Cross-linked resin dispersion separator

Abstract

To provide a separator for a power storage device, which allows high safety to be ensured (an internal maximum heat generation velocity assessed by a nail pricking test to be held down, and a voltage reduction time to be ensured) by achieving both of a shutdown function and a high-temperature film rupture property, and others.SOLUTION: A separator for a power storage device comprises a silane graft-modified polypropylene (A), and a polyethylene (B), of which the mass ratio {(A) / (B)} is 2 / 98 to 80 / 20.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a separator (separator for an electricity storage device) in which silane-grafted modified polypropylene as a crosslinked resin is suitably dispersed in a specific polyethylene. [Background technology]

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

[0003] To ensure battery safety, separators are required to both activate a shutdown function and increase the membrane rupture temperature. For example, Patent Document 1 describes adjusting the high-order physical properties of polyolefin resins contained in lithium-ion battery separators. Furthermore, Patent Document 2 describes adjusting the crystallinity and gel fraction range to suppress heat generation due to short circuits inside the battery using the shutdown function and ensure performance that prevents membrane rupture even if high-temperature areas occur locally within the battery cell (breakdown at 170°C or higher). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 9-216964 [Patent Document 2] International Publication No. 97 / 44839 [Patent Document 3] Japanese Patent Application Publication No. 11-144700 [Patent Document 4] Japanese Patent Application Publication No. 11-172036 [Patent Document 5] Japanese Patent Application Laid-Open No. 2001-176484 [Patent Document 6] Japanese Patent Application Laid-Open No. 2000-319441 [Patent Document 7] Japanese Patent Application Laid-Open No. 2017-203145 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, the output and energy density of lithium-ion batteries for use in mobile devices or automobiles have been increasing. Therefore, separators of high quality (e.g., separators with less resin aggregates) are desired. Safety standards for power storage devices have also become stricter than before, and even greater shutdown functionality and high-temperature membrane rupture resistance are required than those envisioned in Patent Documents 1 to 7.

[0006] In Patent Document 3, if a masterbatch catalyst is used during the extrusion process, the crosslinking reaction of the silane-modified polyethylene may proceed in the extruder, potentially generating resin aggregates in the separator. In addition, in Patent Document 4, the silane crosslinking reaction may not proceed sufficiently, making it difficult to achieve high-temperature membrane rupture resistance. Furthermore, the heat-resistant resin microporous film described in Patent Document 7 is obtained by simply applying a photopolymerizable coating liquid to a film made porous by a dry method. In Example 5 of Patent Document 7, a low-molecular-weight silane coupling agent such as γ-methacryloxypropyltrimethoxysilane is added to the porous film. Meanwhile, since low-molecular-weight silane coupling agents tend to react or bond with plasticizers used in wet methods for making the film porous, it is expected that if a low-molecular-weight silane coupling agent is used in a wet method, bonding between the low-molecular-weight silane coupling agent and the resin of the porous film will be difficult.

[0007] Furthermore, batteries using the separators described in Patent Documents 3 to 7 have poor cycle characteristics, and there is concern that unpredictable side reactions may be induced in the battery during long-term use, resulting in a decrease in battery safety.

[0008] In view of the above problems, the present invention aims to provide a separator for an electricity storage device that can ensure high safety of the electricity storage device (suppression of the maximum internal heat generation rate and ensuring voltage drop time as evaluated by a nail penetration test) by achieving both a shutdown function and high-temperature membrane rupture resistance. The present invention also aims to provide a lithium-ion secondary battery and electricity storage device that include such a separator for an electricity storage device, as well as a method for manufacturing such a separator for an electricity storage device. [Means for solving the problem]

[0009] As a result of extensive research, the present inventors have found that the above-mentioned problems can be solved by using polyethylene and silane-grafted polypropylene as a cross-linked resin in a predetermined ratio, and have thus completed the present invention. [1] A separator for an electricity storage device, wherein the mass ratio {(A) / (B)} of silane-grafted modified polypropylene (A) to polyethylene (B) is 2 / 98 to 80 / 20. [2] The separator for an electricity storage device according to [1], which has a film rupture temperature of 170 to 210°C as measured by thermomechanical analysis (TMA). [3] The following steps: (1) a sheet molding step of extruding silane-grafted modified polypropylene (A), ultra-high molecular weight polyethylene (B), and a plasticizer, cooling and solidifying the extruded polypropylene, and molding a sheet having a mass ratio of (A) to (B) ((A) / (B)) of 1 / 99 to 80 / 20; (2) a stretching step of stretching the sheet to obtain a stretched product; (3) a porous body forming step of extracting a plasticizer from the stretched material to form a porous body; and (4) a heat treatment step of subjecting the porous body to a heat treatment to obtain a heat-treated porous body; A method for producing a separator for an electricity storage device, comprising: [4] The following steps: (5) an affinity treatment step of immersing the heat-treated porous body in an organic solvent that is amphiphilic to water and organic substances to obtain an affinity-treated porous body; (6) a crosslinking step in which the affinity-treated porous body is brought into contact with a mixture of an organometallic catalyst and water or immersed in a basic solution or an acid solution to form a crosslinked porous body having a silane crosslinked structure; and (7) a water-washing and drying step of washing the crosslinked porous body with water and drying it; The method for producing a separator for an electricity storage device according to [3], further comprising: [5] The following steps: (2-A) a preparation step of preparing an exterior body housing a laminate or a wound body of an electrode and the separator for an electricity storage device according to [1] or [2], and a nonaqueous electrolyte; (2-A) a liquid injection step of pouring the nonaqueous electrolyte into the exterior body; A method for manufacturing an electricity storage device comprising: [6] [5] The method for manufacturing an electricity storage device according to [5], wherein the electricity storage device separator is brought into contact with the nonaqueous electrolyte during or after the liquid injection step, thereby initiating a silane crosslinking reaction of the silane-grafted modified polyolefin contained in the electricity storage device separator. [7] The method for producing an electricity storage device according to [5] or [6], wherein the non-aqueous electrolyte solution contains a fluorine-containing lithium salt. [8] The method for producing an electricity storage device according to any one of [5] to [7], wherein the non-aqueous electrolyte solution is an acid solution or a base solution. [9] The following steps: (2-c) a terminal connecting step of connecting a lead terminal to the electrode in the outer casing or the electrode exposed from the outer casing; (2-D) a charge / discharge step of performing at least one cycle of charge / discharge; The method for producing an electricity storage device according to any one of [5] to [8], further comprising:

[10] A lithium ion secondary battery comprising a positive electrode, a negative electrode, the separator for an electricity storage device according to [1] or [2], and a non-aqueous electrolyte solution.

[11] An electricity storage device comprising: a positive electrode; a negative electrode; the separator for an electricity storage device according to [1] or [2]; and a non-aqueous electrolyte solution. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a separator for an electricity storage device that achieves both a shutdown function and high-temperature membrane rupture resistance, thereby ensuring high safety of the electricity storage device (suppressing the maximum internal heat generation rate and ensuring voltage drop time as evaluated by a nail penetration test). Furthermore, according to the present invention, it is possible to provide a lithium ion secondary battery and an electricity storage device that include such a separator for an electricity storage device, and a method for manufacturing such a separator for an electricity storage device. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described, but the present invention is not limited to only this embodiment. The present invention can be modified in various ways without departing from the gist of the present invention. In this specification, unless otherwise specified, the symbol "to" means that the numerical values ​​at both ends thereof are included as the upper and lower limits. Furthermore, in this specification, the upper and lower limits of a numerical range can be combined in any way.

[0012] <Electricity storage device> The separator for an electricity storage device according to this embodiment (hereinafter, sometimes simply referred to as "separator") is used in an electricity storage device. The electricity storage device includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. Examples of this type of electricity storage device include lithium batteries, lithium secondary batteries, lithium ion secondary batteries, sodium secondary batteries, sodium ion secondary batteries, magnesium secondary batteries, magnesium ion secondary batteries, calcium secondary batteries, calcium ion secondary batteries, aluminum secondary batteries, aluminum ion secondary batteries, nickel-metal hydride batteries, nickel-cadmium batteries, electric double layer capacitors, lithium ion capacitors, redox flow batteries, lithium-sulfur batteries, lithium-air batteries, and zinc-air batteries. Among these, from the viewpoint of practicality, lithium batteries, lithium secondary batteries, lithium ion secondary batteries, nickel-metal hydride batteries, and lithium ion capacitors are preferred, and lithium batteries and lithium ion secondary batteries are more preferred.

[0013] <Lithium-ion secondary battery> A lithium ion secondary battery is a storage battery that uses, for example, a lithium transition metal oxide such as lithium cobalt oxide or lithium cobalt composite oxide as a positive electrode, a carbon material such as graphite or black lead as a negative electrode, and a non-aqueous electrolyte. Examples of non-aqueous solvents constituting the non-aqueous electrolyte include ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and mixed solvents thereof. Examples of electrolytes constituting the non-aqueous electrolyte include fluorine-containing lithium salts (LiPF, LiBF, LiN(SOCF), LiSOCF, etc.) and LiBCO. A separator is disposed between the electrodes to prevent contact between the electrodes while allowing lithium (Li) to move between the electrodes.

[0014] <separator> (Polyolefin microporous membrane) The separator according to this embodiment includes the polyolefin microporous membrane according to this embodiment (hereinafter, may be simply referred to as "microporous membrane").

[0015] The microporous membrane contains (A) silane-grafted polypropylene and (B) polyethylene. The mass ratio of the silane-grafted polypropylene to the polyethylene {(A) / (B)} is 2 / 98 to 80 / 20. In other words, the mass ratio of the silane-grafted polypropylene is 2 to 80 mass%, and the mass ratio of the polyethylene is 20 to 98 mass%, based on 100% of the total mass of the silane-grafted polypropylene and the polyethylene. This allows for a high-quality separator (e.g., with less resin aggregates in the separator). Furthermore, the inclusion of such a separator ensures high safety of the electricity storage device (reduced maximum internal heat generation rate and long voltage drop time, as evaluated by a nail penetration test).

[0016] In this regard, the microporous membrane can exhibit high-temperature membrane rupture resistance by constructing a silane crosslinked structure (gelation structure) using silane-grafted polypropylene. This is presumably because the polypropylene dispersed in the polyethylene is favorably linked by the silane crosslinked structure, changing the morphology of the entire membrane and enabling it to maintain its membrane shape even at temperatures exceeding the melting point of polyethylene (e.g., about 130°C to 140°C) and near or even exceeding the melting point of polypropylene (e.g., about 170°C). Therefore, to ensure high-temperature membrane rupture resistance, the mass ratio of the silane-grafted polypropylene is preferably 2.5% by mass or more, more preferably 3% by mass or more, based on 100% by mass of the combined mass of the silane-grafted polypropylene and polyethylene. In other words, the mass ratio of polyethylene is preferably 97.5% by mass or less, more preferably 97% by mass or less.

[0017] On the other hand, by including polyethylene in the microporous membrane, heat setting at high temperatures can be performed while suppressing pore clogging of the resulting microporous membrane. Therefore, from the viewpoint of forming a dense and uniform porous structure, the mass ratio of polyethylene is preferably 20.5% by mass or more, more preferably 21% by mass or more, based on 100% by total mass of silane-grafted modified polypropylene and polyethylene. In other words, the mass ratio of silane-grafted modified polypropylene is preferably 98.5% by mass or less, more preferably 98% by mass or less.

[0018] The weight average molecular weight of the entire microporous membrane is preferably 100,000 or more and 1,200,000 or less, more preferably 150,000 or more and 800,000 or less.

[0019] Based on 100% by mass of the microporous membrane in total, the total mass of the silane-grafted modified polypropylene and polyethylene is preferably 50% by mass or more, more preferably 65% ​​by mass or more, even more preferably 80% by mass or more, and particularly preferably 95% by mass or more, from the viewpoint of reliably exerting the effects of the present invention.

[0020] The microporous membrane may contain known additives such as organometallic catalysts (dehydration condensation catalysts); plasticizers; metal soaps such as calcium stearate or zinc stearate; ultraviolet absorbers; light stabilizers; antistatic agents; antifogging agents; and color pigments.

[0021] ((A) Silane-grafted modified polypropylene) Silane-grafted modified polypropylene has a polypropylene main chain with alkoxysilyl groups grafted onto the main chain. The alkoxysilyl groups are presumably converted to silanol groups through a hydrolysis reaction with water, which then undergoes a crosslinking reaction to form siloxane bonds (see the formula below; the rate at which the TO structure changes to the T1 structure, T2 structure, or T3 structure is arbitrary). The alkoxide substituted on the alkoxysilyl group is not particularly limited, but examples include methoxide, ethoxide, and butoxide. For example, in the formula below, R can be methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, or tert-butyl. [ka]

[0022] In silane-graft-modified polypropylene, the main chain and the graft are connected by a covalent bond. The structure forming such a covalent bond is not particularly limited, but examples thereof include alkyl, ether, glycol, and ester. Before the crosslinking reaction, the silane-graft-modified polypropylene preferably has a silicon-to-carbon ratio (Si / C) of 0.2 to 1.8%, more preferably 0.5 to 1.7%.

[0023] The preferred silane-grafted modified polypropylene has a density of 0.82 to 0.96 g / cm 3 The melt flow rate (MFR) at 230°C is 0.5 to 25 g / min.

[0024] The polypropylene constituting the silane-grafted modified polypropylene may be composed of one type of propylene alone or two or more types of propylene. Two or more types of silane-grafted modified polypropylenes composed of different propylenes may be used in combination.

[0025] The polypropylene constituting the silane-grafted modified polypropylene is preferably a propylene homopolymer. Considering the manufacturing process of the separator according to this embodiment, the silane-grafted modified polypropylene has a modification amount of silanol-containing units of 10% or less, preferably 5% or less, and more preferably 2% or less, based on the total ethylene units in the main chain, prior to the crosslinking treatment step described below. The preferred silane-grafted modified polypropylene has a density of 0.90 to 0.96 g / cm 3 and the melt flow rate (MFR) at 190°C is 0.2 to 5 g / min.

[0026] The crosslinking reaction is accelerated by the use of an organometallic catalyst. In this specification, a resin containing silane-grafted modified polypropylene to which an organometallic catalyst has been added before the sheet forming process (for example, in the stage of a kneading process that is performed as needed) is referred to as a masterbatch resin.

[0027] ((B) Polyethylene) As the polyethylene, one type of polyethylene may be used alone, or two or more types of polyethylene may be used in combination. The polyethylene is preferably an ethylene homopolymer. However, the polyethylene may contain a monomer other than ethylene (another monomer) in an amount of 50% by mass or less or 45% by mass or less of the total mass of ethylene constituting the polyethylene. Examples of the other monomer include propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, and norbornene.

[0028] (any layer) The microporous membrane may be used as a separator by itself, or may have a functional layer on at least one side thereof. Examples of the functional layer include, but are not limited to, an inorganic porous layer containing inorganic particles and a binder, and an adhesive layer containing a thermoplastic polymer.

[0029] <Thermomechanical analysis (TMA)> From the viewpoint of reliably ensuring the high-temperature membrane rupture resistance of the separator and reliably ensuring the safety of the electricity storage device, the separator preferably has a membrane rupture temperature measured by TMA of 170 to 210° C. As described above, in this embodiment, the silane-crosslinked polypropylene is suitably dispersed in the polyethylene, and as a result, the morphology of the entire membrane changes, and the membrane shape can be maintained up to temperatures near or exceeding the melting point of polyethylene (170 to 210° C.).

[0030] Generally, when an unexpected runaway reaction causes heat generation in an electricity storage device, the separator fuses at a low temperature (e.g., 150°C or below), quickly stopping the movement of Li ions and the associated discharge inside or outside the electricity storage device. The electricity storage device is then cooled by the ambient air or a refrigerant, cooling the entire device. This is expected to prevent ignition of the nonaqueous electrolyte or an exothermic decomposition reaction of the electrolyte, ensuring safety. However, even if the runaway reaction occurring in the electricity storage device is not stopped by the fuse and heat generation continues, the separator according to this embodiment can suppress melting and film rupture to a high temperature of 170 to 210°C, thereby reliably ensuring the safety of the electricity storage device. The membrane rupture temperature measured by TMA can be obtained by the method described in the Examples, and can be controlled by changing the composition of the microporous membrane, etc.

[0031] <Separator characteristics> The following describes the properties of the microporous membrane. When the microporous membrane itself is used as a separator, the properties of the microporous membrane are interpreted as the properties of the separator.

[0032] The porosity of the microporous membrane is preferably 20% or more, more preferably 30% or more, and even more preferably 32% or more or 35% or more. A porosity of 20% or more tends to further improve the ability to follow the rapid movement of lithium (Li) ions. On the other hand, the porosity is preferably 90% or less, more preferably 80% or less, and even more preferably 50% or less. A porosity of 90% or less tends to further improve membrane strength and further suppress self-discharge. This porosity can be measured by the method described in the Examples and can be controlled by changing the stretch ratio of the microporous membrane, for example.

[0033] The air permeability of the microporous membrane is preferably 1 sec / 100 cm 3 More than 50sec / 100cm, preferably 3 More preferably, 55 sec / 100 cm 3 More preferably, 70 sec / 100 cm 3 Above, 90sec / 100cm 3 The air permeability is 1 sec / 100 cm 3 By setting the thickness to 400 sec / 100 cm or more, the balance between the membrane thickness, porosity, and average pore size tends to be further improved. 3 Less than 300sec / 100cm, preferably less than 300sec / 100cm 3 Less than 270 sec / 100 cm, more preferably 3 The air permeability is 400 sec / 100 cm or less. 3 The air permeability can be measured by the method described in the Examples and can be controlled by changing the stretching temperature and / or stretch ratio of the microporous membrane.

[0034] The membrane thickness of the microporous membrane is preferably 1.0 μm or more, more preferably 2.0 μm or more, even more preferably 3.0 μm or more, 4.0 μm or more, or 4.5 μm or more. A membrane thickness of 1.0 μm or more tends to further improve membrane strength. On the other hand, the membrane thickness is preferably 500 μm or less, more preferably 100 μm or less, even more preferably 80 μm or less, 22 μm or less, or 19 μm or less. A membrane thickness of 500 μm or less tends to further improve ion permeability. This membrane thickness can be measured by the method described in the Examples and can be controlled by changing the stretch ratio of the microporous membrane, for example.

[0035] When a microporous membrane is used as a separator for a lithium ion secondary battery or as a constituent material thereof, the membrane thickness is preferably 25 μm or less, more preferably 22 μm or less or 20 μm or less, even more preferably 18 μm or less, and particularly preferably 16 μm or less. A membrane thickness of 25 μm or less tends to further improve permeability. In this case, the lower limit of the membrane thickness may be 1.0 μm or more, 3.0 μm or more, 3.5 μm or more, or 4.0 μm or more.

[0036] <Separator manufacturing method> The separator manufacturing method includes the following steps: (1) Sheet forming process; (2) Stretching process; (3) porous body forming step; and (4) Heat treatment process; The method for producing a separator may, if desired, include a kneading step before the sheet-forming step (1) and / or a winding step after the heat treatment step (4).

[0037] (Kneading process) The kneading step is a step of kneading a polyolefin composition containing silane-grafted polypropylene and ultra-high molecular weight polyethylene (UHMWPE) to obtain a kneaded product. The polyolefin composition may contain known additives such as an organometallic catalyst, a plasticizer, metal soaps such as calcium stearate or zinc stearate, an ultraviolet absorber, a light stabilizer, an antistatic agent, an anti-fogging agent, and a coloring pigment.

[0038] ((1) Sheet molding process (extrusion process)) The sheet forming step is a step in which the obtained mixture of the kneaded material and the plasticizer (i.e., a mixture of the silane-grafted modified polypropylene, UHMWPE, and the plasticizer) is extruded, cooled and solidified, and molded into a sheet. A known extruder is used to extrude the mixture. The molding method is not particularly limited, but examples include a method in which the melt-kneaded and extruded molten material is solidified by compression cooling. Examples of cooling methods include a method in which the material is directly contacted with a cooling medium such as cold air or cooling water, and a method in which the material is contacted with a roll or a press cooled with a refrigerant. Among these, the method in which the material is contacted with a roll or a press cooled with a refrigerant is preferred because it provides excellent film thickness controllability.

[0039] The mass ratio {(A) / (B)} of (A) silane-grafted polypropylene and (B) UHMWPE in the sheet is 2 / 98 to 80 / 20. This allows for a high-quality separator (e.g., with less resin aggregates in the separator), and the inclusion of such a separator ensures high safety of the electricity storage device (reduced internal maximum heat generation rate and ensured voltage drop time, as evaluated by a nail penetration test). Here, UHMWPE refers to polyethylene having a weight-average molecular weight of 100,000 or more. Two or more types of UHMWPE having different weight-average molecular weights may be used in combination. The UHMWPE is preferably an ethylene homopolymer. However, it may contain monomers other than ethylene (other monomers) as long as the amount of the other monomers is 50% by mass or less or 45% by mass or less of the total mass of ethylene constituting the UHMWPE. Examples of the other monomers include propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, and norbornene.

[0040] From the viewpoint of ensuring low-temperature shutdown properties at 150°C or less, and from the viewpoint of improving safety by suppressing thermal runaway when the electricity storage device is destroyed while having resistance to film rupture at 170 to 210°C, it is preferable that the silane-grafted modified polypropylene is not a masterbatch resin in the sheet molding process.

[0041] The plasticizer is not particularly limited, but examples thereof include organic compounds that can form a homogeneous solution with polyolefin at temperatures below the boiling point. More specific examples include decalin, xylene, dioctyl phthalate, dibutyl phthalate, stearyl alcohol, oleyl alcohol, decyl alcohol, nonyl alcohol, diphenyl ether, n-decane, n-dodecane, and paraffin oil. Among these, paraffin oil and dioctyl phthalate are preferred. One type of plasticizer may be used alone, or two or more types may be used in combination. The proportion of the plasticizer is not particularly limited, but is preferably 20% by mass or more relative to 100% by mass of the combined total of silane-grafted modified polypropylene and UHMWPE from the perspective of the porosity of the resulting microporous membrane, and is preferably 90% by mass or less from the perspective of viscosity during melt-kneading.

[0042] ((2) Stretching process) The stretching step is a step of stretching the sheet obtained in the sheet forming step to obtain a stretched product. Examples of sheet stretching methods include MD uniaxial stretching using a roll stretching machine, TD uniaxial stretching using a tenter, sequential biaxial stretching using a combination of a roll stretching machine and a tenter, or a combination of a tenter and a tenter, and simultaneous biaxial stretching using a simultaneous biaxial tenter or inflation molding. Simultaneous biaxial stretching is preferred from the viewpoint of obtaining a more uniform film. Note that MD refers to the machine direction of the film, and TD refers to the width direction, i.e., the direction perpendicular to MD. The total areal magnification is preferably 8 times or more, more preferably 15 times or more, and even more preferably 20 times or more or 30 times or more, from the viewpoints of uniformity of the film thickness and a balance between tensile elongation, porosity, and average pore size. A total areal magnification of 8 times or more tends to make it easier to obtain a film with high strength and good thickness distribution. Furthermore, from the viewpoint of preventing breakage, this areal magnification may be 250 times or less.

[0043] ((3) Porous body formation process (extraction process)) The porous body forming step is a step of extracting a plasticizer from the stretched material obtained in the stretching step to form a porous body. The method for extracting the plasticizer is not particularly limited, but examples include immersing the stretched material in an extraction solvent and showering the stretched material with the extraction solvent. The extraction solvent is not particularly limited, but for example, it is preferably a poor solvent for polyolefins and a good solvent for plasticizers, with a boiling point lower than the melting point of polyolefins. Examples of such extraction solvents include, but are not limited to, hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride, 1,1,1-trichloroethane, and fluorocarbons; alcohols such as ethanol and isopropanol; ketones such as acetone and 2-butanone; and alkaline water. The extraction solvents may be used alone or in combination.

[0044] ((4) Heat treatment process) The heat treatment step is a step of heat treating the porous body obtained in the porous body formation step to obtain a heat-treated porous body. The heat treatment method is not particularly limited, but examples include a heat setting method using a tenter or roll stretching machine to perform stretching and relaxation operations. The relaxation operation refers to a reduction operation in MD and / or TD at a predetermined temperature and relaxation rate. The relaxation rate refers to the value obtained by dividing the MD dimension of the membrane after the relaxation operation by the MD dimension of the membrane before the operation; or the value obtained by dividing the TD dimension of the membrane after the relaxation operation by the TD dimension of the membrane before the operation; or, when both MD and TD are relaxed, the value obtained by multiplying the MD relaxation rate by the TD relaxation rate. In the first production method, it is preferable to stretch and relax the porous body in the TD from the viewpoint of obtaining a heat-treated porous body suitable for the subsequent affinity treatment step and crosslinking treatment step.

[0045] <First manufacturing method> Hereinafter, a first manufacturing method for manufacturing an electricity storage device will be described.

[0046] First, the method for producing a separator used in an electricity storage device includes the following steps in addition to the above steps (1) to (4): (5) affinity treatment process; (6) a crosslinking treatment step; and (7) Washing and drying process It is preferable to include the above, and if desired, a winding step may be included after step (7).

[0047] ((5) Affinity Treatment Step) The affinity treatment step involves immersing the heat-treated porous body obtained in the heat treatment step described above in an organic solvent that is amphiphilic to both water and organic substances to obtain an affinity-treated porous body. Specifically, in order to improve the wettability between water and polyolefin, the heat-treated porous body is immersed in an organic solvent that is amphiphilic to both water and organic substances to obtain an affinity-treated porous body. In this embodiment, an amphiphilic organic solvent is disposed inside the affinity-treated porous body (affinity-treated porous body), thereby increasing its affinity with liquids. This may also increase its affinity with materials or catalysts that promote crosslinking reactions during the crosslinking treatment step, for example. The organic solvent used is not particularly limited, but examples include alcohols, acetone, ethylene carbonate, N-methyl-2-pyrrolidone, and dimethyl sulfoxide. The immersion method is also not particularly limited, but examples include immersing the heat-treated porous body in an organic solvent and showering the heat-treated porous body with an organic solvent.

[0048] ((6) Crosslinking Treatment Step) The crosslinking step is a step in which the affinity-treated porous body obtained in the affinity treatment step is brought into contact with a mixture of an organometallic catalyst and water or immersed in a base solution or an acid solution, thereby forming a crosslinked porous body having a silane crosslinking structure. In other words, the crosslinking step is a step in which the alkoxysilyl groups contained inside the obtained affinity-treated porous body are converted into siloxane bonds by silane dehydration condensation reaction (crosslinking reaction).

[0049] In the production of general molded products such as hot water pipes, a Sn-based catalyst is often added to the extruder during the extrusion process. In the manufacturing process of separators for power storage devices, if silane crosslinking is promoted in the extruder during the sheet molding process, gelled areas can lead to production defects and make it difficult to stretch the silane-crosslinked polyolefin in the subsequent stretching process. Therefore, in the first manufacturing method, a silane crosslinking treatment is performed after the stretching, heat treatment, and affinity treatment processes. The resulting silane-crosslinked structure ensures the separator's heat resistance, shape retention, and membrane rupture resistance.

[0050] The metal of the organometallic catalyst may be, for example, at least one selected from the group consisting of scandium, vanadium, copper, zinc, zirconium, palladium, gallium, tin, titanium, iron, nickel, and lead, of which tin, zinc, or palladium is preferred, and tin or zinc is more preferred. Examples of organotin complexes that can be used as catalysts include dibutyltin dilaurate, dibutyltin diacetate, dibutyltin dioctoate, stannous acetate, and stannous caprylate.

[0051] The base solution has a pH greater than 7 and may contain, for example, alkali metal hydroxides, alkaline earth metal hydroxides, alkali metal carbonates, alkali metal phosphates, ammonia, amine compounds, etc. Among these, from the viewpoints of safety and silane crosslinkability of the electricity storage device, alkali metal hydroxides or alkaline earth metal hydroxides are preferred, alkali metal hydroxides are more preferred, and sodium hydroxide is even more preferred.

[0052] The acid solution has a pH of less than 7 and may contain, for example, an inorganic acid, an organic acid, etc. Preferred acids are hydrochloric acid, sulfuric acid, carboxylic acids, or phosphoric acids.

[0053] From the viewpoint of improving safety by suppressing a thermal runaway reaction when the electricity storage device is destroyed, the crosslinking treatment step is preferably carried out by immersing the affinity-treated porous body in a basic solution or an acid solution.

[0054] When the affinity-treated porous body is immersed in a basic solution, from the viewpoint of further improving safety, the temperature of the basic solution is preferably 20°C to 100°C, and / or the pH of the basic solution is preferably 8 to 14. The reagent used for adjusting the pH is not particularly limited, but examples include alkali metal hydroxides and alkaline earth metal hydroxides. From the same viewpoint, it is preferable that the alkaline aqueous solution does not contain amine compounds such as ethylamine, dibutylamine, hexylamine, and pyridine.

[0055] When the affinity-treated porous body is immersed in an acid solution, it is presumed, without wishing to be bound by theory, that the acid acts to catalytically promote the formation of Si-O bonds in the silane-crosslinked polyolefin rather than cleaving the Si-O bonds in the silane-crosslinked polyolefin.

[0056] When the affinity-treated porous body is contacted with a mixture of an organometallic catalyst and water, the content of scandium, vanadium, copper, zinc, zirconium, palladium, gallium, tin, titanium, iron, nickel, or lead in the final microporous membrane is preferably adjusted to a total atomic amount of 0.10 ppm to 200 ppm. In particular, it is more preferable to adjust the zinc or tin content of the microporous membrane to a total atomic amount of 0.10 ppm to 200 ppm. The content of scandium, vanadium, copper, zinc, zirconium, palladium, gallium, tin, titanium, iron, nickel, or lead in the microporous membrane can be adjusted, for example, by a water washing and drying process described below. Such a limited metal content can suppress decomposition of the crosslinked structure in the membrane, thereby making it easier to ensure safety and tending to exhibit good battery cycle characteristics. If the separator contains an excess amount of metal, the eluted ions can penetrate into the positive electrode, changing the structure of the metal clusters that store Li, creating electrical defects throughout the positive electrode and potentially deteriorating cycle performance.

[0057] ((7) Washing and drying process) The water-washing and drying step is a step of washing the crosslinked porous body obtained in the crosslinking step with water and drying it. Preferred conditions for the water-washing and drying step are a water temperature of 20 to 100°C and / or a pH of the washing water of 6 to 8. For example, the inside of the crosslinked porous body can be replaced with water of pH 6 to 8 at a temperature of 20 to 100°C, and then dried. The drying method is not particularly limited, but examples include conveying with heated rolls, blowing with hot air, and heating and drying using an infrared heater. A microporous membrane is obtained through the water-washing and drying step.

[0058] (winding process) In the winding step, the microporous membrane obtained in the water-washing and drying step is slit as necessary and wound around a predetermined core. As described above, the microporous membrane itself may be used as a separator, or a microporous membrane having a functional layer on at least one surface thereof may be used as a separator. When the winding step is carried out after the heat treatment step, the winding step is treated as a step of slitting the heat-treated porous body obtained in the heat treatment step as necessary and winding it around a predetermined core.

[0059] A method for producing an electricity storage device using the separator described above includes, for example, the following steps: (1-a) a step of stacking and / or winding a positive electrode, a separator obtained by the first production method, and a negative electrode to obtain a laminate or a wound body; (1-a) a step of placing the laminate or wound body in an outer casing; (1-c) pouring a non-aqueous electrolyte into the exterior; and (1-d) connecting lead terminals to the positive and negative electrodes; The steps (1-a) to (1-d) can be carried out by methods known in the art, except for using the separator described above. The steps (1-a) to (1-d) can also use the electrodes and non-aqueous electrolyte described above, and can also use positive electrodes, negative electrodes, non-aqueous electrolytes, exterior packages, and charge / discharge devices known in the art.

[0060] <Second manufacturing method> Hereinafter, a second manufacturing method for manufacturing an electricity storage device will be described.

[0061] First, in the second production method, a heat-treated porous body obtained through the above steps (1) to (4) is prepared and can be used as a separator.

[0062] A method for producing an electricity storage device using such a separator includes the following steps: (2-A) a preparation step of preparing an exterior body housing a laminate or wound body of electrodes and separators, and a nonaqueous electrolyte; (2-A) a liquid injection step of pouring a non-aqueous electrolyte into an exterior body; (2-c) a terminal connecting step of connecting a lead terminal to an electrode in the outer casing or an electrode exposed from the outer casing, if desired; (2-D) a charge / discharge step of, if desired, performing at least one charge / discharge cycle; Steps (2-a) to (2-d) can be performed by methods known in the art, except for using a separator (the heat-treated porous body obtained through steps (1) to (4) above). Steps (2-a) to (2-d) can use the electrodes and nonaqueous electrolyte solution described above, and can also use positive electrodes, negative electrodes, nonaqueous electrolyte solutions, exterior bodies, and charge / discharge devices known in the art.

[0063] In the second manufacturing method, it is preferable to bring the separator into contact with the non-aqueous electrolyte during or after step (2-a) to initiate the silane crosslinking reaction of the silane-grafted modified polypropylene. From the viewpoint of ensuring the silane crosslinking reaction of the separator, it is preferable to carry out steps (2-c) and (2-d). It is believed that a substance that catalyzes the silane crosslinking reaction is generated in the non-aqueous electrolyte or on the electrode surface by charge-discharge cycling, thereby achieving the silane crosslinking reaction.

[0064] Although not wishing to be bound by theory, it is believed that the silane graft moieties are converted to silanols by the small amount of moisture contained in the electricity storage device (moisture contained in the electrodes, separator, non-aqueous electrolyte, etc.), undergo a crosslinking reaction, and change into siloxane bonds. Furthermore, when the non-aqueous electrolyte (or the electrolyte in the non-aqueous electrolyte) comes into contact with the electrodes, substances that act as catalysts for the silane crosslinking reaction are generated in the non-aqueous electrolyte or on the electrode surface, dissolve in the non-aqueous electrolyte, and uniformly swell and diffuse into the amorphous parts of the polyolefin where the silane graft moieties are present, thereby uniformly promoting the crosslinking reaction of the separator-containing laminate or wound body. The substance that catalyzes the silane crosslinking reaction may be in the form of an acid solution (e.g., an inorganic or organic acid with a pH < 7), a base solution (e.g., an alkaline solution with a pH > 7), or a film. When the electrolyte contains LiPF6, it may be hydrogen fluoride (HF) generated by the reaction of LiPF6 with water or a fluorine-containing organic substance derived from HF. Here, HF or a fluorine-containing organic substance corresponds to the chemical species generated in the battery according to this embodiment.

[0065] The non-aqueous electrolyte may be any of those listed above as non-aqueous electrolytes that can be used in lithium ion secondary batteries, and is preferably LiPF6, LiN(SO2CF3)2, or LiSO3CF3 from the viewpoint of promoting the cross-linking reaction of the separator.

[0066] By using the separator (the heat-treated porous body obtained through the above steps (1) to (4)), it is possible to provide an electricity storage device assembly kit. The electricity storage device assembly kit includes, for example, the following two elements: (2-a) an exterior housing that houses a laminate or a wound body of a positive electrode, a separator obtained by the second production method, and a negative electrode; and (2-b) a container for storing a non-aqueous electrolyte; When the electricity storage device assembly kit is used, the laminate or wound body of element (2-a) is brought into contact with the non-aqueous electrolyte solution of element (2-b) inside the exterior housing, whereby a silane crosslinking reaction occurs in situ, and an electricity storage device that is both safe and has a high output can be formed.

[0067] In order to promote the cross-linking reaction of the separator, the electricity storage device assembly kit may include, as an accessory (or element (2-c)), a separate container containing a catalyst for cross-linking the alkoxysilyl group to a siloxane bond, such as a mixture of the above-mentioned organometallic catalyst and water, an acid solution, or a base solution.

[0068] An electricity storage device may be manufactured using the electricity storage device assembly kit. A method for manufacturing an electricity storage device using the electricity storage device assembly kit includes, for example, the following steps: (2-A) preparing the electricity storage device assembly kit; (2-B) a step of initiating a silane crosslinking reaction of the silane-grafted modified polyolefin by contacting a separator in the electricity storage device assembly kit (i.e., the heat-treated porous body obtained through the above steps (1) to (4)) with a nonaqueous electrolyte; (2-C) if desired, connecting lead terminals to the positive electrode and the negative electrode in the electricity storage device assembly kit; (2-D) optionally, performing at least one charge / discharge cycle; In steps (2-A) to (2-D), the positive electrode, negative electrode, non-aqueous electrolyte, and outer casing may be those described above or known in the art. From the viewpoint of ensuring that the silane crosslinking reaction of the separator is carried out, it is preferable to carry out steps (2-C) and (2-D).

[0069] In the second manufacturing method, the silane-grafted polyolefin crosslinks upon contact with the nonaqueous electrolyte, which is compatible with conventional manufacturing processes for electricity storage devices, and allows the silane crosslinking reaction to occur after the device is manufactured, improving the safety of the device. [Example]

[0070] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to these examples and comparative examples. Physical properties in the examples were measured by the following methods.

[0071] (1) Weight average molecular weight A calibration curve was created by measuring standard polystyrene under the following conditions using a Waters ALC / GPC 150C (trademark). Chromatograms of the following polymers were also measured under the same conditions, and the weight-average molecular weight of each polymer was 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) Each molecular weight component in the obtained calibration curve was multiplied by 0.43 (Q factor of polyethylene / Q factor of polystyrene=17.7 / 41.3) to obtain a molecular weight distribution curve converted into polyethylene, and the weight average molecular weight was calculated. (Weight average molecular weight of resin composition) The weight average molecular weight was calculated using the Q factor value of the polyolefin with the largest mass fraction, in the same manner as for polyethylene.

[0072] (2) Melt flow rate (MFR) (g / 10 min) Using a melt flow rate measuring instrument (Melt Indexer F-F01) manufactured by Toyo Seiki Seisaku-sha, the weight of the resin extruded for 10 minutes under the conditions of 230°C and a load of 2.16 kg was determined as the MFR value.

[0073] (3) Film thickness (μm) The thickness of the microporous membrane was measured using a micro thickness measuring instrument, KBM (trademark), manufactured by Toyo Seiki Seisaku-sho, at room temperature of 23±2°C and a relative humidity of 60%. Specifically, the thickness was measured at five points at approximately equal intervals across the entire width of the TD, and the average value was obtained.

[0074] (4) Porosity (%) A 10cm x 10cm square sample was cut from the microporous membrane and its volume (cm3 ) and mass (g), and then calculate the density (g / cm 3 ) and the porosity was calculated using the following formula: The density of the mixed composition was determined by calculation from the density of each of the raw materials used and the mixing ratio. Porosity (%) = (volume - mass / density of mixed composition) / volume × 100

[0075] (5) Air permeability and increase in air permeability (sec / 100cm 3 ) In accordance with JIS P-8117 (2009), Toyo Seiki Co., Ltd.'s Gurley air permeability The air permeability of the microporous membrane was measured using a meter, G-B2 (trademark).

[0076] (6) Quantification of resin aggregates in the separator The resin aggregates in the separator are defined as regions having an area of ​​100 μm length x 100 μm width or more and through which light does not pass when the separators obtained in the examples and comparative examples described below are observed with a transmission optical microscope. 2 The number of resin aggregates per unit area was measured.

[0077] (7) Membrane rupture temperature (℃) measured by TMA Using the fixed length mode of Shimadzu Corporation's TMA50 (trademark), the environmental temperature is changed from 25 to 250°C, and the temperature at the moment when the load is completely released is determined as the TMA membrane rupture temperature (the membrane rupture temperature measured by TMA). Specifically, when measuring MD, a microporous membrane with TD 3 mm and MD 13 mm was sampled, both ends of the MD were chucked to a dedicated probe, the chuck distance was set to 10 mm, an initial load of 1.0 g was applied, and the furnace containing the test piece was heated. The temperature at which the load reached 0 g was defined as the TMA membrane rupture temperature. When measuring TD, a microporous membrane with TD 13 mm and MD 3 mm was sampled, and the same procedure as above was performed.

[0078] (8) Battery destruction safety test (nail penetration test) The battery destruction safety test involves driving an iron nail into a battery charged to 4.5V at a speed of 20mm / sec, penetrating it and causing an internal short circuit. This test can clarify the phenomenon that occurs during an internal short circuit by measuring the time-dependent behavior of the battery's voltage drop and the behavior of the battery's surface temperature rise due to an internal short circuit. In addition, in the event of an internal short circuit, the battery may suddenly heat up due to insufficient shutdown function of the separator or rupture at low temperatures, which may result in the non-aqueous electrolyte igniting, causing the battery to smoke and / or explode.

[0079] (Preparation of batteries used in battery destruction safety tests) a. Preparation of the positive electrode A slurry was prepared by dispersing 92.2% by mass of lithium-cobalt composite oxide LiCoO2 as the positive electrode active material, 2.3% by mass each of flake graphite and acetylene black as conductive materials, and 3.2% by mass of polyvinylidene fluoride (PVDF) as a binder in N-methylpyrrolidone (NMP). This slurry was applied to one side of a 20 μm thick aluminum foil that served as a positive electrode current collector using a die coater, dried at 130°C for 3 minutes, and then compression molded using a roll press. At this time, the amount of applied positive electrode active material was 250 g / m 2 , the bulk density of the active material is 3.00 g / cm 3 It was adjusted to be.

[0080] b. Preparation of negative electrode A slurry was prepared by dispersing 96.9% by mass of artificial graphite as the negative electrode active material, and 1.4% by mass of ammonium salt of carboxymethylcellulose and 1.7% by mass of styrene-butadiene copolymer latex as binders in purified water. This slurry was applied to one side of a 12 μm-thick copper foil that served as the negative electrode current collector using a die coater, dried at 120°C for 3 minutes, and then compression-molded using a roll press. The amount of active material applied to the negative electrode was 106 g / m. 2 , the active material bulk density is 1.35 g / cm 3 It was adjusted to be.

[0081] 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.

[0082] d. Battery assembly The separator was cut to a TD of 60 mm and MD of 1000 mm and folded zigzag around the separator. The positive and negative electrodes were alternately stacked between the separators (12 positive electrodes, 13 negative electrodes). The positive electrodes were 30 mm x 50 mm, and the negative electrodes were 32 mm x 52 mm. The zigzag laminate was placed in a laminated bag, and the nonaqueous electrolyte obtained in step c above was poured into it 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 initial charge was performed 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.

[0083] (Maximum heat generation rate) After an iron nail was inserted into the resulting battery, the battery surface temperature was measured over 300 seconds using a thermocouple. From the temperature change graph, the rate at which the temperature change per second was greatest was determined to be the maximum heat generation rate.

[0084] (Voltage drop time) An iron nail was inserted into the resulting battery, and the time required for the voltage to drop from 4.5 V to 3 V was determined as the voltage drop time (3 V drop time).

[0085] (9) Evaluation of cycle characteristics and method for fabricating the battery A battery for evaluating cycle characteristics was fabricated according to the same methods as a. to c. of the battery fabrication method used in the above item "(8) Battery Destruction Safety Test", except that assembly was performed according to d. below. The resulting battery was charged and discharged 100 times in a 60°C atmosphere. The battery was charged at a current of 6.0 mA (1.0 C) up to a battery voltage of 4.2 V, and then the current was reduced from 6.0 mA to maintain 4.2 V. This method of charging for a total of 3 hours was used. The battery was discharged 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 100th cycle and the discharge capacity at the 1st cycle. A high capacity retention rate was evaluated as having good cycle characteristics. d. Battery assembly The separator was cut into 18 mm diameter circles, and the positive and negative electrodes were cut into 16 mm diameter circles. The positive electrode, separator, and negative electrode were stacked in this order, with the active material surfaces of the positive and negative electrodes 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 section (8) Battery Destruction Safety Test (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 this voltage, the current was reduced from 3 mA to maintain the battery voltage at 4.2 V. This was 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.

[0086] (10) Extrusion stability During the extrusion process, the state of the extruded polyolefin composition was observed and evaluated according to the following criteria. ○ (Good): Fluctuations in the extruder current value are within ±0.5 A as an average value over 300 seconds. × (bad): The current fluctuation of the extruder exceeds ±0.5 A in the average value for 300 seconds.

[0087] [Example 1] <Manufacturing method for silane-grafted modified polypropylene> The raw polyolefin used for the silane-grafted modified polypropylene may have a viscosity-average molecular weight (Mv) of 100,000 or more and 1,000,000 or less, a weight-average molecular weight (Mw) of 30,000 or more and 920,000 or less, and a number-average molecular weight of 10,000 or more and 150,000 or less, and may be a propylene or butene copolymerized α-olefin. While the raw polyolefin is melt-kneaded in an extruder, an organic peroxide (di-t-butyl peroxide) is added to generate radicals in the α-olefin polymer chain, and then a trimethoxyalkoxide-substituted vinylsilane is injected to introduce alkoxysilyl groups into the α-olefin polymer by an addition reaction, forming a silane-grafted structure. At the same time, to adjust the radical concentration in the system, an appropriate amount of antioxidant (pentaerythritol tetrakis [3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]) is added to suppress chain reactions (gelation) within the α-olefins. The resulting silane-grafted polyolefin molten resin is cooled in water and processed into pellets, which are then heated and dried at 80°C for two days to remove moisture and unreacted trimethoxyalkoxide-substituted vinylsilane. The residual concentration of unreacted trimethoxyalkoxide-substituted vinylsilane in the pellets is approximately 1500 ppm or less. The silane-grafted modified polyolefin obtained by the above-mentioned production method will be referred to hereinafter or in Table 1 as "silane-grafted modified polypropylene."

[0088] <Preparation of microporous membrane (single layer)> A homopolymer polyethylene (UHMWPE) with a weight-average molecular weight of 1,000,000 and the above silane-grafted modified polypropylene were added in a ratio of polyethylene:silane-grafted modified polypropylene = 95:5 (mass%), and 1 mass% of pentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] was added as an antioxidant. 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 added. -5 m 2 / s) was injected into the extruder cylinder by 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 polyolefin composition was 70% by mass (i.e., the polymer concentration was 30% 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. Subsequently, the melt-kneaded product was extruded through a T-die onto a cooling roll whose surface temperature was controlled to 25° C., thereby obtaining a gel sheet (sheet) having a raw film thickness of 1300 μm.

[0089] (Stretching process) Next, the gel sheet was introduced into a simultaneous biaxial tenter stretching machine and biaxially stretched to obtain a stretched product. The stretching conditions were set as follows: MD magnification 7.0 times, TD magnification 6.0 times (i.e., 7 × 6 times), and biaxial stretching temperature 124°C.

[0090] (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.

[0091] (Heat treatment process) Next, the porous body was introduced into a TD tenter and heat-set at a heat setting temperature of 131°C and a stretch ratio of 1.8 times, and then subjected to a relaxation operation to a TD stretch ratio of 1.7 times, thereby obtaining a heat-treated porous body.

[0092] (Affinity treatment process) Next, the heat-treated porous body was introduced into an ethanol bath (affinity treatment tank) and allowed to remain there while immersed for 60 seconds to perform affinity treatment on the heat-treated porous body, thereby obtaining an affinity-treated porous body.

[0093] (Crosslinking treatment process) Next, the affinity-treated porous body was introduced into a 25% aqueous solution of caustic soda (crosslinking treatment tank) and allowed to remain there while immersed for 60 seconds to perform crosslinking treatment on the affinity-treated porous body, thereby obtaining a crosslinked porous body.

[0094] (Water washing and drying process) The crosslinked porous body was then introduced into water (a water-washing bath) and immersed there for 60 seconds to rinse it, then introduced into a conveyor dryer and dried at 120°C for 60 seconds to obtain a microporous membrane. Thereafter, the ends of the obtained microporous membrane were cut off and taken up into a mother roll having a width of 1,100 mm and a length of 5,000 m.

[0095] [Examples 2 to 5 and Comparative Examples 1 to 3] A microporous membrane was produced in the same manner as in Example 1, except that the conditions were changed as shown in Table 1.

[0096] The microporous membranes obtained in the above examples and comparative examples were subjected to the above evaluations. The evaluation results are shown in Table 1. [Table 1]

[0097] The silane-grafted modified polypropylene (A) in Example 1 was obtained by a modification reaction of a polypropylene having a viscosity average molecular weight of 20,000 with a trimethoxyalkoxide-substituted vinylsilane, and had a density of 0.91 g / cm. 3 and a silane-grafted modified polypropylene having a melt flow rate (MFR) of 18.0 g / min at 230°C.

[0098] In Table 1, "resin composition" indicates the mass ratio relative to 100% by mass of the total of the silane-grafted modified polypropylene (A) and polyethylene (UHMWPE) (B). "Method" indicates the method of silane crosslinking reaction, and in the table, the method is classified as alkaline treatment or acid treatment. Furthermore, the "timing of the crosslinking reaction" indicates whether the silane crosslinking reaction was carried out in the above-mentioned (6) crosslinking treatment step or in the (2-e) step in the above-mentioned second manufacturing method (particularly, whether it was carried out during the first charge / discharge cycle of the energy storage device). Furthermore, "reagent" refers to the reagent used in the above-mentioned (6) crosslinking treatment step, except for Example 5 and Comparative Example 3. In Example 4, a 10% hydrochloric acid solution was used as the reagent instead of the 25% aqueous caustic soda solution used in Example 1. "Temperature" refers to the temperature during the process indicated at the timing of the crosslinking reaction. Furthermore, "pH of the crosslinking treatment tank" and "pH of the water-washing treatment tank" refer to the pH in each tank, and for example, "7 to 12" indicates that the pH is distributed over a range from near the tank inlet to near the outlet.

Claims

1. The following steps: (2-A) a preparation step of preparing an exterior body containing a laminate or wound body of electrodes and a separator for an electricity storage device, the separator having a mass ratio {(A) / (B)} of silane-grafted modified polypropylene (A) to ultra-high molecular weight polyethylene (B) of 2 / 98 to 80 / 20, and a nonaqueous electrolyte; (2-A) a liquid injection step of pouring the nonaqueous electrolyte into the exterior body; Including, the method for producing an electricity storage device, wherein the electricity storage device separator is brought into contact with the nonaqueous electrolyte during or after the liquid injection step, thereby initiating a silane crosslinking reaction of the silane-grafted modified polypropylene (A) contained in the electricity storage device separator.

2. The method for producing an electricity storage device according to claim 1, wherein the electricity storage device separator has a film rupture temperature of 170 to 210°C as measured by thermomechanical analysis (TMA).

3. The method for producing an electricity storage device according to claim 1 or 2, wherein the non-aqueous electrolyte solution contains a fluorine-containing lithium salt.

4. The method for producing an electricity storage device according to any one of claims 1 to 3, wherein the non-aqueous electrolyte solution is an acid solution or a base solution.

5. The following steps: (2-c) a terminal connecting step of connecting a lead terminal to the electrode in the outer casing or the electrode exposed from the outer casing; (2-D) a charge / discharge step of performing at least one charge / discharge cycle; The method for manufacturing an electricity storage device according to any one of claims 1 to 4, further comprising:

6. 6. The method for producing an electricity storage device according to claim 1, wherein the electricity storage device is a lithium ion secondary battery including a positive electrode, a negative electrode, the electricity storage device separator, and the non-aqueous electrolyte solution.

7. 6. The method for producing an electricity storage device according to claim 1, wherein the electricity storage device comprises a positive electrode, a negative electrode, the electricity storage device separator, and the non-aqueous electrolyte solution.

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