JP2017103199A - Separator for non-aqueous electrolyte secondary battery, laminated separator for non-aqueous electrolyte secondary battery, member for non-aqueous electrolyte secondary batter and non-aqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for a non-aqueous electrolyte secondary battery with excellent internal resistance reduction performance even immediately after the non-aqueous electrolyte secondary battery is assembled.SOLUTION: A separator for a non-aqueous electrolyte secondary battery is a porous film which is primarily composed of polyolefin. In the porous film, a phase difference with respect to light having a wavelength of 590 nm is 80 nm or less when impregnated with ethanol and a porosity of 30 to 60%.SELECTED DRAWING: None

Description

  The present invention relates to a separator for a non-aqueous electrolyte secondary battery, a laminated separator for a non-aqueous electrolyte secondary battery, a member for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery.

  Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries are widely used as batteries for personal computers, mobile phones, personal digital assistants, etc. due to their high energy density. Development is underway.

  As a separator in a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery, a microporous film containing polyolefin as a main component is used (Patent Document 1). By using such a separator, a non-aqueous electrolyte secondary battery having both good safety and cycle characteristics can be obtained.

Patent No. 5164296 (issued on March 21, 2013)

  However, when a conventional separator is used, there is a problem that the internal resistance of the battery immediately after the non-aqueous electrolyte secondary battery is assembled is high.

  The present invention has been made in view of such problems, and the object thereof is a separator for a non-aqueous electrolyte secondary battery excellent in internal resistance of the battery immediately after the non-aqueous electrolyte secondary battery is assembled. Another object of the present invention is to provide a non-aqueous electrolyte secondary battery laminated separator, a non-aqueous electrolyte secondary battery member, and a non-aqueous electrolyte secondary battery.

  The present inventor indicates that in a non-aqueous electrolyte secondary battery separator that is not an optical component, the smaller the phase difference with respect to light having a wavelength of 590 nm, the lower the internal resistance of the battery immediately after the non-aqueous electrolyte secondary battery is assembled. For the first time, the present invention has been completed.

  The separator for a non-aqueous electrolyte secondary battery according to the present invention is a porous film mainly composed of polyolefin, and has a phase difference of 80 nm or less with respect to light having a wavelength of 590 nm in a state impregnated with ethanol, and The porosity is 30 to 60%.

  Moreover, the laminated separator for non-aqueous electrolyte secondary batteries according to the present invention includes the above-described separator for non-aqueous electrolyte secondary batteries and a porous layer.

  The nonaqueous electrolyte secondary battery member according to the present invention includes a positive electrode, the nonaqueous electrolyte secondary battery separator or the nonaqueous electrolyte secondary battery laminated separator, and the negative electrode in this order. It is characterized by being arranged.

  In addition, a nonaqueous electrolyte secondary battery according to the present invention includes the separator for a nonaqueous electrolyte secondary battery or the multilayer separator for a nonaqueous electrolyte secondary battery.

  According to the present invention, there is an effect that the internal resistance of the battery immediately after the non-aqueous electrolyte secondary battery is assembled is excellent.

It is a figure which shows the relationship between the molecular chain and pore of resin which comprise a porous film, and phase difference. It is a graph which shows the measurement result of a phase difference and a 10-Hz resistance value in an Example and a comparative example.

  An embodiment of the present invention will be described below, but the present invention is not limited to this. The present invention is not limited to each configuration described below, and various modifications are possible within the scope shown in the claims, and various technical means disclosed in different embodiments are appropriately combined. The obtained embodiment is also included in the technical scope of the present invention. Unless otherwise specified in this specification, “A to B” indicating a numerical range means “A or more and B or less”.

[1. (Separator)
(1-1) Nonaqueous Electrolyte Secondary Battery Separator A nonaqueous electrolyte secondary battery separator according to an embodiment of the present invention is disposed between a positive electrode and a negative electrode in a nonaqueous electrolyte secondary battery. It is a film-like porous film.

  The porous film only needs to be a porous and membrane-like substrate (polyolefin-based porous substrate) mainly composed of a polyolefin-based resin, and has a structure having pores connected to the inside thereof. It is a film in which gas and liquid can permeate from one side to the other side.

  The porous film melts when the battery generates heat, and makes the non-aqueous electrolyte secondary battery separator non-porous, thereby giving a shutdown function to the non-aqueous electrolyte secondary battery separator. . The porous film may be composed of one layer or may be formed from a plurality of layers.

  The film thickness of the porous film may be appropriately determined in consideration of the film thickness of the nonaqueous electrolyte secondary battery member constituting the nonaqueous electrolyte secondary battery, and is preferably 4 to 40 μm. More preferably, it is -30 micrometers, and it is further more preferable that it is 6-15 micrometers.

  The volume-based porosity of the porous film is 30 to 60% so as to increase the amount of electrolyte retained and to reliably prevent the excessive current from flowing (shut down) at a lower temperature. Yes, and preferably 40 to 60%. In addition, the average pore diameter (average pore diameter) of the porous film can provide sufficient ion permeability when used as a separator, and prevents particles from entering the positive electrode and the negative electrode. Therefore, the thickness is preferably 0.3 μm or less, and more preferably 0.14 μm or less.

  Moreover, the phase difference with respect to the light of wavelength 590nm in the state which the porous film impregnated ethanol is 80 nm or less. Preferably they are 5 nm or more and 80 nm or less, More preferably, they are 20 nm or more and 80 nm or less. The birefringence of the porous film is preferably 0.004 or less, more preferably 0.001 or more and 0.004 or less, and further preferably 0.002 or more and 0.004 or less. .

  When the refractive index of light is different between the x-axis direction and the y-axis direction perpendicular to each other in the porous film plane (birefringence), the light incident from the normal direction of the film plane and transmitted through the film is the birefringence Thus, a phase difference is generated between the x-axis direction and the y-axis direction. Such a phase difference is a physical property that is noticed when a film is used as an optical component. However, the present inventors have found for the first time that in a non-aqueous electrolyte secondary battery separator that is not an optical component, the smaller the phase difference, the lower the internal resistance of the battery immediately after the non-aqueous electrolyte secondary battery is assembled. The present invention has been completed.

  That is, as described above, the porous film was used as a separator for a non-aqueous electrolyte secondary battery even when the volume-based porosity of the porous film was set to 30 to 60% in order to increase the retained amount of the electrolytic solution. When assembling a non-aqueous electrolyte secondary battery and injecting the electrolyte, the present inventors paid attention to the fact that the internal resistance of the battery differs depending on the penetration rate of the electrolyte into the separator. And, as described above, by setting the phase difference for light with a wavelength of 590 nm in the porous film to 80 nm or less, the penetration rate of the electrolyte into the separator is increased when the non-aqueous electrolyte secondary battery is assembled, It has been found that the internal resistance of the battery can be lowered.

  The retardation in the porous film depends on the structure of the molecular chains and pores of the resin constituting the porous film. FIG. 1 is a schematic diagram showing the relationship between the phase difference and the structure of the porous film, where (a) shows the structure of the porous film with a relatively small phase difference, and (b) shows the relative phase difference. The structure of a large porous film is shown. As shown in FIG. 1A, in a porous film having a small phase difference, the molecular chains and pores of the resin constituting the film are randomly arranged, and there is almost no anisotropy. On the other hand, as shown in FIG. 1B, in the porous film having a large phase difference, the molecular chains are oriented in a specific direction, and the pores have a shape extending in the same direction.

  When assembling the non-aqueous electrolyte secondary battery, the separator for the non-aqueous electrolyte secondary battery is immersed in the electrolyte while being sandwiched between the positive electrode sheet and the negative electrode sheet. Therefore, the separator for a nonaqueous electrolyte secondary battery absorbs the electrolyte from its end face. At this time, in the case of the porous film shown in FIG. 1B, the electrolytic solution is easily absorbed along the alignment direction of the molecular chain, and the electrolytic solution is difficult to be absorbed in the direction perpendicular to the alignment direction. Therefore, the electrolytic solution is absorbed mainly from the end face perpendicular to the alignment direction of the molecular chains, and the absorbed electrolytic solution penetrates along the alignment direction. As a result, it takes time until the electrolytic solution penetrates the entire separator for the nonaqueous electrolytic secondary battery.

  On the other hand, in the case of the porous film shown in FIG. 1A, since the pores are randomly arranged, the electrolytic solution can be absorbed from any end face, and the electrolytic solution is a non-aqueous electrolytic solution. It can permeate the entire secondary battery separator in a short time.

  Thereby, the internal resistance of the battery immediately after assembling a non-aqueous electrolyte secondary battery can be lowered as the porous film has a smaller phase difference using the porous film as a separator for a non-aqueous electrolyte secondary battery.

The proportion of the polyolefin component in the porous film is required to be 50% by volume or more of the entire porous substrate, preferably 90% by volume or more, and more preferably 95% by volume or more. The polyolefin component of the porous film preferably contains a high molecular weight component having a weight average molecular weight of 5 × 10 ⁵ to 15 × 10 ⁶ . In particular, the inclusion of a polyolefin component having a weight average molecular weight of 1,000,000 or more as the polyolefin component of the porous substrate is preferable because the strength of the entire porous film and the separator for the nonaqueous electrolyte secondary battery is increased.

  Examples of the polyolefin resin constituting the porous film include a high molecular weight homopolymer or copolymer obtained by polymerizing ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and the like. Can do. The porous film may be a layer containing these polyolefin resins alone and / or a layer containing two or more of these polyolefin resins. In particular, high molecular weight polyethylene mainly composed of ethylene is preferable. In addition, a porous base material does not prevent containing components other than polyolefin in the range which does not impair the function of the said layer.

  The air permeability of the porous film is usually in the range of 30 to 500 seconds / 100 cc as a Gurley value, and preferably in the range of 50 to 300 seconds / 100 cc. When the porous film has an air permeability in the above range, sufficient ion permeability can be obtained when used as a separator.

The basis weight of the porous film is strength, film thickness, handling property and weight, and further, in that the weight energy density and volume energy density of the battery when used as a separator of a nonaqueous electrolyte secondary battery can be increased. Usually, a ^{4~20g / m 2, 4~12g / m} 2 is preferred.

Next, the manufacturing method of a porous film is demonstrated.
First, the resin composition used as the raw material of a porous film is manufactured. For example, a polyolefin resin composition is obtained by kneading ultrahigh molecular weight polyethylene, low molecular weight polyethylene having a weight average molecular weight of 10,000 or less, a pore forming agent such as calcium carbonate or a plasticizer, and an antioxidant.

  Subsequently, the resin composition is rolled with a pair of rolling rolls and cooled stepwise while being pulled with a take-up roll having a different speed ratio to form a sheet. And a hole formation agent is removed from the shape | molded sheet | seat, and it extends | stretches so that it may become the set draw ratio.

  Here, by appropriately changing the rolling draw ratio (winding roll speed / rolling roll speed), which is the ratio between the speed of the winding roll and the speed of the rolling roll, and the stretching ratio described above, the level of the porous film The phase difference can be controlled.

(1-2) Laminated separator for non-aqueous electrolyte secondary battery Further, the separator for non-aqueous electrolyte secondary battery of the present invention includes known porous layers such as an adhesive layer, a heat-resistant layer, and a protective layer. Also good. In this specification, a separator provided with a separator for a non-aqueous electrolyte secondary battery and a porous layer is referred to as a laminated separator for a non-aqueous electrolyte secondary battery (hereinafter sometimes referred to as a laminated separator).

  It is more preferable that the separator is subjected to a hydrophilic treatment before the porous layer is formed, that is, before the coating liquid described later is applied. By applying a hydrophilic treatment to the separator, the coating property of the coating liquid is further improved, and therefore a more uniform porous layer can be formed. This hydrophilization treatment is effective when the proportion of water in the solvent (dispersion medium) contained in the coating liquid is high.

  Specific examples of the hydrophilic treatment include known treatments such as chemical treatment with acid or alkali, corona treatment, plasma treatment and the like. Of the hydrophilization treatments, the separator can be hydrophilized in a relatively short time, and the hydrophilization is limited only to the vicinity of the surface of the separator, and the inside of the separator is not altered, so corona treatment is more preferable.

(Porous layer)
The porous layer is preferably a resin layer containing a resin. The resin constituting the porous layer is preferably insoluble in the electrolyte of the non-aqueous electrolyte secondary battery and electrochemically stable in the usage range of the non-aqueous electrolyte secondary battery. When the porous layer is laminated on one side of the separator, the porous layer is preferably the nonaqueous electrolysis of the separator when the separator is used as a member of a nonaqueous electrolyte secondary battery. It is laminated | stacked on the surface facing the positive electrode of a liquid secondary battery, More preferably, it laminates | stacks on the surface which contact | connects the said positive electrode.

  Examples of the resin constituting the porous layer include polyolefins such as polyethylene, polypropylene, polybutene, and ethylene-propylene copolymers; fluorine-containing resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene; vinylidene fluoride -Fluorine-containing rubber such as hexafluoropropylene-tetrafluoroethylene copolymer and ethylene-tetrafluoroethylene copolymer; aromatic polyamide; wholly aromatic polyamide (aramid resin); styrene-butadiene copolymer and its hydride, Rubbers such as methacrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, ethylene propylene rubber, and polyvinyl acetate; polyphenylene ether, polysulfone, polymer Resins having a melting point or glass transition temperature of 180 ° C. or higher such as ether sulfone, polyphenylene sulfide, polyether imide, polyamide imide, polyether amide, and polyester; polyvinyl alcohol, polyethylene glycol, cellulose ether, sodium alginate, polyacrylic acid, poly Water-soluble polymers such as acrylamide and polymethacrylic acid; and the like.

  Specific examples of the aromatic polyamide include poly (paraphenylene terephthalamide), poly (metaphenylene isophthalamide), poly (parabenzamide), poly (metabenzamide), and poly (4,4 ′). -Benzanilide terephthalamide), poly (paraphenylene-4,4'-biphenylenedicarboxylic acid amide), poly (metaphenylene-4,4'-biphenylenedicarboxylic acid amide), poly (paraphenylene-2,6-naphthalenedicarboxylic acid) Acid amide), poly (metaphenylene-2,6-naphthalenedicarboxylic acid amide), poly (2-chloroparaphenylene terephthalamide), paraphenylene terephthalamide / 2,6-dichloroparaphenylene terephthalamide copolymer, metaphenylene Terephthalamide / 2 6-dichloro-para-phenylene terephthalamide copolymer and the like. Of these, poly (paraphenylene terephthalamide) is more preferable.

  Of the above resins, polyolefins, fluorine-containing resins, aromatic polyamides, and water-soluble polymers are more preferable. Especially, when a porous layer is arrange | positioned facing the positive electrode of a nonaqueous electrolyte secondary battery, a fluorine-containing resin is especially preferable. When the fluorine-containing resin is applied, it is easy to maintain various performances such as rate characteristics and resistance characteristics (liquid resistance) of the non-aqueous electrolyte secondary battery due to acidic deterioration during operation of the non-aqueous electrolyte secondary battery. Since water-soluble polymer can use water as a solvent when forming a porous layer, it is more preferable from the viewpoint of process and environmental load, cellulose ether and sodium alginate are more preferable, and cellulose ether is particularly preferable.

  Specific examples of the cellulose ether include carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), carboxyethyl cellulose, methyl cellulose, ethyl cellulose, cyanethyl cellulose, oxyethyl cellulose, etc. CMC and HEC which are excellent in chemical stability are more preferable, and CMC is particularly preferable.

  More preferably, the porous layer contains a filler. Therefore, when the porous layer contains a filler, the resin has a function as a binder resin. The filler is not particularly limited, and may be a filler made of an organic material or a filler made of an inorganic material.

  Specific examples of the filler made of an organic substance include homopolymers of monomers such as styrene, vinyl ketone, acrylonitrile, methyl methacrylate, ethyl methacrylate, glycidyl methacrylate, glycidyl acrylate, and methyl acrylate, or two or more types. Copolymer; polytetrafluoroethylene, tetrafluoroethylene-tetrafluoropropylene copolymer, tetrafluoroethylene-ethylene copolymer, polyvinylidene fluoride and other fluorine-containing resins; melamine resin; urea resin; polyethylene; Examples include fillers made of polypropylene; polyacrylic acid, polymethacrylic acid, and the like.

  Specific examples of fillers made of inorganic materials include calcium carbonate, talc, clay, kaolin, silica, hydrotalcite, diatomaceous earth, magnesium carbonate, barium carbonate, calcium sulfate, magnesium sulfate, barium sulfate, aluminum hydroxide, Examples include fillers made of inorganic substances such as boehmite, magnesium hydroxide, calcium oxide, magnesium oxide, titanium oxide, titanium nitride, alumina (aluminum oxide), aluminum nitride, mica, zeolite, and glass. Only one type of filler may be used, or two or more types may be used in combination.

  Among the above fillers, fillers made of inorganic materials are suitable, fillers made of inorganic oxides such as silica, calcium oxide, magnesium oxide, titanium oxide, alumina, mica, zeolite, aluminum hydroxide, boehmite are more preferable, silica, More preferred is at least one filler selected from the group consisting of magnesium oxide, titanium oxide, aluminum hydroxide, boehmite and alumina, and alumina is particularly preferred. Alumina has many crystal forms such as α-alumina, β-alumina, γ-alumina, and θ-alumina, and any of them can be suitably used. Among these, α-alumina is most preferable because of its particularly high thermal stability and chemical stability.

  The shape of the filler varies depending on the manufacturing method of the organic or inorganic material that is the raw material, the dispersion condition of the filler when producing the coating liquid for forming the porous layer, and the like, spherical, oval, short, The shape may be any shape such as a shape or an indefinite shape having no specific shape.

  When the porous layer contains a filler, the filler content is preferably 1 to 99% by volume, more preferably 5 to 95% by volume of the porous layer. By setting the filler content in the above range, voids formed by contact between fillers are less likely to be clogged with a resin and the like, and sufficient ion permeability can be obtained, and per unit area. The basis weight can be set to an appropriate value.

  In the present invention, a coating liquid for forming a porous layer is usually prepared by dissolving the resin in a solvent and dispersing the filler.

  The solvent (dispersion medium) is not particularly limited as long as it does not adversely affect the porous film, can dissolve the resin uniformly and stably, and can uniformly and stably disperse the filler. Specific examples of the solvent (dispersion medium) include water; lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, and t-butyl alcohol; acetone, toluene, xylene, hexane, N -Methylpyrrolidone, N, N-dimethylacetamide, N, N-dimethylformamide; and the like. The solvent (dispersion medium) may be used alone or in combination of two or more.

The coating liquid may be formed by any method as long as the conditions such as the resin solid content (resin concentration) and the filler amount necessary for obtaining a desired porous layer can be satisfied. Specific examples of the method for forming the coating liquid include a mechanical stirring method, an ultrasonic dispersion method, a high-pressure dispersion method, and a media dispersion method.

  Further, for example, a filler may be dispersed in a solvent (dispersion medium) using a conventionally known disperser such as a three-one motor, a homogenizer, a media type disperser, or a pressure disperser.

  Moreover, the said coating liquid may contain additives, such as a dispersing agent, a plasticizer, surfactant, and a pH adjuster, as components other than the said resin and a filler, in the range which does not impair the objective of this invention. . In addition, the addition amount of an additive should just be a range which does not impair the objective of this invention.

  The method for applying the coating liquid to the separator, that is, the method for forming the porous layer on the surface of the separator that has been subjected to hydrophilic treatment as necessary is not particularly limited. When laminating a porous layer on both sides of a separator, after forming a porous layer on one side of the separator, a sequential laminating method of forming a porous layer on the other side, or a porous layer on both sides of the separator It is possible to apply a simultaneous lamination method for simultaneously forming the layers.

  As a method for forming the porous layer, for example, a method in which the coating liquid is directly applied to the surface of the separator and then the solvent (dispersion medium) is removed; the coating liquid is applied to an appropriate support, and the solvent (dispersion medium) ) Is removed to form a porous layer, and then the porous layer and the separator are pressure-bonded, and then the support is peeled off; after the coating liquid is applied to an appropriate support, the porous film is applied to the coated surface And then removing the solvent (dispersion medium) after peeling off the support; and a method of removing the solvent (dispersion medium) after immersing the separator in the coating liquid and performing dip coating; Is mentioned.

  The thickness of the porous layer is the thickness of the coating film in the wet state after coating, the weight ratio of the resin to the filler, and the solid content concentration of the coating liquid (the sum of the resin concentration and the filler concentration). It can be controlled by adjusting etc. As the support, for example, a resin film, a metal belt, a drum, or the like can be used.

  The method for applying the coating liquid to the separator or the support is not particularly limited as long as it is a method capable of realizing a necessary basis weight and coating area. As a coating method of the coating liquid, a conventionally known method can be employed. As such a method, specifically, for example, gravure coater method, small diameter gravure coater method, reverse roll coater method, transfer roll coater method, kiss coater method, dip coater method, knife coater method, air doctor blade coater method, blade Examples include a coater method, a rod coater method, a squeeze coater method, a cast coater method, a bar coater method, a die coater method, a screen printing method, and a spray coating method.

  As a method for removing the solvent (dispersion medium), a drying method is generally used. Examples of the drying method include natural drying, air drying, heat drying, and reduced pressure drying, and any method may be used as long as the solvent (dispersion medium) can be sufficiently removed. A normal drying apparatus can be used for the drying.

Further, the solvent (dispersion medium) contained in the coating liquid may be replaced with another solvent before drying. As a method for removing the solvent (dispersion medium) after replacing it with another solvent, for example, it is possible to dissolve in the solvent (dispersion medium) contained in the coating liquid and not dissolve the resin contained in the coating liquid. And the separator or support on which the coating solution is applied to form a coating film is immersed in the solvent X, and the solvent in the coating on the separator or the support ( A method of evaporating the solvent X after replacing the dispersion medium) with the solvent X can be mentioned. According to this method, the solvent (dispersion medium) can be efficiently removed from the coating liquid.

  When heating is performed to remove the solvent (dispersion medium) or the solvent X from the coating film of the coating liquid formed on the separator or the support, the pores of the porous film contract and the air permeability is reduced. In order to avoid the decrease in the temperature, it is desirable to carry out at a temperature at which the air permeability of the separator does not decrease, specifically 10 to 120 ° C., more preferably 20 to 80 ° C.

  The film thickness of the porous layer formed by the method described above is 0.5 to 15 μm when the separator is used as a base material and the porous layer is laminated on one or both sides of the separator to form a laminated separator. (Per one side) is preferable, and 2 to 10 μm (per one side) is more preferable.

  When the total thickness of the porous layer is less than 1 μm, internal short circuit due to damage to the nonaqueous electrolyte secondary battery is sufficiently prevented when the laminated separator is used for a nonaqueous electrolyte secondary battery. Can not do it. In addition, the amount of electrolytic solution retained in the porous layer decreases.

  On the other hand, if the total thickness of the porous layer exceeds 30 μm, when the laminated separator is used in a non-aqueous electrolyte secondary battery, the lithium ion permeation resistance increases across the separator, If it repeats, the positive electrode of a nonaqueous electrolyte secondary battery will deteriorate, and a rate characteristic and cycling characteristics will fall. In addition, since the distance between the positive electrode and the negative electrode is increased, the nonaqueous electrolyte secondary battery is increased in size.

  In the following description regarding the physical properties of the porous layer, when the porous layer is laminated on both sides of the separator, the layer is laminated on the surface facing the positive electrode in the laminated separator when a non-aqueous electrolyte secondary battery is used. It refers to at least the physical properties of the porous layer.

The basis weight per unit area (per side) of the porous layer may be appropriately determined in consideration of the strength, film thickness, weight, and handling properties of the laminated separator. In the case of using a laminated separator in a non-aqueous electrolyte secondary battery, it basis weight per unit area of the porous layer is generally to be from 1 to 20 g / m ² and preferably, 2 to 10 g / m ² Is more preferable.

  By setting the basis weight per unit area of the porous layer within these numerical ranges, the weight energy density and volume energy density of the non-aqueous electrolyte secondary battery including the porous layer can be increased. When the basis weight of the porous layer exceeds the above range, the nonaqueous electrolyte secondary battery including the laminated separator becomes heavy.

  The porosity of the porous layer is preferably 20 to 90% by volume, and more preferably 30 to 80% by volume so that sufficient ion permeability can be obtained. The pore diameter of the pores of the porous layer is preferably 1 μm or less, and more preferably 0.5 μm or less. By setting the pore diameter to these sizes, the non-aqueous electrolyte secondary battery including the laminated separator including the porous layer can obtain sufficient ion permeability.

  The air permeability of the laminated separator is preferably a Gurley value of 30 to 1000 sec / 100 mL, and more preferably 50 to 800 sec / 100 mL. When the laminated separator has the above air permeability, sufficient ion permeability can be obtained when the laminated separator is used as a member for a non-aqueous electrolyte secondary battery.

When the air permeability exceeds the above range, it means that the laminated separator has a rough laminated structure due to the high porosity of the laminated separator. As a result, the strength of the separator is lowered, and particularly at high temperatures. There is a risk that the shape stability of the material becomes insufficient. On the other hand, when the air permeability is less than the above range, when the laminated separator is used as a member for a non-aqueous electrolyte secondary battery, sufficient ion permeability cannot be obtained, and the non-aqueous electrolyte is not used. The battery characteristics of the secondary battery may be degraded.

[2. Nonaqueous electrolyte secondary battery)
The nonaqueous electrolyte secondary battery according to the present invention includes the separator or the laminated separator (hereinafter, the separator and the laminated separator may be collectively referred to as a separator or the like). More specifically, the nonaqueous electrolyte secondary battery according to the present invention includes a nonaqueous electrolyte secondary battery member in which a positive electrode, a separator, and the like, and a negative electrode are arranged in this order. That is, the nonaqueous electrolyte secondary battery member is also included in the scope of the present invention. Hereinafter, a lithium ion secondary battery will be described as an example of the nonaqueous electrolyte secondary battery. The constituent elements of the nonaqueous electrolyte secondary battery other than the separator are not limited to the constituent elements described below.

In the non-aqueous electrolyte secondary battery according to the present invention, for example, a non-aqueous electrolyte obtained by dissolving a lithium salt in an organic solvent can be used. Examples of the lithium salt include LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , Li ₂ B ₁₀ Cl ₁₀ , lower aliphatic carboxylic acid lithium salt, LiAlCl _{4 and the} like. The lithium salt may be used alone or in combination of two or more.

Among the lithium salts, at least one selected from the group consisting of LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , and LiC (CF ₃ SO ₂ ) _3. More preferred are fluorine-containing lithium salts.

  Specific examples of the organic solvent constituting the non-aqueous electrolyte include, for example, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one. Carbonates such as 1,2-di (methoxycarbonyloxy) ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl Ethers such as ether, tetrahydrofuran and 2-methyltetrahydrofuran; Esters such as methyl formate, methyl acetate and γ-butyrolactone; Nitriles such as acetonitrile and butyronitrile; N, N-dimethylformamide, N, N-dimethyla Amides such as toamide; Carbamates such as 3-methyl-2-oxazolidone; Sulfur-containing compounds such as sulfolane, dimethyl sulfoxide and 1,3-propane sultone; and a fluorine group introduced into the organic solvent. Fluorine organic solvent; and the like. Only one kind of the organic solvent may be used, or two or more kinds may be used in combination.

  Among the organic solvents, carbonates are more preferable, and a mixed solvent of cyclic carbonate and acyclic carbonate, or a mixed solvent of cyclic carbonate and ethers is more preferable.

  As a mixed solvent of cyclic carbonate and non-cyclic carbonate, ethylene carbonate has a wide operating temperature range and is difficult to decompose even when a graphite material such as natural graphite or artificial graphite is used as the negative electrode active material. More preferred is a mixed solvent containing dimethyl carbonate and ethyl methyl carbonate.

  As the positive electrode, a sheet-like positive electrode in which a positive electrode mixture containing a positive electrode active material, a conductive material, and a binder is usually supported on a positive electrode current collector is used.

  Examples of the positive electrode active material include materials that can be doped / undoped with lithium ions. Specific examples of the material include lithium composite oxides containing at least one transition metal such as V, Mn, Fe, Co, and Ni.

Among the lithium composite oxides, since the average discharge potential is high, lithium composite oxides having an α-NaFeO ₂ type structure such as lithium nickelate and lithium cobaltate, and lithium composites having a spinel type structure such as lithium manganese spinel Oxides are more preferred. The lithium composite oxide may contain various metal elements, and composite lithium nickelate is more preferable.

  Furthermore, the number of moles of at least one metal element selected from the group consisting of Ti, Zr, Ce, Y, V, Cr, Mn, Fe, Co, Cu, Ag, Mg, Al, Ga, In, and Sn The composite lithium nickelate containing the metal element is used such that the ratio of the at least one metal element is 0.1 to 20 mol% with respect to the sum of the number of moles of Ni in the lithium nickelate. It is particularly preferable because of its excellent cycle characteristics when used at a high capacity. Among them, an active material containing Al or Mn and having a Ni ratio of 85% or more, more preferably 90% or more is used in a high capacity of a non-aqueous electrolyte secondary battery including a positive electrode containing the active material. Since it is excellent in cycling characteristics, it is especially preferable.

  Examples of the conductive material include carbonaceous materials such as natural graphite, artificial graphite, cokes, carbon black, pyrolytic carbons, carbon fibers, and fired organic polymer compounds. Only one type of the conductive material may be used. For example, two or more types may be used in combination, such as a mixture of artificial graphite and carbon black.

  Examples of the binder include polyvinylidene fluoride, vinylidene fluoride copolymer, polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer, and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer. , Ethylene-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, thermoplastic resins such as thermoplastic polyimide, polyethylene, and polypropylene, acrylic resin, and styrene-butadiene rubber Can be mentioned. The binder also has a function as a thickener.

  As a method for obtaining the positive electrode mixture, for example, a method of obtaining a positive electrode mixture by pressurizing a positive electrode active material, a conductive material and a binder on a positive electrode current collector; a positive electrode active material, a conductive material using an appropriate organic solvent And a method of obtaining a positive electrode mixture by pasting a material and a binder.

  Examples of the positive electrode current collector include conductors such as Al, Ni, and stainless steel, and Al is more preferable because it is easily processed into a thin film and is inexpensive.

  As a method for producing a sheet-like positive electrode, that is, a method of loading a positive electrode mixture on a positive electrode current collector, for example, a positive electrode active material, a conductive material, and a binder as a positive electrode mixture are added on the positive electrode current collector. Method of pressure molding: After a positive electrode active material, a conductive material and a binder are pasted using an appropriate organic solvent to obtain a positive electrode mixture, the positive electrode mixture is applied to the positive electrode current collector and dried. And a method of pressurizing the obtained sheet-like positive electrode mixture and fixing it to the positive electrode current collector.

  As the negative electrode, a sheet-like negative electrode in which a negative electrode mixture containing a negative electrode active material is usually supported on a negative electrode current collector is used. The sheet-like negative electrode preferably contains the conductive material and the binder.

Examples of the negative electrode active material include materials that can be doped / undoped with lithium ions, lithium metal, and lithium alloys. Specific examples of the material include carbonaceous materials such as natural graphite, artificial graphite, cokes, carbon black, pyrolytic carbons, carbon fibers, and fired organic polymer compounds; Lithium ion doping and dedoping oxides, chalcogen compounds such as sulfides; aluminum (Al), lead (Pb), tin (Sn), bismuth (Bi), silicon (Si), etc. alloyed with alkali metals A cubic intermetallic compound (AlSb, Mg ₂ Si, NiSi ₂ ), a lithium nitrogen compound (Li ₃ -xM _x N (M: transition metal)) or the like that can insert a metal of the above or an alkali metal between lattices is used. be able to. Among the negative electrode active materials, the potential flatness is high, and since the average discharge potential is low, a large energy density can be obtained when combined with the positive electrode. Therefore, the main component is a graphite material such as natural graphite or artificial graphite. A carbonaceous material is more preferable, a mixture of graphite and silicon, a ratio of Si to C being 5% or more is more preferable, and a negative electrode active material having 10% or more is more preferable.

  As a method for obtaining the negative electrode mixture, for example, a method in which the negative electrode active material is pressurized on the negative electrode current collector to obtain the negative electrode mixture; the negative electrode active material is pasted into a paste using an appropriate organic solvent. And the like.

  Examples of the negative electrode current collector include Cu, Ni, stainless steel, and the like. In particular, in a lithium ion secondary battery, it is difficult to form an alloy with lithium, and Cu is more preferable because it is easy to process into a thin film.

  As a method for producing a sheet-like negative electrode, that is, a method of supporting the negative electrode mixture on the negative electrode current collector, for example, a method in which a negative electrode active material to be the negative electrode mixture is pressure-molded on the negative electrode current collector; After the negative electrode active material is made into a paste using an organic solvent to obtain a negative electrode mixture, the negative electrode mixture is applied to the negative electrode current collector and dried to press the sheet-like negative electrode mixture. And a method of fixing to the negative electrode current collector. The paste preferably contains the conductive assistant and the binder.

  After the positive electrode, the separator, etc., and the negative electrode are arranged in this order to form a non-aqueous electrolyte secondary battery member, the non-aqueous electrolyte secondary battery is placed in a container serving as a casing of the non-aqueous electrolyte secondary battery. The non-aqueous electrolyte secondary battery according to the present invention can be manufactured by inserting an electrolyte secondary battery member and then filling the container with the non-aqueous electrolyte and then sealing the container while reducing the pressure. . The shape of the non-aqueous electrolyte secondary battery is not particularly limited, and may be any shape such as a thin plate (paper) type, a disc type, a cylindrical type, and a rectangular column type such as a rectangular parallelepiped. In addition, the manufacturing method of a nonaqueous electrolyte secondary battery is not specifically limited, A conventionally well-known manufacturing method is employable.

<Measurement methods for various physical properties>
Various physical properties of the separators for non-aqueous electrolyte secondary batteries according to the following examples and comparative examples were measured by the following methods.

(1) Porosity A porous film used as a separator for a non-aqueous electrolyte secondary battery is cut into a square with a side length of 8 cm, and the weight of the cut piece: W (g) and thickness: E (cm) Measure. Based on the measured weight (W) and thickness (E), and the true specific gravity ρ (g / cm ³ ) of the porous film,
Porosity = (1-{(W / ρ)} / (8 × 8 × E)) × 100
The porosity of the porous film was calculated according to the formula:

(2) Retardation and birefringence A porous film used as a separator for a non-aqueous electrolyte secondary battery is cut into 4 cm × 4 cm, and 0.5 mL of ethanol is dropped and impregnated with ethanol to obtain a translucent film. It was. At this time, excess ethanol that could not be absorbed was wiped off. And the birefringence index with respect to the light of wavelength 590nm in 25 degreeC of the obtained translucent film was measured using the phase difference measuring apparatus (KOBRA-WPR) by Oji Scientific Instruments, and the phase difference was computed.

(3) Electric resistance A non-aqueous electrolyte secondary battery assembled as described below was applied to a voltage amplitude of 10 mV at 25 ° C. at 25 ° C. with an LCR meter (product name: chemical impedance meter (type 3532-80)) manufactured by Hioki Electric. Then, AC impedance was measured. And the resistance value R of the real part of the measurement frequency of 10 Hz was used as the resistance value immediately after the battery assembly (battery internal resistance value).

<Preparation of separator for non-aqueous electrolyte secondary battery>
The porous film which concerns on Examples 1-4 used as a separator for nonaqueous electrolyte secondary batteries as follows was produced.

Example 1
68% by weight of ultra high molecular weight polyethylene powder (GUR2024, manufactured by Ticona), 32% by weight of polyethylene wax (FNP-0115, manufactured by Nippon Seiki Co., Ltd.) having a weight average molecular weight of 1000, and the total of the ultra high molecular weight polyethylene and polyethylene wax. As 100 parts by weight, antioxidant (Irg1010, manufactured by Ciba Specialty Chemicals) 0.4% by weight, (P168, manufactured by Ciba Specialty Chemicals) 0.1% by weight, sodium stearate 1.3% by weight In addition, calcium carbonate (manufactured by Maruo Calcium Co., Ltd.) having an average pore diameter of 0.1 μm was added so as to be 38% by volume with respect to the total volume. A polyolefin resin composition was obtained by melt-kneading. Then, the polyolefin resin composition was rolled with a pair of rolling rolls having a surface temperature of 150 ° C., and cooled stepwise while being pulled with a winding roll having a different speed ratio. Here, a sheet having a film thickness of about 62 μm was prepared with a rolling draw ratio (winding roll speed / rolling roll speed) of 1.4 times. The sheet was immersed in an aqueous hydrochloric acid solution (hydrochloric acid 4 mol / L, nonionic surfactant 0.5% by weight) to remove calcium carbonate, and then stretched 6.2 times at 105 ° C. A porous film was obtained.

(Example 2)
Ultrahigh molecular weight polyethylene powder (GUR4032, manufactured by Ticona) 68.5% by weight, polyethylene wax having a weight average molecular weight of 1000 (FNP-0115, degree of branching 1 / 1000C, manufactured by Nippon Seiki Co., Ltd.) 31.5% by weight, Anti-oxidant (Irg1010, manufactured by Ciba Specialty Chemicals) 0.4% by weight, (P168, manufactured by Ciba Specialty Chemicals) 0.1 weight based on 100 parts by weight of the total of ultrahigh molecular weight polyethylene and polyethylene wax %, Sodium stearate 1.3% by weight, and calcium carbonate with an average pore size of 0.1 μm (manufactured by Maruo Calcium Co., Ltd.) was added so that the total volume was 36% by volume. Then, the mixture was melt kneaded with a biaxial kneader to obtain a polyolefin resin composition. Then, the polyolefin resin composition was rolled with a pair of rolling rolls having a surface temperature of 150 ° C., and cooled stepwise while being pulled with a winding roll having a different speed ratio. Here, a sheet having a film thickness of about 62 μm was prepared with a rolling draw ratio (winding roll speed / rolling roll speed) of 1.4 times. This sheet was immersed in an aqueous hydrochloric acid solution (hydrochloric acid 4 mol / L, nonionic surfactant 0.5 wt%) to remove calcium carbonate, and subsequently stretched 7 times at 105 ° C. A quality film was obtained.

(Example 3)
Ultra high molecular weight polyethylene powder (GUR4012, manufactured by Ticona) is 80% by weight, polyethylene wax having a weight average molecular weight of 1000 (FNP-0115, degree of branching 1 / 1000C, manufactured by Nippon Seiki Co., Ltd.) is 20% by weight, and this ultra high molecular weight polyethylene And 100 parts by weight of the total amount of polyethylene wax, 0.4% by weight of antioxidant (Irg1010, manufactured by Ciba Specialty Chemicals), 0.1% by weight (P168, manufactured by Ciba Specialty Chemicals), stearic acid After adding 1.3% by weight of sodium, and further adding calcium carbonate (manufactured by Maruo Calcium Co., Ltd.) having an average pore diameter of 0.1 μm so as to be 37% by volume with respect to the total volume, and mixing these with a Henschel mixer in a powder form Then, it was melt-kneaded with a biaxial kneader to obtain a polyolefin resin composition. Then, the polyolefin resin composition was rolled with a pair of rolling rolls having a surface temperature of 150 ° C., and cooled stepwise while being pulled with a winding roll having a different speed ratio. Here, a sheet having a film thickness of about 62 μm was prepared with a rolling draw ratio (winding roll speed / rolling roll speed) of 1.4 times. This sheet was immersed in an aqueous hydrochloric acid solution (hydrochloric acid 4 mol / L, nonionic surfactant 0.5 wt%) to remove calcium carbonate, and subsequently stretched 4 times at 105 ° C. A quality film was obtained.

Example 4
Ultra high molecular weight polyethylene powder (GUR4012, manufactured by Ticona) is 80% by weight, polyethylene wax having a weight average molecular weight of 1000 (FNP-0115, degree of branching 1 / 1000C, manufactured by Nippon Seiki Co., Ltd.) is 20% by weight, and this ultra high molecular weight polyethylene And 100 parts by weight of the total amount of polyethylene wax, 0.4% by weight of antioxidant (Irg1010, manufactured by Ciba Specialty Chemicals), 0.1% by weight (P168, manufactured by Ciba Specialty Chemicals), stearic acid After adding 1.3% by weight of sodium, and further adding calcium carbonate (manufactured by Maruo Calcium Co., Ltd.) having an average pore diameter of 0.1 μm so as to be 37% by volume with respect to the total volume, and mixing these with a Henschel mixer in a powder form Then, it was melt-kneaded with a biaxial kneader to obtain a polyolefin resin composition. Then, the polyolefin resin composition was rolled with a pair of rolling rolls having a surface temperature of 150 ° C., and cooled stepwise while being pulled with a winding roll having a different speed ratio. Here, a sheet having a film thickness of about 62 μm was prepared with a rolling draw ratio (winding roll speed / rolling roll speed) of 1.4 times. The sheet was immersed in an aqueous hydrochloric acid solution (hydrochloric acid 4 mol / L, nonionic surfactant 0.5% by weight) to remove calcium carbonate, and subsequently stretched 5.8 times at 105 ° C. A porous film was obtained.

(Comparative Example 1)
A commercially available polyolefin porous film (polyolefin separator) was used as Comparative Example 1.

(Comparative Example 2)
A commercially available polyolefin porous film (polyolefin separator) different from Comparative Example 1 was used as Comparative Example 2.

(Comparative Example 3)
A commercially available polyolefin porous film (polyolefin separator) different from Comparative Examples 1 and 2 was used as Comparative Example 3.

<Production of non-aqueous electrolyte secondary battery>
Next, a non-aqueous electrolyte secondary battery is prepared using each of the separators for non-aqueous electrolyte secondary batteries made of the porous films of Examples 1 to 4 and Comparative Examples 1 to 3 manufactured as described above. It produced according to.

(Positive electrode)
LiNi _0.5 Mn _0.3 Co _0.2 O ₂ / conductive material / PVDF (weight ratio 92/5/3)
The commercially available positive electrode manufactured by apply | coating to aluminum foil was used. Cut the aluminum foil so that the size of the positive electrode active material layer formed on the positive electrode is 40 mm × 35 mm and the outer periphery of the positive electrode active material layer is 13 mm wide and no positive electrode active material layer is formed. A positive electrode was obtained. The positive electrode active material layer had a thickness of 58 μm and a density of 2.50 g / cm ³ .

(Negative electrode)
A commercially available negative electrode produced by applying graphite / styrene-1,3-butadiene copolymer / sodium carboxymethylcellulose (weight ratio 98/1/1) to a copper foil was used. Cut off the copper foil from the negative electrode so that the size of the portion where the negative electrode active material layer was formed was 50 mm × 40 mm and the outer periphery had a portion with a width of 13 mm and no negative electrode active material layer formed. A negative electrode was obtained. The thickness of the negative electrode active material layer was 49 μm, and the density was 1.40 g / cm ³ .

(assembly)
By laminating (arranging) the positive electrode, the non-aqueous electrolyte secondary battery separator, and the negative electrode in this order in a laminate pouch, a non-aqueous electrolyte secondary battery member was obtained. At this time, the positive electrode and the negative electrode were disposed so that the entire main surface of the positive electrode active material layer of the positive electrode was included in the range of the main surface of the negative electrode active material layer of the negative electrode (overlaid on the main surface).

Subsequently, the non-aqueous electrolyte secondary battery member was put in a bag in which an aluminum layer and a heat seal layer were laminated, and 0.25 mL of the non-aqueous electrolyte was put in this bag. As the non-aqueous electrolyte, an electrolyte at 25 ° C. in which LiPF ₆ having a concentration of 1.0 mol / liter was dissolved in a mixed solvent having a volume ratio of ethyl methyl carbonate, diethyl carbonate and ethylene carbonate of 50:20:30 was used. It was. And the non-aqueous-electrolyte secondary battery was produced by heat-sealing the said bag, decompressing the inside of a bag.

<Measurement results of various physical properties>
Table 1 shows the measurement results of the porosity, birefringence, and retardation of the porous films of Examples 1 to 4 and Comparative Examples 1 to 3. In addition, Table 1 also shows the measurement results of resistance values after assembly in a non-aqueous electrolyte secondary battery using these porous films as separators for non-aqueous electrolyte secondary batteries.

FIG. 2 is a graph in which the measurement results of the examples and comparative examples are plotted with the phase difference shown in Table 1 as the horizontal axis and the 10 Hz resistance as the vertical axis. As shown in Table 1 and FIG. 1, the porous film of Examples 1 to 4 having a porosity of 30 to 60% and a retardation of 80 nm or less was used as a separator for a nonaqueous electrolyte secondary battery. It can be seen that in the water electrolyte secondary battery, the resistance value after assembly is as low as 0.91Ω or less. On the other hand, in Comparative Examples 1 to 3, the porosity is 30 to 60%, but the phase difference is as large as 100 nm or more, and the resistance value after assembly of the nonaqueous electrolyte secondary battery is as high as 0.99Ω or more. You can see that Thus, it was confirmed that the phase difference and the internal resistance of the battery after the assembly of the nonaqueous electrolyte secondary battery were correlated, and a porous film having a porosity of 30 to 60% and a phase difference of 80 nm or less was not used. It was found that the internal resistance of the battery after assembly of the non-aqueous electrolyte secondary battery was excellent by using it as a separator for a water electrolyte secondary battery.

Claims (4)

A porous film mainly composed of polyolefin,
A separator for a non-aqueous electrolyte secondary battery, wherein a phase difference with respect to light having a wavelength of 590 nm in a state impregnated with ethanol is 80 nm or less and a porosity is 30 to 60%.   A laminated separator for a non-aqueous electrolyte secondary battery, comprising the separator for a non-aqueous electrolyte secondary battery according to claim 1 and a porous layer.   The positive electrode, the separator for a non-aqueous electrolyte secondary battery according to claim 1, or the laminated separator for a non-aqueous electrolyte secondary battery according to claim 2, and a negative electrode are arranged in this order. A member for a non-aqueous electrolyte secondary battery.   A non-aqueous electrolyte secondary battery comprising the separator for a non-aqueous electrolyte secondary battery according to claim 1 or a laminated separator for a non-aqueous electrolyte secondary battery according to claim 2.

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