Separator for electrochemical element

Abstract

To provide a separator for an electrochemical element which is excellent in heat resistance and is little deformed when being pressurized under heating environment, and is excellent in mechanical strength.SOLUTION: A separator for an electrochemical element includes a non-woven fabric, and contains two kinds of polyolefin-based core-sheath type composite fibers of a polyolefin-based core-sheath type composite fiber A in which a sheath component is a polyolefin-based resin A having a melting point higher than 135°C and equal to or lower than 150°C, and a polyolefin-based core-sheath type composite fiber B in which a sheath component is a polyolefin-based resin B having a melting point equal to or lower than 135°C, as constituent fibers. The separator for an electrochemical element that is excellent in heat resistance and is little deformed when being pressurized under heating environment, and is excellent in mechanical strength can be provided by containing both of a polyolefin-based resin A having a high melting point, and a polyolefin-based resin B having a low melting point as fusion components.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a separator for an electrochemical element. [Background technology]

[0002] BACKGROUND ART Conventionally, separators for electrochemical elements have been provided and used between the positive and negative electrodes of electrochemical elements (for example, capacitors, primary batteries such as lithium batteries and nickel-metal hydride batteries, secondary batteries, fuel cells, solid-state batteries, air batteries, etc.) to separate the electrodes and prevent short circuits and to facilitate electromotive reactions.

[0003] Since separators for electrochemical elements are required to have ionic conductivity, separators for electrochemical elements using a substrate such as a nonwoven fabric have been studied. Furthermore, polyolefin resin fibers are widely used as constituent fibers for separators for electrochemical elements to provide excellent resistance to electrolytes.

[0004] In such separators for electrochemical elements, fusion fibers containing polyethylene with a low melting point on the fiber surface are widely used to strengthen the bonds between the constituent fibers by fusing them together, thereby realizing electrochemical elements that are less prone to short circuits.

[0005] However, because the melting point of typical polyethylene is low, at 135°C or less, separators for electrochemical elements containing fused fibers containing polyethylene on the fiber surface are prone to deformation in high-temperature environments. Specifically, they are prone to deformation when pressurized by the expansion and contraction of the electrodes accompanying the charging and discharging of the electrochemical element in high-temperature environments such as those generated by the electrochemical element during charging and discharging, or by the vehicle in the case of an electrochemical element installed in a vehicle.

[0006] This causes a problem that the thickness of the separator for an electrochemical element is reduced, the electrolyte retention capacity is reduced, and the electrical resistance of the electrochemical element is increased.

[0007] To address this issue, the present applicant proposed a separator for electrochemical elements containing polyolefin resin fibers, as described in Patent Document 1 (JP 2002-298821 A). Specifically, the proposed separator for electrochemical elements has excellent heat resistance and is resistant to deformation even when subjected to pressure in a hot environment, thanks to a nonwoven fabric fused to the fiber surface with fusible fibers containing a propylene copolymer, such as an ethylene-propylene copolymer, which has a melting point higher than that of polyethylene. In the examples of Patent Document 1, a separator for electrochemical elements is prepared that includes a nonwoven fabric made of polyolefin sheath-core composite fibers, the sheath of which is a propylene copolymer (ethylene-butene-propylene copolymer) with a melting point of 137°C, as the polyolefin resin fiber that fuses the constituent fibers together. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-298821 Summary of the Invention [Problem to be solved by the invention]

[0009] However, polyolefin-based sheath-core conjugate fibers in which the sheath component is a propylene-based copolymer have the problem of being lower in strength than polyolefin-based sheath-core conjugate fibers in which the sheath component is polyethylene.

[0010] As a result, separators for electrochemical elements including nonwoven fabrics in which constituent fibers are fused together with a propylene-based copolymer have low mechanical strength and are therefore prone to tearing due to electrode burrs, etc. Furthermore, electrochemical elements incorporating such separators have the problem of being prone to short circuits.

[0011] The present invention has been made under these circumstances, and an object of the present invention is to provide a separator for an electrochemical element that has excellent heat resistance, undergoes little deformation when subjected to pressure in a heated environment, and has excellent mechanical strength. [Means for solving the problem]

[0012] The invention according to claim 1 of the present invention is "a separator for an electrochemical element comprising a nonwoven fabric, the nonwoven fabric containing, as constituent fibers, polyolefin-based sheath-core composite fiber A, the sheath component of which is polyolefin-based resin A having a melting point higher than 135°C and not higher than 150°C, and polyolefin-based sheath-core composite fiber B, the sheath component of which is polyolefin-based resin B having a melting point not higher than 135°C, the constituent fibers being fused together by the polyolefin-based resin A and the polyolefin-based resin B."

[0013] The invention according to claim 2 of the present invention is "the separator for electrochemical elements according to claim 1, wherein the polyolefin resin A is the only resin contained in the separator for electrochemical elements and having a melting point of more than 135°C and not more than 150°C, and when the melting endothermic peak of the separator for electrochemical elements is measured using a differential scanning calorimeter at a heating rate of 10°C / min, the heat quantity of the melting endothermic peak obtained under a temperature condition of more than 135°C and not more than 150°C is more than 3.9% and less than 9.6% of the total heat quantity of the melting endothermic peak obtained during the measurement."

[0014] The invention according to claim 3 of the present invention is "the separator for electrochemical elements according to claim 1, wherein the polyolefin resin B is the only resin contained in the separator for electrochemical elements and having a melting point of 135°C or less, and when the melting endothermic peak of the separator for electrochemical elements is measured using a differential scanning calorimeter at a heating rate of 10°C / min, the calorific value of the melting endothermic peak obtained under a temperature condition of 135°C or less is more than 13.9% and not more than 50.0% of the calorific value of the entire melting endothermic peak obtained during the measurement."

[0015] The invention of claim 4 of the present invention is "a separator for electrochemical elements according to claim 1, wherein the nonwoven fabric contains, as constituent fibers, polyolefin-based sheath-core composite fiber A, polyolefin-based sheath-core composite fiber B, and also a high-melting point fiber whose main component is polypropylene."

[0016] The invention according to claim 5 of the present invention is "the separator for an electrochemical element according to claim 1, wherein the nonwoven fabric has an average fiber diameter of 16.5 μm or less."

[0017] The invention of claim 6 of the present invention is "the separator for electrochemical elements according to claim 1, wherein the polyolefin resin A is an ethylene-propylene copolymer, and the polyolefin resin B is polyethylene or an ethylene-propylene copolymer." [Effects of the Invention]

[0018] The separator for electrochemical elements according to claim 1 of the present invention is a separator for electrochemical elements comprising a nonwoven fabric, the nonwoven fabric containing two types of polyolefin-based sheath-core conjugated fibers as constituent fibers: polyolefin-based sheath-core conjugated fiber A, the sheath component of which is polyolefin-based resin A having a melting point of more than 135°C and not more than 150°C, and polyolefin-based sheath-core conjugated fiber B, the sheath component of which is polyolefin-based resin B having a melting point of not more than 135°C. The present inventors have found that by including both polyolefin-based resin A having a high melting point and polyolefin-based resin B having a low melting point as fusion components, it is possible to provide a separator for electrochemical elements that achieves both excellent heat resistance and minimal deformation when subjected to pressure in a heated environment, as well as excellent mechanical strength.

[0019] In the separator for electrochemical elements according to claim 2 of the present invention, the polyolefin resin A is the only resin contained in the separator for electrochemical elements and having a melting point of more than 135°C and not more than 150°C, and in the measurement of the melting endothermic peak of the separator for electrochemical elements using a differential scanning calorimeter, the heat quantity of the melting endothermic peak obtained under temperature conditions of more than 135°C and not more than 150°C, which is attributed to the melting of the polyolefin resin A contained in the sheath component of the polyolefin sheath-core composite fiber A, is more than 3.9% and less than 9.6% of the heat quantity of the entire melting endothermic peak obtained during the measurement. When the calorific value of the melting endothermic peak of a separator for electrochemical elements obtained under temperature conditions of higher than 135°C and lower than 150°C is greater than 3.9% of the calorific value of the entire melting endothermic peak obtained during measurement, this means that the separator for electrochemical elements contains, as a constituent fiber, a greater than certain amount of the polyolefin-based sheath-core composite fiber A, which contributes to heat resistance. The applicant has discovered that separators for electrochemical elements having this value greater than 3.9% have improved heat resistance and are less likely to deform when subjected to pressure in a heated environment. Furthermore, when the calorific value of the melting endothermic peak of a separator for electrochemical elements obtained under temperature conditions of higher than 135°C and lower than 150°C is less than 9.6% of the calorific value of the entire melting endothermic peak obtained during measurement, this means that the separator for electrochemical elements contains, as a constituent fiber, less than a certain amount of the polyolefin-based sheath-core composite fiber A, which is resistant to melt deformation and tends not to contribute much to bonding between fibers. The applicant has discovered that separators for electrochemical elements in which this value is less than 9.6% have improved mechanical strength. As described above, the present invention can further provide a separator for an electrochemical element which has excellent heat resistance, undergoes little deformation when subjected to pressure in a heated environment, and has excellent mechanical strength.

[0020] In the separator for electrochemical elements according to claim 3 of the present invention, the only resin contained in the separator for electrochemical elements and having a melting point of 135°C or less is the polyolefin resin B, and in the measurement results of the melting endothermic peak of the separator for electrochemical elements using a differential scanning calorimeter, the heat quantity of the melting endothermic peak obtained under temperature conditions of 135°C or less, which is attributed to the melting of the polyolefin resin B constituting the sheath component of the polyolefin sheath-core composite fiber B, is more than 13.9% and not more than 50.0% of the heat quantity of the entire melting endothermic peak obtained during the measurement. The fact that the calorific value of the melting endothermic peak of a separator for electrochemical elements obtained under a temperature condition of 135°C or less is greater than 13.9% of the calorific value of the entire melting endothermic peak obtained during measurement means that the separator for electrochemical elements contains, as a constituent fiber, a greater than certain amount of the polyolefin-based sheath-core composite fiber B, which can firmly bond fibers together by melt deformation. The applicant has discovered that separators for electrochemical elements in which this value is greater than 13.9% have improved mechanical strength. Furthermore, when the calorific value of the melting endothermic peak of a separator for electrochemical elements obtained under temperature conditions of 135°C or less is 50.0% or less of the calorific value of the entire melting endothermic peak obtained during measurement, this means that the separator for electrochemical elements contains, as a constituent fiber, less than a certain amount of the polyolefin-based sheath-core composite fiber B, which has a low melting point and may cause a decrease in the heat resistance of the separator for electrochemical elements. The applicant has discovered that separators for electrochemical elements in which this value is 50.0% or less have improved heat resistance and are less likely to deform when subjected to pressure in a heated environment. As described above, the present invention can further provide a separator for an electrochemical element which has excellent heat resistance, undergoes little deformation when subjected to pressure in a heated environment, and has excellent mechanical strength.

[0021] In the separator for electrochemical elements according to claim 4 of the present invention, the nonwoven fabric provided in the separator for electrochemical elements contains, in addition to the polyolefin-based sheath-core composite fiber A and the polyolefin-based sheath-core composite fiber B, a high-melting-point fiber mainly composed of polypropylene, which has a higher melting point than the polyolefin resins A and B and is therefore highly heat-resistant. Therefore, the separator for electrochemical elements has better heat resistance and is less likely to deform when subjected to pressure in a heated environment. As described above, the present invention can further provide a separator for an electrochemical element which has excellent heat resistance, undergoes little deformation when subjected to pressure in a heated environment, and has excellent mechanical strength.

[0022] In the separator for an electrochemical element according to claim 5 of the present invention, the nonwoven fabric provided in the separator for an electrochemical element has a small average fiber diameter of 16.5 μm or less. As described above, the present invention can provide a separator for an electrochemical element that has a large specific surface area, is excellent in electrolyte retention even when subjected to pressure, and has high performance.

[0023] In the separator for electrochemical elements according to claim 6 of the present invention, the polyolefin resin A is an ethylene-propylene copolymer, and the polyolefin resin B is polyethylene or an ethylene-propylene copolymer, thereby providing a separator for electrochemical elements that has excellent heat resistance, is less likely to deform when subjected to pressure in a heated environment, and has excellent mechanical strength. DETAILED DESCRIPTION OF THE INVENTION

[0024] The separator for electrochemical elements (hereinafter, sometimes referred to as "separator") of the present invention is a separator with a nonwoven fabric. The nonwoven fabric contains, as constituent fibers, polyolefin-based sheath-core composite fibers A, in which the sheath component is polyolefin-based resin A having a melting point of more than 135°C and not more than 150°C, and polyolefin-based sheath-core composite fibers B, in which the sheath component is polyolefin-based resin B having a melting point of not more than 135°C. The constituent fibers are fused together by the polyolefin-based resins A and B. By including both polyolefin-based resin A with a high melting point and polyolefin-based resin B with a low melting point as fusion components, a separator for electrochemical elements can be provided that achieves both excellent heat resistance, minimal deformation when subjected to pressure in a heated environment, and excellent mechanical strength. Note that, in the present invention, the "melting point" refers to the peak top of the melting endothermic peak observed when heated using a differential scanning calorimeter.

[0025] The polyolefin sheath-core composite fibers A and polyolefin sheath-core composite fibers B contained in the constituent fibers of the nonwoven fabric provided in the separator of the present invention may have the core components arranged concentrically or eccentrically in the fiber cross section, but a concentric arrangement of the core components is preferable because it allows the polyolefin sheath-core composite fibers A and polyolefin sheath-core composite fibers B to be evenly fused. The volume ratio of the core component to the sheath component is not particularly limited, but is preferably 3:7 to 9:1, more preferably 4:6 to 8:2, and even more preferably 5:5 to 7:3, so that the fibers can be firmly fused together and the fiber form can be maintained during fusion.

[0026] The higher the proportion of polyolefin resin A in the sheath component of polyolefin-based sheath-core composite fiber A, the more polyolefin resin A involved in bonding the fibers together in the sheath component, resulting in a separator with excellent mechanical strength.Therefore, the proportion of polyolefin resin A in the sheath component of polyolefin-based sheath-core composite fiber A is preferably 50% or more, more preferably 70% or more, even more preferably 90% or more, and most preferably 100%.

[0027] Similarly, the higher the proportion of polyolefin resin B in the sheath component of polyolefin-based sheath-core composite fiber B, the more polyolefin resin B involved in bonding the fibers together in the sheath component, resulting in a separator with excellent mechanical strength.Therefore, the proportion of polyolefin resin B in the sheath component of polyolefin-based sheath-core composite fiber B is preferably 50% or more, more preferably 70% or more, more preferably 90% or more, and most preferably 100%.

[0028] The polyolefin resin A constituting the sheath component of the polyolefin sheath-core composite fiber A may be any polyolefin resin having a melting point of more than 135°C and not more than 150°C, and may be, for example, a propylene copolymer (ethylene-propylene copolymer, ethylene-butene-propylene copolymer, ethylene-butadiene-propylene copolymer, etc.). The propylene copolymer is a copolymer having, in a linear chain structure, propylene segments with a -(CHCH(CH))- chemical structure and segments with other chemical structures.

[0029] The polyolefin resin A may be a linear polymer or a branched polymer, and may be a block copolymer or a random copolymer, and there are no particular limitations on the three-dimensional structure or the presence or absence of crystallinity.

[0030] The melting point of the polyolefin resin A is higher than 135°C and 150°C or lower so that a separator containing the polyolefin core-sheath composite fiber A has little deformation when subjected to pressure in a heated environment and has excellent heat resistance. However, in order to have even less deformation when subjected to pressure in an even heated environment and have excellent heat resistance, the melting point of the polyolefin resin A is more preferably 136°C or higher, and even more preferably 137°C or higher.

[0031] The resin constituting the core component of the polyolefin-based sheath-core composite fiber A can be a known resin, but is preferably a polyolefin-based resin that has excellent resistance to the electrolyte. Among polyolefin-based resins, polypropylene or polymethylpentene is more preferred, as they have a high melting point, can better maintain the fiber structure, and can further improve the strength of the separator. Among these, polypropylene is even more preferred because of its versatility.

[0032] The melting point of the resin constituting the core component of the polyolefin-based sheath-core composite fiber A is preferably 155°C or higher, more preferably 158°C or higher, and even more preferably 160°C or higher, so that the separator has little deformation when subjected to pressure in a heated environment and excellent heat resistance.

[0033] The polyolefin resin B constituting the sheath component of the polyolefin core-sheath composite fiber B may be any polyolefin resin having a melting point of 135°C or less, and may be, for example, polyethylene or a propylene copolymer (such as an ethylene-propylene copolymer, an ethylene-butene-propylene copolymer, or an ethylene-butadiene-propylene copolymer).

[0034] The polyolefin resin B may be a linear polymer or a branched polymer, and may be a block copolymer or a random copolymer, and there are no particular limitations on the three-dimensional structure or the presence or absence of crystallinity.

[0035] The melting point of the polyolefin resin B is 135° C. or lower so that the separator containing the polyolefin sheath-core composite fiber B containing the polyolefin resin B as the sheath component bonds the constituent fibers together and has excellent mechanical strength, but is more preferably 134° C. or lower, and even more preferably 133° C. or lower. The lower limit of the melting point of the polyolefin resin B is preferably 110° C. or higher so that the separator has excellent heat resistance with little deformation when subjected to pressure in a heated environment.

[0036] The resin constituting the core component of polyolefin-based sheath-core composite fiber B is preferably a polyolefin-based resin, more preferably polypropylene or polymethylpentene, and even more preferably polypropylene, for the same reasons as for polyolefin-based sheath-core composite fiber A.

[0037] The melting point of the resin constituting the core component of the polyolefin-based sheath-core composite fiber B is preferably 155°C or higher, more preferably 158°C or higher, and even more preferably 160°C or higher, so that the separator has little deformation when subjected to pressure in a heated environment and excellent heat resistance.

[0038] The average fiber diameter of the polyolefin-based sheath-core composite fiber A and the polyolefin-based sheath-core composite fiber B is preferably 32 μm or less, more preferably 22 μm or less, and even more preferably 18 μm or less, so that the smaller the average fiber diameter, the larger the specific surface area and the better the electrolyte retention of the separator. The lower limit of the average fiber diameter of the polyolefin-based sheath-core composite fiber A and the polyolefin-based sheath-core composite fiber B is preferably 1.0 μm or more so that the separator has excellent strength. Here, the "average fiber diameter" refers to the arithmetic mean value of the fiber diameters of 10 fibers. Furthermore, when the cross-sectional shape of the fiber is non-circular, the fiber diameter of the fiber is defined as the diameter of a circle having the same area as the cross-sectional area.

[0039] The fiber length of the polyolefin-based sheath-core composite fiber A and the polyolefin-based sheath-core composite fiber B is not particularly limited, but is preferably 0.1 to 100 mm, more preferably 0.2 to 30 mm, and even more preferably 0.3 to 20 mm, so that the fibers are uniformly dispersed and the separator can uniformly retain the electrolyte solution.

[0040] The mass percentage of polyolefin-based sheath-core composite fiber A contained in the nonwoven fabric provided in the separator of the present invention is preferably 10 mass% or more, more preferably 20 mass% or more, and even more preferably 30 mass% or more so that the separator is resistant to deformation when pressure is applied in a heated environment and has excellent heat resistance. The upper limit of the percentage is preferably 90 mass% or less, more preferably 80 mass% or less, and even more preferably 70 mass% or less, because the content of polyolefin-based sheath-core composite fiber B in the separator may become too low, which may result in a deterioration in the mechanical strength of the separator.

[0041] The mass percentage of polyolefin-based sheath-core composite fiber B contained in the nonwoven fabric provided in the separator of the present invention is preferably 10 mass% or more, more preferably 20 mass% or more, and even more preferably 30 mass% or more so that the separator has excellent mechanical strength. The upper limit of the percentage is preferably 90 mass% or less, more preferably 80 mass% or less, and even more preferably 70 mass% or less, because the content of polyolefin-based sheath-core composite fiber A in the separator becomes too low, which may result in poor heat resistance of the separator and cause deformation when the separator is subjected to pressure in a heated environment.

[0042] The nonwoven fabric provided in the separator of the present invention contains polyolefin-based sheath-core composite fiber A and polyolefin-based sheath-core composite fiber B as constituent fibers, but may also contain fibers other than the above two fibers as constituent fibers. For example, the constituent fibers may include fibers with a tensile strength of 5 cN / dtex or more that improve the strength of the separator, ultrafine fibers with a fiber diameter of 5 μm or less that improve the electrolyte retention ability of the separator, and sheath-core composite fibers other than polyolefin-based sheath-core composite fibers A and B that bond the constituent fibers of the separator together and improve the strength of the separator. Note that the constituent fibers of the separator (including polyolefin-based sheath-core composite fiber A and polyolefin-based sheath-core composite fiber B) are preferably composed solely of polyolefin-based resins such as polyethylene, polypropylene, polymethylpentene, ethylene-based copolymers, and propylene-based copolymers because of their excellent resistance to electrolytes. Furthermore, among polyolefin resins, the separator preferably contains high-melting-point fibers primarily composed of polypropylene or polymethylpentene, which have high melting points, because the separator exhibits little deformation when pressurized in a heated environment and has excellent heat resistance, and the separator preferably contains high-melting-point fibers primarily composed of polypropylene because the separator has excellent heat resistance and polypropylene is highly versatile. Note that, in the present invention, "high-melting-point fibers primarily composed of polypropylene" refers to high-melting-point fibers in which the resin contained in the high-melting-point fibers is polypropylene in the highest proportion.

[0043] The mass percentage of the above-mentioned other fibers contained in the nonwoven fabric provided in the separator of the present invention is preferably 80 mass% or less, more preferably 50 mass% or less, and even more preferably 30 mass% or less, so that the separator exhibits little deformation when subjected to pressure in a heated environment, has excellent heat resistance, and is excellent in mechanical strength, due to the polyolefin-based sheath-core composite fiber A and the polyolefin-based sheath-core composite fiber B contained in the nonwoven fabric provided in the separator.

[0044] The smaller the average fiber diameter of the constituent fibers of the nonwoven fabric provided in the separator of the present invention, the larger the specific surface area of ​​the separator, and the higher performance the separator can be with excellent electrolyte retention, so it is preferably 16.5 μm or less, more preferably 16.2 μm or less, and even more preferably 15.9 μm or less. The lower limit of the average fiber diameter of the constituent fibers of the nonwoven fabric provided in the separator is preferably 3.0 μm or more so that the separator has excellent mechanical strength.

[0045] The average fiber diameter (D) of the constituent fibers of the nonwoven fabric provided in the separator is a value calculated by the following formula.

[0046]

number

[0047] where X i is the mass percentage of each fiber in the nonwoven fabric provided in the separator (unit: %), D i is the average fiber diameter of each fiber (unit: μm), ρ i is the density of the resin that makes up each fiber (unit: g / cm 3 ), ρ AV is the average density of each resin that makes up the fiber (unit: g / cm) calculated using the following formula: 3 ) As mentioned above, the "average fiber diameter" of each fiber refers to the arithmetic mean value of the fiber diameters of 10 individual fibers.

[0048]

number

[0049] For example, if the average fiber diameter is D A (μm), resin density ρ A Polyolefin core-sheath composite fiber A is X A mass% and the average fiber diameter is D B (μm), and the resin density is ρ B Polyolefin core-sheath composite fiber B is X B mass% and the average fiber diameter is DC (μm), and the resin density is ρ C X single fibers C When the nonwoven fabric of the separator contains 100% by mass of carbon black, the average fiber diameter (D) of the constituent fibers of the nonwoven fabric of the separator is calculated by the following formula:

[0050]

number

[0051] In addition, the average density ρ AV is a value calculated using the following formula:

[0052]

number

[0053] The resin having a melting point of more than 135°C and less than 150°C contained in the separator of the present invention is preferably only the polyolefin resin A contained in the sheath component of the polyolefin sheath-core composite fiber A. Furthermore, when the resin having a melting point of more than 135°C and less than 150°C contained in the separator of the present invention is only the polyolefin resin A contained in the sheath component of the polyolefin sheath-core composite fiber A, when the separator of the present invention is subjected to melting endothermic peak measurement at a temperature rise rate of 10°C / min using a differential scanning calorimeter, the heat quantity of the melting endothermic peak obtained under temperature conditions of more than 135°C and less than 150°C is preferably more than 3.9% and less than 9.6% of the total heat quantity of the melting endothermic peak obtained during measurement. A heat quantity of the melting endothermic peak obtained under temperature conditions of more than 135°C and less than 150°C that is more than 3.9% of the total heat quantity of the melting endothermic peak obtained during measurement means that the separator contains a greater than a certain amount of the polyolefin sheath-core composite fiber A as a constituent fiber. Therefore, a separator that satisfies the lower limit exhibits little deformation when subjected to pressure in a heated environment, and has improved heat resistance.

[0054] Furthermore, the fact that the calorific value of the melting endothermic peak obtained under temperature conditions of higher than 135°C and lower than 150°C is less than 9.6% of the total calorific value of the melting endothermic peak obtained during measurement means that the separator contains less than a certain amount of the polyolefin-based sheath-core composite fiber A as a constituent fiber. Therefore, a separator that satisfies this upper limit has improved mechanical strength. From the above, when the calorific value of the melting endothermic peak obtained under temperature conditions of higher than 135°C and lower than 150°C is more than 3.9% but less than 9.6% of the total calorific value of the melting endothermic peak obtained during measurement, a separator can be provided that exhibits little deformation when subjected to pressure in a heated environment, has excellent heat resistance, and is excellent in mechanical strength. In order to provide a separator having superior heat resistance and mechanical strength, the calorific value of the melting endothermic peak obtained under a temperature condition of higher than 135°C and lower than 150°C is more preferably 4.2 to 9.3%, and even more preferably 4.5 to 9.0%, of the total calorific value of the melting endothermic peak obtained during measurement.

[0055] The measurement using a differential scanning calorimeter in the present invention is performed in accordance with heat flux differential scanning calorimetry (heat flux DSC) described in JIS K 7121 (2012) "Method for measuring transition temperature of plastics" 4.2(2) using a Q1000 manufactured by TA Instruments under the following (DSC measurement conditions), and a DSC curve is drawn. (DSC measurement conditions) 1. Shape and size of test specimen: A circular separator with a diameter of 6 mm is used as the test specimen. 2. Nitrogen gas flow rate: 50 ml / min 3. Temperature rise: 10℃ / min 4.Measurement start temperature: 0℃

[0056] In order to provide excellent heat resistance to the separator, the resin having a melting point of 135°C or less contained in the separator of the present invention is preferably only the polyolefin resin B contained in the fiber surface of the polyolefin sheath-core composite fiber B. Furthermore, when the resin having a melting point of 135°C or less contained in the separator of the present invention is only the polyolefin resin B contained in the fiber surface of the polyolefin sheath-core composite fiber B, when the separator of the present invention is measured for its melting endothermic peak at a heating rate of 10°C / min using a differential scanning calorimeter, the heat quantity of the melting endothermic peak obtained under a temperature condition of 135°C or less is preferably more than 13.9% and not more than 50.0% of the heat quantity of the entire melting endothermic peak obtained during the measurement.

[0057] When the calorific value of the melting endothermic peak obtained under temperature conditions of 135°C or less is greater than 13.9% of the calorific value of the entire melting endothermic peak obtained during measurement, this means that the separator contains more than a certain amount of the polyolefin sheath-core composite fiber B as a constituent fiber. Therefore, a separator that satisfies the lower limit has improved mechanical strength. Furthermore, when the calorific value of the melting endothermic peak obtained under temperature conditions of 135°C or less is 50.0% or less of the calorific value of the entire melting endothermic peak obtained during measurement, this means that the separator contains less than a certain amount of the polyolefin sheath-core composite fiber B as a constituent fiber. Therefore, a separator that satisfies the upper limit has improved heat resistance, with less deformation when subjected to pressure in a heated environment. From the above, when the calorific value of the melting endothermic peak obtained under temperature conditions of 135°C or less is more than 13.9% but not more than 50.0% of the total calorific value of the melting endothermic peak obtained during measurement, a separator can be provided that shows little deformation when subjected to pressure in a heated environment, has excellent heat resistance, and is excellent in mechanical strength. To provide a separator with even better heat resistance and mechanical strength, the calorific value of the melting endothermic peak obtained under temperature conditions of 135°C or less is more preferably 14.5 to 45.0%, and even more preferably 15.0 to 40.0%, of the total calorific value of the melting endothermic peak obtained during measurement.

[0058] The separator of the present invention comprises a nonwoven fabric. Examples of the nonwoven fabric include dry-laid nonwoven fabrics produced by a carding method or an air-laying method, wet-laid nonwoven fabrics produced by papermaking, and directly spun nonwoven fabrics (melt-blown, spunbond) produced by collecting directly spun fibers. Among these, wet-laid nonwoven fabrics are thinner than other nonwoven fabrics and can realize electrochemical elements with low electrical resistance, so the nonwoven fabric provided in the separator of the present invention is preferably a wet-laid nonwoven fabric.

[0059] The separator of the present invention includes a nonwoven fabric as described above, but may be a separator made of only a nonwoven fabric, or a separator composited with a material other than the nonwoven fabric, such as inorganic particles or a porous film.

[0060] The basis weight of the separator of the present invention is adjusted appropriately, but is preferably 20 to 100 g / m 2 and 25 to 80 g / m 2 and 30 to 70 g / m 2 In the present invention, the "weight" is the weight per square meter of the main surface, which is the widest surface. 2 This refers to the mass per unit mass.

[0061] Furthermore, the thickness of the separator of the present invention is preferably 10 to 250 μm, more preferably 20 to 230 μm, and even more preferably 30 to 200 μm, since a thinner separator tends to reduce the internal resistance of the battery, but if the thickness is too thin, resistance to internal short circuits may be poor. Note that the "thickness" referred to in the present invention refers to the average value of measurements taken at 10 randomly selected points under a load of 147 kPa using an outside micrometer (measurement range: 0 to 25 mm) as specified in JIS B 7502 (2016) "Micrometer" 3.1.

[0062] The porosity of the separator of the present invention at a load of 147 kPa is preferably 50% or more, more preferably 53% or more, and even more preferably 56% or more, since the higher the porosity, the more electrolyte the separator can retain. On the other hand, if the porosity of the separator is too high, the amount of fibers supporting the separator's shape is reduced, making the separator's pores more likely to collapse when pressure is applied, which may result in poor electrolyte retention. Furthermore, short circuits caused by electrode dendrites or burrs penetrating the separator are more likely to occur, which may result in poor short-circuit resistance. Therefore, the porosity is preferably 85% or less, more preferably 83% or less, and even more preferably 81% or less. This "porosity (P) at a load of 147 kPa" (unit: %) refers to the value obtained from the following formula:

[0063]

number

[0064] where Fr n indicates the packing ratio (unit: %) of n component that constitutes the separator, and is the value obtained from the following formula.

[0065]

number

[0066] Here, M is the basis weight of the separator (unit: g / cm 2 ), T is the separator thickness under a load of 147 kPa (unit: cm), Pr n is the mass ratio of n component in the separator, SG n is the density of n component (unit: g / cm 3 ) respectively.

[0067] The higher the penetration strength of the separator of the present invention, the less likely the separator is to be torn by electrode burrs and the like, and the less likely an electrochemical element incorporating this separator is to cause a short circuit. Therefore, it is preferably 1000 gf or more, more preferably 1100 gf or more, and even more preferably 1200 gf or more. Similarly, the higher the penetration strength per unit area of ​​the separator of the present invention, the less likely the separator is to be torn by electrode burrs and the like, and the less likely an electrochemical element incorporating this separator is to cause a short circuit. Therefore, it is preferable that the penetration strength per unit area of ​​the separator of the present invention is 20 gf / (g / m 2 ) or more is preferable, and 21gf / (g / m 2 ) or more is more preferable, and 22gf / (g / m 2 ) or more is more preferable. The "penetration strength" and "penetration strength per unit area weight" can be determined by the following method.

[0068] (1) A separator is placed on a support base having a cylindrical through-hole (inner diameter: 11 mm) so as to cover the cylindrical through-hole, and a fixing material having a cylindrical through-hole (inner diameter: 11 mm) is placed on the separator so as to align with the center of the cylindrical through-hole in the support base, thereby fixing the separator. (2) A needle (radius of curvature at tip: 0.5 mm, diameter: 1 mm, length of protrusion from jig: 2 cm) attached to a handy compression tester (Kato Tech, KES-G5) is inserted vertically into the separator at a speed of 0.1 cm / s, and the force (gf) required for the needle to penetrate is measured. (3) This measurement is carried out 10 times, and the arithmetic mean value is the penetration strength (gf) of the separator. In addition, the basis weight of the separator (g / m 2 ) is divided to obtain the penetration strength per unit area of ​​the separator {(penetration strength of separator (gf)) / (unit area of ​​separator (g / m 2 ))}.

[0069] The separator of the present invention can be produced, for example, as follows.

[0070] First, the above-mentioned polyolefin-based sheath-core conjugate fiber A, the above-mentioned polyolefin-based sheath-core conjugate fiber B, and, if necessary, other fibers such as the above-mentioned high-melting-point fiber are prepared.

[0071] Next, the fibers are blended to form a fiber web. The method for forming this fiber web is not particularly limited, but it can be formed by, for example, a dry method (such as a carding method or an air-laying method) or a wet method. Among these, a wet method is preferred, which can easily produce a fiber web in which the fibers are uniformly dispersed and with little fiber unevenness. As the wet method, it can be formed by a conventionally known method, such as a horizontal fourdrinier method, an inclined wire short net method, a cylinder method, or a combination of a fourdrinier and a cylinder method.

[0072] Next, the polyolefin sheath-core conjugate fibers A and B contained in this fibrous web are fused to bond the constituent fibers of the fibrous web, thereby obtaining a nonwoven fabric. The fusion method is not particularly limited, and examples include a method in which the fibrous web is supported on a conveyor and hot air is blown onto it, or a method in which heat is applied using a calendar. Note that before fusing the polyolefin sheath-core conjugate fibers A and B contained in the fibrous web, the fibrous web may be entangled with a water jet or needles in order to more firmly bond the constituent fibers of the nonwoven fabric.

[0073] Although the nonwoven fabric described above may be used as a separator of the present invention as it is, the fiber surfaces of the polyolefin-based sheath-core composite fibers A and the polyolefin-based sheath-core composite fibers B contained in the constituent fibers of the separator are hydrophobic, and therefore, when the nonwoven fabric described above is used as a separator as it is, the electrolyte retention tends to be low. Therefore, it is preferable to subject the nonwoven fabric described above to a hydrophilization treatment so that the electrolyte retention is excellent by the hydrophilization treatment. Examples of the hydrophilization treatment method include sulfonation treatment, fluorine gas treatment, vinyl monomer graft polymerization treatment, discharge treatment, surfactant treatment, and hydrophilic resin addition treatment.

[0074] Furthermore, the separator of the present invention may be made by carrying inorganic particles such as silica or alumina on the nonwoven fabric, or by combining the separator with a porous film, in addition to the hydrophilic treatment of the nonwoven fabric. [Example]

[0075] The present invention will be specifically described below with reference to examples, but these examples are not intended to limit the scope of the present invention.

[0076] (Polyolefin core-sheath composite fiber A) Polyolefin-based sheath-core composite fiber A (fiber diameter: 17.5 μm, fiber length: 5 mm, core / sheath volume ratio = 6:4, density: 0.91 g / cm) was used. The core component was made of polypropylene (melting point: 160°C) and the sheath component (fusion component) was made of ethylene-propylene copolymer (melting point: 138°C). The ethylene-propylene copolymer covered the fiber surface except for both ends, and the core components were arranged concentrically in the fiber cross section. 3 ) was prepared.

[0077] (Polyolefin core-sheath composite fiber B) Polyolefin-based core-sheath composite fiber B (fiber diameter: 10.6 μm, fiber length: 5 mm, core / sheath volume ratio = 6:4, density: 0.92 g / cm) was used. The core component was made of polypropylene (melting point: 172°C) and the sheath component (fusion component) was made of ethylene-propylene copolymer (melting point: 132°C). The ethylene-propylene copolymer covered the fiber surface except for both ends, and the core components were arranged concentrically in the fiber cross section. 3 ) was prepared.

[0078] (High-melting polypropylene fiber) High-melting-point polypropylene fiber (fiber diameter: 15.4 μm, fiber length: 10 mm, density: 0.93 g / cm) composed solely of polypropylene (melting point: 171 °C). 3 ) was prepared.

[0079] Example 1 A slurry was formed by mixing and dispersing 40 mass% polyolefin-based core-sheath composite fiber A, 30 mass% polyolefin-based core-sheath composite fiber B, and 30 mass% polypropylene high-melting point fiber, and then a fiber web was formed using a wet method (horizontal long-net method). Next, this fiber web was dried at a temperature of 140°C without pressure, and at the same time, the constituent fibers of the fiber web were fused together using the polyolefin resin A contained in the polyolefin core-sheath composite fiber A and the polyolefin resin B contained in the polyolefin core-sheath composite fiber B, thereby producing a nonwoven fabric. Next, the nonwoven fabric was subjected to a fluorine gas treatment using fluorine gas (F2 concentration: 6 vol%, SO2 concentration: 8 vol%, O2 concentration: 12 vol%, N2 concentration: 74 vol%) at a temperature of 40°C to make it hydrophilic, and then the separator (basis weight: 60 g / m 2 The thickness (under a load of 147 kPa): 0.16 mm, porosity (under a load of 147 kPa): 58%, and average fiber diameter: 13.8 μm were produced.

[0080] Example 2 A slurry was formed by mixing and dispersing 20 mass% polyolefin-based core-sheath composite fiber A, 60 mass% polyolefin-based core-sheath composite fiber B, and 20 mass% polypropylene high-melting point fiber, and then a fiber web was formed using a wet method (horizontal long-net method). Thereafter, the nonwoven fabric was produced and hydrophilized in the same manner as in Example 1, and a separator (basis weight: 60 g / m 2 The thickness (under a load of 147 kPa): 0.15 mm, porosity (under a load of 147 kPa): 56%, and average fiber diameter: 12.0 μm were produced.

[0081] Example 3 A slurry was formed by mixing and dispersing 60 mass% polyolefin-based core-sheath composite fiber A, 10 mass% polyolefin-based core-sheath composite fiber B, and 30 mass% polypropylene high-melting point fiber, and then a fiber web was formed using a wet method (horizontal long-net method). Thereafter, the nonwoven fabric was produced and hydrophilized in the same manner as in Example 1, and a separator (basis weight: 60 g / m 2The thickness (at 147 kPa load): 0.16 mm, porosity (at 147 kPa load): 57%, and average fiber diameter: 15.6 μm were produced.

[0082] (Comparative Example 1) A slurry was prepared by mixing and dispersing 70 mass% of polyolefin core-sheath composite fiber A and 30 mass% of polypropylene high-melting fiber, and then a fiber web was formed by a wet method (horizontal fourdrinier method). Thereafter, the nonwoven fabric was produced and hydrophilized in the same manner as in Example 1, and a separator (basis weight: 60 g / m 2 The thickness (at 147 kPa load): 0.15 mm, porosity (at 147 kPa load): 54%, and average fiber diameter: 16.8 μm were produced.

[0083] (Comparative Example 2) Polyolefin core-sheath composite fiber B 70 mass % and polypropylene high melting point fiber 30 mass % were mixed and dispersed to form a slurry, which was then formed into a fiber web by a wet method (horizontal fourdrinier method). Thereafter, the nonwoven fabric was produced and hydrophilized in the same manner as in Example 1, and a separator (basis weight: 40 g / m 2 The thickness (under a load of 147 kPa): 0.14 mm, porosity (under a load of 147 kPa): 70%, and average fiber diameter: 11.5 μm were produced.

[0084] The physical properties of the separators of the examples and comparative examples are shown in Table 1. The various physical properties were evaluated as follows.

[0085] (DSC measurement of separator) The separators of the examples and comparative examples were subjected to DSC measurement under the above-mentioned (DSC measurement conditions), and DSC curves were plotted. Further, from the DSC curve, the calorific value of the entire melting endothermic peak obtained during the DSC measurement, the calorific value of the melting endothermic peak obtained under a temperature condition of 135°C or less, and the calorific value of the melting endothermic peak obtained under a temperature condition of more than 135°C and 150°C or less are calculated; (1) The percentage of the heat energy of the melting endothermic peak obtained at temperatures below 135°C out of the total heat energy of the melting endothermic peak obtained during measurement (heat energy percentage (1)) (2) The percentage of the heat energy of the melting endothermic peak obtained under the temperature condition of 135°C or higher and 150°C or lower out of the total heat energy of the melting endothermic peak obtained during the measurement (heat energy percentage (2)) asked for.

[0086] (Measurement of void ratio after heat pressing) (1) The separator was cut into 5 cm squares. (2) The thickness of the separator under no load was measured by cutting the separator perpendicular to the main surface of the separator, photographing the enlarged cut surface with a stereomicroscope, and calculating the average length in the thickness direction of the separator at 10 points randomly selected from the photograph. (3) The porosity P0 of the separator before hot pressing was calculated from the thickness of the separator without load calculated in (2). (4) The separator was set in a manual hydraulic heat press and heat-pressed at 105°C and 1 MPa for 30 seconds. (5) The thickness of the separator after the heat press without load was measured in the same manner as in (2). (6) The porosity P1 of the separator after hot pressing was calculated from the thickness of the separator without load after hot pressing calculated in (5).

[0087] (Measurement of penetration strength) The penetration strength of the separators of the examples and comparative examples and the penetration strength per unit area of ​​the separator were determined by the above-mentioned method.

[0088] (Measurement of pressurized liquid retention rate) The separators of the examples and comparative examples were cut into circles with a diameter of 30 mm to prepare test pieces, which were then allowed to reach moisture equilibrium at a temperature of 20°C and a relative humidity of 65%, and the mass (M0) was measured for each. Next, the test piece was immersed in a potassium hydroxide solution with a specific gravity of 1.3 (20°C) for 1 hour so that the air in the test piece was replaced with the potassium hydroxide solution, and the potassium hydroxide solution was retained. Next, this test piece was sandwiched between three pieces of filter paper (diameter: 30 mm) on top and bottom, and a pressure of 4.9 MPa was applied for 30 seconds using a pressure pump, after which the mass (M1) of the test piece was measured. The pressurized liquid retention rate was calculated using the following formula: This measurement was performed on four test pieces of one separator, and the arithmetic average of the results was taken as the pressurized liquid retention rate (R p , unit: %.

[0089]

number

[0090] [Table 1]

[0091] Comparing the Example containing both the polyolefin-based sheath-core composite fiber A and the polyolefin-based sheath-core composite fiber B with Comparative Example 1 not containing the polyolefin-based sheath-core composite fiber B, the Example had a higher penetration strength per unit area weight and was a separator with excellent mechanical strength.

[0092] Furthermore, a comparison between the Example containing both the polyolefin-based sheath-core composite fiber A and the polyolefin-based sheath-core composite fiber B and Comparative Example 2 not containing the polyolefin-based sheath-core composite fiber A shows that the Example tends to be able to maintain a higher porosity after heat pressing, and the separator showed less deformation when subjected to pressure in a heated environment, making it a separator with excellent heat resistance.

[0093] From the above, it was found that the separator satisfying the constitution of the present invention shows little deformation when subjected to pressure in a heated environment, has excellent heat resistance, and is excellent in mechanical strength.

[0094] Furthermore, the separator of the example had high mechanical strength, little deformation when pressurized in a heated environment, and excellent heat resistance. This was because the heat quantity of the melting endothermic peak obtained under temperature conditions higher than 135°C and lower than 150°C, which was due to the melting of the ethylene-propylene copolymer contained in the fiber surface of the polyolefin-based composite fusion resin A contained in the separator, was greater than 3.9% and less than 9.6% of the total heat quantity of the melting endothermic peak obtained during measurement.

[0095] Furthermore, the separator of the example had a high mechanical strength, small deformation when pressure was applied in a heated environment, and excellent heat resistance. This was because the heat quantity of the melting endothermic peak obtained under temperature conditions of 135°C or less, which was due to the melting of the ethylene-propylene copolymer contained in the fiber surface of the polyolefin-based core-sheath composite fiber B contained in the separator, was greater than 13.9% and less than 50.0% of the total heat quantity of the melting endothermic peak obtained during measurement.

[0096] Furthermore, the separators of the examples had a small average fiber diameter of 16.5 μm or less in the nonwoven fabric, and therefore had a large specific surface area and were high-performance separators for electrochemical elements with excellent electrolyte retention even when subjected to pressure. [Industrial Applicability]

[0097] The separator of the present invention can be used as a separator for separating electrodes in electrochemical elements such as primary batteries (e.g., lithium batteries, manganese batteries, magnesium batteries, etc.), secondary batteries (e.g., lithium ion batteries, nickel-metal hydride batteries, nickel-cadmium batteries, zinc batteries, redox flow batteries, etc.), and capacitors, regardless of whether the separator is aqueous or non-aqueous.

Claims

1. A separator for an electrochemical element including a nonwoven fabric, the nonwoven fabric including, as constituent fibers, a polyolefin-based sheath-core composite fiber A, the sheath component of which is a polyolefin-based resin A having a melting point of more than 135°C and not more than 150°C, and a polyolefin-based sheath-core composite fiber B, the sheath component of which is a polyolefin-based resin B having a melting point of not more than 135°C, The separator for an electrochemical element, wherein the constituent fibers are fused together by the polyolefin resin A and the polyolefin resin B.

2. the polyolefin resin A is the only resin contained in the separator for an electrochemical element and having a melting point of more than 135°C and not more than 150°C, When the melting endothermic peak of the separator for electrochemical elements is measured using a differential scanning calorimeter at a temperature rise rate of 10°C / min, the calorific value of the melting endothermic peak obtained under a temperature condition of higher than 135°C and lower than 150°C is more than 3.9% and less than 9.6% of the total calorific value of the melting endothermic peak obtained during the measurement. The separator for an electrochemical element according to claim 1 .

3. the polyolefin resin B is the only resin contained in the separator for an electrochemical element and having a melting point of 135°C or less, 2. The separator for electrochemical elements according to claim 1, wherein, when the melting endothermic peak of the separator for electrochemical elements is measured using a differential scanning calorimeter at a heating rate of 10°C / min, the calorific value of the melting endothermic peak obtained under a temperature condition of 135°C or less is more than 13.9% and not more than 50.0% of the total calorific value of the melting endothermic peak obtained during the measurement.

4. The nonwoven fabric contains, as constituent fibers, polyolefin-based sheath-core conjugate fibers A and polyolefin-based sheath-core conjugate fibers B, as well as high-melting-point fibers mainly composed of polypropylene. The separator for an electrochemical element according to claim 1 .

5. The nonwoven fabric has an average fiber diameter of 16.5 μm or less. The separator for an electrochemical element according to claim 1 .

6. the polyolefin resin A is an ethylene-propylene copolymer, The polyolefin resin B is polyethylene or an ethylene-propylene copolymer. The separator for an electrochemical element according to claim 1 .

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