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

Abstract

To provide a separator for a non-aqueous electrolyte secondary battery which is excellent in withstand voltage property when charge / discharge cycles are repeated.SOLUTION: A separator for a non-aqueous electrolyte secondary battery contains a polyolefin porous film, and a heat-resistant porous layer which is stacked on one surface or a both surfaces of the polyolefin porous film and contains a heat-resistant resin, where an absolute value of a thickness change rate before and after a heat shock cycle test is 1.4% or less.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Non-aqueous electrolyte secondary batteries, especially lithium-ion secondary batteries, have a high energy density and are widely used as batteries in personal computers, mobile phones, and portable information terminals. More recently, development has been progressing on them for use in automobiles.

[0003] As an example of a method for evaluating the storage capabilities of such non-aqueous electrolyte secondary batteries, as described in Patent Document 1, a method is known in which a heat shock cycle test is performed, in which the non-aqueous electrolyte secondary battery is stored in a high-temperature environment for a certain period of time, and then the non-aqueous electrolyte secondary battery is stored in a low-temperature environment, and this cycle is repeated a certain number of times, after which the presence or absence of leakage of the non-aqueous electrolyte secondary battery is checked.

[0004] Furthermore, as an example of a separator in such a non-aqueous electrolyte secondary battery, a battery separator is known that is a laminated porous membrane in which a porous layer consisting of an organic filler and a binder resin is provided on at least one side of a polyolefin porous substrate. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2020 / 202744 Brochure [Overview of the project] [Problems that the invention aims to solve]

[0006] However, as shown in Patent Document 1, the heat shock cycle test is a test used to evaluate the performance of a non-aqueous electrolyte secondary battery, and not a test for evaluating the characteristics of a separator.

[0007] Furthermore, conventional battery separators, which are laminated porous films, had room for improvement in their voltage resistance when subjected to repeated charge-discharge cycles. [Means for solving the problem]

[0008] The inventors of the present invention discovered that by performing a heat shock cycle test, which is not normally used to evaluate the characteristics of separators, on the separator alone, they could obtain a novel parameter called the "absolute value of the rate of change in thickness" that correlates with the voltage withstand capability when the separator undergoes repeated charge-discharge cycles, and thus conceived the present invention.

[0009] One aspect of the present invention includes the inventions described in [1] to [6] below. [1] A separator for a non-aqueous electrolyte secondary battery comprising a polyolefin porous film and a heat-resistant porous layer laminated on one or both sides of the polyolefin porous film, The heat-resistant porous layer contains a heat-resistant resin, A separator for non-aqueous electrolyte secondary batteries, wherein the absolute value of the rate of change in thickness before and after a heat shock cycle test, as expressed by the following formula (1), is 1.40% or less. Thickness change rate (%) = {(D0-D1) / D0} × 100 (1) (Here, the heat shock cycle test is performed under the following conditions: high temperature: 85°C, low temperature: -40°C, holding time between high and low temperatures: 30 minutes, temperature transition time: 1 minute, number of cycles: 150 cycles.) In formula (1) above, D0 is the thickness (μm) of the separator for the non-aqueous electrolyte secondary battery before the heat shock cycle test, and D1 is the thickness (μm) of the separator for the non-aqueous electrolyte secondary battery after the heat shock cycle test. [2] The separator for a non-aqueous electrolyte secondary battery according to [1], wherein the heat-resistant porous layer is laminated on both surfaces of the polyolefin porous film. [3] The separator for a non-aqueous electrolyte secondary battery according to [1] or [2], wherein the heat-resistant resin is a nitrogen-containing aromatic resin. [4] The separator for a non-aqueous electrolyte secondary battery according to [3], wherein the nitrogen-containing aromatic resin is an aramid resin. [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 [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]. [Effect of the Invention]

[0010] The separator for a non-aqueous electrolyte secondary battery according to an embodiment of the present invention has an effect of being excellent in withstand voltage characteristics when repeating charge and discharge cycles. [Mode for Carrying Out the Invention]

[0011] An embodiment of the present invention will be described below, but the present invention is not limited thereto. The present invention is not limited to each configuration described below, and various modifications are possible within the scope shown in the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments are also included in the technical scope of the present invention. Unless otherwise specified in this specification, "A to B" representing a numerical range means "A or more and B or less".

[0012] [Embodiment 1: Separator for Non-Aqueous Electrolyte Secondary Battery] The separator for a non-aqueous electrolyte secondary battery according to Embodiment 1 of the present invention includes a polyolefin porous film and a heat-resistant porous layer laminated on one or both surfaces of the polyolefin porous film, and the heat-resistant porous layer contains a heat-resistant resin, and the absolute value of the thickness change rate before and after the heat shock cycle test represented by the following formula (1) is 1.40% or less. Thickness change rate (%) = {(D0 - D1) / D0} × 100 (1) Here, the heat shock cycle test is carried out under the conditions of high temperature: 85°C, low temperature: -40°C, holding time of high temperature and low temperature: 30 minutes, temperature transition time: 1 minute, and number of cycles: 150 cycles. In the formula (1), D0 is the thickness (μm) of the separator for the non-aqueous electrolyte secondary battery before the heat shock cycle test, and D1 is the thickness (μm) of the separator for the non-aqueous electrolyte secondary battery after the heat shock cycle test.

[0013] The separator for a non-aqueous electrolyte secondary battery according to an embodiment of the present invention includes a polyolefin porous film and a heat-resistant porous layer laminated on one or both surfaces of the polyolefin porous film. Hereinafter, the separator for the non-aqueous electrolyte secondary battery is also simply referred to as "separator", the polyolefin porous film is also simply referred to as "porous film", and the "heat-resistant porous layer" is also simply referred to as "porous layer".

[0014] A separator according to one embodiment of the present invention has an absolute value of thickness change rate of 1.4% or less after a heat shock cycle test performed under the conditions of high temperature: 85°C, low temperature: -40°C, holding time for high and low temperatures: 30 minutes, and number of cycles: 150 cycles. Hereinafter, "thickness change rate before and after the heat shock cycle test" will also be simply referred to as "thickness change rate". The absolute value of the thickness change rate depends on the degree of structural change of the separator before and after the heat shock cycle test. The heat-resistant porous layer in the separator has high heat resistance and undergoes less structural change when subjected to repeated temperature changes compared to a porous film. Therefore, the absolute value of the thickness change rate mainly depends on the degree of structural change of the porous film constituting the separator before and after the heat shock cycle test.

[0015] Furthermore, in the separator according to one embodiment of the present invention, a heat-resistant porous layer containing a heat-resistant resin is laminated on one or both sides of a porous film. The heat-resistant resin penetrates into the surface of the porous film on which the heat-resistant porous layer is laminated. As a result, the orientation of the polyolefin crystals near the interface between the porous film and the heat-resistant porous layer is fixed by the penetrating heat-resistant resin. In addition, the structure of the porous film near the interface between the porous film and the heat-resistant porous layer is densified by the penetrating heat-resistant resin. Due to the fixing of the orientation and the densification of the crystal structure, the change in structure when the porous film is subjected to repeated temperature changes is reduced. Therefore, the absolute value of the thickness change rate is a parameter for evaluating the degree of fixing of the orientation and the degree of densification of the crystal structure.

[0016] In a non-aqueous electrolyte secondary battery, the positive and negative electrodes expand and contract during repeated charging and discharging. Furthermore, in a non-aqueous electrolyte secondary battery, the separator is located between the positive and negative electrodes. Therefore, in a conventional non-aqueous electrolyte secondary battery comprising a porous film and a separator including a heat-resistant porous layer laminated on the porous film, the expansion and contraction of the electrodes can cause an external force to be applied to the separator, potentially causing a portion of the heat-resistant porous layer to peel off from the separator, resulting in a decrease in the dielectric strength of the separator.

[0017] On the other hand, the separator according to one embodiment of the present invention has a small absolute value of 1.40% or less for the rate of change in thickness. Therefore, in the separator according to one embodiment of the present invention, the orientation of the polyolefin crystals is suitably fixed near the interface between the porous film and the heat-resistant porous layer, and the structure near the interface between the porous film and the heat-resistant porous layer is suitably densified. Consequently, in the separator according to one embodiment of the present invention, the heat-resistant resin is suitably permeated, and the dielectric strength of the porous film itself is suitably improved. As a result, the separator according to one embodiment of the present invention can maintain sufficient dielectric strength even if a part of the heat-resistant porous layer peels off. Therefore, the separator according to one embodiment of the present invention has the effect of having excellent dielectric strength when repeated charge-discharge cycles are performed.

[0018] The absolute value of the thickness change rate is preferably small from the viewpoint of voltage resistance when the aforementioned charge-discharge cycle is repeated. Specifically, the absolute value of the thickness change rate is preferably 1.38% or less, and more preferably 1.36% or less. Furthermore, the absolute value of the thickness change rate is preferably 0.10% or more, and more preferably 0.15% or more.

[0019] In one embodiment of the present invention, the heat shock cycle test is not particularly limited as long as it can achieve the following conditions: high temperature: 85°C, low temperature: -40°C, holding time between high and low temperatures: 30 minutes, temperature transition time: 1 minute, and number of cycles: 150 cycles. For example, it can be carried out using a commercially available thermal shock testing apparatus. A specific example of the heat shock cycle test is the heat shock cycle test described in the examples. Furthermore, it is preferable to carry out the heat shock cycle test without introducing outside air and under conditions that do not cause condensation.

[0020] In detail, the heat shock test is carried out by a method consisting of the following steps (1) to (6). (1) Place the separator inside the test apparatus, seal the test apparatus, and then heat the inside of the test apparatus so that the temperature inside the apparatus reaches 85°C. (2) Leave the test apparatus for 30 minutes while maintaining the internal temperature at 85°C. (3) After step (2), the inside of the test apparatus is cooled for 1 minute so that the temperature inside the test apparatus becomes -40°C. (4) Leave the test apparatus in place for 30 minutes while maintaining the internal temperature at -40°C. (5) After step (4), the inside of the test apparatus is heated for 1 minute so that the temperature inside the test apparatus reaches 85°C. (6) Repeat steps (2) to (5) until the total number of repetitions (cycles) is 150.

[0021] The heating and cooling methods in steps (1), (3), and (5) are not particularly limited, and methods that are commonly used by those skilled in the art may be employed.

[0022] <Polyolefin porous film> The porous film contains a polyolefin resin and is generally a porous film whose main component is a polyolefin resin. Furthermore, "primarily composed of a polyolefin resin" means that the proportion of polyolefin resin in the porous film is 50% by weight or more, preferably 90% by weight or more, and more preferably 95% by weight or more, of the total material constituting the porous film.

[0023] The porous film has numerous interconnected pores inside, allowing gases and liquids to pass from one side to the other.

[0024] The thickness of the porous film is preferably 4 to 40 μm, and more preferably 5 to 20 μm. If the thickness of the porous film is 4 μm or more, internal short circuits in the battery can be sufficiently prevented. On the other hand, if the thickness of the porous film is 40 μm or less, the size of the non-aqueous electrolyte secondary battery can be prevented from increasing.

[0025] The aforementioned polyolefin resin has a weight-average molecular weight of 5 × 10 5 ~15×10 6 It is more preferable that the polyolefin resin contains high molecular weight components. In particular, it is more preferable that the polyolefin resin contains high molecular weight components with a weight-average molecular weight of 1 million or more, because this improves the strength of the resulting porous film and the separator containing the porous film.

[0026] The polyolefin resin is not particularly limited, but examples 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 homopolymers include polyethylene, polypropylene, and polybutene. Examples of the copolymers include ethylene-propylene copolymers.

[0027] Of these, polyethylene is more preferred because it can prevent excessive current from flowing through the separator at a lower temperature. This prevention of excessive current flow is also called shutdown. Examples of 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 million or more. Of these, ultra-high molecular weight polyethylene with a weight-average molecular weight of 1 million or more is even more preferred.

[0028] The basis weight per unit area of ​​the porous film can be appropriately determined considering strength, film thickness, weight, and handling. However, in order to increase the gravimetric energy density and volumetric energy density of the non-aqueous electrolyte secondary battery, the basis weight should be 4 to 20 g / m². 2 Preferably, it is 4-12 g / m 2 It is more preferable that the amount be 5-10 g / m 2 It is even more preferable that this be the case.

[0029] The air permeability of the porous film is preferably 30 to 500 sec / 100 mL in Gaule values, and more preferably 50 to 300 sec / 100 mL. By having the above air permeability of the porous film, sufficient ion permeability can be obtained.

[0030] The porosity of the porous film is preferably 20 to 80 volume%, and more preferably 30 to 75 volume%, in order to increase the amount of electrolyte it can hold and to reliably prevent excessive current from flowing at lower temperatures. Furthermore, the pore size of the pores in the porous film is preferably 0.3 μm or less, and more preferably 0.14 μm or less, in order to obtain sufficient ion permeability and to prevent particles from entering the positive and negative electrodes.

[0031] <Heat-resistant porous layer> In one embodiment of the present invention, the heat-resistant porous layer is laminated on one or both sides of the polyolefin porous film, and preferably laminated on both sides of the polyolefin porous film. The heat-resistant porous layer contains a heat-resistant resin. The heat-resistant porous layer is preferably an insulating porous layer.

[0032] When a porous layer is laminated on one side of a polyolefin porous film, the porous layer is preferably laminated on the side of the polyolefin porous film facing the positive electrode. More preferably, the porous layer is laminated on the side in contact with the positive electrode.

[0033] Examples of heat-resistant resins that constitute the porous layer include polyolefins; (meth)acrylate resins; fluororesins; polyamide resins; polyimide resins; polyester resins; rubbers; resins with a melting point or glass transition temperature of 180°C or higher; and water-soluble polymers.

[0034] Among the heat-resistant resins mentioned above, polyolefins, polyester resins, acrylate resins, fluororesins, polyamide resins, and water-soluble polymers are preferred. As polyamide resins, fully aromatic polyamides (aramid resins) are preferred. As polyester resins, polyarylates and liquid crystal polyesters are preferred. As fluororesins, polyvinylidene fluoride resins are preferred.

[0035] Furthermore, the heat-resistant resin is preferably a nitrogen-containing aromatic resin. As the nitrogen-containing aromatic resin, aramid resins are more preferably. Examples of the aramid resins include para-aramid and meta-aramid, but para-aramid is even more preferred. Examples of para-aramids include para(paraphenylene terephthalamide), poly(parabenzamide), poly(4,4'-benzanilide terephthalamide), poly(paraphenylene-4,4'-biphenylenedicarboxylic acid amide), poly(paraphenylene-2,6-naphthalenedicarboxylic acid amide), poly(2-chloro-paraphenylene terephthalamide), paraphenylene terephthalamide / 2,6-dichloroparaphenylene terephthalamide copolymer, poly(4,4'-diphenylsulfonyl terephthalamide), paraphenylene terephthalamide / 4,4'-diphenylsulfonyl terephthalamide copolymer, and other para-oriented or para-oriented para-aramids.

[0036] The porous layer may contain a filler. The filler may be an inorganic filler or an organic filler. More preferably, the filler is an inorganic filler consisting of an inorganic oxide such as silica, calcium oxide, magnesium oxide, titanium oxide, alumina, mica, zeolite, aluminum hydroxide, or boehmite. In the porous layer, the filler content may be 10 to 99% by weight or 20 to 75% by weight relative to the total amount of the resin and filler mentioned above.

[0037] In particular, when the heat-resistant resin constituting the porous layer is aramid resin, by setting the filler content within the above-mentioned range of 20-75% by weight, it is possible to suppress the increase in the weight of the separator due to the filler and obtain a separator with good ion permeability.

[0038] In this embodiment, the porous layer is preferably placed between the polyolefin porous film and the positive electrode active material layer of the positive electrode. In the following description of the physical properties of the porous layer, at least the physical properties of the porous layer placed between the polyolefin porous film and the positive electrode active material layer of the positive electrode are referred to when it is used as a non-aqueous electrolyte secondary battery.

[0039] The average thickness of the porous layer is preferably in the range of 0.5 μm to 10 μm per layer, and more preferably in the range of 1 μm to 5 μm, from the viewpoint of ensuring adhesion to the electrodes and high energy density. If the thickness of the porous layer is 0.5 μm or more per layer, internal short circuits due to damage to the non-aqueous electrolyte secondary battery can be sufficiently suppressed, and the amount of electrolyte held in the porous layer can be sufficient. On the other hand, if the thickness of the porous layer exceeds 10 μm per layer, the lithium ion permeability resistance increases in the non-aqueous electrolyte secondary battery, so the positive electrode may deteriorate with repeated cycles. Therefore, the rate characteristics and cycle characteristics of the non-aqueous electrolyte secondary battery may deteriorate. In addition, the distance between the positive and negative electrodes increases, so the internal volume efficiency of the non-aqueous electrolyte secondary battery may decrease.

[0040] The basis weight per unit area of ​​a porous layer can be appropriately determined considering the strength, film thickness, weight, and handling properties of the porous layer. The basis weight per unit area of ​​a porous layer is 0.5 to 20 g / m² per layer. 2 Preferably, it is 0.5 to 10 g / m 2 It is more preferable that the basis weight per unit area of ​​the porous layer is within these numerical ranges, thereby increasing the gravimetric energy density and volumetric energy density of the non-aqueous electrolyte secondary battery. If the basis weight of the porous layer exceeds the above range, the non-aqueous electrolyte secondary battery tends to become heavier.

[0041] The porosity of the porous 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 comprised in the porous 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.

[0042] The air permeability of the laminated separator obtained by laminating the porous layer on the porous film is preferably 30 to 1000 sec / 100 mL in Gurley value, more preferably 50 to 800 sec / 100 mL. By having said air permeability, the laminated separator can obtain sufficient ion permeability in the non-aqueous electrolyte secondary battery.

[0043] <Physical properties of separator for non-aqueous electrolyte secondary battery> The film thickness of the separator according to one embodiment of the present invention is preferably 5.5 μm to 45 μm, more preferably 6 μm to 25 μm.

[0044] The air permeability of said separator is preferably 100 to 350 sec / 100 mL in Gurley value, more preferably 100 to 300 sec / 100 mL.

[0045] The basis weight of said separator is preferably 3.0 to 13.0 g / m 2 and more preferably 5.0 to 9.0 g / m 2 By setting the basis weight per unit area of said separator to these numerical ranges, the weight energy density and the volume energy density of the non-aqueous electrolyte secondary battery can be increased.

[0046] Incidentally, the separator according to one embodiment of the present invention may contain, as necessary, another porous layer other than said porous film and said porous layer, as long as the object of the present invention is not impaired. Examples of said another porous layer include known porous layers such as a heat-resistant layer, an adhesive layer, and a protective layer.

[0047] <Method for manufacturing a separator for non-aqueous electrolyte secondary batteries> (Method for manufacturing polyolefin porous film) The method for producing a porous film is not particularly limited. For example, a sheet-like polyolefin resin composition can be produced by kneading a polyolefin resin with a pore-forming agent such as an inorganic filler or plasticizer, and optionally an antioxidant, and then extruding the mixture. The pore-forming agent can then be removed from the sheet-like polyolefin resin composition using a suitable solvent. Subsequently, a polyolefin porous film can be produced by stretching the polyolefin resin composition from which the pore-forming agent has been removed.

[0048] The inorganic filler mentioned above is not particularly limited and includes inorganic fillers, specifically calcium carbonate, etc. The plasticizer mentioned above is not particularly limited and includes low molecular weight hydrocarbons such as liquid paraffin.

[0049] Specifically, a method for manufacturing a porous film can be described as a method that includes the following steps. (A) A step of kneading ultra-high molecular weight polyethylene, low molecular weight polyethylene with a weight-average molecular weight of 10,000 or less, a pore-forming agent such as calcium carbonate or a plasticizer, and an antioxidant to obtain a polyolefin resin composition. (B) A step of forming a sheet by rolling the obtained polyolefin resin composition with a pair of rolling rollers and cooling it in stages while pulling it with winding rollers with a different speed ratio. (C) A step of removing the pore-forming agent from the obtained sheet using a suitable solvent. (D) A step of stretching the sheet from which the pore-forming agent has been removed at an appropriate stretching ratio.

[0050] (Method for manufacturing porous layers and laminated separators) Examples of methods for manufacturing a porous layer in one embodiment of the present invention and a laminated separator according to one embodiment of the present invention include a method of forming a porous layer on the porous film by applying a coating solution containing the resin contained in the porous layer to one or both sides of the porous film to form a coating layer, and removing the solvent by drying, and a method of forming the porous layer on the porous film by applying a coating solution containing the resin contained in the porous layer to one or both sides of the porous film, depositing the resin contained in the porous layer on the porous film under specific temperature and specific relative humidity conditions to form a coating layer, and then removing the solvent by drying.

[0051] When forming porous layers on both sides of the porous film, (a) the porous layers may be formed simultaneously on both sides of the porous film, or (b) the coating liquid may be applied to one side of the porous film to form a porous layer on that side, and then the coating liquid may be applied to the other side of the porous film to form a porous layer on the other side.

[0052] Furthermore, before applying the coating solution to one or both sides of the porous film, the one or both sides of the polyolefin porous film to be coated with the coating solution can be treated to make it hydrophilic as needed.

[0053] The coating liquid contains a heat-resistant resin contained in the porous layer. The coating liquid may also contain fine particles, which may be contained in the porous layer, as described below. The coating liquid can usually be prepared by dissolving the heat-resistant resin contained in the porous layer in a solvent and dispersing the fine particles. Here, the solvent used to dissolve the heat-resistant resin also serves as a dispersion medium for dispersing the fine particles. Alternatively, the heat-resistant resin may be formed into an emulsion using the solvent.

[0054] The solvent is not particularly limited, as long as it does not adversely affect the polyolefin porous film, uniformly and stably dissolves the heat-resistant resin, and uniformly and stably disperses the fine particles. Specific examples of the solvent include water and organic solvents. The solvent may be used alone or in combination of two or more types.

[0055] The coating solution can be formed by any method as long as it satisfies the conditions such as the resin solid content (resin concentration) and the amount of fine particles necessary to obtain the desired porous layer. Specific methods for forming the coating solution include, for example, mechanical stirring, ultrasonic dispersion, high-pressure dispersion, and media dispersion. Furthermore, the coating solution may contain additives other than the heat-resistant resin and fine particles, such as dispersants, plasticizers, surfactants, and pH adjusters, as long as they do not impair the objectives of the present invention. The amount of additives added should not impair the objectives of the present invention.

[0056] The method of applying the coating solution to a porous film, that is, the method of forming a porous layer on the surface of a porous film, is not particularly limited. Examples of methods for forming a porous layer include: applying the coating solution directly to the surface of a porous film to form a coating layer, and then removing the solvent; applying the coating solution to a suitable support to form a coating layer, removing the solvent to form a porous layer, then pressing this porous layer and the porous film together, and then peeling off the support; and applying the coating solution to a suitable support to form a coating layer, pressing a porous film onto the coating layer, then peeling off the support and then removing the solvent.

[0057] Conventional methods known as the application method for the coating liquid can be employed, specifically, for example, the gravure coater method, the dip coater method, the bar coater method, and the die coater method.

[0058] The most common method for removing the solvent is drying. Alternatively, the solvent contained in the coating solution may be replaced with another solvent before drying.

[0059] (Method for controlling the absolute value of the rate of change in thickness before and after the heat shock cycle test) In one embodiment of the present invention, as a method for controlling the "absolute value of the rate of change in thickness before and after the heat shock cycle test" to a suitable range, for example, a method for suitably controlling the degree of penetration of the heat-resistant resin constituting the heat-resistant porous layer into the porous film can be given by the method shown in (a) and / or (b) below. (a) A method of applying the coating liquid onto the porous film to form a coating layer, drying it to remove the solvent contained in the coating layer, and controlling the tension applied to the porous film to a suitable range before forming the porous layer. (b) A method of applying the coating liquid onto the porous film to form a coating layer, removing the solvent, and then heat-treating (annealing) at a suitable temperature.

[0060] The process of applying the coating liquid onto the porous film to form a coating layer, then drying it to remove the solvent from the coating layer and form the porous layer is usually carried out while applying a specific tension to the porous film. Method (a) is, in detail, a method in which the tension applied to the porous film before drying the coating layer is smaller than the tension applied to the porous film before drying the coating layer in conventional methods for forming porous layers. By reducing the tension, gaps (space) are created in the structure near the surface of the porous film where the coating layer is formed, allowing the heat-resistant resin contained in the coating layer to penetrate. Therefore, method (a) makes it easier for the heat-resistant resin to penetrate the porous film. Consequently, the orientation of the polyolefin crystals can be more fixed and the structure can be made denser near both sides of the porous film. Therefore, the absolute value of the thickness change rate can be reduced and controlled to a suitable range of 1.40% or less. From the viewpoint of reducing the absolute value of the thickness change rate and controlling it to a suitable range, the tension applied to the porous film before drying the coating layer is preferably 0.120 N / mm or less, and more preferably 0.110 N / mm or less. Specifically, when forming the porous layer, it is preferable to control the tension described in the "Water Wash MID" column in Table 2 of the later-described examples to the tension applied to the porous film before drying the coating layer, within the aforementioned preferred range. It is even more preferable to control both the tension described in the "Water Wash MID" column and the tension described in the "Water Wash OUT" column in Table 2 of the later-described examples to the tension applied to the porous film before drying the coating layer, within the aforementioned preferred range.

[0061] Furthermore, from the viewpoint of suppressing the occurrence of wrinkles and the like in the porous film, the tension applied to the porous film before drying the coating layer is preferably 0.080 N / mm or more, and more preferably 0.090 N / mm or more.

[0062] In method (b), the heat treatment is a step of heating the coating layer separately from the drying treatment, after a drying treatment to remove the solvent contained in the coating layer. By performing the heat treatment, the crystal orientation and structural density near the interface between the formed porous layer and the porous film can be suitably controlled. Therefore, the absolute value of the thickness change rate can be reduced and controlled to a suitable range of 1.40% or less.

[0063] In addition, by employing the method shown in (a) and / or the method shown in (b), for example, by laminating the porous layer on both sides of the porous film, the absolute value of the thickness change rate can be made smaller and controlled within a more suitable range. Specifically, by laminating the porous layer on both sides of the porous film, the heat-resistant resin penetrates from both sides of the porous film. Therefore, the orientation of the polyolefin crystals can be more fixed and its structure more densely packed near both sides of the porous film. Hence, by employing the method shown in (a) and / or the method shown in (b), and laminating the porous layer on both sides of the porous film, the absolute value of the thickness change rate can be controlled within a more suitable range.

[0064] [2. Non-aqueous electrolyte secondary battery parts, non-aqueous electrolyte secondary battery] A component for a non-aqueous electrolyte secondary battery according to one embodiment of the present invention is a component for a non-aqueous electrolyte secondary battery in which a positive electrode, the above-mentioned separator or stacked separator, and a negative electrode are arranged in this order. Furthermore, a non-aqueous electrolyte secondary battery according to one embodiment of the present invention includes the above-mentioned separator or stacked separator.

[0065] The component for a non-aqueous electrolyte secondary battery according to one embodiment of the present invention has the effect of improving the reliability of the battery, such as the storage stability of the non-aqueous electrolyte secondary battery, by providing a separator with excellent voltage resistance when repeated charge-discharge cycles are performed, thereby preventing self-discharge and localized material degradation. The non-aqueous electrolyte secondary battery according to one embodiment of the present invention has the effect of providing excellent battery reliability by providing a separator with excellent voltage resistance when repeated charge-discharge cycles are performed.

[0066] A conventionally known manufacturing method can be used as the manufacturing method for a non-aqueous electrolyte secondary battery according to one embodiment of the present invention. For example, a component for a non-aqueous electrolyte secondary battery is formed by arranging a positive electrode, a polyolefin porous film, and a negative electrode in this order. Here, the porous layer may exist between the polyolefin porous film and at least one of the positive electrode and the negative electrode. Next, the component for the non-aqueous electrolyte secondary battery is placed in a container that will serve as the housing for the non-aqueous electrolyte secondary battery. After filling the container with the non-aqueous electrolyte, it is sealed while the pressure is reduced. In this way, a non-aqueous electrolyte secondary battery according to one embodiment of the present invention can be manufactured.

[0067] <Positive electrode> The positive electrode in one embodiment of the present invention is not particularly limited as long as it is generally used as the positive electrode of a non-aqueous electrolyte secondary battery. For example, a positive electrode sheet can be used as the positive electrode, which has 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. The active material layer may further contain a conductive agent.

[0068] Examples of the positive electrode active material include materials that can be doped and dedoped with metal ions such as lithium ions or sodium ions. Specifically, examples of such materials include lithium composite oxides containing at least one transition metal such as V, Mn, Fe, Co, and Ni.

[0069] Examples of the conductive agent include carbonaceous materials such as natural graphite, artificial graphite, coke, carbon black, pyrolytic carbons, carbon fibers, and calcined organic polymer compounds. The conductive agent may be used alone or in combination of two or more types.

[0070] Examples of the aforementioned binders include fluororesins such as polyvinylidene fluoride (PVDF), acrylic resins, and styrene-butadiene rubber. The binder also functions as a thickening agent.

[0071] Examples of the positive electrode current collector include conductors such as Al, Ni, and stainless steel. Among these, Al is more preferred because it is easy to process into a thin film and is inexpensive.

[0072] Examples of methods for manufacturing a positive electrode sheet include a method of pressurizing a positive electrode active material, a conductive agent, and a binder on a positive electrode current collector; a method of making a paste from the positive electrode active material, a conductive agent, and a binder using a suitable organic solvent, applying the paste to the positive electrode current collector, drying it, and then pressing it to fix it to the positive electrode current collector; and so on.

[0073] <Negative electrode> The negative electrode in one embodiment of the present invention is not particularly limited as long as it is one that is generally used as the negative electrode of a non-aqueous electrolyte secondary battery. For example, as the negative electrode, a negative electrode sheet can be used which has 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. The active material layer may further contain a conductive agent.

[0074] Examples of the negative electrode active material include materials that can be doped and dedoped with metal ions such as lithium ions or sodium ions. Examples of such materials include carbonaceous materials. Examples of carbonaceous materials include natural graphite, artificial graphite, coke, carbon black, and pyrolytic carbons.

[0075] Examples of the negative electrode current collector include Cu, Ni, and stainless steel. Cu is more preferred because it is difficult to form alloys with lithium and is easy to process into thin films.

[0076] Examples of methods for manufacturing a negative electrode sheet include a method of pressurizing a negative electrode active material on a negative electrode current collector; a method of making a paste from the negative electrode active material using a suitable organic solvent, coating the paste onto a negative electrode current collector, drying it, and then pressing it to fix it to the negative electrode current collector; and so on. The paste preferably contains the conductive agent and the binder mentioned above.

[0077] <Nonaqueous electrolyte> The non-aqueous electrolyte in one embodiment of the present invention 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 lithium salts include LiClO4, LiPF6, LiAsF6, LiSbF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, and Li2B 10 Cl 10 Examples include lithium salts of lower aliphatic carboxylates and LiAlCl4. The lithium salt may be used alone or in combination of two or more types.

[0078] Examples of organic solvents constituting the non-aqueous electrolyte include carbonates, ethers, esters, nitriles, amides, carbamates, and sulfur-containing compounds, as well as fluorine-containing organic solvents obtained by introducing fluorine groups into these organic solvents. The organic solvent may be used alone or in combination of two or more types. [Examples]

[0079] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0080] [Measurement method] Various measurements in the examples and comparative examples were performed by the following method.

[0081] <Measuring film thickness> In the examples and comparative examples described below, the thickness (film thickness) of the porous film and separator was measured using a high-precision digital length measuring instrument (VL-50) manufactured by Mitutoyo Corporation. In addition, the difference between the thickness of the separator and the thickness of the porous film was calculated and used as the total thickness of the porous layer.

[0082] <Measurement of weight> From the porous films described later in the examples and comparative examples, a square with sides of 8 cm was cut out as a sample, and the weight W1 (g) of the sample was measured. Then, the basis weight of the porous film was calculated according to the following formula (2). Weight basis weight (g / m²) of porous film 2 ) = W1 / (0.08 × 0.08) (2) Similarly, a square with sides of 8 cm was cut from the separator in the examples and comparative examples described later, and its weight W2 (g) was measured. The basis weight of the separator was then calculated according to the following formula (3). Weight of separator (g / m²) 2 ) = W² / (0.08 × 0.08) (3) Furthermore, the difference between the weight basis of the separator and the weight basis of the porous film was calculated to determine the total weight basis of the porous layer.

[0083] <Measurement of air permeability> The air permeability (Gare value) of the separators in the examples and comparative examples described below was measured in accordance with JIS P8117.

[0084] <Measurement of the absolute value of the thickness change retention rate before and after the heat shock cycle test> (Heat shock cycle test) A test sample was prepared by sandwiching the separators described in the examples and comparative examples below between two sheets of paper (ASUKUL, product name Multipaper Super White J, thickness: 0.09 mm), and then attaching the resulting laminate to a glass plate (Nippon Sheet Glass Co., Ltd., product name Ultra Fine Flat Glass (FL110-L4), thickness: 1.1 mm). The test sample was placed inside a thermal shock test apparatus (product name "TSA-71L-A-3" sold by ESPEC Corporation), and a heat shock cycle test (hereinafter also referred to as the "HS test") was performed under the following conditions. ·High temperature: 85℃ • High temperature holding time: 30 minutes ·Low temperature: -40℃ • Low temperature holding time: 30 minutes • Temperature transition time between high and low temperatures: 1 minute • One cycle consists of a high-temperature-low-high temperature change; number of cycles: 150 cycles Furthermore, conditions were set to prevent condensation by not introducing outside air.

[0085] (Measurement of thickness change rate) Using a high-precision digital length measuring instrument manufactured by Mitutoyo Corporation, the film thickness D0 of the separator before the HS test and the film thickness D1 of the separator after the HS test were measured. Using the obtained film thicknesses of the separator before and after the HS test, the rate of change in thickness before and after the HS test was calculated based on the following equation (1), and its absolute value was determined. Thickness change rate (%) = {(D0-D1) / D0} × 100 (1) <Measurement of critical withstand voltage after porous layer delamination> For the examples and comparative examples described below, each porous layer of the separator was peeled off once using tape (3M Scotch Transparent Book Tape (Thick) 845) to prepare the separator as a sample. The limit withstand voltage was measured on the above sample using the IMP3800K impulse insulation tester manufactured by Japan Technart, following the procedure below. (i) The sample to be measured was placed between a cylindrical electrode φ25 mm and a cylindrical electrode φ75 mm in the impulse insulation tester. (ii) By accumulating charge in a capacitor located inside the impulse insulation tester, a voltage was applied to the sample located between the upper and lower electrodes electrically connected to this internal capacitor. The voltage was 0V at the start of the measurement and was then increased linearly, i.e., at a constant rate (25V / sec). (iii) Voltage was applied until dielectric breakdown occurred, i.e., a voltage drop was detected, and the voltage at which the voltage drop was detected was measured. The measured voltage was defined as the limit withstand voltage after porous delamination.

[0086] [Example 1] <Preparation of coating solution> As a resin constituting the porous layer, poly(paraphenylene terephthalamide) (hereinafter referred to as "PPTA"), a type of aramid resin, was synthesized by the following method.

[0087] A 3L separable flask equipped with a stirring blade, thermometer, nitrogen inlet, and powder addition port was used as the synthesis vessel. 2200g of N-methyl-2-pyrrolidone (NMP) was placed in a thoroughly dried flask. 151.07g of calcium chloride powder was added to this, 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-dried under vacuum at 200°C for 2 hours.

[0088] Next, the temperature of the calcium chloride NMP solution was returned to room temperature, and 68.23 g of paraphenylenediamine was added and completely dissolved to obtain solution (1). While maintaining the temperature of solution (1) at 20°C ± 2°C, 124.25 g of terephthalic acid dichloride was added to solution (1) in four portions at approximately 10-minute intervals. Thereafter, stirring was continued at 150 rpm, and the solution (1) was aged for 1 hour while maintaining the temperature at 20°C ± 2°C to obtain aramid polymerization liquid (1) containing 6% by weight of PPTA. The intrinsic viscosity of PPTA contained in aramid polymerization liquid (1) was 1.5 g / dL.

[0089] 100 g of aramid polymerization solution (1) was weighed into a flask, and 6.0 g of alumina A (average particle size: 13 nm) was added to obtain mixture A (1). In mixture A (1), the weight ratio of PPTA to alumina A was 1:1. Next, NMP was added to mixture A (1) so that the solid content was 4.5% by weight, and the mixture was stirred for 240 minutes to obtain mixture B (1). Here, "solid content" refers to the total weight of PPTA and alumina A. Next, 0.73 g of calcium carbonate was added to mixture B (1) and the mixture was stirred for 240 minutes to neutralize the solution and obtain neutralized solution (1). After that, neutralized solution (1) was degassed under reduced pressure to prepare a slurry-like coating solution (1).

[0090] <Fabrication of separators for non-aqueous electrolyte secondary batteries> While conveying a polyethylene porous film (hereinafter also simply referred to as "porous film"), a slurry-like coating liquid (1) was applied to one side of the porous film in the first continuous coating to form a coated film. The thickness and basis weight of the porous film are shown in Table 1 below. Subsequently, while conveying the porous film on which the coated film was formed, PPTA was precipitated on the porous film under the temperature and relative humidity conditions of 75% shown in Table 2 below. Next, a water washing process was performed to wash the coated film on which PPTA had been precipitated with water to remove calcium chloride and solvent. In the water washing process, the porous film on which the coated film was formed was washed by passing it through multiple water washing tanks filled with water. Here, a feed roll was placed between each of the multiple water washing tanks, and the tension applied to the porous film was controlled to the magnitude shown in the "Water Washing MID" column of Table 2 by increasing the rotation speed of the feed roll until the tension applied to the porous film reached a specified tension. Furthermore, by installing a feed roll immediately after the final rinsing tank and controlling the rotation speed of the feed roll, the tension applied to the porous film at the end of the rinsing process was controlled to the magnitude indicated in the "Rinsing OUT" column of Table 2.

[0091] Subsequently, the coated film, from which calcium chloride and solvent had been removed, was dried using heating roller group 1 at heating temperature R1 as shown in Table 2 below, and then dried using heating roller group 2 at heating temperature R2 as shown in Table 2 below. That is, the first drying treatment was performed to continuously dry the coated film. As a result, a single-area layer separator (1) was obtained in which a porous layer was formed on one side of the porous film. Note that the heating temperatures of the first and second rollers of heating roller group 2 were different. Hereinafter, the heating temperature of the first roller is R 2a The heating temperature in the latter half roller is R 2b The first half roller refers to the roller located upstream of the heating roller group 2, and the second half roller refers to the roller located downstream. In other words, the coating film is dried at heating temperature R1, and then heated at heating temperature R 2a Dry at a temperature R 2b Drying was performed using the following method: drying at heating temperature R1 and heating temperature R 2a Drying at the heating temperature R corresponds to the process of removing the solvent from the coated film. 2b Drying at this temperature corresponds to a heat treatment process. That is, heating temperature R 2a Drying at the above temperature R causes a porous layer to precipitate on the porous film, 2b The porous film, which had a porous layer precipitated through drying, was then heat-treated.

[0092] Subsequently, a second continuous coating of the slurry-like coating liquid (1) was applied to the surface of the single-area layer separator opposite to the coated surface. Then, under the same conditions as when the single-area layer separator was made, a coating film with precipitated PPTA was formed on the opposite surface. The calcium chloride and solvent were removed by washing the coating film with water. A second drying treatment was performed on the coating film from which the calcium chloride and solvent had been removed, under the same conditions as when the single-area layer separator was made, to obtain a double-sided laminated separator (1) in which porous layers were formed on both sides of the porous film. The double-sided laminated separator (1) was designated as separator (1).

[0093] [Example 2] A double-sided laminated separator (2) was obtained in the same manner as in Example 1, except that the porous film was changed to a porous film having the thickness and basis weight described in Table 1 below, and the conditions for the PPTA precipitation, water washing, and drying processes were changed as described in Table 2 below. The double-sided laminated separator (2) was designated as separator (2).

[0094] [Example 3] A single-area layer separator (3) was obtained using the same method as in Example 1, except that the conditions for PPTA precipitation, washing, and drying were changed as shown in Table 2 below. In Example 3, the porous layer was formed only on one side of the porous film; that is, the steps from the second continuous coating to the second drying treatment were not performed. The single-area layer separator (3) was referred to as separator (3).

[0095] [Example 4] The porous film was changed to a porous film having the thickness and basis weight described in Table 1 below, and the conditions for PPTA precipitation, washing, and drying were changed as described in Table 2 below, and in the drying process, the heating temperature R 2b Instead of performing the drying process at a heating temperature R 2a A porous film with a precipitated porous layer, obtained by drying, was cut to a size of 210 mm × 297 mm. The cut porous film with the precipitated porous layer was placed in a constant temperature bath and heat-treated (annealed) by heating at a temperature of 130°C for 10 minutes. The same method as in Example 1 was used to obtain a single-area layer separator (4). In Example 3, the porous layer was formed only on one side of the porous film; that is, the steps from the second continuous coating to the second drying treatment were not performed. The single-area layer separator (4) was designated as separator (4).

[0096] [Comparative Example 1] The porous film was changed to a porous film having the thickness and basis weight described in Table 1 below, and the conditions for PPTA precipitation, washing, and drying were changed as described in Table 2 below, and in the drying process, the heating temperature R 2b A comparative single-area layer separator (1) was obtained using the same method as in Example 1, except that the drying step was not performed. In Comparative Example 1, a porous layer was formed only on one side of the porous film, that is, the steps from the second continuous coating to the second drying treatment were not performed. The comparative single-area layer separator (1) was designated as comparative separator (1).

[0097] [Comparative Example 2] A comparative double-sided laminated separator (2) was obtained using the same method as in Example 1, except that the porous film was changed to a porous film having the thickness and basis weight described in Table 1 below, and the conditions for PPTA precipitation, water washing, and drying were changed as described in Table 2 below. The comparative double-sided laminated separator (2) was designated as comparative separator (2).

[0098] [Table 1]

[0099] [Table 2]

[0100] [result] The physical properties of the separators produced in the examples and comparative examples were measured using the method described above, and the results are shown in Table 3 below.

[0101] [Table 3]

[0102] As shown in Table 3, the absolute value of the thickness change rate before and after the HS test was 1.40% or less for separators (1) to (4) described in Examples 1 to 4. On the other hand, the absolute value of the thickness change rate before and after the HS test for comparative separators (1) and (2) described in Comparative Examples 1 and 2 exceeded 1.40%. Furthermore, it was found that separators (1) to (4) had a higher limit withstand voltage after porous layer delamination compared to comparative separators (1) and (2), and maintained sufficient withstand voltage even when the porous layer was partially delaminated after repeated charge-discharge cycles.

[0103] From the above, it was found that the separator according to one embodiment of the present invention exhibits excellent voltage resistance when repeated charge-discharge cycles, due to the absolute value of the thickness change rate before and after the HS test being 1.40%. [Industrial applicability]

[0104] A separator according to one embodiment of the present invention can be used in the manufacture of a non-aqueous electrolyte secondary battery that exhibits excellent voltage resistance and excellent cycle characteristics when repeated charge-discharge cycles are performed.

Claims

1. A separator for a non-aqueous electrolyte secondary battery comprising a polyolefin porous film and a heat-resistant porous layer laminated on both sides of the polyolefin porous film, The heat-resistant porous layer contains a heat-resistant resin, The heat-resistant resin is a nitrogen-containing aromatic resin. A separator for non-aqueous electrolyte secondary batteries, wherein the absolute value of the rate of change in thickness before and after a heat shock cycle test, as expressed by the following formula (1), is 1.40% or less. Thickness change rate (%) = {(D ０ -D １ ) / D ０ } × 100 (1) (Here, the heat shock cycle test is performed under the following conditions: high temperature: 85°C, low temperature: -40°C, holding time between high and low temperatures: 30 minutes, temperature transition time: 1 minute, number of cycles: 150 cycles.) In the above formula (1), D ０ D is the thickness (μm) of the separator for the non-aqueous electrolyte secondary battery before the heat shock cycle test, １ (This is the thickness (μm) of the separator for the non-aqueous electrolyte secondary battery after the heat shock cycle test.)

2. The separator for a non-aqueous electrolyte secondary battery according to Claim 1, wherein the polyolefin porous film comprises a homopolymer or copolymer obtained by polymerizing one or more monomers selected from the group consisting of ethylene, propylene, 1-butene, 4-methyl-1-pentene, and 1-hexene.

3. The separator for a non-aqueous electrolyte secondary battery according to claim 1 or claim 2, wherein the nitrogen-containing aromatic resin is an aramid resin.

4. A component for a non-aqueous electrolyte secondary battery, comprising a positive electrode, a separator for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, and a negative electrode, arranged in this order.

5. A non-aqueous electrolyte secondary battery comprising a separator for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3.

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