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 can provide a battery with excellent capacity retention characteristics after pressurization.SOLUTION: The separator for a non-aqueous electrolyte secondary battery according to the present disclosure includes a polyolefin porous substrate and a heat-resistant layer stacked on top of the polyolefin porous substrate. Under conditions of a temperature of 25°C and a relative humidity of 60 to 70%, a solution composed of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate mixed in a volume ratio of 3:5:2 retains at least 50% of its volume after 300 seconds of holding.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 members of such non-aqueous electrolyte secondary batteries, the development of various separators has been underway. For example, Patent Document 1 discloses a separator for a non-aqueous electrolyte secondary battery comprising a porous film mainly composed of a polyolefin resin, wherein the rate of decrease of diethyl carbonate dropped on the porous film is 15 seconds / mg to 21 seconds / mg, and the spot diameter of diethyl carbonate 10 seconds after dropping on the porous film is 20 mm or more.

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 had room for improvement from the viewpoint of capacity retention characteristics after pressurization. One aspect of the present invention aims to realize a separator for a non-aqueous electrolyte secondary battery that can provide a battery with excellent capacity retention characteristics after pressurization.

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 substrate and a heat-resistant layer laminated on the polyolefin porous substrate, and at a temperature of 25 ° C and a relative humidity of 60 to 70%, ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate are mixed at a volume ratio of 3:5:2, and the solution retention rate after 300 seconds is 50% or more after holding the resulting solution.

Effects 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 that can obtain a battery having excellent capacity retention characteristics after pressurization.

Modes for Carrying Out the Invention

[0008] A first 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〕 A separator for a non-aqueous electrolyte secondary battery according to an embodiment of the present invention includes a polyolefin porous substrate and a heat-resistant layer laminated on the polyolefin porous substrate, and at a temperature of 25 ° C and a relative humidity of 60 to 70%, ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate are mixed at a volume ratio of 3:5:2, and the solution retention rate after 300 seconds is 50% or more after holding the resulting solution.

[0010] Hereinafter, the polyolefin porous substrate will also be simply referred to as the "substrate". Also, 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 substrate and a heat-resistant layer. Therefore, it can also be said that the separator is a laminated separator.

[0011] As a result of intensive research by the present inventors, it has been found that by controlling the liquid retention property of the electrolyte by the separator, a separator capable of obtaining a battery having excellent capacity retention characteristics after pressurization can be realized. In this specification, the liquid retention property of the electrolyte means the property of the separator to hold the electrolyte. It can also be said that the liquid retention property reflects the wettability of the separator. In one embodiment of the present invention, as an index of the liquid retention property of the electrolyte, the solution retention rate measured using a solution simulating the electrolyte is used. It can be said that the higher the solution retention rate, the better the liquid retention property of the electrolyte.

[0012] The present inventors have found that the liquid retention property of the electrolyte can be improved by impregnating a substrate with a polar resin. The more the substrate is impregnated with the polar resin, the more the affinity of the substrate with the electrolyte is improved. Therefore, the electrolyte easily penetrates into the substrate and the electrolyte is difficult to volatilize. Thus, it can be said that the liquid retention property of the electrolyte reflects the degree of impregnation of the polar resin. A battery equipped with a separator having such excellent liquid retention property of the electrolyte has improved capacity retention characteristics. In addition, the more the substrate is impregnated with the polar resin, the higher the strength of the separator. Therefore, since the elastic modulus of the separator in the compression direction increases, it becomes resistant to pressurization. Therefore, by using the separator according to one embodiment of the present invention, a battery having excellent capacity retention characteristics after pressurization can be provided.

[0013] The solution retention rate may be 60% or more, or may be 70% or more. The upper limit of the solution retention rate is not particularly limited, but may be 100% or less.

[0014] The solution retention rate can be measured by the following method. (1) A separator cut out to 80 mm × 80 mm is placed on the scale of a precision balance in a state of being placed on a 100 mm × 100 mm aluminum foil, and the zero point is adjusted. Thereby, the weight of the solution dropped on the separator can be measured hereinafter. (2) Prepare a solution by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 3:5:2, which mimics the electrolyte solution. Under an atmosphere of 25°C and a relative humidity of 60 - 70%, drop 16 μL of the solution from a height of 5 mm onto the center of the separator on a precision balance using a micropipette. (3) Since the solution volatilizes over time, the weight of the solution changes. Therefore, set the point when the weight of the solution reaches 15.0 mg as the 0 - second time point, and measure the weight of the solution 300 seconds later from this point. (4) Divide the weight of the solution after 300 seconds by the weight of the solution at the 0 - second time point, which is 15.0 mg, and express it as a percentage to calculate the solution retention rate.

[0015] The membrane thickness of the separator is preferably 45 μm or less, more preferably 25 μm or less, even more preferably 10 μm or less, particularly preferably 8 μm or less, and most preferably 7 μm or less. If the membrane thickness of the separator is 45 μm or less, the enlargement of the non - aqueous electrolyte secondary battery can be prevented. Also, if the membrane thickness of the separator is 10 μm or less, it is preferable from the viewpoint of ion permeability.

[0016] The membrane thickness of the separator is preferably 3 μm or more, more preferably 4 μm or more, and even more preferably 5 μm or more. If the membrane thickness of the separator is 3 μm or more, the internal short - circuit of the battery can be sufficiently prevented.

[0017] The air permeability of the separator is preferably 30 - 500 sec / 100 mL, more preferably 50 - 300 sec / 100 mL, and even more preferably 70 - 150 sec / 100 mL. By having the above - mentioned air permeability, the separator can obtain sufficient ion permeability in the non - aqueous electrolyte secondary battery. The air permeability represents the value measured by a Wang - type air permeability tester in accordance with JIS P8117.

[0018] <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.

[0019] The polyolefin resin preferably contains a high molecular weight component having a weight average molecular weight of 5×10 5 ~15×10 6 Particularly, it is more preferable that the polyolefin resin contains a high molecular weight component having a weight average molecular weight of 1,000,000 or more, because the strength of the obtained separator is improved.

[0020] 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 an ethylene-propylene copolymer.

[0021] 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 having a weight average molecular weight of 1,000,000 or more. Among these, ultra-high molecular weight polyethylene having a weight average molecular weight of 1,000,000 or more is even more preferable.

[0022] The film thickness of the base material is preferably 40 μm or less, more preferably 20 μm or less, still more preferably 10 μm or less, particularly preferably 7 μm or less, and most preferably 6 μm or less. If the film thickness of the base material is 40 μm or less, enlargement of the non-aqueous electrolyte secondary battery can be prevented. Further, if the film thickness of the base material is 10 μm or less, it is preferable from the viewpoint of ion permeability.

[0023] The film thickness of the base material is preferably 2 μm or more, more preferably 3 μm or more. If the film thickness of the base material is 2 μm or more, internal short circuit of the battery can be sufficiently prevented.

[0024] The weight per unit area of the base material, that is, the weight per unit area can be appropriately determined in consideration of strength, film thickness, weight, and handleability. However, so as to increase the weight energy density and the 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 still more preferably 2 to 10 g / m 2 and particularly preferably 2 to 5 g / m 2

[0025] The base material has a large number of pores connected inside thereof, 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 still more preferably 70 to 150 sec / 100 mL. By the base material having the air permeability, sufficient ion permeability can be obtained. The air permeability represents a value measured with a King Research type air permeability tester in accordance with JIS P8117.

[0026] ​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 holding amount of the electrolytic solution and obtain the 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.

[0027] <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 use range of the battery. The resin may be a polar resin.

[0028] 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; polycarbonate, polyacetal, etc. 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.

[0029] 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-naphthalenedicarboxamide), poly(2-chloro-paraphenylene terephthalamide), a copolymer of paraphenylene terephthalamide / 2,6-dichloroparaphenylene terephthalamide, poly(4,4'-diphenylsulfonyl terephthalamide), a copolymer of paraphenylene terephthalamide / 4,4'-diphenylsulfonyl terephthalamide, and other para-aramids having a para-oriented or para-oriented-like structure. Among these, poly(paraphenylene terephthalamide) is more preferred.

[0030] Examples of the polyester resin include aromatic polyesters such as polyarylate and liquid crystal polyester.

[0031] Examples of the rubbers include styrene-butadiene copolymer and its hydrogenated product, methacrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, ethylene propylene rubber, polyvinyl acetate, and the like.

[0032] 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 with a glass transition temperature of 23°C or lower are also included.

[0033] Examples of the resin with a melting point or glass transition temperature of 180°C or higher include polyphenylene ether, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyetheramide, polyetheretherketone, etc.

[0034] Examples of the water - soluble polymer include polyvinyl alcohol, polyethylene glycol, cellulose ether, sodium alginate, polyacrylic acid, polyacrylamide, polymethacrylic acid, etc.

[0035] The film thickness per layer of the heat - resistant layer is preferably 10 μm or less, more preferably 5 μm or less, still more preferably 1 μm or less, and particularly preferably 0.5 μm or less. If the film thickness per layer of the heat - resistant layer is 10 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.

[0036] 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 or the like of the non-aqueous electrolyte secondary battery can be sufficiently suppressed, and the amount of electrolyte retained in the heat-resistant layer becomes sufficient.

[0037] The weight per unit area of the heat-resistant layer can be appropriately determined in consideration of the strength, film thickness, weight, and handleability of the heat-resistant layer. The weight per unit area of the heat-resistant layer is preferably 2 10 g / m 2 or less, more preferably 2 5 g / m 2 or less, still more preferably 2 1.5 g / m

[0038] or less, and particularly preferably

[0039] 1 g / m 2 or less. Further, the weight per unit area of the heat-resistant layer is preferably 2 0.1 g / m 2 or more. By setting the weight per unit area of the heat-resistant layer within these numerical ranges, the weight energy density and volume energy density of the non-aqueous electrolyte secondary battery can be increased. 2 The weight per unit area per layer of the heat-resistant layer is preferably

[0040] The heat-resistant layer can be a porous layer. The porosity of the heat-resistant layer is preferably 20 to 90% by volume, more preferably 30 to 80% by volume, 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, 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.

[0041] The heat-resistant layer may contain a filler. The filler can be an inorganic filler or an organic filler. As the filler, a filler composed of inorganic oxides such as silica, calcium oxide, magnesium oxide, magnesium hydroxide, titanium oxide, barium sulfate, alumina, mica, zeolite, aluminum hydroxide, or boehmite is preferable, a filler composed of calcium oxide, magnesium oxide, magnesium hydroxide, barium sulfate, or alumina is more preferable, and a filler composed of alumina is 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, so its safety is further improved.

[0042] In the heat-resistant layer, the content of the filler is preferably 99% by weight or less, more preferably 90% by weight or less, even 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 above-mentioned resin and filler. Further, the content of the filler is preferably 10% by weight or more, more preferably 20% by weight or more, based on the weight of the entire heat-resistant layer.

[0043] The average particle diameter of the filler contained in the heat-resistant layer is preferably 1 μm or less, more preferably 800 nm or less, more preferably 500 nm or less, more preferably 100 nm or less, and more preferably 50 nm or less. Here, the average particle diameter of the filler is the average value of the sphere-equivalent particle diameters of 50 fillers. Further, the sphere-equivalent particle diameter of the filler is a value measured by a transmission electron microscope. An example of a specific measurement method is as follows. 1. Use a transmission electron microscope (TEM; JEOL Ltd., transmission electron microscope JEM-2100F) to take pictures at an acceleration voltage of 200 kV and a magnification of 10,000 times using a Gatan Imaging Filter. 2. For the obtained images, use image analysis software (ImageJ) to trace the contours of the particles and measure the spherical equivalent particle sizes of the filler particles (primary particles). 3. Perform the above measurements on 50 filler particles randomly extracted. The arithmetic mean of the spherical equivalent particle sizes of the 50 filler particles is defined as the average particle size of the particles.

[0044] By setting the average particle size of the filler to 1 μm or less, the separator can be made thinner. The lower limit of the average particle size of the filler is not particularly limited, but can be, for example, 5 nm or more.

[0045] <1-3. Another functional layer> The separator may optionally contain 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.

[0046] The other functional layer can 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 can be provided as the outermost layer of the separator.

[0047] For example, the separator further includes an adhesive layer separately from the base material and the heat-resistant layer. In this specification, the adhesive layer means a porous layer having adhesiveness. The adhesive layer can be provided on the surface of the separator that contacts the electrode. Examples of the components contributing to the adhesiveness contained in the adhesive layer include acrylic resins and PVDF.

[0048] The manufacturing method of the base material is not particularly limited. For example, a sheet-like polyolefin resin composition is produced by kneading a polyolefin resin, a pore former such as an inorganic filler or a plasticizer, and optionally an antioxidant or the like, and then extruding. Then, the pore former is removed from the sheet-like polyolefin resin composition with an appropriate solvent. Thereafter, a polyolefin porous film can be manufactured by stretching the polyolefin resin composition from which the pore former has been removed.

[0049] The inorganic filler is not particularly limited, and examples thereof include inorganic fillers, specifically calcium carbonate and the like. The plasticizer is not particularly limited, and examples thereof include low molecular weight hydrocarbons such as liquid paraffin.

[0050] A heat-resistant layer can be formed using a coating liquid obtained by dissolving or dispersing a resin in a solvent. The solvent can also be said to be a dispersion medium for dispersing the resin. Examples of the resin include the above-mentioned resins. 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.

[0051] Examples of the method for forming the heat-resistant layer include a method of directly applying the coating liquid to the surface of the base material and then removing the solvent. Examples of the method for applying the coating liquid to the base material include a bar coater method. Here, the thickness L of the coating bar in the conveyance direction of the base material 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 base material 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 base material is preferably 40 to 60 μm. By controlling the thickness L, the angle θ, and the clearance within the above ranges, the above-mentioned separator can be preferably obtained. This is presumably because by controlling as described above, the resin having polarity contained in the coating liquid can be preferably impregnated into the base material.​

[0052] The solvent preferably has no adverse effect on the base material, 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.

[0053] The coating liquid may contain a filler. The coating liquid may appropriately contain, as components other than the resin and the filler, a dispersant, a plasticizer, a surfactant, a pH adjuster, and the like.

[0054] When the coating liquid contains an aramid resin, the aramid resin can be precipitated by applying moisture to the coating surface. Thus, 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 with a spray or the like, and spraying steam with a nozzle or the like.

[0055] 〔3. Member for Non-aqueous Electrolyte Secondary Battery, Non-aqueous Electrolyte Secondary Battery〕 In a member for a non-aqueous electrolyte secondary battery according to an 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 an 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 a thin plate (paper) type, a disk type, a cylindrical type, a prismatic type such as a rectangular parallelepiped, or the like.

[0056] 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 be present 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 becomes the casing 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.

[0057] <Positive electrode> The positive electrode is not particularly limited as long as it is generally used as the 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.

[0058] Examples of the positive electrode active material include materials capable of doping and dedoping metal ions such as lithium ions or sodium ions. Specific examples of such materials include lithium composite oxides containing at least one kind of transition metal such as V, Mn, Fe, Co, and Ni.

[0059] Examples of the conductive agent include carbonaceous materials such as natural graphite, artificial graphite, cokes, carbon black, pyrolytic carbons, carbon fibers, and fired products of organic polymer compounds. The conductive agent may be used alone or in combination of two or more kinds.

[0060] Examples of the binder include fluorine-based resins such as polyvinylidene fluoride (PVDF), acrylic resins, and styrene-butadiene rubbers. Note that the binder also has a function as a thickening agent.

[0061] 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.

[0062] 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 positive electrode active material, a conductive agent, and a binder into a paste using an appropriate organic solvent, then coating the paste on the positive electrode current collector, drying it, and then pressing it to adhere it to the positive electrode current collector; and the like.

[0063] <Negative electrode> The negative electrode is not particularly limited as long as it is generally used as the 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. The active material layer may further contain a conductive agent.

[0064] Examples of the negative electrode active material include materials capable of doping and dedoping 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.

[0065] Examples of the negative electrode current collector include Cu, Ni, and stainless steel. Cu is more preferable because it is difficult to form an alloy with lithium and is easy to process into a thin film.

[0066] 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, then coating the paste on the negative electrode current collector, drying it, and then pressing it to adhere it to the negative electrode current collector; and the like. The paste preferably contains the above-mentioned conductive agent and the binder.

[0067] <Non-aqueous electrolyte> The non-aqueous electrolyte is not particularly limited as long as it is generally used as the non-aqueous electrolyte of a non-aqueous electrolyte secondary battery. 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, and LiAlCl 4 and the like. Only one kind of the lithium salt may be used, or two or more kinds may be used in combination.

[0068] 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. Only one kind of the organic solvent may be used, or two or more kinds may be used in combination.

[0069] 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.

[0070] 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 a heat-resistant layer laminated on the polyolefin porous substrate, and having a solution retention rate of 50% or more 300 seconds after holding a solution obtained by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 3:5:2 under the conditions of a temperature of 25 ° C and a relative humidity of 60 to 70%. <2> The separator for a non-aqueous electrolyte secondary battery according to <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. <3> The separator for a non-aqueous electrolyte secondary battery according to <2>, wherein the polyamide resin is an aramid resin. <4> The separator for a non-aqueous electrolyte secondary battery according to any one of <1> to <3>, further comprising an adhesive layer separately from the polyolefin porous substrate and the heat-resistant layer. <5>A member for a non-aqueous electrolyte secondary battery, in which a positive electrode, a separator for a non-aqueous electrolyte secondary battery according to any one of <1> to <4>, and a negative electrode are arranged in this order. <6>A non-aqueous electrolyte secondary battery including the separator for a non-aqueous electrolyte secondary battery according to any one of <1> to <4>.

Example

[0071] One embodiment of the present invention will be described below. In this specification, the transport 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.

[0072] 〔Evaluation method〕 [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.

[0073] [Measurement of solution retention rate] Regarding the liquid retention property of the separators manufactured in the examples and comparative examples, evaluation was performed by the following method. (1) A separator cut out to 80 mm × 80 mm was placed on a precision balance with a 100 mm × 100 mm aluminum foil placed thereon, and the zero point was adjusted. (2) A solution obtained by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 3:5:2, which mimics the electrolytic solution, was prepared. In an atmosphere at a temperature of 25°C and a relative humidity of 60 to 70%, the solution was dropped onto the center of the separator on the precision balance from a height of 5 mm with a micropipette in an amount of 16 μL. (3) The point at which the weight of the solution reached 15.0 mg was set as the 0-second time point, and the weight of the solution 300 seconds later was measured. (4) The weight of the solution 300 seconds later was divided by the weight of the solution at the 0-second time point, which was 15.0 mg, and the solution retention rate was calculated by expressing it as a percentage.

[0074] [Difference in capacity retention rate before and after pressurization] First, test non-aqueous electrolyte secondary batteries incorporating the separators described in the examples and comparative examples were fabricated according to the following procedure. 1. A positive electrode was prepared. The positive electrode had a thickness of 49.9 μm, a density of 2.97 g / cm 3 , and a void volume of 24.4 μL. The composition of the positive electrode active material layer was, by weight ratio, LiNi 0.78 Co 0.19 Al 0.03 O 2 : conductive agent: polyvinylidene fluoride = 92:4:4. 2. A negative electrode was prepared. The negative electrode had a thickness of 71.2 μm, a density of 1.45 g / cm 3 , and a void volume of 41.3 μL. The composition of the negative electrode active material layer was, by weight ratio, artificial graphite: styrene-butadiene rubber: carboxymethyl cellulose = 96.5:2.0:1.5. 3. The negative electrode, separator, and positive electrode were laminated in that order to fabricate a member for a non-aqueous electrolyte secondary battery. When a heat-resistant layer was laminated on one side of the separator, the lamination was performed such that the negative electrode active material layer faced the base material side of the separator. Also, the lamination was performed such that the positive electrode active material layer faced the heat-resistant layer side of the separator. 4. The member for a non-aqueous electrolyte secondary battery was stored in a bag formed by laminating an aluminum layer and a heat-sealing layer, and a non-aqueous electrolyte was injected. The injection amount of the non-aqueous electrolyte was 2.8 times the total void volume of the electrodes and the separator. The composition of the non-aqueous electrolyte was vinylene carbonate dissolved in a mixed solvent of ethylene carbonate: ethyl methyl carbonate: diethyl carbonate = 3:5:2 (volume ratio) at a concentration of 1 wt%, and LiPF 6 dissolved at a concentration of 1 mol / L. 5. While reducing the pressure inside the bag, the bag was heat-sealed. Thereby, a test non-aqueous electrolyte secondary battery was fabricated.

[0075] Next, the difference in the capacity retention rate before and after pressurization was measured according to the following procedure. 1. The first charge and discharge cycle was carried out under the conditions of temperature: 25 °C, voltage range: 2.7 - 4.2 V, current value: 0.1C (charging), 0.2C (discharging). Here, 1C is the current value for discharging the rated capacity based on the discharge capacity at the 1-hour rate in 1 hour. The discharge capacity (mAh) at this time was taken as the 0.2C capacity before pressurization. 2. The non-aqueous electrolyte secondary battery was aged by performing charge and discharge for 10 cycles at temperature: 25 °C, voltage range: 2.7 - 4.2 V, current value: 1C (charging), 5C (discharging). 3. Charge and discharge for 1 cycle was carried out under the conditions of temperature: 25 °C, voltage range: 2.5 - 4.2 V, current value: 1C (charging), 7C (discharging). The discharge capacity (mAh) at this time was taken as the 7C capacity before pressurization. 4. Charging was carried out up to 4.2 V under the conditions of temperature: 25 °C, current value: 1C. Then, the non-aqueous electrolyte secondary battery was sandwiched between two SUS plates, and a pressure of 8 MPa was applied with a press. Furthermore, the four corners of the SUS plate were fixed while the pressure was applied, and then the pressure of the press was released. 5. The first charge and discharge cycle was carried out under the conditions of temperature: 25 °C, voltage range: 2.7 - 4.2 V, current value: 0.1C (charging), 0.2C (discharging). The discharge capacity (mAh) at this time was taken as the 0.2C capacity after pressurization. 6. Charge and discharge for 1 cycle was carried out under the conditions of temperature: 25 °C, voltage range: 2.5 - 4.2 V, current value: 1C (charging), 7C (discharging). The discharge capacity (mAh) at this time was taken as the 7C capacity after pressurization. 7. The difference in the capacity retention rate before and after pressurization was calculated by the following formula. Difference in capacity retention rate before and after pressurization = 100 × 7C capacity before pressurization / 0.2C capacity before pressurization - 100 × 7C capacity after pressurization / 0.2C capacity after pressurization.

[0076] 〔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".

[0077] As a container for synthesis, a 3 L separable flask equipped with 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.

[0078] 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 at 20 °C ± 2 °C to obtain an aramid polymerization solution (1) containing 6 wt% of PPTA. The intrinsic viscosity of PPTA contained in the aramid polymerization solution (1) was 1.7 g / dL.

[0079] 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 wt%, 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).

[0080] [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 conveying 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 conveying 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 opposite to the coated surface of the base material to obtain a wound body (1) of a laminated separator having heat-resistant layers formed on both sides of the base material. The wound body (1) of the laminated separator was used as the separator (1).

[0081] 〔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).

[0082] 〔Comparative Example 1〕 The same operations as in Example 1 were performed except that a coating bar with a thickness L of the coating bar in the MD direction of 17.0 mm was used, the angle θ formed between the surface of the base material and the bottom surface (edge part) of the coating bar was set to 3.0°, and the clearance between the coating bar and the base material was changed to 57 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).

[0083] 〔Comparative Example 2〕 The same operations as in Comparative Example 1 were performed except that the clearance between the coating bar and the base material was changed to 50 μm and the heat-resistant layer was formed on only one side, and a laminated separator wound body (4) having a heat-resistant layer formed on one side of the base material was obtained. The laminated separator wound body (4) was used as the separator (4).

[0084] 〔Evaluation Results〕 Table 1 shows the evaluation results and the like of the examples and comparative examples.

[0085]

Table 1

[0086] In Table 1, “aramid impregnation” means that PPTA contained in the heat-resistant layer is impregnated into the base material.

[0087] In Examples 1 and 2 where the solution retention rate after 300 seconds was 50% or more, the difference in the capacity retention rate before and after pressurization was smaller than that in Comparative Examples 1 and 2 where the solution retention rate after 300 seconds was less than 50%. That is, in Examples 1 and 2, a battery excellent in capacity retention characteristics after pressurization could be obtained.

[0088] In addition, the solution retention rate after 300 seconds 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 base material and the bottom surface (edge portion) of the coating bar, the clearance between the coating bar and the base material, and the like.

Industrial Applicability

[0089] 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 a heat-resistant layer laminated on the polyolefin porous substrate, and having a solution retention rate of 50% or more 300 seconds after holding a solution obtained by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 3:5:2 under the conditions of a temperature of 25°C and a relative humidity of 60 to 70%.

2. 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.

3. The separator for a non-aqueous electrolyte secondary battery according to Claim 2, wherein the polyamide resin is an aramid resin.

4. 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.

5. 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 4, and a negative electrode are arranged in this order.

6. A non-aqueous electrolyte secondary battery comprising the separator for a non-aqueous electrolyte secondary battery according to any one of Claims 1 to 4.

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