JP2020161243A - Nonwoven fabric base material for lithium secondary battery separator, and lithium secondary battery separator - Google Patents

Abstract

To provide a nonwoven fabric base material for a lithium secondary battery separator, excellent in heat resistance, high in tensile strength and high in separator productivity.SOLUTION: A nonwoven fabric base material for a lithium secondary battery separator is characterized to contain: stretched polyester fibers; and, as binder fibers, unstretched polyester fibers and wet heat adhesive fibers.SELECTED DRAWING: None

Description

The present invention relates to a non-woven fabric base material and a lithium secondary battery separator used for a lithium secondary battery separator.

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 plates are used.

In particular, a polyolefin-based microporous membrane 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, the polyolefin-based microporous membrane has a problem that it has a low affinity for the electrolytic solution and the electrolytic solution retention is not good. Therefore, it has been attempted to improve the electrolyte retention by using a non-woven fabric or the like instead of the microporous membrane.

Non-woven fabric type mainly composed of fibers made of moist heat-adhesive resin and fibers made of olefin resin for the purpose of preventing internal short circuit, ion permeability and sheet strength as well as electrolyte retention, and is manufactured by a wet papermaking method. Separators are disclosed (see, for example, Patent Document 1). However, the separator of Patent Document 1 whose main fiber is an olefin resin having poor heat resistance melts and shrinks when the battery overheats, loses the function of separating the positive and negative electrodes, and may cause a significant short circuit. ..

Non-woven fabric base material for lithium secondary battery separator (hereinafter, "non-woven fabric base material for lithium secondary battery separator") containing polyethylene terephthalate (PET) fiber as a separator that does not easily melt or shrink even when the battery overheats. Is sometimes abbreviated as "nonwoven fabric base material"), and a separator formed by coating inorganic particles has been proposed (see, for example, Patent Documents 2 to 4).

Patent Document 2 describes a PET non-woven fabric base material containing fibers having a fiber diameter of 0.1 to 10 μm. Patent Document 3 describes a non-woven fabric base material containing crystallized PET fibers and PET fibers for a binder and containing short fibers having an average fiber diameter of 3 μm or less as essential components. Further, Patent Document 4 describes a non-woven fabric base material containing crystallized PET fibers and PET fibers for a binder, containing fibers having a fiber length of 2 mm or less as crystallized PET fibers, and heat-pressure treated by a calendar. There is.

However, the non-woven fabric base materials of Patent Documents 2 to 4 made of only PET fibers have low tensile strength, and when a thin separator is manufactured, there is a possibility that breakage may occur in the manufacturing process.

International Publication No. 2004/038833 Pamphlet Special Table 2005-536857 JP-A-2009-230975 Japanese Unexamined Patent Publication No. 2011-82148

The present invention is intended to solve the above problems. That is, it is an object of the present invention to provide a non-woven fabric base material for a lithium secondary battery separator having excellent heat resistance, high tensile strength, and high separator productivity.

The above problem has been solved by the following invention.

(1) A non-woven fabric base material for a lithium secondary battery separator, which comprises a drawn polyester fiber, an undrawn polyester fiber as a binder fiber, and a moist heat adhesive fiber.

(2) The non-woven fabric base material for a lithium secondary battery separator according to (1), wherein the wet-heat adhesive fiber is an ethylene-vinyl alcohol-based fiber.

(3) The content of the binder fiber is 20 to 60% by mass and the content of the moist heat-adhesive fiber is 3 to 20% by mass with respect to the entire fiber contained in the non-woven fabric base material (1) or (2). Nonwoven fabric substrate for lithium secondary battery separator according to.

(4) The non-woven substrate for the lithium secondary battery separator according to any one of (1) to (3) above, an inorganic particle layer, an organic particle layer, a resin microporous layer, a fine fiber layer, a solid electrolyte, and a gel. A lithium secondary battery separator comprising one or more layers selected from the group consisting of electrolytes or an electrolyte.

According to the non-woven fabric base material of the present invention, it is possible to provide a non-woven fabric base material for a lithium secondary battery separator having excellent heat resistance, high tensile strength, and high separator productivity.

The non-woven fabric base material for a lithium secondary battery separator of the present invention can be a non-woven fabric base material having excellent thermal dimensional stability because the skeleton is formed of stretched polyester fibers having excellent heat resistance. It also contains unstretched polyester fibers and moist heat-adhesive fibers as binder fibers. The unstretched polyester fiber is softened or melted by heat pressure treatment such as a calendar, and firmly adheres to other fibers. On the other hand, the wet-heat adhesive fiber exhibits an adhesive function by flowing or easily deforming from the fiber state at a certain temperature in a wet state, and is a non-woven fabric base material in a wet papermaking method which is a preferable manufacturing method of the non-woven fabric base material of the present invention. The strength of the material can be significantly improved. In the present invention, by containing these two types of binder fibers, a non-woven fabric base material having high tensile strength and high separator productivity can be obtained.

The lithium secondary battery separator nonwoven fabric base material of the present invention is characterized by containing drawn polyester fibers, unstretched polyester fibers as binder fibers, and wet-heat adhesive fibers.

In the present invention, examples of the moist heat-adhesive fiber used as the binder fiber include vinyl alcohol-based fiber and ethylene-vinyl alcohol-based fiber. Vinyl alcohol-based fibers and ethylene-vinyl alcohol-based fibers include vinyl alcohol-based single fibers, ethylene-vinyl alcohol-based single fibers, composite fibers composed of vinyl alcohol and other resins, and ethylene-vinyl alcohol and other resins. Examples thereof include composite fibers. Examples of the composite fiber include a core-sheath type (core-shell type), a parallel type (side-by-side type), and a radial split type.

In the case of composite fibers, the resin to be combined with vinyl alcohol-based fibers and ethylene-vinyl alcohol-based fibers is not particularly limited, and for example, polypropylene, high-density polyethylene (HDPE), low-density polyethylene (LDPE), and linear low-density. Examples thereof include olefin resins such as polyethylene (LLDPE), polyacrylic resins, polyester resins, and polyamide resins. Of these, a core-sheath type composite fiber having an ethylene-vinyl alcohol-based single fiber and a polyester-based fiber as a core and an ethylene-vinyl alcohol-based fiber as a sheath is preferably used.

In the present invention, as the unstretched polyester fiber used as the binder fiber, polyester such as polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polybutylene naphthalate and a copolymer containing the same is spun. Examples thereof include undrawn fibers spun at a speed of 800 to 1,200 m / min.

In the present invention, the average fiber diameter of the binder fiber is preferably 0.1 to 15 μm, more preferably 0.5 to 12 μm, still more preferably 0.7 to 10 μm. If the average fiber diameter is less than 0.1 μm, the strength of the non-woven fabric base material may be insufficient. If the average fiber diameter is thicker than 15 μm, it may be difficult to reduce the thickness.

In the present invention, the "fiber diameter" is the diameter of the fiber obtained by measuring the area of the fiber cross section forming the non-woven fabric base material by scanning electron microscope observation of the non-woven fabric base material cross section and converting it into a perfect circle. The "average fiber diameter" is the average value of the fiber diameters of 50 randomly selected fibers.

In the present invention, the fiber length of the binder fiber is preferably 1 to 15 mm, more preferably 2 to 12 mm, still more preferably 3 to 10 mm. If the fiber length is shorter than 1 mm, the strength of the non-woven fabric base material may be insufficient, and if it is longer than 15 mm, the fibers may become entangled and become lumpy, resulting in uneven thickness.

In the present invention, the content of the binder fiber is preferably 20 to 60% by mass, more preferably 25 to 55% by mass, and 30 to 50% by mass with respect to the total fiber components contained in the non-woven fabric base material. It is more preferably%. If the content of the binder fiber is less than 20% by mass, the strength of the non-woven fabric base material is lowered, and breakage may occur in the manufacturing process of the non-woven fabric base material. On the other hand, when the content of the binder fiber is more than 60% by mass, the surface of the non-woven fabric base material is easily formed into a film, the electrolyte retention property is lowered, and the separator may have a high internal resistance.

In the present invention, the content of the moist heat-adhesive fiber is preferably 3 to 20% by mass, more preferably 4 to 15% by mass, and 5 to 5% by mass with respect to the total fiber components contained in the non-woven fabric base material. More preferably, it is 10% by mass. If the content of the moist heat-adhesive fiber is less than 3% by mass, the strength of the non-woven fabric base material may be insufficient, and if it exceeds 20% by mass, the surface of the non-woven fabric base material is likely to be formed into a film due to melting of the moist heat-adhesive fiber. , There is a risk of becoming a separator with high internal resistance.

In the present invention, the content of the undrawn polyester fiber is preferably 10 to 57% by mass, more preferably 15 to 51% by mass, and 20 to 20% by mass with respect to the total fiber components contained in the non-woven fabric base material. It is more preferably 45% by mass. If the content of the unstretched polyester fiber is less than 10% by mass, the strength of the non-woven fabric base material decreases, and there is a risk of breakage in the manufacturing process of the non-woven fabric base material. If it exceeds 57% by mass of the unstretched polyester fiber, the content of the moist heat-adhesive fiber may be relatively reduced and the strength of the non-woven fabric base material may be insufficient when the content of the binder fiber is within a preferable range. is there.

In the present invention, examples of the polyester of the drawn polyester fiber include polyester having an alkylene terephthalate as the main repeating unit, but polyethylene terephthalate having high heat resistance is preferable. Further, the cross-sectional shape of the fiber is preferably circular. 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 due to the liquid retention property of the electrolytic solution.

In the present invention, the average fiber diameter of the drawn polyester fiber is preferably 0.1 to 10 μm, more preferably 0.5 to 10 μm, still more preferably 0.7 to 8 μm. If the average fiber diameter of the stretched polyester fiber is less than 0.1 μm, the strength of the non-woven fabric base material may be insufficient, and if the average fiber diameter is thicker than 15 μm, it may be difficult to reduce the thickness.

In the present invention, the fiber length of the drawn polyester fiber is preferably 1 to 15 mm, more preferably 2 to 12 mm, still more preferably 3 to 10 mm. If the fiber length is shorter than 1 mm, the strength of the non-woven fabric base material may be insufficient, and if it is longer than 15 mm, the fibers may become entangled and become lumpy, resulting in uneven thickness.

In the present invention, the content of the drawn polyester fiber contained in the non-woven fabric base material is preferably 20 to 80% by mass, more preferably 25 to 75% by mass, and 30 to 70% by mass with respect to the total fibers. It is preferably mass%. When the content of the stretched polyester fiber is less than 20% by mass, wrinkles may occur in the non-woven fabric base material in the step of raising the temperature until the unstretched polyester fiber which is the binder fiber is softened or melted. On the other hand, if the content of the drawn polyester fiber exceeds 80% by mass, the content of the binder fiber, which is a binder fiber, is relatively reduced, so that the strength of the non-woven fabric base material may be insufficient.

In the non-woven fabric base material of the present invention, fibers other than the above-mentioned drawn polyester fibers and binder fibers may be added, if necessary. For example, short fibers and fibrils of solvent-spun cellulose and regenerated cellulose; natural cellulose fibers; pulped and fibrils of natural cellulose fibers; polyolefin, acrylic, total aromatic polyester, total aromatic polyesteramide, polyamide, semi-aromatic polyamide. , Total Aromatic Polyamide, Total Aromatic Polyester, Total Aromatic Polycarbonate, Total Aromatic Polyazomethine, Polyamide, Polyamideimide (PAI), Polyester Ether Ketone (PEEK), Polyphenylene Blast (PPS), Poly-p-Phenylene Contains single fibers and composite fibers made of resins such as benzobisoxazole (PBO), polybenzoimidazole (PBI), and polytetrafluoroethylene (PTFE), and fibrils and splits of single fibers and composite fibers made of these resins. You may. One type of these fibers may be contained, or two or more types may be contained. The 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 non-woven fabric base material, it is particularly preferable to contain a fibrillated product of a total aromatic polyamide.

As a method for producing a non-woven fabric base material, a method of forming a fiber web and adhering, fusing, and entwining the fibers in the fiber web to produce a non-woven fabric can be used. The obtained non-woven fabric may be used as it is, or may be used as a laminated body composed of a plurality of sheets. Examples of the method for producing the fiber web include a dry method such as a card method, an air array method, a spunbond method, and a melt blow method; a wet papermaking method; and an electrostatic spinning method. Of these, a wet papermaking method is preferable as the method for producing the nonwoven fabric base material of the present invention. The fiber web obtained by the wet papermaking method is homogeneous and dense, and can be suitably used as a non-woven fabric base material for a lithium secondary battery separator.

In the wet papermaking method, first, the fibers as a raw material are uniformly dispersed in water, and then a process such as a screen (removal of foreign substances, lumps, etc.) is passed to bring the final fiber concentration to 0.01 to 0.50% by mass. The prepared slurry is machined up to give a wet paper. In order to make the dispersibility of the fibers uniform, chemicals such as dispersants, defoamers, hydrophilic agents, antistatic agents, polymer thickeners, mold release agents, antibacterial agents, and bactericidal agents may be added during the process. is there.

Examples of the paper machine include a paper machine in which a paper machine such as a circular net, a long net, and an inclined wire is installed independently, a combination paper machine in which two or more types of paper machines of the same type or different types are installed online. Can be used. Further, as a method for 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, and a method in which one layer is formed and then fibers are placed on the layer. A method or the like in which the dispersed slurry is cast and laminated can be used. When the slurry in which the fibers are dispersed is cast, the previously formed layer may be in a wet paper state or a dry state. Further, two or more layers can be heat-sealed to form a multilayer structure non-woven fabric.

In the present invention, when a non-woven fabric having a multi-layer structure is produced, it may have a multi-layer structure in which the fiber composition of each layer is the same, or a multi-layer structure in which the fiber composition of each layer is different. In the case of a multi-layer structure, the fiber concentration of the slurry can be lowered by lowering the basis weight of each layer, so that the texture of the non-woven fabric is improved, and as a result, the smoothness and uniformity of the coated surface are improved. Further, even if the formation of each layer is uneven, it can be compensated by laminating. Furthermore, the papermaking speed can be increased, and the operability is improved.

A sheet (base paper) is obtained by drying the wet paper produced by the papermaking net 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 100 to 150 ° C., more preferably 105 to 145 ° C., and even more preferably 110 to 140 ° C. The pressure is preferably 5 to 100 kN / m, more preferably 10 to 80 kN / m.

In the present invention, the non-woven fabric containing the above-mentioned drawn polyester fiber and the undrawn polyester fiber and the moist heat-adhesive fiber as the binder fiber can be used as the non-woven fabric base material as it is. More preferably, the non-woven fabric is passed between the heated metal roll and the elastic roll and subjected to a heating / pressurizing treatment (thermal calendar treatment) to form a non-woven fabric base material. In general, the thermal calendar processing apparatus includes an apparatus composed of a metal heat roll (metal roll) having a heating mechanism and an elastic roll having an elastic body layer 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., more preferably −50 to −20 ° C. with respect to the melting point or softening temperature of the undrawn polyester fiber. .. When the temperature of the metal roll is lower than −60 ° C. with respect to the melting point or softening temperature of the unstretched polyester fiber, the adhesion of the unstretched polyester fiber tends to be insufficient, and the strength of the non-woven fabric base material may decrease. On the other hand, when the temperature of the metal roll is higher than −10 ° C. with respect to the melting point or softening temperature of the unstretched polyester fiber, the non-woven fabric tends to stick to the metal roll, and the surface of the non-woven fabric base material may be defective.

The nip pressure (linear pressure) of the nip during the thermal calendar processing is preferably 9 to 130 kN / m, more preferably 14 to 110 kN / m. The processing speed is preferably 5 to 100 m / min, more preferably 10 to 80 m / min.

In the present invention, the basis weight of the nonwoven fabric base material is preferably from 5 to 20 g / ^{m 2,} more preferably 6~18g / ^{m 2,} more preferably 8~16g / ^{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 nonwoven fabric base material is preferably 10 to 30 μm, more preferably 12 to 27 μm, still more preferably 15 to 25 μm. If the thickness is less than 10 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 nonwoven fabric substrate (basis weight / thickness) is preferably 0.40~0.90g / cm ^3, more preferably 0.45~0.80g / cm ^3, 0.50 More preferably ~ 0.75 g / cm ³ . 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 high internal resistance.

The separator of the present invention comprises the non-woven fabric substrate of the present invention and one or more layers or electrolytes selected from the group consisting of an inorganic particle layer, an organic particle layer, a resin microporous film, a fine fiber layer, a solid electrolyte and a gel-like electrolyte. Contains. The most preferable layer is an inorganic particle layer. A separator containing a non-woven fabric base material and an inorganic particle layer is produced by applying a coating liquid containing an inorganic pigment to the non-woven fabric base material. A separator containing a non-woven fabric base material and an organic particle layer is produced by applying a coating liquid containing organic particles to the non-woven fabric base material. A separator containing a non-woven fabric base material and a resin microporous layer is produced by laminating a resin microporous membrane such as a polyethylene microporous membrane or a polypropylene microporous membrane and a non-woven fabric base material. A separator containing a non-woven fabric base material and a fine fiber layer is produced by forming a fine fiber layer on the non-woven fabric base material, for example, by an electrostatic spinning method. A separator containing a non-woven fabric base material and a solid electrolyte or a gel-like electrolyte is produced by applying a solid electrolyte or a gel-like electrolyte to the non-woven fabric base material. As described above, the non-woven fabric base material of the present invention does not serve as a separator as it is, but is a precursor sheet of a lithium secondary battery separator.

Inorganic particles include alumina, gibsite, boehmite, magnesium oxide, magnesium hydroxide, silica, titanium oxide, barium titanate, zirconium oxide and other inorganic oxides, aluminum nitride and silicon nitride and other inorganic nitrides, aluminum compounds and zeolites. , 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 a coating liquid containing inorganic particles or organic particles is not particularly limited as long as it can uniformly dissolve or disperse a binder, inorganic particles, organic particles, etc., and is, for example, aromatic hydrocarbons such as toluene. Hydrogens, ethers such as tetrahydrofuran, ketones such as methyl ethyl ketone, alcohols such as isopropyl alcohol, N-methyl-2-pyrrolidone (NMP), dimethylacetamide, dimethylformamide, dimethyl sulfoxide, water, etc. are used as necessary. be able to. In addition, these media may be mixed and used if necessary. The medium used is preferably one that does not expand or dissolve the non-woven fabric base material.

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

A porous film can be used as the resin microporous layer. The resin of the porous film is not particularly limited as long as it is a resin capable of forming a film, but a polyolefin resin such as a polyethylene resin and a polypropylene resin is preferable. Examples of the polyethylene-based resin include a resin of a polyethylene-based resin alone such as ultra-low density polyethylene, low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, or ultra high density polyethylene. Further, ethylene propylene copolymer, a mixture of a polyethylene resin and another polyolefin resin, and the like can also be mentioned. Polypropylene-based resins include homopropylene (propylene homopolymer), propylene and ethylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonen or 1-decene. Examples thereof include random copolymers with α-olefins and block copolymers.

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-storing substance as the negative electrode active material, and a metallic lithium secondary battery using metallic lithium as the negative electrode active material.

A lithium storage material is used as the negative electrode active material of the lithium ion secondary battery. Examples of lithium storage substances include carbon-based materials, silicon-based materials, and composite oxides of transition metals and lithium. A carbon-based material is preferably used because it has 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 the carbon-based material 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 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, and sulfur. 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. 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, if necessary. 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. Examples of the polar solvent used in the electrolytic solution of the lithium secondary battery include carbonic acid esters such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methyl 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 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. Unless otherwise specified, all parts and percentages in the examples are based on mass.

<Stretched polyester fiber>
Stretched PET1: Stretched polyester single fiber stretched PET2 made of polyethylene terephthalate with a fiber diameter of 2.3 μm and a fiber length of 3 mm: Stretched polyester monofiber stretched PET3 made of polyethylene terephthalate with a fiber diameter of 3.0 μm and a fiber length of 3 mm: From polyethylene terephthalate A stretched polyester single fiber with a fiber diameter of 5.1 μm and a fiber length of 5 mm.

<Unstretched polyester fiber>
Unstretched PET1: Unstretched polyester single fiber made of polyethylene terephthalate with a fiber diameter of 4.2 μm and a fiber length of 3 mm Unstretched PET2: Unstretched polyester single fiber made of polyethylene terephthalate with a fiber diameter of 6.6 μm and a fiber length of 5 mm

<Moist heat adhesive fiber>
Moist heat-adhesive fiber 1: Single fiber made of ethylene-vinyl alcohol copolymer, fiber diameter 7.2 μm, fiber length 5 mm Moist heat-adhesive fiber 2: Single fiber made of polyvinyl alcohol, fiber diameter 6.3 μm, fiber length 3 mm Fiber Wet Heat Adhesive Fiber 3: The core is polyethylene terephthalate, the sheath component is an ethylene-vinyl alcohol copolymer, the fiber diameter is 11.0 μm, the fiber length is 5 mm, and the core / sheath volume ratio is 50/50. Sheath type composite fiber

<Other fibers>
PP fiber: A stretched olefin single fiber FPA fiber made of polypropylene having a fiber diameter of 5 μm and a fiber length of 5 mm: A para-based total aromatic polyamide fiber having a fiber diameter of 12 μm and a fiber length of 5 mm is treated with a double discifier to further increase the pressure. A fibrillated product having a modified drainage degree of 50 ml, obtained by treating with a homogenizer.

The modified drainage degree is based on JIS P8121-2: 2012 except that an 80-mesh wire mesh with a wire diameter of 0.14 mm and a mesh size of 0.18 mm is used as a sieving plate and the sample concentration is 0.1%. It is the measured value.

Nonwoven fabric substrates for lithium secondary battery separators of Examples 1 to 13 and Comparative Example 1 were produced under the following conditions.

After pouring water into a 2 m ³ dispersion tank, the mixture was mixed at the raw material mixing ratio (%) shown in Table 1, dispersed at a dispersion concentration of 0.2% for 5 minutes, and formed with a tilted wire using a tilting paper machine. The wet paper was hot-pressure dried with a Yankee dryer having a surface temperature of 130 ° C. to prepare a wet non-woven fabric. A wet non-woven fabric is subjected to a metal roll-elastic roll (shore hardness D91) thermal calendar device under the conditions of a metal roll temperature of 195 ° C., a linear pressure of 100 kN / m, a processing speed of 30 m / min, and one nip (nip). Thermal calendar treatment was performed to obtain a non-woven fabric base material for a lithium secondary battery separator of Examples 1 to 13 and Comparative Examples 1 and 2.

A 16-split composite fiber (polypropylene / ethylene-vinyl alcohol split fiber) having a fiber diameter of 14 μm and a fiber length of 5 mm, which is composed of 50% of the PP fiber, a fiber made of polypropylene, and a fiber made of an ethylene-vinyl alcohol copolymer. ) 50% is dispersed in a 2 m ³ dispersion tank at a dispersion concentration of 0.2% for 5 minutes, and wet paper is heated on an inclined wire with a Yankee dryer having a surface temperature of 130 ° C. using an inclined paper making machine. A wet non-woven fabric obtained by pressure drying was produced. The wet non-woven fabric is impregnated with water, and using a thermal calendar device having a metal roll-resin roll (shore hardness D91), the metal roll temperature is 130 ° C., the linear pressure is 100 kN / m, the processing speed is 30 m / min, and one nip. The thermal calendar treatment was performed under the condition of (nip) to obtain a non-woven fabric base material for a lithium secondary battery separator of Comparative Example 2.

The following physical properties were measured and evaluated for the nonwoven fabric substrates of Examples and Comparative Examples, and the results are shown in Table 2.

<Basis weight>
The basis weight of the non-woven fabric 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.

<Tensile strength>
From the non-woven fabric base materials obtained in Examples and Comparative Examples, strip-shaped test pieces having a short side of 50 mm and a long side of 250 mm were cut out so that the MD direction was parallel to the long side, and these test pieces were used as a desktop material tester (a tabletop material tester). Using Orient Tech Co., Ltd., device name: STA-1150), the fabric was stretched under the conditions of a grip interval of 100 mm and a tensile speed of 200 mm / min, and the load value at the time of cutting was taken as the tensile strength and evaluated according to the following criteria.

A: 700 N / m or more B: less than 700 N / m 600 N / m or more C: less than 600 N / m

<Heating dimension maintenance rate>
The non-woven fabric base material cut into 100 mm squares was heat-treated in a dryer at 160 ° C. for 1 hour. Measure the dimensions in the flow direction before and after heat treatment in units of 0.1 mm, and maintain the dimensions after heat treatment with respect to the dimensions before heat treatment (dimensions after heat treatment / dimensions before heat treatment x 100, unit:%) Was calculated and evaluated according to the following criteria.

A: 98% or more B: less than 98% 95% or more C: less than 95%

<Making a separator>
100 parts of boehmite having a volume average particle diameter of 0.9 μm and a BET specific surface area of 5.5 m ² / g is dispersed in 150 parts of water, and a 1% aqueous solution of the carboxymethyl cellulose sodium salt having a viscosity of 200 mPa · s at 25 ° C. Add and stir and mix 75 parts of a 2% aqueous solution, and add and stir and mix 10 parts of a carboxy-modified styrene-butadiene copolymer resin emulsion (solid content concentration 50%) with a glass transition point of -18 ° C and a volume average particle size of 0.2 μm. Finally, adjusting water was added to adjust the solid content concentration to 25% to prepare a coating liquid A.

A reverse gravure coater was used as a coating device on the elastic roll surface of the non-woven fabric base material of Examples and Comparative Examples, and the coating liquid A was applied at a line speed of 30 m / min, and the amount of adhesion as a liquid was 47 g / m ² . One side was coated so that it would be. The coated non-woven fabric base material was dried by blowing hot air at 90 ° C. with a floating air dryer directly connected to the reverse gravure coater to obtain a separator containing the non-woven fabric base material and the inorganic particle layer. As an evaluation of "breakage during separator production", the number of breaks of the non-woven fabric substrate was classified into the following three stages when the separator was wound up by a reeler for 500 m.

A: 0 times B: 1 time C: 2 times or more

<Making evaluation batteries>
Each separator is used, using lithium manganate for the positive electrode, mesocarbon microbeads for the negative electrode, and a 1 mol / L diethyl carbonate / ethylene carbonate (volume ratio 7/3) mixed solvent solution of lithium hexafluorophosphate for the electrolytic solution. A pouch-type lithium ion 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 used for the evaluation of the internal resistance and leakage current as described below.

<Internal resistance>
For each battery, 60mA constant current charge → 4.2V constant voltage charge (1 hour) → constant current discharge at 60mA → when it reaches 2.8V, after performing 5 cycles of break-in charge / discharge in the next cycle sequence. , 60mA constant current charging → 4.2V constant current charging (1 hour) → constant current discharge at 6mA for 30 minutes (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 2.

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 2, the non-woven fabric base material produced in Examples 1 to 13 is a non-woven fabric base material containing stretched polyester fibers, unstretched polyester fibers as binder fibers, and wet-heat adhesive fibers. Therefore, since it is a non-woven fabric base material having high tensile strength and not easily broken when the separator is manufactured, it is a non-woven fabric base material having excellent separator productivity. In addition, it was a non-woven fabric base material having a high heating dimension maintenance rate and excellent heat resistance.

From the comparison between Example 1 and Example 2, the non-woven fabric base material of Example 1 in which the content of the binder fiber is 20% by mass or more with respect to the entire fiber contained in the non-woven fabric base material has a binder fiber content. Compared with the non-woven fabric base material of Example 2 having less than 20% by mass, the number of breaks at the time of producing the separator was smaller, which was a preferable result.

From the comparison between Example 3 and Example 4, the separator of Example 3 produced by using the non-woven fabric base material in which the content of the binder fiber is 60% by mass or less with respect to the entire fibers contained in the non-woven fabric base material is The internal resistance of the battery was lower than that of the separator of Example 4 produced by using the non-woven fabric base material having a binder fiber content of more than 60% by mass, which was a preferable result.

From the comparison between Example 5 and Example 6, the non-woven fabric base material of Example 5 in which the content of the moist heat-adhesive fiber is 3% by mass or more with respect to the entire fiber contained in the non-woven fabric base material is the moist heat-adhesive fiber. The tensile strength was higher than that of the non-woven fabric base material of Example 6 in which the content of the fiber was less than 3% by mass, and the number of breaks during separator production was small, which was a preferable result.

From the comparison between Example 7 and Example 8, the separator of Example 7 using the non-woven fabric base material in which the content of the moist heat-adhesive fiber is 20% by mass or less with respect to the entire fibers contained in the non-woven fabric base material is The internal resistance of the battery was lower than that of the separator of Example 8 produced by using the non-woven fabric base material having a moisture-heat adhesive fiber content of more than 20% by mass, which was a preferable result.

From the comparison between Example 1 and Example 9, the non-woven fabric base material of Example 1 in which the content of the undrawn polyester fiber is 10% by mass or more with respect to the entire fiber contained in the non-woven fabric base material is the undrawn polyester fiber. The tensile strength was higher than that of the separator of Example 9 in which the content of the separator was less than 10% by mass, and the number of breaks during the production of the separator was small, which was a preferable result.

From the comparison between Example 10 and Example 11, the non-woven fabric base material of Example 10 containing unstretched polyester fiber and fiber composed of ethylene-vinyl alcohol as binder fiber has unstretched polyester fiber and polyvinyl alcohol as binder fiber. The tensile strength was higher than that of the separator of Example 11 containing the fiber composed of the fiber, and the number of breaks during the production of the separator was smaller, which was a preferable result.

From the comparison between Example 10 and Example 12, the non-woven fabric base material of Example 12 containing the fibrillated total aromatic polyamide is more than the non-woven fabric base material of Example 10 containing no fibrillated total aromatic polyamide. The heating dimension maintenance rate was high and the heat resistance was excellent.

The non-woven fabric base material of Comparative Example 1 containing no moist heat-adhesive fiber as a binder fiber had a low tensile strength, resulting in a large number of breaks during separator production.

The non-woven fabric base material of Comparative Example 2 composed of olefin fibers and moist heat-adhesive fibers had a very low heating dimension retention rate, resulting in inferior heat resistance.

As a utilization example of the present invention, a non-woven fabric base material for a lithium secondary battery separator and a lithium secondary battery separator are suitable.

Claims (4)

A non-woven fabric base material for a lithium secondary battery separator, which comprises stretched polyester fibers, unstretched polyester fibers as binder fibers, and moist heat-adhesive fibers. The non-woven fabric base material for a lithium secondary battery separator according to claim 1, wherein the wet-heat adhesive fiber is an ethylene-vinyl alcohol-based fiber. The lithium ion according to claim 1 or 2, wherein the content of the binder fiber is 20 to 60% by mass and the content of the wet heat adhesive fiber is 3 to 20% by mass with respect to the entire fiber contained in the non-woven fabric base material. Non-woven fabric base material for next battery separator. Select from the group consisting of the non-woven substrate for the lithium secondary battery separator according to any one of claims 1 to 3 and the group consisting of an inorganic particle layer, an organic particle layer, a resin microporous layer, a fine fiber layer, a solid electrolyte and a gel electrolyte. A lithium secondary battery separator comprising one or more layers or electrolytes.