JP7639314B2 - Battery separator - Google Patents

Description

The present invention relates to a battery separator having a heat-resistant porous layer on at least one side of a polyolefin porous membrane.

Thermoplastic resin porous membranes are widely used as separation, selective permeation and isolation materials for substances, etc. Examples of such membranes include battery separators used in lithium ion secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, polymer batteries, etc., separators for electric double layer capacitors, various filters such as reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes, moisture-permeable waterproof clothing, and medical materials.

In particular, as a separator for a lithium ion secondary battery, it has ion permeability due to impregnation with an electrolyte, is excellent in electrical insulation, electrolyte resistance and oxidation resistance, and can withstand an abnormal temperature rise of 120 to 150°C.
A polyolefin porous membrane is preferably used, which cuts off the electric current at a temperature of about 1000 .ANG. and also has a pore-closing effect to suppress excessive temperature rise.

However, if the temperature continues to rise even after the pores are blocked for some reason, the polyolefin porous membrane may break. This phenomenon is not limited to the case of using polyolefin, and cannot be avoided above the melting point of the resin that constitutes the porous membrane.

In response to this, a heat-resistant separator is used in which a heat-resistant porous layer mainly composed of inorganic particles and a binder resin is coated on the polyolefin porous membrane. By using this heat-resistant separator, the shrinkage of the polyolefin porous membrane due to temperature rise is suppressed by the heat-resistant porous layer.
Among such separators, a separator having a heat-resistant layer formed from inorganic particles, polyvinyl alcohol (PVA), boric acid and/or an organometallic compound having PVA crosslinkability, and a water-soluble compound having a carboxyl group and/or a sulfonic acid group (Patent Document 1), and a lithium-ion battery separator formed by applying a coating liquid containing an alumina hydrate, an aqueous dispersion of a binder polymer, and an aqueous dispersant to a nonwoven fabric and drying the coating liquid, have been proposed (Patent Document 2). However, these separators did not have excellent heat shrinkage resistance and little shedding of the heat-resistant porous layer even when the heat-resistant porous layer was thin.

Patent No. 6022227 Patent Publication No. 2013-84367

The objective of the present invention is to provide a separator that has excellent heat shrink resistance and is less susceptible to shedding of the heat-resistant porous layer, even when the heat-resistant porous layer is thin.

The present inventors have conducted intensive research in light of the conventional technology and have found that the above-mentioned problem can be solved by a battery separator having a heat-resistant porous layer on at least one side of a polyolefin porous membrane, the battery separator being characterized in that the 150°C/1h heat shrinkage rate is 5% or less, the amount of the heat-resistant porous layer that falls off is 0.6 mg or less, and the content of inorganic particles contained in the heat-resistant porous layer is 92 mass% or more and 94.2 mass% or less, assuming that the total composition that forms the heat-resistant porous layer is 100 mass%.
A more preferred embodiment is
(1) The heat-resistant porous layer contains precipitated barium sulfate particles having a BET specific surface area of 11.1 ^m2 /g or more and 30 ^m2 /g or less. ( 2 ) The heat-resistant porous layer has a 180° peel strength of 350 N/m or more. ( 3 ) The heat-resistant porous layer has a thickness of 0.5 μm or more and 4 μm or less.

The battery separator of the present invention, which has a heat-resistant porous layer on at least one side of the polyolefin porous membrane, has excellent heat shrinkage resistance and is less susceptible to shedding of the heat-resistant porous layer, improving the safety and yield of the battery.

Hereinafter, an embodiment of the present invention will be described in detail. Note that the present invention is not limited to the embodiment described below.
[Polyolefin porous membrane]
The thickness of the polyolefin porous membrane in the embodiment of the present invention is not particularly limited as long as it has the function of battery separator, but is preferably 25 μm or less.More preferably, it is 3 μm or more and 20 μm or less, and even more preferably, it is 5 μm or more and 16 μm or less.When the thickness of the polyolefin porous membrane is 25 μm or less, it can achieve both practical membrane strength and pore blocking function, and the area per unit volume of the battery case is not restricted, which is suitable for increasing the capacity of the battery.

The air resistance of the polyolefin porous membrane is 30 sec/100 ml Air or more, 200
sec/100ml Air or less is preferable. More preferably, 40 sec/100ml Air
ir or more, and 150 sec/100 ml Air or less, and more preferably 50 sec/
The air permeability resistance is 100 ml Air or more and 100 sec/100 ml Air or less. If the air permeability resistance is 30 sec/100 cml Air or more, sufficient mechanical strength and insulation are obtained, reducing the possibility of short circuit during charging and discharging of the battery. If the air permeability resistance is 200 sec/100 ml Air or less, the battery has sufficient charge and discharge characteristics, particularly ion permeability (charge and discharge operating voltage) and battery life (closely related to the amount of electrolyte retained), and can fully function as a battery.

The porosity of the polyolefin porous membrane is preferably 20% or more and 70% or less, more preferably 30% or more and 60% or less, and even more preferably 55% or less.
When the ratio is 70% or less, the battery has sufficient charge/discharge characteristics, particularly ion permeability (charge/discharge operating voltage) and battery life (closely related to the amount of electrolyte retained), and can fully exhibit its functions as a battery. Furthermore, sufficient mechanical strength and insulating properties are obtained, reducing the possibility of a short circuit occurring during charge/discharge.

The average pore size of the polyolefin porous membrane has a large effect on the pore blocking performance.
Preferably, the thickness is 1 μm or more and 1.0 μm or less. More preferably, the thickness is 0.02 μm or more and 0.5 μm or less.
The average pore size of the polyolefin porous membrane is less than 0.01 μm, and more preferably 0.03 μm or more and 0.3 μm or less. When the average pore size of the polyolefin porous membrane is less than 0.01 μm, the clogging of the pores occurs due to organic synthesis components when the heat-resistant porous layer is laminated, and the air resistance and electrical resistance are deteriorated. When the average pore size of the polyolefin porous membrane is more than 1 μm, the clogging of the pores occurs due to the heat-resistant porous layer composition, and the air resistance and electrical resistance are deteriorated, or the safety of the battery may be reduced due to the occurrence of a micro-short circuit.

In addition, when the average pore size of the polyolefin porous membrane is 0.01 μm or more and 1.0 μm or less, the anchor effect of the composition can provide sufficient adhesion strength of the heat-resistant porous layer to the polyolefin porous membrane, and when the heat-resistant porous layer is laminated, the air resistance and electrical resistance are not significantly deteriorated, the response of the pore blocking phenomenon to temperature is not slowed, and the pore blocking temperature due to the heating rate is not shifted to a higher temperature side.
The measured value was obtained by the bubble point method defined in I.K 3832:1990.

The polyolefin resin that constitutes the polyolefin porous membrane is not particularly limited, but is preferably polyethylene or polypropylene.In addition, it may be a single material or a mixture of two or more different polyolefin resins, for example, a mixture of polyethylene and polypropylene, or a copolymer of different olefins.Because, in addition to the basic characteristics such as electrical insulation and ion permeability, it has the pore blocking effect that cuts off the current and suppresses excessive temperature rise when the battery temperature rises abnormally.

Among these, polyethylene is particularly preferred from the viewpoint of excellent pore-blocking performance. Hereinafter, the polyolefin resin used in the present invention will be described in detail using polyethylene as an example, but the embodiment of the present invention is not limited thereto.

Examples of polyethylene include ultra-high molecular weight polyethylene, high density polyethylene, medium density polyethylene, and low density polyethylene. There is no particular limitation on the polymerization catalyst, and examples include Ziegler-Natta catalysts, Phillips catalysts, and metallocene catalysts. These polyethylenes may be not only homopolymers of ethylene, but also copolymers containing small amounts of other α-olefins. Examples of α-olefins other than ethylene include propylene,
1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene,
(Meth)acrylic acid, esters of (meth)acrylic acid, styrene, and the like are preferred.

The polyethylene may be a single substance, but is preferably a mixture of two or more kinds of polyethylene. The polyethylene mixture may be a mixture of two or more kinds of ultra-high molecular weight polyethylenes having different weight average molecular weights (Mw), a mixture of similar high density polyethylenes, medium density polyethylenes, and low density polyethylenes, or a mixture of two or more kinds of polyethylenes selected from the group consisting of ultra-high molecular weight polyethylenes, high density polyethylenes, medium density polyethylenes, and low density polyethylenes.

The polyolefin porous membrane is required to have a function of closing the pores when the charge/discharge reaction is abnormal. Therefore, the melting point (softening point) of the resin constituting the membrane is preferably 70° C. or higher and 150° C. or lower. More preferably, it is 80° C. or higher and 140° C. or lower, and even more preferably, it is 100° C. or higher and 130° C. or lower.
When the melting point of the constituent resin is 70° C. or more and 150° C. or less, the pore blocking function does not appear during normal use, causing the battery to become unusable, and safety can be ensured by the pore blocking function appearing in the event of an abnormal reaction.

[Heat-resistant porous layer]
The heat-resistant porous layer of the present invention is provided on at least one side of the polyolefin porous membrane, and contains inorganic particles, water-soluble polymer and additives.When provided on only one side, the process of forming heat-resistant porous layer is reduced, and production cost can be reduced; when provided on both sides, the shrinkage caused by heat of polyolefin porous membrane can be suppressed from both sides, and the shrinkage caused by heat of battery separator can be more effectively reduced.

[Inorganic particles]
The inorganic particles of the present invention are not particularly limited in material as long as they are electrochemically stable. Specifically, sodium oxide, potassium oxide, magnesium oxide, calcium oxide, barium oxide, lanthanum oxide, cerium oxide, strontium oxide, vanadium oxide, SiO ₂
-MgO (magnesium silicate), SiO ₂ -CaO (calcium silicate), hydrotalcite, sodium carbonate, potassium carbonate, magnesium carbonate, calcium carbonate, barium carbonate, lanthanum carbonate, cerium carbonate, basic titanate, basic silicic titanate, basic copper acetate, basic lead sulfate, layered double hydroxide (Mg-Al type, Mg-Fe type, Ni
-Fe type, Li-Al type), layered double hydroxide-alumina silica gel complex, boehmite, alumina, zinc oxide, lead oxide, iron oxide, iron oxyhydroxide, hematite, bismuth oxide, tin oxide, titanium oxide, zirconium oxide and other anion adsorbents, zirconium phosphate, titanium phosphate, apatite, non-basic titanates, niobates, niobium titanates and other cation adsorbents, zeolite, calcium sulfate, magnesium sulfate, aluminum sulfate, gypsum, barium sulfate, alumina trihydrate (AT H), oxide-based ceramics such as fumed silica, precipitated silica, zirconia, and yttria, nitride-based ceramics such as silicon nitride, titanium nitride, and boron nitride, silicon carbide, kaolinite, talc, dickite, nacrite, halloysite, pyro-inorganic particles, layered silicates such as montmorillonite, sericite, amesite, and bentonite, asbestos, diatomaceous earth, glass fiber, synthetic layered silicates such as mica or fluoromica, and zinc borate. These may be used alone or in combination of two or more. Among these, barium sulfate is particularly preferred, and precipitated barium sulfate is more preferred. Specifically, barium sulfate particles are obtained by a method of obtaining barium sulfate by adding sulfuric acid to barium carbonate or barium sulfide (sulfuric acid method), and a method of obtaining barium sulfate by adding sodium sulfate to barium chloride (mirabilite method). By using barium sulfate particles produced by a synthetic method, the volume average particle size of the inorganic particles can be controlled with high precision.
The inorganic particles in the present invention have a BET specific surface area of 6.5 ^{m 2} /g or more and 30 m ² /g or less, preferably 10 m ² /g or more and 25 m ² /g or more, more preferably 15 m ² /g or more and 20 m ² /g or less. If the BET specific surface area is less than 6.5 ^{m 2} /g, the contact points between the inorganic particles in the heat-resistant porous layer are reduced, so that the structure of the heat-resistant porous layer becomes fragile, and it becomes difficult to suppress the shrinkage of the polyolefin porous film at high temperatures, or the heat-resistant porous layer may fall off and be mixed into the battery during the battery manufacturing process, resulting in a high defective rate of the battery. If the BET specific surface area is greater than 30 m ² /g, the gaps between the inorganic particles are reduced, so that the migration path of lithium ions in the battery may become narrow or long, resulting in a high electrical resistance. Furthermore, the inorganic particles may clog the pores of the polyolefin porous film, which may significantly reduce the performance of the battery. When the inorganic particles have a BET specific surface area of 6.5 m ² /g or more and 30 m ² /g or less, the heat-resistant porous layer can be prevented from falling off without impairing the heat shrinkage resistance of the heat-resistant porous layer.

[Water-soluble polymer]
The water-soluble polymer used in the present invention contains at least one functional group selected from the group consisting of amino group, amide group, carbonyl group, carboxyl group, sulfonyl group, phosphoric acid group, hydroxyl group, alkyl group and halogen group in the polymer skeleton. This can exert the effect of binding inorganic particles constituting the heat-resistant porous layer together and the effect of adhering the heat-resistant porous layer to the polyolefin porous membrane. The water-soluble polymer is not particularly limited, but specifically, one or more selected from the group consisting of (meth)acrylic acid copolymer resin, polyacrylamide resin, polyvinyl alcohol resin, polyamide resin, poly(meth)aramid resin, polyethylene glycol resin and cellulose ether resin can be used, among which cellulose ether resin, polyvinyl alcohol resin and polyacrylamide resin are preferred, and cellulose ether resin and polyacrylamide resin are more preferred. These water-soluble polymers may be used alone or in combination of two or more. The water-soluble polymer referred to here is one having a heat resistance of at least 150°C or more. For example, commercially available aqueous solutions can be used. Specific examples of acrylic resins include "Jurymer" (registered trademark) AT-210, ET-410, "Aron" (registered trademark) A-104, AS-2000, and NW-7060 manufactured by Toa Gosei Co., Ltd., "Polystron" (registered trademark) 117, 705, and 1280 manufactured by Arakawa Chemical Co., Ltd., and "Kogam" (registered trademark) series manufactured by Showa Denko K.K. Examples of polyvinyl alcohols include "Kuraray Poval" (registered trademark) 3-98 and 3-88 manufactured by Kuraray Co., Ltd., and "Gosenol" (registered trademark) N-300 and GH-20 manufactured by Mitsubishi Chemical Corporation. Examples of cellulose resins include "Sunrose" (registered trademark) MAC series manufactured by Nippon Paper Industries Co., Ltd.
[Other additives]
The heat-resistant porous layer may appropriately contain a dispersant for improving the dispersion stability of inorganic particles, a thickener and a wetting agent for improving coating properties, and a thermosetting agent and a crosslinking agent for improving heat resistance. Commercially available aqueous solutions or aqueous dispersions can be used. Specific examples of acrylic resins include the "Polysol" series manufactured by Showa Denko K.K. and Nippon Zeon Co., Ltd.
"BM" series manufactured by Toa Gosei Co., Ltd., "Jurimer" (registered trademark) AT-210 manufactured by Toa Gosei Co., Ltd.,
ET-410, "Aron" (registered trademark) A-104, AS-2000, NW-7060,
Examples of the polyvinyl alcohol include "LIOAMLUM" (registered trademark) series manufactured by Toyochem Co., Ltd., TRD202A and TRD102A manufactured by JSR Corporation, "Polystron" (registered trademark) 117, 705 and 1280 manufactured by Arakawa Chemical Co., Ltd., "Kogam" (registered trademark) series manufactured by Showa Denko K.K., and WEM-200U and WEM-3000 manufactured by Taisei Fine Chemical Co., Ltd. Examples of the polyvinyl alcohol include "Kuraray Poval" (registered trademark) 3-98 and 3-88 manufactured by Kuraray Co., Ltd., and "GOHSENOL" (registered trademark) N-300 and GH-20 manufactured by Mitsubishi Chemical Corporation.
[Weight composition ratio of heat-resistant porous layer]
The content of inorganic particles contained in the heat-resistant porous layer in the embodiment of the present invention is 92% by mass or more and 99% by mass or less, with the total composition forming the heat-resistant porous layer being 100% by mass.More preferably, it is 93% by mass or more and 98% by mass or less, and even more preferably, it is 94% by mass or more and 97% by mass or less.If the content of inorganic particles is less than 92% by mass, it may be difficult to suppress the shrinkage of polyolefin porous membrane at high temperature.If the content of inorganic particles is more than 99% by mass, the composition that binds inorganic particles together is small, so that the heat-resistant porous layer may fall off more.

[Average thickness of heat-resistant porous layer]
The average thickness of the heat-resistant porous layer in the embodiment of the present invention is preferably 0.5 μm or more and 4.0 μm or less. More preferably, it is 1.0 μm or more and 3.0 μm or less, and even more preferably, it is 1.5 μm or more and 2.5 μm or less. If the thickness of the heat-resistant porous layer is smaller than 0.5 μm, it may not be possible to suppress the shrinkage of the polyolefin porous film due to heat. If the average thickness of the heat-resistant porous layer is larger than 4.0 μm, the distance between the positive and negative electrodes of the battery cell increases, so that the electrical resistance increases, or the proportion of the battery separator in the volume of the battery increases, and the volumetric energy density of the battery may decrease. If the average thickness of the heat-resistant porous layer is 0.5 μm or more and 4.0 μm or less, the heat-resistant porous layer can suppress the thermal shrinkage of the polyolefin porous film, and the electrical resistance does not increase. Furthermore, the greater the ratio of the thickness of the heat-resistant porous layer to the thickness of the battery separator, the greater the effect of the heat-resistant porous layer in suppressing the thermal shrinkage of the polyolefin porous membrane.

[Method of forming heat-resistant porous layer]
The heat-resistant porous layer of the present invention can be obtained by the following steps.
(a) Preparation of a coating liquid for a heat-resistant porous layer using inorganic particles, a water-soluble polymer, an additive, and a dispersion medium.
(b) A step of applying the coating liquid to at least one surface or both surfaces of a polyolefin porous membrane.
(c) After the coating, the dispersion medium is dried with a dryer to form a heat-resistant porous layer.

In the step (a), it is preferable to use water as the dispersion medium. As long as the dispersion stability of the coating liquid for the heat-resistant porous layer is not impaired, a mixture of water and a hydrophilic solvent such as ethanol, isopropanol, or N-methylpyrrolidone may be used as the dispersion medium. The method for dispersing the coating liquid for the heat-resistant porous layer can be a known method. For example, a ball mill, a bead mill, a planetary ball mill, a vibrating ball mill, a sand mill, a colloid mill, a roll mill, high-speed impeller dispersion, a disperser, a homogenizer, a planetary mixer and a planetary kneader, ultrasonic dispersion, mechanical stirring with a stirring blade, etc. may be used. If the dispersion is insufficient, the effect of suppressing the thermal shrinkage of the polyolefin porous film by the heat-resistant porous layer may be reduced.

In the step (b), the method of applying the heat-resistant porous layer coating liquid to at least one side or both sides of the polyolefin porous membrane can be a known method.For example, direct gravure coating method, reverse gravure coating method, kiss reverse gravure coating method, direct bar coating method, roll brush method, air knife coating method, Mayer bar coating method, pipe doctor method, blade coating method and die coating method can be included, and these methods can be performed alone or in combination.

The battery separator according to the embodiment of the present invention can be used as a battery separator for secondary batteries such as nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc batteries, lithium ion secondary batteries, lithium polymer secondary batteries, and lithium-sulfur batteries, etc. In particular, it is preferable to use it as a separator for lithium ion secondary batteries.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. The measured values in the examples were obtained by the following methods.
In the following, Examples 1 and 5 shall be read as Reference Examples 1 and 5.

1. BET specific surface area (m ² /g)
The measurement was performed using a specific surface area measuring device (Mountec Co., Ltd., fully automatic specific surface area measuring device “Macsorb” (registered trademark) HM Model-1220) in accordance with JIS Z 8830 (2013).

2. Inorganic particle size (μm)
The particle size of the inorganic particles was measured using a laser diffraction particle size distribution measuring device (LA-960V2, manufactured by Horiba, Ltd.) in accordance with JIS Z 8825 (2013) to measure the particle size D50 (μm) at a volume-based cumulative ratio of 50%.
3. Thickness (μm)
The polyolefin porous membrane and the battery separator were measured using a contact type thickness gauge (manufactured by Mitutoyo Corporation, "Litematic" (registered trademark)) under the conditions of a superhard spherical probe φ10.5 mm and a load of 0.15 N, and the thickness was obtained by averaging the measured values at five points. Furthermore, the thickness of the heat-resistant porous layer was obtained by washing the battery separator with the same dispersion medium as the dispersion medium contained in the coating liquid for the heat-resistant porous layer, removing the heat-resistant porous layer, measuring the polyolefin porous membrane with the contact type thickness gauge, and calculating according to the following formula.

Thickness of heat-resistant porous layer = thickness of battery separator - thickness of polyolefin porous film 4. Air permeability resistance (sec/100 ml Air)
According to JIS P 8117, the air resistance was calculated by averaging the measured values of five points using an Oken air resistance meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T). The increase in air resistance due to the heat-resistant porous layer was calculated by washing the battery separator with the same dispersion medium as that contained in the coating liquid for the heat-resistant porous layer, removing the heat-resistant porous layer, measuring the polyolefin porous membrane with the Oken air resistance meter, and calculating the air resistance using the following formula.

Increase in air permeability resistance = air permeability resistance of battery separator - air permeability resistance of polyolefin porous membrane 5. Heat shrinkage rate (%)
The heat shrinkage rates of the battery separator in the MD direction (length direction) and TD direction (width direction) were measured by the following method.
(1) The battery separator is cut into three pieces measuring 100 mm in the MD direction x 100 mm in the TD direction, and using a transparent glass scale (measurement accuracy 0.1 mm), the distance between the midpoints of opposite sides of the battery separator is measured as the length in the MD direction and the length in the TD direction, and these are defined as the initial dimensions (mm).
(2) The battery separator was sandwiched between two sheets of A3 size paper and placed in an oven at 150° C. for 1 hour, after which the battery separator was removed from the oven and left at room temperature for 30 minutes.
(3) Using the glass scale, the distance between the midpoints of the opposite sides of the battery separator was measured again, and this was taken as the dimension after shrinkage (mm). The measurement position was the same as the position where the initial dimension was measured, and if the end of the battery separator was curled, it was spread out and the measurement was performed. From the obtained initial dimension and the dimension after shrinkage, the thermal shrinkage rate (%) in the MD direction and the TD direction was calculated using the following formula.

Heat shrinkage rate (%) = {initial dimension (mm) - dimension after shrinkage (mm)} / initial dimension (mm) x 100
6. Amount of heat-resistant porous layer that fell off (mg)
The amount of the heat-resistant porous layer that had fallen off from the battery separator was measured using a friction tester (manufactured by Trinity Lab Co., Ltd., a static and dynamic friction tester TL201Tt) according to the following method.
(1) The battery separator was cut into a size of 49 mm in the TD direction × 110 mm in the MD direction, and the weight (mg) was measured using an electronic balance (measurement accuracy 0.01 mg).
(2) A 50 mm x 50 mm flat contactor having a single-sided adhesive cushion tape (Edge Cushion Tape TEC-50BK, manufactured by TRUSCO NAKAYAMA CORPORATION) cut to a size of 50 mm x 50 mm attached to its contact surface is fixed to the battery separator so that the cushion tape is in contact with the polyolefin porous membrane surface (in the case of a battery separator having a heat-resistant porous layer on one surface) or the heat-resistant porous layer surface not to be measured (in the case of a battery separator having heat-resistant porous layers on both surfaces).
(3) A piece of polishing film #6000 (film abrasive A3-2SHT, manufactured by 3M ^TM ) cut to a size of 90 mm x 220 mm is fixed horizontally to the sliding portion of the static and dynamic friction measuring device.
(4) The polishing film is brought into contact with the heat-resistant porous layer of the battery separator fixed to the contact, and a weight of 1 kg is placed on the contact. At this time, the contact area between the polishing film and the heat-resistant porous layer is 49 mm x 50 mm.
(5) After the sliding part of the static and dynamic friction tester was slid back and forth five times in the horizontal direction at a speed of 10 mm/sec and a distance of 25 mm, the battery separator was removed from the contact and the weight (mg) was measured using an electronic balance (measurement accuracy 0.01 mg).
(6) From the weights (mg) of the battery separator before and after the test, the amount (mg) of the heat-resistant porous layer that fell off was calculated using the following formula.

Amount of fallen heat-resistant porous layer (mg)=Weight of battery separator before test (mg)−Weight of battery separator after test (mg)
7. 180° peel strength of heat-resistant porous layer (N/m)
The 180° peel strength of the heat-resistant porous layer was measured by the following method using a table-top precision universal testing machine (Autograph AGS-X, manufactured by Shimadzu Corporation).
(1) A battery separator is cut into a piece measuring 25 mm in the TD direction x 100 mm in the MD direction.
(2) A double-sided film tape cut to a size of 20 mm x 90 mm is applied to the polyolefin porous membrane surface of the battery separator (in the case of a battery separator having a heat-resistant porous layer on one side) or the heat-resistant porous layer surface not to be measured (in the case of a battery separator having a heat-resistant porous layer on both sides), and is pressed back and forth once using a rubber roller (manufactured by Tester Sangyo Co., Ltd., SA-1003-B tape pressing roll manual type, roll weight 2 kg, rubber hardness 80 ± 5 Hs). (3) After peeling off the release liner of the double-sided film tape, the battery separator is fixed to a metal plate of 50 mm x 150 mm.
(4) A 15 mm wide single-sided adhesive tape (Nichiban Co., Ltd., "Cellotape" (registered trademark) No. 405) was applied to the heat-resistant porous layer surface of the battery separator fixed to the metal plate, and the tape was pressed back and forth five times using the rubber roller.
(5) The metal plate to which the battery separator was fixed was fixed vertically to the bench-top precision universal testing machine, and within 5 minutes after the single-sided adhesive tape was applied, the single-sided adhesive tape was peeled off in a 180° direction at a speed of 100 mm/sec, and the average strength was determined from a peel distance of 40 mm to 100 mm.
(6) From the obtained peel strength and the width of the single-sided adhesive tape, the 180° peel strength (N/m) of the heat-resistant porous layer was calculated using the following formula.

180° peel strength of heat-resistant porous layer (N/m)=obtained peel strength (N)/width of single-sided adhesive tape (mm)
8. Battery defect rate [Positive electrode production]
An NMP solution containing 1.2 parts by mass of PVDF as a binder was added to 97 parts by mass of lithium cobalt oxide as an active material and 1.8 parts by mass of carbon black to prepare a positive electrode mixture-containing slurry. The positive electrode mixture-containing slurry was uniformly applied to both sides of a positive electrode current collector made of aluminum foil with a thickness of 20 μm and dried to form a positive electrode layer, which was then compression-molded by a roll press machine to set the density of the positive electrode layer excluding the current collector to 3.6 g/cm ³ to prepare a positive electrode.

[Preparation of negative electrode]
An aqueous solution containing 1.0 part by mass of sodium carboxymethylcellulose was added to and mixed with 98 parts by mass of artificial graphite as an active material, and then styrene butadiene latex containing 1.0 part by mass as a solid content as a binder was added and mixed to prepare a negative electrode mixture-containing slurry. This negative electrode mixture-containing slurry was uniformly applied to both sides of a negative electrode current collector made of copper foil with a thickness of 10 μm and dried to form a negative electrode layer, which was then compression-molded by a roll press machine to set the density of the negative electrode layer excluding the current collector to 1.45 g/cm ³ , thereby preparing a negative electrode.
[Preparation of test batteries]
The positive and negative electrodes were tabbed and each microporous membrane was used to prepare a wound body. Next, the wound body was placed in an aluminum laminate bag, and an electrolyte (1.1 mol/L, LiPF ₆ , ethylene carbonate/ethyl methyl carbonate/diethylene carbonate=3/5/2 (volume ratio) to which 0.5 wt% vinylene carbonate and 2 wt% fluoroethylene carbonate were added) was poured, and the bag was sealed with a vacuum sealer. Next, the battery was charged to 10% of its total capacity at a temperature of 35° C. and 0.2 C (C represents the current value at which the battery can be fully charged in 1 hour, and in the case of this battery, it is set to 300 mA), and after storing for 12 hours, one side of the laminate was opened to release gas, and immediately sealed with a vacuum sealer. Next, the battery was charged at a constant current and constant voltage of 0.1 C, 4.35 V, and a cutoff current of 0.05 C at a temperature of 35° C., and then discharged at a constant current of 0.1 C to 3.0 V. This series of charge and discharge was the initial charge and discharge. This was used as a 300 mAh class test battery.

[Battery defect rate (%)]
100 test batteries were produced, and the number of batteries having an initial charge/discharge efficiency of less than 95%, calculated according to the following formula, was taken as the defective rate.

Initial charge/discharge efficiency (%) = initial discharge capacity / initial charge capacity × 100
9. Nail Penetration Test of Battery A nail penetration test was carried out using five of the test batteries in accordance with SAE J2464, and the battery was evaluated according to the following criteria.
No ignition: None of the five pieces ignited. Ignition: All five pieces ignited. (Example 1)
For 100 parts by weight of precipitated barium sulfate (BET specific surface area 6.5 ^m2 /g, particle size D50 = 0.40 μm), 1.0 part by weight (active ingredient) of a polyacrylic acid-based dispersant (Aron (registered trademark) A-6114, manufactured by Toa Gosei Co., Ltd.) was prepared and added to water. Next, the entire amount of the precipitated barium sulfate was added while stirring at 700 rpm with a mixer equipped with a disperser-type blade (Three-One Motor, manufactured by Toki Sangyo Co., Ltd.), and the mixture was stirred at 1000 rpm for 60 minutes to obtain a mixed solution with a solid content of 60% by weight.

The resulting mixture was milled three times using a bead mill disperser (Asada Iron Works, Picomil PCM-LR) and zirconia beads with a bead diameter of 0.5 mm (Toray Industries, Toray Ceram φ0.5 mm) at a bead filling rate of 65% by volume, a peripheral speed of 10 m/sec, and a flow rate of 16 kg/h to obtain a master batch liquid.

While stirring the obtained master batch liquid at 700 rpm with the stirrer, polyacrylamide (Polystron, manufactured by Arakawa Chemical Industries, Ltd.) as a water-soluble polymer, 2.0 parts by weight (active ingredient) of acrylic resin (Polysol AP-4735, manufactured by Showa Denko K.K.) as an additive, 0.2 parts by weight of a wetting agent (SN Wet 366, manufactured by San Nopco Ltd.), and water were added. Thereafter, the mixture was stirred at 700 rpm for 10 minutes to obtain a coating liquid with a solid content of 55% by weight.
The obtained slurry was applied to one side of a polyolefin porous membrane having a thickness of 12 μm by a kiss reverse gravure method and dried at a temperature of 60° C. to obtain a battery separator having a heat-resistant porous layer having a thickness of 3 μm.

The results of evaluating the obtained battery separator in terms of the increase in air permeability resistance, the thermal shrinkage rate at 150°C/1h, the amount of heat-resistant porous layer that fell off, the 180° peel strength of the heat-resistant porous layer, the battery defect rate, and the nail penetration test of the battery are shown in Table 1.

Example 2
A battery separator was obtained in the same manner as in Example 1, except that the precipitated barium sulfate in Example 1 was replaced with precipitated barium sulfate (BET specific surface area: 11.1 ^m2 /g, particle size D50: 0.24 μm), dispersion was performed seven times using a bead mill disperser, and the amount of polyacrylamide was changed to 3.0 parts by weight.

Example 3
A battery separator was obtained in the same manner as in Example 2, except that the thickness of the polyolefin porous membrane in Example 2 was changed to 20 μm.

Example 4
A battery separator was obtained in the same manner as in Example 2, except that the blending amount of polyacrylamide in Example 2 was changed to 1.5 parts by weight, and the blending amount of acrylic resin was changed to 5.0 parts by weight.
Example 5
A battery separator was obtained in the same manner as in Example 1, except that the precipitated barium sulfate in Example 1 was changed to precipitated barium sulfate (BET specific surface area 17.9 ^m2 /g, particle size D50 = 0.15 μm), dispersion was performed 10 times using a bead mill disperser, and the thickness of the heat-resistant porous layer was changed to 2.5 μm.
Example 6
A battery separator was obtained in the same manner as in Example 2, except that the polyacrylamide in Example 2 was changed to carboxymethyl cellulose (SG-L02, manufactured by GL CHEM) and the solid content of the coating liquid was changed to 40% by weight.

(Comparative Example 1)
A battery separator was obtained in the same manner as in Example 1, except that the precipitated barium sulfate in Example 1 was changed to precipitated barium sulfate (BET specific surface area: 5.5 m ² /g, particle size D50: 0.50 μm).

(Comparative Example 2)
A battery separator was obtained in the same manner as in Example 2, except that the precipitated barium sulfate in Example 2 was changed to precipitated barium sulfate (BET specific surface area: 31.0 ^m2 /g, particle size D50: 0.09 μm), dispersion was performed 10 times using a bead mill disperser, and the amount of acrylic resin was changed to 4.0 parts by weight.

(Comparative Example 3)
A battery separator was obtained in the same manner as in Example 4, except that the blending amount of the acrylic resin in Example 4 was changed to 8.0 parts by weight.

(Comparative Example 4)
A battery separator was obtained in the same manner as in Example 1, except that the precipitated barium sulfate in Example 1 was changed to elutriated barium sulfate (BET specific surface area 9.1 m ² /g, particle size D50=0.40 μm).

(Comparative Example 5)
A battery separator was obtained in the same manner as in Example 1, except that the precipitated barium sulfate in Example 1 was replaced with alumina (BET specific surface area 6.5 m ² /g, particle size D50=0.50 μm) and the dispersion using the bead mill disperser was performed 12 times.

The separator of the present invention can be suitably used as a battery separator preferably used in non-aqueous electrolyte batteries such as lithium ion batteries.

Claims (3)

A battery separator having a heat-resistant porous layer on at least one side of a polyolefin microporous membrane, the battery separator having a 150°C/1h heat shrinkage rate of 5% or less and a shedding amount of the heat-resistant porous layer of 0.6 mg or less, the content of inorganic particles contained in the heat-resistant porous layer being 92 mass% or more and 94.2 mass% or less, with the total composition forming the heat-resistant porous layer being 100 mass%, and the heat-resistant porous layer containing precipitated barium sulfate particles having a BET specific surface area of 11.1 ^m2 /g or more and 30 ^m2 /g or less. 2. The battery separator according to claim 1 , wherein the heat-resistant porous layer has a 180° peel strength of 350 N/m or more. 3. The battery separator according to claim 1 , wherein the heat-resistant porous layer has a thickness of 0.5 μm or more and 4.0 μm or less.