Biaxially stretched polyphenylene sulfide film, laminated film, electrode for battery, and battery

Abstract

The objective of the present invention is to provide a biaxially stretched polyphenylene sulfide film and a laminated film which are suitable for production of an electrode for a battery having excellent productivity and battery lifespan. The objective of the present invention is to also provide: an electrode for a battery having excellent productivity and battery lifespan; and said battery. The biaxially stretched polyphenylene sulfide film has a thickness of 0.2 μm to 30 μm, a tensile breaking strength in an MD direction and a tensile breaking strength in a TD direction of 120 MPa or more, respectively, and a ratio of the tensile breaking strength in the MD direction to the tensile breaking strength in the TD direction of 0.7 to 1.3. The laminated film includes the biaxially stretched polyphenylene sulfide film and a metal thin film provided on at least one surface of the biaxially stretched polyphenylene sulfide film. The electrode for a battery includes the laminated film and an electrode material. The battery includes the electrode for a battery.

Description

Biaxially oriented polyphenylene sulfide film, laminated film, battery electrodes, batteries

[0001] The present invention relates to a biaxially stretched polyphenylene sulfide film, a laminated film, a battery electrode, and a battery.

[0002] In recent years, with the miniaturization of electronic devices, there has been a demand for smaller batteries installed in electronic devices. Furthermore, as batteries become smaller, there has been a demand for improved battery capacity and longer battery life. Examples of such electronic devices include battery devices installed in smartphones, tablets, and electric vehicles. The batteries are primarily secondary batteries, such as lithium-ion secondary batteries. Conventional batteries, for example, use a metal foil with a thickness of 10 to 30 μm as an electrode substrate, and have electrodes formed by laminating an electrode material on the metal foil, which are then stacked with a sheet-like separator interposed between them and wound together to form a battery structure. An electrolyte solution is stored inside the battery. Thinning the electrodes and separator is expected to reduce the size of the battery, improve its capacity, and extend its battery life.

[0003] However, there is a limit to how thin the electrodes and separators can be made while maintaining the electrode's bending resistance and puncture resistance to prevent the electrode from being pierced by a nail or other object, which could lead to electrical continuity. Moreover, because the amount of metal used remains the same, the battery weight cannot be reduced.

[0004] As an alternative to metal, Patent Documents 1 and 2 disclose electrodes in which a conductive thin film layer of metal or the like is provided on the surface of a biaxially oriented polyester film, which has excellent mechanical properties and dimensional stability.

[0005] JP-A-10-40919 JP-A 10-40920

[0006] However, polyester films have the problem of having low resistance to electrolytes, which shortens the battery life.

[0007] An object of the present invention is to provide a biaxially stretched polyphenylene sulfide film and a laminate film suitable for producing battery electrodes with excellent productivity and battery life. Another object of the present invention is to provide battery electrodes and batteries with excellent productivity and battery life.

[0008] The present invention provides the following aspects. [Item 1] A biaxially stretched polyphenylene sulfide film having a thickness of 0.2 μm or more and 30 μm or less, a tensile break strength in the MD direction and a tensile break strength in the TD direction each of which is 120 MPa or more, and a ratio of the tensile break strength in the MD direction to the tensile break strength in the TD direction of 0.7 or more and 1.3 or less. [Item 2] The biaxially stretched polyphenylene sulfide film according to Item 1, having a crystallinity of 25% or more as measured by differential scanning calorimetry (DSC). [Item 3] The biaxially stretched polyphenylene sulfide film according to Item 1 or 2, having a ratio of the tensile modulus in the MD direction to the tensile modulus in the TD direction of 0.85 or more and 1.15 or less. [Item 4] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 3, wherein the tensile break strength in the MD direction and the tensile break strength in the TD direction are each 120 MPa or more and 225 MPa or less. [Item 5] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 4, wherein the biaxially stretched polyphenylene sulfide film is a polyphenylene sulfide film that has been sequentially biaxially stretched, and wherein the crystallization temperature of the polyphenylene sulfide film after uniaxial stretching and before biaxial stretching is 104°C or more. [Item 6] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 5, which is used for producing a battery electrode. [Item 7] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 6, wherein the thickness is 0.2 μm or more and 20 μm or less, or 0.2 μm or more and 12 μm or less. [Item 8] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 7, having a thickness of 0.5 μm or more and 10 μm or less. [Item 9] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 8, having a tensile break strength in the MD direction and a tensile break strength in the TD direction of 130 MPa or more or 140 MPa or more, respectively. [Item 10] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 9, having a tensile break strength in the MD direction and a tensile break strength in the TD direction of 240 MPa or less or 225 MPa or less, respectively.[Item 11] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 10, wherein the tensile break strength in the MD direction and the tensile break strength in the TD direction are 215 MPa or less, or 210 MPa or less, respectively. [Item 12] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 11, wherein the ratio of the tensile break strength in the MD direction to the tensile break strength in the TD direction is 0.75 or more and 1.25 or less, or 0.80 or more and 1.20 or less. [Item 13] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 12, wherein the crystallinity measured by differential scanning calorimetry (DSC) is 25.5% or more. [Item 14] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 11, wherein the crystallinity measured by differential scanning calorimetry (DSC) is 40% or less, or 30% or less. [Item 15] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 14, wherein the ratio of the tensile modulus in the MD direction to the tensile modulus in the TD direction is 0.90 or more and 1.10 or less, or 0.95 or more and 1.05 or less. [Item 16] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 15, comprising inert particles. [Item 17] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 16, wherein the inert particles are calcium carbonate particles. [Item 18] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 15, comprising inert particles. [Item 19] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 18, comprising a polyphenylene sulfide resin, wherein the polyphenylene sulfide resin has a melting point of 260°C or more, or 270°C or more. [Item 20] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 19, comprising a polyphenylene sulfide resin, wherein the polyphenylene sulfide resin has a number average molecular weight of 5,000 or more and 20,000 or less, or 8,000 or more and 15,000 or less.[Item 21] The biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 20, wherein the heat shrinkage rate at 180°C for 30 minutes in the MD direction and the heat shrinkage rate at 180°C for 30 minutes in the TD direction are each 4% or less, or 3.5% or less. [Item 22] The biaxially stretched polyphenylene sulfide film according to Item 21, wherein the heat shrinkage rate in the MD direction and the heat shrinkage rate in the TD direction are each 3% or less. [Item 23] A laminated film comprising the biaxially stretched polyphenylene sulfide film according to any one of Items 1 to 22, and a metal thin film provided on at least one surface of the biaxially stretched polyphenylene sulfide film. [Item 24] The laminated film according to Item 23, wherein the metal thin film has a thickness of 1 nm to 1000 nm, or 10 nm to 1000 nm. [Item 25] The laminated film according to Item 24, which is used as an electrode film for a secondary battery. [Item 26] A battery electrode comprising the laminate film according to any one of Items 23 to 25 and an electrode material. [Item 27] ​​The battery electrode according to Item 26, wherein the electrode material is provided on the metal thin film of the laminate film. [Item 28] The battery electrode according to Item 26 or 27, wherein the electrode material comprises at least one of lithium cobalt oxide and graphite. [Item 29] A battery comprising the battery electrode according to Item 28. [Item 30] The battery according to Item 29, which is a lithium ion secondary battery.

[0009] The present invention can provide a biaxially stretched polyphenylene sulfide film and a laminate film suitable for producing battery electrodes with excellent productivity and battery life. The present invention can also provide battery electrodes and batteries with excellent productivity and battery life.

[0010] Hereinafter, the MD direction, i.e., the Machine Direction, may be referred to as the longitudinal direction. In other words, the MD direction and the longitudinal direction are used as synonyms. The TD direction, i.e., the Transverse Direction, may be referred to as the width direction. In other words, the TD and the width direction are used as synonyms.

[0011] The biaxially stretched polyphenylene sulfide film according to an embodiment of the present invention (hereinafter sometimes simply referred to as the "biaxially stretched polyphenylene sulfide film of the present invention") has a thickness of 0.2 μm or more and 30 μm or less, a tensile break strength in the MD direction and a tensile break strength in the TD direction each of which is 120 MPa or more, and a ratio of the tensile break strength in the MD direction to the tensile break strength in the TD direction of 0.7 or more and 1.3 or less. Since the ratio of the tensile break strength in the MD direction to the tensile break strength in the TD direction (i.e., tensile break strength in the MD direction / tensile break strength in the TD direction) is 0.7 or more and 1.3 or less, the battery life can be improved. This is thought to be because wrinkles that may occur due to tension applied to the biaxially stretched polyphenylene sulfide film during battery fabrication can be reduced or prevented. Since the tensile strength in the MD direction and the tensile strength in the TD direction are each 120 MPa or more, the productivity of secondary batteries can be improved. This is because breakage that may occur due to tension applied to the biaxially stretched polyphenylene sulfide film during battery fabrication can be reduced or prevented. In addition, wrinkles that may occur during battery fabrication can be further reduced or prevented. Since the thickness is 30 μm or less, the volumetric energy density of the battery can be improved.

[0012] The biaxially stretched polyphenylene sulfide film of the present invention is described in detail below. The biaxially stretched polyphenylene sulfide film of the present invention contains polyphenylene sulfide (PPS) resin. The biaxially stretched polyphenylene sulfide film of the present invention can be used, for example, as a substrate for a battery electrode (hereinafter sometimes referred to as an "electrode substrate"). A battery has a structure formed by, for example, stacking electrodes with a sheet separator interposed therebetween and winding the stack, and stores an electrolyte solution inside. The battery can be a secondary battery, such as a lithium-ion secondary battery. The battery is installed in, for example, an electronic device. The electronic device is, for example, a battery device installed in a smartphone, tablet, or electric vehicle. A battery fabricated using the biaxially stretched polyphenylene sulfide film of the present invention can have a thinner electrode substrate, thereby improving battery capacity. The biaxially stretched polyphenylene sulfide film of the present invention contains polyphenylene sulfide resin, and therefore has a high melting peak temperature and low heat shrinkage (i.e., dimensional stability) as an electrode substrate. Furthermore, the biaxially stretched polyphenylene sulfide film of the present invention is superior to conventional polyethylene terephthalate films in terms of resistance to electrolytes because it contains a polyphenylene sulfide resin. Furthermore, the biaxially stretched polyphenylene sulfide film of the present invention can prevent or reduce short circuits caused by thermal deformation of the electrode substrate. This can improve battery life. Furthermore, batteries fabricated using the biaxially stretched polyphenylene sulfide film of the present invention can be made lighter than conventional batteries that use metal foil as an electrode substrate, and can improve weight energy density.

[0013] The thickness of the biaxially stretched polyphenylene sulfide film of the present invention is preferably 0.2 μm or more and 30 μm or less. Having such a film thickness, when used as a substrate film for an electrode of an energy storage element, can improve the energy density, particularly the volumetric energy density, of the energy storage element. It can also impart bending resistance. When a thick film exceeding the upper limit is used as an electrode substrate, the film becomes thicker than conventional metal foils, and the energy density, particularly the volumetric energy density, of the energy storage element may not be improved. On the other hand, when the film thickness is too thin to reach the lower limit, film production becomes extremely difficult. Therefore, from the viewpoint of improving the energy density, particularly the volumetric energy density, of the energy storage element and the efficiency of film production, the present invention preferably has the above thickness. A more preferred film thickness is 0.2 μm or more and 20 μm or less, even more preferably 0.2 μm or more and 12 μm or less, and particularly preferably 0.5 μm or more and 10 μm or less. The biaxially stretched polyphenylene sulfide film of the present invention may have a single-layer structure or a two- or more-layer structure.

[0014] The biaxially stretched polyphenylene sulfide film of the present invention preferably has a tensile break strength in the MD direction and the TD direction of 120 MPa or more. The biaxially stretched polyphenylene sulfide film of the present invention preferably has a tensile break strength in the MD direction and the TD direction of 240 MPa or less, and more preferably 225 MPa or less. For example, the tensile break strength in the MD direction and the TD direction is preferably 120 MPa or more and 225 MPa or less. Within this range, the workability of battery production by stacking and winding the electrode material is improved. If it is less than 120 MPa, problems such as breakage may occur during battery production. Therefore, a strength less than 120 MPa is not preferable. If it is greater than 225 MPa, the rigidity of the biaxially stretched polyphenylene sulfide film of the present invention may be excessively high, which may reduce the workability of battery production by winding. Note that the tensile break strengths in the MD direction and the TD direction may be similar or different as long as they are within the above range. For example, the tensile breaking strength in the MD direction and the TD direction may be 130 MPa or more, or 140 MPa or more, respectively. Similarly, the tensile breaking strength in the MD direction and the TD direction may be 215 MPa or less, 210 MPa or less, or 200 MPa or less, respectively.

[0015] In the biaxially stretched polyphenylene sulfide film of the present invention, the ratio of the tensile break strength in the MD direction to the tensile break strength in the TD direction (i.e., tensile break strength in the MD direction / tensile break strength in the TD direction; hereinafter, this may be referred to as "TS-MD / TS-TD") is preferably 0.7 to 1.3. That is, the tensile break strength in the MD direction is preferably 70% to 130% of the tensile break strength in the TD direction. It is even more preferably 75% to 125%, and even more preferably 80% to 120%.

[0016] If the tensile breaking strength in the MD direction is too much larger or too small than the tensile breaking strength in the TD direction (i.e., TS-MD / TS-TD is less than 0.7 or exceeds 1.3) than the above range, that is, if the difference between the tensile breaking strength in one direction and that in the other direction is too large, problems such as breakage or the occurrence of wrinkles in one direction may occur in post-processing steps (for example, coating of electrode material, winding, etc.), which may lead to reduced productivity and a shortened battery life.

[0017] That is, in the present invention, by ensuring that TS-MD / TS-TD is within the above range, problems in processing steps (for example, coating of electrode material, winding, etc.) can be suppressed, and the battery life can be maintained long.

[0018] On the other hand, the ratio of the tensile modulus in the MD direction to the tensile modulus in the TD direction (i.e., tensile modulus in the MD direction / tensile modulus in the TD direction) is preferably 0.85 or more and 1.15 or less. More preferably, it is 0.90 or more and 1.10 or less, and more preferably 0.95 or more and 1.05 or less. By having the ratio of the tensile modulus in the MD direction to the tensile modulus in the TD direction be 0.85 or more and 1.15 or less, it is possible to reduce unintended deformation that may occur in the film when winding the film during battery production, and therefore, the battery life can be further improved.

[0019] The biaxially stretched polyphenylene sulfide film of the present invention preferably has a heat shrinkage rate of 4% or less in both the MD and TD directions at 180°C for 30 minutes. It is more preferably 3.5% or less, and even more preferably 3% or less. If the heat shrinkage rate is too high, excessive thermal deformation of the electrodes of the secondary battery may occur when heat is generated due to internal resistance, potentially resulting in a short circuit. The heat shrinkage rate may be similar or different in the MD and TD directions. The heat shrinkage rate is measured by the following method. A test piece measuring 10 mm in width and 250 mm in length is cut from the biaxially stretched polyphenylene sulfide film. A pair of marks (i.e., a pair of benchmark lines) is made at 200 mm intervals along the length of this test piece, and the distance between the benchmark lines is measured under a tension of 5 gf. This test piece is heat-treated at 180°C for 30 minutes under no load, and the distance between the benchmark lines is then measured under a tension of 5 gf. From these measurement results, the heat shrinkage rate is calculated using the following formula: Heat shrinkage (%) = {(A - B) / A} x 100, where A is the gauge length before heat treatment, and B is the gauge length after heat treatment. Using this procedure, the heat shrinkage in MD and TD is determined.

[0020] The biaxially stretched polyphenylene sulfide film of the present invention can have its surface roughened by adding inactive particles (inactive particles) to the polymer as a roughening agent, as long as the effects of the present invention are not impaired. That is, the biaxially stretched polyphenylene sulfide film of the present invention may contain inactive particles. The surface roughness of the film is preferably a center surface average roughness (SRa) of 5 nm to 70 nm, more preferably 6 nm to 65 nm. Furthermore, the ten-point average roughness (SRz) is preferably 200 nm to 950 nm, more preferably 250 nm to 900 nm. If SRa is less than 5 nm and SRz is less than 200 nm, the transportability of the film is significantly reduced. Furthermore, if SRa exceeds 70 nm and SRz exceeds 950 nm, it is difficult to achieve proper adhesion with the chill roll during vapor deposition. Therefore, by having the above roughness, the present invention can further improve transportability and achieve proper adhesion with the chill roll during vapor deposition.

[0021] The surface roughness of the biaxially stretched polyphenylene sulfide film of the present invention preferably satisfies the following formula (2): 10≦SRz / SRa≦50 (2) When the formula is satisfied, the biaxially stretched polyphenylene sulfide film of the present invention can have good air escape properties when wound around a roll, and wrinkles and creases on the roll can be suppressed.

[0022] The above formula (2) can take the following forms: 10≦SRz / SRa≦40, or 12≦SRz / SRa≦38. By taking such values, the air release properties when the biaxially stretched polyphenylene sulfide film of the present invention is further improved when wound around a roll, and wrinkles and creases on the roll can be more effectively suppressed.

[0023] The surface-roughening agent is preferably fine particles having an average particle size of 0.05 to 5 μm. Preferably, the fine particles are inert particles. The amount added is preferably 500 ppm to 20,000 ppm, more preferably 1,000 ppm to 15,000 ppm, and even more preferably 1,500 ppm to 10,000 ppm. Here, ppm means ppm by mass.

[0024] Examples of surface roughening agents include calcium carbonate, magnesium carbonate, barium carbonate, barium sulfate, calcium phosphate, lithium phosphate, magnesium phosphate, lithium fluoride, aluminum oxide, silicon oxide (silica), titanium oxide, kaolin, talc, carbon black, silicon nitride, boron nitride, and crosslinked polymer fine particles.

[0025] The biaxially stretched polyphenylene sulfide film of the present invention has a melting peak temperature in DSC temperature rise measurement of preferably 250° C. or higher, more preferably 260° C. or higher, and even more preferably 270° C. or higher. The melting peak temperature is, for example, 300° C. or lower. When the film has a melting peak temperature of 250° C. or higher, for example, when used in a secondary battery, when heat is generated due to internal resistance, thermal deformation of the electrode can be suppressed or reduced, and short circuits can also be prevented or reduced.

[0026] The polyphenylene sulfide resin is preferably a homopolymer, but may also be a copolymer, or may be a polymer blend consisting of two or more of these homopolymers and / or copolymers.

[0027] The melting peak temperature of the polyphenylene sulfide resin (hereinafter sometimes referred to as "melting point"), specifically, the melting peak temperature in DSC temperature rise measurement, is preferably 250°C or higher, more preferably 260°C or higher, and even more preferably 270°C or higher. The melting peak temperature of the polyphenylene sulfide resin is, for example, 300°C or lower. When the melting peak temperature is 250°C or higher, for example, when used in a secondary battery, thermal deformation of the electrode can be suppressed or reduced when heat is generated due to internal resistance, and short circuits can also be prevented or reduced.

[0028] The number average molecular weight of the polyphenylene sulfide resin may be, for example, from 5,000 to 20,000, or from 8,000 to 15,000. The number average molecular weight of the polyphenylene sulfide resin is a value calculated in terms of polystyrene using gel permeation chromatography (GPC).

[0029] The content of the polyphenylene sulfide resin in the biaxially stretched polyphenylene sulfide film of the present invention is preferably 98% by mass or more, more preferably 99% by mass or more.

[0030] The biaxially stretched polyphenylene sulfide film of the present invention can be produced according to a conventionally known method. For example, a raw polymer is thoroughly dried under predetermined conditions, then fed into a known melt extrusion device (typically an extruder), and heated to a temperature equal to or higher than the melting point (Tm: °C) of the polymer, particularly to a temperature equal to or higher than Tm and equal to or lower than (Tm + 70) °C, to melt it. During this extrusion process, the raw polymer is melt-kneaded to be homogeneous, and the degree of melt-kneading is adjusted.

[0031] The melt-kneaded polymer is then extruded through a slit die lip into a sheet and rapidly solidified on a rotating cooling drum to obtain a substantially amorphous unstretched sheet. In this case, electrostatic charge application adhesion and / or liquid application adhesion are preferably employed to enhance adhesion to the rotating cooling drum and improve the surface flatness (flatness, smoothness) of the sheet. The electrostatic charge application adhesion method involves applying a DC voltage to a linear electrode stretched in a direction perpendicular to the flow of the sheet extruded from the die to impart static charge to the surface (non-drum side) of the sheet, thereby improving adhesion between the sheet and the rotating cooling drum. The liquid application adhesion method involves uniformly applying a liquid to all or part of the surface of the rotating cooling drum (e.g., the portions in contact with both ends of the sheet) to improve adhesion between the sheet and the rotating cooling drum. In embodiments of the present invention, both methods may be used in combination as necessary. Alternatively, inflation casting or casting may be employed to produce a substantially amorphous unstretched sheet.

[0032] The unstretched sheet thus obtained is then biaxially stretched to form a biaxially stretched film. The stretching method can be a sequential biaxial stretching method (tenter method) or a simultaneous biaxial stretching method (tenter method or tube method). The stretching conditions for the sequential biaxial stretching method are to stretch the unstretched sheet in one direction (MD or TD) by 2.5 to 6.0 times, preferably 3.0 to 5.5 times, at a temperature of (Tg-10) to (Tg+70)°C. Since the temperature-raised crystallization temperature of the film after uniaxial stretching and before biaxial stretching is set within the temperature range described below, this temperature (i.e., the first-stage stretching temperature) is preferably 100°C or higher. Similarly, the first-stage stretching temperature is preferably 115°C or lower. Next, the sheet is stretched 2 to 6 times, preferably 2.5 to 5.5 times, in a direction perpendicular to the first-stage stretching (if the first-stage stretching is in the MD direction, the second-stage stretching is in the TD direction) at a temperature of Tg or higher (Tg + 70°C or lower). Stretching in at least one of the biaxial directions can also be performed in two or more stages. In this case, it is desirable that the final stretching ratio is within the above-mentioned range. After the second-stage stretching, intermediate heat setting may be performed, and the sheet may then be stretched again in the same direction as the first stage and / or the second stage. The unstretched sheet may also be simultaneously biaxially stretched to an areal stretching ratio of 8 to 30 times, preferably 8.5 to 25 times.

[0033] It is preferable that the difference in stretching ratio between the MD direction and the TD direction is small. This is because if the difference in stretching ratio between the MD direction and the TD direction is too large, TS-MD / TS-TD (i.e., the ratio of the tensile breaking strength in the MD direction to the tensile breaking strength in the TD direction) tends to become too large or too small. In addition, the ratio of the tensile modulus in the MD direction to the tensile modulus in the TD direction tends to become too large or too small. The stretching ratio in the TD direction is preferably set in the range of 0.7 to 1.5 times the stretching ratio in the MD direction.

[0034] In order to perform biaxial stretching (i.e., second-stage stretching) so that the difference in stretching ratio relative to the stretching ratio in the uniaxial stretching direction (i.e., the direction of the first-stage stretching) is within the above-mentioned range, the temperature-rise crystallization temperature of the polyphenylene sulfide film after uniaxial stretching and before biaxial stretching is preferably within the following temperature range. That is, the temperature-rise crystallization temperature is preferably 104°C or higher, more preferably 105°C or higher. Furthermore, the temperature-rise crystallization temperature is preferably 115°C or lower, more preferably 110°C or lower. For example, the temperature-rise crystallization temperature is preferably 104°C or higher and 115°C or lower. The temperature-rise crystallization temperature is the temperature at which a resin formed in an insufficiently crystalline state crystallizes when the temperature of the resin is increased.

[0035] The biaxially stretched film thus obtained is then heat-set. The heat-set temperature is preferably 210°C or higher and 280°C or lower, and more preferably 220°C or higher and 280°C or lower. This heat-set is preferably carried out for 1 second to 10 minutes. In this case, the heat-set may be carried out under limited shrinkage or elongation of 20% or less, or under a fixed length, or may be carried out in two or more stages.

[0036] The biaxially stretched polyphenylene sulfide film of the present invention preferably has a density of 1.340 g / dL or more. For example, the density is 1.40 g / dL or less. When the density is within this range, it can be said that crystallization is sufficiently promoted, and therefore, thermal shrinkage can be suppressed.

[0037] In the biaxially stretched polyphenylene sulfide film of the present invention, the crystallinity measured by differential scanning calorimetry (DSC) is preferably 25% or more, more preferably 25.5% or more. The crystallinity is preferably 50% or less, more preferably 40% or less, and even more preferably 30% or less. The upper and lower limits can be appropriately combined, and the crystallinity is preferably, for example, 25% or more and 50% or less. A crystallinity of 25% or more can improve the stability and durability of the film's physical properties, thereby further extending the battery's lifespan. On the other hand, a crystallinity of 50% or less can prevent the film's rigidity from becoming excessively high, thereby preventing excessive deterioration in the workability of battery production by winding. A crystallinity of 25% or more can be achieved, for example, by setting the area ratio of the stretching ratio in the MD direction and the stretching ratio in the TD direction to 8.0 times or more and setting the heat setting temperature of the film after biaxial stretching to 210°C or more.

[0038] The biaxially stretched polyphenylene sulfide film of the present invention may be surface-treated to improve adhesion to a metal thin film. The surface treatment method is not particularly limited, but preferred examples include application of an adhesion-improving layer, corona treatment, plasma treatment, etc.

[0039] The biaxially stretched polyphenylene sulfide film of the present invention can be used to produce battery electrodes, and is preferably used to produce secondary battery electrodes, and more preferably used to produce lithium-ion secondary battery electrodes. To use the biaxially stretched polyphenylene sulfide film of the present invention as a secondary battery electrode film, a metal thin film can be formed on at least one surface of the film. Examples of metals that can form the metal thin film include aluminum, nickel, gold, silver, copper, and cadmium. The thickness of the metal thin film is typically 1 to 1,000 nm, but is preferably 10 to 1,000 nm to suppress internal resistance heat generation in the battery. Examples of methods for forming the metal thin film include vacuum deposition, electroplating, and sputtering.

[0040] The laminated film according to an embodiment of the present invention includes a biaxially stretched polyphenylene sulfide film according to an embodiment of the present invention and a metal thin film provided on at least one of both surfaces (i.e., a pair of both surfaces) of the biaxially stretched polyphenylene sulfide film.

[0041] A battery electrode can be formed by laminating an electrode material on the metal thin film surface of the laminate film according to an embodiment of the present invention. Conventional electrode materials, such as lithium cobalt oxide and graphite, can be used as this electrode material. Furthermore, a battery can be manufactured using this battery electrode by a conventional method. For example, when manufacturing a lithium ion secondary battery, a secondary battery electrode film having a sputtered copper thin film coated with lithium cobalt oxide is used as a positive electrode, and a secondary battery electrode film having an aluminum vapor deposition film coated with graphite is used as a negative electrode. A separator made of a polyethylene microporous film is then interposed between the positive electrode and the negative electrode, and the two are wound together. An organic solvent containing a lithium salt is used as the electrolyte, thereby forming a lithium ion secondary battery.

[0042] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention. Various physical properties and characteristics in the present invention were measured and defined as follows.

[0043] (1) Film Thickness The film was clamped between a spindle detector (K107C manufactured by Anritsu Electric Co., Ltd.), and the thickness was measured at 10 different positions using a digital differential electronic micrometer (K351 manufactured by Anritsu Electric Co., Ltd.), and the average value was calculated to determine the film thickness.

[0044] (2) Tensile Break Strength The film was cut to a width of 10 mm and a length of 150 mm and pulled using a precision universal testing machine (Shimadzu Corporation, Autograph AGS-X) with a chuck distance of 100 mm and a pulling speed of 100 mm / min, and the load at which the film broke was measured. This load was divided by the cross-sectional area of ​​the film specimen (i.e., the thickness of the film specimen x the width of the film specimen), and the stress (MPa) was calculated as the tensile break strength (unit: MPa). The tensile break strain (hereinafter sometimes referred to as "TE") was also calculated. (3) Tensile Modulus The film was cut to a width of 10 mm and a length of 150 mm and pulled using a precision universal testing machine (Shimadzu Corporation, Autograph AGS-X) with a chuck distance of 100 mm and a pulling speed of 100 mm / min, and the maximum value of the slope of the resulting stress-strain curve was calculated as the tensile modulus.

[0045] (4) Film-forming property When a film was produced under the conditions described in the Examples and Comparative Examples, the film-forming property was evaluated according to the following criteria. ∘: No breakage occurred for 60 minutes or more. In other words, continuous film production for 60 minutes or more was possible. Δ: Breakage occurred at least once between 30 minutes and less than 60 minutes. ×: Breakage occurred at least once in less than 30 minutes.

[0046] (5) Melting Peak Temperature (Tm), Heat-Rise Crystallization Temperature 5 mg of polyphenylene sulfide resin was placed in a TA Instruments differential scanning calorimeter DSC250 and heated in a nitrogen gas flow at a heating rate of 10°C / min. The endothermic behavior accompanying the melting of the film was analyzed using first-order and second-order derivatives, and the temperature showing a peak was determined as the melting peak temperature (unit: °C). 5 mg of the film after uniaxial stretching and before biaxial stretching was similarly heated in a TA Instruments differential scanning calorimeter DSC250, and the exothermic behavior accompanying the temperature-rise crystallization of the film was analyzed using first-order and second-order derivatives, and the temperature showing a peak was determined as the heat-rise crystallization temperature (unit: °C).

[0047] (6) Battery Capacity The lithium ion secondary batteries prepared by the method described below were subjected to continuous discharge. When the amount of electricity discharged until the discharge voltage reached 80% of the rated voltage (i.e., discharge capacity) was 2100 mAh or more, it was judged as ◯, when it was 1600 mAh or more but less than 2100 mAh, it was judged as △, and when it was less than 1600 mAh, it was judged as ×.

[0048] (7) Productivity (specifically, productivity of lithium-ion secondary batteries) When 100 lithium-ion secondary batteries were produced by the method described below, the number of lithium-ion secondary batteries in which at least one of breakage and wrinkles occurred was counted from the time of coating the electrode material to the time of winding (i.e., winding up the positive electrode material, separator, and negative electrode material while overlapping them). If the number was less than 9 (i.e., less than 9%), it was judged as ◯, and if the number was 10 or more (i.e., 10% or more), it was judged as ×.

[0049] (8) Lifespan (specifically, lifespan of lithium-ion secondary batteries) A ​​repeated charge-discharge test was carried out on lithium-ion secondary batteries fabricated by the method described below under the condition of 100° C. If the number of deterioration cycles until short-circuiting occurred was 550 or more, the battery was judged as ◯, and if the number of cycles was less than 550, the battery was judged as ×.

[0050] (9) Crystallinity 5 mg of film was placed in a TA Instruments differential scanning calorimeter DSC250 and heated in a nitrogen gas flow at a heating rate of 10°C / min. The exothermic behavior associated with the crystallization due to the temperature rise of the film and the endothermic behavior associated with the melting were analyzed using first and second derivatives to determine the temperatures showing a peak or shoulder, and the temperature-rising crystallization energy Hc1 (unit: J / g) and melting energy Hm (unit: J / g) were obtained. The heat of fusion energy of a perfect crystal was set to 146.2 J / g, and the crystallinity was calculated using the following formula: Crystallinity = (Hm - Hc1) / 146.2 × 100

[0051] (10) Birefringence The refractive index in the longitudinal direction and the refractive index in the width direction of the film after uniaxial stretching and before biaxial stretching were measured, and the birefringence (hereinafter sometimes referred to as "uniaxial stretching Δn") was calculated using the following formula: Birefringence = |Refractive index in longitudinal direction - Refractive index in width direction| The refractive index was calculated using an Abbe refractometer (NAR-4T, manufactured by Atago Co., Ltd., measurement wavelength 589 nm) in accordance with JIS K 7142:2008 "Method for measuring the refractive index of plastics (Method A)."

[0052] Example 1 Polyphenylene sulfide resin and calcium carbonate particles with an average particle size of 0.6 μm were melt-kneaded in a twin-screw extruder to prepare a masterbatch with a calcium carbonate particle concentration of 6000 ppm. The polyphenylene sulfide resin had a Tg of 89°C, a Tm of 278°C, a number-average molecular weight Mn of 9000, and a melt viscosity of 235 Pa·s at 300°C and a shear rate of 122 / s. This masterbatch was dried and crystallized, then melt-extruded through a slit die and rapidly solidified by contacting the resin on a rotating cooling drum at a surface temperature of 30°C using an electrostatic charge application contact method, to obtain an unstretched sheet.

[0053] This unstretched sheet was subjected to sequential biaxial stretching, in which the first stretch was 3.3 times in the longitudinal direction at a stretching temperature of 100°C, and the second stretch was 3.6 times in the transverse direction at a stretching temperature of 100°C. The sheet was then heat-set at 230°C for 30 seconds to obtain a biaxially stretched film with a thickness of 8 μm. The film-forming properties, tensile strength, tensile strain at break (TE), tensile modulus, crystallinity, etc. of the obtained biaxially stretched film are shown in Table 3.

[0054] A thin copper film was formed on both sides of the biaxially stretched film by sputtering, and lithium cobalt oxide was applied on top to form a cathode material.Also, aluminum was vapor-deposited on both sides of the biaxially stretched film, and graphite was applied on top to form a cathode material.

[0055] A separator made of a polyethylene microporous film was interposed between the positive electrode material and the negative electrode material, and the materials were wound together. A lithium ion secondary battery was fabricated using an electrolyte solution prepared by dissolving lithium hexafluorophosphate in a mixed solvent of ethylene carbonate / diethyl carbonate / ethyl acetate.

[0056] The productivity during the series of steps of the obtained battery, the battery capacity, and the battery life are as shown in Table 1.

[0057] [Example 2] A film was formed and a battery was produced in the same manner as in Example 1, except that the heat setting temperature was 240° C. [Example 3] A film was formed and a battery was produced in the same manner as in Example 1, except that the draw ratio in the width direction was 3.4 times and the heat setting temperature was 240° C.

[0058] [Example 4] A film was formed and a battery was produced in the same manner as in Example 1, except that sequential biaxial stretching was performed in the width direction at a stretching temperature of 110°C by 3.5 times as the first stretching and in the longitudinal direction at a stretching temperature of 100°C by 3.0 times as the second stretching, and then heat setting was performed at 250°C.

[0059] Example 5 A film was formed and a battery was fabricated in the same manner as in Example 4, except that the stretching temperature was 105°C, the stretching ratio was 3.5 times in the width direction, and 2.5 times in the longitudinal direction.

[0060] Example 6 A film was formed and a battery was fabricated in the same manner as in Example 5, except that the heat setting temperature was 200°C.

[0061] Example 7 A film was formed and a battery was fabricated in the same manner as in Example 4, except that the thickness of the film was changed to 12 μm.

[0062] Example 8 A film was formed and a battery was fabricated in the same manner as in Example 4, except that the thickness of the film was changed to 4 μm.

[0063] Example 9 A film was formed and a battery was fabricated in the same manner as in Example 4, except that the stretching ratio in the longitudinal direction was 2.5 times at 105°C and the heat setting temperature was 200°C.

[0064] Comparative Example 1: Film formation and battery fabrication were performed in the same manner as in Example 4, except that the width direction stretching temperature was 105°C and the longitudinal stretching ratio was 2.0 times. The tensile break strength MD / TD (i.e., the ratio of the tensile break strength in the MD direction to the tensile break strength in the TD direction) was 0.53, and the tensile modulus MD / TD (i.e., the ratio of the tensile modulus in the MD direction to the tensile modulus in the TD direction) was 0.74. In other words, the balance between the tensile break strength in the longitudinal direction and that in the width direction was poor, and the balance between the tensile modulus in the longitudinal direction and that in the width direction was also poor. Deterioration of battery life was observed.

[0065] [Comparative Example 2] Film formation and battery fabrication were performed in the same manner as in Comparative Example 1, except that the heat setting temperature was 200°C. The tensile breaking strength (MD / TD) was 0.50, and the tensile modulus (MD / TD) was 0.73. In other words, the balance between the tensile breaking strength in the longitudinal direction and that in the width direction was poor, and the balance between the tensile modulus in the longitudinal direction and that in the width direction was also poor. The tensile breaking strength in the MD direction was 115 MPa. Deterioration in battery productivity and battery life was observed.

[0066] [Comparative Example 3] Film production and battery fabrication were performed in the same manner as in Comparative Example 2, except that the primary stretching was performed at 110°C in the longitudinal direction by a factor of 3.0, and the secondary stretching was performed at 100°C in the width direction by a factor of 3.8. The tensile breaking strength (MD / TD) was 0.55, and the tensile modulus (MD / TD) was 0.81. In other words, the balance between the tensile breaking strength in the longitudinal direction and that in the width direction was poor, and the balance between the tensile modulus in the longitudinal direction and that in the width direction was also poor. Deterioration of battery life was observed.

[0067] Comparative Example 4 Film formation and battery production were carried out in the same manner as in Example 4, except that the width direction stretching temperature was 95°C, the longitudinal stretching ratio was 2.0 times, and the heat setting temperature was 200°C. The tensile breaking strength MD / TD was 0.49. In other words, the balance between the tensile breaking strength in the longitudinal direction and that in the width direction was poor. The tensile breaking strength in the MD direction was 108 MPa. Deterioration in battery productivity and battery life was observed.

[0068] In this table, CC in the lubricant column means calcium carbonate particles. The stretch ratio in this table refers to the ratio of the primary stretching ratio to the secondary stretching ratio (i.e., primary stretching ratio / secondary stretching ratio). The area ratio refers to the product of the primary stretching ratio and the secondary stretching ratio. In the stretching method section of this table, MT refers to sequential biaxial stretching in which longitudinal stretching is performed followed by width direction stretching. On the other hand, TM refers to sequential biaxial stretching in which width direction stretching is performed followed by longitudinal direction stretching. In this table, MD / TD listed in the tensile breaking strength column means tensile breaking strength in the MD direction / tensile breaking strength in the TD direction. That is, this means the ratio of the tensile breaking strength in the MD direction to the tensile breaking strength in the TD direction. MD / TD listed in the TE (i.e., tensile breaking strain) column means the ratio of the TE in the MD direction to the TE in the TD direction. MD / TD listed in the tensile modulus column means the ratio of the tensile modulus in the MD direction to the tensile modulus in the TD direction.

[0069] The present invention can provide a biaxially stretched polyphenylene sulfide film, a laminated film, a battery electrode, and a battery, and therefore has industrial applicability.

Claims

1. A biaxially oriented polyphenylene sulfide film having a thickness of 0.2 μm or more and 30 μm or less, a tensile breaking strength in the MD direction and a tensile breaking strength in the TD direction each of which is 120 MPa or more, and a ratio of the tensile breaking strength in the MD direction to the tensile breaking strength in the TD direction of 0.7 or more and 1.3 or less.

2. The biaxially oriented polyphenylene sulfide film according to claim 1, having a crystallinity of 25% or more as measured by differential scanning calorimetry (DSC).

3. The biaxially oriented polyphenylene sulfide film according to claim 1, wherein the ratio of the tensile modulus in the MD direction to the tensile modulus in the TD direction is 0.85 or more and 1.15 or less.

4. The biaxially oriented polyphenylene sulfide film according to claim 1, wherein the tensile breaking strength in the MD direction and the tensile breaking strength in the TD direction are each 120 MPa or more and 225 MPa or less.

5. The biaxially oriented polyphenylene sulfide film according to claim 1, wherein the biaxially oriented polyphenylene sulfide film is a polyphenylene sulfide film that has been sequentially biaxially oriented, and the crystallization temperature of the polyphenylene sulfide film after uniaxial stretching and before biaxial stretching is 104°C or higher.

6. The biaxially stretched polyphenylene sulfide film according to claim 1, which is used to manufacture an electrode for a battery.

7. A laminated film comprising the biaxially oriented polyphenylene sulfide film according to any one of claims 1 to 6, and a metal thin film provided on at least one surface of the biaxially oriented polyphenylene sulfide film.

8. A battery electrode comprising the laminated film according to claim 7 and an electrode material provided on the metal thin film of the laminated film.

9. A battery comprising the battery electrode according to claim 8.

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