TW201843866A - Separator for batteries, electrode body and nonaqueous electrolyte secondary battery - Google Patents

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 non-aqueous electrolyte secondary battery.

Among non-aqueous electrolyte secondary batteries, 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 non-aqueous electrolyte secondary battery include a cylindrical battery, a rectangular battery, and a stacked battery. Generally, these batteries have a structure in which an electrode body and a non-aqueous electrolytic solution are housed in an outer packaging body, in which an electrode system is configured by arranging a positive electrode and a negative electrode through a separator. Examples of the structure of the electrode body include a stacked electrode body in which a positive electrode and a negative electrode are stacked through a separator, and a wound electrode body in which a positive electrode and a negative electrode are wound in a spiral shape through a separator.

Conventionally, as a battery separator, a microporous film made of a polyolefin resin has been mainly used. The microporous membrane made of a polyolefin resin has a so-called shutdown function, so that when the battery abnormally generates heat, the pores of the separator are blocked to prevent current flow, thereby preventing fire and the like.

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 a polyolefin resin. For example, a battery separator is proposed in which a porous layer containing a fluororesin is provided on one or both sides of a layer made of a polyolefin resin. In addition, it is known that inorganic particles are added to the porous layer, thereby preventing the melt-shrinkage of the separator and suppressing the expansion of the short-circuited portion between the electrodes even if sharp metal penetrates the battery due to an accident or the like and causes an emergency short circuit and generates heat.

For example, Patent Document 1 describes an electrode body including a positive electrode, a negative electrode, a three-layer separator, and an adhesive resin layer. The three-layer separator is made of polypropylene / polyethylene / polypropylene and the adhesive resin layer. Disposed between these electrodes and the separator and made of polydifluoroethylene and alumina powder.

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

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

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

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

In addition, Example 23 of Patent Document 6 describes a separator comprising a VdF-HFP copolymer (1.93 million weight average molecular weight, 1.1 mol% of HFP units) and a VdF-HFP copolymer (470,000 weight average molecular weight) HFP unit (4.8 mol%) was dissolved in dimethylacetamide and tripropylene glycol solution, and aluminum hydroxide was added to prepare a coating solution, and this was applied to a polyethylene microporous membrane to form a porous layer separator.

    [Prior technical literature]         [Patent Literature]    

[Patent Document 1] International Publication No. 1999/036981

[Patent Document 2] Japanese Patent No. 5282179

[Patent Document 3] Japanese Patent No. 5282180

[Patent Document 4] Japanese Patent No. 5281281

[Patent Document 5] Japanese Patent No. 5342088

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

In recent years, non-aqueous electrolyte secondary batteries are expected to be used for the development of large-scale applications such as large-scale street signs, lawn mowers, electric vehicles, electric vehicles, hybrid vehicles, and small ships. With this, the popularity of large-scale batteries is also expected. High capacity. Although the above-mentioned Patent Documents 1 to 5 are all for improving the adhesion between a separator including an electrolyte and an electrode, in the case of a large-sized secondary battery, it is required to further improve the adhesion.

The present inventors, as explained below, are roughly divided into two types of adhesion when evaluating the adhesion between the electrode and the separator, that is, the adhesion between the electrode and the separator when dry, and the electrode and the electrode when wet. The adhesiveness of the separator is evaluated, thereby focusing on that the adhesiveness can be more accurately evaluated. Furthermore, it was found that these adhesivenesses can be evaluated by using the peeling strength when dry and the flexural strength when wet as indicators, respectively. .

That is, for example, a wound electrode system is manufactured by winding a positive electrode and a negative electrode with a separator interposed therebetween and applying tension to each member. At this time, with respect to tension, the positive electrode and the negative electrode applied to the metal current collector hardly stretch and shrink, but the separator becomes elongated to some extent in the mechanical direction and is simultaneously wound. If this rolled body is left to stand temporarily, the separator portion will slowly shrink and return to its original length. As a result, a force in a parallel direction occurs at the interface between the electrode and the separator, and the wound electrode body (especially the electrode body wound into a flat shape) easily bends or deforms. Furthermore, by widening or elongated the separator accompanying the increase in the size of the battery, these problems become apparent, and there is a concern that the yield during production may deteriorate. In order to prevent the wound electrode body from being bent or deformed, the adhesiveness between the separator and the electrode is required more than ever. In addition, when the electrode body is transported, if the components are not sufficiently adhered, the electrode and the separator will be peeled off, and the product cannot be transported with good yield. Adhesion problems during transportation will become apparent due to the increase in the size of the battery, which may result in deterioration of productivity. Therefore, the separator is required to be difficult to peel from the electrode and has a high peeling force when dried.

In addition, in order to maintain a high peeling force during drying, the adhesion between the polyolefin microporous membrane and the porous layer is also extremely important. For example, if the damage between the polyolefin microporous membrane and the porous layer occurs before the damage between the porous layer and the electrode occurs, it is not possible to suppress the above-mentioned bending, deformation, and improve the transportability. Therefore, high adhesion between the polyolefin microporous membrane and the porous layer is required for the separator. In the present invention, with regard to this adhesiveness, the peeling force of the tape obtained by the measurement method described later is used as an index. If this value is large, the peeling force during drying can be maintained. In addition, the porous layer can be prevented from falling off during handling of the separator and during transportation after coating, and productivity can be improved.

In addition, compared to square and cylindrical batteries that can apply pressure to the outer package, it is difficult to apply pressure in stacked batteries, and it is more likely to be caused by the swelling and shrinkage of the electrodes accompanying charge and discharge. Free at the interface between the separator and the electrode. As a result, the battery swells, the internal resistance of the battery increases, and the cycle performance decreases. Therefore, the separator is required to adhere to the electrode, and the electrode is located in the battery after the electrolyte is injected. In this specification, this adhesiveness is evaluated using the flexural strength at the time of wetness obtained by the measurement method described later as an index. The method of measuring the flexural strength when wet is described below. It can indicate the difficulty of lateral translation between the stacked electrodes and the separator in an electrode body that is wet with the electrolyte. It can be evaluated according to the actual battery. Adhesion between separator and electrode in a liquid state. It is considered that if this strength is large, battery characteristics such as suppression of battery swelling after repeated charge and discharge can be expected to improve. The peeling force during drying indicates the adhesion of the separator to the interface between the separator and the electrode in a state where the electrolyte does not substantially contain an electrolyte. In addition, the fact that the electrolytic solution is not substantially contained means that the electrolytic solution in the separator is 500 ppm or less.

However, the inventors have found that in the prior art, the adhesion between the electrode and the separator during drying required for the production and transportation of the electrode body, and the adhesion between the electrode and the separator during wetness required after the electrolyte solution was injected. Adhesiveness has a trade-off relationship, it is extremely difficult to satisfy the physical properties of both parties, and there are cases where the adhesiveness is insufficient in the techniques disclosed in the aforementioned patent documents 1 to 5.

In addition, the battery is required to have the following characteristics: even if a severe shock is applied, it is difficult for the convex portion of the electrode active material to penetrate the separator and cause the electrode to short-circuit (hereinafter referred to as short circuit tolerance). However, it is predicted that the thickness of the battery separator will be reduced in the future, and the thinner the separator becomes, the more difficult it will be to ensure the short-circuit tolerance. Although it is known that it is effective to ensure the short-circuit tolerance by containing a certain amount of inorganic particles in the porous layer, when the inorganic particles are contained to such an extent that the short-circuit tolerance is included, the adhesion between the electrode and the separator is reduced. Propensity.

The present invention has been made in view of the above-mentioned circumstances, and an object thereof is to provide both excellent adhesion between an electrode and a separator when dry and adhesion between an electrode and a separator when wet, and a polyolefin microporous membrane and a porous layer. The adhesiveness is also excellent, and further, a battery separator with excellent short-circuit tolerance, and an electrode body and a secondary battery using the same.

As a result of intensive research by the present inventors in order to solve the above-mentioned problems, they have found that the above-mentioned problems can be solved by the following separators, and the present invention has been completed. The separators include at least a polyolefin multilayer microporous membrane and a porous layer, and the polyolefin multilayer The microporous membrane includes a first microporous layer made of a specific first polyolefin resin and a second microporous layer made of a second polyolefin resin, and the porous layer includes a different amount of a mixture with a different structure. Two kinds of fluorine-based resins and a certain amount of inorganic particles.

That is, the present invention is a battery separator comprising a polyolefin microporous membrane and a porous layer provided on at least one side of the polyolefin microporous membrane, the polyolefin microporous membrane system It is made of a polyolefin multilayer microporous film having a three-layer structure of a first microporous layer, a second microporous layer, and a first microporous layer. The first microporous layer is composed of polyethylene and polypropylene. It is made of a first polyolefin resin, and the content of the polypropylene is 10% by mass or more and 50% by mass or less with respect to the entire mass of the first polyolefin resin. The second microporous layer is made of only a polyethylene resin. The porous layer includes a vinylidene fluoride-hexafluoropropylene copolymer (A), a difluoroethylene-hexafluoropropylene copolymer (B), and inorganic particles, and the aforementioned difluoroethylene-hexafluoropropylene copolymer (B) The fluoropropylene copolymer (A) has hexafluoropropylene units of 0.3 mol% to 5.0 mol%, a weight average molecular weight of 900,000 to 2 million, and contains a hydrophilic group. The aforementioned difluoroethylene-hexafluoropropylene copolymerization Object (B) has more than 5 .0 mol% to 8.0 mol% of hexafluoropropylene units, with a weight average molecular weight of 100,000 to 750,000, relative to the aforementioned difluoroethylene-hexafluoropropylene copolymer (A) and the aforementioned difluoroethylene-hexaene The total 100% by mass of the fluoropropylene copolymer (B) includes the aforementioned difluoroethylenevinyl-hexafluoropropylene copolymer (A) in an amount of 86% by mass or more and 98% by mass or less, based on 100 volume of the solid content in the porous layer. %, Containing the aforementioned inorganic particles in an amount of 40% by volume or more and 80% by volume or less.

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

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

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

According to the present invention, it is possible to provide both excellent adhesion between the electrode and the separator when dry and adhesion between the electrode and the separator when wet, and excellent adhesion and short-circuit tolerance between the polyolefin multilayer microporous film and the porous layer. A separator, and an electrode body and a secondary battery using the same.

1‧‧‧Polyolefin microporous membrane

1’‧‧‧Polyolefin multilayer microporous membrane

a‧‧‧The first microporous layer

b‧‧‧ second microporous layer

2‧‧‧ porous layer

4‧‧‧ aluminum foil

5‧‧‧ resin insulator

6‧‧‧ metal ball

10‧‧‧Battery separator

20‧‧‧ Negative electrode (for adhesion evaluation)

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

22‧‧‧ stacked film

30‧‧‧ electrode wound body

31‧‧‧electrode stack

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

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

43‧‧‧ compression jig (upper side)

44‧‧‧ Compression fixture (lower side)

[Fig. 1] Fig. 1 is a schematic diagram showing an example of a battery separator according to this embodiment.

[FIG. 2] FIG. 2 is a schematic diagram showing an example of a battery separator in this embodiment.

3 is a schematic diagram showing a method for evaluating flexural strength when wet.

[Fig. 4] Fig. 4 is a schematic diagram showing an evaluation method of a short-circuit tolerance test.

    [Form of Implementing Invention]    

Hereinafter, embodiments of the present invention will be described with reference to the drawings. Hereinafter, the directions in the figure will be described using the XYZ coordinate system. In this XYZ coordinate system, a plane parallel to the surface (in-plane direction) of the microporous membrane or the separator is set as the XY plane. The direction (thickness direction) perpendicular to the XY plane is set to the Z direction. The X direction, the Y direction, and the Z direction will be described with the arrow direction in the figure being the + direction and the direction opposite to the arrow direction being the-direction. In addition, in the drawings, in order to easily understand each structure, a part of the expression is emphasized or a part of the expression is simplified, and it may be different from the actual structure, shape, proportion, and the like.

    [Configuration of battery separator]    

FIG. 1 and FIG. 2 are diagrams showing an example of a separator in this embodiment. As shown in FIG. 1, a battery separator 10 (hereinafter also referred to as “separator 10”) is provided with a polyolefin microporous membrane 1 and a porous layer stacked on at least one side of the polyolefin microporous membrane 1. Layer 2. As shown in FIG. 2, the polyolefin microporous membrane 1 may be made of a polyolefin multilayer microporous membrane 1 ′. The polyolefin multilayer microporous membrane 1 ′ has a first microporous layer a / a second microporous layer sequentially stacked. b / Three-layer structure of the first microporous layer a. Hereinafter, each layer which comprises a battery separator is demonstrated.

    1.Polyolefin multilayer microporous membrane         (1) The first microporous layer    

The first microporous layer a is made of a first polyolefin resin containing polyethylene and polypropylene. The first polyolefin resin preferably contains polypropylene and polyethylene as main components. In the present specification, polypropylene and polyethylene are used as a main component, and it means that polypropylene and polyethylene are contained in an amount of 95% or more, and preferably 99% by mass or more, with respect to the entire mass of the first polyolefin resin. As the kind of polyethylene, high-density polyethylene is preferably used as a main component from the 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, and 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, and more preferably 7 × 10 ⁵ or less. If the Mw of the high-density polyethylene is in the above range, the stability of the film formation and the finally obtained piercing strength can 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) of the polyolefin multilayer microporous film 1 'and the porous layer 2 can be further improved, and when used as a battery separator Medium, can make the melting temperature even higher. As the type of polypropylene, in addition to homopolymers, block copolymers and random copolymers can also be used. The block copolymer and the random copolymer may contain a copolymer component with an α-olefin other than propylene, and the other α-olefin is preferably ethylene.

The lower limit of Mw of polypropylene is preferably 5 × 10 ⁵ or more, more preferably 6.5 × 10 ⁵ or more, and even more preferably 8 × 10 ⁵ or more. When the Mw of the polypropylene is in the above range, the dispersibility of the polypropylene is not deteriorated when the sheet is formed, and a film having a uniform film thickness can be obtained. The upper limit of Mw of polypropylene is not particularly limited, but is, for example, 2 × 10 ⁶ or less.

The content of polypropylene is preferably 10% by mass or more and 50% by mass or less based on the entire mass of the first polyolefin resin. If the content of polypropylene is more than 50% by mass, there is a possibility that ion permeability may deteriorate. The lower limit of the polypropylene content is preferably 15% by mass or more, and even more preferably 20% by mass or more. In the case where the content of polypropylene is in the above range, it is possible to have both excellent adhesion, good melting characteristics, and ion permeability of the polyolefin multilayer microporous membrane 1 'and the porous layer 2.

    (2) The second microporous layer    

The second microporous layer b is made of only a polyethylene resin. In this specification, the term "only made of a polyethylene resin" means that the polyethylene resin is 99% by mass or more. The reason for this is that contaminating components derived from foreign matter are mixed into the film, or dirt adhered to the production line or device in the manufacturing step of the raw resin or polyolefin microporous film is peeled off and mixed into the film.

Examples of the types of polyethylene that can be used for the second microporous layer b include high-density polyethylene having a density exceeding 0.94 g / cm ³ , medium-density polyethylene having a density in the range of 0.93 to 0.94 g / cm ³ , and low density Low-density polyethylene, linear low-density polyethylene, ultra-high molecular weight polyethylene, and the like at 0.93 g / cm ³ , but from the viewpoint of strength, it is preferable to contain high-density polyethylene and ultra-high molecular weight polyethylene. Polyethylene is not only a homopolymer of ethylene, but also a copolymer containing a small amount of other α-olefins. Examples of the α-olefin include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene. In the polyolefin multilayer microporous film 1 ', it is sometimes difficult to control the physical property unevenness in the width direction depending on the difference in the viscosity of each layer, etc., especially when it is manufactured by a coextrusion method. However, by using ultra-high molecular weight polyethylene in the second microporous layer b, since the molecular network of the entire membrane becomes stable, uneven deformation is difficult to occur, and a multilayer microporous membrane 1 having excellent uniformity in physical properties can be obtained.

The weight average molecular weight (hereinafter referred to as Mw) of the high-density polyethylene is preferably 1 × 10 ⁵ or more, and 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 , and more preferably 7 × 10 ⁵ . When the Mw of the high-density polyethylene is in the above range, it is possible to have both the stability of film formation and the finally obtained perforation strength.

The Mw of the ultra-high molecular weight polyethylene is preferably 1 × 10 ⁶ or more and less than 4 × 10 ⁶ . By using an 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 made finer and the perforation strength can be improved. In addition, if the Mw of the ultra-high molecular weight polyethylene is 4 × 10 ⁶ or more, the viscosity of the molten material becomes too high, and therefore it may be impossible to extrude the resin from the die (mold) and the like to cause a problem in the film forming step. .

The lower limit of the content of the ultra-high molecular weight polyethylene is preferably 5% by mass or more, and more preferably 18% by mass or more, based on 100% by mass of the entire polyethylene resin constituting the second microporous layer b. The upper limit of the content of the ultra-high molecular weight polyethylene is preferably 45% by mass or less, and more preferably 40% by mass or less based on 100% by mass of the entire polyethylene resin. When the content of the ultra-high molecular weight polyethylene is in the above range, it becomes easy to have both perforation strength and gas permeation resistance. Further, if the content of the ultra-high molecular weight polyethylene is within the above-mentioned preferred range, sufficient tensile strength can be obtained even when the thickness of the polyolefin multilayer microporous film 1 'is reduced. The tensile strength of the polyolefin multilayer microporous film 1 'is preferably 100 MPa or more. The upper limit of the tensile strength is not particularly specified.

The ratio of the weight average molecular weight (Mw) to 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, that is, the molecular weight distribution (Mw / Mn), is The range is preferably 5 to 200, and more preferably 10 to 100. If the range of Mw / Mn is the above-mentioned preferable range, it is easy to extrude a solution of polyolefin in the manufacturing step, and even in a case where the thickness of the polyolefin multilayer microporous film 1 'is further thinned, A sufficient mechanical strength can also be obtained. Mw / Mn can be used as a reference for the molecular weight distribution. For example, in the case of a polyolefin resin made of a single substance, the larger the value, the larger the range of the molecular weight distribution. The Mw / Mn of a polyolefin resin made of a single substance can be appropriately adjusted by multi-stage polymerization of polyolefin. The Mw / Mn of the polyolefin resin mixture can be appropriately adjusted by adjusting the molecular weight or mixing ratio of each component.

    (3) Polyolefin multilayer microporous membrane    

The film thickness of the polyolefin multilayer microporous film 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 of the battery, the upper limit is 16 μm or less , More preferably 12 μm or less. In the case where the thickness of the polyolefin multilayer microporous membrane 1 'is in the above-mentioned preferred range, practical film strength and pore blocking function can be maintained, and it is more suitable for increasing the capacity of batteries expected to progress in the future. That is, in the battery separator 10 of this embodiment, even if the thickness of the polyolefin microporous membrane 1 is thin, the layer between the polyolefin multilayer microporous membrane 1 ′ and the porous layer 2 of the separator 10 and between the separator 10 and the electrode The adhesiveness is also excellent, and when the separator 10 is formed into a thin film, its effect is more clearly exhibited.

Here, the thickness ratio of the second microporous layer b to the entire layer (the entirety) of the polyolefin multilayer microporous film 1 'is preferably 30% or more and 90% or less. The lower limit is more preferably 40% or more, and the upper limit is more preferably 80% or less. In the case where the thickness ratio of the second microporous layer b is within the above range, the balance between the melting characteristics, the stability of the permeability in the use range of the separator, and the perforation strength can be made into a good range.

The first microporous layer and the second microporous layer constituting the polyolefin multilayer microporous membrane 1 'may each contain an antioxidant, a heat stabilizer, and an antistatic agent, as long as the effects of the present invention are not impaired. , UV absorbers, even anti-caking agents, fillers or nucleating agents and other additives. In particular, for the purpose of suppressing oxidative degradation due to the thermal history of the polyolefin resin, it is preferable to add an antioxidant. In terms of adjusting or enhancing the characteristics of the polyolefin multilayer microporous membrane 1 ', it is important to appropriately select the type and amount of the antioxidant or heat stabilizer. In this specification, the amounts of these additives are not included in the content of the first polyolefin resin and polyethylene resin.

In the polyolefin multilayer microporous membrane 1 ', it is preferable that the inorganic multilayer particles do not substantially contain inorganic particles. The "substantially free of inorganic particles" means, for example, a content of 50 ppm or less, preferably 10 ppm or less, and most preferably a content below the detection limit when the inorganic element is quantified by fluorescent X-ray analysis. The reason is that even if particles are not actively added to the polyolefin microporous membrane, contaminating components derived from foreign substances may be mixed into the membrane, or adhered to the production line of the raw material resin or polyolefin microporous membrane production process or Dirt of the device is peeled off and mixed into the film.

The upper limit of the air permeability resistance of the polyolefin multilayer microporous membrane 1 'is 300 sec / 100 cm ³ Air or less, preferably 200 sec / 100 cm ³ Air, or more preferably 150 sec / 100 cm ³ Air or less. The microporous polyolefin multilayer film 1 'of the anti-air permeability of not less than the lower limit of 50sec / 100cm ³ Air, preferably less than 70sec / 100cm ³ Air, more preferably less than 100sec / 100cm ³ Air.

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 even more preferably 40% or more. If the air permeability and porosity are in the above-mentioned preferred ranges, there will be sufficient charge and discharge characteristics of the battery, especially in terms of ion permeability (charge and discharge operating voltage) and battery life (closely related to the retention of the electrolyte). , Can fully function as a battery, and because sufficient mechanical strength and insulation can be obtained, the possibility of short-circuiting during charging and discharging is reduced.

Since the average pore diameter of the polyolefin multilayer microporous membrane 1 'greatly affects the pore blocking 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 even more preferably 0.1. μm or more and 0.3 μm or less. If the average pore diameter of the polyolefin multilayer microporous membrane 1 'is in the above-mentioned preferred range, the reaction of the pore blocking phenomenon to temperature will not become slow, and the pore blocking temperature caused by the heating rate will not shift to the high temperature side. .

    2. Manufacturing method of polyolefin multilayer microporous film    

The method for producing a polyolefin multilayer microporous film is not particularly limited as long as it can produce a polyolefin multilayer microporous film 1 'having the above-mentioned characteristics, and a conventionally known method may be used. For example, Japanese Patent No. 2132327 and Japan The method described in the specification of National Patent No. 3347835, International Publication No. 2006/137540, and the like.

As a method for producing a polyolefin multilayer microporous membrane 1 ', it is preferable to include the following steps (1) to (8).

(1) a step of melt-kneading a first polyolefin resin containing polypropylene and a film-forming solvent to prepare a first polyolefin resin solution

(2) the step of melt-kneading the polyethylene resin and the film-forming solvent to prepare a second polyolefin resin solution

(3) a step of co-extruding the first and second polyolefin resin solutions, forming a sheet, and cooling to obtain an extruded body

(4) a step of stretching the extruded body (first stretching step) to obtain a gel-like multilayer sheet

(5) The step of removing the film-forming solvent from the gel-like multilayer sheet to obtain a multilayer sheet

(6) Drying the multilayer sheet to obtain a first extended multilayer sheet

(7) a step of extending the first extended multilayer sheet to obtain a second extended multilayer sheet

(8) A step of heat-treating the second extended multilayer sheet to obtain a polyolefin multilayer microporous film.

In the method for producing a polyolefin multilayer microporous membrane 1 ', for example, in step (3), it is preferable to form a multilayer sheet by simultaneously extruding the first and second polyolefin resin solutions through a multilayer mold under specific conditions. This makes it possible to produce a polyolefin multilayer microporous film that is excellent in close contact between the layers and has excellent melting temperature, mechanical strength, air permeability, and porosity, and a small maximum pore size when used as a battery separator. 1. These characteristics cannot be achieved in a single-layer polyolefin microporous membrane 1. In addition, in addition to using the above-mentioned resin material in steps (1) and (2), by using the appropriate temperature conditions described later in steps (4) and (7), stretching can be achieved even with a thin film thickness. Good porosity and control of fine pore structure.

Each step is described below.

    Step (1) and Step (2): Preparation steps of first and second polyolefin resin solutions    

First, an appropriate film-forming solvent is added to the first polyolefin resin and the polyethylene resin, and then melt-kneaded to prepare the first and second polyolefin resin solutions, respectively. As the melt-kneading method, for example, a method using a biaxial 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.

In the first or second polyolefin resin solution, the blending ratio of the first or second polyolefin resin and the film-forming solvent is not particularly limited, but is 20 to 30 parts by mass relative to the first or second polyolefin resin. Preferably, the film-forming solvent is 70 to 80 parts by mass. If the ratio of the first or second polyolefin resin is within the above range, when the first or second polyolefin resin solution is extruded, it is possible to prevent expansion or neck in at the die exit, and extrusion molding The moldability (self-supporting property) of the body (gel-like molded body) is good.

    Step (3): Step of forming an extruded body    

Next, the first and second polyolefin resin solutions were respectively supplied from an extruder to a die, where the two solutions were combined into a layer shape and extruded into a sheet shape.

The extrusion method may be any of a flat die method and an inflation method. Either method can be a method in which the solution is supplied to individual manifolds and stacked in layers with a lip inlet of a multi-layer mold (multi-manifold method), or the solution is supplied to the mold in a layered flow beforehand. Method (block method). Since the multi-manifold method and the block method are known per se, detailed descriptions thereof are omitted. The clearance of the flat die for multilayer is 0.1 ~ 5mm. The extrusion temperature is preferably 140 to 250 ° C, and the extrusion speed is preferably 0.2 to 15 m / minute. By adjusting the respective 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, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used.

Then, the obtained stacked extruded body is cooled to form an extruded body. As a method of forming the formed body, for example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. The cooling system is preferably performed at a rate of 50 ° C./min or more until at least the gelling temperature. The cooling system is preferably performed until 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 body suitable for stretching is obtained. As the cooling method, a method of contacting a refrigerant such as cold air or cooling water, a method of contacting a cooling roller, or the like can be used, but it is preferable to perform cooling by contacting a roller cooled by the refrigerant.

    Step (4) First extension step    

Next, the obtained extruded molded article is stretched (first stretched) in at least a uniaxial direction to obtain a gelatinous multilayer sheet. Since the extruded product contains a film-forming solvent, it can be uniformly stretched. The extruded body is preferably stretched by a tenter method, a roll method, an inflation method, or a combination thereof at a predetermined magnification after heating. The extension may be a uniaxial extension or a biaxial extension, but a biaxial extension is preferred. In the case of biaxial extension, it may be any of simultaneous biaxial extension, successive extension, and multi-stage extension (for example, a combination of simultaneous biaxial extension and successive extension).

The stretching magnification (area stretching magnification) in this step is preferably 2 times or more, more preferably 3 times or more and 30 times or less in the case of uniaxial stretching. In the case of biaxial stretching, it is preferably 9 times or more, more preferably 16 times or more, and particularly preferably 25 times or more. Moreover, it is preferable that both the long side direction and the transverse side direction (MD and TD directions) are 3 times or more, and the stretching magnifications in the MD direction and the TD direction may be the same as or different from each other. When the stretching magnification is set to 9 times or more, improvement in perforation strength can be expected. In addition, the so-called stretching ratio in this step refers to the area stretching ratio of the microporous film to be supplied to the next step based on the microporous film immediately before the step. The elongation temperature is preferably 90 ° C or higher and 130 ° C or lower, more preferably 110 ° C or higher and 120 ° C or lower, and even more preferably 114 ° C or higher and 117 ° C or lower.

In addition, depending on the desired physical properties, a temperature distribution can be set in the film thickness direction and stretched, thereby obtaining a multilayer microporous film with more excellent mechanical strength. The details of this method are described in Japanese Patent No. 3347854.

    Step (5): Removal of solvent for film formation    

Next, the film-forming solvent was removed from the gel-like multilayer sheet to obtain a multilayer sheet. The removal (cleaning) of the film-forming solvent is performed using a cleaning solvent. In the gel-like multilayer sheet, since the first and second polyolefin phases are phase-separated from the film-forming solvent phase, a porous film can be obtained by removing the film-forming solvent. The obtained porous membrane was made of fibrils forming a fine three-dimensional network structure, and had three-dimensional and irregularly connected pores (voids). The cleaning solvent and the method for removing the film-forming solvent using these solvents are well known, so descriptions are omitted. For example, the method disclosed in Japanese Patent No. 2132327 or Japanese Patent Application Laid-Open No. 2002-256099 can be used.

    Step (6): Dry    

Next, the multilayer sheet is dried to obtain a first extended multilayer sheet. The multilayer microporous membrane is set to 100% by mass (dry weight), and the drying system is preferably performed until the residual cleaning solvent becomes 5% by mass or less, and more preferably, it is performed until the remaining 3% by mass. If the residual cleaning solvent is within the above range, the porosity of the multilayer microporous membrane can be maintained during the second stretching step and the heat treatment step described later, and the deterioration of permeability can be suppressed. The drying temperature is preferably 50 ° C or higher and 80 ° C or lower.

    Step (7): Second extension step    

Next, the first stretched multilayer sheet is stretched (second stretched) to obtain a second stretched multilayer sheet. The dried first stretched multilayer sheet is preferably stretched at least in a uniaxial direction. The stretching of the first stretched multilayer sheet can be performed by a tenter method or the like in the same manner as described above while heating. The extension can be a uniaxial extension or a biaxial extension. In the case of biaxial extension, it may be any of simultaneous biaxial extension and sequential extension.

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

The lower limit of the stretching magnification (area stretching magnification) of the first stretched multilayer sheet in this step toward the uniaxial direction is preferably 1.0 times or more, more preferably 1.1 times or more, and even more preferably 1.2 times or more. The upper limit is preferably set to 1.8 times or less. In the case of uniaxial extension, it is set to 1.0 to 2.0 times in the MD or TD direction. In the case of biaxial extension, the lower limit of the area extension magnification is preferably 1.0 times or more, more preferably 1.1 times or more, and even more preferably 1.2 times or more. The upper limit is preferably 3.5 times or less, and is set to 1.0 to 2.0 times each in the MD direction and the TD direction. The extension magnifications in the MD direction and the TD direction may be the same or different. In addition, the so-called stretch magnification in this step refers to the stretch magnification of the second stretched multi-layer sheet immediately before being supplied to the next step based on the first stretched multi-layer sheet.

    Step (8): Heat treatment    

Next, the second stretched multilayer sheet was heat-treated to obtain a polyolefin multilayer microporous membrane 1 '. By heat-treating the second extended multilayer sheet, the crystals are stabilized and the layered structure is uniformized. As the heat treatment method, a heat fixing treatment and / or a heat relaxation treatment can be used. The so-called heat-fixing treatment is a heat treatment in which the size of the film is maintained and heated at the same time. The so-called thermal relaxation treatment is a heat treatment for thermally shrinking the film in the MD direction or the TD direction during heating. The heat fixing treatment is preferably performed by a tenter method or a roll method. For example, as the thermal relaxation treatment method, a method disclosed in Japanese Patent Application Laid-Open No. 2002-256099 can be mentioned. The heat treatment temperature is more preferably within a range of ± 5 ° C of the second extension temperature of the second extended multilayer sheet, and particularly preferably within a range of ± 3 ° C.

    3. Porous layer    

The porous layer 2 includes two types of a difluoroethylene vinylene-hexafluoropropylene copolymer (VdF-HFP), and inorganic particles. Hereinafter, each component which comprises the porous layer 2 is demonstrated below.

    [Difluoroethylene-hexafluoropropylene copolymer (A)]    

The difluoroethylene-hexafluoropropylene copolymer (A) (hereinafter may be simply referred to as the copolymer (A)) is a copolymer containing a difluoroethylene unit and a hexafluoropropylene unit, and as described later, contains a hydrophilic group. The lower limit of the content of the hexafluoropropylene unit in the copolymer (A) is 0.3 mol%, and preferably 0.5 mol%. In the case where the content of the hexafluoropropylene unit is less than the above range, the polymer crystallinity becomes higher, and the swelling degree of the separator to the electrolyte becomes lower. Therefore, the adhesiveness between the separator and the electrode is reduced, and it may not be possible to obtain sufficiently after the electrolyte is injected. Adhesion between electrode and separator (bending strength when wet). On the other hand, the upper limit of the content of the hexafluoropropylene unit is 5.0 mol%, and preferably 2.5 mol%. In the case where the content of the hexafluoropropylene unit is larger than the above range, the separator is excessively swelled to the electrolytic solution, and the bending strength is reduced when wet.

The lower limit of the weight average molecular weight of the copolymer (A) is 900,000, and preferably 1 million. On the other hand, the upper limit of the weight average molecular weight of the copolymer (A) is 2 million, and more preferably 1.5 million. In the case where 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 does not become extremely long, and the production efficiency can be improved. It can maintain moderate gel strength when it swells in the electrolyte, and can improve its bending strength when wet. The weight average molecular weight of the copolymer (A) is a polystyrene conversion value by gel permeation chromatography.

The copolymer (A) has a hydrophilic group. The copolymer (A) has a hydrophilic group, and can more firmly adhere to an active material existing on the electrode surface or a binder component in the electrode. This reason is not particularly limited, but it is presumed that the adhesion force is increased 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, particularly preferred are carboxylic acid groups and carboxylic acid esters.

As a method for introducing a hydrophilic group into the copolymer (A), a known method can be used. For example, when the copolymer (A) is synthesized, maleic anhydride, maleic acid, and maleic acid can be used. A method of introducing a monomer having a hydrophilic group, such as an ester, a maleic acid monomethyl ester, and the like into the main chain by copolymerization, or a method of introducing as a side chain by grafting. The modification rate of hydrophilic group can be measured by FT-IR, NMR, quantitative titration, etc. For example, in the case of a carboxylic acid group, FT-IR can be used, and a homopolymer is used as a reference, and it can be obtained from the absorption intensity ratio of C-H stretching vibration and C = O stretching vibration of a carboxyl group.

The lower limit of the content of the hydrophilic group of the copolymer (A) is preferably 0.1 mol%, and more preferably 0.3 mol%. On the other hand, the upper limit of the content of the hydrophilic group is preferably 5.0 mol%, and more preferably 4.0 mol%. When the content of the hydrophilic group is more than 5.0 mol%, the crystallinity of the polymer becomes too low, the swelling degree with respect to the electrolytic solution becomes high, and the bending strength is deteriorated when wet. In addition, when the content of the hydrophilic group is within the above range, the effects of increasing the affinity of the inorganic particles contained in the porous layer 2 and the copolymer (A), improving the short-circuit tolerance, and suppressing the fall of the inorganic particles are exerted. This reason is not particularly limited, but it is presumed that the film strength of the porous layer 2 is increased by the copolymer (A) having a hydrophilic group and the inorganic particles, which are the main components of the porous layer 2. The quantitative determination of the hydrophilic group of the difluoroethylene vinylene-hexafluoropropylene copolymer in the porous layer 2 can be obtained by an IR (infrared absorption spectrum) method, an NMR (nuclear magnetic resonance) method, or the like.

As long as the characteristics are not impaired, the copolymer (A) may be a copolymer obtained by further polymerizing a monomer other than difluorovinylene, hexafluoropropylene, and a monomer having a hydrophilic group. Examples of other monomers include monomers such as tetrafluoroethylene, trifluoroethylene, trichloroethylene, and fluoroethylene.

By setting the structure and molecular weight of the copolymer (A) within the above ranges, when the separator 10 is used in a non-aqueous electrolyte secondary battery, it has a high affinity for the non-aqueous electrolyte, and is chemically and physically It has high stability, exhibits flexural strength when wet, and fully maintains its affinity with the electrolyte even when used at high temperatures.

    [Difluoroethylene-hexafluoropropylene copolymer (B)]    

The difluoroethylene vinylene-hexafluoropropylene copolymer (B) (hereinafter, simply referred to as the copolymer (B)) is a copolymer containing a difluoroethylene vinylene unit and a hexafluoropropylene unit. In the copolymer (B), the content of hexafluoropropylene is more than 5.0 mol%, 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%, and preferably 7.5 mol%. In addition, when the content of the hexafluoropropylene unit is more than 8.0 mol%, the electrolyte solution is excessively swelled, and the bending strength is reduced when wet. The copolymer (B) may include a hydrophilic group, but may not include a hydrophilic group.

The weight average molecular weight of the copolymer (B) is 100,000 or more and 750,000 or less. When the weight average molecular weight of the copolymer (B) is in the above range, the affinity to the non-aqueous electrolyte is high, the chemical and physical stability is high, and the excellent adhesion between the separator and the electrode when dried ( Peeling force when drying). This reason is not particularly limited, but it is presumed that the copolymer (B) has fluidity under heating and pressurizing conditions such as a peeling force during drying, and becomes an anchor by penetrating into the porous layer of the electrode ( anchor), whereby the porous layer 2 and the electrode have stable adhesion. That is, in the battery separator 10, the copolymer (B) contributes to the peeling force at the time of drying, and can help prevent bending and deformation of the wound electrode body or the stacked electrode body, and improve the transportability. The copolymer (B) and the copolymer (A) are different resins.

The lower limit of the weight average molecular weight of the copolymer (B) is 100,000, and preferably 150,000. When the weight average molecular weight of the copolymer (B) is lower than the lower limit of the above range, the amount of entanglement of the molecular chains is too small, the strength of the resin becomes weak, and aggregation of the porous layer 2 is liable to occur. 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. In the case where the weight average molecular weight of the copolymer (B) is larger than the upper limit of the above range, in order to obtain the peeling force at the time of drying, it is necessary to increase the pressure temperature in the manufacturing step of the wound body. As a result, the microporous film containing polyolefin as a main component may shrink. When the weight average molecular weight of the copolymer (B) is greater than the upper limit of the above range, the amount of entanglement of the molecular chains may increase, and there is a possibility that it may not flow sufficiently under pressure.

The lower limit of the melting point of the copolymer (B) is preferably 60 ° C, and more preferably 80 ° C. On the other hand, the upper limit of the melting point of the copolymer (B) is preferably 145 ° C, and more preferably 140 ° C. In addition, the melting point (Tm) referred to herein means a peak top temperature of an endothermic peak at a temperature rise measured by a differential scanning calorimetry (DSC).

The copolymer (B) is a copolymer having a difluorovinylene unit and a hexafluoropropylene unit. Like the copolymer (A), the copolymer (B) can be obtained by a suspension polymerization method or the like. The melting point of the copolymer (B) can be adjusted by controlling the crystallinity of a site made of a difluoroethylene unit. For example, when the copolymer (B) contains a monomer other than a difluoroethylene unit, it can be adjusted by controlling the ratio of a difluoroethylene unit. Monomers other than difluoroethylene units may have one or two or more types of tetrafluoroethylene, trifluoroethylene, trichloroethylene, hexafluoropropylene, fluoroethylene maleic anhydride, maleic acid, maleic acid Maleic acid esters, maleic acid monomethyl esters, and the like. Examples thereof include a method of adding the above-mentioned monomers when polymerizing the copolymer (B) and introducing them into the main chain by copolymerization or a method of introducing them as side chains by grafting. The melting point can also be adjusted by controlling the ratio of the head-to-head bond (-CH ₂ -CF ₂ -CF ₂ -CH _2- ) of the difluoroethylene unit.

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

The lower limit of the content of the copolymer (A) is 100% by mass based on the total weight of the copolymer (A) and the copolymer (B), and is preferably 88% by mass. The upper limit of the content of the copolymer (A) is 98% by mass, and more preferably 97% by mass. The upper limit of the content of the copolymer (B) is 14% by mass, and preferably 12% by mass, based on 100% by mass of the total weight of the copolymer (A) and the copolymer (B). The lower limit of the content of the copolymer (B) is 2% by mass and 3% by mass. When the content of the copolymer (A) and the content of the copolymer (B) are within the above-mentioned ranges, the porous layer 2 can have both a high level of flexural strength at the time of wetness and peeling force at the time of drying.

In addition, the porous layer 2 may contain resins other than the copolymer (A) and the copolymer (B) as long as the effect of the present invention is not inhibited. However, as a resin component constituting the porous layer 2, a copolymer ( A) and copolymer (B). When a resin other than the copolymer (A) and the copolymer (B) is included, the content of the copolymer (A) or the copolymer (B) is set to 100% by mass based on 100 mass% of the resin component of the porous layer 2. proportion.

    [Inorganic particles]    

The porous layer 2 contains inorganic particles. The inclusion of particles in the porous layer 2 can increase the short-circuit tolerance in particular and improve the thermal stability.

Examples of the 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, and fluorine Lithium, zeolite, molybdenum sulfide, mica, gibbsite, magnesia, etc. In particular, from the viewpoint of the affinity with the difluoroethylene vinylene-hexafluoropropylene copolymer (A), inorganic particles containing a large amount of OH groups are preferable, and specifically, it is preferable to use a material selected from titanium dioxide and alumina. More than one type of gibbsite.

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

In general, when the porous layer contains inorganic particles having no adhesiveness, the bending strength when wet and the peeling force when dry tend to decrease. However, as described above, the porous layer 2 of the present embodiment has a high adhesion to the electrode by containing a specific fluororesin in a specific ratio and contains inorganic particles in the above range, and resists bending when wet. The balance of strength and peeling force during drying is good, and excellent short-circuit tolerance is obtained.

From the viewpoint of particle shedding, the average particle diameter of the inorganic particles is preferably 1.5 times to 50 times the average flow pore diameter of the polyolefin microporous membrane, and more preferably 2.0 times to 20 times. The average flow pore diameter is measured in accordance with JISK3832 or ASTMF316-86. For example, Perm-Porometer (CFP-1500A, manufactured by PMI Corporation) is used to measure in the order of Dry-up and Wet-up. In Wet-up, pressure is applied to a microporous membrane that is sufficiently saturated with Galwick (trade name) manufactured by PMI Co., which has a known surface tension. Regarding the average flow pore diameter, the pore diameter is converted from the pressure at the point where the curve representing the 1/2 slope of the pressure and flow curve in the Dry-up measurement and the curve in the Wet-up measurement intersect. For the conversion of pressure and pore diameter, the following formula is used.

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 the sliding property with the winding core and particle shedding when the battery is wound, the average particle diameter of the inorganic particles is preferably 0.3 μm to 1.8 μm, more preferably 0.5 μm to 1.5 μm, and even more preferably 0.9 μm. ~ 1.3 μm. The average particle diameter of the particles can be measured by a laser diffraction method or a dynamic light scattering method. For example, it is preferable to use an ultrasonic probe to measure the particles dispersed in an aqueous solution containing a surfactant with a particle size distribution measurement device (manufactured by Nikkiso Co., Ltd., Microtrack HRA). The value of the particle diameter (D50) when 50% was accumulated on the particle side was taken as the average particle diameter. The shape of the particles includes, but is not particularly limited to, a spherical shape, a substantially spherical shape, a plate shape, and a needle shape.

    [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 one side, more preferably 1 μm or more and 2.5 μm or less, and even more preferably 1 μm or more and 2 μm or less. When the film thickness per single surface is 0.5 μm or more, high adhesion to the electrode (bending strength when wet, peeling force when dry) can be ensured. On the other hand, if the film thickness per single surface is 3 μm or less, the winding volume can be reduced, the thickness can be further reduced, and it is more suitable for increasing the capacity of batteries to be developed in the future.

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

    3. Manufacturing method of battery separator    

The manufacturing method of a battery separator is not specifically limited, It can manufacture using a well-known method. An example of a method for manufacturing a battery separator is described below. The method for manufacturing a battery separator may include the following steps (1) to (3) in this order.

(1) a step of obtaining a fluororesin solution, which dissolves a difluoroethylene-hexafluoropropylene copolymer (A) and a difluoroethylene-hexafluoropropylene copolymer (B) in a solvent; (2) obtaining a coating liquid In the step, inorganic particles are added to the fluororesin solution, and the particles are mixed and dispersed. (3) The coating solution is coated on a polyolefin microporous membrane, immersed in a coagulation solution, and washed and dried.

    Step (1): Step of obtaining a fluororesin solution    

The difluoroethylene-hexafluoropropylene copolymer (A) and the difluoroethylene-hexafluoropropylene copolymer (B) were slowly added to the solvent and completely dissolved.

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

    Step (2): Step of obtaining a coating liquid    

In order to obtain a coating liquid, it is important to sufficiently disperse the inorganic particles. Specifically, the particles are added while agitating the fluororesin solution, and then dispersed in a disperser or the like for a predetermined time (for example, about 1 hour) to perform preliminary dispersion. Next, a bead mill or a paint shaker is used. (paint shaker) The particles are dispersed. After this step (dispersion step), the aggregation of the particles is reduced, and the coating liquid is prepared by mixing with a Three-One-Motor with a stirring blade.

    Step (3): the step of applying the coating solution to the microporous membrane and immersing it in the coagulation solution, and then washing and drying    

A coating solution is applied to the microporous membrane, and the coated microporous membrane is immersed in a coagulation solution to make the difluoroethylene-hexafluoropropylene copolymer (A) and the difluoroethylene-hexafluoropropylene copolymer (B) phase Separated, solidified in a state with a three-dimensional mesh structure, washed and dried. Thereby, a battery separator including a microporous membrane and a porous layer on the surface of the microporous membrane can be obtained.

The method for applying the coating liquid to the microporous membrane may be a known method, such as a dip coating method, a reverse roll coating method, a gravure coating method, a kiss coating method, and a roll brush method. method), spray coating method, pneumatic blade coating method, wire bar coating method, pipe doctor method, blade coating method, and die coating method, etc. These methods may be used alone or in combination.

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

Wash with water. For drying, for example, hot air at 100 ° C or lower can be used for drying.

    4.Battery separator    

The battery separator 10 according to this embodiment can be preferably used for any of a battery using an aqueous electrolyte and a battery using a non-aqueous electrolyte, but can be more preferably used for a non-aqueous electrolyte secondary battery. Specifically, it can be preferably used as a separator for secondary batteries such as nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc batteries, lithium secondary batteries, and lithium polymer secondary batteries. Among them, it is preferable to use a separator for a lithium ion secondary battery.

The non-aqueous electrolyte secondary battery has a positive electrode and a negative electrode disposed through 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 may be used, and there may be, for example, an electrode structure (coin type) provided in a disc-shaped positive electrode and a negative electrode, and a plate-shaped positive electrode and negative electrode Electrode structure (stacking type) of alternately stacked electrodes, electrode structure (winding type) of wound positive and negative electrodes in a stacked band shape, and the like. The battery separator in this embodiment can have excellent adhesion 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 a non-aqueous electrolyte secondary battery including a lithium ion secondary battery are not particularly limited, and conventionally known materials can be appropriately used in combination.

In addition, as shown in FIG. 1 (A), the separator 10 for a battery may be stacked with a porous body 2 on one side of the polyolefin microporous membrane 1, or may be stacked with a porous body on both sides of the polyolefin microporous membrane 1. Quality 2.

    5. Physical properties of battery separator    

The bending strength of the separator 10 when wet is preferably 4.0 N or more, more preferably 5.0 N or more, and even more preferably 6.0 N or more. The upper limit of the flexural strength when wet is not particularly specified, but is, for example, 15.0N or less. In the case where the flexural strength is within the above-mentioned preferred range when wet, it is possible to further suppress the partial release at the interface between the separator and the electrode, and it is possible to suppress an increase in internal resistance of the battery and a decrease in battery characteristics. In addition, the bending strength when wet can be measured by a 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 even more preferably 6.0 N / m or more. The upper limit of the peeling force during drying is not particularly specified, but is, for example, 40.0 N / m or less. When the peeling force is within the above-mentioned preferable range during drying, the electrode body can be expected to carry the wound electrode body or the stacked electrode body without scattering. In addition, the peeling force during drying can be measured by a method described in Examples described later.

The separator 10 of this embodiment can have both a high level of bending strength when wet and a peeling force when dry. Specifically, as shown in Examples described later, the separator 10 can satisfy a flexural strength of 4.0 N or more when wet and a peel force of 2.0 N / m or more when dry.

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

The present invention is not limited to the embodiments described above, and various modifications can be made within the scope of the gist thereof.

    [Example]    

Hereinafter, the present invention will be described in further detail through examples, but the implementation aspects of the present invention are not limited to these examples. In addition, the evaluation methods, analysis methods, and materials used in the examples are as follows.

    (1) Film thickness, porosity, and breathability         [Film thickness]    

Using a contact-type film thickness meter ("LITEMATIC" (registered trademark) series 318 manufactured by Mitutoyo Co., Ltd.), the film thickness of the microporous film and the separator was measured. The measurement system uses a super-hard spherical probe Φ9.5mm, and measures 20 points under the condition of weighting 0.01N. The average value of the obtained measurement values is taken as the film thickness.

    [Void ratio]    

The measurement was performed by comparing the weight w _{1 of the} microporous membrane with the equivalent weight w ₂ of the non-voided polymer (a polymer having the same width, length, and composition).

Porosity (%) = (w ₂ -w ₁ ) / w ₂ × 100     [Breathability]    

Using the digital King-type air permeability tester EGO1 manufactured by Asahi Seiko Co., Ltd., the polyolefin microporous film made of the present invention is fixed in a way that there is no wrinkle in the measuring part, and the JIS P-8117 (2009 ) Perform measurement. The sample was made into a 5 cm square, and the measurement point was set to 1 point in the center of the sample. The measured value was taken as the air permeability resistance of the sample [sec]. The same measurement was performed on ten test pieces taken from arbitrary film positions, and the average value of the ten measured values was taken as the air permeability resistance (sec / 100 cm ³ ) of the polyolefin-made stacked microporous film.

    (2) Weight-average molecular weight (Mw) of the difluoroethylene-hexafluoropropylene copolymer (A) and the difluoroethylene-hexafluoropropylene copolymer (B)    

It was determined by the gel permeation chromatography (GPC) method under the following conditions.

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

‧Pipe: 2 Shodex KF-806M manufactured by Showa Denko Corporation

‧Column temperature: 23 ℃

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

‧Solvent flow rate: 0.5ml / min

‧Preparation of sample: Add 4mL of measuring solvent to 2mg of sample, and stir gently at room temperature (visually confirm dissolution)

‧Injection volume: 0.2mL

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

‧Calibration curve: The calibration curve obtained from the use of monodisperse polystyrene standard sample is created using the specified conversion constant.

    (3) Melting point    

A differential scanning thermal analysis device (DSC manufactured by PerkinElmer Co., Ltd.) was used to load 7 mg of resin as a measurement sample into a measuring pan, and the measurement was performed under the following conditions. The temperature was raised at the beginning, and after cooling, the peak top of the endothermic peak at the second temperature rise was taken as the melting point.

‧Heating and cooling rate: ± 10 ℃ / min.

‧Measuring temperature range: 30 ~ 230 ℃.

    (4) Flexural strength when wet    

In general, in the case where a fluororesin binder is used for the positive electrode and a porous layer containing the fluororesin is provided on the separator, the adhesion between the fluororesins is easily diffused. On the other hand, since a negative electrode uses a binder other than a fluororesin and it is difficult for diffusion of the fluororesin to occur, it is more difficult for the negative electrode to obtain adhesion to a separator than a positive electrode. Therefore, in this measurement, evaluation is performed by measuring the bending strength at the time of wetting described below as an index of the adhesion between the separator and the negative electrode.

    (Production of negative electrode)    

An aqueous solution containing 1.5 parts by mass of carmellose was added to 96.5 parts by mass of artificial graphite and mixed, and then 2 parts by mass of styrene butadiene latex was added as a solid content and mixed to prepare a negative electrode mixture. Paste. This slurry containing the negative electrode mixture was uniformly coated on both sides of a negative electrode current collector made of copper foil with a thickness of 8 μm, and dried to form a negative electrode layer. Thereafter, compression molding was performed by a roll press, and The density of the negative electrode layer of the current collector was removed to be 1.5 g / cm ³ to produce a negative electrode.

    (Production of test wound body)    

The produced negative electrode 20 (machine direction 161 mm × width 30 mm) and the produced separator 10 (machine direction 160 mm × width 34 mm) were overlapped, and a metal plate (length 300 mm, width 25 mm, thickness 1 mm) was used as the core. The separator 10 and the negative electrode 20 were wound such that the separator 10 became the inner side, and a metal plate was pulled out to obtain a test wound body 30. The test wound body was approximately 34 mm in length × 28 mm in width.

    (Measurement method of flexural strength when wet)    

Two stacked 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 stacked film 22 formed by fusing three of four sides. . In the glove box, 500 μL of an electrolytic solution in which LiPF ₆ was dissolved at a ratio of 1 mol / L was injected from the opening of the stacked film 22, and ethyl carbonate and ethyl methyl carbonate were mixed at a volume ratio of 3: 7. carbonate) solvent, impregnated the test wound body 30, and sealed one side of the opening with a vacuum sealer.

Next, the test wound body 30 sealed in the stacked film 22 was sandwiched between two gaskets (thickness: 1 mm, 5 cm x 5 cm), and a precision heating and pressing device (manufactured by New East Industries, Ltd., CYPT- 10), pressurize at 98 ° C and 0.6 MPa for 2 minutes, and leave to cool at room temperature. Using a universal testing machine (manufactured by Shimadzu Corporation, AGS-J) in a state of being sealed in the stacked film 22, the flexural strength when wet was measured for the test wound body 30 after pressing. Hereinafter, the details will be described with reference to FIG. 2.

With the 90 ° part facing up, make two aluminum L-shaped angles 41 (thickness 1mm, 10mm × 10mm, and length 5cm) parallel, and arrange the ends aligned, with the 90 ° part as the fulcrum, and The distance was fixed so that it was 15 mm. At a point 7.5 mm in the middle of the distance between the fulcrum points of the two aluminum L-shaped corners 41, align the midpoint of the width direction side of the rolled body for testing (about 28 mm) without exceeding the L-shaped corner 41. As the side in the longitudinal direction, the test wound body 30 is arranged.

Next, as an indenter, a side of the longitudinal direction (about 34 mm) of the rolled body for testing does not exceed the side of the longitudinal direction of the aluminum L-shaped corner 42 (thickness: 1 mm, 10 mm × 10 mm, length: 4 cm). Parallelly, align the 90 ° portion of the aluminum L-shaped angle 42 with the midpoint of the widthwise side of the test roll, and fix the aluminum L-shaped angle 42 to the universal testing machine with the 90 ° portion facing downward. Load cell (load cell capacity 50N). Three test wound bodies were measured at a load speed of 0.5 mm / min, and the average value of the obtained maximum test force was taken as the bending strength at the time of wetness.

    (5) Peeling force during drying         (Production of negative electrode)    

The negative electrode 20 was used in the same manner as in the case of the flexural strength when wet as described above.

    (Production of peeling test piece)    

The negative electrode 20 (70mm × 15mm) produced above was overlapped with the produced separator 10 (90mm in the mechanical direction × 20mm in the width direction), and this was sandwiched by two gaskets (thickness: 0.5mm, 95mm × 27mm). Using a precision heating and pressurizing device (manufactured by Shin Dong Industrial Co., Ltd., CYPT-10), it was pressurized at 90 ° C. and 8 MPa for 2 minutes, and left to cool at room temperature. On the negative side of the stacked body of the negative electrode 20 and the separator 10, stick a double-sided tape made of a width of 1 cm, and make the mechanical direction of the separator parallel to the length direction of the SUS board. One surface is attached to a SUS board (thickness 3mm, length 150mm × width 50mm). This was regarded as a peeling test piece.

    (Measurement method of peeling force during drying)    

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

    (6) Short circuit tolerance test    

The short-circuit tolerance was evaluated using a desktop precision universal testing machine Autograph AGS-X (manufactured by Shimadzu Corporation). First, as shown in FIG. 3 (A), a polypropylene insulator 5 (with a thickness of 0.2 mm) and a negative electrode 21 for a lithium ion battery (total thickness: about 140 μm, substrate: copper foil (thickness about 9 μm)) were stacked and activated. Substance: artificial graphite (particle diameter of about 30 μm), coated on both sides), a separator 10, and a sample stack 31 of aluminum foil 4 (thickness of about 0.1 mm). Next, as shown in FIG. 3 (B), the sample stack 31 is fixed to the compression jig (lower side) 44 of the universal testing machine using a double-sided tape. Next, the aluminum foil 4 and the negative electrode 21 of the sample stack 31 are connected to a circuit made of a capacitor and a clad resistor by 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 and the aluminum foil 4 in the sample stack 31. Next, the compression jig is installed on a universal testing machine, as shown in FIG. 3 (B). Between the two compression jigs 43 and 44, a sample stack 31 containing metal balls 6 is placed at a speed of 0.3 mm. / min. Compression was performed, and the time when the load reached 100 N was regarded as the end of the test. At this time, in the change of the compressive load, the part where the tortuous point appears is the film breaking point of the separator, and the moment when the above circuit is formed through the metal ball and the current is detected is taken as the short-circuit occurrence point. Measure the compressive displacement A (t) when the separator breaks the film by compression and generate a tortuous point of compressive stress, and the instantaneous compressive displacement B (t) in the current flowing in the circuit. ) When the value obtained is 1.1 or more, it means that even if the separator is broken due to foreign matter mixed in the battery, the coating layer composition is adhered to the surface of the foreign matter to maintain insulation, so the short-circuit tolerance is evaluated to be good. On the other hand, in the case where the value obtained by (Expression 1) is greater than 1.0 and less than 1.1, although the film breakage and short-circuit of the separator will not be caused at the same time, in order to prevent the tension or charge / discharge of the wound battery part from occurring at the same time, A short circuit does not occur even when the internal pressure of the battery increases due to the expansion of the electrode, and resistance to a certain degree or more is required. Therefore, the short circuit tolerance is evaluated to be slightly poor. In the case where the value obtained by using (Equation 1) is 1.0, film breakage and short-circuiting of the separator occur at the same time, and since the short-circuit tolerance increase due to the coating layer is not observed, the short-circuit tolerance is evaluated as bad. .

B (t) ÷ A (t) ‧‧‧ (Equation 1).

    (7) Peel strength (tape peel force) of porous layer and polyolefin multilayer microporous film         (Production of peeling test piece)    

The spacers (120 mm in the mechanical direction and 25 mm in the width direction) produced in the examples and comparative examples were placed on a glass plate so that air did not enter. Install the double-sided tape along the mechanical direction of the double-sided tape (100mm in the machine direction x 20mm in the width direction, manufactured by Kiwawa Industrial Co., Ltd., transparent film double-sided tape SFR-2020) and the mechanical direction of the separator. Above, a 2 kg rubber roller (SA-1003-B, manufactured by TESTER SANGYO, manual type, rubber strength 80 ± 5Hs) was used for 5 reciprocating treatments and pressed. A cellophane-tape (manufactured by NICHIBAN Co., Ltd., Cellotape (registered trademark), plant line, No. 405, mechanical direction 100mm × width direction 15mm) was attached to the double-sided tape and the separator in a mechanical direction of about 90 mm. On the separator side of the stacked body, a paper cut into a machine direction of 120 mm × width direction of 25 mm was affixed to the remaining portion of about 10 mm. This was pressed back and forth 5 times with a 2 kg rubber roller. Release the release liner of the double-sided tape and attach it to the SUS board (thickness 3mm, length 150mm x width 50mm) so that the mechanical direction of the separator is parallel to the length direction of the SUS board, and use 2kg The rubber roller is subjected to two reciprocating treatments and pressed. This was regarded as a peeling test piece.

    (Measurement method of tape peeling force)    

Using a universal testing machine (manufactured by Shimadzu Corporation, AGS-J), cut the paper attached to the scotch tape into a machine direction of 120mm × width direction of 25mm, sandwich it on the load cell side chuck, and then SUS The opposite side chuck was sandwiched on the plate side, and a 180-degree peel test was performed at a test speed of 100 mm / minute. The measured values of the stroke from 20 mm to 70 mm in the peel test were averaged, and the averaged value was used as the peel force of the peel test piece. A total of three peeling test pieces were measured, and the average value of the peeling force was taken as the tape peeling force.

In addition, although the porous layer may remain on the polyolefin multilayer microporous film side at the peeling interface, the peeling strength between the porous layer and the polyolefin multilayer microporous film is also calculated in this case.

The peel strength of the porous layer and the polyolefin multilayer microporous film 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 the first polyolefin resin solution    

20% by mass of polypropylene (PP: melting point 162 ° C) with Mw of 2.0 × 10 ⁶ and 80% by mass of high density polyethylene (HDPE: density 0.955g / cm ³ , melting point 135 ° C) with Mw of 5.6 × 10 ⁵ In 100 parts by mass of the obtained first polyolefin resin, an antioxidant [methylene-3- (3,5-di-tert-butyl-4-hydroxyphenyl) -propionate] methane ( tetrakis [methylene-3- (3,5-di-tert-butyl-4-hydroxyphenyl) -propionate] methane) 0.2 parts by mass to prepare a mixture. 25 parts by mass of the obtained mixture was put into a biaxial extruder, and 75 parts by mass of flowing paraffin [35cst (40 ° C)] was supplied from a side feeder of the biaxial extruder at 210 ° C and 250 rpm. Melt-kneading was performed under the conditions to prepare a first polyolefin resin solution.

    (2) Preparation of the second polyolefin resin solution    

40% by mass of ultra high molecular weight polyethylene (UHMwPE) with Mw of 2.0 × 10 ⁶ and 60% by mass of high density polyethylene (HDPE: density of 0.955 g / cm ³ ) with Mw of 5.6 × 10 ⁵ In 100 parts by mass of a polyolefin resin, 0.2 part by mass of an antioxidant [methylene-3- (3,5-di-tert-butyl-4-hydroxyphenyl) -propionate] methane was blended to prepare a mixture. . 25 parts by mass of the obtained mixture was put into another biaxial extruder of the same type as described above, and 75 parts by mass of flowing paraffin [35cSt (40 ° C)] was supplied from a side feed port of the biaxial extruder, so that Melt kneading was performed under the same conditions as described above to prepare a second polyolefin resin solution.

    (3) Extrusion    

The first and second polyolefin resin solutions were supplied from each biaxial extruder to a three-layer T-die (T-DIE), and the first polyolefin resin solution / the second polyolefin resin solution / the first polymer The layer thickness ratio of the olefin resin solution is extruded so that it is output at a speed of 2 m / min by using a cooling roller that has been adjusted to 30 ° C. and then cooled at the same time to obtain a three-layer extrusion. Shaped body.

    (4) First extension, removal of film-forming solvent, drying    

With a tenter stretching machine, the three-layer extruded molded body was simultaneously biaxially stretched at a temperature of 116 ° C in the MD direction and the TD direction by 5 times (first stretching), and then immersed in a temperature-controlled 25 After removing the paraffin wax in a dichloromethane bath at ℃, the mixture was dried in a drying oven adjusted to 60 ° C. to obtain a first extended multilayer sheet.

    (5) Second extension and heat fixing    

Using a batch type stretcher, the first stretched multilayer sheet was stretched 1.4 times (second stretch) in the TD direction at 126 ° C. Next, this film was heat-fixed by a tenter method at 126 ° C to obtain a polyolefin three-layer microporous film A having a thickness of 12 m, a porosity of 46%, and an air permeability of 150 seconds / 100cc.

    [Difluoroethylene-hexafluoropropylene copolymer (A)]    

As the copolymer (A), the copolymer (A1) was synthesized as follows. Using difluoroethylene, hexafluoropropylene and maleic acid monomethyl as starting materials, the suspension polymerization method is used to determine the molar ratio of difluoroethylene / hexafluoropropylene / maleic acid monomethyl The method was 98.0 / 1.5 / 0.5, and the copolymer (A1) was synthesized. The weight average molecular weight of the obtained copolymer (A1) was 1.5 million.

    [Difluoroethylene-hexafluoropropylene copolymer (B)]    

As the copolymer (B), a copolymer (B1) was synthesized as follows. A copolymer (B1) was synthesized by using a suspension polymerization method using difluoroethylene vinylene and hexafluoropropylene as starting materials so that the molar ratio of difluoroethylene vinylene / hexafluoropropylene became 93.0 / 7.0. The weight average molecular weight of the obtained copolymer (B1) was 300,000.

    [Production of battery separator]    

26.5 parts by mass of the copolymer (A1) and 3.5 parts by mass of the copolymer (B1) and 600 parts by mass of N-methyl-2-pyrrolidone (NMP) were mixed, and thereafter, the mixture was stirred with a disperser, and simultaneously The solid content of the porous layer was set to 100% by volume, and alumina particles (average particle diameter: 1.1 μm, density: 4.0g / cc) were added so as to be 51% by volume. Furthermore, a disperser was used for preliminary stirring at 2000 rpm for 1 hour. Next, DYNO-MILL (Dyno over mill Multi Lab (1.46L container, filling rate 80%, Φ0.5mm alumina beads) manufactured by Shinmaru Enterprises) was used under the conditions of a flow rate of 11 kg / hr and a peripheral speed of 10 m / s. Three times, a coating liquid (A) was produced. The obtained coating liquid (A) was applied to both sides of the polyolefin three-layer microporous film A by the same amount by a dip coating method. The coated film was 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 barrier. material. The thickness of the battery separator was 15 μm.

    (Example 2)    

Except that the blending amount of polypropylene was set to 10% by mass and the blending amount of high-density polyethylene was set to 90% by mass in the preparation of the first polyolefin resin solution, the same procedure as in Example 1 was performed to obtain a thickness. Polyolefin three-layer microporous membrane B with 12 μm, porosity 45%, and air resistance 135 seconds / 100cc. 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)    

Except that the blending amount of polypropylene was 45% by mass and the blending amount of high-density polyethylene was 55% by mass in the preparation of the first polyolefin resin solution, the thickness was obtained in the same manner as in Example 1 to obtain a thickness. Polyolefin three-layer microporous membrane C with 12 μm, 48% porosity, and air permeability of 300 seconds / 100cc. 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 the copolymer (B), a copolymer (B2) was synthesized as follows. A copolymer (B2) was synthesized by using a suspension polymerization method using difluoroethylene vinylene and hexafluoropropylene as starting materials so that the molar ratio of difluoroethylene vinylene / hexafluoropropylene became 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) in the preparation of the coating liquid was used.

    (Example 5)    

As the copolymer (B), a copolymer (B3) was synthesized as follows. A copolymer (B3) was synthesized by using a suspension polymerization method using difluoroethylene vinylene and hexafluoropropylene as starting materials so that the molar ratio of difluoroethylene vinylene / hexafluoropropylene became 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) in the preparation of the coating liquid was used.

    (Example 6)    

As the copolymer (A), the copolymer (A2) was synthesized as follows. Using difluoroethylene, hexafluoropropylene and maleic acid monomethyl as starting materials, the suspension polymerization method is used to determine the molar ratio of difluoroethylene / hexafluoropropylene / maleic acid monomethyl The method was 99.0 / 0.5 / 0.5, and the copolymer (A2) was synthesized. The weight average molecular weight of the obtained copolymer (A2) was 1.4 million. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (D) in which the copolymer (A1) was replaced with the copolymer (A2) in the preparation of the coating liquid was used.

    (Example 7)    

As the copolymer (A), the copolymer (A3) was synthesized as follows. Using difluoroethylene, hexafluoropropylene and maleic acid monomethyl as starting materials, the suspension polymerization method is used to determine the molar ratio of difluoroethylene / hexafluoropropylene / maleic acid monomethyl The method was 95.0 / 4.5 / 0.5, and the copolymer (A3) was synthesized. The weight average molecular weight of the obtained copolymer (A3) was 1.7 million. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (E) in which the copolymer (A1) was replaced with the copolymer (A3) in the preparation of the coating liquid was used.

    (Example 8)    

As the copolymer (A), a copolymer (A4) was synthesized as follows. Using difluoroethylene, hexafluoropropylene and maleic acid monomethyl as starting materials, the suspension polymerization method is used to determine the molar ratio of difluoroethylene / hexafluoropropylene / maleic acid monomethyl The method was 98.0 / 1.5 / 0.5, and a copolymer (A4) was synthesized. The weight average molecular weight of the obtained copolymer (A4) was 1.9 million. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (F) in which the copolymer (A1) was replaced with the copolymer (A4) in the preparation of the coating liquid was used.

    (Example 9)    

Except for using the coating liquid (G) in which the blending ratio of the copolymer (A1) and the copolymer (B1) is set to 28.0 parts by mass of the copolymer (A1) and 2.0 parts by mass of the copolymer (B1) in the preparation of the coating solution In the same manner as in Example 1, a battery separator was obtained.

    (Example 10)    

A battery separator was obtained in the same manner as in Example 1 except that a coating liquid (H) was used. The coating liquid (H) was prepared by setting the solid content of the porous layer to 100% by volume during the production of the coating liquid, and oxidizing it. The content of the aluminum particles was set to 40% by volume, the copolymer (A1) was 35.2 parts by mass, the copolymer (B1) was 4.7 parts by mass, and the NMP was changed to 900 parts by mass.

    (Example 11)    

A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (I) was used. The coating liquid (I) was prepared by setting the solid content of the porous layer to 100% by volume during the production of the coating liquid, and oxidizing it. The content of the aluminum particles was set to 75% by volume, the copolymer (A1) was 11.4 parts by mass, the copolymer (B1) was 1.5 parts by mass, and the NMP was changed to 300 parts by mass.

    (Example 12)    

As the copolymer (A), a copolymer (A5) was synthesized as follows. Using difluoroethylene, hexafluoropropylene, and maleic acid monomethyl ester as starting materials, the suspension polymerization method was used to use difluoroethylene / hexafluoropropylene / maleic acid monomethyl ester. The copolymer (A5) was synthesized such that the ratio was 98.4 / 1.5 / 0.1. The weight average molecular weight of the obtained copolymer (A5) was 1.5 million. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (J) in which the copolymer (A1) was replaced with the copolymer (A5) in the preparation of the coating liquid was used.

    (Example 13)    

As the copolymer (A), a copolymer (A6) was synthesized as follows. Using difluoroethylene, hexafluoropropylene and maleic acid monomethyl as starting materials, the suspension polymerization method is used to determine the molar ratio of difluoroethylene / hexafluoropropylene / maleic acid monomethyl The method was 94.5 / 1.5 / 4.0, and a copolymer (A6) was synthesized. The weight average molecular weight of the obtained copolymer (A6) was 1.5 million. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (K) in which the copolymer (A1) was replaced with the copolymer (A6) in the preparation of the coating liquid was used.

    (Example 14)    

The extrusion amounts of the first and second polyolefin resin solutions were adjusted to obtain a polyolefin three-layer microlayer having a layer thickness ratio of 10/80/10, a thickness of 7 μm, a porosity of 37%, and an air permeability of 120 seconds per 100 cm ³ . Porous membrane D. 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 extrusion amounts of the first and second polyolefin resin solutions were adjusted to obtain a polyolefin three-layer microlayer having a layer thickness ratio of 10/80/10, a thickness of 16 μm, a porosity of 45%, and an air permeability of 200 seconds / 100 cm ³ Porous membrane E. 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 using a suspension polymerization method using difluoroethylene vinylene and hexafluoropropylene as starting materials so that the molar ratio of difluoroethylene vinylene / hexafluoropropylene became 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) in the preparation of the coating liquid was used.

    (Example 17)    

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

    (Example 18)    

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

    (Example 19)    

Except for using a coating liquid (O) in which the blending ratio of the copolymer (A1) and the copolymer (B1) is 29.0 parts by mass of the copolymer (A1) and 1.0 part by mass of the copolymer (B1) in the preparation of the coating solution In the same manner as in Example 1, a battery separator was obtained.

    (Comparative example 1)    

Except that polypropylene was not used in the preparation of the first polyolefin resin solution and the blending amount of the high-density polyethylene was set to 100% by mass, the same procedure as in Example 1 was performed to obtain a thickness of 12 μm, a porosity of 44%, Polyolefin three-layer microporous membrane F with air permeability resistance of 100 seconds / 100 cm ³ . 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)    

Except that the blending amount of polypropylene was set to 5% by mass and the blending amount of high-density polyethylene was set to 95% by mass in the preparation of the first polyolefin resin solution, the same procedure as in Example 1 was performed to obtain a thickness. Polyolefin three-layer microporous membrane G of 12 μm, porosity 45%, and air permeability resistance 125 seconds / 100 cm ³ . 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)    

Except that the blending amount of polypropylene was set to 80% by mass and the blending amount of high-density polyethylene was set to 20% by mass in the preparation of the first polyolefin resin solution, the same procedure as in Example 1 was performed to obtain a thickness. Polyolefin three-layer microporous membrane H with 12 μm, porosity 37%, and air permeability resistance of 815 seconds / 100 cm ³ . 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)    

A battery separator was obtained in the same manner as in Example 1 except that the coating solution (P) was used. In the preparation of the coating solution, the coating solution (P) was obtained by mixing 88.3 parts by mass of the copolymer (A1) and the copolymer (B1). 11.7 parts by mass and NMP3500 parts by mass were dissolved and mixed.

    (Comparative example 5)    

A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (Q) was used. In the production of the coating liquid (Q), the solid content of the porous layer was set to 100% by volume to 95%. Alumina particles were added so that the copolymer (A1) was 2.0 parts by mass and the copolymer (B1) was 0.3 parts by mass, and the NMP was changed to 250 parts by mass.

    (Comparative Example 6)    

Except for using the coating liquid (R) in which the blending ratio of the copolymer (A1) and the copolymer (B1) was set to 15.0 parts by mass of the copolymer (A1) and 15.0 parts by mass of the copolymer (B1) in the preparation of the coating solution In the same manner as in Example 1, a battery separator was obtained.

    (Comparative Example 7)    

As the copolymer (A), a copolymer (A7) was synthesized as follows. A copolymer (A7) was synthesized by using a suspension polymerization method with difluoroethylene vinylene and hexafluoropropylene as starting materials so that the molar ratio of difluoroethylene vinylene / hexafluoropropylene became 98.5 / 1.5. The weight average molecular weight of the obtained copolymer (A7) was 1.5 million. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (S) in which the copolymer (A1) was replaced with the copolymer (A7) in the preparation of the coating liquid was used.

    (Comparative Example 8)    

A battery separator was obtained in the same manner as in Example 1 except that the coating solution (T) was used. The coating solution (T) was obtained by replacing the copolymer (A) with polydifluoroethylene (weight) in the preparation of the coating solution. It has an average molecular weight of 1.5 million), 30.0 parts by mass, and was prepared without using the copolymer (B).

    (Comparative Example 9)    

As the copolymer (A), a copolymer (A8) was synthesized as follows. Using difluoroethylene, hexafluoropropylene and maleic acid monomethyl as starting materials, the suspension polymerization method is used to determine the molar ratio of difluoroethylene / hexafluoropropylene / maleic acid monomethyl The method was 98.0 / 1.5 / 0.5, and a copolymer (A8) was synthesized. The weight average molecular weight of the obtained copolymer (A8) was 650,000. A battery separator was obtained in the same manner as in Example 1 except that the copolymer (A1) was replaced with the copolymer (A8) and the NMP was changed to 500 parts by mass of the coating solution (U) in the preparation of the coating solution. .

    (Comparative Example 10)    

As the copolymer (B), a copolymer (B5) was synthesized as follows. A copolymer (B5) was synthesized by using a suspension polymerization method using difluoroethylene vinylene and hexafluoropropylene as starting materials so that the molar ratio of difluoroethylene vinylene / hexafluoropropylene became 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) in which the copolymer (B1) was replaced with the copolymer (B5) in the preparation of the coating liquid was used.

    (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 film used in the above examples and comparative examples are shown in Table 1. The structures of the copolymers (A) and (B) of the porous layer, the weight average molecular weight, and The composition and characteristics of the obtained battery separator are shown in Table 2.

    [Industrial availability]    

The separator for a battery of this embodiment can provide a layer between a polyolefin multilayer microporous membrane and a separator of a porous layer, which satisfies peeling force during drying and flexural strength when wet, when used in a non-aqueous electrolyte secondary battery. A battery separator having excellent adhesiveness and adhesion between a separator and an electrode and excellent short-circuit tolerance. Therefore, even when a larger size and higher capacity of a battery (especially a stacked battery) is required in the future, the battery separator of this embodiment can be suitably used.

Claims (7)

    A battery separator comprising a polyolefin microporous membrane and a porous layer on a surface of at least one side of the polyolefin microporous membrane. The polyolefin microporous membrane is sequentially stacked with a first A microporous layer / second microporous layer / first microporous layer of a three-layer structure of a polyolefin multilayer microporous film, the first microporous layer is made of a first polyolefin resin containing polyethylene and polypropylene And the content ratio of the polypropylene is 10% by mass or more and 50% by mass or less with respect to the entire mass of the first polyolefin resin, the second microporous layer is made of only a polyethylene resin, and the porous layer The difluoroethylene-hexafluoropropylene copolymer (A), the difluoroethylene-hexafluoropropylene copolymer (B), and inorganic particles are included. The difluoroethylene-hexafluoropropylene copolymer ( A) Hexafluoropropylene units having a content of 0.3 mol% to 5.0 mol%, a weight average molecular weight of 900,000 to 2 million, and a hydrophilic group. The difluoroethylene-hexafluoropropylene copolymer (B) has Hexafluoropropylene monomer greater than 5.0mol% and below 8.0mol% The weight average molecular weight is 100,000 or more and 750,000 or less, based on 100% by mass of the total of the difluoroethylene-hexafluoropropylene copolymer (A) and the difluoroethylene-hexafluoropropylene copolymer (B). Containing 86% by mass or more and 98% by mass or less of the difluoroethylenevinyl-hexafluoropropylene copolymer (A), the solid content in the porous layer is 100% by volume, and 40% by volume or more and 80% by volume or less is included. The inorganic particles.         The battery separator according to claim 1, wherein the difluoroethylene-hexafluoropropylene copolymer (A) contains a hydrophilic group of 0.1 mol% or more and 5.0 mol% or less.         The battery separator according to claim 1 or 2, wherein the difluoroethylene-hexafluoropropylene copolymer (B) has a melting point of 60 ° C to 145 ° C.         The battery separator according to any one of claims 1 to 3, wherein the inorganic particles are selected from one or more of titanium dioxide, alumina, and gibbsite.         The battery separator according to any one of claims 1 to 4, wherein the thickness of the polyolefin multilayer microporous film is 3 μm or more and 16 μm or less.         An electrode body comprising a positive electrode, a negative electrode, and the battery separator according to any one of claims 1 to 5.         A non-aqueous electrolyte secondary battery includes the electrode body according to claim 6 and a non-aqueous electrolyte.