Separator for power storage device and manufacturing method thereof

Abstract

To provide a power storage device separator that can provide a power storage device with excellent cycle stability, and a manufacturing method thereof.SOLUTION: In a power storage device separator with a microporous membrane containing polyethylene as a main component, the percentage of the signal strength after a certain period of time in the observation of spin-spin relaxation time (T2 relaxation time) of all protons in pulse NMR measurement using a solid echo method at 120°C or a CPMG method at 180°C is within a certain range.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a separator for an electricity storage device and a method for producing the same. [Background technology]

[0002] Polyolefin microporous membranes exhibit excellent electrical insulation properties or ion permeability and are therefore used as separators for electricity storage devices, such as battery separators, capacitor separators, etc. In recent years, polyolefin microporous membranes have been used particularly as separators for lithium ion secondary batteries, and are being applied not only to small electronic devices such as mobile phones and notebook computers, but also to electric vehicles such as electric cars and small electric motorcycles.

[0003] Patent Document 1 describes a separator for a secondary battery that includes a heat-resistant inorganic layer formed by atomic layer deposition (ALD) on at least one of the first and second surfaces of a porous polymer sheet and on the inner surfaces of the pores. Patent Document 2 describes a two-part adhesive formulation that includes water, a surfactant, and a protected alkylborane complex in a first part and an acrylic monomer and a trialkylborane-substituted initiator in a second part, and that can be applied to low surface energy substrates to produce effective adhesion without the aid of any surface pretreatment.

[0004] Non-Patent Document 1 describes a technique for improving the wettability of a polypropylene (PP) resin separator by modifying it with polyethylene oxide (PEO) through hyperthermal hydrogen-induced crosslinking (HHIC). Non-Patent Documents 2 and 3 describe a technique for modifying polypropylene (PP) resin using borane, since the tertiary carbon in PP is prone to generating radicals. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Announcement No. 2017-84779 [License 2] Special Announcement No. 2007-523248 [Non-licensed literature]

[0006] [Non-licensed Document 1] Changzhen Man, et.al., "Enhanced wetting properties of a polypropylene separator for a lithium-ion battery by hyperthermal hydrogen induced cross-linking of poly(ethylene oxide)", J.Mater.Chem.A, 2014, 2, pp.11980-11986. [Non-licensed Document 2] Mark F. Sonnenschein, et.al., "Physical and Chemical Probes of the Bond Strength between Trialkylboranes and Amines and Their Utility as Stabilized Free Radical Polymerization Catalysts", Macromolecules, 2006, 39, pp.2507-2513. [Non-licensed Document 3] Mark F. Sonnenschein, et.al., "Mechanism of Trialkylborane Promoted Adhesion to Low Surface Energy Plastics", Macromolecules, 2004, 37, pp.7974-7978. [Non-licensed Document 4] Vincent J. McBrierty, "NMR Solid Polymers (Cambridge Solid State Science Series)", Cambridge University Press, Illustrated Edition, August 21, 2008 [Non-Patent Document 5] PJ Hore, et.al., "Introduction to NMR: Essential Tools and Fundamentals (Chemistry Primer Series)", Kagaku Dojin, March 22, 2017 [Non-patent document 6] Hideo Akutsu, Kazuo Shimada, Eiichiro Suzuki, and Yoshifumi Nishimura, "NMR Spectroscopy (Spectroscopy Series)," Kodansha, April 23, 2016 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present disclosure is to provide a separator for an electricity storage device that can provide an electricity storage device with excellent cycle stability, and a method for producing the same. [Means for solving the problem]

[0008] As a result of extensive investigations to solve the above problems, the inventors of the present application have found that the above problems can be solved by a microporous membrane containing polyethylene as a main component (also simply referred to as a "polyethylene microporous membrane") that has a specific membrane thickness, converted strength by mass, air permeability, and porosity, wherein the signal intensity ratio is within a specific range in observation of the spin-spin relaxation time of all protons in pulsed NMR measurement using a solid echo method or a CPMG method. Examples of embodiments of the present disclosure are listed below. [1] A separator for an electricity storage device having a microporous membrane containing polyethylene as a main component, the separator for an electricity storage device having a membrane thickness of 2 to 50 μm, a mass-equivalent strength of 3,000 gf / g to 150,000 gf / g, an air permeability of 10 sec / 100 cc to 500 sec / 100 cc, and a porosity of 25% to 90%, and in observation of the spin-spin relaxation time (T2 relaxation time) of all protons in pulsed NMR measurement using a solid echo method at 120°C, the ratio of signal intensity at 0.2 msec after the start of observation is 0.2% to 40% of the signal intensity at the start of observation, and the ratio of signal intensity at 0.8 msec after the start of observation is 0.05% to 10% of the signal intensity at the start of observation. [2] A separator for an electricity storage device having a microporous membrane containing polyethylene as a main component, the separator for an electricity storage device having a membrane thickness of 2 to 50 μm, a mass-equivalent strength of 3,000 gf / g to 150,000 gf / g, an air permeability of 10 sec / 100 cc to 500 sec / 100 cc, and a porosity of 25% to 90%, and in observation of the spin-spin relaxation time (T2 relaxation time) of all protons in pulsed NMR measurement using the CPMG method at 180°C, the ratio of signal intensity 40 msec after the start of observation is 5% to 30% of the signal intensity at the start of observation, and the ratio of signal intensity 140 msec after the start of observation is 1.5% to 20% of the signal intensity at the start of observation. [3] 3. The separator for an electricity storage device according to item 1 or 2, wherein the polyethylene present in the microporous membrane is grafted with one or more heteroatom-containing functional groups. [4] The one or more heteroatom-containing functional groups may be selected from the following: A type of heteroatom-containing functional group that undergoes self-condensation reactions; A combination of two or more different heteroatom-containing functional groups that undergo a condensation reaction; one or more heteroatom-containing functional groups that form a chelate complex structure via a metal ion; one or more heteroatom-containing functional groups that form any of coordinate bonds, ionic bonds, hydrogen bonds, and covalent bonds with compounds contained in other components in the electricity storage device that are in contact with the surface of the microporous membrane; and One or more heteroatom-containing functional groups with a structure that is stable in one or more stable oxidation or reduction steps 4. The separator for an electricity storage device according to item 3, selected from the group consisting of: [5] 5. The separator for an electricity storage device according to item 4, wherein the one or more heteroatom-containing functional groups have at least one selected from the group consisting of an alkoxysilane group, an alkoxy group, a hydroxyl group, an ester group, a carboxy group, an ether group, and an amine oxide group. [6] A method for producing a separator for an electricity storage device, the method comprising: (1) a step of extruding a resin composition containing polyethylene and a pore-forming material into a sheet using an extruder, followed by cooling and solidifying the extrusion to form a sheet-like molded product; (2) stretching the sheet-like molded body to form a stretched sheet; (3) extracting a pore-forming material from the stretched sheet to form a porous sheet; (4) heat-treating the porous sheet and stretching and relaxing it in the width direction; Including, The above method further comprises the step of reacting the polyethylene with one or more graft molecules having a functional group for bonding to the molecular chain of the polyethylene and a heteroatom-containing functional group, between steps (1) and (2), between steps (2) and (3), between steps (3) and (4), or after step (4), thereby grafting the polyethylene with the heteroatom-containing functional group. [7] 7. The method according to item 6, wherein the functional group for bonding to the molecular chain of the polyethylene comprises a vinyl group. [8] The grafting step includes the following steps: immersing the sheet-like molded article, the stretched sheet, or the porous sheet in a solution containing a solvent, the graft molecule, and an alkylborane or an alkylborane complex; providing oxygen in the solution to react the graft molecules with the polyethylene to graft the polyethylene with the heteroatom-containing functional group; 8. The method according to item 6 or 7, comprising: [9] Item 9. The method according to item 8, further comprising extracting the graft molecules and the alkylborane or alkylborane complex from the sheet-like molded body, the stretched sheet, or the porous sheet after the grafting, and drying the extracted molecules.

[10] 10. The method according to any one of items 6 to 9, wherein the grafting step is carried out after step (4).

[11] 10. The method according to any one of items 6 to 9, wherein the grafting step is carried out between steps (1) and (2).

[12] 10. The method according to any one of items 6 to 9, wherein the grafting step is carried out between steps (2) and (3).

[13] An electricity storage device comprising a positive electrode, a negative electrode, and the electricity storage device separator according to any one of items 1 to 5 between the positive electrode and the negative electrode. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to provide a separator for an electricity storage device that can provide an electricity storage device with excellent cycle stability, and a method for manufacturing the same. Note that the above description should not be considered to disclose all embodiments and all advantages. Further embodiments and advantages thereof will become apparent from the following description and the drawings. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram of a microporous membrane grafted with a type of heteroatom-containing functional group that undergoes self-condensation reaction according to the present disclosure. [Figure 2] FIG. 2 is a schematic diagram of a microporous membrane grafted with two heteroatom-containing functional groups undergoing a condensation reaction according to the present disclosure. [Figure 3] FIG. 3 is a schematic diagram of a microporous membrane grafted with heteroatom-containing functional groups that form chelate complex structures according to the present disclosure. [Figure 4] FIG. 4 is a schematic illustration of a microporous membrane grafted with heteroatom-containing functional groups that form bonds with materials in an electrode according to the present disclosure. [Figure 5] FIG. 5 is a schematic diagram of a microporous membrane grafted with heteroatom-containing functional groups having a stable structure upon one or more stable oxidation or reduction steps according to the present disclosure. [Figure 6] Figure 6(a) shows the results of pulse NMR observation by the CPMG method of the microporous membrane of Example 1. Figure 6(b) shows the results of pulse NMR observation by the solid echo method of the microporous membrane of Example 1. [Figure 7] Figure 7(a) shows the results of pulse NMR observation by the CPMG method of the microporous membrane of Example 2. Figure 7(b) shows the results of pulse NMR observation by the solid echo method of the microporous membrane of Example 2. [Figure 8] Figure 8(a) shows the results of pulse NMR observation by the CPMG method of the microporous membrane of Example 3. Figure 8(b) shows the results of pulse NMR observation by the solid echo method of the microporous membrane of Example 3. [Figure 9] Figure 9(a) shows the results of pulse NMR observation by the CPMG method of the microporous membrane of Example 4. Figure 9(b) shows the results of pulse NMR observation by the solid echo method of the microporous membrane of Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present disclosure will be described in detail for the purpose of illustrating the present disclosure, but the present disclosure is not limited to these embodiments. In the present specification, the upper and lower limits of each numerical range can be combined in any combination.

[0012] <Separator for power storage device> <Graft structure> The separator for an electricity storage device of the present disclosure has a microporous membrane containing polyethylene as a main component. The polyethylene present in the microporous membrane is preferably grafted with one or more heteroatom-containing functional groups. "Grafted" refers to a structure in which the heteroatom-containing functional groups are attached as pendant groups to the polyethylene molecular chain. Grafting the polyethylene with heteroatom-containing functional groups can provide a microporous membrane with various properties, such as thermal stability, electrical properties, and adhesion to electrodes.

[0013] Heteroatoms are atoms other than carbon and hydrogen. Heteroatoms are preferably typical nonmetallic atoms, such as halogen atoms, oxygen atoms, sulfur atoms, nitrogen atoms, phosphorus atoms, and silicon atoms. Halogen atoms include fluorine atoms, chlorine atoms, bromine atoms, and iodine atoms. The polymer high-order structure of a separator primarily composed of polyethylene has low electrostatic polarity and low affinity with electrolytes with high intramolecular polarity. It has been found that by incorporating functional groups containing heteroatoms into appropriate structures in the polymer high-order structure, a partial polar structure can be imparted within the polymer high-order structure, which tends to facilitate swelling and retention of the electrolyte. This allows the electrolyte to swell, resulting in good volume recovery in the compression direction even when the separator is compressed and deformed, and maintaining the separator's uniform pore size structure for a long period of time. This allows for uniform Li-ion conduction, suppresses dendrite growth, and ensures battery safety.

[0014] Preferred examples of the heteroatom-containing functional group include: a single heteroatom-containing functional group that undergoes a self-condensation reaction; a combination of two or more different heteroatom-containing functional groups that undergo a condensation reaction; one or more heteroatom-containing functional groups that form a chelate complex structure via a metal ion; one or more heteroatom-containing functional groups that form any of coordinate bonds, ionic bonds, hydrogen bonds, and covalent bonds with compounds contained in other components in the electricity storage device that are in contact with the surface of the microporous membrane; and one or more heteroatom-containing functional groups that have a stable structure in one or more stable oxidation or reduction steps.

[0015] A heteroatom-containing functional group that undergoes a self-condensation reaction (also referred to as a "self-condensing functional group") refers to a functional group that can react with the same type of functional group, causing one part to leave and the other part to form a new bond. Preferably, a crosslinked structure is formed between polyethylene molecular chains by the self-condensation reaction. Forming a crosslinked structure between polyethylene molecular chains increases the melt viscosity of the microporous membrane, improving the safety of the battery when heat is generated. The crosslinked structure may be formed between polyethylene molecular chains present inside or on the surface of the same microporous membrane.

[0016] Self-condensation may occur spontaneously in the environment within the electrical storage device, or may be triggered by external stimuli, such as heat and light, e.g., ultraviolet light.

[0017] Examples of the self-condensing functional group include functional groups that have both electrophilic and nucleophilic properties, such as carbonyl groups having a hydrogen atom at the α-position, such as acetyl groups and aldehyde groups; and alkoxysilane groups. The self-condensing functional group is preferably an alkoxysilane group, such as a methoxysilane group and an ethoxysilane group, and more preferably a trimethoxysilane group.

[0018] FIG. 1 is a schematic diagram of a PE polymer high-order structure (10) in a microporous membrane grafted with a type of heteroatom-containing functional group (20) that undergoes a self-condensation reaction according to the present disclosure. In FIG. 1, polyethylene molecular chains present in the microporous membrane are grafted with functional groups having trimethoxysilane groups (—Si(OMe)3). The trimethoxysilane groups undergo a dehydration condensation reaction in response to heating as an external stimulus, forming a crosslinked structure (30) between the polyethylene molecular chains. In FIG. 1, n and m are each integers from 0 to 3, and n + m is 3. As shown in FIG. 1, the trimethoxysilane groups grafted to the PE polymer high-order structure in the microporous membrane may form a crosslinked structure in a battery in response to stimulation by an electrolyte and / or electrolytic solution. Furthermore, although not shown, a crosslinked structure may also be formed between polyethylene molecular chains present inside or on the surface of the same microporous membrane.

[0019] A combination of two or more different heteroatom-containing functional groups (also referred to as "condensation functional groups") that undergo a condensation reaction refers to functional groups in which two or more different functional groups react with each other, causing some of the functional groups to leave and the remaining portions to form new bonds. It is preferable that a crosslinked structure be formed in the molecular chains of polyethylene through the self-condensation reaction. Forming a crosslinked structure between the molecular chains of polyethylene increases the melt viscosity of the microporous membrane, improving the safety of the battery when it heats up.

[0020] Self-condensation may occur spontaneously in the environment within the electrical storage device, or may be triggered by external stimuli, such as heat and light, e.g., ultraviolet light.

[0021] Combinations of condensed functional groups include combinations of a functional group having electrophilic properties and a functional group having nucleophilic properties. Examples of functional groups having electrophilic properties include carbonyl groups, such as carboxy groups, ester groups, ketone groups, and aldehyde groups. Examples of functional groups having nucleophilic properties include hydroxyl groups, alkoxy groups, and amine groups. Combinations of condensed functional groups are preferably combinations of an alkoxy group or a hydroxyl group with a carboxy group or an ester group, more preferably a combination of a hydroxyl group with an ester group, and even more preferably a combination of a hydroxyl group with a methyl ester group.

[0022] Figure 2 is a schematic diagram of a PE polymer higher-order structure (10) of a microporous membrane grafted with two types of condensation-reactive heteroatom-containing functional groups (20) according to the present disclosure. In Figure 2, the polyethylene molecular chains present in the microporous membrane are grafted with functional groups having a carboxy group and functional groups having a methyl ester group. The functional groups having a carboxy group and the functional groups having a methyl ester group undergo a condensation reaction by eliminating methanol when heated as an external stimulus, forming a crosslinked structure (30) between the polyethylene molecular chains.

[0023] A heteroatom-containing functional group that forms a chelate complex structure via a metal ion (also referred to as a "chelate complex-forming functional group") is a functional group that can coordinate to a metal ion. The ability to form a chelate complex structure is preferable because it traps metal ions generated within the energy storage device and improves the cycle characteristics of the energy storage device.

[0024] Depending on the number of coordination sites of the metal ion, one chelate complex-forming functional group may be coordinated to one metal ion alone, or multiple chelate complex-forming functional groups may be coordinated to one metal ion. When multiple chelate complex-forming functional groups are coordinated to one metal ion, the multiple chelate complex-forming functional groups may be present in the same microporous membrane.

[0025] The metal ion may be a metal ion having one or more coordination sites (M n+ , n=1 to 3), for example, lithium ion (Li + ), cobalt ions (Co 2+ , Co 3+ ), nickel ions (Ni 2+ ), iron ions (Fe 3+ ), and manganese ions (Mn 2+ ) and the like. The metal ion is lithium ion (Li + ), the ionic resistance at the interface of the microporous membrane tends to be reduced, and the cycle characteristics and output characteristics of the energy storage device tend to be improved.

[0026] Examples of the chelate complex-forming functional group include functional groups that can provide an unshared electron pair that can coordinate to a metal ion, such as an amino group, an amine group, an ether group, a hydroxyl group, a carboxyl group, and a thiol group. Examples of the chelate complex-forming functional group include preferably a functional group having an ether group, more preferably an alkylene glycol group, and even more preferably a methoxyethylene glycol group.

[0027] FIG. 3 is a schematic diagram of a PE polymer higher-order structure (10) of a microporous membrane grafted with heteroatom-containing functional groups (20) that form a chelate complex structure (40) according to the present disclosure. In the upper diagram of FIG. 3, the polyethylene molecular chains present in the microporous membrane are grafted with functional groups (20) having methoxyethylene glycol groups. In FIG. 3, two ether oxygens of one methoxyethylene glycol group are bonded to a metal ion (M n+ ) is coordinated bidentately to the metal ion (M n+ ) in a bidentate coordination relationship. As shown in the lower diagram of Figure 3, the other methoxyethylene glycol groups may form a chelate complex structure (40) in the battery so as to form a crosslinked structure upon stimulation with an electrolyte and / or electrolytic solution. Although not shown, a crosslinked structure may also be formed between polyethylene molecular chains present inside or on the surface of the same microporous membrane.

[0028] Heteroatom-containing functional groups (also referred to as "bond-forming functional groups") that form any of coordinate bonds, ionic bonds, hydrogen bonds, and covalent bonds with compounds contained in other components in the electricity storage device that contact the surface of the microporous membrane are preferred because they can improve adhesion to the electrode and contribute to improving the stability of the electricity storage element. The bond is preferably an ionic bond formed by a carboxyl group. Other components in the electricity storage device that contact the surface of the microporous membrane include a positive electrode and a negative electrode.

[0029] Compounds contained in the positive electrode include a positive electrode active material, a binder, etc. Positive electrode active materials include substances capable of absorbing and releasing metal ions, carbon materials, metal oxides, metal sulfides, metal nitrides, etc. Metal oxides, such as lithium transition metal oxides, are exemplified.

[0030] Compounds contained in the negative electrode include a negative electrode active material, a binder, etc. Examples of the negative electrode active material include substances capable of absorbing and releasing metal ions, such as carbon materials, metal oxides, metal sulfides, and metal nitrides.

[0031] The compound to which the bond-forming functional group is bonded is preferably a compound contained in the positive electrode and / or negative electrode, and more preferably, a compound that is bonded to the positive electrode active material and / or negative electrode active material, such as a metal oxide (MO), via ionic bonding or coordination. x ) The bond-forming functional group is preferably a carboxyl group or the like.

[0032] FIG. 4 is a schematic diagram of a PE polymer higher-order structure (10) in a microporous membrane grafted with heteroatom-containing functional groups (20) that form bonds with materials in an electrode according to the present disclosure. In FIG. 4, the polyethylene molecular chains present in the microporous membrane are grafted with functional groups having methyl ester groups. The methyl ester groups are converted to carboxyl groups by a small amount of hydrofluoric acid (HF) in the electrolyte, and the carboxyl groups are then bonded to metal oxides (MO) in the electrode. x )(50) is ionically bonded to (30).

[0033] Figure 5 is a schematic diagram of a PE polymer high-order structure (10) of a microporous membrane grafted with a heteroatom-containing functional group (20) having a stable structure in one or more stable oxidation or reduction steps according to the present disclosure. A heteroatom-containing functional group having a stable structure in one or more stable oxidation or reduction steps means that the compound can exist stably without decomposition in a structure with a changed oxidation state in the electrochemical oxidation and reduction environment of an energy storage device. In other words, repeated electrochemical oxidation and reduction steps do not cause structural decomposition of the heteroatom-containing functional group, and reproducible, reversible oxidation and reduction waveforms can be observed in cyclic voltammetry (CV) measurements. "One or more oxidation / reduction steps" refers to the existence of one or more oxidation and reduction states. For example, the 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) group can exist in two oxidation and reduction states: a reduced state with a valence of -1 and an oxidation state with a valence of +1, making it a "two-step" oxidation / reduction step.

[0034] Preferred examples of heteroatom-containing functional groups having a stable structure in one or more stable oxidation or reduction steps include nitronyl nitroxide, 4-nitrophenyl nitronyl nitroxide, phenothiazine, dihydrophenazine, and catechol.

[0035] The amount of grafting is preferably about 10 mol% or less, more preferably 2 mol% or less, and even more preferably 1.8 mol% or less, relative to the total molar amount of polyethylene units. Separators with excessive grafting exceeding 10 mol% tend to significantly change the polymer high-order structure due to various chemical reactions occurring within the battery. The amount of grafting is preferably 0.01 mol% or more, more preferably 0.03 mol% or more, and even more preferably 0.1 mol% or more, relative to the total molar amount of polyethylene units. A grafting amount of 0.01 mol% or more is likely to have an effect on the polymer high-order structure. As will be described later, if the entire polymer high-order structure is difficult to unravel, swelling in the electrolyte solution is inhibited. The desired range varies depending on the graft type. Furthermore, when the separator is in the electrolyte solution and the compressive force due to the internal pressure of the battery is released, a high volume recovery rate in the compression direction is desirable. Therefore, it is more important to adjust the mobility of the entire polymer structure of the separator through grafting. Crystalline polymers have a high-order polymer structure composed of crystalline components, amorphous components, and intermediate components. In this high-order structure, the crystalline components are composed of multiple molecular chains, which serve as entanglement points. Furthermore, the amorphous components also serve as entanglement points, with molecular chains intertwining with each other. Such complex entangled polymer structures can swell upon immersion in small molecules (solvents, plasticizers, electrolytes, etc.). This phenomenon is governed by the thermodynamic behavior of the entire high-order structure as it expands due to the small molecules. For example, at high temperatures, the polymer high-order structure undergoes strong molecular motion, which facilitates disentanglement of the entire structure and promotes swelling. Meanwhile, this disclosure focuses on constructing a high-order polymer structure that is easily swollen by electrolytes, not only at high temperatures (the α2 crystal relaxation temperature range of polyethylene measured by viscoelasticity measurements: approximately 100 to 120°C), but also in the battery operating temperature range. When the polymer structure of the separator has a limited range of mobility, the electrolyte solution can easily penetrate the polymer structure, and a separator having a polymer high-order structure that exhibits a high volume recovery rate even when the separator is compressed can be obtained. In the present disclosure, the mobility is quantified using pulsed NMR.The inventors have demonstrated that a separator with a more appropriate mobility structure can exhibit good cycle performance by maintaining uniformity in the pore size of the separator as a whole and uniformity in the electrolyte even during long-term cycling, while also suppressing the formation of dendrites caused by uneven Li-ion conduction, thereby ensuring battery safety during long-term use.

[0036] It is known that the mobility of polyethylene within a separator can be quantified by observing the spin-spin relaxation time (T2 relaxation time) of all protons in the separator. Specifically, the mobility of polyethylene can be divided into three components: low mobility, high mobility, and intermediate mobility. Through extensive experiments, the present inventors have found that by adjusting the signal intensity ratio at a specific time after the start of observation to within a specific range based on the signal intensity at the start of observation using the solid echo method and the Carr-Purcell-Meiboom-Gill (CPMG) method (JEOL JNM-MU25A pulsed NMR instrument, DOS / V Windows version operation manual, pages 2-16-17), separators with high volume recovery rates when compressed in an electrolyte can be obtained. Generally, the physical properties of polyethylene molded products, such as mechanical strength and heat resistance, have been explained by the crystalline structure of polyethylene, which is composed of crystalline and amorphous parts. These studies involve applying external forces to polyethylene, causing significant deformation or destruction, or observing relaxation phenomena when heated over a temperature range significantly wider than that normally used. In other words, the performance of the crystalline structure at the actual conditions and temperatures in which the molded article will be used cannot be directly observed, and therefore accurate analysis is fundamentally impossible. While X-ray structural analysis can also be used to analyze polyethylene molded articles, while it can directly observe the crystalline structure, it cannot provide information on molecular mobility. In response to this, the inventors used pulsed NMR, which can directly observe the mobility of the entire polymer structure, to observe the volume recovery phenomenon due to swelling of the separator when compressed, which occurs due to the interaction between the crystalline structure of polyethylene and the electrolyte. This observation allows us to experimentally identify the appropriate polymeric higher-order structure of polyethylene.

[0037] In the case of the solid echo method, the ratio of signal intensity 0.2 msec after the start of observation is preferably 0.2% to 40%, more preferably 0.5% to 30%, and even more preferably 2.0% to 25%, based on the signal intensity at the start of observation. Furthermore, the ratio of signal intensity 0.8 msec after the start of observation is preferably 0.05% to 10%, more preferably 0.1% to 8.0%, and even more preferably 0.2 to 4.0%, based on the signal intensity at the start of observation. In the case of the CPMG method, the ratio of signal intensity 40 msec after the start of observation is preferably 5% to 30%, more preferably 8% to 28%, and even more preferably 10 to 25%, based on the signal intensity at the start of observation. Furthermore, the ratio of signal intensity 140 msec after the start of observation is preferably 1.5% to 20%, more preferably 2.0% to 18%, and even more preferably 2.5 to 15%, based on the signal intensity at the start of observation. By using such a method, it is possible to determine an appropriate modification amount from the viewpoint of improving battery cycle performance, regardless of the type of grafted chemical species to be modified. The separator of the present disclosure can cause various chemical reactions (crosslinking reaction, metal complex formation reaction, redox reaction) in a battery, and causes some changes in the overall mobility of the polymer higher-order structure observed by pulse NMR before and after the chemical reaction. However, from the viewpoint of effect expression, the above-mentioned ratio of signal intensity before and after the chemical reaction is satisfied.

[0038] <polyethylene> In the present disclosure, the microporous membrane contains polyethylene as a major component. In this specification, "contains polyethylene as a major component" means that the target resin is contained in an amount of 50% by mass or more, based on the total mass of the resin components constituting the microporous membrane. The polyethylene contained in the microporous membrane may be, for example, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, 99% by mass or more, or 100% by mass, based on the total mass of the resin components constituting the microporous membrane.

[0039] In the present disclosure, as long as the microporous membrane contains polyethylene as a main component, it may contain other resins, for example, polyolefin resins other than polyethylene. The polyolefin resin may be a polymer containing a repeating unit of a monomer having 3 to 10 carbon atoms. Examples of the monomer having 3 to 10 carbon atoms include propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. The polyolefin may be a homopolymer, copolymer, or multi-stage polymer, and is preferably a homopolymer. The polyolefin is preferably at least one selected from the group consisting of polypropylene and polybutadiene. The polyolefin resin may be used alone or in combination of two or more. From the viewpoint of improving the heat resistance of the microporous membrane, the polyolefin other than polyethylene may be a mixture of polyethylene and polypropylene.

[0040] The polyethylene is low-density polyethylene (density 0.925 g / cm 3 less than 0.925g / cm), linear low-density polyethylene, medium-density polyethylene (density 0.925g / cm 3 More than 0.942g / cm 3 less than 0.942 g / cm 3 More than 0.970g / cm 3 less than 0.970 g / cm3), and ultra-high molecular weight polyethylene (density 0.970 g / cm3) 3 The polyethylene is preferably an ultra-high molecular weight polyethylene, since it can provide a separator with a low melting point and high strength. The density of the polyethylene is measured according to "D) Density gradient tube method" described in JIS K7112 (1999).

[0041] To exhibit rapid fusing properties, the lower limit of the amount of polyethylene contained in the microporous membrane, based on the total mass of the resin components constituting the microporous membrane, may be preferably 50% by mass or more, for example, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, 99% by mass or more, or 100% by mass. The upper limit of the amount of polyethylene contained in the microporous membrane, based on the total mass of the resin components constituting the microporous membrane, may be preferably 99% by mass or less, for example, 90% by mass or less, 80% by mass or less, or 70% by mass or less.

[0042] Examples of polypropylene include isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene. Examples of copolymers of ethylene and propylene include ethylene-propylene random copolymers and ethylene-propylene rubber.

[0043] From the viewpoint of improving the heat resistance of the microporous membrane, the lower limit of the amount of polypropylene contained in the microporous membrane may be preferably 1% by mass or more, 2% by mass or more, 3% by mass or more, 4% by mass or more, or 5% by mass or more, based on the total mass of the resin components constituting the microporous membrane. The lower limit of the amount of polypropylene contained in the microporous membrane may be preferably 40% by mass or less, for example 35% by mass or less, 30% by mass or less, 25% by mass or less, 20% by mass or less, 15% by mass or less, or 10% by mass or less, based on the total mass of the resin components constituting the microporous membrane.

[0044] <Additives> The microporous membrane may contain any additive. Examples of additives include polymers other than polyolefins; inorganic fillers; phenolic, phosphorus-based, and sulfur-based antioxidants; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers; light stabilizers; antistatic agents; antifogging agents; and coloring pigments. From the viewpoint of shutdown performance and the like, the amount of these additives may be preferably 20 parts by mass or less, for example 10 parts by mass or less, and more preferably 5 parts by mass or less, based on 100 parts by mass of the resin component constituting the microporous membrane.

[0045] <Functional layer> In addition to the microporous membrane, the separator for an electricity storage device may have one or more functional layers on one or both sides of the microporous membrane. Examples of functional layers include a heat-resistant layer containing inorganic particles or a heat-resistant resin such as a crosslinkable polymer, and an adhesive layer containing an adhesive polymer. Methods for laminating the functional layer on the microporous membrane include coating the functional layer on the microporous membrane with a coater such as a gravure coater or a die coater, and laminating by co-extrusion.

[0046] <Microporous membrane thickness> The thickness of the microporous membrane is preferably 0.1 μm or more and 100 μm or less, more preferably 1 μm or more and 50 μm or less, and even more preferably 3 μm or more and 25 μm or less. From the viewpoint of mechanical strength, the thickness of the microporous membrane is preferably 0.1 μm or more, and from the viewpoint of increasing the capacity of the electricity storage device, the thickness of the microporous membrane is preferably 100 μm or less. The thickness of the microporous membrane can be adjusted by controlling the die lip gap, the stretch ratio in the stretching step, etc.

[0047] <Porosity of microporous membrane> The porosity of the microporous membrane is preferably 25% to 95%, more preferably 30% to 65%, and even more preferably 35% to 60%. From the viewpoint of improving ion conductivity, the porosity is preferably 25% or more, and from the viewpoint of voltage resistance characteristics, it is preferably 95% or less. The porosity of the microporous membrane can be adjusted by controlling the mixing ratio of the polyethylene resin composition and the plasticizer, the stretching temperature, the stretching ratio, the heat setting temperature, the stretching ratio during heat setting, and the relaxation rate during heat setting, or by combining these.

[0048] <Air permeability of microporous membrane> The air permeability of the microporous membrane is preferably 10 seconds / 100cc or more, more preferably 50 seconds / 100cc or more, and preferably 1000 seconds / 100cc or less, more preferably 500 seconds / 100cc or less, and even more preferably 300 seconds / 100cc or less. An air permeability of 10 seconds / 100cc or more is preferred from the viewpoint of suppressing self-discharge of the electricity storage device. An air permeability of 1000 seconds / 100cc or less is preferred from the viewpoint of obtaining good charge / discharge characteristics. The air permeability can be adjusted by changing the stretching temperature, stretching ratio, etc.

[0049] <<Method for producing a microporous membrane>> The method for producing a separator for an electricity storage device according to the present disclosure includes the following steps: (1) a step of extruding a resin composition containing polyethylene and a pore-forming material into a sheet using an extruder, followed by cooling and solidifying the extrusion to form a sheet-like molded product; (2) stretching the sheet-like molded body to form a stretched sheet; (3) extracting a pore-forming material from the stretched sheet to form a porous sheet; (4) heat-treating the porous sheet and stretching and relaxing it in the width direction; Includes: The method further includes the step of reacting the polyethylene with one or more graft molecules having a functional group for bonding to the molecular chain of the polyethylene and a heteroatom-containing functional group, between steps (1) and (2), between steps (2) and (3), between steps (3) and (4), or after step (4), thereby grafting the polyethylene with the heteroatom-containing functional group.

[0050] Grafting Step The method for producing a separator for an electricity storage device according to the present disclosure further includes a step of reacting polyethylene with one or more molecules having a functional group for bonding to a molecular chain of the polyethylene and a heteroatom-containing functional group, thereby grafting the polyethylene with the heteroatom-containing functional group. The grafting step can be carried out between steps (1) and (2), between steps (2) and (3), between steps (3) and (4), or after step (4).

[0051] The method for producing a separator for an electricity storage device according to the present disclosure modifies polyethylene in the grafting step, thereby enabling a higher grafting amount (modification amount) to be obtained compared to a method using pre-grafted modified polyethylene as a raw material. Furthermore, even modified polyethylene, which is generally difficult to produce, can be easily modified. Therefore, the production method is simple, low-cost, and has excellent production efficiency.

[0052] In the grafting step, polyethylene is reacted with one or more molecules (hereinafter also referred to as "graft molecules") having a functional group for bonding to the polyethylene molecular chain and a heteroatom-containing functional group. The heteroatom-containing functional group in the graft molecule can be the heteroatom-containing functional group described above in the section "Graft Structure."

[0053] The functional group to be bonded to the polyethylene molecular chain is preferably a functional group having a carbon-carbon unsaturated double bond, more preferably an α-olefin (terminal carbon-carbon unsaturated double bond). The functional group to be bonded to the polyethylene molecular chain is more preferably a vinyl group. Graft molecules having a vinyl group are preferred because they are inexpensive and easy to use.

[0054] The grafting step may include immersing the sheet-like molded article obtained in step (1), the stretched sheet obtained in step (2), the porous sheet obtained in step (3), or the heat-treated porous sheet obtained in step (4) in a solution containing a solvent, graft molecules, and alkylborane or an alkylborane complex; and providing oxygen in the solution to react the graft molecules with polyethylene and graft the polyethylene with a heteroatom-containing functional group.

[0055] The alkylborane or alkylborane complex is preferably a borane compound having a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and examples of the alkyl group having 1 to 10 carbon atoms include a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a heptyl group, an octyl group, a nonyl group, and a decyl group. The alkylborane or alkylborane complex is preferably a trialkylborane having an alkyl group having 1 to 3 carbon atoms, and more preferably triethylborane.

[0056] The solvent used in the grafting process is one that can dissolve the graft molecules and alkylborane or alkylborane complex. Suitable solvents include ether compounds, halogenated compounds, and heterocyclic compounds, with ether compounds including diethyl ether and methyl butyl ether. Examples of halogenated compounds include chloromethane, dichloromethane, and chloroform. Examples of heterocyclic compounds include THF, furan, pyran, pyrrole, thiophene, pyridine, and NMP.

[0057] One method for providing oxygen to a solution is, for example, bubbling gaseous oxygen into the solution. The oxygen concentration in the solution should be 2% to 100%, preferably 10% to 90%. Experiments have shown that air bubbling also exhibits good efficiency. The solution temperature should be -30 to 80°C, more preferably 0 to 70°C.

[0058] By providing oxygen to a solution containing the graft molecule and alkylborane or alkylborane complex, a reaction as shown in the following chemical reaction formula preferably proceeds, although the present disclosure is not limited to this reaction.

[0059] [ka]

[0060] In the above chemical reaction formula, first, triethylborane reacts with oxygen present in the solvent (Sol) to generate peroxyl radicals and ethyl radicals. Next, the ethyl radicals abstract hydrogen atoms from the polyethylene, and the resulting polyethylene radicals attack the α-olefin of the graft molecule to form a bond.

[0061] After the grafting, the method may further comprise extracting the graft molecules and the trialkylborane or trialkylborane complex from the sheet-like molded body, stretched sheet, or porous sheet, followed by drying.

[0062] The solvent used to extract the grafted molecules and the trialkylborane or trialkylborane complex is a dehydrated solvent such as THF, hexane, pentane, benzene, toluene, dichloromethane, 1,2-dichloroethane, ethyl acetate, acetonitrile, DMSO, or DMP.

[0063] <Sheet forming process> In the sheet molding process, a resin composition containing polyethylene and a pore-forming material is extruded into a sheet using an extruder, cooled, and solidified to form a sheet-like molded product. The polyethylene resins described above under "Polyethylene" can be used. Examples of the pore-forming material include plasticizers, inorganic materials, and combinations thereof.

[0064] As the plasticizer, it is preferable to use a non-volatile solvent capable of forming a homogeneous solution at or above the melting point of polyethylene. Examples of such non-volatile solvents include hydrocarbons such as liquid paraffin and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; and higher alcohols such as oleyl alcohol and stearyl alcohol. The plasticizer can be used alone or in combination of two or more. Liquid paraffin is preferred as the plasticizer. Liquid paraffin is highly compatible with polyethylene, and is therefore less likely to cause interfacial peeling between the resin and the plasticizer even when the sheet-shaped molded body is stretched, resulting in more uniform stretching.

[0065] The ratio of polyethylene to plasticizer is within a range that allows them to be uniformly melt-kneaded and molded into a sheet. For example, the amount of plasticizer in the resin composition is preferably 20 to 90% by mass, more preferably 30 to 80% by mass, based on the total mass of the resin composition. When the mass fraction of the plasticizer is 90% by mass or less, moldability is improved when the molten material is molded into a sheet. When the amount of plasticizer is 20% by mass or more, scission of polyethylene molecular chains is unlikely to occur even when the resin composition is stretched at a high ratio, making it easier to form a more uniform and fine pore structure and increasing strength.

[0066] Examples of inorganic materials include oxide ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride ceramics such as silicon nitride, titanium nitride, and boron nitride; ceramics such as silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fiber. The inorganic materials may be used alone or in combination. From the viewpoint of electrochemical stability, silica, alumina, and titania are preferred, and silica is more preferred because it is easily extracted from the sheet-shaped molded body.

[0067] The amount of inorganic material in the resin composition is preferably 5% by mass or more, more preferably 10% by mass or more, based on the total mass of the resin composition, from the viewpoint of obtaining good insulating properties, and is preferably 99% by mass or less, more preferably 95% by mass or less, from the viewpoint of ensuring high strength.

[0068] The resin composition may contain additives other than polyethylene and a pore-forming material. The additives described above in the section "Additives" can be used. From the viewpoint of shutdown performance, the amount of additive in the resin composition is such that the amount of additive in the microporous membrane is preferably 20 parts by mass or less, for example, 10 parts by mass or less, and more preferably 5 parts by mass or less, based on 100 parts by mass of the resin component constituting the microporous membrane.

[0069] Any extruder, such as a single-screw extruder or a twin-screw extruder, can be used as the extruder. The melt-kneaded material is extruded into a sheet using the extruder. Examples of methods for extruding into a sheet include extruding the melt-kneaded material into a sheet using a T-die or the like. The die lip spacing of the T-die is preferably 200 μm or more and 3,000 μm or less, for example, 500 μm or more and 2,500 μm or less. When the die lip spacing is 200 μm or more, the occurrence of smears and the like is reduced, and the impact on film quality, such as streaks or defects, is minimal. This reduces the risk of film breakage in the subsequent stretching process. On the other hand, when the die lip spacing is 3,000 μm or less, the cooling rate is fast, preventing uneven cooling and maintaining the thickness stability of the sheet.

[0070] The molten kneaded material extruded into a sheet is cooled and solidified to obtain a sheet-like molded product. Examples of cooling and solidification methods include contacting the sheet-like molten kneaded material with a thermal conductor and cooling it to a temperature sufficiently lower than the crystallization temperature of the resin component to solidify it. Examples of thermal conductors include metal, water, and air. Metal is preferred as the thermal conductor because of its high thermal conductivity, and from the viewpoint of production efficiency, it is more preferable to use a metal roll. When contacting the extruded sheet-like molten kneaded material with a metal roll, it is even more preferable to sandwich the extruded sheet-like molten kneaded material between at least one pair of rolls, as this further increases thermal conductivity, orients the sheet, increasing film strength, and tends to improve the surface smoothness of the sheet.

[0071] The sheet-shaped body may be optionally rolled. Rolling can be carried out, for example, by a pressing method using a double belt press or the like. By rolling the sheet-shaped body, the orientation of the surface layer portion in particular can be increased. The rolling area ratio is preferably more than 1 time and not more than 3 times, and more preferably more than 1 time and not more than 2 times. When the rolling ratio exceeds 1 time, the surface orientation increases, and the membrane strength of the finally obtained porous membrane tends to increase. On the other hand, when the rolling ratio is 3 times or less, the difference in orientation between the surface layer portion and the central interior is small, and a uniform porous structure tends to be formed in the thickness direction of the membrane.

[0072] <Stretching process> In the stretching step, the sheet-like formed body is stretched to form a stretched sheet. Stretching may be either uniaxial or biaxial. From the viewpoint of increasing the strength of the resulting microporous membrane, biaxial stretching is preferred. When the sheet-like formed body is stretched biaxially at a high ratio, the molecules are oriented in the plane direction, making the resulting microporous membrane less susceptible to tearing and resulting in a microporous membrane with high pin puncture strength. Examples of biaxial stretching methods include simultaneous biaxial stretching, sequential biaxial stretching, multistage stretching, and multiple stretching. From the viewpoints of improved pin puncture strength, stretching uniformity, and shutdown properties, simultaneous biaxial stretching is preferred. From the viewpoint of easy control of plane orientation, sequential biaxial stretching is preferred.

[0073] Simultaneous biaxial stretching refers to a stretching method in which stretching in MD (the machine direction of continuous microporous membrane molding) and stretching in TD (the direction crossing the MD of the microporous membrane at a 90° angle) are carried out simultaneously, and the stretching ratios in each direction may be different. Sequential biaxial stretching refers to a stretching method in which stretching in MD and TD is carried out independently, and while stretching is carried out in MD or TD, the other direction is unconstrained or fixed at a fixed length.

[0074] In the case of biaxial stretching, the areal magnification is preferably 20 to 250 times, more preferably 20 to 100 times, and even more preferably 25 to 70 times. An areal magnification of 20 times or more is preferred in terms of the strength of the microporous membrane. An areal magnification of 200 times or less reduces membrane breakage during the stretching step and is preferred in terms of productivity. The stretching magnifications in each direction are preferably 4 to 10 times in MD and 4 to 10 times in TD, more preferably 5 to 8 times in MD and 5 to 8 times in TD. The stretching magnification is the value obtained by dividing the dimensions of the microporous membrane after stretching by the dimensions of the microporous membrane before stretching. The areal magnification is the value obtained by multiplying the stretching magnification in MD by the stretching magnification in TD.

[0075] <Porous process> In the porosity-forming step, the pore-forming material is extracted from the stretched sheet to form a porous sheet. For example, the extraction method may involve immersing the sheet in an extraction solvent to extract the pore-forming material, followed by drying. The extraction method may be either batchwise or continuous. To prevent the sheet from shrinking, it is preferable to restrain the edges of the sheet during the immersion and drying steps. Furthermore, it is preferable to adjust the amount of pore-forming material remaining in the resulting porous sheet to less than 1% by mass, based on the total mass of the porous sheet.

[0076] The extraction solvent is preferably a poor solvent for polyethylene and a good solvent for the pore-forming material, and has a boiling point lower than the melting point of polyethylene. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; non-chlorine-based halogenated solvents such as hydrofluoroethers and hydrofluorocarbons; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; and ketones such as acetone and methyl ethyl ketone. These extraction solvents may be recovered and reused by operations such as distillation. When an inorganic material is used as the pore-forming material, an alkaline aqueous solution such as sodium hydroxide or potassium hydroxide can be used as the extraction solvent.

[0077] <Heat treatment process> In the heat treatment step, the porous sheet is heat-treated and stretched and relaxed in the width direction. The heat treatment step heat-sets the porous sheet, making it possible to suppress shrinkage of the sheet. The heat treatment includes a stretching operation for adjusting the physical properties and a relaxation operation for reducing the stretching stress. The relaxation operation may be performed after the stretching operation, or the stretching operation may be performed after the relaxation operation. The heat treatment can be performed using a tenter or a roll stretching machine.

[0078] The temperature of the heat treatment, including the stretching or relaxation operation, is preferably within the range of 100°C to 170°C, from the viewpoint of the crystal structure that easily modifies the grafted chemical species. The temperatures of the stretching and relaxation operation within the above range are preferred from the viewpoint of the balance between the reduction in the thermal shrinkage rate and the porosity. The lower limit of the heat treatment temperature is more preferably 110°C or higher, even more preferably 115°C or higher, and the upper limit is more preferably 160°C or lower, even more preferably 150°C or lower, and still more preferably 140°C or lower.

[0079] The stretching operation is an operation of stretching a microporous membrane in MD and / or TD. The stretching operation may be performed in both MD and TD, or in only one of MD and TD. The stretching ratio is the value obtained by dividing the dimension of the microporous membrane after the stretching operation by the dimension of the microporous membrane before the stretching operation. To obtain a microporous membrane with high strength and high porosity, the stretching ratio is preferably 1.1 times or more, more preferably 1.2 times or more, in each of MD and / or TD of the microporous membrane.

[0080] The relaxation operation is a shrinking operation of the microporous membrane in MD and / or TD. The relaxation operation may be performed in both MD and TD, or only in either MD or TD. The relaxation rate refers to the value obtained by dividing the dimension of the microporous membrane after the relaxation operation by the dimension of the microporous membrane before the relaxation operation. When both MD and TD are relaxed, the relaxation rate refers to the value obtained by multiplying the relaxation rate in MD by the relaxation rate in TD. The relaxation rate is preferably 1.0 or less, more preferably 0.97 or less, and even more preferably 0.95 or less.

[0081] The stretching and relaxation operations are preferably carried out in TD. The temperature in the stretching and relaxation operations is preferably lower than the melting point of polyethylene, more preferably in the range of 1°C to 25°C lower than the melting point of polyethylene. A temperature in the stretching and relaxation operations within the above range is preferred from the viewpoint of a balance between reduced thermal shrinkage and porosity of the microporous membrane.

[0082] Post-processing The microporous membrane may be optionally subjected to post-treatments such as hydrophilization treatment with a surfactant or crosslinking treatment with ionizing radiation.

[0083] <Energy storage device> The separator for an electricity storage device of the present disclosure can be used as a separator for an electricity storage device. The electricity storage device of the present disclosure includes a positive electrode, a negative electrode, and the separator for an electricity storage device of the present disclosure between the positive electrode and the negative electrode. Examples of the electricity storage device include a battery, a capacitor, and preferably a lithium-ion secondary battery.

[0084] The electricity storage device contains an electrolyte solution, and the positive electrode, negative electrode, and separator for the electricity storage device are impregnated with the electrolyte solution. The electrolyte solution may contain water, and the water contained in the system after battery fabrication may be water contained in the electrolyte solution or water carried over from components such as the electrodes or separator. The electrolyte solution may contain a non-aqueous solvent. Examples of solvents contained in the non-aqueous solvent of the present disclosure include alcohols such as methanol and ethanol; aprotic solvents, etc. Among these, aprotic solvents are preferred as the non-aqueous solvent.

[0085] Examples of aprotic solvents include cyclic carbonates, fluoroethylene carbonates, lactones, organic compounds having sulfur atoms, chain fluorinated carbonates, cyclic ethers, mononitriles, alkoxy group-substituted nitriles, dinitriles, cyclic nitriles, short-chain fatty acid esters, chain ethers, fluorinated ethers, ketones, and compounds in which some or all of the H atoms of the above aprotic solvents have been substituted with halogen atoms.

[0086] Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, trans-2,3-butylene carbonate, cis-2,3-butylene carbonate, 1,2-pentylene carbonate, trans-2,3-pentylene carbonate, cis-2,3-pentylene carbonate, vinylene carbonate, 4,5-dimethylvinylene carbonate, and vinylethylene carbonate.

[0087] Examples of fluoroethylene carbonate include 4-fluoro-1,3-dioxolan-2-one, 4,4-difluoro-1,3-dioxolan-2-one, cis-4,5-difluoro-1,3-dioxolan-2-one, trans-4,5-difluoro-1,3-dioxolan-2-one, 4,4,5-trifluoro-1,3-dioxolan-2-one, 4,4,5,5-tetrafluoro-1,3-dioxolan-2-one, and 4,4,5-trifluoro-5-methyl-1,3-dioxolan-2-one.

[0088] Examples of lactones include γ-butyrolactone, α-methyl-γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-valerolactone, δ-caprolactone, and ε-caprolactone.

[0089] Examples of organic compounds having a sulfur atom include ethylene sulfite, propylene sulfite, butylene sulfite, pentene sulfite, sulfolane, 3-sulfolene, 3-methyl sulfolane, 1,3-propane sultone, 1,4-butane sultone, 1-propene 1,3-sultone, dimethyl sulfoxide, tetramethylene sulfoxide, and ethylene glycol sulfite.

[0090] Examples of the chain carbonate include ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, dipropyl carbonate, methyl butyl carbonate, dibutyl carbonate, and ethyl propyl carbonate.

[0091] Examples of cyclic ethers include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, and 1,3-dioxane.

[0092] Examples of mononitriles include acetonitrile, propionitrile, butyronitrile, valeronitrile, benzonitrile, and acrylonitrile.

[0093] Examples of alkoxy-substituted nitriles include methoxyacetonitrile and 3-methoxypropionitrile.

[0094] Examples of dinitriles include malononitrile, succinonitrile, methylsuccinonitrile, glutaronitrile, 2-methylglutaronitrile, adiponitrile, 1,4-dicyanoheptane, 1,5-dicyanopentane, 1,6-dicyanohexane, 1,7-dicyanoheptane, 2,6-dicyanoheptane, 1,8-dicyanooctane, 2,7-dicyanooctane, 1,9-dicyanononane, 2,8-dicyanononane, 1,10-dicyanodecane, 1,6-dicyanodecane, 2,4-dimethylglutaronitrile, and ethylene glycol bis(propionitrile) ether.

[0095] Examples of cyclic nitriles include benzonitrile.

[0096] Examples of short-chain fatty acid esters include methyl acetate, methyl propionate, methyl isobutyrate, methyl butyrate, methyl isovalerate, methyl valerate, methyl pivalate, methyl hydroangelate, methyl caproate, ethyl acetate, ethyl propionate, ethyl isobutyrate, ethyl butyrate, ethyl isovalerate, ethyl valerate, ethyl pivalate, ethyl hydroangelate, ethyl caproate, propyl acetate, propyl propionate, propyl isobutyrate, propyl butyrate, propyl isovalerate, propyl valerate, propyl pivalate, propyl hydroangelate, propyl caproate, isopropyl acetate, isopropyl propionate, isopropyl isobutyrate, isopropyl butyrate, isopropyl isovalerate, isopropyl valerate, and isopropyl pivalate. , isopropyl hydroangelate, isopropyl caproate, butyl acetate, butyl propionate, butyl isobutyrate, butyl butyrate, butyl isovalerate, butyl valerate, butyl pivalate, butyl hydroangelate, butyl caproate, isobutyl acetate, isobutyl propionate, isobutyl isobutyrate, isobutyl butyrate, isobutyl isovalerate, isobutyl valerate, isobutyl pivalate, isobutyl hydroangelate, isobutyl caproate, tert-butyl acetate, tert-butyl propionate, tert-butyl isobutyrate, tert-butyl butyrate, tert-butyl isovalerate, tert-butyl valerate, tert-butyl pivalate, tert-butyl hydroangelate, and tert-butyl caproate.

[0097] Examples of the chain ethers include dimethoxyethane, diethyl ether, 1,3-dioxolane, diglyme, triglyme, and tetraglyme. Examples of the fluorinated ethers include those represented by the general formula Rf aa -OR bb (In the formula, Rf aa is an alkyl group containing a fluorine atom, and R bbis an organic group which may contain a fluorine atom). Examples of ketones include acetone, methyl ethyl ketone, and methyl isobutyl ketone.

[0098] Examples of the compounds in which some or all of the H atoms of the above aprotic solvents have been substituted with halogen atoms include compounds in which the halogen atoms are fluorine atoms.

[0099] Examples of fluorinated chain carbonates include methyl trifluoroethyl carbonate, trifluorodimethyl carbonate, trifluorodiethyl carbonate, trifluoroethylmethyl carbonate, methyl 2,2-difluoroethyl carbonate, methyl 2,2,2-trifluoroethyl carbonate, and methyl 2,2,3,3-tetrafluoropropyl carbonate. The fluorinated chain carbonates are represented by the following general formula: R cc -OC(O)OR dd {where, R cc and R dd is CH3, CH2CH3, CH2CH2CH3, CH(CH3)2, and the formula CH2Rf ee (In the formula, Rf ee is an alkyl group having 1 to 3 carbon atoms in which a hydrogen atom is substituted with at least one fluorine atom), and R cc and / or R dd contains at least one fluorine atom. It can be expressed as:

[0100] In addition, examples of fluorinated short-chain fatty acid esters include fluorinated short-chain fatty acid esters such as 2,2-difluoroethyl acetate, 2,2,2-trifluoroethyl acetate, and 2,2,3,3-tetrafluoropropyl acetate. Fluorinated short-chain fatty acid esters are represented by the following general formula: R ff -C(O)OR gg {where, Rff is CH3, CH2CH3, CH2CH2CH3, CH(CH3)2, CF3CF2H, CFH2, CF2H, CF2Rf hh , CFHRf hh , and CH2Rf ii and R is at least one selected from the group consisting of gg are CH3, CH2CH3, CH2CH2CH3, CH(CH3)2, and CH2Rf ii and Rf hh is an alkyl group having 1 to 3 carbon atoms in which a hydrogen atom may be substituted with at least one fluorine atom, and Rf ii is an alkyl group having 1 to 3 carbon atoms in which a hydrogen atom is substituted with at least one fluorine atom, and R ff and / or R gg contains at least one fluorine atom, and R ff If is CF2H, R gg can be expressed as {is not CH3}. [Example]

[0101] Hereinafter, embodiments of the present disclosure will be specifically described with reference to examples and comparative examples, but the present disclosure is not limited to these examples and comparative examples.

[0102] <<Measurement and Evaluation Methods>> <Microporous membrane thickness (μm)> A 10 cm × 10 cm square sample was cut from the microporous membrane, and nine points (3 points × 3 points) were selected in a grid pattern to measure the thickness of the microporous membrane using a microthickness gauge (Type KBM, Toyo Seiki Seisaku-sho, Ltd.) at room temperature of 23 ± 2° C. The average value of the measurements obtained at the nine points was calculated as the thickness (μm) of the microporous membrane.

[0103] <Mass conversion strength (gf / g)> A 10 cm x 10 cm square sample was cut from the microporous membrane, its weight (g) was measured, and the puncture strength (gf) at the center was measured. The mass-equivalent strength (gf / g) was calculated using the following formula. Mass conversion strength (gf / g)=Piercing strength (gf) / weight (g) The puncture strength was determined by conducting a puncture test on the sample film using a handy compression tester "KES-G5" (manufactured by Kato Tech, trademark) under conditions of a needle tip curvature radius of 0.5 mm and a puncture speed of 2 mm / sec.

[0104] <Porosity of microporous membrane (%)> A 10cm x 10cm square sample was cut from the microporous membrane, and its volume (cm 3 The density (g) and mass (g) of the microporous membrane were calculated using these values. 3 ) and the porosity was calculated using the following formula. Porosity (%) = (1 - mass / volume / 0.95) x 100

[0105] <Air permeability of microporous membrane (sec / 100cc)> The air permeability was measured in accordance with JIS P-8117 using a Gurley air permeability meter G-B2 (trademark, inner cylinder mass: 567g) manufactured by Toyo Seiki Seisakusho Co., Ltd. 2 The time (seconds) required for 100 cc of air to pass through the microporous membrane was measured as air permeability (seconds / 100 cc).

[0106] <Volume recovery rate (%) before and after compression in electrolyte> A 10 cm x 10 cm square sample was cut from the microporous membrane and impregnated with the nonaqueous electrolyte described in item c of "Preparation of Evaluation Batteries" below. The sample was compressed at 10 MPa for 3 minutes in a hot press heated to 60°C. The pressure was then released and the sample was allowed to stand for 10 minutes. The electrolyte was then washed off with ethanol and then acetone, and the sample was air-dried. The thickness of the microporous membrane obtained by the above procedure was measured according to the method described above in "Microporous Membrane Thickness (μm)." The average value measured at 25 random points within the sample was used. Volume recovery rate (%) before and after compression in electrolyte = 100 × (thickness after compression in electrolyte swollen state / thickness before compression in electrolyte swollen state)

[0107] <Pulse NMR> The spin-spin relaxation time (T2 relaxation time) of all protons in the microporous membrane was observed using a pulsed NMR measurement device (Minispec MQ20 ​​manufactured by Bruker Japan) under the following conditions. (1) Conditions for solid echography Nuclide: 1H Measurement: T2 Measurement temperature: 120°C (measured 5 minutes after reaching the set temperature) Accumulation count: 256 times Repeat time: 5.0 seconds The maximum observed intensity at the start of measurement (several microseconds after the pulse was turned off) was taken as the reference (100%), and the signal intensity ratios were measured at 0.2 msec and 0.8 msec from the start of measurement for each Example and Comparative Example. When the detected signal intensity was 2.0% or less, the average value of ±0.1 msec before and after was used. (2) Conditions of the CPMG Act Nuclide: 1H Measurement: T2 Measurement temperature: 180°C (measured 5 minutes after reaching the set temperature) Accumulation count: 256 times Pulse duration: 0.1 msec Number of echoes: 800 Repeat time: 5.0 seconds In the CPMG method, the signal intensity ratios at 40 msec and 140 msec from the start of measurement for each example and comparative example were observed in the same manner as in the solid echo method. Pulse NMR can be measured on separators before and after incorporation into a battery. Separators after incorporation into a battery may be in a state where the grafting chemical reaction has progressed. When measuring separators after incorporation into a battery, the separator removed from the battery is washed multiple times with ethanol and acetone to thoroughly remove the electrolyte and electrolyte, and then vacuum dried before measurement. In addition, in the case of separators that are composites of inorganic-containing coating layers, resin binder-containing coating layers, resin composition-containing coating layers, etc., the separator is washed with a solvent, and the PE-based microporous membrane is separated from the layer structure before measurement. Regarding pulse NMR measurements, Non-Patent Documents 4 to 6 can be referenced.

[0108] <Weight average molecular weight (Mw), number average molecular weight (Mn)> A calibration curve was created by measuring standard polystyrene under the following conditions using a Waters ALC / GPC 150C (trademark). Chromatograms were also measured for each of the following polymers under the same conditions, and the weight-average molecular weight and number-average molecular weight of each polymer were calculated based on the calibration curve using the following method. Column: Tosoh GMH6-HT (trademark) x 2 + GMH6-HTL (trademark) x 2 Mobile phase: o-dichlorobenzene Detector: Differential refractometer Flow rate: 1.0ml / min Column temperature: 140℃ Sample concentration: 0.1 wt% (Weight average molecular weight and number average molecular weight of polyethylene and polypropylene) Each molecular weight component in the obtained calibration curve was multiplied by 0.43 (polyethylene Q factor / polystyrene Q factor = 17.7 / 41.3) or 0.64 (polypropylene Q factor / polystyrene Q factor = 26.4 / 41.3) to obtain a molecular weight distribution curve converted into polyethylene or polypropylene, and the weight average molecular weight and number average molecular weight were calculated.

[0109] <Viscosity average molecular weight (Mv)> The intrinsic viscosity [η] at 135°C in decalin solvent was determined based on ASTM-D4020. The Mv of polyethylene was calculated using the following formula. [η]=6.77×10 -4 Mv 0.67 The Mv of polypropylene was calculated using the following formula. [η]=1.10×10 -4 Mv 0.80

[0110] <Preparation of Evaluation Battery> a. Preparation of the positive electrode A slurry was prepared by dispersing 92.2 mass% of lithium-cobalt composite oxide LiCoO2 as the positive electrode active material, 2.3 mass% each of flake graphite and acetylene black as conductive materials, and 3.2 mass% of polyvinylidene fluoride (PVDF) as a binder in N-methylpyrrolidone (NMP). This slurry was applied to one side of a 20 μm thick aluminum foil that served as a positive electrode current collector using a die coater, dried at 130 °C for 3 minutes, and then compression molded using a roll press. At this time, the amount of applied positive electrode active material was 250 g / m 2 , the bulk density of the active material is 3.00 g / cm 3 It was adjusted to be.

[0111] b. Preparation of negative electrode A slurry was prepared by dispersing 81.9% by mass of artificial graphite as the negative electrode active material, 15% by mass of silicon particles (Silgrain e-Si408) manufactured by Elkem, and 1.4% by mass of ammonium salt of carboxymethylcellulose and 1.7% by mass of styrene-butadiene copolymer latex as binders in purified water. This slurry was applied to one side of a 12 μm-thick copper foil that served as the negative electrode current collector using a die coater, dried at 120°C for 3 minutes, and then compression molded using a roll press. The amount of active material applied to the negative electrode was 106 g / m. 2 , the active material bulk density is 1.35 g / cm 3 It was adjusted to be.

[0112] c. Preparation of non-aqueous electrolyte The solution was prepared by dissolving LiPF6 as a solute in a mixed solvent of ethylene carbonate:ethyl methyl carbonate = 1:2 (volume ratio) to a concentration of 1.0 mol / L.

[0113] d. Battery assembly The separators obtained in the Examples and Comparative Examples were cut into 18 mm diameter circles, and the positive and negative electrodes were cut into 16 mm diameter circles. The positive and negative electrodes were stacked in a 28-electrode, separator, and negative electrode set with the active material surfaces facing each other, and then placed in a lidded stainless steel container. The container and lid were insulated, with the container in contact with the copper foil of the negative electrode and the lid in contact with the aluminum foil of the positive electrode. The nonaqueous electrolyte obtained in step c above was poured into the container and sealed. After leaving the battery at room temperature for one day, the battery was charged at a current of 3 mA (0.5 C) in a 25°C atmosphere up to a battery voltage of 4.2 V. After reaching 4.2 V, the current was reduced from 3 mA to maintain the battery voltage at 4.2 V. This method performed the initial charge after battery fabrication for a total of 6 hours. The battery was then discharged at a current of 3 mA (0.5 C) down to a battery voltage of 3.0 V.

[0114] <Battery characteristic retention rate (%) at 1000 cycles> The cycle characteristics were evaluated using the batteries assembled according to steps a to d above. The resulting batteries were charged and discharged 1000 times in a 40°C atmosphere. Charging was performed for a total of 3 hours by charging at a current of 6.0 mA (1.0 C) up to a battery voltage of 4.2 V, and then reducing the current from 6.0 mA to maintain 4.2 V. Discharging was performed at a current of 6.0 mA (1.0 C) down to a battery voltage of 3.0 V. The capacity retention rate was calculated from the discharge capacity at the 1000th cycle and the discharge capacity at the first cycle. A high capacity retention rate was evaluated as having good cycle characteristics.

[0115] <Pass rate (%) of nail penetration test after 1000 cycles> The battery that had undergone the 1000 charge / discharge cycles was placed on an iron plate in a temperature-controlled explosion-proof booth. The temperature in the booth was set to 40°C, and a 3.0 mm diameter iron nail was driven into the center of the battery at a speed of 2 mm / sec, and the nail was maintained in the penetrated state. After the nail penetrated, a thermocouple was installed inside the nail to measure the temperature inside the battery, and the presence or absence of ignition was evaluated. The evaluation was repeated using newly prepared batteries in the same manner, and the number of samples that did not ignite (no ignition) was calculated as a percentage using the following formula: 100 batteries were prepared using each separator for the evaluation. Evaluation result (%) = (100 x number of samples that did not ignite / total number of samples)

[0116] Example 1 [Film production example] (Sheet molding process) The raw polyethylene used was a homopolymer polyethylene (ultra-high molecular weight polyethylene) with a weight-average molecular weight of 1,900,000 and a viscosity-average molecular weight of 2,300,000. 1% by mass of pentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] as an antioxidant was added to the polyethylene, and the mixture was dry-blended using a tumbler blender to obtain a mixture. The resulting mixture was fed into a twin-screw extruder using a feeder under a nitrogen atmosphere. Liquid paraffin (kinematic viscosity at 37.78°C: 7.59 x 10) was also used. -5 m 2 A flow rate of 1000 ppm (1 / s) was injected into the extruder cylinder using a plunger pump. The mixture and liquid paraffin were melt-kneaded in the extruder, and the feeder and pump were adjusted so that the liquid paraffin content in the extruded polyethylene composition was 70% by mass (i.e., so that the polymer concentration was 26% by mass). The melt-kneading conditions were a set temperature of 230°C, a screw rotation speed of 240 rpm, and a discharge rate of 18 kg / h. The melt-kneaded product was then extruded and cast through a T-die onto a cooling roll whose surface temperature was controlled to 20°C, yielding a gel sheet (sheet-shaped molded product) with a film thickness of 1,750 μm.

[0117] (Stretching process) The gel sheet was then introduced into a simultaneous biaxial tenter stretching machine and biaxially stretched to obtain a stretched sheet. The stretching conditions were MD magnification of 7.0x, TD magnification of 7.14x (i.e., 7x7.14x=49.98x≒50x), and the biaxial stretching temperature was 115°C.

[0118] (Porous body formation process) Next, the stretched gel sheet was introduced into a dichloromethane tank and thoroughly immersed in dichloromethane to extract and remove the liquid paraffin, and then the dichloromethane was dried and removed to obtain a porous body (porous sheet).

[0119] (Heat treatment process) Next, the porous body was introduced into a TD tenter for heat setting (HS), where it was subjected to HS at a heat setting temperature of 129°C and a stretch ratio of 2.0 in the TD direction, followed by a relaxation operation of 0.9 in the TD direction (i.e., based on the state before the heat treatment step, it was stretched 2.0 times in the TD direction and then relaxed to 1.8 times).

[0120] (Grafting step) The membrane obtained in the heat treatment step was introduced into a THF solution bath containing trimethoxyvinylsilane (10 wt%) and triethylborane (1 mol / L), and the grafting reaction was carried out while bubbling air from the bottom of the bath. Subsequently, unreacted trimethoxyvinylsilane and the catalyst triethylborane were washed away with THF, and then the THF was dried and removed to obtain a heteroatom-containing functional group-grafted membrane. The resulting microporous membrane was then trimmed and wound into a mother roll measuring 1,100 mm wide and 5,000 m long.

[0121] Examples 2 to 14 A separator for an electricity storage device was obtained in the same manner as in Example 1, except that the conditions were changed as shown in Tables 1 and 2.

[0122] Comparative Examples 1 to 4 Electricity storage device separators were obtained in the same manner as in Example 1, except that the conditions were changed as shown in Table 3. In Comparative Examples 1 and 2, the oxygen concentration was "0%" and borane was "none," so that heteroatom-containing functional groups were not grafted onto the surface of the electricity storage device separator.

[0123] [Table 1]

[0124] [Table 2]

[0125] [Table 3]

[0126] Example 15 [Film production example] (Sheet molding process) The raw polyethylene used was a homopolymer polyethylene (ultra-high molecular weight polyethylene) with a weight-average molecular weight of 2,100,000 and a viscosity-average molecular weight of 2,370,000. 1% by mass of pentaerythrityl tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] as an antioxidant was added to the polyethylene, and the mixture was dry-blended using a tumbler blender to obtain a mixture. The resulting mixture was fed into a twin-screw extruder using a feeder under a nitrogen atmosphere. Liquid paraffin (kinematic viscosity at 37.78°C: 7.59 x 10) was also used. -5 m 2 / s) was injected into the extruder cylinder using a plunger pump. The mixture and liquid paraffin were melt-kneaded in the extruder, and the feeder and pump were adjusted so that the liquid paraffin content in the extruded polyethylene composition was 70% by mass (i.e., so that the polymer concentration was 25% by mass). The melt-kneading conditions were a set temperature of 230°C, a screw rotation speed of 240 rpm, and a discharge rate of 18 kg / h. The melt-kneaded product was then extruded and cast through a T-die onto a cooling roll whose surface temperature was controlled to 20°C, yielding a gel sheet (sheet-shaped molded product) with a film thickness of 1,800 μm.

[0127] (Stretching process) The gel sheet was then introduced into a simultaneous biaxial tenter stretching machine and biaxially stretched to obtain a stretched sheet under the following conditions: MD magnification 7.0x, TD magnification 6.9x (i.e., 7x6.9x=48.3x), and biaxial stretching temperature 115°C.

[0128] (Porous body formation process) Next, the stretched gel sheet was introduced into a dichloromethane tank and thoroughly immersed in dichloromethane to extract and remove the liquid paraffin, and then the dichloromethane was dried and removed to obtain a porous body (porous sheet).

[0129] (Heat treatment process) Next, the porous body was introduced into a TD tenter for heat setting (HS), where it was subjected to HS at a heat setting temperature of 129°C and a stretch ratio of 2.0 in the TD direction, followed by a relaxation operation of 0.9 in the TD direction (i.e., based on the state before the heat treatment step, it was stretched 2.0 times in the TD direction and then relaxed to 1.8 times).

[0130] (Grafting step) The membrane obtained in the heat treatment step was introduced into a THF solution bath containing trimethoxysilane (8 wt%) and triethylborane (1 mol / L), and the grafting reaction was carried out while bubbling air from the bottom of the bath. Subsequently, unreacted trimethoxyvinylsilane and the catalyst triethylborane were washed away with THF, and then the THF was dried and removed to obtain a heteroatom-containing functional group-grafted membrane. The resulting microporous membrane was then trimmed and wound into a mother roll measuring 1,100 mm wide and 5,000 m long.

[0131] Examples 16 to 36 A separator for an electricity storage device was obtained in the same manner as in Example 15, except that the conditions were changed as shown in Tables 4 to 6.

[0132] Comparative Examples 5 and 6 A separator for an electricity storage device was obtained in the same manner as in Example 15, except that the conditions were changed as shown in Table 6.

[0133] [Table 4]

[0134] [Table 5]

[0135] [Table 6] [Industrial Applicability]

[0136] The separator for an electricity storage device according to the present disclosure can be used, for example, as a separator for an electricity storage device, such as a battery or a capacitor, preferably a lithium-ion secondary battery. [Explanation of symbols]

[0137] 10. Higher-order structure of PE polymer in microporous membrane 20 Heteroatom-containing functional groups 30 Crosslinked structure 40 Chelate Complex Structure 50 electrodes

Claims

1. A method for producing a separator for an electricity storage device, the method comprising: (1) a step of extruding a resin composition containing polyethylene and a pore-forming material into a sheet using an extruder, and then cooling and solidifying the extruded resin composition to form a sheet-like molded product; (2) stretching the sheet-like molded body to form a stretched sheet; (3) extracting a pore-forming material from the stretched sheet to form a porous sheet; (4) heat-treating the porous sheet and stretching and relaxing it in the width direction; Including, the method further comprises the step of reacting the polyethylene with one or more graft molecules having a functional group for bonding to a molecular chain of the polyethylene and a heteroatom-containing functional group, between steps (1) and (2), between steps (2) and (3), between steps (3) and (4), or after step (4), thereby grafting the polyethylene with the heteroatom-containing functional group; The grafting step comprises the following steps: immersing the sheet-like molded body, the stretched sheet, or the porous sheet in a solution containing a solvent, the graft molecule, and an alkylborane or an alkylborane complex; providing oxygen in the solution to react the graft molecules with the polyethylene to graft the polyethylene with the heteroatom-containing functional group; Including, The separator for an electricity storage device produced by the method has, in observation of the spin-spin relaxation time (T2 relaxation time) of all protons in pulsed NMR measurement using a solid echo method at 120°C, a signal intensity ratio at 0.2 msec after the start of observation of 0.2% to 40% of the signal intensity at the start of observation, and a signal intensity ratio at 0.8 msec after the start of observation of 0.05% to 10% of the signal intensity at the start of observation.

2. A method for manufacturing a separator for an electricity storage device, the method comprising: (1) a step of extruding a resin composition containing polyethylene and a pore-forming material into a sheet using an extruder, and then cooling and solidifying the extruded resin composition to form a sheet-like molded product; (2) stretching the sheet-like molded body to form a stretched sheet; (3) extracting a pore-forming material from the stretched sheet to form a porous sheet; (4) heat-treating the porous sheet and stretching and relaxing it in the width direction; Including, the method further comprises the step of reacting the polyethylene with one or more graft molecules having a functional group for bonding to a molecular chain of the polyethylene and a heteroatom-containing functional group, between steps (1) and (2), between steps (2) and (3), between steps (3) and (4), or after step (4), thereby grafting the polyethylene with the heteroatom-containing functional group; The grafting step comprises the following steps: immersing the sheet-like molded body, the stretched sheet, or the porous sheet in a solution containing a solvent, the graft molecule, and an alkylborane or an alkylborane complex; providing oxygen in the solution to react the graft molecules with the polyethylene to graft the polyethylene with the heteroatom-containing functional group; Including, The separator for an electricity storage device produced by the method has, in pulsed NMR measurement of the spin-spin relaxation time (T2 relaxation time) of all protons using the CPMG method at 180°C, a signal intensity ratio 40 msec after the start of observation that is 5% to 30% of the signal intensity at the start of observation, and a signal intensity ratio 140 msec after the start of observation that is 1.5% to 20% of the signal intensity at the start of observation.

3. 3. The method of claim 1, wherein the functional group for bonding to the polyethylene molecular chain comprises a vinyl group.

4. The method according to any one of claims 1 to 3, further comprising extracting the graft molecules and the alkylborane or alkylborane complex from the sheet-like molded body, the stretched sheet, or the porous sheet after the grafting, and drying the graft molecules or alkylborane complex.

5. The method according to any one of claims 1 to 4, wherein the grafting step is carried out after step (4).

6. The method according to any one of claims 1 to 4, wherein the grafting step is carried out between steps (1) and (2).

7. The method according to any one of claims 1 to 4, wherein the grafting step is carried out between steps (2) and (3).

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