Separator for non-aqueous secondary battery and non-aqueous secondary battery

Abstract

According to one embodiment of the present invention, a separator for a non-aqueous secondary battery comprises a porous substrate, and a porous layer containing a polyvinylidene fluoride resin and barium sulfate particles, the molecular weight distribution of the polyvinylidene fluoride resin included in the porous layer being 3.5-10 inclusive, the average primary grain size of the barium sulfate particles included in the porous layer being at least 0.01 µm and less than 0.50 µm, and the volume ratio of the barium sulfate particles to the void-excluding volume of the porous layer being greater than 5 vol% and less than 70 vol%.

Description

[Technical Field]

[0001] The present disclosure relates to a separator for a non-aqueous secondary battery and a non-aqueous secondary battery. [Background technology]

[0002] Patent Document 1 describes a heat-resistant porous layer containing barium sulfate particles and an organic synthetic resin component, in which the content of the barium sulfate particles contained in the heat-resistant porous layer is 70% by volume or more and 96% by volume or less of the total volume of the barium sulfate particles and the organic synthetic resin component, and the content of the barium sulfate particles is 1.8 g / m 2 More than 19.8g / m 2 The following battery separator is disclosed.

[0003] Patent Document 2 discloses a separator for a lithium secondary battery that contains an X-ray detectable component, the X-ray detectable component containing a mixture of at least two components selected from the group consisting of metals, metal oxides, metal phosphates, metal carbonates, X-ray fluorescent materials, metal salts, metal sulfates, and mixtures thereof.

[0004] Patent Document 3 discloses a separator for a non-aqueous secondary battery in which the average primary particle size of barium sulfate particles contained in a heat-resistant porous layer is 0.01 μm or more and less than 0.30 μm.

[0005] Patent Document 4 discloses a separator for a non-aqueous secondary battery in which the polyvinylidene fluoride resin contained in the adhesive porous layer has a molecular weight distribution of 3.5 to 10 and a weight average molecular weight of 500,000 to 3,000,000.

[0006] Patent Document 1: International Publication No. 2021 / 029397 Patent Document 2: Japanese Patent Application Laid-Open No. 2021-093376 Patent Document 3: International Publication No. 2019 / 146155 Patent Document 4: International Publication No. 2019 / 054310 Summary of the Invention [Problem to be solved by the invention]

[0007] Misalignment between electrodes and separators inside a battery can cause short circuits and fires. To improve battery safety, there is a need for technology that can detect misalignment between electrodes and separators from outside the battery during the battery manufacturing process.

[0008] The electrode current collectors are generally metal foils that do not transmit X-rays. The position of the electrodes inside the battery can be detected from outside the battery using an imaging method that irradiates X-rays (for example, X-ray CT (X-ray Computed Tomography)). Furthermore, if the separator has low X-ray transmittance, the misalignment between the electrodes and separator can be detected from outside the battery using an imaging method that irradiates X-rays.

[0009] Furthermore, in order to improve the safety and performance of batteries, separators without fine lines or irregularities on the surface are required. Furthermore, as electronic devices become smaller, there is a demand for a reduction in the thickness of the battery.

[0010] It is against this background that the present disclosure has been made. An object of the present disclosure is to provide a separator for a non-aqueous secondary battery in which misalignment with an electrode can be detected by X-ray, the porous layer has an excellent appearance, and the thickness can be reduced by heat pressing. [Means for solving the problem]

[0011] Specific means for solving the above problems include the following aspects. <1> A porous substrate and a porous layer provided on one or both surfaces of the porous substrate, the porous layer containing a polyvinylidene fluoride resin and barium sulfate particles, a molecular weight distribution of the polyvinylidene fluoride resin contained in the porous layer is 3.5 or more and 10 or less, an average primary particle size of the barium sulfate particles contained in the porous layer is 0.01 μm or more and less than 0.50 μm, and a volume ratio of the barium sulfate particles to the volume excluding pores of the porous layer is more than 5 vol% and less than 70 vol%. <2> The weight average molecular weight of the polyvinylidene fluoride resin contained in the porous layer is 500,000 or more and 3,000,000 or less. <1> The non-aqueous secondary battery separator according to claim 1. <3> The basis weight of the porous layer is 2.0 g / m2 in total on both sides of the porous substrate. 2 More than 20.0g / m 2 Below is the <1> or <2> The non-aqueous secondary battery separator according to claim 1. <4> The unit area weight of the barium sulfate particles contained in the porous layer is 0.3 g / m in total on both sides of the porous substrate. 2 More than 19.0g / m 2 Below is the <1> ~ <3> 10. The separator for a non-aqueous secondary battery according to claim 9. <5> a positive electrode, a negative electrode, and a conductive material disposed between the positive electrode and the negative electrode; <1> ~ <4> 10. A non-aqueous secondary battery comprising the separator for a non-aqueous secondary battery according to any one of claims 1 to 9, wherein an electromotive force is generated by doping and undoping of lithium ions. [Effects of the Invention]

[0012] According to the present disclosure, a separator for a nonaqueous secondary battery is provided in which misalignment with an electrode can be detected by X-ray, the porous layer has an excellent appearance, and the thickness can be reduced by heat pressing. DETAILED DESCRIPTION OF THE INVENTION

[0013]

[0023] The following describes embodiments of the present disclosure. These descriptions and examples are intended to illustrate the embodiments and are not intended to limit the scope of the embodiments.

[0014] In the present disclosure, a numerical range indicated using "to" indicates a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in this disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in the numerical ranges described in this disclosure, the upper or lower limit value of that numerical range may be replaced with a value shown in the examples.

[0015] In the present disclosure, the term "step" includes not only an independent step but also a step that cannot be clearly distinguished from other steps as long as the purpose of the step is achieved.

[0016] In the present disclosure, when referring to the amount of each component in a composition, if multiple substances corresponding to each component are present in the composition, the total amount of the multiple substances present in the composition is meant unless otherwise specified. In the present disclosure, the composition may contain multiple types of particles corresponding to each component. When multiple types of particles corresponding to each component are present in the composition, the particle size of each component refers to the value for a mixture of the multiple types of particles present in the composition, unless otherwise specified.

[0017] In the present disclosure, MD (Machine Direction) refers to the longitudinal direction of a porous substrate and a separator manufactured in a long shape, and TD (Transverse Direction) refers to the direction perpendicular to MD in the planar direction of the porous substrate and the separator. In the present disclosure, TD is also referred to as the "width direction."

[0018] In the present disclosure, when the stacking relationship of each layer constituting a separator is expressed as "upper" and "lower," the layer closer to the porous substrate is referred to as "lower," and the layer farther from the porous substrate is referred to as "upper."

[0019] In the present disclosure, the volume of the porous layer excluding pores is referred to as the "solid content volume."

[0020] <Separator for non-aqueous secondary batteries> The separator for a nonaqueous secondary battery according to the present disclosure (also referred to simply as "separator" in the present disclosure) comprises a porous substrate and a porous layer containing a polyvinylidene fluoride resin and barium sulfate particles, the porous layer being provided on one or both sides of the porous substrate and serving as the outermost layer of the separator.

[0021] The description of the porous layer in the present disclosure is a description of the porous layer on each side of the porous substrate. The separator of the present disclosure may have a porous layer containing a polyvinylidene fluoride resin and barium sulfate particles on at least one side of the porous substrate. Examples of embodiments of the separator of the present disclosure include the following embodiments (1) to (3).

[0022] (1) A separator having porous layers containing polyvinylidene fluoride resin and barium sulfate particles on both sides of a porous substrate, wherein the porous layer on one side of the separator and the porous layer on the other side of the separator may be the same or different in components and / or composition. (2) A separator having a porous layer containing polyvinylidene fluoride resin and barium sulfate particles on one surface of a porous substrate, and another layer on the other surface of the porous substrate. (3) A separator having a porous layer containing a polyvinylidene fluoride resin and barium sulfate particles on one side of a porous substrate, and no layer on the other side of the porous substrate (i.e., the surface of the porous substrate is exposed).

[0023] The separator of the present disclosure has a porous layer containing a polyvinylidene fluoride resin and barium sulfate particles. Because barium sulfate has low X-ray transparency, a porous layer containing an appropriate amount of barium sulfate particles can be detected by an imaging method using X-ray irradiation (e.g., X-ray CT).

[0024] The separator of the present disclosure has a porous layer containing a polyvinylidene fluoride resin and barium sulfate particles, and the polyvinylidene fluoride resin contained in the porous layer has a molecular weight distribution of 3.5 or more and 10 or less. In the present disclosure, the molecular weight distribution of a resin refers to the ratio Mw / Mn of the weight average molecular weight (Mw) to the number average molecular weight (Mn).

[0025] When the molecular weight distribution of the polyvinylidene fluoride resin contained in the porous layer is less than 3.5, the polyvinylidene fluoride resin is little deformed by heat pressing in the presence of an electrolyte solution, and therefore the porous layer is less likely to become thin even when heat pressed. From the viewpoint of thinning the porous layer by heat pressing, the molecular weight distribution of the polyvinylidene fluoride resin contained in the porous layer is 3.5 or more, preferably 4.0 or more, more preferably 4.5 or more, and even more preferably 5.0 or more.

[0026] If the molecular weight distribution of the polyvinylidene fluoride resin contained in the porous layer exceeds 10, it is difficult to form the porous layer with high uniformity, and the appearance of the porous layer is inferior. From the viewpoint of excellent appearance of the porous layer, the molecular weight distribution of the polyvinylidene fluoride resin contained in the porous layer is 10 or less, preferably 9.0 or less, more preferably 8.0 or less, and even more preferably 7.0 or less.

[0027] The molecular weight distribution of the polyvinylidene fluoride resin contained in the porous layer is 3.5 or more and 10 or less, preferably 4.0 or more and 9.0 or less, more preferably 4.5 or more and 8.0 or less, and even more preferably 5.0 or more and 7.0 or less, from the viewpoint of achieving an excellent appearance of the porous layer and thinning the porous layer by heat pressing.

[0028] The weight average molecular weight (Mw) of the polyvinylidene fluoride resin contained in the porous layer is preferably 500,000 or more and 3,000,000 or less, more preferably 600,000 or more and 2,000,000 or less, and even more preferably 700,000 or more and 1,000,000 or less, from the viewpoint of excellent appearance of the porous layer.

[0029] The weight-average molecular weight (Mw) and number-average molecular weight (Mn) of the polyvinylidene fluoride resin contained in the porous layer are measured by gel permeation chromatography (GPC) and are polystyrene-equivalent molecular weights. The entire polyvinylidene fluoride resin extracted from the porous layer or the entire polyvinylidene fluoride resin used to form the porous layer is used as a sample. The detailed method for measuring the molecular weight by GPC is as follows. Molecular weight measurements by GPC are performed using a GPC-900 GPC system manufactured by JASCO Corporation, two TSKgel SUPER AWM-H columns manufactured by Tosoh Corporation, N,N-dimethylformamide as the solvent, at a temperature of 40°C and a flow rate of 0.6 mL / min. The polystyrene-equivalent weight average molecular weight (Mw) and number average molecular weight (Mn) obtained are divided by Mn to determine the ratio Mw / Mn, i.e., the molecular weight distribution.

[0030] The separator of the present disclosure has a porous layer containing a polyvinylidene fluoride resin and barium sulfate particles, and the barium sulfate particles contained in the porous layer have an average primary particle size of 0.01 μm or more and less than 0.50 μm.

[0031] If the average primary particle size of the barium sulfate particles is less than 0.01 μm, the barium sulfate particles will aggregate, making it difficult to form a porous layer. Therefore, the average primary particle size of the barium sulfate particles contained in the porous layer is 0.01 μm or more. Furthermore, from the viewpoint of thinning the porous layer by heat pressing, the average primary particle size of the barium sulfate particles contained in the porous layer is 0.01 μm or more. The average primary particle size of the barium sulfate particles contained in the porous layer is preferably 0.05 μm or more, more preferably 0.10 μm or more, and even more preferably 0.15 μm or more.

[0032] If the average primary particle size of the barium sulfate particles contained in the porous layer is 0.50 μm or more, it is difficult to form the porous layer with high uniformity, and the appearance of the porous layer is poor. From the viewpoint of excellent appearance of the porous layer, the average primary particle size of the barium sulfate particles contained in the porous layer is less than 0.50 μm, preferably 0.48 μm or less, more preferably 0.45 μm or less, and even more preferably 0.40 μm or less.

[0033] The average primary particle size of the barium sulfate particles contained in the porous layer is from 0.01 μm to less than 0.50 μm, preferably from 0.05 μm to 0.48 μm, more preferably from 0.10 μm to 0.45 μm, and even more preferably from 0.15 μm to 0.40 μm, from the viewpoint of achieving an excellent appearance of the porous layer and thinning the porous layer by heat pressing.

[0034] The average primary particle size of the barium sulfate particles contained in the porous layer is determined by measuring the long diameter of 100 randomly selected barium sulfate particles during observation using a scanning electron microscope (SEM) and averaging the long diameters of the 100 particles. The sample used for SEM observation is barium sulfate particles, which are the material forming the porous layer, or barium sulfate particles extracted from the porous layer of a separator. There are no limitations on the method for extracting the barium sulfate particles from the porous layer of a separator. Examples of such methods include immersing the porous layer peeled from the separator in an organic solvent that dissolves resin to extract the barium sulfate particles; or heating the porous layer peeled from the separator to approximately 800°C to remove the resin and extract the barium sulfate particles.

[0035] The separator of the present disclosure has a porous layer containing a polyvinylidene fluoride resin and barium sulfate particles, and the volume ratio of the barium sulfate particles to the solid volume of the porous layer is more than 5% by volume and less than 70% by volume.

[0036] If the volume ratio of barium sulfate particles to the solid volume of the porous layer is 5% by volume or less, it is difficult to detect the separator inside the battery from outside the battery using X-rays. From the viewpoint of enabling detection using X-rays, the volume ratio of barium sulfate particles to the solid volume of the porous layer is more than 5% by volume, preferably 20% by volume or more, more preferably 30% by volume or more, and even more preferably 40% by volume or more.

[0037] When the volume ratio of the barium sulfate particles to the solid volume of the porous layer is 70% by volume or more, the porous layer is less likely to become thin even when heat-pressed. From the viewpoint of thinning the porous layer by heat-pressing, the volume ratio of the barium sulfate particles to the solid volume of the porous layer is less than 70% by volume, preferably 68% by volume or less, more preferably 65% ​​by volume or less, and even more preferably 63% by volume or less.

[0038] The volume ratio of barium sulfate particles to the solid volume of the porous layer is more than 5% by volume and less than 70% by volume, preferably 20% by volume or more and 68% by volume or less, more preferably 30% by volume or more and 65% by volume or less, and even more preferably 40% by volume or more and 63% by volume or less, from the viewpoint of enabling detection by X-ray and thinning the porous layer by heat pressing.

[0039] The volume ratio V (vol %) of the barium sulfate particles to the solid content volume of the porous layer is calculated by the following formula. V={(Xa / Da) / (Xa / Da+Xb / Db+Xc / Dc+…+Xn / Dn)}×100 Here, among the constituent materials of the porous layer, the barium sulfate particles are a, and the other constituent materials are b, c, ..., n, the masses of the constituent materials contained in a predetermined area of ​​the porous layer are Xa, Xb, Xc, ..., Xn (g), and the true densities of the constituent materials are Da, Db, Dc, ..., Dn (g / cm 3 ) Xa and the like substituted into the above formula are the mass (g) of the constituent material used to form a porous layer of a predetermined area, or the mass (g) of the constituent material removed from a porous layer of a predetermined area. The Da and other values ​​substituted in the above formula are the true densities (g / cm) of the constituent materials used to form the porous layer.3 ), or the true density (g / cm 3 )

[0040] The weight per unit area of ​​the barium sulfate particles contained in the porous layer is 0.3 g / m2 in total on both sides of the porous substrate. 2 It is preferable that the weight per unit area of ​​the barium sulfate particles is 0.3 g / m or more. 2 If the weight per unit area of ​​the barium sulfate particles contained in the porous layer is 0.5 g / m or more, the separator inside the battery can be easily detected by X-ray from outside the battery. 2 More preferably, 1.0 g / m or more 2 More preferably, 1.5 g / m 2 The above is particularly preferred.

[0041] The unit area weight of the barium sulfate particles contained in the porous layer is 19.0 g / m2 in total on both sides of the porous substrate. 2 It is preferable that the weight per unit area of ​​the barium sulfate particles is 19.0 g / m or less. 2 When the weight per unit area of ​​the barium sulfate particles contained in the porous layer is 17.0 g / m or less, it is easy to form the porous layer with high uniformity and the appearance of the porous layer is more excellent. 2 Less than 15.0 g / m is more preferable. 2 More preferably, 13.0 g / m or less 2 The following are particularly preferred:

[0042] The weight per unit area of ​​the barium sulfate particles contained in the porous layer is 0.3 g / m2 in total on both sides of the porous substrate. 2 More than 19.0g / m 2 Less than 0.5 g / m 2 More than 17.0g / m 2 Less than 1.0 g / m is more preferable. 2 More than 15.0g / m 2 More preferably, 1.5 g / m 2 More than 13.0g / m 2 The following are particularly preferred:

[0043] The unit area weight (g / m) of barium sulfate particles contained in the porous layer 2 ) is the mass of barium sulfate particles contained in a unit area of ​​the porous layer, with the area of ​​the porous layer viewed in plan as the unit.

[0044] The porous substrate and porous layer of the separator of the present disclosure will be described in detail below.

[0045] [Porous base material] In the present disclosure, a porous substrate refers to a substrate having internal pores or voids. Examples of such substrates include microporous membranes; porous sheets made of fibrous materials, such as nonwoven fabrics and paper; and composite porous sheets obtained by laminating one or more other porous layers onto these microporous membranes or porous sheets. In the present disclosure, a microporous membrane is preferred from the viewpoint of thinning and strength of the separator. A microporous membrane refers to a membrane having a large number of internal micropores, a structure in which the micropores are connected, and which allows gas or liquid to pass from one surface to the other.

[0046] The material of the porous substrate is preferably an electrically insulating material, and may be either an organic material or an inorganic material.

[0047] The porous substrate preferably contains a thermoplastic resin to impart a shutdown function to the porous substrate. The shutdown function refers to a function in which, when the battery temperature rises, the constituent materials dissolve and block the pores of the porous substrate, thereby blocking the movement of ions and preventing thermal runaway of the battery. The thermoplastic resin preferably has a melting point of less than 200°C. Examples of thermoplastic resins include polyesters such as polyethylene terephthalate; polyolefins such as polyethylene and polypropylene; and polyolefins are particularly preferred.

[0048] The porous substrate is preferably a microporous membrane containing polyolefin (referred to as a "polyolefin microporous membrane" in the present disclosure). Examples of polyolefin microporous membranes include those used in conventional battery separators, and it is preferable to select one from these that has sufficient mechanical properties and ion permeability.

[0049] From the viewpoint of exhibiting a shutdown function, the polyolefin microporous membrane is preferably a microporous membrane containing polyethylene, and the polyethylene content is preferably 95% by mass or more based on the total mass of the polyolefin microporous membrane.

[0050] The polyolefin microporous film is preferably a microporous film containing polypropylene, from the viewpoint of heat resistance that prevents the film from easily breaking when exposed to high temperatures.

[0051] From the viewpoint of providing a shutdown function and heat resistance that prevents the film from easily rupturing when exposed to high temperatures, the polyolefin microporous film is preferably a polyolefin microporous film containing polyethylene and polypropylene. Examples of polyolefin microporous films containing polyethylene and polypropylene include microporous films in which polyethylene and polypropylene are mixed in one layer. From the viewpoint of achieving both the shutdown function and heat resistance, the microporous film preferably contains 95% by mass or more of polyethylene and 5% by mass or less of polypropylene. Also from the viewpoint of achieving both the shutdown function and heat resistance, a polyolefin microporous film having a laminate structure of two or more layers, at least one layer containing polyethylene and at least one layer containing polypropylene, is preferred.

[0052] The polyolefin contained in the polyolefin microporous membrane preferably has a weight-average molecular weight (Mw) of 100,000 to 5,000,000. When the Mw of the polyolefin is 100,000 or more, the microporous membrane can be imparted with sufficient mechanical properties. On the other hand, when the Mw of the polyolefin is 5,000,000 or less, the microporous membrane has good shutdown properties and is easy to mold.

[0053] Examples of methods for producing a polyolefin microporous membrane include a method in which a molten polyolefin resin is extruded through a T-die to form a sheet, which is crystallized, stretched, and then heat-treated to form a microporous membrane; and a method in which a molten polyolefin resin together with a plasticizer such as liquid paraffin is extruded through a T-die, cooled to form a sheet, stretched, the plasticizer is extracted, and then heat-treated to form a microporous membrane.

[0054] Examples of porous sheets made of fibrous materials include porous sheets such as nonwoven fabrics and papers made of fibrous materials such as polyesters such as polyethylene terephthalate; polyolefins such as polyethylene and polypropylene; heat-resistant resins such as wholly aromatic polyamide, polyamideimide, polyimide, polyethersulfone, polysulfone, polyetherketone, and polyetherimide; and cellulose.

[0055] In the present disclosure, a heat-resistant resin refers to a resin with a melting point of 200°C or higher, or a resin without a melting point but with a decomposition temperature of 200°C or higher. In other words, a heat-resistant resin in the present disclosure refers to a resin that does not melt or decompose in a temperature range below 200°C.

[0056] Examples of composite porous sheets include sheets in which a functional layer is laminated onto a microporous membrane or a porous sheet made of a fibrous material. Such composite porous sheets are preferred because the functional layer allows for additional functionality. Examples of functional layers include, for example, a porous layer made of a heat-resistant resin, or a porous layer made of a heat-resistant resin and an inorganic filler, from the viewpoint of imparting heat resistance. Examples of heat-resistant resins include one or more heat-resistant resins selected from wholly aromatic polyamides, polyamideimides, polyimides, polyethersulfones, polysulfones, polyetherketones, and polyetherimides. Examples of inorganic fillers include metal oxides such as alumina; metal hydroxides such as magnesium hydroxide; and the like. Examples of composite formation methods include coating a microporous membrane or porous sheet with a functional layer, bonding a microporous membrane or porous sheet to a functional layer with an adhesive, and thermocompression bonding a microporous membrane or porous sheet to a functional layer.

[0057] The surface of the porous substrate may be subjected to various surface treatments to improve wettability with the coating liquid for forming the porous layer, as long as the properties of the porous substrate are not impaired. Examples of surface treatments include corona treatment, plasma treatment, flame treatment, and ultraviolet irradiation treatment.

[0058] [Characteristics of porous substrate] The thickness of the porous substrate is preferably 25 μm or less, more preferably 20 μm or less, and even more preferably 15 μm or less from the viewpoint of increasing the energy density of the battery, and is preferably 3 μm or more, more preferably 5 μm or more, and even more preferably 8 μm or more from the viewpoint of the separator production yield and the battery production yield.

[0059] From the viewpoint of suppressing short circuits in the battery, the Gurley value (JIS P8117:2009) of the porous substrate is preferably 20 seconds / 100 mL or more, more preferably 25 seconds / 100 mL or more, even more preferably 60 seconds / 100 mL or more, and particularly preferably 65 seconds / 100 mL or more. The Gurley value (JIS P8117:2009) of the porous substrate is preferably 220 seconds / 100 mL or less, more preferably 200 seconds / 100 mL or less, even more preferably 190 seconds / 100 mL or less, and particularly preferably 150 seconds / 100 mL or less, from the viewpoint of ion permeability and from the viewpoint of preventing the porous structure from being blocked at the boundary between the porous substrate and the porous layer when exposed to high temperatures.

[0060] The porosity of the porous substrate is preferably 20% to 60% from the viewpoint of obtaining appropriate membrane resistance and shutdown function. The porosity ε (%) of the porous substrate is calculated by the following formula. ε={1-Ws / (ds·t)}×100 Here, Ws is the basis weight of the porous substrate (g / m 2 ), ds is the true density of the porous substrate (g / cm 3 ), t is the thickness (μm) of the porous substrate. Basis weight is the mass per unit area.

[0061] The average pore size of the porous substrate is preferably 15 nm to 100 nm from the viewpoint of ion permeability or suppressing short circuits in the battery. The average pore size of the porous substrate is measured using a perm porometer (CFP-1500-A manufactured by PMI) in accordance with ASTM E1294-89.

[0062] [Porous layer] The porous layer has a large number of micropores inside, and the micropores are interconnected, allowing gas or liquid to pass through from one surface to the other.

[0063] The porous layer may be present on only one side of the porous substrate or on both sides of the porous substrate. When the porous layer is present on both sides of the porous substrate, the separator is less likely to curl, resulting in excellent handling during battery production. When the porous layer is present on only one side of the porous substrate, the separator has better ion permeability. In addition, the overall thickness of the separator can be reduced, allowing the production of batteries with higher energy density.

[0064] The porous layer contains at least a polyvinylidene fluoride resin and barium sulfate particles. The porous layer may contain a resin other than the polyvinylidene fluoride resin. The porous layer may contain particles other than the barium sulfate particles. The other particles may be either inorganic particles or organic particles.

[0065] -Polyvinylidene fluoride resin- Examples of polyvinylidene fluoride resins include homopolymers of vinylidene fluoride (i.e., polyvinylidene fluoride); copolymers of vinylidene fluoride and halogen-containing monomers such as hexafluoropropylene, tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, vinyl fluoride, and trichloroethylene; copolymers of vinylidene fluoride and other monomers other than halogen-containing monomers; copolymers of vinylidene fluoride, halogen-containing monomers, and other monomers other than halogen-containing monomers; and mixtures thereof. One type of polyvinylidene fluoride resin may be used alone, or two or more types may be used in combination.

[0066] From the viewpoint of adhesion to electrodes, a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) (VDF-HFP copolymer) is preferred as the polyvinylidene fluoride resin. In the present disclosure, VDF-HFP copolymer includes both a copolymer obtained by polymerizing only VDF and HFP, and a copolymer obtained by polymerizing VDF, HFP, and other monomers. By increasing or decreasing the content of HFP units, the crystallinity, heat resistance, and resistance to dissolution in the electrolyte of the VDF-HFP copolymer can be controlled within appropriate ranges.

[0067] When the porous layer contains a polyvinylidene fluoride resin, the content of the polyvinylidene fluoride resin is preferably 85% by mass to 100% by mass, more preferably 90% by mass to 100% by mass, and even more preferably 95% by mass to 100% by mass, relative to the total amount of all resins contained in the porous layer.

[0068] When porous layers are present on both sides of the porous substrate, the type or amount of polyvinylidene fluoride resin contained in one porous layer may be the same as or different from the type or amount of polyvinylidene fluoride resin contained in the other porous layer.

[0069] -Other resins- The porous layer may contain a resin other than the polyvinylidene fluoride resin, such as wholly aromatic polyamide, polyamideimide, poly-N-vinylacetamide, polyacrylamide, copolymer polyetherpolyamide, polyimide, polyetherimide, acrylic resin, fluorine-containing rubber, styrene-butadiene copolymer, homopolymer or copolymer of vinyl nitrile compounds (such as acrylonitrile and methacrylonitrile), carboxymethyl cellulose, hydroxyalkyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, polyether (such as polyethylene oxide and polypropylene oxide), polysulfone, polyketone, polyether ketone, polyether sulfone, and mixtures thereof.

[0070] The content of other resins contained in the porous layer is preferably 0% by mass to 15% by mass, more preferably 0% by mass to 10% by mass, and even more preferably 0% by mass to 5% by mass, relative to the total amount of resins contained in the porous layer.

[0071] -Barium sulfate particles- The particle shape of the barium sulfate particles contained in the porous layer is not limited, and may be any of spherical, elliptical, plate-like, needle-like, and amorphous. From the viewpoint of suppressing short circuits in the battery, the barium sulfate particles contained in the porous layer are preferably plate-like particles or non-aggregated primary particles.

[0072] The barium sulfate particles contained in the porous layer may be particles whose surfaces have been modified with a silane coupling agent or the like.

[0073] The content of barium sulfate particles in the porous layer is preferably 85% by mass to 100% by mass, more preferably 90% by mass to 100% by mass, and even more preferably 95% by mass to 100% by mass, based on the total amount of inorganic particles in the porous layer.

[0074] When porous layers are present on both sides of the porous substrate, the amount of barium sulfate particles contained in one porous layer may be the same as or different from the amount of barium sulfate particles contained in the other porous layer.

[0075] -Other inorganic particles other than barium sulfate particles- The porous layer may contain inorganic particles other than barium sulfate particles, provided that the volume ratio of the other inorganic particles to the solid volume of the porous layer is preferably 5% by volume or less, more preferably 3% by volume or less, even more preferably 1% by volume or less, and particularly preferably substantially none.

[0076] Examples of other inorganic particles include metal hydroxide particles such as aluminum hydroxide, magnesium hydroxide, calcium hydroxide, chromium hydroxide, zirconium hydroxide, cerium hydroxide, nickel hydroxide, and boron hydroxide; metal oxide particles such as silica, alumina, titania, zirconia, and magnesium oxide; carbonate particles such as calcium carbonate and magnesium carbonate; sulfate particles such as calcium sulfate; and clay minerals such as calcium silicate and talc. As other inorganic particles, metal hydroxide particles or metal oxide particles are preferred from the viewpoints of stability in the electrolyte and electrochemical stability. The other inorganic particles may be surface-modified with a silane coupling agent or the like. The other inorganic particles may be used alone or in combination of two or more.

[0077] The particle shape of the other inorganic particles is not limited, and may be any of spherical, elliptical, plate-like, needle-like, and amorphous. From the viewpoint of suppressing short circuits in the battery, the other inorganic particles contained in the porous layer are preferably plate-like particles or non-aggregated primary particles.

[0078] The average primary particle size of the other inorganic particles is preferably 0.01 μm or more and 5.0 μm or less, and more preferably 0.1 μm or more and 1.0 μm or less.

[0079] -Organic particles- The porous layer may contain organic particles. Examples of organic particles include particles made of crosslinked polymers such as crosslinked poly(meth)acrylic acid, crosslinked poly(meth)acrylic acid ester, crosslinked polysilicone, crosslinked polystyrene, crosslinked polydivinylbenzene, styrene-divinylbenzene copolymer crosslinked product, melamine resin, phenol resin, and benzoguanamine-formaldehyde condensate; and particles made of heat-resistant polymers such as polysulfone, polyacrylonitrile, aramid, and polyacetal. The term "(meth)acrylic" refers to either "acrylic" or "methacrylic." The resin constituting the organic particles may be a mixture, modified product, derivative, copolymer (random copolymer, alternating copolymer, block copolymer, graft copolymer) or crosslinked product of the above-exemplified materials.

[0080] The organic particles may be used alone or in combination of two or more kinds.

[0081] -Other ingredients- The porous layer may contain additives such as a dispersant such as a surfactant, a wetting agent, an antifoaming agent, and a pH adjuster. The dispersant is added to the coating liquid for forming the porous layer for the purpose of improving dispersibility, coatability, or storage stability. The wetting agent, antifoaming agent, and pH adjuster are added to the coating liquid for forming the porous layer for the purpose of, for example, improving compatibility with the porous substrate, suppressing air entrapment in the coating liquid, or adjusting the pH.

[0082] [Characteristics of porous layer] The thickness of the porous layer is preferably 0.5 μm or more on one side, more preferably 1.0 μm or more on one side, and even more preferably 1.5 μm or more on one side, from the viewpoints of ease of X-ray detection of the separator and heat resistance of the battery; and from the viewpoints of ion permeability and energy density of the battery, the thickness is preferably 10.0 μm or less on one side, more preferably 8.0 μm or less on one side, and even more preferably 6.0 μm or less on one side.

[0083] When the porous layer is present on both sides of the porous substrate, the thickness of the porous layer, as the total thickness of both sides of the porous substrate, is preferably 1.0 μm or more, more preferably 2.0 μm or more, and even more preferably 3.0 μm or more, and is preferably 20.0 μm or less, more preferably 16.0 μm or less, and even more preferably 12.0 μm or less.

[0084] When porous layers are present on both sides of a porous substrate, the smaller the difference (μm) between the thickness of one porous layer and the thickness of the other porous layer, the better, and it is preferably 20% or less of the total thickness (μm) of both sides.

[0085] The basis weight (mass per unit area) of the porous layer is set to 2.0 g / m2 in total on both sides of the porous substrate, regardless of whether the porous layer is on one side or both sides of the porous substrate, from the viewpoint of ease of X-ray detection of the separator and heat resistance. 2 More than 2.5g / m is preferable. 2 More preferably, 3.0 g / m or more 2 The above is more preferable. The basis weight (mass per unit area) of the porous layer is set to 20.0 g / m2 in total for both sides of the porous substrate, regardless of whether the porous layer is on one side or both sides of the porous substrate, from the viewpoint of ion permeability, battery energy density, and cycle characteristics. 2 Less than 18.0 g / m is preferred 2 Less than 15.0 g / m is more preferable. 2 The following is even more preferred:

[0086] When the porous layer is on both sides of the porous substrate, the difference between the basis weight of one porous layer and the basis weight of the other porous layer (g / m 2 ) is preferably as small as possible from the viewpoint of suppressing curling of the separator or improving the cycle characteristics of the battery, and the total amount (g / m 2 ) is preferably 20% or less.

[0087] The porosity of the porous layer is preferably 30% or more, more preferably 35% or more, and even more preferably 40% or more from the viewpoint of ion permeability, and is preferably 70% or less, more preferably 65% ​​or less, and even more preferably 60% or less from the viewpoint of the mechanical strength of the porous layer. The porosity ε (%) of the porous layer is calculated by the following formula.

[0088]

number

[0089] The average pore size of the porous layer is preferably 10 nm to 200 nm. If the average pore size is 10 nm or more, when the porous layer is impregnated with an electrolyte solution, the pores are less likely to be clogged even if the resin contained in the porous layer swells. If the average pore size is 200 nm or less, ion migration in the porous layer is highly uniform, and the battery has excellent cycle characteristics and load characteristics.

[0090] The average pore size (nm) of the porous layer is calculated by the following formula, assuming that all pores are cylindrical. d=4V / S In the formula, d is the average pore size (diameter) of the porous layer, and V is the length of the porous layer per 1 m 2 S is the pore volume per 1 m of porous layer 2 represents the pore surface area per unit area. Porous layer 1m 2 The pore volume V per unit area is calculated from the porosity of the porous layer. Porous layer 1m 2 The pore surface area S per unit area is calculated by the following method. First, the specific surface area (m 2 / g) and the specific surface area of ​​the separator (m 2 / g) is calculated from the nitrogen gas adsorption amount by applying the BET equation to the nitrogen gas adsorption method. 2 / g) to each basis weight (g / m 2 ) and multiply each 1m 2 The pore surface area per 1 m of the porous substrate is calculated. 2 The pore surface area per 1m of separator 2 Subtract the pore surface area per m of porous layer 2 Calculate the pore surface area S per unit area. The basis weight is the mass per unit area.

[0091] [Separator characteristics] From the viewpoint of the mechanical strength of the separator, the thickness of the separator is preferably 8 μm or more, more preferably 10 μm or more, and even more preferably 12 μm or more, and from the viewpoint of the energy density of the battery, the thickness is preferably 25 μm or less, more preferably 22 μm or less, and even more preferably 20 μm or less.

[0092] From the viewpoint of suppressing short circuits in the battery, the Gurley value (JIS P8117:2009) of the separator is preferably 50 seconds / 100 mL or more, more preferably 60 seconds / 100 mL or more, even more preferably 70 seconds / 100 mL or more, and particularly preferably 80 seconds / 100 mL or more. From the viewpoint of ion permeability, the Gurley value (JIS P8117:2009) of the separator is preferably 200 seconds / 100 mL or less, more preferably 180 seconds / 100 mL or less, even more preferably 150 seconds / 100 mL or less, and particularly preferably 130 seconds / 100 mL or less.

[0093] From the viewpoint of the battery load characteristics, the separator membrane resistance is set to 1 Ω cm 2 ~10 Ω·cm 2 The membrane resistance of the separator is the resistance value when the separator is impregnated with an electrolytic solution, and is a value measured by an AC method at a temperature of 20°C using 1 mol / L LiBF4-propylene carbonate:ethylene carbonate (mass ratio 1:1) as the electrolytic solution. The lower the membrane resistance value of the separator, the better the ion permeability of the separator.

[0094] [Separator manufacturing method] The separator of the present disclosure can be manufactured, for example, by forming a porous layer on a porous substrate by a wet coating method or a dry coating method. In the present disclosure, the wet coating method is a method in which a coating layer is solidified in a coagulation liquid, and the dry coating method is a method in which a coating layer is solidified by drying. An example embodiment of the wet coating method is described below.

[0095] The wet coating method involves applying a coating liquid containing a resin and a filler onto a porous substrate, immersing the substrate in a coagulating liquid to solidify the coating layer, and then removing the substrate from the coagulating liquid, washing with water, and drying.

[0096] The coating liquid for forming the porous layer is prepared by dissolving or dispersing a polyvinylidene fluoride resin and barium sulfate particles in a solvent. If necessary, other components besides the polyvinylidene fluoride resin and barium sulfate particles may be dissolved or dispersed in the coating liquid.

[0097] The solvent used to prepare the coating liquid includes a solvent that dissolves polyvinylidene fluoride resin (hereinafter also referred to as a "good solvent"), such as polar amide solvents such as N-methylpyrrolidone, dimethylacetamide, and dimethylformamide.

[0098] The solvent used to prepare the coating solution may contain a phase separation agent that induces phase separation in order to form a porous layer with a good porous structure. Therefore, the solvent used to prepare the coating solution may be a mixed solvent of a good solvent and a phase separation agent. The phase separation agent is preferably mixed with the good solvent in an amount that ensures a viscosity appropriate for coating. Examples of the phase separation agent include water, methanol, ethanol, propyl alcohol, butyl alcohol, butanediol, ethylene glycol, propylene glycol, and tripropylene glycol.

[0099] When the solvent used to prepare the coating liquid is a mixed solvent of a good solvent and a phase separation agent, from the viewpoint of forming a good porous structure, the mixed solvent preferably contains 60% by mass or more of the good solvent and 5% by mass to 40% by mass of the phase separation agent.

[0100] The resin concentration in the coating liquid is preferably 1% by mass to 20% by mass from the viewpoint of forming a good porous structure, and the barium sulfate particle concentration in the coating liquid is preferably 0.5% by mass to 50% by mass from the viewpoint of forming a good porous structure.

[0101] The coating liquid may contain a dispersant such as a surfactant, a wetting agent, an antifoaming agent, a pH adjuster, etc. These additives may remain in the porous layer as long as they are electrochemically stable within the range of use of the nonaqueous secondary battery and do not inhibit the reaction within the battery.

[0102] Examples of means for applying the coating liquid to the porous substrate include a Mayer bar, a die coater, a reverse roll coater, a roll coater, a gravure coater, etc. When forming a porous layer on both sides of the porous substrate, it is preferable from the viewpoint of productivity to apply the coating liquid to both sides of the porous substrate simultaneously.

[0103] The coating layer is solidified by immersing the porous substrate with the coating layer formed thereon in a coagulation liquid to induce phase separation in the coating layer while solidifying the resin, thereby obtaining a laminate consisting of the porous substrate and the porous layer.

[0104] The coagulation liquid generally contains the good solvent and phase separation agent used in preparing the coating liquid, as well as water. From the viewpoint of productivity, it is preferable that the mixing ratio of the good solvent and the phase separation agent be the same as the mixing ratio of the mixed solvent used in preparing the coating liquid. From the viewpoints of forming a porous structure and productivity, the water content in the coagulation liquid is preferably 40% by mass to 90% by mass. The temperature of the coagulation liquid is, for example, 20°C to 50°C.

[0105] After the coating layer is solidified in the coagulating liquid, the laminate is lifted out of the coagulating liquid and washed with water. The coagulating liquid is removed from the laminate by washing with water. Furthermore, water is removed from the laminate by drying. The washing with water is carried out, for example, by transporting the laminate in a water bath. The drying is carried out, for example, by transporting the laminate in a high-temperature environment, by blowing air on the laminate, or by bringing the laminate into contact with a heat roll. The drying temperature is preferably 40°C to 80°C.

[0106] The separator of the present disclosure can also be manufactured by a dry coating method, which is a method of applying a coating liquid to a porous substrate and drying the coating layer to volatilize and remove the solvent, thereby forming a porous layer on the porous substrate.

[0107] The separator of the present disclosure can also be produced by a method in which the porous layer is produced as an independent sheet, and this porous layer is then superimposed on a porous substrate and combined with thermocompression bonding or an adhesive. Examples of a method for producing the porous layer as an independent sheet include a method in which the porous layer is formed on a release sheet by applying the above-mentioned wet coating method or dry coating method.

[0108] <Non-aqueous secondary battery> The nonaqueous secondary battery of the present disclosure is a nonaqueous secondary battery that generates electromotive force by doping and dedoping lithium ions, and includes a positive electrode, a negative electrode, and a separator for a nonaqueous secondary battery of the present disclosure. "Doping" refers to occlusion, support, adsorption, or insertion, and refers to the phenomenon in which lithium ions enter the active material of an electrode such as a positive electrode.

[0109] The nonaqueous secondary battery of the present disclosure has a structure in which, for example, a battery element in which a negative electrode and a positive electrode face each other with a separator interposed therebetween is enclosed in an exterior material together with an electrolyte solution. The nonaqueous secondary battery of the present disclosure is suitable for nonaqueous electrolyte secondary batteries, particularly lithium ion secondary batteries.

[0110] Hereinafter, examples of the positive electrode, negative electrode, electrolyte, and exterior material included in the nonaqueous secondary battery of the present disclosure will be described.

[0111] An example of the positive electrode is a structure in which an active material layer containing a positive electrode active material and a binder resin is formed on a current collector. The active material layer may further contain a conductive additive. Examples of the positive electrode active material include lithium-containing transition metal oxides, specifically LiCoO2, LiNiO2, and LiMn 1 / 2 Ni 1 / 2 O2, LiCo 1 / 3 Mn 1 / 3 Ni 1 / 3 O2, LiMn2O4, LiFePO4, LiCo 1 / 2 Ni 1 / 2 O2, LiAl 1 / 4 Ni 3 / 4Examples of the binder resin include polyvinylidene fluoride resin and styrene-butadiene copolymer. Examples of the conductive additive include carbon materials such as acetylene black, ketjen black, and graphite powder. Examples of the current collector include aluminum foil, titanium foil, stainless steel foil, and the like, each having a thickness of 5 μm to 20 μm.

[0112] An example of an embodiment of the negative electrode is a structure in which an active material layer containing a negative electrode active material and a binder resin is formed on a current collector. The active material layer may further contain a conductive additive. Examples of the negative electrode active material include materials capable of electrochemically absorbing lithium ions, such as carbon materials; alloys of lithium with silicon, tin, aluminum, etc.; and Wood's alloy. Examples of the binder resin include polyvinylidene fluoride resins and styrene-butadiene copolymers. Examples of the conductive additive include carbon materials such as acetylene black, ketjen black, graphite powder, and ultrafine carbon fibers. Examples of the current collector include copper foil, nickel foil, stainless steel foil, and the like, each having a thickness of 5 μm to 20 μm. Alternatively, a metallic lithium foil may be used as the negative electrode instead of the above-described negative electrode.

[0113] The electrolyte is a solution in which a lithium salt is dissolved in a non-aqueous solvent. Examples of lithium salts include LiPF6, LiBF4, and LiClO4. Examples of non-aqueous solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, and vinylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and their fluorine-substituted derivatives; and cyclic esters such as γ-butyrolactone and γ-valerolactone. These may be used alone or in combination. A suitable electrolyte solution is a mixture of a cyclic carbonate and a chain carbonate in a mass ratio (cyclic carbonate:chain carbonate) of 20:80 to 40:60, with a lithium salt dissolved in the range of 0.5 mol / L to 1.5 mol / L.

[0114] Examples of the exterior packaging include aluminum laminate film packs, metal cans, etc. The shape of the battery may be rectangular, cylindrical, coin-shaped, etc., and the separator of the present disclosure is suitable for any shape.

[0115] The nonaqueous secondary battery of the present disclosure can be produced by producing a laminate in which the separator of the present disclosure is disposed between a positive electrode and a negative electrode, and then using this laminate by, for example, any of the following methods (1) to (3). In the following description, performing a heat press treatment after impregnating the separator with an electrolyte solution is referred to as "wet heat press," and performing a heat press treatment without impregnating the separator with an electrolyte solution is referred to as "dry heat press."

[0116] (1) The laminate is dry heat pressed to bond the electrodes and separator, and then housed in an exterior packaging (for example, an aluminum laminate film pack; the same applies below). An electrolyte solution is injected into the exterior packaging, and the interior of the exterior packaging is evacuated. The laminate is then wet heat pressed from above the exterior packaging to bond the electrodes and separator and seal the exterior packaging.

[0117] (2) The laminate is placed in an exterior packaging material, an electrolyte solution is injected into it, and the interior of the exterior packaging material is evacuated. After that, the laminate is wet heat pressed onto the exterior packaging material to bond the electrodes and separator and seal the exterior packaging material.

[0118] (3) The laminate is dry heat pressed to bond the electrodes and separator, and then housed in an exterior packaging material. An electrolyte solution is injected into the exterior packaging material, and the interior of the exterior packaging material is evacuated, after which the exterior packaging material is sealed.

[0119] The conditions for the wet heat press in the above manufacturing method are preferably a press temperature of 70°C to 110°C and a press pressure of 0.5 MPa to 2 MPa. The conditions for the dry heat press in the above manufacturing method are preferably a press temperature of 20°C to 100°C and a press pressure of 0.5 MPa to 9 MPa. The press time is preferably adjusted according to the press temperature and press pressure, for example, within the range of 0.5 minutes to 60 minutes.

[0120] When manufacturing a laminate in which a separator is disposed between a positive electrode and a negative electrode, the method of disposing the separator between the positive electrode and the negative electrode may be a method of stacking at least one layer of a positive electrode, a separator, and a negative electrode in this order (so-called stack method), or a method of stacking a positive electrode, a separator, a negative electrode, and a separator in this order and winding them in the length direction. [Example]

[0121] The separator and nonaqueous secondary battery of the present disclosure will be described in more detail below with reference to examples. The materials, amounts used, ratios, processing procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present disclosure. Therefore, the scope of the separator and nonaqueous secondary battery of the present disclosure should not be construed as being limited by the specific examples shown below.

[0122] In the following description, syntheses, treatments, manufacturing, etc. were carried out at room temperature (25°C ± 3°C) unless otherwise specified.

[0123] <Measurement and evaluation methods> The measurement and evaluation methods used in the examples and comparative examples are as follows.

[0124] [Weight-average molecular weight and number-average molecular weight of polyvinylidene fluoride resin] The molecular weight of the polyvinylidene fluoride resin used to form the porous layer was measured by GPC. The molecular weight measurement by GPC was performed using a GPC-900 GPC system manufactured by JASCO Corporation, two TSKgel SUPER AWM-H columns manufactured by Tosoh Corporation, N,N-dimethylformamide as the solvent, at a temperature of 40°C and a flow rate of 0.6 mL / min. The molecular weight in terms of polystyrene was obtained, and the weight-average molecular weight (Mw) and number-average molecular weight (Mn) were calculated. The ratio Mw / Mn, i.e., the molecular weight distribution, was determined by dividing Mw by Mn.

[0125] [Average primary particle size of inorganic particles] The inorganic particles used to form the porous layer were used as samples and observed with an SEM to determine the average primary particle size. More details are as follows. The average primary particle size of the barium sulfate particles contained in the porous layer was determined by measuring the long diameters of 100 randomly selected barium sulfate particles during observation with a scanning electron microscope (SEM) and averaging the long diameters of the 100 particles. The samples used for SEM observation were barium sulfate particles, which are the material that forms the porous layer, or barium sulfate particles extracted from the porous layer of a separator.

[0126] [Volume ratio of inorganic particles] The volume ratio V (vol %) of the inorganic particles to the solid content volume of the porous layer was calculated by the following formula. V={(Xa / Da) / (Xa / Da+Xb / Db+Xc / Dc+…+Xn / Dn)}×100 Here, among the constituent materials of the porous layer, the inorganic particles are a, the other constituent materials are b, c, ..., n, the masses of the constituent materials contained in a predetermined area of ​​the porous layer are Xa, Xb, Xc, ..., Xn (g), and the true densities of the constituent materials are Da, Db, Dc, ..., Dn (g / cm 3 ) Xa and other values ​​substituted into the above formula are the mass (g) of the constituent material used to form the porous layer of a given area. Da and other values ​​substituted into the above formula are the true density (g / cm) of the constituent material used to form the porous layer. 3 )

[0127] [X-ray CT] - Preparation of negative electrode - A negative electrode slurry was prepared by mixing 300 parts by weight of artificial graphite (negative electrode active material), 7.5 parts by weight of an aqueous dispersion containing 40% by weight of a modified styrene-butadiene copolymer (binder resin), 3 parts by weight of carboxymethyl cellulose (thickener), and an appropriate amount of water in a twin-arm mixer. The negative electrode slurry was applied to both sides of a 10 μm-thick copper foil, dried, and pressed to obtain a negative electrode with a negative electrode active material layer on both sides.

[0128] -Preparation of positive electrode- A positive electrode slurry was prepared by mixing 89.5 parts by weight of lithium cobalt oxide powder (positive electrode active material), 4.5 parts by weight of acetylene black (conductive additive), 6 parts by weight of polyvinylidene fluoride (binder resin), and an appropriate amount of N-methyl-2-pyrrolidone in a twin-arm mixer. The positive electrode slurry was applied to both sides of a 20 μm-thick aluminum foil, dried, and pressed to obtain a positive electrode with positive electrode active material layers on both sides.

[0129] -Preparing samples for observation- The positive electrode was cut into a 30mm x 50mm rectangle, the negative electrode was cut into a 30mm x 50mm rectangle, and the separator was cut into a 34mm x 54mm rectangle. The positive electrode, separator, negative electrode, and separator were stacked in this order to produce a laminate with three layers of positive electrode and three layers of negative electrode and five layers of separator. The laminate was inserted into a pack made of aluminum laminate film, and the pack was evacuated and sealed using a vacuum sealer to obtain a sample for observation.

[0130] -X-ray CT- For the X-ray CT, a microfocus X-ray CT system (inspeXio SMX-225CT FPD HR) manufactured by Shimadzu Corporation was used. Images of the thickness direction cross section of the laminate were taken at the edge of the observation sample using an X-ray tube voltage of 220 kV, an X-ray tube current of 100 μA, and an exposure time of 1 sec. The gray value (GV) of the separator was measured from the X-ray CT image and classified as follows: The higher the GV value, the more desirable it is.

[0131] Level 5: GV is 37301 or more Level 4: GV is 36501 or more and 37300 or less Level 3: GV is 35701 or more and 36500 or less Level 2: GV is 35251 or more and 35700 or less Level 1: GV is 35250 or less

[0132] [Separator thickness reduction rate due to heat pressing] -Preparation of measurement samples- The negative and positive electrodes prepared for X-ray CT observation were each cut into a 30 mm x 50 mm rectangle. The separator was cut into a 34 mm x 54 mm rectangle. The positive electrode, separator, negative electrode, and separator were stacked in this order to produce a laminate having three layers of positive and negative electrodes and five layers of separator. The laminate was inserted into a pack made of aluminum laminate film, and an electrolyte (1 mol / L LiPF6-ethylene carbonate:ethyl methyl carbonate [mass ratio 3:7]) was poured into the pack to allow the electrolyte to permeate the laminate. Next, a vacuum sealer was used to create a vacuum inside the pack and temporarily seal it, obtaining a measurement sample.

[0133] -Measuring the thickness of the measurement sample- The measurement sample was heat-pressed in the thickness direction using a heat press machine to bond the electrode and separator. The heat-pressing conditions were a temperature of 75°C or 90°C, a load of 1 MPa, and a pressing time of 2 minutes. The thickness of the measurement sample was measured before and after the heat press using a Mitutoyo micrometer (model number MDC-25SX) at the center of the sample. The thickness of the measurement sample before heat pressing was Tb, and the thickness of the measurement sample after heat pressing was Ta, and the separator thickness reduction rate R was calculated using the following formula: Te in the formula is the thickness of the laminate consisting of three layers of positive electrodes and three layers of negative electrodes. R(%) = (Tb-Ta) / 5 / (Tb-Te) × 100

[0134] The thickness reduction rate of the separator when hot pressed at a temperature of 75°C is preferably 20% or more. The thickness reduction rate of the separator when hot pressed at a temperature of 90°C is preferably 25% or more.

[0135] [Appearance of porous layer] A 48 mm wide transparent adhesive tape (3M, model number 313 3PN) was attached parallel to the transverse direction (TD) to the porous layer on one side of the separator. The transparent adhesive tape was peeled off, and the number of streaks remaining on the adhesive surface of a 200 mm long tape was visually counted.

[0136] <Preparation of separator and battery> [Example 1] -Separator production- A polyvinylidene fluoride resin was dissolved in dimethylacetamide (DMAc) to a resin concentration of 5.0% by mass, and barium sulfate particles were further dispersed by stirring to obtain a coating solution (1). An appropriate amount of the coating solution (1) was placed on a Mayer bar, and the coating solution (1) was applied to both sides of a polyethylene microporous membrane. The coating was performed so that the coating amount was equal on both sides of the polyethylene microporous membrane. The membrane was then immersed in a coagulation liquid (DMAc:water = 50:50 [mass ratio], liquid temperature 40°C) to solidify the coating layer, followed by washing in a water washing tank at 40°C and drying. In this way, a separator having a porous layer formed on both sides of the polyethylene microporous membrane was obtained.

[0137] - Preparation of negative electrode - A negative electrode slurry was prepared by mixing 300 parts by weight of artificial graphite (negative electrode active material), 7.5 parts by weight of an aqueous dispersion containing 40% by weight of a modified styrene-butadiene copolymer (binder resin), 3 parts by weight of carboxymethyl cellulose (thickener), and an appropriate amount of water in a twin-arm mixer. The negative electrode slurry was applied to one side of a 10 μm-thick copper foil, dried, and pressed to obtain a negative electrode with a negative electrode active material layer on one side.

[0138] -Preparation of positive electrode- A positive electrode slurry was prepared by mixing 89.5 parts by weight of lithium cobalt oxide powder (positive electrode active material), 4.5 parts by weight of acetylene black (conductive additive), 6 parts by weight of polyvinylidene fluoride (binder resin), and an appropriate amount of N-methyl-2-pyrrolidone in a twin-arm mixer. The positive electrode slurry was applied to one side of a 20 μm-thick aluminum foil, dried, and pressed to obtain a positive electrode with a positive electrode active material layer on one side.

[0139] -Battery manufacturing- The positive electrode was cut into a 30 mm x 50 mm rectangle, and the negative electrode was cut into a 30 mm x 50 mm rectangle. Lead tabs were welded to each. The separator was cut into a 34 mm x 54 mm rectangle. The positive electrode, separator, and negative electrode were stacked in this order. The stack was inserted into a pack made of aluminum laminate film, and an electrolyte (1 mol / L LiPF6-ethylene carbonate:ethyl methyl carbonate [mass ratio 3:7]) was injected into the pack, allowing the electrolyte to penetrate the stack. Next, a vacuum sealer was used to create a vacuum inside the pack and temporarily seal it. The pack and the stack were then heat-pressed in the stacking direction using a heat press machine to bond the electrode and separator. The heat press conditions were a temperature of 90°C, a load of 1 MPa, and a pressing time of 2 minutes. Next, a vacuum sealer was used to create a vacuum inside the pack and seal it, resulting in a secondary battery.

[0140] [Examples 2 to 9, Comparative Examples 1 to 8] Each separator was produced in the same manner as in Example 1, except that the types and amounts of materials were changed to the specifications shown in Table 1. Then, a secondary battery was produced in the same manner as in Example 1 using each separator.

[0141] Table 1 shows the materials, compositions, physical properties and evaluation results of each separator of Examples 1 to 9 and Comparative Examples 1 to 8. The polyvinylidene fluoride resins used in the examples and comparative examples are all binary copolymers of vinylidene fluoride and hexafluoropropylene.

[0142] [Table 1]

[0143] The disclosure of Japanese Patent Application No. 2022-075140, filed on April 28, 2022, is incorporated herein by reference in its entirety. All publications, patent applications, and technical standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent application, or technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A porous substrate; A porous layer provided on one or both surfaces of the porous substrate and containing a polyvinylidene fluoride resin and barium sulfate particles, the molecular weight distribution of the polyvinylidene fluoride resin contained in the porous layer is 3.5 or more and 10 or less; the barium sulfate particles contained in the porous layer have an average primary particle size of 0.01 μm or more and less than 0.50 μm; The volume ratio of the barium sulfate particles to the volume excluding pores of the porous layer is 55% by volume or more and less than 70% by volume. Separator for non-aqueous secondary batteries.

2. the weight average molecular weight of the polyvinylidene fluoride resin contained in the porous layer is 500,000 or more and 3,000,000 or less; The separator for a non-aqueous secondary battery according to claim 1 .

3. The basis weight of the porous layer is 2.0 g / m in total on both sides of the porous substrate. ２ 20.0g / m or more ２ Below is the The separator for a non-aqueous secondary battery according to claim 1 .

4. The unit area weight of the barium sulfate particles contained in the porous layer is 0.3 g / m in total on both sides of the porous substrate. ２ 19.0g / m or more ２ Below is the The separator for a non-aqueous secondary battery according to claim 1 .

5. A non-aqueous secondary battery comprising: a positive electrode; a negative electrode; and the separator for a non-aqueous secondary battery according to any one of claims 1 to 4, which is disposed between the positive electrode and the negative electrode, and which generates an electromotive force by doping and dedoping of lithium ions.

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