JP2014049222A - Separator for lithium secondary battery, and lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a separator for a lithium secondary battery, excellent in winding properties, high in charge/discharge efficiency, and excellent in high-rate characteristics; and a lithium secondary battery manufactured using the separator.SOLUTION: In a separator for a lithium secondary battery, a porous body comprising inorganic fillers is fastened to a nonwoven fabric base material. The nonwoven fabric base material is a composite nonwoven fabric comprising a spunbond nonwoven fabric and a melt-blown nonwoven fabric. A lithium secondary battery is manufactured using the separator for a lithium secondary battery.

Description

  The present invention relates to a separator for a lithium secondary battery and a lithium secondary battery.

  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 secondary battery typified by a lithium ion battery using an organic electrolyte has attracted attention. This lithium secondary 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, can be obtained. Since an aqueous electrolyte solution cannot be used, a non-aqueous electrolyte 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, the current lithium secondary battery is 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, a porous substrate made of an inorganic filler on a porous substrate such as a commercially available porous film or nonwoven fabric, or a porous film made of an inorganic filler and a binder (for example, Patent Documents 1 to 19) Reference).

  When the porous substrate is a porous film, the separators of Patent Documents 1 to 19 have a problem that battery characteristics when charging / discharging with a large current, that is, high-rate characteristics are low because the through-holes are considerably small. When the porous substrate is a non-woven fabric, there is a problem that pinholes are easily formed because the through-holes are orders of magnitude larger than the porous film. Therefore, in order to prevent the generation of pinholes, the amount of inorganic filler attached must be increased, and there is a problem that it is difficult to reduce the thickness of the entire separator. When the nonwoven fabric is a wet nonwoven fabric, particularly when the thickness of the separator is thinned, the tensile strength becomes insufficient, and there is a problem that the coating property and the winding property are liable to be hindered.

  In addition, wet non-woven fabrics may exist in a state in which many fibers are in close contact with each other due to poor fiber dispersion (also referred to as “bundling”). As a result, gaps necessary for ion movement are blocked, and charge / discharge efficiency and high-rate characteristics are likely to deteriorate.

  If the nonwoven fabric is made by the card method, the fibers are oriented in the flow direction of the web, so the tensile strength in the flow direction becomes extremely strong, it is easy to tear in the width direction, and the coating property is likely to be hindered. there were. When the binder for fixing the inorganic filler to the porous substrate is polyvinyl pyrrolidone or polyurethane, it dissolves in the electrolytic solution and raises the viscosity of the electrolytic solution. Moreover, since the detailed description is not made about the nonwoven fabric used by the Example of patent documents 6-15, it is about the raw material of a nonwoven fabric, a manufacturing method, a structure, a physical property, etc. as a means to improve the characteristic of a lithium secondary battery. There was room for further study.

  When the non-woven fabric is a spunbonded non-woven fabric, there is a problem that the fibers are likely to be unevenly dense and pinholes are likely to occur even after the porous body made of the inorganic filler is fixed. When the nonwoven fabric is a melt-blown nonwoven fabric, it is generally easier to obtain a finer fiber diameter than a spunbonded nonwoven fabric, and it is easy to obtain a dense nonwoven fabric, but the tensile strength and tear strength are weak and there are problems with coating properties and winding properties. .

  In a method in which a mixed solution composed of an inorganic filler and a binder is once coated on a film and the like, and after drying, the coating layer is peeled off from the film to produce a porous film (separator) composed of an inorganic filler and a binder. Although a strong separator can be obtained, it is easy to tear, and there may be a problem in slit workability and winding property.

JP 2008-123996 A JP 2010-123465 A JP 2011-65849 A JP 2011-65850 A JP 2011-154936 A JP 2008-4442 A JP 2008-66094 A JP 2006-164661 A JP 2009-32677 A JP 2009-224341 A Special table 2011-505663 gazette JP 2010-157521 A JP 2012-14994 A Special table 2010-538172 JP 2010-538173 A JP 2008-4439 A Special table 2008-508391 Special table 2008-503049 gazette International Publication No. 2009/96451 Pamphlet

  The subject of this invention is providing the lithium secondary battery which uses the separator for lithium secondary batteries which is excellent in winding property, has high charging / discharging efficiency, and is excellent in a high rate characteristic, and this separator.

  As a result of diligent research to solve the above problems, a separator for a lithium secondary battery in which a porous body made of an inorganic filler is fixed to a nonwoven fabric base material, the nonwoven fabric base material comprising a spunbond nonwoven fabric and a meltblown nonwoven fabric The present inventors have found a lithium secondary battery separator characterized by being a composite nonwoven fabric. Furthermore, the lithium secondary battery characterized by using this separator was discovered.

  The separator of the present invention has a porous body made of an inorganic filler fixed to a nonwoven fabric base material, and the nonwoven fabric base material is a composite nonwoven fabric made of a spunbond nonwoven fabric and a meltblown nonwoven fabric. Has strength and excellent winding properties. In addition, since the fibers of the nonwoven fabric base material are not in close contact with each other and the gap between the fibers is not blocked, ion transfer between the electrodes is not hindered, the charge / discharge efficiency is high, and the lithium secondary battery is excellent in high rate characteristics. Is obtained.

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.

  In the present invention, a composite nonwoven fabric composed of a spunbond nonwoven fabric and a meltblown nonwoven fabric is used as the nonwoven fabric substrate.

  Spunbond non-woven fabric is made by spinning molten thermoplastic resin from a nozzle, discharging the yarn, guiding the spun yarn to a pulling jet directly under the nozzle cap, and pulling it at high speed, and then on the moving conveyor. A web is obtained by depositing while opening, and then the web is integrated and wound by heating, entanglement or the like to obtain a long fiber nonwoven fabric. The pulling speed is preferably 2000 to 7000 m / min. If it is less than 2000 m / min, the molecular orientation of the fiber becomes insufficient, and the tensile strength of the resulting long fiber may decrease. If it exceeds 7000 m / min, yarn breakage during spinning tends to occur. The web may be heated by air-through heating, hot embossing, thermal calendering, or the like that applies hot air heated to a predetermined temperature to the web. Entanglement includes hydroentanglement treatment in which columnar water is applied at a high pressure and fibers are entangled three-dimensionally.

Melt blown nonwoven fabric refers to a melted thermoplastic resin that is discharged from a nozzle and sprayed with a high-speed gas fluid heated from its vicinity to make the thermoplastic resin into fine fibers, which are collected while opening on a moving conveyor. Collect long-fiber nonwoven fabrics. The amount of high-speed gas is preferably 500 to 3000 m ³ / h. If it is less than 500 m ³ / h, the spun fiber diameter may be too large. If it exceeds 3000 m ³ / h, fine fibers may be blown away without being deposited on the conveyor. The obtained long fiber nonwoven fabric may be subjected to air-through heating, hot embossing, thermal calendering, hydroentanglement, and the like.

  As a composite nonwoven fabric composed of a spunbond nonwoven fabric and a meltblown nonwoven fabric, a two-layer structure in which the spunbond nonwoven fabric and the meltblown nonwoven fabric are laminated, a three-layer structure in which a meltblown nonwoven fabric is disposed between two layers of the spunbond nonwoven fabric, and two layers Examples include a three-layer structure in which a spunbond nonwoven fabric is disposed between layers of the melt blown nonwoven fabric, a multilayer structure having four or more layers in which a spunbond nonwoven fabric and a meltblown nonwoven fabric are alternately disposed in layers or in multiple layers. The lamination of the spunbond nonwoven fabric and the meltblown nonwoven fabric can be performed by simultaneously manufacturing and laminating the meltblown nonwoven fabric in the production process of the spunbond nonwoven fabric, or the meltblown nonwoven fabric prepared in advance may be laminated. A nonwoven fabric may be manufactured and laminated simultaneously, or a spunbond nonwoven fabric prepared in advance may be laminated. The average fiber diameter of the spunbond nonwoven fabric and the meltblown nonwoven fabric may be the same or different, but the average fiber diameter of both is different in that it is easy to achieve both inorganic filler support and separator pinhole freeness. Is preferred. These composite nonwoven fabrics are preferably integrated by air-through heating, hot embossing, thermal calendering, and the like, because stronger strength can be obtained.

  The fibers constituting the spunbond nonwoven fabric and the meltblown nonwoven fabric used in the present invention may be one type or two or more types. Further, the fiber itself may be a single fiber composed of a single resin component, a fiber formed by blending two or more types of resin components, or a composite fiber formed by arranging two or more types of resin components. The cross-sectional shape may be any of circular, square, irregular, indeterminate, and hollow. The thermoplastic resin that is the element of the fiber includes polyolefins such as polypropylene, polyethylene, and polymethylpentene, polyesters such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyethylene isophthalate, and polylactic acid, acrylics, and polyamides. , Polyvinyl chloride, polyvinylidene chloride, polyvinyl alcohol, polyurethane, polycarbonate, cellulose, and derivatives and copolymers thereof. The composite fiber is composed of, for example, two types of thermoplastic resins having different melting points, and the arrangement of the two types of thermoplastic resins is a core-sheath type, an eccentric type, a side-by-side type, a sea-island type, an orange type, and a multiple as viewed from the fiber cross section. Examples include a bimetal type, a blend of two or more thermoplastic resins, and a spunbond nonwoven fabric or a meltblown nonwoven fabric. The thermoplastic resin may also contain pigments, inorganic fillers, surfactants, plasticizers, lubricants, antifoaming agents, thickeners, mold release agents, flame retardants, antioxidants, crosslinking agents, and the like. .

  0.5-15 micrometers is preferable, as for the average fiber diameter of the fiber which comprises the spun bond nonwoven fabric used for this invention, 1-10 micrometers is more preferable, and 1-5 micrometers is further more preferable. If it is less than 0.5 μm, the nonwoven fabric is weak and may interfere with the coatability. If it exceeds 15 μm, it may be difficult to reduce the thickness of the separator or a pinhole may occur. In order to adjust the average fiber diameter of the spunbond nonwoven fabric, the die temperature, the melt viscosity of the resin, the single hole discharge amount, the pulling speed, etc. may be adjusted.

The basis weight of the spunbond nonwoven fabric used in the present invention is preferably from 5 to 20 g / ^{m 2,} more preferably 5 to 15 g / ^{m 2,} more preferably 5~12g / ^{m 2.} If it is less than 5 g / m ² , the tensile strength becomes insufficient, which may impair coating properties and winding properties. If it exceeds 20 g / m ² , it may be difficult to reduce the thickness of the separator.

  0.05-15 micrometers is preferable, as for the average fiber diameter of the fiber which comprises the melt blown nonwoven fabric used for this invention, 0.1-10 micrometers is more preferable, and 0.1-5 micrometers is more preferable. In order to adjust the average fiber diameter of the melt blown nonwoven fabric, the die temperature, the melt viscosity of the resin, the single hole discharge amount, the pressure of the high-speed gas, the take-up speed, etc. may be adjusted. The term “take-off” means the speed at which the ejected resin filament is taken up by an ejector having an aspirator function while being cooled and thinned until it is shaken off on a moving conveyor. When the thickness is less than 0.05 μm, the nonwoven fabric is weak and may impair coating properties. If it exceeds 15 μm, it may be difficult to reduce the thickness of the separator or a pinhole may occur.

3-20 g / m < ² > is preferable, as for the basic weight of the melt blown nonwoven fabric used for this invention, 5-15 g / m < ² > is more preferable, and 5-10 g / m < ² > is still more preferable. If it is less than 3 g / m ² , the tensile strength becomes insufficient, which may impair coating properties and winding properties. If it exceeds 20 g / m ² , it may be difficult to reduce the thickness of the separator.

  Examples of the inorganic filler of the porous body made of the inorganic filler in the present invention include inorganic oxides and inorganic hydroxides such as alumina, gibbsite, boehmite, magnesium oxide, magnesium hydroxide, silica, titanium oxide, barium titanate, and zirconium oxide. Inorganic nitrides such as aluminum nitride and silicon nitride, aluminum compounds, zeolite, mica and the like. The shape of the inorganic filler may be any of a spherical shape, a substantially spherical shape, a plate shape, a cube shape, a scale shape, and an indefinite shape.

  The porous body made of an inorganic filler in the present invention means a three-dimensional porous body in which inorganic fillers gather and bind to each other and have a gap between the inorganic fillers. The porous body is prepared by mixing an inorganic filler, a binder (also called a “binder”) and an organic filler having self-adhesive power to form a slurry, contacting the slurry with a nonwoven fabric substrate, and drying the slurry. And fixed to the nonwoven fabric substrate.

  As the binder used in the present invention, ethylene-vinyl acetate copolymer, ethylene-acrylate copolymer, polyvinyl alcohol, styrene-butadiene rubber, nitrile rubber, acrylic rubber, fluorine rubber, epoxy resin, acrylic resin Examples include thermosetting resins such as resins, oxetane resins, novolac resins, resol resins, urea resins, melamine resins, and these can be used alone or in combination of two or more. . Examples of the epoxy resin include copolymers such as glycidyl acrylate, acrylic acid, methyl methacrylate, methacrylic acid, butyl methacrylate, and styrene, but are not limited thereto. Examples of the acrylic resin include, but are not limited to, copolymers such as methyl methacrylate, butyl acrylate, methacrylic acid, and hydroxyethyl methacrylate.

  Examples of the organic filler having a self-adhesive force include polyolefin resins such as polyethylene and polypropylene. Since these can be fixed to the nonwoven fabric base material in the form of particles, they can be bound with the inorganic filler in the form of particles to form a porous body composed of the inorganic filler and polyolefin resin. It can be fixed to. The polyolefin resin is preferably an aqueous dispersion in terms of safety and ease of handling.

  The porous body in the present invention may contain an organic filler having no self-adhesive force. Polyester resins (including aromatic polyesters and wholly aromatic polyesters) such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyethylene isophthalate, polylactic acid, acrylic resins, polyamide resins (aromatic polyamides and wholly aromatics) Polyamides), polyvinyl chloride resins, polyvinylidene chloride resins, polyvinyl alcohol resins, polyurethane resins, polycarbonate resins, cellulose resins, polyimide resins, polyamideimide resins, and fluorine resins. The shape of the organic filler may be spherical, substantially spherical, or indefinite.

The porous body composed of the inorganic filler and the binder in the present invention is prepared by bringing a slurry containing the inorganic filler into contact with the nonwoven fabric substrate and drying it, and adheres to the nonwoven fabric substrate. You may add a thickener, an antifoamer, and an organic solvent to a mixing slurry as needed. 1.00 to 20.00 g / m ² is preferable, 2.00 to 16.00 g / m ² is more preferable, and 3.00 to 13.00 g / m ² is more preferable. If the sticking amount is less than 1.00 g / m ² , pinholes may be generated. When it is more than 20.00 g / m ² , the porous body becomes too thick, and the inorganic filler may fall off or peel off during winding or bending.

  10.0-60.0 mass% is preferable, as for the content rate of the inorganic filler in a separator, 15.0-50.0 mass% is more preferable, and 20.0-40.0 mass% is more preferable. The solid content of the binder in the separator is preferably 1.0 to 30.0% by mass, more preferably 1.5 to 20.0% by mass, and even more preferably 2.0 to 15.0% by mass. . When the content of the inorganic filler is less than 10.0% by mass and the solid content of the binder is more than 30.0% by mass, when pinholes are formed or the binder forms a film, The hole may be partially blocked. If the content of the inorganic filler is more than 60.0% by mass and the solid content of the binder is less than 1.0% by mass, the inorganic filler may fall off or peel off. 0-40 mass% is preferable, as for the content rate of the organic filler with a self-adhesive force in a separator, 0-30 mass% is more preferable, and 0-20 mass% is further more preferable. If it exceeds 40% by mass, the organic fillers bind to each other to form a film, which may hinder ion migration.

  In the method for producing a separator in the present invention, a slurry containing an inorganic filler is brought into contact with a composite nonwoven fabric substrate composed of a spunbond nonwoven fabric and a meltblown nonwoven fabric and dried. Examples of the method for bringing the slurry into contact with the nonwoven fabric substrate include impregnation, coating, spraying, and transfer. Transfer refers to a method in which a slurry is once attached to a film or roll, and then transferred from the film or roll to a nonwoven fabric substrate. In the present invention, coating is preferred because production efficiency is good and it is easy to produce a highly uniform separator. Coating can be performed using a coater such as an air knife coater, a die coater, a roll coater, or a curtain coater. The coated surface may be only one side of the nonwoven fabric substrate or both sides. The number of coatings per side may be one time or two or more times. When coated on the surface of the spunbonded nonwoven fabric, the inorganic filler can easily penetrate into the spunbonded nonwoven fabric, and the advantage that the thickness of the entire separator is not easily increased can be obtained. When applied to the surface of the meltblown nonwoven fabric, the porous body made of an inorganic filler can easily form a uniform layer, and the advantage of greatly suppressing the generation of pinholes can be obtained.

  What is necessary is just to determine a drying temperature by the balance of the heat resistance of a nonwoven fabric base material, and drying efficiency, and 50-180 degreeC is preferable. When the binder is a thermosetting resin, it may be dried to cure the thermosetting resin, and after drying in a short time to remove water once, at a temperature higher than or equal to the drying temperature. The thermosetting resin may be cured by extending the heating time. The heating temperature for curing the thermosetting resin is preferably 150 to 180 ° C., and the heating time is preferably 10 to 30 minutes. After drying, the thickness may be adjusted by calendering as necessary. The calender roll may be heated or at room temperature.

The separator of the present invention preferably has a basis weight of 6.0 to 30.0 g / m ² , more preferably 7.0 to 26.0 g / m ² , and 8.0 to 22.0 g / m ^2. it is more preferably m ^2. When the basis weight is less than 6.0 g / m ² , the separator may have insufficient tensile strength and puncture strength. If it exceeds 30.0 g / m ² , it may be difficult to reduce the thickness of the separator.

  The separator of the present invention preferably has a thickness of 8 to 40 μm, more preferably 10 to 30 μm, and still more preferably 10 to 25 μm. If the thickness is less than 8 μm, the tensile strength and puncture strength of the separator may be insufficient. If it is thicker than 40 μm, the electrode area that can be accommodated in the lithium secondary battery becomes small, and battery characteristics such as capacity may become insufficient.

  When the separator of the present invention is coated on one side, a lithium secondary battery may be produced by winding or laminating the coated surface in contact with either the positive electrode or the negative electrode.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a present Example.

<Substrate 1>
Polyethylene terephthalate is melted at 295 ° C., spun from a nozzle at a base temperature of 300 ° C., taken up while being sucked by an ejector at a spinning speed of 5500 m / min, collected on a moving net conveyor while being disentangled, and long fibers A nonwoven fabric was prepared. Next, the long fiber nonwoven fabric was subjected to a heat calendar treatment at 195 ° C. using a pair of flat rolls to produce a spunbond nonwoven fabric (average fiber diameter 2.0 μm, basis weight 7 g / m ² , thickness 11 μm). did.

<Substrate 2>
Polyethylene terephthalate is melted at 295 ° C., polypropylene is melted at 260 ° C., polyethylene terephthalate is used as the core, polypropylene is used as the sheath (core portion: sheath portion = 80: 20 by mass ratio), and the nozzle temperature is 300 ° C. And spinning with an ejector at a spinning speed of 5000 m / min, and collecting on a moving net conveyor while defibrating to produce a long fiber nonwoven fabric. Next, a spunbonded nonwoven fabric (average fiber diameter: 3.0 μm, basis weight: 8 g / m ² , thickness: 12 μm) was produced by thermal calendering at 120 ° C. using a pair of flat rolls.

<Substrate 3>
2% by mass of polyethylene is added (blended) to a polyester copolymer (melting point: 133 ° C.) obtained by copolymerizing a dicarboxylic acid component mainly composed of terephthalic acid, 1,6-hexanediol, and 1,4-butanediol, The melt was spun at 190 ° C., spun by an ejector at a spinning speed of 4500 m / min, and collected on a moving net conveyor while being disentangled to produce a long fiber nonwoven fabric. Next, the long fiber nonwoven fabric is subjected to thermal calendering treatment at 120 ° C. using a pair of flat rolls to produce a spunbond nonwoven fabric (average fiber diameter 3.5 μm, basis weight 12 g / m ² , thickness 18 μm). did.

<Substrate 4>
Nylon 6 is melted at 290 ° C., hot air at 290 ° C. is blown at 1500 m ³ / h at a pressure of 40 kPa, spun from a nozzle, and collected while disentangled on a moving net conveyor to produce a long fiber nonwoven fabric. did. Next, the long fiber nonwoven fabric was subjected to heat calendering treatment at 180 ° C. using a pair of flat rolls to produce a melt blown nonwoven fabric (average fiber diameter 2.5 μm, basis weight 10 g / m ² , thickness 16 μm). .

<Substrate 5>
Polypropylene is mixed so as to be 20% by mass with respect to polyethylene terephthalate, melted with an extruder, hot air of 290 ° C. is blown at 2000 m ³ / h at a pressure of 50 kPa at a nozzle temperature of 300 ° C., and discharged from the nozzle. Then, the fibers were collected while being disentangled on a moving net conveyor to produce a long fiber nonwoven fabric. Next, the long fiber nonwoven fabric was subjected to heat calendering treatment at 150 ° C. using a pair of flat rolls to produce a melt blown nonwoven fabric (average fiber diameter 0.5 μm, basis weight 5 g / m ² , thickness 8 μm). .

<Substrate 6>
The base material 1 is placed on a continuously driven net conveyor, and a long fiber nonwoven fabric is produced and laminated from the base material 1 according to the production method of <base material 5>, and is laminated at 150 ° C. using a pair of flat rolls. A heat-calender treatment was performed to produce a composite nonwoven fabric (basis weight 12 g / m ² , thickness 18 μm, average fiber diameter of melt-blown nonwoven fabric 0.5 μm) formed by laminating and integrating a spunbond nonwoven fabric and a meltblown nonwoven fabric, and used as a substrate 6 .

<Substrate 7>
The base material 2 is placed on a net conveyor that is continuously driven, and a long-fiber nonwoven fabric is produced and laminated according to the production method of <base material 5> from above, and is laminated at 150 ° C. using a pair of flat rolls. A heat-calender treatment was performed to produce a composite nonwoven fabric (basis weight 13 g / m ² , thickness 18 μm, average fiber diameter of melt-blown nonwoven fabric 0.5 μm) formed by laminating and integrating a spunbond nonwoven fabric and a meltblown nonwoven fabric. .

<Base material 8>
A core-sheath type composite fiber (average fiber diameter: 5.0 μm, fiber length: 38 mm) formed by placing polypropylene as the core component and polyethylene as the sheath component was supplied to a card machine and defibrated to prepare a card web. Subsequently, it was passed between an embossing roll having a crimping area ratio of 15% and a flat roll and heat embossed at 125 ° C. to prepare a nonwoven fabric having a basis weight of 9 g / m ² and a thickness of 15 μm.

<Substrate 9>
Polyester fiber (fineness 0.1 dtex, fiber length 3 mm, manufactured by Teijin Fibers, product name: TM04PN) 60% by mass, unstretched polyester fiber (fineness 0.2 dtex, fiber length 3 mm, manufactured by Teijin Fibers, product name: TK08PN) 40 masses %, A wet nonwoven fabric having a basis weight of 9 g / m ² and a thickness of 15 μm was prepared.

<Substrate 10>
A commercially available porous film made of polyethylene (thickness 12 μm, porosity 40%, average pore diameter 0.4 μm) was used as the substrate 10.

<Coating fluid 1>
While strongly stirring 2578 g of ion-exchanged water, 1520 g of boehmite powder (average primary particle size 0.5 μm) was added little by little and stirred vigorously for a predetermined time to give a styrene-butadiene rubber emulsion (solid content concentration 48.5% by mass). Add 156.7 g, stir vigorously for a predetermined time, and further add 1825 g of carboxymethylcellulose (Daiichi Kogyo Seiyaku, trade name: Cellogen (registered trademark) BSH-12) prepared to a solid content concentration of 2.0 mass%. Then, the mixture was vigorously stirred for a predetermined time to prepare a coating liquid 1 (the solid content of the inorganic filler in the total solid content was 93.1% by mass, the solid content of the binder was 4.7% by mass).

<Coating fluid 2>
Polyvinylidene fluoride was dissolved in N-methyl-2-pyrrolidone so as to have a solid content concentration of 15% by mass. 600 g of this and 1000 g of N-methyl-2-pyrrolidone were put in a container and dissolved uniformly. To this solution, 3000 g of alumina powder (average primary particle size 0.4 μm) was added, and stirred for 1 hour at 2800 rpm using a disper to uniformly disperse coating solution 2 (containing solid content of inorganic filler in the total solid content) The ratio was 97.1% by mass, and the solid content of the binder was 2.9% by mass).

<Coating fluid 3>
200 g of styrene-butadiene rubber emulsion (solid concentration 40% by mass) and 4000 g of ion-exchanged water were placed in a container and uniformly dispersed. To this dispersion, 3920 g of boehmite powder (average primary particle size 1.0 μm) was added and stirred for 5 hours at 2800 rpm using a disper to uniformly disperse coating liquid 3 (solid content of inorganic filler in the total solid content) The content was 98.0% by mass, and the solid content of the binder was 2.0% by mass).

<Coating fluid 4>
Scaly silica water slurry (solid content 15% by mass, aspect ratio 50, average particle size 0.5 μm) 200 g, silica (average primary particle size 3.0 μm) 970 g, styrene-butadiene rubber emulsion (solid content 4) (Mass%) 250 g and ion-exchanged water 1000 g were put in a container, stirred for 1 hour with a stirrer (trade name: Three-One Motor, manufactured by Shinto Kagaku Co., Ltd.), and uniformly dispersed coating liquid 4 (total solid content) The solid content of the inorganic filler was 99.0% by mass, and the solid content of the binder was 1.0% by mass).

<Coating fluid 5>
A polyethylene aqueous dispersion (trade name: Chemipearl (registered trademark) W-700, manufactured by Mitsui Chemicals) was diluted with ion-exchanged water to prepare 3000 g of an aqueous dispersion having a solid content concentration of 20% by mass. Next, 600 g of alumina powder (average primary particle size 0.4 μm) was added and stirred for 1 hour with a stirrer (trade name: Three-One Motor, manufactured by Shinto Kagaku Co., Ltd.), and stirred for 3 hours at 2800 rpm using a disper. The coating liquid 5 (the solid content of the inorganic filler in the total solid content of 50.0% by mass, the solid content of the organic filler of 50.0% by mass) was prepared.

  Table 1 shows the base materials and coating solutions used in Examples and Comparative Examples. “Only S” in Table 1 means that it was applied to the surface of the spunbond nonwoven fabric, “M only” means that it was applied to the surface of the meltblown nonwoven fabric, and “single” means that it was applied to one surface of the substrate. “Both” means coated on both sides of the substrate. “-” In Table 1 means not applicable.

Example 1
The coating liquid 1 was coated on the spunbonded nonwoven fabric surface of the substrate 6 using a micro gravure coater, and dried with a hot air dryer at 120 ° C. to produce the separator of Example 1.

Example 2
The coating liquid 1 was applied to the surface of the meltblown nonwoven fabric of the substrate 6 using a micro gravure coater, and dried with a hot air dryer at 120 ° C. to produce the separator of Example 2.

Example 3
After changing the number of gravure lines of the micro gravure coater, coating the coating liquid 1 on the spunbond nonwoven fabric surface of the substrate 6, drying it with a hot air dryer at 120 ° C., and then coating the coating liquid 1 on the meltblown nonwoven fabric surface. Was applied and dried with a hot air drier at 120 ° C. to produce the separator of Example 3. The total coating amount on both sides of the substrate was adjusted to be about the same as the coating amount of Examples 1 and 2.

Examples 4, 7, and 10
Separators of Examples 4, 7, and 10 were produced in the same manner as Example 1 using a predetermined base material and coating solution.

Examples 5, 8, and 11
Separators of Examples 5, 8, and 11 were produced in the same manner as Example 2 using a predetermined base material and coating solution.

Examples 6, 9, and 12
Separators of Examples 6, 9, and 12 were produced in the same manner as Example 3 using a predetermined base material and coating solution.

Example 13
The coating liquid 5 was applied to the spunbonded nonwoven fabric surface of the substrate 7 using a micro gravure coater, and dried with a hot air dryer at 50 ° C., thereby producing a separator of Example 13.

Example 14
The coating liquid 5 was applied to the melt-blown nonwoven fabric surface of the base material 7 using a micro gravure coater, and dried with a hot air dryer at 50 ° C. to produce a separator of Example 14.

Example 15
After changing the number of gravure lines of the micro gravure coater, coating the coating liquid 5 on the spunbond nonwoven fabric surface of the substrate 7 and drying it with a hot air dryer at 50 ° C., the coating fluid 1 on the meltblown nonwoven fabric surface Was applied and dried with a hot air dryer at 50 ° C. to prepare a separator of Example 15. The total coating amount on both surfaces of the substrate was adjusted to be the same as the coating amounts of Examples 13 and 14.

Comparative Examples 1-3, 7, 9, 11, 13, 15
Using a micro gravure roll, a predetermined coating solution is applied to both surfaces of a predetermined substrate and dried with a hot air dryer at 120 ° C., and Comparative Examples 1 to 3, 7, 9, 11, 13, 15 A separator was produced.

Comparative Examples 4, 6, 8, 10, 12, 14, 16, 17
Using a micro gravure roll, a predetermined coating solution is applied to one side of a predetermined substrate and dried with a hot air dryer at 120 ° C., and Comparative Examples 4, 6, 8, 10, 12, 14, 16, Seventeen separators were produced.

Comparative Example 5
The coating liquid 5 was applied to one side of the nonwoven fabric substrate 3 using a micro gravure roll, and dried with a hot air dryer at 50 ° C. to produce a separator of Comparative Example 5.

Comparative Example 18
The substrate 6 was used as a separator of Comparative Example 18 as it was.

Comparative Example 19
The base material 7 was used as a separator of Comparative Example 19 as it was.

  The average primary particle diameter of alumina, boehmite, scaly silica, and silica used in Examples and Comparative Examples is the particle diameter when the mass ratio is 50% when measured with a laser diffraction particle size distribution analyzer, that is, D50 is meant.

[Evaluation]
The following evaluations were performed on the nonwoven fabric substrates and separators of Examples and Comparative Examples, and the results are shown in Table 2. “-” In Table 2 means not applicable.

<Coating property>
When the coating liquid is applied to the base material, “○” indicates that the base material is not wrinkled or cut, and the coating can be performed without any problem. In the case of cutting, “x” was given.

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

<Fixed amount>
The value obtained by subtracting the basis weight W0 of the original base material from the basis weight W1 of the separator was defined as the fixed amount. The basis weights W0 and W1 were measured according to JIS P8124.

<Content rate 1>
The solid content (mass%) of the binder (binder) in the separator was calculated as the content 1. The value obtained by multiplying the value calculated by <sticking amount> by the solid content of the binder by the basis weight W1 of the separator and multiplying by 100 was shown. The solid content of the binder multiplied by the fixed amount means the solid content of the binder with respect to the total solid mass of the coating liquid.

<Content rate 2>
The content (mass%) of the inorganic filler in the separator was calculated as content 2. Specifically, the value obtained by multiplying the value calculated by <sticking amount> by the content of the inorganic filler by the basis weight W1 of the separator and multiplying by 100 is shown. The content of the inorganic filler multiplied by the fixed amount means the inorganic filler content with respect to the total solid mass of the coating liquid.

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

<Windability>
The separator was cut to a width of 50 mm and a length of 500 mm. The positive electrode and the negative electrode were cut to a width of 45 mm and a length of 490 mm. The separator, the negative electrode, the separator, and the positive electrode were laminated one by one in this order, and this was wound on a winding machine. When the separator was coated on one side, the separator was laminated so that the coated surface was in contact with the positive electrode. The case where the separator was wound without being cut or torn during the winding operation was indicated as “◯”, and the case where the separator was cut or torn was indicated as “x”. For the positive electrode, use is made of a slurry in which an active material lithium cobaltate, a conductive auxiliary agent acetylene black, and a binder polyvinylidene fluoride mixed in a mass ratio of 90: 5: 5 are applied to both sides of an aluminum current collector. It was. The negative electrode used was a slurry in which 97% by mass of natural graphite and 3% by mass of polyvinylidene fluoride were mixed and applied to both surfaces of a copper foil current collector.

[Production of lithium-ion batteries]
<Lithium ion batteries 1-34>
The winding element obtained by winding the electrode and the separator together in the evaluation of <winding property> was put into a battery can, and the electrolyte was injected and sealed to prepare a lithium ion battery. As the electrolytic solution, a mixed solution in which 1 mol / liter of LiPF ₆ was dissolved was used. The mixed solution is a mixture of ethylene carbonate and diethyl carbonate in a mass ratio of 3: 7. The separators of Examples 1, 2, 4, 5, 7, 8, 10, 11, 13, and 14 and Comparative Examples 4 to 6, 8, 10, 12, 14, 16, and 17 have the coated surface in contact with the positive electrode. Placed and wound.

<Lithium ion batteries 35-44>
Except that the separators of Examples 1, 2, 4, 5, 7, 8, 10, 11, 13, and 14 were placed and wound so that the coated surface was in contact with the negative electrode, the same as the lithium ion batteries 1 to 34 Thus, lithium ion batteries 35 to 44 were produced.

  The following evaluation was performed about the lithium ion batteries 1-44, and the result was shown in Table 3.

<Charge / discharge efficiency>
The lithium ion battery was charged at a constant current of up to 4.2 V at 25 ° C. and 1 mA / cm ² , further charged at a constant voltage of 4.2 V, and charged until the current value was attenuated to 0.2 mA or less. The capacity obtained at this time was defined as the initial charge capacity. After completion of charging, the battery was paused for 20 minutes and discharged to 3.0 V at 1 mA / cm ² . The capacity obtained at this time was used as the initial discharge capacity, and the ratio with respect to the initial charge capacity was calculated as the charge / discharge efficiency. Higher charge / discharge efficiency is preferable.

<High rate characteristics>
The lithium ion battery was charged at a constant current of up to 4.2 V at 25 ° C. and 1 mA / cm ² , further charged at a constant voltage of 4.2 V, and charged until the current value was attenuated to 0.2 mA or less. After completion of charging, the battery was paused for 20 minutes and discharged to 3.0 V at 1 mA / cm ² . Next, after charging at a constant voltage of 0.5 A / cm ² , the battery was discharged at 60 ° C. and 1 A / cm ² to 3.0 V, the discharge capacity was measured, the ratio to the initial discharge capacity was calculated, and the high rate characteristic was obtained.

  The separators for lithium secondary batteries of Examples 1 to 15 are formed by fixing a porous body made of an inorganic filler to a nonwoven fabric substrate, and the nonwoven fabric substrate is a composite nonwoven fabric made of a spunbond nonwoven fabric and a meltblown nonwoven fabric. There was no hole, the strength was strong even when the thickness was small, and the coating property and winding property were excellent. Moreover, since there is no binding of fibers and does not hinder ion movement between electrodes, the lithium secondary batteries 1 to 15 and 35 to 44 using the separator have high charge / discharge efficiency and excellent high-rate characteristics. It was.

  In the separators for lithium secondary batteries of Comparative Examples 1 to 5, a porous body made of an inorganic filler is fixed to a nonwoven fabric substrate. However, since the nonwoven fabric base material is a spunbond nonwoven fabric, the strength is strong and the coating property and winding property are good, but there are many pinholes after coating, and the lithium secondary battery using the separator In 16-20, the charge / discharge efficiency was low and the high rate characteristics were inferior.

  In the separators for lithium secondary batteries of Comparative Examples 6 to 11, a porous body made of an inorganic filler is fixed to a nonwoven fabric substrate. However, since the nonwoven fabric base material is a melt blown nonwoven fabric, the lithium secondary batteries 21 to 26 using the separator had good charge / discharge efficiency and high rate characteristics, but the separator strength was weak, and the coatability was Winding property was bad.

  The separators for lithium secondary batteries of Comparative Examples 12 and 13 are obtained by fixing a porous body made of an inorganic filler to a nonwoven fabric substrate. However, since the nonwoven fabric substrate is made by the card method, there are pinholes, the tensile strength in the vertical direction is strong, but it is easy to tear in the width direction, the coating property and the winding property are poor, and the separator is used. The lithium secondary batteries 27 and 28 thus obtained had low charge / discharge efficiency and poor high rate characteristics.

  The separators for lithium secondary batteries of Comparative Examples 14 and 15 are formed by fixing a porous body made of an inorganic filler to a nonwoven fabric substrate. However, since the nonwoven fabric substrate is a wet nonwoven fabric, there is no pinhole, but tensile strength It was difficult to reduce the thickness in order to prevent the film from excessively decreasing. In addition, since the fibers are in close contact with each other and part of the gaps between the fibers are blocked, the ion movement between the electrodes is hindered, and the lithium secondary batteries 29 and 30 using the separator have a slightly lower charge / discharge efficiency. The high rate characteristics were slightly inferior.

  The separators for lithium secondary batteries of Comparative Examples 16 and 17 are not a nonwoven fabric base material, but an inorganic filler layer formed on a porous film made of polyethylene, so there are no pinholes, excellent winding properties, Although the discharge efficiency was high, the lithium secondary batteries 31 and 32 using the separator were inferior in the high rate characteristics.

  The separators for lithium secondary batteries of Comparative Examples 18 and 19 are composed of a single composite nonwoven fabric composed of a spunbond nonwoven fabric and a meltblown nonwoven fabric, and have no winding property because the porous body composed of an inorganic filler is not fixed. The lithium secondary batteries 33 and 34 having pinholes and using the separator have poor charge / discharge efficiency and inferior high-rate characteristics.

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

Claims (2)

  A separator for a lithium secondary battery in which a porous body made of an inorganic filler is fixed to a nonwoven fabric base material, wherein the nonwoven fabric base material is a composite nonwoven fabric made of a spunbond nonwoven fabric and a meltblown nonwoven fabric.   A lithium secondary battery comprising the lithium secondary battery separator according to claim 1.