Microporous membranes for energy storage devices

Abstract

To improve membrane resistance of a microporous film under high temperature, without sacrifice of strength and openness of the microporous film, and to balance device characteristics of a power storage device including the microporous film and high safety in nail piercing test.SOLUTION: Polyolefin contained in microporous film for power storage device has one kind or more of functional group, MFR of 3.0 g / 10 min or less, Mw / Mn of 15 or less, and density of 0.85 g / cm3 or more. In wide-angle X-ray scattering measurement of the microporous film for power storage device, the orientation ratio MD / TD is 1.3 or more, and after the microporous film for power storage device is received in the power storage device, (1) condensation reaction takes place between the functional groups, (2) the functional group reacts on a chemical material in the power storage device, or (3) the functional group reacts on other kind of functional group, and a bridge structure is formed.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a microporous membrane for an electricity storage device. [Background technology]

[0002] Microporous membranes are widely used as separation or permselective separation membranes for various substances, separators, etc., and examples of their applications include microfiltration membranes, separators for fuel cells and capacitors, base materials for functional membranes in which functional materials are filled into the pores to exhibit new functions, separators for electricity storage devices, etc. In particular, polyolefin microporous membranes are suitably used as separators for lithium ion batteries, which are widely used in notebook personal computers, mobile phones, digital cameras, etc.

[0003] To ensure battery safety, it has been proposed to achieve both activation of the shutdown function and an increase in the membrane rupture temperature by forming a crosslinked structure in the separator (Patent Documents 1 to 8). For example, Patent Documents 1 to 6 describe a silane crosslinked structure formed by contacting a separator containing silane-modified polyolefin with water. Patent Document 7 describes a crosslinked structure formed by ring-opening of norbornene by irradiation with ultraviolet light, electron beams, or the like. Patent Document 8 describes a separator insulating layer containing a (meth)acrylic acid copolymer with a crosslinked structure, a styrene-butadiene rubber binder, or the like.

[0004] Lithium-ion batteries use positive and negative electrode materials, electrolytes, and separators. Of these components, separators are required to be inert to electrochemical reactions and surrounding components due to their suitability as insulating materials. Meanwhile, since the early development of lithium-ion battery negative electrode materials, a technology has been established to suppress decomposition of the electrolyte on the negative electrode surface by forming a solid electrolyte interface (SEI) through a chemical reaction during initial charging (Non-Patent Document 1). Even when polyolefin resins are used for separators, oxidation reactions are induced on the positive electrode surface under high voltage, and cases of separator blackening and surface deterioration have been reported. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 9-216964 [Patent Document 2] International Publication No. 97 / 44839 [Patent Document 3] Japanese Patent Application Publication No. 11-144700 [Patent Document 4] Japanese Patent Application Publication No. 11-172036 [Patent Document 5] Japanese Patent Application Laid-Open No. 2001-176484 [Patent Document 6] Japanese Patent Application Laid-Open No. 2000-319441 [Patent Document 7] Japanese Patent Application Laid-Open No. 2011-071128 [Patent Document 8] Japanese Patent Application Laid-Open No. 2014-056843 [Non-patent literature]

[0006] [Non-Patent Document 1] Lithium-ion Secondary Batteries (2nd Edition) Published by Nikkan Kogyo Shimbun Summary of the Invention [Problem to be solved by the invention]

[0007] In recent years, lithium-ion secondary batteries for mobile devices and automobiles have been achieving higher output and higher energy density. Meanwhile, there is a demand for smaller battery cells and stable cycle discharge and charge performance during long-term use. Therefore, strength and porosity are required for the production of microporous membranes that can be used as battery separators. Furthermore, battery safety standards have become stricter than before, and as described in Patent Documents 1 and 2, separators with shutdown functionality and high-temperature membrane rupture properties, as well as stable manufacturing methods for such separators, are desired. In this regard, a shutdown temperature of less than 150°C is desirable, and a higher membrane rupture temperature is desirable.

[0008] However, the crosslinking methods described in Patent Documents 1 to 8 are all performed in-process on the microporous membrane or in batches immediately after the production of the microporous membrane. Therefore, after forming the crosslinked structure described in Patent Documents 1 to 8, the microporous membrane must be coated and slit for use as a separator. The subsequent lamination and winding process with electrodes increases internal stress, which can lead to deformation of the produced electricity storage device. For example, when a crosslinked structure is formed by heating, the internal stress of the separator having the crosslinked structure can increase at room temperature or room temperature. Furthermore, when a crosslinked structure is formed by irradiating a microporous membrane with light such as ultraviolet light or an electron beam, the light irradiation can become non-uniform, resulting in a non-homogeneous crosslinked structure. This is thought to be because the periphery of the crystalline portion of the resin constituting the microporous membrane is easily crosslinked by the electron beam.

[0009] In view of the above problems, an object of the present invention is to improve the membrane rupture resistance of a microporous membrane at high temperatures without impairing the strength and openability of the microporous membrane, and to achieve both good device properties and high safety in a nail penetration test for an electricity storage device using the microporous membrane as a separator. [Means for solving the problem]

[0010] The above problems are solved by the following technical means. [1] A microporous membrane for an electricity storage device comprising a polyolefin, The polyolefin has one or more functional groups, and After being stored in the electricity storage device, (1) the functional groups undergo a condensation reaction with each other, (2) the functional groups react with chemical substances inside the electricity storage device, or (3) the functional groups react with other types of functional groups to form a crosslinked structure, The polyolefin satisfies the following requirements (A) to (C): (A) The melt flow rate (MFR) measured under conditions of a temperature of 230°C and a mass of 2.16 kg is 3.0 g / 10 min or less; (B) the value obtained by dividing the weight average molecular weight Mw by the number average molecular weight Mn (Mw / Mn) is 15 or less; and (C) Density is 0.85 g / cm 3 That's it; and The microporous membrane for an electricity storage device meets the following requirement (D): (D) The ratio of the orientation rate in the transverse direction (TD) to the orientation rate in the machine direction (MD), MD / TD, is 1.3 or more as measured by wide-angle X-ray scattering; A microporous membrane for an electricity storage device, which satisfies the above requirements. [2] Item 2. The microporous membrane for an electricity storage device according to item 1, wherein the crosslinked structure is formed by (1) a condensation reaction between the functional groups. [3] Item 2. The microporous membrane for an electricity storage device according to item 1, wherein the crosslinked structure is formed by (2) the functional group reacting with a chemical substance inside the electricity storage device. [4] 4. The microporous membrane for an electricity storage device according to item 1 or 3, wherein the chemical substance is any one of an electrolyte, an electrolytic solution, an electrode active material, an additive, or a decomposition product thereof contained in the electricity storage device. [5] Item 3. The microporous membrane for an electricity storage device according to item 1, wherein the crosslinked structure is formed by (3) the functional group reacting with another type of functional group. [6] The microporous membrane for an electricity storage device has the following formula (I): R E’X =E' Z / E' Z0 (I) {where, E' Z is the storage modulus measured in a temperature range of 160°C to 300°C after the crosslinking reaction of the microporous membrane for an electricity storage device has progressed in the electricity storage device, and E' Z0 is the storage modulus measured in a temperature range of 160°C to 300°C before the microporous membrane for an electricity storage device is incorporated into the electricity storage device, and E' Z or E' Z0 The conditions for measuring the storage modulus are defined by the following features (i) to (iv). (i) Dynamic viscoelasticity measurement was performed under the following conditions: Atmosphere: Nitrogen Measurement device used: RSA-G2 (TA Instruments) Sample thickness: 5 μm to 50 μm (measurement is performed on one sample regardless of the sample thickness) Measurement temperature range: -50 to 300°C Heating rate: 10℃ / min ·Measurement frequency: 1Hz Deformation mode: Linear tension Initial static tensile load: 0.5N Initial gap distance (at 25°C): 25mm Auto strain adjustment: Enabled (Range: Amplitude 0.05 to 25%, Sine wave load 0.02 to 5N) It was carried out at. (ii) The static tensile load refers to the intermediate value between the maximum stress and the minimum stress in each periodic motion, and the sinusoidal load refers to the oscillating stress centered on the static tensile load. (iii) Sine wave tension mode refers to measuring the vibration stress while performing periodic motion with a fixed amplitude of 0.2%, and the vibration stress was measured by varying the gap distance and static tensile load so that the difference between the static tensile load and the sinusoidal load was within 20%. When the sinusoidal load became 0.02 N or less, the amplitude was amplified so that the sinusoidal load was within 5 N and the increase in amplitude was within 25%, and the vibration stress was measured. (iv) The relationship between the obtained sinusoidal load and amplitude value, and the following equation: σ * =σ0·Exp[i(ωt+δ)], ε * =ε0·Exp(iωt), σ * =E * ε * E * =E'+iE'' (In the formula, σ * : Vibration stress, ε * : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between vibration stress and strain, E * : Complex modulus, E': Storage modulus, E'': Loss modulus Vibration stress: sinusoidal load / initial cross-sectional area Static tensile load: Load at the minimum point of vibration stress at each cycle (minimum point of gap distance at each cycle) Sinusoidal load: the difference between the measured oscillatory stress and the static tensile load The storage modulus is calculated from The mixed storage modulus ratio (R E’x 6. The microporous membrane for an electricity storage device according to any one of items 1 to 5, wherein the ratio of the surface area to the surface area of ​​the microporous membrane is 1.2 to 20 times. [7] A microporous membrane for an electricity storage device comprising a polyolefin, the microporous membrane for an electricity storage device having an amorphous portion crosslinked structure in which amorphous portions of the polyolefin are crosslinked; The polyolefin satisfies the following requirements (A) to (C): (A) The melt flow rate (MFR) measured under conditions of a temperature of 230°C and a mass of 2.16 kg is 3.0 g / 10 min or less; (B) the value obtained by dividing the weight average molecular weight Mw by the number average molecular weight Mn (Mw / Mn) is 15 or less; and (C) Density is 0.85 g / cm 3 That's it; and The microporous membrane for an electricity storage device meets the following requirement (D): (D) The ratio of the orientation rate in the transverse direction (TD) to the machine direction (MD) as measured by wide-angle X-ray scattering is 1.3 or more; A microporous membrane for an electricity storage device, which satisfies the above requirements. [8] Item 8. The microporous membrane for an electricity storage device according to item 7, wherein the amorphous portion is selectively crosslinked. [9] The microporous membrane for an electricity storage device has the following formula (II): R E’mix =E' / E'0(II) In the formula, E' is the storage modulus measured at 160°C to 300°C when the microporous membrane for an electricity storage device has the amorphous crosslinked structure, and E'0 is the storage modulus of a microporous membrane for an electricity storage device that does not have an amorphous crosslinked structure, measured at 160°C to 300°C, and the measurement conditions for the storage modulus E' or E'0 are specified in the following configurations (i) to (iv). (i) Dynamic viscoelasticity measurement was performed under the following conditions: Atmosphere: Nitrogen Measurement device used: RSA-G2 (TA Instruments) Sample thickness: 5 μm to 50 μm (measurement is performed on one sample regardless of the sample thickness) Measurement temperature range: -50 to 300°C Heating rate: 10℃ / min ·Measurement frequency: 1Hz Deformation mode: Linear tension Initial static tensile load: 0.5N Initial gap distance (at 25°C): 25mm Auto strain adjustment: Enabled (Range: Amplitude 0.05 to 25%, Sine wave load 0.02 to 5N) It was carried out at. (ii) The static tensile load refers to the intermediate value between the maximum stress and the minimum stress in each periodic motion, and the sinusoidal load refers to the oscillating stress centered on the static tensile load. (iii) Sine wave tension mode refers to measuring the vibration stress while performing periodic motion with a fixed amplitude of 0.2%, and the vibration stress was measured by varying the gap distance and static tensile load so that the difference between the static tensile load and the sinusoidal load was within 20%. When the sinusoidal load became 0.02 N or less, the amplitude was amplified so that the sinusoidal load was within 5 N and the increase in amplitude was within 25%, and the vibration stress was measured. (iv) The relationship between the obtained sinusoidal load and amplitude value, and the following equation: σ * =σ0·Exp[i(ωt+δ)], ε * =ε0·Exp(iωt), σ * =E * ε * E * =E'+iE'' (In the formula, σ * : Vibration stress, ε * : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between vibration stress and strain, E * : Complex modulus, E': Storage modulus, E'': Loss modulus Vibration stress: sinusoidal load / initial cross-sectional area Static tensile load: Load at the minimum point of vibration stress at each cycle (minimum point of gap distance at each cycle) Sinusoidal load: the difference between the measured oscillatory stress and the static tensile load The storage modulus is calculated from The mixed storage modulus ratio (R E’mix Item 9. The microporous membrane for an electricity storage device according to item 7 or 8, wherein the ratio of the surface area to the surface area of ​​the microporous membrane is 1.2 to 20 times.

[10] The polyolefin has an MFR of 0.25 g / 10 min or more, an Mw / Mn of 4.0 or more, and a density of 1.1 g / cm 3 is less than or equal to, and the ratio of orientation rates MD / TD of the microporous membrane for an electricity storage device is 3.0 or less; 10. The microporous membrane for an electricity storage device according to any one of items 1 to 9.

[11] 11. The microporous membrane for an electricity storage device according to any one of items 1 to 10, wherein the polyolefin is polypropylene.

[12] A microporous membrane for an electricity storage device comprising polypropylene, The polypropylene has one or more functional groups and is β-crystal active; After the microporous membrane for an electricity storage device is housed in the electricity storage device, (1) the functional groups undergo a condensation reaction with each other, (2) the functional groups react with chemical substances inside the electricity storage device, or (3) the functional groups react with other types of functional groups to form a crosslinked structure.

[13] Item 13. The microporous membrane for an electricity storage device according to item 12, wherein the crosslinked structure is formed by (1) a condensation reaction between the functional groups.

[14] Item 13. The microporous membrane for an electricity storage device according to item 12, wherein the crosslinked structure is formed by (2) the functional group reacting with a chemical substance inside the electricity storage device.

[15] Item 15. The microporous membrane for an electricity storage device according to Item 12 or 14, wherein the chemical substance is any one of an electrolyte, an electrolytic solution, an electrode active material, an additive, or a decomposition product thereof contained in the electricity storage device.

[16] Item 13. The microporous membrane for an electricity storage device according to Item 12, wherein the crosslinked structure is formed by (3) the functional group reacting with another type of functional group.

[17] The microporous membrane for an electricity storage device has the following formula (I): R E’X =E' Z / E' Z0 (I) {where, E' Z is the storage modulus measured in a temperature range of 160°C to 300°C after the crosslinking reaction of the microporous membrane for an electricity storage device has progressed in the electricity storage device, and E' Z0 is the storage modulus measured in a temperature range of 160°C to 300°C before the porous membrane for an electricity storage device is incorporated into the electricity storage device, and E' Z or E' Z0 The conditions for measuring the storage modulus are defined by the following features (i) to (iv). (i) Dynamic viscoelasticity measurement was performed under the following conditions: Atmosphere: Nitrogen Measurement device used: RSA-G2 (TA Instruments) Sample thickness: 5 μm to 50 μm (measurement is performed on one sample regardless of the sample thickness) Measurement temperature range: -50 to 300°C Heating rate: 10℃ / min ·Measurement frequency: 1Hz Deformation mode: Linear tension Initial static tensile load: 0.5N Initial gap distance (at 25°C): 25mm Auto strain adjustment: Enabled (Range: Amplitude 0.05 to 25%, Sine wave load 0.02 to 5N) It was carried out at. (ii) The static tensile load refers to the intermediate value between the maximum stress and the minimum stress in each periodic motion, and the sinusoidal load refers to the oscillating stress centered on the static tensile load. (iii) Sine wave tension mode refers to measuring the vibration stress while performing periodic motion with a fixed amplitude of 0.2%, and the vibration stress was measured by varying the gap distance and static tensile load so that the difference between the static tensile load and the sinusoidal load was within 20%. When the sinusoidal load became 0.02 N or less, the amplitude was amplified so that the sinusoidal load was within 5 N and the increase in amplitude was within 25%, and the vibration stress was measured. (iv) The relationship between the obtained sinusoidal load and amplitude value, and the following equation: σ * =σ0·Exp[i(ωt+δ)], ε * =ε0·Exp(iωt), σ* =E * ε * E * =E'+iE'' (In the formula, σ * : Vibration stress, ε * : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between vibration stress and strain, E * : Complex modulus, E': Storage modulus, E'': Loss modulus Vibration stress: sinusoidal load / initial cross-sectional area Static tensile load: Load at the minimum point of vibration stress at each cycle (minimum point of gap distance at each cycle) Sinusoidal load: the difference between the measured oscillatory stress and the static tensile load The storage modulus is calculated from The mixed storage modulus ratio (R E’x 17. The microporous membrane for an electricity storage device according to any one of items 12 to 16, wherein the ratio of the surface area to the surface area of ​​the microporous membrane is 1.2 to 20 times.

[18] A microporous membrane for an electricity storage device comprising polypropylene, wherein the polypropylene is β-crystal active and the microporous membrane for an electricity storage device has an amorphous portion crosslinked structure in which amorphous portions of the polypropylene are crosslinked.

[19] Item 19. The microporous membrane for an electricity storage device according to item 18, wherein the amorphous portion is selectively crosslinked.

[20] The microporous membrane for an electricity storage device has the following formula (II): R E’mix =E' / E'0(II) In the formula, E' is the storage modulus measured at 160°C to 300°C when the microporous membrane for an electricity storage device has the amorphous crosslinked structure, and E'0 is the storage modulus of a microporous membrane for an electricity storage device that does not have an amorphous crosslinked structure, measured at 160°C to 300°C, and the measurement conditions for the storage modulus E' or E'0 are specified in the following configurations (i) to (iv). (i) Dynamic viscoelasticity measurement was performed under the following conditions: Atmosphere: Nitrogen Measurement device used: RSA-G2 (TA Instruments) Sample thickness: 5 μm to 50 μm (measurement is performed on one sample regardless of the sample thickness) Measurement temperature range: -50 to 300°C Heating rate: 10℃ / min ·Measurement frequency: 1Hz Deformation mode: Linear tension Initial static tensile load: 0.5N Initial gap distance (at 25°C): 25mm Auto strain adjustment: Enabled (Range: Amplitude 0.05 to 25%, Sine wave load 0.02 to 5N) It was carried out at. (ii) The static tensile load refers to the intermediate value between the maximum stress and the minimum stress in each periodic motion, and the sinusoidal load refers to the oscillating stress centered on the static tensile load. (iii) Sine wave tension mode refers to measuring the vibration stress while performing periodic motion with a fixed amplitude of 0.2%, and the vibration stress was measured by varying the gap distance and static tensile load so that the difference between the static tensile load and the sinusoidal load was within 20%. When the sinusoidal load became 0.02 N or less, the amplitude was amplified so that the sinusoidal load was within 5 N and the increase in amplitude was within 25%, and the vibration stress was measured. (iv) The relationship between the obtained sinusoidal load and amplitude value, and the following equation: σ * =σ0·Exp[i(ωt+δ)], ε * =ε0·Exp(iωt), σ * =E * ε * E * =E'+iE'' (In the formula, σ * : Vibration stress, ε * : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between vibration stress and strain, E * : Complex modulus, E': Storage modulus, E'': Loss modulus Vibration stress: sinusoidal load / initial cross-sectional area Static tensile load: Load at the minimum point of vibration stress at each cycle (minimum point of gap distance at each cycle) Sinusoidal load: the difference between the measured oscillatory stress and the static tensile load The storage modulus is calculated from The mixed storage modulus ratio (R E’mix 20. The microporous membrane for an electricity storage device according to item 18 or 19, wherein the ratio of the surface area to the surface area of ​​the microporous membrane is 1.2 to 20 times. [twenty one] The microporous membrane for an electricity storage device contains, as the polypropylene, The following requirements (P1) to (P3): (P1) The MFR when measured under conditions of a temperature of 230°C and a mass of 2.16 kg is 2.5 g / 10 min or less; (P2) the value (Mw / Mn) obtained by dividing the weight average molecular weight Mw by the number average molecular weight Mn is 10 or less; and (P3) Density is 0.89 g / cm 3 That's it: A homopolypropylene (A) that satisfies the above formula: a polypropylene (B) that does not satisfy at least one of the requirements (P1) to (P3) and has a functional group; 21. The microporous membrane for an electricity storage device according to any one of items 12 to 20, comprising: [twenty two] Item 22. The microporous membrane for an electricity storage device according to item 21, wherein the content of the polypropylene (B) is 4% by mass or more and 30% by mass or less. [twenty three] Item 23. The microporous membrane for an electricity storage device according to Item 21 or 22, wherein the polypropylene (B) is a silane-modified polypropylene. [twenty four] The homopolypropylene (A) has an MFR of 0.25 g / 10 min or more, an Mw / Mn of 4.9 or more, and a density of 0.96 g / cm 3 24. The microporous membrane for an electricity storage device according to any one of items 21 to 23, wherein: [Effects of the Invention]

[0011] According to the present invention, it is possible to achieve improved resistance to membrane rupture at high temperatures without impairing the strength and pore openability of a microporous membrane for an electricity storage device, and it is possible to achieve both high battery characteristics and high safety in a nail penetration test for an electricity storage device including the microporous membrane as a separator. Furthermore, according to the present invention, it is not necessary to form a crosslinked structure during or immediately after the membrane formation process, which makes it possible to suppress an increase in internal stress in the separator and deformation after fabrication of the electricity storage device, and / or it is possible to impart a crosslinked structure to the separator without using relatively high energy such as light irradiation or heating. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram illustrating a crystalline polymer having a higher-order structure divided into lamellae (crystalline portions), amorphous portions, and intermediate layers between them. [Figure 2] FIG. 1 is a schematic diagram for explaining crystal growth of polyolefin molecules. [Figure 3] FIG. 3 is an example of a graph illustrating the relationship between temperature and storage modulus, comparing the storage modulus of a reference film and a crosslinked film within the temperature range of −50°C to 310°C, and showing the transition temperature between the rubber-like plateau region and the crystalline melt flow region. [Figure 4] FIG. 4 is an example of a graph illustrating the relationship between temperature and loss modulus, comparing the loss modulus of a reference membrane and a crosslinked membrane within the temperature range of −50°C to 310°C, and showing the transition temperature between the rubber-like plateau region and the crystalline melt flow region. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, a mode for carrying out the present invention (hereinafter abbreviated as "embodiment") will be described in detail. Note that the present invention is not limited to the following embodiment, and various modifications can be made within the scope of the gist thereof.

[0014] [Microporous membrane for electricity storage device] The microporous membrane may be formed of one or more types of polyolefin resin, or may be a composite resin membrane containing a polyolefin resin and other resins, and has many fine pores. A microporous membrane containing a polyolefin resin as a main component (hereinafter also referred to as a polyolefin microporous membrane) contains 50% by mass or more of the polyolefin resin relative to the mass of the membrane.

[0015] From the viewpoints of resistance to oxidation-reduction degradation and a dense, uniform porous structure, polyolefin-based microporous membranes are preferably used to form electricity storage devices, more preferably as a constituent material for electricity storage devices, even more preferably as separators for electricity storage devices, and particularly preferably as separators for lithium-ion batteries. In this specification, a separator for an electricity storage device (hereinafter sometimes abbreviated as "separator") refers to a component disposed between multiple electrodes in an electricity storage device and having ion permeability and, if necessary, shutdown properties. The separator includes a microporous membrane and may further include any functional layer, if desired.

[0016] [First and Second Embodiments] The microporous membrane according to the first embodiment contains a polyolefin having one or more functional groups, and after being housed in an electricity storage device, a crosslinked structure is formed by (1) a condensation reaction between the functional groups of the polyolefin, (2) the functional groups of the polyolefin react with chemicals inside the electricity storage device, or (3) the functional groups of the polyolefin react with other types of functional groups. In the first embodiment, the microporous membrane can form a crosslinked structure by any of the reactions (1) to (3) above, thereby maintaining strength and improving membrane rupture resistance at high temperatures of 150°C or higher. For example, when housed in an electricity storage device as a separator, the membrane tends to achieve both device properties and safety.

[0017] Furthermore, in the first embodiment, the polyolefin contained in the microporous membrane satisfies the following requirements (A) to (C): (A) The melt flow rate (MFR) measured under conditions of a temperature of 230°C and a mass of 2.16 kg is 3.0 g / 10 min or less; (B) the value obtained by dividing the weight average molecular weight Mw by the number average molecular weight Mn (Mw / Mn) is 15 or less; and (C) Density is 0.85 g / cm 3 That's it; and the microporous membrane satisfies the following requirement (D): (D) The ratio of the orientation rate in the transverse direction (TD) to the orientation rate in the machine direction (MD), MD / TD, is 1.3 or more as measured by wide-angle X-ray scattering; In the first embodiment, when the polyolefin and the microporous membrane satisfy the requirements (A) to (D), the strength, membrane formability, productivity, and pore openability of the microporous membrane tend to be improved.

[0018] Regarding requirement (A), when the MFR of the polyolefin (PO) is 3.0 g / 10 min or less when measured at a temperature of 230°C and a mass of 2.16 kg, the strength of the resulting microporous membrane is likely to reach an acceptable level. From the same viewpoint, the MFR of the PO is preferably 0.25 to 2.9 g / 10 min, more preferably 0.3 to 2.7 g / 10 min, and even more preferably 0.4 to 2.5 g / 10 min. When measured at a temperature of 230°C and a mass of 2.16 kg, the MFR of the polyolefin (PO) may be 0.5 g / 10 min or more, 0.6 g / 10 min or more, 0.7 g / 10 min or more, 0.8 g / 10 min or more, 0.9 g / 10 min or more, or 1.0 g / 10 min or more. The MFR of the polyolefin (PO) when measured under conditions of a temperature of 230°C and a mass of 2.16 kg may be 2.3 g / 10 min or less, 2.0 g / 10 min or less, 1.8 g / 10 min or less, or 1.5 g / 10 min or less.

[0019] Regarding requirement (B), when the polydispersity (Mw / Mn) of PO is 15 or less, film formability and strength during molding of the resulting PO tend to be compatible. From the viewpoint of compatible film formability and strength, the polydispersity (Mw / Mn) of PO is preferably 4.0 to 13, more preferably 4.9 to 11, and even more preferably 5.2 to 9.0. The polydispersity (Mw / Mn) of PO may be 5.5 or more, 5.8 or more, 6.0 or more, 6.2 or more, or 6.5 or more. The polydispersity (Mw / Mn) of PO may be 8.0 or less, 7.0 or less, or 6.5 or less.

[0020] Regarding requirement (C), the PO density is closely related to the PO crystallinity, so the PO density should be 0.85 g / cm 3 A PO density of 0.85 g / cm or higher improves the productivity of microporous membranes and is particularly effective for dry porosity processes. From the viewpoints of productivity and pore opening, the PO density is 0.85 g / cm or higher. 3 More than 0.88 g / cm is preferable. 3 or more is preferable, or 0.90 g / cm 3 The PO density is preferably 1.1 g / cm or more. 3 Less than 1.0 g / cm is preferred 3 Preferably less than 0.98 g / cm 3 Preferably less than 0.97 g / cm 3 Preferably less than 0.96 g / cm 3 Preferably less than 0.95 g / cm 3 Preferably less than 0.94 g / cm 3 Preferably less than 0.93 g / cm 3 Preferably less than or equal to 0.92 g / cm 3 Preferably, the PO is, for example, polypropylene or polyethylene.

[0021] Regarding requirement (D), when the orientation ratio MD / TD is 1.3 or more during wide-angle X-ray scattering measurement of the microporous membrane, the membrane's microporosity, crosslinked structure, and ion permeability tend to be compatible with desired device characteristics. From the viewpoint of compatibility between membrane properties and device characteristics, the lower limit of the orientation ratio MD / TD is preferably 1.4 or more or 1.5 or more, more preferably 1.6 or more. The upper limit of the orientation ratio MD / TD can be, for example, 4.0 or less, 3.5 or less, or 3.1 or less, depending on the membrane-forming process.

[0022] The microporous membrane according to the second embodiment contains β-crystal activated polypropylene having one or more functional groups, and after being installed in an electricity storage device, (1) the functional groups of the polypropylene undergo a condensation reaction with each other, (2) the functional groups of the polypropylene react with chemicals inside the electricity storage device, or (3) the functional groups of the polypropylene react with other types of functional groups to form a crosslinked structure. In the second embodiment, an unstretched sheet having β-crystals is produced by melt-extrusion of polypropylene having β-crystal activity, and the unstretched sheet is stretched to cause crystal transition to α-crystals, which have a relatively high crystal density, and micropores are formed due to the difference in crystal density between the two to obtain a microporous membrane. The resulting microporous membrane can form a crosslinked structure through any of the above reactions (1) to (3), thereby maintaining strength and improving membrane rupture resistance at high temperatures of 150°C or higher, tending to achieve both the device properties and safety of the electricity storage device. The microporous membrane according to the second embodiment may contain one or more types of polypropylene, and when it contains multiple types of polypropylene, at least one of them has one or more types of functional groups and β-crystal activity.

[0023] In the first and second embodiments, it is believed that functional groups contained in polyolefins such as polypropylene are not incorporated into the crystalline parts of the polyolefin but are crosslinked in the amorphous parts. Therefore, after being housed in an electricity storage device, the microporous membranes according to the first and second embodiments utilize the surrounding environment or chemicals inside the electricity storage device to form a crosslinked structure, thereby suppressing an increase in internal stress or deformation of the fabricated electricity storage device and contributing to safety. On the other hand, if the microporous membrane undergoes a crosslinking reaction and processes such as winding and slitting before being installed in an electricity storage device, the effects of stress such as tension generated during those processes will remain. In this case, if the stress is released after the electricity storage device is assembled, it may be undesirable because it may cause deformation or damage to the electrode wound material due to stress concentration.

[0024] Furthermore, in the first and second embodiments, it is not necessary to form a crosslinked structure during or immediately after the film formation process. This can suppress an increase in internal stress and deformation after fabrication of an electricity storage device when the microporous membrane is used as a separator, and / or can contribute to energy savings by eliminating the need for light irradiation or heating to form a crosslinked structure.

[0025] [Third and Fourth Embodiments] The microporous membrane according to the third embodiment contains a polyolefin and has an amorphous crosslinked structure in which the amorphous parts of the polyolefin are crosslinked. In the third embodiment, the amorphous crosslinked structure of the polyolefin can improve membrane rupture resistance at high temperatures of 150°C or higher, and when the microporous membrane is incorporated into an electricity storage device as a separator, it tends to achieve both good device properties and safety.

[0026] Furthermore, in a third embodiment, the polyolefin contained in the microporous membrane satisfies the following requirements (A) to (C): (A) The melt flow rate (MFR) measured under conditions of a temperature of 230°C and a mass of 2.16 kg is 3.0 g / 10 min or less; (B) the value obtained by dividing the weight average molecular weight Mw by the number average molecular weight Mn (Mw / Mn) is 15 or less; and (C) Density is 0.85 g / cm 3 That's it; and the microporous membrane satisfies the following requirement (D): (D) The ratio of the orientation rate in the transverse direction (TD) to the orientation rate in the machine direction (MD), MD / TD, is 1.3 or more as measured by wide-angle X-ray scattering; In the third embodiment, when the polyolefin and the microporous membrane satisfy requirements (A) to (D), the strength, membrane formability, productivity, pore openness, ion permeability, and electricity storage device properties of the microporous membrane tend to be improved for the same reasons as requirements (A) to (D) described in the first embodiment.

[0027] The microporous membrane according to the fourth embodiment contains polypropylene with β-crystal activity and has an amorphous crosslinked structure in which the amorphous portions of the polypropylene are crosslinked. In the fourth embodiment, an unstretched sheet having β-crystals is produced by melt-extruding polypropylene with β-crystal activity, and the unstretched sheet is stretched to cause crystal transition to α-crystals, which have a relatively high crystal density. Micropores are formed due to the difference in crystal density between the two to produce a microporous membrane. The resulting microporous membrane has an amorphous crosslinked structure, which improves membrane rupture resistance at high temperatures of 150°C or higher, and tends to achieve both the device properties and safety of the electricity storage device. The microporous membrane according to the fourth embodiment may contain one or more types of polypropylene, and when multiple types of polypropylene are contained, at least one of them has β-crystal activity.

[0028] In the third and fourth embodiments, it is believed that functional groups contained in polyolefins such as polypropylene are not incorporated into the crystalline parts of the polyolefin but are crosslinked in the amorphous parts, and therefore the microporous membranes according to the third and fourth embodiments can suppress an increase in internal stress or deformation of the produced electricity storage device while achieving both a shutdown function and high-temperature membrane rupture resistance, compared to conventional crosslinked microporous membranes in which the crystalline parts and their peripheries are easily crosslinked, thereby ensuring the safety of the electricity storage device. From the same viewpoint, the amorphous parts of the polyolefins contained in the microporous membranes according to the third and fourth embodiments are preferably selectively crosslinked, and more preferably are significantly more crosslinked than the crystalline parts.

[0029] [Crosslinking reaction mechanism] In the first, second, third and fourth embodiments, the crosslinking reaction mechanism and the crosslinked structure are not clear, but the present inventors consider the following (A) to (D).

[0030] (A) Crystal structure in polyolefin microporous membranes Polyolefin resins, such as polyethylene, are generally crystalline polymers, as shown in Figure 1, and have a higher-order structure divided into lamellae (crystalline portions) of the crystalline structure, amorphous portions, and intermediate layers between them. In the crystalline portions and the intermediate layers between the crystalline and amorphous portions, the mobility of the polymer chains is low and they cannot be separated, but relaxation phenomena can be observed in the 0°C to 120°C range in solid-state viscoelasticity measurements. On the other hand, in the amorphous portions, the mobility of the polymer chains is very high, and these are observed in the -150°C to -100°C range in solid-state viscoelasticity measurements. This point is related to radical relaxation or radical transfer reactions, crosslinking reactions, etc., which will be described later.

[0031] Furthermore, the polyolefin molecules that make up the crystals are not single; as shown in Figure 2, multiple polymer chains form small lamellae, which then aggregate to form crystals. This phenomenon is difficult to observe directly, but in recent years, academic research using simulations has progressed and clarified it. In this specification, a crystal is the smallest crystal unit measured by X-ray structural analysis, and is a unit that can be calculated as the crystallite size. Thus, even in the crystalline portion (inside the lamella), it is predicted that there are some parts within the crystal that are not constrained and have somewhat high mobility.

[0032] (a) Crosslinking reaction mechanism by electron beam Next, the reaction mechanism of electron beam crosslinking (hereinafter referred to as EB crosslinking) of a polymer is as follows. (i) Electron beam irradiation at tens to hundreds of kGy; (ii) Penetration of the electron beam into the reaction target (polymer) and generation of secondary electrons; (iii) Hydrogen abstraction reaction in polymer chains by secondary electrons and radical generation, (iv) Abstraction of adjacent hydrogen atoms by radicals and migration of active sites, and (v) recombination of radicals to form cross-linking reactions or polyenes. Here, radicals generated in the crystalline region have low mobility, so they persist for long periods of time, and impurities cannot penetrate the crystal, resulting in a low probability of reaction and quenching. Such radical species are called stable radicals and can remain for as long as several months. Their lifetimes can be determined by ESR measurements. As a result, cross-linking reactions within the crystal are thought to be rare. However, in the few unconstrained molecular chains present within the crystal or in the surrounding crystalline-amorphous interlayer, the generated radicals have a somewhat longer lifetime. These radical species are called persistent radicals, and in a mobile environment, cross-linking reactions between molecular chains are thought to proceed with a high probability. On the other hand, in the amorphous region, the mobility of the generated radical species is very high, so the lifetime of the generated radicals is short, and it is thought that not only cross-linking reactions between molecular chains but also polyene reactions within a single molecular chain will proceed with a high probability. As described above, in a microscopic view at the crystal level, it can be assumed that the crosslinking reaction due to EB crosslinking is localized inside the crystal or its periphery.

[0033] (c) Crosslinking reaction mechanism by chemical reaction As mentioned above, polyolefin resins have crystalline and amorphous regions. However, due to steric hindrance, the aforementioned functional groups are not present inside the crystals, but are localized in the amorphous regions. This is generally known, and while units such as methyl groups present in small amounts in polyethylene chains can be incorporated into crystals, grafts that are bulkier than ethyl groups cannot be incorporated ("Basic Polymer Chemistry," published by Tokyo Kagaku Dojin). For this reason, crosslinking points resulting from reactions other than electron beam crosslinking are localized only in the amorphous regions.

[0034] (D) Relationship between differences in cross-linking structure and effects To form a crosslinked structure in the microporous membrane, it is preferable to use a combination of functional groups in the polyolefin resin and a chemical substance contained in the electricity storage device, or to use a chemical substance contained in the electricity storage device as a catalyst. In crosslinking reactions caused by chemical reactions inside the electricity storage device, the morphology of the reaction product varies depending on the raw materials or catalyst used. In research leading to the present invention, the following experiments were conducted to clarify the crosslinked structure and the changes in the physical properties of the microporous membrane that occur due to structural changes.

[0035] Fuse / meltdown property tests were conducted to examine the behavior of chemically crosslinked membranes and membranes that had not undergone EB crosslinking or chemical crosslinking (before crosslinking) during crystalline melting. The results showed that the EB crosslinked membrane exhibited a significantly higher fuse temperature, with the meltdown temperature rising to over 200°C. On the other hand, the chemically crosslinked membrane showed no change in fuse temperature before and after crosslinking, but its meltdown temperature rose to over 200°C. Based on this, it is believed that the fuse characteristics caused by crystalline melting in the EB crosslinked membrane were due to crosslinking around the crystalline region, resulting in an increase in melting temperature and a decrease in melting rate. On the other hand, the chemically crosslinked membrane did not have a crosslinked structure in the crystalline region, which resulted in no change in fuse characteristics. Furthermore, at high temperatures around 200°C, both membranes retained a crosslinked structure after crystalline melting, allowing the entire resin to stabilize in a gel state, resulting in good meltdown properties.

[0036] The above findings are summarized in Table 1 below. [Table 1]

[0037] In the first and third embodiments, (1) the condensation reaction between functional groups of a polyolefin such as polyethylene or polypropylene can be, for example, a reaction via a covalent bond between two or more functional groups A contained in the polyolefin. Also, (3) the reaction between a functional group of a polyolefin and another type of functional group can be, for example, a reaction via a covalent bond between a functional group A and a functional group B contained in the polyolefin.

[0038] In the first and third embodiments, (2) in the reaction between a functional group of a polyolefin such as polyethylene or polypropylene and a chemical substance inside the electricity storage device, for example, functional group A contained in the polyolefin can form a covalent bond or a coordinate bond with any of the electrolyte, electrolytic solution, electrode active material, additive, or decomposition products thereof contained in the electricity storage device. Furthermore, reaction (2) forms a crosslinked structure not only inside the separator but also between the separator and the electrode or between the separator and the solid electrolyte interface (SEI), thereby improving the strength between multiple components of the electricity storage device.

[0039] The microporous membranes according to the first, second, third and fourth embodiments have a crosslinked amorphous structure and a shutdown function and high-temperature resistance to membrane rupture, and are formed by the reaction of a polymer represented by the following formula (I): R E’X =E' Z / E' z0 (I) {where, E' Z is the storage modulus measured in a temperature range of 160°C to 300°C after the crosslinking reaction of the microporous membrane has progressed in the electricity storage device, and E' z0 is the storage modulus measured in the temperature range of 160°C to 300°C before the microporous membrane is incorporated into an electricity storage device.} The mixed storage modulus ratio (R E’x ), and / or the following formula (III): R E’’X =E'' Z / E'' Z0 (III) {in formula, E'' Z is the loss modulus measured in the temperature range of 160°C to 300°C after the crosslinking reaction of the microporous membrane has progressed in the electricity storage device, and E'' Z0 is the loss modulus measured in the temperature range of 160°C to 300°C before the microporous membrane is incorporated into an electricity storage device.} The mixed loss modulus ratio (R E’’x) is preferably 1.2 to 20 times, more preferably 2.0 to 18 times, and even more preferably 3.0 to 16.5 times. Z and E' z0 and E'' Z and E'' z0 The storage modulus E' is the average value of the storage modulus or loss modulus measured within the set temperature range of the measuring device when the widest temperature range is 160°C to 300°C. In the case of a laminated film, the storage modulus E' is measured by removing only the polyolefin microporous film from the laminated film. Z and E' z0 and loss modulus E'' Z and E'' z0 E' shall be measured. Z , E' z0 , E'' Z or E'' z0 The conditions for measuring the elastic modulus are described in the examples.

[0040] The separators according to the first, second, third and fourth embodiments are, from the viewpoint of forming an amorphous crosslinked structure, achieving both a shutdown function and high-temperature resistance to film rupture, a polymer having a structure represented by the following formula (II): R E’mix =E' / E'0(II) {wherein E' is the storage modulus of the microporous membrane having an amorphous crosslinked structure measured at 160°C to 300°C; and E'0 is the storage modulus of a microporous film not having an amorphous crosslinked structure measured at 160°C to 300°C.} The mixed storage modulus ratio (R E’mix ), and / or the following formula (IV): R E’’mix =E'' / E''0(IV) {wherein E'' is the loss modulus measured at 160°C to 300°C when the microporous membrane has an amorphous crosslinked structure, and E''0 is the loss modulus of a microporous membrane not having an amorphous crosslinked structure measured at 160°C to 300°C.} The mixed loss modulus ratio (R E’’mix) is preferably 1.2 to 20 times, more preferably 2.0 to 18 times, and even more preferably 3.0 to 17 times. E' and E'0, and E" and E"0 are the average values ​​of the storage modulus or loss modulus measured within the set temperature range of the measuring device, with the widest temperature range being 160°C to 300°C. In the case of a laminated film, the storage modulus E' and E'0 and the loss modulus E" and E"0 are measured by removing only the polyolefin microporous film from the laminated film. The measurement conditions for the modulus E', E'0, E" or E"0 are described in the Examples.

[0041] The components of the microporous membranes according to the first, second, third and fourth embodiments are described below.

[0042] [Polyolefin] The polyolefin constituting the microporous membrane is not particularly limited, but examples include homopolymers of ethylene or propylene, or copolymers formed from at least two monomers selected from the group consisting of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, and norbornene. Among these, high-density polyethylene, low-density polyethylene, ultra-high molecular weight polyethylene (UHMWPE), polypropylene, polybutene, or a combination thereof is preferred, with polypropylene being more preferred, from the viewpoint of ease of wet or dry porous formation. It is generally known that UHMWPE has a weight-average molecular weight of 1,000,000 or more. The polyolefins may be used alone or in combination of two or more.

[0043] The weight-average molecular weight (Mw) of the polyolefin can be determined arbitrarily so that the polydispersity (Mw / Mn) of the above-described requirement (B) is 15 or less, and from the viewpoints of the heat shrinkability of the microporous membrane and the safety of the electricity storage device, it is preferably 10,000 to 2,000,000, more preferably 20,000 to 1,500,000, and even more preferably 30,000 to 1,000,000. From the same viewpoint, the microporous membrane contains preferably 3 to 25 mass%, more preferably 5 to 20 mass%, of polyolefin having an Mw of 10,000 to 2,000,000 based on the total mass of the raw material polyolefin.

[0044] [Polyolefin having one or more functional groups] From the viewpoints of forming a crosslinked structure, resistance to oxidation-reduction degradation, and a dense, uniform porous structure, the microporous membrane preferably contains, as the polyolefin having one or more functional groups, a functional-group-modified polyolefin or a polyolefin copolymerized with a monomer having a functional group. In this specification, functional-group-modified polyolefin refers to a polyolefin to which a functional group has been bonded after production. The functional group can be bonded to the polyolefin backbone or introduced into a comonomer, and preferably is involved in selective crosslinking of the amorphous portion of the polyolefin. For example, the functional group can be at least one selected from the group consisting of a carboxyl group, a hydroxyl group, a carbonyl group, a polymerizable unsaturated hydrocarbon group, an isocyanate group, an epoxy group, a silanol group, a hydrazide group, a carbodiimide group, an oxazoline group, an acetoacetyl group, an aziridine group, an ester group, an active ester group, a carbonate group, an azide group, a linear or cyclic heteroatom-containing hydrocarbon group, an amino group, a sulfhydryl group, a metal chelate group, and a halogen-containing group.

[0045] [Multiple types of polypropylene] From the viewpoints of the strength, ion permeability, resistance to oxidation-reduction degradation, and dense, uniform porous structure of the microporous membrane, the microporous membrane preferably contains both a polyolefin having one or more functional groups and another polyolefin, and more preferably contains both a first polypropylene (A) and a second polypropylene (B) that can be distinguished from the polypropylene (A) and has functional groups.

[0046] The first polypropylene (A) satisfies the following requirements (P1) to (P3) from the viewpoints of the strength, film-forming property, productivity, and pore openability of the microporous membrane: (P1) The MFR when measured under conditions of a temperature of 230°C and a mass of 2.16 kg is 2.5 g / 10 min or less; (P2) the value (Mw / Mn) obtained by dividing the weight average molecular weight Mw by the number average molecular weight Mn is 10 or less; and (P3) Density is 0.89 g / cm 3 That's it: It is preferable that the homopolypropylene (A) satisfies the following requirement (P1). With respect to the homopolypropylene (A), the MFR is more preferably 0.25 to 2.4 g / 10 min. With respect to the requirement (P2), the dispersity (Mw / Mn) of the homopolypropylene (A) is more preferably 4.9 to 9.0. With respect to the requirement (P3), the density of the homopolypropylene (A) is more preferably 0.90 g / cm. 3 or more and 0.96g / cm 3 or less than 0.90 g / cm 3 or more and 0.93 g / cm 3 The following is the result.

[0047] On the other hand, the second polypropylene (B) having a functional group is a polypropylene that does not satisfy at least one of the above requirements (P1) to (P3) and has at least one functional group, and may be a homopolymer or a copolymer. The polypropylene (B) may have, as a functional group, at least one selected from the group consisting of a carboxyl group, a hydroxyl group, a carbonyl group, a polymerizable unsaturated hydrocarbon group, an isocyanate group, an epoxy group, a silanol group, a hydrazide group, a carbodiimide group, an oxazoline group, an acetoacetyl group, an aziridine group, an ester group, an active ester group, a carbonate group, an azide group, a linear or cyclic heteroatom-containing hydrocarbon group, an amino group, a sulfhydryl group, a metal chelate group, and a halogen-containing group. Among these, silane-modified polypropylene is preferred from the viewpoint of forming a crosslinked structure.

[0048] When polypropylene (A) and (B) are used in combination in a microporous membrane, the content of polypropylene (B) in the microporous membrane is preferably 30% by mass or less, more preferably 4 to 25% by mass, and even more preferably 5 to 20% by mass, from the viewpoint of the balance between strength and crosslinkability.

[0049] [Crosslinking reaction] The crosslinked structure of the microporous membrane contributes to both the shutdown function and high-temperature membrane rupture resistance when used as a separator, and to the safety of the storage device, and is preferably formed in the amorphous portion of the polyolefin. The crosslinked structure can be formed, for example, by a reaction mediated by a covalent bond, a hydrogen bond, or a coordinate bond. Among these, reactions mediated by a covalent bond include the following reactions (I) to (IV): (I) Condensation reaction of multiple identical functional groups (II) Reactions between multiple heterogeneous functional groups (III) Chain condensation reaction of functional groups with electrolytes (IV) Chain condensation reaction of functional groups with additives It is preferable that the compound is at least one selected from the group consisting of: The reaction via a coordinate bond is represented by the following reaction (V): (V) A reaction in which multiple identical functional groups crosslink with eluted metal ions through coordinate bonds. It is preferable that:

[0050] Reaction (I) A schematic scheme and specific examples of reaction (I) are shown below, where A represents the first functional group of the microporous membrane. [ka] {In the formula, R is an alkyl group or heteroalkyl group having 1 to 20 carbon atoms, which may have a substituent.}

[0051] When the functional group A for reaction (I) is a silanol group, the polyolefin contained in the microporous membrane is preferably silane-graft-modified. The silane-graft-modified polyolefin has a polyolefin main chain and a structure having alkoxysilyl grafts on the main chain. Examples of the alkoxide substituted with the alkoxysilyl include methoxide, ethoxide, and butoxide. For example, in the above formula, R can be methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, and tert-butyl. The main chain and the graft are connected by a covalent bond, and examples of such structures include alkyl, ether, glycol, and ester. Considering the process for producing the microporous membrane according to this embodiment, the silane-graft-modified polyolefin preferably has a silicon-to-carbon (Si / C) ratio of 0.2 to 1.8%, more preferably 0.5 to 1.7%, prior to the crosslinking treatment step.

[0052] Reaction (II) A schematic scheme and specific examples of reaction (II) are shown below, where the first functional group of the microporous membrane is A and the second functional group is B. [ka] [ka] [ka] [ka]

[0053] Reactions (I) and (II) can be catalyzed, for example, by a chemical substance within the electrical storage device in which the microporous membrane is incorporated as a separator. The chemical substance can be, for example, an electrolyte, an electrolytic solution, an electrode active material, an additive, or a decomposition product thereof contained in the electrical storage device.

[0054] Reaction (III) A schematic scheme and specific examples of reaction (III) are shown below, where A represents the first functional group of the microporous membrane and Sol represents the electrolyte solution. [ka] [ka] [ka] [ka]

[0055] Reaction (IV) A schematic scheme of reaction (IV) is shown below, where A represents the first functional group of the microporous membrane, B represents the second functional group that is optionally incorporated, and Add represents the additive. [ka]

[0056] From the viewpoint of forming a covalent bond represented by the dotted line in the above scheme, reaction (IV) is preferably a nucleophilic substitution reaction, a nucleophilic addition reaction, or a ring-opening reaction between compound Rx constituting the microporous membrane and compound Ry constituting the additive (Add). Compound Rx may be a polyolefin contained in the microporous membrane, such as polyethylene or polypropylene, and preferably the polyolefin is modified with functional group x, for example, at least one selected from the group consisting of -OH, -NH2, -NH-, -COOH, and -SH.

[0057] Since the multiple compounds Rx are crosslinked via the compound Ry as an additive, it is preferable that the compound Ry has two or more linking reaction units y1. The multiple linking reaction units y1 may have any structure or group, may be substituted or unsubstituted, may contain heteroatoms or inorganic substances, and may be the same or different from each other, as long as they can undergo a nucleophilic substitution reaction, a nucleophilic addition reaction, or a ring-opening reaction with the functional group x of the compound Rx. Furthermore, when the compound Ry has a chain structure, the multiple linking reaction units y1 can each independently be a terminal group, incorporated into the main chain, or a side chain or pendant.

[0058] When reaction (IV) is a nucleophilic substitution reaction, the following description will be given by regarding the functional group x of compound Rx as a nucleophilic group and the linking reaction unit y1 of compound Ry as a leaving group, merely as an example. In this embodiment, however, both the functional group x and the linking reaction unit y1 can become a leaving group depending on their nucleophilicity.

[0059] From the viewpoint of nucleophilic reagent, the functional group x of the compound Rx is preferably an oxygen-based nucleophilic group, a nitrogen-based nucleophilic group, or a sulfur-based nucleophilic group. Examples of oxygen-based nucleophilic groups include hydroxyl groups, alkoxy groups, ether groups, and carboxyl groups, with -OH and -COOH being preferred. Examples of nitrogen-based nucleophilic groups include ammonium groups, primary amino groups, and secondary amino groups, with -NH2 and -NH- being preferred. Examples of sulfur-based nucleophilic groups include -SH and thioether groups, with -SH being preferred.

[0060] When the reaction (IV) is a nucleophilic substitution reaction, the linking reaction unit y1 of the compound Ry, from the viewpoint of the leaving group, may be an alkylsulfonyl group such as CH3SO2- or CH3CH2SO2-; an arylsulfonyl group (-ArSO2-); a haloalkylsulfonyl group such as CF3SO2- or CCl3SO2-; - -, CH3CH2SO3 - -; aryl sulfonate groups (ArSO3 - -);CF3SO3 - -, CCl3SO3 - -; and heterocyclic groups are preferred, and these can be used alone or in combination of two or more. Heteroatoms contained in the heterocycle include nitrogen atoms, oxygen atoms, sulfur atoms, etc., and among these, nitrogen atoms are preferred from the viewpoint of leaving properties. Leaving groups containing a nitrogen atom in the heterocycle include those of the following formulae (y1-1) to (y1-6): [ka] {In the formula, X is a hydrogen atom or a monovalent substituent.} [ka] {In the formula, X is a hydrogen atom or a monovalent substituent.} [ka] {In the formula, X is a hydrogen atom or a monovalent substituent.} [ka] {In the formula, X is a hydrogen atom or a monovalent substituent.} [ka] {In the formula, X is a hydrogen atom or a monovalent substituent.} [ka] {In the formula, X is a hydrogen atom or a monovalent substituent.} A monovalent group represented by the following formula is preferred.

[0061] In formulae (y1-1) to (y1-6), X represents a hydrogen atom or a monovalent substituent. Examples of the monovalent substituent include an alkyl group, a haloalkyl group, an alkoxyl group, and a halogen atom.

[0062] When the reaction (IV) is a nucleophilic substitution reaction and the compound Ry has a chain structure, the compound Ry may contain, in addition to the linking reaction unit y1, the following formulae (y2-1) to (y2-6) as the chain unit y2: [ka] {In the formula, m is an integer of 0 to 20, and n is an integer of 1 to 20.} [ka] {In the formula, n is an integer of 1 to 20.} [ka] {In the formula, n is an integer of 1 to 20.} [ka] {In the formula, n is an integer of 1 to 20.} [ka] {In the formula, X is an alkylene group or an arylene group having 1 to 20 carbon atoms, and n is an integer of 1 to 20.} [ka] {In the formula, X is an alkylene group or an arylene group having 1 to 20 carbon atoms, and n is an integer of 1 to 20.} In addition, when the compound Ry contains a plurality of chain units y2, they may be the same or different, and their arrangement may be block or random.

[0063] In formula (y2-1), m is an integer of 0 to 20, and from the viewpoint of the crosslinked network, preferably 1 to 18. In formulas (y2-1) to (y2-6), n is an integer of 1 to 20, and from the viewpoint of the crosslinked network, preferably 2 to 19 or 3 to 16. In formulas (y2-5) to (y2-6), X is an alkylene group or arylene group having 1 to 20 carbon atoms, and from the viewpoint of the stability of the chain structure, preferably a methylene group, ethylene group, n-propylene group, n-butylene group, n-hexylene group, n-heptylene group, n-octylene group, n-dodecylene group, o-phenylene group, m-phenylene group, or p-phenylene group.

[0064] When reaction (IV) is a nucleophilic substitution reaction, preferred combinations of the functional group x of compound Rx and the linking reaction unit y1 and chain unit y2 of compound Ry are shown in Tables 2 to 4 below.

[0065] [Table 2]

[0066] [Table 3]

[0067] [Table 4]

[0068] As a specific example 1 of the nucleophilic substitution reaction, the functional group x of the polyolefin is -NH2, the linking reaction unit y1 of the additive (compound Ry) is a skeleton derived from succinimide, and the chain unit y2 is -(O-C2H5) nThe reaction scheme for the case where - is shown below. [ka]

[0069] As specific example 2 of a nucleophilic substitution reaction, the reaction scheme is shown below when the functional groups x of the polyolefin are -SH and -NH, the linking reaction unit y1 of the additive (compound Ry) is a nitrogen-containing cyclic skeleton, and the chain unit y2 is o-phenylene. [ka]

[0070] When reaction (IV) is a nucleophilic addition reaction, the functional group x of compound Rx and the linking reaction unit y1 of compound Ry can undergo an addition reaction. In the nucleophilic addition reaction, the functional group x of compound Rx is preferably an oxygen-based nucleophilic group, a nitrogen-based nucleophilic group, or a sulfur-based nucleophilic group. Examples of oxygen-based nucleophilic groups include hydroxyl groups, alkoxy groups, ether groups, and carboxyl groups, with -OH and -COOH being preferred. Examples of nitrogen-based nucleophilic groups include ammonium groups, primary amino groups, and secondary amino groups, with -NH2 and -NH- being preferred. Examples of sulfur-based nucleophilic groups include -SH and thioether groups, with -SH being preferred.

[0071] In the nucleophilic addition reaction, the linking reaction unit y1 of the compound Ry is selected from the group consisting of the following formulae (Ay1-1) to (Ay1-6), from the viewpoint of addition reactivity or availability of raw materials: [ka] [ka] [ka] [ka] {In the formula, R is a hydrogen atom or a monovalent organic group.} [ka] [ka] It is preferable that the group is at least one selected from the group consisting of groups represented by the following formula:

[0072] In formula (Ay1-4), R is a hydrogen atom or a monovalent organic group, and preferably a hydrogen atom, C 1~20 It is an alkyl group, an alicyclic group, or an aromatic group, and more preferably a hydrogen atom, a methyl group, an ethyl group, a cyclohexyl group, or a phenyl group.

[0073] When reaction (IV) is a nucleophilic addition reaction, preferred combinations of the functional group x of compound Rx and the linking reaction unit y1 of compound Ry are shown in Tables 5 and 6 below.

[0074] [Table 5]

[0075] [Table 6]

[0076] As a specific example of a nucleophilic addition reaction, the reaction scheme when the functional group x of the microporous membrane is -OH and the linking reaction unit y1 of the additive (compound Ry) is -NCO is shown below. [ka]

[0077] When reaction (IV) is a ring-opening reaction, the functional group x of compound Rx and the linking reaction unit y1 of compound Ry can undergo a ring-opening reaction, and from the viewpoint of easy availability of raw materials, it is preferable that the cyclic structure on the linking reaction unit y1 side opens. From the same viewpoint, it is more preferable that linking reaction unit y1 is an epoxy group, and it is even more preferable that compound Ry has at least two epoxy groups, and it is even more preferable that it is a diepoxy compound.

[0078] When the reaction (IV) is a ring-opening reaction, the functional group x of the compound Rx is preferably at least one selected from the group consisting of -OH, -NH2, -NH-, -COOH and -SH, and / or the linking reaction unit y1 of the compound Ry is preferably a ring-opening reaction unit represented by the following formula (ROy1-1): [ka] {In the formula, each of the multiple Xs independently represents a hydrogen atom or a monovalent substituent.} In formula (ROy1-1), each of the multiple Xs independently represents a hydrogen atom or a monovalent substituent, and preferably represents a hydrogen atom, C 1~20 The functional group x of the compound Rx and the linking reaction unit y1 of the compound Ry are preferably an alkyl group, an alicyclic group, or an aromatic group, and more preferably a hydrogen atom, a methyl group, an ethyl group, a cyclohexyl group, or a phenyl group. For the epoxy ring-opening reaction, preferred combinations of the functional group x of the compound Rx and the linking reaction unit y1 of the compound Ry are shown in Table 7 below.

[0079] [Table 7]

[0080] Reaction (V) The first functional group of the microporous membrane is A and the metal ion is M. n+ A schematic scheme of reaction (V) and examples of functional group A are shown below. [ka]

[0081] In the above scheme, the metal ion M n+ is preferably eluted from the electricity storage device (hereinafter also referred to as eluted metal ions), for example, Zn 2+ , Mn 2+ , Co 3+ , Ni 2+ and Li + The functional group A may be at least one selected from the group consisting of -COO - The coordinate bond in the case of is shown below. [ka]

[0082] Functional group A is -COOH and the eluted metal ion is Zn 2+ A specific scheme of reaction (V) in the case of is shown below. [ka]

[0083] In the above scheme, the hydrofluoric acid (HF) can be derived from, for example, the electrolyte, electrolytic solution, electrode active material, additive, or decomposition product or water-absorbed product thereof contained in the electricity storage device, depending on the charge / discharge cycle of the electricity storage device.

[0084] (Other ingredients) The microporous membrane may contain, in addition to the polyolefin, known additives such as a dehydration condensation catalyst, a metal soap such as calcium stearate or zinc stearate, an ultraviolet absorber, a light stabilizer, an antistatic agent, an antifogging agent, and a coloring pigment, if desired.

[0085] [Characteristics of microporous membrane] The following properties of the microporous membrane are described for the case where the microporous membrane for an electricity storage device is a flat membrane, but when the microporous membrane for an electricity storage device is in the form of a laminated membrane, they can be measured after removing layers other than the microporous membrane from the laminated membrane.

[0086] The porosity of the microporous membrane is preferably 20% or more, more preferably 30% or more, and even more preferably 39% or more or 42% or more. When the microporous membrane has a porosity of 20% or more, it tends to have better ability to follow the rapid movement of lithium ions when used as a separator in a lithium ion storage device. On the other hand, the porosity of the microporous membrane is preferably 90% or less, more preferably 80% or less, and even more preferably 50% or less. When the porosity of the microporous membrane is 90% or less, it tends to have better membrane strength and to further suppress self-discharge.

[0087] The air resistance of the microporous membrane is preferably 1 second or more, more preferably 50 seconds or more, even more preferably 100 seconds or more, and even more preferably 150 seconds or more or 200 seconds or more per 100 ml of membrane volume. When the air resistance of the microporous membrane is 1 second or more, the balance between membrane thickness, porosity, and average pore size tends to be further improved. Furthermore, the air resistance of the microporous membrane is preferably 450 seconds or less, more preferably 420 seconds or less. When the air resistance of the microporous membrane is 450 seconds or less, ion permeability tends to be further improved.

[0088] The tensile strength of the microporous membrane is preferably 900 to 3000 kg / cm in MD in relation to the ratio of the orientation ratio MD / TD of the requirement (D) described above. 2 , more preferably 1000 to 2500 kg / cm 2 , and more preferably 1210 to 2270 kg / cm 2 and in TD (direction perpendicular to MD, membrane width direction), it is preferably 100 to 1500 kg / cm 2 , more preferably 129 to 1310 kg / cm 2 is.

[0089] The thickness of the microporous membrane is preferably 1.0 μm or more, more preferably 2.0 μm or more, and even more preferably 3.0 μm or more, 4.0 μm or more, or 5.5 μm or more. When the thickness of the microporous membrane is 1.0 μm or more, the membrane strength tends to be further improved. Furthermore, the thickness of the microporous membrane is preferably 500 μm or less, more preferably 100 μm or less, and even more preferably 80 μm or less, 22 μm or less, or 19 μm or less. When the thickness of the microporous membrane is 500 μm or less, the ion permeability tends to be further improved. When the microporous membrane is used as a separator for a lithium ion secondary battery, the thickness of the microporous membrane is preferably 1.0 to 25 μm, more preferably 3.0 to 22 μm, and even more preferably 13 to 18 μm.

[0090] The puncture strength of the microporous membrane is preferably 200 to 500 gf, more preferably 218 to 481 gf or 227 to 450 gf, from the viewpoint of balancing membrane rupture resistance and device safety.

[0091] [Method for manufacturing microporous membrane] The method for producing a microporous membrane includes the following steps: forming a polyolefin resin composition; and A step of forming pores in a polyolefin resin composition; Heat treatment process of the pore-forming material; The following description will be made on the case of a microporous membrane (flat membrane), but this is not intended to exclude forms other than flat membranes.

[0092] The polyolefin resin composition can be produced by melt-kneading a polyolefin resin and other materials using a single-screw or twin-screw extruder.

[0093] The materials to be kneaded in the kneading step can be determined depending on the pore-opening step to be carried out thereafter, since the pore-opening step can be carried out by a known dry method and / or a known wet method.

[0094] Dry methods include a method in which an unstretched sheet containing incompatible particles such as inorganic particles and polyolefin is stretched and extracted to peel the interface between the different materials and form pores, a lamellar pore-forming method, and a β-crystal pore-forming method.

[0095] The lamellar perforation method is a method in which an unstretched sheet having a crystalline lamellar structure is obtained by controlling the melt crystallization conditions during the melt extrusion of a resin to form a sheet, and the resulting unstretched sheet is stretched to cleave the lamellar interfaces and form holes. For example, a circular die extrusion method can be used in the lamellar perforation method. In the circular die extrusion method, for example, a melt-kneaded product of a polypropylene resin composition is blown up mainly in the MD through a circular die to obtain a highly crystalline MD-oriented raw sheet.

[0096] The β-crystal pore-opening method involves producing an unstretched sheet of polypropylene (PP) with β-crystals, which have a relatively low crystal density, during melt extrusion, and then stretching the unstretched sheet to induce crystal transition to α-crystals, which have a relatively high crystal density, thereby forming pores due to the difference in crystal density between the two. Examples of β-crystal nucleating agents that can be used include 1:2,6-naphthalenedicarboxylic acid dicyclohexylamide, and preferably, a β-crystal nucleating agent and an antioxidant are used in combination. In the second and fourth embodiments, the β-crystal pore-opening method is preferred because it allows the use of β-crystal-activated PP.

[0097] As the wet method, a method in which a polyolefin, and optionally other resins and a plasticizer or inorganic material are kneaded using a kneader to form a sheet, which is then stretched as necessary, and the pore-forming material is extracted from the sheet, or a method in which a polyolefin resin composition is dissolved and then immersed in a poor solvent for the polyolefin to solidify the polyolefin and simultaneously remove the solvent, can be used.

[0098] The plasticizer is not particularly limited, but examples thereof include organic compounds that can form a homogeneous solution with polyolefin at temperatures below the boiling point. More specifically, examples thereof include decalin, xylene, dioctyl phthalate, dibutyl phthalate, stearyl alcohol, oleyl alcohol, decyl alcohol, nonyl alcohol, diphenyl ether, n-decane, n-dodecane, and paraffin oil. Among these, paraffin oil and dioctyl phthalate are preferred. The plasticizer may be used alone or in combination of two or more.

[0099] Whether a dry method or a wet method is used, from the viewpoint of maintaining the crosslinkability of the microporous membrane until it is housed in an electricity storage device, the method for producing the microporous membrane preferably does not include a step of contacting the latently crosslinkable polyolefin with a crosslinking agent, other reactive compounds, functional groups of other compounds, a crosslinking-accelerating catalyst, etc. Furthermore, as long as the crosslinkability of the microporous membrane is maintained, the polyolefin resin composition can contain additives such as fluorine-based flow modifiers, waxes, crystal nucleating agents, antioxidants, metal soaps such as metal salts of aliphatic carboxylic acids, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, coloring pigments, etc.

[0100] The heat treatment step of the pore-opened product can be carried out after the stretching step or after pore formation for the purpose of heat setting to suppress shrinkage of the microporous membrane. Examples of heat treatment include a stretching operation carried out in a predetermined temperature atmosphere at a predetermined stretch ratio to adjust physical properties, and / or a relaxation operation carried out in a predetermined temperature atmosphere at a predetermined relaxation ratio to reduce stretching stress. The relaxation operation may be carried out after the stretching operation. These heat treatments can be carried out using a tenter or roll stretching machine.

[0101] [Electricity storage device] The microporous membranes according to the first, second, third, and fourth embodiments can be used in an electricity storage device. Generally, an electricity storage device includes an outer casing, a positive electrode, a negative electrode, a separator disposed between the positive and negative electrodes, and an electrolyte solution. When the microporous membranes according to these embodiments are housed in a device outer casing, the functionally modified polyolefin or functional group graft-copolymerized polyolefin formed during the manufacturing process of the microporous membrane reacts with chemicals contained in the electrolyte solution or additives to form a crosslinked structure, resulting in a crosslinked structure in the manufactured electricity storage device. From the viewpoint of maintaining the crosslinked nature of the microporous membrane until it is housed in the electricity storage device and subsequently improving the safety of the electricity storage device, it is preferable to house the microporous membrane as a separator between the positive and negative electrodes. When the microporous membrane is housed in the electricity storage device as a separator, a crosslinked structure is formed, which allows for compatibility with conventional electricity storage device manufacturing processes and allows for a crosslinking reaction to occur after device manufacture, improving the safety of the electricity storage device.

[0102] Specific examples of the power storage device include lithium batteries, lithium secondary batteries, lithium ion secondary batteries, sodium secondary batteries, sodium ion secondary batteries, magnesium secondary batteries, magnesium ion secondary batteries, calcium secondary batteries, calcium ion secondary batteries, aluminum secondary batteries, aluminum ion secondary batteries, nickel-metal hydride batteries, nickel-cadmium batteries, electric double layer capacitors, lithium ion capacitors, redox flow batteries, lithium-sulfur batteries, lithium-air batteries, zinc-air batteries, etc. Among these, from the viewpoint of practicality, lithium batteries, lithium secondary batteries, lithium ion secondary batteries, nickel-metal hydride batteries, and lithium ion capacitors are preferred, and lithium batteries or lithium ion secondary batteries are more preferred.

[0103] [Lithium-ion secondary battery] Lithium-ion secondary batteries are storage batteries that use a lithium transition metal oxide, such as lithium cobalt oxide or lithium cobalt composite oxide, as the positive electrode, a carbon material, such as graphite, as the negative electrode, and an organic solvent containing a lithium salt, such as LiPF6, as the electrolyte. During charging and discharging, ionized lithium travels back and forth between the electrodes. Furthermore, because the ionized lithium must travel relatively quickly between the electrodes while suppressing contact between the electrodes, a separator is placed between the electrodes. [Example]

[0104] The present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention. The raw materials used and the methods for evaluating various properties of the microporous membrane are as follows.

[0105] [Melt flow rate (MFR) measurement] The melt flow rate (MFR) was measured in accordance with JIS K 7210 under the conditions of 230°C and 2.16 kg for the polypropylene resin (unit: g / 10 min).

[0106] [GPC (Gel Permeation Chromatography) Measurement] A calibration curve was created by measuring standard polystyrene under the following conditions using an Agilent PL-GPC220. Chromatography was also performed on each of the following polymers under the same conditions, and based on the calibration curve, the weight-average molecular weight Mw of each polymer was divided by the number-average molecular weight Mn, as follows: Column: TSKgel GMHHR-H(20) HT (7.8mm I.D. x 30 cm) x 2 Mobile phase: 1,2,4-trichlorobenzene Detector: RI Column temperature: 160℃ Sample concentration: 1mg / ml Calibration curve: Polystyrene

[0107] [Wide-angle X-ray scattering measurements] The measurements were performed using a Rigaku Ultima-IV. Cu-Kα rays were incident on the separator sample, and diffracted light was detected using a D / tex Uitra. The measurement conditions were a distance of 285 mm between the sample and detector, an excitation voltage of 40 kV, and a current of 40 mA. A focused optical system was used, and the slit conditions were DS = 1 / 2°, SS = open, and a vertical slit = 10 mm. The MD / TD orientation ratio was calculated by dividing the measured MD-oriented crystal ratio by the TD-oriented crystal ratio.

[0108] [Thickness (μm)] The thickness of the porous film was measured at room temperature of 23±2°C using a Digimatic Indicator IDC112 manufactured by Mitutoyo Corporation.

[0109] [Porosity (%)] A sample measuring 5 cm x 5 cm was cut out from the porous film, and the porosity was calculated from the volume and mass of the sample using the following formula. Porosity (%) = (Volume (cm 3 ) - mass (g) / density of resin composition (g / cm 3 )) / volume(cm 3 ) x 100

[0110] [Air permeability resistance (sec / 100ml)] The air resistance of the microporous membrane was measured using a Gurley air permeability meter in accordance with JIS P-8117.

[0111] [Piercing strength] A needle with a hemispherical tip and a radius of 0.5 mm was prepared, and a microporous membrane was sandwiched between two plates with openings of 11 mm in diameter, and the needle, microporous membrane, and plates were set in place. A puncture test was performed using an "MX2-50N" made by Imada Co., Ltd., under conditions of a needle tip curvature radius of 0.5 mm, an opening diameter of 11 mm in the microporous membrane holding plate, and a puncture speed of 25 mm / min, bringing the needle into contact with the microporous membrane, and measuring the maximum puncture load (i.e., puncture strength (gf)).

[0112] [Measurement of storage modulus, loss modulus and transition temperature] Dynamic viscoelasticity measurements of the separator were performed using a dynamic viscoelasticity measuring device, and the storage modulus (E'), loss modulus (E''), and the transition temperature between the rubber-like plateau region and the crystalline melt flow region were calculated. The storage modulus change ratio (R E’X ) is calculated by the following formula (I), and the mixed storage modulus ratio (R E’mix ) is calculated by the loss modulus ratio (R E’’X ) is calculated by the mixed loss modulus ratio (R E’’mix ) were calculated according to the following formula (IV). The measurement conditions were as follows (i) to (iv). (i) Dynamic viscoelasticity measurement was performed under the following conditions: Atmosphere: Nitrogen Measurement device used: RSA-G2 (TA Instruments) Sample thickness: 5 μm to 50 μm (measurement is performed on one sample regardless of the sample thickness) Measurement temperature range: -50 to 300°C Heating rate: 10℃ / min ·Measurement frequency: 1Hz Deformation mode: Linear tension Initial static tensile load: 0.5N Initial gap distance (at 25°C): 25mm Auto strain adjustment: Enabled (Range: Amplitude 0.05 to 25%, Sine wave load 0.02 to 5N) It was carried out at. (ii) The static tensile load refers to the intermediate value between the maximum stress and the minimum stress in each periodic motion, and the sinusoidal load refers to the oscillating stress centered on the static tensile load. (iii) Sine wave tension mode refers to measuring the vibration stress while performing periodic motion with a fixed amplitude of 0.2%, and the vibration stress was measured by varying the gap distance and static tensile load so that the difference between the static tensile load and the sinusoidal load was within 20%. When the sinusoidal load became 0.02 N or less, the amplitude was amplified so that the sinusoidal load was within 5 N and the increase in amplitude was within 25%, and the vibration stress was measured. (iv) The relationship between the obtained sinusoidal load and amplitude value, and the following equation: σ * =σ0·Exp[i(ωt+δ)], ε * =ε0·Exp(iωt), σ * =E * ε * E * =E'+iE'' {where, σ * : Vibration stress, ε * : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between vibration stress and strain, E * : Complex modulus, E': Storage modulus, E'': Loss modulus Vibration stress: sinusoidal load / initial cross-sectional area Static tensile load: Load at the minimum point of vibration stress at each cycle (minimum point of gap distance at each cycle) Sinusoidal load: the difference between the measured oscillatory stress and the static tensile load The storage modulus and loss modulus were calculated from the data. E' Z and E' Z0 and E'' Z and E'' Z0 was the maximum value of each storage modulus or each loss modulus at 160°C to 300°C in the dynamic viscoelasticity measurement data. E', E'0, E'' and E''0 were the average values ​​of each storage modulus or each loss modulus at 160°C to 300°C in the dynamic viscoelasticity measurement data. R E’X =E' Z / E' Z0 (I) Comparison before and after injection into the cell R E’mix =E' / E'0(II) Comparison of the presence or absence of amorphous cross-linked structure R E’’X =E'' Z / E'' Z0 (III) Comparison before and after injection into the cell R E’’mix =E'' / E''0(IV) Comparison of the presence or absence of amorphous cross-linked structure

[0113] An example of a graph illustrating the relationship between temperature and storage modulus is shown in Figure 3. As shown in Figure 3, the storage modulus of a reference film (a separator for an electricity storage device that does not have an amorphous crosslinked structure) within the temperature range of -50°C to 310°C is compared with that of the crosslinked film, and the transition temperature between the rubber-like plateau region and the crystalline melting flow region can be confirmed in Figure 3. The transition temperature is defined as the temperature at the intersection of a straight line extending the high-temperature baseline toward the low-temperature side and a tangent line drawn at the inflection point of the curve for the crystalline melting transition region. An example of a graph illustrating the relationship between temperature and loss modulus is shown in Figure 4. Figure 4 compares the loss modulus of a reference film (a separator for an electricity storage device that does not contain a silane-grafted polyolefin) and a crosslinked film within a temperature range of -50°C to 310°C, and shows the transition temperatures determined by the same method as in Figure 3. In this technical field, the storage modulus and loss modulus are determined by the following formula: tanδ=E'' / E' {wherein tan δ represents the loss tangent, E′ represents the storage modulus, and E″ represents the loss modulus.} It is compatible according to The mixed storage modulus ratio (R E’mix ) or mixed loss modulus ratio (R E’’mix In the measurement of E', E', a non-silane-modified polyolefin microporous membrane with a gelation degree of approximately 0% was used as a separator for an electricity storage device that does not have an amorphous crosslinked structure. 0、 Regarding E'' and E''0, if no breakage of the sample (a sudden drop in elastic modulus) was observed between 160°C and 300°C, they were calculated from the average value between 160°C and 300°C, and if breakage of the sample was observed between 160°C and 300°C, they were calculated from the average value from 160°C to the temperature at the breakage point. In this specification, the separator for a storage battery device not having an amorphous crosslinked structure can be a separator manufactured using any one selected from the group consisting of polyethylene:X (viscosity-average molecular weight: 100,000 to 400,000), ultra-high molecular weight PE:Y (viscosity-average molecular weight: 400,000 to 800,000), and ultra-high molecular weight PE:Z (viscosity-average molecular weight: 800,000 to 9,000,000), or a mixture of two or three selected from the group consisting of X, Y, and Z, in any ratio. Polyolefins composed solely of a hydrocarbon skeleton, such as low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), polypropylene (PP), and olefin-based thermoplastic elastomers, may also be added to the mixed composition. More specifically, the separator for a storage battery device not having an amorphous crosslinked structure can refer to a polyolefin-based microporous membrane whose solid content change rate (hereinafter referred to as "gelation degree") before and after heating in a decalin solution at 160°C is 10% or less. When measuring the gelation degree, the solid content means only the resin and does not include other materials such as inorganic substances. On the other hand, the gelation degree of a polyolefin-based microporous film having an amorphous part crosslinked structure such as a silane crosslinked structure is preferably 30% or more, more preferably 70% or more.

[0114] [Tensile test] The tensile strength in the MD and TD directions was measured using an Instron Model 4201 according to the procedure of ASTM-882 and was calculated as the breaking strength.

[0115] [Fuse / Meltdown (F / MD) characteristics] a. Preparation of the positive electrode A slurry was prepared by dispersing 92.2% by mass of lithium-cobalt composite oxide LiCoO2 as the positive electrode active material, 2.3% by mass each of flake graphite and acetylene black as conductive materials, and 3.2% by mass of polyvinylidene fluoride (PVDF) as a binder in N-methylpyrrolidone (NMP). This slurry was applied to one side of a 20 μm thick aluminum foil that served as a positive electrode current collector using a die coater, dried at 130°C for 3 minutes, and then compression molded using a roll press. At this time, the amount of applied positive electrode active material was 250 g / m 2, the bulk density of the active material is 3.00 g / cm 3 It was adjusted to be.

[0116] b. Preparation of negative electrode A slurry was prepared by dispersing 96.9% by mass of artificial graphite as the negative electrode active material, and 1.4% by mass of ammonium salt of carboxymethylcellulose and 1.7% by mass of styrene-butadiene copolymer latex as binders in purified water. This slurry was applied to one side of a 12 μm-thick copper foil that served as the negative electrode current collector using a die coater, dried at 120°C for 3 minutes, and then compression-molded using a roll press. The amount of active material applied to the negative electrode was 106 g / m. 2 , the active material bulk density is 1.35 g / cm 3 It was adjusted to be.

[0117] c. Preparation of non-aqueous electrolyte The solute LiPF6 was dissolved in a 1:2 (volume ratio) mixed solvent of ethylene carbonate and ethyl methyl carbonate to a concentration of 1.0 mol / L. A 200 mm diameter circle was cut out of aluminum foil with a conductive silver paste attached to the back of the positive electrode, separator, and negative electrode. The resulting laminate was then stacked and coated with electrolyte-containing solution (c). The laminate was then clamped in the center between a 600 mm diameter circular aluminum heater, and the aluminum heater was pressurized from above and below to 0.5 MPa using a hydraulic jack. The measurement was completed. The resistance (Ω) between the electrodes was measured while heating the laminate with the aluminum heater at a rate of 2°C / min. The resistance between the electrodes, including the separator fuse, increased until it first exceeded 1000 Ω, which was the fuse temperature (shutdown temperature). Further heating was continued, and the temperature at which the resistance dropped below 1000 Ω was the meltdown temperature (film rupture temperature).

[0118] [Example 1] <Method for producing silane-grafted modified polyolefin> The raw polyolefin used for silane-grafted polyolefins may have a viscosity-average molecular weight (Mv) of 100,000 to 1,000,000, a weight-average molecular weight (Mw) of 30,000 to 920,000, and a number-average molecular weight of 10,000 to 150,000. Propylene or butene copolymerized α-olefins may also be used. While the raw polyolefin is melt-kneaded in an extruder, an organic peroxide (di-t-butyl peroxide) is added to generate radicals within the α-olefin polymer chain. Then, trimethoxyalkoxide-substituted vinylsilane is added to introduce alkoxysilyl groups into the α-olefin polymer through an addition reaction, forming a silane-grafted structure. At the same time, an appropriate amount of antioxidant (pentaerythritol tetrakis [3-(3,5-di-tetrabutyl-4-hydroxyphenyl)propionate]) is added to adjust the radical concentration in the system and suppress chain reactions (gelation) within the α-olefin. The resulting silane-grafted polyolefin molten resin was cooled in water and pelletized, followed by heating and drying at 80°C for 2 days to remove moisture and unreacted trimethoxyalkoxide-substituted vinylsilane. The residual concentration of unreacted trimethoxyalkoxide-substituted vinylsilane in the pellets was approximately 1000 to 1500 ppm. The silane-grafted modified polyolefin obtained by the above-mentioned method is referred to as "silane-modified polypropylene" in the following examples and Table 1.

[0119] <Preparation of microporous membrane (single layer)> A high-molecular-weight polypropylene resin (PP, MFR = 0.25) and the silane-modified polypropylene were dry-blended in a mass ratio of PP:silane-modified polypropylene = 95:5 (mass%), then melted in a 2.5-inch extruder and fed to an annular die using a gear pump. The die temperature was set to 230°C, and the molten polymer was cooled by blown air and then wound up on a roll. The extruded precursor (raw film) had a thickness of 15 μm. The raw film was then annealed at 130°C for 15 minutes. The annealed film was then cold-stretched to 21% at room temperature, then hot-stretched to 158% at 123°C, and relaxed to 128% at 126°C to form micropores, resulting in a microporous membrane. After the stretching and perforation, the physical properties of the microporous membrane were measured. The results are shown in Table 8.

[0120] [Examples 2 to 6, Examples 8 to 10, Comparative Examples 1, 2, and 4] Microporous membranes were obtained in the same manner as in Example 1 except that the raw materials were changed as shown in Table 8, and the obtained microporous membranes were evaluated.

[0121] [Example 7] Polypropylene with an MFR of 2.4 was blended with 0.2% by mass of 1:2,6-naphthalenedicarboxylic acid dicyclohexylamide as a β-crystal nucleating agent and 0.1% by mass of an antioxidant, and the blend was placed in a co-rotating twin-screw extruder and melt-kneaded at a set temperature of 270° C. The resulting strand was cooled and solidified in a water bath and cut with a pelletizer to produce pellets. The resulting β-crystal activated polypropylene pellets and the silane-modified polypropylene were dry-blended in a mass ratio of β-crystal activated polypropylene:silane-modified polypropylene = 95:5 (mass%), then melted in a 2.5-inch extruder and fed to an annular die using a gear pump. The die temperature was set to 230°C, and the molten polymer was cooled by blown air and then wound up on a roll. The extruded precursor (raw film) had a thickness of 15 μm. The raw film was then annealed at 130°C for 15 minutes. The annealed film was then cold-stretched to 21% at room temperature, then hot-stretched to 158% at 123°C, and relaxed to 128% at 126°C to form micropores, resulting in a microporous membrane. After the stretching and perforation, the physical properties of the microporous membrane were measured. The results are shown in Table 8.

[0122] Comparative Example 3 A microporous membrane was obtained in the same manner as in Example 7 except that the raw materials were changed as shown in Table 8, and the obtained microporous membrane was evaluated.

[0123] [Table 8]

Claims

1. A microporous membrane for an electrical storage device comprising both a polyolefin having one or more functional groups and another polyolefin, wherein the microporous membrane for an electrical storage device has an amorphous portion crosslinked structure in which the amorphous portions of the polyolefin having one or more functional groups are crosslinked, and the amorphous portions are selectively crosslinked; The other polyolefin satisfies the following requirements (A) to (C): (A) The melt flow rate (MFR) measured under conditions of a temperature of 230°C and a mass of 2.16 kg is 3.0 g / 10 min or less; (B) the value obtained by dividing the weight average molecular weight Mw by the number average molecular weight Mn (Mw / Mn) is 15 or less; and (C) Density is 0.85 g / cm ３ That's it; and The microporous membrane for an electricity storage device meets the following requirement (D): (D) the ratio of the orientation rate in the transverse direction (TD) to the machine direction (MD) as measured by wide-angle X-ray scattering is 1.3 or more; A microporous membrane for an electricity storage device, which satisfies the above requirements.

2. The microporous membrane for an electricity storage device has the following formula (II): 10 Ｅ’ｍｉｘ H' / H' ０ (10) wherein E' is the storage modulus measured at 160°C to 300°C when the microporous membrane for an electricity storage device has the amorphous crosslinked structure; and E' ０ is the storage modulus of the microporous membrane for an electricity storage device that does not have an amorphous crosslinked structure, measured at 160°C to 300°C, and E' or E' ０ The conditions for measuring the storage modulus are defined by the following features (i) to (iv). (i) Dynamic viscoelasticity measurement was performed under the following conditions: Atmosphere: Nitrogen Measurement device used: RSA-G2 (manufactured by TA Instruments) - Sample film thickness: 5 μm to 50 μm range (measurement is performed on one sample regardless of the sample film thickness) - Measurement temperature range: -50 to 300°C Temperature increase rate: 10°C / min ・Measurement frequency: 1Hz Deformation mode: Linear tension Initial value of static tensile load: 0.5N Initial gap distance (at 25°C): 25 mm Auto strain adjustment: Enabled (range: amplitude 0.05 to 25%, sine wave load 0.02 to 5N) It was carried out at. (ii) The static tensile load refers to the intermediate value between the maximum stress and the minimum stress in each periodic motion, and the sinusoidal load refers to an oscillating stress centered on the static tensile load. (iii) The sinusoidal tensile mode refers to measuring the vibration stress while performing periodic motion at a fixed amplitude of 0.2%, and the vibration stress was measured by varying the gap distance and static tensile load so that the difference between the static tensile load and the sinusoidal load was within 20%. When the sinusoidal load became 0.02 N or less, the amplitude was amplified so that the sinusoidal load was within 5 N and the increase in the amplitude was within 25%, and the vibration stress was measured again. (iv) The relationship between the obtained sinusoidal load and the amplitude value, and the following formula: s ＊ =s ０ ・Exp[i(ωt+d)], e ＊ =e ０ ・Exp(iot) s ＊ =E ＊ ・e ＊ E ＊ =E'++E'' (In the formula, σ ＊ : vibration stress, ε ＊ : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between vibration stress and strain, E ＊ : complex modulus of elasticity, E': storage modulus of elasticity, E'': loss modulus of elasticity Vibration stress: sinusoidal load / initial cross-sectional area Static tensile load: Load at the minimum point of vibration stress at each cycle (minimum point of gap distance at each cycle) Sinusoidal load: the difference between the measured oscillatory stress and the static tensile load The storage modulus is calculated from The mixed storage modulus ratio (R Ｅ’ｍｉｘ 2. The microporous membrane for an electricity storage device according to claim 1, wherein the ratio of the surface area to the surface area of ​​the microporous membrane is 1.2 to 20 times.

3. The other polyolefin has an MFR of 0.25 g / 10 min or more, an Mw / Mn of 4.0 or more, and a density of 1.1 g / cm ３ is less than or equal to, and the ratio of orientation ratios MD / TD of the microporous membrane for an electricity storage device is 3.0 or less; The microporous membrane for an electricity storage device according to claim 1 or 2.

4. The microporous membrane for an electricity storage device according to any one of claims 1 to 3, wherein the polyolefin having one or more functional groups and the other polyolefin are polypropylene.

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