JP7344644B2 - Multilayer porous membrane with polyolefin microporous membrane - Google Patents

Description

TECHNICAL FIELD The present invention relates to a multilayer porous membrane including a polyolefin microporous membrane.

Non-aqueous secondary batteries, represented by lithium-ion secondary batteries (LIBs), have traditionally been widely used as small power sources for portable electronic devices, and in recent years, there have been increasing expectations for their application to large-scale power storage industries for industrial and automotive use. It's increasing. Inside this type of non-aqueous secondary battery, a separator is arranged between the positive and negative electrodes. The separator has the function of preventing direct contact between the positive electrode and the negative electrode and allowing ions to pass through the nonaqueous electrolyte held in the micropores. In particular, microporous polyolefin membranes tend to have excellent ion permeability and are therefore widely used as constituent materials for LIB separators.

For example, Patent Document 1 describes a porous structure formed using a composition containing a water-soluble polymer, water, and barium sulfate having a BET specific surface area of 3.0 m ² /g or more and 6.3 m ² /g or less. A separator for LIB is described in which the layers are laminated to a porous polyolefin substrate. Further, Patent Document 2 describes a separator for LIB in which a heat-resistant layer containing barium sulfate having a Mohs hardness of 6 or less and a resin binder made of polyvinyl alcohol (PVA) is laminated on a laminated porous film made of polyolefin. has been done.

Patent No. 6337512 Patent No. 5697328

Against the backdrop of growing expectations for the application of non-aqueous secondary batteries to the large-scale energy storage industry, separators must not only ensure good ion permeability but also ensure further safety (for example, heat shrinkage at high temperatures). reduction in the rate of increase) is desperately needed.
However, it has long been recognized that there is a trade-off between ensuring good ion permeability and reducing thermal shrinkage, and that it is difficult to achieve both. Patent Documents 1 and 2 also do not specifically consider the aspect of achieving both, and there is room for improvement over the prior art including Patent Documents 1 and 2. Note that such problems are not limited to separators of non-aqueous secondary batteries applied to the large-scale power storage industry, but also exist in separators of non-aqueous secondary batteries used as small power sources.

Therefore, the problem to be solved by the present invention is to provide a multilayer porous membrane that can both ensure good ion permeability and reduce thermal shrinkage, and a separator equipped with such a multilayer porous membrane. It is about providing.

As a result of intensive studies, the present inventors have completed the present invention by discovering that the above problems can be solved by meeting the following requirements. That is, the present invention is as follows.
[1]
A microporous polyolefin membrane, a porous layer containing an ionic inorganic filler and a binder, and provided on at least one side of the microporous polyolefin membrane,
The ionic inorganic filler has a median diameter (D50) of 0.05 to 0.40 μm,
A multilayer porous membrane, wherein the amount of the binder relative to the ionic inorganic filler is 0.1 to 4.0% by mass in terms of solid content.
[2]
The multilayer porous membrane according to [1], wherein the ionic inorganic filler has a BET specific surface area of 7 to 100 m ² /g.
[3]
The multilayer porous membrane according to [1] or [2], wherein the ionic inorganic filler contains barium sulfate.
[4]
The multilayer porous membrane according to any one of [1] to [3], wherein the ratio of the pore diameter of the polyolefin microporous membrane to the volume average particle diameter of the binder is 0.01 to 1.0.
[5]
The multilayer porous film according to any one of [1] to [4], wherein the binder has a volume average particle diameter of 0.05 to 0.3 μm.
[6]
The multilayer porous membrane according to any one of [1] to [5], wherein the ratio of the volume average particle size of the binder to the median diameter (D50) of the ionic inorganic filler is 0.10 to 1.0.
[7]
The multilayer porous membrane according to any one of [1] to [6], wherein the binder contains an acrylic acid ester monomer.
[8]
The multilayer porous film according to any one of [1] to [7], wherein the porous layer has a peel strength of 200 to 1000 g/cm.
[9]
A separator comprising the multilayer porous membrane according to any one of [1] to [8].
[10]
A non-aqueous secondary battery comprising a positive electrode, the multilayer porous membrane according to any one of [1] to [8], and a negative electrode.

According to the multilayer porous membrane according to the present invention, since the median diameter D50 is controlled within a relatively small specific range, a relatively dense porous layer can be realized, and furthermore, even though the amount of binder is kept small, it is possible to Since a strong binding force can be ensured, the thermal shrinkage rate can be reduced. Moreover, since the amount of binder can be kept to a small amount, good ion permeability can be ensured. Therefore, according to the present invention, it is possible to provide a multilayer porous membrane that can both ensure good ion permeability and reduce thermal shrinkage, and a separator equipped with such a multilayer porous membrane. .

Hereinafter, a mode for carrying out the present invention (hereinafter referred to as "this embodiment") will be described. Since the following embodiment is one aspect of the present invention, the present invention is not limited only to the following embodiment. The following embodiments can be modified and implemented as appropriate within the scope of the gist of the present invention.
"(Meth)acrylic" in this specification means "acrylic" and "methacrylic" corresponding thereto. Further, in this specification, "~" means that unless otherwise specified, the numerical values at both ends thereof are included as an upper limit value and a lower limit value.

<Multilayer porous membrane>
The multilayer porous membrane according to this embodiment (hereinafter also simply referred to as "multilayer porous membrane") includes a microporous polyolefin membrane, and a porous layer containing an ionic inorganic filler and a binder. The median diameter D50 of the ionic inorganic filler is 0.05 to 0.40 μm, and the amount of binder to the ionic inorganic filler is 0.1 to 4.0 parts by mass in terms of solid content.
The porous layer is provided on at least one side of the microporous polyolefin membrane. Therefore, both an embodiment in which a porous layer is provided on only one side of a microporous polyolefin membrane and an embodiment in which a porous layer is provided on both sides of a microporous polyolefin membrane are included in the present invention. In addition, "single side" in this specification means the surface facing the positive electrode or the negative electrode when incorporated into a non-aqueous secondary battery.

It has long been recognized that there is a trade-off between ensuring good ion permeability and reducing thermal shrinkage, and that it is difficult to achieve both. That is, in a separator comprising a base material and a porous layer, in order to reduce the thermal shrinkage rate at high temperatures, the particle size of the inorganic filler (alumina, boehmite, etc.) contained in the porous layer is reduced, etc. It is assumed that consideration has been given to increasing the density and increasing the number of binding points between fillers. However, in that case, under the conventional technology, the amount of binder contained in the porous layer has to be increased, which tends to reduce the ion permeability of the separator.
On the other hand, according to the multilayer porous membrane according to the present embodiment, it is possible to ensure both good ion permeability and a reduction in thermal shrinkage rate.

The effects of the multilayer porous membrane can be estimated as follows.
That is, while inorganic fillers (such as alumina or boehmite) that do not have ionic binding properties have their particle surfaces covered with hydroxyl groups, the ionic inorganic filler according to this embodiment has components on its particle surface covered with hydroxyl groups. It has ionic bonding properties. Therefore, by using an ionic inorganic filler, at least a portion of the ions on the particle surface are actively ionized in the solvent, thereby making it easier to generate electric charge on the particle surface. In other words, by using an ionic inorganic filler, the Coulomb force between charges can be made to act more strongly in binding within the porous layer and/or binding between the microporous polyolefin membrane and the porous layer. . Therefore, the binding force per unit binding area can be improved.

As described above, according to the multilayer porous film according to the present embodiment, the particle size of the filler is reduced to increase the number of bonding points between the fillers, and while the amount of binder is kept to a small amount, sufficient bonding between the fillers can be achieved. It is possible to secure adhesion and reduce the heat shrinkage rate. According to such a multilayer porous membrane, it is expected that safety in a nail penetration test for a nonaqueous secondary battery using a separator provided with the multilayer porous membrane will be improved. The above-mentioned effect of being able to secure sufficient binding force while keeping the amount of binder to a small amount is expected to be remarkable when an ionic inorganic filler containing barium sulfate is used.

Moreover, according to the multilayer porous membrane according to the present embodiment, good ion permeability can be ensured since the amount of binder can be kept to a small amount. According to such a multilayer porous membrane, since good ion permeability can be ensured, it is expected that the battery characteristics (rate characteristics) of a non-aqueous secondary battery using a separator provided with the multilayer porous membrane will be improved.

In addition, since the ionic inorganic filler according to the present embodiment has its median diameter D50 controlled within a relatively small specific range, a relatively dense porous layer can be realized, and the bonding points between the fillers can be can be secured in relatively large quantities. These also contribute to reducing the heat shrinkage rate. Since the median diameter D50 is controlled within a relatively small specific range, its BET specific surface area tends to be relatively large. It is also possible to suppress the increase in quantity.
Although the effects of the multilayer porous membrane have been speculated above, the present invention is not limited to the theory conjectured as described above.

The thickness of the multilayer porous membrane is not particularly limited, but is preferably 3.0 μm or more, more preferably 10 μm or more. On the other hand, the thickness of the multilayer porous membrane is preferably 50.0 μm or less, more preferably 30.0 μm or less. It is preferable that the thickness of the multilayer porous membrane is 5.0 μm or more from the viewpoint of the withstand voltage or mechanical strength of the separator. On the other hand, it is preferable from the viewpoint of battery capacity that the thickness of the multilayer porous membrane is 50.0 μm or less. The thickness of the multilayer porous membrane can be controlled by changing the thicknesses of the porous layer and the microporous polyolefin membrane, which will be described later.

The air permeability of the multilayer porous membrane is not particularly limited, but is preferably 500 seconds/100 cm ³ or less, more preferably 300 seconds/100 cm ³ or less, and still more preferably 200 seconds/100 cm ³ or less. It is preferable that the air permeability is 500 seconds/100 cm ³ or less from the viewpoint of obtaining good charge/discharge characteristics. Air permeability is air permeability resistance measured in accordance with JIS P-8117.

The heat shrinkage rate at 150° C. of the multilayer porous membrane is preferably less than 10.0%, more preferably 5.0% or less, and even more preferably 4.0% from the viewpoint of improving safety during nail penetration. % or less. Here, as the thermal contraction rate of the separator, the larger value of both the MD thermal contraction rate and the TD thermal contraction rate is used. The method for measuring the heat shrinkage rate will be described later in the section of Examples. The thermal contraction rate of the separator can be adjusted by appropriately combining the maximum thermal contraction stress of the microporous membrane and the configuration of the porous layer, which will be described later.

The puncture strength of the multilayer porous membrane is not particularly limited, but is preferably 200 gf or more, more preferably 300 gf or more, and still more preferably 350 gf or more. The puncture strength of 200 gf or more is from the viewpoint of suppressing membrane rupture due to fallen active material etc. when the separator is wound together with the electrode, and from the viewpoint of suppressing the fear of short circuit due to expansion and contraction of the electrode due to charging and discharging. It is also preferable. Puncture strength is measured according to the method described in Examples. The puncture strength can be controlled by changing the stretching ratio and/or stretching temperature of the microporous polyolefin membrane.

The multilayer porous membrane itself can be used as a separator. In addition, other layers (for example, a thermoplastic polymer-containing layer that functions as an adhesive layer) can be laminated on the multilayer porous membrane within a range that does not impede the effects of the present invention. Multilayer porous membranes can also be used as separators. Each member that can constitute the multilayer porous membrane will be described below.

[Polyolefin microporous membrane (base material)]
The microporous polyolefin membrane itself may be one that has been conventionally used as a separator. As the polyolefin microporous membrane, a porous membrane with a fine pore size is preferable, and in addition, one having no electronic conductivity, ionic conductivity, and high resistance to non-aqueous electrolytes (organic solvents) is more preferable.
The polyolefin microporous membrane contains a polyolefin resin. The content of the polyolefin resin is preferably 75% by mass or more, more preferably 85% by mass or more, still more preferably 90% by mass or more, still more preferably 95% by mass or more, based on the total mass of the polyolefin microporous membrane. , particularly preferably 98% by mass or more, and may be 100% by mass.

The polyolefin resin is not particularly limited, but may be, for example, a polyolefin resin that can be used in conventional extrusion, injection, inflation, blow molding, and the like. Examples of polyolefin resins include homopolymers containing ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene as monomers, and two or more of these monomers. copolymers, and multistage polymers. These homopolymers, copolymers, and multistage polymers may be used alone or in combination of two or more.

Typical examples of polyolefin resins include, but are not limited to, polyethylene, polypropylene, and polybutene, and more specifically, low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ultra high density polyethylene, etc. Includes molecular weight polyethylene, isotactic polypropylene, atactic polypropylene, ethylene-propylene random copolymer, polybutene, and ethylene propylene rubber. These may be used alone or in combination of two or more.

Among these, polyethylenes such as low-density polyethylene, linear low-density polyethylene, medium-density polyethylene, high-density polyethylene, and ultra-high molecular weight polyethylene are preferred as the polyolefin resin. In particular, high-density polyethylene is preferred because it has a low melting point and high strength, and polyethylene whose density measured according to JIS K 7112 is 0.93 g/cm ³ or more is more preferred. The polymerization catalyst used in producing these polyethylenes is not particularly limited, and examples thereof include Ziegler-Natta catalysts, Phillips catalysts, and metallocene catalysts.
If an abnormal situation such as a short circuit or overcharging occurs in a non-aqueous secondary battery, it may lead to a fire in the worst case. In order to prevent such a situation, non-aqueous secondary batteries are equipped with various safety functions. One of the safety functions is a shutdown function. The shutdown function is a function that suppresses ion conduction in the nonaqueous electrolyte by clogging the micropores in the separator due to thermal melting when the battery generates abnormal heat, stopping the progress of electrochemical reactions. . Generally, the lower the shutdown temperature, the higher the safety. Polyethylene is preferably used as a component of the base material because it has a suitable shutdown temperature.

Moreover, in order to improve the heat resistance of the separator, it is also preferable that the polyolefin microporous membrane contains polypropylene and a polyolefin resin other than polypropylene. The steric structure of polypropylene is not particularly limited, and examples thereof include isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene. Examples of polyolefin resins other than polypropylene are as described above. The polymerization catalyst used in the production of polypropylene is not particularly limited, and examples thereof include Ziegler-Natta catalysts and metallocene catalysts.

The content ratio of polypropylene (polypropylene/polyolefin) to the total mass of polyolefin in the microporous polyolefin membrane is not particularly limited, but from the viewpoint of achieving both heat resistance and good shutdown function, it is preferably 1 to 35% by mass, More preferably 3 to 20% by weight, still more preferably 4 to 10% by weight. From the same point of view, the content ratio of olefin resin other than polypropylene, for example polyethylene (olefin resin other than polypropylene/polyolefin) to the total mass of polyolefin in the microporous polyolefin membrane is preferably 65 to 99% by mass, more preferably 80% by mass. ~97% by weight, more preferably 90~96% by weight.

The viscosity average molecular weight of the polyolefin resin constituting the polyolefin microporous membrane is not particularly limited, but is preferably 30,000 or more and 12 million or less, more preferably 50,000 or more and less than 2 million, and even more preferably 100,000 or more and less than 1 million. be. It is preferable that the viscosity average molecular weight is 30,000 or more because the melt tension during melt molding increases, resulting in better moldability, and the entanglement of polymers tends to further increase strength. On the other hand, when the viscosity average molecular weight is 12 million or less, uniform melt-kneading becomes easy and sheet formability, particularly thickness stability, tends to be excellent, which is preferable. Furthermore, it is preferable that the viscosity average molecular weight is less than 1 million, because the pores tend to be easily blocked when the temperature rises, and a better shutdown function tends to be obtained. The viscosity average molecular weight (Mv) is calculated by the following formula from the intrinsic viscosity [η] measured at a measurement temperature of 135° C. using decalin as a solvent, based on ASTM-D4020.
Polyethylene: [η] = 6.77×10 ^-4 Mv ^0.67 (Chiang's formula)
Polypropylene: [η] = 1.10×10 ^-4 Mv ^0.80
For example, instead of using a polyolefin with a viscosity average molecular weight of less than 1 million, a mixture of a polyolefin with a viscosity average molecular weight of 2 million and a polyolefin with a viscosity average molecular weight of 270,000, the viscosity average molecular weight of which is less than 1 million. Mixtures may also be used.

The polyolefin microporous membrane can contain arbitrary additives. Examples of additives include, but are not limited to, polymers other than polyolefins; inorganic fillers (inorganic particles); phenol-based, phosphorus-based, and sulfur-based antioxidants; metal soaps such as calcium stearate and zinc stearate. UV absorbers; light stabilizers; antistatic agents; antifogging agents; and coloring pigments. The total content of these additives is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, still more preferably 5 parts by mass or less, based on 100 parts by mass of the polyolefin resin in the microporous polyolefin membrane. Of course, the total content of the additives may be 0 parts by mass based on 100 parts by mass of the polyolefin resin in the microporous polyolefin membrane.

The inorganic filler is not particularly limited, but includes, for example, those that have a melting point of 200° C. or higher, have high electrical insulation properties, and are electrochemically stable within the range of use of non-aqueous secondary batteries. Therefore, the inorganic filler as the above-mentioned additive that can be contained in the polyolefin microporous membrane may be an inorganic filler that corresponds to the ionic inorganic filler according to the present embodiment, or an inorganic filler that does not correspond to the ionic inorganic filler. .
Note that specific examples of the ionic inorganic filler according to this embodiment and specific examples of inorganic fillers that do not correspond to the ionic inorganic filler according to this embodiment will be described later.

The porosity of the polyolefin microporous membrane is not particularly limited, but is preferably 20% or more, more preferably 35% or more, and still more preferably 40% or more. On the other hand, the porosity thereof is preferably 90% or less, more preferably 80% or less. It is preferable to set the porosity to 20% or more from the viewpoint of ensuring ion permeability more effectively and reliably. On the other hand, it is preferable to set the porosity to 90% or less from the viewpoint of ensuring puncture strength more effectively and reliably. The porosity can be calculated, for example, from the volume (cm ³ ), mass (g), and membrane density (g/cm ³ ) of a sample of a microporous polyolefin membrane using the following formula:
Porosity = (volume - mass / film density) / volume x 100
It can be found by Here, for example, in the case of a microporous polyolefin membrane made of polyethylene, calculations can be made assuming that the membrane density is 0.95 (g/cm ³ ). The porosity can be controlled by changing the stretching ratio of the microporous polyolefin membrane.

The air permeability of the polyolefin microporous membrane is not particularly limited, but is preferably 10 seconds/100 cm ³ or more, more preferably 50 seconds/100 cm ³ or more. On the other hand, its air permeability is preferably 500 seconds/100 cm ³ or less, more preferably 300 seconds/100 cm ³ or less. It is preferable that the air permeability is 10 seconds/100 cm ³ or more from the viewpoint of suppressing self-discharge of the non-aqueous secondary battery. On the other hand, it is preferable to set the air permeability to 500 seconds/100 cm ³ or less from the viewpoint of obtaining good charge/discharge characteristics. The air permeability can be controlled by changing the stretching temperature and/or stretching ratio of the polyolefin microporous membrane.

The pore diameter (average pore diameter) of the polyolefin microporous membrane is preferably 0.15 μm or less, more preferably 0.10 μm or less. On the other hand, the pore diameter is preferably 0.01 μm or more. Setting the pore diameter to 0.15 μm or less is preferable from the viewpoint of suppressing self-discharge of the non-aqueous secondary battery and suppressing capacity reduction. Pore diameter is measured according to the method described in Examples. The pore diameter can be controlled by changing the stretching ratio when producing the microporous polyolefin membrane.

The thickness of the polyolefin microporous membrane is not particularly limited, but is preferably 2 μm or more, more preferably 5 μm or more. On the other hand, the thickness thereof is preferably 50 μm or less, more preferably 40 μm or less, even more preferably 30 μm or less. It is preferable that the film thickness be 2 μm or more from the viewpoint of improving mechanical strength. On the other hand, setting the film thickness to 50 μm or less is preferable because the volume occupied by the separator in the non-aqueous secondary battery is reduced, which tends to be advantageous in terms of increasing the capacity of the non-aqueous secondary battery. The thickness of the polyolefin microporous membrane is measured according to the method described in Examples. The thickness of the microporous polyolefin membrane can be controlled by changing the stretching ratio of the microporous polyolefin membrane.

The maximum heat shrinkage stress (for example, the maximum heat shrinkage stress at 30 to 200°C) of the polyolefin microporous membrane is not particularly limited, but is preferably 10.0 g or less, more preferably 7.0 g or less, and even more preferably 5 .0g or less. It is preferable that the maximum thermal shrinkage stress is 10.0 g or less. The maximum heat shrinkage stress of the polyolefin microporous membrane is measured according to the method described in Examples. The maximum thermal shrinkage stress of the microporous polyolefin membrane can be controlled by changing the stretching ratio of the microporous polyolefin membrane.

The polyolefin microporous membrane explained above is not limited to a single layer, but can include a plurality of layers. Therefore, for example, a laminate in which a plurality of microporous polyolefin membranes containing different polyolefin resins are laminated is also included in the microporous polyolefin membrane of the present invention.

[Porous layer]
The porous layer according to the present embodiment includes an ionic inorganic filler and a binder, and is provided on at least one side of the microporous polyolefin membrane.

The peel strength of the porous layer is not particularly limited, but is preferably 200 to 1000 g/cm. A high peel strength of the porous layer leads to a high binding force per bonding point between the polyolefin microporous membrane and the filler and a bonding point between the fillers, which tends to improve deformation resistance. . The peel strength of the porous layer is measured according to the method described in Examples. The peel strength of the porous layer can be controlled by changing the type of ionic inorganic filler and/or binder contained in the porous layer, and also by changing the content thereof.

The thickness of the porous layer is not particularly limited, but is preferably 0.5 μm or more, more preferably 1.0 μm or more. On the other hand, the thickness of the porous layer is preferably 10.0 μm or less, more preferably 8.0 μm or less, and even more preferably 6.0 μm. It is preferable that the thickness of the porous layer is 0.5 μm or more because heat resistance tends to be further improved. On the other hand, it is preferable from the viewpoint of ion permeability that the thickness of the porous layer is 10.0 μm or less. In addition, the thickness of the porous layer referred to here is the thickness of the porous layer on at least one surface when the porous layer is provided on both surfaces of the microporous polyolefin membrane. The thickness of the porous layer is measured according to the method described in Examples. The thickness of the porous layer can be controlled by changing the amount of each component constituting the porous layer and the amount of coating liquid applied to form the porous layer.

The air permeability of the porous layer is preferably 100 seconds/100 cm ³ or less, more preferably 50 seconds/100 cm ³ or less. It is preferable that the air permeability is 100 seconds/100 cm ³ or less from the viewpoint of obtaining good charge/discharge characteristics. On the other hand, the air permeability of the porous layer is preferably less than 45 seconds/100 cm ³ from the viewpoint of ensuring good ion permeability. Further, the air permeability of the porous layer may be substantially 0 sec/100 cm ³ .
The air permeability here is calculated from the following formula.
Air permeability of porous layer (sec/100cm ³ ) = Air permeability of multilayer porous membrane - Air permeability of microporous polyolefin membrane

[Ionic inorganic filler]
The ionic inorganic filler according to this embodiment is an inorganic filler having ionic bonding properties. That is, an ionic inorganic filler is an inorganic filler that is made of a combination of a cation and an anion and is electrically neutral due to highly ionic chemical bonds. By including such an ionic inorganic filler as one of its constituent elements, the multilayer porous membrane according to the present embodiment can ensure sufficient binding force while suppressing the amount of binder to a small amount.

The anion of the ionic inorganic filler is not particularly limited, but from the viewpoint of ensuring sufficient binding strength while suppressing the amount of binder to a small amount, it is preferably a sulfate ion, a nitrate ion, a phosphate ion, a carbonate ion, or a halide ion. From the viewpoint of stability inside the battery, sulfate ions, phosphate ions, and halide ions are more preferred, sulfate ions and phosphate ions are more preferred, and sulfate ions are particularly preferred.
The cations of the ionic inorganic filler are not particularly limited, but are preferably alkali metal ions and alkaline earth metal ions, more preferably lithium ions, sodium ions, potassium ions, magnesium ions, calcium ions, strontium ions, and barium. ions, more preferably magnesium ions, calcium ions, and barium ions, particularly preferably barium ions.

Therefore, examples of the ionic inorganic filler according to the present embodiment include lithium sulfate, sodium sulfate, magnesium sulfate, calcium sulfate, barium sulfate, lithium nitrate, sodium nitrate, magnesium nitrate, potassium nitrate, lithium phosphate, sodium phosphate, etc. , magnesium phosphate, potassium phosphate, etc., lithium fluoride, sodium chloride, sodium bromide, lithium carbonate, sodium carbonate, sodium bicarbonate, lithium hexafluorophosphate, lithium tetrafluoroborate, etc. Among them, barium sulfate is most preferred. By using barium sulfate as the ionic inorganic filler, it is expected that the binding force will be further improved while suppressing the amount of binder to a small amount compared to the case of using other ionic inorganic fillers. Therefore, the use of an ionic inorganic filler containing barium sulfate is expected to further improve the peel strength of the porous layer, for example. Ionic inorganic fillers may be used alone or in combination of two or more.

Since so-called metal oxides have higher chemical bonding properties than ionic bonding properties, they are not included in the ionic inorganic filler according to the present embodiment. Therefore, specific examples of inorganic fillers that do not fall under the ionic inorganic filler according to the present embodiment include alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, barium titanate, and iron oxide. Inorganic oxides; inorganic nitrides such as silicon nitride, titanium nitride, and boron nitride; silicon carbide, calcium carbonate, aluminum hydroxide, aluminum hydroxide oxide, barium titanate, talc, kaolinite, dickite, nacrite, halloysite, pyro Ceramics such as phyllite, montmorillonite, sericite, mica, amethyte, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fibers may be mentioned.
Note that the porous layer according to the present embodiment may contain an inorganic filler that does not correspond to the ionic inorganic filler within a range that does not impede the effects of the present invention. The content ratio of the inorganic filler that does not correspond to the ionic inorganic filler according to the present embodiment (inorganic filler that does not correspond to the ionic inorganic filler/porous layer) with respect to the total mass of the porous layer is preferably 50% by mass or less, and 10% by mass or less. It is more preferably less than % by mass, and even more preferably substantially 0% by mass.

The median diameter D50 of the ionic inorganic filler is preferably 0.05 to 0.40 μm. Since the median diameter D50 is controlled within a relatively small specific range, a relatively dense porous layer can be realized and a relatively large number of bonding points with the binder can be secured. Therefore, the above median diameter D50 is preferably 0.05 μm or more, more preferably 0.08 μm or more, and still more preferably 0.10 μm or more. It is preferable that the median diameter D50 is equal to or larger than the above value from the viewpoint of easily preventing the porous layer from becoming too dense and, furthermore, from the viewpoint of easily ensuring good ion permeability. On the other hand, the above median diameter D50 is preferably 0.40 μm or less, more preferably 0.35 μm or less, and still more preferably 0.30 μm or less. It is preferable that the median diameter D50 is equal to or less than the above-mentioned value from the viewpoint of making it easier to reduce the thermal shrinkage rate. The median diameter D50 is measured according to the method described in Examples. The median diameter D50 can be controlled by the particle size of the ionic inorganic filler, its distribution, etc.

Examples of the shape of the ionic inorganic filler include plate-like, scale-like, needle-like, columnar, spherical, polyhedral, and lump-like (block-like) shapes. It's okay. Such an ionic inorganic filler can be obtained by pulverizing a predetermined inorganic raw material using an appropriate pulverizing device such as a ball mill, bead mill, jet mill, or the like. Therefore, by adjusting the degree of pulverization, the particle size of the ionic inorganic filler and its distribution can be controlled.

The BET specific surface area of the ionic inorganic filler is not particularly limited, but it is preferably 7 to 100 m ² /g, more preferably 10 m ² /g or more, even more preferably 12 m ² /g or more. The BET specific surface area of the ionic inorganic filler is calculated by the known BET method. The BET specific surface area of the ionic inorganic filler can be controlled by changing the particle size and distribution of the ionic inorganic filler.

The content ratio of the ionic inorganic filler to the total mass of the porous layer (ionic inorganic filler/porous layer) is not particularly limited, but is preferably 70% by mass or more and less than 100% by mass, and 90% by mass or more and 99% by mass. The following is more preferable, and 95% by mass or more and 98% by mass or less is particularly preferable.

(binder)
The binder according to this embodiment is, for example, a so-called resin binder. The type of resin for the resin binder is not particularly limited, but any resin that is insoluble in the non-aqueous electrolyte and electrochemically stable within the range of use of the non-aqueous secondary battery can be used.

Specific examples of the binder resin according to the present embodiment include, for example, fluorine-containing resins such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene; vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer; Fluororubbers such as ethylene-tetrafluoroethylene copolymers; styrene-butadiene copolymers and their hydrides, acrylonitrile-butadiene copolymers and their hydrides, acrylonitrile-butadiene-styrene copolymers and their hydrides Rubbers such as , methacrylic acid ester-acrylic ester copolymer, styrene-acrylic ester copolymer, acrylonitrile-acrylic ester copolymer, ethylene propylene rubber, PVA, polyvinyl acetate; ethyl cellulose, methyl cellulose, hydroxyethyl cellulose , cellulose derivatives such as carboxymethylcellulose; and resins with a melting point and/or glass transition temperature of 180°C or higher, such as polyphenylene ether, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyamide, and polyester. Can be mentioned. These may be used alone or in combination of two or more.

Specific examples of the resin binder include the following 1) to 6).
1) Conjugated diene polymer: For example, styrene-butadiene copolymer and its hydride, acrylonitrile-butadiene copolymer and its hydride, acrylonitrile-butadiene-styrene copolymer and its hydride;
2) Acrylic polymers: for example, methacrylic ester-acrylic ester copolymers, styrene-acrylic ester copolymers, and acrylonitrile-acrylic ester copolymers, copolymers containing acrylamide monomer units;
3) PVA-based resin: for example, PVA and polyvinyl acetate;
4) Fluororesin: For example, PVdF, polytetrafluoroethylene, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, and ethylene-tetrafluoroethylene copolymer;
5) Cellulose derivatives: for example, ethylcellulose, methylcellulose, hydroxyethylcellulose, and carboxymethylcellulose; and 6) Resins with a melting point and/or glass transition temperature of 180°C or higher, or polymers without a melting point but with a decomposition temperature of 200°C or higher: For example, polyphenylene ether, polysulfone, polyether sulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyamide, and polyester.

Preferably, the binder includes a resin latex binder. As the resin latex binder, for example, a copolymer of an unsaturated carboxylic acid monomer and another monomer copolymerizable with these monomers can be used. Here, examples of the aliphatic conjugated diene monomer include butadiene and isoprene, examples of the unsaturated carboxylic acid monomer include (meth)acrylic acid ester, and examples of other monomers include Examples include styrene. Among these, it is preferable that the binder contains an acrylic acid ester monomer. Examples of acrylic esters include (meth)acrylic acid alkyl esters, such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl amethacrylate; ) Acrylic esters, such as glycidyl acrylate and glycidyl methacrylate; these may be used alone or in combination of two or more.
Although there are no particular limitations on the method of polymerizing such a copolymer, emulsion polymerization is preferred. The emulsion polymerization method is not particularly limited, and known methods can be used. The method of adding monomers and other components is not particularly limited, and any of a batch addition method, a divided addition method, and a continuous addition method can be adopted, and the polymerization method includes one-stage polymerization, Either two-stage polymerization or multi-stage polymerization of three or more stages can be employed.

Here, the binder is preferably a water-soluble polymer or a particulate dispersed polymer. Here, the water-soluble polymer is a polymer whose insoluble matter is less than 0.005 g (1.0% by mass) when 0.5 g of the substance is dissolved in 100 g of water at 25°C. Specifically, examples of the water-soluble polymer include poly(meth)acrylamide, carboxymethylcellulose, methylcellulose, hydroxypropylcellulose, hydroxyethylmethylcellulose, PVA, polycarboxylic acid, and salts thereof.
Further, a particulate dispersion polymer is a polymer that is dispersed in a particle shape in water. Specifically, resin latex binders, PVA, PVdF, etc. can be mentioned. In order to maintain long-term dispersion stability of the particulate dispersion polymer, it is preferable to adjust the pH to a range of 5 to 12 using amines such as ammonia, sodium hydroxide, potassium hydroxide, and dimethylaminoethanol.

The volume average particle diameter of the binder is not particularly limited, but is preferably 0.01 to 0.50 μm. It is more preferably 0.45 μm or less, still more preferably 0.40 μm or less, particularly preferably 0.35 μm or less. It is preferable that the volume average particle diameter is within the above range from the viewpoint of ensuring bonding points with the ionic inorganic filler. The volume average particle size of the binder is measured according to the method described in Examples. The volume average particle diameter of the binder can be controlled by adjusting, for example, the polymerization time, polymerization temperature, raw material composition ratio, raw material input order, pH, and the like.

(Relationship between binder and other components)
The amount of binder relative to the ionic inorganic filler (binder/ionic inorganic filler) is preferably 0.1 to 4.0% by mass in terms of solid content. In this embodiment, sufficient binding force can be ensured while suppressing the amount of binder to a small amount, and as a result, it is possible to reduce the thermal shrinkage rate. Furthermore, since the amount of binder can be kept to a small amount, it is possible to avoid a situation where the ion permeability decreases due to an increase in the amount of binder, thereby ensuring good ion permeability. Therefore, the amount of binder relative to the ionic inorganic filler is preferably 4.0% by mass or less, more preferably 3.0% by mass or less, in terms of solid content. When the amount of the binder is less than or equal to the above value, it is possible to reliably avoid a situation in which the ion permeability decreases.

The ratio of the volume average particle diameter of the binder to the median diameter D50 of the ionic inorganic filler (volume average particle diameter of the binder/median diameter D50 of the ionic inorganic filler is preferably 0.10 to 1.0. Such a ratio is more preferably 0.9 or less, still more preferably 0.8. This ratio is less than or equal to the above value because the volume average particle diameter of the binder is sufficiently larger than the median diameter D50 of the ionic inorganic filler. Although the number of binding points is limited depending on the number of particles of the ionic inorganic filler, it is sufficiently smaller than the median diameter D50 of the ionic inorganic filler. The number of bonding points with the binder tends to increase, and as a result, the thermal shrinkage rate tends to be reduced.On the other hand, this ratio is more preferably 0.12 or more, and still more preferably 0.15 or more. By having such a ratio equal to or higher than the above value, it is possible to prevent the diameter of the binder from becoming too small compared to the gap between the ionic inorganic fillers, and as a result, the binder is likely to exist between the ionic inorganic fillers, The effects of the present invention can be reliably obtained.

The ratio of the pore diameter of the polyolefin microporous membrane to the volume average particle diameter of the binder (pore diameter of the polyolefin microporous membrane/volume average particle diameter of the binder) is preferably from 0.01 to 1.0. This ratio is more preferably 0.05 or more, still more preferably 0.10 or more. When this ratio is at least the above value, the pore diameter of the microporous polyolefin membrane does not become too small, making it easy to ensure good permeability of the microporous polyolefin membrane. On the other hand, this ratio is more preferably 0.90 or less, still more preferably 0.80 or less. When this ratio is below the above value, it becomes easier to prevent the binder from falling into the pores of the polyolefin microporous membrane, and as a result, it becomes easier to ensure good ion permeability.

[Any layer]
The present invention also includes an embodiment in which an arbitrary layer is provided on the multilayer porous membrane described above. Therefore, for a multilayer porous membrane with a porous layer on one side of a microporous polyolefin membrane, there are two types: one in which an arbitrary layer is provided on the surface on the porous layer side, and the other in which an arbitrary layer is provided on the surface on the microporous polyolefin membrane side. The present invention includes both an embodiment in which an arbitrary layer is provided on both surfaces and an embodiment in which an arbitrary layer is provided on both surfaces. In addition, for multilayer porous membranes that have porous layers on both sides of a microporous polyolefin membrane, there are two types: one in which an arbitrary layer is provided on the surface on one porous layer side, and one in which an arbitrary layer is provided on the surface on the other porous layer side. The present invention includes an embodiment in which a layer is provided, an embodiment in which an arbitrary layer is provided between the porous layer and the microporous polyolefin membrane, and an embodiment in which an arbitrary layer is provided on both surfaces.

Optional layers include, for example, a thermoplastic polymer-containing layer that can function as an adhesive layer. The existing form (pattern) of the thermoplastic polymer-containing layer may be, for example, a state in which the thermoplastic polymers are mutually dispersed over the entire surface of the multilayer porous membrane, or a state in which the thermoplastic polymers are present in a sea-island shape. When the thermoplastic polymer is present in a striped pattern, examples of the arrangement pattern include dots, stripes, lattice, stripes, hexagonal, random, etc., and combinations thereof. In addition, examples of the arbitrary layer include a porous layer different from the porous layer according to this embodiment.
The type, total number, existing form (pattern), and thickness of these arbitrary layers are not particularly limited as long as they do not impede the effects of the present invention.

<Separator manufacturing method>
[Method for manufacturing microporous polyolefin membrane]
The method for producing the polyolefin microporous membrane is not particularly limited, and known production methods can be employed. For example, a method in which a polyolefin resin composition and a plasticizer are melt-kneaded, formed into a sheet shape, optionally stretched, and then made porous by extracting the plasticizer; a polyolefin resin containing a polyolefin resin as a main component A method in which the composition is melt-kneaded and extruded at a high draw ratio, and then the polyolefin crystal interface is peeled off by heat treatment and stretching to make it porous; a polyolefin resin composition and an inorganic filler are melt-kneaded and formed into a sheet. After that, the interface between the polyolefin and the inorganic filler is peeled off by stretching to make it porous; and after the polyolefin resin composition is dissolved, it is immersed in a poor solvent for the polyolefin to coagulate the polyolefin and simultaneously remove the solvent. Examples include a method of making the material porous.

Hereinafter, as an example of a method for manufacturing a polyolefin microporous membrane, a method will be described in which a polyolefin resin composition and a plasticizer are melt-kneaded, molded into a sheet shape, and then the plasticizer is extracted. First, a polyolefin resin composition and a plasticizer are melt-kneaded. As a melt-kneading method, for example, a polyolefin resin and other additives as necessary are put into a resin kneading device such as an extruder, a kneader, a laboplast mill, a kneading roll, or a Banbury mixer, and the resin components are heated and melted. An example of this method is to introduce a plasticizer in an arbitrary ratio and knead the mixture. At this time, it is preferable to pre-knead the polyolefin resin, other additives, and plasticizer in a predetermined ratio using a Henschel mixer or the like before introducing the polyolefin resin, other additives, and plasticizer into the resin kneading device. More preferably, only a part of the plasticizer is added during preliminary kneading, and the remaining plasticizer is kneaded while being side-fed to the resin kneading device.

As the plasticizer, a nonvolatile solvent capable of forming a homogeneous solution at a temperature equal to or higher than the melting point of the polyolefin can be used. Specific examples of such nonvolatile solvents include hydrocarbons such as liquid paraffin and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; and higher alcohols such as oleyl alcohol and stearyl alcohol. Among these, liquid paraffin is preferred.

The ratio of the polyolefin resin composition to the plasticizer is not particularly limited as long as they can be uniformly melt-kneaded and molded into a sheet. For example, the mass fraction of the plasticizer in a composition consisting of a polyolefin resin composition and a plasticizer is preferably 30% by mass or more and 80% by mass or less, more preferably 40% by mass or more and 70% by mass or less. Setting the mass fraction of the plasticizer within this range is preferable from the viewpoint of achieving both melt tension during melt molding and formability of a uniform and fine pore structure.

Next, the melt-kneaded product obtained by heating, melting and kneading as described above is formed into a sheet shape. As a method for producing a sheet-like molded product, for example, a melt-kneaded product is extruded into a sheet-like form through a T-die, etc., and cooled to a temperature sufficiently lower than the crystallization temperature of the resin component by contacting with a heat conductor. An example of this method is to solidify the material. Thermal conductors used for cooling and solidification include metals, water, air, and plasticizers themselves, but metal rolls are preferred because of their high heat conduction efficiency. In this case, if the molten mixture is sandwiched between the rolls when it comes into contact with the metal rolls, the heat conduction efficiency will be further increased, the sheet will be oriented, the film strength will be increased, and the surface smoothness of the sheet will also be improved. Therefore, it is more preferable. The die lip interval when extruding into a sheet form from a T-die is preferably 400 μm or more and 3000 μm or less, more preferably 500 μm or more and 2500 μm or less.

It is preferred that the sheet-like molded product thus obtained is then stretched. As the stretching treatment, either uniaxial stretching or biaxial stretching can be suitably used. Biaxial stretching is preferred from the viewpoint of the strength of the resulting microporous membrane. When a sheet-like molded body is stretched biaxially at a high magnification, the molecules are oriented in the plane direction, and the microporous membrane finally obtained becomes difficult to tear and has high puncture strength. Examples of the stretching method include simultaneous biaxial stretching, sequential biaxial stretching, multistage stretching, and multiple stretching. Simultaneous biaxial stretching is preferred from the viewpoint of improving puncture strength, uniformity of stretching, and shutdown properties.

The stretching ratio is preferably in the range of 20 times or more and 100 times or less in area magnification, and more preferably in the range of 25 times or more and 50 times or less. The stretching ratio in each axial direction is preferably in the range of 4 times or more and 10 times or less in the MD direction, 4 times or more and 10 times or less in the TD direction, 5 times or more and 8 times or less in the MD direction, and 5 times in the TD direction. More preferably, it is in a range of 8 times or less. Setting the stretching ratio within this range is preferable because more sufficient strength can be imparted, film breakage in the stretching process can be prevented, and high productivity can be obtained.
Note that the MD direction means, for example, the machine direction when continuously molding a polyolefin microporous membrane, and the TD direction means a direction that crosses the MD direction at an angle of 90°.

The sheet-like molded product obtained as described above may be further rolled. Rolling can be carried out, for example, by a pressing method using a double belt press machine or the like. By rolling, the orientation of the surface layer portion of the sheet-like molded product can be increased. The rolling surface magnification is preferably greater than 1 time and not more than 3 times, more preferably greater than 1 time and not more than 2 times. A rolling ratio within this range is preferable because the strength of the microporous membrane finally obtained can be increased and a porous structure can be formed more uniformly in the thickness direction of the membrane.

Next, the plasticizer is removed from the sheet-like molded body to obtain a microporous membrane. As a method for removing the plasticizer, for example, a method of immersing the sheet-like molded body in an extraction solvent to extract the plasticizer, and thoroughly drying the molded body is exemplified. The method for extracting the plasticizer may be either a batch method or a continuous method. In order to suppress shrinkage of the microporous membrane, it is preferable to restrain the ends of the sheet-like molded body during the series of steps of dipping and drying. Further, the amount of plasticizer remaining in the microporous membrane is preferably less than 1% by mass.

As the extraction solvent, it is preferable to use one that is a poor solvent for the polyolefin resin, a good solvent for the plasticizer, and has a boiling point lower than the melting point of the polyolefin resin. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; hydrofluoroethers and hydrofluorocarbons; Non-chlorinated halogenated solvents; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; and ketones such as acetone and methyl ethyl ketone. Note that these extraction solvents may be recovered and reused by operations such as distillation.

In order to suppress shrinkage of the microporous membrane, heat treatment such as heat setting or thermal relaxation may be performed after the stretching step or after the formation of the microporous membrane. The microporous membrane may be subjected to post-treatments such as hydrophilic treatment using a surfactant or the like, crosslinking treatment using ionizing radiation, or the like.

[Method for forming porous layer]
The method for forming the porous layer is not particularly limited, and any known method can be used. For example, there is a method of applying a coating liquid containing an ionic inorganic filler and a binder to a microporous polyolefin membrane. Alternatively, a raw material containing an ionic inorganic filler and a binder and a raw material for a microporous polyolefin membrane may be laminated and extruded by a coextrusion method, or the microporous polyolefin membrane and the porous layer may be separately produced and then extruded. may be pasted together.

The solvent for the coating solution is preferably one that can uniformly and stably disperse or dissolve the ionic inorganic filler and the binder, such as N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, water, Includes ethanol, toluene, hot xylene, methylene chloride, and hexane.
Various additives such as a dispersant such as a surfactant; a thickener; a wetting agent; an antifoaming agent; a PH adjuster containing an acid or an alkali may be added to the coating liquid.

Methods for dispersing or dissolving the ionic inorganic filler and binder in the medium of the coating solution include, for example, a ball mill, bead mill, planetary ball mill, vibrating ball mill, sand mill, colloid mill, attritor, roll mill, high-speed impeller dispersion, and disperser. , a homogenizer, a high-speed impact mill, ultrasonic dispersion, and mechanical stirring using a stirring blade.

Examples of methods for applying a coating liquid to a polyolefin microporous membrane include a gravure coater method, a small-diameter gravure coater method, a reverse roll coater method, a transfer roll coater method, a kiss coater method, a dip coater method, a knife coater method, and an air doctor coater method. , blade coater method, rod coater method, squeeze coater method, cast coater method, die coater method, screen printing method, and spray coating method.

The method for removing the solvent from the coating film after coating is not particularly limited as long as it does not adversely affect the microporous polyolefin membrane. For example, drying the microporous polyolefin membrane while fixing it at a temperature below the melting point of the material that makes up the microporous polyolefin membrane, drying it under reduced pressure at a low temperature, or solidifying the binder by immersing it in a poor solvent for the binder. One example is a method of simultaneously extracting the solvent. Further, a portion of the solvent may remain as long as it does not significantly affect the device characteristics.

<Separator>
The separator according to this embodiment includes the multilayer porous film described above. The multilayer porous membrane itself can be used as a separator, and a laminate obtained by providing any of the above layers on a multilayer porous membrane can also be used as a separator. Since such a separator includes the multilayer porous membrane described above, it is possible to ensure both good ion permeability and a reduction in thermal shrinkage. Therefore, it can be suitably used in non-aqueous secondary batteries such as LIB.

<Electricity storage device>
The separator according to this embodiment can be suitably used in various power storage devices including non-aqueous secondary batteries. Examples of power storage devices include, but are not limited to, non-aqueous secondary batteries, capacitors, and capacitors. Among them, batteries are preferred, non-aqueous secondary batteries are more preferred, and LIBs are even more preferred, from the viewpoint of more effectively obtaining the benefits of the effects of the present invention.

When manufacturing LIB using a separator, there are no limitations on the positive electrode, negative electrode, and non-aqueous electrolyte, and known ones can be used for each.
As the positive electrode, a positive electrode in which a positive electrode active material layer containing a positive electrode active material is formed on a positive electrode current collector can be suitably used. Examples of the positive electrode current collector include aluminum foil. Examples of the positive electrode active material include lithium-containing composite oxides such as LiCoO ₂ , LiNiO ₂ , spinel-type LiMnO ₄ , and olivine-type LiFePO ₄ . The positive electrode active material layer may appropriately contain a binder, a conductive material, etc. in addition to the positive electrode active material.

As the negative electrode, a negative electrode in which a negative electrode active material layer containing a negative electrode active material is formed on a negative electrode current collector can be suitably used. Examples of the negative electrode current collector include copper foil. Examples of negative electrode active materials include carbon materials such as graphite, non-graphitizable carbon materials, easily graphitizable carbon materials, and composite carbon bodies, as well as silicon, tin, metallic lithium, and various alloy materials.

The nonaqueous electrolyte is not particularly limited, but an electrolyte in which an electrolyte is dissolved in an organic solvent can be used. Examples of organic solvents include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate. Examples of the electrolyte include lithium salts such as LiClO ₄ , LiBF ₄ , and LiPF ₆ .

<Method for manufacturing non-aqueous secondary battery>
The method of manufacturing a non-aqueous secondary battery using a separator is not particularly limited. For example, the following method can be exemplified. First, a vertically elongated separator having a width of 10 to 500 mm (preferably 80 to 500 mm) and a length of 200 to 4000 m (preferably 1000 to 4000 m) is manufactured. Next, the layers are laminated in the order of positive electrode-separator-negative electrode-separator or negative electrode-separator-positive electrode-separator and wound into a circular or flat spiral shape to obtain a wound body. It can be manufactured by housing the wound body in a device can (for example, a battery can) and further injecting an electrolyte. Alternatively, the electrode and the separator may be folded to form a wound body, which is then placed in a device container (for example, an aluminum film), and an electrolyte solution may be poured into the device container.

At this time, the wound body can be pressed. Specifically, a separator, a current collector, and an electrode having an active material layer formed on at least one side of the current collector are arranged such that the thermoplastic polymer-containing layer and the active material layer of the former face each other. A method of stacking and pressing can be exemplified.

The pressing temperature is preferably, for example, 20° C. or higher as a temperature at which adhesiveness can be effectively developed. Further, in order to suppress clogging of holes or thermal shrinkage in the separator due to heat pressing, the pressing temperature is preferably lower than the melting point of the material contained in the microporous membrane, and more preferably 120° C. or lower. The press pressure is preferably 20 MPa or less from the viewpoint of suppressing clogging of holes in the separator. The pressing time may be 1 second or less when using a roll press, or may be surface pressing for several hours, but from the viewpoint of productivity, 2 hours or less is preferable.

Since the non-aqueous secondary battery, especially LIB, manufactured as described above includes the multilayer porous membrane according to the present embodiment, it is possible to ensure both good ion permeability and a reduction in thermal shrinkage rate. I can do it. Therefore, it is expected to exhibit excellent battery characteristics (rate characteristics) and further improve safety (for example, improve the peel strength of the porous layer and the nail insertion strength of the multilayer porous membrane).

EXAMPLES Hereinafter, embodiments of the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples.

Each physical property was evaluated by the following measurement method.
(1) Film thickness (μm)
The thickness of the polyolefin microporous membrane or porous layer was measured at room temperature of 23° C. using a micro-thickness meter (manufactured by Toyo Seiki Co., Ltd., type KBM (trademark)).

(2) Air permeability (sec/100cm ³ )
The air permeability of the polyolefin microporous membrane and the multilayer porous membrane was measured using a Gurley air permeability meter (manufactured by Toyo Seiki Co., Ltd., G-B2 (trademark)) based on JIS P-8117. Then, the porous layer was calculated based on the following formula.
Air permeability of porous layer (sec/100cm ³ ) = Air permeability of multilayer porous membrane - Air permeability of microporous polyolefin membrane

(3) Puncture strength (gf)
Using a handy compression tester "KES-G5" (manufactured by Kato Tech), the multilayer porous membrane was fixed with a sample holder having an opening diameter of 11.3 mm. The puncture strength of the multilayer porous membrane was determined by performing a puncture test on the central part of the fixed multilayer porous membrane in an atmosphere of 25°C under the conditions of a needle tip radius of curvature of 0.5 mm and a puncture speed of 2 mm/sec. (gf) was measured.

(4) Measurement of pore diameter of polyolefin microporous membrane (μm)
Porus Materials, Inc. in the United States applied the measurement principle of the bubble point method (JIS K 3832). The value of the average flow pore diameter measured with a Perm-Porometer manufactured by Co., Ltd. was taken as the pore diameter of the microporous polyolefin membrane. In the measurement, GALWICK manufactured by PMI (surface tension: 15.9 dyn/cm) was used as the impregnating solvent.

(5) Peel strength of porous layer A multilayer porous membrane was pasted onto a slide glass with a width of 76 mm and a length of 126 mm using double-sided tape (Nicetack NWBB-15) with the porous layer side facing upward.
Mending tape (Scotch MP-12) was attached to the surface of the test piece on the porous layer side. The slide glass was fixed in a flat state, one end of the mending tape was folded back, and the mending tape was pulled and peeled off horizontally at a pulling speed of 100 mm/min, and the stress at that time was measured. The measurement was performed twice, and the average value was determined and used as the peel strength.

(6) Heat shrinkage rate (%) at 150℃
The multilayer porous membrane was cut out into a square shape with a side of 100 mm (100 mm along the MD and 100 mm along the TD), and the sample was left standing in a hot air dryer preheated to 150° C. for 1 hour. At this time, the sample was sandwiched between two sheets of copy paper so that the hot air would not directly hit the sample. After the sample was taken out of the oven and cooled, the length (mm) was measured, and the heat shrinkage rate (%) was calculated using the following formula. The measurement was performed along each of the MD and TD, and the larger value was taken as the heat shrinkage rate (%).
Heat shrinkage rate (%) = {(100-length after heating)/100} x 100

(7) Nail penetration test a. Preparation of positive electrode Lithium, nickel, manganese, cobalt composite oxide powder (Li[Ni _1/3 Mn _1/3 Co _1/3 ]O ₂ ) and lithium manganese composite oxide powder (LiMn ₂ O ₄ ) which are positive electrode active materials ) in a mass ratio of 70:30, 85 parts by mass of acetylene black as a conductive aid, 9 parts by mass of PVdF as a binder, and N-methyl-2-pyrrolidone. (NMP) was used as a solvent and mixed uniformly to prepare a positive electrode mixture-containing paste. This positive electrode mixture-containing paste was uniformly applied to both sides of a 20 μm thick current collector made of aluminum foil, dried at 130°C for 3 minutes, and then compression molded using a roll press to reduce the total thickness. The thickness of the positive electrode mixture layer was adjusted to 130 μm, and a positive electrode was produced using aluminum foil as a lead tab.

b. Preparation of Negative Electrode 91 parts by mass of graphite as a negative electrode active material and 9 parts by mass of PVdF as a binder were uniformly mixed using NMP as a solvent to prepare a paste containing a negative electrode mixture. This negative electrode mixture-containing paste was uniformly applied to both sides of a 15 μm thick current collector made of copper foil, dried at 120°C for 3 minutes, and then compression molded using a roll press to reduce the total thickness. The thickness of the negative electrode mixture layer was adjusted to 130 μm, and a negative electrode was produced using copper foil as a lead tab.

c. Preparation of non-aqueous electrolyte As a non-aqueous electrolyte, add LiPF ₆ as a solute to a mixed solvent of ethylene carbonate: ethyl methyl carbonate: dimethyl carbonate = 1:1:1 (volume ratio) at a concentration of 1.0 mol/L. Prepared by dissolving.

d. Cell Preparation An electrode plate laminate was produced by alternately stacking the above 27 positive electrode sheets and 28 negative electrode sheets and separating them with a multilayer porous membrane. The multilayer porous membrane was alternately folded into ninety-nine folds to produce an electrode plate laminate. This electrode plate laminate was housed in an aluminum laminate film, and three sides were heat-sealed. Note that the positive electrode lead tab and the negative electrode lead tab were led out from one side of the laminate film. Furthermore, after drying, the above-mentioned non-aqueous electrolyte was poured into the container, and the remaining one side was sealed. The battery for evaluation thus produced was designed to have a capacity of 10 Ah.

e. Nail penetration evaluation d. The battery obtained was charged with a constant current of 0.3C in an environment of 25 ° C, and after reaching 4.2V, the laminated cell charged with a constant voltage of 4.2V was placed in an explosion-proof booth, An iron nail with a diameter of 2.5 mm was penetrated into the center of the cell at a speed of 3 mm/sec in an environment around 25°C. The nail was maintained in a penetrating state, and those that ignited and exploded within 1 minute were rated ×, those that ignited and exploded within 15 minutes were rated △, and those that did not ignite or explode were rated ○.

(8) Rate characteristics of multilayer porous membrane a. Preparation of positive electrode 91.2 parts by mass of lithium, nickel, manganese, cobalt composite oxide powder (Li[Ni _1/3 Mn _1/3 Co _1/3 ]O ₂ ) as a positive electrode active material, graphite as a conductive additive, A slurry was prepared by dispersing these in N-methylpyrrolidone (NMP). This slurry was applied onto one side of a 20 μm thick aluminum foil serving as a positive electrode current collector using a die coater so that the coating amount of the positive electrode active material was 120 g/m ² . After drying at 130° C. for 3 minutes, compression molding was performed using a roll press machine so that the bulk density of the positive electrode active material was 2.90 g/cm ³ . This was punched out into a circular shape with an area of 2.00 cm ² and used as a positive electrode.

b. Preparation of Negative Electrode Prepare 96.6 parts by mass of artificial graphite as a negative electrode active material, 1.4 parts by mass of ammonium salt of carboxymethylcellulose and 1.7 parts by mass of styrene-butadiene copolymer latex as binders, and mix them with purified water. to prepare a slurry. This slurry was applied onto one side of a 16 μm thick copper foil serving as a negative electrode current collector using a die coater so that the amount of negative electrode active material applied was 53 g/m ² . After drying at 120° C. for 3 minutes, compression molding was performed using a roll press machine so that the bulk density of the negative electrode active material was 1.35 g/cm ³ . This was punched out into a circular shape with an area of 2.05 cm ² and used as a negative electrode.

c. Preparation of non-aqueous electrolyte A non-aqueous electrolyte is prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1:2 (volume ratio) to a concentration of 1.0 mol/L. Prepared.

d. Battery Assembly The negative electrode, multilayer porous membrane, and positive electrode were stacked in this order from the bottom so that the active material surfaces of each of the positive and negative electrodes faced each other. A cell was obtained by accommodating this laminate in a stainless metal container with a lid, in which the main container and the lid were insulated. After drying this cell at 65° C. for 12 hours under reduced pressure, a non-aqueous electrolyte was injected into the container in an argon box and the container was sealed, thereby obtaining a battery for evaluation.

e. Evaluation of rate characteristics d. The battery obtained in step 3 was charged at a constant current of 0.3C in an environment of 25°C, and after reaching 4.2V, it was charged at a constant voltage of 4.2V, and then charged at a constant current of 0.3C. It was discharged to 0V. The total time of constant current charging and constant voltage charging was 8 hours. Note that 1C is the current value at which the battery is discharged in one hour.
Next, the simple battery was charged with a constant current of 1C, and after reaching 4.2V, it was charged with a constant voltage of 4.2V, and discharged to 3.0V with a constant current of 1C. The total time of charging at constant current and charging at constant voltage was 3 hours, and the capacity when discharging at 1C constant current was defined as 1C discharge capacity (mAh).
Next, the simple battery was charged with a constant current of 1C, and after reaching 4.2V, it was charged with a constant voltage of 4.2V, and discharged to 3.0V with a constant current of 10C. The total time of charging at constant current and charging at constant voltage was 3 hours, and the capacity when discharging at 2C constant current was defined as 2C discharge capacity (mAh).
Then, the ratio of 10C discharge capacity to 1C discharge capacity was calculated from the following formula, and this value was taken as the output characteristic.
Output characteristics (%) = (10C discharge capacity/1C discharge capacity) x 10

(9) Maximum heat shrinkage stress (g)
Measurement was performed using TMA50 (trademark) manufactured by Shimadzu Corporation. When measuring the value of MD (TD), a sample cut out to a width of 3 mm in TD (MD) is fixed on a chuck so that the distance between the chucks is 10 mm, and set in a dedicated probe. The initial load was set to 1.0 g, and the sample was heated in a fixed length from 30° C. to 200° C. at a temperature increase rate of 10° C./min. The load (g) generated at that time was measured, and its maximum value was determined. MD and TD were measured, and the larger value was taken as the maximum thermal shrinkage stress (g).

(10) Particle size of ionic inorganic filler (μm)
The particle size distribution of the ionic inorganic filler dispersion or paint slurry was measured using a laser particle size distribution measuring device (Microtrac MT3300EX manufactured by Nikkiso Co., Ltd.). If necessary, the particle size distribution in the above dispersion or slurry coating liquid was adjusted using the particle size distribution of water or the resin binder as a baseline. The particle size at which the cumulative frequency was 50% was defined as the median diameter D50 of the ionic inorganic filler.

(11) Volume average particle diameter of binder (μm)
The volume average particle size (μm) of the binder was measured using a particle size measuring device using a light scattering method (MICROTRACT MUPA150 manufactured by LEED & NORTHRUP).

[Production Example 1] Polyolefin microporous membrane 46.0 parts by mass of homo high density polyethylene with Mv 250,000, 47.0 parts by mass of homo high density polyethylene with Mv 700,000, and 7.0 parts by mass of homopolymer polypropylene with Mv 400,000. were dry blended using a tumbler blender to obtain a polymer mixture. To 100 parts by mass of the obtained polymer mixture, 1.0 parts by mass of pentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] was added as an antioxidant. Then, dry blending was performed again using a tumbler blender to obtain a mixture of polymers, etc. After the resulting mixture of polymers, etc. was replaced with nitrogen, it was fed to a twin-screw extruder by a feeder under a nitrogen atmosphere. Additionally, liquid paraffin (kinematic viscosity at 37.78°C: 7.59×10 ⁻⁵ m ² /s) was injected into the extruder cylinder using a plunger pump. These were melt-kneaded, and the drive of the feeder and pump was controlled so that the liquid paraffin amount ratio in the entire extruded mixture was 62% by mass (resin composition concentration: 38% by mass). Thereby, a melt-kneaded product was obtained. The melt-kneading conditions were a set temperature of 200° C., a screw rotation speed of 100 rpm, and a discharge rate of 230 kg/h.

Subsequently, the obtained melt-kneaded product was extruded and cast through a T-die onto a cooling roll whose surface temperature was controlled to 25° C. to obtain a gel sheet with a thickness of 1200 μm.
Next, the obtained gel sheet was introduced into a simultaneous biaxial tenter stretching machine and biaxially stretched. The stretching conditions were as follows: MD magnification 7.0 times, TD magnification 6.4 times, and temperature set at 123°C. Next, the biaxially stretched sheet was introduced into a methylene chloride bath, and the sheet was sufficiently immersed in methylene to extract and remove liquid paraffin, and then the methylene chloride was removed by drying. Thereafter, the sheet was introduced into a TD tenter and heat-set. The heat setting temperature was 125° C., the TD maximum magnification was 1.5 times, and the relaxation rate was 1.3. Through the above steps, a microporous polyolefin membrane A1 having a thickness of 12 μm was obtained.
The physical properties of the obtained microporous polyolefin membrane A1 were measured by the method described above. The results obtained are shown in Table 1 or Table 2.

[Production Example 2] (Polyolefin microporous membrane)
Polyolefin microporous membrane A2 was obtained in the same manner as in Production Example 1, except that the discharge rate under the above melt-kneading conditions was 180 kg/h, and the thickness of the resulting gel sheet was 1000 μm. The physical properties of the polyolefin microporous membrane A2 were measured by the method described above. The results obtained are shown in Table 1 or Table 2.

[Production Example 3] (Polyolefin microporous membrane)
Polyolefin microporous membrane A3 was obtained in the same manner as in Production Example 1, except that the discharge rate under the above melt-kneading conditions was 260 kg/h, and the thickness of the resulting gel sheet was 1500 μm. The physical properties of the polyolefin microporous membrane A3 were measured by the above method. The results obtained are shown in Table 1 or Table 2.

[Production Example 4] (Polyolefin microporous membrane)
A microporous polyolefin membrane A4 was obtained in the same manner as in Production Example 1, except that the stretching conditions for biaxial stretching were 10.0 times MD magnification and 8.0 times TD magnification. The physical properties of the polyolefin microporous membrane A4 were measured by the above method. The results obtained are shown in Table 1 or Table 2.

[Production Example 5] (Polyolefin microporous membrane)
A microporous polyolefin membrane A5 was obtained in the same manner as in Production Example 1, except that the stretching conditions for biaxial stretching were 10.0 times MD magnification, 8.0 times TD magnification, and 127° C. set temperature. The physical properties of the polyolefin microporous membrane A5 were measured by the above method. The results obtained are shown in Table 1 or Table 2.

[Example 1] (Multilayer porous membrane)
100% by mass of barium sulfate (D50 = 0.35 μm, specific surface area 12 m ² /g), 2.0% by mass of acrylic latex (volume average particle diameter = 0.12 μm) in solid content based on barium sulfate, and polyester. A coating liquid was obtained by preparing ammonium carboxylate in a solid content of 1.0 parts by mass based on barium sulfate, and uniformly dispersing them in water at a concentration of 100% by mass based on barium sulfate. After applying a coating solution to the surface of the above microporous polyolefin membrane A1 using a gravure coater, drying at 60°C to remove water forms a porous layer with a thickness of 3 μm on the microporous polyolefin membrane A1. A multilayer porous membrane having a total membrane thickness of 15 μm and an air permeability of 175 seconds/100 cm ³ was obtained.
The obtained multilayer porous membrane was used as a separator, and its physical properties were measured by the above method. The results obtained are shown in Table 1 or Table 2.

[Examples 2 to 11]
A multilayer porous membrane was obtained in the same manner as in Example 1, except that the particle size or blending amount of the ionic inorganic filler and/or binder was varied as shown in Table 1 or Table 2. The obtained multilayer porous membrane was used as a separator, and its physical properties were measured by the above method. The results obtained are shown in Table 1 or Table 2.

[Examples 12 to 15]
A multilayer porous membrane was obtained in the same manner as in Example 1, except that the thickness or pore diameter of the polyolefin microporous membrane was varied as shown in Table 1 or Table 2. The obtained multilayer porous membrane was used as a separator, and its physical properties were measured by the above method. The results obtained are shown in Table 1 or Table 2.

[Example 16]
A multilayer porous membrane was obtained in the same manner as in Example 1, except that porous layers each having a thickness of 1.5 μm were formed on both sides of the microporous polyolefin membrane by coating twice with a gravure coater. The obtained multilayer porous membrane was used as a separator, and its physical properties were measured by the above method. The results obtained are shown in Table 1 or Table 2.

[Example 17]
80 parts by mass of adhesive resin (acrylic polymer, glass transition temperature 70°C, average particle size 380 nm, electrolyte swelling degree 2.8) and adhesive resin (acrylic polymer, glass transition temperature -6) having different glass transition temperatures. ℃, average particle size 132 nm, electrolyte swelling degree 2.5) and 20% by mass, and water was added to prepare an adhesive resin-containing coating liquid (adhesive resin concentration 3% by mass). This was applied in the form of dots onto the surface of the polyolefin microporous membrane having the porous layer described in Example 1 using a gravure coater. Thereafter, water was removed by drying at 60°C. Furthermore, by applying the coating liquid on the other side in the same manner and drying it, the weight of the adhesive resin is 0.2 g/m ² , the surface coverage is 30%, the average major axis of the dots is 50 μm, and the thickness is 0.5 μm. A multilayer porous membrane having an adhesive layer was obtained.
The obtained multilayer porous membrane was used as a separator, and its physical properties were measured by the above method. The results obtained are shown in Table 1 or Table 2.

[Comparative Examples 1 to 8]
A multilayer porous membrane was obtained in the same manner as in Example 1, except that the composition of the ionic inorganic filler and/or binder was changed as shown in Table 1 or Table 2. The obtained multilayer porous membrane was used as a separator, and its physical properties were measured by the above method. The results obtained are shown in Table 1 or Table 2.

As mentioned above, the heat shrinkage rate at 150°C of the multilayer porous membrane is preferably less than 10.0% from the viewpoint of improving safety during nail penetration, and the air permeability of the porous layer is preferably less than 10.0%. From the viewpoint of ensuring the following, it is preferable that the time is less than 45 seconds/100 cm ³ .
As can be seen from Tables 1 and 2, in Examples 1 to 17, both the heat shrinkage rate at 150° C. of the multilayer porous membrane and the air permeability of the porous layer were within the above ranges. On the other hand, in Comparative Examples 1 to 8, one of the heat shrinkage rate at 150° C. of the multilayer porous membrane and the air permeability of the porous layer was out of the range.
Therefore, according to Examples 1 to 17, it was confirmed that both ensuring good ion permeability and reducing the thermal shrinkage rate could be achieved.

The multilayer porous membrane and separator according to the present invention can ensure good ion permeability and can improve the reduction in thermal shrinkage at high temperatures.
Therefore, the non-aqueous secondary battery using the multilayer porous membrane and separator of the present invention exhibits excellent battery characteristics (rate characteristics) and further improves safety (e.g., peeling of the porous layer It is expected to improve the strength of the membrane, and improve the nail insertion strength of multilayer porous membranes. Therefore, the multilayer porous membrane and separator of the present invention can be used not only in small power supplies for portable electronic devices, such as LIBs installed in electric vehicles (EVs), for which high capacity has been particularly desired in recent years. It can be suitably applied to large-scale power storage industry for industrial use or on-vehicle use.

Claims (10)

A microporous polyolefin membrane, a porous layer containing an ionic inorganic filler and a binder, and provided on at least one side of the microporous polyolefin membrane,
the ionic inorganic filler is barium sulfate,
The binder includes a resin latex binder containing an acrylic ester monomer,
The ionic inorganic filler has a median diameter (D50) of 0.05 to 0.40 μm,
The amount of the binder relative to the ionic inorganic filler is 0.1 to 4.0% by mass in terms of solid content,
A multilayer porous membrane , wherein the porous layer has a peel strength of 200 to 1000 g/cm . The multilayer porous membrane according to claim 1, wherein the ionic inorganic filler has a BET specific surface area of 7 to 100 m ² /g. The multilayer porous membrane according to claim 1 or 2, wherein the ratio of the pore diameter of the microporous polyolefin membrane to the volume average particle diameter of the binder is 0.01 to 1.0. The multilayer porous film according to any one of claims 1 to 3, wherein the binder has a volume average particle size of 0.05 to 0.3 μm. The multilayer porous membrane according to any one of claims 1 to 4, wherein the ratio of the volume average particle size of the binder to the median diameter (D50) of the ionic inorganic filler is 0.10 to 1.0.
A separator comprising the multilayer porous membrane according to any one of claims 1 to 5 . The separator according to claim 6 , having a heat shrinkage rate of 5% or less at 150°C. The separator according to claim 6 or 7 , wherein the microporous polyolefin membrane has a pore diameter of 0.15 μm or less. The separator according to any one of claims 6 to 8 , wherein the microporous polyolefin membrane has a maximum thermal shrinkage stress of 5.0 g or less. A nonaqueous secondary battery comprising a positive electrode, the multilayer porous membrane according to any one of claims 1 to 5 , and a negative electrode.