JP2011168935A - Nonwoven fabric and separator for secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonwoven fabric having many fine spaces, capable of achieving battery capacity increase and thinning the film, and excellent in shut down performance, and to provide a separator for secondary batteries, made of the nonwoven fabric. <P>SOLUTION: There is provided the nonwoven fabric containing 0.5 mass% or more of nanofibers having an average fiber diameter of 10-3,000 nm, wherein an acid modified polyolefin resin is contained in the nonwoven fabric and the content of the acid modified polyolefin resin is 0.1-95 mass%. There is also provided a nonwoven fabric whose nanofiber contains polyvinyl alcohol. Further, a separator for secondary batteries is provided which is made from the nonwoven fabric. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a nonwoven fabric containing nanofibers as fibers constituting the nonwoven fabric and containing an acid-modified polyolefin resin in the nonwoven fabric and a separator for a secondary battery comprising the nonwoven fabric.

  In recent years, with the demand for smaller, lighter and more sophisticated mobile electronic devices such as mobile phones and digital cameras, high-performance batteries have been actively developed. Demand is growing significantly. In particular, lithium-ion batteries are being developed for use in electric vehicles in addition to applications such as mobile phones and notebook computers, and the range of their use is expanding greatly.

In a lithium ion battery, an electrode produced through a separator between a positive electrode and a negative electrode is placed in a container together with an electrolyte (in the case of a lithium ion polymer battery, a gel or all solid electrolyte instead of a liquid electrolyte). It has a structure housed in.
The separator of the lithium ion battery separates the positive electrode and the negative electrode in the battery and retains the electrolytic solution to ensure ionic conductivity between the positive electrode and the negative electrode. Used to shut down the movement of conducted ions. Conventionally, as a separator, a microporous film having a large number of micropores in a film has been often used.
“Shutdown” means that when the temperature in the battery rises, the polymer constituting the separator (microporous membrane) melts to close the communication hole, thereby increasing the electrical resistance of the membrane. This is a phenomenon in which the flow of lithium ions is interrupted.
Therefore, the lithium ion battery separator has (1) a pore diameter of 0.1 μm or less and a complicated path so that even if lithium dentite is generated on the negative electrode side during charging, so as not to cause a short-circuit, (2 ) When the temperature rises due to a micro short circuit, it has the function of shutting down the micropores at around 130 ° C and shutting down the movement of conduction ions. (3) To retain the organic electrolyte and ensure high ionic conductivity. In addition, it is required to have a highly porous structure and to be a thin film, and (4) to be chemically stable with respect to the organic electrolyte.

Conventionally, as a lithium ion secondary battery separator having a large energy density, a polyolefin-based microporous membrane is widely used among microporous membranes. Such polyolefin-based microporous membranes are mainly used because polyolefin can be used in an organic solvent, and polyolefin can be used at an appropriate temperature (around 130 ° C.) when the battery is abnormally heated due to a short circuit or the like. However, due to the recent trend to increase the capacity of lithium ion batteries, separators are required to be made thinner and highly porous. In conventional batteries using a polyolefin-based microporous membrane as a separator, a battery having sufficient battery characteristics cannot be obtained, and a secondary battery separator having a shutdown performance and a high capacity is known. Things are sought.

  Therefore, Patent Document 1 proposes a separator in which a heat-resistant porous layer made of a heat-resistant polymer such as aromatic aramid is laminated on one surface or both surfaces of a polyolefin microporous film by a wet coating method. However, although these separators have shutdown performance, they have not been made highly porous, and it has not been possible to achieve high battery capacity.

  Patent Document 2 proposes a separator composed of a nonwoven fabric made of polyacrylonitrile nanofibers. The non-woven fabric made of nanofibers is a separator having a large number of minute voids because the voids between the nanofibers constituting the pores are holes, and can achieve a high battery capacity. However, polyacrylonitrile has a high melting point, and this separator did not have shutdown performance.

JP 2002-355938 A WO2006 / 049151

  The present invention solves the above-described problems, has a large number of minute voids, can achieve high battery capacity, can be made thin, and has excellent shutdown performance. It is an object of the present invention to provide a non-woven fabric and a separator for a secondary battery comprising the non-woven fabric.

The present inventors have reached the present discovery as a result of studies to solve the above-described problems.
That is, the gist of the present invention is the following (1) to (2).
(1) A nonwoven fabric containing 0.5% by mass or more of nanofibers having an average fiber diameter of 10 to 3000 nm, wherein the nonwoven fabric contains an acid-modified polyolefin resin, and the content of the acid-modified polyolefin resin is 0.1 A nonwoven fabric characterized by being -95 mass%.
(2) A separator for a secondary battery, comprising the nonwoven fabric described in (1).

  Since the nonwoven fabric of the present invention is a nonwoven fabric containing 0.5% by mass or more of nanofibers, it has a large number of minute voids and can be thinned. In addition, since the nonwoven fabric contains an acid-modified polyolefin resin, it is possible to close minute voids at high temperatures, and the secondary battery separator made of the nonwoven fabric of the present invention has excellent shutdown performance. Become. Therefore, the secondary battery separator of the present invention can increase the capacity and thickness of the battery, has an excellent shutdown performance, and can provide a secondary battery that is small, light and excellent in safety. It becomes.

Hereinafter, the present invention will be described in detail. The nonwoven fabric of the present invention is a nonwoven fabric containing 0.5% by mass or more of nanofibers having an average fiber diameter of 10 to 3000 nm, and contains an acid-modified polyolefin resin in the nonwoven fabric.
As the form of the nonwoven fabric of the present invention containing an acid-modified polyolefin resin in the nonwoven fabric, (X) using nanofibers containing acid-modified polyolefin resin, (Y) using nanofibers not containing acid-modified polyolefin resin There is.

When nanofibers containing an acid-modified polyolefin resin are used (in the case of Form X), examples of such nanofibers include the following.
(A) Nanofiber (single type) made of acid-modified polyolefin resin.
(A) A nanofiber (single type) composed of a polymer obtained by mixing a polymer other than the acid-modified polyolefin resin (hereinafter sometimes referred to as other polymer) and the acid-modified polyolefin resin.
(C) Nanofiber (composite type) in which other polymer and acid-modified polyolefin resin are arranged in a core-sheath type, a sea-island type, a side-by-side type, or the like.
(D) A nanofiber obtained by attaching (coating) an acid-modified polyolefin resin to the surface of a nanofiber made of another polymer.

In the case of using nanofibers that do not contain an acid-modified polyolefin resin (in the case of Form Y), the acid-modified polyolefin resin is present in a nonwoven fabric containing nanofibers made of other polymers. For example, acid-modified polyolefin resin adhered to a non-woven fabric containing nanofibers made of another polymer, or acid-modified polyolefin resin in a non-woven fabric containing nanofibers made of another polymer (diameter 0.1 to 10 μm) Degree).
And as a nonwoven fabric containing the nanofiber which consists of other polymers, fibrous materials other than a nanofiber may be included.

  Since the nonwoven fabric of the present invention is composed of nanofibers, it has a large number of minute voids and can be made into a thin film. By containing the acid-modified polyolefin resin, it becomes possible to melt the resin at a high temperature and close the minute voids, and to make a battery separator having excellent shutdown performance.

  The content of the acid-modified polyolefin resin contained in the nonwoven fabric of the present invention is 0.1 to 95% by mass, preferably 5 to 90% by mass, and more preferably 10 to 80% by mass. Is preferred. When the content is less than 0.1% by mass, the fine voids of the nonwoven fabric cannot be sufficiently closed when melted, and a battery separator excellent in shutdown performance cannot be obtained. On the other hand, when the content exceeds 95% by mass, a portion that melts at a high temperature increases, and it becomes difficult to maintain the mechanical strength as a nonwoven fabric.

  First, the acid-modified polyolefin resin contained in the nonwoven fabric of the present invention will be described. As will be described later, since the acid-modified polyolefin resin in the present invention is preferably used as an aqueous dispersion dispersed in an aqueous medium, the acid-modified polyolefin resin is an acid-modified one, and the unsaturated carboxylic acid component (A1) And an olefin component (A2).

  The unsaturated carboxylic acid component (A1) constituting the acid-modified polyolefin resin is introduced by an unsaturated carboxylic acid or an anhydride thereof. The unsaturated carboxylic acid component is a compound having at least one carboxyl group or acid anhydride group in the molecule, and specifically includes acrylic acid, methacrylic acid, maleic acid, maleic anhydride, itaconic acid, anhydrous In addition to itaconic acid, fumaric acid, crotonic acid and the like, unsaturated dicarboxylic acid half ester, half amide and the like can be mentioned. Of these, maleic acid and maleic anhydride are preferable, and maleic anhydride is particularly preferable from the viewpoint of easy dispersion in an aqueous dispersion. In addition, the acid anhydride introduced into the acid-modified polyolefin resin forms an acid anhydride structure in which the adjacent carboxyl groups are dehydrated and cyclized in a dry state of the resin. Then, a part or all of the ring may be opened to take a structure of carboxylic acid or a salt thereof.

  Examples of the olefin component (A2) constituting the acid-modified polyolefin resin include olefins having 2 to 6 carbon atoms such as ethylene, propylene, isobutylene, 1-butene, 1-pentene, 1-hexene, From the viewpoint of resin flexibility, ease of dispersion in an aqueous medium, and adhesiveness, olefins having 2 to 4 carbon atoms such as ethylene, propylene, isobutylene, and 1-butene are preferable, and ethylene and propylene are more preferable. Two or more of these monomers may be used in combination.

  In the present invention, the acid-modified polyolefin resin containing the unsaturated carboxylic acid component (A1) and the olefin component (A2) is, for example, random copolymerization, block copolymerization, graft method, thermal degradation method, etc. It was obtained by going.

  The content of the unsaturated carboxylic acid component (A1) in the acid-modified polyolefin resin is preferably 0.1 to 20% by mass from the viewpoint of satisfying dispersion of the resin in an aqueous medium and resistance to an electrolytic solution. 0.3 to 15% by mass is more preferable, 0.5 to 12% by mass is further preferable, and 1 to 10% by mass is particularly preferable. If the content of the unsaturated carboxylic acid component (A1) is less than 0.1% by mass, it is difficult to disperse the resin in the aqueous medium, and if it exceeds 20% by mass, the resistance to the electrolytic solution may decrease. is there.

  The content of the olefin component (A2) in the acid-modified polyolefin resin may be 50 to 98% by mass in order to sufficiently close the fine voids of the nonwoven fabric when the resin is dispersed and melted in an aqueous medium. Preferably, it is 60-98 mass%, More preferably, it is 70-98 mass%, It is especially preferable that it is 75-95 mass%. If the content of the olefin component (A2) is less than 50% by mass, the fine voids may not be sufficiently blocked during melting. On the other hand, if the content exceeds 98% by mass, the content of the unsaturated carboxylic acid component decreases. Therefore, it may be difficult to disperse the resin in the aqueous medium.

  In addition to the unsaturated carboxylic acid component (A1) and the olefin component (A2), the acid-modified polyolefin resin may contain an acrylic ester or a methacrylic ester component. Specific examples thereof include esters of acrylic acid or methacrylic acid and alcohol, such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, butyl acrylate, and butyl methacrylate. Among them, methyl acrylate, methyl methacrylate, ethyl acrylate, and ethyl methacrylate are preferable, and methyl acrylate and ethyl acrylate are more preferable. The acrylic ester or methacrylic ester component may be copolymerized in the polyolefin resin, and the form thereof is not limited, and examples thereof include random copolymerization, block copolymerization, and graft copolymerization.

  Furthermore, in addition to the above components, the following components may be contained in an amount of 20% by mass or less of the entire acid-modified polyolefin resin. For example, alkenes and dienes having 6 or more carbon atoms such as 1-octene and norbornenes, maleates such as dimethyl maleate, diethyl maleate and dibutyl maleate, (meth) acrylic acid amides, methyl vinyl ether, Alkyl vinyl ethers such as ethyl vinyl ether, vinyl esters such as vinyl formate, vinyl acetate, vinyl propionate, vinyl pivalate, vinyl versatate, and vinyl alcohol obtained by saponifying vinyl esters with basic compounds, 2- Examples include hydroxyethyl acrylate, glycidyl (meth) acrylate, (meth) acrylonitrile, styrene, substituted styrene, vinyl halides, halogenated vinylidenes, carbon monoxide, sulfur dioxide, etc. It can also be.

  The molecular weight of the acid-modified polyolefin resin is preferably 10,000 or more, more preferably 10,000 to 150,000, and further preferably 12,000 to 120,000. It is particularly preferably 15,000 to 100,000, and most preferably 20,000 to 90,000. When the mass average molecular weight is less than 10,000, it is difficult to sufficiently close minute voids of the nonwoven fabric when melted. On the other hand, when the mass average molecular weight exceeds 150,000, it tends to be difficult to disperse the resin in the aqueous medium. In addition, the mass average molecular weight of the resin is obtained by using polystyrene resin as a standard using gel permeation chromatography (GPC).

  Examples of the acid-modified polyolefin resins as described above include Lexpearl EAA series (Nippon Polyethylene), Primacol series (Dow Chemical Japan), Nukurel series (Mitsui / DuPont Polychemical), Bondine series (Arkema) ), Lexpearl ET series (Nippon Polyethylene Co., Ltd.), Yumex series (Sanyo Kasei Co., Ltd.) and the like. In addition, unsaturated carboxylic acids may be used in commercially available resins such as REXTAC (Rexene, USA), Bestplast 408, Bestplast 708 (Huls, Germany), Ubetac APAO (Ube Lexen). Examples include acid-modified polyolefin resins into which an acid component has been introduced.

Next, the nanofiber which comprises the nonwoven fabric of this invention is demonstrated. In the nonwoven fabric of the present invention, there are cases of form X and form Y as described above.
First, the nanofiber containing the acid-modified polyolefin resin in the form X will be described. Specifically, the nanofibers in the above-described forms (a) to (d) are preferable.

  And in any nanofiber of the form of (a)-(d), it is preferable that at least one part of acid-modified polyolefin resin exists in the surface of a nanofiber. The presence of at least a part of the acid-modified polyolefin resin on the surface of the nanofiber facilitates melting at a high temperature, and has an excellent function of closing minute voids in the nonwoven fabric with the melted acid-modified polyolefin resin.

Therefore, when the non-woven fabric is in the form X, the nanofiber is preferably in the form of (a), (c), (d), and considering the strength and productivity of the nanofiber, (c), (d) ) Is preferred.
In the case of the form (c), a core-sheath type in which acid-modified polyolefin is arranged as a sheath component is particularly preferable. In the case of the form (d), as described later, an aqueous dispersion in which an acid-modified polyolefin resin is dispersed in an aqueous medium is used, and this is adhered to the surface of nanofibers made of another polymer by coating or the like. Those are preferred.

  In the nanofibers of the forms (a) to (d), other polymers constituting the nanofiber include polyvinyl alcohol, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polystyrene, nylon, aramid, polyimide, polyamideimide, poly Acrylonitrile, polyarylate, polyvinyl chloride, polyvinyl alcohol, polyethylene oxide, polyurethane, polysulfone, polyvinylidene fluoride, polytetrafluoroethylene, polyfluoroalkoxy fluorine, polycarbonate, polyether ether ketone, polyacrylic acid, polymethacrylic acid, polymethacrylic Methyl acid, phenol resin, melamine resin, polylactic acid, polyglycolic acid, polycaprolactam, polylactic acid-polyglycol Colic acid copolymer, cellulose, cellulose acetate, methylcellulose, cellulose derivatives, chitin, chitin derivatives, chitosan, chitosan derivatives, polyaniline, polypyrrole, polythiophene, polyalkylthiophene, poly (3,4-ethylenedioxythiophene) / poly (4-styrene sulfonic acid), polyparaphenylene vinylene, and the like can be used.

  Next, the non-woven fabric of the present invention in the case of Form Y is one in which an acid-modified polyolefin resin is present in a non-woven fabric made of nanofibers made of other polymers or a fibrous material other than nanofibers. As the other polymer constituting the fibrous material other than the fiber, the same polymer as the other polymer constituting the nanofibers in the above-described forms (a) to (d) can be used.

  As the other polymer constituting the nanofiber as described above, it is preferable to use polyvinyl alcohol because it is excellent in heat resistance, can be easily made into a nanofiber, and can be advantageously obtained in terms of cost.

  The polyvinyl alcohol that can be used in the present invention is a polymer containing 10 mol% or more of vinyl alcohol units, preferably 30 mol% or more, and more preferably 50 mol% or more, and is usually a single weight of vinyl ester or vinyl ether. It can be obtained by hydrolyzing (eg saponification, alcoholysis, etc.) the polymer or copolymer.

  Representative examples of vinyl esters include vinyl acetate, vinyl formate, vinyl propionate, vinyl pivalate, vinyl propionate, vinyl butyrate, vinyl caproate, vinyl caprylate, vinyl caprate, lauric acid. Vinyl, vinyl myristate, vinyl palmitate, vinyl stearate, vinyl cyclohexanecarboxylate, vinyl pivalate, vinyl octylate, vinyl monochloroacetate, divinyl adipate, vinyl methacrylate, vinyl crotrate, vinyl sorbate, vinyl benzoate And vinyl cinnamate. Examples of the vinyl ether include t-butyl vinyl ether and benzyl vinyl ether.

  The saponification degree of polyvinyl alcohol may be appropriately selected depending on the solubility in water, the stability of the aqueous solution, the mechanical strength of the fiber, etc. The saponification degree is preferably 60 mol% or more, and preferably 70 mol% or more. More preferably, 80 mol% or more is further more preferable.

  Moreover, the polyvinyl alcohol may contain the following monomer units. These monomer units include olefins such as propylene, 1-butene and isobutene excluding ethylene; unsaturated acids such as acrylic acid, methacrylic acid, maleic acid, itaconic acid and maleic anhydride, or salts thereof, or a carbon number of 1 Mono- or dialkyl esters up to -18; acrylamides such as acrylamide, N-alkyl acrylamide having 1 to 18 carbon atoms, N, N-dimethylacrylamide, 2-acrylamide propanesulfonic acid or its acid salt or quaternary salt thereof; Methacrylamide such as methacrylamide, C1-C18 N-alkyl methacrylamide, N, N-dimethylmethacrylamide, 2-methacrylamide propane sulfonic acid or salts thereof; N-vinylpyrrolidone, N-vinylformamide, etc. N-vinyl Examples include amides.

  Furthermore, as the average degree of polymerization of polyvinyl alcohol, if the average degree of polymerization is too low, the strength of the nanofibers obtained is lowered, and if it is too high, the solubility in water is lowered and the productivity is lowered. Is preferably 100 to 30000, more preferably 200 to 20000, and even more preferably 300 to 15000.

  The nanofiber in the nonwoven fabric of the present invention has an average fiber diameter of 10 to 3000 nm, preferably 50 to 2000 nm, more preferably 100 to 1000 nm. When the average fiber diameter is less than 10 nm, the mechanical strength of the nanofiber is lowered and the handleability is also lowered. On the other hand, when the average fiber diameter exceeds 3000 nm, it becomes difficult for the nonwoven fabric obtained using the nanofibers to have improved surface area, porosity, etc., which are the characteristics of using the nanofibers.

  In addition, the length of the nanofiber in the present invention is preferably 100 μm or more, more preferably 500 μm or more, and even more preferably 1000 μm or more. When the length of the nanofiber is less than 100 μm, the length of the nanofiber is insufficient, and the entanglement of the fibers when the nonwoven fabric is manufactured becomes insufficient, and it is difficult to maintain the shape of the nonwoven fabric. On the other hand, the length of the nanofiber is preferably 50 cm or less. If the length of the nanofiber exceeds 50 cm, the entanglement between the nanofibers becomes excessive, a nanofiber lump is generated, and the homogeneity of the nonwoven fabric may be lowered.

  And the ratio of the above-mentioned nanofiber contained in the nonwoven fabric of this invention is 0.5 mass% or more of the total mass of a nonwoven fabric in the case of form X and form Y, and 5 mass% or more among them. It is preferable that the amount is 50% by mass or more. If the proportion of nanofibers is less than 0.5% by mass, the resulting nonwoven fabric has a small proportion of nanofibers, and it is difficult to improve the surface area, porosity, etc., which are the characteristics of using nanofibers. It becomes.

Moreover, when things other than the above-mentioned nanofiber are contained in the nonwoven fabric of this invention, it is preferable to contain fibrous materials other than a nanofiber. And in such a nonwoven fabric, it is preferable that 10 mass% or more is a nanofiber in the total mass of a nanofiber and other fibrous materials. When the proportion of the nanofiber is less than 10% by mass, the surface area, the porosity, etc., which are the characteristics of using the nanofiber, tend to be reduced.
As the fibrous material other than nanofibers, those having an average fiber diameter exceeding 3000 nm and having a short fiber (staple fiber) shape are preferred.

  And it is preferable that the nanofiber in this invention was obtained by the electrospinning method. In the electrospinning method, nanofibers can be obtained by charging a spinning solution by applying a high voltage. As a method for charging the spinning solution, it is preferable to connect an electrode connected to a high-voltage power supply device to the spinning solution itself or a container and apply a voltage of 1 to 100 kV, and further, apply a voltage of 2 to 80 kV. It is preferable that a voltage of 5 to 50 kV is applied.

  The type of voltage may be any voltage of direct current or alternating current, and the polarity in the case of direct current may be either an anode or a cathode.

  A case where a metal nozzle is used will be described as an example of a specific manufacturing method. A metal nozzle is attached to the container filled with the spinning solution, and a voltage is applied to the metal nozzle while feeding the solution to the tip of the metal nozzle using a gear pump or the like so that the charged spinning solution faces the metal nozzle. The spinning solution is stretched when the electrostatic attractive force generated between the grounding portion or the deposition portion to which the opposite polarity to the charged polarity of the metal nozzle is applied exceeds the surface tension of the spinning solution.

By being stretched and decreasing in volume, the charge density is increased, and by repulsion by electric repulsion, nanofibers are manufactured and deposited on the deposition part.
And a nanofiber is manufactured and a nonwoven fabric can be obtained by depositing on a deposition part.

  The material and form of the deposition part are not particularly limited, and it is also possible to deposit nanofibers directly on the substrate by placing the substrate between the nozzle and the deposition part or at the same position as the deposition part It is. In this case, a structure in which the nonwoven fabric of the present invention and the substrate are integrated can be obtained.

And when the nonwoven fabric of the present invention is in the form X and the nanofibers containing the acid-modified polyolefin resin are in the forms (a) to (c), the nanofibers are produced by the electrospinning method. In this case, in order to obtain a spinning solution, it is preferable to use an acid-modified polyolefin resin as an aqueous dispersion.
Further, when the nonwoven fabric of the present invention is in the form X and the nanofiber containing the acid-modified polyolefin resin is in the form of (D), the acid-modified polyolefin resin is attached to the nanofiber made of another polymer. As a method of making it, the method of coating the surface of a nanofiber using the aqueous dispersion which disperse | distributed acid-modified polyolefin resin in the aqueous medium is preferable. Examples of such a coating method include a dipping method, a spray coating method, a bar coating method, a screen coating method, and a gravure coating method.

Further, in the case where the nonwoven fabric of the present invention is in the form Y, in order to obtain a nonwoven fabric in which particulate acid-modified polyolefin resin is present in the nonwoven fabric composed of nanofibers composed of other polymers or fibrous materials other than nanofibers, When nanofibers made of a polymer are electrospun, it can be obtained by putting an aqueous dispersion of acid-modified polyolefin into another nozzle and performing electrospinning.
In addition, the case where the nonwoven fabric of the present invention is in the form Y includes nanofibers made of other polymers and nonwoven fabrics made by attaching acid-modified polyolefin resin to nonwoven fabrics made of fibrous materials other than nanofibers. Also when obtaining a nonwoven fabric, it is preferable to adhere to a nonwoven fabric using the aqueous dispersion which disperse | distributed acid-modified polyolefin resin in the aqueous medium. Examples of the attaching method include a dipping method, a spray coating method, and a bar coating method.

  As described above, the acid-modified polyolefin resin in the present invention is used as an aqueous dispersion dispersed in an aqueous medium in any of spinning solution, coating on nanofibers, and adhering to nonwoven fabric. Is preferred. The aqueous dispersion of acid-modified polyolefin resin will be described below.

  When the acid-modified polyolefin resin in the present invention is used as an aqueous dispersion, the acid-modified polyolefin resin and a basic compound are preferably contained in the aqueous medium. The aqueous medium refers to water or a mixture of water and an organic solvent. For example, an aqueous dispersion of an acid-modified polyolefin resin can be obtained by heating and stirring an acid-modified polyolefin resin, a basic compound, and an aqueous medium in a sealed container under pressure.

  In the aqueous dispersion of the acid-modified polyolefin resin, the number average particle diameter of the acid-modified polyolefin resin is preferably 0.01 to 1 μm, more preferably 0.02 to 0.5 μm, and still more preferably 0.04. It is -0.2 micrometer, Most preferably, it is 0.05-0.1 micrometer. When the number average particle size is less than 0.01 μm, the viscosity of the aqueous dispersion may increase and gel may occur. When the number average particle size exceeds 1 μm, the particle size increases, so that the ionic conductivity deteriorates. High capacity tends to be difficult.

  The solid content concentration of the acid-modified polyolefin resin in the aqueous dispersion is preferably 1 to 50% by mass, more preferably 5 to 40% by mass, and particularly preferably 10 to 30% by mass. . When the solid content concentration exceeds 50% by mass, there is a tendency that the viscosity of the dispersion is remarkably increased or the handleability is lowered due to solidification. On the other hand, when the solid content concentration is less than 1% by mass, it is necessary to increase the amount of the aqueous dispersion of the acid-modified polyolefin resin, both when the acid-modified polyolefin resin is contained in the nanofiber, and when it is attached and coated. It becomes difficult to make nanofibers, adhesion is deteriorated, and it is disadvantageous in terms of cost.

  Although it is preferable that the aqueous dispersion contains a basic compound, by containing the basic compound, a part or all of the carboxyl groups in the polyolefin resin are neutralized to form a generated carboxyl. Aggregation between the fine particles can be suppressed by the electric repulsion between the anions, and stability is imparted to the aqueous dispersion.

  Basic compounds include ammonia, triethylamine, N, N-dimethylethanolamine, isopropylamine, aminoethanol, dimethylaminoethanol, diethylaminoethanol, ethylamine, diethylamine, isobutylamine, dipropylamine, 3-ethoxypropylamine, 3- Diethylaminopropylamine, sec-butylamine, propylamine, n-butylamine, 2-methoxyethylamine, 3-methoxypropylamine, 2,2-dimethoxyethylamine, monoethanolamine, morpholine, N-methylmorpholine, N-ethylmorpholine, pyrrole , Pyridine, alkali metal compounds and the like. Among these, ammonia, triethylamine, and N, N-dimethylethanolamine are preferable from the viewpoint of easy dispersion.

  The addition amount of the basic compound is preferably 0.5 to 3.0 times equivalent to the carboxyl group in the acid-modified polyolefin resin, more preferably 0.5 to 2.8 times equivalent, It is especially preferable that it is 0.6-2.5 times equivalent. If it is less than 0.5 times equivalent, the addition effect of a basic compound is not recognized, and if it exceeds 3.0 times equivalent, the stability of the aqueous dispersion may deteriorate.

  As a specific method for producing the aqueous dispersion of the acid-modified polyolefin resin as described above, the above-mentioned acid-modified polyolefin resin, a basic compound, and an aqueous medium composed of water and an organic solvent are used at a temperature of 80 to 280 ° C. Is preferable. By this method, even if it does not contain a non-volatile aqueous dispersion aid such as an emulsifier component or a compound having a protective colloid action, dispersion can be promoted, and a good aqueous dispersion having a fine particle size can be obtained.

  When the aqueous medium is water and an organic solvent, the content of the organic solvent is preferably 50% by mass or less, more preferably 1 to 45% by mass, and more preferably 2 to 40% with respect to the total amount of the aqueous medium. It is more preferable that it is mass%, and it is especially preferable that it is 3-35 mass%. When the amount of the organic solvent exceeds 50% by mass, the stability of the aqueous dispersion may be lowered depending on the organic solvent used.

  Specific examples of the organic solvent include, for example, methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, sec-butanol, tert-butanol, n-amyl alcohol, isoamyl alcohol, sec-amyl alcohol, tert- Alcohols such as amyl alcohol, 1-ethyl-1-propanol, 2-methyl-1-butanol, n-hexanol, cyclohexanol, ketones such as methyl ethyl ketone, methyl isobutyl ketone, ethyl butyl ketone, cyclohexanone, tetrahydrofuran, dioxane, etc. Ethers, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, sec-butyl acetate, 3-methoxybutyl acetate, methyl propionate , Esters such as ethyl propionate, diethyl carbonate, dimethyl carbonate, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, glycol derivatives such as ethylene glycol ethyl ether acetate, 1-methoxy-2-propanol, 1-ethoxy-2-propanol, 3-methoxy-3-methyl-1-butanol, methoxybutanol, acetonitrile, dimethylformamide, dimethylacetamide, diacetone alcohol, ethyl acetoacetate, 1,2 -Dimethyl glycerol, 1, 3- dimethyl glycerol, a trimethyl glycerol, etc. are mentioned. These organic solvents may be used alone or in combination of two or more.

  Among the above organic solvents, ethanol, n-propanol, isopropanol, n-butanol, methyl ethyl ketone, cyclohexanone, tetrahydrofuran, dioxane, ethylene glycol monoethyl ether, ethylene glycol monopropyl are effective because they are highly effective in promoting aqueous formation of resins. Ether, ethylene glycol monobutyl ether, and diethylene glycol monomethyl ether are preferred. Among these, an organic solvent having one hydroxyl group in the molecule is more preferred, and n-propanol, isopropanol, tetrahydrofuran, More preferred are ethylene glycol alkyl ethers.

In addition, the nonwoven fabric of the present invention may be either a wet nonwoven fabric or a dry nonwoven fabric, but it is preferable that the nonwoven fabric is deposited when the nanofibers are electrospun as described above.
And the nonwoven fabric of this invention is suitable to use as a separator for secondary batteries, and a fabric weight is 0.1-10 g / m <2> from a viewpoint of the retainability of electrolyte solution, short circuit prevention property, and ion permeability. It is preferable that it is 0.5-8 g / m <2>, and it is more preferable that it is 1-3 g / m <2>. Moreover, it is preferable that the thickness of a nonwoven fabric is 50 micrometers or less from an ion-permeable viewpoint, More preferably, it is 40 micrometers or less, More preferably, it is 30 micrometers or less. When the thickness is too thin, although it is configured using nanofibers, the current leakage prevention property, short circuit prevention property, and electrolyte retention property tend to deteriorate, so the thickness is 5 μm or more. Is preferred. The basis weight and thickness are both measured according to JIS L 1096.

In addition, the nonwoven fabric of the present invention preferably has an average pore diameter measured by the bubble point method of 0.01 to 10 μm, and particularly has an average pore diameter from the viewpoint of electrolyte retention, short circuit prevention, and ion permeability. 0.01-5 micrometers is preferable and 0.03-2.0 micrometers is further more preferable.
The average pore diameter measured by the bubble point method in the present invention was measured using a palm porometer manufactured by PMI in accordance with JISK 3832.

Next, the separator for a secondary battery of the present invention is made of the nonwoven fabric of the present invention, that is, the one using the nonwoven fabric of the present invention itself or the one used with a base material made of another material (composite structure). Is mentioned.
Examples of the base material that can be used for forming the composite structure include a plate, a film, a woven fabric, a nonwoven fabric, cotton, paper, and a porous material.

In the case where the separator for a secondary battery of the present invention is a composite structure, a non-woven fabric of the present invention is provided on a base material, a base material is provided between two layers of the non-woven fabric of the present invention, The thing of forms, such as a nonwoven fabric of this invention, is mentioned between base materials.
And in order to obtain such a composite structure, a sealer machine, a heat press machine, a heating roll machine, a hot air generator, an ultrasonic welder machine, a high frequency welder machine, a laser machine, a needle punch method, a water punch method It is preferable to make a composite by using a stitch bond method.

  The material constituting the substrate is not particularly limited. Polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polystyrene, nylon, aramid, polyimide, polyamideimide, polyacrylonitrile, polyarylate, polyvinyl chloride , Polyvinyl alcohol, polyethylene oxide, polyurethane, polysulfone, polyvinylidene fluoride, polytetrafluoroethylene, polyfluoroalkoxy fluorine, polycarbonate, polyether ether ketone, polyacrylic acid, polymethacrylic acid, polymethyl methacrylate, phenol resin, melamine resin , Polylactic acid, polyglycolic acid, polycaprolactam, polylactic acid-polyglycolic acid copolymer, cellulose, acetate Examples include roulose, methylcellulose, cellulose derivatives, chitin, chitin derivatives, chitosan, chitosan derivatives, glass, silica, alumina, zeolite, carbon, and metal.

  Hereinafter, the present invention will be described in detail with reference to examples. In addition, the measurement and evaluation method of the various characteristic values mentioned later are as follows.

(1) Average fiber diameter of nanofiber The fiber structure part of the obtained composite fiber structure was observed with a field emission scanning electron microscope (Hitachi, Ltd. S-4000) and described in the image. The average value of the fiber diameters of the 20 nanofibers measured using a caliper based on the scale bar is used as the average fiber diameter.
(2) Configuration of Acid-Modified Polyolefin Resin It was determined by performing ¹ H-NMR analysis (manufactured by Varian, 300 MHz) at 120 ° C. in orthodichlorobenzene (d ₄ ).
(3) Content of unsaturated carboxylic acid component of acid-modified polyolefin resin The acid value of the acid-modified polyolefin resin was measured according to JIS K5407, and the content of unsaturated carboxylic acid was determined from the value.
(4) Fabric weight and thickness of nonwoven fabric The fabric weight and thickness of the obtained nonwoven fabric were measured according to JIS L 1096.
(5) Average pore diameter It measured by said method.
(6) Electrolyte permeability (separator characteristics)
[Before heat treatment]
The obtained non-woven fabric (secondary battery separator) was cut into a strip having a length of 100 mm and a width of 25 mm to obtain a sample. This sample was placed on a Kimwipe, and an organic electrolyte (lithium tetrafluoroborate) was placed on the sample. Was dropped in an ethylene carbonate / diethyl carbonate mixed solvent (mass ratio 1/1) at a concentration of 1 mol / liter and left for 30 seconds. Thirty seconds later, a sample where no wetting of the Kimwipe occurred was marked with ◯, and a sample where wetting of the Kimwipe occurred was marked with ×.
[After heat treatment]
The obtained nonwoven fabric (secondary battery separator) was cut into a strip shape having a length of 100 mm and a width of 25 mm to form a sample, and this sample was placed in a hot air dryer and subjected to heat treatment (at 150 ° C. for 1 hour). Thereafter, evaluation was performed in the same manner as in the case of using the sample before the heat treatment.
(7) Shutdown performance (battery characteristics)
A coin-type battery is produced by sandwiching a graphite electrode (made by Hosen Co., Ltd.), a lithium cobaltate electrode (made by Hosen Co., Ltd.) and a nonwoven fabric (secondary battery separator) obtained between the two electrodes, and conducting a charge / discharge test. went. In the charge / discharge test, 0.2C-4.1V constant current / constant voltage charge was performed in a 25 ° C. environment, and then 0.2C-2.5V constant current discharge was performed to determine the discharge capacity and the charge capacity. And the initial stage charge / discharge efficiency was computed by the following formula.
Initial charge / discharge efficiency (%) = (discharge capacity / charge capacity) × 100
Next, this battery was placed in a hot air dryer and subjected to a heat treatment (1 hour at 150 ° C.), and then a charge / discharge test was performed again under the same conditions as described above. At this time, those that could not be charged were marked as “good” as having shutdown performance, and those that could be charged or discharged were marked as “not having shutdown performance”.

Reference example 1
A three-necked flask equipped with a stirrer was installed in an oil bath with a heater. After adding 160 g of polyvinyl alcohol having an average degree of polymerization of 1800 and a saponification degree of 88% and 840 g of distilled water, the mixture was stirred at room temperature for 15 minutes, and then stirred. The mixture is stirred using an oil bath until the temperature of the aqueous polyvinyl alcohol solution reaches 90 ° C. Stirring was continued for 30 minutes as it was, and then air-cooled to room temperature while stirring to obtain an aqueous polyvinyl alcohol solution.

Reference example 2
A three-necked flask equipped with a stirrer was placed in an oil bath with a heater, 120 g of methyl vinyl ether / maleic anhydride copolymer having a molecular weight of 130,000 and 880 g of distilled water were added, and the mixture was stirred at room temperature for 15 minutes, and then stirred. The mixture is stirred using an oil bath until the temperature of the aqueous methyl vinyl ether / maleic anhydride copolymer solution reaches 70 ° C. Stirring was continued for 120 minutes as it was, and then air-cooled to room temperature while stirring to obtain a methyl vinyl ether / maleic anhydride copolymer aqueous solution.

Reference example 3
Using a stirrer equipped with a 1 L glass container that can be sealed with a heater, 60.0 g of Bondine HX-8290 (manufactured by Arkema) having the composition shown in Table 1 as an acid-modified polyolefin resin, and isopropanol (organic solvent) 48.0 g, 3.9 g of N, N-dimethylethanolamine as a basic compound, and 188.1 g of distilled water were charged in a glass container, stirred, and an aqueous dispersion of milky white uniform acid-modified polyolefin resin “E- 1 "was obtained.

Reference example 4
280 g of propylene-butene-ethylene terpolymer (manufactured by Huls Japan, Bestplast 708, propylene / butene / ethylene = 64.8 / 23.9 / 11.3 mass%) in a four-necked flask And heated and melted in a nitrogen atmosphere. Thereafter, while maintaining the system temperature at 170 ° C., 32.0 g of maleic anhydride as an unsaturated carboxylic acid and 6.0 g of dicumyl peroxide as a radical generator were added over 1 hour with stirring. The reaction was carried out for 1 hour. After completion of the reaction, the obtained reaction product was put into a large amount of acetone to precipitate a resin. The resin was further washed several times with acetone to remove unreacted maleic anhydride, and then dried under reduced pressure in a vacuum dryer to obtain an acid-modified polyolefin resin “P-1” shown in Table 1.
As the acid-modified polyolefin resin, “P-1” is used, “P-1” is 60.0 g, n-propanol (manufactured by Wako Pure Chemical Industries, special grade, boiling point 97 ° C.) 90.0 g, triethylamine (Wako Pure Chemical Industries, Ltd.) 6.2 g of distilled water and 143.8 g of distilled water were charged in a glass container and stirred to obtain a milky-yellow uniform acid-modified polyolefin resin aqueous dispersion “E-2”.

Reference Example 5
120 g of Umex 1010 having the composition shown in Table 1 as the acid-modified polyolefin resin (A) and N, N-dimethylethanol as the basic compound (B) in a hermetically sealed 1 liter glass container equipped with a stirrer and a heater. 12.6 g of amine, 120 g of isopropanol as an organic solvent, and 347 g of distilled water were charged and sealed, followed by stirring to obtain a slightly yellow and translucent uniform aqueous dispersion (solid content concentration 20% by mass).
295 g of the aqueous dispersion and 50 g of distilled water were placed in a 1 L eggplant flask, placed in an evaporator, and the aqueous medium was distilled off by reducing the pressure. When about 100 g of the aqueous medium was distilled off, the heating was terminated and the system was cooled. After cooling, the mixture was filtered under pressure to obtain a slightly yellow translucent uniform acid-modified polyolefin resin aqueous dispersion “E-3”.

Example 1
The aqueous polyvinyl alcohol solution obtained in Reference Example 1, the aqueous dispersion “E-1” of acid-modified polyolefin resin, and distilled water were mixed, and the solid content concentration of polyvinyl alcohol was 8.0% by mass. A spinning solution was prepared by preparing an aqueous solution having a partial concentration of 2.8% by mass.
A syringe equipped with a metal nozzle with an inner diameter of 1 mm is filled with 5 mL of the spinning solution, 15 kV is applied to the metal nozzle to produce nanofibers by electrospinning, and the nanofibers are deposited using an aluminum plate as the deposition part. To obtain a non-woven fabric (form X consisting of nanofibers of (A)). The obtained nonwoven fabric was used as a separator for a secondary battery.

Example 2
The aqueous polyvinyl alcohol solution obtained in Reference Example 1, the aqueous dispersion “E-1” of acid-modified polyolefin resin, and distilled water were mixed, and the solid content concentration of polyvinyl alcohol was 2.8% by mass, and the solid content of the acid-modified polyolefin resin. A nonwoven fabric was obtained in the same manner as in Example 1 except that the aqueous solution had a partial concentration of 8.0% by mass.

Example 3
A nonwoven fabric was obtained in the same manner as in Example 1 except that the aqueous dispersion of the acid-modified polyolefin resin was changed to “E-2”.

Example 4
A nonwoven fabric was obtained in the same manner as in Example 2 except that the aqueous dispersion of the acid-modified polyolefin resin was changed to “E-2”.

Example 5
A nonwoven fabric was obtained in the same manner as in Example 1 except that the aqueous dispersion of the acid-modified polyolefin resin was changed to “E-3”.

Example 6
A nonwoven fabric was obtained in the same manner as in Example 2 except that the aqueous dispersion of the acid-modified polyolefin resin was changed to “E-3”.

Example 7
Using the aqueous solution obtained in Reference Example 1 and the aqueous solution obtained in Reference Example 2, the solid content concentration of polyvinyl alcohol was 8.0% by mass, and the solid content concentration of methyl vinyl ether / maleic anhydride copolymer was 2.0%. A spinning solution was prepared by preparing a mass% aqueous solution.
A syringe equipped with a metal nozzle with an inner diameter of 1 mm is filled with 5 mL of the spinning solution, 15 kV is applied to the metal nozzle to produce nanofibers by electrospinning, and the nanofibers are deposited using an aluminum plate as the deposition part. To obtain a nonwoven fabric. And the nanofiber which can hold | maintain a shape even if immersed in 50 degreeC warm water by heat-processing for 30 minutes at 150 degreeC was obtained. The obtained non-woven fabric was dipped in an aqueous dispersion “E-1” of acid-modified polyolefin resin (solid content concentration 5 mass%), and then the excess aqueous dispersion of acid-modified polyolefin resin was squeezed out with a press roller, and 50 ° C. And then dried in a vacuum at 50 ° C. for 15 hours to obtain a nonwoven fabric (form Y).
In addition, about the ratio of polyvinyl alcohol and acid-modified polyolefin resin in a nonwoven fabric, it computed from the mass change of the nonwoven fabric before and behind immersion in the aqueous dispersion of acid-modified polyolefin resin.

Example 8
A nonwoven fabric was obtained in the same manner as in Example 7, except that the solid content concentration of the aqueous dispersion “E-1” of the acid-modified polyolefin resin was changed to 35 mass%.

Example 9
A nonwoven fabric was obtained in the same manner as in Example 7 except that the aqueous dispersion of the acid-modified polyolefin resin was changed to “E-2”.

Example 10
A nonwoven fabric was obtained in the same manner as in Example 8 except that the aqueous dispersion of the acid-modified polyolefin resin was changed to “E-2”.

Example 11
A nonwoven fabric was obtained in the same manner as in Example 7 except that the aqueous dispersion of the acid-modified polyolefin resin was changed to “E-3”.

Example 12
A nonwoven fabric was obtained in the same manner as in Example 8 except that the aqueous dispersion of the acid-modified polyolefin resin was changed to “E-3”.

Example 13
Using the aqueous solution obtained in Reference Example 1 and the aqueous solution obtained in Reference Example 2, the solid content concentration of polyvinyl alcohol was 8.0% by mass, and the solid content concentration of methyl vinyl ether / maleic anhydride copolymer was 2.0%. A spinning solution was prepared by preparing a mass% aqueous solution.
A syringe equipped with a metal nozzle with an inner diameter of 1 mm was filled with 5 mL of the spinning solution, 15 kV was applied to the metal nozzle, and a stainless steel twill wire mesh with a wire diameter of 0.05 mm, an opening of 0.077 mm, and a 200 mesh was used. Electrospinning was performed. And the nanofiber which can hold | maintain a shape even if immersed in 50 degreeC warm water by heat-processing for 30 minutes at 150 degreeC was obtained. An aqueous dispersion of nanofibers was prepared by adding 0.5 g of nanofibers peeled from the stainless twill wire mesh to 500 g of distilled water and stirring for 10 minutes with a homogenizer with a rotation speed of 5000 rpm. Furthermore, after dispersing 49.5 g of polyethylene terephthalate (PET) fibers having a fiber diameter of 15 μm and a fiber length of 50 mm in an aqueous dispersion of nanofibers, a stainless twill wire mesh having a wire diameter of 0.03 mm, an opening of 0.034 mm, and 400 mesh. And then dried for 24 hours with a dryer at 70 ° C. The obtained wet nonwoven fabric was immersed in an aqueous dispersion “E-1” of acid-modified polyolefin resin (solid concentration 35% by mass), and then an excess aqueous dispersion of acid-modified polyolefin resin was squeezed out with a press roller. The nonwoven fabric (wet nonwoven fabric) which consists of nanofiber, PET fiber, and acid-modified polyolefin resin was obtained by drying for 24 hours with a dryer at ° C. The content of nanofibers, PET fibers, and acid-modified polyolefin resin in the nonwoven fabric is the ratio of nanofibers and PET fibers added to distilled water, and the mass of the nonwoven fabric before and after being immersed in the aqueous dispersion of acid-modified polyolefin resin. Calculated from change.

Example 14
Nanofibers and PET fibers were the same as in Example 13, except that the mass of PET fibers added to distilled water was changed and the mass ratio of nanofibers and PET fibers was changed to that shown in Table 2. And a non-woven fabric made of acid-modified polyolefin resin.

Example 15
Nanofibers and PET fibers were the same as in Example 13, except that the mass of PET fibers added to distilled water was changed and the mass ratio of nanofibers and PET fibers was changed to that shown in Table 2. And a non-woven fabric made of acid-modified polyolefin resin.

Comparative Example 1
Except that nanofibers were deposited by the same method as in Example 7 to produce a nonwoven fabric, and the nonwoven fabric composed of the obtained nanofibers was not immersed in the aqueous dispersion “E-1” of acid-modified polyolefin resin. A nonwoven fabric was obtained in the same manner as in Example 7.

Comparative Example 2
Nanofibers and PET fibers were the same as in Example 13, except that the mass of PET fibers added to distilled water was changed and the mass ratio of nanofibers and PET fibers was changed to that shown in Table 2. And a non-woven fabric made of acid-modified polyolefin resin.

Comparative Example 3
By immersing the aqueous dispersion “E-1” of acid-modified polyolefin resin in a solution diluted 10 times (solid content concentration 3.5% by mass), the mass ratio of nanofibers, PET fibers and acid-modified polyolefin resin is expressed. except for changing such that those wherein, in the same manner as the actual施例14, to obtain a nonwoven fabric made of nanofibers and PET fibers and the acid-modified polyolefin resin.

Comparative Example 4
Separator characteristics and battery characteristics were evaluated using a polypropylene film (porous film) separator (manufactured by Celgard, “Celguard # 2400”, thickness 25 μm, average pore diameter 0.1 μm).

  Table 2 shows the characteristic values of the nonwoven fabrics obtained in Examples 1 to 15 and Comparative Examples 1 to 3, and the evaluation results of the separator for secondary batteries composed of the nonwoven fabric and the separator of Comparative Example 4.

As is clear from Table 2, the secondary battery separator made of the nonwoven fabric obtained in Examples 1 to 15 has high initial charge / discharge efficiency when used as a battery, and excellent performance as a secondary battery separator (high Ion conductivity). Since these secondary battery separators do not allow electrolyte to permeate after heat treatment, and batteries using these secondary battery separators cannot be charged after heat treatment, they have shutdown performance. It was what was.
On the other hand, since the non-woven fabric obtained in Comparative Example 1 was composed only of nanofibers made of polyvinyl alcohol, the secondary battery separator made of this non-woven fabric allows the electrolyte to permeate after heat treatment. The battery using the separator for the secondary battery did not have the shutdown performance because it could be charged and discharged after the heat treatment.
Since the nonwoven fabric obtained in Comparative Example 2 has too little nanofiber content, the surface area, porosity, etc. are reduced, and the secondary battery separator made of this nonwoven fabric allows the electrolyte to permeate after heat treatment, A battery using the separator for a secondary battery did not have shutdown performance because it could be charged and discharged after heat treatment.
Since the non-woven fabric obtained in Comparative Example 3 contained too little acid-modified polyolefin resin, the secondary battery separator made of this non-woven fabric allows the electrolyte to permeate after the heat treatment. The battery used had no shutdown performance because it could be charged and discharged after heat treatment.
In Comparative Example 4, since a polypropylene film was used as a secondary battery separator, the initial charge / discharge efficiency when the battery was made was low, the capacity could not be increased, and the battery characteristics were inferior. It was.

Claims (3)

A nonwoven fabric containing 0.5% by mass or more of nanofibers having an average fiber diameter of 10 to 3000 nm, containing an acid-modified polyolefin resin in the nonwoven fabric, and the content of the acid-modified polyolefin resin is 0.1 to 95 mass %, A non-woven fabric characterized by The nonwoven fabric according to claim 1, wherein the nanofiber contains polyvinyl alcohol. A separator for a secondary battery, comprising the nonwoven fabric according to claim 1.