JP2012123957A - Base material of lithium secondary battery and separator for lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a base material for a lithium secondary battery which ensures excellent heat sealing and excellent coating of an aqueous coating liquid containing filler particles, and in which the filler particles do not fill the base material to close voids in the base material, and to provide a separator for a lithium secondary battery having an excellent electrolyte retention rate and resistance of lithium dendrite.SOLUTION: The base material for a lithium secondary battery consists of a wet type nonwoven fabric containing ultrafine fibers obtained by splitting the split type composite fiber consisting of polyester and polyethylene adjoining each other, and beaten solvent spinning cellulose fiber, and to provide a separator for a lithium secondary battery using that base material.

Description

  The present invention relates to a lithium secondary battery substrate and a lithium secondary battery 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 a high energy density because 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, is obtained. Therefore, a nonaqueous electrolytic solution having sufficient oxidation-reduction resistance is used. Since non-aqueous electrolytes are flammable, there is a risk of ignition and the like, and careful attention is paid to safety in their use. There are several possible cases of exposure to fire and other hazards, but overcharging is particularly dangerous.

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

  When overcharging is prevented by the protection circuit, it is naturally assumed that the protection circuit does not operate well, and it is difficult to say that it is intrinsically safe. The current non-aqueous secondary battery has a safety circuit / PTC element equipped and a separator with a thermal fuse function for the purpose of destroying the battery safely when overcharged. Has been made. However, even if equipped with the above-mentioned means, depending on the overcharge conditions, the safety during overcharge is not guaranteed, and in fact, non-aqueous secondary battery ignition accidents Is still happening.

  As the separator, a film-like porous film made of a polyolefin such as polyethylene is often used. When the temperature inside the battery reaches around 130 ° C., it melts and closes the micropores, thereby transferring lithium ions. Although there is a thermal fuse function (shutdown function) that cuts off the current and shuts off the current, it is suggested that if the temperature further increases due to some situation, the polyolefin itself melts and short-circuits, causing a thermal runaway. In view of this, a heat-resistant separator that does not melt and shrink even at temperatures close to 200 ° C. has been developed.

  As a heat resistant separator, there is a separator made of a nonwoven fabric, and a polyester nonwoven fabric (for example, see Patent Documents 1 and 2) is disclosed. The separator (for example, refer patent documents 3-6) comprised by the wet heat gelling resin which can be gelatinized by heating in presence of moisture, and the nonwoven fabric containing another fiber is disclosed.

  However, since the separators of Patent Documents 1 and 2 both have large holes in the nonwoven fabric, there is a problem that lithium dendrite penetrates through the separator and becomes conductive. Lithium dendrite refers to metallic lithium that deposits on the negative electrode surface when charging and discharging are repeated or overcharged. Lithium dendrite grows gradually, penetrates the separator, reaches the positive electrode, and may cause an internal short circuit. The separators of Patent Documents 3 to 6 have a problem that the gap of the separator becomes insufficient because of the wet heat gelled resin film. When the film is reduced, pinholes are easily formed, and there is a problem that lithium dendrite penetrates the separator. Further, since the wet heat gelation method is complicated, there is a problem that it is difficult to balance the film area of the wet heat gelled resin and the porosity of the separator.

As a means of solving problems such as pinholes in a heat-resistant separator composed only of a nonwoven fabric and achieving higher heat resistance, a base material for a lithium secondary battery such as a nonwoven fabric is impregnated with a slurry containing filler particles or Apply at least one treatment selected from a treatment for coating, a treatment for impregnating or applying a slurry containing a resin, a treatment for laminating and integrating a porous film, and a treatment for impregnating or applying a solid electrolyte or a gel electrolyte. Examples of lithium secondary battery separators are also disclosed (see, for example, Patent Documents 7 to 27). As a base material for a lithium secondary battery, a non-woven fabric (Patent Document 11) having a porosity larger than 50%, a fiber diameter of 1 to 25 μm, and a thickness of 15 to 80 μm, lighter than 50 g / m ^{2 and} thinner than 35 μm A non-woven fabric having a fiber diameter of 0.1 to 10 μm and a porosity of 50 to 97% (Patent Document 12), a porosity of greater than 50%, a fiber diameter of 1 to 25 μm, and a thickness of 15 to 80 μm , A nonwoven fabric in which at least 50% of the pores are 75 to 150 μm (Patent Document 13), a nonwoven fabric having an average fiber diameter of 5 μm or less and an average fiber diameter of 3 μm or less (Patent Document 25), etc. Yes.

  However, all of these lithium secondary battery substrates have large pores in the nonwoven fabric, so that the coating liquid penetrates during coating, and the surface smoothness of the coated surface is poor. Since the filler particles are filled and the pores inside the nonwoven fabric are closed, there is a problem in that the electrolyte solution retention is poor and the internal resistance of the separator is increased. Moreover, when charging / discharging was repeated or overcharged, lithium dendrite was generated on the surface of the negative electrode, and an internal short circuit was likely to occur. In order to prevent the penetration of lithium dendrite, the separator of Patent Document 27 has a melting point of 80 to 130 ° C. when trying to laminate and integrate with a porous substrate having a pore smaller than that of the nonwoven fabric, for example, a porous film by heat sealing. Since the resin A is melted, there is a problem that the heat seal bonding cannot be performed.

  In addition, organic solvents have been the mainstream of coating liquids used when applying filler particles, resins, and solid electrolytes. However, there are problems in the working environment, waste liquid recovery and waste liquid treatment. In recent years, aqueous coating liquids are becoming mainstream because of problems such as complexity, the necessity of making dryers explosion-proof, and the need for coating equipment and storage facilities. However, there is a problem that there is no non-woven fabric that is excellent in the coating property of the aqueous coating liquid and can retain a sufficient gap even after coating. For example, in the base material for a lithium secondary battery described in Patent Documents 7 to 27, a nonwoven fabric mainly composed of inorganic fibers such as glass fibers lacks flexibility, and therefore the base material at the time of coating or battery assembly. May be destroyed. In addition, in the nonwoven fabric mainly composed of organic fibers, especially the nonwoven fabric mainly composed of synthetic fibers, the synthetic fibers themselves can easily repel the aqueous coating liquid, and in order to improve the strength of the nonwoven fabric, heat-fusible fibers are used. When used, a film is formed, and depending on the type and amount of resin and binder contained in the coating liquid, they form a film and the aqueous coating liquid penetrates unevenly, resulting in wetness of the aqueous coating liquid. There was a problem that the nature was bad.

JP 2003-123728 A International Publication No. 2008/130020 Pamphlet JP 2005-317215 A JP 2005-317216 A JP 2005-317217 A International Publication No. 2004/038833 Pamphlet International Publication No. 2007/066768 Pamphlet International Publication No. 2008/143005 Pamphlet International Publication No. 2008/114727 Pamphlet International Publication No. 2009/096451 Pamphlet International Publication No. 2003/073534 (Special Publication 2006-504228) International Publication No. 2004/021476 (Special Table No. 2005-536857) International Publication No. 2004/021499 (Special Table 2005-536658) International Publication No. 2004/021469 Pamphlet (Special Table No. 2005-536858) International Publication No. 2004/021477 (Special Table 2005-536860) International Publication No. 2004/049471 pamphlet (Japanese translations of PCT publication No. 2006-507635) International Publication No. 2004/049472 pamphlet (Japanese Patent Publication No. 2006-507636) International Publication No. 2005/038959 Pamphlet (Special Publication 2007-508669) International Publication No. 2005/038946 Pamphlet (Special Table 2007-508670) International Publication No. 2005/038960 Pamphlet (Japanese Patent Publication No. 2007-509464) International Publication No. 2005/104269 Pamphlet (Special Table 2007-534123) International Publication No. 2006/136472 Pamphlet (Special Table 2008-544457) International Publication No. 2007/028662 Pamphlet (Special Table 2009-507353) JP 2008-208511 A JP 2009-230975 A JP 2005-293891 A JP 2007-157723 A

  The subject of the present invention is excellent in heat sealability, not only in the coating property of a coating solution using an organic solvent as a medium, but also in the coating property of an aqueous coating solution, and when filler particles are applied, To provide a lithium secondary battery base material in which particles are filled in the base material and do not block voids in the base material, and a lithium secondary battery separator excellent in electrolyte retention and lithium dentrite resistance It is in.

  As a result of diligent research to solve the above-mentioned problems, the present inventors include ultrafine fibers obtained by dividing a split-type composite fiber made of polyester and polyethylene, and solvent-spun cellulose fibers that are beaten. The present inventors have found a lithium secondary battery base material comprising a wet nonwoven fabric and a lithium secondary battery separator using the base material.

  The base material for a lithium secondary battery of the present invention is a wet containing an ultrafine fiber obtained by dividing a split type composite fiber in which polyester and polyethylene are adjacent to each other, and a solvent-spun cellulose fiber that is beaten. Because it is made of non-woven fabric, it has excellent heat sealability. In addition, when the intersections between the ultrafine fibers made of polyester and / or the intersections between the ultrafine fibers and the solvent-spun cellulose fibers are adhered with polyethylene, there is no back-through of the coating liquid, and the base material is torn during coating. No cutting and excellent coating properties. In addition, since there are no large through holes in the base material, when filler particles are applied, the filler particles are not filled inside the base material and do not block the voids in the base material. Furthermore, both the ultrafine fiber obtained by splitting a split type composite fiber in which polyester and polyethylene are adjacent to each other and the solvent-spun cellulose fiber formed by beating both have a thin fiber diameter. The film is not covered with a film, and not only the coating property of a coating solution using an organic solvent as a medium but also the wettability of an aqueous coating solution is good. A treatment for impregnating or coating a slurry containing filler particles on a substrate for a lithium secondary battery of the present invention, a treatment for impregnating or applying a slurry containing a resin, a treatment for laminating and integrating a porous film, a solid The separator for a lithium secondary battery of the present invention obtained by performing at least one treatment selected from a treatment of impregnating or coating an electrolyte or a gel electrolyte not only has a thin fiber diameter of the substrate, but also includes filler particles, Since resin or the like is intensively laminated on the surface of the base material, the voids inside the base material are maintained, and an effect that the electrolytic solution retention rate is high is obtained. In addition, even when lithium dendrite is generated on the electrode surface due to repeated charge and discharge, it is possible to prevent the lithium dendrite from conducting between the positive and negative electrodes due to filler particles that are concentrated on the surface of the substrate. Excellent lithium dendrite properties.

  The base material for a lithium secondary battery of the present invention (hereinafter sometimes referred to as “base material”) is a base material for impregnating or coating a slurry containing filler particles, and a slurry containing a resin. A substrate for impregnating or coating, a substrate for laminating and integrating a porous substrate such as a porous film or a nonwoven fabric, a substrate for impregnating or coating a solid electrolyte or a gel electrolyte, It is a precursor sheet of a separator for a lithium secondary battery.

  The separator for a lithium secondary battery of the present invention (hereinafter sometimes referred to as “separator”) is a separator formed by impregnating or coating a slurry containing filler particles on a base material for a lithium secondary battery of the present invention. , A separator formed by impregnating or coating a slurry containing a resin, a separator formed by laminating and integrating a porous base material such as a porous film or a nonwoven fabric, a separator formed by impregnating or applying a solid electrolyte or a gel electrolyte It is. The filler may be either inorganic or organic. Inorganic fillers include inorganic oxides such as alumina, gibbsite, boehmite, magnesium oxide, magnesium hydroxide, silica, titanium oxide, barium titanate, zirconium oxide, and inorganic nitrides such as aluminum nitride and silicon nitride. , Aluminum compounds, zeolite, mica and the like. Examples of the organic filler include polyethylene, polypropylene, polyacrylonitrile, polymethyl methacrylate, polyethylene oxide, polystyrene, polyvinylidene fluoride, ethylene-vinyl monomer copolymer, polyolefin wax, and the like. In the slurry, an ethylene-acrylate copolymer such as an ethylene-vinyl acetate copolymer (EVA, having a structural unit derived from vinyl acetate of 20 to 35 mol%) as a binder, an ethylene-ethyl acrylate copolymer, Rubbers such as styrene-butadiene rubber, fluoro rubber, urethane rubber, ethylene-propylene-diene rubber and derivatives thereof), cellulose derivatives such as carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, polyurethane, epoxy Resin, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, acrylic resin, and the like. These can be used alone or in combination of two or more. . 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. Polyethylene resins include not only ultra-low density polyethylene, low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, or ultra high density polyethylene alone, but also ethylene propylene copolymers. Or a mixture of a polyethylene-based resin and another polyolefin-based resin. Polypropylene resins include homopropylene (propylene homopolymer), or α-, such as propylene and ethylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene or 1-decene. Examples thereof include random copolymers with olefins and block copolymers.

The lithium secondary battery in the present invention means a lithium ion battery or a lithium ion polymer battery. Examples of the negative electrode active material of the lithium secondary battery include carbon materials such as graphite and coke, metallic lithium, aluminum, silica, tin, nickel, and an alloy of lithium and lithium, SiO, SnO, Fe Metal oxides such as ₂ O ₃ , WO ₂ , Nb ₂ O ₅ , Li _4/3 Ti _5/3 O ₄ , and nitrides such as Li _0.4 CoN are used. As the positive electrode active material, lithium cobaltate, lithium manganate, lithium nickelate, lithium titanate, lithium nickel manganese oxide, or lithium iron phosphate is used. Further, the lithium iron phosphate may be a composite with one or more metals selected from manganese, chromium, cobalt, copper, nickel, vanadium, molybdenum, titanium, zinc, aluminum, gallium, magnesium, boron, and niobium.

  As an electrolytic solution for a lithium secondary battery, a solution obtained by dissolving a lithium salt in an organic solvent such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, dimethoxyethane, dimethoxymethane, or a mixed solvent thereof is used. Examples of the lithium salt include lithium hexafluorophosphate and lithium tetrafluoroborate. 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 split type composite fiber used in the present invention is formed by arranging polyester and polyethylene adjacent to each other. Polyethylene may be either high density or low density.

  Examples of the cross-sectional shape of the split composite fiber include a radial type, a layered type, a comb type, and a grid type. Among these, the radial type is preferable because the fiber diameter of the ultrafine fibers can be easily reduced and the voids in the base material are relatively difficult to close. The split type composite fiber is preferably one that can be divided into ultrafine fibers made of polyester and ultrafine fibers made of polyethylene by a method of stirring with a pulper or a mixer or a method of applying a high-pressure water stream. When the split type composite fiber is split by stirring with a pulper or a mixer, a dispersion aid or an antifoaming agent may be used as necessary. The average fiber diameter of the split composite fibers is preferably 3 to 18 μm, more preferably 3 to 16 μm, and even more preferably 6 to 16 μm. If it is less than 3 μm, it may be difficult to divide, and if it is thicker than 18 μm, the major axis of the cross-section of the ultrafine fiber becomes longer, which may block the voids in the substrate.

  The ultrafine fiber obtained by dividing preferably has a minor axis length of 1 to 5 μm in cross section, and more preferably 1 to 3 μm. If the thickness is less than 1 μm, the theoretical flatness of the cross section may become too large and block the voids in the base material, or there may be insufficient adhesion between the intersections between the ultrafine fibers and between the ultrafine fibers and the solvent-spun cellulose fibers. If it exceeds 5 μm, it may be difficult to reduce the thickness of the substrate. The minor axis length means the maximum length in the minor axis direction of the ultrafine fiber cross section. The length of the ultrafine fiber is preferably 0.5 to 10 mm, more preferably 1 to 6 mm, and further preferably 1 to 4 mm. If it is less than 0.5 mm, the ratio of falling out of the screen and flowing into the waste water during wet papermaking may increase, and if it is longer than 10 mm, the ultrafine fibers may rely on each other to form a lump. 1.0-5.0 are preferable and, as for the theoretical flatness of an ultrafine fiber, 1.5-3.0 are more preferable. The theoretical flatness means a value obtained by dividing the maximum length of the major axis of the ultrafine fiber by the minor axis length, and can be calculated from the fiber diameter and the number of divisions of the split type composite fiber. If the theoretical flatness is greater than 5.0, the gaps in the substrate may be blocked, or the intersections between the ultrafine fibers and the adhesion between the ultrafine fibers and the solvent-spun cellulose fibers may be insufficient.

  The solvent-spun cellulose fiber, which is an essential component constituting the base material for a lithium secondary battery of the present invention, means a cellulose fiber obtained by directly dissolving and spinning in an organic solvent without passing through a cellulose derivative. In the present invention, solvent-spun cellulose fibers formed by beating are used. In the present invention, the beating degree of solvent-spun cellulose fiber is expressed by modified freeness. The modified freeness is a freeness measured according to JIS P8121, except that an 80-mesh wire mesh having a wire diameter of 0.14 mm and an aperture of 0.18 mm is used as a sieve plate, and the sample concentration is 0.1%. Means. The modified freeness of the solvent-spun cellulose fiber used in the present invention is preferably 0 to 400 ml, and more preferably 0 to 300 ml. When it exceeds 400 ml, the ratio of the thick fiber diameter increases, and a large through hole may be formed in the base material, and the coating liquid may fall through, or thickness spots and ground spots may be generated.

  The length weighted average fiber length of the solvent-spun cellulose fiber in the present invention is preferably 0.2 to 2.0 mm, more preferably 0.4 to 1.8 mm, and still more preferably 0.5 to 1.5 mm. When the length-weighted average fiber length is less than 0.2 mm, there are cases where the ratio of falling out of the screen and outflowing into the drainage during wet papermaking increases, or fluffing may occur due to rubbing, and if longer than 2.0 mm The fibers may depend on each other and become lumped. It can be determined by utilizing the deflection characteristics obtained by applying laser light to the fiber, and can be measured using a commercially available fiber length measuring device.

  In order to obtain a solvent-spun cellulose fiber beaten in the present invention, solvent-spun cellulose short fibers are dispersed in water at an appropriate concentration, and this is refined by a refiner, a beater, a mill, a grinding device, or a high-speed rotary blade. A rotary blade homogenizer that applies shear force, a double-cylindrical high-speed homogenizer that generates a shear force between a cylindrical inner blade that rotates at high speed and a fixed outer blade, and ultrasonic waves that are refined by ultrasonic shock It may be beaten by adjusting conditions such as the shape of the blade, flow rate, number of treatments, treatment speed, treatment concentration through a crusher, a high-pressure homogenizer, or the like. By these beatings, the solvent-spun cellulose fiber is divided parallel to the fiber long axis and the fiber length is shortened. For this reason, the pores of the base material are formed relatively uniformly, and the coating liquid can be prevented from falling through during coating. In addition, since the back-through of the coating liquid is suppressed, the filler particles do not block the pores inside the substrate.

  In the base material for a lithium secondary battery of the present invention, the mass ratio of the split composite fiber in which polyester and polyethylene are adjacent to each other and the solvent-spun cellulose fiber is preferably 8: 2 to 2: 8, and 7: 3. ~ 4: 6 is more preferable. When the ratio of the split-type conjugate fiber is more than 8: 2, the wet nonwoven fabric may be remarkably shrunk and may be wrinkled or the gap of the base material may be blocked. The water resistance of the resin becomes weak and may be damaged during coating.

  The base material for a lithium secondary battery of the present invention may contain fibers other than split type composite fibers in which polyester and polyethylene are adjacent to each other and solvent-spun cellulose fibers formed by beating. For example, natural cellulose fibers, pulped and fibrillated natural cellulose fibers, solvent-spun cellulose short fibers, short fibers made of synthetic resin (hereinafter sometimes referred to as “synthetic short fibers”), fibrils, pulped products, It may contain a fibrillated product and inorganic fibers. The pulped product or fibrillated product of natural cellulose fiber preferably has a Canadian standard freeness of 0 to 700 ml, more preferably 0 to 500 ml. Examples of the inorganic fiber include glass, alumina, silica, ceramics, and rock wool. When inorganic fiber is contained, it is preferable because the heat-resistant dimensional stability and puncture strength of the base material are improved. Synthetic short fibers include polypropylene, polyethylene, polymethylpentene, polyester, acrylic, polyamide, polyimide, polyvinyl chloride, polyvinylidene chloride, polyvinyl ether, polyvinyl ketone, polyether, polyvinyl alcohol, diene, polyurethane, phenol, melamine, Furan, urea, aniline, unsaturated polyester, fluorine, silicone, polyamideimide, polyetheretherketone, polyphenylene sulfide, polybenzimidazole, poly-p-phenylenebenzobisthiazole, poly-p-phenylenebenzobisoxazole, polytetrafluoro Examples thereof include short fibers made of resins such as ethylene and derivatives thereof, and composite fibers made of two or more kinds of resins. 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. Acrylic in the present invention is composed of a polymer of 100% acrylonitrile, and acrylonitrile is copolymerized with (meth) acrylic acid derivatives such as acrylic acid, methacrylic acid, acrylic acid ester, methacrylic acid ester, and vinyl acetate. It refers to what you let me. Polyamide refers to aliphatic polyamide, semi-aromatic polyamide, and wholly aromatic polyamide. The semi-aromatic polyamide refers to an aromatic polyamide having a fatty chain or the like in a part of the main chain.

  The cross-sectional shape of the synthetic short fiber may be any of a circle, an ellipse, a flat shape, a triangle, a quadrangle, and a polygon. . The average fiber diameter is preferably 0.1 to 12.0 μm, more preferably 0.5 to 8.0 μm, and even more preferably 1.0 to 5.0 μm. When the average fiber diameter of the synthetic short fibers exceeds 12.0 μm, the number of fibers in the thickness direction decreases, so the problem is that the coating liquid may fall through, the thickness is difficult to reduce, or the surface roughness increases. There is. When the thickness is less than 0.1 μm, the effect of adding the synthetic short fiber may be difficult to appear. The average fiber diameter when the cross-sectional shape is other than a circle means the average fiber diameter when converted into a circle having the same area.

  The fiber length of the synthetic short fiber is preferably 0.1 to 10 mm, and more preferably 0.3 to 6 mm. If the fiber length exceeds 10 mm, formation may be poor. On the other hand, when the fiber length is less than 0.1 mm, the mechanical strength of the base material is low, and the base material may be damaged during coating.

  The base material for lithium secondary batteries of the present invention is produced by a wet papermaking method. Specifically, ultrafine fibers obtained by dividing a split type composite fiber in which polyester and polyethylene are adjacent to each other, and solvent-spun cellulose fibers formed by beating are dispersed in water to form a uniform slurry. The wet non-woven fabric is produced by rolling up the paper with a paper machine. You may add chemical | medical agents, such as a dispersion | distribution adjuvant, an antifoamer, a thickener, a peeling agent, and a paper strength enhancer, to a slurry as needed. Examples of the paper machine include a circular paper machine, a long paper machine, an inclined paper machine, an inclined short paper machine, and a composite paper machine of these. In the process of manufacturing a wet nonwoven fabric, a hydroentanglement process may be performed as necessary.

  In the present invention, the wet nonwoven fabric is heat-treated at 135 to 160 ° C. to melt at least a part of the polyethylene, and the intersections between the ultrafine fibers made of polyester and / or the intersections between the ultrafine fibers and the solvent-spun cellulose fibers are adhered. It is preferable. The heat treatment in the present invention is a method in which one side or both sides of a wet nonwoven fabric are brought into contact with a roll heated to a temperature range of 135 to 160 ° C. under no pressure for a predetermined time, and a wet nonwoven fabric between rolls heated to a temperature range of 135 to 160 ° C. For example, a method of pressurizing for a predetermined time using a hot press in a temperature range of 135 to 160 ° C. In the present invention, the winding can be continuously processed, and the fiber shape can be easily maintained without melting and degrading the ultrafine fibers. Therefore, the method is brought into contact with a roll heated under no pressure, or a temperature of 135 to 160 ° C. A method in which a wet nonwoven fabric is pressed between rolls heated to a range is preferable. The roll to be heated may be made of resin or metal. When the temperature is lower than 135 ° C, the adhesion between the ultrafine fibers and / or the intersection between the ultrafine fibers and the solvent-spun cellulose fiber may be weak, and the coatability of the aqueous coating liquid may be insufficient. In some cases, the shrinkage of the wet non-woven fabric is increased and the polyethylene is stuck to a roll or a hot press machine during heat treatment. Whether heat treatment is performed or not, the thickness is adjusted by calendering as necessary.

  4-45 micrometers is preferable, as for the thickness of the base material for lithium secondary batteries of this invention, 10-35 micrometers is more preferable, and 15-30 micrometers is more preferable. If it exceeds 45 μm, the resistance value of the separator may be high, and if it is less than 4 μm, the strength of the base material becomes too weak and may be damaged during handling or coating of the base material.

Density of the lithium secondary battery base material of the present invention is preferably 0.250~0.750g / cm ^3, more preferably 0.300~0.650g / cm ^3, 0.400~0.600g / cm ³ is more preferable. When the density is less than 0.250 g / cm ³ , the coating liquid may be seen through, and when it exceeds 0.750 g / cm ³ , the resistance value of the separator may be increased.

  It is preferable that the base material for lithium secondary batteries of this invention is 0.1-10 micrometers in maximum pore diameter prescribed | regulated by ASTM-F316-86. If it is less than 0.1 μm, the electrolyte solution retention rate may be low, and if it is greater than 10 μm, the coating liquid may fall through, or filler particles may be filled inside the substrate, resulting in voids in the substrate. May be blocked.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a present Example. Table 1 shows the split type composite fibers, synthetic short fibers, and solvent-spun cellulose fibers used in the examples and comparative examples of the present invention. The cross-sectional shape of the split-type conjugate fiber in Table 1 means that the cross-section of the entire split-type conjugate fiber is circular, and the arrangement of polyester and polyethylene is a radial type.

Examples 1, 2, 4-9, 11-16
Slurries 1 to 13 shown in Table 2 were prepared. As shown in Table 3, the slurry corresponding to Examples 1, 2, 4 to 9, and 11 to 16 was wet-paper-made using a double-type circular net paper machine, and dried at a cylinder dryer temperature of 130 ° C. To prepare a wet nonwoven fabric. Next, according to the heat treatment temperature and heat treatment time shown in Table 3, the front and back surfaces of the wet nonwoven fabric were brought into contact with the metal roll and heat-treated, and further calendered to adjust the thickness. Examples 1, 2, 4-9, 11-16 A base material was prepared.

Examples 3 and 10
As shown in Table 3, the slurry corresponding to Examples 3 and 10 was subjected to wet paper making using a double-type circular net paper machine, and dried at a cylinder dryer temperature of 130 ° C. to prepare a wet nonwoven fabric. Subsequently, according to the heat processing temperature and heat processing time which were shown in Table 3, the metal roll was made to contact the front and back of a wet nonwoven fabric, and it heat-processed, and it was set as the base material of Example 3 and 10.

(Comparative Example 1)
Slurry 14 shown in Table 2 was prepared, wet papermaking was performed using a double-lined circular paper machine, and dried at a cylinder dryer temperature of 130 ° C. to prepare a wet nonwoven fabric. Next, according to the heat treatment temperature and heat treatment time shown in Table 3, the metal roll was brought into contact with the front and back surfaces of the wet nonwoven fabric and heat treated, and further calendered to adjust the thickness, thereby producing a substrate of Comparative Example 1.

(Comparative Example 2)
The slurry 15 shown in Table 2 was prepared, wet papermaking was performed using a double-lined circular paper machine, and dried at a cylinder dryer temperature of 130 ° C. to prepare a wet nonwoven fabric. Then, according to the heat treatment temperature and heat treatment time shown in Table 3, the front and back surfaces of the wet nonwoven fabric were brought into contact with the metal roll, heat treated, and further calendered to adjust the thickness, thereby producing a substrate of Comparative Example 2.

(Comparative Examples 3-6)
Slurries 16 to 19 shown in Table 2 were prepared. As shown in Table 3, the slurry corresponding to Comparative Examples 3 to 6 was wet-paper-made using a twin-line paper machine and dried at a cylinder dryer temperature of 130 ° C. to prepare a wet nonwoven fabric. Next, the wet nonwoven fabric was calendered to adjust the thickness, and the substrates of Comparative Examples 3 to 6 were produced.

Example 17
Slurry 2 was subjected to wet papermaking using a double-lined circular paper machine, and dried at a cylinder dryer temperature of 130 ° C. to prepare a wet nonwoven fabric. Next, the front and back surfaces of the wet nonwoven fabric were brought into contact with metal rolls heated to 130 ° C. for 30 seconds each and subjected to heat treatment, and further calendered to adjust the thickness, thereby producing a substrate of Example 17.

Example 18
Slurry 2 was subjected to wet papermaking using a double-lined circular paper machine, and dried at a cylinder dryer temperature of 130 ° C. to prepare a wet nonwoven fabric. Subsequently, the front and back surfaces of the wet nonwoven fabric were brought into contact with metal rolls heated to 165 ° C. for 5 seconds each and subjected to heat treatment, and further calendered to adjust the thickness, whereby a substrate of Example 18 was produced.

(Comparative Example 7)
The slurry 20 shown in Table 2 was prepared, wet paper was made using a combined paper machine of a circular paper machine and a short paper machine, and dried at a cylinder dryer temperature of 135 ° C. to produce a wet nonwoven fabric. Next, the wet nonwoven fabric was exposed to a mixed gas of fluorine: oxygen: nitrogen = 1: 73: 26 in a volume ratio for 1 minute. Thereafter, it was washed with hot water at 60 ° C. and dried by passing it through a 70 ° C. atmosphere with a hot air dryer. The wet nonwoven fabric is sprayed with water to impregnate 100% by mass and passed through a pair of metal rolls heated to 130 ° C. at a linear pressure of 500 N / cm at a speed of 3.3 m / min to gel the ethylene-vinyl alcohol copolymer. A base material of Comparative Example 7 was produced. The core-sheath composite fiber in Table 2 is formed by arranging polypropylene in the core and high-density polyethylene in the sheath, and the cross-sectional area ratio between the core and the sheath is 50:50.

(Comparative Example 8)
Slurry 21 shown in Table 2 was prepared, wet papermaking was performed using a circular paper machine, and dried at a cylinder dryer temperature of 140 ° C. to prepare a wet nonwoven fabric. Next, calendar treatment was performed to adjust the thickness, and a base material of Comparative Example 8 was produced.

  The raw material symbols in Table 2 correspond to the symbols in Table 1.

[Preparation of separator]
1000 g of silicon oxide (average particle size: 1 μm), 1000 g of water, and 100 g of polyvinyl butyral are placed in a container and dispersed by stirring for 1 hour with a stirrer (trade name: Three-One Motor, manufactured by Shinto Kagaku Co., Ltd.). A coating solution was prepared. The aqueous coating liquid was applied to the base materials of Examples and Comparative Examples using an applicator and dried with a drier to obtain a separator having a silicon oxide layer with a thickness of 3 μm per side.

[Evaluation]
The following evaluation was performed about the base material of an Example and a comparative example, and the separator produced using this base material, and the result was shown in Table 3.

<Thickness of base material>
The thickness was measured according to JIS P8118, and the average value was calculated.

<Density of base material>
The basis weight of the substrate was measured in accordance with JIS P8124, and the value obtained by dividing the basis weight by the thickness and multiplying by 100 was taken as the density.

<Heat treatment>
The state of the base material when the base material was heat-treated was observed. ◎ when there was almost no shrinkage and no cutting, there was some shrinkage, but when the base material was not cut and it was able to heat treatment smoothly, ○, it was not cut, but the shrinkage was large, A case where thickness unevenness or partial sticking occurred on the roll was Δ, a case where the substrate was severely shrunk and the substrate was cut, or a case where the base material was stuck on the roll and peeled off was indicated as x.

<Coating property A>
In [Preparation of Separator], the state of the substrate when applied to the substrate was observed. ○ When the base material is cut, torn, or cracked, and can be applied without any problem ○, When the base material is frequently cut, torn or cracked, the coating is hindered did.

<Wettability>
In [Production of Separator], the wettability of the substrate when applied to the substrate was evaluated. The case where the coating liquid soaked into the entire base material immediately was marked with ◯, and the case where the whole or a part of the substrate surface repels the coating liquid and the coating liquid soaked was marked with x.

<Back-through>
In the manufacturing process of the separator, ○ when the coating liquid did not penetrate the substrate at all, slightly through, but when there was no trouble such as the back side sticking to the roll of the coating device, Δ, In the case where the back surface sticks to the roll and the smooth coating cannot be performed, the case was evaluated as x.

<Coating property B>
A solution (coating solution using an organic solvent as a medium) prepared by dissolving 100 g of polyvinylidene fluoride (mass average molecular weight 300000) in 1800 g of N-methyl-2-pyrrolidone was prepared. Coating was performed using an applicator. At this time, there was no cutting, tearing or cracking of the base material, and the coating could be performed without any problems. ○ When cutting, tearing or cracking of the base material frequently occurred and hindered coating × It was.

<Heat sealability>
A base material and a polypropylene porous film are cut to an appropriate length with a width of 50 mm, both are stacked one by one, and a pair of upper and lower pressure contact bars (width) whose surface set at 140 ° C. is coated with polytetrafluoroethylene (width) Using a heat sealer having a length of 1 cm and a length of 30 cm, the heat sealer was pressed at a pressure of 0.2 MPa for 1 second and heat sealed. The case where the adhesive strength after heat sealing was strong was indicated by ◯, the case where the adhesive strength was weak was indicated by △, and the case where the adhesive force was not adhered was indicated by ×.

<Gap>
The cross section of the separator was observed with an electron microscope, and voids inside the substrate were confirmed. A when there is almost no filling of silicon oxide inside the substrate and there are almost no voids inside the substrate, part of the silicon oxide is filled inside the substrate, but the electrolyte can be retained sufficiently B when the void remains, C when filled with silicon oxide and almost clogging the void, and D when there is little silicon oxide filling, but the substrate itself has insufficient void did. A is the best, B is practically no problem, and C and D are practically problematic.

<Electrolytic solution retention>
The separator was cut to 100 mm width × 100 mm, immersed in the electrolyte solution for 1 minute, suspended for 1 minute to cut off the excess electrolyte solution, and the mass W1 of the separator was measured. The value obtained by subtracting the mass W0 of the separator before holding the electrolytic solution from W1 and dividing the value W2 by W0 and multiplying by 100 was defined as the electrolytic solution retention rate (%). As the electrolytic solution, a mixed solution in which 1 mol / l of LiPF ₆ was dissolved was used. The mixed solution is ethylene carbonate and diethyl carbonate in a mass ratio of 3: 7.

<Dendrite resistance>
A metal lithium foil was placed on one side of the separator and a positive electrode was placed on the opposite side of the separator and laminated, and an electrolyte was injected to prepare 100 laminate cells. The battery was charged at a constant current of up to 3.6 V at 0.5 mA / cm ² and further overcharged by applying 3.6 V for 24 hours. The case where an abnormal current flowed during this overcharge was regarded as an internal short circuit, the overcharge was stopped, the laminate cell was opened, and the generation state of lithium dendrite was confirmed. Dendritic resistance was defined as the proportion of cells that generated lithium dendrite and passed through the substrate due to overcharging. It means that it is excellent in dendrite resistance, so that this ratio is small. For the positive electrode, a slurry in which a lithium cobaltate as an active material, acetylene black as a conductive additive, and polyvinylidene fluoride as a binder are mixed at a mass ratio of 90: 5: 5 is applied to both surfaces of an aluminum current collector. Using. The electrolytic solution is the same as that described in the evaluation of <Electrolytic solution retention>.

  The base materials for lithium secondary batteries of Examples 1 to 16 contain ultrafine fibers obtained by dividing a split type composite fiber in which polyester and polyethylene are adjacent to each other, and solvent-spun cellulose fibers that are beaten. Since the wet non-woven fabric is made of polyester, the ultrafine fibers made of polyester and / or the intersections of the ultrafine fibers and solvent-spun cellulose fibers are bonded with polyethylene, and the surface of the substrate and the inside thereof are not covered with a film. The wetness of the aqueous coating liquid containing particles is good, there is no see-through of the aqueous coating liquid, and the aqueous coating liquid is excellent in coating properties. Filler particles are filled inside the base material to close the voids in the base material. The heat sealability was excellent. Since the base material for lithium secondary batteries of Examples 1 to 16 is obtained by heat-treating the wet nonwoven fabric at 135 to 160 ° C., the heat treatment is smoothly performed, and the ultrafine fibers made of polyester and / or the ultrafine fibers and The intersection point of solvent-spun cellulose fiber is bonded with polyethylene, and the ultrafine fiber made of polyester maintains the fiber shape. It was excellent. The lithium secondary battery separators produced in Examples 1 to 16 had a high electrolyte solution retention rate because voids inside the substrate remained. Moreover, since the filler was intensively supported on the surface of the substrate, the lithium dendrite resistance was excellent.

  The base material for a lithium secondary battery of Example 17 is composed of an ultrafine fiber obtained by dividing a split type composite fiber in which polyester and polyethylene are adjacent to each other, and a solvent-spun cellulose fiber formed by beating, and has a heat seal property. Although the heat treatment temperature was less than 135 ° C., there was no shrinkage or cutting during the heat treatment, but the adhesion between the ultrafine fibers made of polyester and the intersection of the ultrafine fibers and solvent-spun cellulose fibers was insufficient. Therefore, the coating solution using an organic solvent as a medium was able to be applied, but the coating property of the aqueous coating solution was poor. Therefore, when an aqueous coating liquid was used, a separator coated with filler particles could not be produced.

  The base material for the lithium secondary battery of Example 18 is composed of an ultrafine fiber obtained by dividing a split type composite fiber in which polyester and polyethylene are adjacent to each other, and a solvent-spun cellulose fiber formed by beating, and is heat-sealable. However, since the heat treatment temperature exceeded 160 ° C., the amount of heat became excessive, the base material contracted greatly, and a part of the base material stuck to the roll. However, the wettability of the aqueous coating liquid containing the filler particles is good, there is no back-through of the aqueous coating liquid, and the aqueous coating liquid is excellent in coating properties. The gap was not blocked. The separator for the lithium secondary battery produced in Example 18 had a high electrolyte solution retention rate because voids inside the substrate remained. Moreover, since the filler was intensively supported on the surface of the substrate, the lithium dendrite resistance was excellent.

  On the other hand, the base material for the lithium secondary battery of Comparative Example 1 is composed only of ultrafine fibers obtained by dividing a split type composite fiber in which polyester and polyethylene are adjacent to each other. The aqueous coating liquid had good coating properties and excellent heat sealability, but the aqueous coating liquid had poor wettability. Moreover, since the solvent-spun cellulose fiber formed by beating was not contained, there were few voids inside the substrate. The lithium secondary battery separator using the lithium secondary battery base material of Comparative Example 1 had good dendrite resistance but poor electrolyte solution retention.

  The base material for the lithium secondary battery of Comparative Example 2 is composed of ultrafine fibers obtained by dividing a split type composite fiber in which polyester and polyethylene are adjacent to each other, and other synthetic short fibers. Although the wettability and coating property were good and the heat sealing property was excellent, since it did not contain the solvent-spun cellulose fiber formed by beating, large through-holes existed and the aqueous coating liquid broke through. Further, the filler particles were filled in the base material, and the pores inside the base material were closed. The separator using the base material for the lithium secondary battery of Comparative Example 2 had poor electrolytic solution retention and dendrite resistance.

  Since the base material for the lithium secondary battery of Comparative Example 3 is composed of 100% solvent-spun cellulose fiber formed by beating, there is no heat sealability, and a coating solution using an organic solvent as a medium could be applied. When applied to the liquid, it collapsed and the aqueous coating liquid could not be applied. Therefore, a separator coated with filler particles could not be produced.

  The base material for the lithium secondary battery of Comparative Example 4 is composed of solvent-spun cellulose fiber and synthetic short fiber (polyester short fiber) formed by beating, and a split type composite fiber in which polyester and polyethylene are adjacent to each other is divided. Because it does not contain ultrafine fibers, the heat-sealability is insufficient, and the coating liquid using an organic solvent as a medium could be applied, but when it is applied to the aqueous coating liquid, it collapses and the aqueous coating liquid is applied. I couldn't. Therefore, a separator coated with filler particles could not be produced.

  The base material for the lithium secondary battery of Comparative Example 5 is composed of a solvent-spun cellulose fiber and a synthetic short fiber (acrylic short fiber) formed by beating, and a split type composite fiber in which polyester and polyethylene are adjacent to each other is divided. The coating liquid using an organic solvent as a medium can be applied because it does not contain ultrafine fibers, but it can collapse when applied to an aqueous coating liquid, and the aqueous coating liquid can be applied. could not. Therefore, a separator coated with filler particles could not be produced.

  The base material for the lithium secondary battery of Comparative Example 6 is composed of a solvent-spun cellulose fiber and a synthetic short fiber (fully aromatic polyamide short fiber) formed by beating, and a split type composite fiber in which polyester and polyethylene are adjacent to each other Because it does not contain fine fibers formed by splitting, there is no heat-sealability, and coating liquids that use organic solvents as a medium could be applied, but when applied to aqueous coating liquids, they collapsed and applied aqueous coating liquids I couldn't. Therefore, a separator coated with filler particles could not be produced.

  The base material for the lithium secondary battery of Comparative Example 7 was adhered to the metal roll in the heat treatment for forming the gel film of the ethylene-vinyl alcohol copolymer, and delamination occurred. Couldn't get. Moreover, since the gel film was formed on the surface and inside of the substrate, the coating property was good, but the wettability of the aqueous coating liquid was poor, and there were few voids inside the substrate. In addition, since there were no finer fibers than the split composite fiber and the synthetic short fiber, a large through hole existed, and the aqueous coating liquid partially penetrated. The separator using the base material for a lithium secondary battery of Comparative Example 7 had a poor electrolyte solution retention and poor dendrite resistance.

  The base material for the lithium secondary battery of Comparative Example 8 does not contain the ultrafine fiber obtained by dividing the split type composite fiber in which polyester and polyethylene are adjacent to each other, and the solvent-spun cellulose fiber formed by beating. Because it is composed only of short fibers, heat sealability and coating properties were good, but wettability of aqueous coating liquid was poor and there was a large through hole, so there was a back-through of the coating liquid, and the substrate was used The separator made was poor in dendrite resistance. Moreover, since the inside of the base material was filled with the filler, there were few voids inside the base material, and the separator using the base material had a poor electrolyte solution retention rate.

  The base material for lithium secondary batteries of this invention can be used conveniently for lithium ion secondary batteries, such as a lithium ion secondary battery and a lithium ion polymer secondary battery.

Claims (2)

  A lithium secondary battery comprising ultrafine fibers obtained by dividing a split-type composite fiber in which polyester and polyethylene are adjacent to each other, and a wet nonwoven fabric containing solvent-spun cellulose fibers formed by beating Substrate for use.   A process for impregnating or applying a slurry containing filler particles, a process for impregnating or applying a slurry containing a resin, and a porous film are integrally laminated on the lithium secondary battery substrate according to claim 1. A separator for a lithium secondary battery obtained by performing at least one treatment selected from a treatment and a treatment of impregnating or applying a solid electrolyte or a gel electrolyte.