JP5512461B2 - Microporous film and battery separator - Google Patents

Description

  The present invention relates to a microporous film and a battery separator.

Microporous films, especially polyolefin microporous films, are used in microfiltration membranes, battery separators, capacitor separators, fuel cell materials, etc., and are particularly suitable for use as lithium ion secondary battery separators. ing. In recent years, lithium ion secondary batteries have been used for small electronic devices such as mobile phones and notebook personal computers, and are also being applied to in-vehicle applications such as hybrid electric vehicles.
In in-vehicle applications, durability can be cited as a characteristic particularly required for lithium ion secondary batteries. Durability is battery performance such as how much the initial electric capacity can be maintained, for example, when a battery is used for a long period of time, and is generally evaluated as a cycle characteristic that repeats a charge / discharge cycle in general. .
Lithium-ion secondary batteries have an electrical capacity that decreases when charging and discharging cycles are repeated due to various factors. How to prevent this decrease in electrical capacity is an important issue in improving battery durability.
As one of the factors that decrease the electric capacity, it has been pointed out that the battery separator is deformed and the electrolyte dies out due to the expansion and contraction of the electrode due to charging and discharging, but the conventional battery separator is used for a longer period of time required for in-vehicle use. Under the conditions, the decrease in electric capacity was particularly serious, which was a problem.
Patent Document 1 discloses a technical example for improving the cycle characteristics of a battery, focusing on the compression deformation rate of the battery separator.

JP 2000-212322 A

However, although the microporous film described in Patent Document 1 has a compressive deformation rate of a certain value or more, the compressive deformation rate alone cannot cope with both the expansion and contraction behavior of the electrode, and it can be used for a longer period of time. There is a problem that the durability under the conditions cannot be satisfied.
This invention is made | formed in view of the said situation, and when it uses as a battery separator, it makes it a subject to provide the microporous film which can improve the durability of a battery significantly.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have dramatically improved battery cycle characteristics when a microporous film having specific compression characteristics and porosity is used as a battery separator. As a result, the present invention has been completed.

That is, the present invention is as follows.
[1]
A microporous film containing a thermoplastic resin, wherein the compressive deformation stress is 10 to 40 MPa, the compression deformation recovery rate is 35 to 90%, and the porosity is 30 to 60%.
[2]
The microporous film according to the above [1], wherein the thermoplastic resin is a thermoplastic resin composition containing a polyolefin resin.
[3]
The said thermoplastic resin composition is a microporous film of the said [2] description which contains 5-90 mass parts of (b) polyphenylene ether resin with respect to 100 mass parts of (a) polypropylene resin.
[4]
The microporous film according to any one of the above [1] to [3], which has a laminated structure including at least two or more different microporous film layers.
[5]
The battery separator containing the microporous film in any one of said [1]-[4].

  The microporous film of the present invention can drastically improve the cycle characteristics of the battery when used as a battery separator.

Hereinafter, a mode for carrying out the present invention (hereinafter referred to as “the present embodiment”) will be described in detail. In addition, this invention is not limited to this following Embodiment, It can implement in various changes within the range of the summary.
The microporous film of the present embodiment is a microporous film containing a thermoplastic resin, and has a compressive deformation stress of 10 to 40 MPa, a compressive deformation recovery rate of 35 to 90%, and a porosity of 30 to 60%. It has been adjusted.

[Thermoplastic resin]
The microporous film of the present embodiment contains a thermoplastic resin. The thermoplastic resin is not particularly limited, but homopolymers and copolymers of olefin monomers such as ethylene, propylene, 1-butene, pentene, 1-hexene, heptene, octene, 4-methyl-1-pentene, Alternatively, an olefin resin (eg, polyethylene (PE), polypropylene (PP), ethylene / propylene copolymer, ethylene / 1-butene copolymer, propylene / 1-butene copolymer, etc.) that is a blend is used. be able to.
The thermoplastic resin is preferably a thermoplastic resin composition containing a polyolefin resin from the viewpoint of stability during battery use.

  Furthermore, the thermoplastic resin composition in the present embodiment preferably contains 5 to 90 parts by mass of (b) polyphenylene ether resin, more preferably 10 to 80 parts per 100 parts by mass of (a) polypropylene resin. It is preferable that the thermoplastic resin composition contains 20 parts by mass, more preferably 20 to 65 parts by mass. Setting the content ratio of the polyphenylene ether resin within the above range is preferable from the viewpoint of stretchability of the obtained microporous film.

Examples of (a) polypropylene resin (hereinafter sometimes abbreviated as “PP”) include homopolymers, random copolymers, and block copolymers. From the viewpoints of physical properties and applications of the resulting microporous film, homopolymers can be used. It is preferable that
The polymerization catalyst used for obtaining the polypropylene resin is not particularly limited, and examples thereof include a Ziegler-Natta catalyst and a metallocene catalyst. The stereoregularity of the polypropylene resin is not particularly limited, and isotactic or syndiotactic polypropylene resin is used.
A polypropylene resin is used individually by 1 type or in mixture of 2 or more types. Polypropylene resin may have any crystallinity and melting point, and is a blend of two types of polypropylene resins with different properties depending on the properties and applications of the resulting microporous film. It may be.

  The melt flow rate (hereinafter also referred to as “MFR”) of the polypropylene resin in the present embodiment (measured under a load of 230 ° C. and 2.16 kg according to ASTM D1238. The same shall apply hereinafter) is preferably 0.00. It is 1 to 100 g / 10 minutes, more preferably 0.1 to 80 g / 10 minutes. When the MFR of the polypropylene resin is 0.1 g / 10 min or more, the microporous film tends to be easily processed, and when it is 100 g / 10 min or less, the strength of the microporous film tends to be improved. is there.

  In addition to the above-described polypropylene resin, the polypropylene resin in the present embodiment includes JP-A-44-15422, JP-A-52-30545, JP-A-6-313078, and JP-A-2006-83294. It may be a known modified polypropylene resin as described in 1. Furthermore, the polypropylene resin in the present embodiment may be a mixture of any proportion of the above-described polypropylene resin and the above-described modified polypropylene resin.

  Examples of the (b) polyphenylene ether resin (hereinafter sometimes abbreviated as “PPE”) include those having a repeating unit represented by the following general formula (1).

Here, in formula (1), R ¹ , R ² , R ³ and R ⁴ are each independently a hydrogen atom, a halogen atom, a lower alkyl group having 1 to 7 carbon atoms, a phenyl group, a haloalkyl group, or an aminoalkyl. And a group selected from the group consisting of a group, a hydrocarbonoxy group, and a halohydrocarbonoxy group in which at least two carbon atoms separate a halogen atom and an oxygen atom.

  Specific examples of PPE include poly (2,6-dimethyl-1,4-phenylene ether), poly (2-methyl-6-ethyl-1,4-phenylene ether), and poly (2-methyl-6). -Phenyl-1,4-phenylene ether) and poly (2,6-dichloro-1,4-phenylene ether). Further, as PPE, for example, polyphenylene ether copolymer such as a copolymer of 2,6-dimethylphenol and other phenols (for example, 2,3,6-trimethylphenol and 2-methyl-6-butylphenol). Examples include coalescence. Among these, from the viewpoint of miscibility with polyolefin resin, in particular, poly (2,6-dimethyl-1,4-phenylene ether), 2,6-dimethylphenol and 2,3,6-trimethylphenol. And poly (2,6-dimethyl-1,4-phenylene ether) are more preferable.

  Moreover, it does not specifically limit regarding the manufacturing method of PPE, If it is PPE obtained by a well-known manufacturing method, it can be used by this Embodiment.

  As the PPE in the present embodiment, the above-mentioned PPE and a styrene monomer and / or an α, β-unsaturated carboxylic acid or a derivative thereof (for example, an ester compound or an acid anhydride compound) are used in the presence of a radical generator. Alternatively, a known modified PPE obtained by reacting at a temperature of 80 to 350 ° C. in a molten state, a solution state or a slurry state in the absence can also be used. Furthermore, the mixture of the above-mentioned PPE and the above-mentioned modified PPE in any ratio may be used.

  The reduced viscosity of PPE in the present embodiment is preferably 0.15 to 2.50, more preferably 0.30 to 2.00. When the reduced viscosity of PPE is in the above range, the miscibility with the olefin resin tends to be good.

  As PPE in this Embodiment, what added polystyrene, high impact polystyrene, syndiotactic polystyrene, and / or the rubber reinforced syndiotactic polystyrene etc. to PPE other than the above-mentioned PPE is used suitably. .

In addition to the polypropylene resin and the polyphenylene ether resin, the thermoplastic resin composition preferably further contains an admixture. The admixture is not particularly limited, but a hydrogenated block copolymer is preferable. This hydrogenated block copolymer is a block copolymer comprising at least one polymer block mainly composed of a structural unit of vinyl aromatic compound and at least one polymer block mainly composed of a structural unit of a conjugated diene compound. It is a block copolymer obtained by hydrogenating a polymer. For example, a hydrogenated product of a styrene-butadiene block copolymer can be used. The content of the admixture is preferably 1 to 20% by mass, more preferably 1 to 15% by mass, based on the entire thermoplastic resin composition.
The admixture such as the styrene-butadiene block copolymer can be produced by a known method, and the amount of bound styrene, (number) average molecular weight, 1,2-vinyl bond amount, 3,4-vinyl bond amount. The physical properties such as can be adjusted as appropriate.

  In addition to the above components, the thermoplastic resin composition as a raw material in the present embodiment has additional components such as a filler as required, as long as the effects exhibited by the present invention are not impaired. (Inorganic particles such as calcium carbonate, barium sulfate, alumina, titanium oxide, talc), olefin elastomers, antioxidants, metal deactivators, heat stabilizers, flame retardants (organic phosphate ester compounds, ammonium polyphosphate) Flame retardants such as organic compounds, aromatic halogen flame retardants, silicone flame retardants), fluorine polymers, plasticizers (low molecular weight polyethylene, epoxidized soybean oil, polyethylene glycol, fatty acid esters, etc.), antimony trioxide, etc. Agent, weather resistance (light) improver, slip agent, inorganic or organic filler and reinforcing material (polyacrylonitrile fiber, carbon black, Calcium acid, conductive metal fibers, conductive carbon black, etc.), various colorants, mold release agents and the like may be added. The content of the additional component is preferably 0.01 to 5% by mass, more preferably 0.01 to 1% by mass, based on the entire thermoplastic resin composition.

[Production method of microporous film]
Although it does not specifically limit as a manufacturing method of the microporous film of this Embodiment, For example, it can manufacture by the method containing each process of the following (A)-(D).
(A) A step of taking a molten thermoplastic resin composition at a draw ratio of 10 to 300 to obtain a film,
(B) a step of heat-treating the film obtained in the step (A) at 100 ° C. to 160 ° C.,
(C) a cold stretching step of stretching the film obtained in the step (B) at a temperature of −20 ° C. or higher and lower than 100 ° C.,
(D) A heat stretching step of stretching the film obtained in the step (C) at a temperature of 100 ° C. or higher and lower than 150 ° C.
Hereinafter, (A) a sheet forming step, (B) a heat treatment step, (C) a cold drawing step, and (D) a heat drawing step, respectively.

(A) As a method of forming the thermoplastic resin composition into a sheet (film) in the sheet forming step, a sheet forming method such as T-die extrusion molding, inflation molding, calender molding, Skyf method or the like can be employed. Among these, T-die extrusion molding is preferable from the viewpoints of physical properties and applications required for the microporous film obtained in the present embodiment.
On the other hand, in the (C) cold stretching step and (D) hot stretching step, a method of stretching and relaxing in a uniaxial direction and / or a biaxial direction in one step or two or more steps by a roll, a tenter, an autograph or the like. Can be adopted. Among these, from the viewpoints of physical properties and applications required for the microporous film obtained in the present embodiment, two or more stages of uniaxial stretching with a roll are preferable.
Below, although an example is given and demonstrated about the manufacturing method of the microporous film of this Embodiment, this Embodiment is not limited to this example.
[(A) Sheet forming step]
[Step (A)]
Step (A) is a step of obtaining a molten film of the thermoplastic resin composition at a draw ratio of 10 to 300.
In the sheet molding step, a thermoplastic resin composition containing the above-mentioned (a) polypropylene resin, (b) polyphenylene ether resin, and (c) an admixture as necessary is supplied to an extruder, and is 200 ° C to 350 ° C. The melt kneading is preferably performed at a temperature of 260 ° C to 320 ° C. By pelletizing the kneaded material thus obtained, pellets of a thermoplastic resin composition in which a polyphenylene ether resin is dispersed in a polypropylene resin are obtained. Next, the obtained pellets are supplied to an extruder and extruded from a T-die at a temperature of 200 ° C. to 350 ° C., preferably 260 ° C. to 320 ° C., and the obtained film is 20 to 150 ° C., preferably It casts on a 50 to 120 degreeC roll, and solidifies by cooling.
Or the above-mentioned (a) polypropylene resin, (b) polyphenylene ether resin, and the resin composition containing (c) admixture as needed is supplied to an extruder, 200 to 350 degreeC, Preferably it is 260 degreeC. Melt-knead at a temperature of ~ 320 ° C. Thereby, a kneaded material in which the polyphenylene ether resin is dispersed in the polypropylene resin is obtained. The kneaded product is directly extruded into a film form from a T-shaped die without being once formed into a pellet, and the obtained film is cast on a roll and cooled and solidified. In the present embodiment, the roll temperature is particularly preferably 100 to 150 ° C, and more preferably 120 to 150 ° C.

  In the sheet forming step, the molten thermoplastic resin composition is discharged from a T-shaped die to form a sheet-like melt, and this sheet-like melt is drawn at a draw ratio of 10 to 300, preferably 50 to 250, more preferably. It is formed into a film-like molded body by taking it out at 130 to 200. Setting the draw ratio in the above range is a viewpoint of forming two kinds of micropores having different micropores formed in the sea part which is a matrix region and a microhole due to separation of the sea part / island part interface in the stretching process described later. To preferred. By setting the draw ratio to 10 or more, micropores are easily formed in the sea, and by setting the draw ratio to 300 or less, a stable film-like molded product tends to be formed.

[(B) Heat treatment step]
Step (B) is a step of heat-treating the film obtained in step (A) at 100 ° C to 160 ° C.
(B) In the heat treatment step, the film-like molded body obtained in the step (A) is held at a temperature of 100 ° C. to 160 ° C. for a predetermined time. The heat treatment method for the film-shaped molded body is not particularly limited. For example, the method of bringing the film into contact with a heated roll, the method of exposing to a heated gas phase before winding the film on the roll, and the film on the core Examples thereof include a method of exposing in a wound heating gas phase or a heating liquid phase, and a method of combining them. In addition, it is preferable to adjust the temperature of the heat treatment to the above range because micropores are easily formed in the sea part which is a matrix region in the stretching step described later. By setting the heat treatment temperature to 100 ° C or higher, the lamellar crystals of the polypropylene resin tend to grow easily, and by setting the heat treatment temperature to 160 ° C or lower, the lamellar crystals of the polypropylene resin tend to exist stably without melting. .

[(C) cold drawing step]
Step (C) is a cold stretching step in which (C) the film obtained in the step (B) is stretched at a temperature of −20 ° C. or higher and lower than 100 ° C.
(C) In the cold drawing step, the film obtained in the heat treatment step (B) is extruded at a temperature of −20 ° C. or higher and lower than 100 ° C., preferably 0 ° C. or higher and lower than 50 ° C. (hereinafter, “ MD direction ”) is preferably 1.1 times to less than 2.0 times, and preferably in the width direction (hereinafter referred to as“ TD direction ”) is preferably 1.0 times to 2.0 times. A first stretching is performed. Thereby, a 1st stretched film is obtained. More preferably, the temperature and the draw ratio in the first stretching are 1.1 to 2.0 times in the MD direction at a temperature of 0 ° C. or more and less than 50 ° C., and uniaxial stretching is preferred. When stretched at −20 ° C. or higher, the risk of film breakage tends to be reduced, and when stretched at less than 100 ° C., the resulting microporous film has a high porosity and low air permeability. There is a tendency.

[(D) Thermal stretching step]
Step (D) is a heat stretching step in which the film obtained in the step (C) is stretched at a temperature of 100 ° C. or higher and lower than 150 ° C.
(D) In the hot stretching step, the first stretched film obtained in the (C) cold stretching step is at a temperature of 100 ° C. or higher and lower than 150 ° C., preferably 110 ° C. or higher and lower than 140 ° C., in the MD direction. Preferably, the second stretching is performed 1.1 times to less than 5.0 times, and preferably 1.0 times to 5.0 times in the TD direction. When stretched at 100 ° C. or higher, the risk of the film breaking tends to be reduced, and when stretched at less than 150 ° C., the porosity of the resulting microporous film tends to be high and the air permeability tends to be low. It is in.

  From the viewpoint of physical properties and applications required for the microporous film of the present embodiment, it is preferable to stretch two or more stages under the conditions described above. If the stretching process is one stage, the stretched film obtained may not satisfy the required physical properties.

[(E) Heat setting step]
In addition to the steps (A) to (D) described above, the method for producing a microporous film of the present embodiment includes (E) a step of performing a heat treatment on the stretched film obtained in the step (D) ( A heat setting step).
This heat setting method includes a method of heat shrinking to such an extent that the length of the microporous film after heat setting is reduced by 3 to 50% with respect to the length of the microporous film before heat setting (hereinafter referred to as this method). The method is also referred to as “relaxation”), and a method of heat setting so that the dimension in the stretching direction does not change. By this heat setting, it is possible to obtain a microporous film having much better dimensional stability, that is, having a small heat shrinkage rate.
The heat setting step is a step in which heat treatment is performed in order to prevent shrinkage in the stretching direction of the microporous film due to residual stress. Preferably, the heat setting step is performed at a temperature of 130 ° C. or higher and lower than 200 ° C. with respect to the stretched film of the step (D). Heat treatment.
The heat setting temperature is 130 ° C. or more and less than 200 ° C., preferably 150 to 180 ° C. When the heat setting temperature is 130 ° C. or higher, the thermal shrinkage tends to decrease, and when it is lower than 200 ° C., the air permeability tends to decrease. Further, from the viewpoint of further improving the balance between the permeability and heat shrinkage of the microporous film, in the heat setting step, it is preferable to perform heat setting at the final stage temperature in the heat stretching step.

[Physical properties of microporous film]

  The compressive deformation stress of the microporous film in the present embodiment is 10 to 40 MPa, more preferably 15 to 38 MPa, from the viewpoint of appropriateness at the time of battery preparation and influence on the electrode. Setting the compressive deformation stress to 40 MPa or less suppresses the resistance of the battery separator against the expansion of the electrode during the charge / discharge cycle of the battery and avoids the collapse of the electrode structure, for example, the bonded structure of the electrode active material and the binder. This tends to suppress battery capacity reduction. Further, setting the compressive deformation stress to 10 MPa or more tends to prevent the battery separator from being crushed when a battery is created or during a charge / discharge cycle, and tends to suppress an increase in the resistance of the battery separator. . This also tends to suppress a decrease in battery capacity.

  Examples of means for adjusting the compressive deformation stress to the above range include controlling the draw ratio and the cooling process in the step (A). Specifically, increasing the compressive deformation stress includes increasing the draw ratio or increasing the temperature of the cooling roll. To decrease the compressive stress, decreasing the draw ratio or decreasing the cooling roll temperature For example, it can be lowered.

  The compression deformation recovery rate of the microporous film in the present embodiment is 35 to 90%, and more preferably 45 to 90%, from the viewpoint of battery separator characteristics and battery characteristics. Setting the compression deformation recovery rate to 90% or less suppresses changes in film properties during long-term use, and setting the compression deformation recovery rate to 35% or more cannot follow the expansion and contraction of the electrode during charge and discharge. In addition, the battery separator is liable to withstand the phenomenon that the battery capacity is reduced due to the electrolyte withering of the battery separator.

  Examples of means for adjusting the compression deformation recovery rate to the above range include adjusting the heat setting conditions in the step (E). Specifically, increasing the compression deformation recovery rate includes increasing the relaxation amount (relaxation rate), and decreasing it reduces the relaxation amount (relaxation rate).

The porosity of the microporous film in the present embodiment is 30 to 60%, preferably 35% to 60%, more preferably 45% to 60%. By setting the porosity of the microporous film to 30% or more, sufficient ion permeability can be ensured when the microporous film is used for battery applications. On the other hand, by setting the porosity to 60% or less, the microporous film can ensure sufficient mechanical strength.
The porosity of the microporous film can be adjusted within the above range by appropriately setting the composition of the thermoplastic resin composition, the stretching temperature, the stretching ratio, and the like. More specifically, in order to increase the porosity, for example, the stretching ratio can be increased, and in order to decrease the porosity, the stretching ratio can be decreased.

The average pore size of the microporous film in the present embodiment is preferably 0.01 to 0.50 μm, more preferably 0.05 to 0.50 μm. Here, as the average pore diameter of the microporous film, a mode diameter obtained by measurement with a mercury porosimeter is employed. By setting the average pore size of the microporous film within the above range, a microporous film excellent in the balance between the electric resistance value and the film strength as a battery separator tends to be obtained.
The average pore diameter of the microporous film can be adjusted within the above range by controlling the stretching ratio, stretching temperature, and the like.

  5-40 micrometers is preferable and, as for the film thickness of the microporous film in this Embodiment, 10-30 micrometers is more preferable. When the film thickness of the microporous film is 5 μm or more, sufficient mechanical strength tends to be obtained, and when the film thickness is 40 μm or less, resistance as a battery separator is reduced, so that sufficient battery performance is obtained. There is a tendency.

The air permeability of the microporous film in the present embodiment is preferably 10 to 5000 seconds / 100 cc, more preferably 50 to 1000 seconds / 100 cc, and further preferably 100 to 500 seconds / 100 cc. Setting the air permeability of the microporous film to 5000 seconds / 100 cc or less can contribute to ensuring sufficient ion permeability of the microporous film. On the other hand, setting the air permeability to 10 seconds / 100 cc or more is preferable from the viewpoint of obtaining a more uniform microporous film having no defects.
The air permeability of the microporous film can be adjusted within the above range by appropriately setting the composition of the thermoplastic resin composition, the stretching temperature, the stretching ratio, and the like.

[Laminated structure]
Next, the microporous film having a laminated structure including at least two different microporous film layers in the present embodiment will be described. In the present embodiment, the microporous film can have a laminated structure composed of at least two different microporous film layers. Examples of the laminating method include a method of laminating by a co-extrusion method in the sheet forming step, a method of laminating after forming the sheet, a method of laminating after making porous, and the like. The layer structure can be freely selected as long as it satisfies the film characteristics in the present embodiment. Even if each layer does not satisfy the film characteristics of the present embodiment, it is sufficient that the entire layered structure is satisfied.

Surprisingly, it has been found that when the microporous film of the present embodiment is used as a battery separator, the cycle characteristics, which are indicators of durability, are dramatically improved.
The reason is not clear, but the battery separator has an appropriate compressive deformation stress and compressive deformation recovery rate, which causes electrode deterioration such as peeling of the binder when the electrode expands and contracts when the charge / discharge cycle is repeated. Therefore, the battery separator can maintain the shape in the thickness direction, and also can maintain the liquid retaining ability of the electrolytic solution. Therefore, the electric capacity can be maintained.
The microporous film of the present embodiment has a good balance of film properties, and can be suitably used as a battery separator, more specifically as a lithium ion secondary battery separator. In addition, it is used as various separation membranes.

  In addition, each physical property in this specification can be measured according to the method described in the following Examples, 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 raw material used and the evaluation method of various characteristics are as follows.

[raw materials]
(1) Polypropylene resin as component (a) A resin having a melt flow rate measured by JIS K-7120 (temperature 230 ° C.) of 0.4 g / 10 min was used.
(2) Polyphenylene ether resin as component (b) A resin having a reduced viscosity of 0.54 obtained by oxidative polymerization of 2,6-xylenol was used.
(3) Admixture of component (c) Polystyrene (i) -hydrogenated polybutadiene-polystyrene (ii), having a bonded styrene content of 43%, a number average molecular weight of 95,000, and polybutadiene before hydrogenation Total amount of 1,2-vinyl bond and 3,4-vinyl bond 80%, polystyrene (i) number average molecular weight 30,000, polystyrene (ii) number average molecular weight 10,000, polybutadiene part hydrogenation rate A 99.9% hydrogenated styrene-butadiene block copolymer was used.

[Evaluation method]
(1) Compression characteristics The compression characteristics were measured under the following conditions using a dynamic ultra-micro hardness meter (Shimadzu Corporation: DUH-211S) using an indentation depth setting load-unloading test mode.
A microporous film sample is cut into a 1 cm square, fixed on the sample stage with an adhesive (water glue), and a flat indenter (made of diamond) having a diameter of 50 μm at a speed of 4.4413 mN / sec from the surface of the microporous film. After pushing down to a depth of 3 μm (no hold time), it was pulled back to the initial microporous film surface position at a speed of 4.4413 mN / sec. The load applied to the planar indenter during this measurement was measured.
(1-1) Compressive deformation stress (MPa)
It was calculated by the following formula.
Compressive deformation stress (MPa) = Maximum load (N) during pressing / Cross sectional area of flat indenter (mm ² )
(1-2) Compression deformation recovery rate (%)
It was calculated by the following formula.
Compression deformation recovery rate (%) = (depth (μm) / push-in depth (3 μm) at which the load is zero when pulled back) × 100

(2) Film thickness (μm)
It measured with the dial gauge (Ozaki Seisakusho: "PEACOCK No.25" (trademark)).

(3) Porosity (%)
A 10 cm square sample was taken and calculated from the volume and mass using the following equation.
Porosity (%) = (volume (cm ³ ) −mass (g) / density of polymer composition) / volume (cm ³ ) × 100

(4) Air permeability (sec / 100cc)
It measured with the Gurley type air permeability meter based on JIS P-8117.

(5) Battery cycle test (capacity maintenance rate (%))
(5-1) Preparation of Nonaqueous Electrolytic Solution Prepared by dissolving LiPF ₆ as a solute at a concentration of 1.0 mol / liter in ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio) as a mixed solvent. .
(5-2) Production of positive electrode 90 parts by mass of Li (Co _1/3 Ni _1/3 Mn _1/3 ) O ₂ as a positive electrode active material, 4 parts by mass of acetylene black as a conductive agent, 2 parts by mass of graphite, A positive electrode slurry was prepared by dispersing 4 parts by mass of polyvinylidene fluoride (PVdF) as a binder and N-methyl-2-pyrrolidone (NMP) as a solvent. This positive electrode slurry was applied to both sides of a 20 μm thick aluminum foil serving as a positive electrode current collector with a coater, dried at 80 ° C. and 120 ° C. for 7 minutes, respectively, and then compression molded with a roll press. At this time, the coating amount of the positive electrode was 116 g / m ² and the bulk density was 2.58 g / cm ³ . This was cut in accordance with the battery width to form a strip. An uncoated portion is formed on the belt-like positive electrode, and an aluminum lead is connected to the current collector end of the uncoated portion.
(5-3) Production of Negative Electrode 97 parts by mass of natural graphite as a negative electrode active material, 1 part by mass of carboxymethyl cellulose as a thickener, and 2 parts by mass of styrene / butadiene copolymer latex as a binder are dispersed in purified water. Thus, a negative electrode slurry was prepared. This negative electrode slurry was applied to both sides of a 18 μm thick copper foil serving as a negative electrode current collector with a coater, dried at 80 ° C. and 100 ° C. for 5 minutes, respectively, and then compression molded with a roll press. At this time, the coating amount of the negative electrode was 51 g / m ² and the bulk density was 1.30 g / cm ³ . This was cut in accordance with the battery width to form a strip. The strip-shaped negative electrode has an uncoated portion, and a nickel lead was connected to the end of the current collector of the uncoated portion.
(5-4) Production of Battery A microporous film is used as a battery separator, and a belt-like positive electrode and a belt-like negative electrode are stacked in the order of a belt-like negative electrode, a battery separator, a belt-like positive electrode, and a battery separator, and are wound in a spiral shape a plurality of times. A mold laminate was produced. This cylindrical laminated body was sandwiched between ring-shaped insulators at both ends, accommodated in a stainless steel container, a nickel lead led out from the negative electrode current collector was connected to the bottom of the container, and led out from the positive electrode current collector An aluminum lead was connected to the container lid terminal. Further, the non-aqueous electrolyte was poured into the container and sealed.
The lithium ion secondary battery produced as described above was designed to have a diameter of 18 mm and a height of 65 mm (18650 size cylindrical battery) with a nominal discharge capacity of 1500 mAh.
(5-5) Cycle test After charging the above battery to 4.2 V at a constant current of 1 C (1500 mA) under the condition of a temperature of 50 ° C., the final voltage is 4.2 V and a constant voltage for a total of 3 hours. The battery was charged and discharged to a final voltage of 3.0 V with a constant current of 1C. The cycle was repeated with this charging / discharging as one cycle, and the ratio of the discharge capacity after 300 cycles to the initial discharge capacity was expressed as the capacity retention rate.

[Example 1]
(A) 100 parts by mass of polypropylene resin, (b) 11 parts by mass of polyphenylene ether resin, (c) 3 parts by mass of admixture were set at a temperature of 260 to 320 ° C. and a screw rotation speed of 300 rpm, Using a twin-screw extruder having two raw material supply ports (located at the approximate center of the extruder), polyphenylene ether resin is supplied from the first raw material supply port of the extruder, and polypropylene resin and admixture are supplied from the second raw material supply port. The mixture was supplied to an extruder and melt kneaded to obtain a thermoplastic resin composition as pellets.
The thermoplastic resin composition pellets obtained as described above were introduced into a single screw extruder set at 20 mm in diameter, L / D = 30, and 260 ° C. via a feeder, and the lip thickness set at the tip of the extruder. After extruding from a 5 mm T-die, the molten resin was immediately drawn into a molten roll with 25 ° C cold air and cooled to 110 ° C at a draw ratio of 200 to form a precursor film.
This precursor film was heat-treated at 150 ° C. for 3 hours, and uniaxially stretched 1.2 times in the MD direction at a temperature of 25 ° C., and then this stretched film was further 2.5 times in the MD direction at a temperature of 130 ° C. The film was uniaxially stretched, further relaxed 0.9 times in the MD direction at 155 ° C., and heat fixed to obtain a microporous film. The film thickness, porosity, air permeability, compressive deformation stress, and compressive deformation recovery rate of the obtained microporous film were measured, and the results are shown in Table 1. In addition, Table 1 shows the battery evaluation results when the obtained microporous film was used as a battery separator.

[Example 2]
A microporous film was obtained in the same manner as in Example 1 except that the lip thickness of the T die was 3 mm and the draw ratio was 100. The film thickness, porosity, air permeability, compressive deformation stress, and compressive deformation recovery rate of the obtained microporous film were measured, and the results are shown in Table 1. In addition, Table 1 shows the battery evaluation results when the obtained microporous film was used as a battery separator.

[Example 3]
A microporous film was obtained in the same manner as in Example 1 except that the lip thickness of the T die was 3 mm, the draw ratio was 150, and the cast roll temperature was 130 ° C. The film thickness, porosity, air permeability, compressive deformation stress, and compressive deformation recovery rate of the obtained microporous film were measured, and the results are shown in Table 1. In addition, Table 1 shows the battery evaluation results when the obtained microporous film was used as a battery separator.

[Example 4]
(A) 100 parts by mass of polypropylene resin, (b) 11 parts by mass of polyphenylene ether resin, (c) 3 parts by mass of admixture were set at a temperature of 260 to 320 ° C. and a screw rotation speed of 300 rpm, Using a twin-screw extruder having two raw material supply ports (located at the approximate center of the extruder), polyphenylene ether resin is supplied from the first raw material supply port of the extruder, and polypropylene resin and admixture are supplied from the second raw material supply port. The mixture was supplied to an extruder and melt kneaded to obtain a thermoplastic resin composition as pellets.
The thermoplastic resin composition pellets obtained as described above were introduced into a single screw extruder set at 20 mm in diameter, L / D = 30, and 260 ° C. via a feeder, and the lip thickness set at the tip of the extruder. After extruding from a 1.2-mm T-die, the precursor film A was formed by drawing it at a draw ratio of 110 with a cast roll immediately cooled to 130 ° C. by applying cold air at 25 ° C. to the molten resin.
Further, high-density polyethylene (density 0.960, melt index 1.0) was introduced into a single-screw extruder set at 20 mm in diameter, L / D = 30, 180 ° C. via a feeder, and installed at the tip of the extruder. After extruding from a T-die having a lip thickness of 3 mm, a precursor film B was formed by immediately drawing the melted resin at a draw ratio of 280 with a cast roll that had been cooled to 130 ° C. by applying cold air at 25 ° C.
Precursor film A and precursor film B are unwound at an unwinding speed of 4.0 m / min in a configuration of A / B / A, respectively, and led to a heating roll, where the thermocompression bonding temperature is 130 ° C., the wire The film was thermocompression bonded at a pressure of 2.0 kg / cm, then led to a 25 ° C. cooling roll at the same speed and wound to obtain a laminated film. This laminated film was annealed for 1 hour in a hot air circulating oven heated to 130 ° C.
Next, the annealed laminated film was uniaxially stretched 1.3 times in the MD direction at a temperature of 25 ° C., and then the stretched laminated film was uniaxially stretched 2.5 times in the MD direction at a temperature of 125 ° C. A laminated microporous film was obtained. The laminated microporous film was heat-fixed by relaxing it at a temperature of 125 ° C. by a factor of 0.8. The resulting laminated microporous film was measured for film thickness, porosity, air permeability, compressive deformation stress, and compressive deformation recovery rate, and the results are shown in Table 1. Table 1 shows the battery evaluation results when the obtained laminated microporous film was used as a battery separator.

[Example 5]
A microporous film was obtained in the same manner as in Example 1 except that the relaxation rate during heat setting was 0.8 times. The film thickness, porosity, air permeability, compressive deformation stress, and compressive deformation recovery rate of the obtained microporous film were measured, and the results are shown in Table 1. In addition, Table 1 shows the battery evaluation results when the obtained microporous film was used as a battery separator.

[Example 6]
A microporous film was obtained in the same manner as in Example 1 except that the relaxation rate during heat setting was 0.95 times. The film thickness, porosity, air permeability, compressive deformation stress, and compressive deformation recovery rate of the obtained microporous film were measured, and the results are shown in Table 1. In addition, Table 1 shows the battery evaluation results when the obtained microporous film was used as a battery separator.

[Comparative Example 1]
7 parts by mass of ultra high molecular weight polyethylene (viscosity average molecular weight 2,000,000), 28 parts by mass of high density polyethylene (viscosity average molecular weight 280,000), and 0.3 parts by mass of an antioxidant are mixed with this composition Then, it was put into a twin screw extruder through a feeder. Further, 65 parts by mass of liquid paraffin (kinematic viscosity of 75.9 cSt at 37.78 ° C.) was injected into the extruder by side feed, kneaded, extruded from a T die installed at the tip of the extruder, and immediately cooled to 25 ° C. The mixture was cooled and solidified with a cast roll to form a precursor film having a thickness of 1.5 mm.
This precursor film was stretched 7 times in the MD direction and TD direction at 120 ° C. by a simultaneous biaxial stretching machine, and then this stretched film was immersed in methyl ethyl ketone, extracted and removed from liquid paraffin, dried and microporous. A characteristic film was obtained. Furthermore, this was heat-set at 125 ° C. to obtain a microporous film. The film thickness, porosity, air permeability, compressive deformation stress, and compressive deformation recovery rate of the obtained microporous film were measured, and the results are shown in Table 1. In addition, Table 1 shows the battery evaluation results when the obtained microporous film was used as a battery separator.

[Comparative Example 2]
Polypropylene (density 0.910, melt index 1.0) was introduced into a single screw extruder set at 20 mm in diameter, L / D = 30, 200 ° C. via a feeder, and a lip thickness of 5 mm installed at the tip of the extruder. After extruding from a T-die, the precursor film was formed by drawing it at a draw ratio of 200 with a cast roll immediately cooled to 130 ° C. by applying 25 ° C. cold air to the molten resin.
This precursor film was heat-treated at 150 ° C. for 3 hours, and uniaxially stretched 1.2 times in the MD direction at a temperature of 25 ° C., and then this stretched film was further 2.5 times in the MD direction at a temperature of 60 ° C. The film was uniaxially stretched and further relaxed 0.9 times at 155 ° C. to perform heat setting to obtain a microporous film. The film thickness, porosity, air permeability, compressive deformation stress, and compressive deformation recovery rate of the obtained microporous film were measured, and the results are shown in Table 1. In addition, Table 1 shows the battery evaluation results when the obtained microporous film was used as a battery separator.

[Comparative Example 3]
12 parts by mass of ultra high molecular weight polyethylene having an average molecular weight of 2,000,000, 12 parts by mass of high density polyethylene having an average molecular weight of 280,000, 16 parts by mass of linear low density polyethylene having an average molecular weight of 150,000, dioctyl phthalate (DOP) 42 After mixing and granulating 4 parts by mass and 17.6 parts by mass of finely divided silica, a precursor film having a thickness of 90 μm was formed by kneading and extruding with a twin screw extruder equipped with a T-die.
DOP and fine silica were extracted from this precursor film with methylene chloride and aqueous sodium hydroxide solution to form a microporous film. Two microporous films were stacked and heated to 118 ° C., stretched 5.3 times in the MD direction, and then stretched 1.8 times in the TD direction to obtain a microporous film. The film thickness, porosity, air permeability, compressive deformation stress, and compressive deformation recovery rate of the obtained microporous film were measured, and the results are shown in Table 1. In addition, Table 1 shows the battery evaluation results when the obtained microporous film was used as a battery separator.

  From the results in Table 1, when the microporous film of the present embodiment (Examples 1 to 6) was used as a battery separator, it was possible to dramatically improve the cycle characteristics of the battery.

  The microporous film of the present invention has industrial applicability as a battery separator, more specifically as a lithium ion secondary battery separator.

Claims (5)

  A microporous film containing a thermoplastic resin, wherein the compressive deformation stress is 10 to 40 MPa, the compression deformation recovery rate is 35 to 90%, and the porosity is 30 to 60%.   The microporous film according to claim 1, wherein the thermoplastic resin is a thermoplastic resin composition containing a polyolefin resin.   The said thermoplastic resin composition is a microporous film of Claim 2 containing 5-90 mass parts of (b) polyphenylene ether resin with respect to 100 mass parts of (a) polypropylene resin.   The microporous film according to any one of claims 1 to 3, having a laminated structure including at least two different microporous film layers.   The battery separator containing the microporous film of any one of Claims 1-4.