JP2018160357A - Synthetic resin microporous film, separator for power storage device and power storage device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a synthetic resin microporous film capable of achieving use as a separator for power storage and high resistance to oxidation degradation even when a charging final voltage of the power storage device.SOLUTION: The synthetic resin microporous film includes: synthetic resin microporous base material film with a microporous part; a skin layer which is formed at least part of a surface of the microporous film and containing engineering plastic; antioxidant 0.001-10 pts.mass relative to the synthetic resin microporous base material film 100 pts.mass.SELECTED DRAWING: None

Description

  The present invention relates to a synthetic resin microporous film, a separator for an electricity storage device, and an electricity storage device.

  Conventionally, power storage devices such as lithium ion batteries, capacitors, and capacitors have been used.

  For example, a lithium ion secondary battery is generally configured by disposing a positive electrode, a negative electrode, and a separator in an electrolytic solution. The positive electrode is formed by applying lithium cobalt oxide or lithium manganate to the surface of the aluminum foil. The negative electrode is formed by applying carbon to the surface of the copper foil. And the separator is arrange | positioned so that a positive electrode and a negative electrode may be partitioned off, and the electrical short circuit between electrodes is prevented.

As a lithium ion secondary battery, Patent Document 1 includes a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator interposed between the positive electrode and the negative electrode. The separator includes a polyolefin layer, an oxidation resistance The oxidation-resistant layer contains an oxidation-resistant polymer, the oxidation-resistant polymer does not contain a —CH ₂ — group in the main chain, and contains a —CH ₂ (CH ₃ ) — group. A lithium ion secondary battery is disclosed in which the oxidation-resistant layer faces the positive electrode without being included.

JP 2007-227361 A

  The separator includes a polyolefin layer and an oxidation resistant layer. While this oxidation resistant layer has a thickness of 1 to 16 μm as described in paragraph No. [0018] of Patent Document 1, the size of the micropores formed in the polyolefin layer is usually 1 μm or less. It is.

  In the separator, if the oxidation-resistant layer is also formed inside the polyolefin layer, that is, on the wall surface of the micropore, the micropore of the polyolefin layer is blocked by the oxidation-resistant layer, and the lithium ion secondary battery Does not function as a separator. Therefore, the separator has only an oxidation-resistant layer formed on one side of the microporous membrane (polyolefin layer).

  Since the separator has only an oxidation-resistant layer formed on only one side of the polyolefin layer, the separator is inferior in high-potential resistance on the inside of the polyolefin layer and on the surface where the oxidation-resistant layer is not formed. Therefore, when the end-of-charge voltage is increased for the purpose of improving the energy density of the lithium ion secondary battery, there arises a problem that the separator is oxidized and deteriorated and the mechanical strength is easily lowered.

  INDUSTRIAL APPLICABILITY The present invention can be used as a separator for an electricity storage device, and is a synthetic resin microporous film that hardly undergoes oxidative degradation even when the end-of-charge voltage of the electricity storage device is set high, and an electricity storage device separator using the same And an electricity storage device.

The synthetic resin microporous film of the present invention is
A synthetic resin microporous substrate film having micropores;
A coating layer formed on at least part of the surface of the synthetic resin microporous film and containing engineering plastic,
An antioxidant is contained in a proportion of 0.001 part by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the synthetic resin microporous substrate film.

  The synthetic resin microporous substrate film includes micropores penetrating in the thickness direction of the film. The fine pores can impart excellent ion permeability to the synthetic resin microporous film. Thereby, the synthetic resin microporous film can transmit ions such as lithium ions in the thickness direction.

  The synthetic resin microporous substrate film contains a synthetic resin, preferably contains a thermoplastic resin, and more preferably contains an olefin resin. As the olefin resin, ethylene resin and propylene resin are preferable, and propylene resin is more preferable.

  Examples of the propylene-based resin include homopolypropylene and copolymers of propylene and other olefins. When a synthetic resin microporous film is produced by the stretching method, homopolypropylene is preferable. Propylene-type resin may be used independently, or 2 or more types may be used together. The copolymer of propylene and another olefin may be a block copolymer or a random copolymer. The content of the propylene component in the propylene-based resin is preferably 50% by mass or more, and more preferably 80% by mass or more.

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

  Examples of the ethylene-based resin include ultra-low density polyethylene, low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ultra high density polyethylene, and an ethylene-propylene copolymer. Further, the ethylene-based resin microporous film may contain other olefin-based resin as long as it contains an ethylene-based resin. The content of the ethylene component in the ethylene-based resin is preferably more than 50% by mass, more preferably 80% by mass or more.

  The weight average molecular weight of the olefin resin is not particularly limited, but is preferably 30,000 to 500,000, and more preferably 50,000 to 480,000. Although the weight average molecular weight of propylene-type resin is not specifically limited, 250,000-500,000 are preferable and 280,000-480,000 are more preferable. Although the weight average molecular weight of ethylene-type resin is not specifically limited, 30,000-250,000 are preferable and 50,000-200,000 are more preferable. According to the olefin resin having a weight average molecular weight within the above range, it is possible to provide an olefin resin microporous substrate film having excellent film forming stability and having uniform micropores. .

  The molecular weight distribution (weight average molecular weight Mw / number average molecular weight Mn) of the olefin resin is not particularly limited, but is preferably 5 to 30, and more preferably 7.5 to 25. The molecular weight distribution of the propylene-based resin is not particularly limited, but is preferably 7.5 to 12, and more preferably 8 to 11. The molecular weight distribution of the ethylene-based resin is not particularly limited, but is preferably 5.0 to 30, and more preferably 8.0 to 25. According to the olefin resin having a molecular weight distribution within the above range, an olefin resin microporous substrate film having a high surface opening ratio and excellent mechanical strength can be provided.

  Here, the weight average molecular weight and number average molecular weight of the olefin resin are polystyrene-converted values measured by a GPC (gel permeation chromatography) method. Specifically, 6 to 7 mg of an olefin resin is sampled, the collected olefin resin is supplied to a test tube, and 0.05% by mass of BHT (dibutylhydroxytoluene) is contained in the test tube. A diluted solution is prepared by adding a DCB (orthodichlorobenzene) solution and diluting the olefin-based resin concentration to 1 mg / mL.

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

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

  Although melting | fusing point of an olefin resin is not specifically limited, 130-170 degreeC is preferable and 133-165 degreeC is more preferable. Although melting | fusing point of propylene-type resin is not specifically limited, 160-170 degreeC is preferable and 160-165 degreeC is more preferable. Although melting | fusing point of ethylene-type resin is not specifically limited, 130-140 degreeC is preferable and 133-139 degreeC is more preferable. According to the olefinic resin having a melting point within the above range, it is possible to provide an olefinic resin microporous substrate film that has excellent film-forming stability and suppresses a decrease in mechanical strength at high temperatures. it can.

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

As an index indicating the crystallinity of homopolypropylene, there is an isotactic pentad fraction (mmmm fraction) measured by ¹³ C-NMR method. The isotactic pentad fraction measured by the ¹³ C-NMR method of homopolypropylene is a side chain with respect to the main chain formed by a carbon-carbon bond composed of any five consecutive propylene units. The ratio of the three-dimensional structure in which all of the methyl groups are located in the same direction occupies the whole molecular chain of homopolypropylene.

The isotactic pentad fraction measured by the ¹³ C-NMR method of homopolypropylene is preferably 90% or more, and more preferably 95% or more. By setting the isotactic pentad fraction to 90% or higher, the synthetic resin microporous substrate film has uniformly formed micropores.

  The synthetic resin microporous substrate film includes micropores. It is preferable that the micropore part penetrates in the film thickness direction, and this can impart excellent air permeability to the synthetic resin microporous film. Such a synthetic resin microporous film can transmit ions such as lithium ions in the thickness direction.

  The method for producing the synthetic resin microporous substrate film is not particularly limited, and a conventionally known wet method or stretching method is used. The wet method is, for example, a step of obtaining a synthetic resin film by molding a resin composition containing a synthetic resin and a filler or a plasticizer, and by extracting the filler or the plasticizer from the synthetic resin film. And a step of obtaining a synthetic resin microporous substrate film in which micropores are formed. On the other hand, the stretching method has a step of obtaining a synthetic resin microporous substrate film in which micropores are formed by uniaxially stretching or biaxially stretching the synthetic resin film.

  As the synthetic resin microporous substrate film, the synthetic resin microporous substrate film produced by the stretching method is easy to form a coating layer on the wall surface of the micropore, and the synthetic resin microporous substrate film has excellent high potential resistance. A film can be obtained.

As a method for producing a synthetic resin microporous substrate film by a stretching method, specifically,
(1) A step of obtaining a synthetic resin film by extruding a synthetic resin, a step of generating and growing a lamellar crystal in the synthetic resin film, and a micropore by stretching the synthetic resin film and separating the lamellar crystals A step of obtaining a synthetic resin microporous substrate film in which a part is formed;
(2) A step of obtaining a synthetic resin film by extruding a synthetic resin composition containing a synthetic resin and a filler, and an interface between the synthetic resin and the filler by uniaxially or biaxially stretching the synthetic resin film And (3) a synthetic resin and an extractable material (for example, a filler or a plasticizer). A step of obtaining a synthetic resin film by extruding the synthetic resin composition contained therein, a step of forming a micropore by extracting an extractable substance from the synthetic resin film with a solvent, and a synthetic resin having the micropore formed therein And a method of obtaining a synthetic resin microporous substrate film by stretching the film.

  Especially, since the effect by this invention can be exhibited especially, the lamella extending | stretching method by the method of (1) is preferable.

As a method for producing a synthetic resin microporous substrate film, preferably the following steps,
An extrusion step of obtaining a synthetic resin film by melt-kneading and extruding the synthetic resin in an extruder;
Curing process for curing the synthetic resin film,
A stretching step of uniaxially stretching the synthetic resin film after the curing step;
An annealing step for annealing the synthetic resin film after the stretching step;
Is used.

  First, in the extrusion step, a synthetic resin film is preferably obtained by supplying a synthetic resin to an extruder, melt-kneading, and extruding from a T die attached to the tip of the extruder.

  Next, in the curing process, the synthetic resin film obtained by the extrusion process described above is cured. This curing process is performed in order to grow the lamellar crystals formed in the synthetic resin film in the extrusion process. As a result, it is possible to promote the formation of a laminated lamellar structure in which crystallized portions (lamellar) and amorphous portions are alternately arranged in the extrusion direction of the synthetic resin film. Instead, it is possible to generate a crack in the non-crystal part between the lamella crystals and form a minute through hole (micro hole part) starting from this crack.

  The curing step is preferably performed by curing the synthetic resin film obtained by the extrusion step at (Tm-30) to (Tm-5) ° C. for 1 minute or more. The Tm is the melting point of the synthetic resin.

  Next, in the stretching step, the synthetic resin film after the curing step is uniaxially stretched. Uniaxial stretching is preferably performed in the extrusion direction of the synthetic resin film. In the stretching process, the lamella crystal part in the synthetic resin film is hardly melted, and the lamella crystals are separated from each other by stretching, whereby the non-crystalline part is stretched to form microfibrils while forming microfibrils. be able to. Microfibrils are formed so as to connect adjacent lamellar crystal parts. Thereby, the micropore part is separated and formed by the lamellar crystal part adjacent to each other in the extrusion direction (stretching direction) of the film and the microfibril adjacent to each other in the width direction of the film.

The stretching process includes the following processes:
A first stretching step in which the synthetic resin film after the curing step is uniaxially stretched at a stretching ratio of 1.2 to 1.6 times at a surface temperature of −20 ° C. or more and less than 100 ° C .;
A second stretching step in which the synthetic resin film stretched in the first stretching step is uniaxially stretched at a surface magnification of 1.2 to 3.0 times at a surface temperature of 100 to 150 ° C;
It is preferably carried out by a method having In the first stretching step and the second stretching step, uniaxial stretching is preferably performed in the extrusion direction of the synthetic resin film.

  In the present invention, the stretch ratio of the synthetic resin film is a value obtained by dividing the length of the synthetic resin film in the stretching direction after stretching by the length of the synthetic resin film in the same direction as the stretching direction before stretching. Say.

  Next, in the annealing step, the synthetic resin film after the stretching step, preferably after the second stretching step is annealed. According to the annealing step, residual strain generated in the synthetic resin film due to the stretching applied in the above-described stretching step can be alleviated, and thereby the thermal shrinkage of the synthetic resin microporous substrate film can be reduced.

  The annealing step is performed in the extrusion direction while heating the synthetic resin film after the stretching step, preferably after the second stretching step, at a surface temperature of (Tm-35) ° C. or higher and below the melting point of the synthetic resin. It is preferably performed by shrinking at a shrinkage rate of ˜25%.

  The shrinkage rate of the synthetic resin film refers to the shrinkage length of the synthetic resin film in the stretching direction after the annealing step divided by the length of the synthetic resin film in the stretching direction after the stretching step, preferably after the second stretching step. And the value multiplied by 100.

  The synthetic resin microporous film has a coating layer formed on at least a part of the surface of the synthetic resin microporous substrate film. The coating layer is preferably formed on the entire surface of the synthetic resin microporous substrate film. Furthermore, since the coating layer is more excellent in the high potential resistance of the synthetic resin microporous film, it is preferably formed on the wall surface of the micropores of the synthetic resin microporous substrate film. The synthetic resin microporous film has an excellent high potential resistance by having a coating layer. That is, even when a high potential is applied to the synthetic resin microporous film, the coating layer prevents the synthetic resin microporous film from being oxidatively deteriorated, and the electricity storage device can set the charge end voltage high. Can be configured. The surface of the synthetic resin microporous substrate film excludes the portion corresponding to the opening end of the microporous portion from the entire surface of both surfaces of the synthetic resin microporous substrate film when the microporous portion is assumed to be solid. The part that was done. The both surfaces of the synthetic resin microporous substrate film refer to the surface having the largest area and the surface opposite to this surface.

The coating layer formed on the surface of the synthetic resin microporous substrate film and the wall surface of the micropores can be confirmed in the following manner. FIB (Focused Ion Beam) processing is performed on the sample of the synthetic resin microporous film, the cross section of the synthetic resin microporous film is exposed, and elemental analysis is performed by FE-TEM / EDS. Depending on whether oxygen and nitrogen derived from polyamideimide are uniformly distributed on the surface excluding the newly created surface by FIB processing, a coating layer is formed on the surface of the synthetic resin microporous substrate film and the wall surface of the micropores. It can be confirmed whether or not it is formed.
The observation conditions of FE-TEM are as follows, for example.
FE-TEM: JEM-ARM200F
Accelerating voltage: 200kV
STEM-HAADF observation EDS element map measurement

  The coating layer contains engineering plastic. The engineering plastic is not particularly limited, for example, vinylidene fluoride-hexafluoropropylene copolymer, polyamideimide, polyacetal, polyamide, polycarbonate, modified polyphenylene ether, polybutylene terephthalate, polysulfone, polyethersulfone, polyphenylenesulfide, Examples include polyether ether ketone, polyimide, polyether imide, and vinylidene fluoride-hexafluoropropylene copolymer and polyamide imide are preferable.

  The thickness of the coating layer is preferably 1 to 100 nm, and more preferably 5 to 50 nm. If the thickness is within the above range, the synthetic resin microporous film is uniformly coated on the surface of the synthetic resin microporous film while maintaining high ion permeability of the synthetic resin microporous film. Even when added, it is possible to further suppress a decrease in mechanical strength due to oxidative degradation of the synthetic resin microporous film. In addition, the thickness of a film layer means the value which measured the cross section of the synthetic resin microporous film using the FE-TEM element distribution mapping method.

  The content of the coating layer in the synthetic resin microporous film is preferably 0.05 to 80 parts by mass, more preferably 0.1 to 75 parts by mass with respect to 100 parts by mass of the synthetic resin resin microporous substrate film. 3 to 70 parts by mass are more preferable, and 0.5 to 67 parts by mass are particularly preferable. When the content of the coating layer is within the above range, the synthetic resin microporous film exhibits excellent high potential resistance while maintaining excellent air permeability.

  The method for forming the coating layer on the surface of the synthetic resin microporous substrate film and the wall surface of the micropore is not particularly limited. For example, an engineering plastic solution containing engineering plastic is applied to the surface of the synthetic resin microporous substrate film. After coating, the engineering plastic solution is impregnated into the micropores of the synthetic resin microporous substrate film and applied also to the wall surface of the micropores, and then the coating layer is removed by removing the solvent contained in the engineering plastic solution The method of forming is mentioned.

  The engineering plastic solution is obtained by dissolving engineering plastic in a solvent. The solvent is not particularly limited as long as the engineering plastic can be dissolved, but the engineering plastic solution can be uniformly applied to the wall surface of the microporous portion of the synthetic resin microporous substrate film. It is preferable to use a solvent having high affinity for the synthetic resin contained in the material film. When the synthetic resin microporous substrate film contains an olefin-based resin, the solvent having high affinity for the synthetic resin is preferably one or more solvents selected from the group consisting of ethyl acetate, xylene and toluene. . In addition, a solvent may be used independently or 2 or more types may be used together. Whether or not the affinity for the synthetic resin is high may be determined on the basis of a solubility parameter.

  In the solvent constituting the engineering plastic solution, the content of the solvent having a high affinity for the synthetic resin contained in the synthetic resin microporous substrate film is preferably 10% by mass or more, and 15% by mass or more. More preferred is 20% by mass or more.

  In the solvent constituting the engineering plastic solution, a solvent other than a solvent having a high affinity for the synthetic resin contained in the synthetic resin microporous substrate film may be contained. Examples of such a solvent include alcohols such as methanol and ethanol, N-methylpyrrolidone, and dimethylacetamide.

  The content of the engineering plastic in the engineering plastic solution is preferably 1 to 5% by mass, more preferably 1 to 4% by mass, and 2 to 3% by mass when the total amount of the engineering plastic and the solvent is 100% by mass. Particularly preferred. By setting the engineering plastic content in the engineering plastic solution within the above range, the surface of the synthetic resin microporous substrate film and the wall surface of the micropores are maintained while maintaining good ion permeability of the micropores. The layer can be formed uniformly. Therefore, the synthetic resin microporous film is excellent in excellent high potential resistance and ion permeability.

  The synthetic resin microporous film contains an antioxidant. When the end-of-charge voltage of the electricity storage device is set to be as high as 4.4 V or more using a graphite negative electrode, for example, the synthetic resin microporous film may be exposed to a high potential and cause oxidative degradation. Furthermore, the synthetic resin microporous film may be deteriorated by the gas (mainly containing oxygen) generated by the decomposition of the electrolyte solution on the electrode surface and the electrolyte solution itself. Such oxidative deterioration of the synthetic resin microporous film can be suppressed to some extent by the presence of the above-described coating layer, but by further adding an antioxidant to the synthetic resin microporous film, Even when a high potential is applied, it is possible to constitute a power storage device that can remarkably suppress a decrease in mechanical strength due to oxidative deterioration of a synthetic resin microporous film and set a high end-of-charge voltage. it can.

  The mechanism by which the decrease in mechanical strength due to the oxidative deterioration of the synthetic resin microporous film can be remarkably suppressed by containing an antioxidant in the synthetic resin microporous film has not been clearly elucidated. Presumed to be due to the mechanism.

Under conditions where the end-of-charge voltage of the electricity storage device is set high, oxygen generated from the active material is superoxide anion radical (commonly referred to as “superoxide”), hydroxyl radical, hydrogen peroxide, singlet oxygen, etc. Of active oxygen. These active oxygens react with the synthetic resin of the synthetic resin microporous film to generate peroxide radicals (ROO.). In contrast to this ROO ·, a primary antioxidant (AOH) represented by a hindered phenolic antioxidant is converted to peroxide (ROOH) by the reaction of (1) below. It is considered that the reactivity of ROO · is lowered and the oxidative deterioration of the synthetic resin microporous film is suppressed. In addition, secondary antioxidants (B) typified by phosphorous antioxidants and sulfur antioxidants change ROOH into more inactive alcohol (ROH) by the reaction (2) below. It is considered that the oxidative deterioration of the synthetic resin microporous film is suppressed. For this reason, it is thought that the oxidative deterioration of the synthetic resin microporous film can be remarkably suppressed by using the primary antioxidant and the secondary antioxidant in combination.
ROO ・ + AOH → ROOH + AO ・ （1）
ROOH + B → ROH + B = O (2)

  The content of the antioxidant in the synthetic resin microporous film is 0.001 to 10 parts by mass, preferably 0.01 to 9 parts by mass with respect to 100 parts by mass of the synthetic resin microporous substrate film. 05-8 mass parts is more preferable, and 0.1-7 mass parts is especially preferable. When the content of the antioxidant is 0.001 parts by mass or more, the high potential resistance of the synthetic resin microporous film is improved. Generation | occurrence | production of the gel resulting from antioxidant as the content of antioxidant is 10 mass parts or less is suppressed, and the synthetic resin microporous film is excellent in ion permeability.

  The antioxidant is not particularly limited, and examples thereof include phenolic antioxidants such as hindered phenolic antioxidants, monophenolic antioxidants, bisphenolic antioxidants, and polyphenolic antioxidants, and hindered amines. Antioxidants, phosphorus antioxidants, sulfur antioxidants, benzotriazole antioxidants, benzophenone antioxidants, triazine antioxidants, salicylate antioxidants, etc. An inhibitor and a phosphorus-based antioxidant are preferred.

  Examples of the hindered phenol antioxidant include 2,4-bis [(laurylthio) methyl] -o-cresol, 1,3,5-tris (3,5-di-tert-butyl-4-hydroxybenzyl). ), 1,3,5-tris (4-t-butyl-3-hydroxy-2,6-dimethylbenzyl), 2,4-bis- (n-octylthio) -6- (4-hydroxy-3,5) -Di-t-butylanilino) -1,3,5-triazine, pentaerythritol tetrakis [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate, 2,6-di-t-butyl- 4-nonylphenol, 2,2′-isobutylidene-bis- (4,6-dimethyl-phenol), 4,4′-butylidene-bis- (2-tert-butyl-5-methylphenol), 2,2 ′ Thio-bis- (6-t-butyl-4-methylphenol), 2,5-di-t-amyl-hydroquinone, 2,2′thiodiethylbis- (3,5-di-t-butyl-4- Hydroxyphenyl) -propionate, 1,1,3-tris- (2′-methyl-4′-hydroxy-5′-t-butylphenyl) -butane, 2,2′-methylene-bis- (6- (1 -Methyl-cyclohexyl) -p-cresol), 2,4-dimethyl-6- (1-methyl-cyclohexyl) -phenol, N, N-hexamethylenebis (3,5-di-t-butyl-4-hydroxy -Hydrocinnamamide) and the like.

  Examples of the hindered amine antioxidant include bis (2,2,6,6-tetramethyl-4-piperidyl) sebacate and bis (N-methyl-2,2,6,6-tetramethyl-4-piperidyl). Sebacate, N, N′-bis (2,2,6,6-tetramethyl-4-piperidyl) -1,6-hexamethylenediamine, 2-methyl-2- (2,2,6,6-tetramethyl) -4-piperidyl) amino-N- (2,2,6,6-tetramethyl-4-piperidyl) propionamide, tetrakis (2,2,6,6-tetramethyl-4-piperidyl) (1,2, 3,4-butanetetracarboxylate, poly [{6- (1,1,3,3-tetramethylbutyl) imino-1,3,5-triazine-2,4-diyl} {(2,2,6 , 6-Tetramethyl-4- Peridyl) imino} hexamethyl {(2,2,6,6-tetramethyl-4-piperidyl) imino}], poly [(6-morpholino-1,3,5-triazine-2,4-diyl) {(2 , 2,6,6-tetramethyl-4-piperidyl) imino} hexamethine {(2,2,6,6-tetramethyl-4-piperidyl) imino}], dimethyl succinate and 1- (2-hydroxyethyl) Polycondensate with -4-hydroxy-2,2,6,6-tetramethylpiperidine, N, N′-4,7-tetrakis [4,6-bis {N-butyl-N- (1,2, 2,6,6-pentamethyl-4-piperidyl) amino} -1,3,5-triazin-2-yl] -4,7-diazadecane-1,10-diamine.

  Examples of phosphorus antioxidants include tris (isodecyl) phosphite, tris (tridecyl) phosphite, phenylisooctylphosphite, phenylisodecylphosphite, phenyldi (tridecyl) phosphite, diphenylisooctylphosphite, diphenyl Isodecyl phosphite, diphenyltridecyl phosphite, triphenyl phosphite, tris (nonylphenyl) phosphite, 4,4 ′ isopropylidenediphenol phosphite, trisnonylphenyl phosphite, trisdinonylphenyl phosphite, tris (2,4-di-t-butylphenyl) phosphite, tris (biphenyl) phosphite, distearyl pentaerythritol diphosphite Di (2,4-di-t-butylphenyl) pentaerythritol diphosphite, di (nonylphenyl) pentaerythritol diphosphite, phenylbisphenol A pentaerythritol diphosphite, tetratridecyl 4,4′-butylidenebis ( 3-methyl-6-tert-butylphenol) diphosphite, hexatridecyl 1,1,3-tris (2-methyl-4-hydroxy-5-tert-butylphenyl) butane triphosphite, 3,5-diphosphite -T-butyl-4-hydroxybenzyl phosphite diethyl ester, sodium bis (4-t-butylphenyl) phosphite, sodium-2,2-methylene-bis (4,6-di-t-butylphenyl) -phosphite 1,3-bis (diphenoxyphosphonoxy ) -Benzene, ethylbisphosphite (2,4-ditert-butyl-6-methylphenyl) and the like.

  Examples of the sulfur-based antioxidant include 2,2-thio-diethylenebis [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate], 2,4-bis [(octylthio) methyl. ] -O-cresol, 2,4-bis [(laurylthio) methyl] -o-cresol, and the like.

  Examples of the benzotriazole antioxidant include oligomer type and polymer type compounds having a benzotriazole structure.

  Examples of the benzophenone antioxidant include 2-hydroxy-4-methoxybenzophenone, 2,4-dihydroxybenzophenone, 2-hydroxy-4-n-octoxybenzophenone, 4-dodecyloxy-2-hydroxybenzophenone, 2- Hydroxy-4-octadecyloxybenzophenone, 2,2′dihydroxy-4-methoxybenzophenone, 2,2′dihydroxy-4,4′-dimethoxybenzophenone, 2,2 ′, 4,4′-tetrahydroxybenzophenone, 2- Hydroxy-4-methoxy-5sulfobenzophenone, 2-hydroxy-4-methoxy-2′-carboxybenzophenone, 2-hydroxy-4-chlorobenzophenone and the like can be mentioned.

  Examples of the triazine antioxidant include 2,4-bis (allyl) -6- (2-hydroxyphenyl) 1,3,5-triazine.

  Examples of the salicylate-based antioxidant include phenyl salicylate, p-octylphenyl salicylate, p-tertbutylphenyl salicylate, and the like.

  The antioxidant is preferably contained in the synthetic resin constituting the synthetic resin microporous base film, but is preferably contained also in the wall surface of the microporous portion of the synthetic resin microporous film. .

  The method for adding the antioxidant to the synthetic resin microporous film is not particularly limited. For example, (1) a method of applying the antioxidant to the synthetic resin microporous film; (2) in the liquid antioxidant (3) A coating solution is prepared by dissolving or dispersing an antioxidant in a solvent, and immersing the synthetic resin microporous film into the synthetic resin microporous film. Is applied to the synthetic resin microporous film, and then the solvent is removed by heating the synthetic resin microporous film; (4) a coating solution is prepared by dissolving or dispersing the antioxidant in the solvent; A method of immersing the synthetic resin microporous film in the interior and applying the coating solution in the synthetic resin microporous film, and then heating the synthetic resin microporous film to remove the solvent; During production, synthesis A method for producing a synthetic resin film by supplying an antioxidant together with fat to an extruder and producing a synthetic resin microporous film by stretching the synthetic resin film; (6) An antioxidant in an engineering plastic solution After the addition, an engineering plastic solution is applied to the surface of the synthetic resin microporous substrate film, and a method of removing the solvent contained in the engineering plastic solution is exemplified. Especially, the method of said (3)-(6) is preferable and the method of (5) is more preferable. According to these methods, an antioxidant can be uniformly contained in the synthetic resin microporous film.

  In the methods (3), (4) and (6) above, the heating temperature of the synthetic resin microporous film and the synthetic resin microporous substrate film for removing the solvent should be set according to the type and boiling point of the solvent used. Can do. The heating temperature of the synthetic resin microporous film and the synthetic resin microporous substrate film for removing the solvent is preferably 50 to 140 ° C, more preferably 70 to 130 ° C. By setting the heating temperature within the above range, the applied solvent can be efficiently removed while reducing thermal shrinkage and blockage of the micropores of the synthetic resin microporous film and the synthetic resin microporous substrate film. it can.

  In the methods (3), (4) and (6), the heating time for the synthetic resin microporous film and the synthetic resin microporous substrate film for removing the solvent is not particularly limited, and the type of solvent used It can be set by the boiling point. The heating time of the synthetic resin microporous film and the synthetic resin microporous substrate film for removing the solvent is preferably 0.02 to 60 minutes, and more preferably 0.1 to 30 minutes.

  The coating solution can smoothly flow on the wall surface of the microporous part in the synthetic resin microporous film, thereby not only the surface of the synthetic resin microporous film but also the wall surface of the microporous part of the synthetic resin microporous film and The antioxidant can penetrate into the synthetic resin and the coating layer constituting the synthetic resin microporous film through the wall surface. As a result, the antioxidant can be applied and permeated throughout the synthetic resin microporous film.

  The solvent used in the coating solution is not particularly limited as long as it can dissolve or disperse the antioxidant. For example, alcohols such as methanol, ethanol, propanol, isopropyl alcohol, acetone, methyl ethyl ketone, methyl isobutyl ketone, etc. Examples include ketones, ethers such as tetrahydrofuran and dioxane, ethyl acetate, chloroform and acetonitrile. Of these, ethyl acetate, tetrahydrofuran, acetonitrile, and methyl ethyl ketone are preferable. These solvents can be removed smoothly after the coating solution is applied to the surface of the synthetic resin microporous film. Furthermore, the solvent has low reactivity with an electrolyte solution constituting an electricity storage device such as a lithium ion secondary battery, and is excellent in safety.

  0.1-5 mass% is preferable and, as for content of antioxidant in a coating liquid, 0.5-3 mass% is more preferable. By making content of antioxidant into the said range, antioxidant can be uniformly contained in a synthetic resin microporous film, without obstruct | occluding the micropore part of a synthetic resin microporous film. Therefore, it is possible to produce a synthetic resin microporous film with improved high potential resistance without reducing air permeability.

  The thickness of the synthetic resin microporous film is preferably 5 to 40 μm, and more preferably 5 to 30 μm.

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

  The air permeability of the synthetic resin microporous film is preferably 30 to 600 sec / 100 mL, more preferably 50 to 600 sec / 100 mL, and particularly preferably 70 to 300 sec / 100 mL. A synthetic resin microporous film having an air permeability within the above range is excellent in both mechanical strength and ion permeability.

  The air permeability of the synthetic resin microporous film was determined by measuring the air permeability at 10 arbitrary locations of the synthetic resin microporous film in accordance with JIS P8117 in an atmosphere of a temperature of 23 ° C. and a relative humidity of 65%. , And the value obtained by calculating the arithmetic mean value.

  The surface opening ratio of the synthetic resin microporous film is preferably 30 to 55%, and more preferably 30 to 50%. A synthetic resin microporous film having a surface opening ratio within the above range is excellent in both mechanical strength and ion permeability.

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

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

  The maximum major axis of the open end of the micropore in the synthetic resin microporous film is preferably 1 μm or less, and more preferably 100 nm to 800 nm. A synthetic resin microporous film having a maximum long diameter of 1 μm or less at the opening end of the micropore portion is excellent in mechanical strength and has uniform ion permeability. Such a synthetic resin microporous film can reduce the occurrence of minute internal short circuits (dendritic short circuits) due to the growth of dendrites (dendritic crystals).

  The average major axis of the open ends of the micropores in the synthetic resin microporous film is preferably 500 nm or less, and more preferably 200 nm to 500 nm. A synthetic resin microporous film having an average major axis at the opening end of the micropores of 500 nm or less has a uniform ion permeability, which can reduce the occurrence of minute internal short circuits (dendritic shorts).

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

  The major axis of the opening end of each micropore appearing in the obtained photograph is measured. Among the long diameters of the opening ends in the microhole portion, the maximum long diameter is set as the maximum long diameter of the opening end of the microhole portion. The arithmetic mean value of the major axis of the open end in each micropore is defined as the average major axis of the open end of the micropore. The major axis of the open end of the microhole is defined as the diameter of a perfect circle having the smallest diameter that can surround the open end of the microhole. Micropores that exist across the imaging range and the non-imaging range are excluded from the measurement target.

  The porosity of the synthetic resin microporous film is preferably 35 to 65%, more preferably 40 to 60%. A synthetic resin microporous film having a porosity in the above range is excellent in both mechanical strength and ion permeability.

In addition, the porosity of a synthetic resin microporous film can be measured in the following way. First, a synthetic resin microporous film is cut to obtain a flat square test piece having a length of 10 cm and a width of 10 cm. Next, assuming the weight W (g) and thickness T (cm) of the test piece, the apparent density ρ (g / cm ³ ) is calculated by the following formula (1). In addition, the thickness of a test piece measures 15 thickness of a test piece using a dial gauge (for example, signal ABS Digimatic indicator by Mitutoyo Corporation), and makes it the arithmetic mean value. Then, using this apparent density ρ (g / cm ³ ) and the density ρ ₀ (g / cm ³ ) of the synthetic resin itself, the porosity P (%) of the synthetic resin microporous film based on the following formula (2) Can be calculated.
Apparent density ρ (g / cm ³ ) = W / (100 × T) (1)
Porosity P [%] = 100 × [(ρ ₀ −ρ) / ρ ₀ ] (2)
The density ρ ₀ (g / cm ³ ) of the synthetic resin itself is a value calculated using the volume (arithmetic average value of 10 measurement results) and weight of the sample obtained using a pycnometer. The density ρ ₀ (g / cm ³ ) of the synthetic resin itself can be measured, for example, under the following conditions.
Device name: M-Ultrapyc 1200e
Gas used: He
Purge method (time): Flow (3 minutes)
Pressure: 17.0 psig
Measurement temperature: 25 ° C
Number of measurements: 10 times

The pore density of the synthetic resin microporous film is preferably 15 / μm ² or more, and more preferably 17 / μm ² or more. A synthetic resin microporous film having a pore density of 15 / μm ² or more is excellent in ion permeability.

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

  Synthetic resin microporous film uses non-aqueous electrolyte secondary batteries such as lithium ion batteries, non-aqueous electrolyte primary batteries such as lithium primary batteries, and non-aqueous electrolytes such as capacitors (such as electric double layer capacitors). It can be suitably used as a separator for power storage devices such as chemical elements and capacitors.

  Since the synthetic resin microporous film has excellent high potential resistance, oxidative degradation hardly occurs even when the charge end voltage of the electricity storage device is set high. Therefore, the obtained electricity storage device can be charged and discharged at a high current density.

  The electricity storage device is not particularly limited as long as it includes a synthetic resin microporous film as a separator, and includes a positive electrode, a negative electrode, a separator including a synthetic resin microporous film, and an electrolytic solution (preferably a nonaqueous electrolytic solution). Contains. A synthetic resin microporous film is arrange | positioned between a positive electrode and a negative electrode, and can prevent the electrical short circuit between electrodes by this. In addition, the electrolytic solution is filled at least in the micropores of the synthetic resin microporous film, whereby ions such as lithium ions can move between the electrodes during charging and discharging.

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

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

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

  The synthetic resin microporous film of the present invention has an excellent high potential resistance because a coating layer containing an engineering plastic is formed on the surface of the synthetic resin microporous substrate film and an antioxidant is contained. Even if a high potential is applied to the synthetic resin microporous film, it is suppressed from oxidative degradation. Therefore, the synthetic resin microporous film can be suitably used as a separator for an electricity storage device, and oxidative degradation does not occur even when the end-of-charge voltage of the electricity storage device is set to a high value such as 4.4 V or higher. It is difficult to occur (high potential resistance), and the obtained electricity storage device can be charged and discharged at a high current density.

  Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited thereto.

  Details of the engineering plastics and antioxidants used in the examples are described below.

(Antioxidant)
Antioxidant 1 (hindered phenol-based antioxidant, trade name “IRGANOX1010” manufactured by BASF)
・ Antioxidant 2 (phosphorus antioxidant, trade name “IRGAFOS 168” manufactured by BASF)
・ Antioxidant 3 (hindered amine antioxidant, product name “TINUVIN622SF” manufactured by BASF)
(Engineering plastic)
Polyamideimide 1 (trade name “Bilomax HR15ET” manufactured by Toyobo Co., Ltd., number average molecular weight: 6000)
Polyamideimide 2 (trade name “Bilomax HR16NN” manufactured by Toyobo Co., Ltd., number average molecular weight: 30000)
Polyvinylidene fluoride 1 (vinylidene fluoride-hexafluoropropylene copolymer, trade name “KYNAR (registered trademark) 740” manufactured by Arkema, number average molecular weight: 156000)
Polyvinylidene fluoride 2 (vinylidene fluoride-hexafluoropropylene copolymer, trade name “KYNAR FLEX (registered trademark) 2801” manufactured by Arkema, number average molecular weight: 380000)

(Examples 1-6)
1. Production of homopolypropylene microporous film (extrusion process)
Table 1 shows 100 parts by mass of homopolypropylene (weight average molecular weight 413000, number average molecular weight 44300, melting point 163 ° C., isotactic pentad fraction (mmmm fraction) 97%) and the antioxidants shown in Table 1. After being melt-kneaded at a resin temperature of 200 ° C., extruded into a film form from a T die attached to the tip of the extruder, cooled until the surface temperature reaches 30 ° C., and long A scale-like homopolypropylene film (thickness 30 μm, width 200 mm) was obtained. The extrusion rate was 12 kg / hour, the film forming speed was 22 m / min, and the draw ratio was 70. In Table 1, Example 2 means that antioxidant 1 and antioxidant 2 were used in combination.

(Curing process)
Next, a homopolypropylene film roll was obtained by winding a long homopolypropylene film around a core in a roll shape. While maintaining this homopolypropylene film roll so that its axial direction is horizontal, the homopolypropylene film roll was installed while rotating at a rotational speed of 0.1 rpm in the circumferential direction around the axial core of the core. It was allowed to stand for 24 hours in a hot air oven where the ambient temperature was 155 ° C.

(First stretching process)
Next, the homopolypropylene film is continuously unwound from the cured homopolypropylene film roll at an unwinding speed of 0.5 m / min so that the surface temperature of the homopolypropylene film becomes 23 ° C. and 50% / min. The film was uniaxially stretched using a uniaxial stretching apparatus only in the extrusion direction at a stretching ratio of 1.2 times at a stretching speed of.

(Second stretching step)
Subsequently, the homopolypropylene film was uniaxially stretched only in the extrusion direction at a stretching ratio of 2 times at a stretching rate of 42% / min so that the surface temperature was 120 ° C. using a uniaxial stretching apparatus (second stretching step). .

(Annealing process)
Thereafter, the homopolypropylene film was supplied to the hot air furnace, and the homopolypropylene film was allowed to run for 1 minute so that the surface temperature was 130 ° C. and no tension was applied to the homopolypropylene film. The film was annealed to obtain a homopropylene microporous substrate film (thickness 25 μm) having an elongated shape and a micropore. The shrinkage ratio of the homopolypropylene film in the annealing process was 5%.

The open end of the micropores in the homopolypropylene microporous substrate film had a maximum major axis of 610 nm and an average major axis of 360 nm. The homopolypropylene microporous substrate film had a surface opening ratio of 38% and a pore density of 15 / μm ² .

2. Preparation of coating layer In a solvent having the composition shown in Table 1, the engineering plastic shown in Table 1 is added and dissolved so that the solid content concentration is the amount shown in Table 1, and then ethyl acetate is added. An engineering plastic solution was prepared. The content of the engineering plastic in the engineering plastic solution was 2.4% by mass. The solvent of the engineering plastic solution was composed of 40% by mass of ethanol, 40% by mass of toluene and 20% by mass of ethyl acetate.

  The engineering plastic solution was applied to the entire surface of the homopolypropylene microporous substrate film using a wire bar coater (number No. 8). The engineering plastic solution was impregnated into the micropores of the homopolypropylene microporous substrate film and exuded over the entire other surface of the homopolypropylene microporous substrate film. As a result, the engineering plastic solution was uniformly applied to the entire surface of the homopolypropylene microporous substrate film and the entire wall surface of the micropores.

  Thereafter, the solvent in the engineering plastic solution was evaporated and removed in a vacuum environment at 80 ° C., so that the surface of the homopolypropylene microporous substrate film and the entire wall surface of the micropores were made of engineering plastic. A film layer was formed to obtain a homopolypropylene microporous film.

(Comparative Example 1)
The homopolypropylene microporous substrate film obtained in Example 1 was used as a homopolypropylene microporous film.

(Comparative Example 2)
In Example 1, a homopolypropylene microporous film was obtained in the same manner as in Example 1 except that the antioxidant was not supplied to the extruder.

  The obtained homopolypropylene microporous film was measured for air permeability, thickness, porosity, and pore density in the manner described above, and the results are shown in Table 1.

  Table 1 shows the content of the coating layer with respect to 100 parts by mass of the homopolypropylene microporous substrate film for the obtained homopolypropylene microporous film.

  About the obtained homopolypropylene microporous film, high potential resistance (tensile strength maintenance rate) and heat shrinkage were measured in the following manner, and the results are shown in Table 1.

[High potential resistance (Tensile strength maintenance rate)]
1) Preparation of positive electrode High-potential positive electrode (LiNi _0.5 Mn _1.5 O ₄ , Tanaka Chemical Co., Ltd.) powder, acetylene black, and polyvinylidene fluoride (PVdF) as a positive electrode active material in N-methyl-2-pyrrolidone (NMP) A slurry was prepared by dispersing. In the slurry, the high-potential positive electrode (LiNi _0.5 Mn _1.5 O ₄ , Tanaka Chemical Co., Ltd.) powder contained 90 mass%, acetylene black 5 mass%, and polyvinylidene fluoride (PVdF) 5 mass%. The obtained slurry was applied onto an aluminum foil having a thickness of 20 μm, dried at a temperature of 140 ° C. for 30 minutes, and then pressed to obtain a positive electrode sheet. Thereafter, a positive electrode sheet having a positive electrode active material layer formed on one surface was punched to obtain a flat rectangular positive electrode having a length of 50 mm and a width of 100 mm.

2) Production of negative electrode A slurry was prepared by dispersing 92% by mass of natural graphite powder and 8% by mass of polyvinylidene fluoride (PVdF) as negative electrode active materials in N-methyl-2-pyrrolidone. The slurry contained 92% by mass of natural graphite powder and 8% by mass of polyvinylidene fluoride (PVdF). The obtained slurry was applied onto a copper foil having a thickness of 15 μm, dried under reduced pressure at a temperature of 80 ° C. for 30 minutes, and then pressed to obtain a negative electrode sheet. Thereafter, a negative electrode sheet having a negative electrode active material layer formed on one surface thereof was punched out to obtain a negative electrode having a plane rectangular shape of 55 mm long × 105 mm wide.

3) Production of Separator A rectangular rectangular separator measuring 60 mm in length and 110 mm in width was obtained by punching out the synthetic resin microporous film produced in Examples and Comparative Examples.

4) Cell preparation The laminated body was obtained by laminating | stacking a positive electrode and a negative electrode through a separator. Thereafter, a tab lead was joined to each electrode by ultrasonic welding. After storing the laminate in an exterior material made of aluminum laminate foil, the exterior material was heat sealed to obtain a laminate element.

Next, after drying the laminated body element at 60 ° C. under reduced pressure for 24 hours, the electrolyte solution was injected at room temperature and normal pressure in a glove box (dew point −70 ° C. or lower) filled with Ar gas. did. As the electrolytic solution, a LiPF ₆ solution (1 mol / L) containing ethylene carbonate (E) and dimethyl carbonate (D) at a volume ratio (E: D) of 3: 7 was used as a solvent. After injecting an electrolyte solution into the laminate element, sealing under reduced pressure was performed to produce a lithium ion secondary battery for evaluation.

5) Battery evaluation A 0.2 C constant current obtained by measuring the lithium ion secondary battery produced as described above in the voltage range of 4.0 to 4.9 V based on the capacity estimated from the amount of active material in the positive electrode. The battery was charged and discharged five times, and it was confirmed that the lithium ion secondary battery operated normally. Thereafter, constant voltage charging was continued at 4.9 V for 900 hours.

  After charging at a constant voltage, the separator was taken out from the lithium ion secondary battery and washed with a dimethyl carbonate (DMC) solvent. The separator after washing was naturally dried at 25 ° C. for 3 hours.

6) Judgment of resistance to high potential Three flat rectangular specimens are prepared by punching from the partial surface in contact with the separator electrode after drying to a length (MD direction) of 90 mm and a width (TD direction) of 10 mm. did. Next, in accordance with JIS Z7161, the tensile strength in the MD (extrusion direction) of the three test pieces after the battery test was measured, and the arithmetic average value of the tensile strengths of the three test pieces was “after the battery test”. Of tensile strength ”. From a synthetic resin microporous film before being used as a separator of a lithium ion secondary battery, three flat rectangular test pieces were prepared by punching so as to be 90 mm long (MD direction) × 10 mm wide (TD direction). The tensile strength before the battery test was measured in the same manner as described above. The tensile strength maintenance rate was calculated based on the following formula, and those having a tensile strength maintenance rate exceeding 10% were judged to have high potential resistance. In the synthetic resin microporous film of Comparative Example 1, since the test piece after the battery test was cracked during the measurement of the tensile strength, the tensile strength maintenance ratio could not be calculated.
Tensile strength retention rate (%) = 100 × tensile strength after battery test
/ Tensile strength before battery test

[Heat shrinkage]
From the synthetic resin microporous film, 5 test pieces each having a flat rectangular shape measuring 2 cm in length and 10 cm in width were cut out. At this time, the lateral direction of the test piece was made parallel to the length direction (extrusion direction) of the synthetic resin microporous film. Next, a marked line having a length of 8 cm (L ₀ ) is drawn in the lateral direction of the test piece, the test piece is heated at 130 ° C. for 1 hour and left at room temperature for 30 minutes, and then the length of the marked line (L ₁ ) Was measured. And the heat-shrinkage rate was computed based on the following formula, and the arithmetic mean value of five samples was made into the heat-shrinkage rate.
Thermal contraction rate (%) = 100 × (L ₀ −L ₁ ) / L ₀

Claims (6)

A synthetic resin microporous substrate film having micropores;
A coating layer formed on at least part of the surface of the synthetic resin microporous film and containing engineering plastic,
A synthetic resin microporous film comprising an antioxidant in a proportion of 0.001 part by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the synthetic resin microporous substrate film.   2. The synthetic resin microporous film according to claim 1, wherein the coating layer is formed on at least a part of the wall surface of the microporous portion of the synthetic resin microporous substrate film.   The synthetic resin microporous film according to claim 1 or 2, wherein the coating layer contains a vinylidene fluoride-hexafluoropropylene copolymer and / or a polyamideimide.   The said coating layer is contained in the ratio of 0.05 to 80 mass parts with respect to 100 mass parts of synthetic resin microporous base films, The any one of Claims 1-3 characterized by the above-mentioned. The synthetic resin microporous film according to Item.   An electrical storage device separator comprising the synthetic resin microporous film according to any one of claims 1 to 4.   An electricity storage device using the electricity storage device separator according to claim 5.