TW201834298A - Battery separator, electrode body, and nonaqueous electrolyte secondary battery - Google Patents

Abstract

The present invention addresses the problem of providing a battery separator having excellent adhesiveness and short-circuit resistance. The present invention pertains to a battery separator provided with a polyolefin fine porous membrane, and a porous layer stacked on at least one surface of the polyolefin fine porous membrane, wherein: the porous layer includes a vinylidene fluoride - hexafluoropropylene copolymer (A), a vinylidene-fluoride hexafluoropropylene copolymer (B), and inorganic particles; the vinylidene fluoride - hexafluoropropylene copolymer (A) has 0.3-5.0 mol% of a hexafluoropropylene unit, has a weight-average molecular weight of 900,000-2,000,000, and includes a hydrophilic group; and the vinylidene fluoride - hexafluoropropylene copolymer (B) has more than 5.0 mol% but not more 8.0 mol% of a hexafluoropropylene unit and has a weight-average molecular weight of 100,000-750,000.

Description

 Battery separator, electrode body and nonaqueous electrolyte secondary battery  

The present invention relates to a separator for a battery, an electrode body, and a nonaqueous electrolyte secondary battery.

Non-aqueous electrolyte secondary batteries, particularly lithium ion secondary batteries, are widely used in small electronic devices such as mobile phones or mobile information terminals. Examples of the form of the nonaqueous electrolyte secondary battery include a cylindrical battery, a prismatic battery, and a laminated battery. In general, these batteries have a structure in which an electrode body in which a positive electrode and a negative electrode are disposed through a separator, and a non-aqueous electrolyte are housed in an exterior body. The structure of the electrode body is, for example, a laminated electrode body in which a positive electrode and a negative electrode are laminated through a separator, and a wound electrode body in which a positive electrode and a negative electrode are wound into a separator through a separator.

At present, as a separator for batteries, a microporous film containing a polyolefin resin is mainly used. Since the microporous membrane containing a polyolefin resin has a so-called shutdown function, it is possible to suppress the flow of electric current and prevent ignition or the like by clogging the pores of the separator when the battery is abnormally heated.

In recent years, in the case of a separator for a battery, an attempt has been made to improve battery characteristics by providing a layer other than a polyolefin resin on one or both sides of a layer containing a polyolefin resin. For example, a separator for a battery in which a porous layer containing a fluororesin is provided on one or both sides of a layer containing a polyolefin resin has been proposed. Further, it is known that by adding inorganic particles to the porous layer, even when a sharp metal penetrates the battery due to an accident or the like, causing sudden short-circuit and heat generation, it is possible to prevent melt shrinkage of the separator and suppress short circuit between the electrodes. The expansion of the ministry.

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

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

Further, in Example 1 of Patent Document 3, a separator is disclosed which dissolves PVdF (weight average molecular weight: 500,000) and VdF-HFP copolymer (weight average molecular weight: 400,000, HFP unit: 5 mol%) in dimethyl A solution of hydrazine and tripropylene glycol was applied to the polyethylene microporous membrane to form a porous layer.

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

Further, in Example 1 of Patent Document 5, a separator is disclosed 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 methyl acetamide and tripropylene glycol was applied to the polyethylene microporous membrane to form a porous layer.

Further, in Example 23 of Patent Document 6, a separator is produced which is a VdF-HFP copolymer (weight average molecular weight: 1.93 million, HFP unit: 1.1 mol%) and a VdF-HFP copolymer (weight average molecular weight: 470,000). HFP unit 4.8 mol%) was dissolved in a solution of dimethylacetamide and tripropylene glycol, and a coating liquid of aluminum hydroxide was further added thereto, and this was applied to a polyethylene microporous film to form a porous layer.

 [Previous Technical Literature]    [Patent Literature]  

[Patent Document 1] Japan Re-Form 1999-036981

[Patent Document 2] Japanese Patent No. 5282179

[Patent Document 3] Japanese Patent No. 5282180

[Patent Document 4] Japanese Patent No. 5282181

[Patent Document 5] Japanese Patent No. 5342088

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

In recent years, non-aqueous electrolyte secondary batteries have been expected to be used for large-scale applications such as large-sized tablet computers, lawn mowers, electric motor vehicles, electric vehicles, hybrid electric vehicles, and small ships. It is speculated that large batteries will become popular. It is speculated that further capacity needs to be increased. In the above-mentioned Patent Documents 1 to 5, the adhesion between the separator containing the electrolytic solution and the electrode is improved. However, when the secondary battery is increased in size, further improvement in adhesion is required.

As described below, the inventors of the present invention have focused on the adhesion between the electrode and the separator at the time of drying and the adhesion between the electrode and the separator at the time of wetness when evaluating the adhesion between the electrode and the separator. The two types of adhesion were evaluated separately to evaluate the adhesion more correctly, and it was further found that these adhesions can be evaluated by using the peeling force at the time of drying and the bending strength at the time of wetting as indicators.

In other words, for example, the wound electrode system is produced by passing the positive electrode and the negative electrode through a separator and applying tension to each member while winding. At this time, the positive electrode or the negative electrode applied to the metal current collector hardly expands and contracts with respect to the tension, but the separator is wound while being elongated to some extent in the machine direction. If the winding body is temporarily placed, the spacer portion gradually contracts to return to the original length. As a result, a force in the parallel direction is generated in the boundary surface between the electrode and the separator, and the wound electrode body (especially the electrode body which is flatly wound) is liable to be warped or skewed. In addition, these problems become apparent due to the widening or the lengthening of the separator due to the increase in the size of the battery, and there is a concern that the yield at the time of production deteriorates. In order to suppress the occurrence of deflection or skew of the wound electrode body, the separator is required to have more adhesion to the electrode than ever before. In addition, when the electrode body is transported, if the respective members are not sufficiently adhered to each other, the electrode and the separator are peeled off, and the yield cannot be conveyed satisfactorily. The problem of the adhesion at the time of transportation becomes apparent due to the increase in the size of the battery, and the yield is deteriorated. Therefore, it is required that the separator is difficult to peel off from the electrode and the peeling force is high at the time of drying.

Further, in the case of a laminated battery, it is difficult to apply pressure compared to an angular or cylindrical battery to which a pressure is applied from an exterior body, and the swelling of the electrode due to charging and discharging is caused. Shrinkage is prone to partial detachment at the interface of the separator and the electrode. As a result, the battery expands, the resistance inside the battery increases, and the cycle performance is lowered. Therefore, the adhesion of the separator to the electrodes in the battery after the electrolyte is injected is required. In the present specification, the adhesion is measured by the bending strength at the time of wetness obtained by the measurement method described later as an index. It is considered that if the strength is large, it is expected to suppress an increase in battery characteristics such as battery expansion after repeated charge and discharge. Moreover, the bending strength at the time of wetting in this specification shows the adhesiveness of the separator and the electrode in the state in which the separator contains an electrolyte solution. The peeling force at the time of drying indicates the adhesion to the boundary surface of the separator and the electrode in a state where the separator substantially does not contain the electrolyte. Further, the fact that the electrolyte solution is substantially absent means that the electrolyte 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 spacer during drying required for the manufacture or transportation of the electrode body, and the electrode and the spacer during the wetness required after the electrolyte is injected are found. There is a substitution relationship in the adhesiveness, and it is extremely difficult to satisfy the physical properties of both parties; and the techniques disclosed in the above Patent Documents 1 to 5 have insufficient adhesion.

Further, it is required that even if a sharp impact is applied to the battery, the protruding portion of the electrode active material penetrates the separator, and it is difficult to short-circuit the electrode (hereinafter referred to as short-circuit resistance). However, when it is predicted that the film thickness of the separator for a battery will be thinned in the future, the thinner the thickness of the separator, the more difficult it is to ensure short-circuit resistance. It is known that it is effective to contain a certain amount or more of inorganic particles in order to ensure short-circuit resistance. However, when inorganic particles having a degree of short-circuit resistance can be ensured, the adhesion between the electrode and the separator is lowered. tendency.

In view of the above, it is an object of the present invention to provide a separator for a battery which is excellent in adhesion between an electrode and a separator during drying and adhesion between an electrode and a separator during wetness, and which is excellent in short circuit resistance, and a separator for use thereof. Electrode body and secondary battery.

In order to solve the above problems, the present inventors have intensively studied and found that it is possible to solve the above problem by providing a separator having a porous layer containing two kinds of fluorine-based resins having different structures, a blending ratio thereof, and a specific amount of inorganic particles. The subject matter further completes the present invention.

That is, the present invention relates to a separator for a battery comprising a polyolefin microporous membrane and a separator for a battery laminated on at least one surface of the polyolefin microporous membrane, wherein the porous layer contains vinylidene fluoride- a hexafluoropropylene copolymer (A), a copolymer of vinylidene fluoride-hexafluoropropylene (B), and inorganic particles, wherein the vinylidene fluoride-hexafluoropropylene copolymer (A) has 0.3 mol% or more and 5.0 mol. The hexafluoropropylene unit having a weight average molecular weight of 900% or more and 2,000,000 or less and containing a hydrophilic group, and the vinylidene fluoride-hexafluoropropylene copolymer (B) has more than 5.0 mol% and 8.0 mol% or less. The hexafluoropropylene unit has a weight average molecular weight of 100,000 or more and 750,000 or less, and is a total of the above-mentioned vinylidene fluoride-hexafluoropropylene copolymer (A) and the above-mentioned vinylidene fluoride-hexafluoropropylene copolymer (B). 100% by mass, the 86 parts by mass or more and 98% by mass or less of the vinylidene fluoride-hexafluoropropylene copolymer (A) is contained in an amount of 40% by volume or more based on 100% by volume of the solid content in the porous layer. The above inorganic particles are at most 5% by volume.

Further, the vinylidene fluoride-hexafluoropropylene copolymer (A) preferably contains 0.1 mol% or more and 5.0 mol% or less of a hydrophilic group.

Further, it is preferred that the vinylidene fluoride-hexafluoropropylene copolymer (B) has a melting point of 60 ° C or more and 145 ° C or less.

Further, the inorganic particles are preferably one or more selected from the group consisting of titanium dioxide, aluminum oxide and diaspore.

Moreover, it is preferable that the thickness of the polyolefin microporous film is 3 μm or more and 16 μm or less.

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

Further, the present invention is a nonaqueous electrolyte secondary battery comprising the electrode body of the present invention and a nonaqueous electrolyte.

According to the present invention, it is possible to provide a battery separator which is excellent in both the adhesion between the electrode and the separator during drying and the adhesion between the electrode and the separator at the time of wetness, and which is excellent in short-circuit resistance, and an electrode using the same Body and secondary battery.

1‧‧‧Polyolefin microporous membrane

2‧‧‧Porous layer

4‧‧‧Aluminum foil

5‧‧‧Resin insulator

6‧‧‧metal ball

10‧‧‧Battery separator

20‧‧‧Negative electrode (for evaluation of adhesion)

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

22‧‧‧Laminated film

30‧‧‧Electrical winding body

31‧‧‧electrode laminate

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

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

43‧‧‧Compression fixture (upper side)

44‧‧‧Compression fixture (lower side)

Fig. 1 is a view showing an example of a separator for a battery of the embodiment.

Fig. 2 is a schematic view showing a method of evaluating the bending strength at the time of wetting.

Fig. 3 is a schematic view showing an evaluation method of the short circuit resistance test.

 [Formation of the Invention]  

Hereinafter, embodiments of the present invention will be described with reference to the drawings. Hereinafter, the direction in the drawing will be described using the XYZ coordinate system. In this XYZ coordinate system, a surface parallel to the surface (in-plane direction) of the microporous film or the separator is referred to as an XY plane. Further, a direction (thickness direction) perpendicular to the XY plane is set to the Z direction. The X direction, the Y direction, and the Z direction are respectively indicated by the direction of the arrow in the figure being the + direction, and the direction opposite to the direction of the arrow being the - direction. In addition, in the drawings, in order to make it easy to understand each configuration, there is a case where a part of the configuration is emphasized or simplified, and the actual structure, shape, scale, and the like are different.

Fig. 1 is a view showing an example of a separator for a battery of the embodiment. As shown in FIG. 1, the battery separator 10 (hereinafter, simply referred to as "spacer 10") includes a polyolefin microporous membrane 1 and a porous layer 2 laminated on at least one surface of the polyolefin microporous membrane 1. Hereinafter, each layer constituting the separator for a battery will be described.

[1] Polyolefin microporous membrane

The polyolefin microporous film 1 is a microporous film containing a polyolefin resin. The polyolefin microporous membrane 1 is not particularly limited, and a polyolefin microporous membrane used for a known separator for a battery can be used. Further, in the present specification, the microporous film means a film having a void that connects the inside. Hereinafter, an example of the polyolefin microporous membrane 1 will be described, but the polyolefin microporous membrane used in the present invention is not limited thereto.

[Polyolefin resin]

The polyolefin resin constituting the polyolefin microporous membrane 1 (hereinafter, simply referred to as "microporous membrane 1") may be ethylene, propylene, 1-butene or 4-methyl-1-penta A homopolymer, a two-stage polymer, a copolymer, or a mixture thereof in which a olefin, 1-hexene or the like is polymerized. Among them, as the polyolefin resin, a polyethylene resin is preferably used as a main component. The content of the polyethylene resin is preferably 70% by mass or more, more preferably 90% by mass or more, and still more preferably 100% by mass based on 100% by mass of the total mass of the polyolefin resin in the microporous film 1. In the polyolefin resin, various additives such as an antioxidant and an inorganic filler may be added as needed within the range which does not impair the effects of the present invention.

The film thickness of the polyolefin microporous membrane 1 is not particularly limited, and is preferably 3 μm or more and 16 μm or less, more preferably 5 μm or more and 12 μm or less, and still more preferably 5 μm or more and 10 μm from the viewpoint of increasing the capacity of the battery. the following. When the film thickness of the polyolefin microporous film is within the above preferred range, it is possible to maintain a practical film strength and pore clogging function, and it is more suitable for a battery having a higher capacity in the future. In the battery separator 10 of the present embodiment, even when the thickness of the polyolefin microporous membrane 1 is small, the interlayer of the polyolefin microporous membrane 1 and the porous layer 2 having excellent separator 10 and the separator 10 can be obtained. The adhesion between the electrodes exerts the effect more clearly when the spacer 10 is thinned.

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

[Method for Producing Polyolefin Microporous Membrane]

The method for producing the microporous membrane 1 is not particularly limited as long as it can produce a polyolefin microporous membrane having desired properties, and a conventionally known method can be used. For the method of producing the microporous membrane 1, for example, the method described in Japanese Patent No. 2132327, Japanese Patent No. 3347835, and International Publication No. 2006/137540 can be used. Hereinafter, an example of a method of producing the microporous membrane 1 will be described. Further, the method of producing the microporous membrane 1 is not limited to the following method.

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

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

(2) a step of extruding the aforementioned polyolefin solution and cooling it to form a gel-like sheet

(3) a first stretching step of stretching the aforementioned gel-like sheet

(4) a step of removing a solvent for film formation from the stretched gel-like sheet

(5) a step of drying the sheet after removing the solvent for film formation

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

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

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

Hereinafter, each step will be described separately.

(1) Preparation steps of polyolefin solution

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

In the polyolefin solution, the blending ratio of the polyolefin resin and the solvent for film formation is not particularly limited, but it is preferably from 70 to 80 parts by mass based on 20 to 30 parts by mass of the polyolefin resin. When the ratio of the polyolefin resin is within the above range, expansion or contraction of the die outlet can be prevented when the polyolefin solution is extruded, and the formability and self-supportability of the extrusion molded body (gel-like molded body) are improved. .

(2) Step of forming a gelatinous sheet

The polyolefin solution was supplied from the extruder to the mold and extruded into a sheet. A plurality of polyolefin solutions of the same or different composition may be supplied from an extruder to a mold where the layers are layered and extruded into a sheet.

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

The gel-like sheet is formed by cooling the obtained extruded molded body. As a method of forming the gel-like sheet, a method disclosed in, for example, Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Cooling is preferably carried out at a rate of 50 ° C/min or more to at least the gelation temperature. Cooling is preferably carried out to below 25 °C. The microphase of the polyolefin separated by the solvent for film formation can be fixed by cooling. When the cooling rate is within the above range, the degree of crystallization is maintained within an appropriate range, and it becomes a gel-like sheet suitable for stretching. As the cooling method, a method of bringing into contact with a refrigerant such as cold air or cooling water, a method of contacting with a cooling roll, or the like can be used, and it is preferable to perform cooling by bringing it into contact with a roller that has been cooled by a refrigerant.

(3) The first stretching step

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

The stretching ratio (area stretching ratio) in this step is preferably 9 times or more, more preferably 16 times or more, and particularly preferably 25 times or more. Further, the stretching ratios in the machine direction (MD) and the width direction (TD) may be the same as each other or may be different from each other. Further, the draw ratio in this step means the area draw ratio of the microporous film immediately before the next step, based on the microporous film immediately before the step.

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

By the above stretching, the polyethylene sheet (lamella) is cleaved, and the polyethylene phase is refined to form a plurality of fibrils. The fibrils form a three-dimensionally irregularly connected mesh structure. By stretching, the mechanical strength is increased and the pores are enlarged. When the stretching is carried out under appropriate conditions, the through-pore diameter can be controlled, and even a thinner film thickness has a high porosity.

It is also possible to perform stretching by providing a temperature distribution in the film thickness direction in accordance with desired physical properties, whereby a microporous film excellent in mechanical strength can be obtained. The details of the method are described in Japanese Patent No. 3347854.

(4) Removal of solvent for film formation

The solvent for film formation is removed (cleaned) using a cleaning solvent. Since the polyolefin phase is separated from the solvent phase for film formation, if the solvent for film formation is removed, a fibril containing a fine three-dimensional network structure and having pores (voids) which are three-dimensionally irregularly connected can be obtained. Quality membrane. The method of washing the solvent and the method of removing the solvent for film formation using the solvent are well known, and thus the description thereof is omitted. The method disclosed in, for example, Japanese Patent No. 2132327 or Japanese Patent Laid-Open Publication No. 2002-256099 can be used.

(5) Drying

The microporous membrane after removing the solvent for film formation is dried by a heat drying method or an air drying method. The drying temperature is preferably not less than the crystal dispersion temperature (Tcd) of the polyolefin resin, and particularly preferably 5 ° C or more lower than Tcd. In the drying, the microporous membrane is made into 100% by mass (dry weight), and it is preferably carried out until the residual cleaning solvent is 5% by mass or less, and more preferably 3% by mass or less. When the remaining cleaning solvent is in the above range, the porosity of the microporous film can be maintained and the deterioration of the permeability can be suppressed when the stretching step and the heat treatment step of the microporous film in the subsequent stage are performed.

(6) Second stretching step

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

(7) Heat treatment

Further, the dried microporous film can be subjected to heat treatment. By heat treatment, the crystallization is stabilized and the sheet is homogenized. As the heat treatment method, a heat setting treatment and/or a heat relaxation treatment can be used. The heat setting treatment refers to a heat treatment in which heating is performed while maintaining the size of the film. The heat relaxation treatment refers to a heat treatment for thermally shrinking the film in the mechanical direction or the width direction in the heating film. The heat setting treatment is preferably carried out by a tenter method or a roll method. For example, as a thermal relaxation treatment method, a method disclosed in Japanese Laid-Open Patent Publication No. 2002-256099 can be cited. The heat treatment temperature is preferably in the range of Tcd to Tm of the polyolefin resin, more preferably in the range of ±5 ° C of the stretching temperature of the microporous film, and particularly preferably in the second stretching temperature of the microporous film ± 3 Within the range of °C.

(8) Cross-linking treatment, hydrophilization treatment

Further, the microporous film after bonding or after stretching can be further subjected to a crosslinking treatment and a hydrophilization treatment. For example, the microporous film is subjected to a crosslinking treatment by irradiating an ionizing radiation such as an α line, a β line, a γ line, or an electron beam. In the case of irradiating an electron beam, it is preferably an electron beam amount of 0.1 to 100 Mrad, preferably an acceleration voltage of 100 to 300 kV. By the crosslinking treatment, the melting temperature of the microporous membrane rises. Further, the hydrophilization treatment can be carried out by monomer grafting, surfactant treatment, corona discharge, or the like. The monomer grafting is preferably carried out after the crosslinking treatment.

[2] porous layer

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

[Flyvinylidene fluoride-hexafluoropropylene copolymer (A)]

The vinylidene fluoride-hexafluoropropylene copolymer (A) (hereinafter, simply referred to as the copolymer (A)) is a copolymer comprising a vinylidene fluoride unit and a hexafluoropropylene unit, and as described later, contains a hydrophilic group. base. The content of the hexafluoropropylene unit in the copolymer (A) is 0.3 mol% or more, preferably 0.5 mol% or more. When the content of the hexafluoropropylene unit is smaller than the above range, the crystallinity of the polymer becomes high, and the degree of swelling of the separator with respect to the electrolytic solution is lowered. Therefore, the adhesion between the separator and the electrode is lowered, and the electrolyte solution cannot be sufficiently obtained. The subsequent adhesion of the electrode to the separator (bending strength at wet). On the other hand, the content of the hexafluoropropylene unit is 5.0 mol% or less, more preferably 2.5 mol% or less. When the content of the hexafluoropropylene unit exceeds the above range, the separator may excessively swell the electrolyte solution and may lower the bending strength when wet.

The weight average molecular weight of the copolymer (A) is 900,000 or more, preferably 1,000,000 or more. On the other hand, the weight average molecular weight of the copolymer (A) is 2,000,000 or less, more preferably 1.5,000,000 or less. When the weight average molecular weight of the copolymer (A) is within the above range, in the step of forming the porous layer, the time during which the copolymer (A) is dissolved in the solvent does not become extremely long, the production efficiency can be improved, or the production efficiency can be improved. Maintaining moderate rubber strength when swelled in the electrolyte can increase the bending strength during wetting. Further, the weight average molecular weight of the copolymer (A) is based on a polystyrene equivalent value of gel permeation chromatography.

The copolymer (A) has a hydrophilic group. The copolymer (A), by having a hydrophilic group, becomes more firmly adhered to the binder component present in the active material or electrode present on the surface of the electrode. The reason for this is not clear, and it is presumed that the force is increased by hydrogen bonding. Examples of the hydrophilic group include a hydroxyl group, a carboxylic acid group, a sulfonic acid group, and a salt thereof. Among them, a carboxylic acid group or a carboxylic acid ester is particularly preferred.

As a method of 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, maleic acid ester, and horse can be used. A method of introducing a monomer having a hydrophilic group such as methyl monomethyl ester into a main chain by a copolymerization method, or a method of introducing a side chain by grafting. The hydrophilic group modification ratio can be measured by FT-IR, NMR, quantitative titration or the like. For example, in the case of a carboxylic acid group, FT-IR can be used, and the absorption intensity ratio of C-H stretching vibration and C=O stretching vibration of a carboxyl group can be obtained based on a homopolymer.

The content of the hydrophilic group of the copolymer (A) is preferably 0.1 mol% or more, more preferably 0.3 mol% or more. On the other hand, the content of the hydrophilic group is preferably 5.0 mol% or less, more preferably 4.0 mol% or less. By setting the content of the hydrophilic group to 5.0 mol% or less, it is possible to suppress the crystallinity of the polymer from becoming too low, and the degree of swelling of the electrolytic solution is increased, and the bending strength at the time of wetting is deteriorated. In addition, when the content of the hydrophilic group is in the above range, the affinity between the inorganic particles contained in the porous layer 2 and the copolymer (A) is increased, the short-circuit resistance is improved, and the inorganic particles are prevented from falling off. effect. The reason for this is not clear, and it is presumed that the film strength of the porous layer 2 is increased by the copolymer (A) having a hydrophilic group as a main component of the porous layer 2 and inorganic particles. The quantification of the hydrophilic group of the vinylidene fluoride-hexafluoropropylene copolymer in the porous layer 2 can be determined by an IR (infrared absorption spectrum) method, an NMR (nuclear magnetic resonance) method, or the like.

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

The structure of the copolymer (A), by setting the molecular weight within the above range, the separator 10, in the case of a nonaqueous electrolyte secondary battery, has high affinity for a nonaqueous electrolyte, chemical and physical stability It is high and exhibits bending strength when wet, and can maintain sufficient affinity with the electrolyte even when used at high temperatures.

[Flyvinylidene fluoride-hexafluoropropylene copolymer (B)]

The vinylidene fluoride-hexafluoropropylene copolymer (B) (hereinafter, simply referred to as the copolymer (B)) is a copolymer comprising a vinylidene fluoride unit and a hexafluoropropylene unit. The content of hexafluoropropylene in the copolymer (B) is more than 5.0 mol%, more preferably 6.0 mol% or more, still more preferably 7.0 mol% or more. When the content of the hexafluoropropylene unit is 5.0 mol% or less, the adhesion between the separator and the electrode at the time of drying (peeling force at the time of drying) may not be sufficiently obtained. On the other hand, the content on the upper limit side is 8.0 mol% or less, more preferably 7.5 mol% or less. Further, when the content of the hexafluoropropylene unit exceeds 8.0 mol%, there is a case where the electrolytic strength is excessively swollen and the bending strength is lowered when wet. Further, the copolymer (B) may contain a hydrophilic group, but may not contain a hydrophilic group.

The weight average molecular weight of the copolymer (B) is from 100,000 to 750,000. When the weight average molecular weight of the copolymer (B) is within the above range, the affinity for the nonaqueous electrolyte is high, and the chemical and physical stability is high, and excellent adhesion between the separator and the electrode during drying can be obtained (drying) When peeling force). The reason for this is not clear, and it can be presumed that the copolymer (B) has fluidity under heat and pressure conditions such as exhibiting peeling force upon drying, and enters the porous layer of the electrode to become a anchor, whereby the porous layer 2 It has a strong adhesion to the electrode. In other words, in the battery separator 10, the copolymer (B) can contribute to peeling force during drying, and contributes to prevention of deflection or skew of the wound electrode body or the laminated electrode body or improvement of conveyability. Further, the copolymer (B) and the copolymer (A) are different resins.

The weight average molecular weight of the copolymer (B) is 100,000 or more, preferably 150,000 or more. When the weight average molecular weight of the copolymer (B) is less than the lower limit of the above range, the amount of entanglement of the molecular chain is too small, so that the resin strength is weak and the aggregation failure of the porous layer 2 is likely to occur. On the other hand, the weight average molecular weight of the copolymer (B) is preferably 750,000 or less, more preferably 700,000 or less. When the weight average molecular weight of the copolymer (B) exceeds the upper limit of the above range, in order to obtain the peeling force at the time of drying, it is necessary to increase the press temperature in the production step of the wound body. When this is done, there is a shrinkage of the microporous film containing polyolefin as a main component. In addition, when the weight average molecular weight of the copolymer (B) exceeds the upper limit of the above range, the amount of entanglement of the molecular chain increases, and there is a possibility that the molecular chain does not sufficiently flow under the pressing conditions.

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

The copolymer (B) is a copolymer having a vinylidene fluoride unit and a hexafluoropropylene unit. The copolymer (B) can be obtained by a suspension polymerization method or the like in the same manner as the copolymer (A). The melting point of the copolymer (B) can be adjusted by controlling the crystallinity of a portion containing a vinylidene fluoride unit. For example, when the copolymer (B) contains a monomer other than the vinylidene fluoride unit, the melting point can be adjusted by controlling the ratio of the vinylidene fluoride unit. The monomer other than the vinylidene fluoride unit may have one or more kinds of tetrafluoroethylene, trifluoroethylene, trichloroethylene, hexafluoropropylene, fluorovinyl maleic anhydride, maleic acid, maleic acid ester, Monomethyl maleate and the like. A method of adding the above monomer when the copolymer (B) is polymerized, a method of introducing a main chain by copolymerization, or a method of introducing a side chain by grafting may be mentioned. Further, the melting point can be adjusted by controlling the ratio of the head-to-Head bond (-CH ₂ -CF ₂ -CF ₂ -CH ₂ -) of the vinylidene fluoride unit.

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

The content of the copolymer (A) is 86% by mass or more, and more preferably 88% by mass or more based on 100% by mass based on the total weight of the copolymer (A) and the copolymer (B). The content of the copolymer (A) is preferably 98% by mass or less, and more preferably 97% by mass or less. In addition, the content of the copolymer (B) is 14% by mass or less, preferably 12% by mass or more based on 100% by mass based on the total weight of the copolymer (A) and the copolymer (B). Further, the content of the copolymer (B) is 2% by mass or more and 3% by mass or more. When the content of the copolymer (A) and the content of the copolymer (B) are within the above range, the porous layer 2 can achieve both excellent wet bending strength and peeling force during drying at a high level.

Further, the porous layer 2 can contain a resin other than the copolymer (A) and the copolymer (B), and the resin component constituting the porous layer 2 preferably contains a copolymer (A) insofar as the effect of the present invention is not impaired. And copolymer (B). Further, when a resin other than the copolymer (A) and the copolymer (B) is contained, the content of the copolymer (A) or the copolymer (B) is set to be 100% by mass relative to the resin component of the porous layer 2. %proportion.

[Inorganic Particles]

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

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

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

In general, when the inorganic layer is not contained in the porous layer, the bending strength at the time of wetness and the peeling force at the time of drying tend to be lowered. However, as described above, when the porous layer 2 of the present embodiment contains a specific fluororesin in a specific ratio and contains inorganic particles in the above range, it has a high adhesion force to the electrode, and the bending strength and the drying are wet. The balance of the peeling force is improved, and excellent short circuit resistance can be obtained.

The average particle diameter of the inorganic particles is preferably 1.5 times or more and 50 times or less, more preferably 2.0 times or more and 20 times or less, of the average pore diameter of the polyolefin microporous film. The average flow pore size is measured in accordance with JIS K3832, and can be determined by using a Perm-Porometer (for example, CFP-1500A manufactured by PMI Co., Ltd.) in the order of Dry-up and Wet-up. . Specifically, the pressure at a point indicating the 1/2 slope of the pressure and flow rate curves and the curve intersecting the curve measured by Wet-up in the Dry-up measurement is converted into the pore diameter. The pressure and the aperture are converted using the following 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.

The average particle diameter of the inorganic particles is preferably from 0.3 μm to 1.8 μm, more preferably from 0.5 μm to 1.5 μm, from the viewpoint of slidability of the wound core or particle detachment when the cell is wound. Good is 0.9μm~1.3μm. The average particle diameter of the particles can be measured using a laser diffraction method or a dynamic light scattering type measuring device. For example, it is preferable to use an ultrasonic probe to measure particles dispersed in an aqueous solution in which a surfactant is added by a particle size distribution measuring apparatus (Microtrac HRA, manufactured by Nikkiso Co., Ltd.), from the small particle side converted by volume. The value of the particle diameter (D50) at the time of cumulative 50% was taken as the average particle diameter. The shape of the particles is not particularly limited as long as it is a true spherical shape, a substantially spherical shape, a plate shape, or 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 surface, more preferably 1 μm or more and 2.5 μm or less, and still more preferably 1 μm or more and 2 μm or less. When the thickness of each single film is 0.5 μm or more, high adhesion to the electrode (bending strength at the time of wetting and peeling force at the time of drying) can be ensured. On the other hand, when the thickness of the single-face film is 3 μm or less, the winding volume can be suppressed, and the film can be further thinned, which is more suitable for increasing the capacity of the battery which may be developed in the future.

The porosity of the porous layer 2 is preferably 30% or more and 90% or less, more preferably 40% or more and 70% or less. When the porosity of the porous layer 2 is within the above range, it is possible to prevent the resistance of the separator from rising, to flow a large current, and to maintain the film strength.

[3] Method for manufacturing battery separator

The method for producing the separator for a battery is not particularly limited, and it can be produced by a known method. Hereinafter, an example of a method of producing a separator for a battery will be described. The method for producing a separator for a battery can include the following steps (1) to (3) in order.

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

(2) A step of adding inorganic particles to a fluorine-based resin solution, mixing and dispersing them to obtain a coating liquid

(3) a step of applying a coating liquid to a polyolefin microporous membrane, immersing it in a coagulating liquid, and washing and drying

(1) Step of obtaining a fluororesin solution

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

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

(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 stirring the fluororesin solution, and are stirred by a dispersor or the like for a predetermined period of time (for example, about 1 hour) to carry out preliminary dispersion, and then the particles are obtained by using a bead mill or a paint shaker. The dispersion step (dispersion step), the aggregation of the particles was reduced, and further, the coating liquid was prepared by mixing with a Three-One Motor equipped with a stirring blade.

(3) a step of applying a coating liquid to a microporous membrane, immersing it in a coagulating liquid, and washing and drying

The coating liquid is applied to the microporous membrane, and the coated microporous membrane is immersed in the coagulating liquid to obtain a vinylidene fluoride-hexafluoropropylene copolymer (A) and a vinylidene fluoride-hexafluoropropylene copolymer (B). The phase separation is carried out, and it is solidified in a state having a three-dimensional network structure, and is washed and dried. Thereby, a separator for a battery including a microporous membrane and a porous layer on the surface of the microporous membrane can be obtained. The method of applying the coating liquid to the microporous film may be a known method, and examples thereof include a dip coating method, a reverse roll coating method, a gravure coating method, a kiss coating method, a roll brush method, a spray method, and an air knife. These methods can be carried out individually or in combination by a coating method, a Meyer bar coating method, a pipe doctor method, a knife coating method, a die coating method, and the like.

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

In terms of cleaning, water can be used. Drying can be carried out by using, for example, hot air of 100 ° C or lower.

[4] Physical properties of battery separators

Battery separator

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

In the nonaqueous electrolyte secondary battery, the positive electrode and the negative electrode are disposed through a separator, and the separator contains an electrolyte (electrolyte). The structure of the non-aqueous electrolyte electrode is not particularly limited, and a conventionally known structure can be used. For example, it is possible to have an electrode structure (coin type) in which a disk-shaped positive electrode and a negative electrode are opposed to each other, and an alternately laminated flat plate shape. The electrode structure (layered type) of the positive electrode and the negative electrode, the electrode structure (winding type) of the strip-shaped positive electrode and the negative electrode on which the laminated layer is wound, and the like. The separator for a battery of the present embodiment can have excellent adhesion between the separator and the electrode in any of the battery structures.

The current collector, the positive electrode, the positive electrode active material, the negative electrode, the negative electrode active material, and the electrolytic solution used in the nonaqueous electrolyte secondary battery, such as a lithium ion secondary battery, are not particularly limited, and can be suitably used in combination with a conventionally known material.

Further, as shown in FIG. 1(A), the battery separator 10 may be porous 2 in one area of the polyolefin microporous membrane 1, or porous 2 in the two-layer layer of the polyolefin microporous membrane 1. Further, although the polyolefin microporous membrane 1 is one layer in Fig. 1, it may be a laminate of two or more layers. Further, the battery separator 10 may further laminate the polyolefin microporous membrane 1 and other layers other than the porous material 2.

The wet strength at the time of wetting of the separator for a battery is preferably 4.0 N or more, more preferably 5.0 N or more, still more preferably 6.0 N or more. The upper limit of the bending strength at the time of wetting is not particularly limited, and is, for example, 15.0 N or less. When the bending strength at the time of wetting is within the above preferred range, partial detachment at the interface between the separator and the electrode can be further suppressed, and an increase in internal resistance of the battery and a decrease in battery characteristics can be suppressed. Further, the bending strength at the time of wetting can be measured by the method described in the examples below.

The peeling force at the time of drying of the separator for a battery is preferably 2.0 N/m or more, more preferably 5.0 N/m or more, still more preferably 6.0 N/m or more. The upper limit of the peeling force at the time of drying is not particularly limited, and is, for example, 40.0 N/m or less. When the peeling force at the time of drying is in the above-described preferred range, it is expected that the wound electrode body or the laminated electrode body can be conveyed without the electrode body being dispersed. Further, the peeling force during drying can be measured by the method described in the examples below.

The separator for a battery of the present embodiment can achieve both a high strength and a bending strength at the time of wetting and a peeling force during drying. Specifically, as shown in the examples below, the separator for a battery can satisfy a bending strength of 4.0 N or more at the time of wetness and a peeling force of 2.0 N/m or more at the time of drying.

Further, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit and scope of the invention.

 [Examples]  

Hereinafter, the present invention will be described in further detail by way of examples, but the embodiments of the invention are not limited thereto. Further, various methods and materials for the evaluation method and analysis used in the examples are as follows.

(1) Film thickness

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

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

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

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

‧Tube: shodex KF-806M made by Showa Denko Co., Ltd. 2

‧column temperature: 23 ° C

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

‧Solvent flow rate: 0.5ml/min

‧ Sample preparation: 4 mL of the measurement solvent was added to 2 mg of the sample, and the mixture was smoothly stirred at room temperature (visual confirmation of dissolution)

‧ Injection volume: 0.2mL

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

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

(3) melting point

Using a differential scanning calorimeter (DSC manufactured by PerkinElmer Co., Ltd.), 7 mg of a resin was placed in the measurement crucible as a sample for measurement, and the measurement was carried out under the following conditions. After the initial temperature rise and cooling, the peak of the endothermic peak at the time of the second temperature rise was taken as the melting point.

‧ Heating and cooling rate: ±10 °C/min.

‧ Measurement temperature range: 30~230 °C.

(4) Flexural strength when wet

In general, when a porous layer containing a fluororesin is provided on the separator as a binder of a fluororesin, it is easy to ensure adhesion by mutual diffusion of fluororesins. On the other hand, in the negative electrode, a binder other than the fluororesin is used, and diffusion of the fluorine-based resin is hard to occur. Therefore, it is difficult to obtain adhesion to the separator as compared with the positive electrode. Therefore, in the measurement, the wet bending strength described below was measured and evaluated 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 carboxymethylcellulose was added to 96.5 parts by mass of artificial graphite, and further mixed, and 2 parts by mass of styrene butadiene latex as a solid component was further added and mixed to prepare a slurry containing a negative electrode mixture. . This slurry containing a negative electrode mixture was uniformly coated on both surfaces of a negative electrode current collector including a copper foil having a thickness of 8 μm, and dried to form a negative electrode layer, and then compression-molded by a roll press to eliminate inclusion of a set. The negative electrode layer of the electric body had a density of 1.5 g/cm ³ to prepare a negative electrode.

(2) Production of test winding body

The negative electrode 20 (machine direction: 161 mm × width direction: 30 mm) prepared as described above was superposed on the produced separator 10 (machine direction: 160 mm × width direction: 34 mm), and a metal plate (length: 300 mm, width: 25 mm, thickness: 1 mm) was used as a core. The separator 10 and the negative electrode 20 were wound up so that the separator 10 was inside, and the metal plate was pulled out to obtain a test wound body 30. The test winding body had a length of about 34 mm and a width of about 28 mm.

(Method for measuring bending strength when wet)

Two laminated films (length: 70 mm, width: 65 mm, thickness: 0.07 mm) containing polypropylene were placed, and the test wound body 30 was placed in a three-layered bag-shaped laminated film 22 in which three sides were welded. LiPF _{6 was} dissolved in 500 μL of an electrolytic solution in which a solvent of ethylene carbonate and ethyl methyl carbonate was mixed at a volume ratio of 3:7, and was injected from the opening of the laminated film 22 in a glove box. This was impregnated into the test winding body 30, and one side of the opening was sealed with a vacuum sealing machine.

Then, the test wound body 30 in which the laminated film 22 was sealed was sandwiched between two sheets of a gasket (thickness: 1 mm, 5 cm × 5 cm) by a precision heating and pressing device (CYPT, manufactured by Shinto Industries Co., Ltd.) -10) Pressurization at 98 ° C and 0.6 MPa for 2 minutes, and cooling at room temperature. In the state in which the laminated film 22 was sealed, the bending strength of the wetness was measured by using a universal testing machine (AGS-J, manufactured by Shimadzu Corporation). Hereinafter, details will be described with reference to Fig. 2 .

Two aluminum L-shaped angle members 41 (thickness: 1 mm, 10 mm × 10 mm, length: 4 cm) are arranged in parallel with the 90° portion facing upward and the ends are aligned, and the 90° portion is used as a fulcrum and the distance between the fulcrums is 15 mm. Fix it. The test winding body 30 is disposed in such a manner that the midpoint of the side (about 28 mm) in the width direction of the test winding body is aligned with the 7.5 mm point in the middle of the distance between the fulcrums of the two aluminum L-shaped angle members 41. It does not exceed the side in the longitudinal direction of the L-shaped angle member 41.

Then, as the indenter, the side in the longitudinal direction of the test wound body (about 34 mm) does not exceed the side in the longitudinal direction of the aluminum L-shaped angle material 42 (thickness: 1 mm, 10 mm × 10 mm, length: 4 cm) and becomes parallel. The 90° portion of the aluminum L-shaped angle member 42 is aligned with the midpoint of the side in the width direction of the test winding body, and the aluminum L-shaped angle member 42 is fixed to the universal testing machine with the 90° portion facing downward. Load cell (load sensor capacity 50N). The average value of the maximum test force obtained by measuring the three test winding bodies at a load speed of 0.5 mm/min was taken as the bending strength at the time of wetting.

(5) Peeling force during drying

(production of negative electrode)

The same negative electrode 20 as in the case of the above-described bending strength at the time of wetting was used.

(production of peeling test piece)

The negative electrode 20 (70 mm × 15 mm) prepared as described above was superposed on the produced separator 10 (machine direction: 90 mm × width direction: 20 mm), and was sandwiched between two sheets of adhesive pads (thickness: 0.5 mm, 95 mm × 27 mm). The mixture was pressurized at 90 ° C and 8 MPa for 2 minutes by a precision heating and pressurizing apparatus (manufactured by Shinto Industries Co., Ltd., CYPT-10), and cooled at room temperature. On the negative electrode side of the laminate of the negative electrode 20 and the separator 10, a double-sided tape having a width of 1 cm was attached, and the other surface of the double-sided tape was attached to the SUS so that the mechanical direction of the separator was parallel to the longitudinal direction of the SUS plate. Plate (thickness 3 mm, length 150 mm x width 50 mm). This was taken as a peeling test piece.

(Method for measuring peeling force during drying)

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

(6) Short-circuit resistance test

The evaluation of the short-circuit resistance was carried out using a desktop precision universal testing machine AutoGraph AGS-X (manufactured by Shimadzu Corporation). First, as shown in Fig. 3(A), a polypropylene insulator 5 (thickness: 0.2 mm) and a lithium ion battery negative electrode 21 (total thickness: about 140 μm, substrate: copper foil (thickness: about 9 μm) were laminated and activated. Substance: Sample laminate 31 of artificial graphite (particle size of about 30 μm), double-sided coating), separator 10, and aluminum foil 4 (thickness: about 0.1 mm). Next, as shown in FIG. 3(B), the sample laminated body 31 is fixed to the compression jig (lower side) 44 of the universal testing machine with a double-sided tape. Next, the aluminum foil 4 and the negative electrode 21 of the sample laminate 31 described above were connected by a cable to a circuit including a capacitor and a clad resistor. 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 laminate 31. Next, a compression jig was attached to the universal testing machine, and as shown in Fig. 3(B), a sample laminate 31 containing the metal balls 6 was placed between the two compression jigs 43, 44, and compressed at a speed of 0.3 mm/min. When the load reaches 100N, the test is over. At this time, in the compression load change, the portion where the inflection point appears is used as the film breaking point of the spacer, and the moment at which the current is detected by the metal ball forming the above-described circuit is used as the short-circuit generating point. The compression displacement A(t) when the separator is broken by the compression and the inflection point of the compressive stress, and the compression displacement B(t) at the moment when the current flows into the circuit are measured by the following (Formula 1). When the value is 1.1 or more, it means that even if the separator breaks due to foreign matter mixed in the battery, the coating layer composition can be adhered to the surface of the foreign matter to maintain the insulation. Therefore, the short circuit resistance is evaluated as good. On the other hand, in the case where the value obtained by the formula 1 is more than 1.0 and less than 1.1, the film rupture and the short circuit of the separator do not occur at the same time, but the electrode is used even in the tension or charge and discharge accompanying the winding of the battery member. In the case where the internal pressure of the battery which is inflated is increased, short-circuiting does not occur, and a certain level of resistance is required. Therefore, the short-circuit resistance is evaluated as being slightly inferior. When the value obtained by the formula 1 is 1.0, the short circuit occurs simultaneously with the breakage of the separator, and the short circuit resistance due to the coating layer is not improved. Therefore, the short circuit resistance is evaluated as defective.

B(t)÷A(t)...(Formula 1)

(Example 1)

[Copolymer (A)]

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

[Copolymer (B)]

As the copolymer (B), the copolymer (B1) was synthesized in the following manner. The vinylidene fluoride and hexafluoropropylene were used as a starting material, and the copolymer (B1) was synthesized by a suspension polymerization method in such a manner that the molar ratio of vinylidene fluoride/hexafluoropropylene became 93.0/7.0. The obtained copolymer (B1) had a weight average molecular weight of 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 then the mixture was stirred with a disperser to make the porous The alumina particles (having an average particle diameter of 1.1 μm and a density of 4.0 g/cm ³ ) were added so as to have a solid content of the layer of 100% by volume and 51% by volume, and further stirred at 2000 rpm for 1 hour with a disperser. Then, DYNO-MILL (DYNO-MILL Multi Lab (1.46 L container, filling rate 80%, Φ0.5 mm alumina beads) manufactured by SHINMARU ENTERPRISES) was used, and the treatment was carried out under the conditions of a flow rate of 11 kg/hr and a peripheral speed of 10 m/s. The coating liquid (A) was prepared three times. The obtained coating liquid (A) was applied in an equal amount to both surfaces of a polyethylene microporous film having a thickness of 7 μm, a porosity of 40%, and an airtightness of 100 seconds/100 cm ³ by a dip coating method. The coated film was immersed in an aqueous solution (coagulating liquid) containing 10% by mass of N-methyl-2-pyrrolidone (NMP), washed with pure water, and then dried at 50 ° C to obtain a separator for a battery. The separator for a battery has a thickness of 10 μm.

(Example 2)

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

(Example 3)

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

(Example 4)

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

(Example 5)

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

(Example 6)

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

(Example 7)

In the preparation of the coating liquid, a coating liquid in which the blend ratio of the copolymer (A1) and the copolymer (B1) is 28.0 parts by mass of the copolymer (A1) and 2.0 parts by mass of the copolymer (B1) is used ( In the same manner as in Example 1, except for G), a separator for a battery was obtained.

(Example 8)

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

(Example 9)

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

(Embodiment 10)

As the copolymer (A), the copolymer (A5) was synthesized in the following manner. Using vinylidene fluoride, hexafluoropropylene and monomethyl maleate as starting materials, the molar ratio of vinylidene fluoride/hexafluoropropylene/monomethyl maleate was 98.4/1.5 by suspension polymerization. The copolymer (A5) was synthesized in a manner of /0.1. The obtained copolymer (A5) had a weight average molecular weight of 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) was used in the production of the coating liquid.

(Example 11)

As the copolymer (A), the copolymer (A6) was synthesized in the following manner. Using vinylidene fluoride, hexafluoropropylene and monomethyl maleate as starting materials, the molar ratio of vinylidene fluoride/hexafluoropropylene/monomethyl maleate was 94.5/1.5 by suspension polymerization. The copolymer (A6) was synthesized in a manner of /4.0. The obtained copolymer (A6) had a weight average molecular weight of 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) was used in the production of the coating liquid.

(Embodiment 12)

A separator for a battery was obtained in the same manner as in Example 1 except that a polyethylene microporous membrane having a thickness of 5 μm, a porosity of 35%, and an air impermeability of 150 seconds/100 cm ³ was used as the polyolefin microporous membrane. The separator for the battery has a thickness of 8 μm.

(Example 13)

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

(Example 14)

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

(Example 15)

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

(Embodiment 16)

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

(Example 17)

In the preparation of the coating liquid, a coating liquid in which the blend 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) is used ( In the same manner as in Example 1, except for O), a separator for a battery was obtained.

(Comparative Example 1)

In addition to the coating liquid (P) in which 88.3 parts by mass of the copolymer (A1), 11.7 parts by mass of the copolymer (B1), and 500 parts by mass of NMP were dissolved and mixed in the preparation of the coating liquid, and Examples 1 A separator for a battery was obtained in the same manner.

(Comparative Example 2)

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

(Comparative Example 3)

In the preparation of the coating liquid, a coating liquid in which the blend ratio of the copolymer (A1) and the copolymer (B1) is 15.0 parts by mass of the copolymer (A1) and 15.0 parts by mass of the copolymer (B1) is used ( In the same manner as in Example 1, except for R), a separator for a battery was obtained.

(Comparative Example 4)

As the copolymer (A), the copolymer (A7) was synthesized in the following manner. The vinylidene fluoride and hexafluoropropylene were used as a starting material, and the copolymer (A7) was synthesized by a suspension polymerization method in such a manner that the molar ratio of vinylidene fluoride/hexafluoropropylene became 98.5/1.5. The obtained copolymer (A7) had a weight average molecular weight of 1.5 million. A separator for a battery was obtained in the same manner as in Example 1 except that the coating liquid (S) in which the copolymer (A1) was replaced with the copolymer (A7) was used in the production of the coating liquid.

(Comparative Example 5)

In the preparation of the coating liquid, a coating liquid (T) prepared by replacing the copolymer (A1) with polyvinylidene fluoride (weight average molecular weight: 1.5 million) by 30.0 parts by mass and without using the copolymer (B) is used. A separator for a battery was obtained in the same manner as in Example 1.

(Comparative Example 6)

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

(Comparative Example 7)

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

(Comparative Example 8)

A separator for a battery was obtained in the same manner as in Comparative Example 1, except that a polyethylene microporous membrane having a thickness of 5 μm, a porosity of 35%, and a gas impermeability of 150 seconds/100 cm ³ was used as the polyolefin microporous membrane. The separator for the battery has a thickness of 8 μm.

The structure and weight average molecular weight of the copolymer (A) and the copolymer (B) used in the above examples and comparative examples, or the composition of the coating liquid, and the properties of the obtained separator for a battery are shown in Table 1.

 [Industrial availability]  

When the separator for a battery of the present embodiment is used for a nonaqueous electrolyte secondary battery, it can satisfy the peeling force during drying and the bending strength at the time of wetting, and the interlayer adhesion of the separator of the polyolefin multilayer microporous film and the porous layer, A separator for batteries which is excellent in adhesion between a separator and an electrode and which is excellent in short-circuit resistance. Therefore, the battery separator of the present embodiment can be applied even when the battery (particularly, the laminated battery) is required to be larger and higher in capacity.

Claims (7)

 A separator for a battery comprising a polyolefin microporous membrane and a separator for a battery laminated on at least one surface of the polyolefin microporous membrane, the porous layer comprising a vinylidene fluoride-hexafluoropropylene copolymer (A), a copolymer (B) of vinylidene fluoride-hexafluoropropylene, and inorganic particles, and the vinylidene fluoride-hexafluoropropylene copolymer (A) has a hexafluoro group of 0.3 mol% or more and 5.0 mol% or less. The propylene unit has a weight average molecular weight of 900,000 or more and 2,000,000 or less and contains a hydrophilic group, and the vinylidene fluoride-hexafluoropropylene copolymer (B) has more than 5.0 mol% and 8.0 mol% or less of hexafluoropropylene unit. The weight average molecular weight is 100,000 or more and 750,000 or less, and is 100% by mass based on 100% by mass of the total of the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B). 86 parts by mass or more and 98% by mass or less of the vinylidene fluoride-hexafluoropropylene copolymer (A) contains 40% by volume or more and 80% by volume or less based on 100% by volume of the solid content in the porous layer. Inorganic particles.    The separator for a battery according to claim 1, wherein the vinylidene fluoride-hexafluoropropylene copolymer (A) contains 0.1 mol% or more and 5.0 mol% or less of a hydrophilic group.    The separator for a battery according to claim 1 or 2, wherein the vinylidene fluoride-hexafluoropropylene copolymer (B) has a melting point of 60 ° C or more and 145 ° C or less.    The separator for a battery according to any one of claims 1 to 3, wherein the inorganic particles are one or more selected from the group consisting of titanium dioxide, aluminum oxide and diaspore.    The separator for a battery according to any one of claims 1 to 4, wherein the polyolefin microporous film has a thickness of 3 μm or more and 16 μm or less.    An electrode body comprising a positive electrode, a negative electrode, and a separator for a battery according to any one of claims 1 to 5.    A nonaqueous electrolyte secondary battery comprising the electrode body of claim 6 and a nonaqueous electrolyte.