JP2019212436A - Base material for lithium ion battery separator and lithium ion battery separator - Google Patents

Abstract

To provide a base material for a lithium ion battery separator, in which adhesiveness between the base material for the lithium ion battery separator and a coated layer containing inorganic particles is high and coatability of an aqueous coating solution is excellent, and also to provide the lithium ion battery separator using the base material for the lithium ion battery separator.SOLUTION: In a base material for a lithium ion battery separator formed by containing a fibrillated solvent-spun cellulose fiber and a synthetic resin short fiber, the percentage content of the fibrillated solvent-spun cellulose fiber is 2% by mass or more and 25% by mass or less with respect to all the fiber components contained in the base material, and as a synthetic resin short fiber, a core-sheath type composite fiber is included, in which a resin having a melting point of 160° C or more is used as a core component and a polyethylene resin is used as a sheath component.SELECTED DRAWING: None

Description

  The present invention relates to a lithium ion battery separator base material (hereinafter, “lithium ion battery separator base material” may be abbreviated as “base material”) and a lithium ion battery separator (hereinafter referred to as “lithium ion battery separator”). (Sometimes abbreviated as “separator”).

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

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

  Moreover, when overcharging is prevented by the protection circuit, it is naturally assumed that the protection circuit does not operate well, and it is difficult to say that it is intrinsically safe. Current lithium-ion batteries are equipped with safety valves, PTC elements, separators with a thermal fuse function, etc. for the purpose of destroying the battery safely when overcharged, because the protection circuit is broken. ing. However, even if equipped with the above-mentioned means, depending on the overcharge conditions, the safety during overcharge is not necessarily ensured, in fact, the lithium ion battery ignition accident is currently But it is happening.

  As the lithium ion battery separator, a porous film made of polyolefin such as polyethylene or polypropylene is often used. The porous film made of polyolefin has a thermal fuse function (shutdown function) that prevents lithium ions from moving and shuts off the current by melting and closing the micropore when the temperature inside the battery reaches around 130 ° C. There is. However, it has been suggested that if the temperature further rises for some reason, the polyolefin itself may melt and short-circuit, causing thermal runaway.

  Therefore, in order to improve heat resistance, a first layer having a water contact angle of 125 ° or more and a second layer having a water contact angle of 120 ° C. or less are provided, and at least one surface has an outermost layer. The nonwoven fabric base material as the second layer, and at least one of the raw fibers includes 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 by 20 ° C. than the low melting point component, A battery separator (for example, refer to Patent Document 1) having a nonwoven fabric base material that is a composite fiber formed in (1) and an insulating layer containing inorganic particles on the base material is disclosed. In these separators, it is necessary to subject the fiber raw material to a water repellent treatment or a hydrophilic treatment, and there is a problem that the work process becomes complicated and the cost increases. Moreover, when a chemical | medical agent is used for a water repellent process or a hydrophilic treatment, there existed a problem which decomposes | disassembles in a battery and deteriorates a battery characteristic.

  Further, a lithium ion, which is a non-woven fabric used as a support to be coated, has an inorganic particle layer formed on the surface of a non-woven fabric containing a polyolefin low-stretch composite fiber having an elongation of 40% or less. A secondary battery separator (see, for example, Patent Document 2) is disclosed. Since these separators are composed only of polyolefin-based fibers, there has been a problem that heat fusion between the fibers proceeds, films are easily formed, and internal resistance tends to deteriorate. In addition, the adhesiveness between the nonwoven fabric and the inorganic particle layer is poor, and there is a problem that repelling is likely to occur when an aqueous coating liquid is used. In the case of solvent coating, there is a problem that the cost is increased.

JP 2017-45663 A JP2013-204154A

  An object of the present invention is to provide a lithium ion battery separator substrate having high adhesion between a lithium ion battery separator substrate and a coating layer containing inorganic particles, and having excellent aqueous coating solution coating properties, and the lithium ion battery separator. An object of the present invention is to provide a lithium ion battery separator that uses a base material for a battery.

  As a result of diligent research to solve the above problems, the following invention has been found.

(1) In the base material for a lithium ion battery separator containing fibrillated cellulose fibers and synthetic resin short fibers, the content of fibrillated natural cellulose fibers is 2 with respect to the total fiber components contained in the base material. A lithium ion battery separator comprising a core-sheath type composite fiber having a core component of a resin having a melting point of 160 ° C. or higher as a synthetic resin short fiber and a polyethylene resin as a sheath component. Substrate for use.

(2) The base material for a lithium ion battery separator according to the above (1), wherein the core component of the core-sheath type composite fiber is a polypropylene resin, and the average fiber diameter is 6 μm or less.

(3) A lithium ion battery separator comprising the lithium ion battery separator substrate according to (1) or (2) above and a coating layer containing inorganic particles.

  The base material for a lithium ion battery separator of the present invention has high adhesiveness with a coating layer containing inorganic particles, and can achieve the effect of being excellent in the coating property of an aqueous coating liquid.

  In the present invention, the base material for a lithium ion battery separator is a lithium ion battery separator that is combined with a coating layer containing inorganic particles, a porous film, a solid (gelled) electrolyte, and the like. This is a precursor sheet. The substrate of the present invention alone does not become a lithium ion battery separator. In terms of heat resistance, a separator having a substrate and a coating layer containing inorganic particles is most preferable.

  In the present invention, as inorganic particles contained in the coating layer, alumina such as α-alumina, β-alumina and γ-alumina; alumina hydrate such as boehmite; magnesium compound such as magnesium oxide and magnesium hydroxide are used. Can do. Among these, α-alumina, alumina hydrate, and magnesium hydroxide are preferably used in terms of high stability to the electrolyte used in the lithium ion battery.

  The porous film is not particularly limited as long as it is a resin capable of forming a film, but polyolefin resins such as polyethylene resins and polypropylene resins are preferable. Examples of polyethylene resins include not only polyethylene resins such as ultra low density polyethylene, low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, and ultra high density polyethylene, but also ethylene-propylene copolymers, Examples thereof include a mixture of a polyethylene resin and another polyolefin resin. Examples of the polypropylene resin include homopropylene (propylene homopolymer), ethylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene and other α-olefins. Examples thereof include a random copolymer with propylene or a block copolymer.

The lithium ion battery in the present invention means a lithium ion secondary battery or a lithium ion polymer secondary battery. The negative electrode active material of the lithium ion battery is not limited in any way, but it is preferable to use a negative electrode active material having an equilibrium potential for absorbing and releasing lithium ions of 1 V (vs Li ⁺ / Li) or less. By using such a negative electrode active material, a battery having a large potential difference between the positive and negative electrodes, that is, a large amount of energy that can be stored, can be obtained. As the negative electrode active material satisfying this condition, for example, a carbon material such as graphite, hard carbon, low crystalline carbon, graphite coated with amorphous carbon, carbon nanotube, or a mixture thereof can be used. In addition to carbon materials, alloys of lithium and one or more metals selected from metallic lithium, aluminum, silica, tin, nickel, and lead, SiO, SnO, Fe ₂ O ₂ , WO ₂ , Nb ₂ O ₅ A metal oxide such as Li _4/3 Ti _5/3 O ₄ or a nitride such as Li _0.4 CoN is used.

The positive electrode active material is not particularly limited as long as it can reversibly occlude and release lithium ions. For example, lithium cobalt oxide (LiCoO _2), lithium nickelate (LiNiO _2), spinel-type lithium manganate (LiMn ₂ O _4), and is represented by the general formula _{LiNi x Co y Mn z O 2} (x + y + z = 1) And composite metal oxides such as lithium vanadium compounds (LiV ₂ O ₅ ) and olivine-type LiMPO ₄ (wherein M represents Co, Ni, Mn, or Fe).

Examples of electrolytes for lithium ion batteries include organic solvents such as propylene carbonate, ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dimethoxyethane, dimethoxymethane, γ-butyrolactone (BL), and mixed solvents thereof. A solution in which a lithium salt is dissolved in a solvent is used. Examples of the lithium salt include lithium hexafluorophosphate (LiPF ₆ ) and lithium tetrafluoroborate (LiBF ₄ ). As solid electrolyte, what melt | dissolved lithium salt in gel-like polymers, such as polyethyleneglycol, its derivative (s), polymethacrylic acid derivative, polysiloxane, its derivative (s), polyvinylidene fluoride, is used.

  The base material for lithium ion battery separators of the present invention is a base material for lithium ion battery separators comprising fibrillated cellulose fibers and synthetic resin short fibers, and fibrils relative to all fiber components contained in the base materials. The content of the modified natural cellulose fiber is 2% by mass or more and 25% by mass or less.

  As a raw material of the base material for the lithium ion battery separator of the present invention, natural cellulose fibers include wood fibers such as conifers and hardwoods, seed hair fibers such as cotton linters, Bombax cotton, cabbages, bamboo fibers, sugarcane fibers, hemp, Examples include gin-skin fibers such as Kozo and Mitsumata, and leaf fibers such as Manila hemp.

  Among these natural cellulose fibers, cellulose derived from pulp such as wood fibers and seed hair fibers which are easily adjusted to an appropriate fiber diameter by fibrillation is preferable. The natural cellulose fiber is preferably high-purity cellulose having a low lignin or hemicellulose content and an α-cellulose content of 98 to 100% by mass.

  In the present invention, the average fiber diameter of the fibrillated natural cellulose fiber is not more than a micron order. That is, the average fiber diameter is 0.15 to 30 μm, preferably 0.2 to 10 μm, more preferably 0.25 to 5 μm, and particularly preferably 0.25 to 1 μm. The average fiber diameter is an average value of 50 fiber diameters obtained by measuring 50 fiber diameters randomly selected from fibers forming the base material by scanning electron microscope observation of the surface or cross section of the base material. .

  The length weighted average fiber length of the fibrillated natural cellulose fiber is preferably 0.2 mm or more and 3.0 mm or less, more preferably 0.2 mm or more and 2.0 mm or less, and 0.2 mm or more and 1.6 mm or less. More preferably. When the length-weighted average fiber length is 0.2 mm or more, it prevents an increase in the ratio of falling off the screen and draining into drainage during wet papermaking, providing excellent adhesion between the substrate and the coating layer. Excellent liquid coatability. In addition, since the length-weighted average fiber length is 3.0 mm or less, the fibers can be prevented from becoming tangled and becoming lumpy, and as a result, the occurrence of uneven thickness can be prevented. Excellent coatability.

  The length-weighted average fiber length of the fibrillated natural cellulose fiber of the present invention was measured using Kajaani Fiber Lab V3.5 (manufactured by Metso Automation). In Kajaani Fiber Lab V3.5 (manufactured by Metso Automation), for each fiber passing through the detection unit, the total true length (L) of the bent fiber and the shortest length (l) of both ends of the bent fiber are determined. Can be measured. The “length-weighted average fiber length” is a length-weighted average fiber length calculated from the projected fiber length obtained by measuring the shortest length (l) of both ends of the bent fiber.

  The effect of being beaten (fibrillated) is that the fibers form a fine structure inside the separator substrate, which improves the electrolyte retention, reduces the pores, and causes internal short circuits. Is difficult to occur. In addition, a dense network structure is formed by synthetic resin short fibers and fibrillated natural cellulose fibers, which can prevent the repelling of water-based coating liquids, provides a base material with excellent coating properties and good adhesion to inorganic particles. It becomes easy.

  In the present invention, as a method for producing fibrillated natural cellulose fiber, a raw material fiber is dispersed in a solvent to prepare a dispersion. Examples of the solvent for dispersion include water and organic solvents, but water is preferable from the viewpoint of productivity and cost. For dispersion, a homodisper, a three-one motor, or the like is used. The concentration of the dispersion is preferably from 0.01 to 5 mass%, more preferably from 0.1 to 3 mass%. The fibrillated natural cellulose fiber gives a high pressure by passing a small-diameter orifice by applying a pressure difference of 20 to 200 MPa to this dispersion, and rapidly decelerates it by colliding it with the wall surface. It is obtained by repeating 5 to 100 times using a high-pressure homogenizer to which is added.

  In the present invention, the fibrillated natural cellulose fiber may be fibrillated only with a high-pressure homogenizer. However, the fibrillated natural cellulose fiber is treated with a combination of a high-pressure homogenizer and other devices such as a refiner, a beater, and a grinding device to be fibrillated. It may be good.

  The content of fibrillated natural cellulose fibers is 2% by mass or more and 25% by mass or less with respect to the total fiber components contained in the base material of the present invention. 5 mass% or more is more preferable, and 10 mass% or more is further more preferable. Moreover, 20 mass% or less is more preferable, and 15 mass% or less is further more preferable. When the content of the fibrillated natural cellulose fiber is less than 2% by mass, the mechanical strength of the base material is increased. However, when an aqueous coating liquid is applied, repelling is likely to occur and the coating property is deteriorated. In addition, the surface of the substrate is easily formed into a film, and the adhesion between the substrate and the coating layer is deteriorated. On the other hand, when the content of the fibrillated natural cellulose fiber exceeds 25% by mass, if the basis weight of the substrate is low, the mechanical strength is deteriorated. In addition, since the coating liquid easily penetrates, the coating liquid passes through, and the coating property of the aqueous coating liquid deteriorates and the adhesion between the base material and the coating layer deteriorates.

  In the present invention, the synthetic resin short fiber includes a core-sheath type composite fiber having a resin having a melting point of 160 ° C. or more as a core component and a polyethylene resin as a sheath component. Hereinafter, unless otherwise specified, “core-sheath type composite fiber having a resin having a melting point of 160 ° C. or more as a core component and a polyethylene resin as a sheath component” may be abbreviated as “core-sheath type composite fiber”.

  In this invention, it is preferable that the ratio of a core-sheath-type composite fiber is 75-98 mass% with respect to all the fiber components contained in this base material, It is more preferable that it is 80-95 mass%, 85- More preferably, it is 90 mass%. When the base material contains the core-sheath type composite fiber, the melting point of the sheath component of the core-sheath type composite fiber makes the bonding point between the fibers strong, and the effect of improving the mechanical strength of the base material is obtained. . In addition, in a sheet-like base material, the core-sheath composite fiber existing on the sheet surface is melted, whereby the adhesion of the sheet surface becomes strong, and the effect of suppressing the fluff on the surface can be obtained.

  When the ratio of the core-sheath type composite fiber is less than 75% by mass, the fibrillated natural cellulose fiber increases, so that the affinity with the aqueous coating solution becomes too high, and the coating solution is easy to break through. May deteriorate, or the adhesion between the substrate and the coating layer may deteriorate. On the other hand, when the ratio of the core-sheath type composite fiber is more than 98% by mass, the adhesion point between the core-sheath type composite fibers increases and the mechanical strength becomes strong, but the sheet surface is easily formed into a film. When applied, repelling is likely to occur, and the coatability may deteriorate or the adhesion between the substrate and the coating layer may deteriorate. In addition, the internal resistance is likely to deteriorate due to the film formation on the sheet surface, and the internal short circuit and self-discharge characteristics are likely to deteriorate due to the expansion of the pore diameter.

  In the present invention, the resin having a melting point of 160 ° C. or higher used as the core component of the core-sheath composite fiber includes polyester, acrylic, polypropylene, wholly aromatic polyester, wholly aromatic polyester amide, polyamide, semi-aromatic polyamide, wholly aromatic. Polyamide, wholly aromatic polyether, wholly aromatic polycarbonate, polyimide, polyamideimide (PAI), polyetheretherketone (PEEK), polyphenylene sulfide (PPS), poly-p-phenylenebenzobisoxazole (PBO), polybenzo Examples include resins such as imidazole (PBI), polytetrafluoroethylene (PTFE), and ethylene-vinyl alcohol copolymer.

  These core-sheath type composite fibers may be used alone or in combination of two or more. Among these, the core component is preferably polyester, acrylic, polypropylene, wholly aromatic polyester, wholly aromatic polyester amide, polyamide, semi-aromatic polyamide, wholly aromatic polyamide, more preferably polyester, acrylic, polypropylene, and polypropylene. Particularly preferred from the viewpoint of spinning.

  When the melting point of the resin used as the core component is 160 ° C. or higher, the shape of the core portion can be maintained. The melting point of the resin is more preferably 163 ° C. or higher. The melting point is a value measured according to JIS K7121: 2012.

  In the present invention, by including a core-sheath type composite fiber using a polyethylene resin in the sheath part, it is easy to form a network structure by being intertwined uniformly with the fibrillated natural cellulose fiber, and melted by applying heat, and adhesive strength A lithium ion battery separator substrate with higher surface smoothness, excellent denseness and mechanical strength, excellent adhesion between the substrate and the coating layer, and aqueous coating solution coating be able to.

  The melting point of the polyethylene resin of the sheath component is preferably 115 ° C. or more from the viewpoint of the effect of suppressing excessive film formation on the substrate surface, and it is 140 ° C. or less to improve the adhesion of the core-sheath composite fiber. It is preferable in terms of effect. The melting point is a value measured according to JIS K7121: 2012.

  The average fiber diameter of the core-sheath type composite fiber is preferably 6 μm or less, more preferably 1.0 to 6.0 μm, further preferably 1.5 to 5.8 μm, and particularly preferably 2.0 to 5.5 μm. When the average fiber diameter is less than 1.0 μm, the fibers are too thin and the substrate is easily formed into a film. On the other hand, as the average fiber diameter is larger than 6 μm, the number of fibers per mass is decreased, so that the bonded portion between the fibers is decreased, and the mechanical strength of the substrate may be decreased. Furthermore, when the substrate is made to have a low thickness of less than 20.0 μm, the maximum pore diameter is enlarged, the aqueous coating liquid can easily penetrate, and the coatability is deteriorated, or the adhesion between the substrate and the coating layer is increased. May decrease. More preferably, by setting the average fiber diameter to 1.0 to 6.0 μm, the base material can be made to have a desired thickness, and the denseness can be made sufficient. It is possible to improve the adhesiveness and the coating property of the aqueous coating liquid.

  The fiber diameter of the core-sheath type composite fiber is determined by measuring the cross-sectional area of each of 40 fibers randomly selected from the fibers forming the base material by scanning electron microscope observation of the cross section of the base material. It is an average value of 40 fiber diameters when the fiber diameter is calculated assuming that it is a perfect circle. In the present invention, it is preferable that the fiber diameter of all the core-sheath type composite fibers is 6.0 μm or less.

  In the present invention, the base material may contain not only fibrillated natural cellulose fibers and core-sheath type composite fibers but also synthetic resin short fibers other than the core-sheath type composite fibers. Synthetic resin short fibers other than core-sheath type composite fibers include polyolefin, polyester, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyamide, acrylic, polyvinyl chloride, polyvinylidene chloride, polyvinyl ether, polyvinyl ketone, and polyether. And non-fibrillated short fibers made of synthetic resins such as polyvinyl alcohol, diene, polyurethane, phenol, melamine, furan, urea, aniline, unsaturated polyester, fluorine, silicone, and derivatives thereof. By including synthetic resin short fibers other than the core-sheath type composite fiber, the tensile strength and puncture strength of the substrate can be increased.

  The synthetic resin short fiber other than the core-sheath type composite fiber may be a fiber (single fiber) made of a single resin, or may be a composite fiber made of two or more kinds of resins. Moreover, the synthetic resin short fiber contained in the base material of the present invention may be one kind or a combination of two or more kinds. Examples of the composite fiber include a core-sheath type, an eccentric type, a side-by-side type, a sea-island type, an orange type, and a multiple bimetal type.

  The fineness of the synthetic resin short fibers other than the core-sheath type composite fiber is preferably 0.01 dtex or more and 0.6 dtex or less, and more preferably 0.02 dtex or more and 0.3 dtex or less. When the fineness exceeds 0.6 dtex, the number of fibers in the thickness direction is reduced, so that the pore size distribution of the base material is widened. As a result, the aqueous coating liquid is easily deflated and the coating property is deteriorated. It is easy to do, and the adhesiveness of a base material and a coating layer may also deteriorate. On the other hand, when the fineness is less than 0.01 dtex, the fiber becomes very expensive and it becomes difficult to stably produce the fiber, or when the substrate is produced by a wet papermaking method, the dehydrating property may be lowered.

  The fiber length of the synthetic resin short fiber is preferably 1 mm or more and 10 mm or less, and more preferably 1 mm or more and 5 mm or less. If the fiber length exceeds 10 mm, formation may be poor. On the other hand, when the fiber length is less than 1 mm, the mechanical strength of the base material is lowered, and the base material may be damaged when the coating layer is formed. In addition, the preferable fiber length in the core-sheath type composite fiber of 6.0 micrometers or less is also the same range as the above.

  The base material for lithium ion battery separators of the present invention may contain fibers other than fibrillated natural cellulose fibers and synthetic resin short fibers. Examples thereof include fibrids made of synthetic resin, pulped products made of synthetic resin, and inorganic fibers. Examples of the inorganic fiber include glass, alumina, silica, ceramics, and rock wool. Regenerated cellulose may be contained as the cellulose fiber.

  The thickness of the base material for a lithium ion battery separator of the present invention is preferably 6 μm or more, more preferably 8 μm or more, and further preferably 10 μm or more. Moreover, 20 micrometers or less are preferable, 18 micrometers or less are more preferable, and 15 micrometers or less are still more preferable. Even when the thickness of the base material is within the above range, the base material of the present invention can keep the internal resistance low, and can maintain the tensile strength necessary in the coating process and electrode lamination process. In addition, the workability in each process is not impaired. If the thickness of the substrate exceeds 20 μm, the internal resistance of the substrate may become too high. In addition, the battery may not be able to have a high capacity. If the thickness of the base material is less than 6 μm, the strength of the base material becomes too weak, and there is a risk of damage during handling or coating of the base material.

Density of the lithium ion battery separator base material of the present invention, 0.25 g / cm ³ or more 0.65 g / cm ³ or less are preferred, 0.30 g / cm ³ or more 0.50 g / cm ³ or less is more preferable. If the density is less than 0.25 g / cm ³ , the strength of the base material becomes too weak and may be damaged during handling or coating of the base material. If the density exceeds 0.65 g / cm ³ , When the material is made into a film, the adhesiveness between the base material and the coating layer is deteriorated, the coating property of the aqueous coating liquid is deteriorated, or the internal resistance of the base material is sometimes too high.

  It is preferable that the base material for lithium ion battery separators of this invention is a wet nonwoven fabric manufactured by the wet papermaking method. In the wet papermaking method, fibers are dispersed in water to form a uniform papermaking slurry, and this papermaking slurry is rolled up with a papermaking machine to produce a wet nonwoven fabric. Examples of the paper machine include a circular net paper machine, a long net paper machine, an inclined paper machine, an inclined short net paper machine, and a composite machine of these. In the process of manufacturing a wet nonwoven fabric, hydroentanglement treatment may be performed as necessary. As the wet nonwoven fabric processing, heat treatment, calendar treatment, thermal calendar treatment, or the like may be performed.

  In the present invention, the average particle size of the inorganic particles is preferably 0.02 μm or more and 2.00 μm or less, and more preferably 0.10 μm or more and 1.00 μm or less. When the average particle diameter exceeds 2.00 μm, it may be difficult to form the coating layer with an appropriate thickness, or the surface unevenness may be increased. On the other hand, when the average particle diameter is less than 0.02 μm, it is necessary that the inorganic particles easily fall off from the separator base material and that the binder should be increased in order to prevent the dropout. In addition, the average particle diameter said by this invention represents the volume average particle diameter measured by the laser diffraction scattering method.

  In the present invention, the coating layer can contain a binder. Various organic polymers can be used as the binder. Examples include styrene-butadiene copolymer elastomers (styrene butadiene rubber), acrylonitrile-butadiene copolymer elastomers, (meth) acrylate polymer elastomers, styrene- (meth) acrylate polymer elastomers, polyvinylidene fluoride heavy Various organic polymers such as coalescence can be used.

  In the present invention, the content of the binder contained in the coating layer is preferably 2% by mass or more and 200% by mass or less with respect to the total amount of inorganic particles. 5 mass% or more and 50 mass% or less are especially preferable. If the amount of the binder is too small, the inorganic particles may easily fall off from the substrate. On the other hand, if the amount of the binder is too large, the coating layer becomes too dense and the ion permeability may be lowered.

  A separator having a coating layer containing a substrate and inorganic particles can be produced by forming a coating layer on at least one surface of the substrate. As a method of forming a coating layer on at least one surface of a substrate, a slurry (coating solution) for forming a coating layer in which each component constituting the coating layer is dispersed or dissolved in a medium such as water or an organic solvent is used. The method of preparing and coating this on a base material is mentioned.

  The medium for preparing the slurry for forming the coating layer is not particularly limited as long as it can uniformly dissolve or disperse the binder and inorganic particles. For example, aromatic hydrocarbons 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 or the like can be used as necessary. The medium used is preferably a medium that does not expand the base material or a medium that does not dissolve the base material.

  In order to form the coating layer, various coating apparatuses can be used as an apparatus for coating the substrate with the coating liquid. For example, various coaters such as a gravure coater, a die coater, a lip coater, a blade coater, a curtain coater, an air knife coater, a rod coater, a roll coater, a kiss touch coater, and a dip coater can be used.

Although the coating amount of the coating layer depends on the substrate, the dry coating amount per side of the substrate is preferably 1 g / m ² or more and 30 g / m ² or less, and preferably 2 g / m ² or more and 20 g / m ² or less. Is more preferably 3 g / m ² or more and 15 g / m ² or less. When the coating amount of the coating layer is too small, when the coating layer is formed, the coating liquid may penetrate into the substrate, and the coating layer may not be formed on the surface of the substrate. Moreover, when there is too much coating amount, by filling up the pore of a base material too much, ion permeability may be inhibited and battery characteristics may worsen.

  In the coating layer of the present invention, in addition to the inorganic particles and the binder, various dispersants such as polyacrylic acid and sodium carboxymethyl cellulose, various thickeners such as hydroxyethyl cellulose, sodium carboxymethyl cellulose, and polyethylene oxide, various wetting agents Various additives such as preservatives and antifoaming agents can be added as necessary. In general, a non-aqueous coating liquid using an organic solvent as a medium has a low surface tension, and an aqueous coating liquid using water as a medium has a high surface tension. Since the base material of the present invention has high acceptability of the coating liquid, both the non-aqueous coating liquid and the aqueous coating liquid can be applied without any problem, but in the present invention, an aqueous system using only water as a medium. It is preferable to use a coating liquid.

  EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples. In the examples, all percentages (%) and parts are based on mass unless otherwise specified. The coating amount is a dry coating amount.

Example 1
<Preparation of base material>
The core component is polypropylene resin, the sheath component is polyethylene resin, the fineness of 0.2 dtex (average fiber diameter 5.6 μm), the core-sheath type composite fiber 98 parts by weight of the fiber length 3 mm, and the concentration of cotton linter pulp to 2% by mass The mixture was dispersed in water so that the clearance was gradually narrowed using a double disc refiner, and the treatment was repeated 10 times, followed by 20 treatments using a high-pressure homogenizer at 49 MPa, and an average fiber diameter of 0. 2 parts by mass of fibrillated natural cellulose fibers having a length of 3 μm and a weight-weighted average fiber length of 0.22 mm are dispersed in water by a pulper to prepare a uniform papermaking slurry having a concentration of 0.5% by mass, and a circular net type papermaking A wet paper web was obtained using a machine and dried with a cylinder dryer having a surface temperature of 135 ° C. to obtain a sheet. One of the rolls is a chrome-plated steel roll, the other roll is a resin roll of hardness Shore D92, and the obtained sheet is heat calendered by a heat calender device having a surface temperature of the steel roll of 128 ° C. Thus, a base material having a basis weight of 6 g / m ² and a thickness of 15 μm was produced.

<Preparation of coating liquid>
100 parts by mass of boehmite having a volume average particle size of 0.9 μm and a specific surface area of 5.5 m ² / g, 120% by mass of a 1% by mass aqueous solution of a carboxymethyl cellulose sodium salt having a viscosity of 200 mPa · s at 25 ° C. Next, 300 parts by mass of a 1% by mass aqueous solution of a carboxymethyl cellulose sodium salt having a viscosity of 7000 mPa · s at a viscosity of 7000 mPa · s, a styrene butadiene rubber (SBR) system for a lithium ion battery. A coating solution was prepared by mixing and stirring 10 parts by weight of a binder (manufactured by JSR Corporation, trade name: TRD2001) (solid content concentration: 48% by mass).

<Preparation of separator>
On one side of the base material, a coating liquid was applied and dried so as to have a coating amount of 10 g / m ² with a kiss reverse gravure coater, thereby preparing a separator.

Example 2
90 parts by mass of a core-sheath type composite fiber having a fineness of 0.2 dtex (average fiber diameter of 5.6 μm) and a fiber length of 3 mm, in which the core component is a polypropylene resin and the sheath component is a polyethylene resin, and the average fiber diameter used in Example 1 A substrate having a basis weight of 6 g / m ² and a thickness of 15 μm was prepared in the same manner as in Example 1 except that 10 parts by mass of fibrillated natural cellulose fibers having a length-weighted average fiber length of 0.22 mm was used. . Next, the coating liquid used in Example 1 was coated and dried by the same method as in Example 1 so that the coating amount was 10 g / m ² , thereby producing a separator.

Example 3
80 parts by mass of a core-sheath type composite fiber having a fineness of 0.2 dtex (average fiber diameter of 5.6 μm) and a fiber length of 3 mm, and a mean fiber diameter used in Example 1, in which the core component is polypropylene resin and the sheath component is polyethylene resin A substrate having a basis weight of 7 g / m ² and a thickness of 18 μm was prepared in the same manner as in Example 1 except that 20 parts by mass of fibrillated natural cellulose fibers having a length-weighted average fiber length of 0.22 mm was used. . Next, the coating liquid used in Example 1 was coated and dried by the same method as in Example 1 so that the coating amount was 10 g / m ² , thereby producing a separator.

Example 4
The core component is a polypropylene resin and the sheath component is a polyethylene resin. Fineness 0.2 dtex (average fiber diameter 5.6 μm), core-sheath type composite fiber 75 parts by mass of fiber length 3 mm, and average fiber diameter used in Example 1 A substrate having a basis weight of 8 g / m ² and a thickness of 20 μm was prepared in the same manner as in Example 1 except that 25 parts by mass of fibrillated natural cellulose fibers having a length-weighted average fiber length of 0.22 mm was used. . Next, the coating liquid used in Example 1 was coated and dried by the same method as in Example 1 so that the coating amount was 10 g / m ² , thereby producing a separator.

Example 5
90 parts by mass of a core-sheath type composite fiber having a fineness of 0.5 dtex (average fiber diameter of 8.1 μm) and a fiber length of 3 mm, in which the core component is a polypropylene resin and the sheath component is a polyethylene resin, and the average fiber diameter used in Example 1 A substrate having a basis weight of 8 g / m ² and a thickness of 20 μm was prepared in the same manner as in Example 1 except that 10 parts by mass of fibrillated natural cellulose fibers having a length-weighted average fiber length of 0.22 mm was used. . Next, the coating liquid used in Example 1 was coated and dried by the same method as in Example 1 so that the coating amount was 10 g / m ² , thereby producing a separator.

Example 6
The core component is a polypropylene resin and the sheath component is a polyethylene resin. Fineness 0.2 dtex (average fiber diameter 5.6 μm), core-sheath type composite fiber 70 mass parts with a fiber length of 3 mm, and 0.3 dtex (average fiber diameter 6. 7μm), 10 parts by mass of polypropylene short fiber having a fiber length of 3 mm, and cotton linter pulp are dispersed in water to a concentration of 2% by mass, and repeatedly treated 5 times while gradually narrowing the clearance using a double disc refiner. Then, using a high-pressure homogenizer, the treatment was repeated 20 times under the condition of 49 MPa, except that 20 parts by mass of fibrillated natural cellulose fibers having an average fiber diameter of 0.4 μm and a length-weighted average fiber length of 0.25 mm were carried out. A substrate having a basis weight of 7 g / m ² and a thickness of 18 μm was produced in the same manner as in Example 1. Next, the coating liquid used in Example 1 was coated and dried by the same method as in Example 1 so that the coating amount was 10 g / m ² , thereby producing a separator.

Comparative Example 1
The same method as in Example 1 except that the core component is polypropylene resin and the sheath component is polyethylene resin, and the core-sheath composite fiber is 100 parts by mass with a fineness of 0.2 dtex (average fiber diameter 5.6 μm) and a fiber length of 3 mm. A substrate having a basis weight of 8 g / m ² and a thickness of 20 μm was prepared. Next, the coating liquid used in Example 1 was coated and dried by the same method as in Example 1 so that the coating amount was 10 g / m ² , thereby producing a separator.

Comparative Example 2
98.6 parts by mass of core-sheath type composite fiber having a fineness of 0.2 dtex (average fiber diameter 5.6 μm) and a fiber length of 3 mm, in which the core component is polypropylene resin and the sheath component is polyethylene resin, and the average used in Example 1 A basis having a basis weight of 6 g / m ² and a thickness of 15 μm in the same manner as in Example 1 except that 1.4 parts by mass of fibrillated natural cellulose fiber having a fiber diameter of 0.3 μm and a length-weighted average fiber length of 0.22 mm was used. A material was prepared. Next, the coating liquid used in Example 1 was coated and dried by the same method as in Example 1 so that the coating amount was 10 g / m ² , thereby producing a separator.

Comparative Example 3
The core component is a polypropylene resin and the sheath component is a polyethylene resin. Fineness of 0.2 dtex (average fiber diameter 5.6 μm), 74 parts by mass of a core-sheath type composite fiber having a fiber length of 3 mm, and the average fiber diameter used in Example 1 A base material having a basis weight of 8 g / m ² and a thickness of 20 μm was prepared in the same manner as in Example 1 except that 26 parts by mass of fibrillated natural cellulose fiber having a length-weighted average fiber length of 0.22 mm was used. . Next, the coating liquid used in Example 1 was coated and dried by the same method as in Example 1 so that the coating amount was 10 g / m ² , thereby producing a separator.

  The following physical properties were measured and evaluated for the lithium ion battery separator substrates and lithium ion battery separators of Examples and Comparative Examples, and the results are shown in Tables 1 and 2.

<Base weight of base material and coating amount of coating layer>
Based on JIS P8124, the basic weight of the base material and the separator was measured. The coating amount of the coating layer was calculated by subtracting the basis weight of the substrate from the basis weight of the separator.

<Thickness of base material and separator>
The thickness at the time of 5N load was measured using the outside micrometer prescribed | regulated to JISB7502.

<Adhesiveness between substrate and coating layer>
Cut out 5 test pieces each measuring 100 mm in width and 100 mm in the flow direction from each separator, and lightly affix a cellophane tape of 10 mm in width and 50 mm in length from the top of the separator coating layer in the flow and width directions. The stainless steel cylinder was reciprocated twice. Thereafter, the cellophane tape was removed from the coating layer, the separator was visually observed, and evaluated according to the following evaluation criteria.

○: A coating layer remains on the substrate surface.
Δ: About half of the coating layer is peeled off from the substrate.
X: Almost all of the coating layer is peeled off from the substrate.

<Coating properties of coating liquid>
The coating layer surface of each separator was visually observed and evaluated according to the following evaluation criteria.

○: The coating layer surface is uniform and there is no repellency.
Δ: The surface of the coating layer is not repelled, but unevenness and pinholes are slightly seen.
X: Repelling is observed on the surface of the coating layer.

  As shown in Table 1, the base materials produced in Examples 1 to 6 contain fibrillated cellulose fibers and synthetic resin short fibers, and fibrillated natural cellulose with respect to all fiber components contained in the base material. The fiber content is 2% by mass or more and 25% by mass or less, and the synthetic resin short fiber includes a core-sheath type composite fiber having a resin having a melting point of 160 ° C. or more as a core component and a polyethylene resin as a sheath component. The separator which has the base material of Examples 1-6 and the coating layer containing an inorganic particle had high adhesiveness of a base material and a coating layer, and was excellent in the applicability | paintability of aqueous | water-based coating liquid.

  When compared in Examples 1 to 6, the base material of Example 5 is a case where the fiber diameter of the core-sheath type composite fiber having polypropylene resin as the core component and polyethylene resin as the sheath component exceeds 6 μm and is 8 μm. There was no problem in the adhesion between the base material and the coating layer, and no repelling of the aqueous coating liquid was observed, but the aqueous coating liquid easily entered the base material, and irregularities and pinholes were faintly seen.

  Since the base material of Comparative Example 1 did not contain fibrillated natural cellulose fibers, the aqueous coating liquid hardly penetrated into the base material, the adhesion between the base material and the coating layer was poor, and the coating layer was easily peeled off from the base material. Moreover, the phenomenon which repels a water-system coating liquid partially on the base material was seen, and the part which cannot be coated remained.

  The base material of the comparative example 2 is a case where the content rate of a fibrillated natural cellulose fiber is less than 2 mass%. For this reason, the aqueous coating liquid hardly penetrates into the substrate, and the coating layer is easily peeled off from the substrate. In addition, although no repelling of the aqueous coating liquid was observed, slight unevenness was observed on the surface of the coating layer.

  The base material of the comparative example 3 is a case where the content rate of fibrillated natural cellulose fiber exceeds 25 mass%. The aqueous coating liquid is likely to soak into the substrate, and the aqueous coating liquid comes through a little. For this reason, the adhesion between the substrate and the coating layer was deteriorated, and although the water-based coating liquid was not repelled, irregularities and pinholes were observed on the coating layer surface.

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

Claims (3)

  In the base material for a lithium ion battery separator comprising a fibrillated cellulose fiber and a short synthetic resin fiber, the content of the fibrillated natural cellulose fiber is 2% by mass or more based on the total fiber components contained in the base material. A base material for a lithium ion battery separator, comprising a core-sheath type composite fiber having a core component of a resin having a melting point of 160 ° C. or more as a synthetic resin short fiber and a polyethylene resin as a sheath component. .   The base material for a lithium ion battery separator according to claim 1, wherein the core component of the core-sheath composite fiber is a polypropylene resin, and the average fiber diameter is 6 µm or less.   A lithium ion battery separator comprising the lithium ion battery separator substrate according to claim 1 and a coating layer containing inorganic particles.