JP2017082127A - Olefinic resin microporous film, separator for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an olefinic resin microporous film having resistance to pressure.SOLUTION: Provided is an olefinic resin microporous film with a micropore part, characterized in that a thickness change rate when the pressure of 35 MPa is acted to a thickness direction is 10% or lower and also porosity is 40 to 60%, thus an olefinic resin microporous film having excellent resistance to the pressure in the thickness direction, and in which the reduction of thickness caused by pressure is reduced can be provided.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an olefin resin microporous film, a separator for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery.

  Conventionally, lithium ion secondary batteries have been used as power sources for portable electronic devices. This lithium ion secondary battery is generally configured by disposing a positive electrode, a negative electrode, and a separator in an electrolytic solution. The positive electrode is formed by applying lithium cobalt oxide or lithium manganate to the surface of an aluminum foil. The negative electrode is formed by applying carbon to the surface of a copper foil. And the separator is arrange | positioned so that a positive electrode and a negative electrode may be partitioned off, and the short circuit with a positive electrode and a negative electrode is prevented.

  When charging a lithium ion secondary battery, lithium ions are released from the positive electrode and enter the negative electrode. On the other hand, when the lithium ion secondary battery is discharged, lithium ions are released from the negative electrode and move to the positive electrode. Such charging / discharging is repeated in the lithium ion secondary battery. Therefore, the separator used in the lithium ion secondary battery is required to be able to transmit lithium ions satisfactorily.

  An olefin resin microporous film is used as a separator of a lithium ion secondary battery (for example, Patent Document 1).

  When the separator is installed in the battery, high pressure is applied to the separator. In particular, a pressure of about 20 MPa is applied to a cylindrical battery or a square battery. Furthermore, since the electrode expands and contracts due to repeated charging and discharging, it is assumed that a higher pressure is applied when the electrode expands.

International Publication No. 2006/106783

  However, the conventional olefin-based resin microporous film cannot withstand the pressure and has a problem that the microporous is deformed. When the pore diameter of the olefin-based resin microporous film is deformed, the amount of the electrolytic solution retained inside the separator is reduced, causing a problem that the battery is withered during charging and discharging, and the battery life is shortened.

  In addition, if the olefin resin microporous film cannot withstand the pressure in the battery and becomes thin, the olefin resin microporous film is broken by the needle-shaped crystal lithium dendrite generated during charging and discharging, and the positive electrode and the negative electrode are short-circuited. There is a fear. Furthermore, when the foreign material is contained, the risk of short circuit due to the foreign material increases due to the thin olefin resin microporous film.

  The present invention provides an olefin resin microporous film that is resistant to pressure. Furthermore, this invention provides the separator for nonaqueous electrolysis secondary batteries and the nonaqueous electrolysis secondary battery using the said olefin resin microporous film.

  The olefin-based resin microporous film of the present invention is an olefin-based resin microporous film having micropores, and has a thickness change rate of 10% or less when a pressure of 35 MPa is applied in the thickness direction and has a porosity. Is 40 to 60%.

  According to the present invention, it is possible to provide an olefin-based resin microporous film that has excellent resistance to pressure in the thickness direction and has reduced thickness reduction due to pressure. According to this olefin-based resin microporous film, the electrolytic solution can be sufficiently retained even after repeated charge and discharge, a decrease in discharge capacity due to liquid withering is suppressed, and excellent mechanical strength is maintained. A lithium ion secondary battery in which a short circuit due to dendrites and foreign matters is prevented can be provided.

It is the model side view which showed an example of the manufacturing apparatus used for the extrusion process and the conveyance process in the method of this invention.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely using an Example, this invention is not limited by these Examples.

  The olefin resin microporous film has micropores. The olefin resin microporous film preferably includes an olefin resin stretched film having micropores. The stretched olefin resin film is a stretched film in which micropores are formed by stretching the olefin resin film.

  The olefin resin microporous film contains an olefin resin. As the olefin resin, ethylene resin and propylene resin are preferable, and propylene resin is more preferable.

  Examples of the propylene-based resin include homopolypropylene and copolymers of propylene and other olefins. When the olefin resin microporous film is produced by the stretching method described later, homopolypropylene is preferable. Propylene-type resin may be used independently, or 2 or more types may be used together. The copolymer of propylene and another olefin may be a block copolymer or a random copolymer.

  Examples of the olefin copolymerized with propylene include α such as ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-nonene and 1-decene. -An olefin etc. are mentioned, Ethylene is preferable.

  The weight average molecular weight of the olefin resin is preferably 250,000 to 500,000, and more preferably 280,000 to 480,000. According to the olefin resin having a weight average molecular weight within the above range, the film forming stability is excellent, the micropores are uniformly formed, and the pressure resistance in the thickness direction is excellent. It is possible to provide an olefin-based resin microporous film in which a decrease in thickness due to the above is reduced.

  The molecular weight distribution (weight average molecular weight Mw / number average molecular weight Mn) of the olefin resin is preferably 7.5 to 12, and more preferably 8 to 11. According to the olefin resin having a molecular weight distribution within the above range, it has a high surface opening ratio, excellent ion permeability, excellent resistance to pressure in the thickness direction, and is attributed to pressure. An olefin-based resin microporous film with reduced thickness reduction can be provided.

  Here, the weight average molecular weight and number average molecular weight of the olefin-based resin are polystyrene-converted values measured by a GPC (gel permeation chromatography) method. Specifically, after collecting 6 to 7 mg of olefin resin, supplying the collected olefin resin to a test tube, o-DCB containing 0.05% by weight of BHT (dibutylhydroxytoluene) in the test tube ( (Orthodichlorobenzene) solution is added and diluted so that the concentration of the olefin resin is 1 mg / mL to prepare a diluted solution.

  The dilution liquid is shaken for 1 hour at 145 ° C. and a rotation speed of 25 rpm using a dissolution filter, and the olefin resin is dissolved in the o-DCB solution to obtain a measurement sample. Using this measurement sample, the weight average molecular weight and number average molecular weight of the olefin resin can be measured by the GPC method.

The weight average molecular weight and the number average molecular weight in the olefin resin can be measured, for example, with the following measuring apparatus and measurement conditions.
Product name "HLC-8121GPC / HT" manufactured by TOSOH
Measurement conditions Column: TSKgelGMHHR-H (20) HT x 3
TSKguardcolumn-HHR (30) HT x 1
Mobile phase: o-DCB 1.0 mL / min
Sample concentration: 1 mg / mL
Detector: Bryce refractometer
Reference material: Polystyrene (Molecular weight: 500-8420000, manufactured by TOSOH)
Elution conditions: 145 ° C
SEC temperature: 145 ° C

  160-170 degreeC is preferable and, as for melting | fusing point of olefin resin, 160-165 degreeC is more preferable. According to the olefin resin having a melting point within the above range, it has excellent film forming stability and has excellent resistance to pressure in the thickness direction even at high temperatures, and thickness reduction due to pressure. It is possible to provide a microporous olefin-based resin film having a reduced content.

  In the present invention, the melting point of the olefin-based resin can be measured using a differential scanning calorimeter (for example, a device name “DSC220C”, manufactured by Seiko Instruments Inc.) according to the following procedure. First, 10 mg of an olefin resin is heated from 25 ° C. to 250 ° C. at a heating rate of 10 ° C./min, and held at 250 ° C. for 3 minutes. Next, the olefin-based resin is cooled from 250 ° C. to 25 ° C. at a temperature decrease rate of 10 ° C./min, and held at 25 ° C. for 3 minutes. Subsequently, the olefin resin is reheated from 25 ° C. to 250 ° C. at a rate of temperature increase of 10 ° C./min, and the temperature at the top of the endothermic peak in this reheating step is defined as the melting point of the olefin resin.

  The olefin resin microporous film includes micropores formed by stretching (preferably uniaxial stretching) the olefin resin film. The minute hole portion penetrates in the film thickness direction.

  The porosity of the olefin resin microporous film is 40 to 60%, preferably 45 to 55%. By setting the porosity to 40% or more, the olefin-based resin microporous film can hold a large amount of electrolyte in the micropores, and as a result, the olefin resin is charged and discharged with the lithium ion secondary battery. Generation | occurrence | production of the part which an electrolyte solution runs short in a system resin microporous film can be reduced. By setting the porosity of the olefin-based resin microporous film to 60% or less, the olefin-based resin microporous film has excellent resistance to pressure in the thickness direction and reduced reduction in thickness due to pressure, and can do.

In addition, the porosity of an olefin resin microporous film can be measured in the following way. First, a test piece having a square shape (area 100 cm ² ) measuring 10 cm in length and 10 cm in width is obtained by cutting the olefin-based resin microporous film. Next, the weight W (g) and thickness T (cm) of the test piece are measured, and the apparent density ρ (g / cm ³ ) is calculated by the following formula (I). In addition, the thickness of a test piece measures 15 thickness of a test piece using a dial gauge (for example, signal ABS Digimatic indicator by Mitutoyo Corporation), and makes it the arithmetic mean value. The weight of the test piece can be measured using a balance (for example, a product name “AB204-S” manufactured by METTLER TOLEDO). Then, using this apparent density ρ (g / cm ³ ) and the density ρ ₀ (g / cm ³ ) of the olefin resin itself, the porosity P (% of the olefin resin microporous film based on the following formula (II) ) Can be calculated.
Apparent density ρ (g / cm ³ ) = W / (100 × T) (I)
Porosity P [%] = 100 × [(ρ ₀ −ρ) / ρ ₀ ] (II)

  The average major axis of the open ends of the micropores in the olefin-based resin microporous film is preferably 100 nm to 500 nm, more preferably 200 nm to 500 nm, and particularly preferably 250 to 350 nm. According to the micropores having an average major axis at the open end within the above range, the electrolyte can be sufficiently taken into the olefin-based resin microporous film by a capillary phenomenon and has excellent resistance to pressure in the thickness direction. The olefin-based resin microporous film having a reduced thickness reduction due to pressure can be obtained.

  The maximum major axis of the open ends of the micropores in the olefin-based resin microporous film is preferably 100 nm to 1 μm, more preferably 100 nm to 800 nm, and particularly preferably 500 to 800 nm. According to the micropore part having the maximum major axis of the open end within the above range, the electrolyte can be sufficiently taken into the olefin-based resin microporous film by a capillary phenomenon and has excellent resistance to pressure in the thickness direction. The olefin-based resin microporous film having a reduced thickness reduction due to pressure can be obtained.

  In addition, the maximum major axis and the average major axis of the open ends of the micropores in the olefin resin microporous film are measured as follows. First, the surface of the olefin resin microporous film is coated with carbon. Next, 10 arbitrary positions on the surface of the olefin resin microporous film are photographed at a magnification of 10,000 using a scanning electron microscope. The photographing range is a plane rectangular range of 9.6 μm long × 12.8 μm wide on the surface of the olefin resin microporous film.

  The major axis of the opening end of each micropore appearing in the obtained photograph is measured. The major axis of the open end of the microhole is defined as the diameter of a perfect circle having the smallest diameter that can surround the open end of the microhole. Micropores that exist across the imaging range and the non-imaging range are excluded from the measurement target. And let the largest long diameter among the long diameters of the opening end in the micropore part which has appeared in the photograph be the maximum long diameter of the opening end of the micropore part of the olefin resin microporous film. The arithmetic mean value of the major axis of the open end in each micropore portion appearing in the photograph is defined as the average major axis of the micropore portion of the olefin-based resin microporous film.

  The surface opening ratio of the olefin resin microporous film is preferably 25 to 55%, more preferably 30 to 50%. By setting the surface opening ratio to 25% or more, the electrolytic solution can be sufficiently taken into the olefin resin microporous film by capillary action. By setting the surface opening ratio to 55% or less, it is possible to suppress a decrease in mechanical strength of the olefin-based resin microporous film due to the formation of excessive micropores, and it has excellent resistance to pressure in the thickness direction. It is possible to obtain an olefin-based resin microporous film having a reduced thickness reduction due to pressure.

  The surface opening ratio of the olefin resin microporous film can be measured in the following manner. First, in an arbitrary portion of the surface of the olefin-based resin microporous film, a measurement portion having a plane rectangular shape of 9.6 μm in length and 12.8 μm in width is determined, and this measurement portion is photographed at a magnification of 10,000 times.

Next, each micropore formed in the measurement portion is surrounded by a rectangle whose one of the long side and the short side is parallel to the extending direction. The rectangle is adjusted so that both the long side and the short side have the minimum dimension. Let the area of the said rectangle be an opening area of each micropore part. The total opening area S (μm ² ) of the micropores is calculated by summing the opening areas of the micropores. This is the total opening area S of the minute hole ([mu] m ²⁾ of 122.88μm ² (9.6μm × 12.8μm) surface porosity values multiplied by 100 and divided by the (%). In addition, about the micropore part which exists across the measurement part and the part which is not a measurement part, only the part which exists in a measurement part among micropores is set as a measuring object.

The pore density of the olefin resin microporous film is preferably 15 / μm ² or more, more preferably 16 / μm ² or more, and particularly preferably 17 / μm ² or more. By setting the pore density to 15 pores / μm ² or more, the electrolyte can be uniformly absorbed, and even when pressure is applied, the olefin resin fine particles can be prevented from partially draining. A porous film can be provided.

In addition, the pore density of an olefin resin microporous film is measured in the following way. First, in an arbitrary portion of the surface of the olefin-based resin microporous film, a measurement portion having a plane rectangular shape of 9.6 μm in length and 12.8 μm in width is determined, and this measurement portion is photographed at a magnification of 10,000 times. Then, the number of micropores is measured in the measurement part, and the pore density can be calculated by dividing this number by 122.88 μm ² (9.6 μm × 12.8 μm).

  5-100 micrometers is preferable and, as for the thickness of an olefin resin microporous film, 10-50 micrometers is more preferable.

  In the present invention, the thickness of the olefin resin microporous film can be measured according to the following procedure. That is, the arbitrary 10 places of an olefin resin microporous film are measured using a dial gauge, and let the arithmetic mean value be the thickness of an olefin resin microporous film.

  When the pressure of 35 MPa in the olefin resin microporous film is applied in the thickness direction, the thickness change rate is 10% or less, preferably 8% or less, and more preferably 6% or less. When the thickness change rate is 10% or less, the film has excellent resistance to pressure in the thickness direction. Therefore, even when a high pressure is applied in the thickness direction, the electrolytic solution can be sufficiently retained, and a short circuit between the positive electrode and the negative electrode due to dendrites and foreign matters can be effectively prevented.

The thickness change rate when the pressure of 35 MPa in the olefin resin microporous film is applied in the thickness direction is measured in the following manner. A plane square (one side: 6 cm) test piece is cut out from the olefin resin microporous film. The thickness is measured using a dial gauge at 10 arbitrary positions of the test piece in a state where no pressure is applied to the test piece in the thickness direction, and the arithmetic average value of these thicknesses is defined as the initial thickness T ₀ . Next, a flat square-shaped (one side: 6 cm) stainless steel plate was superimposed on both surfaces of the test piece, and then a planar square-shaped (one side: 6 cm) rubber plate was superimposed on each stainless steel plate to obtain a laminate. Is made. The laminate is sandwiched in the thickness direction using a pair of press plates, and a pressure of 35 MPa is applied to the test piece in the thickness direction for 2 minutes. Next, the pressure of 35 MPa applied to the test piece is removed. The thickness was measured using a dial gauge at any position 10 places of the test piece immediately after removal of the pressure, the arithmetic mean of their thickness and the compression thickness T _1. Based on the following formula, the thickness change rate when a pressure of 35 MPa in the olefin resin microporous film is applied in the thickness direction is calculated.
Thickness change rate _{(%) = 100 × │T 0} -T 1 │ / T 0

  The air permeability of the olefin-based resin microporous film is preferably 50 to 600 sec / 100 mL, more preferably 100 to 300 sec / 100 mL, and particularly preferably 100 to 150 sec / 100 mL. The olefin-based resin microporous film having an air permeability within the above range has many micropores penetrating in the thickness direction, and such micropores absorb a large amount of electrolyte to the inside. Even when pressure is applied, it is possible to prevent the electrolyte from withstanding.

  The air permeability of the olefin-based resin microporous film is a value measured in accordance with JIS P8117 in an atmosphere at a temperature of 23 ° C. and a relative humidity of 65%.

  In the olefin-based resin microporous film, the rate of change in air permeability when a pressure of 35 MPa is applied in the thickness direction is preferably 35% or less, more preferably 30% or less, and 28% or less. More preferably. When the rate of change in air permeability is 35% or less, it has excellent resistance to pressure in the thickness direction. Therefore, even when a high pressure is applied in the thickness direction, the electrolytic solution can be sufficiently retained, and a short circuit between the positive electrode and the negative electrode due to dendrites and foreign matters can be effectively prevented.

In the olefin resin microporous film, the rate of change in air permeability when a pressure of 35 MPa is applied in the thickness direction is measured as follows. A plane square (one side: 6 cm) test piece is cut out from the olefin resin microporous film. In a state where no pressure is applied to the test piece in the thickness direction, the air permeability of the test piece is measured according to JIS P8117 in an atmosphere of a temperature of 23 ° C. and a relative humidity of 65%. Let the measured air permeability be the initial air permeability P ₀ . A pressure of 35 MPa is applied to the test piece in the thickness direction for 2 minutes in the same manner as when measuring the rate of change in thickness when a pressure of 35 MPa is applied in the thickness direction of the olefin resin microporous film. Next, the pressure of 35 MPa applied to the test piece is removed, and the air permeability of the test piece immediately after the pressure is removed is measured in the same manner as described above. The measured air permeability is defined as a compressed air permeability P ₁ . Based on the following formula, in the olefin-based resin microporous film, the rate of change in air permeability when a pressure of 35 MPa is applied in the thickness direction is calculated.
The rate of change of air permeability _{(%) = 100 × │P 0} -P 1 │ / P 0

  Next, an example of the manufacturing method of the olefin resin microporous film of this invention is demonstrated. In FIG. 1, an example of the manufacturing apparatus used for the extrusion process and cooling process in the method of this invention is shown. In the manufacturing apparatus of FIG. 1, a T die 2 is attached to the front end of the extruder 1. A first cooling roll 3 and a second cooling roll 4 are disposed below the T die 2. The first cooling roll 3 is disposed on the most upstream side in the transport direction of the olefin resin film A. And the 2nd cooling roll 4 is arrange | positioned at predetermined intervals in the downstream in the film conveyance direction of the 1st cooling roll 3.

  The T-die 2 has an extrusion port 2a that opens downward. The T die 2 is configured to extrude the molten olefin resin supplied from the extruder 1 toward the outer peripheral surface of the first cooling roll 3 while forming the molten olefin resin from the extrusion port 2a into the olefin resin film A. . The first cooling roll 3 is configured to rotate at a predetermined peripheral speed so that the olefin resin film A is brought into close contact with the outer peripheral surface of the first cooling roll 3 while being cooled and solidified. Has been. Then, the second cooling roll 4 is rotated at a predetermined peripheral speed so that the olefin-based resin film A fed from the first cooling roll 3 is brought into close contact with the outer peripheral surface of the second cooling roll 4, The olefin-based resin film A is configured to be cooled. In order to bring the olefin resin film A extruded from the T die 2 into close contact with the first cooling roll 3, an air flow is blown out from an air knife (not shown), and the olefin resin film A is blown by the wind pressure of the air flow. The first cooling roll 3 is pressed, a nip roll (not shown) is disposed in contact with the first cooling roll 3, and the olefin resin film A is pressed against the first cooling roll 3 by the nip roll. May be.

(Extrusion process)
In the method of the present invention, first, an extrusion process for obtaining an olefin resin film A is performed by melt-kneading an olefin resin in the extruder 1 and extruding it from a T die 2 attached to the tip of the extruder 1. .

  The temperature of the olefin resin when the olefin resin is melt-kneaded in the extruder 1 is preferably (Tm + 20) ° C. to (Tm + 100) ° C., more preferably (Tm + 25) ° C. to (Tm + 80) ° C., and (Tm + 25) ° C. ~ (Tm + 50) ° C is particularly preferred. By setting the temperature of the olefin resin at the time of melt-kneading within the above range, the orientation of the olefin resin can be improved and the generation of lamellae can be promoted. “Tm” means the melting point (° C.) of the olefin resin.

  50-300 are preferable, as for the draw ratio at the time of extruding an olefin resin from the extruder 1 to a film form, 65-250 are more preferable, and 70-250 are especially preferable. By setting the draw ratio to 50 or more, the tension applied to the olefin resin can be improved. As a result, the olefin resin can be sufficiently oriented to promote the production of lamellae. Moreover, the film-forming stability of an olefin resin film can be improved by making a draw ratio into 300 or less. This makes it possible to obtain an olefin-based resin microporous film having a uniform thickness and width.

  The draw ratio refers to a value obtained by dividing the clearance of the lip of the T die by the thickness of the olefin resin film extruded from the T die. The clearance of the T die lip is measured by measuring the clearance of the lip of the T die at 10 locations or more using a clearance gauge in accordance with JIS B7524 (for example, JIS clearance gauge manufactured by Nagai Gauge Manufacturing Co., Ltd.), and the arithmetic average thereof. This can be done by determining the value. The thickness of the olefin resin film extruded from the T die is 10 or more in the thickness of the olefin resin film extruded from the T die using a dial gauge (for example, signal ABS Digimatic Indicator manufactured by Mitutoyo Corporation). It can be performed by measuring and calculating the arithmetic mean value.

  The film forming speed of the olefin resin film is preferably 10 to 300 m / min, more preferably 15 to 250 m / min, and particularly preferably 15 to 30 m / min. By setting the film forming speed of the olefin resin film to 10 m / min or more, the tension applied to the olefin resin can be improved. As a result, the olefin resin molecules can be sufficiently oriented to promote the formation of lamellae. Moreover, the film-forming stability of an olefin resin film can be improved by making the film-forming speed | rate of an olefin resin film into 300 m / min or less. This makes it possible to obtain an olefin-based resin microporous film having a uniform thickness and width.

(Cooling process)
Next, the olefin-based resin film A extruded from the T-die 2 is sequentially supplied and transported to a plurality of cooling rolls 3 and 4 arranged at a predetermined interval in the transport direction of the olefin-based resin film A. Thus, a cooling step for cooling the olefin-based resin film A is performed.

  Although the surface temperature of the 1st cooling roll 3 is (Tm-20) degreeC-(Tm-120) degreeC, (Tm-25) degreeC-(Tm-100) degreeC are preferable, (Tm-30)-( Tm−90) ° C. is more preferable. By setting the surface temperature of the first cooling roll 3 within the above range, lamellae can be sufficiently generated in the olefin-based resin film A.

  Although two cooling rolls 3 and 4 are used in FIG. 1, the number of cooling rolls may be two or more, preferably 2 to 5, and more preferably 2.

Between the cooling rolls adjacent to each other in the transport direction of the olefin resin film A, the surface temperature of the downstream cooling roll is set to be 10 ° C. or lower than the surface temperature of the upstream cooling roll. The difference (T _d −T _u ) between the surface temperature [T _u (° C.)] of the downstream cooling roll and the surface temperature [T _d (° C.)] of the upstream cooling roll is 10 ° C. or more. 20 degreeC or more is preferable, 50 degreeC or more is more preferable, and 50-90 degreeC is especially preferable. Thus, the olefin resin film A can be gradually cooled by lowering the surface temperature of the cooling roll toward the downstream side in the transport direction, and thereby a lamella is highly generated in the olefin resin film A. be able to.

  As shown in FIG. 1, when two cooling rolls 3 and 4 are used, the second cooling roll 4 is installed adjacent to the downstream side in the film transport direction of the first cooling roll 3, and the upstream cooling roll As the first cooling roll 3, the second cooling roll 4 is used as the downstream cooling roll.

  The surface temperature of the second cooling roll 4 is preferably (Tm-140) ° C. to (Tm-110) ° C., and preferably (Tm-140) ° C. to (Tm-120) ° C. By setting the surface temperature of the second cooling roll 4 within the above range, lamellae can be sufficiently generated in the olefin-based resin film A.

  In the cooling step, the olefin-based resin film A is cooled while being carried on the surface of each cooling roll. The contact time between the olefin resin film A and each cooling roll is preferably 0.01 to 2 seconds. By making contact time in the said range, the surface temperature of the olefin resin film A can be made into the same surface temperature as each cooling roll.

(Curing process)
Next, the olefin resin film obtained by the cooling process described above is cured. The curing process of the olefin resin is performed to grow the lamella formed in the olefin resin film in the cooling process. This makes it possible to form a laminated lamella structure in which crystallized portions (lamellar) and amorphous portions are alternately arranged in the extrusion direction of the olefin resin film. In particular, by setting the curing temperature to a relatively low temperature, the crystallized portion (lamella) can be highly grown in the film thickness direction. Therefore, by stretching such an olefin-based resin film in a stretching process described later, it is possible to form micropores penetrating in the thickness direction of the film.

  The curing temperature of the olefin-based resin film is preferably (Tm-30) ° C to (Tm-5) ° C, and more preferably (Tm-25) ° C to (Tm-10) ° C. By setting the curing temperature of the olefin resin film to a temperature of (Tm-30) ° C. or higher, crystallization of the olefin resin film can be sufficiently promoted. Moreover, collapse of the lamella structure by relaxation of the molecular orientation of an olefin resin can be reduced by making the curing temperature of an olefin resin film below the temperature of (Tm-5) degreeC.

  In addition, the curing temperature of an olefin resin film is the surface temperature of an olefin resin film. However, when the surface temperature of the olefin resin film cannot be measured, for example, when the olefin resin film is cured in a roll shape, the curing temperature of the olefin resin film is the atmospheric temperature and To do. For example, when curing is performed in a state where the olefin-based resin film is wound into a roll inside a heating apparatus such as a hot stove, the temperature inside the heating apparatus is set as the curing temperature.

  The curing of the olefin-based resin film may be performed while the olefin-based resin film is running, or may be performed in a state where the olefin-based resin film is wound into a roll.

  When curing is performed while running the olefin resin film, the curing time of the olefin resin film is preferably 1 minute or more, and more preferably 5 minutes to 60 minutes.

  When the olefin-based resin film is cured in a rolled state, the curing time is preferably 1 hour or longer, and more preferably 15 hours or longer. By curing the olefin-based resin film in a state wound in such a curing time, the curing of the olefin-based resin film as a whole can be performed sufficiently with the curing temperature described above. Thereby, a lamella can be fully grown in an olefin resin film. Moreover, from a viewpoint of reducing the thermal deterioration of an olefin resin film, the curing time is preferably 35 hours or less, and more preferably 30 hours or less.

  If the olefin resin film is cured in a roll shape, the olefin resin film is unwound from the olefin resin film roll after the curing process, and the stretching process and the annealing process described later are performed. Good.

(Stretching process)
Next, the extending | stretching process of extending | stretching the unstretched olefin resin film after a curing process is performed. The unstretched olefin resin film is preferably stretched uniaxially. This stretching step preferably includes a first stretching step and a second stretching step following the first stretching step.

(First stretching step)
In the first stretching step, the olefin resin film after the curing step is preferably subjected to uniaxial stretching at a stretching ratio of 1.03 to 1.5 times at a surface temperature of −20 ° C. or higher and lower than 100 ° C. In the first stretching step, the olefin resin film is preferably uniaxially stretched only in the extrusion direction. In the first stretching step, the lamellae in the olefin-based resin film are hardly melted, and by separating the lamellae by stretching, the fine cracks are efficiently generated independently in the non-crystalline portion between the lamellae. A large number of micropores are reliably formed starting from this crack.

  In the first stretching step, the surface temperature of the olefin resin film is preferably −20 ° C. or higher and lower than 100 ° C., more preferably 0 to 80 ° C., and particularly preferably 10 to 40 ° C. By setting the surface temperature of the olefin-based resin film to −20 ° C. or higher, breakage of the olefin-based resin film during stretching can be reduced. Moreover, a crack can be generated in the amorphous part between lamellae by setting the surface temperature of the olefin resin film to less than 100 ° C.

  In the first stretching step, the stretching ratio of the olefin resin film is preferably 1.03 to 1.5 times, and more preferably 1.05 to 1.3 times. By setting the draw ratio to 1.03 times or more, micropores can be formed in the non-crystalline part between lamellae. Moreover, a micropore part can be formed uniformly by making a draw ratio into 1.5 times or less.

  In addition, in this invention, the draw ratio of an olefin resin film means the value which remove | divided the length of the olefin resin film after extending | stretching by the length of the olefin resin film before extending | stretching.

  The stretching speed in the first stretching step of the olefin resin film is preferably 20% / min or more, more preferably 20 to 500% / min, and particularly preferably 20 to 70% / min. By setting the stretching speed to 20% / min or more, the micropores can be formed uniformly in the non-crystalline part between lamellae. By setting the stretching speed to 500% / min or less, it is possible to suppress breakage of the olefin-based resin film in the first stretching step.

  In addition, in this invention, the extending | stretching speed of an olefin resin film means the change rate of the dimension in the extending | stretching direction of the olefin resin film per unit time.

  The method for stretching the olefin resin film in the first stretching step is not particularly limited as long as the olefin resin film can be uniaxially stretched. For example, a plurality of rolls having different peripheral speeds are used for the olefin resin film. Examples thereof include a method of uniaxially stretching at a predetermined temperature using a stretching apparatus.

(Second stretching step)
In the second stretching step, the olefinic resin film after the first stretching step is subjected to a uniaxial stretching process at a surface temperature of 100 to 150 ° C. and a stretching ratio of 1.2 to 2.2 times. A stretched film is obtained. Also in the second stretching step, the olefin resin film is preferably uniaxially stretched only in the extrusion direction. By performing such a stretching process in the second stretching step, a large number of micropores formed in the olefin resin film in the first stretching step can be grown.

  In the second stretching step, the surface temperature of the olefin resin film is preferably 100 to 150 ° C, and more preferably 110 to 140 ° C. By setting the surface temperature of the olefin resin film to 100 ° C. or higher, the micropores formed in the olefin resin film in the first stretching step can be highly grown. Moreover, the obstruction | occlusion of the micropore part formed in the olefin resin film in a 1st extending | stretching process can be reduced by making the surface temperature of an olefin resin film into 150 degrees C or less.

  In the second stretching step, the stretching ratio of the olefin resin film is preferably 1.2 to 2.2 times, more preferably 1.5 to 2 times. By setting the draw ratio of the olefin resin film to 1.2 times or more, the micropores formed in the olefin resin film during the first drawing step can be grown. Thereby, the olefin resin microporous film having excellent air permeability can be provided. Moreover, it becomes possible by suppressing the draw ratio of an olefin resin film to 2.2 times or less to block | close the micropore part formed in the olefin resin film in a 1st extending | stretching process.

  In the second stretching step, the stretching speed of the olefin resin film is preferably 500% / min or less, more preferably 400% / min or less, and particularly preferably 15 to 60% / min. By setting the stretching speed of the olefin resin film within the above range, micropores can be uniformly formed in the olefin resin film.

  The method for stretching the olefin resin film in the second stretching step is not particularly limited as long as the olefin resin film can be uniaxially stretched. For example, a plurality of rolls having different peripheral speeds are used for the olefin resin film. Examples thereof include a method of uniaxially stretching at a predetermined temperature using a stretching apparatus.

(Annealing process)
Next, an olefin-based resin microporous film is manufactured by performing an annealing process for annealing the olefin-based resin stretched film after the stretching process, preferably after the second stretching process. This annealing process is performed in order to relieve the residual strain generated in the stretched olefin resin film in the stretching process described above, and to suppress the heat shrinkage caused by heating in the resulting olefin resin microporous film.

  110-175 degreeC is preferable and, as for the surface temperature of the olefin resin stretched film in an annealing process, 130-170 degreeC is more preferable. Furthermore, it is preferable that the surface temperature of the olefin resin stretched film in the annealing step is equal to or higher than the surface temperature of the olefin resin film in the second stretch step. By setting the surface temperature of the stretched olefin resin film to 110 ° C. or higher, the strain remaining in the stretched olefin resin film can be sufficiently relaxed. Moreover, the obstruction | occlusion of the micropore part formed at the extending process can be suppressed by making the surface temperature of an olefin resin stretched film into 175 degrees C or less.

  The shrinkage ratio of the stretched olefin resin film in the annealing step is preferably 25% or less. By setting the shrinkage ratio of the stretched olefin resin film to 25% or less, the occurrence of sagging of the stretched olefin resin film can be reduced and the stretched olefin resin film can be annealed uniformly.

  The shrinkage rate of the olefin resin film is obtained by dividing the shrinkage length of the olefin resin stretched film in the stretching direction during the annealing step by the length of the olefin resin stretched film in the stretching direction after the second stretching step. The value multiplied by 100.

  According to the method of the present invention, as described above, the micropores penetrating in the thickness direction of the film can be formed in the olefin resin film. Therefore, the olefin resin microporous film obtained by the method of the present invention has a high porosity and includes micropores that are uniformly formed three-dimensionally. Such an olefin-based resin microporous film has excellent resistance to pressure in the thickness direction and has a small thickness change rate even when pressure is applied in the thickness direction. Therefore, even if the olefin-based resin microporous film is incorporated into a secondary battery in which a high pressure is applied to the separator in the battery, such as a cylindrical battery or a square battery, the thickness direction change caused by the pressure The rate has been reduced. Therefore, the olefin-based resin microporous film can always sufficiently hold the electrolytic solution, always maintain excellent mechanical strength, and effectively prevent a short circuit between the positive electrode and the negative electrode due to dendrite. The olefin-based resin microporous film does not deform even when high pressure is applied to the microporous film due to the expansion and contraction of the electrode accompanying charging / discharging of the battery, and can maintain stable battery performance over a long period of time. A lithium ion secondary battery can be provided.

  The olefin resin microporous film of the present invention is suitably used as a separator for a nonaqueous electrolyte secondary battery. Examples of the non-aqueous electrolyte secondary battery include a lithium ion secondary battery.

  The non-aqueous electrolyte secondary battery is not particularly limited as long as it contains the olefin resin microporous film of the present invention as a separator, and includes a positive electrode, a negative electrode, a separator including an olefin resin microporous film, and non-aqueous electrolysis. Contains liquid. The olefin resin microporous film is disposed between the positive electrode and the negative electrode, thereby preventing an electrical short circuit between the electrodes. In addition, the nonaqueous electrolytic solution is filled at least in the micropores of the olefin resin microporous film, whereby lithium ions can move between the electrodes during charging and discharging.

  The positive electrode is not particularly limited, but preferably includes a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector. The positive electrode active material layer preferably includes a positive electrode active material and voids formed between the positive electrode active materials. When the positive electrode active material layer includes voids, the voids are also filled with the non-aqueous electrolyte. The positive electrode active material is a material capable of occluding and releasing lithium ions, and examples of the positive electrode active material include lithium cobaltate and lithium manganate. Examples of the current collector used for the positive electrode include aluminum foil, nickel foil, and stainless steel foil. The positive electrode active material layer may further contain a binder, a conductive auxiliary agent, and the like.

  The negative electrode is not particularly limited, but preferably includes a negative electrode current collector and a negative electrode active material layer formed on at least one surface of the negative electrode current collector. The negative electrode active material layer preferably includes a negative electrode active material and voids formed between the negative electrode active materials. When the negative electrode active material layer contains voids, the voids are also filled with the non-aqueous electrolyte. The negative electrode active material is a material capable of occluding and releasing lithium ions. Examples of the negative electrode active material include graphite, carbon black, acetylene black, and ketjen black. Examples of the current collector used for the negative electrode include copper foil, nickel foil, and stainless steel foil. The negative electrode active material layer may further contain a binder, a conductive auxiliary agent, and the like.

A nonaqueous electrolytic solution is an electrolytic solution in which an electrolyte salt is dissolved in a solvent that does not contain water. Examples of the non-aqueous electrolyte used in the lithium ion secondary battery include a non-aqueous electrolyte obtained by dissolving a lithium salt in an aprotic organic solvent. Examples of the aprotic organic solvent include a mixed solvent of a cyclic carbonate such as propylene carbonate and ethylene carbonate and a chain carbonate such as diethyl carbonate, methyl ethyl carbonate, and dimethyl carbonate. Examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , and LiN (SO ₂ CF ₃ ) ₂ .

[Example 1]
(Extrusion process)
A homopolypropylene film A was obtained using the production apparatus shown in FIG. First, homopolypropylene (weight average molecular weight 413000, molecular weight distribution 9.3, melting point 163 ° C.) is supplied to the extruder 1, melt-kneaded at a resin temperature of 200 ° C., and a T-die 2 attached to the tip of the extruder 1. Was extruded into a film. The extrusion rate was 9 kg / hour, the film formation rate was 22 m / min, and the draw ratio was 83.

(Cooling process)
The homopolypropylene film A extruded from the T die 2 is supplied to the outer peripheral surface of the first cooling roll 3 and taken up so as to be wound according to the rotation of the first cooling roll 3. The homopolypropylene film A was continuously conveyed by sending the homopolypropylene film A to the downstream side in the film conveyance direction in a state of being in close contact over the lower arc length. In addition, the surface temperature of the 1st cooling roll 3 was previously adjusted to 90 degreeC.

  Next, the homopolypropylene film A fed from the first cooling roll 3 is supplied to the second cooling roll 4 and taken up so as to be wound in accordance with the rotation of the second cooling roll 4. The propylene-based resin film A was continuously conveyed by sending the homopolypropylene film A to the downstream side in the film conveying direction in a state of being in close contact with the upper arc length of 4. In addition, the surface temperature of the 2nd cooling roll 4 was previously adjusted to 25 degreeC.

  The surface temperature of the homopolypropylene film A sent out from the second cooling roll 4 was 25 ° C. The long homopolypropylene film A after cooling had a thickness of 30 μm and a width of 200 mm.

(Curing process)
The obtained long homopolypropylene film A (length: 50 m) was wound around a cylindrical core having an outer diameter of 3 inches in a roll shape to obtain a homopolypropylene film roll. The homopolypropylene film roll was allowed to stand for 24 hours in a hot air oven in which the atmospheric temperature of the place where the roll was installed was 145 ° C. At this time, the temperature of the homopolypropylene film A was entirely the same as the temperature inside the hot stove from the surface of the roll to the inside.

(First stretching step)
Next, the homopolypropylene film A is continuously unwound from the roll, and after the surface temperature of the homopolypropylene film A is set to 20 ° C., the homopolypropylene film A is sequentially passed over the first stretching roll and the second stretching roll, By rotating the peripheral speed so as to be larger than the peripheral speed of the first stretching roll, the homopolypropylene film A is stretched at a stretching rate of 1.2 times at a stretching speed of 50% / min only in the transport direction (extrusion direction). Uniaxial stretching was performed.

(Second stretching step)
Next, the homopolypropylene film A fed from the second stretching roll is supplied into the heating furnace, and after the surface temperature of the homopolypropylene film A is set to 120 ° C., the seven stretching films installed in the heating furnace Homopolypropylene film A is wound on each of the rolls up and down and zigzag in the transport direction, and the respective peripheral speeds of the stretching rolls are rotated so as to increase sequentially in the transport direction of the homopolypropylene film A. The film was uniaxially stretched only in the transport direction at a stretching ratio of 2.0 times at a stretching rate of 42% / min, thereby obtaining a homopolypropylene stretched film.

(Annealing process)
Next, the homopolypropylene stretched film is sequentially supplied to the first roll and the second roll disposed above and below in the hot air oven so that the surface temperature of the homopolypropylene stretched film becomes 155 ° C. The homopolypropylene stretched film was annealed by being transported in a hot stove for 4 minutes without applying tension to obtain a homopolypropylene microporous film (thickness: 25 μm, basis weight: 11 g / m ² ). The shrinkage ratio of the homopolypropylene stretched film in the annealing step was 5%.

[Comparative Example 1]
A homopolypropylene microporous film was obtained in the same manner as in Example 1 except that the surface temperature of the first cooling roll 3 in the cooling step was 25 ° C. and the atmospheric temperature in the hot stove in the curing step was 144 ° C.

[Comparative Example 2]
A polyethylene microporous film was obtained in the same manner as in Example 1 except that polyethylene (weight average molecular weight 170000, molecular weight distribution 31.2, melting point 130 ° C.) was used instead of homopolypropylene.

[Comparative Example 3]
The polypropylene resin was put into a twin-screw extruder, melted and kneaded at a set temperature of 300 ° C., extruded, cooled and solidified with a casting roll at 120 ° C. to produce a polypropylene resin film. The polypropylene resin film is stretched 4 times in the longitudinal direction using a longitudinal stretching machine, then stretched twice in the transverse direction at 105 ° C. by a transverse stretching machine, and then subjected to an annealing treatment at 125 ° C. Thus, a polypropylene resin microporous film was obtained.

[Evaluation]
With respect to the homopolypropylene microporous film, polyethylene microporous film and polypropylene resin microporous film prepared in Examples and Comparative Examples, the porosity was measured according to the method described above.

For the homopolypropylene microporous film, polyethylene microporous film, and polypropylene resin microporous film of Examples and Comparative Examples, the rate of change in thickness when a pressure of 35 MPa was applied in the thickness direction was measured according to the method described above. In Table 1, it is simply expressed as “thickness change rate”. Table 1 shows the initial thickness T ₀ and the compressed thickness T ₁ of the homopolypropylene microporous film, the polyethylene microporous film, and the polypropylene resin microporous film.

For the homopolypropylene microporous film, polyethylene microporous film, and polypropylene resin microporous film of Examples and Comparative Examples, the rate of change in air permeability was measured according to the method described above when a pressure of 35 MPa was applied in the thickness direction. did. In Table 1, it is simply expressed as “change rate of air permeability”. Table 1 shows the initial air permeability P ₀ and the compressed air permeability P ₁ of the homopolypropylene microporous film, the polyethylene microporous film, and the polypropylene resin microporous film.

[Battery characteristics evaluation]
According to the following procedures, lithium ion batteries were prepared using the homopolypropylene microporous film, polyethylene microporous film, and polypropylene resin microporous film of Examples and Comparative Examples as separators, and cycle characteristics were evaluated.

(Production of lithium ion battery)
For positive electrode formation by mixing and stirring 92% by weight of LiMn ₂ O ₄ (average particle size: 26 μm) as a positive electrode active material, 4% by weight of carbon black as a conductive additive, and 4% by weight of polyvinylidene fluoride as a binder resin A composition was prepared, and this positive electrode forming composition was applied to one surface of an aluminum foil as a positive electrode current collector using a comma coater and dried to prepare a positive electrode active material layer. Thereafter, a positive electrode current collector having a positive electrode active material layer formed on one surface was punched into a plane rectangle of 30 mm length × 60 mm width to obtain a positive electrode.

  Next, a composition for forming a negative electrode was prepared by mixing and stirring 91% by weight of graphite particles as a negative electrode active material, 5% by weight of carbon black as a conductive additive, and 4% by weight of polyvinylidene fluoride as a binder resin. Prepare an electrolytic copper foil with one surface roughened by an electrolysis method as a negative electrode current collector, apply the composition for forming a negative electrode on the roughened surface of this electrolytic copper foil using a comma coater, and dry it. Thus, a negative electrode active material layer was produced. Thereafter, a negative electrode current collector having a negative electrode active material layer formed on one surface was punched into a plane rectangle of 30 mm length × 60 mm width to obtain a negative electrode.

Next, the positive electrode, the microporous film, and the negative electrode are stacked so that the positive electrode active material layer and the negative electrode active material layer face each other with the microporous film therebetween, and a laminate is formed, and a tab is arranged on each electrode. After setting, the laminate was dried under reduced pressure at 80 ° C. for 12 hours. The laminate was dried under reduced pressure. The laminate was housed in an exterior material and an electrolyte was injected in an argon gas atmosphere, and then the exterior material was sealed under reduced pressure to produce a lithium ion battery. Incidentally, the electrolyte, ethylene carbonate and ethyl methyl carbonate 3: 7 LiPF ₆ solution a mixed solution obtained by mixing at a volume ratio as a solvent (1 mol / L) was used.

  The lithium ion secondary battery produced as described above was placed in a constant temperature bath at 25 ° C., and charging / discharging was repeated at a current corresponding to 1C. “◎” when the capacity retention rate after 1000 cycles (discharge capacity after 100 × 1000 cycles / initial discharge capacity) is 85% or more, “◯” when 78% or more and less than 85%, 68 % And less than 78% were evaluated as “Δ”, and less than 68% as “x”.

  The olefin resin microporous film of the present invention is suitably used as a separator for a nonaqueous electrolyte secondary battery.

1 Extruder 2 T-die
2a Extrusion port 3 First cooling roll 4 Second cooling roll A Propylene resin film

Claims (4)

An olefin-based resin microporous film having micropores, wherein the thickness change rate when a pressure of 35 MPa is applied in the thickness direction is 10% or less and the porosity is 40 to 60%. Olefin-based resin microporous film. The olefin-based resin microporous film according to claim 1, wherein the olefin-based resin microporous film includes an olefin-based resin stretched film having micropores. A separator for a non-aqueous electrolyte secondary battery comprising the olefin-based resin microporous film according to claim 1. A nonaqueous electrolyte secondary battery comprising: a negative electrode; a positive electrode; a separator for a nonaqueous electrolyte secondary battery according to claim 3; and a nonaqueous electrolyte.

Images