KR102210007B1 - Battery separator, electrode body and non-aqueous electrolyte secondary battery - Google Patents

Abstract

An object of the present invention is to provide a battery separator having excellent adhesion and short-circuit resistance. The present invention comprises a polyolefin microporous membrane and a porous layer laminated on at least one side of the polyolefin microporous membrane, and the porous layer comprises a vinylidene fluoride-hexafluoropropylene copolymer (A), and a vinylidene fluoride-hexafluoro. The low-propylene copolymer (B) and the inorganic particles are included, and the vinylidene fluoride-hexafluoropropylene copolymer (A) has a hexafluoropropylene unit of 0.3 mol% or more and 5.0 mol% or less, and the weight average molecular weight is It is 900,000 or more and 2 million or less, and contains a hydrophilic group, and the vinylidene fluoride-hexafluoropropylene copolymer (B) has more than 5.0 mol% and less than 8.0 mol% of hexafluoropropylene units, and has a weight average molecular weight This is a battery separator of 100,000 or more and 750,000 or less.

Description

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

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

Non-aqueous electrolyte secondary batteries, especially lithium ion secondary batteries, are widely used in small electronic devices such as mobile phones and portable information terminals. Examples of the form of the nonaqueous electrolyte secondary battery include a cylindrical battery, a prismatic battery, and a laminate battery. In general, these batteries have a configuration in which an electrode body in which a positive electrode and a negative electrode are disposed through a separator, and a non-aqueous electrolyte solution are accommodated in an exterior body. 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 the positive electrode and the negative electrode are wound in a vortex shape through a separator.

Conventionally, a microporous membrane mainly made of a polyolefin resin is used as a battery separator. Since the microporous membrane made of a polyolefin resin has a so-called shutdown function, the pores of the separator are blocked during abnormal heat generation of the battery, thereby suppressing the flow of current and preventing ignition and the like.

In recent years, in battery separators, attempts have been made to improve battery characteristics by forming a layer other than polyolefin resin on one or both surfaces of a layer made of a polyolefin resin. For example, a battery separator in which a porous layer containing a fluororesin is formed on one or both surfaces of a layer made of a polyolefin resin has been proposed. In addition, by adding inorganic particles to the porous layer, it is known that even when a sharp metal penetrates into the battery due to an accident or the like and causes a sudden short circuit and generates heat, the melt shrinkage of the separator is prevented, and the expansion of the short circuit portion between the electrodes is suppressed.

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

In addition, in Example 1 of Patent Document 2, a VdF-HFP copolymer (0.6 mol% HFP unit) and a VdF-HFP copolymer (480,000 weight average molecular weight, 4.8 mol% HFP unit) were prepared in a dimethylacetamide and tripropylene glycol solution. Dissolved in, and coated on a polyethylene microporous membrane, a separator in which a porous layer was formed is described.

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

Further, in Example 1 of Patent Document 4, PVdF (weight average molecular weight 700,000) and VdF-HFP copolymer (weight average molecular weight 470,000, HFP unit 4.8 mol%) were dissolved in a solution of dimethylacetamide and tripropylene glycol, A separator in which a porous layer was formed by coating this on a polyethylene microporous membrane is described.

Further, in Example 1 of Patent Document 5, PVdF (weight average molecular weight 350,000) and VdF-HFP copolymer (weight average molecular weight 270,000, HFP copolymerization 4.8 mol%) were dissolved in a solution of dimethylacetamide and tripropylene glycol, A separator in which a porous layer was formed by coating this on a polyethylene microporous membrane is described.

In addition, in Example 23 of Patent Document 6, the VdF-HFP copolymer (weight average molecular weight 193 million, HFP units 1.1 mol%) and the VdF-HFP copolymer (weight average molecular weight 470,000, HFP units 4.8 mol%) were used in dimethylacetase. A coating liquid dissolved in an amide and tripropylene glycol solution and further added with aluminum hydroxide is prepared, and this is applied to a polyethylene microporous membrane to form a porous layer.

Japanese patent republished 1999-036981 Japanese Patent No.5282179 Japanese Patent No. 5282180 Japanese Patent No.5282181 Japanese Patent No. 5342088 International Publication No. 2016/152863

Recently, non-aqueous electrolyte secondary batteries are expected to be developed for large-sized applications such as large tablets, mowing machines, electric motorcycles, electric vehicles, hybrid vehicles, and small ships, and accordingly, the spread of large-sized batteries is expected, and higher capacity is also assumed. do. All of the above Patent Documents 1 to 5 have improved adhesion between a separator containing an electrolyte and an electrode, but when the secondary battery is enlarged, a new improvement in adhesion is required.

In the case of evaluating the adhesion between the electrode and the separator as described below, the inventors of the present invention further evaluated by distinguishing and evaluating the adhesion between the electrode and the separator during drying and the adhesion between the electrode and the separator when wet. Focusing on the ability to accurately evaluate the adhesiveness, it was found that the adhesiveness of these can be evaluated by using the peel strength upon drying and the bending strength upon wetness as indicators, respectively.

That is, for example, a wound electrode body is manufactured by winding a positive electrode and a negative electrode through a separator while applying tension to each member. At this time, the positive electrode or the negative electrode coated on the metal current collector hardly expands and contracts with respect to the tension, but the separator is wound while extending to some extent in the machine direction. If this winding body is left for a while, the separator part gradually shrinks and tries to return to its original length. As a result, a force in a parallel direction is generated at the interface between the electrode and the separator, and the wound electrode body (particularly, the electrode body wound flatly) is liable to be warped or deformed. In addition, these problems become present due to the widening or lengthening of the separator due to the increase in the size of the battery, and there is a concern about deterioration in yield during production. In order to suppress the occurrence of warpage or deformation of the wound electrode body, the separator is required to have more adhesion to the electrode than so far. In addition, when conveying the electrode body, the electrode and the separator are peeled off unless the respective members are sufficiently adhered to each other, and thus, it cannot be conveyed with good yield. The problem of adhesion at the time of conveyance becomes present due to an increase in the size of the battery, and there is a concern that the yield is deteriorated. Therefore, the separator is difficult to peel from the electrode, and a high peeling force during drying is required.

In addition, in a laminate type battery, it is difficult to apply pressure compared to a prismatic or cylindrical battery in which pressure is applied from an exterior body, and partial glass at the interface between the separator and the electrode is likely to occur due to swelling and contraction of the electrode following charging and discharging. As a result, it leads to expansion of the battery, an increase in resistance inside the battery, and a decrease in cycle performance. Therefore, the separator is required for adhesion to the electrode in the battery after the electrolyte is injected. In this specification, the wet-time bending strength obtained by the measurement method described later for this adhesiveness is evaluated as an index. When this strength is large, it is thought that improvement of battery characteristics, such as suppression of expansion of the battery after repeated charging and discharging, is expected. In addition, the wet bending strength as used herein refers to the adhesion between the separator and the electrode in a state in which the separator contains an electrolyte. Peeling force upon drying refers to adhesion to the interface between the separator and the electrode in a state in which the separator does not contain an electrolyte substantially. In addition, substantially not containing an electrolytic solution means that the electrolytic solution in the separator is 500 ppm or less.

However, the inventors have argued that in the prior art, there is a trade-off relationship between the adhesion between the electrode and the separator upon drying required for manufacturing or transporting the electrode body, and the adhesion between the electrode and the separator upon wet required after injection of the electrolyte. In the case where it is very difficult to satisfy the physical properties, and in the techniques disclosed in Patent Documents 1 to 5 described above, it has been found that there is a case where the adhesiveness is insufficient.

In addition, even when a sudden impact is applied to the battery, the convex portion of the electrode active material penetrates the separator and the electrode is difficult to short-circuit (hereinafter, referred to as short-circuit resistance) is required. However, since it is predicted that the film thickness of the battery separator becomes thinner in the future, it becomes difficult to secure short-circuit resistance as the thickness of the separator decreases. In order to ensure short-circuit resistance, it is known that it is effective to contain a certain amount or more of inorganic particles in the porous layer, but when inorganic particles are contained in a sufficient amount to ensure short-circuit resistance, the adhesion between the electrode and the separator tends to decrease. .

The present invention is in view of the above circumstances, and is excellent in both the adhesion between the electrode and the separator during drying, and the adhesion between the electrode and the separator when wet, and has excellent short-circuit resistance, and an electrode body and secondary battery using the same. It aims to provide.

As a result of intensive research in order to solve the above problems, the present inventors have solved the above problems by a separator having two types of fluorine-based resins having different structures, a mixing ratio thereof, and a porous layer containing a specific amount of inorganic particles. What could be done was found and the present invention was completed.

That is, the present invention is a battery separator comprising a polyolefin microporous membrane and a porous layer laminated on at least one surface of the polyolefin microporous membrane,

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 mol% or more and 5.0 mol% or less hexafluoropropylene units, has a weight average molecular weight of 900,000 or more and 2 million or less, and contains a hydrophilic group,

The vinylidene fluoride-hexafluoropropylene copolymer (B) has more than 5.0 mol% and less than 8.0 mol% of hexafluoropropylene units, and has a weight average molecular weight of 100,000 or more and 750,000 or less,

The vinylidene fluoride-hexafluoropropylene copolymer (A) based on 100% by mass of the total of the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B) It is a battery separator containing 86% by mass or more and 98% by mass or less, and 40% by volume or more and 80% by volume or less of the inorganic particles with respect to 100% by volume of solid content in the porous layer.

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

In addition, the vinylidene fluoride-hexafluoropropylene copolymer (B) preferably has a melting point of 60°C or more and 145°C or less.

In addition, the inorganic particles are preferably at least one selected from titanium dioxide, alumina and boehmite.

Further, it is preferable that the thickness of the polyolefin microporous membrane 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 battery separator 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.

(Effects of the Invention)

Advantageous Effects of Invention According to the present invention, there are provided 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 when wet, and has excellent short-circuit resistance, and an electrode body and a secondary battery using the same.

1 is a schematic diagram showing an example of a battery separator of this embodiment.
2 is a schematic diagram showing a method of evaluating bending strength when wet.
3 is a schematic diagram showing an evaluation method of a short-circuit resistance test.

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, the surface parallel to the surface (in-plane direction) of the microporous membrane or separator is set as the XY plane. In addition, the direction perpendicular to the XY plane (thickness direction) is the Z direction. Each of the X-direction, Y-direction and Z-direction will be described assuming that the direction of the arrow in the drawing is the + direction, and the direction opposite to the direction of the arrow is the-direction. In addition, in the drawings, in order to make each configuration easier to understand, a part is emphasized or a part is simplified, and the actual structure, shape, scale, etc. may differ.

1 is a diagram showing an example of a battery separator according to the present embodiment. As shown in Fig. 1, the battery separator 10 (hereinafter sometimes abbreviated as ``separator 10'') is laminated on at least one side of a polyolefin microporous membrane 1 and a polyolefin microporous membrane 1 It has a porous layer (2). Hereinafter, each layer constituting the battery separator will be described.

[1] polyolefin microporous membranes

The polyolefin microporous membrane 1 is a microporous membrane containing a polyolefin resin. The polyolefin microporous membrane 1 is not particularly limited, and a polyolefin microporous membrane used for a known battery separator can be used. In addition, in the present specification, the microporous membrane means a membrane having pores connected therein. 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]

As a polyolefin resin constituting the polyolefin microporous membrane (1) (hereinafter sometimes abbreviated as ``microporous membrane (1)''), ethylene, propylene, 1-butene, 4-methyl 1-pentene, 1-hexene, etc. are polymerized. One homopolymer, a two-stage polymer, a copolymer, or a mixture thereof. Among them, as the polyolefin resin, it is preferable to have a polyethylene resin 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 with respect to 100% by mass of the total mass of the polyolefin resin in the microporous membrane (1). If necessary, various additives such as antioxidants and inorganic fillers may be added to the polyolefin resin within a range that does not impair the effects of the present invention.

The film thickness of the polyolefin microporous membrane 1 is not particularly limited, but from the viewpoint of increasing the capacity of the battery, it is preferably 3 µm or more and 16 µm or less, more preferably 5 µm or more and 12 µm or less, and still more preferably 5 µm or more. It is 10 μm or less. When the film thickness of the polyolefin microporous film is in the above-described preferred range, practical film strength and pore-clogging function can be maintained, and it is more suitable for higher capacity of a battery which is expected to proceed in the future. That is, in the battery separator 10 of the present embodiment, even if the thickness of the polyolefin microporous membrane 1 is thin, the interlayer of the polyolefin microporous membrane 1 and the porous layer 2 of the separator 10, and the separator 10 and electrodes The adhesion to the liver can be excellent, and when the separator 10 is made thin, the effect is more clearly exhibited.

The air permeability 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 it is preferably 30% or more and 70% or less. The average pore diameter of the polyolefin microporous membrane 1 is not particularly limited, but from the viewpoint of the pore blocking performance, it is preferably 0.01 μm or more and 1.0 μm or less.

[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. As a method of manufacturing the microporous membrane 1, for example, a 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 for manufacturing the microporous membrane 1 will be described. In addition, the manufacturing method of the microporous membrane 1 is not limited to the following method.

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

(1) Process of preparing a polyolefin solution by melt-kneading the polyolefin resin and a film-forming solvent

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

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

(4) Step of removing the film-forming solvent from the gel-like sheet after stretching

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

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

(7) Process of heat-treating the sheet after drying

(8) Step of crosslinking and/or hydrophilizing the sheet after the stretching step

Hereinafter, each process is demonstrated respectively.

(1) Preparation process of polyolefin solution

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

The blending ratio of the polyolefin resin and the film-forming solvent in the polyolefin solution is not particularly limited, but it is preferably 70 to 80 parts by mass of the film-forming solvent with respect to 20 to 30 parts by mass of the polyolefin resin. When the ratio of the polyolefin resin is within the above range, when extruding the polyolefin solution, swelling and neck-in at the die exit can be prevented, and the moldability and self-support of the extruded molded body (gel-shaped molded body) become good.

(2) Formation process of gel-like sheet

The polyolefin solution is fed from an extruder to a die and extruded into a sheet shape. A plurality of polyolefin solutions of the same or different composition may be fed from an extruder to one die, laminated therein in a layer shape, and extruded in a sheet shape.

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

The obtained extruded body is cooled to form a gel-like sheet. As a method for forming a gel-like sheet, for example, the method disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. It is preferable to perform cooling at a rate of 50°C/min or more to at least the gelation temperature. It is preferable to perform cooling to 25 degreeC or less. The microphase of the polyolefin separated by the solvent for film formation by cooling can be fixed. When the cooling rate is within the above range, the degree of crystallinity is maintained in an appropriate range to obtain a gel-like sheet suitable for stretching. As the cooling method, a method of contacting a coolant such as cold air or cooling water, a method of contacting a cooling roll, or the like can be used, but it is preferable to cool by contacting a roll cooled with a coolant.

(3) 1st drawing process

Next, the obtained gel-like sheet is stretched in at least one axial direction. Since the gel-like sheet contains a film-forming solvent, it can be stretched uniformly. After heating, the gel-like sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof. The stretching may be uniaxial or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching, and multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used.

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

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

Cleavage occurs between the polyethylene lamellas by the above stretching, and the polyethylene phase is refined to form a number of fibrils. The fibrils form a three-dimensional, irregularly connected mesh structure. The mechanical strength is improved and the pores are enlarged by stretching. However, when stretching is performed under appropriate conditions, it is possible to control the through-hole diameter and to have a high porosity even with a thin film thickness.

It may be stretched by forming a temperature distribution in the film thickness direction according to the desired physical properties, whereby a microporous film excellent in mechanical strength is obtained. Details of the method are described in Japanese Patent No. 3347854.

(4) Removal of solvent for film formation

The film-forming solvent is removed (washed) using a cleaning solvent. Since the polyolefin phase is phase-separated from the film-forming solvent phase, when the film-forming solvent is removed, a porous film consisting of fibrils forming a fine three-dimensional mesh structure and having holes (voids) that are irregularly communicated in three dimensions is obtained. Lose. The cleaning solvent and a method of removing the solvent for film formation using the cleaning solvent are well known and thus description thereof will be omitted. For example, the method disclosed in Japanese Patent Application Laid-Open No. 2132327 or Japanese Patent Application Laid-Open No. 2002-256099 can be used.

(5) drying

The microporous membrane from which the film-forming solvent has been removed is dried by a heat drying method or an air drying method. The drying temperature is preferably not more than the crystal dispersion temperature (Tcd) of the polyolefin resin, and particularly preferably 5°C or more lower than Tcd. Drying is preferably performed until the residual washing solvent is 5% by mass or less, and more preferably 3% by mass or less, using the microporous membrane as 100% by mass (dry mass). When the residual washing solvent is within the above range, the porosity of the microporous membrane is maintained and deterioration of the permeability is suppressed when the stretching process and the heat treatment process of the microporous membrane at the later stage are performed.

(6) 2nd drawing process

It is preferable to stretch the microporous membrane after drying in at least one axial direction. Stretching of the microporous membrane can be performed by a tenter method or the like as described above while heating. The stretching may be uniaxial or biaxial stretching. In the case of biaxial stretching, any of simultaneous biaxial stretching and sequential stretching may be used. The stretching temperature in this step is not particularly limited, but usually 90 to 135°C is preferable, and more preferably 95 to 130°C. In the case of uniaxial stretching, the stretching ratio (area stretching ratio) of the microporous membrane in the present step in the uniaxial direction is preferably 1.0 to 2.0 times in the machine direction or the width direction. In the case of biaxial stretching, the area stretching ratio is preferably 1.0 times the lower limit, more preferably 1.1 times, and still more preferably 1.2 times. 3.5 times the upper limit is suitable. The machine direction and the width direction are respectively 1.0 to 2.0 times, and the draw ratios in the machine direction and the width direction may be the same or different. In addition, the draw ratio in this process refers to the draw ratio of the microporous film just before being provided to the next process based on the microporous film immediately before this process.

(7) heat treatment

In addition, the microporous membrane after drying can be subjected to heat treatment. The crystal is stabilized by heat treatment, and the lamella is homogenized. As the heat treatment method, a heat setting treatment and/or a heat relaxation treatment can be used. The heat setting treatment is a heat treatment that heats while keeping the film dimensions unchanged. The heat relaxation treatment is a heat treatment in which the film is thermally contracted in the machine direction or the width direction during heating. It is preferable to perform heat setting treatment by a tenter method or a roll method. For example, the method disclosed in Japanese Patent Laid-Open No. 2002-256099 is mentioned as a heat relaxation treatment method. The heat treatment temperature is preferably within the range of Tcd to Tm of the polyolefin resin, more preferably within the range of the stretching temperature ±5°C of the microporous membrane, and particularly preferably within the range of the second stretching temperature ±3°C of the microporous membrane.

(8) crosslinking treatment, hydrophilic treatment

Further, the microporous membrane after bonding or stretching may be further subjected to a crosslinking treatment and a hydrophilic treatment. For example, a crosslinking treatment is performed by irradiating the microporous membrane with ionizing radiation such as α-rays, β-rays, γ-rays, and electron beams. In the case of irradiation of an electron beam, an electron dose of 0.1 to 100 Mrad is preferable, and an acceleration voltage of 100 to 300 kV is preferable. The meltdown temperature of the microporous membrane increases by crosslinking treatment. In addition, the hydrophilization treatment can be performed by monomer graft, surfactant treatment, corona discharge, or the like. It is preferable to perform monomer graft after crosslinking treatment.

[2] porous layers

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

[Vylidene fluoride-hexafluoropropylene copolymer (A)]

The vinylidene fluoride-hexafluoropropylene copolymer (A) (hereinafter, it may be abbreviated only as the copolymer (A)) is a copolymer containing vinylidene fluoride units and hexafluoropropylene units, as described below. Together, it contains a hydrophilic group. 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 less than the above range, the polymer crystallinity increases and the swelling degree of the separator to the electrolyte decreases, so the adhesion between the separator and the electrode decreases, and the adhesion between the electrode and the separator after injection of the electrolyte (wet Bending strength) may not be sufficiently obtained. 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 swell excessively with respect to the electrolytic solution, resulting in a decrease in bending strength when wet.

The weight average molecular weight of the copolymer (A) is 900,000 or more, preferably 1 million or more. On the other hand, the weight average molecular weight of the copolymer (A) is 2 million or less, more preferably 1.5 million or less. When the weight average molecular weight of the copolymer (A) is within the above range, the time for dissolving the copolymer (A) in the solvent in the process of forming the porous layer is not extremely long, and the production efficiency can be increased or swelled in the electrolyte solution. When made, suitable gel strength can be maintained, and bending strength can be improved when wet. In addition, the weight average molecular weight of the copolymer (A) is a polystyrene conversion value by gel permeation chromatography.

Copolymer (A) has a hydrophilic group. Since the copolymer (A) has a hydrophilic group, it becomes possible to more firmly adhere to the active material present on the electrode surface or the binder component in the electrode. Although the reason for this is not clear, it is presumed that it is because the adhesive strength is improved 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 these, a carboxylic acid group and a carboxylic acid ester are particularly preferred.

As a method of introducing a hydrophilic group into the copolymer (A), a known method can be used. For example, maleic anhydride, maleic acid, maleic acid ester, maleic acid monomethyl ester, etc. can be used in the synthesis of the copolymer (A). A method of introducing a monomer having a hydrophilic group into the main chain by copolymerization, a method of introducing it as a side chain by grafting, or the like can be used. Hydrophilic group denaturation rate can be measured by FT-IR, NMR, quantitative titration, or the like. For example, in the case of a carboxylic acid group, FT-IR can be used to calculate the absorption strength ratio of C-H stretching vibration and C=O stretching vibration of a carboxyl group based on a homopolymer.

The content of the hydrophilic group in the copolymer (A) is preferably 0.1 mol% or more, and 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, the polymer crystallinity is too low and the degree of swelling to the electrolyte is increased, so that the deterioration of the bending strength during wet can be suppressed. In addition, when the content of the hydrophilic group is within the above range, the affinity between the inorganic particles contained in the porous layer 2 and the copolymer (A) increases, improves short-circuit resistance, and also exhibits the effect of suppressing the dropping of the inorganic particles. do. Although this reason is not clear, it is presumed that the film strength of the porous layer 2 increases by the copolymer (A) having a hydrophilic group, which is the main component of the porous layer 2, and inorganic particles. 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 obtained by further polymerizing other monomers other than vinylidene fluoride, hexafluoropropylene, and a monomer having a hydrophilic group within a range that does not impair the properties. As another monomer, for example, a monomer such as tetrafluoroethylene, trifluoroethylene, trichloroethylene, and vinyl fluoride can be mentioned.

When the structure and molecular weight of the copolymer (A) are within the above range, the separator 10 has high affinity for a non-aqueous electrolyte solution, high chemical and physical stability, and improves bending strength when wetted when used for a non-aqueous electrolyte secondary battery. It is expressed, and the affinity with the electrolyte is sufficiently maintained even when used under high temperature.

[Vylidene fluoride-hexafluoropropylene copolymer (B)]

The vinylidene fluoride-hexafluoropropylene copolymer (B) (hereinafter, simply abbreviated as copolymer (B)) is a copolymer containing vinylidene fluoride units and hexafluoropropylene units. The content of hexafluoropropylene in the copolymer (B) is more than 5.0 mol%, more preferably 6.0 mol% or more, and still more preferably 7.0 mol% or more. When the content of the hexafluoropropylene unit is 5.0 mol% or less, the adhesion between the separator and the electrode during drying (peeling force during drying) may not be sufficiently obtained. On the other hand, the content on the upper limit side is 8.0 mol% or less, more preferably 7.5 mol% or less. In addition, when the content of the hexafluoropropylene unit exceeds 8.0 mol%, it may swell excessively with respect to the electrolytic solution, resulting in a decrease in the bending strength when wet. Further, the copolymer (B) may contain a hydrophilic group, but may not contain it.

Copolymer (B) has a weight average molecular weight of 100,000 or more and 750,000 or less. When the weight average molecular weight of the copolymer (B) is within the above range, the affinity to the non-aqueous electrolyte solution is high, the chemical and physical stability is high, and excellent adhesion between the separator and the electrode during drying (peeling force during drying) is obtained. Lose. Although the reason for this is not clear, the copolymer (B) exhibits fluidity under heating and pressurizing conditions that exhibit peeling force upon drying, and becomes an anchor by entering the porous layer of the electrode, whereby the porous layer 2 and the electrode It can be estimated that it is because it has strong adhesiveness between. That is, in the battery separator 10, the copolymer (B) contributes to the peeling force upon drying, and may contribute to the prevention of warpage and deformation of the wound electrode body or the laminated electrode body, and improvement of transportability. In addition, the copolymer (B) is a resin different from the copolymer (A).

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, since the amount of entanglement of the molecular chains is too small, the resin strength is weakened, and cohesive 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, and more preferably 700,000 or less. When the weight average molecular weight of the copolymer (B) exceeds the upper limit of the above range, it is necessary to increase the press temperature in the manufacturing process of the wound body in order to obtain peeling force upon drying. If so, the microporous membrane containing the polyolefin as a main component may shrink. 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 chains increases, and there is a fear that it may not be able to sufficiently flow under press conditions.

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

The copolymer (B) is a copolymer having a vinylidene fluoride unit and a hexafluoropropylene unit. The copolymer (B) can be obtained by a suspension polymerization method or the like similarly to the copolymer (A). The melting point of the copolymer (B) can be adjusted by controlling the crystallinity of the portion consisting of vinylidene fluoride units. For example, when a monomer other than a vinylidene fluoride unit is contained in the copolymer (B), the melting point can be adjusted by controlling the ratio of the vinylidene fluoride unit. Monomers other than the vinylidene fluoride unit include tetrafluoroethylene, trifluoroethylene, trichloroethylene, hexafluoropropylene, vinyl fluoride maleic anhydride, maleic acid, maleic acid ester, monomethyl maleate, etc. or You may have two or more. When the copolymer (B) is polymerized, a method of adding the above monomer and introducing it into the main chain by copolymerization, or a method of introducing it as a side chain by grafting may be mentioned. Further, the melting point may be adjusted by controlling the ratio of the head-to-head bonds (-CH ₂ -CF ₂ -CF ₂ -CH ₂ -) of the vinylidene fluoride unit.

[Content 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 with respect to 100% by mass of the total weight of the copolymer (A) and the copolymer (B). The upper limit of the content of the copolymer (A) is 98% by mass or less, and more preferably 97% by mass or less. In addition, the content of the copolymer (B) is 14 mass% or less, preferably 12 mass% or less with respect to 100 mass% of the total weight of the copolymer (A) and the copolymer (B). In addition, 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 ranges, the porous layer 2 can have excellent bending strength when wet and peeling force when drying at a high level.

In addition, the porous layer 2 may contain resins other than the copolymer (A) and the copolymer (B) within a range that does not impair the effects of the present invention, but the resin component constituting the porous layer 2 is It is preferably composed of the polymer (A) and the copolymer (B). In addition, 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 based on 100% by mass of the resin component of the porous layer (2). Make it a ratio.

[Weapon Particles]

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

Examples of inorganic particles include calcium carbonate, calcium phosphate, amorphous silica, crystalline glass particles, kaolin, talc, titanium dioxide, alumina, silica-alumina composite oxide particles, barium sulfate, calcium fluoride, lithium fluoride, zeolite, molybdenum sulfide, mica, Boehmite, magnesium oxide, and the like. In particular, from the viewpoint of affinity with the vinylidene fluoride-hexafluoropropylene copolymer (A), inorganic particles containing a large amount of OH groups are preferred, and specifically, at least one selected from titanium dioxide, alumina, and boehmite is used. It is desirable.

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 to be. On the other hand, the content of the inorganic particles is 40% by volume or more, more preferably 45% by volume or more, still more preferably 50% by volume or more, and still more preferably 51% by volume or more. In addition, the content of the inorganic particles contained in the porous layer 2 was calculated by calculating the density of the copolymer (A) and the copolymer (B) as 1.77 g/cm 3.

In general, when inorganic particles having no adhesiveness are included in the porous layer, the bending strength when wet and the peeling force during drying tend to decrease. However, the porous layer 2 according to the present embodiment contains a specific fluororesin in a specific ratio as described above, so that when the inorganic particles are contained in the above range, it has high adhesion to the electrode, bending strength when wet, and drying. At the time, the balance of the peel force becomes good, and excellent short-circuit resistance can be obtained.

From the viewpoint of particle dropout, the average particle diameter of the inorganic particles is preferably 1.5 times or more and 50 times or less of the average flow pore diameter of the polyolefin microporous membrane, and more preferably 2.0 times or more and 20 times or less. The average flow pore diameter is measured according to JISK3832, and can be obtained by measuring in the order of Dry-up and Wet-up using a Perm-Porometer (for example, PMI, CFP-1500A). Specifically, in dry-up measurement, the hole diameter is converted from the pressure at the point where the pressure and flow rate curves intersect with the curve representing the slope of 1/2 of the pressure and the flow rate curve. Conversion of the pressure and the hole diameter uses 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 an integer.

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, further preferably 0.9 µm to 1.3 µm, from the viewpoint of slipping property with the winding core during cell winding and particle dropping. Μm. The average particle diameter of the particles can be measured using a laser diffraction method or a dynamic light scattering method. For example, particles dispersed in an aqueous solution containing a surfactant using an ultrasonic probe are measured with a particle size distribution measuring device (manufactured by Nikkoo Co., Ltd., Microtrac HRA), and accumulated by 50% from the small particle side in terms of volume. It is preferable to make the value of the particle diameter (D50) at the time of becoming the average particle diameter. The shape of the particles may include a spherical shape, a substantially spherical shape, a plate shape, and a needle shape, but is not particularly limited.

[Physical properties of the porous layer]

The film thickness of the porous layer 2 is preferably 0.5 µm or more and 3 µm or less per side, more preferably 1 µm or more and 2.5 µm or less, and still more preferably 1 µm or more and 2 µm or less. When the film thickness per side is 0.5 μm or more, high adhesion to the electrode (bending strength when wet, peeling force when dry) can be secured. On the other hand, if the film thickness per side is 3 µm or less, the volume to be wound can be suppressed, the film can be made thinner, and it is more suitable for increasing the capacity of the battery which will proceed in the future.

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

[3] Method for producing a battery separator

The method of manufacturing the battery separator is not particularly limited, and it can be manufactured using a known method. Hereinafter, an example of a method for manufacturing a battery separator will be described. The manufacturing method of the battery separator may sequentially include the following steps (1) to (3).

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

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

(3) The process of applying a coating solution to a polyolefin microporous membrane, immersing it in a coagulating solution, washing, and drying

(1) Step of obtaining a fluororesin solution

Vinylidene fluoride-hexafluoropropylene copolymer (A) and vinylidene fluoride-hexafluoropropylene copolymer (B) are slowly added to a solvent to completely dissolve.

The solvent is not particularly limited as long as it is capable of dissolving the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B) and miscible with the coagulation solution. From the viewpoint of solubility and low volatility, the solvent is preferably N-methyl-2-pyrrolidone.

(2) Process of obtaining coating liquid

In order to obtain a coating liquid, it is important to sufficiently disperse the inorganic particles. Specifically, a step of adding particles while stirring the fluororesin solution, pre-dispersing by stirring with a disper or the like for a certain period of time (for example, about 1 hour), and then dispersing the particles using a bead mill or a paint shaker ( A dispersion step) to reduce agglomeration of particles, and to prepare a coating solution by mixing with a three-one motor equipped with a stirring blade.

(3) The process of applying the coating solution to the microporous membrane, immersing it in the coagulation solution, washing and drying

The coating solution was applied to the microporous membrane, and the coated microporous membrane was immersed in the coagulation solution to phase-separate the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B). , It is solidified in a state of having a three-dimensional mesh structure, washed, and dried. Thereby, a microporous membrane and a battery separator including a porous layer on the surface of the microporous membrane are obtained. The method of applying the coating liquid to the microporous membrane may be a known method, for example, dip coating method, reverse roll coating method, gravure coating method, kiss coating method, roll brush method, spray coating method, air knife A coating method, a Mayer bar coating method, a pipe doctor method, a blade coating method, and a die coating method, and the like, and these methods can be used alone or in combination.

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

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

[4] properties of battery separators

Battery separator

The battery separator 10 of this embodiment can be suitably used for either a battery using an aqueous electrolyte or a battery using a nonaqueous electrolyte, but can be more suitably used for a nonaqueous 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 it as a separator for a lithium ion secondary battery.

In a nonaqueous electrolyte secondary battery, a positive electrode and a negative electrode are 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 can be used, for example, an electrode structure in which a disk-shaped positive electrode and a negative electrode are arranged to face each other (coin type), and a plate-shaped positive electrode and negative electrode are alternately It may have a stacked electrode structure (stacked type), an electrode structure in which a stacked strip-shaped positive electrode and negative electrode are wound (wound type), and the like. The battery separator of this embodiment can have excellent adhesion between the separator and the electrode in any battery structure.

A current collector, a positive electrode, a positive electrode active material, a negative electrode, a negative electrode active material, and an electrolyte solution used in non-aqueous electrolyte secondary batteries including lithium ion secondary batteries, etc. are not particularly limited, and conventionally known materials may be suitably used in combination.

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

The wet-time bending strength of the battery separator is preferably 4.0 N or more, more preferably 5.0 N or more, and still more preferably 6.0 N or more. The upper limit of the bending strength during wet is not particularly determined, but is, for example, 15.0 N or less. When the bending strength when wet is within the above-described preferred range, partial glass at the interface between the separator and the electrode can be further suppressed, and an increase in battery internal resistance and a decrease in battery characteristics can be suppressed. In addition, the bending strength when wet can be measured by the method described in Examples to be described later.

The peeling force upon drying of the battery separator 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 upon drying is not particularly determined, but is, for example, 40.0 N/m or less. When the peeling force upon drying is within the preferred range, it is expected that the wound electrode body or the laminated electrode body can be conveyed without disturbing the electrode body. In addition, the peeling force upon drying can be measured by the method described in Examples to be described later.

The battery separator of this embodiment can achieve both the bending strength when wet and the peeling force when drying at a high level. Specifically, as shown in Examples to be described later, the battery separator may have a bending strength of 4.0N or more when wet and a peeling force of 2.0N/m or more when dry.

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

Example

Hereinafter, the present invention will be described in more detail by examples, but embodiments of the present invention are not limited to these examples. In addition, the evaluation method used in the Example, each method of analysis, and the material 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 ("LITEMATIC" (registered trademark) series 318 manufactured by Mitutoyo Corporation). For the measurement, 20 points were measured under the condition of a weight of 0.01 N using a superhard spherical measuring instrument φ 9.5 mm, and the average value of the obtained measured values was taken as the film thickness.

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

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

Measurement device: GPC-150C manufactured by Waters Corporation

·Column: SHOWA DENKO K.K. Produced shodex KF-806M 2ea

·Column temperature: 23℃

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

Solvent flow rate: 0.5ml/min

Sample preparation: 4 mL of the measurement solvent was added to 2 mg of the material, and gently stirred at room temperature (dissolution was visually recognized).

・Injection volume: 0.2mL

Detector: differential refractive index detector RI (RI-8020 type sensitivity 16 manufactured by TOSOH CORPORATION)

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

(3) melting point

7 mg of resin was put into a measurement pan with a differential scanning calorimetry apparatus (DSC manufactured by PerkinElmer Co., Ltd.), and it was measured under the following conditions as a sample for measurement. After heating and cooling for the first time, the peak top of the endothermic peak at the second temperature increase was taken as the melting point.

·Temperature increase and cooling rate: ±10℃/min.

·Measurement temperature range: 30~230℃.

(4) Bending strength when wet

In general, a fluororesin binder is used for the positive electrode, and when a porous layer containing a fluororesin is provided on the separator, adhesion is easily secured by mutual diffusion between the fluororesins. On the other hand, a binder other than a fluorine resin is used for the negative electrode, and since diffusion of the fluorine-based resin is difficult to occur, the adhesion of the negative electrode to the separator is difficult to be obtained compared to the positive electrode. So, in this measurement, it evaluated as an index of the adhesiveness between a separator and a negative electrode by measuring the bending strength at the time of wetting demonstrated below.

(Production of negative play)

An aqueous solution containing 1.5 parts by mass of carboxymethyl cellulose was added and mixed in 96.5 parts by mass of artificial graphite, and 2 parts by mass of styrene butadiene latex were added and mixed as a solid content to obtain a slurry containing a negative electrode mixture. This negative electrode mixture-containing slurry was uniformly applied to both surfaces of a negative electrode current collector made of copper foil having a thickness of 8 μm, dried to form a negative electrode layer, and then compression-molded by a roll press to form the density of the negative electrode layer excluding the current collector. Was made into 1.5 g/cm 3, and a negative electrode was produced.

(Production of test winding body)

A metal plate (length 300 mm, width 25 mm, thickness 1 mm) is stacked by stacking the negative electrode 20 (mechanical direction 161 mm × width direction 30 mm) and the produced separator 10 (machine direction 160 mm × width direction 34 mm). Then, the separator 10 and the negative electrode 20 were wound up so that the separator 10 was inside, and the metal plate was removed to obtain a winding body 30 for testing. The test winding body was about 34 mm long and about 28 mm wide.

(Measurement method of bending strength when wet)

Two laminate films made of polypropylene (length 70 mm, width 65 mm, thickness 0.07 mm) were stacked, and three of the four sides were welded together, and the test winding body 30 was placed in a bag-shaped laminate film 22. 500 μL of an electrolyte solution obtained by dissolving LiPF ₆ at a ratio of 1 mol/L in a solvent in which ethylene carbonate and ethyl methyl carbonate were mixed in a volume ratio of 3: 7 was injected in a glove box from the opening of the laminate film 22, and the test winding body 30 Was impregnated in the vacuum sealer to seal one side of the opening.

Next, the test winding body 30 enclosed in the laminate film 22 was sandwiched by two gaskets (thickness 1 mm, 5 cm x 5 cm), and a precision heating and pressing device (manufactured by SINTOKOGIO, LTD., CYPT-10) was used. It pressurized at 98 degreeC and 0.6 MPa for 2 minutes, and it stood to cool at room temperature. While encapsulating in the laminate film 22, the bending strength at the time of wetness was measured using a universal testing machine (manufactured by Shimadzu Corporation, AGS-J) with respect to the test wound body 30 after pressing. Hereinafter, the details will be described with reference to FIG. 2.

Two aluminum L-shaped angles (41) (thickness 1mm, 10mm×10mm, length 5cm) are arranged in parallel so that the 90° part is on the top, and the ends are aligned, and the distance between the points is set at 90° as a point. It was fixed to be 15 mm. By aligning the midpoint of the side (about 28mm) in the width direction of the test winding body to the middle of the distance between the points of the two aluminum L-shaped angles 41, pushed from the longitudinal side of the L-shaped angle 41 The test winding body 30 was arrange|positioned so that it might not come out.

Subsequently, as an indenter, the aluminum L-shaped angle 42 (thickness 1 mm, 10 mm × 10 mm, length 4 cm) in the longitudinal direction of the test winding body (approximately 34 mm) is not pushed out from the side in the length direction. Align the 90° part of the aluminum L-shaped angle 42 to the midpoint of the side in the width direction of the winding body, and insert the aluminum L-shaped angle 42 to the load cell (load cell capacity 50N) of the universal testing machine so that the 90° part is down. Fixed. 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 wet.

(5) Peeling force when drying

(Production of negative play)

The same negative electrode 20 as in the case of the bending strength at the time of wet was used.

(Preparation of peeling test piece)

The negative electrode 20 (70mm×15mm) prepared above and the prepared separator 10 (90mm in machine direction×20mm in width direction) were superimposed, and then inserted with two gaskets (thickness 0.5mm, 95mm×27mm). It pressurized for 2 minutes at 90 degreeC and 8 MPa with a precision heating pressurization apparatus (SINTOKOGIO, LTD. make, CYPT-10), and it stood to cool at room temperature. A double-sided tape having a width of 1 cm is applied to the negative electrode side of the laminate of the negative electrode 20 and the separator 10, and the other side of the double-sided tape is placed on a SUS plate (thickness 3 mm, length 150 mm × width 50 mm). The direction and the longitudinal direction of the SUS plate were attached so that they were parallel. This was made into 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 inserted into the chuck on the load cell side, and a 180 degree peel test was performed at a test speed of 300 mm/min. The value obtained by averaging the measured values from the stroke 20 mm to 70 mm during the peel test was taken 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 in terms of width was taken as peeling force upon drying (N/m).

(6) Short circuit immunity test

The evaluation of the short-circuit resistance was performed using a table-top precision universal testing machine autograph AGS-X (manufactured by Shimadzu Corporation). First, as shown in Fig. 3(A), an insulator 5 made of polypropylene (thickness 0.2 mm), a negative electrode 21 for lithium ion batteries (total thickness: about 140 μm, base material: copper foil (about 9 μm thickness), Active material: artificial graphite (particle diameter of about 30 µm), double-sided coating), separator 10, and aluminum foil 4 (thickness of about 0.1 mm) were laminated to prepare a sample laminate 31. Next, as shown in Fig. 3(B), the sample laminate 31 was fixed to the compression jig (lower side) 44 of the universal testing machine with double-sided tape. Next, the aluminum foil 4 and the negative electrode 21 of the sample laminate 31 were connected to a circuit composed of a capacitor and a clad resistor with a cable. The capacitor was charged to about 1.5 V, and a metal ball 6 (material: chromium (SUJ-2)) having a diameter of about 500 µm was disposed between the separator in the sample laminate 31 and the aluminum foil 4. Subsequently, a compression jig is attached to the universal testing machine, and as shown in Fig. 3(B), the sample laminate 31 including the metal ball 6 is placed between the compression jigs 43 and 44, and the speed is 0.3 It was compressed at mm/min., and the test was finished when the load reached 100 N. At this time, the portion where the inflection point appeared in the change in the compressive load was used as the breakup point of the separator, and the moment when the circuit was formed through the metal sphere and the current was detected was used as the point of occurrence of a short. A value obtained by measuring the compressive displacement A(t) when the separator breaks due to compression and an inflection point in compressive stress, and the compressive displacement B(t) at the moment of current flowing through the circuit, and obtained by the following (Equation 1) When is 1.1 or more, it was evaluated that the short-circuit resistance was good because it means that insulation was maintained by adhering the coating layer composition to the surface of the foreign material even if the separator was broken by foreign matter mixed in the battery. On the other hand, if the value obtained by Equation 1 is greater than 1.0 and less than 1.1, the separator does not break and short circuit occurs at the same time, but also in the increase of the internal pressure of the battery due to the tension applied to the winding of the battery member or the expansion of the electrode during charging and discharging. In order not to cause a short circuit, it was evaluated that the short-circuit resistance was slightly poor because a certain or higher resistance was required. When the numerical value obtained by Equation 1 was 1.0, short-circuiting occurred at the same time as the separation of the separator, and improved short-circuit resistance by the coating layer was not observed, so that the short-circuit resistance was evaluated as poor.

B(t)÷A(t) ···(Equation 1)

(Example 1)

[Copolymer (A)]

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

[0104]

[Copolymer (B)]

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

[Manufacture 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 alumina particles (average particle diameter) while stirring with a disper. 1.1 µm, density 4.0 g/cm 3) was added so that the solid content of the porous layer was 100% by volume to be 51% by volume, and further stirred with a disper for 1 hour at 2000 rpm. Then, the coating solution was treated three times under conditions of a flow rate of 11 kg/hr and a circumferential speed of 10 m/s using a Dino Mill (Dino Mill Multi Lab (1.46 L container, filling rate 80%, φ0.5 mm alumina beads) manufactured by Shinmaru Enterprises Corporation). (A) was produced. The obtained coating liquid (A) was applied in an equal amount by a dip coating method on both sides of a polyethylene microporous membrane having a thickness of 7 µm, a porosity of 40%, and an air permeability resistance of 100 seconds/100 cm 3. The film after application was immersed in an aqueous solution (coagulant) containing 10% by mass of N-methyl-2-pyrrolidone (NMP), washed with pure water, and dried at 50°C to obtain a battery separator. The thickness of the battery separator was 10 µm.

(Example 2)

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

(Example 3)

As the copolymer (B), the copolymer (B3) was synthesized as follows. Using vinylidene fluoride and hexafluoropropylene as starting materials, a copolymer (B3) was synthesized by suspension polymerization so that the molar ratio of vinylidene fluoride/hexafluoropropylene was 92.0/8.0. The weight average molecular weight of the obtained copolymer (B3) was 350,000. In preparation of the coating liquid, 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.

(Example 4)

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

(Example 5)

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

(Example 6)

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

(Example 7)

In the preparation of the coating solution, the examples except that the coating solution (G) in which the mixing ratio of the copolymer (A1) and the copolymer (B1) was 28.0 parts by mass of the copolymer (A1) and 2.0 parts by mass of the copolymer (B1) was used. In the same manner as in 1, a battery separator was obtained.

(Example 8)

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

(Example 9)

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

(Example 10)

As the copolymer (A), a copolymer (A5) was synthesized as follows. A copolymer (a copolymer using vinylidene fluoride, hexafluoropropylene, and monomethyl maleate as starting materials as starting materials so that the molar ratio of vinylidene fluoride/hexafluoropropylene/monomethyl maleate is 98.4/1.5/0.1 by a suspension polymerization method ( A5) was synthesized. The weight average molecular weight of the obtained copolymer (A5) was 1.5 million. In preparation of the coating liquid, 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.

(Example 11)

As the copolymer (A), a copolymer (A6) was synthesized as follows. Using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials, a copolymer in which the molar ratio of vinylidene fluoride/hexafluoropropylene/maleic acid monomethyl ester is 94.5/1.5/4.0 by suspension polymerization ( A6) was synthesized. The weight average molecular weight of the obtained copolymer (A6) was 1.5 million. In preparation of the coating liquid, 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.

(Example 12)

A battery separator 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 permeability of 150 seconds/100 cm 3 was used as the polyolefin microporous membrane. The thickness of the battery separator was 8 µm.

(Example 13)

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

(Example 14)

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

(Example 15)

In the preparation of the coating solution, the alumina particles were replaced with plate-shaped 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 31.5 parts by mass of the copolymer (A1) and the copolymer (B1). A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (M) which was set to 4.2 parts by mass was used.

(Example 16)

In the preparation of the coating solution, the alumina particles were changed to an average particle diameter of 0.4 µm and titania particles (density of 4.23 g/cm 3 ), and the coating solution (N) was made into 25.3 parts by mass of the copolymer (A1) and 3.4 parts by mass of the copolymer (B1). ), except for using, in the same manner as in Example 1, to obtain a battery separator.

(Example 17)

In the preparation of the coating solution, the examples except that the coating solution (O) was used in which the mixing ratio of the copolymer (A1) and the copolymer (B1) was 29.0 parts by mass of the copolymer (A1) and 1.0 parts by mass of the copolymer (B1). In the same manner as in 1, a battery separator was obtained.

(Comparative Example 1)

In the preparation of the coating solution, except for using the coating solution (P) obtained by dissolving and mixing 88.3 parts by mass of the copolymer (A1), 11.7 parts by mass of the copolymer (B1), and 3500 parts by mass of NMP, A separator was obtained.

(Comparative Example 2)

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

(Comparative Example 3)

In the preparation of the coating solution, the examples except that the coating solution (R) in which the mixing ratio of the copolymer (A1) and the copolymer (B1) was 15.0 parts by mass of the copolymer (A1) and 15.0 parts by mass of the copolymer (B1) was used. In the same manner as in 1, a battery separator was obtained.

(Comparative Example 4)

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

(Comparative Example 5)

In the preparation of the coating solution, examples except that the copolymer (A) was replaced with 30.0 parts by mass of polyvinylidene fluoride (weight average molecular weight 1.5 million), and the coating solution (T) prepared without using the copolymer (B) was used. In the same manner as in 1, a battery separator was obtained.

(Comparative Example 6)

As the copolymer (A), a copolymer (A8) was synthesized as follows. Using vinylidene fluoride, hexafluoropropylene, and monomethyl maleate as starting materials, a copolymer so that the molar ratio of vinylidene fluoride/hexafluoropropylene/monomethyl maleate is 98.0/1.5/0.5 by suspension polymerization ( A8) was synthesized. The weight average molecular weight of the obtained copolymer (A8) was 650,000. In preparation of the coating liquid, 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 coating liquid (U) in which the NMP was changed to 500 parts by mass was used.

(Comparative Example 7)

As the copolymer (B), a copolymer (B5) was synthesized as follows. Copolymer (B5) was synthesized so that the molar ratio of vinylidene fluoride/hexafluoropropylene was 93.0/7.0 by suspension polymerization using vinylidene fluoride and hexafluoropropylene as starting materials. The weight average molecular weight of the obtained copolymer (B5) was 70,000. In preparation of the coating liquid, 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) was used.

(Comparative Example 8)

A battery separator 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 an air permeability of 150 seconds/100 cm 3 was used as the polyolefin microporous membrane. The thickness of the battery separator was 8 µm.

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

(Industrial availability)

When the battery separator of this embodiment is used for a nonaqueous electrolyte secondary battery, it satisfies the peel strength during drying and the bending strength when wet, the adhesion between the layers of the polyolefin multilayer microporous membrane and the separator of the porous layer, and adhesion between the separator and the electrode. It is possible to provide a battery separator having excellent properties and excellent short-circuit resistance. Therefore, the battery separator of the present embodiment can be suitably used even when a larger size and higher capacity of a battery (especially, a laminate type battery) is required in the future.

1 Polyolefin microporous membrane 2 Porous layer
4 Aluminum foil 5 Resin insulator
6 Metal sphere 10 Battery separator
20 negative electrode (for adhesion evaluation) 21 negative electrode (for short-circuit resistance evaluation)
22 Laminate film 30 Electrode winding body
31 Electrode stack 41 Aluminum L-shaped angle (lower side)
42 Aluminum L-shaped angle (top side) 43 Compression jig (top side)
44 Compression jig (lower side)

Claims (7)

A battery separator comprising a polyolefin microporous membrane and a porous layer laminated on at least one surface of the polyolefin microporous membrane,
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 mol% or more and 5.0 mol% or less of hexafluoropropylene units, has a weight average molecular weight of 900,000 or more and 2 million or less, and 0.1 mol% of a hydrophilic group. It contains more than 5.0 mol% or less,
The vinylidene fluoride-hexafluoropropylene copolymer (B) has more than 5.0 mol% and less than 8.0 mol% of hexafluoropropylene units, and has a weight average molecular weight of 100,000 or more and 750,000 or less,
The vinylidene fluoride-hexafluoropropylene copolymer (A) based on 100% by mass of the total of the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B) A battery separator containing 86% by mass or more and 98% by mass or less, and 40% by volume or more and 80% by volume or less of the inorganic particles based on 100% by volume of solid content in the porous layer. delete The method of claim 1,
The vinylidene fluoride-hexafluoropropylene copolymer (B) has a melting point of 60°C or more and 145°C or less. The method of claim 1 or 3,
A battery separator wherein the inorganic particles are at least one selected from titanium dioxide, alumina, and boehmite. The method of claim 1 or 3,
A battery separator having a thickness of the polyolefin microporous membrane of 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 claim 1 or 3. A nonaqueous electrolyte secondary battery comprising the electrode body according to claim 6 and a nonaqueous electrolyte.

Images