JP2012133898A - Base material for 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 with no pattern transfer of a papermaking web nor pin hole, and with good suitability to coating of a coating liquid containing filler particles, and to provide a separator for a lithium secondary battery which has a good electrolytic solution keeping rate, and an excellent resistance against lithium dendrite.SOLUTION: The base material for a lithium secondary battery comprises: an ultrafine fiber formed by dividing a division type composite fiber with polyester and polyethylene adjacent to each other; and a wet type nonwoven fabric containing a synthetic short fiber. The separator for a lithium secondary battery is formed by use of the 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. Current non-aqueous secondary batteries have a protection circuit that breaks when overcharged, and is equipped with a safety valve and PTC element for the purpose of safely destroying the battery when overcharged. Thermal separators are used for lithium secondary battery separators. The device which has a function is made | formed. 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 a separator for a lithium secondary battery, a film-like porous film made of polyolefin such as polyethylene is often used, and when the temperature inside the battery is around 130 ° C., it melts and closes the micropores. There is a thermal fuse function (shutdown function) that prevents the movement of lithium ions and cuts off the current, but if the temperature rises further due to some situation, the polyolefin itself may melt and short-circuit, suggesting the possibility of thermal runaway Has been. Therefore, there is a need for a heat-resistant separator that does not melt and shrink even at temperatures close to 200 ° C.

  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 are not considered at all about the fiber cross-sectional shape that is a pinhole forming factor, the pattern of the papermaking net is transferred to the wet nonwoven fabric, and the density of the fiber density can be uneven, There was a problem that the fiber did not get on the intersection of the wires constituting the net and that part became a pinhole. Furthermore, there is a problem that the lithium dendrite penetrates through the separator due to the presence of the pinhole. 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 25). 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, these lithium secondary battery substrates do not take into account the fiber cross-sectional shape, which is a factor for forming pinholes, so that the pattern of the papermaking net is transferred to a wet non-woven fabric, and the density of the fiber density is uneven. Then, there is a problem that the fiber does not get on the intersection of the wires constituting the net and the portion becomes a pinhole. Therefore, when the coating liquid containing filler particles is applied, the back surface will pass through, and the problem of poor surface smoothness of the coated surface, filler particles are filled inside the nonwoven fabric, and the pores inside the nonwoven fabric are blocked, There was a problem that the electrolyte solution retention was poor and the internal resistance of the separator was increased. In addition, there is a problem that the filler does not stay in the sparse part of the fiber and remains as a hole. Furthermore, there is a problem that the lithium dendrite penetrates through the separator due to the presence of the pinhole.

  In the base materials for lithium secondary batteries described in Patent Documents 7 to 27, the nonwoven fabric mainly composed of inorganic fibers such as glass fibers lacks flexibility, so the base material is destroyed during coating or battery assembly. May be. Moreover, in order to improve the strength of the nonwoven fabric, a film is formed when heat-fusible fibers are used, and depending on the type and amount of resin and binder contained in the coating liquid, they form a film, There was a problem that the penetration of the coating liquid was uneven and the wettability of the coating liquid was poor.

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 object of the present invention is to provide a lithium secondary battery substrate that has no pattern transfer on the papermaking network, no pinholes, good wettability and good coatability of the coating liquid, and lithium that has excellent electrolyte retention and lithium dendrite resistance. The object is to provide a separator for a secondary battery.

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

  The base material for a lithium secondary battery of the present invention is composed of a wet nonwoven fabric containing ultrafine fibers and synthetic short fibers obtained by dividing a split type composite fiber in which polyester and polyethylene are adjacent to each other. There is no paper pattern transfer and no pinholes. Moreover, since the base material surface and the inside are not covered with a film, the wettability and coating property of a coating liquid are good. The separator for the lithium secondary battery according to the present invention is such that the filler particles are intensively laminated on the surface of the base material, so that the voids inside the base material are maintained, the electrolytic solution retention rate is high, and the lithium dendrite is repeatedly charged and discharged. Even when is generated on the electrode surface, the filler particles concentrated on the substrate surface can prevent the lithium dendrite from conducting between the positive and negative electrodes, and is excellent in lithium dendrite resistance.

  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 any of high density, medium density, low density, ultra high density, ultra low density, and linear low density.

  Examples of the cross-sectional shape of the split type composite fiber include a radial type, a layered type, a comb type, and a grid type. 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 3.0 to 18.0 μm, and more preferably 6.0 to 16.0 μm. If it is less than 3.0 μm, it may be difficult to divide. If it is thicker than 18.0 μm, the long axis of the cross-section of the ultrafine fiber after division becomes long, so that the gap in the substrate may be blocked.

  The ultrafine fibers obtained by splitting preferably have a minor axis length of the cross section of 1.0 to 5.0 μm, and more preferably 1.0 to 3.0 μm. If the thickness is less than 1.0 μm, the theoretical flatness of the cross section becomes too large and the gaps in the base material are blocked, or the bonding between the ultrafine fibers and the intersection between the ultrafine fibers and the synthetic short fibers may be insufficient. . If it exceeds 5.0 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 the thickness is less than 0.5 mm, there is a case where the ratio of falling out of the screen and outflowing into drainage during wet papermaking increases. If it is longer than 10 mm, the fine fibers may depend 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, the intersections between the ultrafine fibers, the intersections between the synthetic short fibers, or the adhesion between the intersections of the ultrafine fibers and the synthetic short fibers may be insufficient. is there.

  Synthetic short fibers constituting the base material for lithium secondary battery of the present invention include polypropylene, polyethylene, polymethylpentene, polyester, acrylic, polyamide, polyimide, polyvinyl chloride, polyvinylidene chloride, polyvinyl ether, polyvinyl ketone, poly Ether, polyvinyl alcohol, diene, polyurethane, phenol, melamine, furan, urea, aniline, unsaturated polyester, fluorine, silicone, polyamideimide, polyetheretherketone, polyphenylene sulfide, polybenzimidazole, poly-p-phenylenebenzobisthiazole , Poly-p-phenylenebenzobisoxazole, polytetrafluoroethylene, short fibers composed of 100% resin such as derivatives thereof, or composite composed of two or more kinds of resins Wei, and the like. 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 that the coating liquid can be seen through, the thickness becomes 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.

  In the base material for a lithium secondary battery of the present invention, the mass ratio of the split composite fiber and the synthetic short fiber in which polyester and polyethylene are adjacent to each other is preferably 8: 2 to 2: 8, and 7: 3 4: 6 is more preferable. If the ratio of the split-type composite fibers is more than 8: 2, the wet nonwoven fabric may be remarkably shrunk and may be wrinkled or the gaps in the substrate may be blocked. If the ratio is less than 2: 8, pattern transfer of the papermaking net may occur, or the strength of the substrate may be weakened and 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 synthetic short fibers are dispersed in water to form a uniform slurry, which is then sprinkled with a paper machine. To make a wet nonwoven fabric. You may add chemical | medical agents, such as a dispersing aid, an antifoamer, a thickener, and a release agent, 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 combining these.

  In the present invention, the wet nonwoven fabric is heat-treated at 135 to 160 ° C. to melt a part or all of polyethylene, the intersection of ultrafine fibers made of polyester, the intersection of synthetic short fibers, the ultrafine fibers and the synthetic short fibers It is preferable to bond the intersections. 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. The method of pressurizing through, the method of pressurizing for a predetermined time using a hot press machine in the temperature range of 135-160 degreeC, the combination of these methods, etc. can carry out. The roll to be heated may be made of resin or metal. When the temperature is lower than 135 ° C., the intersection between the ultrafine fibers, the intersection between the synthetic short fibers, and the intersection between the ultrafine fibers and the synthetic short fibers are weak, and the tensile strength and the puncture strength may be insufficient. If it exceeds 160 ° C., the shrinkage of the wet nonwoven fabric may increase and deform, or the polyethylene may stick to the roll or hot press machine during the heat treatment. Whether heat treatment is performed or not, the thickness is adjusted by calendering as necessary.

  The wet nonwoven fabric in the present invention contains any one of the core-sheath type, the eccentric type, and the side-by-side type composite fiber in a part of the synthetic short fiber, and one of the components of the composite fiber has the same melting point or lower than that of polyethylene. In the case of having a melting point, the intersection between the ultrafine fiber made of polyester and the composite fiber and / or the intersection between the composite fibers is also bonded by the component of the composite fiber. Also when it contains a composite fiber, it is preferable to heat-process at 135-160 degreeC, and if it heat-processes in this temperature range, the production | generation of a film | membrane can be suppressed.

  4-45 micrometers is preferable, as for the thickness of the base material for lithium secondary batteries of this invention, 8-40 micrometers is more preferable, and 10-35 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 0.250~0.700g / 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 0.700 g / cm ³ , the resistance value of the separator may be increased.

  The base material for a lithium secondary battery of the present invention preferably has a maximum pore diameter of 3 to 15 μm as defined by ASTM-F316-86, more preferably 3 to 10 μm, and further preferably 3 to 8 μm. preferable. When the thickness is less than 3 μm, the voids of the base material are closed by the ultrafine fibers, and the electrolyte solution retention rate may be lowered. If it is larger than 15 μm, the coating liquid may be seen through, or filler particles may be filled inside the base material, which may block the voids in the base material.

  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 and synthetic short 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. In Table 1, “PP” means polypropylene, “EVOH” means ethylene-vinyl alcohol copolymer, “PET” means polyethylene terephthalate, and “PE” means high density polyethylene. The cross-sectional area ratio of the core part and the sheath part of the core-sheath type composite fibers of F10 and F11 is 50:50, and the core part is polypropylene.

Example 1
Slurry 1 shown in Table 2 was prepared. Slurry 1 was subjected to wet papermaking using a double-lined circular paper machine, and dried at a Yankee dryer temperature of 140 ° 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 heat-treated by contacting the front and back surfaces of the wet nonwoven fabric, and then passed through a pair of metal rolls heated to 135 ° C. at a linear pressure of 500 N / cm. The base material of Example 1 was produced.

Examples 4, 9, 12, 13
Slurries 4, 9, 12, and 13 shown in Table 2 were prepared. As shown in Table 3, wet papermaking was performed on the slurry 4, 9, 12, and 13 using a double-type circular net paper machine, and dried at a Yankee dryer temperature of 140 ° 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 are brought into contact with the metal roll for heat treatment, and further passed between a pair of metal rolls under the conditions of temperature and linear pressure shown in Table 3. It heat-processed and the base material of Example 4, 9, 12, 13 was produced.

Examples 2, 3, 5-8, 10, 11, 14, 15
Slurries 2, 3, 5-8, 10, 11, 14, and 15 shown in Table 2 were prepared. As shown in Table 3, slurry 2, 3, 5-8, 10, 11, 14, and 15 were wet-made using a double-type circular net paper machine, and dried at a Yankee dryer temperature of 140 ° C. A wet nonwoven fabric was prepared. 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 2, 3, 5-8, 10, The base materials of 11, 14, and 15 were produced.

Example 16
The slurry 4 was subjected to wet paper making using a double-type circular net paper machine, and dried at a Yankee dryer temperature of 140 ° 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 base material of Example 16.

Example 17
The slurry 4 was subjected to wet paper making using a double-type circular net paper machine, and dried at a Yankee dryer temperature of 140 ° C. to prepare a wet nonwoven fabric. Subsequently, the front and back surfaces of the wet nonwoven fabric were brought into contact with a metal roll heated to 165 ° C. for 5 seconds each and heat-treated, and further calendered to adjust the thickness, thereby producing a substrate of Example 17.

(Comparative Example 1)
Slurry 16 shown in Table 2 was prepared, wet papermaking was performed using a double-lined circular paper machine, and dried at a Yankee dryer temperature of 140 ° 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)
Slurry 17 shown in Table 2 was prepared, wet papermaking was performed using a twin-type circular net paper machine, and dried at a Yankee dryer temperature of 130 ° C. to prepare a wet nonwoven fabric. Next, a thermal calendar process was performed at 130 ° C. and a linear pressure of 500 N / cm, and a base material of Comparative Example 2 was produced.

(Comparative Example 3)
Slurry 18 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 Yankee 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 100% by mass of water 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 form an ethylene-vinyl alcohol copolymer as a gel film. Thus, a base material of Comparative Example 3 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, 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.). An aqueous 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 1 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.

<Net pattern transfer>
The substrate was visually checked to determine whether there was a pattern transfer on the papermaking net. The case where there was a pattern transfer was set as “Yes”, and the case where there was no pattern transfer was set as “No”.

<Pinhole>
Light was applied from the back side of the substrate, and it was visually determined whether or not there was a pinhole in the substrate. When there is a pinhole, it is “Yes”, and when there is no pinhole, it is “No”.

<Heat treatment>
The state of the base material when the base material was heat-treated was observed. ○ when the heat treatment can be performed smoothly without cutting the base material, the cutting was not performed, but the shrinkage is large, the thickness unevenness of the base material, or the case where some sticking occurs on the roll, Δ, remarkably contracted, The case where the substrate was cut, or the case where it was stuck to a roll and peeled off was marked as x.

<Wettability>
In the production of the separator, the wettability of the coating liquid when applied to the substrate was evaluated. ○ when the coating liquid soaks quickly and uniformly into the substrate, △ when the coating liquid soaks evenly into the substrate but takes time to soak, △, the substrate surface soaks the coating liquid uniformly The case where there was not was set as x.

<Coating property>
In the production of the 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 problems, fluffing and tearing may occur, and when the base material can be applied by weakening the tension (tension) The case where any of Δ, substrate cutting, tearing and cracking frequently occurred and hindered the coating was evaluated as x.

<Back-through>
In the production of the separator, ○ when the coating liquid did not penetrate the substrate at all, slightly penetrated, but Δ when the back surface did not interfere with the roll of the coating device When the back surface sticked to the roll and the trouble such as the inability to perform smooth coating occurred, x was marked.

<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, and a portion of the silicon oxide is filled inside the substrate, but the gap can hold the electrolyte sufficiently B is the case where C remains, C is the case where the silicon oxide is filled and the gap is almost closed, and D is the case where there is almost no silicon oxide filling, but the base itself is insufficient. A is the best, B is practically no problem, and C and D are practically problematic.

<Electrolytic solution retention>
The separator was cut to a width of 100 mm × 100 mm, dipped 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>.

  Since the base materials for lithium secondary batteries of Examples 1 to 15 are composed of 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 synthetic short fibers. The papermaking net pattern was not transferred, and there was no pinhole. Also, the intersections of the ultrafine fibers made of polyester, the intersections of the synthetic short fibers, and the intersections of the ultrafine fibers and the synthetic short fibers are bonded with the components of polyethylene and / or core-sheath type composite fibers, and the surface of the substrate and the inside Since the film is not covered with a film, the wettability of the coating liquid containing the filler particles is good and the coating property is excellent. The filler particles were not filled inside the base material and did not close the voids of the base material. In the base materials for lithium secondary batteries of Examples 1 to 15, since the wet nonwoven fabric is heat-treated at 135 to 160 ° C., the heat treatment is smoothly performed, and the ultrafine fibers made of polyester maintain the fiber shape. Since it forms a framework, cutting and tearing did not occur during coating, and the coating property was excellent. The separators for lithium secondary batteries produced by applying filler particles to the substrates of Examples 1 to 15 of the present invention had a high electrolyte solution retention rate because voids inside the substrate remained. Moreover, since the filler was intensively supported on the base material surface, it was excellent in dendrite resistance. Since the base material for lithium secondary batteries of Examples 14 and 15 had a large fiber diameter of the synthetic short fiber, the hole diameter of the base material was slightly increased, and the dendrite resistance was slightly inferior.

  Since the base material for the lithium secondary battery of Example 16 is composed of ultrafine fibers obtained by dividing a split type composite fiber in which polyester and polyethylene are adjacent to each other and synthetic short fibers, the papermaking net pattern is not transferred. There was no pinhole, but because the heat treatment temperature was less than 135 ° C., there was a point where the adhesion between the ultrafine fibers made of polyester, the intersection between the synthetic fibers, and the intersection between the ultrafine fibers and the synthetic fiber was insufficient, The wettability of the coating liquid was good, but the coatability was slightly bad. The lithium secondary battery separator produced by applying the filler to the base material of Example 16 had a high electrolyte solution retention rate because the voids inside the base material remained. Moreover, since the filler was intensively supported on the base material surface, it was excellent in dendrite resistance.

  Since the base material for the lithium secondary battery of Example 17 is composed of ultrafine fibers obtained by dividing a split type composite fiber in which polyester and polyethylene are adjacent to each other and synthetic short fibers, the papermaking net pattern is not transferred. However, since the heat treatment temperature exceeded 160 ° C., the amount of heat became excessive, the base material contracted greatly, a part of the base material stuck to the roll, delaminated, and pinholes were generated. Therefore, the wettability of the coating liquid containing the filler particles is slightly inferior, and the coating liquid is slightly behind, but the coating liquid is excellent in coating properties, and the filler particles are filled inside the base material and the voids of the base material are filled. Did not occlude. The separator for a lithium secondary battery produced by applying a filler to the base material of Example 17 had a high electrolyte solution retention ratio, but was slightly inferior in dendrite resistance.

  On the other hand, since the base material for the lithium secondary battery of Comparative Example 1 is composed of only ultrafine fibers obtained by dividing a split type composite fiber in which polyester and polyethylene are adjacent to each other, the papermaking net pattern can be transferred. In addition, there was no pinhole, the coating liquid did not show through, and the coating property was good, but the wettability of the coating solution was poor, and there were few voids inside the substrate. The lithium secondary battery separator produced by applying a filler to the base material of Comparative Example 1 had good dendrite resistance but poor electrolyte retention.

  Although the base material for lithium secondary batteries of Comparative Example 2 is composed of synthetic short fibers, it does not contain ultrafine fibers formed by dividing split composite fibers in which polyester and polyethylene are adjacent to each other. There were many pinholes transferred. For this reason, the wettability and the coatability of the coating liquid were good, but the coating liquid slipped through, the filler particles were filled inside the base material, and the pores inside the base material were blocked. The lithium secondary battery separator produced by applying a filler to the base material of Comparative Example 2 had poor electrolyte retention and dendrite resistance.

  In the base material for the lithium secondary battery of Comparative Example 3, since the gel film of the ethylene-vinyl alcohol copolymer was formed on the surface and inside of the base material, there was no pattern transfer of the papermaking network. There were few voids and the wettability of the coating liquid was poor. In addition, there was a pinhole, and there was slight penetration of the coating liquid. The separator for a lithium secondary battery produced by applying a filler to the base material of Comparative Example 3 had a poor electrolyte retention rate and poor dendrite resistance.

  The base material for lithium ion 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 base material for a lithium secondary battery, characterized by comprising an ultrafine fiber obtained by dividing a split type composite fiber in which polyester and polyethylene are adjacent to each other, and a wet nonwoven fabric containing a synthetic short fiber.   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.