JP7176249B2 - Porous film, secondary battery separator and secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to a porous film, a secondary battery separator, and a secondary battery.

Secondary batteries such as lithium-ion batteries are widely used in portable digital devices such as smartphones, tablets, mobile phones, laptops, digital cameras, digital video cameras, and portable game consoles, as well as in portable devices such as power tools, electric motorcycles, and power-assisted bicycles. It is widely used in equipment and automotive applications such as electric vehicles, hybrid vehicles, and plug-in hybrid vehicles.

Lithium ion batteries generally have a secondary battery separator and an electrolyte interposed between a positive electrode in which a positive electrode active material is laminated on a positive electrode current collector and a negative electrode in which a negative electrode active material is laminated on a negative electrode current collector. have a configuration.

Polyolefin-based porous substrates are used as secondary battery separators. The characteristics required for secondary battery separators are the ability to contain electrolyte in the porous structure and enable ion movement, and the ability to close the porous structure by melting when the lithium ion battery overheats. It has a shutdown characteristic that stops the power generation by stopping the movement of ions.

Furthermore, in the manufacturing process of the secondary battery, in order to maintain the laminate when transporting the laminate in which the positive electrode, the separator, and the negative electrode are laminated, or the wound laminate of the positive electrode, the separator, and the negative electrode is cylindrically shaped, When inserting into a rectangular can, etc., the laminate is hot-pressed before insertion. is placed in a can to increase the energy density, and in the laminate type, to prevent the shape from deforming after being inserted into the exterior material, the adhesiveness between the separator and the electrode before impregnation with the electrolyte is It has been demanded.

On the other hand, lithium ion batteries are also required to have excellent battery characteristics such as high output and long life, and are required to exhibit good battery characteristics without deteriorating battery characteristics.

In response to these demands, in Patent Document 1, by laminating a heat-resistant porous layer containing a particulate organic binder and an inorganic filler, both ion permeability and adhesion to electrodes are achieved. In Patent Document 2, by laminating an adhesive layer formed on a heat-resistant layer, both adhesion to electrodes and blocking resistance are achieved. Further, in Patent Document 3, when a force curve based on the pressing force is created using an atomic force microscope (AFM), a thermoplastic layer is laminated that defines the deflection amount of the cantilever calculated from the force curve. It is said that the adhesiveness with the electrode and the high-temperature storage characteristics are improved.

Japanese Patent No. 564378 Japanese Patent No. 6191597 JP 2017-147050 A

As described above, the adhesion between the electrode and the separator is required by the heat press process in the manufacturing process of the secondary battery. In addition, excellent battery characteristics are also required, and it is necessary to achieve both adhesion and battery characteristics.

SUMMARY OF THE INVENTION An object of the present invention is to provide a porous film having adhesion to an electrode and excellent battery characteristics.

Accordingly, the present inventors have made extensive studies in order to provide a porous film that has adhesiveness to electrodes and excellent battery characteristics. As a result, in the prior arts such as Patent Documents 1 to 3, by providing an adhesive layer, the adhesive layer has adhesiveness to the electrode active material. The present inventors have found that filling the voids in the material or the separator reduces the porosity and the ion transport rate, thereby degrading the battery characteristics, leading to the present invention.

In order to solve the above problems, the porous film of the present invention has the following configuration.

[1] A porous film in which a porous layer A is laminated on at least one side of a porous substrate, the porous film satisfying the following (1) to (3).
(1) the air permeability of the porous film is 50 sec/100 cm ³ or more and 1000 sec/100 cm ³ or less;
(2) the air permeability of the porous film after heat treatment at 60°C for 30 minutes is 1.05 times or more the air permeability before heat treatment;
(3) After immersing the porous film subjected to the heat treatment of (2) in a solvent composed of at least one of ethylene carbonate, dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, and propylene carbonate at 25° C. for 24 hours Air permeability is 0.95 times or less before immersion.

[2] The porous film according to [1], wherein the porous film has an air permeability of 1000 sec/100 cm ³ or less after being immersed in the solvent of (3).

[3] The porous film according to [1] or [2], wherein the porous layer A contains at least one resin selected from the group consisting of acrylic resins, styrene resins, fluororesins, and olefin resins. .

[4] The porous film according to any one of [1] to [3], wherein the porous layer A has a thickness of 0.05 μm or more and 5 μm or less.

[5] The porous film according to any one of [1] to [4], wherein the porous layer A contains inorganic particles.

[6] The porous film according to any one of [1] to [5], wherein a porous layer B containing inorganic particles is laminated between the porous substrate and the porous layer A.

[7] The porous film according to any one of [1] to [6], wherein the porous layer A is laminated on both sides of the porous substrate.

[8] A secondary battery separator comprising the porous film according to any one of [1] to [7].

[9] A secondary battery using the secondary battery separator according to [8].

According to the present invention, a porous film in which a porous layer A is laminated on at least one side of a porous base material, and a porous film that satisfies the following (1) to (3) is used as an electrode and and excellent battery characteristics, it is possible to provide a porous film having adhesiveness to electrodes and excellent battery characteristics. By using the porous film of the present invention, it is possible to provide a secondary battery with high productivity, high capacity, high output, and long life.
(1) the air permeability of the porous film is 50 sec/100 cm ³ or more and 1000 sec/100 cm ³ or less;
(2) the air permeability of the porous film after heat treatment at 60°C for 30 minutes is 1.05 times or more the air permeability before heat treatment;
(3) The porous film subjected to the heat treatment of (2) above is immersed in a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate at 25 ° C. for 24 hours. 0.95 times or less.

A porous film in which a porous layer A is laminated on at least one side of a porous substrate, and the porous film satisfies the following (1) to (3), and has adhesiveness to an electrode. By forming a porous film having excellent battery characteristics, the porous film has adhesiveness to an electrode and excellent battery characteristics.
(1). the air permeability of the porous film is 50 sec/100 cm ³ or more and 1000 sec/100 cm ³ or less;
(2). The air permeability of the porous film after heat treatment at 60° C. for 30 minutes is 1.05 times or more the air permeability before heat treatment,
(3). The porous film subjected to the heat treatment of (2) was immersed in a solvent comprising at least one of dimethyl carbonate, ethylmethyl carbonate and diethyl carbonate at 25°C for 24 hours, and the air permeability was 0.00% before the immersion. 95 times or less.

The present invention will be described in detail below.

[Porous film]
(air permeability)
The porous film of the present invention has an air permeability of 50 sec/100 cm ³ or more and 1000 sec/100 cm ³ or less. It is preferably 80 sec/100 cm ³ or more and 800 sec/100 cm ³ or less, more preferably 100 sec/100 cm ³ or more and 500 sec/100 cm ³ or less, and still more preferably 100 sec/100 cm ³ or more and 300 sec/100 cm ³ or less. When the air permeability is 50 sec/100 cm ³ or more, sufficient mechanical properties can be obtained. In addition, when it is 1000 sec/100 cm ³ or less, it is possible to prevent impregnation of the electrolytic solution from deteriorating. If the air permeability is less than 50 sec/100 cm ³ , sufficient mechanical properties may not be obtained. Moreover, when it is larger than 1000 sec/100 cm ³ , the impregnating property of the electrolytic solution may deteriorate.

The air permeability of the porous film after heat treatment at 60° C. for 30 minutes is at least 1.05 times the air permeability before heat treatment. It is preferably 1.1 times or more, more preferably 1.2 times or more, still more preferably 1.5 times or more, and most preferably 2.0 times or more. The increase in air permeability due to heat treatment indicates that the resin contained in the porous layer A is softened by heating and film formation is promoted. When the air permeability is 1.05 times or more the air permeability before the heat treatment, the adhesiveness with the electrode is sufficient. If it is less than 1.05 times, the adhesion to the electrode may not be sufficient.

After immersing the heat-treated porous film in a solvent comprising at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate at 25° C. for 24 hours, the air permeability is 0.95 times or less that before immersion. be. It is preferably 0.9 times or less, more preferably 0.7 times or less, and still more preferably 0.5 times or less. When it is 0.95 times or less, it is easy to make porous by immersion in a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, sufficient ion mobility can be obtained, and battery characteristics are improved. can be improved. If it is more than 0.95 times, the pore formation by solvent immersion is insufficient, and sufficient ion mobility may not be obtained, resulting in deterioration of battery characteristics. Moreover, the air permeability of the porous film after being immersed in the solvent is preferably 1000 sec/100 cm ³ or less. It is more preferably 500 sec/100 cm ³ or less, still more preferably 300 sec/100 cm ³ or less. When it is 1000 sec/100 cm ³ or less, sufficient ion mobility can be obtained and battery characteristics can be improved. If it is larger than 1000 sec/100 cm ³ , sufficient ion mobility cannot be obtained, and battery characteristics may deteriorate.

[Porous layer A]
(organic resin)
The porous layer A in the present invention contains an organic resin as a main component. The organic resin in the present invention is preferably a film-forming resin that forms a film of the organic resin itself by heat treatment. The film-forming resin referred to herein is a resin having a minimum film-forming temperature of 20° C. or higher and 100° C. or lower and a glass transition temperature of 15° C. or higher and 100° C. or lower. Here, the minimum film-forming temperature is the lowest temperature at which a crack-free uniform film is formed when the resin emulsion is dried, for example, according to the provisions of "JIS K6828-2 Determination of whitening temperature and minimum film-forming temperature". Let temperature be

A resin having a minimum film-forming temperature of 15° C. or higher and 100° C. or lower and a glass transition temperature of 15° C. or higher and 100° C. or lower is preferable because high adhesiveness with the electrode can be obtained. A hot press process is often used for the process of adhering the electrode and the porous film, and in this case, the minimum film-forming temperature is 15° C. or higher and 100° C. or lower, and the glass transition temperature is 15° C. or higher and 100° C. or lower. Part of the porous layer enters the gaps between the active materials of the electrodes by resin, heat, or press, and exhibits an anchoring effect, thereby enabling adhesion to the electrodes, which is preferable. In the case of a resin having a minimum film-forming temperature higher than 100° C., sufficient adhesiveness to electrodes may not be obtained.

Organic resins constituting the porous layer of the present invention include acrylic resins, olefin resins such as polyethylene and polypropylene, styrene resins, crosslinked polystyrene, methyl methacrylate-styrene copolymers, polyimides, fluororesins, melamine resins, phenol resins, Polyacrylonitrile, silicone resin, urethane resin, polycarbonate, carboxymethyl cellulose resin, etc., may be mentioned, and only one of these may be used, or a plurality thereof may be used in combination. Among these, acrylic resins, olefin resins, styrene resins, fluororesins, urethane resins, and polyacrylonitrile are preferably used, and more preferably acrylic resins, styrene resins, and fluororesins, from the viewpoint of adhesion to electrodes. . These organic resins A may be used singly or in combination of two or more if necessary.

Although the shape of the organic resin constituting the porous layer of the present invention is not particularly limited, it is preferably in the form of particles from the viewpoint of improving battery performance by making it porous. The shape of the particles is not particularly limited, and may be spherical, polygonal, flattened, fibrous, or the like. It is preferable, and the closer to a true sphere is particularly preferable. In the case of particles, the average particle diameter of the particles is preferably 0.01 μm or more and 5 μm or less, more preferably 0.05 μm or more and 3 μm or less, and still more preferably 0.1 μm or more and 1 μm or less. When the average particle size is 0.01 μm or more, the porous structure can be prevented from becoming dense, and the battery characteristics can be improved. Further, when the thickness is 5 μm or less, it is possible to prevent the thickness of the entire porous film from becoming thick, thereby improving the battery characteristics. If the average particle size is less than 0.01 μm, the porous structure may become dense and the battery characteristics may deteriorate. On the other hand, if the thickness is larger than 5 μm, the thickness of the entire porous film becomes thick, and the battery characteristics may deteriorate.

In addition, the average particle diameter of the particles draws a square or rectangle with the smallest area that completely surrounds the particles observed by microscopic observation of the porous layer surface, that is, the edges of the particles are in contact with the four sides of the square or rectangle. Draw a square or rectangle with a square or rectangle, and measure the particle size of each of 100 randomly selected particles as the length of one side in the case of a square and the length of the long side (major axis diameter) in the case of a rectangle. The average value was taken as the average particle size.

(Inorganic particles)
The porous layer A of the present invention may contain inorganic particles. When the porous layer contains inorganic particles, it is possible to impart thermal dimensional stability and suppress short circuits due to foreign matter.

Specific examples of inorganic particles include inorganic oxide particles such as aluminum oxide, boehmite, silica, titanium oxide, zirconium oxide, iron oxide and magnesium oxide; inorganic nitride particles such as aluminum nitride and silicon nitride; Poorly soluble ionic crystal particles such as barium chloride and barium sulfate may be used. One type of these particles may be used, or two or more types may be mixed and used.

The average particle diameter of the inorganic particles used is preferably 0.05 μm or more and 5.0 μm or less. It is more preferably 0.10 μm or more and 3.0 μm or less, and still more preferably 0.20 μm or more and 1.0 μm or less. When the thickness is 0.05 μm or more, the porous layer can be prevented from becoming dense, and the air permeability can be lowered. In addition, since the pore diameter is increased, it is possible to prevent impregnation of the electrolytic solution from being deteriorated and improve productivity. When the thickness is 5.0 μm or less, sufficient dimensional stability can be obtained, and the film thickness of the porous layer can be reduced, thereby improving battery characteristics. If the thickness is less than 0.05 μm, the porous layer becomes dense, which may increase the air permeability. In addition, since the pore diameter becomes small, the impregnability of the electrolytic solution is lowered, which may affect the productivity. If it is larger than 5.0 μm, sufficient dimensional stability may not be obtained, and the film thickness of the porous layer may increase, resulting in deterioration of battery characteristics.

In addition, the average particle diameter of the particles draws a square or rectangle with the smallest area that completely surrounds the particles observed by microscopic observation of the porous layer surface, that is, the edges of the particles are in contact with the four sides of the square or rectangle. Draw a square or rectangle with a square or rectangle, and measure the particle size of each of 100 randomly selected particles as the length of one side in the case of a square and the length of the long side (major axis diameter) in the case of a rectangle. The average value was taken as the average particle size.

The shape of the particles used may be spherical, plate-like, needle-like, rod-like, elliptical, etc. Any shape may be used. Among them, a spherical shape is preferable from the viewpoint of surface modification properties, dispersibility, and coatability.

When inorganic particles are contained, the content in the entire porous layer A is preferably 50% by mass or more, more preferably 70% by mass or more, and still more preferably 80% by mass or more. When the content of the inorganic particles in the entire porous layer A is 50% by mass or more, thermal dimensional stability and suppression of short circuits due to foreign matter are sufficient.

(binder)
The porous layer A of the present invention may contain a binder resin in order to bind the organic resin and inorganic particles constituting the porous layer A. As the binder resin, a resin that is insoluble in the electrolyte of the battery and that is electrochemically stable within the range of use of the battery is preferred.

For example, polyamide, polyamideimide, polyimide, polyetherimide, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, polysulfone, polyketone, polyetherketone, polycarbonate, polyacetal, polyvinyl alcohol, polyethylene glycol , cellulose ether, sodium alginate, acrylic acid, acrylamide, methacrylic acid and the like. These binder resins may be used singly or in combination of two or more if necessary.

(Formation of porous layer A)
The porous film of the present invention is a porous film in which a porous layer A is laminated on at least one side of a porous substrate, and the porous film satisfies the following (1) to (3). A porous film having adhesiveness to an electrode and excellent battery characteristics can be obtained by a method for producing a porous film having adhesiveness to an electrode and excellent battery characteristics. , the method is described below.
(1) the air permeability of the porous film is 50 sec/100 cm ³ or more and 1000 sec/100 cm ³ or less;
(2) the air permeability of the porous film after heat treatment at 60°C for 30 minutes is 1.05 times or more the air permeability before heat treatment;
(3) The porous film subjected to the heat treatment of (2) above is immersed in a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate at 25 ° C. for 24 hours. 0.95 times or less.

The organic resin constituting the porous layer A is dispersed at a predetermined concentration to prepare a water-based dispersion coating liquid. The aqueous dispersion coating liquid is prepared by dispersing, suspending, or emulsifying an organic resin in a solvent. At least water is used as a solvent for the water-based dispersion coating liquid, and a solvent other than water may be added. The solvent other than water is not particularly limited as long as it does not dissolve the organic resin and can disperse, suspend or emulsify the organic resin in a solid state. Examples include organic solvents such as methanol, ethanol, 2-propanol, acetone, tetrahydrofuran, methyl ethyl ketone, ethyl acetate, N-methylpyrrolidone, dimethylacetamide, dimethylformamide and dimethylformamide. From the viewpoints of low environmental load, safety, and economy, water-based emulsions in which an organic resin is emulsified in water or a mixture of water and alcohol are preferred.

In addition, binders, film-forming aids, dispersants, thickeners, stabilizers, antifoaming agents, leveling agents, and the like may be added to the coating liquid, if necessary. A film-forming aid is added to adjust the film-forming property of the organic resin and improve the adhesion to the porous substrate. Specifically, propylene glycol, diethylene glycol, ethylene glycol, butyl cellosolve acetate, butyl cellosolve, cellosolve acetate, texanol and the like. These film-forming aids may be used singly or in combination of two or more if necessary. The amount of film-forming aid added is preferably 0.1% by mass or more and 10% by mass or less, more preferably 1% by mass or more and 8% by mass or less, and still more preferably 2% by mass or more and 6% by mass with respect to the total amount of the coating liquid. % or less. By making it 0.1% by mass or more, sufficient film-forming properties can be obtained, and by making it 10% by mass or less, when the coating liquid is applied to a porous substrate, the coating liquid becomes porous. It is possible to prevent impregnation of the base material and improve productivity. If it is less than 0.1% by mass, sufficient film-forming properties may not be obtained, and if it is more than 10% by mass, the coating liquid becomes porous when applied to a porous substrate. The base material is impregnated, and productivity may decrease.

As a method for dispersing the coating liquid, a known method may be used. Ball mills, bead mills, sand mills, roll mills, homogenizers, ultrasonic homogenizers, high-pressure homogenizers, ultrasonic devices, paint shakers and the like. A plurality of these mixing and dispersing machines may be combined for stepwise dispersion.

Next, the obtained coating liquid is applied onto the porous substrate and dried to laminate a porous layer. As a coating method, a known method may be used. For example, dip coating, gravure coating, slit die coating, knife coating, comma coating, kiss coating, roll coating, bar coating, spray coating, dip coating, spin coating, screen printing, inkjet printing, pad printing and other kinds of printing etc. is available. The coating method is not limited to these, and the coating method may be selected according to preferable conditions such as the organic resin, binder, dispersant, leveling agent, solvent to be used, and base material. In order to improve coatability, for example, the porous substrate may be subjected to surface treatment such as corona treatment or plasma treatment.

The content of the organic resin A in the porous layer A is preferably 1% by mass or more and 100% by mass or less, more preferably 5% by mass or more and 100% by mass or less, based on 100% by mass of the entire porous layer. More preferably, it is 10% by mass or more and 100% by mass or less. When the content of the organic resin A in the porous layer A is 1% by mass or more, sufficient adhesiveness to the electrode can be obtained. If the content of the organic resin A in the porous layer A is less than 1% by mass, sufficient adhesion to the electrode may not be obtained.

The film thickness of the porous layer A is preferably 0.05 μm or more and 5 μm or less. More preferably, it is 0.10 μm or more and 3 μm or less. More preferably, it is 0.2 μm or more and 1 μm or less. The film thickness of the porous layer A as used herein refers to the film thickness of the porous layer A in the case of a porous film having the porous layer A on one side of the porous substrate. In the case of a porous film having a porous layer A inside, it means the sum of the film thicknesses of both porous layers A. When the film thickness of the porous layer A is 0.05 μm or more, sufficient adhesiveness to the electrode can be obtained. Further, when the particle size is 5 μm or less, a sufficient porous structure can be obtained, and the battery characteristics can be improved. Moreover, it is advantageous in terms of cost. If the film thickness of the porous layer A is less than 0.05 μm, sufficient adhesiveness to the electrode may not be obtained. On the other hand, when the thickness is more than 5 μm, a sufficient porous structure may not be obtained, and the battery characteristics may deteriorate. Moreover, it may be disadvantageous in terms of cost.

[Porous layer B]
In the porous film of the present invention, a porous layer B containing inorganic particles may be laminated between the porous substrate and the porous layer A. For the porous layer B, the same inorganic particles, binders and other additives as those for the porous layer A may be used. Moreover, the porous layer B may be formed on one side or both sides.

The inorganic particles contained in the porous layer B are 50% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more. When the inorganic particles contained in the porous layer B are 50% by mass or more, sufficient thermal dimensional stability can be obtained, and short circuits caused by foreign matter can be suppressed. If the inorganic particles contained in the porous layer B are less than 50% by mass, sufficient thermal dimensional stability and suppression of short circuits due to foreign matter may not be achieved.

The method of laminating the porous layer B is not particularly limited. a method of immersing the porous substrate in the working solution, performing dip coating, and then removing the solvent; and the like.

When the porous layer B is laminated between the porous substrate and the porous layer A, the porous layer A may be laminated after the porous layer B is laminated on the porous substrate. After applying the coating liquid for the porous layer B, the coating liquid for the porous layer A may be further applied and dried to form a laminate. The porous layer B and the porous layer A may be coated and laminated at the same time.

The film thickness of the porous layer B is 0.5 μm or more, preferably 1 μm or more, more preferably 2 μm or more. When the film thickness of the porous layer B is 0.5 μm or more, sufficient thermal dimensional stability can be obtained, and short circuits due to foreign matter can be suppressed. If the film thickness of the porous layer B is less than 0.5 μm, sufficient thermal dimensional stability and suppression of short circuits due to foreign matter may not be achieved.

In addition, the film thickness of the porous layer B is obtained by observing the cross section with a microscope, and measuring the thickness from the interface between the porous base material and the porous layer B to the interface between the porous layer A and the porous layer B in the observation area. Measured as a distance, observed and measured at each of five randomly selected locations, and the average value was taken as the film thickness of the porous B film.

[Porous substrate]
In the present invention, the porous substrate refers to a substrate having pores inside. In the present invention, the porous substrate includes, for example, a porous film having pores therein, a nonwoven fabric, or a porous film sheet made of a fibrous material. The material constituting the porous substrate is preferably composed of a resin that is electrically insulating, electrically stable, and stable in the electrolytic solution. From the viewpoint of imparting a shutdown function, a thermoplastic resin having a melting point of 200° C. or less is preferable. The shutdown function here is a function that, when the lithium-ion battery overheats abnormally, closes the porous structure by melting with heat, stops the movement of ions, and stops power generation.

Examples of thermoplastic resins include polyolefin-based resins, and the porous substrate is preferably a polyolefin-based porous substrate. Further, the polyolefin-based porous substrate is more preferably a polyolefin-based porous substrate having a melting point of 200° C. or lower. Specific examples of polyolefin-based resins include polyethylene, polypropylene, copolymers thereof, and mixtures thereof. Examples include single-layer porous substrates containing 90% by mass or more of polyethylene, polyethylene and polypropylene. A multi-layered porous substrate consisting of

As a method for producing a porous base material, a method of forming a polyolefin resin into a sheet and then stretching it to make it porous, or a method of dissolving the polyolefin resin in a solvent such as liquid paraffin to form a sheet and then extracting the solvent. and a method of making it porous.

The thickness of the porous substrate is preferably 3 μm or more and 50 μm or less, more preferably 5 μm or more and 30 μm or less. When the thickness of the porous substrate is 50 µm or more, the internal resistance of the porous substrate can be reduced. Moreover, when the thickness of the porous substrate is 3 μm or more, the production is facilitated, and sufficient mechanical properties can be obtained. If the thickness of the porous substrate exceeds 50 μm, the internal resistance of the porous substrate may increase. In addition, when the thickness of the porous substrate is less than 3 μm, manufacturing becomes difficult, and sufficient mechanical properties may not be obtained.

The thickness of the porous substrate can be measured by microscopic observation of the cross section. When the porous layer is laminated, the perpendicular distance between the interface between the porous substrate and the porous layer is measured as the thickness of the porous substrate. Observations and measurements were made on each of five randomly selected locations, and the average value was taken as the thickness of the porous substrate.

The air permeability of the porous substrate is preferably 50 sec/100 cm ³ or more and 1,000 sec/100 cm ³ or less. More preferably 50 sec/100 cm ³ or more and 500 sec/100 cm ³ or less. When the air permeability is 1,000 sec/100 cm ³ or less, sufficient ion mobility can be obtained and battery characteristics can be improved. can. When it is 50 seconds/100 cm ³ or more, sufficient mechanical properties can be obtained. If the air permeability is more than 1,000 sec/100 cm ³ , sufficient ion mobility cannot be obtained, and battery characteristics may deteriorate.

[Secondary battery]
The porous film of the present invention can be suitably used as a separator for secondary batteries such as lithium ion batteries. A lithium-ion battery has a configuration in which a secondary battery separator and an electrolyte are interposed between a positive electrode in which a positive electrode active material is laminated on a positive electrode current collector and a negative electrode in which a negative electrode active material is laminated on a negative electrode current collector. there is

_The positive electrode is obtained by laminating a positive electrode material composed of an active material, a binder _resin , and a conductive aid _on a current collector. Lithium-containing transition metal oxides having a layered structure, spinel-type manganese oxides such as LiMn ₂ O ₄ , and iron-based compounds such as LiFePO ₄ can be used. A resin having high oxidation resistance may be used as the binder resin. Specific examples include fluorine resins, acrylic resins, styrene-butadiene resins, and the like. Carbon materials such as carbon black and graphite are used as conductive aids. As the current collector, a metal foil is suitable, and aluminum is often used in particular.

The negative electrode is obtained by laminating a negative electrode material composed of an active material and a binder resin on a current collector, and the active material includes carbon materials such as artificial graphite, natural graphite, hard carbon and soft carbon, tin and silicon. , metal materials such as Li, and lithium titanate (Li ₄ Ti ₅ O ₁₂ ). A fluorine resin, an acrylic resin, a styrene-butadiene resin, or the like is used as the binder resin. As the current collector, metal foil is suitable, and copper foil is often used in particular.

The electrolytic solution serves as a field for transferring ions between the positive electrode and the negative electrode in the secondary battery, and is made by dissolving the electrolyte in an organic solvent. Examples of electrolytes include LiPF ₆ , LiBF ₄ , and LiClO ₄ , and LiPF ₆ is preferably used from the viewpoint of solubility in organic solvents and ionic conductivity. Examples of the organic solvent include ethylene carbonate, propylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, etc. Two or more of these organic solvents may be used in combination.

As a method for producing a secondary battery, first, an active material and a conductive agent are dispersed in a binder solution to prepare an electrode coating liquid, and this coating liquid is applied onto a current collector, and the solvent is dried. Thus, a positive electrode and a negative electrode are obtained. The film thickness of the coating film after drying is preferably 50 μm or more and 500 μm or less. A secondary battery separator is placed between the obtained positive electrode and negative electrode so as to be in contact with the active material layer of each electrode, enclosed in an outer packaging material such as an aluminum laminate film, and after injecting an electrolytic solution, the negative electrode lead and the Install a safety valve and seal the exterior material. The secondary battery thus obtained has high adhesion to the electrode, has excellent battery characteristics, and can be manufactured at low cost.

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited by these examples. The measurement methods used in this example are shown below.

[Measuring method]
(1) Initial air permeability Select one place randomly extracted from each of three samples of 100 mm × 100 mm size, and measure the Oken type air permeability measurement device (EG01-5-1MR manufactured by Asahi Seiko Co., Ltd.). was used to measure in accordance with JIS P 8117 (2009), and the average value was defined as air permeability (sec/100 cm ³ ).

(2) Air permeability change rate after heat treatment Three samples of 100 mm × 100 mm size were each subjected to heat treatment at 60 ° C. for 30 minutes, and each sample was randomly selected and one place was selected. Using a degree measuring device (EG01-5-1MR manufactured by Asahi Seiko Co., Ltd.), the air permeability was measured according to JIS P 8117 (2009), and the average value was taken as the air permeability (sec/100 cm ³ ). Using the obtained air permeability and initial air permeability, the air permeability change rate after heat treatment was calculated from the following formula.
Air permeability change rate after heat treatment=Air permeability after heat treatment/Initial air permeability (3) Air permeability change rate after immersion in solvent The sample was completely immersed in a solvent composed of at least one kind of diethyl carbonate at 25° C. for 24 hours. After that, take out the sample, dry it, select one place randomly extracted from each sample, and use the Oken type air permeability measuring device (EG01-5-1MR manufactured by Asahi Seiko Co., Ltd.), It was measured in accordance with JIS P 8117 (2009), and the average value was taken as air permeability (sec/100 cm ³ ). Using the obtained air permeability and the air permeability after heat treatment, the rate of change in air permeability after solvent immersion was calculated from the following equation.

Change rate of air permeability after immersion in solvent = Air permeability after immersion in solvent / Air permeability after heat treatment (4) Film thickness of porous layer A Observed with a microscope (S-800 manufactured by Hitachi, Ltd., accelerating voltage 26 kV), the highest point from the interface with the porous substrate is the thickness, and if it is on one side, it is only on one side, and if it is on both sides, both sides. The film thickness of the porous layer A was obtained by measuring the total thickness. Five locations randomly selected from a 100 mm x 100 mm size sample were measured and averaged.

(5) Adhesiveness to Electrodes The active material is Li(Ni _5/10 Mn _2/10 Co _3/10 )O ₂ , the binder is vinylidene fluoride resin, and the conductive aid is acetylene black and graphite positive electrode 15 mm×100 mm. The porous film is placed so that the active material and the porous layer are in contact with each other, hot pressed at 0.5 MPa, 80° C., 0.2 m/min with a hot roll press, and manually peeled off using tweezers. The adhesive strength was evaluated in the following four stages. Similarly, the adhesive strength between the negative electrode and the porous film, in which the active material is graphite, the binder is vinylidene fluoride resin, and the conductive aid is carbon black, was also measured, and the positive electrode and the negative electrode were each evaluated to obtain the adhesive strength.
・Adhesion strength ◎: The electrode and the porous film side were separated with a strong force ・Adhesion strength ○: The electrode and the porous film were separated with a slightly strong force ・Adhesion strength △: The electrode and the porous film were separated with a weak force ・Adhesion strength ×: The electrode and the porous film were separated with extremely weak force.

(6) Battery production The positive electrode sheet contains 92 parts by mass of Li(Ni _5/10 Mn _2/10 Co _3/10 )O ₂ as a positive electrode active material, and 2.5 parts by mass of acetylene black and graphite as positive electrode conductive aids. Each positive electrode slurry was prepared by dispersing 3 parts by mass of polyvinylidene fluoride as a positive electrode binder in N-methyl-2-pyrrolidone using a planetary mixer. (Coating basis weight: 9.5 mg/cm ² ).

This positive electrode sheet was cut into a size of 40 mm×40 mm. At this time, a current-collecting tab bonding portion without an active material layer was cut out to a size of 5 mm×5 mm outside the active material surface. An aluminum tab having a width of 5 mm and a thickness of 0.1 mm was ultrasonically welded to the tab bonding portion.

For the negative electrode sheet, 98 parts by mass of natural graphite as a negative electrode active material, 1 part by mass of carboxymethyl cellulose as a thickener, and 1 part by mass of a styrene-butadiene copolymer as a negative electrode binder are dispersed in water using a planetary mixer. The resulting negative electrode slurry was coated on a copper foil, dried, and rolled to prepare a negative electrode slurry (coating basis weight: 5.5 mg/cm ² ).

This negative electrode sheet was cut into a size of 45 mm×45 mm. At this time, a current-collecting tab bonding portion without an active material layer was cut out to a size of 5 mm×5 mm outside the active material surface. A copper tab having the same size as the positive electrode tab was ultrasonically welded to the tab bonding portion.

Next, the porous film was cut into a size of 55 mm×55 mm, and the positive electrode and the negative electrode were superimposed on both sides of the porous film so that the active material layer separated the porous film, and the positive electrode coated portion was entirely opposed to the negative electrode coated portion. Arranged to obtain an electrode group. The positive electrode/negative electrode/porous film was sandwiched between a sheet of aluminum laminate film of 90 mm×200 mm, the long sides of the aluminum laminate film were folded, and the two long sides of the aluminum laminate film were heat-sealed to form a bag.

An electrolytic solution prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate:diethyl carbonate=1:1 (volume ratio) at a concentration of 1 mol/liter was used. 1.5 g of the electrolytic solution was poured into a bag-shaped aluminum laminate film, and the short sides of the aluminum laminate film were heat-sealed while being impregnated under reduced pressure to obtain a laminate type battery.

(7) Discharge load characteristics Discharge load characteristics were tested according to the following procedure, and evaluated by the discharge capacity retention rate.

Using the laminate type battery, the discharge capacity when discharged at 0.5 C at 25 ° C. and the discharge capacity when discharged at 10 C were measured. discharge capacity)×100 to calculate the discharge capacity retention rate. Here, the charging conditions were constant current charging at 0.5 C and 4.3 V, and the discharging conditions were constant current discharge at 2.7 V. Five laminate-type batteries were produced, and the average of the measurement results of the three batteries after removing the maximum and minimum discharge capacity retention rates was taken as the capacity retention rate. When the discharge capacity retention rate is less than 55%, it is x, when it is 55% or more and less than 65%, it is ◯, and when it is 65% or more, it is ⊚.

(8) Charge-Discharge Cycle Characteristics Charge-discharge cycle characteristics were tested according to the following procedure, and evaluated by the discharge capacity retention rate.

<1st to 300th cycles>
Charging and discharging were set as one cycle, charging conditions were constant current charging at 2C and 4.3V, and discharging conditions were constant current discharging at 2C and 2.7V, and charging and discharging were repeated 300 times at 25°C.

<Calculation of discharge capacity maintenance rate>
The discharge capacity retention rate was calculated by (discharge capacity at 300th cycle)/(discharge capacity at 1st cycle)×100. Five laminate-type batteries were produced, and the average of the measurement results of the three batteries after removing the maximum and minimum discharge capacity retention rates was taken as the capacity retention rate. When the discharge capacity retention rate was less than 60%, it was rated as x;

(Example 1)
A water-based emulsion coating liquid in which film-forming particles having a minimum film-forming temperature of 40°C, a glass transition temperature of 50°C, and an average particle diameter of 0.15 µm are dispersed by emulsion polymerization as a main component of methacrylic acid/acrylic acid ester. adjusted. This coating liquid was coated on both sides of a polyethylene porous substrate (thickness 7 μm, air permeability 110 seconds/100 cm ³ ) using a wire bar, and placed in a hot air oven (drying temperature set 50° C.). The film was dried until the solvent was volatilized to form a porous layer A, thereby obtaining the porous film of the present invention. Regarding the obtained porous film, the initial air permeability, the air permeability change rate after heat treatment, the air permeability change rate after immersion in a solvent, the film thickness of the porous layer A, the adhesion to the electrode, the discharge load characteristics and Table 1 shows the measurement results of cycle characteristics.

(Example 2)
A porous film of the present invention was obtained in the same manner as in Example 1, except that film-forming particles having a minimum film-forming temperature of 45°C and a glass transition temperature of 70°C were used.

(Example 3)
A porous film of the present invention was obtained in the same manner as in Example 1, except that film-forming particles having a minimum film-forming temperature of 60°C and a glass transition temperature of 85°C were used.

(Example 4)
A porous film of the present invention was obtained in the same manner as in Example 1, except that the film thickness of the porous layer A was 5.0 μm.

(Example 5)
A porous film of the present invention was obtained in the same manner as in Example 1, except that the film thickness of the porous layer A was 2.0 μm.

(Example 6)
A porous film of the present invention was obtained in the same manner as in Example 1, except that the film thickness of the porous layer A was 0.05 μm.

(Example 7)
A coating liquid B was prepared by dispersing 95% by mass of alumina particles (average particle diameter of 0.4 μm) as inorganic particles and 5% by mass of acrylic resin as a binder in water. This coating solution was applied onto a polyethylene porous substrate (thickness 7 μm, air permeability 110 sec/100 cm ³ ) using a wire bar, and the solvent contained was was dried until volatilized to form a porous layer B. After that, the coating liquid prepared in Example 1 was applied onto the porous layer B and dried in a hot air oven (drying setting temperature 50°C) until the contained solvent was volatilized, and the porous layer A was formed. to obtain the porous film of the present invention.

(Comparative example 1)
Except for using film-forming particles having a minimum film-forming temperature of 35° C. and a glass transition temperature of 45° C., which are composed of at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate and which are highly swellable in a solvent. , in the same manner as in Example 1 to obtain a porous film.

(Comparative example 2)
The coating liquid B used in Example 7 was applied onto a polyethylene porous substrate (thickness 7 μm, air permeability 110 sec/100 cm ³ ) using a wire bar, and dried in a hot air oven (drying temperature set 50° C.). and dried until the contained solvent was volatilized to form a porous layer B and obtain a porous film.

(Comparative Example 3)
A porous film was obtained in the same manner as in Example 1, except that film-forming particles having a minimum film-forming temperature of 5°C and a glass transition temperature of 10°C were used.

(Comparative Example 4)
A porous film was obtained in the same manner as in Example 1, except that film-forming particles having a minimum film-forming temperature of 80°C and a glass transition temperature of 95°C were used.

From Table 1, each of Examples 1 to 7 is a porous film in which a porous layer A is laminated on at least one side of a porous substrate, and the porous film satisfies the following (1) to (3). Since it is a film, sufficient electrode adhesion and good battery characteristics can be obtained.
(1) the air permeability of the porous film is 50 sec/100 cm ³ or more and 1000 sec/100 cm ³ or less;
(2) the air permeability of the porous film after heat treatment at 60°C for 30 minutes is 1.05 times or more the air permeability before heat treatment;
(3) The porous film subjected to the heat treatment of (2) above is immersed in a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate at 25 ° C. for 24 hours. 0.95 times or less.

On the other hand, in Comparative Example 1, the rate of change in air permeability after immersion in the solvent increased, suggesting that the porous film contained diethyl carbonate and swelled. As a result, the permeability of the porous film deteriorates, and good battery characteristics cannot be obtained. Comparative Example 2 has good battery characteristics, but does not provide sufficient adhesion to electrodes. In Comparative Example 3, since the initial air permeability is high, the air permeability after immersion in the solvent increases, and good battery characteristics cannot be obtained. Further, in Comparative Example 4, since the air permeability change rate after the heat treatment is low, sufficient adhesiveness to the electrode cannot be obtained.

Claims (9)

A porous film in which a porous layer A is laminated on at least one side of a porous substrate,
The porous layer A is mainly composed of an organic resin, and the organic resin is methacrylic acid/acrylic acid ester,
The minimum film-forming temperature of the organic resin is 40° C. or higher and 100° C. or lower, and the glass transition temperature is 50° C. or higher and 85 ° C. or lower,
A porous film that satisfies the following (1) to (3).
(1) the air permeability of the porous film is 50 sec/100 cm ³ or more and 1000 sec/100 cm ³ or less;
(2) the air permeability of the porous film after heat treatment at 60°C for 30 minutes is 1.05 times or more the air permeability before heat treatment;
(3) The porous film subjected to the heat treatment of (2) above is immersed in a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate at 25 ° C. for 24 hours. 0.95 times or less. 2. The porous film according to claim 1, wherein the organic resin is in the form of particles and has an average particle size of 0.01 [mu]m or more and 5 [mu]m or less. 3. The porous film according to claim 1, wherein the porous film has an air permeability of 1000 sec/100 cm ³ or less after being immersed in the solvent of (3). The porous film according to any one of claims 1 to 3, wherein the porous layer A has a thickness of 0.05 µm or more and 5 µm or less. 5. The porous film according to any one of claims 1 to 4, wherein the porous layer A contains inorganic particles. 6. The porous film according to any one of claims 1 to 5, wherein a porous layer B containing inorganic particles is laminated between the porous substrate and the porous layer A. 7. The porous film according to any one of claims 1 to 6, wherein the porous layer A is laminated on both sides of the porous substrate. A secondary battery separator comprising the porous film according to any one of claims 1 to 7. A secondary battery comprising the secondary battery separator according to claim 8 .