JP7498571B2 - Microporous membrane for power storage devices - Google Patents

Description

The present invention relates to a microporous membrane for an electricity storage device.

Microporous membranes are widely used as separation or selective permeation separation membranes for various substances, and as separators, etc. Examples of their applications include precision filtration 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, and separators for electricity storage devices. In particular, polyolefin microporous membranes are preferably used as separators for lithium ion batteries, which are widely used in notebook personal computers, mobile phones, digital cameras, etc.

In order to ensure the safety of the battery, it has been proposed to achieve both the activation of the shutdown function and an improvement in the film 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 silane-modified polyolefin-containing separator with water. Patent Document 7 describes a crosslinked structure formed by ring-opening of norbornene by irradiation with ultraviolet light, electron beams, etc. Patent Document 8 describes that the insulating layer of the separator contains a (meth)acrylic acid copolymer having a crosslinked structure, a styrene-butadiene rubber binder, etc.

The components used for lithium-ion batteries include positive and negative electrode materials, electrolytes, and separators. Of these components, the separator is required to be inert to electrochemical reactions or surrounding components due to its suitability as an insulating material. On the other hand, since the beginning of the development of negative electrode materials for lithium-ion batteries, a technology has been established to suppress the decomposition of the electrolyte on the negative electrode surface by forming a solid electrolyte interface (SEI) through a chemical reaction during the first charge (Non-Patent Document 1). Even if polyolefin resin is used for the separator, oxidation reactions are induced on the positive electrode surface under high voltage, and cases of blackening and surface deterioration of the separator have been reported.

Japanese Patent Application Laid-Open No. 9-216964 International Publication No. WO 97/44839 Japanese Patent Application Laid-Open No. 11-144700 Japanese Patent Application Laid-Open No. 11-172036 JP 2001-176484 A JP 2000-319441 A JP 2011-071128 A JP 2014-056843 A

Lithium-ion Secondary Battery (2nd Edition) Published by Nikkan Kogyo Shimbun

In recent years, the output and energy density of lithium-ion secondary batteries for use in mobile devices or vehicles have been increasing, while there is a demand for smaller battery cells and stable cycle charge/discharge performance during long-term use. For this reason, strength and porosity are required for the manufacture of microporous membranes that can be used as battery separators. Furthermore, the standard for battery safety has become stricter than before, and as described in Patent Documents 1 and 2, there is a demand for separators that have a shutdown function and high-temperature membrane rupture properties, and a stable manufacturing method thereof. In this regard, it is desirable for the shutdown temperature to be lower than 150°C, and it is desirable for the membrane rupture temperature to be higher.

However, the crosslinking methods described in Patent Documents 1 to 8 are all performed in-process or in a batch immediately after the preparation of the microporous membrane. Therefore, after the formation of the crosslinked structure described in Patent Documents 1 to 8, the microporous membrane must be coated and slit in order to be used as a separator, and the internal stress increases in the subsequent lamination and winding process with the electrodes, which may cause 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 may increase at room temperature or room temperature. In addition, when a crosslinked structure is formed by irradiating a microporous membrane with light such as ultraviolet light or an electron beam, the light irradiation may become non-uniform, resulting in a non-homogeneous crosslinked structure. This is thought to be because the periphery of the crystalline part of the resin constituting the microporous membrane is easily crosslinked by the electron beam.

In view of the above problems, the present invention aims to improve the resistance to rupture of a microporous membrane at high temperatures without impairing the strength and porosity of the microporous membrane, and to achieve both device characteristics and high safety in a nail penetration test for an electricity storage device that uses a microporous membrane as a separator.

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 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;
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) the density is 0.85 g/ ^cm3 or more;
and the microporous membrane for an electricity storage device satisfies the following requirement (D):
(D) the ratio of the orientation rate in the width direction (TD) to the machine direction (MD) as measured by wide-angle X-ray scattering, MD/TD, is 1.3 or more;
A microporous membrane for an electricity storage device, comprising:
[2]
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]
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]
2. 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)
{In the formula, _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 in the electricity storage device, and the measurement conditions for the storage modulus _E'Z or _E'Z0 are specified in the following configurations (i) to (iv).
(i) Dynamic viscoelasticity measurement was performed under the following conditions:
Atmosphere: Nitrogen Measurement equipment used: RSA-G2 (manufactured by 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°C/min
Measurement frequency: 1Hz
Deformation mode: Sine wave tension mode (Linear tension)
Initial value of static tensile load: 0.5N
Initial gap distance (at 25°C): 25mm
· Auto strain adjustment: Enabled (range: amplitude value 0.05 to 25%, sine wave load 0.02 to 5N)
It was carried out in.
(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 oscillatory stress centered on the static tensile load.
(iii) The sinusoidal tensile mode refers to measuring the vibration stress while performing periodic motion with a fixed amplitude of 0.2%, and in this case, the gap distance and the static tensile load were changed so that the difference between the static tensile load and the sinusoidal load was within 20% to measure the vibration stress. When the sinusoidal load became 0.02 N or less, the amplitude value was amplified so that the sinusoidal load was within 5 N and the increase in the amplitude value was within 25%, and the vibration stress was measured.
(iv) The relationship between the obtained sinusoidal load and the amplitude value, and the following formula:
σ ^* = _σ0 ·Exp[i(ωt+δ)],
ε ^* = ε ₀ Exp(iωt),
σ ^* =E ^* ·ε ^*
E ^* =E'+iE''
(In the formula, σ ^* : oscillatory stress, ε ^* : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between oscillatory stress and strain, E ^* : complex elastic modulus, E': storage elastic modulus, E'': loss elastic modulus. Oscillatory stress: sinusoidal load/initial cross-sectional area. Static tensile load: load at the minimum point of oscillatory stress in each cycle (minimum point of gap distance in each cycle). Sine wave load: difference between measured oscillatory stress and static tensile load.)
The storage modulus is calculated from
6. The microporous membrane for an electricity storage device according to any one of items 1 to 5, wherein a mixed storage modulus ratio (R _E′x ) defined by:
[7]
A microporous membrane for an electricity storage device comprising a polyolefin, the microporous membrane for an electricity storage device having an amorphous part crosslinked structure in which an amorphous part of the polyolefin is 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) the density is 0.85 g/ ^cm3 or more;
and the microporous membrane for an electricity storage device satisfies the following requirement (D):
(D) the ratio of the orientation rate in the width direction (TD) to the machine direction (MD) is 1.3 or more, as measured by wide-angle X-ray scattering;
A microporous membrane for an electricity storage device, comprising:
[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 portion crosslinked structure, and _E'0 is the storage modulus measured at 160°C to 300°C of the microporous membrane for an electricity storage device that does not have an amorphous portion crosslinked structure, and the measurement conditions for the storage modulus of 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 equipment 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
Heating rate: 10°C/min
Measurement frequency: 1Hz
Deformation mode: Sine wave tension mode (Linear tension)
Initial value of static tensile load: 0.5N
Initial gap distance (at 25°C): 25mm
· Auto strain adjustment: Enabled (range: amplitude value 0.05 to 25%, sine wave load 0.02 to 5N)
It was carried out in.
(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 oscillatory stress centered on the static tensile load.
(iii) The sinusoidal tensile mode refers to measuring the vibration stress while performing periodic motion with a fixed amplitude of 0.2%, and in this case, the gap distance and the static tensile load were changed so that the difference between the static tensile load and the sinusoidal load was within 20% to measure the vibration stress. When the sinusoidal load became 0.02 N or less, the amplitude value was amplified so that the sinusoidal load was within 5 N and the increase in the amplitude value was within 25%, and the vibration stress was measured.
(iv) The relationship between the obtained sinusoidal load and the amplitude value, and the following formula:
σ ^* = _σ0 ·Exp[i(ωt+δ)],
ε ^* = ε ₀ Exp(iωt),
σ ^* =E ^* ·ε ^*
E ^* =E'+iE''
(In the formula, σ ^* : oscillatory stress, ε ^* : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between oscillatory stress and strain, E ^* : complex elastic modulus, E': storage elastic modulus, E'': loss elastic modulus. Oscillatory stress: sinusoidal load/initial cross-sectional area. Static tensile load: load at the minimum point of oscillatory stress in each cycle (minimum point of gap distance in each cycle). Sine wave load: difference between measured oscillatory stress and static tensile load.)
The storage modulus is calculated from
9. The microporous membrane for an electricity storage device according to item 7 or 8, wherein a mixed storage modulus ratio (R _E′mix ) defined by:
[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/ ^cm3 or less; and the orientation ratio MD/TD of the microporous membrane for an electricity storage device is 3.0 or less.
Item 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)
{In the formula, _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 in the electricity storage device, and the measurement conditions for the storage modulus _E'Z or _E'Z0 are specified in the following configurations (i) to (iv).
(i) Dynamic viscoelasticity measurement was performed under the following conditions:
Atmosphere: Nitrogen Measurement equipment 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
Heating rate: 10°C/min
Measurement frequency: 1Hz
Deformation mode: Sine wave tension mode (Linear tension)
Initial value of static tensile load: 0.5N
Initial gap distance (at 25°C): 25mm
· Auto strain adjustment: Enabled (range: amplitude value 0.05 to 25%, sine wave load 0.02 to 5N)
It was carried out in.
(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 oscillatory stress centered on the static tensile load.
(iii) The sinusoidal tensile mode refers to measuring the vibration stress while performing periodic motion with a fixed amplitude of 0.2%, and in this case, the gap distance and the static tensile load were changed so that the difference between the static tensile load and the sinusoidal load was within 20% to measure the vibration stress. When the sinusoidal load became 0.02 N or less, the amplitude value was amplified so that the sinusoidal load was within 5 N and the increase in the amplitude value was within 25%, and the vibration stress was measured.
(iv) The relationship between the obtained sinusoidal load and the amplitude value, and the following formula:
σ ^* = _σ0 ·Exp[i(ωt+δ)],
ε ^* = ε ₀ Exp(iωt),
σ ^* =E ^* ·ε ^*
E ^* =E'+iE''
(In the formula, σ ^* : oscillatory stress, ε ^* : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between oscillatory stress and strain, E ^* : complex elastic modulus, E': storage elastic modulus, E'': loss elastic modulus. Oscillatory stress: sinusoidal load/initial cross-sectional area. Static tensile load: load at the minimum point of oscillatory stress in each cycle (minimum point of gap distance in each cycle). Sine wave load: difference between measured oscillatory stress and static tensile load.)
The storage modulus is calculated from
Item 17. The microporous membrane for an electricity storage device according to any one of items 12 to 16, wherein a mixed storage modulus ratio (R _E′x ) defined by:
[18]
A microporous membrane for an electricity storage device comprising polypropylene, the polypropylene being β-crystal active, and the microporous membrane for an electricity storage device having an amorphous portion crosslinked structure in which an amorphous portion of the polypropylene is 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 portion crosslinked structure, and _E'0 is the storage modulus measured at 160°C to 300°C of the microporous membrane for an electricity storage device that does not have an amorphous portion crosslinked structure, and the measurement conditions for the storage modulus of 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 equipment used: RSA-G2 (manufactured by 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°C/min
Measurement frequency: 1Hz
Deformation mode: Sine wave tension mode (Linear tension)
Initial value of static tensile load: 0.5N
Initial gap distance (at 25°C): 25mm
· Auto strain adjustment: Enabled (range: amplitude value 0.05 to 25%, sine wave load 0.02 to 5N)
It was carried out in.
(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 oscillatory stress centered on the static tensile load.
(iii) The sinusoidal tensile mode refers to measuring the vibration stress while performing periodic motion with a fixed amplitude of 0.2%, and in this case, the gap distance and the static tensile load were changed so that the difference between the static tensile load and the sinusoidal load was within 20% to measure the vibration stress. When the sinusoidal load became 0.02 N or less, the amplitude value was amplified so that the sinusoidal load was within 5 N and the increase in the amplitude value was within 25%, and the vibration stress was measured.
(iv) The relationship between the obtained sinusoidal load and the amplitude value, and the following formula:
σ ^* = _σ0 ·Exp[i(ωt+δ)],
ε ^* = ε ₀ Exp(iωt),
σ ^* =E ^* ·ε ^*
E ^* =E'+iE''
(In the formula, σ ^* : oscillatory stress, ε ^* : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between oscillatory stress and strain, E ^* : complex elastic modulus, E': storage elastic modulus, E'': loss elastic modulus. Oscillatory stress: sinusoidal load/initial cross-sectional area. Static tensile load: load at the minimum point of oscillatory stress in each cycle (minimum point of gap distance in each cycle). Sine wave load: difference between measured oscillatory stress and static tensile load.)
The storage modulus is calculated from
20. The microporous membrane for an electricity storage device according to item 18 or 19, wherein a mixed storage modulus ratio (R _E′mix ) defined by:
[21]
The microporous membrane for an electricity storage device contains, as the polypropylene,
The following requirements (P1) to (P3):
(P1) The MFR, 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) the density is 0.89 g/ ^cm3 or more:
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:
[22]
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.
[23]
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.
[24]
^24. The microporous membrane for an electricity storage device according to any one of items 21 to 23, wherein 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 or less.

According to the present invention, it is possible to improve the resistance to membrane rupture at high temperatures without impairing the strength and porosity of the microporous membrane for an electricity storage device, and it is possible to achieve both the battery characteristics of an electricity storage device equipped with the microporous membrane as a separator and high safety in a nail penetration test. Furthermore, according to the present invention, since it is not necessary to form a crosslinked structure during or immediately after the membrane production process, it is possible to suppress an increase in the internal stress of the separator and deformation after the production 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.

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

The following provides a detailed explanation of the form for carrying out the present invention (hereinafter, abbreviated as "embodiment"). Note that the present invention is not limited to the following embodiment, and can be modified in various ways within the scope of the gist of the invention.

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

From the viewpoint of resistance to oxidation-reduction degradation and a dense and uniform porous structure, the polyolefin-based microporous membrane is preferably used to form an electricity storage device, more preferably used as a constituent material of the electricity storage device, even more preferably used as a separator for the electricity storage device, and particularly preferably used as a separator for a lithium ion battery. In this specification, a separator for an electricity storage device (hereinafter sometimes abbreviated as "separator") refers to a member that is disposed between multiple electrodes in an electricity storage device and has ion permeability and, if necessary, shutdown properties. The separator includes a microporous membrane, and may further include any functional layer as desired.

[First and second embodiments]
The microporous membrane according to the first embodiment includes a polyolefin having one or more functional groups, and after being housed in an electricity storage device, (1) the functional groups of the polyolefin undergo a condensation reaction with each other, (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 to form a crosslinked structure. In the first embodiment, the microporous membrane can form a crosslinked structure by any of the above reactions (1) to (3), thereby maintaining strength and improving membrane rupture resistance at high temperatures of 150° C. or higher, and tends to achieve both device characteristics and safety when housed in an electricity storage device, for example as a separator.

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) the density is 0.85 g/ ^cm3 or more;
and the microporous membrane satisfies the following requirement (D):
(D) the ratio of the orientation rate in the width direction (TD) to the machine direction (MD) as measured by wide-angle X-ray scattering, MD/TD, is 1.3 or more;
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.

Regarding requirement (A), if the MFR of the polyolefin (PO) is 3.0 g/10 min or less when measured under conditions of a temperature of 230° C. and a mass of 2.16 kg, the strength of the obtained microporous film 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. 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 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.

Regarding requirement (B), when the PO dispersity (Mw/Mn) is 15 or less, there is a tendency that both film formability and strength can be achieved during PO molding. From the viewpoint of achieving both film formability and strength, the PO dispersity (Mw/Mn) is preferably 4.0 to 13, more preferably 4.9 to 11, and even more preferably 5.2 to 9.0. The PO dispersity (Mw/Mn) may be 5.5 or more, 5.8 or more, 6.0 or more, 6.2 or more, or 6.5 or more. The PO dispersity (Mw/Mn) may be 8.0 or less, 7.0 or less, or 6.5 or less.

Regarding requirement (C), since PO density is closely related to PO crystallinity, when the PO density is 0.85 g/cm ³ or more, the productivity of the microporous film is improved, and it is particularly effective for the dry porosity method. From the viewpoint of productivity and pore opening, the PO density is preferably 0.85 g/cm ³ or more, preferably 0.88 g/cm ³ or more, or preferably 0.90 g/cm ³ or more. In addition, the PO density is preferably 1.1 g/cm ³ or less, preferably 1.0 g/cm ³ or less, preferably 0.98 g/cm ³ or less, preferably 0.97 g/cm ³ or less, preferably 0.96 g/cm ³ or less, preferably 0.95 g/cm ³ or less, preferably 0.94 g/cm ³ or less, preferably 0.93 g/cm ³ or less, or preferably less than 0.92 g/cm ^3. Examples of PO include polypropylene and polyethylene.

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 match the desired device characteristics. From the viewpoint of compatibility between the membrane properties and the device characteristics, the lower limit of the orientation ratio MD/TD is preferably 1.4 or more or 1.5 or more, and 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 production process.

The microporous membrane according to the second embodiment includes a β-crystal active polypropylene having one or more functional groups, and after being stored 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 during melt extrusion of polypropylene having β-crystal activity, and the unstretched sheet produced is stretched to cause crystal transition to α-crystals with 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 obtained microporous membrane can form a crosslinked structure by any of the above reactions (1) to (3), which can maintain strength and improve resistance to membrane rupture at high temperatures of 150°C or higher, and tends to balance the device characteristics 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 multiple types of polypropylene are contained, at least one of them has one or more functional groups and β-crystal activity.

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 polyolefins 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 form a crosslinked structure by utilizing the surrounding environment or chemicals inside the electricity storage device, thereby suppressing an increase in internal stress or deformation of the produced 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 incorporated into an electricity storage device, the effects of stress such as tension generated during those processes remain. In this case, if the stress is released after the electricity storage device is assembled, it is considered that this may cause deformation or damage to the electrode windings due to stress concentration, which is not preferable.

In addition, in the first and second embodiments, it is not necessary to form a crosslinked structure during or immediately after the film-forming process, so that when the microporous film is used as a separator, it is possible to suppress an increase in internal stress and deformation after the production of an electricity storage device, and/or contribute to energy savings without using light irradiation or heating to form a crosslinked structure.

[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 part of the polyolefin is crosslinked. In the third embodiment, the amorphous crosslinked structure of the polyolefin can improve resistance to membrane rupture 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 device characteristics and safety.

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) the density is 0.85 g/ ^cm3 or more;
and the microporous membrane satisfies the following requirement (D):
(D) the ratio of the orientation rate in the width direction (TD) to the machine direction (MD) as measured by wide-angle X-ray scattering, MD/TD, is 1.3 or more;
In the third embodiment, when the polyolefin and the microporous membrane satisfy requirements (A) to (D), the strength, membrane formability, productivity, pore opening, and ion permeability of the microporous membrane, as well as the electricity storage device properties, tend to be improved for the same reasons as those of requirements (A) to (D) described in the first embodiment.

The microporous membrane according to the fourth embodiment contains polypropylene having β-crystal activity, and has an amorphous crosslinked structure in which the amorphous part of the polypropylene is crosslinked. In the fourth embodiment, an unstretched sheet having β-crystals is produced during melt extrusion of polypropylene having β-crystal activity, and the unstretched sheet produced is stretched to cause crystal transition to α-crystals having a relatively high crystal density, and micropores are formed due to the difference in crystal density between the two to obtain a microporous membrane. Since the obtained microporous membrane has an amorphous crosslinked structure, it is possible to improve the membrane rupture resistance at high temperatures of 150°C or higher, and there is a tendency to achieve both the device characteristics and safety of the power storage device. The microporous membrane according to the fourth embodiment may contain one or more types of polypropylene, and when it contains multiple types of polypropylene, at least one of them has β-crystal activity.

In the third and fourth embodiments, it is believed that the functional groups contained in the polyolefin such as polypropylene are not incorporated into the crystalline portion of the polyolefin, but are crosslinked in the amorphous portion. 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, as compared with conventional crosslinked microporous membranes in which the crystalline portion and its periphery are easily crosslinked, and thus can ensure the safety of the electricity storage device. From the same viewpoint, the amorphous portion of the polyolefin contained in the microporous membranes according to the third and fourth embodiments is preferably selectively crosslinked, and more preferably is significantly crosslinked compared to the crystalline portion.

[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 as follows (A) to (D).

(A) Crystal structure in polyolefin microporous film Polyolefin resins, such as polyethylene, are generally crystalline polymers, as shown in FIG. 1, and have a higher-order structure divided into lamellae (crystalline parts) of the crystal structure, amorphous parts, and intermediate layers between them. In the crystalline parts and the intermediate layers between the crystalline parts and amorphous parts, 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 viscoelasticity measurements. On the other hand, the mobility of the polymer chains in the amorphous parts is very high, and they are observed in the -150°C to -100°C range in solid viscoelasticity measurements. This point is related to radical relaxation or radical migration reactions, crosslinking reactions, etc., which will be described later.

In addition, the polyolefin molecules that make up the crystals are not single, but rather, 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 has progressed through simulations and has become clearer. 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. In this way, even in the crystalline portion (inside the lamella), it is predicted that there are parts that are not constrained and have somewhat high mobility.

(a) Crosslinking Reaction Mechanism by Electron Beam The reaction mechanism of electron beam crosslinking (hereinafter referred to as EB crosslinking) of a polymer is as follows.
(i) Irradiation with an electron beam of several tens to several 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 generation of radicals;
(iv) Abstraction of adjacent hydrogen by radicals and migration of active sites, and (v) recombination of radicals to form crosslinking reactions or polyenes.
Here, the radicals generated in the crystal portion have poor mobility, so they exist for a long period of time, and impurities cannot enter the crystal, so the probability of reaction and quenching is low. Such radical species are called stable radicals, and remain for a long period of time, such as several months, and their lifetimes are clarified by ESR measurement. As a result, crosslinking reactions in the crystal are considered to be poor. However, in the unconstrained molecular chains or the peripheral crystal-amorphous intermediate layer portions that are present in small amounts inside the crystal, the generated radicals have a somewhat long lifetime. Such radical species are called persistent radicals, and in an environment with mobility, crosslinking reactions between molecular chains are considered to proceed with a high probability. On the other hand, since the mobility of the amorphous portion is very high, the generated radical species have a short lifetime, and it is considered that not only crosslinking reactions between molecular chains but also polyene reactions within a single molecular chain proceed with a high probability.
As described above, in a microscopic view at the crystal level, it can be assumed that the crosslinking reaction caused by EB crosslinking is localized inside the crystal or in its periphery.

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

(E) Relationship between Differences in Crosslinking Structure and Effects In order to form a crosslinking structure in a microporous membrane, it is preferable to use a combination of a functional group in a polyolefin resin and a chemical substance contained in an electricity storage device, or to use a chemical substance contained in an electricity storage device as a catalyst. In a crosslinking reaction caused by a chemical reaction inside an electricity storage device, the morphology of the reaction product differs depending on the raw material or catalyst used. In the research leading to the present invention, the phenomenon was elucidated by the following experiments in order to clarify the crosslinking structure and the change in the physical properties of the microporous membrane due to the change in structure.

A fuse/meltdown characteristic test was conducted to examine the behavior of both films during crystal melting, one that had not been EB crosslinked or chemically crosslinked (before) and one that had been chemically crosslinked. As a result, the fuse temperature of the EB crosslinked film was significantly higher, with the meltdown temperature rising to 200°C or higher. On the other hand, the fuse temperature of the chemically crosslinked film remained unchanged before and after crosslinking, and it was confirmed that the meltdown temperature rose to 200°C or higher. From this, it is believed that the fuse characteristics caused by crystal melting in the EB crosslinked film were due to crosslinking around the crystals, which caused the melting temperature to rise and the melting rate to decrease. On the other hand, it was concluded that the chemically crosslinked film did not have a crosslinked structure in the crystals, so there was no change in the fuse characteristics. In addition, in the high temperature range of around 200°C, both films have a crosslinked structure after crystal melting, so the entire resin can be stabilized in a gel state, resulting in good meltdown characteristics.

The above findings are summarized in Table 1 below.

In the first and third embodiments, (1) the condensation reaction between functional groups of polyolefins such as polyethylene and 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.

In the first and third embodiments, (2) in the reaction between the functional group of a polyolefin such as polyethylene or polypropylene and a chemical substance inside the electricity storage device, for example, the 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, additives, or decomposition products thereof contained in the electricity storage device. In addition, according to reaction (2), a cross-linked structure can be formed 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.

The microporous membranes according to the first, second, third and fourth embodiments have, from the viewpoints of forming an amorphous part crosslinked structure, achieving both a shutdown function and high-temperature membrane rupture resistance, a polymerizable compound represented by the following formula (I):
R _{E 'X} = E ' _Z / E ' _Z0 (I)
{In the formula, _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 a temperature range of 160°C to 300°C before the microporous membrane is incorporated into the electricity storage device.}
and/or a mixed storage modulus ratio (R _E′x ) defined by the following formula (III):
R E _{'' X} = E '' _Z / E '' _{Z 0} (III)
{In the formula, E″ _Z is the loss 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 loss modulus measured in a temperature range of 160° C. to 300° C. before the microporous membrane is incorporated into the electricity storage device.}
The mixed loss modulus ratio (R _E''x ) defined by is preferably 1.2 to 20 times, more preferably 2.0 to 18 times, and even more preferably 3.0 to 16.5 times. E' _Z and E' _z0 , and E'' _Z and E'' _z0 are respectively average values of 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 addition, in the case of a laminated film, only the polyolefin microporous film is removed from the laminated film to measure the storage modulus E' _Z and E' _z0 and the loss modulus E'' _Z and E'' _z0 . The measurement conditions for the modulus E' _Z , E' _z0 , E'' _Z or E'' _z0 are described in the Examples.

In the separators according to the first, second, third and fourth embodiments, from the viewpoints of forming an amorphous part crosslinked structure, achieving both a shutdown function and high-temperature resistance to film rupture, etc., a polyimide having a structure represented by the following formula (II):
R _E'mix = E'/E' ₀ (II)
{In the formula, E' is the storage modulus of a microporous membrane having an amorphous crosslinked structure measured at 160°C to 300°C, and _E'0 is the storage modulus of a microporous membrane not having an amorphous crosslinked structure measured at 160°C to 300°C.}
and/or a mixed storage modulus ratio (R _E′mix ) defined by the following formula (IV):
R _{E ″ mix} ＝ E ″ / E ″ ₀ (IV)
{In the formula, 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 measured at 160°C to 300°C of a microporous membrane that does not have an amorphous crosslinked structure.}
The mixed loss modulus ratio (R _E''mix ) defined by 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 respectively average values 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' 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.

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

[Polyolefin]
The polyolefin constituting the microporous membrane is not particularly limited, and examples thereof 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, from the viewpoint of ease of making the membrane porous in a wet or dry manner, high density polyethylene, low density polyethylene, ultra-high molecular weight polyethylene (UHMWPE), polypropylene, polybutene, or a combination thereof is preferred, and polypropylene is more preferred. In general, it is known that the weight average molecular weight of UHMWPE is 1,000,000 or more. The polyolefin may be used alone or in combination of two or more kinds.

The weight average molecular weight (Mw) of the polyolefin can be arbitrarily determined so that the dispersity (Mw/Mn) of requirement (B) described above 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 raw material polyolefin.

[Polyolefins having one or more functional groups]
From the viewpoints of forming a crosslinked structure, resistance to oxidation-reduction deterioration, and dense and uniform porous structure, the microporous membrane preferably contains a functional group-modified polyolefin or a polyolefin copolymerized with a monomer having a functional group as a polyolefin having one or more functional groups. In this specification, functional group-modified polyolefin refers to a polyolefin to which a functional group is bonded after production. The functional group is bonded to the polyolefin skeleton or can be introduced into a comonomer, and is preferably involved in selective crosslinking of the amorphous part of the polyolefin, and 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.

[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, it is preferable that the microporous membrane 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.

The first polypropylene (A) satisfies the following requirements (P1) to (P3) from the viewpoints of the strength, film formability, productivity, and pore openability of the microporous membrane:
(P1) The MFR, 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) the density is 0.89 g/ ^cm3 or more:
It is preferable that the homopolypropylene (A) satisfies the following requirement (P1). With respect to the requirement (P2), the homopolypropylene (A) has a polydispersity (Mw/Mn) of more preferably 4.9 to 9.0. With respect to the requirement (P3), the homopolypropylene (A) has a density of more preferably 0.90 g/cm 3 or more and 0.96 g/cm ³ or ^less , or 0.90 g/cm ³ or more and 0.93 g/cm ³ or less.

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 at least one functional group selected from the group consisting of, for example, 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, and among these, it is preferable that the polypropylene is a silane-modified polypropylene from the viewpoint of forming a crosslinked structure.

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.

[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 via a covalent bond, a hydrogen bond, or a coordinate bond. Among them, the reaction via a covalent bond is represented by the following reactions (I) to (IV):
(I) a condensation reaction of a plurality of identical functional groups; (II) a reaction between a plurality of different functional groups; (III) a chain condensation reaction between a functional group and an electrolyte; and (IV) a chain condensation reaction between a functional group and an additive.
The reaction via a coordinate bond is represented by the following reaction (V):
(V) A reaction in which a plurality of identical functional groups crosslink via coordinate bonds with eluted metal ions is preferred.

｛式中、Ｒは、置換基を有していてもよい炭素数１～２０のアルキル基又はヘテロアルキル基である。｝ Reaction (I)
A schematic scheme and specific examples of reaction (I) are shown below, where the first functional group of the microporous membrane is A.

{In the formula, R is an alkyl group or heteroalkyl group having 1 to 20 carbon atoms, which may have a substituent.}

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 structure in which the main chain is a polyolefin and the main chain has an alkoxysilyl as a graft. 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, tert-butyl, and the like. In addition, the main chain and the graft are connected by a covalent bond, and examples of the structure include alkyl, ether, glycol, and ester. Considering the manufacturing process of the microporous membrane according to this embodiment, the silane-graft modified polyolefin preferably has a silicon to carbon ratio (Si/C) of 0.2 to 1.8%, and more preferably 0.5 to 1.7%, before the crosslinking treatment step.

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.

Reactions (I) and (II) can be catalyzed, for example, catalytically promoted, by a chemical inside the energy storage device in which the microporous membrane is incorporated as a separator. The chemical can be, for example, an electrolyte, an electrolytic solution, an electrode active material, an additive, or any of their decomposition products contained in the energy storage device.

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

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

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, -NH ₂ , -NH-, -COOH, and -SH.

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 y _1. The multiple linking reaction units y ₁ may be 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. In addition, when the compound Ry has a chain structure, the multiple linking reaction units y ₁ may each independently be an end group, be incorporated into the main chain, or be a side chain or pendant.

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 be a leaving group depending on their nucleophilicity.

From the viewpoint of the nucleophilic agent, 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 the oxygen-based nucleophilic group include a hydroxyl group, an alkoxy group, an ether group, and a carboxyl group, among which -OH and -COOH are preferred. Examples of the nitrogen-based nucleophilic group include an ammonium group, a primary amino group, and a secondary amino group, among which _-NH2 and -NH- are preferred. Examples of the sulfur-based nucleophilic group include, for example, -SH and a thioether group, among which -SH is preferred.

｛式中、Ｘは、水素原子又は１価の置換基である。｝

｛式中、Ｘは、水素原子又は１価の置換基である。｝

｛式中、Ｘは、水素原子又は１価の置換基である。｝

｛式中、Ｘは、水素原子又は１価の置換基である。｝

｛式中、Ｘは、水素原子又は１価の置換基である。｝

｛式中、Ｘは、水素原子又は１価の置換基である。｝
で表される１価の基が好ましい。 When the reaction (IV) is a nucleophilic substitution reaction, from the viewpoint of the leaving group, the linking reaction unit y ₁ of the compound Ry is preferably an alkylsulfonyl group such as CH ₃ SO ₂ - or CH ₃ CH ₂ SO ₂ -; an arylsulfonyl group (-ArSO ₂ -); a haloalkylsulfonyl group such as CF ₃ SO ₂ - or CCl ₃ SO ₂ -; an alkylsulfonate group such as CH ₃ SO ₃ ^- - or CH ₃ CH ₂ SO ₃ ^- -; an arylsulfonate group (ArSO ₃ ^- -); a haloalkylsulfonate group such as CF ₃ SO ₃ ^- - or CCl ₃ SO ₃ ^- -; and a heterocyclic group, which can be used alone or in combination of a plurality of kinds. Examples of the heteroatom contained in the heterocycle include a nitrogen atom, an oxygen atom, and a sulfur atom, and among them, a nitrogen atom is preferable from the viewpoint of leaving property. Examples of the leaving group containing a nitrogen atom in the heterocycle include the following formulae (y ₁ -1) to (y ₁ -6):

{In the formula, X is a hydrogen atom or a monovalent substituent.}

{In the formula, X is a hydrogen atom or a monovalent substituent.}

{In the formula, X is a hydrogen atom or a monovalent substituent.}

{In the formula, X is a hydrogen atom or a monovalent substituent.}

{In the formula, X is a hydrogen atom or a monovalent substituent.}

{In the formula, X is a hydrogen atom or a monovalent substituent.}
A monovalent group represented by the following formula is preferred.

In formulae (y ₁ -1) to (y ₁ -6), X is 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.

｛式中、ｍは、０～２０の整数であり、かつｎは、１～２０の整数である。｝

｛式中、ｎは、１～２０の整数である。｝

｛式中、ｎは、１～２０の整数である。｝

｛式中、ｎは、１～２０の整数である。｝

｛式中、Ｘは、炭素数１～２０のアルキレン基、又はアリーレン基であり、かつｎは、１～２０の整数である。｝

｛式中、Ｘは、炭素数１～２０のアルキレン基、又はアリーレン基であり、かつｎは、１～２０の整数である。｝
で表される２価の基から成る群から選択される少なくとも１つを有することが好ましい。また、化合物Ｒｙに複数の鎖状ユニットｙ_２が含まれる場合には、それらは、互いに同一でも異なっていてもよく、それらの配列はブロックでもランダムでもよい。 When the reaction (IV) is a nucleophilic substitution reaction and the compound Ry has a chain structure, the compound Ry has, in addition to the linking reaction unit _y1 , the following formulae ( _y2-1 ) to ( _y2-6 ) as the chain unit _y2 :

{wherein m is an integer from 0 to 20, and n is an integer from 1 to 20.}

{wherein n is an integer from 1 to 20.}

{wherein n is an integer from 1 to 20.}

{wherein n is an integer from 1 to 20.}

{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 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 from each other, and their arrangement may be block or random.

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

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.

As a specific example 1 of a nucleophilic substitution reaction, the reaction scheme is shown below in which 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} ) _n- .

As a specific example 2 of the nucleophilic substitution reaction, the reaction scheme is shown below in which the functional groups x of the polyolefin are -SH and _-NH2 , the linking reaction unit _y1 of the additive (compound Ry) is a nitrogen-containing cyclic skeleton, and the chain unit _y2 is o-phenylene.

When the reaction (IV) is a nucleophilic addition reaction, the functional group x of the compound Rx and the linking reaction unit y ₁ of the compound Ry can undergo an addition reaction. In the nucleophilic addition reaction, 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 the oxygen-based nucleophilic group include a hydroxyl group, an alkoxy group, an ether group, and a carboxyl group, among which -OH and -COOH are preferred. Examples of the nitrogen-based nucleophilic group include an ammonium group, a primary amino group, and a secondary amino group, among which -NH ₂ and -NH- are preferred. Examples of the sulfur-based nucleophilic group include, for example, -SH and a thioether group, among which -SH is preferred.

｛式中、Ｒは、水素原子又は１価の有機基である。｝

で表される基から成る群から選択される少なくとも１つであることが好ましい。 In the nucleophilic addition reaction, the linking reaction unit y ₁ of the compound Ry is represented by the following formulae (Ay ₁ -1) to (Ay ₁ -6) from the viewpoint of addition reactivity or availability of raw materials:

In the formula, R is a hydrogen atom or a monovalent organic group.

It is preferable that the alkyl group is at least one selected from the group consisting of groups represented by the following formula:

In formula (Ay ₁ -4), R is a hydrogen atom or a monovalent organic group, preferably a hydrogen atom, a C _1-20 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.

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.

As a specific example of a nucleophilic addition reaction, a reaction scheme in which 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.

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 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, 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.

｛式中、複数のＸは、それぞれ独立に、水素原子又は１価の置換基である。｝
で表される少なくとも２つの基であることが好ましい。式（ＲＯｙ_１－１）において、複数のＸは、それぞれ独立に、水素原子又は１価の置換基であり、好ましくは、水素原子、Ｃ_１～２０アルキル基、脂環式基、又は芳香族基であり、より好ましくは、水素原子、メチル基、エチル基、シクロヘキシル基又はフェニル基である。エポキシ開環反応について、化合物Ｒｘの官能基ｘと化合物Ｒｙの連結反応ユニットｙ_１の好ましい組み合わせを下記表７に示す。 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, -NH ₂ , -NH-, -COOH and -SH, and/or the linking reaction unit y ₁ of the compound Ry is represented by the following formula (ROy ₁ -1):

In the formula, each of the multiple Xs is independently a hydrogen atom or a monovalent substituent.
In formula (ROy ₁ -1), each of the multiple Xs independently represents a hydrogen atom or a monovalent substituent, preferably a hydrogen atom, a C _1-20 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 compound Rx and the linking reaction unit y ₁ of compound Ry are shown in Table 7 below.

Reaction (V)
A schematic scheme of reaction (V) and examples of the functional group A are shown below, where the first functional group of the microporous membrane is A and the metal ion is Mn ^+.

In the above scheme, the metal ion M ⁿ⁺ is preferably one that has been eluted from the electricity storage device (hereinafter also referred to as eluted metal ion), and can be, for example, at least one selected from the group consisting of Zn ²⁺ , Mn ²⁺ , Co ³⁺ , Ni ²⁺ and Li ⁺ . Examples of coordinate bonds when the functional group A is ^-COO- are shown below.

A specific scheme of reaction (V) in the case where the functional group A is —COOH and the eluted metal ion is Zn ²⁺ is shown below.

In the above scheme, hydrofluoric acid (HF) can be derived from, for example, the electrolyte, electrolytic solution, electrode active material, additives, or their decomposition products or water absorption products contained in the electricity storage device, depending on the charge/discharge cycle of the electricity storage device.

(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.

[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 laminate membrane, the properties can be measured after removing layers other than the microporous membrane from the laminate membrane.

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 compliance with the rapid movement of lithium ions when used as a separator for 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 microporous membrane has a porosity of 90% or less, it tends to have better membrane strength and better suppress self-discharge.

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 the membrane thickness, the porosity, and the average pore size tends to be further improved. In addition, 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, the ion permeability tends to be further improved.

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

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.

From the viewpoint of achieving a balance between membrane rupture resistance and device safety, the puncture strength of the microporous membrane is preferably 200 to 500 gf, and more preferably 218 to 481 gf or 227 to 450 gf.

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

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

The materials to be kneaded in the kneading process can be determined depending on the subsequent pore opening process, since the pore opening process can be carried out by known dry and/or wet methods.

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 off the interface between the different materials to form holes, a lamellar hole-opening method, and a β-crystal hole-opening method.

The lamellar hole opening method is a method in which an unstretched sheet having a crystalline lamellar structure is obtained by controlling the melt crystallization conditions during sheeting by melt extrusion of a resin, and the lamellar interface is opened by stretching the unstretched sheet obtained to form holes. In the lamellar hole opening method, for example, a circular die extrusion method can be used. In the circular die extrusion method, for example, a melt kneaded product of a polypropylene resin composition can be blown up mainly in the MD from a circular die to obtain a highly crystalline MD-oriented raw sheet.

The β crystal pore opening method is a method in which an unstretched sheet having β crystals with a relatively low crystal density is produced during melt extrusion of polypropylene (PP), and the unstretched sheet produced is stretched to cause crystal transition to α crystals with a relatively high crystal density, forming holes due to the difference in crystal density between the two. As a β crystal nucleating agent, for example, 1:2,6-naphthalenedicarboxylic acid dicyclohexylamide can be used, 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 β crystal active PP can be used.

Wet methods include using a kneader to knead polyolefin, optionally with other resins, and a plasticizer or inorganic material, forming the mixture into a sheet, stretching it as necessary, and then extracting the pore-forming material from the sheet; or dissolving a polyolefin resin composition and then immersing it in a poor solvent for the polyolefin to solidify the polyolefin and simultaneously remove the solvent.

The plasticizer is not particularly limited, but examples thereof include organic compounds that can form a homogeneous solution with polyolefin at a temperature 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, paraffin oil, etc. Among these, paraffin oil and dioctyl phthalate are preferred. The plasticizer may be used alone or in combination of two or more types.

Whether a dry method or a wet method is used, from the viewpoint of maintaining the crosslinking property of the microporous membrane until it is housed in the electricity storage device, it is preferable that the method of manufacturing the microporous membrane does not include a step of contacting the potentially crosslinkable polyolefin with a crosslinking agent, other reactive compounds, functional groups of other compounds, a crosslinking-promoting catalyst, etc. Furthermore, as long as the crosslinking property of the microporous membrane is maintained, the polyolefin resin composition may contain additives such as fluorine-based flow modifiers, waxes, crystal nucleating materials, antioxidants, metal soaps such as metal salts of aliphatic carboxylates, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, coloring pigments, etc.

The heat treatment process of the pore-opening material can be carried out for the purpose of heat setting after the stretching process or after pore formation in order to suppress shrinkage of the microporous membrane. Examples of heat treatment include a stretching operation carried out in a predetermined temperature atmosphere and at a predetermined stretching rate for the purpose of adjusting physical properties, and/or a relaxation operation carried out in a predetermined temperature atmosphere and at a predetermined relaxation rate for the purpose of reducing stretching stress. A relaxation operation may be carried out after the stretching operation. These heat treatments can be carried out using a tenter or roll stretching machine.

[Electricity storage device]
The microporous membranes according to the first, second, third and fourth embodiments can be used in an electricity storage device. In general, an electricity storage device includes an exterior body, a positive electrode, a negative electrode, a separator disposed between the positive and negative electrodes, and an electrolyte. When the microporous membranes according to these embodiments are housed in a device exterior body, the functional group-modified polyolefin or functional group-grafted copolymerized polyolefin formed during the manufacturing process of the microporous membrane reacts with the chemicals contained in the electrolyte or additives to form a crosslinked structure, so that the manufactured electricity storage device has a crosslinked structure. From the viewpoint of maintaining the crosslinking property of the microporous membrane until it is housed in the electricity storage device and improving the safety of the electricity storage device thereafter, it is preferable that the microporous membrane is disposed between the positive and negative electrodes as a separator. When the microporous membrane is housed in the electricity storage device as a separator, a crosslinked structure is formed, so that the safety of the electricity storage device can be improved by causing a crosslinking reaction after the device is manufactured while being compatible with the manufacturing process of the conventional electricity storage device.

Specific examples of power storage devices 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-hydrogen batteries, nickel-cadmium batteries, electric double layer capacitors, lithium ion capacitors, redox flow batteries, lithium-sulfur batteries, lithium-air batteries, and zinc-air batteries. Among these, from the viewpoint of practicality, lithium batteries, lithium secondary batteries, lithium ion secondary batteries, nickel-hydrogen batteries, and lithium ion capacitors are preferred, and lithium batteries and lithium ion secondary batteries are more preferred.

[Lithium-ion secondary battery]
A lithium ion secondary battery is a storage battery that uses a lithium transition metal oxide such as lithium cobalt oxide or lithium cobalt composite oxide as a positive electrode, a carbon material such as graphite or graphite as a negative electrode, and an organic solvent containing a lithium salt such as _LiPF6 as an electrolyte. When charging and discharging a lithium ion secondary battery, ionized lithium travels back and forth between the electrodes. In addition, since it is necessary for the ionized lithium to move between the electrodes relatively quickly while suppressing contact between the electrodes, a separator is placed between the electrodes.

The present invention will be explained 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 deviate from the gist of the invention. The raw materials used and the methods for evaluating the various properties of the microporous membrane are as follows.

[Measurement of Melt Flow Rate (MFR)]
The melt flow rate (MFR) was expressed as a value measured on a polypropylene resin under conditions of 230° C. and 2.16 kg in accordance with JIS K 7210 (unit: g/10 min).

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

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

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

[Porosity (%)]
A sample measuring 5 cm×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 )×100

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

[Puncture strength]
A needle with a hemispherical tip with a radius of 0.5 mm was prepared, and a microporous membrane was sandwiched between two plates with an opening with a diameter (dia.) of 11 mm, and the needle, microporous membrane, and plates were set. A puncture test was performed using "MX2-50N" manufactured by Imada Co., Ltd. under conditions of a needle tip curvature radius of 0.5 mm, an opening diameter of the microporous membrane holding plate of 11 mm, and a puncture speed of 25 mm/min, and the needle and microporous membrane were brought into contact with each other to measure the maximum puncture load (i.e., puncture strength (gf)).

[Measurement of storage modulus, loss modulus and transition temperature]
Dynamic viscoelasticity of the separator was measured using a dynamic viscoelasticity measuring device, and the storage modulus (E'), loss modulus (E''), and transition temperature between the rubber-like plateau region and the crystalline melt flow region were calculated. The storage modulus change ratio (R _E'X ) was calculated according to the following formula (I), the mixed storage modulus ratio (R _E'mix ) according to the following formula (II), the loss modulus ratio (R _E''X ) according to the following formula (III), and the mixed loss modulus ratio (R _E''mix ) 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 equipment used: RSA-G2 (manufactured by 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°C/min
Measurement frequency: 1Hz
Deformation mode: Sine wave tension 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 value 0.05 to 25%, sine wave load 0.02 to 5N)
It was carried out in.
(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 oscillatory stress centered on the static tensile load.
(iii) The sinusoidal tensile mode refers to measuring the vibration stress while performing periodic motion with a fixed amplitude of 0.2%, and in this case, the gap distance and the static tensile load were changed so that the difference between the static tensile load and the sinusoidal load was within 20% to measure the vibration stress. When the sinusoidal load became 0.02 N or less, the amplitude value was amplified so that the sinusoidal load was within 5 N and the increase in the amplitude value was within 25%, and the vibration stress was measured.
(iv) The relationship between the obtained sinusoidal load and the amplitude value, and the following formula:
σ ^* = _σ0 ·Exp[i(ωt+δ)],
ε ^* = ε ₀ Exp(iωt),
σ ^* =E ^* ·ε ^*
E ^* =E'+iE''
{wherein, σ ^* : oscillatory stress, ε ^* : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between oscillatory stress and strain, E ^* : complex elastic modulus, E': storage elastic modulus, E'': loss elastic modulus, oscillatory stress: sinusoidal load/initial cross-sectional area, static tensile load: load at the minimum point of oscillatory stress in each cycle (minimum point of gap distance in each cycle), sinusoidal load: difference between measured oscillatory stress and static tensile load}
The storage modulus and loss modulus were calculated from the above.
_E'Z , _E'Z0 , and E''Z, and _E''Z0 were defined as the maximum values 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} defined as 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 introduction into the cell R _E'mix = E'/E' ₀ (II) Comparison of the presence or absence of a crosslinked structure in the amorphous region R _E''X = E'' _Z / E'' _Z0 (III) Comparison before and after introduction into the cell R _E''mix = E''/E'' ₀ (IV) Comparison of the presence or absence of a crosslinked structure in the amorphous region

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 not having an amorphous crosslinked structure) in 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 to the low-temperature side and a tangent drawn at the inflection point of the curve of the crystalline melting change portion.
An example of a graph for explaining the relationship between temperature and loss modulus is shown in Figure 4. In Figure 4, the loss modulus of a reference film (a separator for an electricity storage device that does not contain a silane-grafted modified polyolefin) in the temperature range of -50°C to 310°C is compared with that of a crosslinked film, and the transition temperatures determined in the same manner as in Figure 3 are shown. In this technical field, the storage modulus and the loss modulus are expressed by the following formula:
tan δ=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.
In the measurement of the mixed storage elastic modulus ratio (R _E'mix ) or the mixed loss elastic modulus ratio (R _E''mix ), a non-silane-modified polyolefin-based microporous membrane with a gelation degree of about 0% was used as a separator for an electricity storage device not having an amorphous crosslinked structure. In addition, E', E'0 _, E'' and _E''0 were calculated from the average value from 160°C to 300°C when no breakage of the sample (sudden drop in elastic modulus) was observed at 160°C to 300°C, and when breakage of the sample was observed at 160°C to 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 an electric storage device not having an amorphous crosslinked structure may be a separator manufactured with a composition in which 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 million) or two or three selected from the group consisting of X, Y and Z are mixed in any ratio. In addition, polyolefins composed only of a hydrocarbon skeleton, such as low-density polyethylene: LDPE, linear low-density polyethylene: LLDPE, polypropylene: PP, and olefin-based thermoplastic elastomers, may be added to the mixed composition. More specifically, the separator for an electric storage device not having an amorphous crosslinked structure may mean a polyolefin-based microporous film in which the rate of change in solid content before and after heating at 160°C in a decalin solution (hereinafter referred to as "gelation degree") is 10% or less. In addition, 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.

[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 determined as the breaking strength.

[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 of flaky graphite and acetylene black as the conductive material, and 3.2% by mass of polyvinylidene fluoride (PVDF) as the binder in N-methylpyrrolidone (NMP). This slurry was applied to one side of an aluminum foil having a thickness of 20 μm as a positive electrode current collector with a die coater, dried at 130 ° C for 3 minutes, and then compression molded with a roll press machine. At this time, the amount of active material applied to the positive electrode was adjusted to 250 g / ^m2 , and the bulk density of the active material was adjusted to 3.00 g / ^cm3 .

b. Preparation of negative electrode A slurry was prepared by dispersing 96.9% by mass of artificial graphite as a 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 a binder in purified water. This slurry was applied to one side of a copper foil having a thickness of 12 μm, which serves as a negative electrode current collector, using a die coater, dried at 120° C. for 3 minutes, and then compression molded using a roll press. At this time, the amount of active material applied to the negative electrode was adjusted to 106 g/m ² and the bulk density of the active material was adjusted to 1.35 g/cm ³ .

c. Preparation of non-aqueous electrolyte A solute _LiPF6 was dissolved in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio) to prepare a solution with a concentration of 1.0 mol/L. The positive electrode, separator, and negative electrode, in which a resistance measurement wire was attached to the back of the aluminum foil with conductive silver paste, were cut out into a circular shape with a diameter of 200 mm, and the separator and negative electrode were stacked together. The electrolyte-containing electrolyte solution was added to the resulting laminate and the whole was dyed. The laminate was sandwiched in the center with a circular aluminum heater with a diameter of 600 mm, and the aluminum heater was pressurized to 0.5 MPa from above and below with a hydraulic jack, completing the preparation for measurement. The resistance (Ω) between the electrodes was measured while the laminate was heated with the aluminum heater at a temperature rise rate of 2 ° C. / min. The temperature at which the resistance between the electrodes rose together with the fuse of the separator and the resistance exceeded 1000Ω for the first time was taken as the fuse temperature (shutdown temperature). Heating is continued, and the temperature at which the resistance falls to 1000 Ω or less is taken as the meltdown temperature (film rupture temperature).

[Example 1]
<Method for producing silane-grafted polyolefin>
The raw polyolefin used for the silane graft modified polyolefin may have a viscosity average molecular weight (Mv) of 100,000 or more and 1,000,000 or less, a weight average molecular weight (Mw) of 30,000 or more and 920,000 or less, and a number average molecular weight of 10,000 or more and 150,000 or less, and may be a propylene or butene copolymerized α-olefin. While melt-kneading the raw polyolefin in an extruder, an organic peroxide (di-t-butyl peroxide) is added to generate radicals in the α-olefin polymer chain, and then a trimethoxyalkoxide-substituted vinylsilane is injected to introduce an alkoxysilyl group into the α-olefin polymer by an addition reaction, forming a silane graft structure. At the same time, in order to adjust the radical concentration in the system, an appropriate amount of an antioxidant (pentaerythritol tetrakis [3-(3,5-di-tetra-butyl-4-hydroxyphenyl) propionate]) is added to suppress the chain reaction (gelation) in the α-olefin. The obtained silane-grafted polyolefin molten resin is cooled in water and pelletized, and then heated and dried 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 is about 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.

<Preparation of microporous membrane (single layer)>
A high molecular weight polypropylene resin (PP, MFR = 0.25) and the above 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 temperature of the die was set at 230 ° C, and the molten polymer was cooled by blown air and then wound on a roll. The extruded precursor (raw film) had a thickness of 15 μm, and 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 to obtain a microporous membrane. After the above stretching and perforation, the physical properties of the microporous membrane were measured. The results are shown in Table 8.

[Examples 2 to 6, Examples 8 to 10, Comparative Examples 1, 2, and 4]
A microporous membrane was 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 membrane was evaluated.

[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 tank and cut with a pelletizer to prepare pellets.
The obtained β-crystal active polypropylene pellets and the above silane-modified polypropylene were dry-blended in a mass ratio of β-crystal active 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 on a roll. The extruded precursor (raw film) had a thickness of 15 μm, and 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 to obtain a microporous membrane. After the above stretching and perforation, the physical properties of the microporous membrane were measured. The results are shown in Table 8.

[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.

Claims (18)

A microporous membrane for an electricity storage device comprising a polyolefin,
the polyolefin has one or more functional groups, and after being 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;
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) the density is 0.85 g/ ^cm3 or more;
and the microporous membrane for an electricity storage device satisfies the following requirement (D):
(D) the ratio of the orientation rate in the width direction (TD) to the machine direction (MD) as measured by wide-angle X-ray scattering, MD/TD, is 1.3 or more;
A microporous membrane for an electricity storage device, which satisfies the above requirements. The microporous membrane for an electricity storage device according to claim 1, wherein the crosslinked structure is formed by (1) a condensation reaction between the functional groups. The microporous membrane for an electricity storage device according to claim 1, wherein the crosslinked structure is formed by (2) reacting the functional group with a chemical substance inside the electricity storage device. The microporous membrane for an electricity storage device according to claim 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. The microporous membrane for an electricity storage device according to claim 1, wherein the crosslinked structure is formed by (3) reacting the functional group with another type of functional group. The microporous membrane for an electricity storage device has the following formula (I):
R _E'X = _E'Z / _E'Z0 (I)
{In the formula, _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 in the electricity storage device, and the measurement conditions for the storage modulus _E'Z or _E'Z0 are specified in the following configurations (i) to (iv).
(i) Dynamic viscoelasticity measurement was performed under the following conditions:
Atmosphere: Nitrogen Measurement equipment used: RSA-G2 (manufactured by 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°C/min
Measurement frequency: 1Hz
Deformation mode: Sine wave tension mode (Linear tension)
Initial value of static tensile load: 0.5N
Initial gap distance (at 25°C): 25mm
· Auto strain adjustment: Enabled (range: amplitude value 0.05 to 25%, sine wave load 0.02 to 5N)
It was carried out in.
(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 oscillatory stress centered on the static tensile load.
(iii) The sinusoidal tensile mode refers to measuring the vibration stress while performing periodic motion with a fixed amplitude of 0.2%, and in this case, the gap distance and the static tensile load were changed so that the difference between the static tensile load and the sinusoidal load was within 20% to measure the vibration stress. When the sinusoidal load became 0.02 N or less, the amplitude value was amplified so that the sinusoidal load was within 5 N and the increase in the amplitude value was within 25%, and the vibration stress was measured.
(iv) The relationship between the obtained sinusoidal load and the amplitude value, and the following formula:
σ ^* = _σ0 ·Exp[i(ωt+δ)],
ε ^* = ε ₀ Exp(iωt),
σ ^* =E ^* ·ε ^*
E ^* =E'+iE''
(In the formula, σ ^* : oscillatory stress, ε ^* : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between oscillatory stress and strain, E ^* : complex elastic modulus, E': storage elastic modulus, E'': loss elastic modulus. Oscillatory stress: sinusoidal load/initial cross-sectional area. Static tensile load: load at the minimum point of oscillatory stress in each cycle (minimum point of gap distance in each cycle). Sine wave load: difference between measured oscillatory stress and static tensile load.)
The storage modulus is calculated from
The microporous membrane for an electricity storage device according to any one of claims 1 to 5, wherein a mixed storage modulus ratio (R _E'x ) defined by: 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/ ^cm3 or less; and the orientation ratio 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 any one of claims 1 to 6 . The microporous membrane for an electricity storage device according to any one of claims 1 to 7 , wherein the polyolefin is polypropylene. 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. The microporous membrane for an electricity storage device according to claim 9 , wherein the crosslinked structure is formed by (1) a condensation reaction between the functional groups. The microporous membrane for an electricity storage device according to claim 9 , wherein the crosslinked structure is formed by (2) the functional group reacting with a chemical substance inside the electricity storage device. The microporous membrane for an electricity storage device according to claim 9 or 11 , 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. The microporous membrane for an electricity storage device according to claim 9 , wherein the crosslinked structure is formed by (3) the functional group reacting with another type of functional group. The microporous membrane for an electricity storage device has the following formula (I):
R _E'X = _E'Z / _E'Z0 (I)
{In the formula, _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 in the electricity storage device, and the measurement conditions for the storage modulus _E'Z or _E'Z0 are specified in the following configurations (i) to (iv).
(i) Dynamic viscoelasticity measurement was performed under the following conditions:
Atmosphere: Nitrogen Measurement equipment used: RSA-G2 (manufactured by 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°C/min
Measurement frequency: 1Hz
Deformation mode: Sine wave tension 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 value 0.05 to 25%, sine wave load 0.02 to 5N)
It was carried out in.
(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 oscillatory stress centered on the static tensile load.
(iii) The sinusoidal tensile mode refers to measuring the vibration stress while performing periodic motion with a fixed amplitude of 0.2%, and in this case, the gap distance and the static tensile load were changed so that the difference between the static tensile load and the sinusoidal load was within 20% to measure the vibration stress. When the sinusoidal load became 0.02 N or less, the amplitude value was amplified so that the sinusoidal load was within 5 N and the increase in the amplitude value was within 25%, and the vibration stress was measured.
(iv) The relationship between the obtained sinusoidal load and the amplitude value, and the following formula:
σ ^* = _σ0 ·Exp[i(ωt+δ)],
ε ^* = ε ₀ Exp(iωt),
σ ^* =E ^* ·ε ^*
E ^* =E'+iE''
(In the formula, σ ^* : oscillatory stress, ε ^* : strain, i: imaginary unit, ω: angular frequency, t: time, δ: phase difference between oscillatory stress and strain, E ^* : complex elastic modulus, E': storage elastic modulus, E'': loss elastic modulus. Oscillatory stress: sinusoidal load/initial cross-sectional area. Static tensile load: load at the minimum point of oscillatory stress in each cycle (minimum point of gap distance in each cycle). Sine wave load: difference between measured oscillatory stress and static tensile load.)
The storage modulus is calculated from
The microporous membrane for an electricity storage device according to any one of claims 9 to 13 , wherein a mixed storage modulus ratio (R _E'x ) defined by: The microporous membrane for an electricity storage device contains, as the polypropylene,
The following requirements (P1) to (P3):
(P1) The MFR, 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) the density is 0.89 g/ ^cm3 or more:
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;
The microporous membrane for an electricity storage device according to any one of claims 9 to 14 , comprising: The microporous membrane for an electricity storage device according to claim 15 , wherein a content of the polypropylene (B) is 4 mass% or more and 30 mass% or less. The microporous membrane for an electricity storage device according to claim 15 or 16 , wherein the polypropylene (B) is a silane-modified polypropylene. The microporous membrane for an electricity storage device according to any one of claims 15 to 17 , wherein 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/ ^cm3 or less.

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