WO2018124176A1

WO2018124176A1 - Battery separator, electrode body, and nonaqueous electrolyte secondary battery

Info

Publication number: WO2018124176A1
Application number: PCT/JP2017/046887
Authority: WO
Inventors: 辻本　潤; 水野　直樹
Original assignee: 東レ株式会社
Priority date: 2016-12-27
Filing date: 2017-12-27
Publication date: 2018-07-05
Also published as: CN109661736A; TWI750288B; KR102210007B1; JP7229775B2; KR20190042715A; JPWO2018124176A1; TW201834298A; CN109661736B

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 non-aqueous electrolyte secondary battery

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

Non-aqueous electrolyte secondary batteries, especially lithium ion secondary batteries, are widely used in small electronic devices such as mobile phones and portable information terminals. Examples of the form of the nonaqueous electrolyte secondary battery include a cylindrical battery, a square battery, and a laminate battery. In general, these batteries have a configuration in which an electrode body in which a positive electrode and a negative electrode are arranged via a separator, and a nonaqueous electrolytic solution are housed in an exterior body. Examples of the structure of the electrode body include a laminated electrode body in which a positive electrode and a negative electrode are stacked via a separator, and a wound electrode body in which the positive electrode and the negative electrode are spirally wound via a separator.

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

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

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

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

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

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

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

Further, in Example 23 of Patent Document 6, a VdF-HFP copolymer (weight average molecular weight 1.93 million, HFP unit 1.1 mol%) and a VdF-HFP copolymer (weight average molecular weight 470,000, HFP unit 4. 8 mol%) is dissolved in a dimethylacetamide and tripropylene glycol solution, and a coating solution is prepared by adding aluminum hydroxide, and this is applied to a polyethylene microporous membrane to form a separator having a porous layer. Are listed.

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

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

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

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

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

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

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

The present invention has been made in view of the above circumstances, and is excellent in both the adhesion between the electrode and the separator during drying and the adhesion between the electrode and the separator during wetness, and excellent in short circuit resistance. An object is to provide a separator, an electrode body using the separator, and a secondary battery.

In order to solve the above-mentioned problems, the present inventors have conducted extensive research, and as a result, a separator having a porous layer containing two types of fluorine-based resins having different structures and their blending ratio and a specific amount of inorganic particles is used. The present inventors have found that the problem can be solved and have completed the present invention.

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 not less than 0.3 mol% and not more than 5.0 mol% of hexafluoropropylene units, and has a weight average molecular weight of not less than 900,000 and not more than 2 million, And includes a hydrophilic group,
The vinylidene fluoride-hexafluoropropylene copolymer (B) has more than 5.0 mol% and not more than 8.0 mol% hexafluoropropylene units, and has a weight average molecular weight of 100,000 to 750,000,
The total amount of the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B) is 100% by mass with respect to the vinylidene fluoride-hexafluoropropylene copolymer ( A) is contained in an amount of 86% by mass or more and 98% by mass or less, and the inorganic particles are contained in an amount of 40% by volume or more and 80% by volume or less with respect to 100% by volume of the solid content in the porous layer.
It is a battery separator.

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

The vinylidene fluoride-hexafluoropropylene copolymer (B) preferably has a melting point of 60 ° C. or higher and 145 ° C. or lower.

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

The thickness of the polyolefin microporous membrane is preferably 3 μm or more and 16 μm or less.

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

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

According to the present invention, a battery separator excellent in both the adhesion between the electrode and the separator at the time of drying and the adhesion between the electrode and the separator at the time of wetness and excellent in short circuit resistance, and the same are used. An electrode body and a secondary battery are provided.

FIG. 1 is a schematic diagram illustrating an example of a battery separator according to the present embodiment. FIG. 2 is a schematic diagram showing a method for evaluating the bending strength when wet. FIG. 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, directions in the figure will be described using an XYZ coordinate system. In the XYZ coordinate system, a plane parallel to the surface (in-plane direction) of the microporous film or the separator is defined as an XY plane. The direction perpendicular to the XY plane (thickness direction) is the Z direction. In each of the X direction, the Y direction, and the Z direction, the direction of the arrow in the figure is the + direction, and the direction opposite to the arrow direction is the − direction. Further, in the drawings, in order to make each configuration easy to understand, a part of the structure is emphasized or a part of the structure is simplified, and an actual structure, shape, scale, or the like may be different.

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

[1] Polyolefin microporous membrane The polyolefin microporous membrane 1 is a microporous membrane containing a polyolefin resin. The polyolefin microporous membrane 1 is not particularly limited, and a polyolefin microporous membrane used for a known battery separator can be used. In the present specification, the microporous membrane means a membrane having voids connected to the inside. Hereinafter, although an example of the polyolefin microporous membrane 1 is demonstrated, the polyolefin microporous membrane used for this invention is not limited to this.

[Polyolefin resin]
Examples of the polyolefin resin constituting the polyolefin microporous membrane 1 (hereinafter sometimes abbreviated as “microporous membrane 1”) include ethylene, propylene, 1-butene, 4-methyl 1-pentene, 1-hexene and the like. Examples thereof include a polymerized homopolymer, a two-stage polymer, a copolymer, or a mixture thereof. Among these, 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. is there. You may add various additives, such as antioxidant and an inorganic filler, to the polyolefin resin in the range which does not impair the effect of this invention as needed.

The thickness of the polyolefin microporous membrane 1 is not particularly limited, but is preferably 3 μm or more and 16 μm or less, more preferably 5 μm or more and 12 μm or less, and further preferably 5 μm or more and 10 μm or less from the viewpoint of increasing the battery capacity. It is. When the film thickness of the polyolefin microporous film is within the above-mentioned preferable range, practical film strength and pore blocking function can be retained, which is suitable for increasing the capacity of the battery which is expected to advance in the future. That is, the battery separator 10 of the present embodiment is provided between the polyolefin microporous film 1 and the porous layer 2 of the separator 10 and between the separator 10 and the electrode, even if the polyolefin microporous film 1 is thin. The adhesiveness can be excellent, and when the separator 10 is thinned, the effect is more clearly exhibited.

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

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

The manufacturing method of the microporous membrane 1 can include the following steps (1) to (5), and can further include the following steps (6) to (8).
(1) Step of melt-kneading the polyolefin resin and a film-forming solvent to prepare a polyolefin solution (2) Step of extruding the polyolefin solution and cooling to form a gel-like sheet (3) Stretching the gel-like sheet The first stretching step (4) The step of removing the film-forming solvent from the stretched gel-like sheet (5) The step of drying the sheet after the film-forming solvent is removed (6) The dried sheet Second stretching step to stretch (7) Step of heat treating the dried sheet (8) Step of crosslinking and / or hydrophilizing the sheet after the stretching step.

Hereinafter, each step will be described.

(1) Preparation Step of Polyolefin Solution After adding an appropriate film-forming solvent to the polyolefin resin, it is melt-kneaded to prepare a polyolefin solution. As a melt-kneading method, for example, a method using a twin-screw extruder described in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Since the melt-kneading method is well-known, description is abbreviate | 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 weight of the film-forming solvent with respect to 20 to 30 parts by weight of the polyolefin resin. When the ratio of the polyolefin resin is within the above range, swell and neck-in can be prevented at the die exit when extruding the polyolefin solution, and the moldability and self-supporting property of the extruded molded body (gel-shaped molded body) are improved.

(2) Gel-like sheet forming step A polyolefin solution is fed from an extruder to a die and extruded into a sheet. A plurality of polyolefin solutions having the same or different compositions may be fed from an extruder to a single die, where they are laminated in layers and extruded into sheets.

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 adjusted by adjusting each extrusion amount of the polyolefin solution. As an extrusion method, for example, methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used.

A gel-like sheet is formed by cooling the obtained extruded product. As a method for forming the gel-like sheet, for example, methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Cooling is preferably performed at a rate of 50 ° C./min or more at least up to the gelation temperature. Cooling is preferably performed to 25 ° C. or lower. By cooling, the polyolefin microphase separated by the film-forming solvent can be immobilized. When the cooling rate is within the above range, the crystallization degree is maintained in an appropriate range, and a gel-like sheet suitable for stretching is obtained. As a cooling method, a method of contacting with a cooling medium such as cold air or cooling water, a method of contacting with a cooling roll, or the like can be used, but it is preferable that the cooling is performed by contacting a roll cooled with a cooling medium.

(3) 1st extending process Next, the obtained gel-like sheet | seat is extended | stretched at least to a uniaxial direction. Since the gel-like sheet contains a film-forming solvent, it can be stretched uniformly. The gel-like sheet is preferably stretched at a predetermined ratio after heating by a tenter method, a roll method, an inflation method, or a combination thereof. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching, and multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used.

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. Moreover, the draw ratios in the machine direction (MD) and the width direction (TD) may be the same or different. In addition, the draw ratio in this process means the area draw ratio of the microporous film immediately before being used for the next process on the basis of the microporous film immediately before this process.

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

The stretching as described above causes cleavage between polyethylene lamellae, the polyethylene phase becomes finer, and a large number of fibrils are formed. Fibrils form a three-dimensional irregularly connected network structure. Stretching improves the mechanical strength and enlarges the pores. However, when stretching is performed under appropriate conditions, the through-hole diameter can be controlled, and a high porosity can be achieved even with a thinner film thickness.

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

(4) Removal of film-forming solvent The film-forming solvent is removed (washed) using a cleaning solvent. Since the polyolefin phase is phase-separated from the film-forming solvent phase, removing the film-forming solvent consists of fibrils that form a fine three-dimensional network structure, and pores (voids) that communicate irregularly in three dimensions. A porous membrane having the following is obtained. Since the cleaning solvent and the method for removing the film-forming solvent using the same are known, the description thereof is omitted. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Laid-Open No. 2002-256099 can be used.

(5) Drying The microporous film 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 higher 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 cleaning solvent is 5% by mass or less, more preferably 3% by mass or less, with the microporous membrane being 100% by mass (dry mass). When the residual cleaning solvent is within the above range, the porosity of the microporous membrane is maintained when the subsequent microporous membrane stretching step and heat treatment step are performed, and deterioration of permeability is suppressed.

(6) Second stretching step It is preferable to stretch the dried microporous membrane in at least a uniaxial direction. The microporous membrane can be stretched by the 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, either simultaneous biaxial stretching or sequential stretching may be used. The stretching temperature in this step is not particularly limited, but is usually preferably 90 to 135 ° C, more preferably 95 to 130 ° C. In this step, the stretching ratio (area stretching ratio) in the uniaxial direction of stretching of the microporous membrane is preferably 1.0 to 2.0 times in the machine direction or the width direction in the case of uniaxial stretching. In the case of biaxial stretching, the area stretching ratio is preferably 1.0 times the lower limit, more preferably 1.1 times, and even more preferably 1.2 times. The upper limit is preferably 3.5 times. The stretching ratio in the machine direction and the width direction may be the same or different from each other in the machine direction and the width direction. In addition, the draw ratio in this process means the draw ratio of the microporous film just before being provided to the next process on the basis of the microporous film immediately before this process.

(7) Heat treatment Moreover, the microporous film after drying can be heat-treated. The crystal is stabilized by heat treatment, and the lamella is made uniform. As the heat treatment method, heat setting treatment and / or heat relaxation treatment can be used. The heat setting treatment is a heat treatment in which heating is performed while keeping the dimensions of the film unchanged. The thermal relaxation treatment is a heat treatment that heat-shrinks the film in the machine direction or the width direction during heating. The heat setting treatment is preferably performed by a tenter method or a roll method. For example, as a thermal relaxation treatment method, a method disclosed in Japanese Patent Application Laid-Open No. 2002-256099 can be given. The heat treatment temperature is 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 and hydrophilization treatment Further, a crosslinking treatment and a hydrophilization treatment can also be performed on the microporous membrane after bonding or stretching. For example, the microporous membrane is subjected to a crosslinking treatment by irradiation with ionizing radiation such as α rays, β rays, γ rays, and electron beams. In the case of electron beam irradiation, 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 is increased by the crosslinking treatment. The hydrophilic treatment can be performed by monomer grafting, surfactant treatment, corona discharge, or the like. Monomer grafting is preferably performed after the crosslinking treatment.

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

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

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, in the step of forming the porous layer, the time for dissolving the copolymer (A) in the solvent is not extremely long, and the production efficiency is increased. Can maintain an appropriate gel strength when swollen in the electrolyte, and can improve the bending strength when wet. In addition, the weight average molecular weight of a copolymer (A) is a polystyrene conversion value by a gel permeation chromatography.

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

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

The 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 prevent the polymer crystallinity from becoming too low and the degree of swelling with respect to the electrolytic solution to become high and the bending strength when wet is deteriorated. Further, when the content of the hydrophilic group is within the above range, the affinity between the inorganic particles contained in the porous layer 2 and the copolymer (A) is increased, the short circuit resistance is improved, and the inorganic particles are removed. It also has an inhibitory effect. Although this reason is not certain, it is guessed that the film strength of the porous layer 2 is increased by the copolymer (A) having a hydrophilic group as the main component of the porous layer 2 and the inorganic particles. The quantification of the hydrophilic group of the vinylidene fluoride-hexafluoropropylene copolymer in the porous layer 2 can be determined by IR (infrared absorption spectrum) method, NMR (nuclear magnetic resonance) method or the like.

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

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

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

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

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

The 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 less, more preferably 140 ° C. or less. In addition, melting | fusing point (Tm) here is the temperature of the peak top of the endothermic peak at the time of temperature rising measured by the differential scanning calorimetry (DSC) method.

The copolymer (B) is a copolymer having a vinylidene fluoride unit and a hexafluoropropylene unit. The copolymer (B) can be obtained by a suspension polymerization method or the like, similar to the copolymer (A). The melting point of the copolymer (B) can be adjusted by controlling the crystallinity of the site composed of vinylidene fluoride units. For example, when the copolymer (B) contains a monomer other than the vinylidene fluoride unit, the melting point can be adjusted by controlling the ratio of the vinylidene fluoride unit. Monomers other than vinylidene fluoride units are tetrafluoroethylene, trifluoroethylene, trichloroethylene, hexafluoropropylene, fluorinated vinyl maleic anhydride, maleic acid, maleic acid ester, maleic acid monomethyl ester, etc. You may have more. Examples thereof include a method in which the monomer is added when the copolymer (B) is polymerized and introduced into the main chain by copolymerization, or a method in which it is introduced as a side chain by grafting. Further, the melting point may be adjusted by controlling the ratio of Head-to-Head bonds (—CH ₂ —CF ₂ —CF ₂ —CH ₂ —) of vinylidene fluoride units.

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

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

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

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

The content of the inorganic particles contained in the porous layer 2 is 80% by volume or less, preferably 70% by volume or less, more preferably 60% by volume with respect to 100% by volume of the solid content volume of the porous layer 2. It is as follows. 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 further preferably 51% by volume or more. The content of the inorganic particles contained in the porous layer 2 was calculated by calculating the density of the copolymer (A) and the copolymer (B) as 1.77 g / cm ³ .

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

From the viewpoint of particle dropout, the average particle size of the inorganic particles is preferably 1.5 times or more and 50 times or less, more preferably 2.0 times or more and 20 times or less of the average flow pore size of the polyolefin microporous membrane. It is. The average flow pore size is measured according to JISK3832, and can be determined by measuring in the order of Dry-up and Wet-up using a palm porometer (for example, CFP-1500A manufactured by PMI). Specifically, the pore diameter is converted from the pressure at the point where the curve showing the 1/2 slope of the pressure / flow rate curve in the Dry-up measurement and the curve of the Wet-up measurement intersect. The following formula is used for conversion of pressure and pore diameter.
d = C · γ / P
In the above formula, “d (μm)” is the pore diameter of the microporous membrane, “γ (mN / m)” is the surface tension of the liquid, “P (Pa)” is the pressure, and “C” is a constant.

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

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

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

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

(1) Step of obtaining a fluororesin solution The vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B) are gradually added to a solvent and completely dissolved.

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

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

(3) The step of applying the coating solution to the microporous membrane, immersing it in the coagulation solution, washing and drying The coating solution is applied to the microporous membrane, and the applied microporous membrane is immersed in the coagulation solution and fluorinated The vinylidene-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B) are phase-separated, coagulated in a state having a three-dimensional network structure, washed and dried. As a result, a microporous membrane and a battery separator having a porous layer on the surface of the microporous membrane are obtained. The method of applying the coating liquid to the microporous film may be a known method, for example, dip coating method, reverse roll coating method, gravure coating method, kiss coating method, roll brush method, spray coating method, Examples thereof include an air knife coating method, a Mayer bar coating method, a pipe doctor method, a blade coating method, and a die coating method, and these methods can be used alone or in combination.

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

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

[4] Physical Properties of Battery Separator Battery Separator The battery separator 10 of the present embodiment can be suitably used for both a battery using an aqueous electrolyte and a battery using a nonaqueous electrolyte. The secondary battery can be preferably used. Specifically, it can be preferably used as a separator for secondary batteries such as nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc batteries, lithium secondary batteries, and lithium polymer secondary batteries. Especially, it is preferable to use as a separator of a lithium ion secondary battery.

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

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

As shown in FIG. 1A, the battery separator 10 may be formed by laminating the porous 2 on one surface of the polyolefin microporous membrane 1, and porous on both surfaces of the polyolefin microporous membrane 1. Even if two are laminated. In FIG. 1, the polyolefin microporous membrane 1 is a single layer, but may be a laminate of two or more layers. In addition, the battery separator 10 may further include a layer other than the polyolefin microporous membrane 1 and the porous 2.

The wet 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 when wet is not particularly defined, but is, for example, 15.0 N or less. When the bending strength when wet is within the above preferable range, partial release at the interface between the separator and the electrode can be further suppressed, and the increase in battery internal resistance and the decrease in battery characteristics can be suppressed. In addition, the bending strength when wet can be measured by the method described in Examples described later.

The peel strength during drying of the battery separator is preferably 2.0 N / m or more, more preferably 5.0 N / m or more, and still more preferably 6.0 N / m or more. Although the upper limit of the peeling force at the time of drying is not particularly defined, it is, for example, 40.0 N / m or less. When the peeling force at the time of drying is within the preferable range, it is expected that the wound electrode body or the laminated electrode body can be transported without the electrode body being scattered. In addition, the peeling force at the time of drying can be measured by the method as described in the below-mentioned Example.

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

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

EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, the embodiment of this invention is not limited to these Examples. The evaluation methods, analysis methods and materials used in the examples are as follows.

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

(2) Weight average molecular weight (Mw) of vinylidene fluoride-hexafluoropropylene copolymer (A) and vinylidene fluoride-hexafluoropropylene copolymer (B)
It calculated | required by the gel permeation chromatography (GPC) method on the following conditions.
・ Measurement device: GPC-150C manufactured by Waters Corporation
・ Column: 2 shodex KF-806M manufactured by Showa Denko KK ・ Column temperature: 23 ° C
Solvent (mobile phase): 0.05M lithium chloride added N-methyl-2-pyrrolidone (NMP)
・ Solvent flow rate: 0.5 ml / min ・ Sample preparation: 4 mL of measurement solvent was added to 2 mg of the sample, and gently stirred at room temperature (dissolution was visually confirmed).
・ Injection volume: 0.2mL
・ Detector: Differential refractive index detector RI (RI-8020 type sensitivity 16 manufactured by Tosoh Corporation)
-Calibration curve: Created from a calibration curve obtained using a monodisperse polystyrene standard sample, using a polyethylene conversion coefficient (0.46).

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

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

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

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

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

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

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

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

(5) Peel strength during drying (preparation of negative electrode)
The same negative electrode 20 as in the case of the bending strength when wet was used.
(Preparation of peel test piece)
The negative electrode 20 (70 mm × 15 mm) created above and the produced separator 10 (machine direction 90 mm × width direction 20 mm) are stacked, and this is sandwiched between two gaskets (thickness 0.5 mm, 95 mm × 27 mm), Pressurization was performed at 90 ° C. and 8 MPa for 2 minutes with a precision heating and pressurizing apparatus (CYPT-10, manufactured by Shinto Kogyo Co., Ltd.), and the mixture was allowed to cool at room temperature. A double-sided tape having a width of 1 cm is attached to the negative electrode side of the laminate of the negative electrode 20 and the separator 10, and the other side of the double-sided tape is attached to a SUS plate (thickness 3 mm, length 150 mm × width 50 mm). The pasting was performed so that the machine direction and the SUS plate length direction were parallel. This was made into the peeling test piece.
(Measurement method of peel strength during drying)
A separator 10 was sandwiched between load cell side chucks using a universal testing machine (AGS-J, manufactured by Shimadzu Corporation), and a 180 degree peel test was performed at a test speed of 300 mm / min. A value obtained by averaging measured values from a stroke of 20 mm to 70 mm during the peel test was defined as the peel strength of the peel test piece. A total of three peel test pieces were measured, and a value obtained by converting the average peel force into a width was defined as a peel force during drying (N / m).

(6) Short-circuit resistance test The short-circuit resistance was evaluated using a desktop precision universal testing machine Autograph AGS-X (manufactured by Shimadzu Corporation). First, as shown in FIG. 3A, a polypropylene insulator 5 (thickness 0.2 mm), a lithium ion battery negative electrode 21 (total thickness: about 140 μm, base material: copper foil (thickness: about 9 μm), active Material: Sample laminate 31 was prepared by laminating artificial graphite (particle size: about 30 μm, double-sided coating), separator 10, and aluminum foil 4 (thickness: about 0.1 mm). Next, as shown in FIG. 3B, the sample laminate 31 was fixed to the compression jig (lower side) 44 of the universal testing machine with double-sided tape. Next, the aluminum foil 4 and the negative electrode 21 of the sample laminate 31 were connected to a circuit composed of a capacitor and a clad resistor with a cable. The capacitor was charged to about 1.5 V, and a metal ball 6 (material: chromium (SUJ-2)) having a diameter of about 500 μm was placed between the separator in the sample laminate 31 and the aluminum foil 4. Next, a compression jig is attached to the universal testing machine, and the sample laminate 31 including the metal balls 6 is placed between both compression jigs 43 and 44 as shown in FIG. / Min. The test was terminated when the load reached 100 N. At this time, the part where the inflection point appeared in the change in compressive load was taken as the film breaking point of the separator, and the moment when the circuit was formed and the current was detected via the metal sphere was taken as the short-circuit occurrence point. The compression displacement A (t) when the separator breaks due to compression and an inflection point occurs in the compression stress, and the compression displacement B (t) at the moment when the current flows through the circuit are measured. If the numerical value obtained in 1.1 is 1.1 or more, even if the separator breaks due to foreign matter mixed in the battery, it means that the insulation is maintained by the coating layer composition adhering to the surface of the foreign matter, Short-circuit resistance was evaluated as good. On the other hand, when the numerical value obtained by Equation 1 is greater than 1.0 and less than 1.1, the separator film breakage and short circuit do not occur at the same time, but the tension applied to the winding of the battery member or the expansion of the electrode during charge / discharge In order to prevent a short circuit from occurring even when the internal pressure of the battery increases, a certain level of resistance is required, and thus the short circuit resistance was evaluated as slightly poor. When the numerical value obtained by Equation 1 is 1.0, a short circuit occurred at the same time as the film breakage of the separator, and no improvement in the short circuit resistance by the coating layer was observed, so the short circuit resistance was evaluated as poor.
B (t) ÷ A (t) (Formula 1)
(Example 1)
[Copolymer (A)]
As copolymer (A), copolymer (A1) was synthesized as follows. The molar ratio of vinylidene fluoride / hexafluoropropylene / maleic acid monomethyl ester was 98.0 / 1.5 / 0.5 by suspension polymerization using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials. Thus, a copolymer (A1) was synthesized. The weight average molecular weight of the obtained copolymer (A1) was 1,500,000.

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

[Preparation of battery separator]
26.5 parts by mass of copolymer (A1) and 3.5 parts by mass of copolymer (B1) and 600 parts by mass of N-methyl-2-pyrrolidone (NMP) were mixed, and then stirred with a disper. Alumina particles (average particle size 1.1 μm, density 4.0 g / cm ³ ) were added so that the solid content of the porous layer was 100% by volume to 51% by volume, and further pre-stirred at 2000 rpm with a disper for 1 hour. did. Next, using a dyno mill (Dynomill Multilab (1.46 L container, filling rate 80%, φ0.5 mm alumina beads) made by Shinmaru Enterprises) under the conditions of a flow rate of 11 kg / hr and a peripheral speed of 10 m / s. It processed 3 times and produced the coating liquid (A). The obtained coating liquid (A) was applied in an equal amount on both surfaces of a polyethylene microporous film having a thickness of 7 μm, a porosity of 40%, and an air resistance of 100 seconds / 100 cm ³ by a dip coating method. The coated film is immersed in an aqueous solution (coagulation solution) containing 10% by mass of N-methyl-2-pyrrolidone (NMP), washed with pure water, and then dried at 50 ° C. to obtain a battery separator. It was. The thickness of the battery separator was 10 μm.

(Example 2)
As copolymer (B), copolymer (B2) was synthesized as follows. A copolymer (B2) was synthesized by suspension polymerization using vinylidene fluoride and hexafluoropropylene as starting materials so that the molar ratio of vinylidene fluoride / hexafluoropropylene was 94.5 / 5.5. The weight average molecular weight of the obtained copolymer (B2) was 280,000. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (B) in which the copolymer (B1) was replaced with the copolymer (B2) was used in the preparation of the coating liquid.

(Example 3)
As the copolymer (B), a copolymer (B3) was synthesized as follows. A copolymer (B3) was synthesized by suspension polymerization using vinylidene fluoride and hexafluoropropylene as starting materials so that the molar ratio of vinylidene fluoride / hexafluoropropylene was 92.0 / 8.0. The weight average molecular weight of the obtained copolymer (B3) was 350,000. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (C) in which the copolymer (B1) was replaced with the copolymer (B3) was used in the preparation of the coating liquid.

Example 4
As copolymer (A), copolymer (A2) was synthesized as follows. The molar ratio of vinylidene fluoride / hexafluoropropylene / maleic acid monomethyl ester was 99.0 / 0.5 / 0.5 by suspension polymerization using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials. Thus, a copolymer (A2) was synthesized. The weight average molecular weight of the obtained copolymer (A2) was 1,400,000. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (D) in which the copolymer (A1) was replaced with the copolymer (A2) was used in the preparation of the coating liquid.

(Example 5)
As copolymer (A), copolymer (A3) was synthesized as follows. The molar ratio of vinylidene fluoride / hexafluoropropylene / maleic acid monomethyl ester was 95.0 / 4.5 / 0.5 by suspension polymerization using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials. Thus, a copolymer (A3) was synthesized. The weight average molecular weight of the obtained copolymer (A3) was 1,700,000. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (E) in which the copolymer (A1) was replaced with the copolymer (A3) was used in the preparation of the coating liquid.

(Example 6)
As copolymer (A), copolymer (A4) was synthesized as follows. The molar ratio of vinylidene fluoride / hexafluoropropylene / maleic acid monomethyl ester was 98.0 / 1.5 / 0.5 by suspension polymerization using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials. Thus, a copolymer (A4) was synthesized. The weight average molecular weight of the obtained copolymer (A4) was 1,900,000. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (F) in which the copolymer (A1) was replaced with the copolymer (A4) was used in the preparation of the coating liquid.

(Example 7)
In the preparation of the coating liquid, the blending ratio of the copolymer (A1) and the copolymer (B1) was 28.0 parts by mass of the copolymer (A1) and 2.0 parts by mass of the copolymer (B1). A battery separator was obtained in the same manner as in Example 1 except that the working liquid (G) was used.

(Example 8)
In the preparation of the coating liquid, the content of alumina particles was set to 40% by volume with the solid content of the porous layer being 100% by volume, and 35.2 parts by mass of the copolymer (A1) and the copolymer (B1). A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (H) in which 4.7 parts by mass and NMP was changed to 900 parts by mass was used.

Example 9
In the preparation of the coating liquid, the content of alumina particles is 75% by volume with the solid content of the porous layer being 100% by volume, and 11.4 parts by mass of copolymer (A1) and copolymer (B1). A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (I) in which 1.5 parts by mass and NMP was changed to 300 parts by mass was used.

(Example 10)
As copolymer (A), copolymer (A5) was synthesized as follows. The molar ratio of vinylidene fluoride / hexafluoropropylene / maleic acid monomethyl ester was 98.4 / 1.5 / 0.1 by suspension polymerization using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials. Thus, a copolymer (A5) was synthesized. The weight average molecular weight of the obtained copolymer (A5) was 1,500,000. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (J) was used instead of the copolymer (A5) in the preparation of the coating liquid.

(Example 11)
As copolymer (A), copolymer (A6) was synthesized as follows. The molar ratio of vinylidene fluoride / hexafluoropropylene / maleic acid monomethyl ester was 94.5 / 1.5 / 4.0 by suspension polymerization using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials. Thus, a copolymer (A6) was synthesized. The weight average molecular weight of the obtained copolymer (A6) was 1,500,000. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (K) in which the copolymer (A1) was replaced with the copolymer (A6) was used in the preparation of the coating liquid.

(Example 12)
A battery separator was obtained in the same manner as in Example 1 except that a polyethylene microporous film having a thickness of 5 μm, a porosity of 35%, and a gas permeability of 150 seconds / 100 cm ³ was used as the polyolefin microporous film. The thickness of the battery separator was 8 μm.

(Example 13)
A battery separator was obtained in the same manner as in Example 1 except that a polyethylene microporous membrane having a thickness of 12 μm, a porosity of 45%, and an air resistance 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. A copolymer (B4) was synthesized by suspension polymerization using vinylidene fluoride and hexafluoropropylene as starting materials so that the molar ratio of vinylidene fluoride / hexafluoropropylene was 93.0 / 7.0. The weight average molecular weight of the obtained copolymer (B1) was 700,000. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (L) in which the copolymer (B1) was replaced with the copolymer (B4) was used in the preparation of the coating liquid.

(Example 15)
In preparation of the coating liquid, the alumina particles are replaced with plate-like boehmite particles (density 3.07 g / cm ³ ) having an average particle diameter of 1.0 μm and an average thickness of 0.4 μm, and the copolymer (A1) 31.5 mass. Battery separator was obtained in the same manner as in Example 1 except that the coating liquid (M) was used in an amount of 4.2 parts by weight of the copolymer (B1).

(Example 16)
In the preparation of the coating liquid, the alumina particles are replaced with an average particle size of 0.4 μm, titania particles (density 4.23 g / cm ³ ), 25.3 parts by mass of copolymer (A1), and copolymer (B1) 3. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (N) having 4 parts by mass was used.

(Example 17)
In the preparation of the coating solution, the blending ratio of the copolymer (A1) and the copolymer (B1) was 29.0 parts by mass of the copolymer (A1) and 1.0 part by mass of the copolymer (B1). A battery separator was obtained in the same manner as in Example 1 except that the working liquid (O) was used.

(Comparative Example 1)
In preparation of the coating liquid, except that the coating liquid (P) in which 88.3 parts by mass of the copolymer (A1), 11.7 parts by mass of the copolymer (B1) and 3500 parts by mass of NMP were dissolved and mixed was used. Obtained a battery separator in the same manner as in Example 1.

(Comparative Example 2)
In the preparation of the coating liquid, alumina particles were added so that the solid content of the porous layer was 100% by volume to 95% by volume, and 2.0 parts by mass of the copolymer (A1), 0. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (Q) in which 3 parts by mass and NMP was changed to 250 parts by mass was used.

(Comparative Example 3)
In the preparation of the coating liquid, the blending ratio of the copolymer (A1) and the copolymer (B1) was 15.0 parts by mass of the copolymer (A1) and 15.0 parts by mass of the copolymer (B1). A battery separator was obtained in the same manner as in Example 1 except that the working liquid (R) was used.

(Comparative Example 4)
As copolymer (A), copolymer (A7) was synthesized as follows. A copolymer (A7) was synthesized by suspension polymerization using vinylidene fluoride and hexafluoropropylene as starting materials so that the molar ratio of vinylidene fluoride / hexafluoropropylene was 98.5 / 1.5. The weight average molecular weight of the obtained copolymer (A7) was 1,500,000. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (S) in which the copolymer (A1) was replaced with the copolymer (A7) was used in the preparation of the coating liquid.

(Comparative Example 5)
In the preparation of the coating liquid, the coating liquid (T) prepared without using the copolymer (B) by replacing the copolymer (A) with 30.0 parts by weight of polyvinylidene fluoride (weight average molecular weight 1,500,000) A battery separator was obtained in the same manner as in Example 1 except that was used.

(Comparative Example 6)
As copolymer (A), copolymer (A8) was synthesized as follows. The molar ratio of vinylidene fluoride / hexafluoropropylene / maleic acid monomethyl ester was 98.0 / 1.5 / 0.5 by suspension polymerization using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials. Thus, a copolymer (A8) was synthesized. The weight average molecular weight of the obtained copolymer (A8) was 650,000. A battery was prepared in the same manner as in Example 1 except that in the preparation of the coating liquid, the copolymer (A1) was replaced with the copolymer (A8) and the coating liquid (U) in which NMP was changed to 500 parts by mass was used. A separator was obtained.

(Comparative Example 7)
As copolymer (B), copolymer (B5) was synthesized as follows. A copolymer (B5) was synthesized by suspension polymerization using vinylidene fluoride and hexafluoropropylene as starting materials so that the molar ratio of vinylidene fluoride / hexafluoropropylene was 93.0 / 7.0. The weight average molecular weight of the obtained copolymer (B5) was 70,000. A battery separator was obtained in the same manner as in Example 1 except that the coating liquid (V) was used instead of the copolymer (B5) in the preparation of the coating liquid.

(Comparative Example 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 a gas permeability resistance 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 copolymers (B) used in the above Examples and Comparative Examples, the composition of the coating solution, and the characteristics of the battery separator obtained.

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

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

Claims

A separator for a battery comprising a polyolefin microporous membrane and a porous layer 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 not less than 0.3 mol% and not more than 5.0 mol% of hexafluoropropylene units, and has a weight average molecular weight of not less than 900,000 and not more than 2 million, And includes a hydrophilic group,
The vinylidene fluoride-hexafluoropropylene copolymer (B) has more than 5.0 mol% and not more than 8.0 mol% hexafluoropropylene units, and has a weight average molecular weight of 100,000 to 750,000,
The total amount of the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride-hexafluoropropylene copolymer (B) is 100% by mass with respect to the vinylidene fluoride-hexafluoropropylene copolymer ( A) is contained in an amount of 86% by mass or more and 98% by mass or less, and the inorganic particles are contained in an amount of 40% by volume or more and 80% by volume or less with respect to 100% by volume of the solid content in the porous layer.
Battery separator.
The battery separator according to claim 1, wherein the vinylidene fluoride-hexafluoropropylene copolymer (A) contains 0.1 mol% or more and 5.0 mol% or less of a hydrophilic group.
The battery separator according to claim 1 or 2, wherein the vinylidene fluoride-hexafluoropropylene copolymer (B) has a melting point of 60 ° C or higher and 145 ° C or lower.
The battery separator according to any one of claims 1 to 3, wherein the inorganic particles are at least one selected from titanium dioxide, alumina, and boehmite.
The battery separator according to any one of claims 1 to 4, wherein the polyolefin microporous membrane has a thickness of 3 µm to 16 µm.
An electrode body comprising a positive electrode, a negative electrode, and the battery separator according to any one of claims 1 to 5.
A nonaqueous electrolyte secondary battery comprising the electrode body according to claim 6 and a nonaqueous electrolyte.