JP2019212490A - 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 which is high in adhesiveness between the base material for the lithium ion battery separator and a coated layer containing inorganic particles, excellent in coatability of an aqueous coating solution and also excellent in storage characteristics at high temperature even when containing a polyethylene terephthalate resin 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 synthetic resin short fiber, the synthetic resin short fiber is contained, which consists of a polyethylene terephthalate resin and a core-sheath composite fiber with an average fiber diameter of 6 μm or less in which a resin with a melting point of 160°C or more is used as a core component and a polyethylene resin is used as a sheath component, are contained as the synthetic resin.The percentage content of the core-sheath composite fiber is 70 to 95% by mass based on all fiber components contained in the base material.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.

On the other hand, a polyvinylidene fluoride polymer porous membrane (for example, Patent Document 3) based on a polyethylene terephthalate (PET) nonwoven fabric has been proposed as a separator having high safety and heat resistance during overcharge. However, when a separator including such a polyethylene terephthalate nonwoven fabric and an electrolytic solution are combined with an electrolyte LiPF ₆ dissolved in a carbonate-based solvent such as ethylene carbonate or diethyl carbonate, for example, it is reduced on the negative electrode surface. In a system in which a reaction is caused to form a radical and this radical exists, a decomposition reaction by transesterification proceeds between the PET and the solvent, so that there is a problem that the PET deteriorates and the storage characteristics at high temperature deteriorates. It is disclosed. For this problem, in a lithium ion secondary battery having a separator using a PET nonwoven fabric, vinylene carbonate, vinyl acetate, vinyl butyrate, vinyl hexanate, etc. are added to the electrolyte to suppress the decomposition reaction of PET. A method (for example, Patent Documents 4 and 5) has been proposed.

  However, the method of adding vinylene carbonate or the like to the electrolytic solution suppresses the decomposition reaction at high temperature of the separator using the PET nonwoven fabric and is effective for improving the storage characteristics, but the battery characteristics are not added to vinylene carbonate or the like. In comparison, there was a problem of a decrease.

JP 2017-45663 A JP2013-204154A International Publication No. 01/67536 Pamphlet JP 2005-293891 A JP 2003-187867 A

  The problem of the present invention is that the adhesion between the base material for lithium ion battery separator and the coating layer containing inorganic particles is high, the coating property of the aqueous coating liquid is excellent, and even when the polyethylene terephthalate resin is included, the storage property at high temperature An object of the present invention is to provide an excellent base material for a lithium ion battery separator and a lithium ion battery separator using the base material for a lithium ion battery separator.

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

(1) In a base material for a lithium ion battery separator containing a synthetic resin short fiber, the synthetic fiber short fiber has an average fiber diameter of 6 μm or less using a resin having a melting point of 160 ° C. or more as a core component and a polyethylene resin as a sheath component. Core-sheath type composite fiber and synthetic resin short fiber made of polyethylene terephthalate resin, and the content of the core-sheath type composite fiber is 70 to 95% by mass with respect to the total fiber components contained in the base material A base material for a lithium ion battery separator.

(2) The base material for a lithium ion battery separator according to (1), wherein an average fiber diameter of the synthetic resin short fibers made of the polyethylene terephthalate resin is 5.5 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 lithium ion battery separator of the present invention has high adhesiveness with a coating layer containing inorganic particles, excellent coating properties of aqueous coating liquid, and excellent storage characteristics at high temperatures even when it contains a polyethylene terephthalate resin. The effect can be achieved.

  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 a lithium ion battery separator of the present invention is a base material for a lithium ion battery separator comprising a synthetic resin short fiber. The synthetic resin short fiber is a resin having a melting point of 160 ° C. or more as a core component, and a polyethylene resin. A core-sheath type composite fiber having an average fiber diameter of 6 μm or less as a sheath component and a synthetic resin short fiber made of polyethylene terephthalate resin, the content of the core-sheath type composite fiber with respect to all the fiber components contained in the base material 70 to 95% by mass.

  In the present invention, the core-sheath type composite 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. 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”. Further, “synthetic resin short fiber made of polyethylene terephthalate resin” is sometimes abbreviated as “PET fiber”.

  In the present invention, the ratio of the core-sheath type composite fiber is 70 to 95% by mass, more preferably 80 to 95% by mass, and more preferably 85 to 95% by mass with respect to the total fiber components contained in the substrate. More preferably. When the base material contains the core-sheath type composite fiber, the melting point 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. Moreover, in the base material which is sheet-like, the fusion of the core-sheath type composite fiber existing on the sheet surface strengthens the adhesion of the sheet surface, and the effect of strengthening the fiber fixation on the surface is obtained. In addition, even when PET fiber is included in the base material, the shape of the base material can be maintained, so that an effect that storage characteristics at high temperatures are hardly deteriorated can be obtained.

  When the ratio of the core-sheath type composite fiber is less than 70% by mass, the adhesion point between the fibers does not increase, and the effect of improving the mechanical strength may be lowered. When the PET fiber increases, the affinity with the water-based coating liquid becomes too high, and the coating liquid tends to get through and the coating property deteriorates, or the adhesion between the substrate and the coating layer may deteriorate. is there. Moreover, when it contains PET fiber and it preserve | saves at high temperature, it becomes impossible to maintain the shape of a base material, and a preservation | save characteristic may deteriorate. On the other hand, when the ratio of the core-sheath type composite fiber is more than 95% by mass, the adhesion point between the core-sheath type composite fibers increases and the mechanical strength increases, but the sheet surface is easily formed into a film, and an aqueous coating solution is applied. In such a case, the repellency is likely to occur and the coatability may be deteriorated or the adhesion between the substrate and the coating layer may be deteriorated. In addition, the internal resistance is likely to deteriorate due to the film formation on the sheet surface, and the internal short circuit may be easily deteriorated due to the expansion of the pore diameter.

  In the present invention, resins having a melting point of 160 ° C. or higher used as the core component of the core-sheath type composite fiber include polyester, acrylic, polypropylene (PP), wholly aromatic polyester, wholly aromatic polyester amide, polyamide, and semi-aromatic polyamide. , Wholly aromatic polyamide, wholly aromatic polyether, wholly aromatic polycarbonate, polyimide, polyamideimide (PAI), polyetheretherketone (PEEK), polyphenylene sulfide (PPS), poly-p-phenylenebenzobisoxazole (PBO) And resins such as polybenzimidazole (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 used as the core component is 160 ° C. or higher, and 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, compared with other synthetic resin short fibers, the fibers contained in the base material are intertwined uniformly to form a network structure. Easy to melt by applying heat, can increase the adhesive strength, excellent in denseness and mechanical strength, excellent adhesion between the base and coating layers and aqueous coating liquid, and contains PET fibers However, it is possible to obtain a base material for a lithium ion battery separator that can maintain storage characteristics at high temperatures.

  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 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. And the coating property of the aqueous coating liquid can be improved, and the shape and mechanical strength as a substrate can be maintained.

  In the present invention, the average fiber diameter is determined by measuring the cross-sectional area of each of 40 fibers randomly selected from the fibers forming the base material by observing the cross section of the base material with a scanning electron microscope. It is an average value of the 40 fiber diameters when the fiber diameter is calculated assuming that it is a circle. In the present invention, the fiber diameter of all the core-sheath type composite fibers is preferably 6 μm or less.

  In the present invention, the base material may contain a synthetic resin short fiber other than the core-sheath composite fiber and the PET fiber. Synthetic short fibers other than core-sheath composite fibers and PET fibers include polyolefin, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyamide, acrylic, polyvinyl chloride, polyvinylidene chloride, polyvinyl ether, polyvinyl ketone, poly Non-fibrillated short fibers made of synthetic resins such as ether, polyvinyl alcohol, diene, polyurethane, phenol, melamine, furan, urea, aniline, unsaturated polyester, fluorine, silicone, and derivatives thereof.

  The base material of the present invention includes a PET fiber having a feature that the fiber diameter can be reduced and the melting point is high, as a synthetic resin short fiber. By using PET fibers, the thickness can be reduced, the tensile strength and puncture strength of the substrate can be increased, and the dimensional stability of the substrate can be improved. Moreover, the affinity with the aqueous coating liquid can be increased, and the coating becomes easy.

  The content of the PET fiber is preferably 5% by mass or more and 30% by mass or less with respect to all the fiber components contained in the base material of the present invention. Moreover, 7 mass% or more and 20 mass% or less are more preferable, and 10 mass% or more and 15 mass% or less are further more preferable. When the content of the PET fiber is less than 5% by mass, it is difficult to obtain an effect of improving the mechanical strength, puncture strength, and dimensional stability of the substrate. On the other hand, when the content of the PET fiber exceeds 30% by mass, the shape and mechanical strength of the substrate during high temperature storage may not be maintained.

  The average fiber diameter of the PET fibers contained in the substrate of the present invention is preferably 5.5 μm or less, more preferably 0.9 μm or more and 5.5 μm or less, and even more preferably 1.5 μm or more and 3.5 μm or less. When the average fiber diameter exceeds 5.5 μm, the pore diameter distribution of the base material cannot be narrowed, and as a result, the aqueous coating liquid may easily penetrate through. Moreover, the adhesiveness and coating property of a base material and a coating layer may deteriorate. On the other hand, when the average fiber diameter is less than 0.9 μm, the sea component of the sea-island fiber is dissolved and the island component is taken out to produce the PET fiber, so that the fiber becomes very expensive. Since crystallization often does not progress, resistance to the electrolyte solution may be low. Moreover, since the number of fibers increases too much, the mechanical strength may decrease.

  In the present invention, the fineness of the synthetic resin short fibers other than the core-sheath composite fiber and the PET fiber will be described. The fineness is preferably 0.01 dtex or more and 0.3 dtex or less, and more preferably 0.02 dtex or more and 0.1 dtex or less. When the fineness exceeds 0.3 dtex, the number of fibers in the thickness direction is reduced, so that the pore diameter 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 stable production of the fiber may be difficult.

  The fiber length of the synthetic resin short fiber of the present invention 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.

  The base material of the present invention may contain fibrillated heat-resistant fibers as fibers other than synthetic resin short fibers. Fibrylated heat-resistant fibers include wholly aromatic polyamide, wholly aromatic polyester, polyimide, polyamideimide, polyether ether ketone, polyphenylene sulfide, polybenzimidazole, poly-p-phenylenebenzobisthiazole, poly-p-phenylenebenzo. A fibrillated fiber made of a heat resistant resin such as bisoxazole or polytetrafluoroethylene is used. Among these, wholly aromatic polyamides, which are easily fibrillated and have high affinity with the electrolytic solution and aqueous coating solution, are preferable.

  In the present invention, the modified drainage degree of the fibrillated heat-resistant fiber is preferably 0 ml or more and 400 ml or less, preferably 0 ml or more and 350 ml or less, more preferably 0 ml or more and 300 ml or less. When the modified freeness is 400 ml or less, it becomes easy to form a dense network structure with the synthetic resin short fibers, so that the mechanical strength of the base material is easily improved and the shape of the base material is easily maintained. Further, the coating property of the aqueous coating liquid and the adhesiveness with the coating layer are good. On the other hand, when the modified freeness is less than 0 ml, fibrillation of the fibrillated heat-resistant fiber proceeds too much, and the number of fine fibers joined by the core-sheath type composite fiber increases, so that the tensile strength may decrease. is there. As fibrillation of fibrillated heat-resistant fibers progresses, modified freeness continues to decrease. Even after the modified freeness reaches 0 ml, when the fiber is further fibrillated, the fibers pass through the mesh, and the modified freeness starts to increase. In the present invention, the state in which the modified freeness starts to increase in reverse is referred to as “the modified freeness is less than 0 ml”.

  In the present invention, the modified freeness refers to JIS P8121-2: 2012 except that an 80 mesh wire net having a wire diameter of 0.14 mm and an aperture of 0.18 mm is used as a sieve plate and the sample concentration is 0.1%. It is a value measured according to

  In the fibrillated heat resistant fiber, the mass weighted average fiber length is preferably 0.02 mm or more and 1.50 mm or less. The length weighted average fiber length is preferably 0.02 mm or more and 1.00 mm or less. When the average fiber length is shorter than the preferred range, the fibrillated heat resistant fiber may fall off from the substrate. When the average fiber length is longer than the preferred range, the coating property of the aqueous coating liquid and the smoothness of the coating layer may be lowered, or the adhesiveness with the coating layer containing inorganic particles may be lowered. Moreover, disaggregation of the fibers is deteriorated, and dispersion failure is likely to occur.

  When the fibrillated heat-resistant fiber has the above-described mass-weighted average fiber length and length-weighted average fiber length, a dense network structure is formed between the fibrillated heat-resistant fiber and the synthetic resin short fiber, It is possible to prevent the water-based coating liquid from being repelled, and to easily obtain a substrate having excellent coating properties and good adhesion to inorganic particles.

  In the present invention, the mass-weighted average fiber length and length-weighted average fiber length of the fibrillated heat-resistant fiber are the mass weight measured in the projected fiber length (Proj) mode using Kajaani FiberLab V3.5 (manufactured by Metso Automation). An average fiber length (L (w)) and a length weighted average fiber length (L (l)).

  The average fiber width of the fibrillated heat resistant fiber is preferably from 0.5 μm to 30.0 μm, more preferably from 3.0 μm to 25.0 μm, and even more preferably from 5.0 μm to 20.0 μm. When the average fiber width exceeds 30.0 μm, the coating property of the aqueous coating liquid and the adhesion between the base material and the coating layer may deteriorate, and when the average fiber width is less than 0.5 μm, the base material falls off the base material. There is a case.

  In the present invention, the average fiber width of the fibrillated heat-resistant fiber is a fiber width (Fiber Width) measured using Kajaani Fiber Lab V3.5 (manufactured by Metso Automation).

  Fibrilized heat-resistant fiber consists of refiner, beater, mill, milling device, rotary homogenizer that gives shearing force by high-speed rotary blade, high-speed rotating cylindrical inner blade and fixed outer blade Double cylindrical high-speed homogenizer that generates shearing force between them, ultrasonic crusher that is refined by ultrasonic impact, and a high-speed by passing a small-diameter orifice by applying a pressure difference of at least 20 MPa to the fiber suspension Then, it can be obtained by processing using a high-pressure homogenizer or the like that applies a shearing force or cutting force to the fiber by colliding with this and rapidly decelerating.

  When the fibrillated heat-resistant fiber is incorporated in the base material of the present invention, a dense network structure is formed with the synthetic resin short fiber, so that the mechanical strength of the base material is reduced little during high temperature storage and the shape is maintained. And storage characteristics can be kept good. When the fibrillated heat-resistant fiber is contained, the content with respect to all the fiber components contained in the base material of the present invention is preferably 2% by mass or more and 20% by mass or less, more preferably 5% by mass or more and 15% by mass or less. 7 mass% or more and 10 mass% or less are still more preferable.

  The base material for a lithium ion battery separator of the present invention may contain fibers other than synthetic resin short fibers and fibrillated heat resistant fibers. Examples thereof include cellulose fibers, pulped and fibrillated cellulose fibers, fibrils made of synthetic resins, pulped products made of synthetic resins, and inorganic fibers. Examples of the inorganic fiber include glass, alumina, silica, ceramics, and rock wool. The cellulose fiber may be either natural cellulose or regenerated cellulose.

  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. If the average particle size is too large, it may be difficult to form the coating layer with an appropriate thickness, or the surface irregularities may be large. On the other hand, if the average particle diameter is too small, the inorganic particles are liable to fall off from the separator substrate, and the binder must 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 Examples, percentages (%) and parts are all 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 and the sheath component is polyethylene resin, the fineness of 0.2 dtex (average fiber diameter 5.6 μm), the core-sheath type composite fiber 95 parts by mass and the fineness 0.06 dtex (average fiber diameter 2. 5 μm), 5 parts by weight of stretched PET fiber having a fiber length of 3 mm, are dispersed in water by a pulper to prepare a uniform papermaking slurry having a concentration of 0.5% by weight, and a wet paper web is formed using a circular net type papermaking machine. Obtained and dried by a cylinder dryer having a surface temperature of 135 ° C. to obtain a sheet. The obtained sheet is a steel roll whose one roll is chrome-plated, the other roll is a resin roll of hardness Shore D92, and the surface temperature of the steel roll is heat calendered by a heat calender device having a surface temperature of 128 ° C. A base material having a basis weight of 7 g / m ² and a thickness of 17 μm was prepared.

<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
A core component is a polypropylene resin and a sheath component is a polyethylene resin, a fineness of 0.2 dtex (average fiber diameter of 5.6 μm), a core-sheath type composite fiber having a fiber length of 3 mm, 85 parts by mass, and a fineness of 0.06 dtex (average fiber diameter of 2 0.5 μm), and 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 the stretched PET fiber was 15 parts by mass with a fiber length of 3 mm. 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
The core component is a polypropylene resin and the sheath component is a polyethylene resin, the fineness of 0.2 dtex (average fiber diameter 5.6 μm), the core-sheath type composite fiber 70 mass parts of the fiber length 3 mm, the fineness 0.06 dtex (average fiber diameter 2 0.5 μm), and 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 the stretched PET fiber was 30 parts by mass with a fiber length of 3 mm. 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, the fineness of 0.2 dtex (average fiber diameter 5.6 μm), the core-sheath type composite fiber 70 mass parts of the fiber length 3 mm, the fineness 0.06 dtex (average fiber diameter 2 0.5 μm), 25 parts by mass of stretched PET fiber having a fiber length of 3 mm, and a pulp-like product (average fiber length of 1.7 mm, average fiber diameter of 10 μm) of wholly aromatic polyamide fiber, using a high-pressure homogenizer, modified drainage 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 5 parts by mass of the fibrillated heat-resistant fiber fibrillated to a degree of 50 ml 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
The core component is a polypropylene resin and the sheath component is a polyethylene resin, the fineness of 0.2 dtex (average fiber diameter 5.6 μm), the core-sheath type composite fiber 70 mass parts of the fiber length 3 mm, the fineness 0.06 dtex (average fiber diameter 2 0.5 μm), 20 mass parts of stretched PET fiber having a fiber length of 3 mm, and a pulp-like product (average fiber length of 1.7 mm, average fiber diameter of 10 μm) of wholly aromatic polyamide fiber, using a high-pressure homogenizer, modified drainage 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 10 parts by mass of the fibrillated heat-resistant fiber fibrillated to a degree of 50 ml 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, the fineness of 0.2 dtex (average fiber diameter 5.6 μm), the core-sheath type composite fiber 70 mass parts of the fiber length 3 mm, the fineness 0.06 dtex (average fiber diameter 2 0.5 μm), 10 parts by mass of stretched PET fiber having a fiber length of 3 mm, and a pulp-like product (average fiber length of 1.7 mm, average fiber diameter of 10 μm) of wholly aromatic polyamide fiber, using a high-pressure homogenizer, modified drainage 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 20 parts by mass of the fibrillated heat-resistant fiber fibrillated to a degree of 350 ml 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 7
The core component is polypropylene resin and the sheath component is polyethylene resin. Fineness 0.2 dtex (average fiber diameter 5.6 μm), core-sheath type composite fiber 70 mass parts with fiber length 3 mm, fineness 0.3 dtex (average fiber diameter 5 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 the stretched PET fiber was 30 parts by mass with a fiber length of 5 mm. 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 8
A core component is a polypropylene resin, and a sheath component is a polyethylene resin, a fineness of 0.2 dtex (average fiber diameter of 5.6 μm), a core-sheath type composite fiber having a fiber length of 3 mm, 70 parts by mass, and a fineness of 0.4 dtex (average fiber diameter of 6 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 the stretched PET fiber was 30 parts by mass with a fiber length of 5 mm. 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
The core component is polypropylene resin and the sheath component is polyethylene resin. Fineness of 0.2 dtex (average fiber diameter of 5.6 μm), 96 parts by mass of core-sheath type composite fiber having a fiber length of 3 mm, and fineness of 0.06 dtex (average fiber diameter of 2 0.5 μm), and 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 4 parts by mass of drawn PET fiber having a fiber length of 3 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.

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), core-sheath type composite fiber 69 parts by mass of fiber length 3 mm, fineness 0.06 dtex (average fiber diameter 2 0.5 μm), and 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 31 parts by mass of stretched PET fiber having a fiber length of 3 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.

Comparative Example 4
A core component is a polypropylene resin and a sheath component is a polyethylene resin. Fineness of 0.5 dtex (average fiber diameter of 8.1 μm), core-sheath type composite fiber having a fiber length of 3 mm, 70 parts by mass, and fineness of 0.06 dtex (average fiber diameter of 2 0.5 μm), and 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 the stretched PET fiber was 30 parts by mass with a fiber length of 3 mm. 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.

<Production of evaluation battery>
Using each separator (31 mm x 56 mm), the positive electrode (coating part is 25 mm x 50 mm) on the lithium salt of nickel / manganese / cobalt composite oxide (NMC111 positive electrode material), the negative electrode (coating part is 27 mm x 52 mm) and graphite Using a 1 mol / L, DEC / EC (volume ratio 7/3) mixed solvent solution of LiPF _{6 as} the electrolytic solution, the surface provided with the coating layer containing inorganic particles of each separator was opposed to the negative electrode, A lithium ion battery for evaluation pouch having a design capacity of 30 mAh was produced, and the following self-discharge was evaluated.

<Evaluation of voltage drop>
For each battery, after various charging such as 6 mA constant current charging → 4.2 V constant voltage charging (72 hours), constant current discharging up to 2.8 V at 30 mA → 30 mA constant current charging → 4.2 constant voltage charging (1 5) with a break-in charge of time). The voltage after the last charge (V ₀ ), the voltage after 120 hours at 50 ° C. (V ₁₂₀ ) was measured, and the difference between V ₀ and V ₁₂₀ (hereinafter sometimes referred to as “voltage drop”) was determined. . The smaller the voltage drop, the better the storage characteristics at high temperature (50 ° C.).

  As shown in Table 1, the base materials produced in Examples 1 to 8 are base materials containing synthetic resin short fibers, and the core-sheath type composite fibers having an average fiber diameter of 6 μm or less are used as the synthetic resin short fibers. And a PET fiber, and the content of the core-sheath composite fiber is 70 to 95% by mass with respect to the total fiber components contained in the substrate. The separator which has the base material of Examples 1-8 and the coating layer containing an inorganic particle had high adhesiveness of a base material and a coating layer, and was excellent in the coating property of aqueous | water-based coating liquid. Moreover, it was excellent in storage characteristics at high temperatures.

  Although the base material of Examples 4-6 is a case where the fibrillated heat-resistant fiber is contained in addition to the core-sheath type composite fiber and the PET fiber, the tendency to improve the storage characteristics at high temperature was observed. .

  The base material of Example 8 is a case where the average fiber diameter of the PET fiber exceeds 5.5 μm, but compared with Examples 3 and 7 in which the average fiber diameter of the PET fiber is 5.5 μm or less, the water-based coating Although no repelling of the liquid was observed, since the aqueous coating liquid easily entered the substrate, irregularities and pinholes were observed, and the storage characteristics at high temperatures tended to deteriorate.

  The base material of Comparative Example 1 is a case where the core-sheath type composite fiber having an average fiber diameter of 6 μm or less is 100% by mass, but the aqueous coating liquid hardly penetrates the base material, and the adhesion between the base material and the coating layer is poor. The coating layer was easily peeled off from the substrate. 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. As a result, the storage characteristics at high temperatures deteriorated.

  The base material of Comparative Example 2 is a case where the content of the core-sheath type composite fiber having an average fiber diameter of 6 μm or less is more than 95% by mass. For this reason, the aqueous coating liquid hardly penetrates into the substrate, and the coating layer is easily peeled off from the substrate. Further, although no repelling of the aqueous coating liquid was observed, pinholes were observed on the surface of the coating layer. As a result, the storage characteristics at high temperatures deteriorated.

  The base material of Comparative Example 3 is a case where the content of the core-sheath type composite fiber having an average fiber diameter of 6 μm or less is less than 70% by mass. Although the adhesion between the base material and the coating layer did not deteriorate, the aqueous coating liquid was likely to soak into the base material, and the aqueous coating liquid began to penetrate slightly. Although no repelling of the aqueous coating liquid was observed, irregularities and pinholes were observed on the surface of the coating layer. Therefore, when compared with Example 3, the storage characteristics at high temperatures deteriorated.

  The base material of Comparative Example 4 is a case where the average 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. Although it deteriorated and no repelling of the aqueous coating liquid was observed, pinholes were observed in the coating layer. In addition, the number of fibers of the core-sheath type composite fiber decreased, and the immobilization of the PET fiber decreased, so that the storage characteristics at high temperature deteriorated.

  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 lithium ion battery separators containing synthetic resin short fibers, as the synthetic resin short fibers, a core sheath having an average fiber diameter of 6 μm or less using a resin having a melting point of 160 ° C. or more as a core component and a polyethylene resin as a sheath component. Characterized in that the content of the core-sheath type composite fiber is 70 to 95% by mass with respect to the total fiber components contained in the base material, and the synthetic resin short fiber made of polyethylene terephthalate resin. A base material for a lithium ion battery separator.   The base material for a lithium ion battery separator according to claim 1, wherein an average fiber diameter of the synthetic resin short fibers made of the polyethylene terephthalate resin is 5.5 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.