JP2021144838A - Base material for lithium secondary battery separator - Google Patents

Abstract

To provide a base material for a lithium secondary battery separator in which a coating liquid does not easily strike through.SOLUTION: A base material for a lithium secondary battery separator made of a non-woven fabric containing at least a main synthetic fiber and a binder synthetic fiber as synthetic fibers contains a non-fibrillated synthetic fiber having a fiber length of 0.2 mm or less.SELECTED DRAWING: None

Description

The present invention relates to a substrate used for a lithium secondary battery separator. Hereinafter, the "lithium battery separator base material" may be abbreviated as "base material".

For electrochemical elements such as lithium secondary batteries, aluminum electrolytic capacitors, electric double layer capacitors, and lithium ion capacitors, electrochemical element separators for preventing contact between electrode plates are used.

In particular, a polyolefin-based microporous film such as polypropylene or polyethylene is often used as a lithium secondary battery separator (hereinafter, "lithium secondary battery separator" may be abbreviated as "separator"). These microporous membranes close the pores by melting the membrane due to heat, and utilize the shutdown effect that insulates the positive electrode and the negative electrode to give the separator safety. However, when such an olefin-based microporous membrane is used as a separator, there is a problem that when the battery overheats, it melts and shrinks, the function of separating the positive and negative electrodes is lost, and a significant internal short circuit occurs. ..

As a heat-resistant separator that does not easily melt or shrink even when the battery overheats, a separator made by coating inorganic particles such as alumina, silica, and boehmite on a base material made of woven fabric or non-woven fabric has been proposed. (See, for example, Patent Document 1). However, in the separator of this technology, when the amount of coating of inorganic particles is increased in order to improve heat resistance, the coating liquid exudes to the back surface of the base material (hereinafter, the phenomenon is referred to as "the back of the coating liquid". There is a problem that it is difficult to obtain a uniform separator because the roll of the coating apparatus is soiled and the dry solid material of the coating liquid adhering to the roll is reattached to the separator.

As a separator having high heat resistance and preventing strike-through of the coating liquid, non-fibrillated fibers having a fiber length of 0.10 to 1.20 mm and a fiber diameter of 1.0 μm or less are contained together with inorganic particles and an organic polymer. Separators using a coating liquid have been proposed (see, for example, Patent Document 2). However, the fibers contained in the coating liquid may adhere to the coating apparatus, and the amount of the coating liquid adhered to the corresponding portion may be insufficient or excessive. Therefore, the technique described in Patent Document 2 has a problem that it is difficult to form a uniform coating layer and linear defects called "streaks" are likely to occur in the insulating layer.

Further, a non-woven fabric group comprising a first layer having a water contact angle of 125 ° or more and a second layer having a water contact angle of 120 ° or less, and at least one surface having an outermost layer as a second layer. The material and at least one of the raw material fibers are composite fibers formed of a low melting point component made of a polyolefin resin and a high melting point component made of a thermoplastic resin having a melting point higher than the low melting point component by 20 ° C. or more. A battery separator having a non-woven fabric base material and an insulating layer containing inorganic particles in the base material has been proposed (see, for example, Patent Document 3). This separator needs to be subjected to a water-repellent treatment or a hydrophilic treatment on the fiber raw material, which causes a problem that the work process becomes complicated and the cost becomes high. In addition, when a chemical is used for the water-repellent treatment or the hydrophilic treatment, it may be decomposed in the battery and the battery characteristics may be deteriorated.

Further, as a base material having high heat resistance and preventing composites such as inorganic particles and resins from falling off, polyester-based ultrashort fibers having an average fiber diameter of 5 μm or less and a fiber length of 2 mm or less are contained as essential components. A base material characterized by the above has been proposed (see, for example, Patent Document 4). In the examples of Patent Document 4, the base material in which the content ratio of the polyester-based ultrashort fibers having a fiber length of 2.0 to 0.05 mm is 0.7 to 33% by mass with respect to the total fiber components contained in the base material is used. It is manufactured, and the impregnation coating method is used in the production of the separator. However, when the fiber length of the polyester-based ultrashort fiber exceeds 0.2 mm, the effect of preventing strike-through in the surface coating may be insufficient. Further, when the content ratio of the polyester-based ultrashort fibers of 0.2 mm or less is large, the internal resistance of the separator may increase.

Japanese Unexamined Patent Publication No. 2008-4443 Japanese Unexamined Patent Publication No. 2015-258069 JP-A-2017-45663 Japanese Unexamined Patent Publication No. 2011-82148

An object of the present invention is to provide a base material for a lithium secondary battery separator in which the coating liquid does not easily strike through.

The above problem has been solved by the following invention.

(1) As the synthetic fiber, a base material for a lithium secondary battery separator made of a non-woven fabric containing at least a main synthetic fiber and a binder synthetic fiber contains a non-fibrillated synthetic fiber having a fiber length of 0.2 mm or less. A base material for a lithium secondary battery separator.

(2) Main synthetic fiber, binder Synthetic fiber and non-fibrillated synthetic fiber having a fiber length of 0.2 mm or less The total length of the non-fibrillated synthetic fiber having a fiber length of 0.2 mm or less The base material for the lithium secondary battery separator according to (1) above, wherein the content is 0.1% or more and less than 2.0%.

The base material for a lithium secondary battery separator of the present invention is a base material for a lithium secondary battery separator made of a non-woven fabric containing at least a main synthetic fiber and a binder synthetic fiber as synthetic fibers, and has a fiber length of 0.2 mm or less. By containing the non-woven synthetic fiber, the effect of preventing strike-through of the coating liquid can be achieved.

It is a fiber length distribution figure of the slurry containing the main synthetic fiber, the binder synthetic fiber, and the synthetic fiber of a fiber length 0.2 mm or less. It is a microscope photograph of a main synthetic fiber and a synthetic fiber which is not fibrillated and has a fiber length of 0.2 mm or less.

The base material for the lithium secondary battery separator of the present invention is a base material for a lithium secondary battery separator from a non-woven fabric containing at least a main synthetic fiber and a binder synthetic fiber as synthetic fibers, and has a fiber length of 0.2 mm or less. It is characterized by containing synthetic fibers that are not fibrillated. In the present specification, "non-fibrillated synthetic fiber having a fiber length of 0.2 mm or less" may be referred to as "short fiber having a fiber length of 0.2 mm or less".

The base material for a lithium secondary battery separator of the present invention is preferably produced by producing a sheet by a wet fabrication method and then hot-pressing the sheet with a thermal roll. The base material of the present invention is a non-woven fabric containing a main synthetic fiber and a binder synthetic fiber as synthetic fibers. It also contains short fibers of 0.2 mm or less.

FIG. 1 shows the fiber length of a fiber slurry containing a main synthetic fiber, a binder synthetic fiber, and a short fiber of 0.2 mm or less. It is a fiber length distribution map measured by the fiber quality analyzer (FQA-360) manufactured by the company. The total length of the short fibers of 0.2 mm or less was 0.5% with respect to the total length of the main synthetic fiber, the binder synthetic fiber and the short fibers of 0.2 mm or less.

FIG. 2 is a photograph of a main synthetic fiber and a short fiber of 0.2 mm or less taken with a microscope at a magnification of 200 times. The fibers in the circle are synthetic fibers having a fiber length of 0.2 mm or less, and the other fibers are main synthetic fibers.

The main synthetic fiber in the present invention is a fiber that forms the skeleton of the base material. Resins constituting the main synthetic fiber include polyolefin-based, polyester-based, polyvinyl acetate-based, ethylene-vinyl acetate copolymer-based, polyamide-based, acrylic-based, polyvinyl chloride-based, polyvinylidene chloride-based, polyvinyl ether-based, and polyvinyl. Examples thereof include resins such as ketones, polyethers, polyvinyl alcohols, polyesteramides, polyether ether ketones, and ethylene-vinyl alcohol copolymers. Derivatives of these resins can also be used. Among these resins, it is preferable to use a polyester resin, an acrylic resin, or a polyolefin resin in order to improve the adhesiveness with the coating layer. Further, in order to improve the heat resistance of the separator, it is preferable to use a polyester resin, an acrylic resin, or a polyamide resin.

The diameter of the main synthetic fiber is not particularly limited, but is preferably 0.1 to 15 μm, more preferably 0.5 to 10 μm, and particularly preferably 1 to 8 μm. When the diameter of the main synthetic fiber is less than 0.1 μm, the base material becomes too dense, the coating layer does not easily penetrate into the base material, and the adhesiveness between the coating layer and the base material becomes insufficient. Sometimes. If the diameter of the main synthetic fiber is larger than 15 μm, it may be difficult to reduce the thickness, or the base material may be easily strike-through.

The fiber length of the main synthetic fiber of the present invention is not particularly limited, but is preferably more than 0.2 mm and 10 mm or less, more preferably 1 to 6 mm, and further preferably 3 to 5 mm. The cross-sectional shape of the main synthetic fiber is preferably circular, and the cross-sectional aspect ratio (fiber cross-sectional major axis / fiber cross-sectional minor axis) of the fiber before dispersion in water in the papermaking process is preferably 1.0 to less than 1.2. .. When the fiber cross-sectional aspect ratio is 1.2 or more, the fiber dispersibility may be lowered, or the uniformity of the base material and the smoothness of the coated surface may be adversely affected by the occurrence of entanglement or entanglement of the fibers. However, fibers having irregular cross sections such as T-type, Y-type, and triangular can also be contained within a range that does not impair other characteristics such as fiber dispersibility in order to prevent strike-through and surface smoothness.

The aspect ratio (fiber length / diameter) of the main synthetic fiber is preferably 10 to 4500, more preferably 20 to 2000, and further preferably 40 to 1500. When the aspect ratio is less than 10, the dispersibility of the fibers is good, but the fibers may fall off from the base material, or the fibers may pierce the papermaking wire and the peelability from the wire may deteriorate. On the other hand, when the aspect ratio exceeds 4500, although it contributes to the formation of a three-dimensional network of fibers, the uniformity of the base material and the smoothness of the surface may be adversely affected by the occurrence of entanglement and entanglement of the fibers.

In the present invention, the content ratio of the main synthetic fiber is preferably 30% by mass or more and less than 90% by mass, and more preferably 40% by mass or more and less than 80% by mass with respect to the total fiber components contained in the base material. It is preferably 50% by mass or more and less than 70% by mass. When the content ratio of the main synthetic fiber is less than 30% by mass, wrinkles may occur on the base material in the step of raising the temperature until the binder synthetic fiber is softened or melted. Further, when the content of the main synthetic fiber is 90% by mass or more, the strength of the base material may be insufficient.

The base material for a lithium secondary battery separator of the present invention contains a binder synthetic fiber. By incorporating a step of raising the temperature until the binder synthetic fiber is softened or melted into the base material manufacturing process, the binder synthetic fiber improves the mechanical strength of the base material. For example, the base material can be produced by a wet manufacturing method, and the binder synthetic fiber can be softened or melted in a subsequent drying step.

Examples of the binder synthetic fiber include core-sheath fibers (core-shell type), parallel fibers (side-by-side type), composite fibers such as radial split fibers, and undrawn fibers. Since the composite fiber is difficult to form a film, the mechanical strength can be improved while maintaining the voids of the base material. More specifically, a combination of polypropylene (core) and polyethylene (sheath), a combination of polypropylene (core) and ethylene vinyl alcohol (sheath), a combination of high melting point polyester (core) and low melting point polyester (sheath), polyester, etc. Undrawn fibers can be mentioned. In addition, single fibers (zen'yu type) composed only of low melting point resins such as polyethylene and polypropylene, and hot water-soluble binders such as polyvinyl alcohols are easy to form a film in the drying process of the base material, but have characteristics. Can be used as long as it does not interfere with. In the present invention, a combination of a high melting point polyester (core) and a low melting point polyester (sheath), and undrawn polyester fibers can be preferably used.

The diameter of the binder synthetic fiber is not particularly limited, but is preferably 0.1 to 15 μm, more preferably 0.5 to 10 μm, and further preferably 1 to 8 μm. If the diameter of the binder synthetic fiber is less than 0.1 μm, the strength of the base material may be insufficient. If the diameter of the binder synthetic fiber is larger than 15 μm, it may be difficult to reduce the thickness. In addition to the role of improving the mechanical strength of the base material, the binder synthetic fiber also plays a role of forming a uniform three-dimensional network together with the main synthetic fiber. Further, in the step of raising the temperature until the binder synthetic fiber is softened or melted, the smoothness of the base material can also be improved, and it is more effective when the step is accompanied by pressurization.

The fiber length of the binder synthetic fiber is not particularly limited, but is preferably more than 0.2 mm and 10 mm or less, more preferably 1 to 6 mm, still more preferably 3 to 5 mm. The cross-sectional shape of the binder synthetic fiber is preferably circular, but fibers having irregular cross-sections such as T-shaped, Y-shaped, and triangular fibers also have a range that does not impair other characteristics in order to prevent strike-through and smoothness of the coated surface. Can be contained.

The aspect ratio (fiber length / diameter) of the binder synthetic fiber is preferably 10 to 4500, more preferably 20 to 2000, and further preferably 40 to 1500. When the aspect ratio is less than 10, the dispersibility of the fibers is good, but the fibers may fall off from the base material, or the fibers may pierce the papermaking wire and the peelability from the wire may deteriorate. On the other hand, when the aspect ratio exceeds 4500, although it contributes to the formation of a three-dimensional network of fibers, the uniformity of the base material and the smoothness of the surface may be adversely affected by the occurrence of entanglement and entanglement of the fibers.

In the present invention, the content ratio of the binder synthetic fiber is preferably 10% by mass or more and less than 70% by mass, and more preferably 20% by mass or more and less than 60% by mass with respect to the total fiber components contained in the base material. It is preferably 30% by mass or more and less than 50% by mass. If the content ratio of the binder synthetic fiber is less than 10% by mass, the strength of the base material is lowered, and there is a possibility that breakage may occur in the manufacturing process of the base material. On the other hand, when the content of the binder synthetic fiber is 70% by mass or more, the surface of the base material is easily formed into a film, the holding property of the electrolytic solution is lowered, and the separator may have a high internal resistance.

In the present invention, the short fibers of 0.2 mm or less are fibers that supplement the skeleton formation of the base material. Resins constituting short fibers of 0.2 mm or less include polyolefin-based, polyester-based, polyvinyl acetate-based, ethylene-vinyl acetate copolymer-based, polyamide-based, acrylic-based, polyvinyl chloride-based, polyvinyl chloride-based, and polyvinyl chloride. Examples thereof include resins such as ether-based, polyvinyl-ketone-based, polyether-based, polyvinyl alcohol-based, polyesteramide-based, polyether ether-ketone-based, and ethylene-vinyl alcohol copolymer-based resins. Derivatives of these resins can also be used. Among these resins, it is preferable to use a polyester resin, an acrylic resin, or a polyolefin resin in order to improve the adhesiveness with the coating layer. Further, in order to improve the heat resistance of the separator, it is preferable to use a polyester resin, an acrylic resin, or a polyamide resin.

In the present invention, as will be described later, since the short fiber of 0.2 mm or less is a main synthetic fiber or a binder synthetic fiber shortened by cutting in the fiber manufacturing process or the wet fabrication method, the main synthetic fiber or the binder The preferred diameter of the synthetic fiber and the preferred diameter of the short fiber of 0.2 mm or less are in the same range.

In the present invention, the fiber length of a short fiber of 0.2 mm or less is 0.2 mm or less, preferably 0.05 to 0.2 mm, and more preferably 0.1 to 0.2 mm. The cross-sectional shape of short fibers of 0.2 mm or less is preferably circular, but short fibers of 0.2 mm or less having irregular cross-sections such as T-type, Y-type, and triangle can also be used to prevent strike-through of the coating liquid and to prevent strike-through of the coating layer. Can be included for surface smoothness. The short fibers of 0.2 mm or less exist in the non-woven fabric in contact with the main synthetic fiber and the binder synthetic fiber, and control the voids of the base material. By being present in the voids between the binder synthetic fibers and at the contact points between the main synthetic fibers and the binder synthetic fibers, the voids are subdivided to prevent strike-through of the coating liquid and improve the surface smoothness of the coating layer.

The short fibers of 0.2 mm or less are main synthetic fibers or binder synthetic fibers cut by the clearance between the stirring blade and the container, which are rotated by a device such as a pulper that disperses in water in the wet manufacturing method. Further, it is a main synthetic fiber or a binder synthetic fiber that has been cut short in the fiber manufacturing process.

In the base material of the present invention, the content ratio of the short fibers of 0.2 mm or less is "the total length of the short fibers of 0.2 mm or less with respect to the total length of the main synthetic fiber, the binder synthetic fiber and the short fibers of 0.2 mm or less. It is represented by "ratio of length". The content ratio of the short fibers of 0.2 mm or less is preferably 0.1% or more and less than 2.0%, more preferably 0.2% or more and less than 1.5%, and further preferably 0.2%. More than 1.0%. If the content ratio of short fibers of 0.2 mm or more is less than 0.1%, the effect of preventing strike-through of the coating liquid may not be obtained. On the other hand, when it is 2.0% or more, short fibers of 0.2 mm or less are likely to fall off from the base material and cause contamination, which may reduce the separator performance. Content ratio of short fibers of 0.2 mm or less in the present invention (ratio of total length of short fibers of 0.2 mm or less to total length of main synthetic fiber, binder synthetic fiber and short fiber of 0.2 mm or less) The fiber length of the pulper-dispersed slurry was changed to OpTest Equipment Inc. It is a value calculated based on the length-weighted fiber length data measured by a fiber quality analyzer (FQA-360) manufactured by the same company.

In the base material of the present invention, fibers other than the above-mentioned main synthetic fiber, binder synthetic fiber, and short fiber may be added, if necessary. For example, short fibers and fibrils of solvent-spun cellulose and regenerated cellulose, pulped and fibrillated natural cellulose fibers, polyolefins, acrylics, total aromatic polyesters, total aromatic polyesteramides, polyamides, semi-aromatic polyamides, and total aromatics. Polyamide, total aromatic polyether, total aromatic polycarbonate, total aromatic polyazomethine, polyimide, polyamideimide (PAI), polyetheretherketone (PEEK), polyphenylene sulfide (PPS), poly-p-phenylene benzobisoxazole ( It may contain a pulped product, a fibrillated product, or a divided product made of a resin such as PBO), polybenzoimidazole (PBI), or polytetrafluoroethylene (PTFE). These fibers may contain one kind or two or more kinds. Semi-aromatic refers to those having, for example, a fat chain in a part of the main chain. The total aromatic polyamide may be either para-type or meta-type. Among these fibers, in order to improve the heat resistance of the base material, it is particularly preferable to contain a fibrillated product of a total aromatic polyamide.

The method for producing the base material for the lithium secondary battery separator of the present invention will be described. In the base material for a lithium secondary battery separator of the present invention, it is preferable that after a sheet is produced by a wet fabrication method, the sheet is thermally pressure-processed by a thermal roll.

In the wet papermaking method, the fibers were first uniformly dispersed in water, and then the final fiber concentration was adjusted to 0.01 to 0.50% by mass through steps such as a screen (removal of foreign substances, lumps, etc.). The slurry is made with a paper machine to obtain wet paper. In the process, chemicals such as dispersants, antifoaming agents, hydrophilic agents, antistatic agents, polymer thickeners, mold release agents, antibacterial agents, and bactericidal agents may be added.

As the papermaking method, for example, a papermaking method such as a long net, a circular net, or an inclined wire type can be used. Use a paper machine having at least one paper making method selected from these paper making methods, or a combination paper machine in which two or more paper machines of the same type or different types selected from these paper making methods are installed online. can do. Further, in the case of producing a non-woven fabric having a multi-layer structure of two or more layers, a laminating method in which wet papers made by each paper machine are laminated, or after forming one sheet, the sheet is placed on the sheet. A method of casting a slurry in which fibers are dispersed can be used.

A sheet is obtained by drying the wet paper produced by the paper machine with a Yankee dryer, an air dryer, a cylinder dryer, a suction drum type dryer, an infrared type dryer, or the like. When the wet paper is dried, it is brought into close contact with a heat roll such as a Yankee dryer and heat-pressure dried, so that the smoothness of the adhered surface is improved. Hot pressure drying means drying by pressing wet paper against the hot roll with a touch roll or the like. The surface temperature of the heat roll is preferably 80 to 180 ° C, more preferably 100 to 160 ° C, and even more preferably 110 to 160 ° C. The pressure is preferably 5 to 100 kN / m, more preferably 10 to 80 kN / m.

In the present invention, a non-woven fabric containing the above-mentioned main synthetic fiber, binder synthetic fiber and short fiber of 0.2 mm or less can be used as a base material as it is. More preferably, the non-woven fabric is passed between the heated metal roll and the elastic roll, and is subjected to a heating / pressurizing treatment (thermal calendar treatment) to be used as a base material. Generally, as a thermal calendar processing device, there is a device composed of a metal heat roll (metal roll) having a heating mechanism and an elastic roll in which an elastic body layer is adhered to an outer peripheral surface of a metal core. This is for the purpose of expressing the mechanical strength of the non-woven fabric and adjusting the thickness by bringing the two rolls into contact with each other substantially in parallel, sandwiching the non-woven fabric between the two rolls, and rotating and running while applying a high nip pressure. It is used.

In the present invention, the metal roll temperature during the thermal calendar treatment is preferably −60 to −10 ° C., preferably −50 to −20 ° C. with respect to the melting point (melting temperature) or softening temperature of the binder synthetic fiber. Is more preferable.

For example, when the melting point of the binder synthetic fiber is 240 ° C., the temperature of the metal roll and the surface temperature of the base material are preferably 180 to 230 ° C., more preferably 190 to 220 ° C. If the temperature of the metal roll is lower than −60 ° C. with respect to the melting point or softening temperature of the binder synthetic fiber, the adhesion of the binder synthetic fiber tends to be insufficient, and the strength of the base material may decrease. On the other hand, if the temperature of the metal roll is higher than −10 ° C. with respect to the melting point or softening temperature of the binder synthetic fiber, the non-woven fabric tends to adhere to the metal roll, and there is a possibility that defects may occur on the surface of the base material.

The nip pressure during the thermal calendar treatment is preferably 19 to 180 kN / m, more preferably 39 to 150 kN / m. The processing speed is preferably 3 to 150 m / min, more preferably 5 to 100 m / min.

In the present invention, the basis weight of the base material is preferably from 5 to 20 g / ^{m 2,} more preferably 6~18g / ^{m 2,} more preferably 7~16g / ^{m 2.} If it exceeds 20 g / m ² , it may be difficult to thin the separator, and if ^{it is less than 5 g / m 2} , it may be difficult to obtain sufficient strength. The basis weight is measured based on the method specified in JIS P 8124: 2011 (paper and paperboard-basis weight measurement method).

In the present invention, the thickness of the base material is preferably 8 to 30 μm, more preferably 10 to 25 μm, and even more preferably 12 to 20 μm. If the thickness is less than 8 μm, the strength may be insufficient. On the other hand, if the thickness exceeds 30 μm, it becomes difficult to thin the separator.

In the present invention, the density of the base material (basis weight / thickness) is preferably 0.40~0.90g / cm ^3, more preferably 0.45~0.80g / cm ^3, 0.50~ 0.75 g / cm ³ is even more preferred. If the density is ^{less than 0.40 g / cm 3} , the strength may be insufficient. On the other hand, if the density ^{exceeds 0.90 g / cm 3} , the separator may have a high internal resistance.

The base material for a lithium secondary battery separator of the present invention is preferably used for a separator containing a base material and one or more selected from the group containing inorganic particles, organic particles, fine fibers, solid electrolytes and gel-like electrolytes. .. This separator can be produced by a method of applying a coating liquid containing inorganic particles, organic particles or fine fibers to a base material, or a method of applying a solid electrolyte or a gel-like electrolyte to a base material. As described above, the base material of the present invention does not serve as a separator as it is, and is a precursor sheet of a lithium secondary battery separator.

Inorganic particles include alumina, gibsite, boehmite, magnesium oxide, magnesium hydroxide, silica, titanium dioxide, barium titanate, zirconium oxide and other inorganic oxides, aluminum nitride and silicon nitride and other inorganic nitrides, aluminum compounds and zeolite. , Mica, etc.

Examples of the organic particles include particles such as polyethylene, polypropylene, polyacrylonitrile, polymethylmethacrylate, polyethylene oxide, polystyrene, polyvinylidene fluoride, ethylene-vinyl monomer copolymer, and polyolefin wax.

The medium for preparing the coating liquid containing the inorganic particles and the organic particles is not particularly limited as long as it can uniformly dissolve or disperse the inorganic particles, the organic particles and the like, and for example, an aromatic hydrocarbon such as toluene. , Ethers such as tetrahydrofuran, ketones such as methyl ethyl ketone, alcohols such as isopropyl alcohol, N-methyl-2-pyrrolidone (NMP), dimethylacetamide, dimethylformamide, dimethylsulfoxide, water, etc., as necessary. Can be done. Further, these media may be mixed and used if necessary. A medium that does not expand the base material or a medium that does not dissolve the base material is preferable.

As a method of applying a coating liquid containing inorganic particles or organic particles on a base material, for example, various coating methods such as blades, rods, reverse rolls, lips, dies, curtains, air knives, flexo, screens, offsets, etc. , Various printing methods such as gravure and inkjet, transfer methods such as roll transfer and film transfer, pulling methods such as dipping, and the like can be selected and used as necessary.

In the present invention, the lithium secondary battery refers to a secondary battery in which lithium ions move between positive and negative electrodes during charging and discharging. The lithium secondary battery includes a lithium ion secondary battery using a lithium occlusion material as a negative electrode active material, and a metallic lithium secondary battery using metallic lithium as a negative electrode active material.

A lithium occlusion material is used as the negative electrode active material of the lithium ion secondary battery. Examples of lithium occlusion substances include carbon-based materials, silicon-based materials, and composite oxides of transition metals and lithium. Carbon-based materials are preferably used because they have a good balance between the amount of lithium that can be occluded per mass and the difficulty of deterioration due to absorption and release of lithium. Examples of carbon-based materials include graphite such as natural graphite and artificial graphite; amorphous carbon such as hard carbon, soft carbon and mesoporous carbon; and nanocarbon materials such as carbon nanotubes and graphene. Silicon-based materials are preferably used because they have a large amount of lithium that can be occluded per mass. Examples of the silicon-based material include silicon, silicon monoxide (SiO), and silicon dioxide (SiO _2). Lithium titanate, which is one of the composite oxides of a transition metal and lithium, is preferably used because it is unlikely to deteriorate due to absorption and release of lithium.

Examples of the negative electrode of the lithium ion secondary battery include an electrode obtained by coating a metal foil with a negative electrode material containing the above-mentioned negative electrode active material. If necessary, the negative electrode material can be mixed with a binder such as polyvinylidene fluoride or a styrene-butadiene copolymer; a conductive agent such as carbon black or a nanocarbon material; a dispersant; a thickener or the like. Examples of the metal used for the metal foil include copper and aluminum.

Examples of the positive electrode active material of the lithium secondary battery include a composite oxide of a transition metal and lithium, a composite salt having an olivine structure of a transition metal and lithium, sulfur and the like. Examples of the composite oxide of the transition metal and lithium include a composite oxide of one or more transition metals and lithium selected from cobalt, nickel, and manganese. Typical metals such as aluminum and magnesium; transition metals such as titanium and chromium can be further compounded with these composite oxides. Examples of the composite salt having an olivine structure of a transition metal and lithium include a composite salt having an olivine structure of one or more transition metals selected from iron and manganese and lithium.

Examples of the positive electrode of the lithium secondary battery include an electrode obtained by coating a metal foil with a positive electrode material containing the above-mentioned positive electrode active material. If necessary, the positive electrode material can be mixed with a binder such as polyvinylidene fluoride or an acrylic acid ester copolymer; a conductive agent such as carbon black or a nanocarbon material; a dispersant; a thickener or the like. Examples of the metal used for the metal foil include aluminum and the like.

Examples of the electrolytic solution of the lithium secondary battery include a solution in which a lithium salt is dissolved in a polar solvent and a solution in which a lithium salt is dissolved in an ionic liquid. As the polar solvent used for the electrolytic solution of the lithium secondary battery, carbonic acid esters such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC) and ethylmethyl carbonate (EMC); ethyl acetate, propyl acetate, etc. Examples thereof include fatty acid esters such as ethyl propionate. Examples of the lithium salt used in the electrolytic solution of the lithium secondary battery include lithium hexafluorophosphate (LiPF ₆ ) and lithium tetrafluoroborate (LiBF ₄ ). As the solid electrolyte or gel-like electrolyte, a polymer in which a lithium salt or a lithium salt and an electrolytic solution are dissolved in a polymer such as polyethylene glycol or a derivative thereof, a polymethacrylic acid derivative, polysiloxane or a derivative thereof, or polyvinylidene fluoride is used.

Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to the present examples.

(Example 1)
Main synthetic fiber (stretched polyester fiber, diameter 2.3 μm, fiber length 3 mm), binder synthetic fiber (unstretched polyester fiber, diameter 4.1 μm, fiber length 3 mm, melting point 245 ° C) in a blending ratio of 60:40. Fibers having a solid content of 2 kg were ^{put into a 2 m 3} pulper (dispersion container) ^{together with 1 m 3} of dispersed water and dispersed for 10 minutes to prepare a slurry. The fiber length of the slurry was determined by OpTest Equipment Inc. It was measured with a fiber quality analyzer (FQA-360) manufactured by the company. The content ratio of short fibers of 0.2 mm or less was 1.5%. A wet paper was formed from the slurry with an inclined wire paper machine, and then hot-pressure dried with a Yankee dryer having a surface temperature of 150 ° C. to obtain a non-woven fabric.

Using a thermal calendar device having a metal roll-elastic roll (shore hardness D91) configuration, the obtained non-woven fabric is subjected to a metal roll temperature of 200 ° C., a linear pressure of 50 kN / m, a processing speed of 50 m / min, and one nip (nip). A base material for a lithium secondary battery separator was obtained by performing a thermal calendar treatment under the conditions of. The surface in contact with the elastic roll was used as the coated surface.

(Example 2)
Main synthetic fiber (stretched polyester fiber, diameter 2.3 μm, fiber length 3 mm), binder synthetic fiber (unstretched polyester fiber, diameter 4.1 μm, fiber length 3 mm, melting point 245 ° C) in a blending ratio of 60:40. Fibers having a solid content of 2 kg were ^{put into a 2 m 3} pulper (dispersion container) ^{together with 1 m 3} of dispersed water and dispersed for 10 minutes to prepare a slurry. The slurry 18 strained with a net mesh was removed leaving only the dispersed water fibers, new water 1 m ³ was added made a re-dispersed slurry tone construed the fibers. When the fiber length of the redispersed slurry was measured, the content ratio of short fibers of 0.2 mm or less was 0.5%. A wet paper was formed from the redispersed slurry with an inclined wire paper machine, and then hot-pressure dried with a Yankee dryer having a surface temperature of 150 ° C. to obtain a non-woven fabric.

Using a thermal calendar device having a metal roll-elastic roll (shore hardness D91) configuration, the obtained non-woven fabric is subjected to a metal roll temperature of 200 ° C., a linear pressure of 50 kN / m, a processing speed of 50 m / min, and one nip (nip). A base material for a lithium secondary battery separator was obtained by performing a thermal calendar treatment under the conditions of. The surface in contact with the elastic roll was used as the coated surface.

(Example 3)
Main synthetic fiber (stretched polyester fiber, diameter 2.3 μm, fiber length 3 mm), binder synthetic fiber (unstretched polyester fiber, diameter 4.1 μm, fiber length 3 mm, melting point 245 ° C) in a blending ratio of 60:40. Fibers having a solid content of 2 kg were ^{put into a 2 m 3} pulper (dispersion container) ^{together with 1 m 3} of dispersed water and dispersed for 10 minutes to prepare a slurry. The slurry 18 strained with a net mesh was removed leaving only the dispersed water fibers, new water 1 m ³ was added made a re-dispersed slurry tone construed the fibers. Leaving only fibers to remove dispersed water strained then redispersed slurry 18 mesh net, were forms regulating the retrocession dispersed slurry construed fibers newly water 1 m ³ was added. When the fiber length of the redispersed slurry was measured, the content ratio of short fibers of 0.2 mm or less was 0.1%. A wet paper was formed from the redispersed slurry with an inclined wire paper machine, and then hot-pressure dried with a Yankee dryer having a surface temperature of 150 ° C. to obtain a non-woven fabric.

Using a thermal calendar device having a metal roll-elastic roll (shore hardness D91) configuration, the obtained non-woven fabric is subjected to a metal roll temperature of 200 ° C., a linear pressure of 50 kN / m, a processing speed of 50 m / min, and one nip (nip). A base material for a lithium secondary battery separator was obtained by performing a thermal calendar treatment under the conditions of. The surface in contact with the elastic roll was used as the coated surface.

(Example 4)
Main synthetic fiber (stretched polyester fiber, diameter 2.3 μm, fiber length 3 mm), binder synthetic fiber (unstretched polyester fiber, diameter 4.1 μm, fiber length 3 mm, melting point 245 ° C) in a blending ratio of 60:40. Fibers having a solid content of 2 kg were ^{put into a 2 m 3} pulper (dispersion container) ^{together with 1 m 3} of dispersed water and dispersed for 5 minutes to prepare a slurry. The slurry 18 strained with a net mesh was removed leaving only the dispersed water fibers, new water 1 m ³ was added made a re-dispersed slurry tone construed the fibers. Leaving only fibers to remove dispersed water strained then redispersed slurry 18 mesh net, were forms regulating the retrocession dispersed slurry construed fibers newly water 1 m ³ was added. When the fiber length of the redispersed slurry was measured, the content ratio of short fibers of 0.2 mm or less was 0.06%. A wet paper was formed from the redispersed slurry with an inclined wire paper machine, and then hot-pressure dried with a Yankee dryer having a surface temperature of 150 ° C. to obtain a non-woven fabric.

Using a thermal calendar device having a metal roll-elastic roll (shore hardness D91) configuration, the obtained non-woven fabric is subjected to a metal roll temperature of 200 ° C., a linear pressure of 50 kN / m, a processing speed of 50 m / min, and one nip (nip). A base material for a lithium secondary battery separator was obtained by performing a thermal calendar treatment under the conditions of. The surface in contact with the elastic roll was used as the coated surface.

(Example 5)
Main synthetic fiber (stretched polyester fiber, diameter 2.3 μm, fiber length 3 mm), binder synthetic fiber (unstretched polyester fiber, diameter 4.1 μm, fiber length 3 mm, melting point 245 ° C) in a blending ratio of 60:40. Fibers having a solid content of 2 kg were ^{put into a 2 m 3} pulper (dispersion container) ^{together with 1 m 3} of dispersed water, and dispersed for 30 minutes to prepare a slurry. When the fiber length of the slurry was measured, the content ratio of short fibers was 2.8%. A wet paper was formed from the slurry with an inclined wire paper machine, and then hot-pressure dried with a Yankee dryer having a surface temperature of 150 ° C. to obtain a non-woven fabric.

Using a thermal calendar device having a metal roll-elastic roll (shore hardness D91) configuration, the obtained non-woven fabric is subjected to a metal roll temperature of 200 ° C., a linear pressure of 50 kN / m, a processing speed of 50 m / min, and one nip (nip). A base material for a lithium secondary battery separator was obtained by performing a thermal calendar treatment under the conditions of. The surface in contact with the elastic roll was used as the coated surface.

(Comparative Example 1)
Main synthetic fiber (stretched polyester fiber, diameter 2.3 μm, fiber length 3 mm), binder synthetic fiber (unstretched polyester fiber, diameter 4.1 μm, fiber length 3 mm, melting point 245 ° C) in a blending ratio of 60:40. Fibers having a solid content of 2 kg were ^{put into a 2 m 3} pulper (dispersion container) ^{together with 1 m 3} of dispersed water and dispersed for 5 minutes to prepare a dispersed slurry. Dispersed slurry 18 strained with a net mesh leaving only fibers to remove the dispersing water, new water added 1 m ³ after form a re-dispersed slurry tone construed fibers, redispersed slurry 18 mesh leaving only fibers to remove dispersed water straining in the network, and form regulating the retrocession dispersed slurry construed fibers newly water 1 m ³ was added. The redispersed slurry was strained with an 18-mesh net to remove dispersed water leaving fibers, and fresh water was added twice to prepare the final slurry. When the fiber length of the final slurry was measured, the content ratio of short fibers was 0.0%. After forming a wet paper with a circular net paper machine, the final slurry was hot-pressure dried with a Yankee dryer having a surface temperature of 150 ° C. to obtain a non-woven fabric.

Using a thermal calendar device having a metal roll-elastic roll (shore hardness D91) configuration, the obtained non-woven fabric is subjected to a metal roll temperature of 200 ° C., a linear pressure of 50 kN / m, a processing speed of 50 m / min, and one nip (nip). A base material for a lithium secondary battery separator was obtained by performing a thermal calendar treatment under the conditions of. The surface in contact with the elastic roll was used as the coated surface.

The lithium secondary battery separator base materials obtained in Examples and Comparative Examples were evaluated for basis weight, thickness, air permeability, and coating liquid strike-through, and the results are shown in Table 1.

(Basis weight)
The basis weight of the base material was measured according to JIS P8124: 2011.

(Thickness)
The thickness under 5N load was measured using an outer micrometer specified in JIS B7502: 2016.

(Breathability)
The air permeability by the Frazier method was measured according to JIS L 1096: 2010.

<Making a separator>
100 parts by mass of boehmite having a volume average particle diameter of 0.9 μm and a BET specific surface area of 5.5 g / m ² is dispersed in 150 parts by mass of water, and a carboxy of a 1 mass% aqueous solution having a viscosity of 3000 mPa · s at 25 ° C. Add 75 parts by mass of a 2% by mass aqueous solution of methyl cellulose sodium salt, stir and mix, and carboxy-modified styrene-butadiene copolymer resin emulsion (solid content concentration 50% by mass) with a glass transition point of -18 ° C and a volume average particle size of 0.2 μm. A mass portion was added and mixed by stirring, and finally, adjusting water was added to adjust the solid content concentration to 30% by mass to prepare a coating liquid A.

(Evaluation of strike-through of coating liquid)
A reverse gravure coater was used as a coating device on the elastic roll surface of the base material of Examples and Comparative Examples, and the coating liquid A was applied at a line speed of 50 m / min, and the wet coating amount was 40 g / m ^2. One side was coated as shown. The coated base material was dried by blowing hot air at 90 ° C. with a floating air dryer directly connected to a reverse gravure coater to obtain a separator. The strike-through evaluation of the coating liquid was classified into the following three stages according to the state of adhesion of the coating liquid to the guide roll of the coating device and the inside of the floating air dryer.

1: There is almost no adhesion of the coating liquid to the inside of the guide roll or floating air dryer.
2: The coating liquid adheres to the inside of the guide roll or floating air dryer, but it is not retransferred to the separator.
3: The strike-through coating liquid adheres to the inside of the guide roll or the floating air dryer, and the obtained separator has surface non-uniformity due to retransfer.

<Making evaluation batteries>
Using each separator, lithium manganate is used for the positive electrode, mesocarbon microbeads are used for the negative electrode, and 1 mol / L diethyl carbonate / ethylene carbonate (volume ratio 7/3) mixed solvent solution of lithium hexafluorophosphate is used for the electrolytic solution. A pouch-type lithium secondary battery for evaluation having a design capacity of 30 mAh was prepared so that the inorganic particle layer of No. 1 was opposed to the negative electrode, and was subjected to the following evaluation of internal resistance.

(Internal resistance evaluation)
For each battery, 60mA constant current charging → 4.2V constant voltage charging (1 hour) → constant current discharge at 60mA → when it reaches 2.8V, after performing 5 cycles of break-in charging and discharging in the next cycle sequence. , 60mA constant current charging → 4.2V constant current charging (1 hour) → 6mA constant current discharge (discharge amount 3mAh) → Measure the voltage just before the end of discharge (voltage a) → 60mA constant current charging → 4.2V Constant voltage charging (1 hour) → Constant current discharge at 90mA for 2 minutes (Discharge amount 3mAh) → Measure the voltage (voltage b) just before the end of discharge, and perform internal resistance Ω = (voltage a-voltage b) / (90mA) The internal resistance was calculated by the formula of -6mA). The results are shown in Table 1.

A: Internal resistance less than 4.0Ω B: Internal resistance 4.0Ω or more and less than 5.0Ω C: Internal resistance 5.0Ω or more

As shown in Table 1, the base material for the lithium secondary battery separator of Examples 1 to 5 is made of a non-woven fabric containing at least a main synthetic fiber and a binder synthetic fiber as synthetic fibers, and has a fiber length of 0.2 mm. Since it is characterized by containing the following non-fibrillated synthetic fibers, good results were obtained in the strike-through evaluation. Further, the separators prepared using the base materials of Examples 1 to 5 had low internal resistance and were good.

From the comparison between Example 1 and Example 5, the separator prepared using the base material of Example 1 in which the content ratio of short fibers of 0.2 mm or less is less than 2.0% is short of 0.2 mm or less. The fiber content ratio was 2.0% or more with respect to the total fiber components contained in the base material, and the internal resistance was lower than that of the separator prepared using the base material of Example 5, which was good.

From the comparison between Example 3 and Example 4, the separator prepared using the base material of Example 3 in which the content ratio of the short fibers of 0.2 mm or less is 0.1% or more is 0.2 mm or less. The fiber content ratio was better than that of the separator prepared using the base material of Example 4 in which the content ratio of the fibers was less than 0.1% with respect to the total fiber components contained in the base material, with less strike-through.

In the base material for the lithium secondary battery separator of Comparative Example 1 which does not contain short fibers of 0.2 mm or less, the coating liquid that has been pierced through has adhered to the inside of the guide roll and the floating air dryer, and is re-applied to the obtained separator. Surface non-uniformity was caused by transfer.

The base material for a lithium secondary battery separator of the present invention can be used not only for a lithium secondary battery separator but also for a lithium ion polymer battery separator, a lithium ion capacitor separator, and the like, and further, a metal ion battery using a metal other than lithium. It can also be used as a separator, a metal ion polymer battery separator, a metal ion capacitor separator, and the like.

Claims (2)

As the synthetic fiber, a base material for a lithium secondary battery separator made of a non-woven fabric containing at least a main synthetic fiber and a binder synthetic fiber contains a non-fibrillated synthetic fiber having a fiber length of 0.2 mm or less. A characteristic base material for a lithium secondary battery separator. Main synthetic fiber, binder Synthetic fiber and non-fibrillated synthetic fiber with fiber length of 0.2 mm or less The total length of non-fibrillated synthetic fiber with fiber length of 0.2 mm or less is 0. The base material for a lithium secondary battery separator according to claim 1, which is 1% or more and less than 2.0%.

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