. Of these, the short side 45 mm × the long side end 10 mm is a margin for connecting tabs. After ultrasonically welding an aluminum tab with a width of 5 mm, a length of 30 mm, and a thickness of 100 μm to an arbitrary position of the margin of the cut aluminum sheet, the entire margin including the weld is covered with Capton (registered trademark) tape. Insulation treatment was performed. The porous membrane of the sample was cut out to 55 mm × 55 mm.

The cut out measurement electrode 1, measurement electrode 2, and sample are dried under reduced pressure in a desiccator (decompression degree 0.09 MPa, 80 ° C.) for 12 hours, and then in the order of measurement electrode 1 / sample / measurement electrode 2. Stacked. At this time, the entire effective measurement area of 40 mm × 40 mm of the measurement electrode 1 was arranged so as to face the measurement electrode 2 with the sample film interposed therebetween. Next, the above (electrode / sample / electrode) laminate was sandwiched between aluminum laminated films and heat-sealed to form a bag shape, leaving one side of the aluminum laminated film. Inject 1.5 g of an electrolytic solution prepared by dissolving _{LiPF 6} as a solute in a mixed solvent of ethylene carbonate: diethyl carbonate = 3: 7 (volume ratio) to a concentration of 1 mol / L into a bag-shaped aluminum laminate film. A laminated cell was prepared by heat-sealing the short side portion of the aluminum laminated film while impregnating it under reduced pressure. Two types of such cells were prepared with two or four samples between the electrodes.

For each cell produced as described above, the AC impedance was measured under the conditions of a voltage amplitude of 10 mV and a frequency of 10 to 5,000 Hz in an atmosphere of 25 ° C., and the AC resistance (Ω) was obtained from the Core-Cole plot. The obtained AC resistance was plotted against the number of samples, and the AC resistance per sample was calculated from the inclination when the plots were connected by a straight line. By multiplying the effective measurement area 16cm ² to AC resistance obtained was calculated film resistance normalized (Ω · cm ^2). For the AC resistance values used in the above plot, 5 cells were created for each of the two types of evaluation cells with different numbers of samples, and 3 measured values excluding the maximum and minimum AC resistance values were used. The averaged values were used respectively.

(4) Heat shrinkage rate in the in-plane direction The sample is placed in the longitudinal direction (MD) 50 mm × width direction (TD) 50 mm (here, MD is the film forming direction of the film, and TD is the direction orthogonal to it. The sample was cut into (1) and heat-treated in a hot air oven at a predetermined temperature (150 ° C. or 200 ° C.) for 1 hour with substantially no tension applied, and then cooled to 25 ° C. The treated sample was sandwiched between glass plates having a thickness of 3 mm, and the dimensions of the portion having the largest dimensional change in each of MD and TD were measured and used as the treated dimension L (mm). Using the obtained L, the heat shrinkage rate (%) of MD and TD was calculated by the following formula. The measurement was performed 5 times, and the average value was calculated for each of MD and TD.

In-plane heat shrinkage rate (%) = ((50-L) / 50) × 100
(5) Heat shrinkage rate in the thickness direction A sample was cut out in a length direction of 50 mm × a width direction of 50 mm, and the thickness D ₀ (μm) before treatment was measured by the method described in (1). This sample was heat-treated in a hot air oven at 500 ° C. for 10 minutes with this sample sandwiched between two stainless steel plates (SUS316, thickness 1 mm, 200 mm square), and then cooled to 25 ° C. The sample thickness D after the treatment was measured in the same manner as above, and the heat shrinkage rate (%) was calculated by the following formula.

Heat shrinkage rate in the thickness direction (%) = ((D ₀ −D) / D ₀ ) × 100
(Example 1)
Dehydrated N-methyl-2-pyrrolidone (NMP) was charged with 2-chloro-1,4-phenylenediamine equivalent to 80 mol% and 4,4'-diaminodiphenyl ether corresponding to 20 mol% with respect to the total amount of diamine. Dissolved. The aromatic polyamide (A) was polymerized by adding 2-chloroterephthaloyl chloride corresponding to 99 mol% with respect to the total amount of diamine as acid dichloride and stirring the mixture. The obtained polymerization solution was neutralized with 97 mol% lithium carbonate with respect to the total amount of acid dichloride, and the pH was further adjusted to 10.0 with 15 mol% diethanolamine and 25 mol% triethanolamine to aroma. A solution having a group polyamide concentration of 10% by mass was obtained. The pH was measured by collecting 10 g of the above solution in a beaker, adding 40 g of pure water, stirring at 25 ° C. for 10 minutes using a stirrer, and then measuring the extracted water using a pH meter. The log viscosity η _inh of the obtained aromatic polyamide was 2.5 dl / g.

Next, alumina particles (average particle size 0.4 μm) and NMP for dilution are added to the obtained aromatic polyamide solution, pre-dispersed with a stirrer, and kneaded with a bead mill to achieve the aroma in the film-forming stock solution. The content of the alumina particles was adjusted to 90% by mass with respect to the total amount of the group polyamide and the alumina particles.

The film-forming stock solution obtained above is continuously applied in the form of a film on a stainless steel (SUS316) belt as a support, and the coating film is applied in a temperature-controlled humidity-controlled air having a temperature of 50 ° C. and a relative humidity of 85% RH. Was treated until it could be peeled off from the support. Next, the coating film was peeled off from the support and introduced into a water bath at 30 ° C. to extract the solvent, neutralizing salt and the like. Subsequently, the obtained porous film in a water-containing state was introduced into a tenter having a temperature of 280 ° C., and the film-forming direction (MD) was a constant length in the tenter chamber, and the film was stretched 1.05 times in the width direction (TD). A heat-resistant porous film having a thickness of 10 μm was obtained by subjecting it to a high-temperature heat treatment for 1 minute and continuously winding it around the MD.

Table 1 shows the evaluation results of the obtained heat-resistant porous membrane.

(Example 2)
The diamine for obtaining the aromatic polyamide (B) is 2-chloro-1,4-phenylenediamine corresponding to 50 mol% and 4,4'-diaminodiphenyl ether corresponding to 50 mol% with respect to the total amount of diamine. A heat-resistant porous film was obtained in the same manner as in Example 1 except for the above. Table 1 shows the evaluation results of the obtained heat-resistant porous membrane.

(Example 3)
Examples except that the acid dichloride for obtaining the aromatic polyamide (C) is 2-chloroterephthaloyl chloride corresponding to 30 mol% and terephthaloyl chloride corresponding to 69 mol% with respect to the total amount of diamine. A heat-resistant porous film was obtained in the same manner as in 1. Table 1 shows the evaluation results of the obtained heat-resistant porous membrane.

(Example 4)
The diamines for obtaining the aromatic polyamide (D) were 2-chloro-1,4-phenylenediamine corresponding to 50 mol% and 4,4'-diaminodiphenyl ether corresponding to 50 mol% with respect to the total amount of diamine. A heat-resistant porous film was obtained in the same manner as in Example 1 except that the acid dichloride was terephthaloyl chloride corresponding to 99 mol% with respect to the total amount of diamine. Table 1 shows the evaluation results of the obtained heat-resistant porous membrane.

(Example 5)
Same as Example 1 except that the diamine for obtaining the aromatic polyamide (E) is 2,2'-ditrifluoromethyl-4,4'-diaminobiphenyl, which corresponds to 100 mol% with respect to the total amount of diamine. A heat-resistant porous film was obtained. Table 1 shows the evaluation results of the obtained heat-resistant porous membrane.

(Example 6)
A heat-resistant porous film was obtained in the same manner as in Example 1 except that the content of the alumina particles was 80% by mass with respect to the total amount of the aromatic polyamide and the alumina particles in the film-forming stock solution. Table 1 shows the evaluation results of the obtained heat-resistant porous membrane.

(Example 7)
A heat-resistant porous film was obtained in the same manner as in Example 1 except that the content of the alumina particles was 70% by mass with respect to the total amount of the aromatic polyamide and the alumina particles in the film-forming stock solution. Table 1 shows the evaluation results of the obtained heat-resistant porous membrane.

(Example 8)
The weight average molecular weight (Mw) of 2.0 × 10 ⁶ ultra high molecular weight polyethylene 20 wt% of high density polyethylene 70 wt% of the Mw of 3.5 × 10 ^5, and Mw of 3.8 × 10 ⁴ polybutylenes A resin composition consisting of 10% by mass of terephthalate was charged into a twin-screw extruder. Liquid paraffin was melt-kneaded from the side feeder of this twin-screw extruder at 230 ° C. so that the mass ratio of resin composition: liquid paraffin was 3: 7, and a resin solution was prepared in the extruder. Subsequently, this resin solution was extruded from a T-die installed at the tip of the extruder and cooled while being taken up by a cooling roll whose temperature was adjusted to 0 ° C. to form a gel-like sheet. The obtained gel-like sheet was simultaneously biaxially stretched at 115 ° C. using a tenter stretching machine so that both MD and TD were 5-fold, and a stretched film was obtained. The obtained stretched membrane was introduced into a washing tank containing dichloromethane whose temperature was adjusted to 25 ° C. for washing. Finally, it was introduced into a tenter, and both MD and TD were heat-fixed at a constant length and a constant width at 125 ° C. for 2 minutes, and then continuously wound to obtain a porous polyolefin film. The obtained porous polyolefin membrane had a thickness of 8 μm and a Gale air permeability of 90 seconds / 100 ml.

Next, the film-forming stock solution containing the aromatic polyamide and the inorganic particles obtained in the same manner as in Example 1 was continuously applied in a film form on both sides of the polyolefin porous film by a dip coat, and a water bath at 30 ° C. was applied. Introduced to. Subsequently, the obtained water-containing film is heat-treated in a hot air oven at 80 ° C. for 10 minutes to form a heat-resistant porous film having a thickness of 2 μm on each side (total thickness of 4 μm on both sides) on both sides of the polyolefin porous film. The formed laminated porous film was obtained. Table 1 shows the evaluation results of the obtained laminated porous membrane.

(Example 9)
A laminated porous film was obtained in the same manner as in Example 8 except that the thickness of the heat-resistant porous film formed on both sides of the polyolefin porous film was 4 μm on each side (total thickness on both sides was 8 μm). Table 1 shows the evaluation results of the obtained laminated porous membrane.

(Example 10)
A laminated porous film was obtained in the same manner as in Example 8 except that the membrane-forming stock solution containing the aromatic polyamide and the inorganic particles obtained in the same manner as in Example 7 was used. Table 1 shows the evaluation results of the obtained laminated porous membrane.

(Example 11)
In the dehydrated N-methyl-2-pyrrolidone, 2,2'-ditrifluoromethyl-4,4'-diaminobiphenyl corresponding to 100 mol% of the total amount of diamine as a diamine was dissolved at room temperature. To this, 4,4'-(hexafluoroisopropyridene) diphthalic anhydride corresponding to 100 mol% with respect to the total amount of diamine was added over 30 minutes, and after the total amount was added, stirring was performed for about 2 hours. A precursor of aromatic polyimide (G) was polymerized. Further, the pH was adjusted to 10.0 with triethanolamine to obtain a solution having a precursor concentration of 10% by mass. The log viscosity η _inh of the obtained aromatic polyimide precursor was 2.5 dl / g.

After that, a heat-resistant porous film was obtained in the same manner as in Example 1. Table 1 shows the evaluation results of the obtained heat-resistant porous membrane.

(Example 12)
A laminated porous film was obtained in the same manner as in Example 8 except that a film-forming stock solution containing an aromatic polyimide precursor and inorganic particles obtained in the same manner as in Example 11 was used. Table 1 shows the evaluation results of the obtained laminated porous membrane.

(Example 13)
A laminated porous film was obtained in the same manner as in Example 8 except that the membrane-forming stock solution containing the aromatic polyamide and the inorganic particles obtained in the same manner as in Example 5 was used. Table 1 shows the evaluation results of the obtained laminated porous membrane.

(Example 14)
The film-forming stock solution containing the aromatic polyamide and the inorganic particles obtained in the same manner as in Example 7 was continuously applied in a film form on both sides of the polyolefin porous film obtained in the same manner as in Example 8 by dip coating. bottom. Next, the treatment was carried out in a temperature-controlled humidity-controlled air having a temperature of 90 ° C. and a relative humidity of 95% RH for 10 seconds, and then introduced into a water bath at 30 ° C. Subsequently, the obtained water-containing film is heat-treated in a hot air oven at 80 ° C. for 10 minutes to form a heat-resistant porous film having a thickness of 2 μm on each side (total thickness of 4 μm on both sides) on both sides of the polyolefin porous film. The formed laminated porous film was obtained. Table 1 shows the evaluation results of the obtained laminated porous membrane.

(Example 15)
A heat-resistant porous film was obtained in the same manner as in Example 1 except that the pH of the aromatic polyamide solution was set to 7.0. Table 1 shows the evaluation results of the obtained heat-resistant porous membrane.

(Comparative Example 1)
The diamine for obtaining the aromatic polyamide (F) was 4,4'-diaminodiphenyl ether corresponding to 100 mol% with respect to the total amount of diamine, and the acid dichloride was terephthalo corresponding to 99 mol% with respect to the total amount of diamine. A heat-resistant porous film was obtained in the same manner as in Example 1 except that it was an ilchloride. The surface of the obtained heat-resistant porous film was roughened due to the aggregation of inorganic particles, and powder was found to fall off. Table 1 shows the evaluation results of the obtained heat-resistant porous membrane.

(Comparative Example 2)
A heat-resistant porous film in the same manner as in Example 1 except that the aromatic polyamide (F) is used and the content of the alumina particles is 70% by mass with respect to the total amount of the aromatic polyamide and the alumina particles in the film-forming stock solution. Got Table 1 shows the evaluation results of the obtained heat-resistant porous membrane.

(Comparative Example 3)
A heat-resistant porous film was obtained in the same manner as in Example 1 except that the content of the alumina particles was 50% by mass with respect to the total amount of the aromatic polyamide and the alumina particles in the film-forming stock solution. Table 1 shows the evaluation results of the obtained heat-resistant porous membrane.

(Comparative Example 4)
A laminated porous film was obtained in the same manner as in Example 8 except that a film-forming stock solution containing an aromatic polyamide and inorganic particles obtained in the same manner as in Comparative Example 2 was used. Table 1 shows the evaluation results of the obtained laminated porous membrane.

(Comparative Example 5)
A laminated porous film was obtained in the same manner as in Example 8 except that a film-forming stock solution containing an aromatic polyamide and inorganic particles obtained in the same manner as in Comparative Example 3 was used. Table 1 shows the evaluation results of the obtained laminated porous membrane.

The heat-resistant porous film of the present invention is characterized in that it has a high content of inorganic particles and is excellent in dispersibility and supportability, so that the film resistance is low. Therefore, good output characteristics and cycle characteristics can be obtained when used as a separator for a secondary battery. Further, since it has excellent compression resistance and heat resistance, it is possible to maintain the insulation between the positive and negative electrodes even if pressure is applied to the inside of the battery for some reason or the battery is continuously exposed to high temperature. Further, the laminated porous membrane in which the heat-resistant porous membrane of the present invention is formed on at least one of the porous membrane base materials containing a polyolefin resin has the above-mentioned characteristics of the heat-resistant porous membrane in addition to the shut-down function of the base material. Since it is a laminated porous membrane having excellent low resistance, compression resistance, and heat resistance, it can be suitably used as a separator for a secondary battery for a wide range of applications.

Claims (4)

A laminated porous film in which a porous film is formed on at least one surface of a porous film base material containing a polyimide resin, wherein the porous film contains an aromatic heat-resistant resin and inorganic particles, and is an inorganic particle. aromatic content is 60 to 99 wt%, have a Fang aromatic heat resistant resin is an electron withdrawing group on the aromatic ring, and a logarithmic viscosity (η _inh) 1.5~7.0dl / g A laminated porous film which is a polyamic acid which is a group polyamide, an aromatic polyimide or a precursor thereof. Porous membrane thickness direction of 500 ° C. of thermal shrinkage is -0.5～15.0%, the laminated porous film according to claim 1. The laminated porous film according to claim 1 or 2 , wherein the film resistance at 25 ° C. is 1.6 to 9.6 Ωcm ^2. The laminated porous membrane according to any one of claims 1 to 3, wherein the heat shrinkage rate at 150 ° C. in at least one direction in the in-plane direction is −0.5 to 20%.