JP5463154B2 - Laminated microporous membrane and separator for non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a laminated microporous membrane and a separator for a nonaqueous electrolyte secondary battery comprising the laminated microporous membrane.

  Polyolefin-based microporous membranes are used for microfiltration membranes, battery separators, capacitor separators, fuel cell materials, and the like, and in particular, lithium ion secondary battery separators. In recent years, lithium ion secondary batteries have been applied to small electronic devices such as mobile phones and laptop computers, as well as electric vehicles and small electric motorcycles. In particular, notebook PCs and mobile phone equipment, which will continue to expand rapidly worldwide, will be produced and used in various countries. It is required to meet the characteristics. Under such circumstances, multilayer separators have been proposed so far for the purpose of combining functions.

For example, Patent Document 1 discloses a polyolefin-based microporous membrane that requires polyethylene having a weight average molecular weight of 500,000 or more and is excellent in gas permeability and puncture strength (resin) And an unstretched film original fabric obtained by extrusion molding of a liquid solvent is stretched, and two or more films obtained by solvent extraction are overlapped and further co-stretched).
Patent Document 2 discloses a battery separator made of a porous laminated film comprising a porous polypropylene layer having a weight average molecular weight of 500,000 or more and a porous layer made of a material having a melting point of 100 to 135 ° C. The air permeability (Gurley seconds) and strength (needle penetration strength) at 25 ° C. are described.
Patent Document 3 discloses a battery separator having a three-layer structure of PP / PE / PP by a stretch opening method as a separator having excellent mechanical strength, and it is described that the porosity is usually 30 to 65%. ing. In the examples, an example in which the porosity of the PP layer is 41% and the porosity of the PE layer is 44% is disclosed.
Patent Document 4 discloses a separator having a high porosity (porosity) and a high strength as a power storage device separator having a porosity of 80% or more containing inorganic powder of 50% by mass or more.
Patent Document 5 discloses a polyethylene resin porous film in which thickness, air permeability, pin puncture strength, and surface roughness are adjusted to specific ranges, and the surface of the film is appropriately roughened. Therefore, it is described that it is suitable as a separator for a non-aqueous electrolyte battery.

JP 2003-103624 A JP 09-219184 A JP 2000-048794 A JP 2007-095440 A JP-A-11-060791

However, the separators described in Patent Documents 1 to 5 still have room for improvement from the viewpoint of achieving both cycle performance and safety (overcharge).
In view of the above circumstances, an object of the present invention is to provide a laminated microporous membrane capable of realizing a secondary battery having both good safety and good cycleability when used as a separator.

  As a result of intensive studies on the above problems, the inventors of the present invention have a laminated microporous structure having a specific air permeability, piercing strength, average pore diameter, and liquid absorption index adjusted to a specific range. The present inventors have found that a film can solve the above problems and have completed the present invention.

That is, the present invention is as follows.
[1]
A laminated microporous membrane having an air permeability in terms of membrane thickness of 20 μm of 300 seconds / 100 cc or less, a puncture strength in terms of membrane thickness of 20 μm of 300 g or more, and an average pore diameter D of all layers of 0.02 μm or more and 0.1 μm or less. Te, absorbent index (H30) is Ri der more than 7mm,
When the curvature of the laminated microporous membrane is T (−), the average pore diameter is D (μm), and the porosity is P (%), the F value defined by the following formula (1) is 0.25. A laminated microporous membrane satisfying ≦ F ≦ 0.9 .
F = T / (D * P) (1)
[2]
It consists of at least two layers of (B layer) different from (A layer) and (A layer), and the porosity (porosity A) of the (A layer) is 35 to 80%. The laminated microporous membrane according to the above [1], wherein the porosity (porosity B) is in the range of 30 to 60% and (porosity A)-(porosity B) is in the range of 3 to 40%.
[3]
The layered microparticle according to the above [2], wherein the porosity (porosity A) of the (A layer) is 40 to 70%, and the porosity (porosity B) of the (B layer) is 30 to 50%. Porous membrane.
[4]
The laminated microporous membrane according to any one of the above [1] to [3], comprising a polyolefin composition as a main component .
[5 ]
The laminated microporous membrane according to any one of [2] to [ 4 ], wherein the thickness of the (B layer) is 35% or more with respect to the entire laminated microporous membrane.
[ 6 ]
The laminated microporous membrane according to any one of [2] to [ 5 ], wherein the (A layer) and the (B layer) are laminated by a coextrusion method.
[ 7 ]
The separator for nonaqueous electrolyte secondary batteries which consists of a lamination | stacking microporous film in any one of said [1]-[ 6 ].

  According to the present invention, it is possible to provide a laminated microporous membrane capable of realizing a secondary battery having both good safety and good cycleability when used as a separator.

  Hereinafter, a mode for carrying out the present invention (hereinafter abbreviated as “the present embodiment”) will be described in detail. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.

  The laminated microporous membrane of the present embodiment has an air permeability equivalent to a membrane thickness of 20 μm of 300 seconds / 100 cc or less, a puncture strength equivalent to a membrane thickness of 20 μm of 300 g or more, and an average pore diameter D of all layers of 0.02 to 0. A laminated microporous membrane satisfying 1 μm or less and having a liquid absorbency index (H30) of 7 mm or more.

  In the laminated microporous membrane of the present embodiment, the air permeability and puncture strength in terms of membrane thickness of 20 μm, the average pore diameter of all layers, and the liquid absorption index are adjusted to the specific ranges, respectively. By functioning synergistically, it is possible to achieve both battery safety and cycleability when used as a separator. Hereinafter, each physical property of the laminated microporous film will be described.

  The laminated microporous membrane of the present embodiment has an air permeability equivalent to a film thickness of 20 μm of 300 seconds / 100 cc or less and a puncture strength equivalent to a film thickness of 20 μm of 300 g or more. When the air permeability and puncture strength are within the above ranges, the battery cycle and film strength are compatible when used as a separator.

  The air permeability in terms of a film thickness of 20 μm is preferably 50 seconds / 100 cc or more from the viewpoint of mechanical strength and self-discharge, and preferably 250 seconds / 100 cc or less from the viewpoint of battery cycle characteristics and rate characteristics. The air permeability in terms of the film thickness of 20 μm is more preferably from 70 seconds / 100 cc to 210 seconds / 100 cc, and even more preferably from 100 seconds / 100 cc to 210 seconds / 100 cc. Here, the air permeability in terms of the film thickness of 20 μm is a value measured with a Gurley air permeability meter “G-B2” (trademark, manufactured by Toyo Seiki Seisakusho Co., Ltd.) in accordance with JIS P-8117. Further, the method for adjusting the air permeability to the above range depends on the production method of the microporous membrane, but a so-called “wet method” in which a resin and a plasticizer are used as raw materials, and the plasticizer is extracted and made porous after film formation. In the case of, the method of adjusting the mixing ratio of resin and plasticizer, the draw ratio and temperature in a film forming process, the method of adjusting the draw ratio and temperature in a heat setting process, etc. are mentioned. In addition, a crystalline resin is used without using a plasticizer, and the interface between the amorphous parts between the lamellae and the interface between the resin and the inorganic filler such as calcium carbonate is cleaved by longitudinal stretching at a low temperature to make it porous. In the case of the “dry method”, a method of controlling the crystallization of the lamella by adjusting the draft ratio and the stretching speed can be mentioned.

The puncture strength in terms of film thickness of 20 μm is 300 g or more, and if it is within this range, the separator will not be broken due to abnormal protrusions or foreign matter of the electrode active material when used as a separator for a lithium ion secondary battery. Overchargeability is improved. From the viewpoint of strength during battery assembly, the weight is preferably 400 g or more, and more preferably 500 g or more.
In addition, although there is no limitation in particular as an upper limit, it is 1000 g or less, for example. Here, the puncture strength in terms of the film thickness of 20 μm is puncture using a handy compression tester “KES-G5” (trade name, manufactured by Kato Tech) under the conditions of a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / sec. The value obtained by conducting a test. Examples of the method for adjusting the puncture strength to the above range include a method of changing the orientation state of the microporous film. Specifically, in either case of the wet method or the dry method, the draw ratio or temperature The method of adjusting is mentioned. Another method is to use a high-strength resin as the raw material resin itself. For example, in the case of polyolefin, mixing ultrahigh molecular weight polyethylene having a weight average molecular weight of 500,000 or more, preferably 1,000,000 or more. Etc.

  In the laminated microporous membrane of the present embodiment, the average pore diameter D of all layers is 0.02 μm or more and 0.1 μm or less. When the average pore diameter D is within the above range, it is preferable from the viewpoint of achieving both ion permeability and fine short-circuit prevention. The average pore diameter D is preferably 0.03 [mu] m or more from the viewpoint of ion permeability, and preferably 0.09 [mu] m or less from the viewpoint of prevention of slight short circuit. Here, the average pore diameter of all layers can be determined by the measurement method described in Examples described later. In addition, as a method for adjusting the average pore diameter to the above range, in the case of a wet method, a method of adjusting a combination of a resin and a plasticizer and using a resin that has a larger dispersion diameter of the plasticizer, Examples include a method using a poor poor solvent, and examples of the film forming conditions include a method of adjusting a stretching ratio in the stretching step.

  Further, the laminated microporous membrane of the present embodiment has a liquid absorbency index (H30) of 7 mm or more. As a result, the cycle performance of the battery is improved and the electrolyte solution absorbability at the time of battery production is also increased, which contributes to a significant increase in production speed. The liquid absorption index is more preferably 8 mm or more, and further preferably 10 mm or more. Here, the liquid absorbency index can be obtained by the measurement method described in Examples described later. Moreover, as a method of adjusting the liquid absorbency index to the above range, a method of adjusting the F value described later and the like can be mentioned.

  The laminated microporous membrane of the present embodiment preferably has a porosity in the range of 30 to 70%. If the porosity is within the above range, the balance between membrane strength and permeability tends to be better. In particular, the balance of self-discharge suppression effect, prevention of fine short-circuiting, and cycle characteristics when used as a separator for a secondary battery or the like is good. Further, the porosity is preferably 30 to 59% from the viewpoint of film strength, and preferably 30 to 49% from the viewpoint of suppressing fine short-circuiting. Here, the porosity can be obtained by the measurement method described in Examples described later. Examples of the method for adjusting the porosity to the above range include a method for adjusting the mixing ratio of the raw material resin and the plasticizer in the case of the wet method, and a method for adjusting the draw ratio in the case of the dry method.

Further, the laminated microporous membrane of the present embodiment is composed of at least two layers of (A layer) and (B layer) different from (A layer), and the porosity (porosity A) of (A layer) Is preferably 35 to 80%, the porosity (porosity B) of (B layer) is within the range of 30 to 60%, and (porosity A)-(porosity B) is within the range of 3 to 40%. Here, (A layer) is a liquid absorption layer mainly responsible for liquid absorption, and (B layer) mainly functions as a main layer responsible for basic performance such as strength and permeability.
Note that “different” in the case of “different from (A layer) (B layer)” may be a difference in raw materials, or a difference in physical properties such as porosity and air permeability.

  When the porosity A is within the above range, the liquid absorbency is good, and the possibility that the (A layer) will be poor during the formation of the laminated microporous film is reduced, and the film can be favorably formed. It is in. The porosity A is more preferably in the range of 40 to 70%, and in this range, the film forming property tends to be further improved. The porosity A is more preferably in the range of 45% to 65%. When the porosity is within this range, the strength of the (A layer) alone is sufficiently added, so that high-speed film formation is possible and handling properties are also improved. Tend to improve. Here, the handling property indicates the ease of handling when the operator installs the laminated microporous membrane on a predetermined pass line of the battery winding machine. It refers to the independence of the waist and membranes related to the elastic modulus.

  Moreover, when the porosity B is in the above range, the balance between the permeability and strength of the main layer (B layer) is good, and it is good for existing battery winding machines when used for secondary battery separators. Tend to be produced. The gas efficiency B is more preferably in the range of 30 to 50%, and in this range, a microporous membrane having a better balance between cycleability and strength tends to be obtained.

  Further, when the difference of (porosity A) − (porosity B) is in the range of 3 to 40%, the balance between liquid absorption and film homogeneity tends to be good. (Porosity A) − (Porosity B) is preferably 5% or more, more preferably 8% or more, from the viewpoint of liquid absorbency. When (porosity A) − (porosity B) is in this range, the porosity of the liquid absorbing layer (A layer) is sufficiently large and the liquid absorbing property is increased, and the main layer (B layer) The strength tends to be sufficient. Further, (porosity A) − (porosity B) is preferably 30% or less, more preferably 25% or less from the viewpoint of the homogeneity of the film. When (porosity A)-(porosity B) is within this range, in the coextrusion which is a preferred embodiment in the method for producing a laminated microporous film to be described later, in the die of (A layer) (B layer) As a result, there is a tendency that good merging can be performed without causing a defect phenomenon that affects the homogeneity of the film, such as inter-layer disturbance.

The laminated microporous membrane of the present embodiment has an F defined by the following formula (1) when the curvature is T (−), the average pore diameter is D (μm), and the porosity is P (%). It is preferable that the value satisfies 0.25 ≦ F ≦ 0.9.
F = T / (D * P) (1)
When the F value is within the above range, the liquid absorbency is improved and the liquid absorbency index (H30) tends to be easily adjusted to 7 mm or more. As apparent from the definition, the F value is a parameter related to the permeation resistance of the membrane. The F value is preferably 0.9 or less, more preferably 0.8 or less, from the viewpoint of liquid absorbency and ion permeability. On the other hand, if the F value is too small, the strength of the film tends to decrease, so it is preferably 0.25 or more, more preferably 0.4 or more.

  The laminated microporous membrane of the present embodiment may be subjected to a hydrophilization treatment or a corona treatment that increases the affinity for the electrolytic solution. However, if the F value is within the above range, the absorption can be performed without the hydrophilization treatment. There is a tendency to achieve improved liquidity.

  In order to adjust the F value within the preferable range described above, the average pore diameter, porosity, and curvature of the laminated microporous membrane are adjusted by a known method, and of course, the above-mentioned prescribed porosity (A layer) ) And (B layer) can also be used for adjustment.

  Furthermore, the average pore diameter DA of (A layer) is preferably 0.03 μm or more, more preferably 0.05 μm or more, from the viewpoint of liquid absorbency. The average pore diameter DA is preferably 0.15 μm or less, more preferably 0.09 μm or less from the viewpoint of strength. Similarly, the average pore diameter DB of the B layer is preferably 0.03 to 0.1 μm, and more preferably 0.04 to 0.09 μm from the viewpoints of ion permeability, fine short-circuit preventing property, and strength.

  The range of the curvature of the laminated microporous membrane of the present embodiment is preferably 1.0 to 3.0, and within this range, the above-described F value can be easily balanced. The range of the curvature is more preferably 1.5 to 2.5. Here, the curvature can be obtained by the measurement method described in Examples described later. In addition, as a method for adjusting the curvature to the above range, in the case of a wet method, a method for adjusting stretching conditions such as a stretching ratio and a stretching temperature, a method for appropriately selecting a combination of a resin and a plasticizer, and the like can be given. It is done.

  In the laminated microporous membrane of the present embodiment, the thickness of (B layer) is preferably 35% or more with respect to the entire laminated microporous membrane. When the thickness of (B layer) is in the above range, the balance between the strength and permeability of the entire laminated microporous membrane tends to be good. The thickness of (B layer) is more preferably 50% or more, and still more preferably 60% or more. The upper limit is not particularly limited, but is preferably less than 100%.

  Examples of the layer configuration of the laminated microporous membrane of the present embodiment include, but are not particularly limited to, two layers, three layers, or more multilayers. For example, in the case of three layers, (B layer) / (A layer) / (B layer), (A layer) / (B layer) / (A layer), etc. are preferable. The laminated microporous film may have a multilayer structure having more than three layers.

  Each layer of the laminated microporous membrane of the present embodiment is usually made of a resin composition containing a thermoplastic resin. The resin composition preferably contains a polyolefin as a main component from the viewpoints of moldability and solvent resistance to the electrolytic solution. Examples of the polyolefin include polyethylene and polypropylene. The term “main component” means that the proportion of the specific component in the matrix component containing the specific component is preferably 50% by mass or more, more preferably 80% by mass or more, and 100% by mass. It means you may.

  Examples of the polyethylene include high density polyethylene, ultra high molecular weight polyethylene, linear low density polyethylene, high pressure method low density polyethylene, and mixtures thereof. Among these, from the viewpoint of reducing thermal shrinkage when used as a separator, it is preferable to use linear high density polyethylene, ultrahigh molecular weight polyethylene, or a mixture thereof by ion polymerization. The ultra-high molecular weight polyethylene here refers to those having a viscosity average molecular weight of 500,000 or more. The proportion of ultra high molecular weight polyethylene in the total polyethylene is preferably 5 to 50% by mass, and more preferably 9 to 40% by mass from the viewpoint of dispersibility.

  The viscosity average molecular weight (Mv) of polyethylene (when multiple types of polyethylene are used, the total viscosity average molecular weight) is preferably 200,000 or more from the viewpoint of improving the strength of the laminated microporous membrane, Preferably it is 300,000 or more. The upper limit of the viscosity average molecular weight (Mv) is preferably 10 million or less, more preferably 5 million or less, from the viewpoints of extrusion moldability and stretchability.

  The molecular weight distribution (Mw / Mn) of polyethylene is preferable from the viewpoint of improving the kneadability when mixing and kneading inorganic particles, etc., and suppressing the generation of granular defects in which the inorganic particles are secondarily aggregated. Is 4 or more, more preferably 6 or more.

  Examples of polypropylene include propylene homopolymers such as isotactic polypropylene (IPP), syndiotactic polypropylene, and atactic polypropylene, and propylene and comonomer such as ethylene, butene, and α-olefin having 5 or more carbon atoms. Examples thereof include a random copolymer (RPP), a block copolymer (BPP), and a terpolymer obtained by copolymerization. Among them, IPP having high crystallinity is preferable when it is desired to have heat resistance, and RPP and BPP that are easily stretched are preferable for the purpose of imparting strength.

  The viscosity average molecular weight (Mv) of polypropylene is easy to melt and knead. As a result, when it is used as a film, it tends to improve fish-eye defects, and is preferably 1 million or less, more preferably 700,000 or less. More preferably, it is 600,000 or less. Further, from the viewpoint of film strength, it is preferably 100,000 or more, more preferably 200,000 or more.

Also preferred is a resin composition obtained by blending 0.5-30% by mass of polypropylene, BPP, or RPP with reduced stereoregularity using a metallocene catalyst or the like with respect to IPP. Thereby, the permeability tends to be improved when a microporous film mainly composed of polypropylene is molded by a wet method described later.
The molecular weight distribution (Mw / Mn) of polypropylene is preferably 5-20.

  In this Embodiment, you may mix an inorganic filler with the resin composition of each layer. Examples of the inorganic filler that can be used include alumina (for example, α-alumina), silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, iron oxide, and other oxide ceramics; silicon nitride, nitriding Nitride ceramics such as titanium and boron nitride; silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, candy Ceramics such as sight, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; glass fiber etc. may be used, and these may be used alone or in combination. Also good. As a ratio for which an inorganic filler accounts in the resin composition of each layer, Preferably it is 10-90 mass%, More preferably, it is 20-55 mass%.

  In the laminated microporous membrane of the present embodiment, various additives such as an antioxidant, a nucleating agent, a dispersion aid, and an antistatic agent may be blended as necessary.

  Examples of the antioxidant include phenolic antioxidants such as “Irganox 1010”, “Irganox 1076”, and “BHT” (both are trademarks, manufactured by Ciba Specialty Chemicals), phosphorus-based and sulfur-based antioxidants. A secondary antioxidant, a hindered amine weathering agent, and the like can be used alone or in accordance with the purpose. In particular, a combination of a phenolic antioxidant and a phosphorus antioxidant is preferably used. Specifically, pentaerythrityl-tetrakis [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate], octadecyl-3- (3,5-di-t-butylhydroxyphenyl) propionate 1,3,5-trimethyl-2,4,6-tris (3,5-di-t-butylhydroxybenzyl) benzene, tris (2,4-di-t-butylphenyl) phosphite, tetrakis (2 , 4-di-t-butylphenyl) -4,4′-biphenylene phosphite is preferred. In addition, 6- [3- (3-t-butyl-4-hydroxy-5-methylphenyl) propoxy] -2,4,8,10-tetra-t-butyldibenzo [d, f] [1,3, 2] Dioxyphosphine and the like are also suitable. The blending amount of the antioxidant is preferably 100 ppm to 10000 ppm with respect to the resin constituting the laminated microporous membrane, and the ratio is preferably 1/3 to 3/1 in the case of a combined phenol / phosphorus system. It is.

  In the present embodiment, when polypropylene is used as the resin constituting the laminated microporous membrane, it is preferable to use a crystal nucleating agent in order to control its crystallinity and control the formation of micropores. It is preferable when producing a porous membrane. Although it does not specifically limit as a kind of nucleating agent, Carboxylic acid metal salts, such as general benzyl sorbitol type | system | group, a phosphoric acid metal salt, aluminum t-butylbenzoate, etc. are mentioned. Specific examples thereof include bis (p-ethylbenzylidene) sorbitol, bis (4-methylbenzylidene) sorbitol, bis (3,4-dimethylbenzylidene) sorbitol, bisbenzylidenesorbitol, and the like. The blending amount of the nucleating agent is preferably 100 ppm or more based on the amount of polypropylene from the viewpoint that crystallization proceeds rapidly and moldability is easy, although it depends on the desired crystallization conditions. From the viewpoint of preventing excessive bleeding due to the nucleating agent, it is preferably 10,000 ppm or less. A more preferable amount of the nucleating agent is 100 to 2,000 ppm with respect to polypropylene. In the production method of microporous membranes using ordinary plasticizers, when liquid paraffin or phthalate ester plasticizers are used, polyethylene tends to exhibit permeability, but polypropylene has smaller pores than polyethylene. The permeability tends to be inferior. As a means for eliminating the permeability of this polypropylene, a method of adjusting the pores to an appropriate size is effective, and the phase separation speed is adjusted by using a nucleating agent, so that an appropriate pore structure can be easily formed.

  In addition, as a dispersion aid for polypropylene and polyethylene, for example, a hydrogenated styrene-butadiene elastomer, an elastomer copolymerized with ethylene and propylene, or the like is used as necessary. Although the compounding quantity of these adjuvants is not specifically limited, Preferably 1-10 mass parts is used with respect to 100 mass parts of total amounts of a polypropylene and polyethylene.

  Antistatic agents include amines such as alkyldiethanolamine and hydroxyalkylethanolamine, amine esters such as stearyl diethanolamine monofatty acid ester, alkyloamides such as lauric acid diethanolamide and stearic acid diethanolamide, glycerin and diethanolamine. Mono-fatty acid esters of glycerin, anionic antistatic agents such as alkylbenzene sulfonic acids, polyoxyethylene alkyl ethers, and the like may be used alone or in combination. The blending amount of these antistatic agents is not particularly limited, but is preferably about 500 to 10,000 ppm with respect to the resin constituting the laminated microporous film.

The method for producing the laminated microporous membrane of the present embodiment may be a known method such as a co-extrusion method, a method of laminating each layer individually and then laminating them, and the above-described (A layer) and (B layer). In the case of a laminated microporous film containing, it is preferably produced by a method comprising the following (Step 1) and (Step 2).
(Step 1) The resin composition A of (A layer) and the resin composition B of (B layer) are extruded in a molten state to form a laminated film in which (A layer) and (B layer) are laminated. Laminated film forming process (Process 2) After the laminated film forming process, the laminated microporous film forming process in which both the (A layer) and (B layer) are microporous. A third layer other than (B layer) may be included.

  In (Step 1), first, the resin composition A of (A layer) and the resin composition B of (B layer) are respectively kneaded. As a method of kneading the resin composition A or B, the raw material resin and, optionally, a plasticizer is preliminarily kneaded with a Henschel mixer, a tumbler mixer, or the like, and the kneaded product is put into an extruder, and in the extruder Examples of the method include introducing a plasticizer at a desired ratio as required, while heating and melting, and further kneading. Such a method tends to be able to obtain a sheet having a better dispersibility of the resin composition, and is preferable from the viewpoint that each layer can be stretched without breaking even at a high magnification. (Step 2) is a step (wet method) in which a plasticizer and an inorganic filler are blended in the resin composition A and the resin composition B, respectively, and a plasticizer and an inorganic filler are extracted after film formation to form a multilayer microporous film. ), A plasticizer and an inorganic filler may be added to the resin composition A and the resin composition B. In the case where (Step 2) is a step of opening using a crystal interface between the resin composition A and the resin composition B or an interface between the inorganic filler and the resin composition (dry method), the resin composition Opening can be carried out without adding a plasticizer to A and the resin composition B.

  When the resin composition A and the resin composition B contain a plasticizer, the resin component concentration in the resin composition A is preferably 25 to 50% by mass, and the resin component concentration in the resin composition B is preferably 30 to 55% by mass. (The resin component concentration may be abbreviated as “PC” (polymer concentration).) The difference (PCB−PCA) between the PC of (B layer) and the PC of (A layer) is preferably 3 to 20 mass. %. Setting the ratio in the above range is preferable from the viewpoint of adjusting the physical properties of the laminated microporous film to the specific range of the present embodiment.

  As the melt extruder used in the above (Step 1), it is preferable to use a twin-screw extruder, which can impart a strong shear, thereby further improving dispersibility. More preferably, L / D of the screw of a twin screw extruder is about 20-70, More preferably, it is 30-60. The screw may be provided with a full flight portion and generally a kneading portion such as a kneading disk or a rotor.

  The die attached to the tip of the extruder is not particularly limited, but a circular die, a T die, or the like is used. When using inorganic particles or a resin composition that tends to deteriorate, those that have taken measures to suppress wear and adhesion due thereto, for example, Teflon (registered trademark) processing, ceramic processing, Nickel processing, molybdenum processing, and hard chrome coating are preferably used.

  It is preferable to use a coextrusion die in the step (1) for obtaining a laminated film. In the case of a T die, a coat hanger type multi-manifold die in which the molten resin is spread in a film shape inside the die and then the layers are joined together Is particularly preferable from the viewpoint of thickness control. However, it is also possible to use a feed block die or a crosshead die. In the case of a circular die, a spiral die, and in the case of a multilayer film having five or more layers, a stack die is preferable from the viewpoint of preventing thermal deterioration, and is particularly preferable when it is desired to increase the adhesive strength between layers.

  In the above (Step 1), the resin composition A and the resin composition B are both extruded in a molten state, and the two are stacked and laminated preferably in the die, but laminated outside the die. May be.

  Here, in (Step 1), the melt viscosity at the extrusion temperature of the resin composition A and the extrusion temperature of the resin composition B when the resin composition A and the resin composition B are both extruded in a molten state. The ratio with the melt viscosity at is preferably 1/5 to 5/1, more preferably 1/2 to 2/1. Setting the ratio in the above range is preferable from the viewpoint of suppressing interface disturbance at the time of resin merging and suppressing uneven thickness.

  The molten resin extruded from the die is introduced into, for example, a casting apparatus, and may be formed by bank molding or bankless molding. The thick original fabric obtained in the casting process can be used as the original fabric before stretching. Thereafter, the film is stretched for imparting high mechanical strength and a balance between physical and longitudinal properties. As the stretching method, biaxial stretching is preferable, and simultaneous biaxial stretching and sequential biaxial stretching are more preferable. The stretching temperature varies depending on the resin composition used, but is generally a temperature in the range between the Vicat softening point and the melting point of the main resin. From the viewpoint of film strength, the draw ratio is preferably in the range of 3 to 200 times, preferably 20 to 60 times in terms of area magnification.

  (Step 2) is a laminated microporous film forming step in which both the (A layer) and (B layer) are microporous after the laminated film forming step, and as described above, is performed by a wet method or a dry method. . The extraction of the plasticizer and the inorganic filler may be performed by immersing the film in an extraction solvent and then sufficiently drying the film. When extracting only the plasticizer, the extraction solvent is preferably a poor solvent for the polyolefin and the inorganic filler and a good solvent for the plasticizer, and the boiling point is preferably lower than the melting point of the polyolefin. Examples of such extraction solvents include chlorinated solvents such as methylene chloride and 1,1,1-trichloroethane; ketones such as methyl ethyl ketone and acetone; hydrofluorocarbons, hydrofluoroethers, cyclic hydrofluorocarbons, perocarbons, perfluoros. Halogenous organic solvents such as ether; ethers such as diethyl ether and tetrahydrofuran; hydrocarbons such as n-hexane and cyclohexane; alcohols such as methanol and isopropyl alcohol. Among the above, methylene chloride is particularly preferable. Two or more of these extraction solvents may be used. The extraction step may be before or after the stretching step, or may be multistage extraction using a plurality of extraction tanks. Examples of the extraction solvent for the inorganic filler include alkaline water.

  Moreover, you may add heat fixation by heating extending | stretching as needed for adjustment of film | membrane physical properties, such as a film thickness and an air permeability, or the heat shrink prevention of a film. Examples of the stretching after extraction of the plasticizer and the inorganic filler include uniaxial stretching, simultaneous biaxial stretching, and sequential biaxial stretching, and simultaneous biaxial stretching and sequential biaxial stretching are preferable. The stretching temperature varies depending on the resin composition used, but is generally a temperature in the range between the Vicat softening point and the melting point of the main resin. The draw ratio is preferably more than 1 time and 10 times or less in terms of area magnification.

  Further, when heat treatment for dimensional stabilization is performed, from the viewpoint of reducing film shrinkage in a high temperature atmosphere, for example, using a biaxial stretching machine, a uniaxial stretching machine, or both, 100 ° C. or more and 150 ° C. or less. Heat treatment can be performed. Preferably, it is carried out by relaxing the magnification and / or stress in the width direction, the length direction, or both directions at a temperature below the melting point of the main resin.

  The laminated microporous film thus obtained may be appropriately subjected to corona treatment or electron beam crosslinking treatment, and may be coated with an inorganic layer or an organic layer.

  The laminated microporous membrane of the present embodiment preferably has a three-dimensional network structure in which pores are three-dimensionally complicated. The three-dimensional network structure means a structure in which the surface has a vein shape and the film structure of a cross section from an arbitrary three-dimensional coordinate axis direction is a sponge shape. Leaf vein is a state in which fibrils form a network structure. These can be confirmed by observing the surface and cross section with a scanning electron microscope. The fibril diameter of the three-dimensional network structure is preferably 0.01 μm or more and 0.3 μm or less, and this can also be observed with a scanning electron microscope.

The laminated microporous membrane in the present embodiment is suitably used as a separator for a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery. In addition, it can be used as various separation membranes.
The various parameters described above are measured according to the measurement methods in the examples described later unless otherwise specified.

  Next, the present embodiment will be described more specifically with reference to examples and comparative examples. However, the present embodiment is not limited to the following examples unless it exceeds the gist. In addition, the physical property in an Example was measured with the following method. Judgment was performed on necessary items, and ◎, ○, and Δ were accepted, and x was rejected.

(1) The thickness of each layer and the total thickness (μm)
The thickness of each layer constituting the laminate was measured by cross-sectional observation with a general scanning electron microscope (S4100, manufactured by Hitachi, Ltd.).
The total thickness of each layer was taken as the total thickness.

(2) Porosity (%)
(Porosity of all layers)
The weight per unit area W (g / cm ² ) and the average density ρ (g / cm ³ ) of the components (resin and additive) constituting the microporous membrane were calculated from the mass of the 100 mm square microporous membrane sample. The following formula was calculated from the thickness d (cm).
Total layer porosity = (W / (d * ρ)) * 100 (%)
(Porosity of each layer)
A cross-sectional SEM photograph was taken, and the porosity of each layer (porosity C) was determined based on the area ratio between the void portion and the resin portion in each layer. The total sum of the calculated porosity of each layer and the thickness of the layer is defined as porosity D. If the porosity D is different from the total layer porosity described above, the efficiency of each layer is corrected as follows: did.
Porosity of each layer = porosity C * total layer porosity / porosity D

(3) Air permeability (sec / 100cc)
Based on JIS P-8117, it was measured with a Gurley type air permeability meter “G-B2” (trademark, manufactured by Toyo Seiki Seisakusho Co., Ltd.).
In addition, the value in a table | surface is the air permeability of 20 micrometers conversion calculated by the proportional calculation on the basis of total thickness.

(4) Puncture strength (g)
Using a handy compression tester “KES-G5” (trade name, manufactured by Kato Tech Co., Ltd.), the puncture test was performed under the conditions of a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / sec.
In addition, the value in a table | surface is the puncture strength of 20 micrometers conversion calculated by the proportional calculation on the basis of total thickness.

(5) Average pore diameter (μm), curvature The fluid inside the capillary is known to follow a Knudsen flow when the mean free path of the fluid is larger than the capillary pore diameter, and a Poiseuille flow when it is small. Therefore, it is assumed that the air flow in the measurement of the permeability of the microporous membrane follows the Knudsen flow, and the water flow in the measurement of the permeability of the microporous membrane follows the Poiseuille flow.
In this case, the average pore diameter D (μm) and the curvature T (dimensionless) are the air permeation rate constant R _gas (m ³ / (m ² · sec · Pa)) and the water permeation rate constant R _liq (m ³ / (m ² · sec · Pa)), air molecular velocity ν (m / sec), water viscosity η (Pa · sec), standard pressure P _s (= 101325 Pa), porosity ε (%), membrane From the thickness L (μm), it can be obtained using the following equation.
D = 2ν × (R _liq / R _gas ) × (16η / 3P _s ) × 10 ⁶
T = (D × (ε / 100) × ν / (3L × P _s × R _gas )) ^1/2
Here, R _gas is obtained from the air permeability (sec) using the following equation.
R _gas = 0.0001 / (air permeability × (6.424 × 10 ⁻⁴ ) × (0.01276 × 101325))
R _liq is determined from the water permeability (cm ³ / (cm ² · sec · Pa)) using the following equation.
R _liq = water permeability / 100
In addition, water permeability is calculated | required as follows. A microporous membrane previously immersed in alcohol is set in a stainless steel permeation cell having a diameter of 41 mm, and after the alcohol in the membrane is washed with water, water is allowed to permeate at a differential pressure of about 50000 Pa, and 120 seconds have elapsed. The water permeation amount per unit time, unit pressure, and unit area was calculated from the water permeation amount (cm ³ ) at the time, and this was taken as water permeability.
Ν is a gas constant R (= 8.314), an absolute temperature T (K), a circumference ratio π, and an average molecular weight M of air (= 2.896 × 10 ⁻² kg / mol), using the following formula. Desired.
ν = ((8R × T) / (π × M)) ^1/2

(6) Liquid absorbency index H30 (mm) and liquid absorbency Laminated microporous membrane is sampled in a strip shape of MD100mm and TD10mm, and the upper 5mm of the strip is fixed to a stand etc. so that the MD direction is vertical. Left to stand. At this time, the lower 95 mm of the strip was suspended vertically and floated in the air. Under the condition of 23 ° C., the following electrolyte solution reagent was immersed from the lower end of the strip to a portion of 10 mm, and the liquid height at which the reagent rose after 30 minutes was measured based on the immersion time. The liquid height can be easily determined by changing the color of the porous film from white to translucent. The determination was performed as follows. The immersion in the reagent was performed in a glass bottle to avoid the influence of wind and the like.
Electrolytic solution simulation reagent: Mixed at a ratio of ethylene carbonate / propylene carbonate / dimethyl ether = 3/1/6.
A: Increased by 9 mm or more.
A: Less than 9 mm and increased by 7 mm or more.
X: It rose only less than 7 mm.

(7) Overcharge A 50mm * 50mm film sample is sandwiched between Φ35mm electrodes with a clean surface and a voltage is applied to the electrodes to increase the voltage value when a 0.5mA current flows and sparks. did. This measurement was measured at least 15 times in the plane of the sample film, and the average value was recorded. The average value is 1.8 KV or more, ◎, less than 1.8 KV, 1.0 KV or more, ◯, less than 1.0 KV, 0.8 KV or more, Δ, and less than 0.8 KV, ×.

(8) Cyclicity After producing an electrode and an electrolytic solution as shown below, an evaluation battery was produced using the electrode and an electrolytic solution, and its cycle characteristics were evaluated.
(I) Production of positive electrode 100 parts by mass of lithium cobalt composite oxide LiCoO ₂ as a positive electrode active material, 2.5 parts by mass of flake graphite and acetylene black as a conductive agent, and polyvinylidene fluoride (PVDF) 3 as a binder A slurry was prepared by dispersing 5 parts by mass in N-methylpyrrolidone (NMP). This slurry was applied to both surfaces of a 20 μm thick aluminum foil serving as a positive electrode current collector with a die coater, dried at 130 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material coating amount of the positive electrode was 250 g / m ² and the active material bulk density was 3.00 g / cm ³ . This was cut in accordance with the battery width to form a strip.
(Ii) Production of negative electrode As a negative electrode active material, 90 parts by mass of graphitized mesophase pitch carbon fiber (MCF) and 10 parts by mass of flake graphite, 1.4 parts by mass of ammonium salt of carboxymethyl cellulose as a binder and styrene-butadiene A slurry was prepared by dispersing 1.8 parts by mass of polymer latex in purified water. This slurry was applied to both sides of a 12 μm thick copper foil serving as a negative electrode current collector with a die coater, dried at 120 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material application amount of the negative electrode was 106 g / m ² , and the active material bulk density was 1.35 g / cm ³ . This was cut in accordance with the battery width to form a strip.
(Iii) Preparation of Nonaqueous Electrolyte Solution It was prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio) to a concentration of 1.0 mol / liter.
(Iv) Production of battery for evaluation Electrode plate laminate by winding microporous membrane separator to be evaluated, strip-shaped positive electrode, and strip-shaped negative electrode in a spiral shape by overlapping the strip-shaped negative electrode, separator, strip-shaped positive electrode and separator in this order. Was made. After pressing this electrode plate laminate into a flat shape, it is housed in an aluminum container, the aluminum lead is led out from the positive electrode current collector and the nickel lead is led out from the negative electrode current collector to the bottom of the container. The battery wound body was fabricated by welding to a battery.
(V) Cycle characteristics The above-described non-aqueous electrolyte solution was injected into the battery roll for evaluation produced as described above and sealed to produce a lithium ion battery.
The battery was charged to a charge end voltage of 4.2 V with a charge current of 1 A under a temperature of 25 ° C., and discharged to a discharge end voltage of 3 V with a charge current of 1 A. Charging / discharging was repeated with this as one cycle, and the capacity retention after 500 cycles with respect to the initial capacity was expressed as cycle characteristics.

(9) Viscosity average molecular weight Mv
Based on ASTM-D4020, the intrinsic viscosity [η] at 135 ° C. in a decalin solvent was determined. Mv of polyethylene was calculated by the following formula.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67
For polypropylene, Mv was calculated by the following formula.
[Η] = 1.10 × 10 ⁻⁴ Mv ^0.80

[Example 1]
A production example of a laminated microporous membrane having a three-layer configuration of (A layer) / (B layer) / (A layer) is shown. The raw material resins used in the examples are shown in Table 1. In Table 1, HDPE represents high density polyethylene having a viscosity average molecular weight (MV) of 300,000, UHMWPE represents ultrahigh molecular weight polyethylene having MV of 1,000,000, and PP represents isotactic homopolypropylene having MV of 500,000.
The raw material resin (resin component) was blended at the blending ratio (parts by mass) shown in Table 1. 0.5 parts by mass of bis (P-ethylbenzylidene) sorbitol as a nucleating agent and tetrakis- [methylene- (3 ′, 5′-di-t-butyl-4) as an antioxidant with respect to 100 parts by mass of the raw material resin '-Hydroxyphenyl) propionate] 0.3 parts by mass of methane and 10 parts by mass of liquid paraffin (kinematic viscosity at 37.8 ° C., density of 868 kg / m ³ ) as a plasticizer were blended. These raw materials were stirred with a Henschel mixer to prepare the raw materials.
Next, the raw material of the (A) layer and the raw material of the (B) layer were respectively charged into separate twin screw extruders (caliber 44 mm, L / D = 49). Liquid paraffin was injected into the middle part of the cylinders of both extruders so as to be 68% by mass in the (A) layer and 62% by mass in the (B) layer.
The amount of extrusion of both surface layers ((A) layer) and intermediate layer ((B) layer) is adjusted so that the thickness ratio of the (A) layer and the (B) layer becomes the thickness ratio shown in Table 2 at the die outlet. Set to.
A 250 mesh screen was disposed between the extruder and the die. The die used was a T-die capable of multi-manifold coextrusion. In the die, the surface layer was divided into approximately equal parts and stacked on both sides of the intermediate layer. The melted film raw material coming out of the die was cooled and solidified with a cast roll.
The sheet was stretched by a simultaneous biaxial stretching machine at 120 ° C. at an area magnification of 45 times, then immersed in methylene chloride, extracted after removing liquid paraffin, dried, and further subjected to 125 ° C. by a tenter stretching machine. The film was stretched 1.5 times in the transverse direction, and the stretched sheet was relaxed in the width direction by 7% at 130 ° C. and heat-treated. As a result, a microporous film of 18 μm (surface layer / intermediate layer / surface layer = 3 μm / 12 μm / 3 μm) was obtained in all layers having the same composition in the two surface layers and different two-layer three-layer structures in the intermediate layer. Table 2 shows the physical properties of the obtained microporous membrane.

[Examples 2-11 and 13-15, Comparative Examples 1-3]
A laminated microporous membrane was molded and evaluated in the same manner as in Example 1 while changing each layer configuration and structural factors described in Tables 2 and 3. Silica (R972 manufactured by Aerosil Co., Ltd.) was added as an inorganic filler to the raw material resins of compositions 6 and 7 used in Examples 14 and 15. Moreover, although the quantity of the added liquid paraffin differed in each case, it was in the range of 40 mass%-80 mass%. The simultaneous biaxial stretching machine was stretched at an area magnification of 45 times within a range of 115 ° C to 130 ° C. Liquid paraffin is extracted and dried, then dried, and further stretched 1.1 to 2.0 times in the transverse direction at 120 to 130 ° C. by a tenter stretching machine, and this stretched sheet is relaxed in the width direction by 7% at 130 ° C. The heat treatment was performed. The physical properties of the obtained microporous membrane are shown in Tables 2 and 3.

[Example 12]
(Layer A) was disposed in the intermediate layer, (Layer B) was disposed in the surface layer, and a laminated microporous membrane was obtained in the same manner as in Example 1.

[Comparative Example 4]
This is an example of film formation by the stretch opening method, and (A layer) and (B layer) were separately extruded and then stacked. Specifically, the layer composition shown in Table 3 (Layer A) (Layer B) was extruded separately without the addition of a plasticizer, and a take-up raw fabric was created with a draft ratio of 200. It heat-processed and integrated so that it might become the arrangement | positioning of Table 3 (A layer) (B layer). Then, it formed into a film by cold extending | stretching and hot extending | stretching. The evaluation results are shown in Table 3.

  ADVANTAGE OF THE INVENTION According to this invention, when used as a separator, the multilayer microporous film which can implement | achieve the secondary battery which has favorable safety | security and favorable cycling property can be provided. Further, the application of the present invention can provide a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery (LIB) having excellent oxidation resistance under a high voltage.

Claims (7)

A laminated microporous membrane having an air permeability in terms of membrane thickness of 20 μm of 300 seconds / 100 cc or less, a puncture strength in terms of membrane thickness of 20 μm of 300 g or more, and an average pore diameter D of all layers of 0.02 μm or more and 0.1 μm or less. Te, absorbent index (H30) is Ri der more than 7mm,
When the curvature of the laminated microporous membrane is T (−), the average pore diameter is D (μm), and the porosity is P (%), the F value defined by the following formula (1) is 0.25. A laminated microporous membrane satisfying ≦ F ≦ 0.9 .
F = T / (D * P) (1)   It consists of at least two layers of (B layer) different from (A layer) and (A layer), and the porosity (porosity A) of the (A layer) is 35 to 80%. The laminated microporous membrane according to claim 1, wherein the porosity (porosity B) is in the range of 30 to 60% and (porosity A)-(porosity B) in the range of 3 to 40%.   The laminated microporous structure according to claim 2, wherein the porosity (porosity A) of the (A layer) is 40 to 70%, and the porosity (porosity B) of the (B layer) is 30 to 50%. film.   The laminated microporous membrane according to any one of claims 1 to 3, mainly comprising a polyolefin composition. The laminated microporous membrane according to any one of claims 2 to 4 , wherein the thickness of the (B layer) is 35% or more with respect to the entire laminated microporous membrane. The laminated microporous membrane according to any one of claims 2 to 5 , wherein the (A layer) and the (B layer) are laminated by a coextrusion method. The separator for nonaqueous electrolyte secondary batteries which consists of a lamination | stacking microporous film of any one of Claims 1-6 .