Polyolefin microporous membrane, separator film for power storage devices using same, and power storage device

Abstract

Disclosed is a polyolefin microporous membrane which has a polyethylene resin layer that contains a polyethylene resin and a polypropylene resin layer that contains a polypropylene resin, wherein: the content ratio of the polyethylene resin in the total mass of the polyolefin microporous membrane is 5 mass% to 45 mass%; the porosity of the polyolefin microporous membrane is 42% to 62%; the surface aperture ratio of the polypropylene resin layer is 5% to 30%; and the surface aperture ratio of the polyethylene resin layer is 5% to 20.6%.

Description

Polyolefin microporous membrane, separator film for power storage device using same, and power storage device

[0001] The present invention relates to a polyolefin microporous membrane and an electricity storage device.

[0002] In recent years, electricity storage devices with high energy density, high electromotive force, and low self-discharge have become widespread. Examples of electricity storage devices include lithium ion secondary batteries and lithium ion capacitors. In lithium ion secondary batteries and lithium ion capacitors, a separator film is interposed between the positive electrode and the negative electrode to prevent short-circuiting due to contact between the positive electrode and the negative electrode. A polyolefin microporous membrane is used as the separator film.

[0003] When the internal temperature of an electricity storage device rises above a predetermined temperature due to an abnormal current or the like, a separator film made of a polyolefin microporous membrane can block the pores in the porous membrane, preventing ions from flowing between the electrodes and increasing electrical resistance. This stops the function of the electricity storage device and prevents risks such as fire due to excessive temperature rise. The function of preventing risks such as fire due to excessive temperature rise is extremely important for separator films for electricity storage devices, and is generally referred to as non-porous or shutdown (hereinafter referred to as SD).

[0004] An example of a polyolefin microporous membrane used as a battery separator is the polyolefin microporous membrane described in Patent Document 1. Patent Document 1 discloses a polyolefin microporous membrane having a three-layer laminate separator film sandwiched between two outer layers made of polypropylene and an inner layer made of polyethylene, characterized by a thickness of 7.0 to 50 μm, a porosity of 30 to 70%, and a surface open area of ​​10 to 30%.

[0005] JP 2017-141428 A

[0006] In recent years, separators in electricity storage devices are required to have even lower resistance and improved safety while maintaining a thin film. Patent Document 1 describes air permeability, but does not describe low resistance performance related to liquid resistance. The present invention has been made in view of the above circumstances, and aims to provide a polyolefin microporous membrane that maintains a thin film while exhibiting low resistance and excellent safety. Another aim of the present invention is to provide an electricity storage device that has the above polyolefin microporous membrane interposed between electrodes and has low resistance and excellent safety.

[0007] To solve the above problems, the present inventors have conducted extensive research focusing on the material, content ratio, porosity, and surface open area of ​​a polyolefin microporous membrane.

[0008] [1] A polyolefin microporous membrane having 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 accounts for 5% to 45% by mass of the total mass of the polyolefin microporous membrane, the porosity of the polyolefin microporous membrane is 42% to 62%, and the surface porosity of the polypropylene-based resin layer is 5% to 30% and 5% to 20.6%. [2] The polyolefin microporous membrane according to [1], wherein the polyethylene-based resin layer is an intermediate layer. [3] The polyolefin microporous membrane according to [1] or [2], wherein the polyolefin microporous membrane has a laminate structure in which the polyethylene-based resin layer and the polypropylene-based resin layer are laminated, and the laminate structure is a three-layer structure in which polypropylene-based resin layers are present on both sides of the polyethylene-based resin layer. [4] The polyolefin microporous membrane according to any one of [1] to [3], wherein the polyolefin microporous membrane has a pin puncture strength (N / μm) and a porosity (%) that satisfy the following formula (I): 100 × pin puncture strength / (100 − porosity) ≥ 0.44 (1). [5] The polyolefin microporous membrane has an electrical resistance of 0.06 (Ω cm ２ / μm) or less. [6] The polyolefin microporous membrane of any of [1] to [5], having a tortuosity of 1 or more and 2 or less. [7] The polyolefin microporous membrane of any of [1] to [6], having a surface porosity of the polypropylene-based resin layer of 6% to 25%. [8] The polyolefin microporous membrane of any of [1] to [7], having a surface porosity of the polyethylene-based resin layer of 7% to 19%. [9] The polyolefin microporous membrane of any of [1] to [8], having a higher surface porosity of the polyethylene-based resin layer than that of the polypropylene-based resin layer.

[10] The polyolefin microporous membrane of [9], having a difference between the surface porosity of the polyethylene-based resin layer and the surface porosity of the polypropylene-based resin layer of 1% to 8%.

[11] A separator film for an electricity storage device, comprising the polyolefin microporous membrane of any of [1] to

[10] .

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

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

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

[12] .

[0009] According to the present invention, by adjusting the specific material, content, porosity, and surface open area of ​​the polyolefin microporous membrane, it is possible to provide a polyolefin microporous membrane that has shutdown properties while simultaneously improving low resistance and pin puncture strength.

[0010] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the following embodiments.

[0011] <Polyolefin microporous membrane> The polyolefin microporous membrane of the present invention contains at least a polypropylene (hereinafter sometimes referred to as PP)-based resin and a polyethylene (hereinafter sometimes referred to as PE)-based resin as resin materials. The polyolefin microporous membrane has a multilayer structure.

[0012] <Polyethylene Resin Content> From the viewpoint of enhancing pin puncture strength while maintaining the shutdown property described below, the polyethylene resin content in the total mass of the polyolefin microporous membrane has a lower limit of 5% by mass or more, preferably 9% by mass or more, more preferably 11% by mass or more, and particularly preferably 13% by mass or more. The upper limit is 45% by mass or less, preferably 38% by mass or less, more preferably 32% by mass or less, and particularly preferably 29% by mass or less.

[0013] <Porosity> The porosity of the polyolefin microporous membrane of the present invention is 42% or more and 62% or less, from the viewpoint of ensuring sufficient mechanical strength of the polyolefin microporous membrane and further ensuring sufficient ion mobility while suppressing short circuits. It 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, it is preferably 60% or less, more preferably 58% or less, and even more preferably 56% or less. A porosity within the above range allows for the production of an electricity storage device with good performance. Here, the porosity refers to the porosity of the entire polyolefin microporous membrane.

[0014] <Polypropylene-based resin and polypropylene-based resin layer> The polypropylene-based resin is a polymer contained in the polypropylene-based resin layer as a main component in a proportion of 80% by mass or more. In the present invention, such polymers may 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 may also contain these additives. The content of the polypropylene-based resin in the entire polypropylene-based resin layer is preferably 85% by mass or more, more preferably 90% by mass or more. The upper limit is not particularly limited, but is preferably 100% by mass or less.

[0015] It is preferable to use a polypropylene-based resin having high stereoregularity. The pentad fraction 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 limited, but is preferably 100% or less.

[0016] 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. When the polypropylene-based resin has a zero-shear viscosity of 10,000 Pa·s or higher, a polyolefin microporous membrane can be obtained that maintains the moldability of the PP during shutdown. Furthermore, by using a polypropylene-based resin having a zero-shear viscosity of 100,000 Pa·s or lower, the shape retention characteristics of the laminate structure are improved. As a result, the shape and pores of the polypropylene-based resin layer can be reliably maintained even when the interior of an electrical storage device using the polyolefin microporous membrane as a separator is subjected to an environment exceeding 200°C. The zero-shear viscosity of the polypropylene-based resin at 200°C is more preferably 13,000 to 80,000 Pa·s, even more preferably 14,000 to 50,000 Pa·s, and particularly preferably 14,500 to 40,000 Pa·s.

[0017] The lower limit of the crystalline melting peak temperature (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 lower, more preferably 173° C. or lower, even more preferably 170° C. or lower, and most preferably 169° C. or lower. When the melting point of the polypropylene-based resin is 155° C. or higher, a polyolefin microporous membrane with excellent processability can be obtained.

[0018] <Polyethylene Resin and Polyethylene Resin Layer> The polyethylene resin is a polymer contained in the polyethylene resin layer as a main component at a ratio of 80% by mass or more. In the present invention, such a polymer may be used alone as the polyethylene resin, or a mixture of two or more types may be used. In addition, polyethylene resins generally contain additives such as surfactants, antioxidants, plasticizers, flame retardants, and colorants for various purposes. The polyethylene resin used in the present invention may also contain these additives. The polyethylene resin preferably accounts for 85% by mass or more, more preferably 90% by mass or more, of the entire polyethylene resin layer. The upper limit is not particularly limited, but is preferably 100% by mass or less.

[0019] The density of polyethylene resin is 0.950 g / cm ３ 0.970g / cm or more ３ The polyethylene resin preferably has a density of 0.960 g / cm or less. ３ The above high density polyethylene is more preferable, but medium density polyethylene may also be used.

[0020] The zero shear viscosity of the polyethylene 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 higher is preferred because it provides the polyethylene resin layer with sufficient strength. Furthermore, a zero shear viscosity of 90,000 Pa·s or lower is preferred because it reduces the impact on the shape of the polypropylene resin layer. The zero shear viscosity of the polyethylene resin at 200°C is more preferably 20,000 to 70,000 Pa·s, and even more preferably 25,000 to 50,000 Pa·s.

[0021] The melting point of the polyethylene 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 polyolefin microporous membrane of the present invention, shutdown occurs at temperatures equal to or higher than the melting point of the polyethylene resin. That is, molten polyethylene resin flows into the pores of the polypropylene resin layer, blocking ion movement. It is preferable that the melting point of the PE resin used in the polyolefin microporous membrane is 140°C or lower, because shutdown can be initiated early. It is not easy to use a polyethylene resin with a melting point below 100°C as a raw material for the polyolefin microporous membrane.

[0022] <Surface porosity of polypropylene-based resin layer> The surface porosity of the polypropylene-based resin layer of the present invention is 5% or more and 30% or less, from the viewpoint of ensuring sufficient ion mobility and effectively suppressing short circuits. Preferably 5% or more, more preferably 6% or more, and even more preferably 7% or more. On the other hand, preferably 28% or less, more preferably 25% or less, even more preferably 20% or less, and particularly preferably 19% or less. The surface porosity of both surfaces of the polypropylene-based resin layer (i.e., the surface porosity of one surface and the surface porosity of the other surface) may be the same or different as long as they are within the above range. When the polyolefin microporous membrane of the present invention has multiple polypropylene-based resin layers, it is sufficient that the surface porosity of at least one of the multiple polypropylene-based resin layers is within the above range, but it is preferable that the surface porosity of all polypropylene-based resin layers included in the polyolefin microporous membrane of the present invention is within the above range. For example, when the polyolefin microporous membrane of the present invention has two polypropylene-based resin layers, the surface porosity of at least one polypropylene-based resin layer may be within the above range, but it is preferable that the surface porosity of both polypropylene-based resin layers be within the above range.

[0023] <Surface porosity of polyethylene resin layer> The surface porosity of the polyethylene resin layer of the present invention is 5% or more and 20.6% or less. Preferably, it is 6% or more, more preferably 7% or more, even more preferably 9% or more, and particularly preferably 12% or more. On the other hand, it is preferably 19.5% or less, more preferably 19% or less, and even more preferably 18.5% or less. When it is in the above range, it is possible to simultaneously reduce electrical resistance and improve pin puncture strength while ensuring ion mobility and shutdown properties. The surface porosity of both surfaces of the polyethylene resin layer (i.e., the surface porosity of one surface and the surface porosity of the other surface) may be the same or different as long as they are within the above range. When the polyolefin microporous membrane of the present invention has multiple polyethylene resin layers, it is sufficient that the surface porosity of at least one of the multiple polyethylene resin layers is in the above range, but it is preferable that the surface porosity of all polyethylene resin layers included in the polyolefin microporous membrane of the present invention is in the above range.

[0024] In the polyolefin microporous membrane of the present invention, the surface porosity of the polyethylene resin layer is preferably higher than that of the polypropylene resin layer, and such an embodiment can further increase the pin puncture strength even when the porosities are approximately the same. When the surface porosity of the polyethylene resin layer is higher than that of the polypropylene resin layer, the difference between the surface porosity of the polyethylene resin layer and the surface porosity of the polypropylene resin layer is preferably 1% or more, more preferably 2% or more, even more preferably 3% or more, and is preferably 8% or less, more preferably 7% or less, and even more preferably 6% or less.

[0025] <Laminated structure with polyethylene-based resin layer as intermediate layer> From the viewpoint of improving shutdown properties and pin puncture strength, the polyolefin microporous membrane of the present invention preferably has a laminated structure with a polyethylene-based resin layer as an intermediate layer. A polypropylene-based resin layer may be present on both sides of the polyethylene-based resin layer, or a polypropylene-based resin layer may be present on one surface and a polyethylene-based resin layer may be present on the other surface. The laminated structure referred to here simply requires that a polyethylene-based resin layer be an intermediate layer, and that a polypropylene-based resin layer be present on at least one surface of the polyethylene-based resin layer. The total number of layers in the laminated structure may be 4 or more.

[0026] <Three-layer laminate structure> From the viewpoints of improving shutdown properties and pin puncture strength, the polyolefin microporous membrane of the present invention preferably has a three-layer laminate structure in which polypropylene-based resin layers are present on both sides of a polyethylene-based resin layer, i.e., a three-layer laminate structure in which polypropylene-based resin layer / polyethylene-based resin layer / polypropylene-based resin layer are laminated in this order.

[0027] <Regarding Formula (I) for Pin Puncture Strength and Porosity> The pin puncture strength and porosity of the polyolefin microporous membrane of the present invention can be measured by the measurement methods described below, and the pin puncture strength (N / μm) and porosity (%) per μm of membrane thickness preferably satisfy the following formula (I): 100 × pin puncture strength / (100 − porosity) ≧ 0.44 (I) When formula (I) is satisfied, the pin puncture strength of the membrane for an electricity storage device separator film can be improved while maintaining good ionic conductivity. The left side of formula (I) is preferably 0.44 or more, more preferably 0.45 or more, even more preferably 0.47 or more, and even more preferably 0.48 or more. The upper limit of the value of "100 × pin puncture strength / (100 − porosity)" is not particularly limited, but is preferably 0.54 (i.e., 100 × pin puncture strength / (100 − porosity) ≦ 0.54). Formula (I) is a value indicating the pin puncture strength per thickness equivalent to the substantial resin component excluding the pore portion, and in formula (I), the unit of pin puncture strength is "N / μm" and the unit of porosity is "%".

[0028] <Electrical Resistance (ER)> The polyolefin microporous membrane of the present invention has an electrical resistance of 0.06 (Ω·cm) in order to achieve good ionic conductivity. ２ The electrical resistance referred to here is the electrical resistance per 1 μm of film thickness. Electrical resistance (Ω cm ２ / μm) is more preferably 0.057 or less, even more preferably 0.055 or less, and even more preferably 0.05 or less, and the electrical resistance (Ω cm ２ The lower limit of the surface area (μm) is not particularly limited, but is preferably 0.01 or more. Within the above range, the electrical resistance can be reduced while maintaining the shutdown property and pin puncture strength. The electrical resistance is measured by filling the polyolefin microporous membrane with an electrolyte and measuring the resistance of the separator filled with the electrolyte. In this case, the electrolyte is LiPF 4 at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of EC / EMC = 3 / 7. ６　 A polyolefin microporous membrane is impregnated with an electrolyte solution containing the above dissolved therein, and the membrane is saturated with the electrolyte solution to be used as a measurement sample.

[0029] <Film Thickness> The thinner the overall thickness of the polyolefin microporous membrane of the present invention, the more compact the electricity storage device can be, which is preferable. If the overall thickness of the polyolefin microporous membrane is too thick, when used as a separator in an electricity storage device, the volume ratio of the separator increases, resulting in a smaller capacity per unit volume of the electricity storage device, which is undesirable. The overall thickness of the polyolefin microporous membrane is preferably 30 μm or less, more preferably 25 μm or less, and even more preferably 20 μm or less. By setting the thickness of the laminated structure to 20 μm or less, the capacity of an electricity storage device using this polyolefin microporous membrane as a separator can be increased. On the other hand, it 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. A thickness of 7 μm or more provides good mechanical strength as a separator interposed between electrodes, making the membrane less likely to rupture. Therefore, electricity storage devices using the polyolefin microporous membrane as a separator are less likely to short-circuit. The thickness of the polyolefin microporous membrane can be determined by image analysis of an image of a cross section of the microporous membrane taken with a scanning electron microscope (SEM), or by using a dot-type thickness measuring device.

[0030] <Air permeability> The lower limit of the air permeability (Gurley value) of the polyolefin microporous membrane of the present invention is preferably 80 s / 100 cc or more, more preferably 90 s / 100 cc or more, even more preferably 95 s / 100 cc or more, still more preferably 100 s / 100 cc or more, and particularly preferably 130 s / 100 cc or more. The upper limit of the air permeability (Gurley value) is preferably 500 s / 100 cc or less, more preferably 400 s / 100 cc or less, even more preferably 300 s / 100 cc or less, and even more preferably 250 s / 100 cc or less.

[0031] A polyolefin microporous membrane with an air permeability of 500 s / 100 cc or less is preferred because ion migration is less likely to be suppressed or hindered when used as a separator in an electricity storage device. Conversely, a polyolefin microporous membrane with an air permeability of 80 s / 100 cc or more can prevent excessive ion migration and a rapid temperature rise in the event of a failure in the electricity storage device when used as a separator in an electricity storage device.

[0032] <Tensile strength> The tensile strength of the polyolefin microporous membrane in the MD direction has a lower limit of preferably 130 MPa or more, more preferably 150 MPa or more, even more preferably 170 MPa or more, and even more preferably 180 MPa or more, and an upper limit of preferably 250 MPa or less, more preferably 230 MPa or less, even more preferably 210 MPa or less, and even more preferably 200 MPa or less. A tensile strength in the MD direction within the above range maintains good ionic conductivity, while a tensile strength of 150 MPa or more tends to further suppress breakage during slitting or winding of the electricity storage device, or to further suppress short circuits due to foreign matter in the electricity storage device. On the other hand, the tensile strength of the microporous membrane in the TD (direction perpendicular to the MD, membrane width direction) direction is preferably 5 MPa or more, more preferably 6 MPa or more, and even more preferably 7 MPa or more, and an upper limit of preferably 20 MPa or less, more preferably 18 MPa or less, and even more preferably 16 MPa or less.

[0033] <Tensile elongation> The tensile elongation of the polyolefin microporous membrane is preferably 45% or more, more preferably 50% or more, particularly preferably 60% or more, in both MD and TD. An MD tensile elongation of 45% or more can reduce the possibility of the separator breaking when the battery is deformed by external force, etc. Furthermore, when the polyolefin microporous membrane and the electrode are laminated in the presence of minute foreign matter, the polyolefin microporous membrane is distorted in the minute sections that are generated, causing pinholes and reducing the possibility of battery failure due to micro-short circuits. The upper limit of the MD and / or TD tensile elongation of the polyolefin microporous membrane is not particularly limited.

[0034] <Tortuosity> The upper limit of the tortuosity of the polyolefin microporous membrane is preferably 2 or less, more preferably 1.8 or less, even more preferably 1.6 or less, and even more 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 even more preferably 1.35 or more. The tortuosity can be explained as an indicator of the tortuous path that ions take from one side of the porous membrane through the pores of the membrane body to the opposite side of the membrane. A low tortuosity as described above promotes greater or faster movement of ions and electrolytes through the porous battery separator membrane during charge / discharge cycles of a lithium ion battery compared to a high tortuosity. A tortuosity within the above range can improve pin puncture strength.

[0035] <Shutdown property> When the internal temperature of an electricity storage device rises above a predetermined temperature due to an abnormal current or the like, a separator film made of a polyolefin microporous membrane can block the pores of the porous membrane, preventing ions from flowing between the electrodes and increasing electrical resistance. This stops the function of the electricity storage device, preventing risks such as fire due to excessive temperature rise. The function of preventing risks such as fire due to excessive temperature rise is extremely important for separator films for electricity storage devices, and is generally referred to as non-porous or shutdown (hereinafter referred to as SD). The shutdown property of the polyolefin microporous membrane of the present invention refers to the fact that when the polyolefin microporous membrane is heated to a temperature above the melting point of the polyethylene resin as described above, all or part of the pores in the polypropylene resin layer are blocked in this temperature range, causing the resistance of the separator film, measured as impedance, to increase; if the resistance exceeds 1000 Ω, the shutdown property is determined to be present.

[0036] <Puncture Strength, Puncture Elongation> In this specification, the puncture strength refers to the stress of a film sample against the force applied by fixing the periphery of the film sample and piercing a needle of a specific size from the outer surface of the film sample in the thickness direction of the film sample. The puncture elongation refers to the distance (elongation) that the needle travels from the time the needle contacts the film sample until a hole is made when the periphery of the film sample is fixed and pierced from the outer surface of the film sample in the thickness direction of the film sample. The methods for measuring the puncture elongation and puncture strength are described in the examples below.

[0037] From the viewpoints of separator productivity and secondary battery safety, the puncture strength of the polyolefin microporous membrane is preferably 2 N or more, more preferably 3 N or more, and although the upper limit is not particularly limited, it is preferably 10 N or less. From the viewpoints of puncture strength, wettability to non-aqueous solvents, and voltage resistance, the puncture elongation of the microporous film 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, and although the upper limit is not particularly limited, it is preferably 10 mm or less.

[0038] <Method for producing polyolefin microporous membrane> The method for producing the polyolefin microporous membrane of the present invention will be described below. The polyolefin microporous membrane of the present invention is preferably produced by a dry process in which no solvent is used during production.

[0039] [Precursor Film] The precursor film of the present invention comprises a polyethylene-based resin layer (hereinafter referred to as PE layer) containing the above-mentioned polyethylene-based resin and a polypropylene-based resin layer (hereinafter referred to as PP layer) containing the above-mentioned polypropylene-based resin. The precursor film may have any configuration, as long as it has a PP layer on top of a PE layer. Examples of such configurations include three-layer structures such as PE layer / PP layer / PE layer, PP layer / PE layer / PP layer, and five-layer structures such as PP layer / PE layer / PP layer / PE layer / PP layer, and PE layer / PP layer / PE layer / PP layer / PE layer / PP layer. Among these, a three-layer structure in which a pair of PP layers containing a polypropylene-based resin are disposed on both sides of a PE layer containing a polyethylene-based resin, i.e., a PP layer / PE layer / PP layer, is preferred. Since the laminate structure of the precursor film and the laminate structure of the polyolefin microporous membrane are essentially the same, the laminate structure of the precursor film can be determined depending on the laminate structure of the resulting polyolefin microporous membrane.

[0040] [Method for producing precursor film] The precursor film of the present invention can be produced by a method of producing a film by collectively extruding the above-mentioned polyethylene-based resin and the above-mentioned polypropylene-based resin into a film, i.e., a method for producing a film by co-extrusion. For example, in the case of producing a precursor film having a PE layer as an intermediate layer and PP layers provided on both sides thereof, the precursor film can be produced by co-extrusion of the polyethylene-based resin and the polypropylene-based resin so that the PE layer is the intermediate layer and the PP layer is formed on both sides thereof.

[0041] When co-extruding a polyethylene resin and a polypropylene resin, the apparatus used for co-extrusion is not particularly limited, and conventionally known apparatuses can be used.Such co-extrusion apparatuses include, for example, a co-extruder equipped with a circular die, a co-extruder equipped with a feed block or a multi-manifold T-die, etc.In addition, as a method for forming a raw sheet, melt molding using a T-die is also suitable.When forming the PP film and PE film, which are raw sheets used for a polyolefin microporous membrane, by melt molding using a T-die, etc., the PP film and the PE film may be formed separately.

[0042] When co-extrusion is performed using a co-extruder, the die temperature is preferably 185 to 240°C, more preferably 190 to 235°C, and even more preferably 195 to 230°C. By setting the die temperature to 185°C or higher, a precursor film can be formed without breaking during film formation. Furthermore, by setting the die temperature to 240°C or lower, deterioration of the polypropylene-based resin due to heat can be reduced, and deterioration in the properties of the final porous film can be suppressed.

[0043] [Method for producing microporous polyolefin membrane] The microporous polyolefin membrane of the present invention is produced by stretching the precursor film of the present invention described above to make it porous.

[0044] When a precursor film is stretched to form pores, it is preferable to preliminarily heat-treat the precursor film before stretching it to form pores. Preliminary heat treatment can crystallize the polyethylene-based resin and polypropylene-based resin that make up the precursor film, making the precursor film more suitable for stretching to form pores, and thus more efficiently forming pores by stretching. Furthermore, the crystallinity of the precursor film can be controlled by the heat-treatment conditions, and controlling the crystallinity by the heat-treatment conditions can adjust the pore opening characteristics of the porous film obtained by stretching it to form pores.

[0045] Examples of the heat treatment method include a method in which the precursor film is brought into contact with a preheated roll, or a method in which the precursor film is passed through an environment heated to a predetermined temperature, and any appropriate method may be used.

[0046] 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 setting the heat treatment temperature within this range, the crystallinity of the precursor film can be more appropriately increased, and the air permeability of the resulting porous film can be made sufficiently low.

[0047] In the manufacturing method of the present invention, for example, a laminated film obtained by thermocompression bonding (lamination process) a laminate obtained by laminating a PP film and a PE film obtained by melt molding using a T-die may be used as the precursor film.

[0048] [Laminating Process] In the laminating process, a laminate obtained by laminating raw rolls of PP film, PE film, and PP film in this order is thermocompression-bonded. In the present invention, for example, each film to be thermocompression-bonded is unwound from a raw roll stand, and passed between heated rolls with the PP film disposed on both sides of the PE film. This results in a laminated film in which the PP film is thermocompression-bonded to both sides of the PE film.

[0049] [Stretching Step] The precursor film thus heat-treated is stretched to make it porous (stretching porosity), thereby obtaining a porous film.

[0050] The stretching method used for perforation by stretching is not particularly limited, but examples thereof include uniaxial stretching in the machine direction (MD), uniaxial stretching in the width direction (TD) substantially perpendicular to the machine direction, sequential biaxial stretching in which stretching is performed in the machine direction (MD) and then in the width direction (TD), simultaneous biaxial stretching in which stretching is performed in the machine direction (MD) and the width direction (TD) substantially simultaneously, and tubular biaxial stretching. Of these, it is advisable to adopt a stretching method suitable for the purpose.

[0051] A specific method for making the film porous by stretching is, for example, to obtain a porous film by stretching a heat-treated precursor film at a low temperature in a low-temperature stretching zone and then stretching it at a higher temperature in a high-temperature stretching zone to make the film porous. Stretching by only one of low-temperature stretching and high-temperature stretching may not sufficiently make both the polypropylene-based resin and the polyethylene-based resin porous, and a porous film may not be formed.

[0052] The temperature for low-temperature stretching is not particularly limited, but is preferably −20° C. or higher and +50° C. or lower, and more preferably +20° C. or higher and +40° C. or lower. If the low-temperature stretching temperature is too low, the precursor film is likely to break during stretching, which is undesirable. On the other hand, if the low-temperature stretching temperature is too high, the polyethylene resin in the precursor film is less likely to be perforated, which is undesirable.

[0053] The stretching ratio of the low-temperature stretching is not particularly limited, but is preferably in the range of 3% or more and 200% or less, more preferably 5% or more and 100% or less. When the stretching ratio of the low-temperature stretching is 3% or more, a porous film with a sufficiently low Gurley value is easily obtained. On the other hand, when the stretching ratio of the low-temperature stretching exceeds 200%, the generated crazes turn into cracks, which cause film rupture. Therefore, the stretching ratio of the low-temperature stretching is preferably 200% or less.

[0054] The temperature of the high-temperature stretching is preferably 70° C. or higher and 150° C. or lower, more preferably 80° C. or higher and 145° C. or lower. By setting the temperature of the high-temperature stretching within this range, the high-temperature stretching can sufficiently increase the porosity, and a porous film with sufficiently low air permeability can be easily obtained.

[0055] The stretching ratio of the high-temperature stretching is not particularly limited, but is preferably in the range of 100% to 400%. If the stretching ratio of the high-temperature stretching is too low, the Gurley value of the porous film may not be sufficiently low. If the stretching ratio of the high-temperature stretching is too high, the air permeability of the porous film may be too low.

[0056] Through the above steps, the polyolefin microporous membrane of the present invention having a polyethylene resin layer containing a polyethylene resin and a polypropylene resin layer containing a polypropylene resin is obtained.

[0057] <Separator film for electricity storage device> The separator film for electricity storage device of this embodiment of the present invention has the polyolefin microporous membrane of the present invention. The separator film for electricity storage device of the present invention may consist solely of the polyolefin microporous membrane of the present invention. That is, the polyolefin microporous membrane of the present invention may be used as it is as a separator film for electricity storage device without any additional processing.

[0058] The separator film for an electricity storage device of the present invention may be any film as long as it has the polyolefin microporous membrane of the present invention. Therefore, the separator film for an electricity storage device of the present invention may have, for example, 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 polyolefin microporous membrane. The heat-resistant porous layer, adhesive layer, and functional layer may each be a single layer, or may be a laminate of multiple layers. Furthermore, the heat-resistant porous layer, adhesive layer, and functional layer may each be provided individually as a layer having a single function, or may be provided as a layer that combines the functions of at least two layers selected from the heat-resistant porous layer, adhesive layer, and functional layer.

[0059] The heat-resistant porous layer, adhesive layer, and functional layer may all be formed using known materials. The heat-resistant porous layer may be, for example, a layer made of heat-resistant fine particles and an organic binder. The adhesive layer may be, for example, a layer made of an organic material such as a fluorine-based resin. The functional layer may be, for example, a layer containing organic fine particles and a binder.

[0060] The heat-resistant porous layer, adhesive layer, and functional layer can all be formed by applying a predetermined coating liquid. For example, when forming a separator film for an electricity storage device having a heat-resistant porous layer, an adhesive layer, and a functional layer in this order on one or both sides of a polyolefin microporous membrane, it can be formed by the following method. A coating liquid containing a mixture of heat-resistant fine particles and an organic binder is applied to one or both sides of the polyolefin microporous membrane to form a heat-resistant porous layer. Next, an organic material such as a fluorine-based resin is applied to the heat-resistant porous layer to form an adhesive layer. Next, a coating liquid containing a mixture of organic fine particles and a binder is applied to the adhesive layer. This results in a separator film for an electricity storage device having a heat-resistant porous layer, an adhesive layer, and a functional layer in this order on one or both sides of the polyolefin microporous membrane.

[0061] [Heat-resistant porous layer] Next, the heat-resistant porous layer that may be provided in the separator film for an electricity storage device of the present invention will be described in detail. The heat-resistant porous layer contains heat-resistant fine particles, and preferably contains an organic binder. The heat-resistant porous layer contains heat-resistant fine particles, thereby improving the heat resistance of the separator film for an electricity storage device. The heat-resistant porous layer may be a single layer, or may be a multilayer in which multiple layers are laminated.

[0062] (Heat-resistant fine particles) The heat resistance of the heat-resistant fine particles is preferably 200° C. or higher, more preferably 300° C. or higher, and even more preferably 400° C. or higher. Here, "heat resistance" means that no change in shape, such as deformation, is visually observed at least at that temperature.

[0063] The heat-resistant fine particles are preferably inorganic fine particles having electrical insulation properties. Specific examples of heat-resistant fine particles include iron oxide, silica (SiO ２ ), alumina (Al ２ O ３ ), TiO ２ , magnesia, boehmite, BaTiO ２inorganic nitride fine particles such as aluminum nitride and silicon nitride; sparingly soluble ionic crystal fine particles such as calcium fluoride, barium fluoride and barium sulfate; covalently bonded crystal fine particles such as silicon and diamond; and clay fine particles such as montmorillonite.

[0064] Here, the inorganic oxide fine particles may be fine particles of substances derived from mineral resources such as boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, mica, or artificial products thereof. Furthermore, the inorganic compounds constituting these inorganic fine particles may be element-substituted or solid-solutioned as necessary. Furthermore, the inorganic fine particles may be surface-treated. Furthermore, the heat-resistant fine particles may be metal, SnO ２ The particles may be electrically insulating particles obtained by coating the surface of a conductive material, such as a conductive oxide such as tin-indium oxide (ITO), or a carbonaceous material such as carbon black or graphite, with an electrically insulating material (for example, the inorganic oxides described above).

[0065] Organic fine particles may also be used as the heat-resistant fine particles. Specific examples of organic fine particles include fine particles of crosslinked polymers such as polyimide, melamine resin, phenolic resin, aromatic polyamide resin, crosslinked polymethyl methacrylate (crosslinked PMMA), crosslinked polystyrene (crosslinked PS), polydivinylbenzene (PDVB), and benzoguanamine-formaldehyde condensates; and fine particles of heat-resistant polymers such as thermoplastic polyimides. The organic resin (polymer) constituting these organic fine particles may be a mixture, modified product, derivative, copolymer (random copolymer, alternating copolymer, block copolymer, graft copolymer), or crosslinked product (in the case of the heat-resistant polymer) of the above-mentioned materials.

[0066] The heat-resistant fine particles may be one of the above-mentioned examples or a combination of two or more of them. As the heat-resistant fine particles, inorganic fine particles and organic fine particles can be used as described above, and they may be used appropriately depending on the application.

[0067] As the heat-resistant fine particles, it is particularly preferable to use boehmite. For example, the boehmite used has an average particle size of preferably 0.001 μm or more, more preferably 0.1 μm or more, and preferably 15 μm or less, more preferably 3 μm or less. The average particle size of the heat-resistant fine particles can be defined as the number average particle size measured using, for example, a laser scattering particle size distribution meter (for example, "LA-920" manufactured by HORIBA) after dispersing the heat-resistant fine particles in a medium that does not dissolve them.

[0068] The shape of the heat-resistant fine particles may be, for example, nearly spherical or plate-like. From the viewpoint of preventing short circuits, the heat-resistant fine particles are preferably plate-like particles. Typical examples of plate-like heat-resistant fine particles include alumina and boehmite.

[0069] (Organic Binder) The organic binder is contained in the heat-resistant porous layer to bind the heat-resistant fine particles contained as a main component to each other and to bind the heat-resistant fine particles to the polyolefin microporous membrane. There are no particular limitations on the organic binder as long as it can effectively bond the heat-resistant fine particles to each other and to the heat-resistant fine particles and the polyolefin microporous membrane, and is electrochemically stable and stable to the electrolyte of the electricity storage device.

[0070] Examples of organic binders include ethylene-vinyl acetate copolymers (EVA, containing 20 to 35 mol% structural units derived from vinyl acetate), ethylene-acrylic acid copolymers such as ethylene-ethyl acrylate copolymers (EEA), fluororesins [such as polyvinylidene fluoride (PVDF)], fluorine-based rubbers, styrene-butadiene rubber (SBR), water-soluble cellulose derivatives such as carboxymethyl cellulose (CMC) and hydroxyethyl cellulose (HEC), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinylpyrrolidone (PVP), poly-N-vinylacetamide, cross-linked acrylic resins, polyurethanes, epoxy resins, polyimides, etc. These organic binders may be used alone or in combination of two or more.

[0071] Among these organic binders, heat-resistant resins having a heat resistance of 150°C or higher are preferred, and it is particularly preferred to use highly flexible materials such as ethylene-vinyl acetate copolymer (EVA), ethylene-acrylic acid copolymer, ethylene-ethyl acrylate copolymer (EEA), polyvinyl butyral (PVB), fluorine-based rubber, styrene-butadiene rubber (SBR), etc. It is also preferred to use, as the organic binder, a cross-linked acrylic resin (self-cross-linking acrylic resin) with a low glass transition temperature that has a cross-linked structure containing butyl acrylate as the main component.

[0072] The heat-resistant porous layer contains heat-resistant fine particles as a main component. "Containing as a main component" means that the heat-resistant fine particles account for 70% by mass or more of the total mass of the constituent components of the heat-resistant porous layer. The amount of heat-resistant fine particles in the heat-resistant porous layer is preferably 80% by mass or more, more preferably 85% by mass or more, of the total mass of the constituent components of the heat-resistant porous layer. When the heat-resistant porous layer contains heat-resistant fine particles as a main component, the spacing between the heat-resistant fine particles becomes appropriate, and thermal shrinkage of a separator film for an electricity storage device having a polyolefin microporous membrane can be effectively suppressed. A suitable upper limit of the amount of heat-resistant fine particles in the heat-resistant porous layer is, for example, 99% by mass or less of the total mass of the constituent components of the heat-resistant porous layer.

[0073] The content of the organic binder in the heat-resistant porous layer is preferably 1.1 to 30 parts by mass per 100 parts by mass of the heat-resistant fine particles. If the content of the organic binder is 30 parts by mass or less, the pores of the heat-resistant porous layer are not filled with the organic binder, which is preferable because it does not impair the function of the separator. If the content of the organic binder is 1.1 parts by mass or more, the effect of including the organic binder becomes significant.

[0074] 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. When the heat-resistant porous layer is 0.5 μm or more, the meltdown prevention effect due to the heat-resistant porous layer is sufficiently obtained. Furthermore, when the heat-resistant porous layer is 10 μm or less, defects are less likely to occur in the heat-resistant porous layer, which is preferable. Furthermore, when the heat-resistant porous layer is 10 μm or less, an excessively thick heat-resistant porous layer can be prevented from increasing the amount of electrolyte injected in an electricity storage device using the separator film for an electricity storage device, which can prevent an increase in battery manufacturing costs and a decrease in energy density per volume and per mass, which is preferable.

[0075] When the average thickness of the polyolefin microporous membrane is a (μm) and the average thickness of the heat-resistant porous layer is b (μm), the thickness ratio a / b is preferably 0.5 to 20, more preferably 1 to 10. When the value of a / b is 20 or less, the thickness of the heat-resistant porous layer relative to the polyolefin microporous membrane is not too thick, and an increase in the amount of electrolyte solution injected into the polyolefin microporous membrane having the heat-resistant porous membrane, which would result in a decrease in energy density, can be prevented. When the value of a / b is 0.5 or more, the meltdown prevention effect achieved by having the heat-resistant porous layer is sufficiently obtained.

[0076] The Gurley value (air permeability) of a polyolefin microporous membrane having a heat-resistant porous layer on one or both sides is not particularly limited, but is preferably 80 to 700 seconds / 100 cc, more preferably 90 to 650 seconds / 100 cc, and even more preferably 100 to 600 seconds / 100 cc. When the Gurley value is 700 seconds / 100 cc or less, the polyolefin microporous membrane having a heat-resistant porous layer can sufficiently function when used as a separator for an electricity storage device. When the polyolefin microporous membrane is used as a separator for an electricity storage device, a Gurley value of 80 seconds / 100 cc or more is preferred because it facilitates ensuring uniformity of the internal reaction.

[0077] <Electricity storage device> The electricity storage device of the present invention includes at least a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode. It may also include a nonaqueous electrolyte impregnated in the separator. The electricity storage device of the present invention has the polyolefin microporous membrane or the separator film for an electricity storage device of the present invention as the separator.

[0078] The polyolefin microporous membrane or separator film for an electricity storage device of the present invention can be used, for example, as a separator for the following first and second electricity storage devices. In particular, it is preferably used as a separator for a lithium ion battery (first electricity storage device) or a lithium ion capacitor (second electricity storage device) that uses a lithium salt as the electrolyte salt, more preferably for a lithium ion battery, and even more preferably for a lithium ion secondary battery.

[0079] The shape of the separator used in the electricity storage device may be adjusted appropriately depending on, for example, the shape of the lithium ion secondary battery, etc. Similarly, the shapes of the positive electrode and negative electrode of the electricity storage device may also be adjusted appropriately depending on, for example, the shape of the lithium ion secondary battery, etc.

[0080] [Lithium-ion secondary battery (first power storage device)] The lithium-ion secondary battery as the power storage device of the present invention has a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. [Non-aqueous electrolyte] The non-aqueous electrolyte used in the power storage device of the present invention contains, for example, an electrolyte salt and a non-aqueous solvent. The non-aqueous electrolyte may be in a liquid state or a gel state.

[0081] [Non-aqueous solvent] Suitable non-aqueous solvents used in the non-aqueous electrolyte solution include cyclic carbonates and chain esters. The non-aqueous solvent preferably contains a chain ester, more preferably a chain carbonate, and most preferably both a cyclic carbonate and a chain carbonate, since this synergistically improves the electrochemical properties over a wide temperature range, particularly at high temperatures. The term "chain ester" is used as a concept that includes chain carbonates and chain carboxylic acid esters.

[0082] The cyclic carbonate may be one or more selected from ethylene carbonate (EC), propylene carbonate (PC), and vinylene carbonate (VC), and a combination of EC and PC is particularly preferred.

[0083] The non-aqueous solvent preferably contains ethylene carbonate and / or propylene carbonate, since this increases the stability of the coating formed on the electrode and improves the high-temperature, high-voltage cycle characteristics. The content of ethylene carbonate and / or propylene carbonate 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.

[0084] Suitable examples of the chain ester include ethyl methyl carbonate (EMC) as an asymmetric chain carbonate, dimethyl carbonate (DMC) and diethyl carbonate (DEC) as symmetric chain carbonates, and ethyl propionate (EP) as a chain carboxylic acid ester. The chain ester can be a combination of EMC and an asymmetric chain ester containing an ethoxy group, such as EP.

[0085] The content of the chain ester is not particularly limited, but is preferably in the range of 60 to 90% by volume relative to the total volume of the nonaqueous solvent. A content of 60% by volume or more is preferred because the viscosity of the nonaqueous electrolyte does not become too high. A content of 90% by volume or less is preferred because the electrical conductivity of the nonaqueous electrolyte decreases, reducing the risk of a deterioration in electrochemical properties over a wide temperature range, especially at high temperatures.

[0086] The volume ratio of EP among the chain esters in the non-aqueous solvent is preferably 1% by volume or more, more preferably 2% by volume or more. The upper limit is preferably 30% by volume or less, more preferably 20% by volume or less. The asymmetric chain carbonate more preferably has an ethyl group, and ethyl methyl carbonate is particularly preferred.

[0087] From the viewpoint of improving electrochemical properties over a wide temperature range, particularly at high temperatures, the ratio of cyclic carbonate to chain ester (volume ratio) is preferably 10:90 to 45:55, more preferably 15:85 to 40:60, and particularly preferably 20:80 to 35:65.

[0088] [Electrolyte Salt] The electrolyte salt used in the electricity storage device of the present invention is preferably a lithium salt. ６ , LiBF ４ , LiN(SO ２ F) ２ , LiN(SO ２ CF ３ ) ２ Preferably, one or more selected from the group consisting of LiPF ６ , LiBF ４ and LiN(SO ２ F) ２ More preferably, one or more selected from LiPF ６ It is most preferred to use

[0089] [Production of Non-Aqueous Electrolyte] The non-aqueous electrolyte used in the electricity storage device of the present invention can be obtained, for example, by adding an electrolyte salt to a non-aqueous solvent, and, if necessary, adding a composition prepared by mixing a compound such as a solubilizing agent with the non-aqueous electrolyte at a specific mixing ratio, and mixing them. The compound to be added to the non-aqueous electrolyte is preferably purified in advance to a degree that does not significantly reduce productivity, and the amount of impurities is preferably as small as possible.

[0090] [Positive Electrode] The positive electrode of the 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 a positive electrode active material, a conductive agent, and a binder. As the positive electrode active material, for example, a composite metal oxide containing lithium and one or more elements selected from the group consisting of cobalt, manganese, and nickel is used. These positive electrode active materials may be used alone or in combination of two or more.

[0091] Examples of such lithium composite metal oxides include LiCoO ２ , LiCo １－ｘ Mｘ O ２ (wherein M is one or more elements selected from Sn, Mg, Fe, Ti, Al, Zr, Cr, V, Ga, Zn, and Cu), LiMn ２ O ４ , LiNiO ２ , LiCo １－ｘ Ni ｘ O ２ , LiCo １／３ Ni １／３ Mn １／３ O ２ , LiNi ０．５ Mn ０．３ Co ０．２ Mn ０．３ O ２ , LiNi ０．８ Mn ０．１ Co ０．１ O ２ , LiNi ０．８ Co ０．１５ Al ０．０５ O ２ , Li ２ MnO ３ and LiMO ２ (M is a transition metal such as Co, Ni, Mn, or Fe) and LiNi solid solutions. １／２ Mn ３／２ O ４ One or more selected from the following are preferred.

[0092] The conductive agent for the positive electrode is not particularly limited as long as it is an electron-conductive material that does not undergo chemical changes. Examples include graphite such as natural graphite (e.g., flake graphite), artificial graphite, and one or more types of carbon black selected from acetylene black. Examples of binders include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer (NBR), and carboxymethyl cellulose (CMC). Examples of the positive electrode current collector include aluminum foil and stainless steel plate.

[0093] The positive electrode can be produced, for example, by the following method: A positive electrode active material, a conductive agent, and a binder are mixed, and a solvent is added and kneaded to form a positive electrode mixture. The positive electrode mixture is then applied to a positive electrode current collector, dried, pressure-molded, and heat-treated under predetermined conditions to form a positive electrode mixture layer.

[0094] [Negative Electrode] The negative electrode of a lithium ion secondary battery has a current collector and a negative electrode mixture layer formed on the current collector. The negative electrode mixture layer contains a negative electrode active material, a conductive agent, and a binder. The negative electrode active material can be lithium metal, a lithium alloy, a carbon material capable of absorbing and releasing lithium, tin (element), a tin compound, silicon (element), a silicon compound, or Li ４ Ti ５ O １２ These lithium titanate compounds can be used either alone or in combination of two or more.

[0095] Among these, it is more preferable to use highly crystalline carbon materials such as artificial graphite and natural graphite in terms of their lithium ion absorption and desorption capabilities. As the negative electrode active material, it is particularly preferable to use artificial graphite particles having a block structure in which a plurality of flat graphitic fine particles are aggregated or bonded non-parallel to each other, or particles obtained by spheroidizing flake natural graphite by repeatedly applying mechanical actions such as compressive force, frictional force, and shear force.

[0096] The conductive agent and binder used in the negative electrode can be the same as those used in the positive electrode. Copper foil or the like is used as the negative electrode current collector. The negative electrode can be produced, for example, by the following method. A negative electrode active material is mixed with a conductive agent and a binder, and a solvent is added and kneaded to form a negative electrode mixture. The negative electrode mixture is then applied to the negative electrode current collector, dried, pressure-molded, and heat-treated under specified conditions to form a negative electrode mixture layer.

[0097] The structure of the lithium ion secondary battery as one of the power storage devices of the present invention is not particularly limited, and for example, a coin-type battery, a cylindrical battery, a prismatic battery, a laminated battery, or the like can be applied.

[0098] [Wound-Type Lithium-Ion Secondary Battery] A wound-type lithium-ion secondary battery, which is an example of a lithium-ion secondary battery, has a configuration in which, for example, an electrode body is housed in a battery case together with a non-aqueous electrolyte. The electrode body is composed of a positive electrode, a negative electrode, and a separator. At least a portion of the non-aqueous electrolyte is impregnated into the electrode body.

[0099] The wound lithium-ion secondary battery includes a positive electrode comprising a long sheet-shaped positive electrode current collector and a positive electrode composite layer containing a positive electrode active material and disposed on the positive electrode current collector. The negative electrode comprises a long sheet-shaped negative electrode current collector and a negative electrode composite layer containing a negative electrode active material and disposed on the negative electrode current collector. The separator, like the positive electrode and negative electrode, is formed in a long sheet shape. The positive electrode and negative electrode are wound into a cylindrical shape with the separator interposed therebetween.

[0100] The battery case includes a cylindrical case body with a bottom and a lid that closes the opening of the case body. The lid and case body are made of, for example, metal and are insulated from each other. The lid is electrically connected to the positive electrode current collector, and the case body is electrically connected to the negative electrode current collector. The lid may also serve as the positive electrode terminal, and the case body may also serve as the negative electrode terminal.

[0101] Lithium-ion secondary batteries can be charged and discharged at temperatures between −40 and 100° C., preferably between −10 and 80° C. Furthermore, measures to prevent an increase in the internal pressure of wound lithium-ion secondary batteries can be taken, such as providing a safety valve on the battery lid or making cuts in the battery case body, gasket, or other components. Furthermore, as a safety measure to prevent overcharging, a current-breaking mechanism that senses the internal pressure of the battery and cuts off the current can be provided on the lid.

[0102] [Manufacturing Method of Wound Lithium-ion Secondary Battery] An example of a manufacturing method of a wound lithium-ion secondary battery is described below. First, a positive electrode, a negative electrode, and a separator are each prepared. Next, they are stacked and wound into a cylindrical shape to assemble an electrode body. Next, the electrode body is inserted into a case body, and a non-aqueous electrolyte is poured into the case body. This impregnates the electrode body with the non-aqueous electrolyte. After the non-aqueous electrolyte is poured into the case body, a lid is placed on the case body, and the lid and case body are sealed. Note that the shape of the electrode body after winding is not limited to a cylindrical shape. For example, after winding the positive electrode, separator, and negative electrode, pressure may be applied from the side to form a flattened shape.

[0103] The lithium ion secondary battery described above can be used as a secondary battery for various applications. For example, it can be mounted on a vehicle such as an automobile and suitably used as a power source for a driving source such as a motor that drives the vehicle. The type of vehicle is not particularly limited, but examples include hybrid vehicles, plug-in hybrid vehicles, electric vehicles, and fuel cell vehicles. Such a lithium ion secondary battery can be used alone or by connecting multiple batteries in series and / or parallel.

[0104] "Laminated Lithium-ion Secondary Battery" While the wound lithium-ion secondary battery has been described above, the present invention is not limited to this and may be applied to a laminated lithium-ion secondary battery. A laminated lithium-ion secondary battery is, for example, a battery in which positive electrodes and negative electrodes are alternately stacked with separators interposed therebetween and laminated (sealed).

[0105] A laminated lithium-ion secondary battery can be manufactured by the following method. For example, a positive or negative electrode is sandwiched between a pair of separators and packaged. In the present invention, the positive electrode is a bagged electrode. The separator is slightly larger than the electrode. The electrode body is sandwiched between the pair of separators, and the tab protruding from the electrode end is made to protrude outward from the separator. The side edges of the stacked pair of separators are joined together to form a bag. One electrode and the other electrode packaged with this separator are alternately stacked and impregnated with an electrolyte, thereby producing a laminated battery. At this time, the separators and electrodes may be compressed in the thickness direction to reduce the thickness.

[0106] [Lithium-ion capacitor (second power storage device)] Another power storage device of the present invention is a lithium-ion capacitor. The lithium-ion capacitor of the present invention has a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The separator uses the polyolefin microporous membrane or separator film for a power storage device of the present invention. The lithium-ion capacitor can store energy by utilizing the intercalation of lithium ions into the negative electrode, which is a carbon material such as graphite. Examples of the positive electrode include those that utilize an electric double layer between an activated carbon electrode and an electrolyte, and those that utilize the doping / dedoping reaction of a π-conjugated polymer electrode. The non-aqueous electrolyte contains at least LiPF ６ Lithium salts such as

[0107] The electricity storage device of the present invention has low resistance and excellent safety due to the polyolefin microporous membrane of the present invention being sandwiched between electrodes, and can be suitably installed in various products such as vehicles, aircraft, and power tools.

[0108] The present invention will now be described in more detail with reference to examples, but the present invention is not limited to these examples.

[0109] (Evaluation Experiment Method) The polyolefin microporous membranes produced by the methods described below, their raw materials, and raw sheets were evaluated for the following items by the methods described below.

[0110] [Film Thickness Measurement] Five tape-shaped test pieces with a lengthwise (MD) dimension of 50 mm across the entire width were prepared from the sample. The five test pieces were stacked, and the thickness was measured at 25 measurement points at equal intervals in the width direction using an electric micrometer (Millitron 1240 stylus 5 mmφ (flat surface, stylus pressure 0.75 N) manufactured by Fineprüf GmbH). 1 / 5 of the measured value was taken as the thickness per sheet at each point, and the average value was calculated to be the film thickness.

[0111] [Surface porosity] The surface of the polypropylene resin layer and the surface of the polyethylene resin layer of the polyolefin microporous membrane were observed with a scanning electron microscope (SEM), and the images were binarized to calculate the surface porosity by image analysis.The surface porosity was evaluated as a percentage by calculating the total area of ​​the pores through binarization and dividing it by the area subjected to image analysis.In each example and comparative example, the surface porosity of both surfaces of the polypropylene resin layer was almost the same, and the surface porosity of both surfaces of the polyethylene resin layer was also almost the same.Furthermore, the surface porosity between each of the two polypropylene resin layers was also almost the same.

[0112] [Electrical Resistance (ER)] Electrical resistance ER is determined by filling a polyolefin microporous membrane with an electrolyte solution and measuring the resistance (Ω cm) of the separator filled with the electrolyte solution. ２ The unit of electrical resistance is Ω cm ２ The separator resistance is characterized by cutting small pieces of separator from the finished material and placing them between two platinum electrodes. LiPF was dissolved in a solvent mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 3:7 (EC / EMC) to give a concentration of 1 mol / L. ６　The separator is impregnated and saturated with an electrolyte solution containing the dissolved platinum. The resistance (R) of the separator in ohms (Ω) is measured by a four-probe AC ​​impedance method. To reduce measurement errors at the platinum electrode / separator interface, multiple measurements are required by adding more layers. The electrical (ionic) resistance (R) of the separator saturated with the electrolyte is then calculated based on the multiple layer measurements. ｓ ) (Ω) into the formula R s = ρ s t / A (in the formula, ρ s is the ionic resistivity of the separator (Ω cm), and A is the area of ​​the electrode (cm ２ ) and t is the thickness of the separator (cm). s The ratio of ρ / A is the calculated slope of the change in separator resistance (ΔR) with respect to the multilayer (Δδ), where slope = ρ s / A = ΔR / Δδ. s The resistance value per 1 μm obtained by dividing by the film thickness is the electrical resistance ER (Ω cm ２ / μm).

[0113] [Porosity] Two 100 mm x 100 mm test pieces were taken using a mold along both end faces at both ends in the width direction of the sample. The mass of each of the two test pieces taken was measured to the nearest 0.1 mg. The porosity was calculated from the measured mass using the following formula: Porosity (%) = [1 - {w / (L1 x L2 x t) x ρ}] x 100 w: Mass of test piece (g) L1: Vertical length of test piece (cm) L2: Horizontal length of test piece (cm) t: Thickness of test piece (cm) ρ: Density of test piece (g / cm ３ )

[0114] [Puncture Strength, Puncture Elongation] A microporous film having a thickness of T (μm) was punctured at a speed of 90 mm / min with a 1 mm diameter needle having a spherical tip (radius of curvature R: 0.5 mm), and the maximum load was measured. The measured maximum load, L (N), was converted to the maximum load P (N / μm) per 1 μm of film thickness using the formula: P = L / T, and this was taken as the puncture strength. The elongation at the maximum load was taken as the puncture elongation. The value of the following formula (I) was calculated from the measured puncture strength and the porosity measured above (in Tables 1 and 2, the calculated value is shown as the value on the left side of formula (I)). 100 × puncture strength / (100 - porosity) ≧ 0.44 ... (I)

[0115] [Shutdown Characteristics] The shutdown characteristics were confirmed using an electrical resistance measurement cell. A mixed solvent was prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of EC / EMC = 3 / 7. LiPF was added to the mixed solvent to a concentration of 1 mol / L. ６　 A polyolefin microporous membrane was impregnated with an electrolyte solution containing the above-mentioned compound dissolved therein to prepare a separator sample. The separator sample impregnated with the electrolyte was sandwiched between nickel electrodes and heated at a rate of 10°C / min. Electrical resistance ER was measured using a resistance measuring device (LCR Hitester (manufactured by Hioki Electric Industry Co., Ltd.)) at a measurement frequency of 1 kHz. When the polyolefin microporous membrane was heated to a temperature above the melting point of the PE resin, all or part of the pores in the PP resin layer were blocked in this temperature range, causing the resistance of the separator film, measured as impedance, to increase. If the resistance exceeded 1000Ω, it was determined that the separator had shutdown properties. In Tables 1 and 2, polyolefin microporous membranes that had shutdown properties were marked with 'Good', and polyolefin microporous membranes that did not had shutdown properties were marked with 'Poor'.

[0116] [Tensile strength] Measurement was carried out in accordance with ASTM D-822. A tensile test was carried out using a 100 N load cell in a tensile tester under the conditions of a chuck distance of 50 mm and a crosshead speed of 50 mm / min. The tensile strength was calculated based on the load W (kg) at the time of breaking the test piece, the cross-sectional area S (mm ２The tensile strength (kg / mm ２ ) = W / S

[0117] [Tensile elongation] The tensile elongation was measured by measuring the gauge length L of the test piece before the test. ０ The tensile elongation (%) was calculated from the gauge length L (mm) at break using the following formula: Tensile elongation (%) = (L - L ０ ) / L ０ × 100 The tensile strength in the MD direction was rounded to the nearest integer. The tensile elongation was rounded to the nearest integer. The average values ​​of each measurement were evaluated as the tensile strength and tensile elongation.

[0118] [Air permeability] A test piece measuring 80 mm in the MD direction and the entire width was taken from a polyolefin microporous membrane, and measurements were performed at three points, namely the center and the left and right ends (50 mm inward from the end faces), using a B-type Gurley densometer (manufactured by Toyo Seiki Co., Ltd.) in accordance with JIS P8117. The average value of the three points was evaluated as the air permeability value.

[0119] Tortuosity (τ) was calculated using the following formula, where A is the area of ​​the membrane (cm ２ ), R is the membrane resistance in ohm-cm (Ω-cm), ε is the porosity, t is the membrane thickness in cm, and ζ is the electrolyte resistance in Ω-cm. τ = (A × R × ε / (t × ζ)) １／２

[0120] [Zero Shear Viscosity] Shear dynamic viscoelasticity measurements of molten PP resin were performed using a TA Instruments ARES rheometer (model: ARES). A cone-parallel plate (cone angle 0.1 rad) geometry was used. Dynamic viscoelasticity measurements were performed at four temperatures: 220°C, 200°C, 180°C, and 160°C, under conditions of a frequency range of 400 to 0.01 s-1 (5 points per decade) and a strain of 0.1 (10%). A master curve was created using the measurement data at 200°C as the reference. The viscosity value that became constant in the frequency range of 0.01 s-1 or less was defined as the zero shear viscosity. In this specification, the creation of a master curve based on the measurement data at 200°C and calculation of the zero shear viscosity based on this master curve is referred to as the "200°C condition." The zero shear viscosity of the PP resin used in Examples 1 to 16 and Comparative Examples 1 to 4 at 200°C was 16,000 Pa·s. The zero shear viscosity of the PE resin used in Examples 1 to 16 and Comparative Examples 1 to 4 at 200°C was 34,000 Pa·s.

[0121] Example 1 An example of a method for producing the polyolefin microporous membrane of Example 1 is described below. The method for producing the polyolefin microporous membrane of Example 1 is not limited to the method described below, and other methods may be used. For example, the polyolefin microporous membrane of Example 1 may be produced by using an unstretched PP raw film or PE raw film and performing a lamination step and a stretching step.

[0122] [Film Production of Raw Fabric] A precursor film having a three-layer structure including a PE layer (a three-layer structure of PP layer / PE layer / PP layer) was obtained by co-extruding a polypropylene resin and a polyethylene resin using a co-extruder equipped with a multi-manifold die at a die temperature (co-extrusion temperature) of 180 to 230°C, with the PE layer as an intermediate layer and PP layers formed on both sides of the intermediate layer. The PE layer content in the precursor film was 12.8% by mass.

[0123] [Stretching Process] The obtained 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 then low-temperature stretched in a cold stretching zone. The low-temperature stretched precursor film was then high-temperature stretched in a hot stretching zone at 130 to 135°C at a stretch ratio of 180 to 300% (maximum stretch ratio), and then heat-relaxed to a stretch ratio of 100 to 200% (final stretch ratio). A porous film was then produced by heat-setting at a constant temperature. In this example, the precursor film was stretched uniaxially in the machine direction (MD). The results are shown in Table 1.

[0124] Table 1 also shows the PE content, membrane thickness, surface open area of ​​the PP layer and PE layer, ER, porosity, pin puncture strength, pin puncture strength per thickness equivalent to the substantial resin component excluding pores (Equation (1)), pin puncture elongation, shutdown property, tensile strength, tensile elongation, air permeability, and bending degree of the polyolefin microporous membrane of Example 1.

[0125] Examples 2 to 16, Comparative Example 1 Polyolefin microporous membranes were produced in the same manner as in Example 1, except that the membrane thickness and PE content were changed to those shown in Table 1 by adjusting the extrusion rate of the resin during melt extrusion. The results are shown in Tables 1 and 2.

[0126] Comparative Examples 2 to 4 Using the same polypropylene resin as in Example 1, an unstretched polypropylene film (PP raw roll) was obtained using a T-die. The unstretched PP raw roll was then thermocompressed to a desired thickness and stretched in the same manner as in Example 1 to produce a polyolefin microporous membrane. The results are shown in Table 2.

[0127]

[0128]

[0129] As shown in Table 1, in Examples 1 to 16, porous films were obtained in which the PE content was controlled to 5% to 45% by mass, the surface porosity of the PP layer was controlled to 5% to 30%, and the surface porosity of the PE layer was controlled to 5% to 20.6%. The porous films thus obtained had an ER of 0.06 Ω cm ２ / μm or less, the formula (I) of the pin puncture strength per thickness equivalent to the substantial resin component excluding the pore portion was 0.44 or more, the pin puncture elongation was 5.8 mm or more, shutdown properties were present, and the bending degree was 1.6 or less. From these results, it can be said that the porous films obtained in Examples 1 to 16 have both low resistance and high pin puncture strength and have shutdown properties, and therefore can be suitably used, for example, as separators for lithium ion batteries.

[0130] On the other hand, in Comparative Example 1, which is a porous film in which the PE content was controlled to more than 45% by mass, the surface porosity of the PP was 10.0% and the surface porosity of the PE exceeded 20%, but an effective pore path length that could become through-holes was not formed, and the ER was 0.06 Ω cm ２ The ER was 0.046 Ω cm in Comparative Examples 2, 3, and 4, which were PP porous films not containing PE. ２ Although the resistance was low at 0.35 / μm or less, the pin puncture strength formula (I) per thickness equivalent to the substantial resin component excluding the pore portion was 0.35 or less, the pin puncture elongation was 4.1 mm or less, and furthermore, the film did not have shutdown properties, resulting in unsatisfactory results.

Claims

1. A polyolefin microporous membrane having a polyethylene resin layer containing a polyethylene resin and a polypropylene resin layer containing a polypropylene resin, wherein the content of the polyethylene resin in the total mass of the polyolefin microporous membrane is 5% to 45% by mass, the porosity of the polyolefin microporous membrane is 42% to 62%, the surface porosity of the polypropylene resin layer is 5% to 30%, and the surface porosity of the polyethylene resin layer is 5% to 20.6%.

2. The microporous polyolefin membrane according to claim 1, wherein the polyethylene resin layer is an intermediate layer.

3. The polyolefin microporous membrane according to claim 1 or 2, wherein the polyolefin microporous membrane has a laminated structure in which the polyethylene-based resin layer and the polypropylene-based resin layer are laminated together, and the laminated structure is a three-layer laminated structure in which polypropylene-based resin layers are present on both sides of the polyethylene-based resin layer.

4. The polyolefin microporous membrane according to claim 1 or 2, wherein the pin puncture strength (N / μm) and porosity (%) of the polyolefin microporous membrane satisfy the following formula (I): 100×pin puncture strength / (100−porosity)≧0.44 (I).

5. The electrical resistance of the polyolefin microporous membrane is 0.06 (Ω cm ２ 3. The microporous polyolefin membrane according to claim 1 or 2, wherein the microporous polyolefin membrane has a surface area of ​​1 / μm or less.

6. The microporous polyolefin membrane according to claim 1 or 2, wherein the tortuosity of the microporous polyolefin membrane is 1 or more and 2 or less.

7. The microporous polyolefin film according to claim 1 or 2, wherein the surface porosity of the polypropylene-based resin layer is 6% to 25%.

8. The microporous polyolefin membrane according to claim 1 or 2, wherein the surface porosity of the polyethylene resin layer is 7% to 19%.

9. The microporous polyolefin membrane according to claim 1 or 2, wherein the surface porosity of the polyethylene-based resin layer is higher than the surface porosity of the polypropylene-based resin layer.

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

11. A separator film for an electricity storage device, comprising the polyolefin microporous membrane according to claim 1 or 2.

12. An electricity storage device comprising the separator film for an electricity storage device according to claim 11, a positive electrode, and a negative electrode.

13. A vehicle, an aircraft, or an electric tool equipped with the electricity storage device according to claim 12.

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