JP2015050043A - Lithium ion secondary battery separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery separator using a nonwoven fabric as a base material, having inorganic particles applied on at least one of surfaces of the base material and excellent in workability when manufactured, strength and uniformity.SOLUTION: The lithium ion secondary battery separator uses a nonwoven fabric including synthetic resin short fibers as a base material and has inorganic particles applied on at least one of surfaces of the base material. A nonwoven fabric subjected to wet paper making using a degassed fiber slurry, more preferably, a nonwoven fabric subjected to wet paper making after degassing white water generated in the step that the nonwoven fabric is subjected to wet paper making and further more preferably, a nonwoven fabric subjected to wet paper making after including synthetic resin ultrashort fibers having 450 ml or less of a modified freeness measured using an 80-mesh wire net at a concentration of 0.1 mass% are used as the base material.

Description

  The present invention relates to a lithium ion secondary battery separator (hereinafter sometimes abbreviated as “separator”) that can be suitably used for lithium ion secondary batteries such as lithium ion secondary batteries and lithium ion polymer secondary batteries.

  With the recent spread of portable electronic devices and higher performance, secondary batteries having high energy density are desired. As this type of battery, a lithium ion secondary battery using an organic electrolyte (non-aqueous electrolyte) has attracted attention. This lithium ion secondary battery has an energy density of about 3.7 V, which is about three times that of an alkaline secondary battery, which is a conventional secondary battery, and thus has a high energy density. Since a water-based electrolyte cannot be used, a non-aqueous electrolyte having sufficient oxidation-reduction resistance is used. Since non-aqueous electrolytes are flammable, there is a risk of ignition and the like, and careful attention is paid to safety in their use. There are several possible cases of exposure to fire and other hazards, but overcharging is particularly dangerous.

  In order to prevent overcharging, current non-aqueous secondary batteries are charged at a constant voltage and a constant current, and the battery is equipped with a precise IC (protection circuit). The cost required for this protection circuit is large, and it is a factor that increases the cost of non-aqueous secondary batteries.

  When overcharging is prevented by the protection circuit, it is naturally assumed that the protection circuit does not operate well, and it is difficult to say that it is intrinsically safe. The current lithium-ion secondary battery breaks the protective circuit when overcharged, and is equipped with a safety valve / PTC element and a device that has a thermal fuse function in order to safely destroy the battery when overcharged. ing. However, even if equipped with the above-mentioned means, depending on the overcharge conditions, the safety during overcharge is not guaranteed reliably, and in fact, a lithium ion secondary battery ignition accident Is still happening.

  As the separator, a film-like porous film made of polyolefin such as polyethylene is often used. This separator has a thermal fuse function (shutdown function) that prevents the movement of lithium ions and shuts off the current by melting and closing the micropores when the temperature inside the battery is near 130 ° C. It has been suggested that if the temperature further increases due to some situation, the polyolefin itself may melt and short-circuit, causing thermal runaway. In view of this, a heat-resistant separator that does not melt and shrink even at temperatures close to 200 ° C. has been developed.

  As heat-resistant separators, there are non-woven fabrics composed of polyester fibers, and non-woven fabrics in which aramid fibers, which are heat-resistant fibers, are blended with polyester-based fibers. References 1-3). On the other hand, examples in which a nonwoven fabric or a woven fabric is used as a base material, and various composite treatments are performed on these base materials to form a lithium ion secondary battery separator have been reported. For example, separators made by laminating a film-like porous film made of polyolefin on a nonwoven fabric substrate composed of polyester fibers and composited, or applying inorganic particles, resin, etc. to a substrate such as nonwoven fabric or woven fabric There has been reported a separator having heat resistance by performing a composite treatment (see, for example, Patent Documents 4 to 6). However, the non-woven fabric used as the base material has large pores and low surface smoothness, so the surface variation when combined by surface coating is large, and composites such as filler particles and resins There was a quality problem such as being likely to drop off.

  In order to improve the denseness of the base material, a base material made of a nonwoven fabric in which the average fiber diameter of the synthetic resin short fibers constituting the base material is reduced and fibers having a specific fiber diameter and fiber length are contained has been reported. (For example, refer to Patent Documents 7 and 8). However, depending on the conditions for producing the base material, the dispersibility of the fibers may deteriorate, and the uniformity of the base material may be impaired. Moreover, the base material which mix | blended the fibrillated fiber as a fiber which comprises a base material, and improved the denseness more is also reported (for example, refer patent documents 9-11). However, in the base material blended with fibrillated fibers, the strength of the base material may be insufficient depending on the processing conditions of the composite treatment, and as a result, the strength and uniformity of the separator may be insufficient, A further improvement in the strength of the substrate is desired.

JP 2003-123728 A JP 2007-317675 A JP 2006-19191 A JP 2005-293891 A JP 2005-536857 A JP 2007-157723 A JP 2009-230975 A JP 2011-82148 A JP 2012-3873 A JP 2011-187346 A International Publication No. 2011/46066 Pamphlet

  An object of the present invention is a lithium ion secondary battery separator in which a nonwoven fabric is used as a base material and inorganic particles are coated on at least one surface of the base material. It is in providing the lithium ion secondary battery separator excellent in the.

(1) It is characterized in that a nonwoven fabric containing synthetic resin short fibers and wet-made using a deaerated fiber slurry is used as a base material, and inorganic particles are applied to at least one surface of the base material surface. A lithium ion secondary battery separator,
(2) The lithium ion secondary battery separator according to (1) above, wherein the white water generated in the process of wet papermaking is degassed and used for wet papermaking,
(3) A modified freeness measured in accordance with JIS P8121 is 450 ml except that an 80 mesh wire net having a wire diameter of 0.14 mm and an aperture of 0.18 mm is used as a sieve plate and the sample concentration is 0.1% by mass. The lithium ion secondary battery separator according to (1) or (2), wherein the nonwoven fabric contains the following synthetic resin ultrashort fibers:
I found.

  In the lithium ion secondary battery separators (1) to (3) of the present invention, inorganic particles are applied to at least one surface of a substrate. A lithium ion secondary battery separator (1) according to the present invention is characterized in that the base material is a nonwoven fabric containing a synthetic resin short fiber and wet-paper-made using a deaerated fiber slurry. Compared to a lithium ion secondary battery separator, the processability when inorganic particles are applied to the substrate surface is good, and the strength and uniformity are excellent. Further, the base material (2) for the lithium ion secondary battery separator of the present invention, which is used for degassing white water generated in the process of wet paper making of the nonwoven fabric and wet paper making, has inorganic particles on the surface of the base material. Workability when applied is better, and strength and uniformity are better.

  The lithium ion secondary battery separator (3) of the present invention conforms to JIS P8121, except that an 80 mesh wire net having a wire diameter of 0.14 mm and an aperture of 0.18 mm is used as a sieve plate, and the sample concentration is 0.1% by mass. By including a synthetic resin ultrashort fiber having a modified freeness measured in 450 ml or less, the workability, strength, and uniformity when the inorganic particles are applied to the substrate surface are further improved.

  Hereinafter, the lithium ion secondary battery separator of the present invention will be described in detail.

  The lithium ion secondary battery separator of the present invention comprises a synthetic resin short fiber, a non-woven fabric made by wet paper making using a deaerated fiber slurry, and inorganic particles on at least one surface of the substrate surface Is applied.

  In the present invention, the base material is prepared by dispersing synthetic resin short fibers in water, then diluting and dispersing stepwise, feeding the fiber slurry to the wire part, dehydrating the fiber slurry in the wire part, and wet paper making Manufactured by. Thereafter, the wet paper is squeezed by the press part and dried by the dryer part. When mechanical action is applied to the synthetic resin short fibers and dispersed in water, air is entrained and many bubbles are generated. In order to eliminate these bubbles, an antifoaming agent and an antifoaming agent are added, but even if relatively large bubbles can be erased, very small bubbles cannot be erased. In addition, when a fiber slurry in which synthetic resin short fibers are dispersed is fed or dehydrated by a wire part of a paper machine, a pressure reducing action is applied to the fiber slurry, so that small bubbles or slurry present in the slurry are added. The gas dissolved in it expands to generate new bubbles. As a result, foam-derived pinholes are generated in the base material, and the uniformity of the base material is impaired. A base material having a pinhole tends to concentrate stress and strain on the pinhole portion, and as a result, the mechanical strength of the base material is inferior. In addition, when inorganic particles are applied to the surface of the substrate, the coating liquid escapes from the pinhole portion of the substrate to the opposite surface, deteriorating workability during coating, and the uniformity of the separator Impairs sex.

  In order to solve the above-mentioned problems, by using a fiber slurry that has been degassed, wet paper making a nonwoven fabric as a base material, a base material excellent in workability and strength can be obtained. A lithium ion secondary battery separator excellent in uniformity can be provided.

  In this invention, the deaeration process performed when manufacturing a base material is performed in the preparation process of a fiber slurry. For example, it is carried out alone or in combination in a dispersion process of synthetic resin short fibers as raw materials, a dilution process of synthetic resin short fiber slurries, and a process of feeding raw materials to the wire part of a paper machine. The degree of deaeration treatment is preferably at least a degree of treatment that reduces the amount of dissolved gas in the slurry. As a method for deaeration treatment in the present invention, there are a method for treating a fiber slurry in which a synthetic resin short fiber slurry and water are mixed, and a method for mixing a synthetic resin short fiber slurry and deaerated diluted water. As an apparatus for performing a deaeration process, for example, a decurator (registered trademark, manufactured by Andritz) is mixed as an apparatus for degassing a fiber slurry in which a synthetic resin short fiber slurry and water are mixed. As a device to be used, a POM system (manufactured by Aikawa Tekko Co., Ltd.), a deaeration pump (manufactured by Yokota Seisakusho), a vacuum deaerator (registered trademark, manufactured by Matsubo Co., Ltd.) or the like can be applied.

In the present invention, when producing a base material, white water generated in the process of wet paper making of the nonwoven fabric is degassed and used for wet paper making, for the following reasons, workability, strength, uniform It is possible to provide a substrate and a separator with better properties.
(1) It is white water that circulates that is considered to have the largest amount of water in the wet papermaking process, and reducing the dissolved gas in the white water has a great effect of reducing pinholes in the base material.
(2) It is thought that many oil agents, surfactants, etc. adhering to the fiber which is easy to generate | occur | produce a bubble are contained, and generation | occurrence | production of a bubble can be suppressed by reducing the dissolved gas in white water.
(3) When the whole fiber slurry is deaerated, fiber twist or the like may occur. By treating white water with a small amount of fiber, generation of defects due to fiber twist can be suppressed.

  Furthermore, in the present invention, the nonwoven fabric used as the base material is JIS P811, except that an 80 mesh wire net having a wire diameter of 0.14 mm and an aperture of 0.18 mm is used as the sieve plate, and the sample concentration is 0.1% by mass. By incorporating synthetic resin ultra-short fibers (hereinafter sometimes abbreviated as “synthetic resin ultra-short fibers”) having a modified freeness measured in accordance with 450 ml or less, the uniformity of the substrate is further improved. When the inorganic particles are applied, it is possible to prevent the coating liquid from coming off on the opposite surface.

  In the present invention, the freeness of the synthetic resin ultrashort fibers is JIS P811, except that an 80 mesh wire mesh having a wire diameter of 0.14 mm and an aperture of 0.18 mm is used as a sieve plate and the sample concentration is 0.1% by mass. It measured by the method according to. When a synthetic resin ultrashort fiber having a modified drainage degree higher than 450 ml is blended, the fineness and uniformity of the base material may not be improved. It may fall out on the opposite side.

  The fiber length of the synthetic resin ultrashort fiber blended with the base material is preferably 0.3 to 1.0 mm, and more preferably 0.3 to 0.6 mm. If it is less than 0.3 mm, the synthetic resin ultrashort fibers may fall off the wire during wet paper making, and the denseness and uniformity may not change. If the thickness exceeds 1.0 mm, the uniformity may not change, or defects may occur due to fiber twist.

  In the present invention, the preferable content of the synthetic resin ultrashort fiber is 1 to 30% by mass, and more preferably 5 to 20% by mass with respect to the entire nonwoven fabric. If it is less than 1% by mass, the denseness and uniformity may not be changed as compared with the case where the synthetic resin ultrashort fibers are not contained. Although it may exceed 30% by mass, the improvement effect is saturated when it exceeds 30% by mass.

The basis weight of the base material of the present invention is preferably 6.0 to 30.0 g / m ² . When it exceeds 30.0 g / m ² , the resistance value when used as a separator increases. If it is less than 6.0 g / m ² , it will be difficult to obtain uniformity. More preferably, it is 8.0-20.0 g / m < ² >. The basis weight means a basis weight based on a method defined in JIS P 8124 (paper and paperboard—basis weight measurement method).

  The synthetic resin short fibers may be heat-bonded short fibers (short fibers for binders) or non-heat-bonded short fibers. When used as a heat-sealing short fiber, a core-sheath type, an eccentric type, a side-by-side type, a sea-island type, an orange type, a multiple bimetal type composite fiber, or a single component type can be mentioned, but it is said that uniformity is obtained In particular, the single component type is preferable.

  Synthetic resin short fibers and resins constituting the synthetic resin ultrashort fibers include polyolefin resins, polyester resins, polyvinyl acetate resins, ethylene-vinyl acetate copolymer resins, polyamide resins, acrylic resins, and polychlorinated resins. Vinyl resin, polyvinylidene chloride resin, polyvinyl ether resin, polyvinyl ketone resin, polyether resin, polyvinyl alcohol resin, diene resin, polyurethane resin, phenol resin, melamine resin, furan resin, Examples include urea resins, aniline resins, unsaturated polyester resins, alkyd resins, fluorine resins, and silicone resins. Of these, polyester resins, acrylic resins, and polyolefin resins are preferable from the viewpoint of strength characteristics. In particular, a polyester resin is preferable.

  Examples of the polyester resin include polyethylene terephthalate, polybutylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, and polyethylene isophthalate. These may be used alone or in combination of two or more. Among these, when used for a base material for a lithium ion secondary battery separator, a polyethylene terephthalate resin having excellent heat resistance is preferable.

  In the present invention, as a method for producing a nonwoven fabric, a method in which a fiber web is formed and the fibers in the fiber web are bonded, fused, and entangled can be used. The obtained nonwoven fabric may be used as it is or may be used as a laminate comprising a plurality of sheets. A wet papermaking method is used as a method for producing the fiber web. The wet papermaking method is a method in which fibers are dispersed in water to form a uniform papermaking slurry, and this papermaking slurry is used to obtain a fiber web using a papermaking machine having at least one of a wire such as a circular net, a long net, and an inclined type. It is.

  As a method for producing a nonwoven fabric from a fibrous web, a hydroentanglement method, a needle punch method, a binder adhesion method, or the like can be used. In particular, when the wet method is used with emphasis on uniformity, it is preferable to bond the heat-bonded short fibers by performing a binder bonding method. A uniform nonwoven fabric is formed from a uniform web by the binder bonding method. It is preferable to adjust the thickness or make the thickness uniform by applying pressure to the non-woven fabric that has been wet-papered in this way with a calendar or the like. However, it is preferable to pressurize at a temperature at which the heat-bonded short fibers do not form a film (a temperature lower by 20 ° C. or more than the melting point or softening point of the heat-bonded short fibers).

  In the present invention, the fiber length of the synthetic resin short fibers other than the synthetic resin ultrashort fibers is preferably more than 2 mm and 7 mm or less, more preferably 3 to 6 mm, and still more preferably 3 to 5 mm. When the fiber length exceeds 7 mm, there is a problem that dispersion of fibers in the production of the nonwoven fabric may be difficult due to the balance with the average fiber diameter, a formation defect occurs, and a good nonwoven fabric cannot be produced. May occur. On the other hand, when the fiber length of the synthetic resin short fiber is 2 mm or less, the strength necessary for the base material may not be exhibited.

  In the present invention, the fineness of the synthetic resin short fibers is preferably 0.007 to 1.3 dtex, more preferably 0.02 to 1.1 dtex, and further preferably 0.04 to 0.8 dtex. When the fineness of the synthetic resin short fibers exceeds 1.3 dtex, the number of fibers in the thickness direction decreases, and thus the required denseness of the base material may not be ensured. When the fineness of the synthetic resin short fiber is less than 0.007 dtex, stable production of the fiber becomes difficult.

  The lithium ion secondary battery separator of the present invention is formed by coating inorganic particles on at least one surface of the substrate surface. When inorganic particles are applied, a binder resin may be applied together in some cases. As inorganic particles, alumina such as α-alumina, β-alumina, and γ-alumina, hydrated alumina such as boehmite, magnesium oxide, calcium oxide, and the like can be used. Among these, α-alumina or alumina hydrate is preferably used in terms of high stability to the electrolyte used in the lithium ion battery. As the binder resin, various synthetic resins such as styrene-butadiene resin, acrylic ester resin, methacrylic ester resin, and fluororesin such as polyvinylidene fluoride can be used.

  In the present invention, the coating liquid used when the inorganic particles are applied includes, in addition to the inorganic particles and the binder resin, various dispersants such as polyacrylic acid and sodium carboxymethyl cellulose, hydroxyethyl cellulose, sodium carboxymethyl cellulose, and polyethylene. Various thickeners such as oxide, various additives such as various wetting agents, preservatives and antifoaming agents can be blended as necessary. Among these additives, agents such as thickeners and wetting agents can be suitably used for adjusting the degree of penetration of the coating liquid in the present invention.

  In the present invention, there is no particular limitation on the coating method when the inorganic particles are applied. For example, conventionally known air doctor coater, blade coater, knife coater, rod coater, squeeze coater, impregnation coater, gravure coater, kiss roll Examples include a coater, a die coater, a reverse roll coater, a transfer roll coater, and a spray coater.

In the present invention, the coating amount of inorganic particles provided on at least one surface of the substrate surface (including binder resin and additives) is preferably 1.0 to 20.0 g / m ^{2, and} more preferably 4.0 to 4.0. 15.0 g / m ² is more preferable. If the coating amount is less than 1.0 g / m ² , the surface of the non-woven fabric cannot be sufficiently covered, the pore diameter becomes large, and a short circuit may occur. On the other hand, when the coating amount exceeds 20.0 g / m ² , it may be difficult to reduce the thickness of the separator.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a present Example. All parts and percentages in the examples are based on mass unless otherwise specified.

<Manufacture of non-woven fabric>
In Examples 1 to 12 and Comparative Examples 1 and 2, the nonwoven fabric used as a base material prepared a fiber-blended slurry described in Table 1, and subjected to a fiber slurry deaeration process according to the method described in Table 1. . The fiber slurry that has been degassed is made up by a wet method using an inclined wire, and a polyethylene terephthalate (PET) short fiber for binder is bonded to the binder by a cylinder dryer at 120 ° C. to express the sheet strength. A nonwoven fabric having 10 g / m ² or 8 g / m ² and a width of 50 cm was produced. Next, a calendar process was performed at a roll temperature of 190 ° C. to produce a nonwoven fabric having a basis weight of 10 g / m ² and a thickness of 17 μm, or a basis weight of 8 g / m ² and a thickness of 14 μm.

<Manufacture of lithium ion secondary battery separator>
100 parts of boehmite having a volume average particle diameter of 2.3 μm and a specific surface area of 3 m ² / g are sufficiently mixed with 120 parts of a 0.3% aqueous solution of carboxymethylcellulose sodium salt having a viscosity of 200 mPa · s at 25 ° C. Then, 300 parts of a 0.5% aqueous solution of carboxymethyl cellulose sodium salt having a viscosity of 7000 mPa · s at 25 ° C. of the 1% by mass aqueous solution and carboxy-modified styrene having a glass transition point of 5 ° C. and a volume average particle size of 0.2 μm -A coating solution was prepared by mixing and stirring 10 parts of a butadiene resin (SBR) emulsion (solid content concentration 50%). Using the gravure coater, this coating liquid was coated on the one surface of the base material and dried so that the dry coating amount was 15 g / m ² , and Examples 1 to 12 were used. The lithium ion secondary battery separator of Comparative Examples 1-2 was manufactured.

<Evaluation>
The following evaluation was performed about the base material and lithium ion secondary battery separator which were obtained by the Example and the comparative example, and the result was shown in Table 2.

Strength:
The base material of an Example and a comparative example was cut and aligned in 50 mm width strip shape. The test piece was mounted on a 40 mmφ fixed frame installed on a tabletop material testing machine (trade name: STA-1150, manufactured by Orientec Co., Ltd.), and the tip was rounded (curvature: 1.6) with a diameter of 1.0 mm. The metal needle (manufactured by Orientec Co., Ltd.) was lowered at a constant speed of 50 mm / min. The maximum load (g) at this time was measured and used as the puncture strength. Measure puncture strength at 5 or more locations for one sample, and the lowest puncture strength among all measured values is excellent in strength if it is 1.0 N or more, and practical if it is 0.5 or more and less than 1.0 N If it can be used above and less than 0.5N, it is considered weak in strength.

Uniformity:
The uniformity of the separators of the examples and comparative examples was visually evaluated with respect to the separator having an area of 150 mm width and 1000 mm length by the following index. Practically, it was judged that it could be used if it was “3” or more.
“5”: Both surfaces of the separator are excellent in uniformity.
“4”: Very small protrusions and irregularities are occasionally seen on both sides of the separator.
“3”: Very small protrusions and irregularities are seen on both sides of the separator.
“2”: Small protrusions and irregularities are seen on both sides of the separator.
“1”: Clear protrusions and irregularities are seen on both sides of the separator.

Processability:
In the examples and comparative examples, the working efficiency when coating the coating liquid on the surface of the nonwoven fabric used as the substrate using a gravure coater was evaluated by the following index as workability. Practically, it was judged that it could be used if it was “3” or more.
“5”: The coating liquid does not penetrate the nonwoven fabric, and the coating roll of the coater is not soiled.
“4”: The coating liquid does not penetrate the nonwoven fabric, but sometimes it is necessary to clean the coating roll of the coater.
“3”: The coating solution sometimes penetrates the nonwoven fabric. Operation is possible while washing the coater's backing roll.
“2”: The coating liquid penetrates part of the nonwoven fabric, and the coating roll of the coater partially becomes white. Occasionally stop and roll cleaning is required.
“1”: The coating liquid penetrates through the nonwoven fabric, the coating roll of the coater becomes white, and the operability is considerably poor. Stop frequently and need roll cleaning.

  By comparing Examples 1 to 4 and 6 with Comparative Example 1 and using a nonwoven fabric obtained by wet papermaking the deaerated fiber slurry, the inorganic particles are coated on the substrate. It can be seen that a lithium ion secondary battery separator having excellent workability and excellent strength and uniformity can be provided.

  By comparing Examples 1 to 3 with Examples 4 and 5, the step of dispersing the fibers (corresponding to the degassing treatment “raw material” in Table 1) and the step of diluting the fibers (degassing treatment “Dilution in Table 1” In this case, the substrate is coated with inorganic particles by degassing in one of the steps of feeding the fiber to the head box (corresponding to the degassing treatment “whole” in Table 1). It can be seen that a lithium ion secondary battery separator having excellent processability and excellent strength and uniformity can be provided. In addition, the white water generated from the paper machine is degassed in the process in which the nonwoven fabric is subjected to wet paper making, and it is used during wet paper making. It can be seen that a lithium ion secondary battery separator superior in strength and uniformity can be provided. Furthermore, a lithium ion secondary battery separator that can be used practically is provided from the viewpoints of workability when the inorganic particles are coated on the substrate, strength of the separator, and uniformity even when the basis weight of the substrate is lowered. can do.

  By comparing Examples 4, 5, 7, 8 and Comparative Example 2 with Examples 9 to 12, a fiber slurry containing synthetic resin ultrashort fibers having a modified freeness of 450 ml or less was deaerated. It can be seen that by wet papermaking, the processability when the inorganic particles are coated on the base material is further improved, and a lithium ion secondary battery separator further excellent in strength and uniformity can be provided. Moreover, even if the basis weight of the substrate is lowered, it is possible to provide an excellent lithium ion secondary battery separator from the viewpoints of workability when the inorganic particles are applied to the substrate, the strength of the separator, and uniformity. it can.

  The base material for lithium ion secondary battery separators and the lithium ion secondary battery separator of the present invention can be suitably used for lithium ion secondary batteries such as lithium ion secondary batteries and lithium ion polymer secondary batteries.

Claims (3)

  Lithium comprising a synthetic resin short fiber, a non-woven fabric made by wet papermaking using a deaerated fiber slurry, and having inorganic particles coated on at least one surface of the substrate surface Ion secondary battery separator.   2. The lithium ion secondary battery separator according to claim 1, wherein the separator is used when white paper generated in a process of wet paper making is subjected to deaeration treatment and wet paper making.   Synthesis with a modified freeness of 450 ml or less measured according to JIS P8121, except that an 80-mesh wire mesh with a wire diameter of 0.14 mm and an aperture of 0.18 mm was used as the sieve plate, and the sample concentration was 0.1% by mass. The lithium ion secondary battery separator according to claim 1 or 2, wherein the nonwoven fabric contains resin ultrashort fibers.