TWI750288B - Battery separator, electrode body and non-aqueous electrolyte secondary battery - Google Patents

Abstract

An object of the present invention is to provide a battery separator excellent in adhesiveness and short-circuit resistance.

The present invention relates to a battery separator comprising a polyolefin microporous membrane and a porous layer laminated on at least one side of the polyolefin microporous membrane, wherein the porous layer comprises a vinylidene fluoride-hexafluoropropylene copolymer (A), The vinylidene fluoride-hexafluoropropylene copolymer (B), and inorganic particles, the vinylidene fluoride-hexafluoropropylene copolymer (A) has 0.3 mol% or more and 5.0 mol% or less of hexafluoropropylene units, and the weight average The molecular weight is 900,000 or more and 2,000,000 or less, and contains a hydrophilic group, the vinylidene fluoride-hexafluoropropylene copolymer (B) has more than 5.0 mol% and 8.0 mol% of hexafluoropropylene units, and the weight average molecular weight is 100,000 Above and below 750,000.

Description

Battery separator, electrode body, and non-aqueous electrolyte secondary battery

The present invention relates to a battery separator, an electrode body, and a non-aqueous electrolyte secondary battery.

Non-aqueous electrolyte secondary batteries, especially lithium ion secondary batteries, are widely used in small electronic devices such as mobile phones and mobile information terminals. As a form of a nonaqueous electrolyte secondary battery, a cylindrical battery, a corner battery, a laminated battery, etc. are mentioned, for example. In general, these batteries have a structure in which a positive electrode and a negative electrode are disposed through an electrode body through a separator, and a non-aqueous electrolyte solution is accommodated in an exterior body. The structure of the electrode body includes, for example, a laminated electrode body in which a positive electrode and a negative electrode are laminated through a separator, and a wound electrode body in which the positive electrode and the negative electrode are wound through a separator in a spiral shape.

At present, as a separator for a battery, a microporous film containing a polyolefin resin is mainly used. Since the microporous membrane made of polyolefin resin has a so-called shutdown function, it can suppress the flow of electric current by blocking the pores of the separator when the battery is abnormally heated, thereby preventing fire and the like.

In recent years, in the field of battery separators, attempts have been made to improve battery characteristics by providing layers other than polyolefin resins on one or both sides of a layer containing a polyolefin resin. For example, a battery separator in which a porous layer containing a fluororesin is provided on one side or both sides of a layer containing a polyolefin resin has been proposed. In addition, it is known that by incorporating inorganic particles into the porous layer, even when a sharp metal penetrates the battery due to an accident or the like, causing a sudden short circuit and heat generation, the melting shrinkage of the separator can be prevented, and the short circuit between the electrodes can be suppressed. expansion of the department.

For example, Patent Document 1 describes an electrode body comprising a positive electrode, a negative electrode, and a polypropylene. Polyethylene. A three-layer separator of polypropylene, and an adhesive resin layer containing polyvinylidene fluoride and alumina powder disposed between these electrodes and the separator.

Furthermore, Example 1 of Patent Document 2 describes a separator comprising a VdF-HFP copolymer (HFP unit 0.6 mol %) and a VdF-HFP copolymer (weight average molecular weight 470,000 HFP unit 4.8 mol %) ) was dissolved in a solution of dimethylacetamide and tripropylene glycol, and applied to a polyethylene microporous membrane to form a porous layer.

Furthermore, Example 1 of Patent Document 3 describes a separator in which PVdF (weight average molecular weight 500,000) and VdF-HFP copolymer (weight average molecular weight 400,000, HFP unit 5 mol %) are dissolved in dimethyl A solution of ethylacetamide and tripropylene glycol was applied to a polyethylene microporous membrane to form a porous layer.

Furthermore, Example 1 of Patent Document 4 describes a separator in which PVdF (weight average molecular weight 700,000) and VdF-HFP copolymer (weight average molecular weight 470,000, HFP unit 4.8 mol %) are dissolved in dimethyl A solution of ethylacetamide and tripropylene glycol was applied to a polyethylene microporous membrane to form a porous layer.

In addition, Example 1 of Patent Document 5 describes a separator in which PVdF (weight average molecular weight 350,000) and VdF-HFP copolymer (weight average molecular weight 270,000, HFP unit 4.8 mol %) were dissolved in dimethyl A solution of ethylacetamide and tripropylene glycol was applied to a polyethylene microporous membrane to form a porous layer.

Furthermore, Example 23 of Patent Document 6 describes a separator prepared by combining a VdF-HFP copolymer (weight average molecular weight 1.93 million, HFP unit 1.1 mol %) and a VdF-HFP copolymer (weight average molecular weight 470,000 , HFP unit 4.8 mol%) was dissolved in dimethylacetamide and tripropylene glycol solution, further added the coating liquid of aluminum hydroxide, and coated it on the polyethylene microporous membrane to form a porous layer.

[Prior Art Literature] [Patent Literature]

[Patent Document 1] Japanese Re-List No. 1999-036981

[Patent Document 2] Japanese Patent No. 5282179

[Patent Document 3] Japanese Patent No. 5282180

[Patent Document 4] Japanese Patent No. 5282181

[Patent Document 5] Japanese Patent No. 5342088

[Patent Document 6] International Publication No. 2016/152863

In recent years, non-aqueous electrolyte secondary batteries have been expected to be used in large-scale applications such as large-scale tablet computers, lawn mowers, electric vehicles, electric vehicles, hybrid vehicles, and small ships. It is presumed that a further increase in capacity is required. All of the above-mentioned Patent Documents 1 to 5 aim to improve the adhesion between a separator including an electrolyte solution and an electrode, but when the size of the secondary battery is increased, further improvement in adhesion is required.

As will be described below, the inventors of the present invention have focused on evaluating the adhesion between the electrode and the separator by evaluating the adhesion between the electrode and the separator when dry and the adhesion between the electrode and the separator when wet. The two types of adhesive properties were evaluated separately to more accurately evaluate the adhesive properties, and it was found that these adhesive properties could be evaluated by using the peeling force when dry and the flexural strength when wet as indexes, respectively.

That is, for example, a wound electrode system is produced by winding the positive electrode and the negative electrode through a separator to apply tension to each member. At this time, the positive electrode or negative electrode applied to the metal current collector hardly expands and contracts with respect to the tension, but the separator is wound while being stretched to some extent in the machine direction. When the wound body is temporarily placed, the spacer portion gradually shrinks and returns to the original length. As a result, a force in a parallel direction is generated at the boundary surface between the electrode and the separator, and the wound electrode body (especially the flatly wound electrode body) tends to be bent or skewed. In addition, these problems become apparent due to the widening or elongated separators accompanying the enlargement of the battery, and there is a concern that the yield during production will deteriorate. In order to suppress the occurrence of deflection and distortion of the wound electrode body, the separator is required to have more adhesion to the electrode than ever before. In addition, when the electrode body is transported, unless each member is sufficiently bonded, the electrode and the separator are peeled off, and it cannot be transported with good yield. The problem of adhesion at the time of transportation becomes obvious due to the increase in size of the battery, and there is concern that the yield rate will deteriorate. Therefore, the separator is required to be difficult to peel from the electrode and to have a high peeling force during drying.

In addition, in the case of laminated batteries, it is difficult to apply pressure compared with angle-type and cylindrical-type batteries in which pressure is applied by the outer casing, due to the swelling of electrodes accompanying charging and discharging. Partial release at the interface between the separator and the electrode easily occurs due to shrinkage. As a result, the swelling of the battery, the increase in the internal resistance of the battery, and the decrease in the cycle performance are caused. Therefore, the adhesiveness between the separator and the electrodes in the battery after the injection of the electrolyte solution is required. In this specification, this adhesiveness is evaluated using the flexural strength at the time of wet obtained by the measurement method mentioned later as an index. It is considered that when this strength is large, improvement in battery characteristics such as suppression of battery swelling after repeated charge and discharge can be expected. In addition, the bending strength when wet in this specification means the adhesiveness of a separator and an electrode in the state in which the separator contains an electrolyte solution. The peeling force at the time of drying represents the adhesiveness to the boundary surface between the separator and the electrode in a state where the separator does not substantially contain an electrolyte solution. Moreover, substantially no electrolyte solution means that the electrolyte solution in a separator is 500 ppm or less.

However, the inventors found that, in the prior art, the adhesiveness between the electrode and the separator during drying, which is required for the manufacture or transportation of the electrode body, and the bonding between the electrode and the separator during the moistening required after the injection of the electrolyte solution There is a trade-off relationship between the adhesion of the two, and it is extremely difficult to satisfy the physical properties of both sides; and the technologies disclosed in the above-mentioned Patent Documents 1 to 5 have insufficient adhesion.

In addition, even if a sudden impact is applied to the battery, the protruding portion of the electrode active material penetrates the separator and is required to have a property that it is difficult to short-circuit the electrode (hereinafter, referred to as short-circuit resistance). However, when the film thickness of the battery separator is expected to be reduced in the future, it will be difficult to ensure short-circuit resistance as the separator is thinner. It is known that it is effective to contain a certain amount or more of inorganic particles in the porous layer in order to ensure short-circuit resistance. However, when the inorganic particles are contained to a degree that can ensure short-circuit resistance, there is a possibility that the adhesion between the electrode and the separator may be reduced. tendency.

The present invention has been made in view of the above-mentioned matters, and an object of the present invention is to provide a separator for batteries which is excellent in both the adhesiveness of the electrode and the separator when dry and the adhesiveness of the electrode and the separator when wet, and is excellent in short-circuit resistance, and a separator using the same. electrode bodies and secondary batteries.

In order to solve the above-mentioned problems, the inventors of the present invention have made intensive and repeated studies, and as a result found that the above-mentioned problems can be solved by a separator including a porous layer containing two kinds of fluorine-based resins with different structures, their blending ratios, and a specific amount of inorganic particles. The subject of the present invention has been completed.

That is, the present invention relates to a battery separator comprising a polyolefin microporous membrane and a porous layer laminated on at least one side of the polyolefin microporous membrane, wherein the porous layer contains vinylidene fluoride- A hexafluoropropylene copolymer (A), a vinylidene fluoride-hexafluoropropylene copolymer (B), and inorganic particles, the vinylidene fluoride-hexafluoropropylene copolymer (A) having a content of 0.3 mol % or more and 5.0 mol % or less of hexafluoropropylene units, the weight average molecular weight is 900,000 or more and 2,000,000 or less, and contains a hydrophilic group, and the vinylidene fluoride-hexafluoropropylene copolymer (B) has more than 5.0 mol % and 8.0 mol % or less of A hexafluoropropylene unit having a weight average molecular weight of 100,000 or more and 750,000 or less, relative to the sum of the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B) 100 mass %, containing 86 mass % or more and 98 mass % or less of the vinylidene fluoride-hexafluoropropylene copolymer (A), with respect to 100 volume % of the solid content in the porous layer, containing 40 volume % or more, 80 volume % volume % or less of the aforementioned inorganic particles.

Moreover, it is preferable that the said vinylidene fluoride-hexafluoropropylene copolymer (A) contains 0.1 mol% or more and 5.0 mol% or less of hydrophilic groups.

Moreover, it is preferable that the said vinylidene fluoride-hexafluoropropylene copolymer (B) system melting|fusing point is 60 degreeC or more and 145 degrees C or less.

Moreover, it is preferable that the said inorganic particle is 1 or more types selected from titania, alumina, and diaspore.

Moreover, it is preferable that the thickness of the said polyolefin microporous membrane is 3 micrometers or more and 16 micrometers or less.

Further, the present invention is an electrode body comprising a positive electrode, a negative electrode, and the battery separator of the present invention.

Further, the present invention is a non-aqueous electrolyte secondary battery including the electrode body of the present invention and a non-aqueous electrolyte.

According to the present invention, it is possible to provide a separator for a battery which is excellent in both the adhesion between the electrode and the separator when dry and the adhesion between the electrode and the separator when wet, and which is excellent in short-circuit resistance, and an electrode using the same. body and secondary battery.

1‧‧‧Polyolefin Microporous Film

2‧‧‧Porous layer

4‧‧‧Aluminum foil

5‧‧‧Resin insulators

6‧‧‧Metal Ball

10‧‧‧Isolators for batteries

20‧‧‧Negative electrode (for adhesion evaluation)

21‧‧‧Negative electrode (for short-circuit resistance evaluation)

22‧‧‧Laminated film

30‧‧‧Electrode winding body

31‧‧‧Electrode Laminate

41‧‧‧Aluminum L-shaped angle (lower side)

42‧‧‧Aluminum L-shaped angle (upper side)

43‧‧‧Compression fixture (upper side)

44‧‧‧Compression fixture (lower side)

Fig. 1 is a schematic diagram showing an example of the battery separator of the present embodiment.

FIG. 2 is a schematic diagram showing a method for evaluating the flexural strength when wet.

FIG. 3 is a schematic diagram showing the evaluation method of the short-circuit resistance test.

[Form of implementing the invention]

Hereinafter, embodiments of the present invention will be described with reference to the drawings. Hereinafter, the directions in the drawings will be described using the XYZ coordinate system. In this XYZ coordinate system, the plane parallel to the surface (in-plane direction) of the microporous membrane or the separator is referred to as the XY plane. In addition, the direction (thickness direction) perpendicular|vertical to XY plane is made into Z direction. The X direction, the Y direction, and the Z direction are respectively described as the + direction in the arrow direction in the figure, and the - direction in the opposite direction to the arrow direction. In addition, in the drawings, in order to facilitate understanding of each configuration, a part of the structure is emphasized or a part of the structure is simplified, and it may be different from the actual structure, shape, scale, and the like.

FIG. 1 is a view showing an example of the battery separator of the present embodiment. As shown in FIG. 1 , a battery separator 10 (hereinafter, abbreviated as “separator 10 ”) includes a polyolefin microporous membrane 1 and a porous layer 2 laminated on at least one surface of the polyolefin microporous membrane 1 . Hereinafter, each layer constituting the battery separator will be described.

[1] Polyolefin microporous membrane

The polyolefin microporous membrane 1 is a microporous membrane containing a polyolefin resin. The polyolefin microporous membrane 1 is not particularly limited, and polyolefin microporous membranes used for known battery separators can be used. In addition, in this specification, the microporous membrane means a membrane having voids connecting the inside. Hereinafter, an example of the polyolefin microporous membrane 1 will be described, but the polyolefin microporous membrane used in the present invention is not limited to this.

[Polyolefin resin]

Examples of the polyolefin resin constituting the polyolefin microporous film 1 (hereinafter, abbreviated as "microporous film 1") include ethylene, propylene, 1-butene, and 4-methyl-1-pentene. Homopolymers, 2-stage polymers, copolymers, or mixtures thereof etc. polymerized with ene, 1-hexene, and the like. Among them, as the polyolefin resin, a polyethylene resin is preferably used as the main component. The content of the polyethylene resin is preferably 70% by mass or more, more preferably 90% by mass or more, and even more preferably 100% by mass with respect to 100% by mass of the total mass of the polyolefin resin in the microporous film 1 . To the polyolefin resin, if necessary, various additives such as antioxidants and inorganic fillers can be added within a range that does not impair the effects of the present invention.

The film thickness of the polyolefin microporous membrane 1 is not particularly limited, but from the viewpoint of increasing the capacity of the battery, it is preferably 3 μm or more and 16 μm or less, more preferably 5 μm or more and 12 μm or less, still more preferably 5 μm or more and 10 μm the following. When the film thickness of the polyolefin microporous film is within the above-mentioned preferred range, practical film strength and pore blocking function can be maintained, which is more suitable for the increase in battery capacity expected to be developed in the future. That is, in the battery separator 10 of the present embodiment, even if the thickness of the polyolefin microporous membrane 1 is thin, an excellent separator 10 can be obtained between the polyolefin microporous membrane 1 and the porous layer 2, and between the separator 10 and the porous layer 2. The adhesiveness between electrodes exhibits its effect more clearly when the separator 10 is thinned.

The air impermeability of the polyolefin microporous film 1 is not particularly limited, but is preferably 50 sec/100 cm ³ Air or more and 300 sec/100 cm ³ Air or less. The porosity of the polyolefin microporous membrane 1 is not particularly limited, but is preferably 30% or more and 70% or less. The average pore diameter of the polyolefin microporous membrane 1 is not particularly limited, but from the viewpoint of pore blocking performance, it is preferably 0.01 μm or more and 1.0 μm or less.

[Manufacturing method of polyolefin microporous membrane]

The method for producing the microporous membrane 1 is not particularly limited as long as a polyolefin microporous membrane having desired properties can be produced, and a conventionally known method can be used. As the production method of the microporous membrane 1, for example, the methods described in Japanese Patent No. 2132327, Japanese Patent No. 3347835, International Publication No. 2006/137540, and the like can be used. Hereinafter, an example of the manufacturing method of the microporous membrane 1 is demonstrated. In addition, the manufacturing method of the microporous membrane 1 is not limited to the following method.

The manufacturing method of the microporous membrane 1 may include the following steps (1) to (5), and may further include the following steps (6) to (8).

(1) The step of preparing the polyolefin solution by melt-kneading the aforementioned polyolefin resin and the solvent for film formation

(2) The step of extruding the aforementioned polyolefin solution and cooling to form a colloidal sheet

(3) The first stretching step of stretching the gel sheet

(4) Step of removing the solvent for film formation from the stretched jelly sheet

(5) Step of drying the sheet from which the film-forming solvent has been removed

(6) The second stretching step of stretching the dried sheet

(7) The step of heat-treating the dried sheet

(8) A step of subjecting the sheet after the stretching step to a crosslinking treatment and/or a hydrophilization treatment.

Hereinafter, each step will be described separately.

(1) Preparation steps of polyolefin solution

After adding an appropriate film-forming solvent to the polyolefin resin, each is melt-kneaded to prepare a polyolefin solution. As a melt-kneading method, for example, a method using a twin-screw extruder described in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Since the melt-kneading method is well known, the description is omitted.

In the polyolefin solution, the blending ratio of the polyolefin resin and the film-forming solvent is not particularly limited, but is preferably 70 to 80 parts by mass of the film-forming solvent relative to 20 to 30 parts by mass of the polyolefin resin. When the ratio of the polyolefin resin is within the above range, expansion and shrinkage at the die outlet can be prevented when the polyolefin solution is extruded, and the moldability and self-supporting properties of the extrusion molded body (gel-like molded body) are improved. .

(2) Forming steps of gelatinous sheet

The polyolefin solution is supplied from the extruder to the die and extruded into a sheet. A plurality of polyolefin solutions of the same or different compositions can be supplied from an extruder to a die, where they are laminated into layers and extruded into sheets.

The extrusion method may be either a flat die method or a blow molding method. The extrusion temperature is preferably 140 to 250°C, and the extrusion speed is preferably 0.2 to 15 m/min. The film thickness can be adjusted by adjusting each extrusion amount of the polyolefin solution. As the extrusion method, for example, methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used.

A gelatinous sheet is formed by cooling the obtained extrusion molding. As a method of forming the jelly sheet, for example, methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Cooling is preferably performed to at least the gelling temperature at a rate of 50°C/min or more. Cooling is preferably performed to 25°C or lower. By cooling, the microphase of the polyolefin separated by the solvent for film formation can be immobilized. When the cooling rate is within the above range, the degree of crystallinity is maintained within an appropriate range, and a jelly sheet suitable for stretching is obtained. As the cooling method, a method of contacting with a refrigerant such as cold air and cooling water, a method of contacting a cooling roll, etc. can be used, and cooling is preferably performed by contacting a roll cooled with a refrigerant.

(3) 1st stretching step

Next, the obtained jelly-like sheet is stretched in at least uniaxial direction. Since the jelly sheet contains the film-forming solvent, it can be stretched uniformly. The gel sheet is preferably heated and then stretched at a predetermined ratio by a tenter method, a roll method, a blow molding method, or a combination thereof. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching, and multi-stage stretching (eg, a combination of simultaneous biaxial stretching and sequential stretching) may be used.

The draw ratio (area draw ratio) in this step is preferably 9 times or more, more preferably 16 times or more, and particularly preferably 25 times or more. In addition, the stretching ratios in the machine direction (MD) and the width direction (TD) may be the same or different from each other. In addition, the draw ratio in this step refers to the area draw ratio of the microporous film immediately before the next step, with the microporous film immediately before the step as a reference.

The stretching temperature in this step is preferably set within the range of the crystal dispersion temperature of the polyolefin resin (Tcd) ~ Tcd+30°C, more preferably within the crystal dispersion temperature (Tcd)+5°C ~ The crystal dispersion temperature (Tcd )+28°C, and it is particularly preferable to set it within the range of Tcd+10°C to Tcd+26°C. For example, in the case of polyethylene, the stretching temperature is preferably 90 to 140°C, and more preferably 100 to 130°C. The crystal dispersion temperature (Tcd) was obtained by measuring the temperature characteristics of dynamic viscoelasticity according to ASTM D4065.

By the above-mentioned stretching, the polyethylene lamella is cleaved, the polyethylene phase is refined, and many fibrils are formed. The fibrils form a three-dimensionally irregularly connected mesh structure. By stretching, the mechanical strength is improved, and the pores are enlarged at the same time. If stretching is performed under appropriate conditions, the through hole diameter can be controlled, and even a thinner film thickness has a high porosity.

Depending on desired physical properties, a microporous film excellent in mechanical strength can be obtained by stretching the film while setting a temperature distribution in the film thickness direction. The details of the method are described in Japanese Patent No. 3347854.

(4) Removal of the solvent for film formation

The removal (washing) of the solvent for film formation is performed using a washing solvent. Since the polyolefin phase is phase-separated from the film-forming solvent, when the film-forming solvent is removed, a porous structure including fibrils forming a fine three-dimensional network structure and having pores (voids) that are irregularly connected three-dimensionally can be obtained quality membrane. Since the cleaning solvent and the method of removing the film-forming solvent using the same are well known, the description is omitted. For example, the method disclosed in Japanese Patent No. 2132327 or Japanese Patent Laid-Open No. 2002-256099 can be used.

(5) Drying

The microporous membrane from which the solvent for film formation has been removed is dried by a heat drying method or an air drying method. The drying temperature is preferably lower than or equal to the crystal dispersion temperature (Tcd) of the polyolefin resin, and particularly preferably lower than Tcd by 5°C or more. The drying system is performed until the microporous membrane is 100% by mass (dry weight), preferably until the residual cleaning solvent becomes 5% by mass or less, more preferably 3% by mass or less. When the residual cleaning solvent is within the above range, the porosity of the microporous film can be maintained during the stretching step and the heat treatment step of the microporous film in the latter stage, and the deterioration of the permeability can be suppressed.

(6) Second stretching step

Preferably, the dried microporous membrane is stretched in at least uniaxial direction. The stretching of the microporous film can be performed by a tenter method or the like in the same manner as above while heating. Stretching may be uniaxial stretching or biaxial stretching. In the case of biaxial stretching, any of simultaneous biaxial stretching and sequential stretching may be used. The stretching temperature in this step is not particularly limited, but is usually preferably 90 to 135°C, more preferably 95 to 130°C. In the case of uniaxial stretching, the stretching ratio in the uniaxial direction (area stretching ratio) of the microporous membrane in this step is preferably 1.0 to 2.0 in the machine direction or the width direction. times. In the case of biaxial stretching, the lower limit of the area stretching ratio is preferably 1.0 times, more preferably 1.1 times, and even more preferably 1.2 times. The upper limit is preferably 3.5 times, 1.0 to 2.0 times in the machine direction and the width direction, respectively, and the stretching ratios in the machine direction and the width direction may be the same or different from each other. In addition, the draw ratio in this step refers to the draw ratio of the microporous membrane immediately before the next step, based on the microporous membrane immediately before the present step.

(7) Heat treatment

In addition, the dried microporous membrane can be heat-treated. By heat treatment, the crystals are stabilized and the lamellae are uniformized. As a heat treatment method, a heat setting treatment and/or a heat relaxation treatment can be used. The heat setting treatment refers to a heat treatment in which heating is performed while keeping the dimensions of the film unchanged. The thermal relaxation treatment refers to a heat treatment of thermally shrinking the film in the machine direction or the width direction in heating the film. The heat setting treatment is preferably performed by a tenter method or a roll method. For example, as a thermal relaxation treatment method, the method disclosed in Japanese Patent Application Laid-Open No. 2002-256099 can be mentioned. The heat treatment temperature is preferably in the range of Tcd~Tm of the polyolefin resin, more preferably in the range of ±5°C of the stretching temperature of the microporous film, particularly preferably in the range of the second stretching temperature of the microporous film ±3 within the range of °C.

(8) Cross-linking treatment, hydrophilization treatment

In addition, a crosslinking treatment and a hydrophilization treatment can be further performed on the microporous membrane after bonding or stretching. For example, the crosslinking treatment is performed by irradiating the microporous membrane with ionizing radiation such as α rays, β rays, γ rays, and electron rays. In the case of irradiating electron beams, the amount of electron beams is preferably 0.1 to 100 Mrad, and an accelerating voltage of 100 to 300 kV is preferred. By the cross-linking treatment, the melting temperature of the microporous membrane increases. In addition, the hydrophilization treatment can be performed by monomer grafting, surfactant treatment, corona discharge, or the like. The monomer grafting is preferably carried out after the crosslinking treatment.

[2] Porous layer

The porous layer 2 contains two kinds of vinylidene fluoride-hexafluoropropylene copolymers (VdF-HFP), and inorganic particles. Hereinafter, each component constituting the porous layer 2 will be described below.

[Vinylidene fluoride-hexafluoropropylene copolymer (A)]

The vinylidene fluoride-hexafluoropropylene copolymer (A) (hereinafter, it may be simply referred to as the copolymer (A)) is a copolymer containing a vinylidene fluoride unit and a hexafluoropropylene unit, and as described later, contains a hydrophilic base. The content of the hexafluoropropylene units in the copolymer (A) is 0.3 mol % or more, preferably 0.5 mol % or more. When the content of the hexafluoropropylene unit is smaller than the above range, the crystallinity of the polymer becomes high, and the degree of swelling of the separator with respect to the electrolyte solution becomes low, so that the adhesion between the separator and the electrode decreases, and there is a possibility that the injected electrolyte solution cannot be sufficiently obtained. Adhesion (flexural strength when wet) between the electrode and the separator after that. On the other hand, the content of the hexafluoropropylene units is 5.0 mol % or less, more preferably 2.5 mol % or less. When the content of the hexafluoropropylene unit exceeds the above-mentioned range, the separator may swell excessively with respect to the electrolyte solution, and the flexural strength may decrease when wetted.

The weight average molecular weight of the copolymer (A) is 900,000 or more, preferably 1,000,000 or more. On the other hand, the weight average molecular weight of the copolymer (A) is 2 million or less, more preferably 1.5 million or less. When the weight average molecular weight of the copolymer (A) is in the above range, in the step of forming the porous layer, the time for dissolving the copolymer (A) in the solvent does not become extremely long, and the production efficiency can be improved, or the production efficiency can be improved. Maintaining a moderate adhesive strength when swollen in an electrolyte solution can improve the flexural strength when wet. In addition, the weight average molecular weight of the copolymer (A) is a polystyrene conversion value based on gel permeation chromatography.

The copolymer (A) has a hydrophilic group. Since the copolymer (A) has a hydrophilic group, it becomes possible to more firmly adhere to the active material present on the electrode surface or the binder component in the electrode. The reason for this is not clear, but it is presumed that the adhesion force is improved by hydrogen bonding. As a hydrophilic group, a hydroxyl group, a carboxylic acid group, a sulfonic acid group, these salts, etc. are mentioned, for example. Among them, a carboxylic acid group and a carboxylic acid ester are particularly preferred.

As a method of introducing a hydrophilic group into the copolymer (A), a known method can be used. For example, when synthesizing the copolymer (A), a method of introducing maleic anhydride, maleic acid, maleic acid A method of introducing a monomer having a hydrophilic group such as monomethyl lylate to the main chain, or a method of introducing it as a side chain by grafting, etc. The hydrophilic group modification rate can be measured by FT-IR, NMR, quantitative titration, or the like. For example, in the case of a carboxylic acid group, FT-IR can be used to obtain the ratio of the absorption intensities of the C-H stretching vibration and the C=O stretching vibration of the carboxyl group on the basis of the homopolymer.

The content of the hydrophilic group in the copolymer (A) is preferably 0.1 mol % or more, more preferably 0.3 mol % or more. On the other hand, the content of the hydrophilic group is preferably 5.0 mol % or less, more preferably 4.0 mol % or less. By setting the content of the hydrophilic group to 5.0 mol% or less, it can be suppressed that the crystallinity of the polymer becomes too low, the degree of swelling to the electrolyte solution becomes high, and the flexural strength deteriorates when wet. In addition, when the content of the hydrophilic group is within the above range, the affinity of the inorganic particles contained in the porous layer 2 with the copolymer (A) is increased, the short-circuit resistance is improved, and the detachment of the inorganic particles is suppressed. Effect. The reason for this is not clear, but it is presumed that the membrane strength of the porous layer 2 is increased by the copolymer (A) having a hydrophilic group and the inorganic particles, which are the main components of the porous layer 2 . The quantitative determination of the hydrophilic group of the vinylidene fluoride-hexafluoropropylene copolymer in the porous layer 2 can be determined by an IR (infrared absorption spectroscopy) method, an NMR (nuclear magnetic resonance) method, or the like.

The copolymer (A) may be a copolymer in which other monomers other than vinylidene fluoride, hexafluoropropylene, and monomers having a hydrophilic group are further polymerized within the range of non-destructive properties. Examples of other monomers include monomers such as tetrafluoroethylene, trifluoroethylene, trichloroethylene, and vinyl fluoride.

The structure of the copolymer (A), by setting the molecular weight within the above-mentioned range, the separator 10, when used in a non-aqueous electrolyte secondary battery, has high affinity for the non-aqueous electrolyte, and is chemically and physically stable. High, exhibits flexural strength when wet, and can maintain sufficient affinity with electrolyte even when used at high temperature.

[Vinylidene fluoride-hexafluoropropylene copolymer (B)]

The vinylidene fluoride-hexafluoropropylene copolymer (B) (hereinafter, it may be simply referred to as the copolymer (B)) is a copolymer containing a vinylidene fluoride unit and a hexafluoropropylene unit. The content of hexafluoropropylene in the copolymer (B) is more than 5.0 mol %, more preferably 6.0 mol % or more, and still more preferably 7.0 mol % or more. When the content of the hexafluoropropylene unit is 5.0 mol % or less, the adhesion between the separator and the electrode at the time of drying (peeling force at the time of drying) may not be sufficiently obtained. On the other hand, the content on the upper limit side is 8.0 mol % or less, more preferably 7.5 mol % or less. In addition, when the content of the hexafluoropropylene unit exceeds 8.0 mol %, the flexural strength may decrease when the electrolyte solution is swelled excessively and wetted. In addition, the copolymer (B) may contain a hydrophilic group, but may not contain a hydrophilic group.

The weight average molecular weight of the copolymer (B) is 100,000 or more and 750,000 or less. When the weight average molecular weight of the copolymer (B) is in the above range, the affinity for the non-aqueous electrolyte is high, the chemical and physical stability is high, and excellent adhesion between the separator and the electrode during drying can be obtained (drying). peeling force). The reason for this is not clear, but it is presumed that the copolymer (B) has fluidity under heating and pressure conditions such as the peeling force at the time of drying, enters the porous layer of the electrode, and becomes an anchor, whereby the porous layer 2 Because of the strong adhesion to the electrodes. That is, in the battery separator 10 , the copolymer (B) can contribute to the peeling force during drying, and can contribute to the prevention of deflection and distortion of the wound electrode body or the laminated electrode body, and the improvement of transportability. In addition, the copolymer (B) and the copolymer (A) are different resins.

The weight average molecular weight of the copolymer (B) is 100,000 or more, preferably 150,000 or more. When the weight average molecular weight of the copolymer (B) is less than the lower limit of the above-mentioned range, the amount of entanglement of molecular chains is too small, so the resin strength becomes weak, and cohesion failure of the porous layer 2 is likely to occur. On the other hand, the weight average molecular weight of the copolymer (B) is preferably 750,000 or less, more preferably 700,000 or less. When the weight average molecular weight of the copolymer (B) exceeds the upper limit of the above-mentioned range, it is necessary to increase the pressing temperature in the production process of the wound body in order to obtain the peeling force at the time of drying. In this way, the microporous membrane containing polyolefin as a main component may shrink. Moreover, when the weight average molecular weight of a copolymer (B) exceeds the upper limit of the said range, the amount of entanglement of molecular chains increases, and there exists a possibility that it may become unable to fully flow under pressing conditions.

The melting point of the copolymer (B) is preferably 60°C or higher, more preferably 80°C or higher. On the other hand, the melting point of the copolymer (B) is preferably 145°C or lower, more preferably 140°C or lower. In addition, the melting point (Tm) referred to here is the temperature of the peak-top of the endothermic peak at the time of temperature increase measured by the differential scanning calorimetry (DSC) method.

The copolymer (B) is a copolymer having a vinylidene fluoride unit and a hexafluoropropylene unit. The copolymer (B) can be obtained by a suspension polymerization method or the like similarly to the copolymer (A). The melting point of the copolymer (B) can be adjusted by controlling the crystallinity of the site containing the vinylidene fluoride unit. For example, when a monomer other than the vinylidene fluoride unit is contained in the copolymer (B), the melting point can be adjusted by controlling the ratio of the vinylidene fluoride unit. The monomers other than the vinylidene fluoride unit may have one or more of tetrafluoroethylene, trifluoroethylene, trichloroethylene, hexafluoropropylene, fluorovinyl maleic anhydride, maleic acid, maleic acid ester, Methyl maleate, etc. The above-mentioned monomers are added at the time of polymerizing the copolymer (B), and the method of introducing the main chain by copolymerization, or the method of introducing as a side chain by grafting is exemplified. In addition, the melting point can be adjusted by controlling the ratio of Head-to-Head bonding (-CH ₂ -CF ₂ -CF ₂ -CH _{2 -) of vinylidene fluoride units.}

[Content of Copolymer (A) and Copolymer (B)]

The content of the copolymer (A) is 86% by mass or more, more preferably 88% by mass or more, based on 100% by mass of the total weight of the copolymer (A) and the copolymer (B). The upper limit of the content of the copolymer (A) is 98% by mass or less, more preferably 97% by mass or less. Moreover, content of a copolymer (B) is 14 mass % or less with respect to 100 mass % of total weights of a copolymer (A) and a copolymer (B), Preferably it is 12 mass % or more. Moreover, content of the copolymer (B) is 2 mass % or more and 3 mass % or more. When the content of the copolymer (A) and the content of the copolymer (B) are within the above ranges, the porous layer 2 can achieve both excellent wet flexural strength and dry peel strength at a high level.

In addition, the porous layer 2 may contain resins other than the copolymer (A) and the copolymer (B) within a range that does not impair the effects of the present invention, and the resin component constituting the porous layer 2 preferably contains the copolymer (A). ) and copolymer (B). Moreover, in the case of containing resins other than the copolymer (A) and the copolymer (B), the content of the copolymer (A) or the copolymer (B) is set to be based on 100 mass of the resin component of the porous layer 2 %proportion.

[inorganic particles]

The porous layer 2 contains inorganic particles. By including inorganic particles in the porous layer 2 , the short-circuit resistance can be particularly improved, and the improvement of thermal stability can be expected.

Examples of inorganic particles include calcium carbonate, calcium phosphate, amorphous silica, crystalline glass particles, kaolin, talc, titanium dioxide, alumina, silica-alumina composite oxide particles, barium sulfate, fluoride Calcium, lithium fluoride, zeolite, molybdenum sulfide, mica, diaspore, magnesium oxide, etc. In particular, from the viewpoint of affinity with the vinylidene fluoride-hexafluoropropylene copolymer (A), inorganic particles containing many OH groups are preferably used, One or more selected from diaspore.

The content of the inorganic particles contained in the porous layer 2 is 80 vol % or less, preferably 70 vol % or less, more preferably 60 vol % or less with respect to 100 vol % of the solid content volume of the porous layer 2 . On the other hand, the content of the inorganic particles is 40% by volume or more, more preferably 45% by volume or more, still more preferably 50% by volume or more, and still more preferably 51% by volume or more. In addition, the content of the inorganic particles contained in the porous layer 2 was calculated with the density of the copolymer (A) and the copolymer (B) set to 1.77 g/cm ^{3 .}

In general, when inorganic particles having no adhesiveness are contained in the porous layer, the bending strength when wet and the peeling force when dry tend to decrease. However, as described above, the porous layer 2 of the present embodiment contains a specific fluororesin in a specific ratio, and when inorganic particles are contained within the above-mentioned range, the porous layer 2 has high adhesion force to the electrode, flexural strength when wet, and dryness. In this case, the balance of peeling force becomes better, and excellent short-circuit resistance can be obtained.

From the viewpoint of particle shedding, the average particle diameter of the inorganic particles is preferably 1.5 times or more and 50 times or less, more preferably 2.0 times or more and 20 times or less, of the mean flow pore size of the polyolefin microporous membrane. The mean flow pore diameter is measured in accordance with JISK3832, and can be obtained by measuring in the order of Dry-up and Wet-up using a Perm-Porometer (for example, CFP-1500A, manufactured by PMI Corporation). . Specifically, the pore diameter was converted from the curve representing the 1/2 slope of the pressure and flow rate curve in the Dry-up measurement, and the pressure at the point where the curve intersected with the curve in the Wet-up measurement. The following formula was used for the conversion of pressure and pore diameter.

d=C‧γ/P

In the above formula, "d(μm)" is the pore size of the microporous membrane, "γ(mN/m)" is the surface tension of the liquid, "P(Pa)" is the pressure, and "C" is a constant.

The average particle size of the inorganic particles is preferably from 0.3 μm to 1.8 μm, more preferably from 0.5 μm to 1.5 μm, and even more preferably from the viewpoint of sliding property with the winding core or particle detachment when the battery cell is wound. Preferably, it is 0.9 μm to 1.3 μm. The average particle diameter of the particles can be measured using a measuring apparatus of a laser diffraction method or a dynamic light scattering method. For example, it is preferable to measure the particles dispersed in the aqueous solution to which the surfactant is added by using an ultrasonic probe and a particle size distribution analyzer (Microtrac HRA, manufactured by Nikkiso Co., Ltd.), from the volume-converted small particle side. The value of the particle diameter (D50) at the time of accumulation of 50% was set as the average particle diameter. The shape of the particles includes a true spherical shape, a substantially spherical shape, a plate shape, and a needle shape, and is not particularly limited.

[Properties of Porous Layer]

The film thickness of the porous layer 2 is preferably 0.5 μm or more and 3 μm or less per single side, more preferably 1 μm or more and 2.5 μm or less, and even more preferably 1 μm or more and 2 μm or less. When the thickness per single film is 0.5 μm or more, high adhesion to the electrode (flexural strength when wet and peeling force when dry) can be ensured. On the other hand, if the thickness per single film is 3 μm or less, the winding volume can be suppressed, and the film can be further thinned, which is more suitable for the high capacity of the battery that may be developed in the future.

The porosity of the porous layer 2 is preferably 30% or more and 90% or less, and more preferably 40% or more and 70% or less. When the porosity of the porous layer 2 is within the above range, the resistance of the separator can be prevented from rising, a large current can flow, and the film strength can be maintained.

[3] Manufacturing method of battery separator

The manufacturing method of the separator for batteries is not specifically limited, It can manufacture using a well-known method. Hereinafter, an example of the manufacturing method of the separator for batteries is demonstrated. The manufacturing method of the separator for batteries can include the following steps (1)-(3) in this order.

(1) Step of obtaining a fluororesin solution in which the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B) are dissolved in a solvent

(2) Step of adding inorganic particles to a fluororesin solution, mixing, and dispersing to obtain a coating solution

(3) Steps of applying the coating liquid to the polyolefin microporous membrane, immersing it in the coagulation liquid, washing and drying

(1) Step of obtaining fluororesin solution

The vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B) were gradually added to the solvent and completely dissolved.

The solvent is not particularly limited as long as it can dissolve the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B) and can be mixed with the coagulation liquid. From the viewpoint of solubility and low volatility, the solvent is preferably N-methyl-2-pyrrolidone.

(2) Step of obtaining coating liquid

In order to obtain a coating liquid, it is important to sufficiently disperse the inorganic particles. Specifically, pre-dispersion is performed by adding particles while stirring the aforementioned fluororesin solution and stirring with a disper or the like for a predetermined time (for example, about 1 hour), and then using a bead mill or a paint shaker to disperse the particles In the dispersing step (dispersing step), the aggregation of particles is reduced, and further mixing is performed by a Three-One Motor with a stirring blade to prepare a coating liquid.

(3) Steps of applying the coating liquid to the microporous membrane, immersing it in the coagulation liquid, washing and drying

The coating liquid is applied to the microporous membrane, and the coated microporous membrane is immersed in the coagulation liquid, and the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B) are Phase-separated, solidified in a state having a three-dimensional mesh structure, washed and dried. Thereby, the separator for batteries provided with the microporous membrane and the porous layer on the surface of the microporous membrane can be obtained. The method for applying the coating liquid to the microporous membrane may be a known method, and examples thereof include dip coating, reverse roll coating, gravure coating, kiss coating, roll brushing, spray coating, and air knife coating. A coating method, a Meyer bar coating method, a pipe doctor method, a knife coating method, a die coating method, or the like can be implemented alone or in combination.

The coagulation liquid is preferably an aqueous solution containing water as a main component, preferably an aqueous solution containing 1 to 20 mass % of a good solvent of the copolymer (A) and the copolymer (B), and more preferably an aqueous solution containing 5 to 15 mass %. As a good solvent, N-methyl-2-pyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide are mentioned. The immersion time in the coagulation liquid is preferably 3 seconds or more. There is no upper limit, and 10 seconds is sufficient.

For cleaning, water can be used. Drying can be performed using, for example, hot air at 100° C. or lower.

[4] Physical properties of battery separators

Battery separator

The battery separator 10 of the present embodiment can be suitably used for either a battery using an aqueous electrolyte solution or a battery using a non-aqueous electrolyte, but can be more suitably used for a non-aqueous electrolyte secondary battery. Specifically, it can be preferably used as a separator for secondary batteries such as nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc batteries, lithium secondary batteries, and lithium polymer secondary batteries. Among them, it is preferably used as a separator for lithium ion secondary batteries.

In the non-aqueous electrolyte secondary battery, the positive electrode and the negative electrode are arranged through a separator, and the separator contains an electrolytic solution (electrolyte). The structure of the non-aqueous electrolyte electrode is not particularly limited, and conventionally known structures can be used. The electrode structure of the positive electrode and the negative electrode (laminated type), the electrode structure of the laminated strip-shaped positive electrode and the negative electrode (winding type), etc. are wound. The separator for a battery of the present embodiment can have excellent adhesion between the separator and the electrode in any battery structure.

The current collector, positive electrode, positive electrode active material, negative electrode, negative electrode active material, and electrolyte used in non-aqueous electrolyte secondary batteries including lithium ion secondary batteries are not particularly limited, and conventionally known materials can be appropriately used in combination.

Furthermore, as shown in FIG. 1(A) , the battery separator 10 may have the porous layer 2 on one area of the polyolefin microporous membrane 1 or may have the porous layer 2 on both sides of the polyolefin microporous membrane 1 . In addition, in FIG. 1, although the polyolefin microporous membrane 1 is one layer, it may be a laminated body of two or more layers. In addition, the battery separator 10 may further laminate layers other than the polyolefin microporous membrane 1 and the porous layer 2 .

The wet flexural strength of the battery separator is preferably 4.0 N or more, more preferably 5.0 N or more, and even more preferably 6.0 N or more. The upper limit of the flexural strength when wet is not particularly specified, but is, for example, 15.0 N or less. When the flexural strength when wet is within the above-mentioned preferred range, part of the interface between the separator and the electrode can be further suppressed from dissociating, and the increase in the internal resistance of the battery and the deterioration in the battery characteristics can be suppressed. In addition, the bending strength when wet can be measured by the method described in the Example mentioned later.

The peeling force at the time of drying of the battery separator is preferably 2.0 N/m or more, more preferably 5.0 N/m or more, and still more preferably 6.0 N/m or more. The upper limit value of the peeling force during drying is not particularly limited, but is, for example, 40.0 N/m or less. When the peeling force at the time of drying is within the above-mentioned preferred range, it can be expected that the wound electrode body or the laminated electrode body can be conveyed in a state in which the electrode body is not dispersed. In addition, the peeling force at the time of drying can be measured by the method described in the Example mentioned later.

The battery separator of the present embodiment can achieve both the flexural strength when wet and the peeling strength when dry at a high level. Specifically, as shown in the examples described later, the battery separator can satisfy the flexural strength when wet of 4.0 N or more and the peeling force when dry of 2.0 N/m or more.

In addition, the present invention is not limited to the above-described embodiment, and can be implemented with various modifications within the scope of the gist.

[Example]

Hereinafter, the present invention will be described in further detail by way of examples, but the embodiments of the present invention are not limited to these examples. In addition, the evaluation method used in the Example, the various methods of analysis, and the material are as follows.

(1) Film thickness

The film thicknesses of the microporous film and the separator were measured using a contact film thickness meter ("Lightomatic" (registered trademark) series 318 manufactured by Mitutoyo Co., Ltd.). The measurement system uses a super hard spherical probe of Φ9.5 mm, measures 20 points under the condition of a weight of 0.01 N, and sets the average value of the obtained measurement values as the film thickness.

(2) Weight average molecular weight (Mw) of vinylidene fluoride-hexafluoropropylene copolymer (A) and vinylidene fluoride-hexafluoropropylene copolymer (B)

It was calculated|required by the gel permeation chromatography (GPC) method under the following conditions.

‧Measuring device: GPC-150C manufactured by Waters Corporation

‧Pipe column: shodex KF-806M2 made by Showa Denko Co., Ltd.

‧Column temperature: 23℃

‧Solvent (mobile phase): 0.05M N-methyl-2-pyrrolidone (NMP) added with lithium chloride

‧Solvent flow rate: 0.5ml/min

‧Sample preparation: Add 4 mL of measurement solvent to 2 mg of sample, and stir smoothly at room temperature (visually confirm the dissolution)

‧Injection volume: 0.2mL

‧Detector: Differential refractive index detector RI (manufactured by Tosoh, type RI-8020, sensitivity 16)

• Calibration curve: A calibration curve obtained using a monodisperse polystyrene standard sample was prepared using a polyethylene conversion factor (0.46).

(3) Melting point

Using a differential scanning calorimeter (DSC manufactured by PerkinElmer Co., Ltd.), 7 mg of resin was placed in a measurement crucible as a sample for measurement, and the measurement was performed under the following conditions. After the first heating and cooling, the peak top of the endothermic peak at the second heating was used as the melting point.

‧Heating and cooling speed: ±10℃/min.

‧Measurement temperature range: 30~230℃.

(4) Bending strength when wet

Generally, a fluororesin binder is used for the positive electrode, and when a porous layer containing a fluororesin is provided on the separator, the mutual diffusion of the fluororesins makes it easy to ensure adhesion. On the other hand, since a binder other than a fluororesin is used for the negative electrode, the diffusion of the fluorine-based resin hardly occurs, and therefore, the negative electrode is more difficult to obtain adhesion to the separator than that of the positive electrode. Therefore, in this measurement, evaluation was performed as an index of the adhesiveness between the separator and the negative electrode by measuring the flexural strength when wet as described below.

(production of negative electrode)

The aqueous solution containing 1.5 parts by mass of carboxymethyl cellulose was added to 96.5 parts by mass of artificial graphite and mixed, and the styrene butadiene latex of 2 parts by mass as solid content was further added and mixed, and the slurry containing the negative electrode mixture was made . The slurry containing the negative electrode mixture was uniformly coated on both sides of a negative electrode current collector including a copper foil with a thickness of 8 μm, and dried to form a negative electrode layer. The density of the negative electrode layer of the electric body was 1.5 g/cm ³ , and a negative electrode was produced.

(2) Preparation of the winding body for the test

The negative electrode 20 (machine direction 161mm x width direction 30mm) prepared above is overlapped with the prepared separator 10 (machine direction 160mm x width direction 34mm), and a metal plate (length 300mm, width 25mm, thickness 1mm) is used as a winding core Then, the separator 10 and the negative electrode 20 were wound up so that the separator 10 would be inside, and the metal plate was pulled out to obtain the wound body 30 for testing. The wound body for the test was about 34 mm in length and about 28 mm in width.

(Measuring method of flexural strength when wet)

Two sheets of polypropylene-containing laminate films (length 70 mm, width 65 mm, thickness 0.07 mm) were stacked, and the roll body 30 for testing was placed in a bag-shaped laminate film 22 having three sides welded on four sides. _{LiPF 6 was} dissolved in 500 μL of an electrolyte solution containing a solvent in which ethyl ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 3:7 at a ratio of 1 mol/L, and injected from the opening of the laminated film 22 in a glove box. This was impregnated in the winding body 30 for testing, and one side of the opening was sealed with a vacuum sealer.

Next, the test roll body 30 encapsulated in the laminated film 22 was sandwiched between two sheets of gaskets (thickness 1 mm, 5 cm×5 cm), and the test roll 30 was sealed with a precision heating and pressing apparatus (manufactured by Shinto Industries, Ltd., CYPT). -10) Pressurize at 98° C. and 0.6 MPa for 2 minutes, and cool at room temperature. The flexural strength when wet was measured using a universal testing machine (manufactured by Shimadzu Corporation, AGS-J) with respect to the test roll body 30 after pressurization while keeping the state enclosed in the laminated film 22 . The details are described below with reference to FIG. 2 .

Two aluminum L-shaped angle bars 41 (thickness 1mm, 10mm×10mm, length 4cm) are arranged in parallel with the 90° part facing up and the ends are aligned, and the 90° part is used as a fulcrum, and the distance between the fulcrums is 15mm. to be fixed. The winding body 30 for testing was arranged so that the midpoint of the side (about 28 mm) in the width direction of the winding body for testing was aligned with a position of 7.5 mm in the middle of the distance between the fulcrums of the two aluminum L-shaped angle members 41, And it does not exceed the side of the longitudinal direction of the L-shaped angle material 41.

Next, as an indenter, the side in the longitudinal direction (about 34 mm) of the winding body for testing was parallel to the side in the longitudinal direction of the aluminum L-shaped angle material 42 (thickness 1 mm, 10 mm×10 mm, length 4 cm). The 90° part of the aluminum L-shaped angle bar 42 is aligned with the midpoint of the side in the width direction of the winding body for testing, and the aluminum L-shaped angle bar 42 is fixed to the end of the universal testing machine so that the 90° part faces downward. Load cell (load cell capacity 50N). The average value of the maximum test force obtained by measuring three winding bodies for testing at a load speed of 0.5 mm/min was taken as the wet bending strength.

(5) Peeling force when dry

(production of negative electrode)

The negative electrode 20 having the same flexural strength when wet was used as described above.

(Preparation of peeling test piece)

The negative electrode 20 (70mm×15mm) made above and the separator 10 (machine direction 90mm×width direction 20mm) made above were overlapped, and were sandwiched with 2 sheets of adhesive pads (thickness 0.5mm, 95mm×27mm), It pressurized at 90 degreeC and 8 MPa for 2 minutes by the precision heating press apparatus (made by Shinto Industrial Co., Ltd., CYPT-10), and cooled at room temperature. A double-sided tape with a width of 1 cm was affixed to the negative side of the laminate of the negative electrode 20 and the separator 10, and the other side of the double-sided tape was affixed to the SUS so that the machine direction of the separator was parallel to the longitudinal direction of the SUS plate. Plate (thickness 3mm, length 150mm x width 50mm). This was used as a peeling test piece.

(Measuring method of peeling force during drying)

Using a universal testing machine (manufactured by Shimadzu Corporation, AGS-J), the separator 10 was clamped in the load cell side jig, and a 180-degree peel test was performed at a test speed of 300 mm/min. The value obtained by averaging the measured values from a stroke of 20 mm to 70 mm in the peeling test was used as the peeling force of the peeling test piece. A total of three peel test pieces were measured, and a value obtained by converting the average value of peel force to width was used as peel force (N/m) at the time of drying.

(6) Short circuit resistance test

The evaluation of the short-circuit resistance was implemented using a desktop precision universal testing machine AutoGraph AGS-X (manufactured by Shimadzu Corporation). First, as shown in FIG. 3(A) , a polypropylene insulator 5 (thickness: 0.2 mm), a negative electrode 21 for lithium ion batteries (total thickness: about 140 μm, base material: copper foil (thickness: about 9 μm), and an active Substances: artificial graphite (particle size about 30 μm), double-sided coating), separator 10 , sample laminate 31 of aluminum foil 4 (thickness about 0.1 mm). Next, as shown in FIG.3(B), the sample laminated body 31 was fixed to the compression jig (lower side) 44 of the universal testing machine with the double-sided tape. Next, the aluminum foil 4 and the negative electrode 21 of the above-mentioned sample laminate 31 were connected to a circuit including a capacitor and a clad resistor with a cable. The capacitor was charged to about 1.5 V, and a metal ball 6 (material: chromium (SUJ-2)) with a diameter of about 500 μm was placed between the separator and the aluminum foil 4 in the sample laminate 31 . Next, a compression jig is installed in the universal testing machine, and as shown in FIG. 3(B), the sample laminate 31 including the metal balls 6 is placed between the two compression jigs 43 and 44, and the compression is performed at a speed of 0.3 mm/min. , when the load reaches 100N, it is regarded as the end of the test. At this time, in the change of the compressive load, the part where the inflection point appeared was used as the film breaking point of the separator, and the moment when the electric circuit was formed through the metal ball and the current was detected was used as the short circuit generation point. The compressive displacement A(t) at the moment when the compressive stress produces an inflection point and the compressive displacement B(t) at the moment when the current flows into the circuit is measured, and the following (Equation 1) is used to obtain the When the numerical value is 1.1 or more, it means that even if the separator is broken due to foreign matter mixed in the battery, the coating layer composition adheres to the surface of the foreign matter to maintain insulation, and thus the short-circuit resistance is evaluated as good. On the other hand, when the numerical value obtained by Equation 1 is larger than 1.0 and smaller than 1.1, membrane rupture and short-circuiting of the separator do not occur simultaneously, but in order to protect the electrode even under tension or charge/discharge associated with winding the battery member Even if the internal pressure of the battery increases due to the expansion of the battery, a short circuit does not occur, and a certain or more resistance is required, so the short circuit resistance is evaluated as slightly unsatisfactory. When the numerical value obtained by Equation 1 was 1.0, the short-circuit resistance was evaluated as unsatisfactory since the short-circuit resistance occurred simultaneously with the film rupture of the separator, and the short-circuit resistance improvement by the coating layer was not seen.

B(t)÷A(t)...(Equation 1)

(Example 1)

[Copolymer (A)]

As the copolymer (A), the copolymer (A1) was synthesized in the following manner. Using vinylidene fluoride, hexafluoropropylene and monomethyl maleate as starting materials, the molar ratio of vinylidene fluoride/hexafluoropropylene/monomethyl maleate was 98.0/1.5 by suspension polymerization. The copolymer (A1) was synthesized in the manner of /0.5. The weight average molecular weight of the obtained copolymer (A1) was 1.5 million.

[Copolymer (B)]

As the copolymer (B), the copolymer (B1) was synthesized in the following manner. Using vinylidene fluoride and hexafluoropropylene as starting materials, the copolymer (B1) was synthesized by the suspension polymerization method so that the molar ratio of vinylidene fluoride/hexafluoropropylene was 93.0/7.0. The weight average molecular weight of the obtained copolymer (B1) was 300,000.

[Production of separator for battery]

26.5 parts by mass of the copolymer (A1), 3.5 parts by mass of the copolymer (B1), and 600 parts by mass of N-methyl-2-pyrrolidone (NMP) were mixed, and then stirred with a disperser to disperse the porous material. ^{Alumina particles (average particle diameter 1.1 μm, density 4.0 g/cm 3} ) were added so that the solid content of the layer was 100 vol % and 51 vol %, and preliminarily stirred at 2000 rpm for 1 hour with a disperser. Next, using DYNO-MILL (DYNO-MILL Multi Lab manufactured by SHINMARU ENTERPRISES (1.46L container, 80% filling rate, Φ0.5mm alumina beads)), the treatment was carried out under the conditions of a flow rate of 11kg/hr and a peripheral speed of 10m/s Three times, a coating liquid (A) was prepared. The obtained coating solution (A) was applied on both sides of a polyethylene microporous membrane having a thickness of 7 μm, a porosity of 40%, and an airtightness of 100 seconds/100 cm ^{3 by a dip coating method.} The coated film was immersed in an aqueous solution (coagulation liquid) containing 10 mass % of N-methyl-2-pyrrolidone (NMP), washed with pure water, and then dried at 50° C. to obtain a battery separator. The thickness of the battery separator was 10 μm.

(Example 2)

As the copolymer (B), the copolymer (B2) was synthesized in the following manner. Using vinylidene fluoride and hexafluoropropylene as starting materials, the copolymer (B2) was synthesized by the suspension polymerization method so that the molar ratio of vinylidene fluoride/hexafluoropropylene was 94.5/5.5. The weight average molecular weight of the obtained copolymer (B2) was 280,000. A battery separator was obtained in the same manner as in Example 1, except that the coating liquid (B) obtained by replacing the copolymer (B1) with the copolymer (B2) was used in the preparation of the coating liquid.

(Example 3)

As the copolymer (B), the copolymer (B3) was synthesized in the following manner. Using vinylidene fluoride and hexafluoropropylene as starting materials, the copolymer (B3) was synthesized by a suspension polymerization method so that the molar ratio of vinylidene fluoride/hexafluoropropylene was 92.0/8.0. The weight average molecular weight of the obtained copolymer (B3) was 350,000. A battery separator was obtained in the same manner as in Example 1, except that the coating liquid (C) obtained by replacing the copolymer (B1) with the copolymer (B3) was used in the preparation of the coating liquid.

(Example 4)

As the copolymer (A), the copolymer (A2) was synthesized in the following manner. Using vinylidene fluoride, hexafluoropropylene and monomethyl maleate as starting materials, the molar ratio of vinylidene fluoride/hexafluoropropylene/monomethyl maleate was 99.0/0.5 by suspension polymerization. The copolymer (A2) was synthesized in the manner of /0.5. The weight average molecular weight of the obtained copolymer (A2) was 1.4 million. A battery separator was obtained in the same manner as in Example 1, except that the coating liquid (D) obtained by replacing the copolymer (A1) with the copolymer (A2) was used in the preparation of the coating liquid.

(Example 5)

As the copolymer (A), the copolymer (A3) was synthesized in the following manner. Using vinylidene fluoride, hexafluoropropylene and monomethyl maleate as starting materials, the molar ratio of vinylidene fluoride/hexafluoropropylene/monomethyl maleate was 95.0/4.5 by suspension polymerization. The copolymer (A3) was synthesized in the manner of /0.5. The weight average molecular weight of the obtained copolymer (A3) was 1.7 million. A battery separator was obtained in the same manner as in Example 1, except that the coating liquid (E) obtained by replacing the copolymer (A1) with the copolymer (A3) was used in the preparation of the coating liquid.

(Example 6)

As the copolymer (A), the copolymer (A4) was synthesized in the following manner. Using vinylidene fluoride, hexafluoropropylene and monomethyl maleate as starting materials, the molar ratio of vinylidene fluoride/hexafluoropropylene/monomethyl maleate was 98.0/1.5 by suspension polymerization. The copolymer (A4) was synthesized in the manner of /0.5. The weight average molecular weight of the obtained copolymer (A4) was 1.9 million. A battery separator was obtained in the same manner as in Example 1, except that the coating liquid (F) obtained by replacing the copolymer (A1) with the copolymer (A4) was used in the preparation of the coating liquid.

(Example 7)

Except for the preparation of the coating liquid, a coating liquid ( Except G), it carried out similarly to Example 1, and obtained the separator for batteries.

(Example 8)

Except for the preparation of the coating liquid, the solid content of the porous layer was 100% by volume, the content of the alumina particles was 40% by volume, the copolymer (A1) was changed to 35.2 parts by mass, and the copolymer (B1) was changed to 35.2 parts by mass. ) was changed to 4.7 parts by mass and NMP was changed to 900 parts by mass of the coating liquid (H), and the same procedure as in Example 1 was carried out to obtain a battery separator.

(Example 9)

Except for the preparation of the coating liquid, the solid content of the porous layer was 100% by volume, the content of the alumina particles was 75% by volume, the copolymer (A1) was changed to 11.4 parts by mass, and the copolymer (B1) was changed to 11.4 parts by mass. ) was changed to 1.5 parts by mass and NMP was changed to 300 parts by mass of the coating liquid (I), and the same procedure as in Example 1 was carried out to obtain a battery separator.

(Example 10)

As the copolymer (A), the copolymer (A5) was synthesized in the following manner. Using vinylidene fluoride, hexafluoropropylene and monomethyl maleate as starting materials, the molar ratio of vinylidene fluoride/hexafluoropropylene/monomethyl maleate was 98.4/1.5 by suspension polymerization. The copolymer (A5) was synthesized in the manner of /0.1. The weight average molecular weight of the obtained copolymer (A5) was 1.5 million. A battery separator was obtained in the same manner as in Example 1, except that the coating liquid (J) obtained by replacing the copolymer (A1) with the copolymer (A5) was used in the preparation of the coating liquid.

(Example 11)

As the copolymer (A), the copolymer (A6) was synthesized in the following manner. Using vinylidene fluoride, hexafluoropropylene and monomethyl maleate as starting materials, the molar ratio of vinylidene fluoride/hexafluoropropylene/monomethyl maleate was 94.5/1.5 by suspension polymerization. The copolymer (A6) was synthesized in the manner of /4.0. The weight average molecular weight of the obtained copolymer (A6) was 1.5 million. A battery separator was obtained in the same manner as in Example 1, except that the coating liquid (K) obtained by replacing the copolymer (A1) with the copolymer (A6) was used in the preparation of the coating liquid.

(Example 12)

A battery separator was obtained in the same manner as in Example 1, except that a polyethylene microporous film having a thickness of 5 μm, a porosity of 35%, and an airtightness of 150 seconds/100 cm ^{3 was used as the polyolefin microporous film.} The thickness of the battery separator was 8 μm.

(Example 13)

A battery separator was obtained in the same manner as in Example 1, except that a polyethylene microporous film having a thickness of 12 μm, a porosity of 45%, and an air-impermeability of 95 seconds/100 cm 3 was used as the polyolefin microporous film. The thickness of the battery separator was 15 μm.

(Example 14)

As the copolymer (B), the copolymer (B4) was synthesized in the following manner. Using vinylidene fluoride and hexafluoropropylene as starting materials, the copolymer (B4) was synthesized by the suspension polymerization method so that the molar ratio of vinylidene fluoride/hexafluoropropylene was 93.0/7.0. The weight average molecular weight of the obtained copolymer (B4) was 700,000. A battery separator was obtained in the same manner as in Example 1, except that the coating liquid (L) obtained by replacing the copolymer (B1) with the copolymer (B4) was used in the preparation of the coating liquid.

(Example 15)

Except for the preparation of the coating liquid, the alumina particles were replaced by plate-shaped diaspore particles (density 3.07 g/cm3) with an average particle diameter of 1.0 μm and an average thickness of 0.4 μm (density 3.07 g/cm 3 ), and the copolymer (A1) was set to 31.5 mass A battery separator was obtained in the same manner as in Example 1, except that the copolymer (B1) was 4.2 parts by mass of the coating liquid (M).

(Example 16)

Except for the preparation of the coating liquid, the alumina particles were replaced by titanium oxide particles with an average particle size of 0.4 μm (density 4.23 g/cm ³ ), the copolymer (A1) was set to 25.3 parts by mass, and the copolymer (B1) was set to 25.3 parts by mass. A separator for batteries was obtained in the same manner as in Example 1, except that the amount of the coating liquid (N) was 3.4 parts by mass.

(Example 17)

Except for the preparation of the coating liquid, a coating liquid ( O), it carried out similarly to Example 1, and obtained the separator for batteries.

(Comparative Example 1)

In the preparation of the coating liquid, the coating liquid (P) obtained by dissolving and mixing 88.3 parts by mass of the copolymer (A1), 11.7 parts by mass of the copolymer (B1), and 3500 parts by mass of NMP was used. 1 The separator for batteries was obtained in the same manner.

(Comparative Example 2)

Except for the preparation of the coating liquid, alumina particles were added so that the solid content of the porous layer was 100% by volume and 95% by volume, the copolymer (A1) was changed to 2.0 parts by mass, and the copolymer ( B1) was changed into 0.3 mass part and NMP was changed into the coating liquid (Q) of 250 mass parts, it carried out similarly to Example 1, and obtained the separator for batteries.

(Comparative Example 3)

Except for the preparation of the coating liquid, a coating liquid ( Except R), it carried out similarly to Example 1, and obtained the separator for batteries.

(Comparative Example 4)

As the copolymer (A), the copolymer (A7) was synthesized in the following manner. Using vinylidene fluoride and hexafluoropropylene as starting materials, the copolymer (A7) was synthesized by the suspension polymerization method so that the molar ratio of vinylidene fluoride/hexafluoropropylene was 98.5/1.5. The weight average molecular weight of the obtained copolymer (A7) was 1.5 million. A battery separator was obtained in the same manner as in Example 1, except that the coating liquid (S) obtained by replacing the copolymer (A1) with the copolymer (A7) was used in the preparation of the coating liquid.

(Comparative Example 5)

Except for the preparation of the coating liquid, a coating liquid (T) prepared without using the copolymer (B) was used by replacing the copolymer (A1) with 30.0 parts by mass of polyvinylidene fluoride (weight average molecular weight: 1.5 million). Otherwise, it carried out similarly to Example 1, and obtained the separator for batteries.

(Comparative Example 6)

As the copolymer (A), the copolymer (A8) was synthesized in the following manner. Using vinylidene fluoride, hexafluoropropylene and monomethyl maleate as starting materials, the molar ratio of vinylidene fluoride/hexafluoropropylene/monomethyl maleate was 98.0/1.5 by suspension polymerization. The copolymer (A8) was synthesized in the manner of /0.5. The weight average molecular weight of the obtained copolymer (A8) was 650,000. A battery was obtained in the same manner as in Example 1, except that in the preparation of the coating liquid, the coating liquid (U) in which the copolymer (A1) was replaced by the copolymer (A8) and the NMP was changed to 500 parts by mass was used. Use spacer.

(Comparative Example 7)

As the copolymer (B), the copolymer (B5) was synthesized in the following manner. Using vinylidene fluoride and hexafluoropropylene as starting materials, the copolymer (B5) was synthesized by the suspension polymerization method so that the molar ratio of vinylidene fluoride/hexafluoropropylene was 93.0/7.0. The weight average molecular weight of the obtained copolymer (B5) was 70,000. A battery separator was obtained in the same manner as in Example 1, except that the coating liquid (V) obtained by replacing the copolymer (B1) with the copolymer (B5) was used in the preparation of the coating liquid.

(Comparative Example 8)

A battery separator was obtained in the same manner as in Comparative Example 1, except that a polyethylene microporous film having a thickness of 5 μm, a porosity of 35%, and an airtightness of 150 seconds/100 cm ^{3 was used as the polyolefin microporous film.} The thickness of the battery separator was 8 μm.

Table 1 shows the structures and weight average molecular weights of the copolymers (A) and copolymers (B) used in the above Examples and Comparative Examples, the composition of the coating liquid, and the properties of the obtained battery separators.

[Industrial Availability]

The separator for a battery of the present embodiment can satisfy the peeling force when dry, the bending strength when wet, the interlayer adhesion between the polyolefin multilayer microporous film and the separator of the porous layer, And a separator for batteries with excellent adhesion between the separator and the electrode, and excellent short-circuit resistance. Therefore, the separator for a battery of the present embodiment can be applied even when the battery (particularly, a laminated battery) is required to increase in size and capacity in the future.

1‧‧‧Polyolefin Microporous Film

2‧‧‧Porous layer

10‧‧‧Isolators for batteries

Claims (6)

A battery separator comprising a polyolefin microporous membrane and a porous layer laminated on at least one side of the polyolefin microporous membrane, the porous layer comprising a vinylidene fluoride-hexafluoropropylene copolymer (A), a vinylidene fluoride-hexafluoropropylene copolymer (B), and inorganic particles, the vinylidene fluoride-hexafluoropropylene copolymer (A) having 0.3 mol % or more and 5.0 mol % or less of hexafluoroethylene A propylene unit having a weight average molecular weight of 900,000 or more and 2,000,000 or less, and containing a hydrophilic group of 0.1 mol % or more and 5.0 mol % or less, and the vinylidene fluoride-hexafluoropropylene copolymer (B) has more than 5.0 mol %, 8.0 mol% or less of hexafluoropropylene units with a weight average molecular weight of 100,000 or more and 750,000 or less, relative to the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer The total 100 mass % of (B) contains 86 mass % or more and 98 mass % or less of the vinylidene fluoride-hexafluoropropylene copolymer (A), with respect to 100 volume % of the solid content in the porous layer, containing 40 mass % volume % or more and 80 volume % or less of the inorganic particles. The battery separator according to claim 1, wherein the vinylidene fluoride-hexafluoropropylene copolymer (B) has a melting point of 60°C or higher and 145°C or lower. The battery separator according to claim 1 or 2, wherein the inorganic particles are at least one selected from titanium dioxide, aluminum oxide, and diaspore. The battery separator according to claim 1 or 2, wherein the polyolefin microporous film has a thickness of 3 μm or more and 16 μm or less. An electrode body having a positive electrode, a negative electrode, and as claimed in items 1 to 4 The separator for batteries according to any one of them. A non-aqueous electrolyte secondary battery comprising the electrode body according to claim 5, and a non-aqueous electrolyte.

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