Microporous polyolefin membrane, separator film for energy storage devices that use this, and energy storage device

Abstract

A microporous polyolefin membrane is provided comprising a polyethylene-based resin layer containing a polyethylene-based resin and a polypropylene-based resin layer containing a polypropylene-based resin, wherein the polyethylene-based resin content of the total weight of the microporous polyolefin membrane is 5% to 45% by weight, the porosity of the microporous polyolefin membrane is 42% to 62%, and the surface opening ratio of the polypropylene-based resin layer is 5% to 30% and the surface opening ratio of the polyethylene-based resin layer is 5% to 20.6%.

Description

Technical field

[0001] The present invention relates to a microporous polyolefin membrane and an energy storage device. State of the art

[0002] In recent years, energy storage devices with high energy density, high electromotive force, and low self-discharge have become increasingly popular. Examples of energy storage devices include lithium-ion secondary batteries and lithium-ion capacitors. In lithium-ion secondary batteries and lithium-ion capacitors, a separator film is inserted between the positive and negative electrodes to prevent them from coming into contact and causing a short circuit. A microporous polyolefin membrane is used as the separator film.

[0003] If the internal temperature of the energy storage device rises above a predetermined temperature due to an abnormal current or the like, a separator film made of a microporous polyolefin membrane can block and close the pores in the microporous membrane, preventing ions from flowing between the electrodes and increasing the electrical resistance.

[0004] Accordingly, the energy storage device's function is stopped, and the risk of ignition or similar events due to excessive temperature rise can be prevented, thus ensuring safety. The function of preventing the risk of ignition or similar events due to excessive temperature rise is extremely important for separator films used in energy storage devices and is generally referred to as pore closure or shutdown (hereinafter referred to as SD).

[0005] For example, a microporous polyolefin membrane used for a battery separator is described in patent document 1. Patent document 1 discloses a microporous polyolefin membrane with a three-layer separator film having a sandwich structure, in which both outer layers consist of polypropylene and the inner layer of polyethylene, wherein the membrane has a thickness of 7.0 to 50 µm, a porosity of 30 to 70%, and a surface opening ratio of 10 to 30%. State of the art document (patent document)

[0006] Patent Document 1: JP-A-2017-141428 Summary of the invention Problems to be solved by the invention

[0007] In recent years, separators in energy storage devices have been required to exhibit lower resistance and improved safety while maintaining a low thickness. Although patent document 1 describes air permeability, it does not describe low resistance with respect to fluid resistance.

[0008] The present invention was made in view of the above circumstances, and one objective of the present invention is to provide a microporous polyolefin membrane which has low resistance and excellent safety while maintaining a thin thickness.

[0009] Another objective of the present invention is to provide an energy storage device with low resistance and excellent safety, in which the aforementioned microporous polyolefin membrane is arranged between the electrodes.

[0010] To solve the aforementioned problems, the inventors involved conducted extensive investigations focusing on the material, composition, porosity, and surface area opening ratio of the microporous polyolefin membrane. That is to say, the present invention relates to the following points. [1] A microporous polyolefin membrane with a polyethylene-based resin layer containing a polyethylene-based resin and a polypropylene-based resin layer containing a polypropylene-based resin, wherein the content of the polyethylene-based resin in the total weight of the microporous polyolefin membrane is 5 wt% to 45 wt%, the porosity of the microporous polyolefin membrane is 42% to 62%, and The surface opening ratio of the resin layer on polypropylene is 5% to 30%, and the surface opening ratio of the resin layer on polyethylene is 5% to 20.6%. [2] The microporous polyolefin membrane according to [1], wherein the polyethylene-based resin layer is an intermediate layer. [3] The microporous polyolefin membrane according to [1] or [2], wherein the microporous polyolefin membrane has a layered structure in which the polyethylene-based resin layer and the polypropylene-based resin layer are layered, and The layer structure is a three-layer structure in which the polypropylene-based resin layers are present on both sides of the polyethylene-based resin layer. [4] The microporous polyolefin membrane according to one of [1] to [3], wherein the microporous polyolefin membrane has a puncture resistance (N / µm) and a porosity (%) which satisfy the following formula (I). 100×Puncture resistance / (100−Porosity)≥0.44 [5] The microporous polyolefin membrane according to one of [1] to [4], wherein the microporous polyolefin membrane has an electrical resistance of 0.06 (Ω·cm 2 / µm) or less. [6] The microporous polyolefin membrane according to one of [1] to [5], wherein the microporous polyolefin membrane has a tortuosity of 1 or more and 2 or less. [7] The microporous polyolefin membrane according to one of [1] to [6], wherein the surface opening ratio of the polypropylene-based resin layer is 6% to 25%. [8] The microporous polyolefin membrane according to one of points [1] to [7], wherein the surface opening ratio of the polyethylene-based resin layer is 7% to 19%. [9] The microporous polyolefin membrane according to one of [1] to [8], wherein the surface opening ratio of the polyethylene-based resin layer is greater than the surface opening ratio of the polypropylene-based resin layer.

[10] The microporous polyolefin membrane according to [9], wherein the difference between the surface opening ratio of the polyethylene-based resin layer and the surface opening ratio of the polypropylene-based resin layer is 1% or more and 8% or less.

[11] A separator film for an energy storage device comprising the microporous polyolefin membrane according to one of [1] to

[10] .

[12] An energy storage device comprising the separator film for an energy storage device according to

[11] , a positive electrode and a negative electrode.

[13] A vehicle, an aircraft or a power tool equipped with the energy storage device according to

[12] . Effects of the invention

[0011] According to the present invention, it is possible to provide a microporous polyolefin membrane which can simultaneously improve low resistance and puncture resistance while also exhibiting shutdown properties by adjusting the material, content, porosity and surface opening ratio of the microporous polyolefin membrane. Description of the embodiments

[0012] The following describes in detail embodiments of the present invention.

[0013] However, the present invention is not limited to the following embodiments. <Mikroporöse Polyolefinmembran>

[0014] A microporous polyolefin membrane according to the present invention comprises at least one polypropylene-based resin (hereinafter sometimes referred to as PP) and one polyethylene-based resin (hereinafter sometimes referred to as PE) as resin materials. The microporous polyolefin membrane has a multilayer structure. <Gehalt an Harz auf Polyethylenbasis>

[0015] From the perspective of increasing puncture resistance while maintaining the shutdown properties described below, the lower limit of the polyethylene-based resin content in the total weight of the microporous polyolefin membrane is 5% by weight or more, preferably 9% by weight or more, more preferably 11% by weight or more, and particularly preferably 13% by weight or more. The upper limit is 45% by weight or less, preferably 38% by weight or less, more preferably 32% by weight or less, and particularly preferably 29% by weight or less. <Porosität>

[0016] The porosity of the microporous polyolefin membrane of the present invention is 42% or more and 62% or less, from the perspective of ensuring sufficient mechanical strength of the microporous polyolefin membrane and furthermore ensuring sufficient ion mobility while simultaneously suppressing short circuits. The porosity of the microporous polyolefin membrane is preferably 43% or more, more preferably 44% or more, even more preferably 45% or more, and particularly preferably 49% or more. On the other hand, the porosity of the microporous polyolefin membrane is preferably 60% or less, more preferably 58% or less, and even more preferably 56% or less. If the porosity is within the aforementioned range, a high-performance energy storage device can be obtained. Here, porosity refers to the porosity of the entire microporous polyolefin membrane. <Harz auf Polypropylenbasis und Harzschicht auf Polypropylenbasis>

[0017] The polypropylene-based resin is a polymer that is the main component of a polypropylene-based resin layer at a concentration of 80% by weight or more. Within the scope of the present invention, such a polymer can be used alone or in combination as the polypropylene-based resin. Furthermore, polypropylene-based resins generally contain additives such as surfactants, antioxidants, plasticizers, flame retardants, and colorants for various purposes. The polypropylene-based resin used in the present invention can also contain these additives. The content of the polypropylene-based resin in the total polypropylene-based resin layer is preferably 85% by weight or more, more preferably 90% by weight or more, and the upper limit of this is not particularly restricted, but is preferably 100% by weight or less.

[0018] A polypropylene-based resin with high stereoregularity is preferably used. The pentad content of the polypropylene-based resin is preferably 80% or more, more preferably 90% or more, and even more preferably 94% or more. The upper limit is not particularly restricted, but is preferably 100% or less.

[0019] The zero-shear viscosity ηPP (Pa·s) of the polypropylene-based resin at 200°C is preferably in the range of 10,000 to 100,000 Pa·s. If the polypropylene-based resin has a zero-shear viscosity of 10,000 Pa·s or more, a microporous polyolefin membrane can be obtained that is capable of maintaining the formability of PP during shutdown. Furthermore, by using a polypropylene-based resin with a zero-shear viscosity of 100,000 Pa·s or less, the dimensional stability properties of the layered structure are good. Consequently, the shape and porosity of the polypropylene-based resin layer can be reliably maintained even when the interior of an energy storage device using the microporous polyolefin membrane as a separator is exposed to an environment above 200°C. The zero shear viscosity of the polypropylene-based resin at 200°C is preferably 13,000 to 80,000 Pa·s, more preferably 14,000 to 50.000 Pa·s, and particularly preferably 14,500 to 40,000 Pa·s.

[0020] The lower limit of the crystalline melting point of the polypropylene-based resin, measured by a differential scanning calorimeter (DSC), is preferably 155°C or higher, more preferably 157°C or higher, even more preferably 159°C or higher, and most preferably 160°C or higher. The upper limit is preferably 175°C or less, more preferably 173°C or less, even more preferably 170°C or less, and most preferably 169°C or less. When the melting point of the polypropylene-based resin is 155°C or higher, a microporous polyolefin membrane with excellent processability can be obtained. <Harz auf Polyethylenbasis und Harzschicht auf Polyethylenbasis>

[0021] The polyethylene-based resin is a polymer that is the main component of a polyethylene-based resin layer in a proportion of 80% by weight or more. Within the scope of the present invention, such a polymer can be used alone or in combination as the polyethylene-based resin. Furthermore, polyethylene-based resins generally contain additives such as surfactants, antioxidants, plasticizers, flame retardants, and colorants for various purposes. The polyethylene-based resin used in the present invention can also contain these additives. The content of the polyethylene-based resin in the total polyethylene-based resin layer is preferably 85% by weight or more, more preferably 90% by weight or more, and the upper limit of this is not particularly restricted, but is preferably 100% by weight or less.

[0022] The density of the polyethylene-based resin is preferably 0.950 g / cm³.3 or more and 0.970 g / cm² 3 or less. The polyethylene-based resin is high-density polyethylene with a density of 0.960 g / cm³. 3 or more preferably, but medium-density polyethylene can also be used.

[0023] The zero-shear viscosity of the polyethylene-based resin at 200°C is preferably in the range of 15,000 to 90,000 Pa·s. A zero-shear viscosity of 15,000 Pa·s or more provides sufficient strength to the polyethylene-based resin layer, which is preferred. A zero-shear viscosity of 90,000 Pa·s or less reduces the influence on the shape of the polypropylene-based resin layer, which is preferred. A zero-shear viscosity of the polyethylene-based resin at 200°C is preferably 20,000 to 70,000 Pa·s, and even more preferably 25,000 to 50,000 Pa·s.

[0024] The melting point of the polyethylene-based resin is preferably 100°C or higher and 140°C or lower, more preferably 110°C or higher and 138°C or lower, and even more preferably 120°C or higher and 137°C or lower. In the microporous polyolefin membrane of the present invention, shutdown occurs at a temperature equal to or higher than the melting point of the polyethylene-based resin. That is, the molten polyethylene-based resin flows into the pores of the polypropylene-based resin layer, thus blocking the movement of ions. If the melting point of the PE resin used for the microporous polyolefin membrane is 140°C or lower, shutdown can be initiated earlier, which is preferred. It is not easy to use a polyethylene-based resin with a melting point of less than 100°C as a raw material for a microporous polyolefin membrane. <Oberflächenöffnungsverhältnis der Harzschicht auf Polypropylenbasis>

[0025] The surface opening ratio of the polypropylene-based resin layer of the present invention is 5% or more and 30% or less, from the point of view of ensuring sufficient ion mobility and effective suppression of short circuits.

[0026] The surface open area ratio of the polypropylene-based resin layer is preferably 5% or more, more preferably 6% or more, and even more preferably 7% or more. Alternatively, the surface open area ratio of the polypropylene-based resin layer is preferably 28% or less, more preferably 25% or less, even more preferably 20% or less, and particularly preferably 19% or less. The surface open area ratio of the two surfaces of the polypropylene-based resin layer (i.e., the surface open area ratio of one surface and the surface open area ratio of the other surface) can be the same or different, as long as they are within the range mentioned above.If the microporous polyolefin membrane of the present invention comprises a plurality of polypropylene-based resin layers, it is sufficient that at least one of the plurality of polypropylene-based resin layers has a surface opening ratio within the above range, but it is preferred that all polypropylene-based resin layers contained in the microporous polyolefin membrane of the present invention have a surface opening ratio within the above range. If the microporous polyolefin membrane of the present invention comprises, for example, two polypropylene-based resin layers, the surface opening ratio of at least one polypropylene-based resin layer may be within the above range, but it is preferred that the surface opening ratio of both polypropylene-based resin layers be within the above range. <Oberflächenöffnungsverhältnis der Harzschicht auf Polyethylenbasis>

[0027] The surface opening ratio of the polyethylene-based resin layer of the present invention is 5% or more and 20.6% or less.

[0028] The surface open area ratio of the polyethylene-based resin layer is preferably 6% or more, more preferably 7% or more, even more preferably 9% or more, and particularly preferably 12% or more. Alternatively, the surface open area ratio of the polyethylene-based resin layer is preferably 19.5% or less, more preferably 19% or less, and even more preferably 18.5% or less. By ensuring the aforementioned range, it is possible to simultaneously reduce electrical resistance and improve puncture resistance while maintaining ion mobility and shutdown properties. The surface open area ratio of the two surfaces of the polyethylene-based resin layer (i.e., the surface open area ratio of one surface and the surface open area ratio of the other surface) can be the same or different, as long as they are within the aforementioned range.If the microporous polyolefin membrane of the present invention comprises a plurality of polyethylene-based resin layers, it is sufficient that at least one of the plurality of polyethylene-based resin layers has a surface opening ratio within the above range, but it is preferred that all polyethylene-based resin layers contained in the microporous polyolefin membrane of the present invention have a surface opening ratio within the above range.

[0029] In the microporous polyolefin membrane of the present invention, the surface open-cell ratio of the polyethylene-based resin layer is preferably higher than the surface open-cell ratio of the polypropylene-based resin layer. By choosing such an embodiment, the puncture resistance can be further increased, even if the porosities are approximately the same. If the surface open-cell ratio of the polyethylene-based resin layer is higher than that of the polypropylene-based resin layer, the difference between the surface open-cell ratios of the polyethylene-based and polypropylene-based resin layers is preferably 1% or more, more preferably 2% or more, more preferably 3% or more, and is more preferably 8% or less, more preferably 7% or less, and more preferably 6% or less. <Schichtstruktur mit Harzschicht auf Polyethylenbasis als Zwischenschicht>

[0030] From the perspective of improving shutdown properties and puncture resistance, it is preferred that the microporous polyolefin membrane of the present invention has a layered structure with a polyethylene-based resin layer as an intermediate layer. The polypropylene-based resin layer can be present on both sides of the polyethylene-based resin layer, or the polypropylene-based resin layer can be present on one surface and the polyethylene-based resin layer on the remaining surface. The layered structure referred to herein can have the polyethylene-based resin layer as an intermediate layer, and the polypropylene-based resin layer can be present on at least one side of the polyethylene-based resin layer.

[0031] The total number of layers in the layered structure can be four or more. <Dreilagige Schichtstruktur>

[0032] From the perspective of improving shutdown properties and puncture resistance, the microporous polyolefin membrane of the present invention preferably has a three-layer structure in which the polypropylene-based resin layers are present on both sides of the polyethylene-based resin layer. That is to say, it is preferred that the microporous polyolefin membrane of the present invention has a three-layer structure in which the polypropylene-based resin layer / the polyethylene-based resin layer / the polypropylene-based resin layer are layered in that order. <Formel (I) für Durchstoßfestigkeit und Porosität>

[0033] The puncture resistance (pinhole strength) and the porosity of the microporous polyolefin membrane of the present invention can be determined using the measurement method described below, and it is preferred that the puncture resistance (N / µm) per 1 µm membrane thickness and the porosity (%) satisfy the following formula (I). 100×Puncture resistance / (100−Porosity)≥0.44

[0034] If formula (I) is satisfied, the puncture resistance of the separator film for an energy storage device can be improved while maintaining good ionic conductivity. The value on the left-hand side of formula (I) is preferably 0.44 or more, more preferably 0.45 or more, more preferably 0.47 or more, and furthermore preferably 0.48 or more. The upper limit of the value “100 × puncture resistance / (100-porosity)” is not particularly restricted, but is preferably 0.54 (that is, 100 × puncture resistance / (100-porosity) ≤ 0.54). Formula (I) is a value that gives the puncture resistance per unit thickness corresponding to the essential resin component without the pore content, and in formula (I) the unit of puncture resistance is “N / µm” and the unit of porosity is “%”. <Elektrischer Widerstand (ER)>

[0035] To achieve good ionic conductivity, it is preferred that the microporous polyolefin membrane of the present invention has an electrical resistance of 0.06 (Ω·cm). 2 / µm) or less. The electrical resistance referred to here is the electrical resistance per 1 µm of film thickness. The electrical resistance (Ω·cm) 2 / µm) is preferably 0.057 or less, more preferably 0.055 or less and furthermore preferably 0.05 or less, and the lower limit of the electrical resistance (Ω·cm) 2The resistance (µm) is not particularly limited, but is preferably 0.01 or greater. Within the aforementioned range, the electrical resistance can be reduced while maintaining shutdown properties and puncture resistance. The electrical resistance is measured by filling the microporous polyolefin membrane with an electrolyte and measuring the resistance of the electrolyte-filled separator. In this case, an electrolyte solution is used in which LiPF6 is dissolved at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of EC / EMC = 3 / 7 and introduced into the microporous polyolefin membrane, which can be used as a sample, until it is saturated. <foliendicke>

[0036] The thinner the overall layer thickness of the microporous polyolefin membrane of the present invention, the more compact an energy storage device can be, which is preferred. If the microporous polyolefin membrane has a large overall thickness when used as a separator in an energy storage device, the volume fraction of the separator increases, which reduces the capacity per unit volume of the energy storage device, which is undesirable.

[0037] The thickness of the entire microporous polyolefin membrane is preferably 30 µm or less, more preferably 25 µm or less, and even more preferably 20 µm or less. By adjusting the thickness of the layered structure to 20 µm or less, the capacity of an energy storage device using the microporous polyolefin membrane as a separator can be increased. On the other hand, the thickness of the entire microporous polyolefin membrane is preferably 7 µm or more, more preferably 8 µm or more, even more preferably 8.5 µm or more, and particularly preferably 9 µm or more. When the thickness of the layered structure is 7 µm or more, the separator arranged between the electrodes exhibits good mechanical strength, making the microporous polyolefin membrane less likely to rupture. Therefore, an energy storage device using the microporous polyolefin membrane as a separator is less prone to short circuits.The thickness of the microporous polyolefin membrane can be determined by image analysis of a cross-section of the microporous membrane recorded with a scanning electron microscope (SEM) or by using a point-type thickness gauge. <Luftdurchlässigkeit>

[0038] The lower limit of the air permeability (Gurley value) of the microporous polyolefin membrane of the present invention is preferably 80 s / 100 cm. 3 or more, preferably 90 s / 100 cm 3 or more, preferably 95 s / 100 cm 3 or more, preferably 100 s / 100 cm 3 or more, and especially preferred 130 s / 100 cm 3 or more. The upper limit of air permeability (Gurley value) is preferably 500 s / 100 cm². 3 or less, preferably at 400 s / 100 cm 3 or less, preferably at 300 s / 100 cm 3 or less, and preferably at 250 s / 100 cm 3 or less.

[0039] If the microporous polyolefin membrane has an air permeability of 500 s / 100 cm 3 If the microporous polyolefin membrane has an air permeability of 80 s / 100 cm², it is less likely that ion mobility will be suppressed or hindered when used as a separator for an energy storage device, which is preferred. However, if the microporous polyolefin membrane has an air permeability of 80 s / 100 cm², the ion mobility is less likely to be suppressed or hindered. 3 or more, when using the microporous polyolefin membrane as a separator for an energy storage device, it can prevent the temperature from rising too quickly in the event of a power storage device failure, because the ions move too quickly. <zugfestigkeit>

[0040] The tensile strength of the microporous polyolefin membrane in the MD direction has a lower limit of preferably 130 MPa or more, more preferably 150 MPa or more, more preferably 170 MPa or more, and furthermore preferably 180 MPa or more, and an upper limit of preferably 250 MPa or less, more preferably 230 MPa or less, more preferably 210 MPa, and furthermore preferably 200 MPa or less. If the tensile strength in the MD direction is within the above range, good ionic conductivity is maintained, whereas at a tensile strength of 150 MPa or more, breakage during cutting or winding of an energy storage device is more likely to be suppressed, or short circuits due to foreign matter or the like within an energy storage device are more likely to be suppressed.On the other hand, the tensile strength of the microporous membrane in the TD direction (direction perpendicular to the MD, membrane width direction) is preferably 5 MPa or more, more preferably 6 MPa or more and more preferably 7 MPa or more, wherein the upper limit is preferably 20 MPa or less, more preferably 18 MPa or less and more preferably 16 MPa or less. <zugdehnung>

[0041] The tensile strain of the microporous polyolefin membrane is preferably 45% or more, more preferably 50% or more, and particularly preferably 60% or more in both the MD and TD directions. If the tensile strain in the MD direction is 45% or more, the possibility of separator breakage when the battery is deformed by an external force can be reduced. Furthermore, it can prevent the microporous polyolefin membrane from deforming in minute sections that arise when the microporous polyolefin membrane and the electrodes are layered in the presence of minute foreign particles, thereby reducing the possibility of pinholes forming that could lead to battery failure due to micro-short circuits. The upper limit of the tensile strain in the MD and / or TD direction of the microporous polyolefin membrane is not particularly restricted. <Tortuosität>

[0042] The upper limit of the tortuosity of the microporous polyolefin membrane is preferably 2 or less, more preferably 1.8 or less, even more preferably 1.6 or less, and furthermore preferably 1.55 or less. The lower limit is preferably 1 or more, more preferably 1.2 or more, even more preferably 1.3 or more, and furthermore preferably 1.35 or more. Tortuosity can be described as a measure of the convoluted path that ions take from one side of the porous membrane through the pores in the main part of the membrane to the opposite side of the membrane. A low tortuosity, as described above, promotes greater or faster ion and electrolyte movement through the porous battery separator membrane during the charge-discharge cycles of a lithium-ion battery compared to a high tortuosity. Within the aforementioned tortuosity range, puncture resistance can be improved. <shutdown-eigenschaften>

[0043] A separator film made of microporous polyolefin membrane can block and close the pores in the porous membrane to prevent ions from flowing between the electrodes and increasing electrical resistance when the internal temperature of an energy storage device rises above a predetermined temperature due to an abnormal current or other reasons.

[0044] Accordingly, the function of an energy storage device is stopped, and the risk of ignition or similar hazards due to excessive temperature increases can be prevented. The function of preventing the risk of ignition or similar hazards due to excessive temperature increases is extremely important for separator films used in energy storage devices and is generally referred to as pore closure or shutdown (hereinafter referred to as SD).

[0045] The shutdown properties of the microporous polyolefin membrane of the present invention are such that when the microporous polyolefin membrane is heated to a temperature equal to or higher than the melting point of the polyethylene-based resin, all or part of the pores in the polypropylene-based resin layer are blocked and closed in this temperature range, resulting in an increase in the resistance of the separator film, measured as impedance, and the membrane is considered to have shutdown properties when the resistance exceeds 1000 Ω. <Durchstoßfestigkeit, Einstichdehnung>

[0046] In this specification, puncture strength (needle puncture strength) means the stress a membrane specimen can withstand against a force applied by fixing the circumference of the membrane specimen and inserting a needle of a predetermined size from the outer surface of the membrane specimen in the direction of the thickness of the membrane specimen. Penetration strain is the distance (elongation) a needle of a predetermined size travels from the point where it contacts the membrane specimen until a hole is formed, when the circumference of the membrane specimen is fixed and the needle is inserted from the outer surface of the membrane specimen in the direction of the thickness of the membrane specimen. The methods for measuring penetration strain and puncture strength are explained in the following examples.

[0047] The puncture resistance of the microporous polyolefin membrane, considering the effectiveness of the separator and the safety of the secondary battery, is preferably 2 N or more, more preferably 3 N or more. The upper limit is not particularly restricted, but is preferably 10 N or less. The insertion elongation of the microporous membrane, considering puncture resistance, wettability with non-aqueous solvents, and dielectric strength, is preferably 2.5 mm or more, more preferably 3 mm or more, even more preferably 3.5 mm or more, particularly preferably 4 mm or more, and most preferably 5 mm or more. The upper limit is not particularly restricted, but is preferably 10 mm or less. <Verfahren zur Herstellung einer mikroporösen Polyolefinmembran>

[0048] The process for producing the microporous polyolefin membrane of the present invention will now be described.

[0049] The microporous polyolefin membrane of the present invention is preferably produced in a dry process in which no solvent is used during production. [Precursor slide]

[0050] The precursor film of the present invention comprises a polyethylene-based resin layer (hereinafter referred to as the PE layer) containing the polyethylene-based resin mentioned above, and a polypropylene-based resin layer (hereinafter referred to as the PP layer) containing the polypropylene-based resin mentioned above. The structure of the precursor film is not particularly restricted, but can be any structure with a PP layer on a PE layer, and examples include a three-layer structure such as PE layer / PP layer / PE layer and PP layer / PE layer / PP layer, a five-layer structure such as PP layer / PE layer / PP layer / PE layer / PP layer or PE layer / PP layer / PE layer / PP layer and PE layer / PP layer / PE layer / PE layer / PP layer / PE layer

[0051] Among these, a preferred configuration consists of a pair of PP layers containing a polypropylene-based resin arranged on both sides of a PE layer containing a polyethylene-based resin; i.e., a three-layer structure of PP layer / PE layer / PP layer is preferred. The layer structure of the precursor film and the layer structure of the microporous polyolefin membrane are essentially identical, so that the layer structure of the precursor film can be determined depending on the layer structure of the resulting microporous polyolefin membrane. [Method for producing a precursor film]

[0052] The precursor film of the present invention can be produced by a method for forming a film by co-extrusion of the aforementioned polyethylene-based resin and the aforementioned polypropylene-based resin into a film, i.e., a method for forming a film by co-extrusion. For example, in the production of a precursor film with a PE layer as an intermediate layer and a PP layer on both sides thereof, the film can be produced by co-extrusion of a polyethylene-based resin and a polypropylene-based resin, such that the PE layer is the intermediate layer and a PP layer is formed on both sides thereof.

[0053] When the polyethylene-based resin and the polypropylene-based resin are coextruded, the equipment used for coextrusion is not particularly restricted, and any conventionally known device can be used. Examples of such equipment for coextrusion include a coextruder incorporating a circular die, a coextruder incorporating a feed block or a multi-T die, and the like. Furthermore, melt forming with a T-die is also suitable as a method for web formation. If the PP film and the PE film, which are to be used as film webs for the microporous polyolefin membrane, are formed by melt forming with a T-die or similar device, the PP film and the PE film can be formed separately.

[0054] When coextrusion is carried out using a coextruder, the die temperature is preferably 185 to 240°C, more preferably 190 to 235°C, and even more preferably 195 to 230°C. Setting the die temperature to 185°C or higher allows the precursor film to be formed without cracking during film formation. Furthermore, setting the die temperature to 240°C or lower reduces heat degradation of the polypropylene-based resin and suppresses deterioration of the properties of the final porous film. [Method for producing a microporous polyolefin membrane]

[0055] The microporous polyolefin membrane of the present invention can be produced by stretching and porosity of the precursor film of the present invention described above.

[0056] When a precursor film is stretched to become porous, it is preferable to subject it to heat treatment prior to this process. Pre-heating the precursor film allows the polyethylene-based and polypropylene-based resins that comprise it to crystallize, thus improving its suitability for porosity formation through stretching and enabling more efficient porosity formation. Furthermore, the crystallinity of the precursor film can be controlled by the heat treatment conditions, and this control allows for adjustment of the pore opening properties of the porous film obtained through stretching and porosity formation.

[0057] Examples of heat treatment methods include a process in which the precursor film is brought into contact with a preheated roller, or a process in which the precursor film is passed through an environment heated to a predetermined temperature, with any suitable method being used.

[0058] The heat treatment temperature is preferably 110°C or higher and 145°C or lower, more preferably higher than 128°C and 140°C or lower, and even more preferably 130°C or higher and 136°C or lower. By adjusting the heat treatment temperature within the aforementioned range, the crystallinity of the precursor film can be suitably increased and the air permeability of the resulting porous film can be sufficiently low.

[0059] In the manufacturing process according to the present invention, for example a layered film obtained by hot pressing (lamination process) of a laminate obtained by laminating a PP film and a PE film using a T-nozzle by melt forming can be used as a precursor film. [Laminating process]

[0060] In the lamination process, the PP film, the PE film, and the PP film are layered as film webs in that order to form a laminate, which is then hot-pressed together. According to the present invention, for example, each film to be hot-pressed is unwound from a roll stand and passed between heated rollers, with the PP films being arranged on each side of the PE film. In this way, a layered film is created in which the PP films are bonded to both sides of the PE film by hot pressing. [Extension procedure]

[0061] The heat-treated precursor film is then stretched to make it porous (stretch porosity), resulting in a porous film.

[0062] Although the stretching process is not particularly limited with respect to stretch porosity, examples include uniaxial stretching in the machine direction (MD), uniaxial stretching in the width direction almost perpendicular to the machine direction (TD), successive biaxial stretching to stretch in the machine direction (MD) and subsequently in the width direction (TD), simultaneous biaxial stretching to stretch in the machine direction (MD) and in the width direction (TD) almost simultaneously, tubular biaxial stretching, or the like. It is advisable to select a suitable stretching method depending on the intended application.

[0063] Examples of a specific stretching and porosity process include a process in which the heat-treated precursor film undergoes low-temperature stretching in a low-temperature stretching zone, the precursor film is subsequently stretched in a higher-temperature stretching zone at a temperature higher than that of the low-temperature stretching, and the precursor film is porous to obtain a porous film. Neither the polypropylene-based nor the polyethylene-based resin can be made fully porous by stretching at low or high temperature alone, and the porous film may not be formed.

[0064] Although the temperature of the low-temperature drawing process is not particularly restricted, it is preferably -20°C or more and +50°C or less, and more preferably +20°C or more and +40°C or less. If the temperature of the low-temperature drawing process is too low, the precursor film will tear easily during drawing, which is undesirable. Conversely, if the temperature for low-temperature drawing is too high, hardly any open pores will form in the polyethylene-based resin in the precursor film, so such a temperature is not preferred.

[0065] Although the sizing ratio of the low-temperature stretching process is not particularly limited, it is preferably in the range of 3% or more and 200% or less, and more preferably in the range of 5% or more and 100% or less. If the sizing ratio of the low-temperature stretching process is 3% or more, the porous film with a sufficiently low Gurley value is easily obtained. However, if the sizing ratio during low-temperature stretching is more than 200%, hairline cracks appear, which then develop into cracks and lead to film rupture. For this reason, the sizing ratio during low-temperature stretching is preferably 200% or less.

[0066] The temperature of the high-temperature drawing process is preferably 70°C or more and 150°C or less, and more preferably 80°C or more and 145°C or less. The porosity achieved during high-temperature drawing can be sufficiently reduced by adjusting the temperature of the high-temperature drawing process to this range, and a porous film with a sufficiently low air permeability is easily obtained.

[0067] The elongation ratio during high-temperature stretching is not particularly limited, but preferably lies in the range of 100% or more and 400% or less. If the elongation ratio during high-temperature stretching is too low, the Gurley value of the porous film may not be sufficiently low. Conversely, if the elongation ratio during high-temperature stretching is too high, the air permeability of the porous film may become too low.

[0068] The above steps yield the microporous polyolefin membrane of the present invention comprising a polyethylene-based resin layer containing a polyethylene-based resin and a polypropylene-based resin layer containing a polypropylene-based resin. <Separatorfolie für Energiespeichervorrichtung>

[0069] The separator film for an energy storage device according to the present invention includes the microporous polyolefin membrane from an embodiment of the present invention.

[0070] The separator film for an energy storage device according to the present invention can only be made from a microporous polyolefin membrane of the present invention. That is, the microporous polyolefin membrane of the present invention can be used as is, without further processing, as a separator film for an energy storage device.

[0071] The separator film for an energy storage device of the present invention can be any film, as long as it comprises the microporous polyolefin membrane of the present invention. Therefore, the separator film for an energy storage device of the present invention can, for example, have at least one layer selected from a heat-resistant porous layer, an adhesive layer, and a functional layer on one or both sides of the microporous polyolefin membrane. The heat-resistant porous layer, the adhesive layer, and the functional layer can each be a single layer or a laminate of several layers.Furthermore, the heat-resistant porous layer, the adhesive layer and the functional layer can be provided individually as layers, each with a single function, or as one layer that combines the functions of at least two layers selected from the heat-resistant porous layer, the adhesive layer and the functional layer.

[0072] The heat-resistant porous layer, the adhesive layer, and the functional layer can all be formed from known layers.

[0073] The heat-resistant porous layer can, for example, be a layer of heat-resistant fine particles and an organic binder.

[0074] The adhesive layer can, for example, be a layer of an organic material such as a fluorine-based resin.

[0075] An example of a functional layer is a layer of organic fine particles and a binder.

[0076] The heat-resistant porous layer, the adhesive layer, and the functional layer can all be formed by a process for applying a predetermined coating fluid.

[0077] For example, if a separator film for an energy storage device is formed with a heat-resistant porous layer, an adhesive layer, and a functional layer in that order on one or both sides of the microporous polyolefin membrane, it can be formed by the method shown below.

[0078] A coating fluid containing a mixture of heat-resistant fine particles and an organic binder is applied to one or both surfaces of the microporous polyolefin membrane to form a heat-resistant porous layer. An organic material, such as a fluorine-based resin, is then applied to the heat-resistant porous layer to form an adhesive layer. Finally, a coating fluid containing a mixture of fine organic particles and a binder is applied to the adhesive layer.

[0079] In this way, a separator film for an energy storage device can be obtained with a heat-resistant porous layer, an adhesive layer and a functional layer in that order on one or both sides of the microporous polyolefin membrane. [Heat-resistant porous layer]

[0080] The following describes in detail the heat-resistant porous layer that can be provided on the separator film for an energy storage device of the present invention.

[0081] The heat-resistant porous layer contains heat-resistant fine particles and preferably an organic binder. This heat-resistant porous layer improves the heat resistance of the separator film for an energy storage device. The heat-resistant porous layer can be a single layer or a multilayer layer consisting of several layers. (Heat-resistant fine particles)

[0082] The heat resistance of the heat-resistant fine particles is preferably 200 °C or more, more preferably 300 °C or more, and even more preferably 400 °C or more.

[0083] The term "heat-resistant" means that at least at this temperature no change in shape, such as deformation, can be visually observed.

[0084] Inorganic fine particles with electrical insulating properties are preferred as heat-resistant fine particles. These may include, in particular, fine inorganic oxide particles such as iron oxide, silicon dioxide (SiO2), aluminum oxide (Al2O3), TiO2, magnesium oxide, boehmite, and BaTiO2; fine inorganic nitride particles such as aluminum nitride and silicon nitride; sparingly soluble ionic fine crystal particles such as calcium fluoride, barium fluoride, and barium sulfate; covalent fine crystal particles such as silicon and diamond; and fine clay particles such as montmorillonite.

[0085] The fine inorganic oxide particles can be fine particles derived from mineral raw materials such as boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, and mica, or they can be synthetically produced from these materials. Furthermore, the inorganic compounds that comprise these fine inorganic particles can be elemental substitutes or exist in solid solution, depending on the requirements.

[0086] Furthermore, the inorganic fine particles can undergo surface treatment. Additionally, the fine inorganic particles are formed by coating the surface of a conductive material, e.g., metal, SnO2, a conductive oxide such as tin indium oxide (ITO), a carbon-containing material such as carbon black and graphite (for example, the aforementioned inorganic oxide or similar), to achieve electrical insulation properties.

[0087] Furthermore, fine organic particles can be used as heat-resistant fine particles. Specific examples of fine organic particles include polyimide, melamine resin, phenolic resin, aromatic polyamide resin, cross-linked polymethyl methacrylate (cross-linked PMMA), cross-linked polystyrene (cross-linked PS), polydivinylbenzene (PDVB), benzoguanamine-formaldehyde condensation products; fine particles of a cross-linked polymer such as thermoplastic polyimide; and fine particles of a heat-resistant polymer such as thermoplastic polyimide. The organic resin (polymer) from which these fine organic particles are composed can be a mixture, a modified product, a derivative, a copolymer (a statistical copolymer, an alternating copolymer, a block copolymer, or a graft copolymer), or a cross-linked product (in the case of the aforementioned heat-resistant polymer).

[0088] The substances mentioned above can be used individually or in combination as heat-resistant fine particles. Inorganic and organic fine particles, as described above, can be used as heat-resistant fine particles; however, they can be used in a suitable manner depending on the application.

[0089] Boehmite is particularly preferred as a heat-resistant fine particle. For example, boehmite with an average particle size of preferably 0.001 µm or more, more preferably 0.1 µm or more, more preferably 15 µm or less, and more preferably 3 µm or less is used.

[0090] The average particle size of the heat-resistant fine particles can be measured, for example, with a laser scattering particle size distribution meter (for example, “LA-920”, manufactured by HORIBA) by dispersing the heat-resistant fine particles in a medium in which the heat-resistant fine particles are not dissolved.

[0091] The shape of the heat-resistant fine particles can be, for example, nearly spherical or plate-like. From the perspective of preventing short circuits, it is preferable for the heat-resistant fine particles to be plate-shaped. Representative examples of plate-shaped heat-resistant fine particles are aluminum oxide and boehmite. (Organic binder)

[0092] The organic binder is contained in the heat-resistant porous layer to allow the heat-resistant fine particles, which are the main component, to adhere to each other and to bind the heat-resistant fine particles to the microporous polyolefin membrane.

[0093] Regarding the organic binder, there is no particular restriction as long as the heat-resistant fine particles can adhere well to each other and to the microporous polyolefin membrane, it is electrochemically stable, and, in the case of use as a separator for an energy storage device, it is stable against an electrolyte solution.

[0094] Examples of organic binders that can be used include ethylene-vinyl acetate copolymer (EVA, ethylene-vinyl acetate copolymer with 20 to 35 mol% of a vinyl acetate-derived structural unit), ethylene-acrylic acid copolymer such as ethylene-ethyl acrylate copolymer (EEA), soluble cellulose derivatives such as fluorinated resin [like polyvinylidene fluoride (PVDF)], fluorinated rubber, styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC) and hydroxyethylcellulose (HEC), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinylpyrrolidone (PVP), poly-N-vinylacetamide, cross-linked acrylic resin, polyurethane, epoxy resin, polyimide, and the like. These organic binders can be used alone, or two or more of them can be used in combination.

[0095] Among the organic binders mentioned above, a heat-resistant resin with a heat resistance of 150°C or higher is preferred, and in particular ethylene-vinyl acetate copolymer (EVA), ethylene-acrylic acid copolymer, ethylene-ethyl acrylate copolymer (EEA), polyvinyl butyral (PVB), fluorocarbon rubber, styrene-butadiene rubber (SBR), and the like are even more preferred. Furthermore, a cross-linked acrylic resin with a low glass transition temperature (self-crosslinking acrylic resin) having a structure in which butyl acrylate is cross-linked as the main component is also preferred.

[0096] The heat-resistant porous layer contains heat-resistant fine particles as a major component. "Include as a major component" means that the heat-resistant fine particles are present in a proportion of 70% by weight or more of the total amount of the constituent components of the heat-resistant porous layer.

[0097] The content of heat-resistant fine particles in the heat-resistant porous layer is preferably 80% by weight or more, more preferably 85% by weight or more, based on the total weight of the constituent components of the heat-resistant porous layer. The heat shrinkage of the entire porous film, including the microporous polyolefin membrane, can be satisfactorily suppressed if the heat-resistant fine particles are included as the main component in the heat-resistant porous layer. For example, the preferred upper limit of the content of heat-resistant fine particles in the heat-resistant porous layer is 99% by weight or less, based on the total weight of the constituent components of the heat-resistant porous layer.

[0098] The organic binder content in the heat-resistant porous layer is preferably 1.1 to 30 parts by weight, based on 100 parts by weight of the heat-resistant fine particles. If the organic binder content is 30 parts by weight or less, the pores of the heat-resistant porous layer are not filled with the organic binder, which is preferred because it does not impair the separator function. If the organic binder content is 1.1 parts by weight or more, the effect of the organic binder becomes significant.

[0099] The thickness of the heat-resistant porous layer is not particularly limited, but is preferably 0.5 µm or more, more preferably 1 µm or more, and even more preferably 2 µm or more. The thickness of the heat-resistant porous layer is preferably 10 µm or less, more preferably 8 µm or less, and even more preferably 6 µm or less. If the heat-resistant porous layer has a thickness of 0.5 µm or more, the effect of preventing melting due to the presence of the heat-resistant porous layer can be sufficiently achieved. Furthermore, if the heat-resistant porous layer has a thickness of 10 µm or less, defects in the heat-resistant porous layer are less likely to occur, which is preferred.Furthermore, if the heat-resistant porous layer is 10 µm or less, it is possible to prevent an increase in the content of the electrolyte solution injected into an energy storage device with the separator film for an energy storage device, which would otherwise be caused by an excessively thick heat-resistant porous layer, thereby increasing the manufacturing costs of the battery and reducing the energy density per volume and per weight, which is preferred.

[0100] If the average thickness of the microporous polyolefin membrane is a (µm) and the average thickness of the heat-resistant porous layer is b (µm), the ratio of the thicknesses a / b is preferably 0.5 or more and 20 or less, more preferably 1 or more and 10 or less. If the value of a / b is 20 or less, the thickness of the heat-resistant porous layer is not too thick relative to the microporous polyolefin membrane, and an increase in the electrolyte solution content injected into the microporous polyolefin membrane with the heat-resistant porous membrane can be prevented, thus avoiding a decrease in energy density. If the value of a / b is 0.5 or more, the heat-resistant porous layer can sufficiently prevent melting.

[0101] The Gurley value (air permeability) of the microporous polyolefin membrane with a heat-resistant porous layer on one or both sides is not particularly limited, but is preferably 80 to 700 seconds / 100 cm². 3 , preferably 90 to 650 seconds / 100 cm 3 and even more preferred 100 to 600 seconds / 100 cm 3 If the Gurley value is 700 seconds / 100 cm 3 If the Gurley value is 80 seconds / 100 cm² or less, the microporous polyolefin membrane with a heat-resistant porous layer can exhibit sufficient functionality when used as a separator for an energy storage device. 3 or more is preferred because it is easy to ensure the uniformity of the internal reaction when used as a separator for an energy storage device. <energiespeichervorrichtung>

[0102] The energy storage device of the present invention comprises at least one positive electrode, one negative electrode, and a separator arranged between the positive and negative electrodes. The energy storage device may also contain a non-aqueous electrolyte solution introduced into the separator.

[0103] The energy storage device of the present invention has the microporous polyolefin membrane of the present invention or the separator film for an energy storage device of the present invention as a separator.

[0104] The microporous polyolefin membrane or separator film of the present invention, or energy storage device of the present invention, can be used, for example, as a separator for the first and second energy storage devices described below. The microporous polyolefin membrane or separator film of the present invention is preferably used as a separator for a lithium-ion battery (first energy storage device) or a lithium-ion capacitor (second energy storage device) that uses a lithium salt as the electrolyte, preferably for a lithium-ion battery, and even more preferably for a lithium-ion secondary battery.

[0105] The shape of the separator used in the energy storage device can be adapted as needed, for example, depending on the shape of the lithium-ion secondary battery. Likewise, the shapes of the positive and negative electrodes of the energy storage device can be adapted to the shape of the lithium-ion secondary battery, and so on. [Lithium-ion secondary battery (first energy storage device)]

[0106] The lithium-ion secondary battery as an energy storage device of the present invention comprises a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte solution. [Non-aqueous electrolyte solution]

[0107] The non-aqueous electrolyte solution used in the energy storage device of the present invention contains, for example, an electrolyte salt and a non-aqueous solvent.

[0108] The non-aqueous electrolyte solution can be in a liquid state or in a gel state. [Non-aqueous solvent]

[0109] Preferred examples of the non-aqueous solvent used for the non-aqueous electrolyte solution include cyclic carbonates and chain esters. Since the electrochemical properties synergistically improve over a wide temperature range, particularly at high temperatures, a chain ester is preferably included, a chain carbonate is more preferably included, and both a cyclic carbonate and a chain carbonate are most preferably included. The term "chain ester" is used as a concept encompassing both a chain carbonate and a chain carboxylate ester.

[0110] Examples of cyclic carbonates include one or more selected from ethylene carbonate (EC), propylene carbonate (PC) and vinylene carbonate (VC), with the combination of EC and PC being particularly preferred.

[0111] It is preferred that the non-aqueous solvent contains ethylene carbonate and / or propylene carbonate, as this increases the stability of a coating formed on an electrode and improves the high-temperature and high-voltage cycle properties. The ethylene carbonate and / or propylene carbonate content is preferably 3% by volume or more, more preferably 5% by volume or more, and even more preferably 7% by volume or more, based on the total volume of the non-aqueous solvent. The upper limit is preferably 45% by volume or less, more preferably 35% by volume or less, and even more preferably 25% by volume or less.

[0112] Preferred examples of chain esters include ethyl methyl carbonate (EMC) as an asymmetric chain carbonate, dimethyl carbonate (DMC) and diethyl carbonate (DEC) as symmetric chain carbonates, and ethyl propionate (hereinafter EP) as a chain carboxylate ester. Among the chain esters, asymmetric chain esters such as EMC and EP, which contain an ethoxy group, can be combined.

[0113] Although the chain ester content is not particularly limited, it is preferably used in the range of 60 to 90% by volume, based on the total volume of the non-aqueous solvent. If the chain ester content is 60% by volume or more, the viscosity of the non-aqueous electrolyte solution is not too high, which is preferred. If the content is 90% by volume or less, the non-aqueous electrolyte solution can hardly degrade with respect to its electrical conductivity and thus its electrochemical properties over a wide temperature range, especially at high temperatures, which is preferred.

[0114] The volume fraction of EP among the chain esters is preferably 1% by volume or more, and more preferably 2% by volume or more in the non-aqueous solvent. The upper limit is preferably 30% by volume or less, and more preferably 20% by volume or less. The asymmetric chain carbonate preferably has an ethyl group, and ethyl methyl carbonate is particularly preferred.

[0115] The ratio of cyclic carbonate to chain ester, which is the ratio cyclic carbonate:chain ester (volume ratio), is preferably in the range of 10:90 to 45:55, more preferably in the range of 15:85 to 40:60 and particularly preferably in the range of 20:80 to 35:65 with regard to improving the electrochemical properties over a wide temperature range, especially at high temperatures. [Electrolyte salt]

[0116] Preferred examples of an electrolyte salt used for the energy storage device of the present invention are lithium salts. One or more lithium salts selected from the group consisting of LiPF6, LiBF4, LiN(SO2F)2 and LiN(SO2CF3)2 are preferred, with one or more selected from the group consisting of LiPF6, LiBF4 and LiN(SO2F)2 being more preferred, and LiPF6 being even more preferred. [Preparation of a non-aqueous electrolyte solution]

[0117] The non-aqueous electrolyte solution used in the energy storage device of the present invention can be prepared, for example, by adding an electrolyte salt to a non-aqueous solvent and, if necessary, by adding a composition prepared by mixing a compound, such as a dissolving agent, with the non-aqueous electrolyte solution in a specific mixing ratio, and by mixing the same.

[0118] The compound to be added to the non-aqueous electrolyte solution is preferably pre-purified to contain as few impurities as possible, within a range that does not significantly reduce performance. [Positive electrode]

[0119] The positive electrode of a lithium-ion secondary battery has a positive electrode current collector and a positive electrode composite layer formed on the positive electrode current collector. The positive electrode composite layer contains an active material for the positive electrode, a conductive material, and a binder.

[0120] For example, a mixed metal oxide containing one or more elements from the group consisting of cobalt, manganese, nickel, and lithium is used as the active material for the positive electrode. These active materials for the positive electrode can be used alone or in combination with one or more of them.

[0121] Examples of such lithium mixed metal oxides include LiCoO2, LiCo 1-x M x O2 (where M is one or more elements selected from Sn, Mg, Fe, Ti, Al, Zr, Cr, V, Ga, Zn and Cu), LiMn2O4, LiNiO2, LiCo 1-x Ni x O2, LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3 O2, LiNi 0,5 Mn 0,3 Co 0,2 Mn 0,3 O2, LiNi 0,8 Mn 0,1 Co 0,1 O2, LiNi 0,8 Co 0,15 Al 0,05 O2, a solid solution of Li2MnO3 and LiMO2 (where M is a transition metal such as Co, Ni, Mn and Fe), and LiNi 1 / 2 Mn 3 / 2 O4, one.

[0122] The conductive material for the positive electrode is not particularly restricted, as long as it is an electron-conducting material that does not undergo chemical changes. For example, one or more types of carbon black, selected from graphite such as natural graphite (like flake graphite) and synthetic graphite, and carbon black can be used.

[0123] Examples of binders that can be used include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), a styrene-butadiene copolymer (SBR), an acrylonitrile-butadiene copolymer (NBR), or carboxymethylcellulose (CMC).

[0124] The current collector of the positive electrode can consist of aluminum foil, a stainless steel plate, or similar materials.

[0125] The positive electrode can be manufactured, for example, using the following method. The active material of the positive electrode, the conductive material, and the binder are mixed, a solvent is added, and the mixture is kneaded to produce the positive electrode compound. This compound is then applied to a positive electrode current collector, dried, molded under pressure, and heat-treated under predetermined conditions to form a layer of the positive electrode compound. [Negative electrode]

[0126] The negative electrode of a lithium-ion secondary battery has a current collector and a negative electrode composite layer formed on the current collector. The negative electrode composite layer contains an active material for the negative electrode, a conductive material, and a binder.

[0127] Suitable active materials for the negative electrode include lithium metals and lithium alloys; as well as carbon materials, tin (simple substance), tin compounds, silicon (simple substance), silicon compounds, and lithium titanate compounds such as Li4Ti5O. 12 or similar substances that can encapsulate and release lithium, used alone or in combination with two or more.

[0128] Among these, highly crystalline carbon materials such as synthetic graphite and natural graphite are preferable in terms of their ability to trap and release lithium ions.

[0129] Artificial graphite particles are preferably used as the active material for the negative electrode. These particles have a block structure in which a large number of flat, fine graphite particles are aggregated or bonded non-parallel to each other, or particles obtained by spheroidizing natural graphite flakes through repeated application of mechanical forces such as pressure, friction, and shear.

[0130] The conductive material and binder used for the negative electrode can be the same as those used for the positive electrode.

[0131] The current collector of the negative electrode can be made from copper foil or similar material.

[0132] The negative electrode can be manufactured, for example, using the following method. The active material of the negative electrode, the conductive material, and the binder are mixed. A solvent is then added, and the mixture is kneaded to produce the negative electrode compound. This compound is then applied to a negative electrode current collector, dried, die-cast, and heat-treated under specified conditions to form a composite layer for the negative electrode.

[0133] The structure of the lithium-ion secondary battery as one of the energy storage devices of the present invention is not particularly limited, and for example a button battery, a cylindrical battery, a prismatic battery, a layered battery or the like can be used. [Winded lithium-ion secondary battery]

[0134] A wound-type lithium-ion secondary battery, which is an example of a lithium-ion secondary battery, has a structure in which an electrode assembly, along with the non-aqueous electrolyte solution, is contained, for example, in a battery casing. The electrode assembly consists of a positive electrode, a negative electrode, and a separator. At least a portion of the non-aqueous electrolyte solution is incorporated into the electrode assembly.

[0135] The wound lithium-ion secondary battery contains, as the positive electrode, a long, sheet-like positive electrode current collector and a positive electrode composite layer containing an active material for the positive electrode and applied to the positive electrode current collector. The wound-type lithium-ion secondary battery contains, as the negative electrode, a long, flat current collector for the negative electrode and a negative electrode composite layer containing an active material for the negative electrode and applied to the negative electrode current collector.

[0136] The separator is shaped similarly to the positive and negative electrodes, in the form of a long arc. The positive and negative electrodes are wound into a cylindrical shape, with the separator positioned between them.

[0137] The battery casing consists of a cylindrical body with a base and a lid that closes the opening of the body. The lid and the body are made of metal, for example, and are insulated from each other. The lid is electrically connected to the current collector of the positive electrode, and the body is electrically connected to the current collector of the negative electrode. The lid can also serve as a terminal for a positive electrode, and the body can also serve as a terminal for a negative electrode.

[0138] The lithium-ion secondary battery can be charged and discharged in a temperature range of -40 to 100°C, preferably in the range of -10 to 80°C. As a measure against an internal pressure increase in the wound-type lithium-ion secondary battery, a method for providing a safety valve on the battery cover or a method for creating an interruption in an element such as the battery casing or seal can also be adopted. A current-interruption mechanism that detects the internal pressure of a battery and interrupts the current can also be provided in a cover as a safety measure to prevent overcharging. [Method for manufacturing a wound-type lithium-ion secondary battery]

[0139] As an example, a process for manufacturing a lithium-ion secondary battery is described below.

[0140] First, a positive electrode, a negative electrode, and a separator are manufactured separately. Then, an electrode assembly is assembled by layering and cylindrical winding. The electrode assembly is then inserted into a housing, and a non-aqueous electrolyte solution is injected into the housing. This impregnates the electrode assembly with the non-aqueous electrolyte solution. After the injection of the non-aqueous electrolyte solution into the housing, the housing is covered with a lid, and the lid and housing are sealed.

[0141] The shape of the electrode arrangement after winding is not limited to a cylindrical shape. For example, a positive electrode, a separator, and a negative electrode can be wound and then flattened by applying pressure from the side.

[0142] The lithium-ion secondary battery mentioned above can be used as a secondary power source for various purposes. For example, vehicles such as cars are equipped with this battery, which can be used as an energy source for propulsion systems like motors. While the types of vehicles are not particularly limited, examples include hybrid cars, plug-in hybrid cars, electric cars, fuel cell vehicles, and the like. Such a lithium-ion secondary battery can be used alone, or multiple batteries can be connected in series and / or parallel. [Layered lithium-ion secondary battery]

[0143] A wound lithium-ion secondary battery has been described above, but the present invention is not limited to this and can also be applied to a layered lithium-ion secondary battery.

[0144] A layered lithium-ion secondary battery, for example, is a battery in which positive and negative electrodes are stacked alternately with separators in between and then layered (sealed).

[0145] The layered lithium-ion secondary battery can be manufactured according to the following method. For example, a positive electrode or a negative electrode is inserted and packaged between two separators. In the present invention, the positive electrode is a packaged electrode. The separator is slightly larger than the electrodes. The electrode body is embedded between a pair of separators, and the tabs extending from the ends of the electrode project outwards from the separators. A layered battery can be manufactured by joining the side edges of a pair of stacked separators together and packaging them in a bag, and then alternately stacking one electrode and the other electrode packaged with this separator and impregnating them with an electrolyte solution. At this point, the separator and the electrodes can be compressed in the direction of thickness to reduce the thickness. [Lithium-ion capacitor (second energy storage device)]

[0146] Another energy storage device within the meaning of the present invention is a lithium-ion capacitor.

[0147] The lithium-ion capacitor of the present invention comprises a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte solution. The microporous polyolefin membrane or separator film of the present invention is used as the separator for an energy storage device of the present invention. Lithium-ion capacitors can store energy by utilizing the incorporation of lithium ions into a carbon material such as graphite, which serves as the negative electrode. Examples of the positive electrode include those that use an electrical double layer between an activated carbon electrode and an electrolyte solution, and those that utilize the doping / dedoping reaction of a π-conjugated polymer electrode. The non-aqueous electrolyte solution contains at least one lithium salt such as LiPF6.

[0148] The energy storage device of the present invention exhibits low resistance and excellent safety, since the microporous polyolefin membrane of the present invention is embedded between the electrodes, and can be suitably incorporated into various products such as vehicles, aircraft and power tools. EXAMPLES

[0149] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. (Experimental procedures for evaluation)

[0150] The microporous polyolefin membrane, its raw materials and film webs, produced according to the procedure described below, were evaluated for the following points according to the procedure described below. (Measurement of film thickness)

[0151] Five ribbon-shaped test specimens with a total width of 50 mm in the longitudinal direction (MD direction) were produced from the sample. The five test specimens were stacked, and the thickness was measured at 25 equally spaced measuring points in the width direction using an electronic micrometer (Millitron 1240 Stylus 5 mmφ (flat surface, pen pressure 0.75 N), manufactured by Feinpruf GmbH). At each point, one-fifth of the measured value was taken as the thickness per sheet, and the average value was calculated as the film thickness. [Surface opening ratio]

[0152] The surface of the polypropylene-based resin layer and the surface of the polyethylene-based resin layer of the microporous polyolefin membrane were examined using a scanning electron microscope (SEM). The images were binarized, and the surface open area ratio was calculated using image analysis. The surface open area ratio was then evaluated as a percentage by calculating the total pore area through binarization and dividing it by the area subjected to image analysis.

[0153] In all examples and comparisons, the surface open-cell ratio of both surfaces of the polypropylene-based resin layer was approximately equal, and the surface open-cell ratio of both surfaces of the polyethylene-based resin layer was also approximately equal. Furthermore, the surface open-cell ratio between each of the two polypropylene-based resin layers was also approximately equal.

[0154] The electrical resistance ER is defined as the resistance value (Ω·cm). 2 / µm) of a separator filled with an electrolyte solution in a microporous polyolefin membrane. The unit of electrical resistance is Ω·cm. 2 / µm. The separator's resistance is characterized by cutting small pieces of the separator from the obtained material and then placing them between two platinum electrodes. The separator was impregnated and saturated with an electrolyte solution containing LiPF6 at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of EC / EMC = 3 / 7. The separator's resistance (R) in ohms (Ω) is measured using a four-probe AC ​​impedance method. To reduce the measurement error at the platinum electrode-separator interface, multilayer measurements are required by adding further layers. The electrical (ionic) resistance (R) was determined from these multilayer measurements. s ) in Ohm (Ω) of the separator saturated with electrolyte solution according to equation R s = ρ s t / A is calculated, where ρ s the specific ion resistance (Ω·cm) of the separator, A the area (cm²) 2 ) of the electrode and t the thickness (cm) of the separator. The ratio ρ s / A is the calculated slope of the change in separator resistance (ΔR) with respect to the multilayer (Δδ), given by slope = ρ s / A = ΔR / Δδ. The resistance value per 1 µm, which is obtained by dividing the resulting R s The resistance resulting from the film thickness is expressed as ER (Ω·cm). 2 / µm) defined. [Porosity]

[0155] Two 100 mm × 100 mm test specimens were extracted from the sample using a form, along both end faces in the width direction. The weight of each specimen was measured to an accuracy of 0.1 mg. The porosity was calculated from the measured weight using the following formula. Porosity(%)=[1−{w / (L1×L2×t)×ρ}]×100 w: Weight of the test specimen (g) L1: Vertical length of the test specimen (cm) L2: Horizontal length of the test specimen (cm) t: Thickness of the test specimen (cm) ρ: Density of the test specimen (g / cm³) 3 ) (Puncture resistance, insertion elongation)

[0156] A needle with a diameter of 1 mm and a spherical tip (radius of curvature R: 0.5 mm) is inserted through a microporous membrane of thickness T (µm) at a speed of 90 mm / min. The maximum stress upon penetration at this speed was measured. The measured value of the maximum stress L (N) was converted into the maximum stress P (N / µm) per 1 µm of membrane thickness using the following formula: P = L / T, which was then used as the puncture resistance (needle penetration resistance). The elongation at maximum stress was used as the insertion strain (needle penetration strain).

[0157] Furthermore, the value of the following formula (I) was calculated from the measured puncture resistance and the porosity measured above (in Tables 1 and 2, the calculated value is given as the value on the left side of formula (I)). 100×Puncture resistance / (100−Porosity)≥0.44 [Shutdown properties]

[0158] The shutdown properties were verified using an electrical resistance measuring cell. A mixed solvent was prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of EC / EMC = 3 / 7. An electrolyte solution containing LiPF6 dissolved in the mixed solvent at a concentration of 1 mol / L was introduced into a microporous polyolefin membrane to create a separator test specimen. The electrolyte-impregnated separator test specimen was embedded between nickel electrodes, and the temperature was increased at a rate of 10°C / min. The electrical resistance ER was measured using a resistance meter (LCR HiTester (manufactured by Hioki Electric Industry Co., Ltd.)) at a measurement frequency of 1 kHz.When the temperature of the microporous polyolefin membrane rises to or above the melting point of the PE resin, all or some of the pores in the PP resin layer become blocked and closed within this temperature range. This increases the resistance of the separator film, measured as impedance, and when it exceeds 1000 Ω, the separator film is found to exhibit shutdown properties. In Tables 1 and 2, microporous polyolefin membranes exhibiting shutdown properties are marked with “○”, and those without are marked with “×”. [Tensile strength]

[0159] The measurements were performed in accordance with ASTM D-822. A tensile test was carried out using a tensile testing machine with a 100 N load cell under conditions of a clamping distance of 50 mm and a crosshead speed of 50 mm / min. The tensile strength was determined from the load W (kg) at the moment of failure of the specimen and the cross-sectional area S (mm²). 2 , the thickness is the mean value of the foil thickness measurements) of the test specimen calculated according to the following formula. Tensile strength (kg / mm2) = W / S [Tensile elongation]

[0160] The tensile strain was calculated according to the following formula from the gauge length L0 (mm) of the test specimen before the test and the gauge length L (mm) at the time of tearing. Tensile strain (%) = (L−L0) / L0×100

[0161] The tensile strength in the MD direction was rounded to the nearest whole number by rounding up to one decimal place. The tensile elongation was rounded to the nearest whole number by rounding down to one decimal place.

[0162] The mean values ​​of the measured values ​​were evaluated as tensile strength and tensile elongation. [Air permeability]

[0163] A test specimen with a full width of 80 mm in the MD direction was taken from the microporous polyolefin membrane and measurements were taken at three points—in the middle, and at the left and right ends (50 mm inward from the end face)—using a Gurley densometer type B (manufactured by Toyo Seiki Co., Ltd.) according to JIS P8117. The average of the three values ​​was determined as the value for air permeability. [Tortuosity]

[0164] The tortuosity (τ) is calculated according to the following formula, where A is the area (cm²). 2 ) of the membrane, R is the resistance (Ω·cm) of the membrane, ε is the porosity, t is the thickness (cm) of the membrane and ζ is the resistance (Ω·cm) of the electrolyte solution. τ=(A×R×ε / (t×ζ) )1 / 2 [Zero shear viscosity]

[0165] The shear-dynamic viscoelasticity of molten PP resin was measured using an ARES rheometer (model: ARES) manufactured by TA Instruments. The geometry used was a cone-parallel plate (cone angle 0.1 rad). Dynamic viscoelasticity measurements were performed at four temperatures: 220°C, 200°C, 180°C, and 160°C, under conditions with a frequency range of 400 to 0.01 s. -1 (5 points per decade) and a strain of 0.1 (10%) were performed, and a master curve was created using the measurement data at 200°C as a standard. The viscosity value, which is measured in the frequency range of 0.01 s -1 or less constant, was assumed to be the zero shear viscosity. In this description, a master curve is created based on the measurement data mentioned above at 200°C, and the zero shear viscosity is calculated based on this master curve, which is referred to as the "200°C condition." The zero shear viscosity of the PP resin used in Examples 1 to 16 and Comparison Examples 1 to 4 was 16,000 Pa·s at 200°C. The zero shear viscosity of the PE resin used in Examples 1 to 16 and Comparison Examples 1 to 4 was 34,000 Pa·s at 200°C. (Example 1)

[0166] The following is an example of a process for producing the microporous polyolefin membrane of Example 1. The process for producing the microporous polyolefin membrane of Example 1 is not limited to the process described below, and other processes may also be used. For example, the microporous polyolefin membrane of Example 1 can also be produced using an unstretched PP or PE web film and by performing a lamination step and a stretching step. [Production of foil webs]

[0167] Using a polypropylene-based resin and a polyethylene-based resin, a coextruder equipped with a multi-diete was used to coextrude the two resins at a die temperature (coextrusion temperature) of 180 to 230°C. The PE layer formed the middle layer, with PP layers on both sides, to produce a precursor film consisting of a three-layer structure including a PE layer (a three-layer structure of PP layer / PE layer / PP layer). The PE layer content in the precursor film was 12.8% by weight. [Extension procedure]

[0168] The resulting precursor film was then heat-treated in a temperature range of 128.0°C to 136.0°C, and the heat-treated precursor film was subsequently drawn in a low-temperature cold-drawing zone. The low-temperature drawn precursor film was then hot-drawn in a hot-drawing zone at 130 to 135°C with a draw ratio of 180 to 300% (maximum draw ratio) and subsequently thermally relaxed to a draw ratio of 100 to 200% (final draw ratio). Afterward, the film was heat-set at a specific temperature to produce a porous film. In this example, the precursor film was drawn uniaxially in the machine direction (MD). The results are shown in Table 1.

[0169] Table 1 also shows the PE content, membrane thickness, surface opening ratio of the PP layer and the PE layer, ER, porosity, puncture resistance, puncture resistance per unit thickness corresponding to the substantial resin fraction without pores (formula (I)), puncture elongation, shutdown properties, tensile strength, tensile elongation, air permeability and tortuosity of the microporous polyolefin membrane of Example 1. (Examples 2 to 16, comparison example 1)

[0170] A microporous polyolefin membrane was produced in the same manner as in Example 1, except that the membrane thickness and PE content were changed to the values ​​given in Table 1 by adjusting the amount of resin extruded during melt extrusion. The results are shown in Tables 1 and 2. (Comparative examples 2 to 4)

[0171] The same polypropylene-based resin as in Example 1 was used, and an unstretched polypropylene (PP) sheet was produced using a T-nozzle. This unstretched PP sheet was then bonded to the desired thickness by thermocompression and subsequently stretched in the same manner as in Example 1 to produce a microporous polyolefin membrane. The results are shown in Table 2. [Table 1] [Table 2]

[0172] As shown in Table 1, porous films were obtained according to Examples 1 to 16, in which the PE content was adjusted to 5% to 45% by weight and the surface open area ratio of the PP layer to 5% to 30%, while the surface open area ratio of the PE layer was adjusted to 5% to 20.6%. The porous film thus obtained had an ER of 0.06 Ω·cm². 2 / µm or less, a puncture resistance per unit thickness corresponding to the essential non-porous resin component (formula (I)) of 0.44 or more, a penetration elongation of 5.8 mm or more, shutdown properties, and a tortuosity of 1.6 or less. From these results, it can be deduced that the porous films obtained in Examples 1 to 16 exhibit both low resistance and high puncture resistance and shutdown properties and can therefore be used, for example, as separators for lithium-ion batteries.

[0173] On the other hand, in comparative example 1, which involved a porous film in which the PE content was adjusted to more than 45 wt%, the surface opening ratio of the PP was 10.0% and the surface opening ratio of the PE exceeded 20%, but no effective pore length that could become through holes was formed, resulting in an ER of more than 0.06 Ω·cm 2 / µm and a tortuosity greater than 1.6. Furthermore, the puncture strength per unit thickness, corresponding to the main resin component excluding the pore content (formula (I)), was 0.44 or less, which is an inadequate result. Furthermore, in comparison examples 2, 3 and 4, which are porous PP films that do not contain PE, the ER was 0.046 Ω·cm 2 / µm or less, the puncture resistance per unit thickness corresponding to the actual resin component without the pore content (formula (I)) was 0.35 or less, the penetration elongation was 4.1 mm or less, and they also lacked shutdown properties, leading to unsatisfactory results. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] JP-A-2017-141428

[0006] < / energiespeichervorrichtung> < / zugdehnung> < / zugfestigkeit> < / foliendicke>

Claims

A microporous polyolefin membrane comprising a polyethylene-based resin layer containing a polyethylene-based resin and a polypropylene-based resin layer containing a polypropylene-based resin, wherein the polyethylene-based resin content of the total weight of the microporous polyolefin membrane is 5% to 45% by weight, the porosity of the microporous polyolefin membrane is 42% to 62%, and the surface opening ratio of the polypropylene-based resin layer is 5% to 30% and the surface opening ratio of the polyethylene-based resin layer is 5% to 20.6%. The microporous polyolefin membrane according to claim 1, wherein the polyethylene-based resin layer is an intermediate layer. The microporous polyolefin membrane according to claim 1 or 2, wherein the microporous polyolefin membrane has a layered structure in which the polyethylene-based resin layer and the polypropylene-based resin layer are layered, and the layered structure is a three-layered structure in which the polypropylene-based resin layers are present on both sides of the polyethylene-based resin layer. The microporous polyolefin membrane according to claim 1 or 2, wherein the microporous polyolefin membrane has a puncture resistance (N / µm) and a porosity (%) satisfying the following formula (I): 100 × puncture resistance / (100 − porosity a ¨ t ) ≥ 0.44 The microporous polyolefin membrane according to claim 1 or 2, wherein the microporous polyolefin membrane has an electrical resistance of 0.06 (Ω·cm2 / µm) or less. The microporous polyolefin membrane according to claim 1 or 2, wherein the microporous polyolefin membrane has a tortuosity of 1 or more and 2 or less. The microporous polyolefin membrane according to claim 1 or 2, wherein the surface opening ratio of the polypropylene-based resin layer is 6% to 25%. The microporous polyolefin membrane according to claim 1 or 2, wherein the surface opening ratio of the polyethylene-based resin layer is 7% to 19%. The microporous polyolefin membrane according to claim 1 or 2, wherein the surface opening ratio of the polyethylene-based resin layer is higher than the surface opening ratio of the polypropylene-based resin layer. The microporous polyolefin membrane according to claim 9, wherein the difference between the surface opening ratio of the polyethylene-based resin layer and the surface opening ratio of the polypropylene-based resin layer is 1% or more and 8% or less. A separator film for an energy storage device comprising the microporous polyolefin membrane according to claim 1 or 2. An energy storage device comprising the separator film for an energy storage device according to claim 11, a positive electrode and a negative electrode. A vehicle, an aircraft or a power tool equipped with the energy storage device according to claim 12.