Porous sheet, method for producing same, separator for nonaqueous secondary battery, and nonaqueous secondary battery

Abstract

This porous sheet includes fibers containing particles of an organic foaming agent and resin. The ratio D (P) / D (F) of the particle diameter D (P) of the particles to the average small diameter D (F) of the fibers is 6.0 or less.

Description

Perforated sheet and manufacturing method thereof, separator for non-aqueous secondary battery, and non-aqueous secondary battery

[0001] The present invention relates to a porous sheet and a method for producing the same, a separator for a non-aqueous secondary battery, and a non-aqueous secondary battery.

[0002] It is known that a slurry composition containing a binder and a melamine compound is applied to a microporous polypropylene film substrate, and then dried to form a functional layer, and the resulting laminate of the substrate and functional layer is used as a separator for a lithium ion secondary battery (see Patent Document 1).It is also known that a nonwoven fabric is produced by electrospinning a material containing polyacrylonitrile and calcium carbonate, and the nonwoven fabric is used as a separator for a lithium ion secondary battery (see Patent Document 2).

[0003] International Application No. 2020 / 040163 (Corresponding Publication: U.S. Patent Application Publication No. 2021 / 0184313) JP 2018-28986 A

[0004] In the technology of Patent Document 1, in a lithium ion secondary battery using a laminate of a substrate and a functional layer as a separator, the internal resistance may increase over time. Also, in the technology of Patent Document 2, the foaming of calcium carbonate contained in the separator suppresses heat generation when an internal short circuit occurs in the lithium ion secondary battery, but if the heat generation can be suppressed more effectively, it would be preferable in terms of further improving the safety of non-aqueous secondary batteries such as lithium ion secondary batteries.

[0005] Therefore, there is a need for a porous sheet that, when used as a separator for a non-aqueous secondary battery, can reduce the increase in internal resistance of the non-aqueous secondary battery and further suppress heat generation during an internal short circuit; a separator for a non-aqueous secondary battery including the porous sheet; a non-aqueous secondary battery in which the increase in internal resistance is reduced and heat generation during an internal short circuit is further suppressed; and a method for producing the porous sheet.

[0006] The present inventors have speculated that when a separator contains an organic foaming agent, the organic foaming agent comes into contact with the electrodes and is decomposed by an oxidation reaction or a reduction reaction, and the decomposition products increase the internal resistance of non-aqueous secondary batteries. Based on this speculation, the present inventors have conducted extensive research to solve the above-mentioned problems, and have completed the present invention. Specifically, the present invention provides the following.

[0007] <1> A porous sheet comprising fibers containing resin and organic foaming agent particles, wherein the ratio D(P) / D(F), of the particle diameter D(P) of the particles to the average small diameter D(F) of the fibers, is 6.0 or less. <2> The porous sheet according to <1>, wherein the organic foaming agent has a thermal decomposition temperature of 140°C or higher and 400°C or lower. <3> The porous sheet according to <1> or <2>, wherein the particle diameter D(P) is 0.1 μm or higher and 3.0 μm or lower. <4> The porous sheet according to any one of <1> to <3>, wherein the average small diameter D(F) is 0.1 μm or higher and 1 μm or lower. <5> A separator for a non-aqueous secondary battery, comprising the porous sheet according to any one of <1> to <4>. <6> A non-aqueous secondary battery, comprising a positive electrode, a negative electrode, and the separator for a non-aqueous secondary battery according to <5>. <7> A method for producing the porous sheet according to any one of <1> to <4>, comprising: a step of preparing a liquid composition containing the resin and particles of the organic blowing agent; and a step of performing electrospinning using the liquid composition as a material.

[0008] According to the present invention, there are provided a porous sheet which, when used as a separator for a non-aqueous secondary battery, can reduce an increase in the internal resistance of the non-aqueous secondary battery and further suppress heat generation in the event of an internal short circuit; a separator for a non-aqueous secondary battery including the porous sheet; a non-aqueous secondary battery in which an increase in internal resistance is reduced and heat generation in the event of an internal short circuit is further suppressed; and a method for producing the porous sheet.

[0009] Fig. 1 is a partial schematic diagram of a fiber having irregularities when viewed from the radial direction, and Fig. 2 is a partial schematic diagram of a fiber having no irregularities when viewed from the radial direction.

[0010] The present invention will be described in detail below with reference to embodiments and examples. However, the present invention is not limited to the embodiments and examples shown below, and can be implemented with any modifications within the scope of the claims of the present invention and their equivalents. The components of the embodiments shown below can be combined as appropriate. In addition, in the drawings, the same components are designated by the same reference numerals, and their description may be omitted.

[0011] In the following description, unless otherwise specified, the directions of elements as "parallel," "vertical," and "orthogonal" may include an error within a range that does not impair the effects of the present invention, for example, within a range of ±3°, ±2°, or ±1°.

[0012] In the following description, unless otherwise specified, the term "sheet" is not limited to a material that is a constituent of the sheet, and may be composed of a material that contains, for example, a resin. Furthermore, unless otherwise specified, the term "sheet" refers to a member having a shape in which the dimension in one direction (thickness) is smaller than the length and width dimensions when viewed from that direction (thickness direction).

[0013] In the following description, a "long" sheet refers to a sheet having a length that is 5 times or more its width, preferably 10 times or more its width, and specifically refers to a sheet having a length that can be wound into a roll for storage or transportation. There is no particular upper limit to the length of the sheet, and it can be, for example, 100,000 times or less its width.

[0014] In the following description, unless otherwise specified, a polymer being "water-soluble" means that when 0.5 g of the polymer is dissolved in 100 g of water at 25°C, the insoluble content is less than 1.0 mass %, and a polymer being water-insoluble means that when 0.5 g of the polymer is dissolved in 100 g of water at 25°C, the insoluble content is 1.0 mass % or more.

[0015] 1. Overview of the Perforated Sheet The perforated sheet according to one embodiment of the present invention comprises fibers containing resin and organic foaming agent particles, and the ratio D(P) / D(F) of the particle diameter D(P) of the particles to the average small diameter D(F) of the fibers is 6.0 or less. When the perforated sheet has the above-described configuration, when the perforated sheet is used as a battery separator, an increase in the internal resistance of the battery can be suppressed, and heat generation in the event of an internal short circuit in the battery can be further suppressed, thereby improving the safety of the battery.

[0016] The reason why the porous sheet according to this embodiment exhibits the above-described effects when used as a battery separator is presumed to be as follows: The organic foaming agent may be decomposed by oxidation or reduction upon contact with an electrode. The decomposition products of the organic foaming agent may degrade the input / output characteristics of the battery. For example, the decomposition products of the organic foaming agent may increase the internal resistance of the battery. It is believed that the greater the ratio D(P) / D(F), the greater the proportion of organic foaming agent particles exposed on the fiber surface. The greater the proportion of organic foaming agent particles exposed on the fiber surface, the greater the proportion of the organic foaming agent decomposed upon contact with the electrode, further degrading the input / output characteristics of the battery. When the ratio D(P) / D(F) is below a predetermined value, the proportion of organic foaming agent particles exposed on the fiber surface decreases, and the proportion of organic foaming agent particles present in the fiber wrapped in resin increases significantly. The organic foaming agent particles wrapped in resin may avoid contact with the electrode. Therefore, if the proportion of organic foaming agent particles present in the fiber while being wrapped in resin increases significantly, it is believed that the deterioration of the battery's input / output characteristics, especially the increase in internal resistance, is suppressed. On the other hand, the organic foaming agent decomposes and foams due to heat generated by a short circuit inside the battery, reducing the ionic conductivity of the separator and inhibiting the progress of the battery reaction, which is thought to suppress further heat generation. In addition, the foaming of the organic foaming agent causes the pressure inside the battery to increase quickly, allowing the internal pressure release mechanism that a battery normally has to function quickly, thereby improving the safety of the battery.

[0017] The porous sheet is preferably a nonwoven fabric, which can be easily formed from particle-containing fibers. A nonwoven fabric is a fibrous molded product in which fibers are oriented in one direction or randomly and bonded by adhesion or entanglement.

[0018] The porous sheet may be in the form of a laminate with another layer. For example, the porous sheet may be laminated with a positive electrode, a negative electrode, or disposed between a positive electrode and a negative electrode.

[0019] <1.1. Ratio D(P) / D(F)> The ratio D(P) / D(F) is usually 6 or less, preferably 3 or less, more preferably 1 or less, even more preferably 0.6 or less, and even more preferably 0.5 or less, and is usually greater than 0, preferably 0.1 or greater. When the ratio D(P) / D(F) is equal to or less than the upper limit, the proportion of organic foaming agent particles exposed from the fibers is reduced, which can suppress decomposition of the organic foaming agent due to contact with the electrodes. As a result, the increase in internal resistance of the battery can be reduced. Furthermore, because the organic foaming agent particles can be widely dispersed throughout the porous sheet, if a short circuit occurs inside a battery containing the porous sheet as a separator, the organic foaming agent may generate gas even if the temperature rise in the battery is localized. As a result, heat generation throughout the battery can be suppressed, thereby improving battery safety.

[0020] (Particle diameter D(P)) The particle diameter D(P) of the organic foaming agent particles means the volume-based particle diameter (D50) (median diameter) at which the cumulative volume calculated from the smallest diameter side reaches 50% in the volume-based particle diameter distribution measured by a laser diffraction method. The particle diameter D(P) can be measured in detail by the method described later in the Examples section.

[0021] The particle diameter D(P) is preferably 0.1 μm or more, more preferably 0.2 μm or more, even more preferably 0.3 μm or more, and preferably 3.0 μm or less, more preferably 2 μm or less, even more preferably 1 μm or less. When the particle diameter D(P) is above the lower limit, the dispersibility of the organic foaming agent particles in the fiber material is further improved, and when it is below the upper limit, the degree of localization of the organic foaming agent in the porous sheet can be further reduced. Therefore, when the particle diameter D(P) is within the above range, the organic foaming agent can effectively exert its foaming function throughout the porous sheet.

[0022] The particle diameter D(P) of the particles of the organic blowing agent can be adjusted by adjusting the production conditions such as the granulation conditions of the organic blowing agent.

[0023] (Average small diameter D(F) of fiber) When the fiber has irregularities when viewed from the radial direction, the small diameter of the fiber refers to the diameter of the recessed portion of the fiber. In other words, when the fiber has irregularities, the fiber diameter along the longitudinal direction of the fiber alternates between maximum and minimum, and the diameter of the recessed portion of the fiber refers to the smallest fiber diameter. In this case, the average small diameter of the fiber is the average diameter of the recessed portions of the fiber, that is, the average of the smallest fiber diameters. When the fiber does not have irregularities when viewed from the radial direction, the small diameter of the fiber refers to the diameter at any location on the fiber. In this case, the average small diameter of the fiber refers to the average of diameters at multiple arbitrary locations on the fiber. The small diameter of the fiber will be explained below using figures.

[0024] FIG. 1 is a partial schematic diagram of a fiber having irregularities when viewed from the radial direction. As shown in FIG. 1 , fiber F100 has, in order along the longitudinal direction D100 of the fiber, a convex portion A101, a concave portion B101, a convex portion A102, a concave portion B102, a convex portion A103, and a concave portion B103. Fiber F100 has a structure in which convex portions and concave portions are alternately repeated along the longitudinal direction D100 of the fiber. Fiber F100 has, in order along the longitudinal direction D100 of the fiber, a fiber diameter L101 that is a maximum value, a fiber diameter S101 that is a minimum value, a fiber diameter L102 that is a maximum value, a fiber diameter S102 that is a minimum value, a fiber diameter L103 that is a maximum value, and a fiber diameter S103 that is a minimum value. The minimum values ​​of fiber diameter S101, fiber diameter S102, and fiber diameter S103 are small diameters of fiber F100.

[0025] Fig. 2 is a partial schematic diagram of a fiber without irregularities when viewed from the radial direction. As shown in Fig. 2, the fiber F200 has no irregularities, i.e., there are no local minimums or maximums in the transition of the fiber diameter along the longitudinal direction D200 of the fiber. In this case, the fiber diameter S200 at any point of the fiber F200 is the small diameter of the fiber F200.

[0026] The average small diameter D(F) of the fibers contained in the porous sheet can be determined by observing any location of the porous sheet from the thickness direction using a scanning or transmission electron microscope (for example, a field emission scanning electron microscope (Regulus 8230 manufactured by Hitachi High-Tech Corporation)), measuring the small diameters of the fibers at 20 points within the observed range, and calculating the arithmetic mean value of the obtained small diameters at 20 points. The observed range can be, for example, 20 μm × 20 μm.

[0027] The average small diameter D(F) of the fibers is preferably 0.1 μm or more, more preferably 0.2 μm or more, even more preferably 0.3 μm or more, and preferably 1 μm or less, more preferably 0.9 μm or less, even more preferably 0.8 μm or less. When the average small diameter D(F) of the fibers is equal to or greater than the lower limit, the mechanical strength of the porous sheet can be improved. When the average small diameter D(F) of the fibers is equal to or less than the upper limit, the short-circuit prevention function as a battery separator can be more stably exhibited.

[0028] The average small diameter D(F) of the fibers can be adjusted by appropriately adjusting the conditions in the fiber production method. For example, when the fibers are produced by electrospinning, the average small diameter D(F) can be adjusted by appropriately adjusting the conditions such as the viscosity of the liquid composition used as the electrospinning material and the discharge speed of the liquid composition.

[0029] As an example of adjusting the conditions for electrospinning, the average small diameter D(F) of the fibers can be increased by increasing the solids concentration of the liquid composition, decreasing the applied voltage, shortening the distance between the nozzle and the collector, or increasing the inner diameter of the nozzle.

[0030] <1.2. Resin> The resin contained in the fiber typically contains a polymer. The polymer typically has a molecular weight distribution. The weight average molecular weight Mw of the polymer that can be contained in the resin is not particularly limited, but is, for example, 10,000 or more, such as 15,000 or more, and is, for example, 10,000,000 or less, such as 1,000,000 or less. The weight average molecular weight can be measured by gel permeation chromatography (GPC).

[0031] The resin for the porous sheet of this embodiment is preferably a resin that can be used as a component of a battery. The resin has resistance to oxidation-reduction reactions to a degree that allows it to be used as a component of a battery, and is generally less susceptible to oxidation-reduction than organic foaming agents.

[0032] The thermal decomposition temperature of the resin is preferably 200°C or higher, more preferably 300°C or higher, and from the viewpoint of the heat resistance of the resin, the higher the better, but it may be, for example, 500°C or lower.

[0033] The glass transition temperature of the polymer that can be contained in the resin is not particularly limited, but is, for example, −120° C. or higher, for example, −50° C. or higher, for example, 0° C. or higher, for example, 10° C. or higher, and for example, 200° C. or lower. The glass transition temperature of the polymer can be measured by differential scanning calorimetry (DSC) at a temperature rise rate of 5° C. / min.

[0034] The polymers that can be contained in the resin may be used alone or in combination of two or more kinds in any ratio.

[0035] The resin may contain a water-soluble polymer or a water-insoluble polymer.

[0036] Examples of water-soluble polymers include cellulose derivatives; polyacrylic acid compounds such as polyacrylic acid and its salts (such as sodium salts); and polyvinyl alcohol.

[0037] Examples of the cellulose derivative include nonionic cellulose derivatives, anionic cellulose derivatives, and cationic cellulose derivatives.

[0038] Examples of nonionic cellulose derivatives include microcrystalline cellulose; alkyl celluloses such as methyl cellulose, methyl ethyl cellulose, and ethyl cellulose; and hydroxyalkyl celluloses such as hydroxyethyl cellulose, hydroxybutyl methyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose stearoxy ether, carboxymethyl hydroxyethyl cellulose, alkyl hydroxyethyl cellulose, and nonoxynyl hydroxyethyl cellulose.

[0039] Examples of anionic cellulose derivatives include those obtained by substituting the nonionic cellulose derivatives with various derivative groups and their salts (sodium salts, ammonium salts, etc.). Specific examples include sodium cellulose sulfate, carboxymethyl cellulose (CMC), and salts thereof.

[0040] Further, examples of cationic cellulose derivatives include low-nitrogen hydroxyethyl cellulose dimethyl diallyl ammonium chloride (polyquaternium-4), O-[2-hydroxy-3-(trimethylammonio)propyl]hydroxyethyl cellulose chloride (polyquaternium-10), O-[2-hydroxy-3-(lauryldimethylammonio)propyl]hydroxyethyl cellulose chloride (polyquaternium-24), and the like.

[0041] Examples of water-insoluble polymers include polyamides, polyimides, polyamideimides, and polyvinylidene fluoride.

[0042] These polymers may be used singly or in combination of two or more kinds in any ratio.

[0043] In addition to the polymer, the resin may further contain optional components. Examples of optional components that may be contained in the resin in addition to the polymer include surfactants, antioxidants, leveling agents, viscosity modifiers, and dispersants. These optional components may be used alone or in combination of two or more in any ratio. The content of the polymer in the resin is preferably 70% by mass or more, more preferably 80% by mass or more, and is usually 100% by mass or less, and may be 98% by mass or less, or may be 95% by mass or less, based on the total mass of the resin being 100% by mass.

[0044] The resins may be used singly or in combination of two or more kinds in any ratio.

[0045] The resin content in the porous sheet is preferably 30% by mass or more, more preferably 40% by mass or more, and is usually less than 100% by mass, preferably 80% by mass or less, more preferably 70% by mass or less, assuming that the total mass of the components constituting the porous sheet is 100% by mass.

[0046] 1.3. Organic Foaming Agent Particles The organic foaming agent is an organic compound that generates a gas upon thermal decomposition, preferably a compound that generates a non-flammable gas upon thermal decomposition. Examples of non-flammable gases that can be generated by the organic foaming agent include nitrogen gas and carbon dioxide gas.

[0047] The thermal decomposition temperature of organic foaming agents tends to be lower than that of inorganic foaming agents. On the other hand, compared to inorganic foaming agents, organic foaming agents tend to have lower resistance to oxidation and reduction reactions. In the porous sheet of this embodiment, by setting the ratio D(P) / D(F) within a predetermined range, the organic foaming agent can be prevented from coming into contact with the electrode and being decomposed by oxidation or reduction reactions, thereby suppressing an increase in the internal resistance of the battery due to the decomposition products of the organic foaming agent. Therefore, an organic foaming agent whose thermal decomposition temperature tends to be lower than that of inorganic foaming agents can be used as the foaming agent. Therefore, when the porous sheet is used as a battery separator, both improved safety due to suppression of heat generation in the battery and suppression of an increase in internal resistance can be achieved.

[0048] The thermal decomposition temperature of the organic blowing agent is preferably 140°C or higher, more preferably 150°C or higher, even more preferably 180°C or higher, even more preferably 200°C or higher, even more preferably 210°C or higher, and is preferably 400°C or lower, more preferably 380°C or lower, even more preferably 360°C or lower.

[0049] When the thermal decomposition temperature of the organic foaming agent is equal to or higher than the lower limit, the stability of the organic foaming agent contained in the porous sheet is improved, and when the porous sheet is used as a battery separator, changes in battery characteristics due to temperature rise can be reduced.When the thermal decomposition temperature of the organic foaming agent is equal to or lower than the upper limit, when the porous sheet is used as a battery separator, the temperature at which the porous sheet begins to generate gas is lowered, and the progress of the battery reaction can be suppressed at a lower temperature, and heat generation in the battery during a short circuit can be effectively suppressed.

[0050] The thermal decomposition temperature of the organic blowing agent can be determined by measuring the weight of the organic blowing agent using a thermogravimetric differential thermal analyzer while heating it from 25° C. to 500° C. at a heating rate of 10° C. / min under a nitrogen atmosphere, and determining the temperature at which the measured weight becomes 70% of the weight at the start of the measurement (25° C.) (30% weight loss temperature). Specifically, the thermal decomposition temperature of the organic blowing agent can be measured by the method described later in the Examples section.

[0051] The organic blowing agent is usually a compound that does not have a molecular weight distribution. The molecular weight of the organic blowing agent is preferably 30 or more, more preferably 40 or more, and preferably 2000 or less, more preferably 1000 or less.

[0052] Examples of organic foaming agents include compounds that generate nitrogen gas upon thermal decomposition. Examples of compounds that generate nitrogen gas upon thermal decomposition include melamine compounds; ２ ), a hydrazo group (—NH—NH—), and a nitroso group; and derivatives and salts thereof. Examples of compounds containing at least one selected from the group consisting of an amino group, an azo group, a hydrazino group, a hydrazo group, and a nitroso group include azobisisobutyronitrile, p-toluenesulfonylhydrazide, 5-methyl-1H-benzotriazole, oxybisbenzenesulfonylhydrazide (OBSH), trihydrazinotriazine, azodicarbonamide, hydrazodicarbonamide (HDCA), dinitrosopentamethylenetetramine (DPT), p-toluenesulfonylsemicarbazide, and p,p'-oxybisbenzenesulfonylsemicarbazide. Among these, preferred organic foaming agents are melamine compounds and compounds containing an azo group, with melamine compounds and azodicarbonamide being more preferred, and melamine compounds being even more preferred due to their low reactivity with the electrolyte.

[0053] The melamine compound refers to melamine, melamine derivatives, and salts thereof. Examples of the melamine compound include the compound represented by the following formula (I) and salts thereof. Examples of the salts include cyanurates and sulfates.

[0054]

[0055] In formula (I), each of the multiple A's independently represents a hydroxy group or -N(R １ ) R ２ where R １ and R ２each independently represents a hydrogen atom or a monovalent hydrocarbon group which may be substituted with a hydroxy group. １ If there are multiple R １ may be the same or different, R ２ If there are multiple R ２ may be the same or different.

[0056] Here, R １ or R ２ When the number of carbon atoms in the monovalent hydrocarbon group, which may be substituted with a hydroxy group, is 2 or more, one or more carbon atom-carbon atom single bonds that the monovalent hydrocarbon group may have may be substituted with a group represented by "-O-". １ or R ２ The monovalent hydrocarbon group which may be substituted with a hydroxy group, represented by the following formula (I), preferably has 1 or more and 5 or less carbon atoms.

[0057] Among these, the melamine compound serving as the organic blowing agent is preferably melamine (a compound in which all A's in formula (I) are amino groups); ammeline (a compound in which two A's are amino groups and one A's is a hydroxy group in formula (I)); ammelide (a compound in which two A's are hydroxy groups and one A's is an amino group in formula (I)); or salts thereof with cyanuric acid, more preferably melamine and melamine cyanurate, and even more preferably melamine cyanurate.

[0058] The organic foaming agent is more preferably one or more selected from the group consisting of melamine, melamine cyanurate, and azodicarbonamide, more preferably melamine cyanurate and / or azodicarbonamide, and particularly preferably melamine cyanurate from the viewpoint of low reactivity with the electrolyte solution.

[0059] The organic foaming agent may be used alone or in combination of two or more kinds in any ratio. For example, as the organic foaming agent, one melamine compound may be used, or two or more melamine compounds may be used in combination in any ratio.

[0060] The content of the organic foaming agent in the porous sheet is preferably 30% by mass or more, more preferably 40% by mass or more, and is usually less than 100% by mass, preferably 80% by mass or less, more preferably 70% by mass or less, based on 100% by mass of the total mass of the components constituting the porous sheet.

[0061] In the porous sheet, the content ratio of the organic foaming agent when the total mass of the organic foaming agent and resin is 100% by mass (organic foaming agent / (organic foaming agent + resin) × 100 (mass%)) is preferably 49% by mass or more, more preferably 50% by mass or more, even more preferably 55% by mass or more, even more preferably 60% by mass or more, and preferably 90% by mass or less, more preferably 85% by mass or less, and even more preferably 80% by mass or less. When the content ratio in the porous sheet (organic foaming agent / (organic foaming agent + resin) × 100 (mass%)) is above the lower limit, the foaming function of the porous sheet is further enhanced, and heat generation during an internal short circuit in the battery can be more effectively suppressed. When the content ratio in the porous sheet (organic foaming agent / (organic foaming agent + resin) × 100 (mass%)) is below the upper limit, the mechanical strength of the porous sheet is superior.

[0062] <1.4. Optional Components> The fibers contained in the porous sheet may contain optional components in addition to the resin and the organic foaming agent particles. For example, the fibers may contain an inorganic foaming agent. Examples of inorganic foaming agents include sodium bicarbonate and calcium carbonate. For example, the fibers may contain a solvent. For example, when a porous sheet is formed by a method including electrospinning, solvents contained in the spinning material may remain in the fibers. The content of optional components other than the resin and the organic foaming agent particles is, relative to the total mass of the components constituting the porous sheet as 100% by mass, for example, 10% by mass or less, for example, 5% by mass or less, for example, 3% by mass or less, and usually 0% by mass or more, and may even be 0% by mass. For example, the content of the inorganic foaming agent in the fibers is, relative to the total mass of the components constituting the porous sheet as 100% by mass, for example, 10% by mass or less, for example, 5% by mass or less, for example, 3% by mass or less, and usually 0% by mass or more, and may even be 0% by mass.

[0063] <1.5. Properties of porous sheet> The thickness of the porous sheet is preferably 1 μm or more, more preferably 3 μm or more, since this improves the mechanical strength and allows the porous sheet to stably exhibit its function as a separator for a nonaqueous secondary battery, and is preferably 35 μm or less, more preferably 30 μm or less, since this allows the thickness of the laminate of the porous sheet and the electrodes to be reduced and the nonaqueous secondary battery to be miniaturized.

[0064] The porosity of the porous sheet is preferably 30% or more, more preferably 40% or more, from the viewpoint of ion permeability, and is preferably 90% or less, more preferably 85% or less, from the viewpoint of suppressing short circuits between electrodes. Here, the porosity of the porous sheet means the ratio (%) of pores when the total volume of the porous sheet is taken as 100%.

[0065] <1.6. Method for producing porous sheet> The porous sheet is not particularly limited and can be produced by any method. Since fibers can be easily formed using a liquid composition containing particles as a material, the method for producing the porous sheet preferably includes the following steps (1) and (2). Step (1): A step of preparing a liquid composition containing the resin and particles of the organic blowing agent. Step (2): A step of performing electrospinning using the liquid composition as a material. Steps (1) and (2) are usually performed in this order. The method for producing the porous sheet may include any step in addition to steps (1) and (2). Below, a method for producing the porous sheet, including steps (1) and (2), will be described.

[0066] (Step (1)) The liquid composition prepared in step (1) contains the resin and particles of the organic blowing agent. The liquid composition usually further contains a solvent. The solvent may be a dispersion medium capable of dispersing the components of the liquid composition.

[0067] Examples of solvents that can be contained in the liquid composition include water; alcohol solvents such as methanol, ethanol, and isopropyl alcohol; ketone solvents such as acetone and methyl ethyl ketone; aliphatic hydrocarbon solvents such as hexane; alicyclic hydrocarbon solvents such as cyclohexane, methylcyclohexane, ethylcyclohexane, and decalin; aromatic hydrocarbon solvents such as benzene and toluene; amide solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMAc); dimethyl sulfoxide; and mixtures thereof.

[0068] When the liquid composition contains a solvent, a solvent that does not or hardly reacts with the organic foaming agent under the electrospinning conditions in step (2) described below can be appropriately selected and used.

[0069] The liquid composition may contain, as optional components, a surfactant, a dispersant, and the like.

[0070] The solids concentration in the liquid composition (i.e., the concentration of the remaining components after removing the solvent from the liquid composition) can be appropriately adjusted so as to achieve the desired D(P) / D(F) range in the electrospinning in step (2), and is, for example, 3% by mass or more, for example, 5% by mass or more, for example, 20% by mass or less, for example, 15% by mass or less, assuming the liquid composition to be 100% by mass.

[0071] The liquid composition may be prepared by any procedure, for example, by mixing or dispersing the resin, particles of the organic blowing agent, and optional components such as a solvent, if necessary, in any order.

[0072] When mixing or dispersing the components of the liquid composition, for example, ultrasonic mixing or dispersion and / or stirring using a stirrer (for example, "Three-One Motor" manufactured by Shinto Scientific Co., Ltd.) can be performed. As a device for ultrasonic mixing or dispersion, for example, an ultrasonic cleaner can be used.

[0073] The resin may be dissolved in the solvent by heating the solvent.

[0074] (Step (2)) In step (2), electrospinning is performed using the liquid composition prepared in step (1). Electrospinning is a spinning method in which a voltage is applied between a resin and a plate-shaped collector, causing the resin to turn into thin fibers due to an electric charge and then depositing them on the collector. In electrospinning, a liquid containing a resin is typically ejected from a small hole such as a nozzle tip opening, and a voltage is applied to form droplets of the liquid containing the resin. In this case, the application of voltage causes the droplets to deform into a conical shape called a Taylor cone, and the resin is ejected toward the collector in the form of thin fibers, thereby achieving spinning. Electrospinning can produce fibers in which particles of an organic blowing agent are well dispersed in the resin. Electrospun fibers essentially consist of resin, particles of an organic blowing agent, and other optional solid components.

[0075] As the electrospinning device for step (2), a commercially available device, for example, a product named "NANON" (manufactured by MEC Co., Ltd.) can be used.

[0076] In step (2), the voltage applied to the nozzle and collector is preferably 10 kV or more, more preferably 15 kV or more, and is preferably 30 kV or less, more preferably 27 kV or less. By applying a voltage within this range, spinning from the nozzle can be stabilized, and scattering of droplets in a non-fibrous form and intermittent fiber formation can be suppressed.

[0077] In step (2), the inter-electrode distance, i.e., the distance from the nozzle tip to the collector, is preferably 5 mm or more, more preferably 100 mm or more, and is preferably 300 mm or less, more preferably 200 mm or less. By keeping the inter-electrode distance within this range, stable spinning can be achieved.

[0078] In step (2), the discharge rate of the liquid composition from the nozzle is preferably 0.01 mL / hr or more, more preferably 0.1 mL / hr or more, and is preferably 3 mL / hr or less, more preferably 1.5 mL / hr or less. By setting the discharge rate within this range, scattering of droplets in a non-fibrous form can be suppressed, and droplets of the liquid composition at the tip of the nozzle during spinning can be stabilized, thereby achieving stable spinning.

[0079] In step (2), the relative humidity of the spinning environment is preferably 20% or higher, and preferably 40% or lower. By keeping the relative humidity within this range, the solvent can be stably volatilized, and stable spinning can be achieved. When performing step (2), dehumidification is often required to achieve such a relative humidity. In the dehumidification step, air is often heated, and therefore the temperature of the spinning environment in step (2) can be normal room temperature or higher, for example, in the range of 20°C to 35°C.

[0080] A porous sheet in the form of a nonwoven fabric can be produced by depositing electrospun fibers on a flat, plate-shaped collector. The shape of the collector is not limited to a flat plate, and may be any shape suitable for obtaining a porous sheet of the desired shape. For example, a long porous sheet can be continuously produced by using a collector in the form of a drum, belt, or the like, and performing electrospinning on the peripheral surface while rotating it. A positive electrode and a negative electrode may be used as the collector, and the porous sheet may be obtained in the form of a laminate with the positive electrode or a laminate with the negative electrode.

[0081] In electrospinning, any device may be used as the device for extruding the liquid composition from the nozzle, and for example, a device such as a pump or syringe capable of sending the liquid composition to the nozzle may be used.

[0082] (Optional Step) The manufacturing method of this embodiment may include an optional step in addition to step (1) and step (2). For example, the structure containing the resin and particles of the organic blowing agent deposited on the collector after step (2) may be a porous sheet product, or the structure may be further subjected to an optional step to form a porous sheet. For example, a step of heating the structure to remove the solvent remaining therein may be performed.

[0083] <1.7. Uses of the porous sheet> The porous sheet can be used for any application. The porous sheet can be preferably used as a functional layer for a non-aqueous secondary battery, more preferably as a separator for a non-aqueous secondary battery. The porous sheet may be used alone as a separator for a non-aqueous secondary battery, or may be used in combination with any component as a separator for a non-aqueous secondary battery. Therefore, a separator for a non-aqueous secondary battery according to one embodiment of the present invention includes the porous sheet and, if necessary, any component.

[0084] 2. Nonaqueous Secondary Battery A nonaqueous secondary battery according to one embodiment of the present invention includes a positive electrode, a negative electrode, and a separator for a nonaqueous secondary battery. In addition to the positive electrode, negative electrode, and separator, the nonaqueous secondary battery of this embodiment may also include components that are typically included in nonaqueous secondary batteries, such as an electrolyte solution.

[0085] <2.1. Positive Electrode and Negative Electrode> The positive electrode and negative electrode are not particularly limited, and any known positive electrode and negative electrode that can be used in a non-aqueous secondary battery may be used.

[0086] 2.2. Separator The separator included in the nonaqueous secondary battery of this embodiment includes the porous sheet. The separator may include any component other than the porous sheet. Examples of the optional component other than the porous sheet include an adhesive layer, a heat-resistant layer, and a conductive layer.

[0087] <2.3. Electrolyte> A non-aqueous secondary battery usually contains an electrolyte. As the electrolyte, an organic electrolyte solution in which a supporting electrolyte is dissolved in an organic solvent is usually used. When the non-aqueous secondary battery is a lithium ion secondary battery, an example of the supporting electrolyte is a lithium salt. An example of the lithium salt is LiPF ６, LiAsF ６ , LiBF ４ , LiSbF ６ , LiAlCl ４ , LiClO ４ , C.F. ３ SO ３ Li, C ４ F ９ SO ３ Li, CF ３ COOLi, (CF ３ CO) ２ NLi, (CF ３ SO ２ ) ２ NLi, (C ２ F ５ SO ２ Among them, LiPF is the most popular because it is easily soluble in solvents and shows a high degree of dissociation. ６ , LiClO ４ , C.F. ３ SO ３ Li is preferred. As the supporting electrolyte, one type may be used alone, or two or more types may be used in combination at any ratio.

[0088] As the organic solvent, which is a component of the electrolyte solution, a solvent capable of dissolving the supporting electrolyte can be appropriately selected. Suitable examples of the organic solvent when the nonaqueous secondary battery is a lithium ion secondary battery include carbonates such as dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate (BC), ethyl methyl carbonate (EMC), and vinylene carbonate (VC); esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; sulfur-containing compounds such as sulfolane and dimethyl sulfoxide; and mixtures thereof. Among these, carbonates are preferred because of their high dielectric constant and wide stable potential range.

[0089] 2.4. Other Optional Elements The nonaqueous secondary battery may include optional elements in addition to the positive electrode, negative electrode, separator, and electrolyte solution. For example, the nonaqueous secondary battery may be provided with an overcurrent protection element such as a fuse or a PTC element, an expanded metal, a lead plate, or the like, as needed, to prevent an internal pressure increase in the battery, overcharging or overdischarging, etc. The shape of the nonaqueous secondary battery may be any of a coin type, a button type, a sheet type, a cylindrical type, a rectangular type, a flat type, etc.

[0090] 2.5. Characteristics of Non-Aqueous Secondary Battery Because the non-aqueous secondary battery contains the porous sheet, internal resistance is reduced and heat generation during a short circuit inside the battery is suppressed, improving safety. The internal resistance of a non-aqueous secondary battery can be evaluated, for example, by placing a charged non-aqueous secondary battery in a thermostatic chamber at 60°C and storing it for one week, followed by an IV resistance test. Furthermore, heat generation suppression during an internal short circuit in a non-aqueous secondary battery can be evaluated, for example, by a test in which a nail is pierced through the non-aqueous secondary battery to forcibly short-circuit it.

[0091] 2.6. Manufacturing Method of Non-Aqueous Secondary Battery The manufacturing method of the non-aqueous secondary battery of this embodiment is not particularly limited. The non-aqueous secondary battery can be manufactured using the above-described components by a known manufacturing method of a non-aqueous secondary battery. For example, the non-aqueous secondary battery of this embodiment can be manufactured by stacking a positive electrode and a negative electrode with the porous sheet serving as the secondary battery separator interposed therebetween, rolling or folding this as necessary according to the battery shape, placing it in a battery container, injecting an electrolyte into the battery container, and sealing it.

[0092] The present invention will be described in detail below with reference to examples. However, the present invention is not limited to the examples shown below, and can be implemented with any modifications within the scope of the claims of the present invention and their equivalents.

[0093] In the following description, the units "%" and "parts" that represent amounts are by mass unless otherwise specified. Furthermore, the operations described below were carried out at room temperature (20°C ± 15°C) and atmospheric pressure (1 atm) unless otherwise specified.

[0094] <Evaluation Method> <Thermal Decomposition Temperature of Organic Foaming Agent> In thermogravimetric analysis using a thermogravimetric differential thermal analyzer (manufactured by Hitachi High-Tech Science Corporation, product name "TG / DTA7200"), the weight of the organic foaming agent was measured while the temperature was increased from 25°C to 500°C at a heating rate of 10°C / min in a nitrogen atmosphere, and the temperature at which the measured weight became 70% of the weight at the start of the measurement (25°C) (30% weight loss temperature) was defined as the thermal decomposition temperature of the organic foaming agent.

[0095] <Particle diameter D(P): median diameter on a volume basis> The particle diameter D(P) (median diameter) of particles of the organic blowing agent was measured by a laser diffraction method as follows. First, a measurement sample was prepared by adjusting the solids concentration of the prepared aqueous dispersion containing the organic blowing agent to 0.1% by mass. Then, in the particle size distribution (volume basis) measured using a laser diffraction particle size distribution analyzer (manufactured by Beckman Coulter, Inc., product name "LS-13 320"), the particle diameter D50 at which the cumulative volume calculated from the smallest diameter side reached 50% was taken as the particle diameter D(P) of the particles of the organic blowing agent.

[0096] <Average small diameter D(F) of fibers> The porous sheets obtained by the procedures of each Example and Comparative Example were observed using a field emission scanning electron microscope ("Regulus 8230" manufactured by Hitachi High-Tech Corporation). The observation magnification was 5000 times, and the observation range was 20 μm × 20 μm. In the observation range, a total of 20 small diameters were measured at any location for any plurality of fibers, and the arithmetic average value thereof was taken as the average small diameter D(F) of the fibers contained in the porous sheet.

[0097] <Initialization> After injecting the electrolyte, the lithium ion secondary battery was left to stand at 25°C for 5 hours. Next, it was charged to a cell voltage of 3.65 V at 25°C using a constant current method at 0.2 C, and then aging treatment was performed at 60°C for 12 hours. Then, it was discharged to a cell voltage of 3.00 V using a constant current method at 25°C using a constant current method at 0.2 C. Thereafter, it was subjected to CC-CV charging at 0.2 C (upper limit cell voltage 4.25 V), and CC discharging to 3.00 V using a constant current method at 0.2 C. This charge and discharge at 0.2 C was repeated three times.

[0098] <IV Resistance Test After One Week of Storage> (Storage Test) The initialized lithium ion secondary battery was discharged at a constant current of 0.2 C at a temperature of 25°C until the cell voltage reached 3.00 V. Then, CC-CV charging was performed at 0.2 C (upper limit cell voltage 4.25 V, final current 0.01 C). The charged battery was placed in a thermostatic chamber at 60°C and stored for one week. Then, the temperature was returned to 25°C, and the following IV resistance test was performed.

[0099] (IV Resistance Test) In an environment of 25°C, the battery was discharged to 3.00 V at a constant current of 0.2 C. Thereafter, the battery was charged at 0.2 C to a state of charge (SOC) of 50% and allowed to stand for 600 seconds. The voltage at 600 seconds was measured as V ０ Then, 0.5C (=I ０．５ ) constant current method for 10 seconds, and the voltage at the 10th second is V ０．５ After that, the battery was charged with the same amount of electricity as the previous discharge at a constant current of 0.2 C. Next, the battery was charged at 1.0 C (= I １．０ ) constant current method for 10 seconds, and the voltage at the 10th second is V １．０ Then, the battery was charged with the same amount of electricity as the previous discharge at a constant current of 0.2 C. Next, the battery was charged at 1.5 C (= I １．５ ) constant current method for 10 seconds, and the voltage at the 10th second is V １．５ (I ０．５ , V ０．５ ), (I １．０ , V １．０ ), (I １．５ , V １．５ ) was plotted on an XY graph, and the slope b of the regression line was determined using the following formula, which was taken as DCR (direct current resistance).

[0100]

[0101] The DCR relative value of each example and comparative example was calculated when the DCR of Example 1 was set to 100, and evaluated according to the following criteria. The smaller the DCR relative value, the smaller the IV resistance of the lithium ion secondary battery, indicating a reduction in the internal resistance after the storage test. A: DCR relative value is 103 or less B: DCR relative value is greater than 103 and less than 105 C: DCR relative value is greater than 105 and less than 110 D: DCR relative value is greater than 110

[0102] <Heat suppression test during internal short circuit (forced internal short circuit test)> After injecting the electrolyte, the lithium ion secondary batteries in the examples and comparative examples were left standing at 25°C for 5 hours. Next, they were charged to a cell voltage of 3.65 V using a constant current method at 0.2 C at 25°C, and then aged for 12 hours at 60°C. Then, they were discharged to a cell voltage of 3.00 V using a constant current method at 0.2 C at 25°C. Then, they were subjected to CC-CV charging (upper limit cell voltage 4.35 V) using a constant current method at 0.2 C, and CC discharge to 3.00 V using a constant current method at 0.2 C. This 0.2 C charge-discharge cycle was repeated three times. Then, they were charged to 4.35 V (cutoff condition: 0.02 C) using a constant voltage-constant current (CC-CV) method at a charge rate of 0.2 C in an atmosphere of 25°C. Thereafter, an iron nail having a diameter of 1 mm and a length of 10 cm was penetrated near the center of the lithium ion secondary battery at a speed of 3 m / min to forcibly short-circuit the battery. This forcible short-circuit was performed on five lithium ion secondary batteries (test specimens) each produced by the same procedure, and the number of test specimens that did not burst or catch fire was used to evaluate the battery performance according to the following criteria. The greater the number of test specimens that did not burst or catch fire, the better the lithium ion secondary battery's ability to suppress heat generation during an internal short circuit. A: 4 or 5 test specimens that did not burst or catch fire B: 3 test specimens that did not burst or catch fire C: 2 test specimens that did not burst or catch fire D: 1 or 0 test specimens that did not burst or catch fire

[0103] Example 1 1-1 Preparation of Slurry Composition 1 for Porous Sheet Melamine cyanurate particles (volume-based median diameter 300 nm, thermal decomposition temperature: 350°C) as an organic foaming agent, a polyvinyl alcohol aqueous solution with a solids concentration of 15% as a water-soluble polymer, and a carboxymethyl cellulose aqueous solution with a solids concentration of 2% as a dispersant were added to a vessel so that the solids mass ratio was 50:50:2, and then ion-exchanged water as a solvent was added so that the slurry solids concentration was 10%, and the mixture was stirred for 60 minutes with a stirrer ("Three-One Motor" manufactured by Shinto Scientific Co., Ltd.) to prepare slurry composition 1 as a liquid composition.

[0104] <1-2: Electrospinning> Electrospinning was performed using slurry composition 1 as a spinning material. Electrospinning was performed using an electrospinning device (trade name "NANON", manufactured by MEC Co., Ltd.). Specifically, approximately 2.5 mL of slurry composition 1 was filled into a 5 mL syringe-shaped container, and a 22G (inner diameter 0.4 mm) metal nozzle was attached to the tip of the container and placed facing a flat plate-shaped collector. Aluminum foil was laid on the surface of the collector, and the following spinning conditions were set. Then, a voltage was applied between the nozzle and the collector, and electrospinning was performed until the film thickness reached 20 μm. The electrospinning conditions were as follows:

[0105] Applied voltage: 20 kV Discharge rate of liquid composition: 0.5 mL / hr Distance from nozzle tip to collector (distance between electrodes): 150 mm Nozzle: 22G (inner diameter 0.4 mm) Nozzle length: 15 mm Temperature inside the device: 30°C Relative humidity inside the device: 30%

[0106] By the above electrospinning, a porous sheet having a thickness of 20 μm was formed on the collector, and the obtained porous sheet was used as a separator for a lithium ion secondary battery.

[0107] <1-3: Preparation of Negative Electrode> 63 parts of styrene, 34 parts of 1,3-butadiene, 2 parts of itaconic acid, 1 part of 2-hydroxyethyl acrylate, 0.3 parts of t-dodecyl mercaptan as a molecular weight modifier, 5 parts of sodium dodecylbenzenesulfonate as an emulsifier, 150 parts of ion-exchanged water, and 1 part of potassium persulfate as a polymerization initiator were added to a 5 MPa pressure vessel equipped with a stirrer, and after thorough stirring, the mixture was heated to 55°C to initiate polymerization. The reaction was stopped by cooling when the monomer consumption reached 95.0%. A 5% aqueous sodium hydroxide solution was added to the aqueous dispersion containing the polymer obtained in this way, and the pH was adjusted to 8. Subsequently, unreacted monomer was removed by heated vacuum distillation. The mixture was then cooled to a temperature of 30°C or below to obtain an aqueous dispersion containing a binder for a negative electrode (a binder composition for a negative electrode).

[0108] A planetary mixer was charged with 48.75 parts of artificial graphite (theoretical capacity 360 mAh / g) as a negative electrode active material, 48.75 parts of natural graphite (theoretical capacity 360 mAh / g), and 1 part of carboxymethyl cellulose (solids equivalent). The mixture was then diluted with ion-exchanged water to a solids concentration of 60%, and then kneaded for 60 minutes at a rotation speed of 45 rpm. Then, 1.5 parts of the negative electrode binder composition obtained above (solids equivalent) were added, and the mixture was kneaded for 40 minutes at a rotation speed of 40 rpm. Ion-exchanged water was added to the mixture to a viscosity of 3000±500 mPa·s (measured with a Brookfield viscometer at 25°C and 60 rpm), thereby preparing a negative electrode slurry composition.

[0109] The negative electrode slurry composition was applied to the surface of a 15 μm thick copper foil current collector using a comma coater in an amount of 11±0.5 mg / cm after drying. ２ The copper foil coated with the negative electrode slurry composition was then transported at a speed of 400 mm / min through an oven at 80°C for 2 minutes and then through an oven at 110°C for 2 minutes to dry the negative electrode slurry composition on the copper foil, thereby obtaining a negative electrode blank having a negative electrode composite layer formed on the current collector. The negative electrode composite layer side of the prepared negative electrode blank was then roll-pressed under a temperature of 25±3°C and a load of 11 t (tons), resulting in a negative electrode composite layer having a density of 1.60 g / cm. ３ A negative electrode of 1000 .mu.m was obtained.

[0110] <1-4: Preparation of Positive Electrode> NMC622 (LiNi) as a positive electrode active material was added to a planetary mixer. ０．6 Mn ０．2 Co ０．2 O ２96.0 parts of cellulose acetate, 2.0 parts of carbon black (manufactured by Denka, trade name "Li-100") as a conductive material, in terms of solids content, and 2.0 parts of polyvinylidene fluoride (manufactured by Solvay, trade name "Solef (registered trademark) 5130") were added and mixed. Furthermore, N-methylpyrrolidone (NMP) was gradually added, and the mixture was stirred and mixed at a temperature of 25±3°C and a rotation speed of 60 rpm to obtain a positive electrode slurry composition with a viscosity of 3,600 mPa s (measured with a Brookfield viscometer, 25±3°C, 60 rpm (rotor M4)). The positive electrode slurry composition was applied to a 20 μm-thick aluminum foil current collector using a comma coater in a coating amount of 18±0.5 mg / cm after drying. ２ The cathode slurry composition on the aluminum foil was then dried by transporting the aluminum foil at a speed of 0.5 m / min through an oven at 90°C for 2 minutes and then through an oven at 120°C for 2 minutes, thereby obtaining a cathode raw sheet having a cathode composite layer formed on the current collector. Thereafter, the cathode composite layer side of the prepared cathode raw sheet was roll-pressed under a temperature of 25±3°C and a load of 14 t (tons), and the density of the cathode composite layer was 3.30 g / cm ３ A positive electrode of 1000 .mu.m was obtained.

[0111] <1-5: Preparation of Secondary Battery> A laminated cell (equivalent to an initial design discharge capacity of 3 Ah) was prepared using the negative electrode, positive electrode, and separator obtained above. First, a separator was placed between the negative electrode and the positive electrode to obtain a laminate. The obtained laminate was placed in an aluminum package and vacuum dried at 60°C for 10 hours. Then, a 1.0 M LiPF 6 solution was added as an electrolyte. ６ A solution (solvent: a mixed solvent of ethylene carbonate (EC) / diethyl carbonate (DEC) = 3 / 7 (volume ratio), additive: containing 2 volume % vinylene carbonate (solvent ratio)) was filled into the battery. Furthermore, in order to seal the opening of the aluminum packaging, the aluminum packaging was closed by heat sealing at a temperature of 150°C, thereby producing a lithium ion secondary battery. For this lithium ion secondary battery, the IV resistance after a storage test and the heat generation suppression during an internal short circuit were evaluated using the methods described above.

[0112] Example 2 Slurry composition 2 was prepared in the same manner as in the preparation of slurry composition 1, except that ion-exchanged water was added so that the solids concentration was 7%. Using slurry composition 2, a porous sheet (separator) was prepared in the same manner as in Example 1, except that the electrospinning conditions were changed to an applied voltage of 27 kV, a distance from the nozzle tip to the collector (distance between electrodes) of 180 mm, and a nozzle of 27G (inner diameter 0.2 mm). A lithium ion secondary battery was obtained in the same manner as in Example 1, except for the above-mentioned items, and the IV resistance after a storage test and the heat generation suppression during an internal short circuit were evaluated.

[0113] Example 3 Slurry composition 3 was prepared in the same manner as in the preparation of slurry composition 1, except that ion-exchanged water was added so that the solids concentration was 12%. Using slurry composition 3, a porous sheet (separator) was prepared in the same manner as in Example 1, except that the electrospinning conditions were changed to an applied voltage of 17 kV, a distance from the nozzle tip to the collector (distance between electrodes) of 120 mm, and a nozzle of 18G (inner diameter 0.9 mm). A lithium ion secondary battery was obtained in the same manner as in Example 1, except for the above-mentioned items, and the IV resistance after a storage test and the heat generation suppression during an internal short circuit were evaluated.

[0114] Example 4 Slurry composition 4 was prepared in the same manner as in preparation of slurry composition 1, except that the organic foaming agent particles were changed to melamine cyanurate particles (median diameter on a volume basis of 3 μm, thermal decomposition temperature: 350° C.) and the stirring time with the stirrer was changed to 20 minutes. Electrospinning was performed using slurry composition 4. A lithium ion secondary battery was obtained in the same manner as in Example 1, except for the above-mentioned points, and the IV resistance after a storage test and the heat generation suppression during an internal short circuit were evaluated.

[0115] Example 5 Slurry composition 5 was prepared in the same manner as in Slurry composition 1, except that the melamine cyanurate particles used as the organic blowing agent were replaced with azodicarbonamide particles (median diameter on a volume basis: 0.3 μm, thermal decomposition temperature: 220° C.). Electrospinning was carried out using Slurry composition 5. A lithium ion secondary battery was obtained in the same manner as in Example 1, except for the above-mentioned points, and the IV resistance after a storage test and the heat generation suppression during an internal short circuit were evaluated.

[0116] Example 6 Melamine cyanurate particles (volume-based median diameter: 300 nm, thermal decomposition temperature: 350°C) as organic blowing agent particles were added to a dimethylacetamide solution of polyvinylidene fluoride so that the mass ratio of the melamine cyanurate particles to polyvinylidene fluoride was 50:50, and dimethylacetamide as a solvent was further added so that the final slurry solids concentration was 12%, and the resulting mixture was stirred with a stirrer. Slurry composition 6 was prepared in the same manner as in the preparation of slurry composition 1, except that polyvinylidene fluoride is a water-insoluble polymer. A lithium ion secondary battery was obtained in the same manner as in Example 1, and the IV resistance after a storage test and the heat generation suppression during an internal short circuit were evaluated.

[0117] Comparative Example 1 Slurry composition 7 was prepared in the same manner as in preparation of slurry composition 1, except that the organic foaming agent particles were changed to melamine cyanurate particles (median diameter on a volume basis of 5 μm, thermal decomposition temperature: 350° C.) and the stirring time with the stirrer was changed to 20 minutes. A lithium ion secondary battery was obtained in the same manner as in Example 1, except for the above-mentioned points, and the IV resistance after a storage test and the heat generation suppression during an internal short circuit were evaluated.

[0118] Comparative Example 2 Slurry composition 8 was prepared in the same manner as in the preparation of slurry composition 1, except that the melamine cyanurate particles used as the organic foaming agent were replaced with calcium carbonate particles (volume-based median diameter: 3 μm, thermal decomposition temperature: 700° C.). A lithium ion secondary battery was obtained in the same manner as in Example 1, except for the above-mentioned points, and the IV resistance after a storage test and the heat generation suppression during an internal short circuit were evaluated.

[0119] <Results> The results are shown in the table below. In the table below, the abbreviations have the following meanings: "PVA": Polyvinyl alcohol "PVDF": Polyvinylidene fluoride "MC": Melamine cyanurate "ADCA": Azodicarbonamide "Ca carbonate": Calcium carbonate "Resistance": IV resistance test after leaving for 1 week "Heat suppression": Heat suppression test during internal short circuit

[0120]

[0121]

[0122] From the above results, it can be seen that the lithium ion secondary batteries according to the Examples are excellent in suppressing heat generation during an internal short circuit and have reduced internal resistance after a storage test. On the other hand, the lithium ion secondary battery according to Comparative Example 1 has a separator (porous sheet) with a D(P) / D(F) ratio of greater than 6.0, and is therefore inferior in suppressing heat generation during an internal short circuit and in reducing internal resistance after a storage test. Furthermore, the lithium ion secondary battery according to Comparative Example 2 contains particles in the separator (porous sheet) that are not organic foaming agent particles, and is therefore inferior in suppressing heat generation during an internal short circuit.

[0123] F100, F200: Fiber D100, D200: Longitudinal direction of fiber A101, A102, A103: Convex portions B101, B102, B103: Concave portions

Claims

1. A porous sheet comprising fibers containing particles of a resin and an organic foaming agent, wherein the ratio D(P) / D(F) of the particle diameter D(P) of the particles to the average small diameter D(F) of the fibers is 6.0 or less.

2. The porous sheet according to claim 1, wherein the organic foaming agent has a thermal decomposition temperature of 140°C or higher and 400°C or lower.

3. The porous sheet according to claim 1, wherein the particle diameter D(P) is 0.1 μm or more and 3.0 μm or less.

4. The porous sheet according to claim 1, wherein the average small diameter D(F) is 0.1 μm or more and 1 μm or less.

5. A separator for a non-aqueous secondary battery comprising the porous sheet according to any one of claims 1 to 4.

6. A non-aqueous secondary battery comprising a positive electrode, a negative electrode, and the separator for non-aqueous secondary batteries according to claim 5.

7. A method for producing a porous sheet according to any one of claims 1 to 4, comprising the steps of: preparing a liquid composition containing the resin and particles of the organic foaming agent; and performing electrospinning using the liquid composition as a material.

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