WO2018168835A1

WO2018168835A1 - Separator for batteries, electrode body and nonaqueous electrolyte secondary battery

Info

Publication number: WO2018168835A1
Application number: PCT/JP2018/009674
Authority: WO
Inventors: 辻本　潤; 水野　直樹
Original assignee: 東レ株式会社
Priority date: 2017-03-17
Filing date: 2018-03-13
Publication date: 2018-09-20
Also published as: TW201843866A; KR20190088567A; JPWO2018168835A1; CN110249449B; JP7330885B2; KR102231395B1; TWI744505B; CN110249449A

Abstract

The present invention addresses the problem of providing a separator for batteries, which has excellent bondability and short circuit resistance. The present invention is a separator for batteries, which is provided with a polyolefin microporous membrane and a porous layer that is arranged on at least one surface of the polyolefin microporous membrane, and which is configured such that: the polyolefin microporous membrane is composed of a polyolefin multilayer microporous membrane that has a three-layer structure wherein a first microporous layer, a second microporous layer and another first microporous layer are laminated in this order; the first microporous layers are formed from a first polyolefin resin that contains a polyethylene and a polypropylene; the content of the polypropylene is from 10% by mass to 50% by mass (inclusive) relative to the total mass of the first polyolefin resin; the second microporous layer is formed only from a polyethylene resin; and the porous layer contains a vinylidene fluoride-hexafluoropropylene copolymer (A), a vinylidene fluoride-hexafluoropropylene copolymer (B) and inorganic particles.

Description

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

The present invention relates to a battery separator, an electrode body, and a nonaqueous 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 portable information terminals. Examples of the form of the nonaqueous electrolyte secondary battery include a cylindrical battery, a square battery, and a laminate battery. In general, these batteries have a configuration in which an electrode body in which a positive electrode and a negative electrode are arranged via a separator, and a nonaqueous electrolytic solution are housed in an exterior body. Examples of the structure of the electrode body include a laminated electrode body in which a positive electrode and a negative electrode are stacked via a separator, and a wound electrode body in which the positive electrode and the negative electrode are spirally wound via a separator.

Conventionally, microporous membranes mainly made of polyolefin resin are used as battery separators. Since the microporous membrane made of polyolefin resin has a so-called shutdown function, the current flow can be suppressed and ignition can be prevented by closing the pores of the separator when the battery is abnormally heated.

In recent years, in battery separators, attempts have been made to improve battery characteristics by providing layers other than polyolefin resin on one or both sides of a layer made of polyolefin resin. For example, a battery separator in which a porous layer containing a fluororesin is provided on one or both surfaces of a layer made of a polyolefin resin has been proposed. In addition, by adding inorganic particles to the porous layer, even if sharp metal penetrates the battery due to an accident, etc., even if it causes a sudden short circuit and generates heat, it prevents melting shrinkage of the separator and suppresses the expansion of the short circuit part between the electrodes. It has been known.

For example, Patent Document 1 includes a positive electrode, a negative electrode, a three-layer separator made of polypropylene, polyethylene, and polypropylene, and an adhesive resin layer made of polyvinylidene fluoride and alumina powder disposed between the electrode and the separator. An electrode body provided with is described.

In Example 1 of Patent Document 2, VdF-HFP copolymer (HFP unit 0.6 mol%) and VdF-HFP copolymer (weight average molecular weight 470,000, HFP unit 4.8 mol%) were used. There is described a separator in which a porous layer is formed by dissolving in a dimethylacetamide and tripropylene glycol solution and applying this to a polyethylene microporous membrane.

In Example 1 of Patent Document 3, PVdF (weight average molecular weight 500,000) and VdF-HFP copolymer (weight average molecular weight 400,000, HFP unit 5 mol%) were dissolved in dimethylacetamide and tripropylene glycol solution. A separator in which a porous layer is formed by applying this to a polyethylene microporous film is described.

In Example 1 of Patent Document 4, PVdF (weight average molecular weight: 700,000) and VdF-HFP copolymer (weight average molecular weight: 470,000, HFP unit: 4.8 mol%) were mixed in dimethylacetamide and tripropylene glycol solution. A separator is described in which a porous layer is formed by dissolving the polymer in a polyethylene microporous film.

In Example 1 of Patent Document 5, PVdF (weight average molecular weight 350,000) and VdF-HFP copolymer (weight average molecular weight 270,000, HFP copolymer 4.8 mol%) were mixed with dimethylacetamide and tripropylene glycol. A separator is disclosed in which a porous layer is formed by dissolving in a solution and coating it on a polyethylene microporous membrane.

Further, in Example 23 of Patent Document 6, 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%) is dissolved in a dimethylacetamide and tripropylene glycol solution, and a coating solution is prepared by adding aluminum hydroxide, and this is applied to a polyethylene microporous membrane to form a separator having a porous layer. Are listed.

International Publication No. 1999/036981 Japanese Patent No. 5282179 Patent No. 5282180 Patent No. 5282181 Patent No. 53442088 International Publication No. 2016/152863

In recent years, non-aqueous electrolyte secondary batteries are expected to be deployed for large applications such as large tablets, mowers, electric motorcycles, electric cars, hybrid cars, small ships, etc. Higher capacity is also expected. Patent Documents 1 to 5 all improve the adhesion between the separator containing the electrolytic solution and the electrode. However, when the secondary battery is enlarged, further improvement in the adhesion is required.

As will be described below, the present inventors evaluated the adhesion between the electrode and the separator when the adhesion between the electrode and the separator during drying and the adhesion between the electrode and the separator when wet. Focusing on the fact that the adhesiveness can be more accurately evaluated by distinguishing and evaluating two types of adhesiveness, and further, using these adhesivenesses as indicators of peel strength when dry and bending strength when wet, respectively. And found that it can be evaluated.

That is, for example, the wound electrode body is manufactured by winding a positive electrode and a negative electrode while applying tension to each member via a separator. At this time, the positive electrode and the negative electrode applied to the metal current collector hardly expand or contract with respect to the tension, but the separator is wound while extending to some extent in the machine direction. When this wound body is left for a while, the separator portion is gradually contracted to return to the original length. As a result, a force in the parallel direction is generated at the boundary surface between the electrode and the separator, and the wound electrode body (particularly, the electrode body wound flatly) is likely to bend and distort. Furthermore, these problems become apparent due to the widening and lengthening of the separator accompanying the increase in the size 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 adhesiveness with the electrode than ever before. Further, when the electrode body is transported, the electrodes and the separator are peeled off unless the respective members are sufficiently bonded, and cannot be transported with a high yield. The problem of adhesion at the time of transportation becomes obvious due to the increase in size of the battery, and there is a concern that the yield may deteriorate. Therefore, the separator is required to have a high peeling force during drying that is difficult to peel off from the electrode.

Also, in order to maintain a high peeling force during drying, the adhesion between the polyolefin microporous film and the porous layer is also extremely important. For example, if the polyolefin microporous film and the porous layer break before the porous layer and the electrode break, it is not possible to suppress the above-described deflection and distortion and improve the transportability. For this reason, the separator is required to have high adhesion between the polyolefin microporous membrane and the porous layer. In this invention, the tape peeling force obtained by the measuring method mentioned later about this adhesiveness was made into the parameter | index. When this value is large, the peeling force during drying is maintained. In addition, it is expected that the yield of the porous layer will be improved by preventing the porous layer from falling off during handling of the separator and during transportation after coating.

Furthermore, in laminated batteries, it is difficult to apply pressure compared to prismatic and cylindrical batteries that can be pressurized with an outer package, and partial swelling at the interface between the separator and electrode due to electrode swelling / shrinkage associated with charge / discharge. Is easily released. As a result, the battery swells, the resistance inside the battery increases, and the cycle performance decreases. Therefore, the separator is required to have adhesiveness with the electrode in the battery after injecting the electrolytic solution. In this specification, this adhesiveness is evaluated using as an index the wet bending strength obtained by the measurement method described later. The measurement method of the bending strength when wet described later can show the difficulty of lateral displacement between the laminated electrode and the separator in the electrode body wet with the electrolyte, and the separator in a state containing the electrolyte Adhesiveness with an electrode can be evaluated according to an actual battery. If this strength is large, it can be considered that improvement of battery characteristics such as suppression of battery swelling after repeated charge and discharge is expected. The peeling force at the time of drying represents the adhesiveness to the interface between the separator and the electrode when the separator does not substantially contain the electrolyte. In addition, that electrolyte solution does not contain substantially means that the electrolyte solution in a separator is 500 ppm or less.

However, in the prior art, the inventors have required adhesion between the electrode and the separator during drying, which is required for manufacturing and transporting the electrode body, and between the electrode and the separator when wet, which is required after injecting the electrolyte. There is a trade-off relationship with the adhesiveness of the material, it is extremely difficult to satisfy both physical properties, and the techniques disclosed in Patent Documents 1 to 5 described above may have insufficient adhesiveness. I found out.

Furthermore, the battery is required to have a characteristic in which the convex portion of the electrode active material penetrates the separator and the electrode is not easily short-circuited (hereinafter referred to as short-circuit resistance) even when a sudden impact is applied. However, in the future, the battery separator is expected to have a thin film thickness. However, as the thickness of the separator decreases, it becomes difficult to ensure short circuit resistance. In order to ensure short circuit resistance, it is known that it is effective to contain a certain amount or more of inorganic particles in the porous layer, but when including inorganic particles that can ensure short circuit resistance, There is a tendency for the adhesion of the resin to decrease.

The present invention has been made in view of the above circumstances, and is excellent in both the adhesion between the electrode and the separator when dried and the adhesion between the electrode and the separator when wet, and the polyolefin microporous membrane and the porous interlayer Another object of the present invention is to provide a battery separator that is excellent in the adhesive property of the battery and has excellent short circuit resistance, and an electrode body and a secondary battery using the battery separator.

As a result of intensive studies to solve the above problems, the inventors of the present invention have developed a first microporous layer made of a specific first polyolefin resin and a second microporous layer made of a second polyolefin resin. The above-mentioned problem can be solved by a separator including at least a polyolefin multilayer microporous membrane containing a porous layer containing two types of fluororesins having different structures in a specific amount and a specific amount of inorganic particles As a result, the present invention has been completed.

That is, the present invention is a battery separator comprising a polyolefin microporous membrane and a porous layer on at least one surface of the polyolefin microporous membrane,
The polyolefin microporous membrane comprises a polyolefin multilayer microporous membrane having a three-layer structure in which the first microporous layer / second microporous layer / first microporous layer are laminated in this order.
The first microporous layer is made of a first polyolefin resin containing polyethylene and polypropylene, and the content of the polypropylene is 10% by mass or more and 50% by mass with respect to the total mass of the first polyolefin resin. %, And the second microporous layer is made of only polyethylene resin,
The porous layer includes a vinylidene fluoride-hexafluoropropylene copolymer (A), a vinylidene fluoride-hexafluoropropylene copolymer (B), and inorganic particles,
The vinylidene fluoride-hexafluoropropylene copolymer (A) has not less than 0.3 mol% and not more than 5.0 mol% of hexafluoropropylene units, and has a weight average molecular weight of not less than 900,000 and not more than 2 million, And includes a hydrophilic group,
The vinylidene fluoride-hexafluoropropylene copolymer (B) has more than 5.0 mol% and not more than 8.0 mol% hexafluoropropylene units, and has a weight average molecular weight of 100,000 to 750,000,
The total amount of the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B) is 100% by mass with respect to the vinylidene fluoride-hexafluoropropylene copolymer ( A) is contained in an amount of 86% by mass or more and 98% by mass or less, and the inorganic particles are contained in an amount of 40% by volume or more and 80% by volume or less with respect to 100% by volume of the solid content in the porous layer.
It is a battery separator.

The vinylidene fluoride-hexafluoropropylene copolymer (A) preferably contains 0.1 mol% or more and 5.0 mol% or less of a hydrophilic group. The vinylidene fluoride-hexafluoropropylene copolymer (B) preferably has a melting point of 60 ° C. or higher and 145 ° C. or lower. The inorganic particles are preferably at least one selected from titanium dioxide, alumina, and boehmite.

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

The present invention is also a non-aqueous electrolyte secondary battery comprising the electrode body of the present invention and a non-aqueous electrolyte.

According to the present invention, both the adhesion between the electrode and the separator at the time of drying and the adhesion between the electrode and the separator at the time of wetness are excellent, and the adhesion between the polyolefin multilayer microporous membrane and the porous layer, and A separator excellent in short circuit resistance, and an electrode body and a secondary battery using the separator are provided.

FIG. 1 is a schematic diagram illustrating an example of a battery separator according to the present embodiment. FIG. 2 is a schematic view showing an example of the battery separator of the present embodiment. FIG. 3 is a schematic diagram showing a method for evaluating the bending strength when wet. FIG. 4 is a schematic diagram showing an evaluation method for a short-circuit resistance test.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. Hereinafter, directions in the figure will be described using an XYZ coordinate system. In the XYZ coordinate system, a plane parallel to the surface (in-plane direction) of the microporous film or the separator is defined as an XY plane. The direction perpendicular to the XY plane (thickness direction) is the Z direction. In each of the X direction, the Y direction, and the Z direction, the direction of the arrow in the figure is the + direction, and the direction opposite to the arrow direction is the − direction. Further, in the drawings, in order to make each configuration easy to understand, a part of the structure is emphasized or a part of the structure is simplified, and an actual structure, shape, scale, or the like may be different.

[Configuration of battery separator]
FIG.1 and FIG.2 is a figure which shows an example of the separator which concerns on this embodiment. As shown in FIG. 1, a battery separator 10 (hereinafter sometimes abbreviated as “separator 10”) includes a polyolefin microporous membrane 1 and a porous layer laminated on at least one surface of the polyolefin microporous membrane 1. Layer 2. As shown in FIG. 2, the polyolefin microporous membrane 1 includes a polyolefin multilayer microporous structure having a three-layer structure in which a first microporous layer a / second microporous layer b / first microporous layer a are laminated in this order. It can consist of membrane 1 '. Hereinafter, each layer constituting the battery separator will be described.

1. Polyolefin multilayer microporous membrane (1) First microporous layer The first microporous layer a is composed of a first polyolefin resin containing polyethylene and polypropylene. The first polyolefin resin is preferably composed mainly of polypropylene and polyethylene. In this specification, having polypropylene and polyethylene as main components means that polypropylene and polyethylene are contained in an amount of 95% or more, preferably 99% by mass or more, based on the total mass of the first polyolefin resin. As a kind of polyethylene, it is preferable that a high density polyethylene is a main component from a viewpoint of strength. The lower limit of the weight average molecular weight (hereinafter referred to as Mw) of the high-density polyethylene is preferably 1 × 10 ⁵ or more, more preferably 2 × 10 ⁵ or more. The upper limit of the Mw of the high density polyethylene is preferably 8 × 10 ⁵ or less, more preferably 7 × 10 ⁵ or less. When the Mw of the high-density polyethylene is in the above range, the stability of the film formation and the finally obtained puncture strength can both be achieved.

In the polyolefin multilayer microporous membrane 1 ′, it is important that the first microporous layer a contains polypropylene. When polypropylene is added to the first microporous layer a, the peel strength (adhesiveness) between the polyolefin multilayer microporous membrane 1 'and the porous layer 2 is further improved, and when used as a battery separator, meltdown The temperature can be further improved. As the type of polypropylene, a block copolymer and a random copolymer can be used in addition to the homopolymer. The block copolymer and random copolymer can contain a copolymer component with an α-olefin other than propylene, and ethylene is preferable as the other α-olefin.

The lower limit of Mw of polypropylene is preferably 5 × 10 ⁵ or more, more preferably 6.5 × 10 ⁵ or more, and still more preferably 8 × 10 ⁵ or more. When the Mw of the polypropylene is within the above range, a film having a uniform film thickness can be obtained without deteriorating the dispersibility of the polypropylene during sheet formation. In addition, although the upper limit of Mw of a polypropylene is not specifically limited, For example, it is 2 * 10 < ⁶ > or less.

The content of polypropylene is preferably 10% by mass or more and 50% by mass or less with respect to the total mass of the first polyolefin resin. When the content of polypropylene exceeds 50% by mass, ion permeability may be deteriorated. The lower limit of the polypropylene content is preferably 15% by mass or more, and more preferably 20% by mass or more. When the content of polypropylene is in the above range, both excellent adhesion between the polyolefin multilayer microporous membrane 1 ′ and the porous layer 2, and good meltdown characteristics and ion permeability can be achieved.

(2) Second microporous layer The second microporous layer b is made of only a polyethylene resin. In this specification, being made of only a polyethylene resin means that the polyethylene resin is 99% by mass or more. This is because contaminants derived from foreign substances and dirt adhering to the raw resin or polyolefin microporous membrane manufacturing process lines and equipment may be peeled off and mixed into the membrane.

The type of polyethylene used in the second microporous layer b, high density polyethylene such as density exceeding 0.94 g / cm ^3, density polyethylene in the range density of 0.93 ~ 0.94g / cm ³ , Low density polyethylene having a density lower than 0.93 g / cm ³ , linear low density polyethylene, ultrahigh molecular weight polyethylene, and the like. From the viewpoint of strength, high density polyethylene and ultra high molecular weight polyethylene may be contained. preferable. The polyethylene may be not only an ethylene homopolymer but also a copolymer containing a small amount of another α-olefin. Examples of the α-olefin include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene and the like. In the case of the polyolefin multilayer microporous membrane 1 ′, particularly when it is produced by a coextrusion method, it may be difficult to control uneven physical properties in the width direction due to a difference in viscosity between the layers. By using ultrahigh molecular weight polyethylene, the molecular network of the entire membrane is strengthened, so that non-uniform deformation hardly occurs and the multilayer microporous membrane 1 having excellent physical property uniformity can be obtained.

Here, the weight average molecular weight (hereinafter referred to as Mw) of the high density polyethylene is preferably 1 × 10 ⁵ or more, more preferably 2 × 10 ⁵ or more. The upper limit of the weight average molecular weight of the high density polyethylene is preferably 8 × 10 ⁵ , more preferably 7 × 10 ⁵ . When the Mw of the high-density polyethylene is in the above range, the stability of the film formation and the finally obtained puncture strength can both be achieved.

The Mw of the ultra high molecular weight polyethylene is preferably 1 × 10 ⁶ or more and less than 4 × 10 ⁶ . By using ultra high molecular weight polyethylene having an Mw of 1 × 10 ⁶ or more and less than 4 × 10 ⁶ , the pores and fibrils can be miniaturized and the puncture strength can be increased. In addition, when the Mw of the ultra high molecular weight polyethylene is 4 × 10 ⁶ or more, the viscosity of the melt becomes too high, and thus there may be a problem in the film forming process such that the resin cannot be extruded from the die. .

The content of the ultrahigh molecular weight polyethylene is preferably 5% by mass or more, more preferably 18% by mass or more, with respect to 100% by mass of the entire polyethylene resin constituting the second microporous layer b. . The upper limit of the content of ultrahigh molecular weight polyethylene is preferably 45% by mass or less, more preferably 40% by mass or less, based on 100% by mass of the entire polyethylene resin. When the content of the ultrahigh molecular weight polyethylene is in the above range, it is easy to obtain both puncture strength and air resistance. Further, when the content of the ultrahigh molecular weight polyethylene is within the above preferable range, sufficient tensile strength can be obtained even when the thickness of the polyolefin multilayer microporous membrane 1 ′ is reduced. The tensile strength of the polyolefin multilayer microporous membrane 1 'is preferably 100 MPa or more. There is no particular upper limit on the tensile strength.

The molecular weight distribution (Mw / Mn) which is the ratio of the weight average molecular weight (Mw) and the number average molecular weight (Mn) of the polyolefin resin of the first microporous layer a and the polyethylene resin of the second microporous layer b is respectively It is preferably in the range of 5 to 200, more preferably 10 to 100. When the range of Mw / Mn is the above preferable range, it is easy to extrude the polyolefin solution in the production process, and even when the thickness of the polyolefin multilayer microporous membrane 1 'is further reduced, sufficient machinery can be obtained. Strength is obtained. Mw / Mn is used as a measure of molecular weight distribution. For example, in the case of a polyolefin resin composed of a single substance, the larger this value, the wider the molecular weight distribution. Mw / Mn of a single polyolefin resin can be appropriately adjusted by multistage polymerization of polyolefin. Moreover, Mw / Mn of the mixture of polyolefin resin can be suitably adjusted by adjusting the molecular weight and mixing ratio of each component.

(3) Polyolefin multilayer microporous membrane The thickness of the polyolefin multilayer microporous membrane 1 ′ is not particularly limited, but the lower limit is 3 μm or more, more preferably 5 μm or more, and even more preferably 7 μm or more. From the viewpoint of increasing the capacity, it is 16 μm or less, more preferably 12 μm or less. When the film thickness of the polyolefin multilayer microporous membrane 1 ′ is in the above preferred range, practical membrane strength and pore blocking function can be retained, which is more suitable for increasing the capacity of the battery, which is expected to advance in the future. That is, the battery separator 10 of the present embodiment has the polyolefin microporous membrane 1 having a small thickness, the separator 10 between the polyolefin multilayer microporous membrane 1 'and the porous layer 2, and between the separator 10 and the electrode. And when the separator 10 is thinned, the effect is more clearly exhibited.

Here, the thickness ratio of the second microporous layer b is preferably 30% or more and 90% or less with respect to the entire layer (the whole) of the polyolefin multilayer microporous membrane 1 ′. The lower limit is more preferably 40% or more, and the upper limit is more preferably 80% or less. When the thickness ratio of the second microporous layer b is within the above range, the meltdown characteristics, the stability of the permeability in the usage range of the separator, and the balance of the puncture strength can be in a favorable range.

In the first microporous layer and the second microporous layer constituting the polyolefin multilayer microporous membrane 1 ′, an antioxidant, a heat stabilizer, an antistatic agent, Various additives such as an ultraviolet absorber, an antiblocking agent, a filler, or a nucleating agent may be contained. In particular, it is preferable to add an antioxidant for the purpose of suppressing oxidative deterioration due to the thermal history of the polyolefin resin. Appropriate selection of the type and amount of antioxidants and heat stabilizers is important for adjusting or enhancing the properties of the polyolefin multilayer microporous membrane 1 '. In addition, in this specification, the addition amount of these additives is not included in content of said 1st polyolefin resin and polyethylene resin.

Further, it is preferable that the polyolefin multilayer microporous membrane 1 ′ does not substantially contain inorganic particles. “Substantially free of inorganic particles” means, for example, a content of 50 ppm or less, preferably 10 ppm or less, most preferably the detection limit or less when inorganic elements are quantified by fluorescent X-ray analysis. Even if particles are not positively added to the polyolefin microporous membrane, contaminants derived from foreign substances and raw material resin or dirt attached to the line and equipment in the polyolefin microporous membrane manufacturing process are peeled off. It is because it may be mixed in.

The air resistance of this multi-layer, microporous polyolefin membrane 1 'has an upper limit of 300 sec / 100 cm ³ Air or less, preferably 200 sec / 100 cm ³ Air, more preferably at most 150 sec / 100 cm ³ Air. The lower limit of the air resistance of the multi-layer, microporous polyolefin membrane 1 ', 50 sec / 100 cm ³ Air or more, preferably 70 sec / 100 cm ³ Air or more, more preferably 100 sec / 100 cm ³ Air or more.

The upper limit of the porosity of the polyolefin multilayer microporous membrane 1 ′ is preferably 70% or less, more preferably 60% or less, and even more preferably 55% or less. The lower limit of the porosity is preferably 30% or more, more preferably 35% or more, and further preferably 40% or more. When the air permeability resistance and the porosity are within the above preferred ranges, sufficient battery charge / discharge characteristics, particularly ion permeability (charge / discharge operating voltage) and battery life (closely related to the amount of electrolyte retained) In this case, the function as a battery can be sufficiently exerted, and sufficient mechanical strength and insulation can be obtained, so that the possibility of a short circuit during charge / discharge is reduced.

Since the average pore diameter of the polyolefin multilayer microporous membrane 1 ′ greatly affects the pore closing performance, it is preferably 0.01 μm or more and 1.0 μm or less, more preferably 0.05 μm or more and 0.5 μm or less, and still more preferably 0.8. 1 μm or more and 0.3 μm or less. When the average pore diameter of the polyolefin multilayer microporous membrane 1 ′ is within the above preferred range, the response to the temperature of the pore clogging phenomenon does not become slow, and the pore clogging temperature due to the temperature rise rate does not shift to a higher temperature side. .

2. Production method of polyolefin multilayer microporous membrane The production method of polyolefin multilayer microporous membrane is not particularly limited as long as the polyolefin multilayer microporous membrane 1 'having the above-described properties can be produced, and conventionally known methods can be used. For example, the methods described in the specifications of Japanese Patent No. 2132327 and Japanese Patent No. 3347835, International Publication No. 2006/137540, and the like can be used.

The method for producing the polyolefin multilayer microporous membrane 1 ′ preferably includes the following steps (1) to (8).
(1) Step of preparing a first polyolefin resin solution by melt-kneading a first polyolefin resin containing polypropylene and a film-forming solvent (2) Melting and kneading a polyethylene resin and a film-forming solvent; Step (3) of preparing the polyolefin resin solution (3) Steps of coextruding the first and second polyolefin resin solutions to form a sheet and then cooling to obtain an extruded product (4) Stretching the extruded product (first (1) stretching step) to obtain a gel-like multilayer sheet (5) removing the film-forming solvent from the gel-like multilayer sheet and obtaining the multilayer sheet (6) drying the multilayer sheet, Step of obtaining (7) Step of drawing the first stretched multilayer sheet to obtain a second stretched multilayer sheet (8) Step of heat treating the second stretched multilayer sheet to obtain a polyolefin multilayer microporous membrane.

In the production method of the polyolefin multilayer microporous membrane 1 ′, for example, in the step (3), it is preferable that the first and second polyolefin resin solutions are simultaneously extruded by a multilayer die under a specific condition to form a multilayer sheet. . Thereby, when used as a battery separator, it has excellent meltdown temperature, mechanical strength, air permeability resistance and porosity, and a polyolefin multi-layer microporous membrane having a small maximum pore diameter. 1 can be manufactured. These characteristics cannot be achieved by the single-layer polyolefin microporous membrane 1. Further, in the step (1) and the step (2), after using the above-described resin material, in the step (4) and the step (7), the film is stretched under an appropriate temperature condition to be described later. Good porosity and fine pore structure control can be achieved.

Hereinafter, each step will be described.

Step (1) and Step (2): Preparation Steps of First and Second Polyolefin Resin Solution First, an appropriate film-forming solvent is added to the first polyolefin resin and polyethylene resin, respectively, and then melt-kneaded. First and second polyolefin resin solutions are prepared, respectively. As a melt-kneading method, for example, a method using a twin-screw extruder described in the specifications of Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Since the melt-kneading method is well-known, description is abbreviate | omitted.

The mixing ratio of the first or second polyolefin resin and the film-forming solvent in the first or second polyolefin resin solution is not particularly limited, but is 20 to 30 parts by mass of the first or second polyolefin resin. On the other hand, the film forming solvent is preferably 70 to 80 parts by mass. When the ratio of the first or second polyolefin resin is within the above range, swell and neck-in can be prevented at the die outlet when the first or second polyolefin resin solution is extruded, and an extruded molded body (gel-shaped molded body) ) And formability and self-supporting property are improved.

Step (3): Extrusion Forming Step Next, the first and second polyolefin resin solutions are respectively fed from an extruder to one die, where the two solutions are combined in layers and extruded into a sheet.

The extrusion method may be either a flat die method or an inflation method. In either method, the solution is supplied to separate manifolds and stacked in layers at the lip inlet of a multilayer die (multiple manifold method), or the solution is supplied to the die in a layered flow in advance (block method) ) Can be used. Since the multi-manifold method and the block method itself are known, a detailed description thereof will be omitted. The gap of the multi-layer flat die is 0.1 to 5 mm. The extrusion temperature is preferably 140 to 250 ° C., and the extrusion speed is preferably 0.2 to 15 m / min. By adjusting the extrusion amounts of the first and second polyolefin resin solutions, the film thickness ratio of the first and second microporous layers can be adjusted.

As the extrusion method, for example, methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used.

Next, an extruded molded body is formed by cooling the obtained laminated extruded molded body. As a forming method of the formed body, for example, methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Cooling is preferably performed at a rate of 50 ° C./min or more at least up to the gelation temperature. Cooling is preferably performed to 30 ° C. or lower. By cooling, the microphases of the first and second polyolefins separated by the film-forming solvent can be fixed. When the cooling rate is within the above range, the crystallinity is maintained in an appropriate range, and an extruded product suitable for stretching is obtained. As a cooling method, a method of contacting with a cooling medium such as cold air or cooling water, a method of contacting with a cooling roll, or the like can be used, but it is preferable that the cooling is performed by contacting with a roll cooled with a cooling medium.

Step (4) First Stretching Step Next, the obtained extruded molded body is stretched at least in a uniaxial direction (first stretching) to obtain a gel-like multilayer sheet. Since the extruded product contains a film-forming solvent, it can be stretched uniformly. The extruded body is preferably stretched at a predetermined ratio after heating by a tenter method, a roll method, an inflation 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 multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used.

In the case of uniaxial stretching, the stretching ratio (area stretching ratio) in this step is preferably 2 times or more, and more preferably 3 times or more and 30 times or less. In the case of biaxial stretching, 9 times or more is preferable, 16 times or more is more preferable, and 25 times or more is particularly preferable. Further, it is preferably 3 times or more in both the longitudinal direction and the transverse direction (MD and TD directions), and the draw ratios in the MD direction and the TD direction may be the same or different. When the draw ratio is 9 times or more, improvement of puncture strength can be expected. In addition, the draw ratio in this process means the area draw ratio of the microporous film immediately before being used for the next process on the basis of the microporous film immediately before this process. The stretching temperature is preferably 90 ° C. or higher and 130 ° C. or lower, more preferably 110 ° C. or higher and 120 ° C. or lower, and still more preferably 114 ° C. or higher and 117 ° C. or lower.

Depending on the desired physical properties, the film may be stretched by providing a temperature distribution in the film thickness direction, whereby a multilayer microporous film having further excellent mechanical strength can be obtained. Details of the method are described in Japanese Patent No. 3347854.

Step (5): Removal of film-forming solvent Next, the film-forming solvent is removed from the gel-like multilayer sheet to obtain a multilayer sheet. Removal (cleaning) of the film-forming solvent is performed using a cleaning solvent. In the gel-like multilayer sheet, the first and second polyolefin phases are phase-separated from the film-forming solvent phase. Therefore, when the film-forming solvent is removed, a porous film is obtained. The obtained porous film is composed of fibrils forming a fine three-dimensional network structure, and has pores (voids) that communicate irregularly three-dimensionally. Since the cleaning solvent and the method for removing the film-forming solvent using the same are known, the description thereof is omitted. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Laid-Open No. 2002-256099 can be used.

Step (6): Drying Next, the multilayer sheet is dried to obtain a first stretched multilayer sheet. Drying is preferably carried out until the residual cleaning solvent is 5% by mass or less, more preferably 3% by mass or less, with the multilayer microporous membrane being 100% by mass (dry weight). When the residual cleaning solvent is within the above range, the porosity of the multilayer microporous membrane is maintained when the second stretching step and heat treatment step described below are performed, and deterioration of permeability is suppressed. The drying temperature is preferably 50 ° C. or higher and 80 ° C. or lower.

Step (7): Second Stretching Step Next, the first stretched multilayer sheet is stretched (second stretch) to obtain a second stretched multilayer sheet. The first stretched multilayer sheet after drying is preferably stretched in at least a uniaxial direction. The first stretched multilayer sheet can be stretched by the tenter method or the like while heating. The stretching may be uniaxial stretching or biaxial stretching. In the case of biaxial stretching, either simultaneous biaxial stretching or sequential stretching may be used.

The stretching temperature in this step is not particularly limited, but is usually 90 to 135 ° C, more preferably 95 to 130 ° C.

The lower limit of the stretching ratio (area stretching ratio) in the uniaxial direction of stretching of the first stretched multilayer sheet in this step is preferably 1.0 times or more, more preferably 1.1 times or more, and further preferably 1 .2 times or more. The upper limit is preferably 1.8 times or less. In the case of uniaxial stretching, it is 1.0 to 2.0 times in the MD direction or TD direction. In the case of biaxial stretching, the lower limit of the area stretching ratio is preferably 1.0 times or more, more preferably 1.1 times or more, and still more preferably 1.2 times or more. The upper limit is preferably 3.5 times or less, and 1.0 to 2.0 times in each of the MD direction and the TD direction, and the draw ratios in the MD direction and the TD direction may be the same or different. In addition, the draw ratio in this process means the draw ratio of the 2nd extending | stretching multilayer sheet just before using for the following process on the basis of a 1st extending | stretching multilayer sheet.

Step (8): Heat Treatment Next, the second stretched multilayer sheet is heat treated to obtain a polyolefin multilayer microporous membrane 1 ′. By heat-treating the second stretched multilayer sheet, the crystals are stabilized and the lamella is made uniform. As the heat treatment method, heat setting treatment and / or heat relaxation treatment can be used. The heat setting treatment is a heat treatment in which heating is performed while keeping the dimensions of the film unchanged. The thermal relaxation treatment is a heat treatment that heat-shrinks the film in the MD direction or the TD direction during heating. The heat setting treatment is preferably performed by a tenter method or a roll method. For example, as a thermal relaxation treatment method, a method disclosed in Japanese Patent Application Laid-Open No. 2002-256099 can be given. The heat treatment temperature is more preferably within the range of ± 5 ° C. of the second stretching temperature of the second stretched multilayer sheet, and particularly preferably within the range of ± 3 ° C.

3. Porous layer The porous layer 2 contains two types of vinylidene fluoride-hexafluoropropylene copolymers (VdF-HFP) and inorganic particles. Hereinafter, each component which comprises the porous layer 2 is demonstrated below.

[Vinylidene fluoride-hexafluoropropylene copolymer (A)]
The vinylidene fluoride-hexafluoropropylene copolymer (A) (hereinafter sometimes simply referred to as copolymer (A)) is a copolymer containing vinylidene fluoride units and hexafluoropropylene units. As described later, it contains a hydrophilic group. The lower limit of the content of hexafluoropropylene units in the copolymer (A) is 0.3 mol%, preferably 0.5 mol%. When the content of the hexafluoropropylene unit is smaller than the above range, the polymer crystallinity becomes high and the degree of swelling of the separator with respect to the electrolytic solution decreases, so that the adhesion between the separator and the electrode decreases, and the electrode after injection of the electrolytic solution In some cases, sufficient adhesion between the separator and the separator (wet strength when wet) cannot be obtained. On the other hand, the upper limit of the content of hexafluoropropylene units is 5.0 mol%, more preferably 2.5 mol%. When the content of the hexafluoropropylene unit exceeds the above range, the separator may swell excessively with respect to the electrolytic solution, and the bending strength when wet may be reduced.

The lower limit of the weight average molecular weight of the copolymer (A) is 900,000, preferably 1,000,000. On the other hand, the upper limit of the weight average molecular weight of the copolymer (A) is 2 million, more preferably 1.5 million. When the weight average molecular weight of the copolymer (A) is within the above range, in the step of forming the porous layer, the time for dissolving the copolymer (A) in the solvent is not extremely long, and the production efficiency is increased. Can maintain an appropriate gel strength when swollen in the electrolyte, and can improve the bending strength when wet. In addition, the weight average molecular weight of a copolymer (A) is a polystyrene conversion value by a gel permeation chromatography.

The copolymer (A) has a hydrophilic group. Since the copolymer (A) has a hydrophilic group, the copolymer (A) can be more firmly bonded to the active material existing on the electrode surface and the binder component in the electrode. The reason for this is not particularly limited, but it is presumed that the adhesive force is improved by hydrogen bonding. Examples of the hydrophilic group include a hydroxyl group, a carboxylic acid group, a sulfonic acid group, and salts thereof. Among these, carboxylic acid groups and carboxylic acid esters are particularly preferable.

As a method for introducing a hydrophilic group into the copolymer (A), a known method can be used. For example, in the synthesis of the copolymer (A), maleic anhydride, maleic acid, maleic ester, malein A method of introducing a monomer having a hydrophilic group such as acid monomethyl ester into the main chain by copolymerization or a method of introducing it as a side chain by grafting can be used. 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, it can be determined from the absorption intensity ratio of C—H stretching vibration and C═O stretching vibration of a carboxyl group based on a homopolymer using FT-IR.

The lower limit of the hydrophilic group content of the copolymer (A) is preferably 0.1 mol%, more preferably 0.3 mol%. On the other hand, the upper limit of the hydrophilic group content is preferably 5.0 mol%, more preferably 4.0 mol%. When the content of the hydrophilic group exceeds 5.0 mol%, the polymer crystallinity becomes too low, the degree of swelling with respect to the electrolytic solution becomes high, and the bending strength when wet is deteriorated. Further, when the content of the hydrophilic group is within the above range, the affinity between the inorganic particles contained in the porous layer 2 and the copolymer (A) is increased, the short circuit resistance is improved, and the inorganic particles are removed. It also has an inhibitory effect. Although this reason is not specifically limited, it is guessed that the film | membrane intensity | strength of the porous layer 2 increases with the copolymer (A) which has a hydrophilic group which is the main component of the porous layer 2, and an inorganic particle. The quantification of the hydrophilic group of the vinylidene fluoride-hexafluoropropylene copolymer in the porous layer 2 can be determined by IR (infrared absorption spectrum) method, NMR (nuclear magnetic resonance) method or the like.

The copolymer (A) is a copolymer obtained by further polymerizing other monomers other than vinylidene fluoride, hexafluoropropylene, and a monomer having a hydrophilic group, as long as the characteristics are not impaired. Also good. Examples of other monomers include monomers such as tetrafluoroethylene, trifluoroethylene, trichloroethylene, and vinyl fluoride.

By setting the structure and molecular weight of the copolymer (A) within the above ranges, the separator 10 has a high affinity for the nonaqueous electrolyte when used in a nonaqueous electrolyte secondary battery, and is chemically and physically Stability is high, it exhibits bending strength when wet, and the affinity with the electrolyte is sufficiently maintained even when used at high temperatures.

[Vinylidene fluoride-hexafluoropropylene copolymer (B)]
The vinylidene fluoride-hexafluoropropylene copolymer (B) (hereinafter sometimes simply referred to as copolymer (B)) is a copolymer containing vinylidene fluoride units and hexafluoropropylene units. . The content of hexafluoropropylene in the copolymer (B) exceeds 5.0 mol%, more preferably 6.0 mol% or more, and even 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 during drying (peeling force during drying) may not be sufficiently obtained. On the other hand, the upper limit is 8.0 mol%, more preferably 7.5 mol%. On the other hand, when the content of the hexafluoropropylene unit exceeds 8.0 mol%, it may swell excessively with respect to the electrolytic solution, and the bending strength when wet may decrease. The copolymer (B) may contain a hydrophilic group or not.

The copolymer (B) has a weight average molecular weight of 100,000 to 750,000. When the weight average molecular weight of the copolymer (B) is in the above range, it has high affinity for the non-aqueous electrolyte, high chemical and physical stability, and excellent separator and electrode during drying. Adhesiveness (peeling force when dried) is obtained. The reason for this is not particularly limited, but the copolymer (B) has fluidity under heating and pressure conditions that develop a peeling force during drying, and becomes an anchor by entering the porous layer of the electrode. It can be presumed that the layer 2 and the electrode have strong adhesiveness. That is, in the battery separator 10, the copolymer (B) contributes to the peeling force at the time of drying, and can contribute to the deflection of the wound electrode body and the laminated electrode body, the prevention of distortion, and the improvement of the transportability. The copolymer (B) is a resin different from the copolymer (A).

The lower limit of the weight average molecular weight of the copolymer (B) is 100,000, preferably 150,000. When the weight average molecular weight of the copolymer (B) is below the lower limit of the above range, the amount of entanglement of the molecular chains is too small, so that the resin strength becomes weak and the porous layer 2 is liable to cohesive failure. On the other hand, the upper limit of the weight average molecular weight of the copolymer (B) is preferably 750,000, more preferably 700,000. When the weight average molecular weight of a copolymer (B) exceeds the upper limit of the said range, in order to obtain the peeling force at the time of drying, it is necessary to raise the press temperature in the manufacturing process of a winding body. If it does so, there exists a possibility that the microporous film | membrane which has polyolefin as a main component may shrink | contract. Moreover, when the weight average molecular weight of a copolymer (B) exceeds the upper limit of the said range, there exists a possibility that the amount of molecular chain entanglement may increase and it cannot fully flow on press conditions.

The lower limit of the melting point of the copolymer (B) is preferably 60 ° C, more preferably 80 ° C. On the other hand, the upper limit of the melting point of the copolymer (B) is preferably 145 ° C, more preferably 140 ° C. In addition, melting | fusing point (Tm) here is the temperature of the peak top of the endothermic peak at the time of temperature rising 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, similar to the copolymer (A). The melting point of the copolymer (B) can be adjusted by controlling the crystallinity of the site composed of vinylidene fluoride units. For example, when the copolymer (B) contains a monomer other than the vinylidene fluoride unit, the melting point can be adjusted by controlling the ratio of the vinylidene fluoride unit. Monomers other than vinylidene fluoride units are tetrafluoroethylene, trifluoroethylene, trichloroethylene, hexafluoropropylene, fluorinated vinyl maleic anhydride, maleic acid, maleic acid ester, maleic acid monomethyl ester, etc. You may have more. Examples thereof include a method in which the monomer is added when the copolymer (B) is polymerized and introduced into the main chain by copolymerization, or a method in which it is introduced as a side chain by grafting. Further, the melting point may be adjusted by controlling the ratio of Head-to-Head bonds (—CH ₂ —CF ₂ —CF ₂ —CH ₂ —) of vinylidene fluoride units.

[Contents of Copolymer (A) and Copolymer (B)]
The content of the copolymer (A) is 86% by mass, more preferably 88% by mass, with respect to 100% by mass of the total weight of the copolymer (A) and the copolymer (B). is there. The upper limit of the content of the copolymer (A) is 98% by mass, more preferably 97% by mass. The upper limit of the content of the copolymer (B) is 14% by mass, preferably 12% by mass with respect to 100% by mass of the total weight of the copolymer (A) and the copolymer (B). It is. The lower limit of the content of the copolymer (B) is 2% by mass, which is 3% by mass. When the content of the copolymer (A) and the content of the copolymer (B) are within the above ranges, the porous layer 2 has both excellent bending strength when wet and peeling strength when drying at a high level. Can do.

The porous layer 2 can contain a resin other than the copolymer (A) and the copolymer (B) as long as the effects of the present invention are not impaired. The copolymer (A) and the copolymer (B) are preferably used. In addition, when resin other than a copolymer (A) and a copolymer (B) is included, content of the said copolymer (A) or the said copolymer (B) is 100 mass of resin components of the porous layer 2. % As a percentage.

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

Inorganic particles include calcium carbonate, calcium phosphate, amorphous silica, crystalline glass particles, kaolin, talc, titanium dioxide, alumina, silica-alumina composite oxide particles, barium sulfate, calcium fluoride, lithium fluoride, zeolite , Molybdenum sulfide, mica, boehmite, magnesium oxide and the like. In particular, from the viewpoint of affinity with the vinylidene fluoride-hexafluoropropylene copolymer (A), inorganic particles containing a large amount of OH groups are preferred, and specifically, selected from titanium dioxide, alumina, and boehmite. It is preferable to use more than one type.

The content of the inorganic particles contained in the porous layer 2 is 80% by volume, preferably 70% by volume, more preferably 60% by volume with respect to 100% by volume of the solid content volume of the porous layer 2. It is. On the other hand, the lower limit of the content of inorganic particles is 40% by volume, more preferably 45% by volume, still more preferably 50% by volume, and most preferably 51% by volume. The content of the inorganic particles contained in the porous layer 2 was calculated by calculating the density of the copolymer (A) and the copolymer (B) as 1.77 g / cm ³ .

Generally, when inorganic particles having no adhesiveness are contained in the porous layer, the bending strength when wet and the peeling force when drying tend to decrease. However, as described above, the porous layer 2 according to the present embodiment contains a specific fluororesin in a specific ratio, so that when the inorganic particles are contained in the above range, the porous layer 2 has a high adhesive force to the electrode, The balance between the bending strength when wet and the peeling force when drying is good, and excellent short-circuit resistance can be obtained.

From the viewpoint of particle dropout, the average particle size 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 average flow pore size of the polyolefin microporous membrane. It is. The average flow pore size was measured according to JISK3832 and ASTM F316-86, and for example, measured in the order of Dry-up and Wet-up using a palm porometer (PMI, CFP-1500A). For the wet-up, pressure was applied to a microporous membrane sufficiently immersed in Galwick (trade name) manufactured by PMI with a known surface tension, and the pore size converted from the pressure at which air began to penetrate was defined as the maximum pore size. For the average flow pore size, the pore size was converted from the pressure at the point where the curve showing a 1/2 slope of the pressure / flow curve in the Dry-up measurement and the curve of the Wet-up measurement intersected. The following formula was used for conversion of pressure and pore diameter.
Formula: d = C · γ / P
In the above formula, “d (μm)” is the pore diameter of the microporous membrane, “γ (mN / m)” is the surface tension of the liquid, “P (Pa)” is the pressure, and “C” is a constant.

From the viewpoint of slipperiness with the winding core during cell winding and particle dropping, the average particle size of the inorganic particles is preferably 0.3 μm to 1.8 μm, more preferably 0.5 μm to 1.5 μm, still more preferably. 0.9 μm to 1.3 μm. The average particle diameter of the particles can be measured using a laser diffraction method or dynamic light scattering method measuring device. For example, particles dispersed in an aqueous solution containing a surfactant using an ultrasonic probe were measured with a particle size distribution measuring apparatus (manufactured by Nikkiso Co., Ltd., Microtrac HRA) and accumulated 50% from the small particle side in terms of volume. The value of the particle size (D50) at the time is preferably the average particle size. Examples of the shape of the particles include a true spherical shape, a substantially spherical shape, a plate shape, and a needle shape, but are not particularly limited.

[Physical 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 side, more preferably 1 μm or more and 2.5 μm or less, and further preferably 1 μm or more and 2 μm or less. When the film thickness per side is 0.5 μm or more, high adhesion to the electrode (bending strength when wet, peel strength when drying) can be secured. On the other hand, if the film thickness per side is 3 μm or less, the winding volume can be suppressed and the film can be made thinner, which is more suitable for increasing the capacity of batteries that will be developed in the future.

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

3. Manufacturing method of battery separator The manufacturing method of the battery separator is not particularly limited, and can be manufactured using a known method. Hereinafter, an example of a method for manufacturing a battery separator will be described. The battery separator manufacturing method can include the following steps (1) to (3) in sequence.
(1) A step of obtaining a fluororesin solution in which a vinylidene fluoride-hexafluoropropylene copolymer (A) and a vinylidene fluoride-hexafluoropropylene copolymer (B) are dissolved in a solvent. (2) Inorganic in the fluororesin solution. A step of adding particles, mixing and dispersing to obtain a coating solution (3) A step of applying the coating solution to the polyolefin microporous membrane, immersing it in a coagulation solution, washing and drying.

Step (1): Step of obtaining a fluororesin solution The vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B) are gradually added to a 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 is miscible with the coagulation liquid. From the viewpoint of solubility and low volatility, the solvent is preferably N-methyl-2-pyrrolidone.

Step (2): Step of obtaining a coating solution In order to obtain a coating solution, it is important to sufficiently disperse inorganic particles. Specifically, particles are added while stirring the fluororesin solution and pre-dispersed by stirring with a disper for a certain time (for example, about 1 hour), and then dispersed using a bead mill or paint shaker. Through the step (dispersing step), the aggregation of particles is reduced, and further mixed with a three-one motor with a stirring blade to prepare a coating solution.

Step (3): A step of applying the coating liquid to the microporous film, immersing it in the coagulating liquid, washing and drying, applying the coating liquid to the microporous film, immersing the applied microporous film in the coagulating liquid The vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B) are phase-separated, coagulated in a state having a three-dimensional network structure, washed and dried. As a result, a microporous membrane and a battery separator having a porous layer on the surface of the microporous membrane are obtained.

The method of applying the coating liquid to the microporous film may be a known method, for example, dip coating method, reverse roll coating method, gravure coating method, kiss coating method, roll brush method, spray coating method, Examples thereof include an air knife coating method, a Mayer bar coating method, a pipe doctor method, a blade coating method, and a die coating method, and these methods can be used alone or in combination.

The coagulation liquid preferably contains water as a main component, and is preferably an aqueous solution containing 1 to 20% by mass of a good solvent for the copolymer (A) and the copolymer (B), more preferably 5 to 15% by mass. It is an aqueous solution. Examples of the good solvent include N-methyl-2-pyrrolidone, N, N-dimethylformamide, and N, N-dimethylacetamide. The immersion time in the coagulation liquid is preferably 3 seconds or more. The upper limit is not limited, but 10 seconds is sufficient.

Water can be used for cleaning. Drying can be performed using, for example, hot air of 100 ° C. or less.

4). Battery Separator The battery separator 10 of the present embodiment can be suitably used for both a battery using an aqueous electrolyte and a battery using a non-aqueous electrolyte, but is preferably used for a non-aqueous electrolyte secondary battery. it can. 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. Especially, it is preferable to use as a separator of a lithium ion secondary battery.

In a non-aqueous electrolyte secondary battery, a positive electrode and a negative electrode are arranged via a separator, and the separator contains an electrolytic solution (electrolyte). The structure of the non-aqueous electrolyte electrode is not particularly limited, and a conventionally known structure can be used. For example, an electrode structure (coin type) in which a disk-like positive electrode and a negative electrode are opposed to each other, a flat plate-like structure An electrode structure (stacked type) in which positive and negative electrodes are alternately stacked, an electrode structure in which stacked belt-like positive and negative electrodes are wound (winding type), and the like can be used. The battery separator of this embodiment can have excellent adhesiveness between the separator and the electrode in any battery structure.

The current collector, the positive electrode, the positive electrode active material, the negative electrode, the negative electrode active material, and the electrolyte used in the non-aqueous electrolyte secondary battery including a lithium ion secondary battery are not particularly limited, and a conventionally known material is appropriately selected. They can be used in combination.

As shown in FIG. 1A, the battery separator 10 may be formed by laminating the porous 2 on one surface of the polyolefin microporous membrane 1, and porous on both surfaces of the polyolefin microporous membrane 1. 2 may be laminated.

5). Physical Properties of Battery Separator The wet bending strength of the separator 10 is preferably 4.0 N or more, more preferably 5.0 N or more, and still more preferably 6.0 N or more. The upper limit of the bending strength when wet is not particularly defined, but is, for example, 15.0 N or less. When the bending strength when wet is within the above preferable range, partial release at the interface between the separator and the electrode can be further suppressed, and the increase in battery internal resistance and the decrease in battery characteristics can be suppressed. In addition, the bending strength when wet can be measured by the method described in Examples described later.

The peeling force when the separator 10 is dried 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. Although the upper limit of the peeling force at the time of drying is not particularly defined, it is, for example, 40.0 N / m or less. When the peeling force at the time of drying is within the preferable range, it is expected that the wound electrode body or the laminated electrode body can be transported without the electrode body being scattered. In addition, the peeling force at the time of drying can be measured by the method as described in the below-mentioned Example.

The separator 10 of the present embodiment can achieve both a high bending strength when wet and a high peel strength when drying. Specifically, the separator 10 can satisfy a bending strength when wet of 4.0 N or more and a peel strength when dry of 2.0 N / m or more, as shown in Examples described later.

A preferable upper limit of the air resistance of the separator 10 is 350 sec / 100 cm ³ Air, preferably 250 sec / 100 cm ³ Air, and more preferably 200 sec / 100 cm ³ Air. The lower limit is 50 sec / 100 cm ³ Air, preferably 70 sec / 100 cm ³ Air, more preferably 100 sec / 100 cm ³ Air.

In addition, this invention is not limited to said embodiment, It can implement in various deformation | transformation within the range of the summary.

Hereinafter, the present invention will be described in more detail by way of examples, but the embodiments of the present invention are not limited to these examples. The evaluation methods, analysis methods and materials used in the examples are as follows.

(1) Film thickness, porosity, air resistance [film thickness]
Using a contact-type film thickness meter (“Lightmatic” (registered trademark) series 318 manufactured by Mitutoyo Corporation), the film thicknesses of the microporous film and the separator were measured. In the measurement, 20 points were measured under the condition of a weight of 0.01 N using a carbide spherical measuring element φ9.5 mm, and the average value of the obtained measured values was taken as the film thickness.

[Porosity]
The weight w _{1 of the} microporous membrane was compared with the weight w _{2 of the} polymer without pores equivalent to the weight (a polymer having the same width, length and composition), and the measurement was performed by the following equation.
Porosity (%) = (w ₂ −w ₁ ) / w ₂ × 100
[Air permeability resistance]
Using the digital type Oken type air permeability resistance tester EGO1 manufactured by Asahi Seiko Co., Ltd., the polyolefin laminated microporous membrane of the present invention is fixed so that wrinkles do not enter the measurement part. 8117 (2009). The sample was 5 cm square, the measurement point was one point in the center of the sample, and the measured value was the air resistance [seconds] of the sample. The same measurement was performed on 10 test pieces taken from arbitrary film positions, and the average value of the 10 measured values was defined as the air resistance of the polyolefin microporous membrane (sec / 100 cm ³ ).

(2) Weight average molecular weight (Mw) of vinylidene fluoride-hexafluoropropylene copolymer (A) and vinylidene fluoride-hexafluoropropylene copolymer (B)
It calculated | required by the gel permeation chromatography (GPC) method on the following conditions.
・ Measurement device: GPC-150C manufactured by Waters Corporation
・ Column: 2 shodex KF-806M manufactured by Showa Denko KK ・ Column temperature: 23 ° C
Solvent (mobile phase): 0.05M lithium chloride added N-methyl-2-pyrrolidone (NMP)
・ Solvent flow rate: 0.5 ml / min ・ Sample preparation: 4 mL of measurement solvent was added to 2 mg of the sample, and gently stirred at room temperature (dissolution was visually confirmed).
・ Injection volume: 0.2mL
・ Detector: Differential refractive index detector RI (RI-8020 type sensitivity 16 manufactured by Tosoh Corporation)
-Calibration curve: Prepared from a calibration curve obtained using a monodisperse polystyrene standard sample, using a predetermined conversion constant.

(3) Melting point With a differential scanning calorimeter (DSC manufactured by PerkinElmer Co., Ltd.), 7 mg of resin was put in a measurement pan to make a measurement sample, and measurement was performed under the following conditions. After the temperature was first raised and cooled, the peak top of the endothermic peak during the second temperature rise was taken as the melting point.
・ Temperature increase and cooling rate: ± 10 ° C / min.
-Measurement temperature range: 30-230 ° C.

(4) Bending strength when wet Generally, when a fluororesin binder is used for the positive electrode and a porous layer containing the fluororesin is provided on the separator, adhesion is easily secured by mutual diffusion between the fluororesins. . On the other hand, a binder other than a fluororesin is used for the negative electrode, and the diffusion of the fluororesin hardly occurs. Therefore, the negative electrode is less likely to have adhesion with the separator than the positive electrode. Therefore, in this measurement, the wet bending strength described below was measured and evaluated as an index of adhesiveness between the separator and the negative electrode.

(Preparation of negative electrode)
An aqueous solution containing 1.5 parts by mass of carboxymethylcellulose was added to 96.5 parts by mass of artificial graphite and mixed, and further 2 parts by mass of styrene butadiene latex as a solid content was added and mixed to obtain a negative electrode mixture-containing slurry. This negative electrode mixture-containing slurry is uniformly applied to both surfaces of a negative electrode current collector made of a copper foil having a thickness of 8 μm and dried to form a negative electrode layer. The density of the negative electrode layer excluding the body was 1.5 g / cm ³ to produce a negative electrode.

(Preparation of test winding)
The negative electrode 20 (machine direction 161 mm × width direction 30 mm) produced above and the produced separator 10 (machine direction 160 mm × width direction 34 mm) are stacked, and a metal plate (length 300 mm, width 25 mm, thickness 1 mm) is laminated. The separator 10 and the negative electrode 20 were wound so that the separator 10 was inside as a winding core, and the metal plate was pulled out to obtain a test winding body 30. The test winding was about 34 mm long and about 28 mm wide.

(Method of measuring bending strength when wet)
Two laminated films made of polypropylene (length 70 mm, width 65 mm, thickness 0.07 mm) were stacked, and the test roll 30 was placed in a bag-shaped laminate film 22 in which three of the four sides were welded. . 500 μL of an electrolytic solution in which LiPF ₆ was dissolved at a ratio of 1 mol / L in a solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 3: 7 was injected from the opening of the laminate film 22 in a glove box, The rotating body 30 was impregnated, and one side of the opening was sealed with a vacuum sealer.

Next, the test roll 30 enclosed in the laminate film 22 is sandwiched between two gaskets (thickness 1 mm, 5 cm × 5 cm), and placed in a precision heating and pressing apparatus (CYPT-10, manufactured by Shinto Kogyo Co., Ltd.). The mixture was pressurized at 98 ° C. and 0.6 MPa for 2 minutes and allowed to cool at room temperature. With the test roll 30 after being pressurized, the bending strength when wet was measured using a universal testing machine (manufactured by Shimadzu Corporation, AGS-J) while encapsulated in the laminate film 22. Details will be described below with reference to FIG.

Two aluminum L-shaped angles 41 (thickness 1 mm, 10 mm x 10 mm, length 5 cm) are arranged in parallel so that the 90 ° part is on top, with the ends aligned, and the fulcrum with the 90 ° part as a fulcrum The distance was fixed to 15 mm. The length direction of the L-shaped angle 41 by aligning the midpoint of the width direction of the test winding body (about 28 mm) with the 7.5 mm point which is the middle of the distance between the fulcrums of the two aluminum L-shaped angles 41 The test winding body 30 was arranged so as not to protrude from the sides of the test piece.

Next, the length direction side (about 34 mm) of the test winding body does not protrude from the length direction side of the aluminum L-shaped angle 42 (thickness 1 mm, 10 mm × 10 mm, length 4 cm) as an indenter. The 90 ° portion of the aluminum L-shaped angle 42 is aligned with the midpoint of the side in the width direction of the test winding body, and the aluminum L-shaped angle 42 is placed so that the 90 ° portion is down. It was fixed to the load cell (load cell capacity 50N) of the universal testing machine. The average value of the maximum test force obtained by measuring the three test winding bodies at a load speed of 0.5 mm / min was defined as the bending strength when wet.

(5) Peeling force during drying (Preparation of negative electrode)
The same negative electrode 20 as in the case of the bending strength when wet was used.

(Preparation of peel test piece)
The negative electrode 20 produced above (70 mm × 15 mm) and the produced separator 10 (machine direction 90 mm × width direction 20 mm) are stacked and sandwiched between two gaskets (thickness 0.5 mm, 95 mm × 27 mm), Pressurization was performed at 90 ° C. and 8 MPa for 2 minutes with a precision heating and pressurizing apparatus (CYPT-10, manufactured by Shinto Kogyo Co., Ltd.), and the mixture was allowed to cool at room temperature. A double-sided tape having a width of 1 cm is attached to the negative electrode side of the laminate of the negative electrode 20 and the separator 10, and the other side of the double-sided tape is attached to a SUS plate (thickness 3 mm, length 150 mm × width 50 mm). The pasting was performed so that the machine direction and the SUS plate length direction were parallel. This was made into the peeling test piece.

(Measurement method of peel strength during drying)
A separator 10 was sandwiched between load cell side chucks using a universal testing machine (AGS-J, manufactured by Shimadzu Corporation), and a 180 degree peel test was performed at a test speed of 300 mm / min. A value obtained by averaging measured values from a stroke of 20 mm to 70 mm during the peel test was defined as the peel strength of the peel test piece. A total of three peel test pieces were measured, and a value obtained by converting the average peel force into a width was defined as a peel force during drying (N / m).

(6) Short-circuit resistance test The short-circuit resistance was evaluated using a desktop precision universal testing machine Autograph AGS-X (manufactured by Shimadzu Corporation). First, as shown in FIG. 3A, a polypropylene insulator 5 (thickness 0.2 mm), a lithium ion battery negative electrode 21 (total thickness: about 140 μm, base material: copper foil (thickness: about 9 μm), active Material: Sample laminate 31 was prepared by laminating artificial graphite (particle size: about 30 μm, double-sided coating), separator 10, and aluminum foil 4 (thickness: about 0.1 mm). Next, as shown in FIG. 3B, the sample laminate 31 was fixed to the compression jig (lower side) 44 of the universal testing machine with double-sided tape. Next, the aluminum foil 4 and the negative electrode 21 of the sample laminate 31 were connected to a circuit composed of 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)) having a diameter of about 500 μm was placed between the separator in the sample laminate 31 and the aluminum foil 4. Next, a compression jig is attached to the universal testing machine, and the sample laminate 31 including the metal balls 6 is placed between both compression jigs 43 and 44 as shown in FIG. / Min. The test was terminated when the load reached 100 N. At this time, the part where the inflection point appeared in the change in compressive load was taken as the film breaking point of the separator, and the moment when the circuit was formed and the current was detected via the metal sphere was taken as the short-circuit occurrence point. The compression displacement A (t) when the separator breaks due to compression and an inflection point occurs in the compression stress, and the compression displacement B (t) at the moment when the current flows through the circuit are measured. If the numerical value obtained in 1.1 is 1.1 or more, even if the separator breaks due to foreign matter mixed in the battery, it means that the insulation is maintained by the coating layer composition adhering to the surface of the foreign matter, Short-circuit resistance was 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, the film breakage and short circuit of the separator do not occur at the same time, but the tension applied to the battery member or the electrode during charging / discharging In order to prevent a short circuit from occurring even when the internal pressure of the battery increases due to expansion, a certain level of resistance is required. When the numerical value calculated by (Equation 1) is 1.0, a short circuit occurred at the same time as the film breakage of the separator, and no improvement in the short circuit resistance by the coating layer was observed, so the short circuit resistance was evaluated as poor. .
B (t) ÷ A (t) (Formula 1).

(7) Peel strength between the porous layer and polyolefin multilayer microporous membrane (tape peel strength)
(Preparation of peel test piece)
The separators (machine direction: 120 mm × width direction: 25 mm) prepared in Examples and Comparative Examples were installed on a glass plate so that air did not enter. Double-sided tape (machine direction 100mm x width direction 20mm, manufactured by Seiwa Sangyo Co., Ltd., transparent film double-sided tape SFR-2020) is installed so that the machine direction of the separator and the machine direction of the separator are aligned. The sample was subjected to 5 reciprocating treatments using a roller (SA-1003-B, tester industry, manual type, rubber strength 80 ± 5 Hs). A cellophane tape (manufactured by Nichiban Co., Ltd., cello tape (registered trademark), plant-based, No. 405, machine direction: 100 mm × width direction: 15 mm) is pasted on the separator side of the laminate of the double-sided tape and the separator, and the rest is left. A piece of paper cut in a machine direction of 120 mm and a width direction of 25 mm was attached to a site of about 10 mm. This was pressure-bonded 5 times with a 2 kg rubber roller. Remove the release liner from the double-sided tape, attach it to a SUS plate (thickness 3 mm, length 150 mm x width 50 mm) so that the machine direction of the separator and the length direction of the SUS plate are parallel, and reciprocate twice with a 2 kg rubber roller. Crimped. This was made into the peeling test piece.

(Measurement method of tape peeling force)
Using a universal testing machine (AGS-J, manufactured by Shimadzu Corporation), the paper cut on the cellophane tape and cut in the machine direction 120 mm x width direction 25 mm is sandwiched between the load cell side chuck, and the SUS plate side is placed on the opposite lower chuck A 180 degree peel test was carried out at a test speed of 100 mm / min. A value obtained by averaging measured values from a stroke of 20 mm to 70 mm during the peel test was defined as the peel strength of the peel test piece. A total of three peel test pieces were measured, and the average peel force was defined as the tape peel force.

In this case, the porous layer surface may remain on the polyolefin multilayer microporous membrane side at the peeling interface. In this case, the peel strength between the porous layer and the polyolefin multilayer microporous membrane was calculated.
The peel strength between the porous layer and the polyolefin multilayer microporous membrane is preferably 0.15 N / mm or more, more preferably 0.20 N / mm or more, and most preferably 0.25 N / mm or more.

Example 1
(1) Preparation of First Polyolefin Resin Solution 20% by mass of polypropylene (PP: melting point 162 ° C.) having a Mw of 2.0 × 10 ⁶ and high density polyethylene (HDPE: density of 0.6) having an Mw of 5.6 × 10 ⁵ . To 100 parts by mass of the first polyolefin resin consisting of 80% by mass (955 g / cm ³ , melting point 135 ° C.), the antioxidant tetrakis [methylene-3- (3,5-ditertiarybutyl-4-hydroxyphenyl) -propionate] A mixture was prepared by blending 0.2 part by mass of methane. 25 parts by mass of the obtained mixture was charged into a twin screw extruder, 75 parts by mass of liquid paraffin [35 cst (40 ° C.)] was supplied from the side feeder of the twin screw extruder, and melt kneaded under the conditions of 210 ° C. and 250 rpm. Thus, a first polyolefin resin solution was prepared.

(2) Preparation of second polyolefin resin solution 40% by mass of ultra high molecular weight polyethylene (UHMwPE) having Mw of 2.0 × 10 ⁶ and high density polyethylene (HDPE: density of 0.955 g) having Mw of 5.6 × 10 ⁵ / Cm ³ ) 100 parts by mass of a second polyolefin resin composed of 60% by mass with 0.2 parts by mass of the antioxidant tetrakis [methylene-3- (3,5-ditertiarybutyl-4-hydroxyphenyl) -propionate] methane Parts were blended to prepare a mixture. 25 parts by mass of the obtained mixture was charged into another twin screw extruder of the same type as described above, and 75 parts by mass of liquid paraffin [35 cSt (40 ° C.)] was supplied from the side feeder of the twin screw extruder. A second polyolefin resin solution was prepared by melt-kneading under the same conditions.

(3) Extrusion The first and second polyolefin resin solutions are supplied from each twin-screw extruder to the three-layer T-die, and the first polyolefin resin solution / second polyolefin resin solution / first polyolefin resin solution. The film was extruded so as to have a layer thickness ratio of 10/80/10, and cooled while being drawn at a take-up speed of 2 m / min with a cooling roll adjusted to 30 ° C. to obtain a three-layer extrudate.

(4) First stretching, removal of film-forming solvent, and drying Three-layer extrusion molded body was simultaneously biaxially stretched (first stretching) by 5 times at 116 ° C. in both MD and TD directions by a tenter stretching machine. Subsequently, the paraffin was removed by immersion in a methylene chloride bath adjusted to 25 ° C., and then dried in a drying furnace adjusted to 60 ° C. to obtain a first stretched multilayer sheet.

(5) Second stretching and heat setting The first stretched multilayer sheet was stretched 1.4 times (second stretching) in the TD direction at 126 ° C using a batch stretching machine. Next, this membrane was heat-set at 126 ° C. by a tenter method to obtain a polyolefin three-layer microporous membrane A having a thickness of 12 μm, a porosity of 46%, and an air resistance of 150 seconds / 100 cc.

[Vinylidene fluoride-hexafluoropropylene copolymer (A)]
As copolymer (A), copolymer (A1) was synthesized as follows. The molar ratio of vinylidene fluoride / hexafluoropropylene / maleic acid monomethyl ester was 98.0 / 1.5 / 0.5 by suspension polymerization using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials. Thus, a copolymer (A1) was synthesized. The weight average molecular weight of the obtained copolymer (A1) was 1,500,000.

[Vinylidene fluoride-hexafluoropropylene copolymer (B)]
As copolymer (B), copolymer (B1) was synthesized as follows. A copolymer (B1) was synthesized by suspension polymerization using vinylidene fluoride and hexafluoropropylene as starting materials 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.

[Preparation of battery separator]
26.5 parts by mass of copolymer (A1) and 3.5 parts by mass of copolymer (B1) and 600 parts by mass of N-methyl-2-pyrrolidone (NMP) were mixed, and then stirred with a disper. Alumina particles (average particle size 1.1 μm, density 4.0 g / cc) were added so that the solid content of the porous layer was 100% by volume to be 51% by volume, and further pre-stirred at 2000 rpm with a disper for 1 hour. . Next, using a dyno mill (Dynomill Multilab (1.46 L container, filling rate 80%, φ0.5 mm alumina beads) made by Shinmaru Enterprises) under the conditions of a flow rate of 11 kg / hr and a peripheral speed of 10 m / s. It processed 3 times and produced the coating liquid (A). The obtained coating liquid (A) was applied to both sides of the polyolefin three-layer microporous membrane A in an equal amount by a dip coating method. The coated film is immersed in an aqueous solution (coagulation solution) containing 10% by mass of N-methyl-2-pyrrolidone (NMP), washed with pure water, and then dried at 50 ° C. to obtain a battery separator. It was. The thickness of the battery separator was 15 μm.

(Example 2)
In the preparation of the first polyolefin resin solution, the thickness was 12 μm, the porosity was 45%, in the same manner as in Example 1 except that the blending amount of polypropylene was 10% by mass and the blending amount of high-density polyethylene was 90% by mass. A polyolefin three-layer microporous membrane B having an air resistance of 135 seconds / 100 cc was obtained. A battery separator was obtained in the same manner as in Example 1 except that the polyolefin three-layer microporous membrane B was used.

(Example 3)
In the preparation of the first polyolefin resin solution, the thickness was 12 μm, the porosity was 48%, in the same manner as in Example 1 except that the blending amount of polypropylene was 45 mass% and the blending amount of high-density polyethylene was 55 mass%. A polyolefin three-layer microporous membrane C having an air resistance of 300 seconds / 100 cc was obtained. A battery separator was obtained in the same manner as in Example 1 except that the polyolefin three-layer microporous membrane C was used.

Example 4
As copolymer (B), copolymer (B2) was synthesized as follows. A copolymer (B2) was synthesized by suspension polymerization using vinylidene fluoride and hexafluoropropylene as starting materials 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) in which the copolymer (B1) was replaced with the copolymer (B2) was used in the preparation of the coating liquid.

(Example 5)
As the copolymer (B), a copolymer (B3) was synthesized as follows. A copolymer (B3) was synthesized by suspension polymerization using vinylidene fluoride and hexafluoropropylene as starting materials 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) in which the copolymer (B1) was replaced with the copolymer (B3) was used in the preparation of the coating liquid.

(Example 6)
As copolymer (A), copolymer (A2) was synthesized as follows. The molar ratio of vinylidene fluoride / hexafluoropropylene / maleic acid monomethyl ester was 99.0 / 0.5 / 0.5 by suspension polymerization using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials. Thus, a copolymer (A2) was synthesized. The weight average molecular weight of the obtained copolymer (A2) was 1,400,000. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (D) in which the copolymer (A1) was replaced with the copolymer (A2) was used in the preparation of the coating liquid.

(Example 7)
As copolymer (A), copolymer (A3) was synthesized as follows. The molar ratio of vinylidene fluoride / hexafluoropropylene / maleic acid monomethyl ester was 95.0 / 4.5 / 0.5 by suspension polymerization using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials. Thus, a copolymer (A3) was synthesized. The weight average molecular weight of the obtained copolymer (A3) was 1,700,000. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (E) in which the copolymer (A1) was replaced with the copolymer (A3) was used in the preparation of the coating liquid.

(Example 8)
As copolymer (A), copolymer (A4) was synthesized as follows. The molar ratio of vinylidene fluoride / hexafluoropropylene / maleic acid monomethyl ester was 98.0 / 1.5 / 0.5 by suspension polymerization using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials. Thus, a copolymer (A4) was synthesized. The weight average molecular weight of the obtained copolymer (A4) was 1,900,000. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (F) in which the copolymer (A1) was replaced with the copolymer (A4) was used in the preparation of the coating liquid.

Example 9
In the preparation of the coating liquid, the blending ratio of the copolymer (A1) and the copolymer (B1) was 28.0 parts by mass of the copolymer (A1) and 2.0 parts by mass of the copolymer (B1). A battery separator was obtained in the same manner as in Example 1 except that the working liquid (G) was used.

(Example 10)
In the preparation of the coating liquid, the content of alumina particles was set to 40% by volume with the solid content of the porous layer being 100% by volume, and 35.2 parts by mass of the copolymer (A1) and the copolymer (B1). A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (H) in which 4.7 parts by mass and NMP was changed to 900 parts by mass was used.

(Example 11)
In the preparation of the coating liquid, the content of alumina particles is 75% by volume with the solid content of the porous layer being 100% by volume, and 11.4 parts by mass of copolymer (A1) and copolymer (B1). A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (I) in which 1.5 parts by mass and NMP was changed to 300 parts by mass was used.

(Example 12)
As copolymer (A), copolymer (A5) was synthesized as follows. The molar ratio of vinylidene fluoride / hexafluoropropylene / maleic acid monomethyl ester was 98.4 / 1.5 / 0.1 by suspension polymerization using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials. Thus, a copolymer (A5) was synthesized. The weight average molecular weight of the obtained copolymer (A5) was 1,500,000. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (J) was used instead of the copolymer (A5) in the preparation of the coating liquid.

(Example 13)
As copolymer (A), copolymer (A6) was synthesized as follows. The molar ratio of vinylidene fluoride / hexafluoropropylene / maleic acid monomethyl ester was 94.5 / 1.5 / 4.0 by suspension polymerization using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials. Thus, a copolymer (A6) was synthesized. The weight average molecular weight of the obtained copolymer (A6) was 1,500,000. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (K) in which the copolymer (A1) was replaced with the copolymer (A6) was used in the preparation of the coating liquid.

(Example 14)
The amount of extrusion of the first and second polyolefin resin solutions was adjusted, and a polyolefin three-layer fine layer having a layer thickness ratio of 10/80/10, a thickness of 7 μm, a porosity of 37%, and an air resistance of 120 seconds / 100 cm ³ A porous membrane D was obtained. A battery separator was obtained in the same manner as in Example 1 except that the polyolefin three-layer microporous membrane D was used. The thickness of the battery separator was 10 μm.

(Example 15)
The amount of extrusion of the first and second polyolefin resin solutions was adjusted, and the layer thickness ratio was 10/80/10, the thickness was 16 μm, the porosity was 45%, and the air permeability resistance was 200 seconds / 100 cm ^3. A porous membrane E was obtained. A battery separator was obtained in the same manner as in Example 1 except that the polyolefin three-layer microporous membrane E was used. The thickness of the battery separator was 19 μm.

(Example 16)
As the copolymer (B), a copolymer (B4) was synthesized as follows. A copolymer (B4) was synthesized by suspension polymerization using vinylidene fluoride and hexafluoropropylene as starting materials 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 700,000. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (L) in which the copolymer (B1) was replaced with the copolymer (B4) was used in the preparation of the coating liquid.

(Example 17)
In preparation of the coating liquid, the alumina particles are replaced with plate-like boehmite particles (density 3.07 g / cm ³ ) having an average particle diameter of 1.0 μm and an average thickness of 0.4 μm, and the copolymer (A1) 31.5 mass. Battery separator was obtained in the same manner as in Example 1 except that the coating liquid (M) was used in an amount of 4.2 parts by weight of the copolymer (B1).

(Example 18)
In preparation of the coating liquid, the alumina particles are replaced with an average particle diameter of 0.4 μm, titania particles (density 4.23 g / cc), 25.3 parts by mass of the copolymer (A1), and the copolymer (B1) 3.4. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (N) in mass parts was used.

(Example 19)
In the preparation of the coating solution, the blending ratio of the copolymer (A1) and the copolymer (B1) was 29.0 parts by mass of the copolymer (A1) and 1.0 part by mass of the copolymer (B1). A battery separator was obtained in the same manner as in Example 1 except that the working liquid (O) was used.

(Comparative Example 1)
In the preparation of the first polyolefin resin solution, the thickness was 12 μm, the porosity was 44%, and the air resistance was the same as in Example 1 except that polypropylene was not used and the blending amount of the high-density polyethylene was 100% by mass. A polyolefin three-layer microporous membrane F of 100 seconds / 100 cm ³ was obtained. A battery separator was obtained in the same manner as in Example 1 except that the polyolefin three-layer microporous membrane F was used.

(Comparative Example 2)
In the preparation of the first polyolefin resin solution, the thickness was 12 μm, the porosity was 45%, in the same manner as in Example 1 except that the blending amount of polypropylene was 5 mass% and the blending amount of high-density polyethylene was 95 mass%. A polyolefin three-layer microporous membrane G having an air resistance of 125 sec / 100 cm ³ was obtained. A battery separator was obtained in the same manner as in Example 1 except that the polyolefin three-layer microporous membrane G was used.

(Comparative Example 3)
In the preparation of the first polyolefin resin solution, the thickness was 12 μm, the porosity was 37%, in the same manner as in Example 1 except that the blending amount of polypropylene was 80% by mass and the blending amount of high-density polyethylene was 20% by mass. A polyolefin three-layer microporous membrane H having an air resistance of 815 sec / 100 cm ³ was obtained. A battery separator was obtained in the same manner as in Example 1 except that the polyolefin three-layer microporous membrane H was used.

(Comparative Example 4)
In preparation of the coating liquid, except that the coating liquid (P) in which 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 were dissolved and mixed was used. Obtained a battery separator in the same manner as in Example 1.

(Comparative Example 5)
In the preparation of the coating liquid, alumina particles were added so that the solid content of the porous layer was 100% by volume to 95% by volume, and 2.0 parts by mass of the copolymer (A1), 0. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (Q) in which 3 parts by mass and NMP was changed to 250 parts by mass was used.

(Comparative Example 6)
In the preparation of the coating liquid, the blending ratio of the copolymer (A1) and the copolymer (B1) was 15.0 parts by mass of the copolymer (A1) and 15.0 parts by mass of the copolymer (B1). A battery separator was obtained in the same manner as in Example 1 except that the working liquid (R) was used.

(Comparative Example 7)
As copolymer (A), copolymer (A7) was synthesized as follows. A copolymer (A7) was synthesized by suspension polymerization using vinylidene fluoride and hexafluoropropylene as starting materials 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 1,500,000. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (S) in which the copolymer (A1) was replaced with the copolymer (A7) was used in the preparation of the coating liquid.

(Comparative Example 8)
In the preparation of the coating liquid, the coating liquid (T) prepared without using the copolymer (B) by replacing the copolymer (A) with 30.0 parts by weight of polyvinylidene fluoride (weight average molecular weight 1,500,000) A battery separator was obtained in the same manner as in Example 1 except that was used.

(Comparative Example 9)
As copolymer (A), copolymer (A8) was synthesized as follows. The molar ratio of vinylidene fluoride / hexafluoropropylene / maleic acid monomethyl ester was 98.0 / 1.5 / 0.5 by suspension polymerization using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials. Thus, a copolymer (A8) was synthesized. The weight average molecular weight of the obtained copolymer (A8) was 650,000. A battery was prepared in the same manner as in Example 1 except that in the preparation of the coating liquid, the copolymer (A1) was replaced with the copolymer (A8) and the coating liquid (U) in which NMP was changed to 500 parts by mass was used. A separator was obtained.

(Comparative Example 10)
As copolymer (B), copolymer (B5) was synthesized as follows. A copolymer (B5) was synthesized by suspension polymerization using vinylidene fluoride and hexafluoropropylene as starting materials 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) was used instead of the copolymer (B5) in the preparation of the coating liquid.

(Comparative Example 11)
A battery separator was obtained in the same manner as in Comparative Example 1 except that the polyolefin three-layer microporous membrane D was used. The thickness of the battery separator was 10 μm.

The composition and characteristics of the polyolefin multilayer microporous membrane used in the above Examples and Comparative Examples are shown in Table 1, and the structure of the copolymer (A) and copolymer (B) of the porous layer, the weight average molecular weight, coating The composition of the liquid and the characteristics of the obtained battery separator are shown in Table 2.

The battery separator of the present embodiment, when used in a non-aqueous electrolyte secondary battery, satisfies the peeling force during drying and the bending strength when wet, and the adhesion between the separators of the polyolefin multilayer microporous membrane and the porous layer. And the separator for batteries which is excellent in the adhesiveness between a separator and an electrode, and is excellent in short circuit tolerance can be provided. Therefore, the battery separator according to the present embodiment can be suitably used even when a larger size and a higher capacity of a battery (particularly a laminate type battery) are required in the future.

DESCRIPTION OF SYMBOLS 1 ... Polyolefin microporous film 1 '... Polyolefin multilayer microporous film a ... 1st microporous layer b ... 2nd microporous layer 2 ... Porous layer 4 ... Aluminum foil 5 ... Resin insulator 6 ... Metal sphere 10 ... Battery separator 20 ... negative electrode (for adhesion evaluation)
21 ... Negative electrode (for short circuit resistance evaluation)
22 ... Laminate film 30 ... Electrode roll 31 ... Electrode laminate 41 ... Aluminum L-shaped angle (lower side)
42 ... Aluminum L-shaped angle (upper side)
43 ... Compression jig (upper side)
44 ... Compression jig (lower side)

Claims

A separator for a battery comprising a polyolefin microporous membrane and a porous layer on at least one surface of the polyolefin microporous membrane,
The polyolefin microporous membrane comprises a polyolefin multilayer microporous membrane having a three-layer structure in which the first microporous layer / second microporous layer / first microporous layer are laminated in this order.
The first microporous layer is made of a first polyolefin resin containing polyethylene and polypropylene, and the content of the polypropylene is 10% by mass or more and 50% by mass with respect to the total mass of the first polyolefin resin. %, And the second microporous layer is made of only polyethylene resin,
The porous layer includes a vinylidene fluoride-hexafluoropropylene copolymer (A), a vinylidene fluoride-hexafluoropropylene copolymer (B), and inorganic particles,
The vinylidene fluoride-hexafluoropropylene copolymer (A) has not less than 0.3 mol% and not more than 5.0 mol% of hexafluoropropylene units, and has a weight average molecular weight of not less than 900,000 and not more than 2 million, And includes a hydrophilic group,
The vinylidene fluoride-hexafluoropropylene copolymer (B) has more than 5.0 mol% and not more than 8.0 mol% hexafluoropropylene units, and has a weight average molecular weight of 100,000 to 750,000,
The total amount of the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B) is 100% by mass, based on the vinylidene fluoride-hexafluoropropylene copolymer ( A) is contained in an amount of 86% by mass or more and 98% by mass or less, and the inorganic particles are contained in an amount of 40% by volume or more and 80% by volume or less with respect to 100% by volume of the solid content in the porous layer.
Battery separator.
The battery separator according to claim 1, wherein the vinylidene fluoride-hexafluoropropylene copolymer (A) contains 0.1 mol% or more and 5.0 mol% or less of a hydrophilic group.
The battery separator according to claim 1 or 2, 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 any one of claims 1 to 3, wherein the inorganic particles are at least one selected from titanium dioxide, alumina, and boehmite.
The battery separator according to any one of claims 1 to 4, wherein the polyolefin multilayer microporous membrane has a thickness of 3 µm to 16 µm.
An electrode body comprising a positive electrode, a negative electrode, and the battery separator according to any one of claims 1 to 5.
A nonaqueous electrolyte secondary battery comprising the electrode body according to claim 6 and a nonaqueous electrolyte.