Separator for non-aqueous electrolyte secondary battery, member for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

To provide a separator for a non-aqueous electrolyte secondary battery that has both heat resistance and low resistance.SOLUTION: The separator for a non-aqueous electrolyte secondary battery according to the present disclosure includes a polyolefin porous substrate and heat-resistant layers stacked on both surfaces of the polyolefin porous substrate. The total membrane thickness is 8.0 μm or less. A parameter X calculated from the following formula (1) is 1.0 or less. X=(A'+A") / {C×(A' / A")2}...(1) (In the formula, A' and A" are the weight basis [g / m2] of the heat-resistant layers, A'≤A", and C is the weight basis [g / m2] of the polyolefin porous substrate.)SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a separator for a non-aqueous electrolyte secondary battery, a member for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery.

Background Art

[0002] Non-aqueous electrolyte secondary batteries, particularly lithium-ion secondary batteries, are widely used as batteries for personal computers, mobile phones, portable information terminals, in-vehicle applications, etc. because of their high energy density.

[0003] As a member of the non-aqueous electrolyte secondary battery, the development of a separator with excellent heat resistance has been underway. For example, as in Patent Document 1, a laminated separator in which a porous layer containing a heat-resistant resin is laminated on a base material is known.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the above-described conventional technologies have room for further improvement from the viewpoint of achieving both heat resistance and low resistance. One aspect of the present invention aims to realize a separator for a non-aqueous electrolyte secondary battery having both heat resistance and low resistance.

Means for Solving the Problems

[0006] In order to solve the above problems, a separator for a non-aqueous electrolyte secondary battery according to one aspect of the present invention includes a polyolefin porous base material and heat-resistant layers laminated on both surfaces of the polyolefin porous base material, has a total film thickness of 8.0 μm or less, and a parameter X obtained from the following formula (1) is 1.0 or less. X = (A'+A") / {C×(A' / A") 2} ···(1) (In the formula, A' and A" are the weights per unit area [g / m 2 of the respective heat-resistant layers, A'≤A", and C is the weight per unit area [g / m 2 of the polyolefin porous base material.) [Advantages of the Invention]

[0007] According to one aspect of the present invention, it is possible to provide a separator for a non-aqueous electrolyte secondary battery having both heat resistance and low resistance. [Embodiments for Carrying Out the Invention]

[0008] An embodiment of the present invention will be described below, but the present invention is not limited thereto. Unless otherwise specified in this specification, "A to B" representing a numerical range means "A or more and B or less".

[0009] [1. Separator for Non-Aqueous Electrolyte Secondary Battery] The separator for a non-aqueous electrolyte secondary battery according to an embodiment of the present invention includes a polyolefin porous base material and heat-resistant layers laminated on both surfaces of the polyolefin porous base material, has a total film thickness of 8.0 μm or less, and a parameter X obtained from the following formula (1) is 1.0 or less. X = (A'+A") / {C×(A' / A") 2} ···(1) (In the formula, A' and A" are the weights per unit area [g / m 2 of the respective heat-resistant layers, A'≤A", and C is the weight per unit area [g / m 2 of the polyolefin porous base material.) Hereinafter, the polyolefin porous base material will also be simply referred to as the "base material". Further, the separator for a non-aqueous electrolyte secondary battery will also be simply referred to as the "separator". The separator is a laminate including a base material and heat-resistant layers. Therefore, it can also be said that the separator is a laminated separator.

[0010] The weight per unit area of the two heat-resistant layers laminated on both sides of the polyolefin porous substrate may be the same or different. When the weight per unit area of the two heat-resistant layers is different, A' represents the smaller weight per unit area, and A'' represents the larger weight per unit area.

[0011] The inventors of the present invention were conducting research on thin film separators, which are in increasing demand for consumer applications, and found that there is room for improvement from the perspective of achieving both heat resistance and low resistance in the prior art. As a result of intensive research by the inventors, it has been found that a separator having both heat resistance and low resistance can be realized by controlling the weight per unit area of the substrate and the weight per unit area of the heat-resistant layer so as to satisfy a specific formula.

[0012] It can be said that the parameter X is an index indicating the balance between the heat resistance and the connectivity of the separator having heat-resistant layers on both sides. When the weight per unit area A' and A'' of the heat-resistant layers of the separator are large, X tends to increase. The larger X is, the thicker the heat-resistant layer becomes, so the heat resistance increases, but the connectivity tends to decrease. Also, when the difference between the weight per unit area A' and A'' of the heat-resistant layers is large, since A' / A'' becomes small, X tends to increase. In this case, it is considered that the connectivity of the heat-resistant layer on the side with the larger weight per unit area is low. If the connectivity is low, the resistance increases, which may cause a deterioration in battery characteristics. If X is 1.0 or less, the connectivity does not become too low, and the balance between the heat-resistant layers on both sides is also good, so both connectivity and heat resistance can be achieved. From this perspective, X is preferably 0.8 or less, more preferably 0.5 or less.

[0013] On the other hand, when the weight per unit area A' and A'' of the heat-resistant layers of the separator are small, X tends to decrease. The smaller the value of X, the thinner the heat-resistant layer becomes, so the connectivity increases, but the heat resistance tends to decrease. If the heat resistance is low, the shrinkage amount at high temperatures tends to increase. From this perspective, X is preferably 0.05 or more, more preferably 0.1 or more, and still more preferably 0.2 or more.

[0014] The total film thickness of the separator is preferably 7.0 μm or less, more preferably 6.8 μm or less, from the viewpoint of ion permeability. The total film thickness of the separator is preferably 3.0 μm or more, more preferably 4.0 μm or more, and even more preferably 5.0 μm or more. If the film thickness of the separator is 3.0 μm or more, internal short circuit of the battery can be sufficiently prevented.

[0015] The air permeability of the separator is preferably 30 to 500 sec / 100 mL, more preferably 50 to 300 sec / 100 mL, and even more preferably 70 to 150 sec / 100 mL. By having the above air permeability, the separator can obtain sufficient ion permeability in a non-aqueous electrolyte secondary battery. The air permeability represents a value measured with a Wang Research air permeability tester in accordance with JIS P8117.

[0016] As described above, if the heat-resistant layer becomes thicker, parameter X increases, but the ion permeability may decrease and the rate characteristics may deteriorate. In the separator, from the viewpoints of improving heat resistance, connectivity, and rate characteristics, it is preferable to control the following parameter Y in consideration of the porosity D of the heat-resistant layer in addition to parameter X. The separator preferably has a parameter Y obtained from the following formula (2) of less than 1.6, more preferably less than 1.3, and even more preferably less than 1.0. Parameter Y may be 0.1 or more, or may be 0.3 or more. Y = X / (D / 100) ···(2) (In the formula, X is the value obtained from the above formula (1), and D is the porosity [%] of the heat-resistant layer.) The porosity D [%] of the heat-resistant layer can be obtained from the following formula (3) for each component i constituting the heat-resistant layer, using the mass Wi [g / cm 2 of component i per unit area, the density di [g / cm 3 of component i, and the thickness t [cm] of the heat-resistant layer. D = {1 - (ΣWi / di) × (1 / t)} × 100 ···(3) In addition, when the porosity of the heat-resistant layers on both sides is different, the average value of these is taken as the value of the porosity D [%].

[0017] <1-1. Polyolefin Porous Substrate> The polyolefin porous substrate means a porous substrate mainly composed of a polyolefin resin. "Mainly composed of a polyolefin resin" means that the proportion of the polyolefin resin in the substrate is 50% by weight or more, preferably 90% by weight or more, and more preferably 95% by weight or more of the total materials constituting the substrate. The substrate can be a polyolefin porous film.

[0018] The polyolefin resin preferably contains a high molecular weight component with a weight average molecular weight of 5×10 5 ~15×10 6 More preferably, it contains a high molecular weight component with a weight average molecular weight of 1,000,000 or more. In particular, when the polyolefin resin contains a high molecular weight component with a weight average molecular weight of 1,000,000 or more, the strength of the resulting separator is improved, which is more preferable.

[0019] The polyolefin resin is not particularly limited, and examples thereof include thermoplastic resins such as homopolymers or copolymers obtained by polymerizing monomers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, and 1-hexene. Examples of the homopolymer include polyethylene, polypropylene, and polybutene. Examples of the copolymer include ethylene-propylene copolymer.

[0020] Among these, polyethylene is more preferable because it can prevent an excessive current from flowing through the separator at a lower temperature. Note that preventing the flow of this excessive current is also referred to as shutdown. Examples of the polyethylene include low-density polyethylene, high-density polyethylene, linear polyethylene (ethylene-α-olefin copolymer), and ultra-high molecular weight polyethylene with a weight average molecular weight of 1,000,000 or more. Among these, ultra-high molecular weight polyethylene with a weight average molecular weight of 1,000,000 or more is even more preferable.

[0021] The film thickness of the base material is preferably 7.0 μm or less, more preferably 6.0 μm or less, and even more preferably 5.5 μm or less from the viewpoint of ion permeability. The film thickness of the base material is preferably 2.0 μm or more, and more preferably 3.0 μm or more. If the film thickness of the base material is 2.0 μm or more, internal short circuit of the battery can be sufficiently prevented.

[0022] The weight per unit area of the base material, that is, the weight per unit area (i.e., C), can be appropriately determined in consideration of strength, film thickness, weight, and handleability. However, in order to increase the weight energy density and volume energy density of the non-aqueous electrolyte secondary battery, the weight per unit area is preferably 2 to 20 g / m 2 and more preferably 2 to 12 g / m 2 and even more preferably 2 to 10 g / m 2 and particularly preferably 2 to 5 g / m 2

[0023] The base material has a large number of pores connected therein, and it is possible to allow gas and liquid to pass from one surface to the other surface. The air permeability of the base material is preferably 30 to 500 sec / 100 mL, more preferably 50 to 300 sec / 100 mL, and even more preferably 70 to 150 sec / 100 mL. By having the above air permeability, sufficient ion permeability can be obtained. The air permeability represents a value measured by a Kawabata air permeability tester in accordance with JIS P8117.

[0024] ​The porosity of the base material is preferably 20 to 80% by volume, more preferably 30 to 75% by volume, so as to increase the retention amount of the electrolytic solution and obtain a function of reliably preventing excessive current from flowing at a lower temperature. Also, from the viewpoint of strength, the porosity of the base material is preferably 60% by volume or less, more preferably 50% by volume or less. The pore diameter of the pores in the base material is preferably 0.3 μm or less, more preferably 0.14 μm or less, so as to obtain sufficient ion permeability and prevent particles from entering the positive electrode and the negative electrode.

[0025] <1-2. Heat-resistant layer> The heat-resistant layer means a layer having a melting temperature higher than that of the base material. The heat-resistant layer may contain a resin having heat resistance. The resin may be a resin having a melting point or a glass transition temperature higher than that of the resin constituting the base material. The resin is preferably insoluble in the electrolytic solution of the non-aqueous electrolyte secondary battery and electrochemically stable within the operating range of the battery.

[0026] Examples of the resin include polyolefin resins; (meth)acrylate resins; fluorine-containing resins; polyamide resins; polyimide resins; polyester resins; rubbers; resins having a melting point or a glass transition temperature of 180°C or higher; water-soluble polymers; polycarbonates, polyacetals, and the like. Among these resins, one or more resins selected from the group consisting of polyolefin resins, (meth)acrylate resins, fluorine-containing resins, polyamide resins, polyester resins, and water-soluble polymers are more preferable.

[0027] As the polyamide resin, aramid resins such as aromatic polyamide and wholly aromatic polyamide are preferred. Examples of the aramid resin include para-aramid and meta-aramid, with para-aramid being preferred. Examples of para-aramid include poly(paraphenylene terephthalamide), poly(parabenzamide), poly(4,4'-benzylanilide terephthalamide), poly(paraphenylene-4,4'-biphenylene dicarboxamide), poly(paraphenylene-2,6-naphthalene dicarboxamide), poly(2-chloro-paraphenylene terephthalamide), a copolymer of paraphenylene terephthalamide / 2,6-dichloroparaphenylene terephthalamide, poly(4,4'-diphenylsulfonyl terephthalamide), and a copolymer of paraphenylene terephthalamide / 4,4'-diphenylsulfonyl terephthalamide, etc., para-aramids having a para-oriented or para-oriented-like structure. Among these, poly(paraphenylene terephthalamide) is more preferred.

[0028] Examples of the polyester resin include aromatic polyesters such as polyarylate and liquid crystal polyester.

[0029] Examples of the rubbers include styrene-butadiene copolymer and its hydride, methacrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, ethylene propylene rubber, polyvinyl acetate, etc.

[0030] Examples of the fluororesin include polyvinylidene fluoride (PVDF), polytetrafluoroethylene, vinylidene fluoride - hexafluoropropylene copolymer, tetrafluoroethylene - hexafluoropropylene copolymer, tetrafluoroethylene - perfluoroalkyl vinyl ether copolymer, vinylidene fluoride - tetrafluoroethylene copolymer, vinylidene fluoride - trifluoroethylene copolymer, vinylidene fluoride - trichloroethylene copolymer, vinylidene fluoride - vinyl fluoride copolymer, vinylidene fluoride - hexafluoropropylene - tetrafluoroethylene copolymer, ethylene - tetrafluoroethylene copolymer, etc. Among the above fluororesins, fluororubbers having a glass transition temperature of 23°C or lower are also included.

[0031] Examples of the resin having a melting point or a glass transition temperature of 180°C or higher include polyphenylene ether, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyetheramide, and polyetheretherketone, etc.

[0032] Examples of the water - soluble polymer include polyvinyl alcohol, polyethylene glycol, cellulose ether, sodium alginate, polyacrylic acid, polyacrylamide, polymethacrylic acid, etc.

[0033] The film thickness per layer of the heat - resistant layer is preferably 1.0 μm or less, more preferably 0.8 μm or less, still more preferably 0.5 μm or less, and particularly preferably 0.3 μm or less. If the film thickness per layer of the heat - resistant layer is 1.0 μm or less, in the non - aqueous electrolyte secondary battery, the permeation resistance of metal ions can be suppressed, so that the deterioration of rate characteristics and cycle characteristics can be suppressed. In addition, an increase in the distance between the positive electrode and the negative electrode can also be suppressed, so that the decrease in the internal volume efficiency of the non - aqueous electrolyte secondary battery can be suppressed.

[0034] The film thickness per layer of the heat-resistant layer is preferably 0.1 μm or more. When the film thickness per layer of the heat-resistant layer is 0.1 μm or more, internal short circuits due to breakage of the non-aqueous electrolyte secondary battery or the like can be sufficiently suppressed, and the amount of electrolyte retained in the heat-resistant layer becomes sufficient.

[0035] The weight per unit area of each layer of the heat-resistant layer (i.e., each of A' and A") is preferably 0.1 to 5 g / m 2 more preferably 0.1 to 1 g / m 2 even more preferably 0.1 to 0.5 g / m 2 even more preferably 0.1 to 0.3 g / m 2 and particularly preferably 0.1 to 0.3 g / m.

[0036] The heat-resistant layer can be a porous layer. The porosity of the heat-resistant layer (i.e., D) is preferably 20 to 90% and more preferably 30 to 80% so as to obtain sufficient ion permeability. Further, the pore diameter of the pores in the heat-resistant layer is preferably 1.0 μm or less, and more preferably 0.5 μm or less. By setting the pore diameter of the pores to these sizes, the non-aqueous electrolyte secondary battery can obtain sufficient ion permeability.

[0037] The heat-resistant layer may contain a filler. The filler can be an inorganic filler or an organic filler. As the filler, fillers composed of inorganic oxides such as silica, calcium oxide, magnesium oxide, magnesium hydroxide, titanium oxide, barium sulfate, alumina, mica, zeolite, aluminum hydroxide, or boehmite are preferable, fillers composed of calcium oxide, magnesium oxide, magnesium hydroxide, barium sulfate, and alumina are more preferable, and fillers composed of alumina are even more preferable. A non-aqueous electrolyte secondary battery provided with a heat-resistant layer containing a filler having a high thermal conductivity has excellent heat dissipation in the battery, and thus its safety is further improved.

[0038] In the heat-resistant layer, the filler content is preferably 99% by weight or less, more preferably 90% by weight or less, still more preferably 70% by weight or less, and particularly preferably 60% by weight or less, based on the weight of the entire heat-resistant layer, that is, the total weight of the resin and filler described above. Further, the filler content is preferably 10% by weight or more, more preferably 20% by weight or more, based on the weight of the entire heat-resistant layer.

[0039] The average particle size of the filler contained in the heat-resistant layer is preferably 1 μm or less, more preferably 800 nm or less, still more preferably 500 nm or less, still more preferably 100 nm or less, and particularly preferably 50 nm or less. Here, the average particle size of the filler is the average value of the sphere-equivalent particle sizes of 50 fillers. Further, the sphere-equivalent particle size of the filler is a value measured by a transmission electron microscope. A specific measurement method is illustrated as follows. 1. Using a transmission electron microscope (TEM; JEOL Ltd., transmission electron microscope JEM-2100F), at an acceleration voltage of 200 kV, and using a Gatan Imaging Filter for the imaging magnification, take images at 10,000 times magnification. 2. For the obtained image, using image analysis software (ImageJ), trace the contour of the particles and measure the sphere-equivalent particle size of the filler particles (primary particles). 3. Perform the above measurement on 50 randomly selected filler particles. The arithmetic mean of the sphere-equivalent particle sizes of the 50 filler particles is taken as the average particle size of the particles.

[0040] By setting the average particle size of the filler to 1 μm or less, the separator can be thinned. The lower limit value of the average particle size of the filler is not particularly limited, but can be, for example, 5 nm or more.

[0041] The true density of the material constituting the heat-resistant layer is preferably 1.5 to 3 g / m 3 and more preferably 2 to 3 g / m 3 The materials contained in the heat-resistant layers on both sides may be the same or different, but are preferably the same.

[0042] <1-3. Another functional layer> The separator may optionally include another functional layer different from the base material and the heat-resistant layer, as long as the object of the present invention is not impaired. Examples of the other functional layer include known porous layers such as an adhesive layer and a protective layer.

[0043] The other functional layer may be provided on one or both sides of the separator. When the separator has heat-resistant layers on both sides of the base material, the other functional layer may be provided on the heat-resistant layers on both sides or on the heat-resistant layer on one side. When the separator has a heat-resistant layer on only one side of the base material, the other functional layer may be provided on the heat-resistant layer or on the surface of the base material where the heat-resistant layer is not provided. The other functional layer may be provided as the outermost layer of the separator.

[0044] For example, the separator further includes an adhesive layer in addition to the base material and the heat-resistant layer. In this specification, the adhesive layer means a porous layer having adhesiveness. The adhesive layer may be provided on the surface of the separator that contacts the electrode. Examples of the components contributing to the adhesiveness included in the adhesive layer include acrylic resins and PVDF.

[0045] 〔2. Method for manufacturing a separator for a non-aqueous electrolyte secondary battery〕 The method for manufacturing the base material is not particularly limited. For example, a sheet-like polyolefin resin composition is produced by kneading a polyolefin-based resin, a pore-forming agent such as an inorganic filler or a plasticizer, and optionally an antioxidant or the like, and then extruding. Then, the pore-forming agent is removed from the sheet-like polyolefin resin composition using an appropriate solvent. Thereafter, a polyolefin porous film can be manufactured by stretching the polyolefin resin composition from which the pore-forming agent has been removed.

[0046] The inorganic filler is not particularly limited, and examples thereof include inorganic fillers, specifically calcium carbonate. The plasticizer is not particularly limited, and examples thereof include low-molecular-weight hydrocarbons such as liquid paraffin.

[0047] A heat-resistant layer can be formed using a coating liquid obtained by dissolving or dispersing a resin in a solvent. Incidentally, the solvent can also be said to be a dispersion medium for dispersing the resin. Examples of the resin include the resins described above. Examples of the method for forming the coating liquid include a mechanical stirring method, an ultrasonic dispersion method, a high-pressure dispersion method, a media dispersion method, and the like.

[0048] Examples of the method for forming the heat-resistant layer include a method of directly applying the coating liquid to the surface of a substrate and then removing the solvent. Examples of the method for applying the coating liquid to the substrate include a bar coater method. Here, the thickness L of the coating bar in the conveyance direction of the substrate is preferably 5.0 to 10.0 mm, more preferably 5.0 to 7.0 mm. Further, the angle θ formed between the surface of the substrate and the bottom surface (edge portion) of the coating bar is preferably more than 0.0° and less than 3.0°, more preferably more than 0.0° and 1.0° or less. Furthermore, the clearance between the coating bar and the substrate is preferably 40 to 60 μm. By controlling the thickness L, the angle θ, and the clearance within the above ranges, the above-described separator can be suitably obtained.

[0049] The solvent preferably has no adverse effect on the substrate, dissolves the resin uniformly and stably, and disperses the filler uniformly and stably. Examples of the solvent include N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide, N,N-dimethylformamide, acetone, and water.

[0050] The coating liquid may contain a filler. The coating liquid may appropriately contain a dispersant, a plasticizer, a surfactant, a pH adjuster, and the like as components other than the resin and the filler.

[0051] When the coating liquid contains an aramid resin, the aramid resin can be deposited by applying moisture to the coating surface. Thereby, a heat-resistant layer may be formed. Specific methods for applying moisture to the coating surface are not particularly limited, and examples include exposing to an atmosphere with high humidity, spraying water using a spray or the like, and spraying water vapor using a nozzle or the like.

[0052] 〔3. Member for non-aqueous electrolyte secondary battery, non-aqueous electrolyte secondary battery〕 In the member for a non-aqueous electrolyte secondary battery according to one embodiment of the present invention, a positive electrode, the above-described separator, and a negative electrode are arranged in this order. Further, a non-aqueous electrolyte secondary battery according to one embodiment of the present invention includes the above-described separator. The shape of the non-aqueous electrolyte secondary battery is not particularly limited, and may be, for example, a thin plate (paper) type, a disk type, a cylindrical type, a prismatic type such as a rectangular parallelepiped, or the like.

[0053] For example, a member for a non-aqueous electrolyte secondary battery can be formed by arranging a positive electrode, the above-described separator, and a negative electrode in this order. Here, the heat-resistant layer may exist between the base material and at least one of the positive electrode and the negative electrode. Next, the member for the non-aqueous electrolyte secondary battery is placed in a container that serves as a housing of the non-aqueous electrolyte secondary battery. After filling the inside of the container with the non-aqueous electrolyte, it is sealed while reducing the pressure. Thereby, a non-aqueous electrolyte secondary battery can be manufactured.

[0054] <Positive electrode> The positive electrode is not particularly limited as long as it is generally used as a positive electrode of a non-aqueous electrolyte secondary battery. For example, as the positive electrode, a positive electrode sheet having a structure in which an active material layer containing a positive electrode active material and a binder is formed on a positive electrode current collector can be used. Note that the active material layer may further contain a conductive agent.

[0055] Examples of the positive electrode active material include materials capable of doping and dedoping metal ions such as lithium ions or sodium ions. Specifically, examples of such materials include lithium composite oxides containing at least one kind of transition metal such as V, Mn, Fe, Co, and Ni.

[0056] Examples of the conductive agent include carbonaceous materials such as natural graphite, artificial graphite, cokes, carbon black, pyrolytic carbons, carbon fibers, and fired bodies of organic polymer compounds. Only one type of the conductive agent may be used, or two or more types may be used in combination.

[0057] Examples of the binder include fluorine-based resins such as polyvinylidene fluoride (PVDF), acrylic resins, and styrene-butadiene rubber. Note that the binder also has a function as a thickening agent.

[0058] Examples of the positive electrode current collector include conductors such as Al, Ni, and stainless steel. Among them, Al is more preferable because it is easy to process into a thin film and is inexpensive.

[0059] Examples of the method for manufacturing the positive electrode sheet include a method of pressure molding a positive electrode active material, a conductive agent, and a binder on a positive electrode current collector; a method of making a paste of a positive electrode active material, a conductive agent, and a binder using a suitable organic solvent, coating the paste on a positive electrode current collector, drying it, and then pressing it to adhere it to the positive electrode current collector; and the like.

[0060] <Negative electrode> The negative electrode is not particularly limited as long as it is generally used as a negative electrode of a non-aqueous electrolyte secondary battery. For example, as the negative electrode, a negative electrode sheet having a structure in which an active material layer containing a negative electrode active material and a binder is formed on a negative electrode current collector can be used. Note that the active material layer may further contain a conductive agent.

[0061] Examples of the negative electrode active material include materials capable of doping and de-doping metal ions such as lithium ions or sodium ions. Examples of such materials include carbonaceous materials. Examples of the carbonaceous materials include natural graphite, artificial graphite, cokes, carbon black, and pyrolytic carbons.

[0062] Examples of the negative electrode current collector include Cu, Ni, stainless steel, etc. Cu is more preferable because it is difficult to form an alloy with lithium and is easy to process into a thin film.

[0063] Examples of the method for manufacturing the negative electrode sheet include: a method of pressure-molding the negative electrode active material on the negative electrode current collector; a method of making the negative electrode active material into a paste using a suitable organic solvent, applying the paste to the negative electrode current collector, drying it, and then pressing it to adhere to the negative electrode current collector; etc. The paste preferably contains the above-mentioned conductive agent and the binder.

[0064] <Non-aqueous electrolyte> The non-aqueous electrolyte is not particularly limited as long as it is a non-aqueous electrolyte generally used in non-aqueous electrolyte secondary batteries. As the non-aqueous electrolyte, 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 , lithium salts of lower aliphatic carboxylic acids, LiAlCl 4 , etc. The lithium salt may be used alone or in combination of two or more.

[0065] Examples of the organic solvent include carbonates, ethers, esters, nitriles, amides, carbamates, sulfur-containing compounds, and fluorine-containing organic solvents obtained by introducing a fluorine group into these organic solvents, etc. The organic solvent may be used alone or in combination of two or more.

[0066] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims. Embodiments obtained by appropriately combining technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0067] One embodiment of the present invention may include the following configuration. <1>A separator for a non-aqueous electrolyte secondary battery, comprising a polyolefin porous substrate and heat-resistant layers laminated on both surfaces of the polyolefin porous substrate, having a total film thickness of 8.0 μm or less and a parameter X obtained from the following formula (1) of 1.0 or less. X = (A'+A") / {C×(A' / A") 2} ···(1) (In the formula, A' and A" are the weight per unit area [g / m 2 of each of the heat-resistant layers, A'≤A", and C is the weight per unit area [g / m 2 of the polyolefin porous substrate.) <2>The separator for a non-aqueous electrolyte secondary battery according to <1>, wherein a parameter Y obtained from the following formula (2) is less than 1.6. Y = X / (D / 100) ···(2) (In the formula, X is the value obtained from the formula (1), and D is the porosity [%] of the heat-resistant layer.) <3>The separator for a non-aqueous electrolyte secondary battery according to <1> or <2>, wherein the heat-resistant layer contains one or more resins selected from the group consisting of polyolefin resins, (meth)acrylate resins, fluorine-containing resins, polyamide resins, polyester resins, and water-soluble polymers. <4>The separator for a non-aqueous electrolyte secondary battery according to <3>, wherein the polyamide resin is an aramid resin. <5>The separator for a non-aqueous electrolyte secondary battery according to any one of <1> to <4>, further comprising an adhesive layer separately from the polyolefin porous substrate and the heat-resistant layer. <6>A member for a non-aqueous electrolyte secondary battery, in which a positive electrode, the separator for a non-aqueous electrolyte secondary battery according to any one of <1> to <5>, and a negative electrode are arranged in this order. A non-aqueous electrolyte secondary battery comprising the separator for non-aqueous electrolyte secondary batteries according to any one of <7><1> to <5>.

Example

[0068] One embodiment of the present invention will be described below. In this specification, the conveyance direction in the manufacture of the separator is also referred to as the MD direction, and the direction that is horizontal to the surface of the separator and perpendicular to the MD direction is also referred to as the TD direction.

[0069] 〔Evaluation method〕 [Areal weight] A square sample of 8 cm × 8 cm was cut out from the base material (polyethylene porous film). The weight of this sample was measured and designated as W1 [g]. The areal weight of the base material was calculated according to the following formula (4). Areal weight of the base material [g / m 2 =W1 [g] / (0.08 × 0.08) ··· (4) A square sample of 8 cm × 8 cm was cut out from the separator. The weight of this sample was measured and designated as W2 [g]. The areal weight of the separator was calculated according to the following formula (5). Areal weight of the separator [g / m 2 =W2 [g] / (0.08 × 0.08) ··· (5) The areal weight of the heat-resistant layer was calculated by subtracting the areal weight of the base material from the areal weight of the separator.

[0070] [Air permeability] The air permeability of the separator cut out to a size of 60 mm × 60 mm was measured using a digital type Oka research air permeability tester EGO1 manufactured by Asahi Seiko Co., Ltd. in accordance with JIS P8117.

[0071] [Film thickness] The film thickness (total film thickness) of the separator and the film thickness of the polyethylene porous film (base material film thickness) were measured using a high-precision digital length measuring instrument (VL-50) manufactured by Mitutoyo Corporation.

[0072] [True density] The weight fraction of each component i is w i, taking the true density as ρ i The true density of the material constituting the heat-resistant layer was calculated as = 1 / Σ(w i / ρ i ). Since the following Examples and Comparative Examples had the same composition, the true density of the material constituting the heat-resistant layer was the same. Specifically, the true density of the aramid resin was 1.44 g / cm 3 , and the true density of alumina was 3.27 g / cm 3 . Therefore, the true density of the material constituting the heat-resistant layer was 2.00 g / cm 3 . As the true density of the aramid resin, the density described in Takashi No-ma, "Trends in the Development of Synthetic Fibers" Special Issue: Characteristics and Applications of Aramid Fibers, Journal of the Fiber Society (Fiber and Industry), Vol. 56, No. 8, pp. 241-247, 2000 was used. As the true density of alumina, the density described in the product information disclosed by the manufacturer was used.

[0073] [Heat Shrinkage Rate] A square sample of 8 cm (MD direction) × 8 cm (TD direction) was cut out from the separator. A square of 6 cm (MD direction) × 6 cm (TD direction) was drawn on the sample. The lengths of two sides parallel to the MD direction of the drawn square were measured to the second decimal place in cm units, and the average value L1 was calculated. Next, the sample was sandwiched between papers and placed in an oven heated to 130°C. After 1 hour, the sample was taken out of the oven, and the lengths of two sides parallel to the MD direction of the drawn square were measured to the second decimal place in cm units, and the average value L2 was calculated. Using L1 and L2, the heat shrinkage rate in the MD direction was calculated from the following formula. Heat shrinkage rate [%] = {(L1 - L2) / L1} × 100.

[0074] [AC Resistance] The AC resistance of the separator was measured according to the following procedure. 1. Six separators of 6 cm (MD direction) × 4.7 cm (TD direction) were cut out. 2. Six separators were stacked and sandwiched between two aluminum electrodes to fabricate a laminate. The aluminum electrodes had a thickness of 25 μm and an electrode area of 4.5 cm × 3.0 cm. Nickel tabs with seals were ultrasonically welded to the aluminum electrodes. This process was carried out in a dry box (dew point: -50°C or lower). 3. 250 μL of non-aqueous electrolyte was injected into the laminate, and an outer aluminum laminate was vacuum-sealed. In this way, a test cell was fabricated. The composition of the non-aqueous electrolyte was a mixed solvent of ethylene carbonate:ethyl methyl carbonate:diethyl carbonate = 3:5:2 (volume ratio), in which LiPF 6 was dissolved to a concentration of 1 mol / L. This process was carried out in a dry box (dew point: -50°C or lower). 4. A fixing weight (5 cm × 6 cm × 8 cm, approximately 2.2 kg) was placed on the laminate part of the test cell. 5. The AC impedance was measured with the weight placed. Thereby, the AC resistance [Ω·cm 2 of the separator was measured. An LCR meter (IM3536, Hioki Electric Co., Ltd.) was used for the measurement of the AC impedance. The measurement conditions were an AC voltage of 10 mV and a measurement frequency of 20 kHz to 200 kHz. In the obtained Cole-Cole plot, the value of the series equivalent resistance (real axis) when the reactance (imaginary axis) became 0 was taken as the AC resistance [Ω·cm 2 of the separator.

[0075] 〔Preparation of Coating Liquid (1)〕 As the resin constituting the heat-resistant layer (porous layer), poly(paraphenylene terephthalamide), which is a kind of aramid resin, was synthesized by the following method. Hereinafter, poly(paraphenylene terephthalamide) is referred to as "PPTA".

[0076] As a container for synthesis, a separable flask with a capacity of 3 L having a stirring blade, a thermometer, a nitrogen inlet tube, and a powder addition port was used. 2200 g of N-methyl-2-pyrrolidone (NMP) was charged into the sufficiently dried flask. 151 g of calcium chloride powder was added thereto, and the temperature was raised to 100 °C to completely dissolve it, obtaining a calcium chloride NMP solution. The calcium chloride powder used was pre-vacuum dried at 200 °C for 2 hours.

[0077] Next, the temperature of the calcium chloride NMP solution was returned to room temperature, 68.23 g of paraphenylenediamine was added, and it was completely dissolved to obtain solution (1). While maintaining the temperature of solution (1) at 20 °C ± 2 °C, 124.61 g of terephthaloyl dichloride was added to solution (1). Then, while continuing stirring, solution (1) was aged for 1 hour while maintaining the temperature of solution (1) at 20 °C ± 2 °C, obtaining an aramid polymerization solution (1) containing 6% by weight of PPTA. The intrinsic viscosity of PPTA contained in the aramid polymerization solution (1) was 1.7 g / dL.

[0078] 100 g of the aramid polymerization solution (1) was weighed into a flask, 6.0 g of alumina A (average particle size: 13 nm) was added to obtain a mixture A (1). In the mixture A (1), the weight ratio of PPTA to alumina A was 1:1. Next, NMP was added to the mixture A (1) so that the solid content became 4.5% by weight, and it was stirred for 240 minutes to obtain a mixture B (1). Here, the "solid content" refers to the total weight of PPTA and alumina A. Next, 0.73 g of calcium carbonate was added to the mixture B (1) and stirred for 240 minutes to neutralize the solution, obtaining a neutralized solution (1). Then, the neutralized solution (1) was defoamed under reduced pressure to prepare a slurry-like coating solution (1).

[0079] [Example 1] A polyethylene porous film with a film thickness of 5.3 μm and a porosity of 45% by volume was used as the base material. While transporting the base material, the base material was coated with a coating liquid (1) to form a coating film. At this time, the thickness L of the coating bar in the MD direction was 6.0 mm, the angle θ formed between the surface of the base material and the bottom surface (edge part) of the coating bar was 1.0°, and the clearance between the coating bar and the base material was 45 μm. Then, while transporting the base material on which the coating film was formed, the base material was passed through a precipitation tank set at 50 °C and a relative humidity of 70% to precipitate PPTA. Next, the coating film deposited on the base material was washed with water to remove calcium chloride and the solvent. Then, a drying treatment was performed to obtain a laminated separator having a heat-resistant layer formed on one side of the base material. Subsequently, the same operation was performed on the surface of the base material opposite to the coated surface, and a wound body (1) of a laminated separator having heat-resistant layers formed on both sides of the base material was obtained. The wound body (1) of the laminated separator was used as the separator (1). The porosity of the heat-resistant layer was 75%.

[0080] 〔Example 2〕 The same operations as in Example 1 were performed except that the clearance between the coating bar and the base material was changed to 41 μm, and a wound body (2) of a laminated separator having heat-resistant layers formed on both sides of the base material was obtained. The wound body (2) of the laminated separator was used as the separator (2). The porosity of the heat-resistant layer was 80%.

[0081] 〔Example 3〕 The same operations as in Example 1 were performed except that the clearance between the coating bar and the base material was changed to 48 μm, and a wound body (3) of a laminated separator having heat-resistant layers formed on both sides of the base material was obtained. The wound body (3) of the laminated separator was used as the separator (3). The porosity of the heat-resistant layer was 75%.

[0082] 〔Comparative Example 1〕 A coating bar with a thickness L of 17.0 mm in the MD direction was used, the angle θ formed between the surface of the substrate and the bottom surface (edge part) of the coating bar was set to 3.0°, and the clearance between the coating bar and the substrate was changed to 64 μm. Otherwise, the same operations as in Example 1 were performed to obtain a wound laminated separator (4) with heat-resistant layers formed on both sides of the substrate. The wound laminated separator (4) was used as the separator (4). The porosity of the heat-resistant layer was 71%.

[0083] 〔Comparative Example 2〕 The same operations as in Comparative Example 1 were performed except that the clearance between the coating bar and the substrate was changed to 55 μm to obtain a wound laminated separator (5) with heat-resistant layers formed on both sides of the substrate. The wound laminated separator (5) was used as the separator (5). The porosity of the heat-resistant layer was 68%.

[0084] 〔Comparative Example 3〕 The same operations as in Comparative Example 1 were performed except that the clearance between the coating bar and the substrate was changed to 60 μm to obtain a wound laminated separator (6) with heat-resistant layers formed on both sides of the substrate. The wound laminated separator (6) was used as the separator (6). The porosity of the heat-resistant layer was 72%.

[0085] 〔Comparative Example 4〕 The same operations as in Comparative Example 1 were performed except that the clearance between the coating bar and the substrate was changed to 62 μm to obtain a wound laminated separator (7) with heat-resistant layers formed on both sides of the substrate. The wound laminated separator (7) was used as the separator (7). The porosity of the heat-resistant layer was 67%.

[0086] 〔Comparative Example 5〕 The same operations as in Comparative Example 1 were performed except that a polyethylene porous film with a film thickness of 10.7 μm and a porosity of 43% by volume was used as the substrate and the clearance between the coating bar and the substrate was changed to 80 μm to obtain a wound laminated separator (8) with heat-resistant layers formed on both sides of the substrate. The wound laminated separator (8) was used as the separator (8). The porosity of the heat-resistant layer was 67%.

[0087] 〔Evaluation Results〕 The evaluation results of the examples and comparative examples are shown in Table 1.

[0088]

Table 1

[0089] From the viewpoint of safety, it is preferable that the heat shrinkage rate is less than 4%. Also, from the viewpoint of battery performance, the AC resistance is 2.6 Ω·cm 2 or less, and it is preferable that the air permeability is 150 s / 100 mL or less. Comparing Examples 1 to 3 and Comparative Examples 2 to 4 in which the total film thickness of the separator is 8.0 μm or less, in Examples 1 to 3 where parameter X is 1.0 or less, compared to Comparative Examples 2 to 4 where parameter X exceeds 1.0, while maintaining the heat shrinkage rate at less than 4%, the air permeability can be reduced, and the AC resistance can also be maintained or reduced to 2.6 Ω·cm 2 or less. In Comparative Examples 1 and 5 where the total film thickness of the separator exceeds 8.0 μm, the air permeability and AC resistance were larger than those in the examples.

[0090] Note that parameter X could be controlled by adjusting the thickness L of the coating bar in the MD direction, the angle θ formed between the surface of the substrate and the bottom surface (edge portion) of the coating bar, the clearance between the coating bar and the substrate, etc.

Industrial Applicability

[0091] One aspect of the present invention can be used for non-aqueous electrolyte secondary batteries.

Claims

1. A separator for a non-aqueous electrolyte secondary battery, comprising a polyolefin porous substrate and heat-resistant layers laminated on both surfaces of the polyolefin porous substrate, having a total film thickness of 8.0 μm or less and a parameter X obtained from the following formula (1) of 1.0 or less. X = (A' + A") / {C × (A' / A")} ２}...(1) (In the formula, A' and A" are the weight per unit area [g / m ２ of each of the heat-resistant layers, A' ≤ A", and C is the weight per unit area [g / m ２ of the polyolefin porous base material.)

2. The separator for a non-aqueous electrolyte secondary battery according to Claim 1, wherein a parameter Y obtained from the following formula (2) is less than 1.

6. Y = X / (D / 100) ··· (2) (In the formula, X is the value obtained from the formula (1), and D is the porosity [%] of the heat-resistant layer.)

3. The separator for a non-aqueous electrolyte secondary battery according to Claim 1, wherein the heat-resistant layer contains one or more resins selected from the group consisting of polyolefin resins, (meth)acrylate resins, fluorine-containing resins, polyamide resins, polyester resins, and water-soluble polymers.

4. The separator for a non-aqueous electrolyte secondary battery according to Claim 3, wherein the polyamide resin is an aramid resin.

5. The separator for a non-aqueous electrolyte secondary battery according to Claim 1, further comprising an adhesive layer separately from the polyolefin porous substrate and the heat-resistant layer.

6. A member for a non-aqueous electrolyte secondary battery, in which a positive electrode, the separator for a non-aqueous electrolyte secondary battery according to any one of Claims 1 to 5, and a negative electrode are arranged in this order.

7. A non-aqueous electrolyte secondary battery comprising the separator for a non-aqueous electrolyte secondary battery according to any one of Claims 1 to 5.

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