JP7551328B2 - Laminated separator for power storage device, and power storage device - Google Patents

Description

The present invention relates to a laminated separator for an electricity storage device and an electricity storage device using the separator.

Electricity storage devices are used in a wide range of applications, including as power sources for consumer devices, in-vehicle power sources, and stationary power sources. Among electricity storage devices, lithium-ion secondary batteries have excellent characteristics such as high energy density and high durability, and in recent years their use as a power source for electric vehicles has rapidly expanded.

In lithium-ion secondary batteries, a polyolefin microporous membrane is generally used as the separator. The separator is placed between the positive and negative electrodes and prevents electronic conduction due to direct contact or short-circuiting between the positive and negative electrodes, while allowing ionic conduction through the electrolyte held in the microporous membrane.

Large batteries are generally used as lithium-ion secondary batteries for electric vehicles. The use of large batteries increases the energy density when multiple batteries are connected to construct a storage module, and since the number of connected batteries can be reduced compared to small batteries, the productivity of the storage module is also improved. However, large batteries have poor heat dissipation properties due to their size, and if the battery generates abnormal heat due to an internal short circuit or the like, the temperature at the center of the battery can easily reach 400°C or more, which can easily lead to serious accidents such as fire.

Furthermore, batteries for electric vehicles are often arranged in high density to make effective use of the limited space inside the vehicle. Therefore, if one battery catches fire, it can easily affect other batteries. Therefore, it is necessary to maintain a high level of safety for all batteries installed in electric vehicles.

In this context, separators are required to have the property of maintaining sufficient electrical insulation and maintaining the safety of the battery even when the temperature inside the battery becomes high. In recent years, various attempts have been made to improve the insulation of separators at high temperatures, but the most widely used method is to form an inorganic layer mainly made of inorganic particles on a substrate such as a polyolefin microporous film by coating. For example, in Patent Document 1, an inorganic layer mainly made of boehmite is formed on a polyolefin microporous film by coating, to produce a laminated separator consisting of a polyolefin layer and an inorganic layer.

On the other hand, techniques for forming inorganic layers by methods other than coating have also been proposed. For example, in Patent Document 2, a laminated separator consisting of two polyolefin layers and one inorganic layer, with the inorganic layer containing up to 98% by mass of silica, is produced by a co-extrusion method. In addition, Patent Document 3 discloses a technique for producing a laminated separator consisting of two polyolefin layers and one inorganic layer, with the inorganic layer containing up to 97% by mass of barium sulfate, by a co-extrusion method.

International Publication No. 2014/208454 JP 2017-80977 A International Publication No. 2019/235112

However, as shown in Comparative Example 1 described later, the laminated separator described in Patent Document 1 has a film resistance value that decreases as the temperature increases at temperatures above the melting point of the polyolefin resin, and the separator is insufficient in terms of maintaining insulation at high temperatures. In addition, this technology requires separate manufacturing processes for the polyolefin layer and the inorganic layer, which results in a large number of steps and low productivity.

In addition, as shown in Comparative Example 2 described later, the laminated separator described in Patent Document 2 does not have sufficient density of the inorganic layer, and the property of maintaining insulation at high temperatures is not satisfactory. Furthermore, there is room for improvement in terms of battery durability and output characteristics.

The laminated separator described in Patent Document 3 has an insufficient density in the inorganic layer, and there is room for improvement in terms of maintaining insulation at high temperatures, as shown in Comparative Example 3 described below.

In view of the above circumstances, the present invention aims to provide a laminated separator for an electricity storage device that has excellent insulating properties at high temperatures, and an electricity storage device that includes the separator.

As a result of intensive research aimed at achieving the above object, the present inventors have found that the above-mentioned problems can be solved by a laminate separator for an electricity storage device, which has a porous layer A containing a polyolefin resin and a porous layer B containing a large amount of inorganic particles and exhibits a specific resistance behavior at high temperatures, and have completed the present invention. One embodiment of the present invention is exemplified below.
[1]
A laminated separator for an electricity storage device, comprising: a porous layer A containing a polyolefin resin; and a porous layer B containing inorganic particles, wherein the content of the inorganic particles in the porous layer B is 60% by mass or more and less than 100% by mass; the separator has a membrane resistance at 200°C of 200 Ω· ^cm2 or more and exceeds a membrane resistance at 170°C; and at least one layer of the porous layer A and at least one layer of the porous layer B are laminated.
[2]
2. The laminate separator for an electricity storage device according to item 1, wherein the porosity of the porous layer B is 30% or more and 70% or less.
[3]
3. The laminate separator for an electricity storage device according to item 1 or 2, wherein at least two layers of the porous layer A and at least one layer of the porous layer B are laminated.
[4]
3. The laminate separator for an electricity storage device according to item 1 or 2, wherein at least one layer of the porous layer A and at least two layers of the porous layer B are laminated.
[5]
5. The laminate separator for an electricity storage device according to any one of items 1 to 4, wherein both surface layers are the porous layer A or the porous layer B.
[6]
6. The laminate separator for an electricity storage device according to any one of items 1 to 5, wherein the inorganic particles are surface-treated inorganic particles.
[7]
7. The laminate separator for an electrical storage device according to any one of items 1 to 6, wherein the inorganic particles have an average particle size of 31 nm or more and 1000 nm or less.
[8]
8. The laminate separator for an electricity storage device according to any one of items 1 to 7, wherein the inorganic particles are one or more selected from the group consisting of boehmite, alumina, titania, magnesium oxide, zinc oxide, calcium sulfate, aluminum sulfate, barium sulfate, magnesium hydroxide, lithium phosphate, sodium phosphate, and magnesium phosphate.
[9]
9. The laminate separator for an electrical storage device according to any one of items 1 to 8, wherein the porous layer A contains polyethylene having a viscosity average molecular weight of 500,000 or more and 5,000,000 or less.
[10]
10. The laminate separator for an electrical storage device according to any one of items 1 to 9, wherein the porous layer B contains a polyolefin having a viscosity average molecular weight of 200,000 or more and 10,000,000 or less.
[11]
11. The laminate separator for an electricity storage device according to any one of items 1 to 10, wherein the laminate separator for an electricity storage device has an air permeability of 30 sec/100 ml or more and 150 sec/100 ml or less.
[12]
Item 12. An electricity storage device comprising: a positive electrode; a negative electrode; and the laminate separator for an electricity storage device according to any one of items 1 to 11.

The present invention provides a laminated separator for an electricity storage device that can maintain the safety of a large battery even if the battery generates abnormal heat, and an electricity storage device that includes the separator.

FIG. 2 is a schematic diagram of a measuring device for film resistance value. FIG. 1B is a schematic diagram illustrating overlapping of a plurality of members in the measurement device of FIG. 1A. 1 is a graph showing the relationship between temperature and film resistance value.

The following is a detailed description of the embodiment of the present invention (hereinafter, abbreviated as "present embodiment"). Note that the present invention is not limited to the following embodiment, and can be modified in various ways within the scope of the gist of the invention.

<<Porous Layer A>>
The laminate separator for an electricity storage device according to this embodiment has a polyolefin resin-containing porous layer A. The laminate separator contains at least one porous layer A, and may contain two or more porous layers.

The thickness of each layer of the porous layer A is preferably 1 μm or more, more preferably 2 μm or more, and even more preferably 3 μm or more. When the thickness of each layer of the porous layer A is 1 μm or more, the mechanical strength and voltage resistance of the laminated separator are improved. Furthermore, the thickness of each layer of the porous layer A is preferably 17 μm or less, more preferably 15 μm or less, and even more preferably 13 μm or less. When the thickness of each layer of the porous layer A is 17 μm or less, the energy density of the electricity storage device can be increased.

From the viewpoint of increasing the ion permeability of the laminated separator, the porosity of the porous layer A is preferably 30% or more, more preferably 35% or more, and even more preferably 40% or more. On the other hand, from the viewpoint of maintaining the structure of the laminated separator, the porosity is preferably 75% or less, more preferably 70% or less, and even more preferably 65% or less.

The thickness ratio of the porous layer A to the entire laminate separator is preferably 5% or more, more preferably 10% or more, and even more preferably 15% or more, taking into consideration the physical strength of the laminate separator. If the thickness ratio of the porous layer A is 5% or more, the laminate separator is given fuse characteristics and the insulating properties at high temperatures are improved, which is preferable. On the other hand, from the viewpoint of increasing the ion permeability of the laminate separator, the thickness ratio of the porous layer A is preferably 90% or less, more preferably 85% or less, and even more preferably 80% or less. If the thickness ratio of the porous layer A is 90% or less, the dimensional stability of the laminate separator at high temperatures is improved, and the insulating properties at high temperatures are improved, which is preferable.

The method for producing the porous layer A is not particularly limited, and various methods can be used. Specifically, a stretching process such as uniaxial stretching or biaxial stretching; a process for removing a third component other than the polyolefin resin and the ionic compound by a method such as extraction, volatilization, or decomposition can be used. As the third component, plasticizers such as liquid paraffin and dioctyl phthalate; solvents such as acetone and water can be used alone or in combination.

From the viewpoint of reducing the manufacturing cost of the laminated separator and improving the quality such as the uniformity of the film thickness, the method of producing the porous layer A preferably includes at least a stretching step, more preferably includes a stretching step and a third component removal step, even more preferably includes a stretching step and a third component extraction step, and particularly preferably includes a biaxial stretching step and a plasticizer extraction step. When the porous layer A is produced by these methods, not only can the porous layer A be formed uniformly, but it also becomes easy to adjust the physical properties such as the thickness and porosity of the entire laminated separator for the electricity storage device.

<Polyolefin resin>
The laminated separator for an electricity storage device according to this embodiment has a porous layer A containing a polyolefin resin. The polyolefin resin used in this embodiment is a polyolefin resin used in normal extrusion, injection, inflation, blow molding, and the like, and is a polymer containing an olefin hydrocarbon as a monomer component, such as a homopolymer, copolymer, or multistage polymer of ethylene, propylene, 1-butene, 2-butene, 4-methyl-1-pentene, 1-hexene, or 1-octene. Polyolefins selected from the group consisting of these homopolymers, copolymers, and multistage polymers can also be used alone or in combination.

Specific examples of polyolefin resins include low-density polyethylene (density 0.910 g/cm3 or ^more and less than 0.930 g/ ^cm3 ), linear low-density polyethylene (density 0.910 g/ ^cm3 or more and less than 0.940 g/ ^cm3 ), medium-density polyethylene (density 0.930 g/ ^cm3 or more and less than 0.942 g/ ^cm3 ), high-density polyethylene (density 0.942 g/ ^cm3 or more), ultra-high molecular weight polyethylene (density 0.910 g/ ^cm3 or more and 0.970 g/ ^cm3 or less), isotactic polypropylene, syndiotactic polypropylene, atactic polypropylene, polybutene, polymethylpentene, and ethylene propylene rubber.

The proportion of polyolefin resin in porous layer A is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more. When the proportion of polyolefin resin is 70% by mass or more, the formation of porous layer A is facilitated, and physical strength and fuse characteristics can be imparted to porous layer A. In addition, this proportion is preferably 100% by mass or less, and more preferably 100% by mass. When the proportion of polyolefin resin is 100% by mass or less, the uniformity of porous layer A tends to be increased.

The polyolefin resin preferably contains high-density polyethylene. The proportion of high-density polyethylene in the polyolefin resin is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, and may be 100% by mass. When the proportion of high-density polyethylene in the polyolefin resin is 70% by mass or more, the mechanical strength of the laminated separator is increased due to the entanglement of the high-density polyethylene molecular chains.

The polyolefin resin preferably contains a high molecular weight polyethylene. The proportion of the high molecular weight polyethylene in the polyolefin resin is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and may be 100% by mass. The viscosity average molecular weight (Mv) of the high molecular weight polyethylene is preferably 500,000 or more and 5,000,000 or less, more preferably 600,000 or more and 4,000,000 or less, and even more preferably 700,000 or more and 3,000,000 or less. The viscosity average molecular weight can be determined by measuring the intrinsic viscosity of the decalin solvent at 135 ° C. based on ASTM-D4020 and calculating from a calculation formula corresponding to the polyolefin resin.
For polyethylene, Mv can be calculated by the following formula:
[η]=6.77× ^10-4 Mv ^0.67
For polypropylene, Mv can be calculated by the following formula:
[η]=1.10×10 ^-4 Mv ^0.80

The use of a polyolefin resin with a viscosity average molecular weight of 500,000 or more is preferable from the viewpoint of maintaining high melt tension and ensuring good moldability when melt-kneading the polyolefin resin composition, and from the viewpoint of imparting sufficient entanglement to the molecular chains of the polyolefin resin and enhancing the mechanical strength of the laminated separator. On the other hand, adjusting the viscosity average molecular weight to 5,000,000 or less is preferable from the viewpoint of improving the stability of extrusion molding of the polyolefin resin.

From the viewpoint of improving the heat resistance of the laminated separator, it is preferable that the polyolefin resin contains polypropylene. The proportion of polypropylene in the polyolefin resin is preferably 1 mass% or more, more preferably 3 mass% or more, and even more preferably 5 mass% or more. The proportion of polypropylene in the polyolefin resin is preferably 30 mass% or less, more preferably 25 mass% or less, and even more preferably 20 mass% or less. Adjusting the proportion of polypropylene to 1 mass% or more is preferable from the viewpoint of improving the heat resistance of the laminated separator. On the other hand, from the viewpoint of improving the uniformity of the laminated separator, it is preferable that the proportion of polypropylene is 30 mass% or less.

The polyolefin resin used in this embodiment can be mixed with additives such as phenol-based, phosphorus-based, and sulfur-based antioxidants; metal soaps such as calcium stearate and lithium stearate; ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and color pigments, as necessary, within the scope of the advantages of the present invention.

<<Porous layer B>>
The laminate separator for an electricity storage device according to this embodiment has a porous layer B containing a large amount of inorganic particles. The laminate separator includes at least one porous layer B, and may include two or more porous layers.

The thickness of each layer of the porous layer B is preferably 1 μm or more, more preferably 2 μm or more, and even more preferably 3 μm or more. When the thickness of each layer of the porous layer B is 1 μm or more, the insulating properties of the laminated separator at high temperatures are improved. Furthermore, the thickness of each layer of the porous layer B is preferably 17 μm or less, more preferably 15 μm or less, and even more preferably 13 μm or less. When the thickness of each layer of the porous layer B is 17 μm or less, the energy density of the electricity storage device can be increased.

From the viewpoint of increasing the ion permeability of the laminated separator, the porosity of the porous layer B is preferably 30% or more, more preferably 35% or more, even more preferably 40% or more, and particularly preferably 45% or more. On the other hand, from the viewpoint of maintaining the insulating properties of the laminated separator at high temperatures, the porosity of the porous layer B is preferably 70% or less, more preferably 68% or less, even more preferably 66% or less, and particularly preferably 64% or less.

The thickness ratio of the porous layer B to the entire laminate separator is preferably 5% or more, more preferably 10% or more, and even more preferably 15% or more, from the viewpoint of improving the insulating properties of the laminate separator at high temperatures. If the thickness ratio of the porous layer B is 5% or more, the insulating properties of the laminate separator at high temperatures are improved, which is preferable. On the other hand, from the viewpoint of facilitating the manufacture of the laminate separator, the thickness ratio of the porous layer B is preferably 95% or more, more preferably 90%, and even more preferably 85% or less.

The method for producing the porous layer B is not particularly limited, and various methods can be used. Specifically, a stretching process such as uniaxial stretching or biaxial stretching; a process for removing a third component other than the polyolefin resin and the ionic compound by a method such as extraction, volatilization, or decomposition can be used. As the third component, plasticizers such as liquid paraffin and dioctyl phthalate; and solvents such as acetone and water can be used alone or in combination.

From the viewpoint of reducing the manufacturing cost of the laminated separator and improving the quality such as the uniformity of the film thickness, the method of producing the porous layer B preferably includes at least a stretching step, more preferably includes a stretching step and a third component removal step, even more preferably includes a stretching step and a third component extraction step, and particularly preferably includes a biaxial stretching step and a plasticizer extraction step. When the porous layer B is produced by these methods, not only can the porous layer B be formed uniformly, but it also becomes easy to adjust the physical properties such as the thickness and porosity of the entire laminated separator for the electricity storage device.

The content of inorganic particles in the porous layer B is 60% by mass or more, preferably 65% by mass or more, more preferably 70% by mass or more, and even more preferably 75% by mass or more. When the content of inorganic particles is 60% by mass or more, the insulating properties of the laminated separator at high temperatures tend to be improved. In addition, the content of inorganic particles is less than 100% by mass, preferably 99.5% by mass or less, more preferably 99% by mass or less, and even more preferably 98.5% by mass or less. When the content of inorganic particles is less than 100% by mass, the inorganic particles are prevented from falling off from the porous layer B, and the uniformity of the porous layer B can be improved.

Porous layer B may contain a polyolefin resin. When porous layer B contains a polyolefin resin, inorganic particles are bonded to each other, and falling off of inorganic particles from porous layer B can be suppressed. Examples of polyolefin resins include homopolymers, copolymers, and multi-stage polymers of ethylene, propylene, 1-butene, 2-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene, and from the viewpoint of improving moldability, polyethylene and polypropylene are preferable. One type of polyolefin resin may be used alone, or two or more types may be used in combination.

The polyolefin resin in the porous layer B preferably has a specific molecular weight from the viewpoint of ensuring the moldability and physical strength of the porous layer B. The viscosity average molecular weight (Mv) of the polyolefin resin in the porous layer B is preferably 200,000 or more and 10,000,000 or less, more preferably 300,000 or more and 8,000,000 or less, and even more preferably 400,000 or more and 6,000,000 or less.

By using a polyolefin resin with a viscosity average molecular weight of 200,000 or more, it is possible to maintain high melt tension and ensure good moldability when melt-kneading the composition constituting the porous layer B. Furthermore, since sufficient entanglement is imparted to the polyolefin molecular chains, it becomes possible to stretch the porous layer B uniformly when stretched. On the other hand, by using a polyolefin resin with a viscosity average molecular weight of 10,000,000 or less, it is possible to increase the dimensional stability of the porous layer B at high temperatures, thereby improving the insulating properties of the laminated separator at high temperatures.

The content of polyolefin resin in the porous layer B is preferably more than 0% by mass, more preferably 0.1% by mass or more, even more preferably 0.5% by mass or more, and particularly preferably 1% by mass or more. When the content of polyolefin resin exceeds 0% by mass, the uniformity of the porous layer B is increased, and the falling off of inorganic particles from the porous layer B tends to be suppressed. In addition, the content of this polyolefin resin is preferably 40% by mass or less, more preferably 20% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less. When the content of polyolefin resin is 40% by mass or less, the dimensional stability of the porous layer B at high temperatures can be improved.

Inorganic particles
The laminated separator for an electricity storage device according to this embodiment has a porous layer B containing inorganic particles. The inorganic particles used in this embodiment are particulate substances mainly composed of elements other than carbon. It is desirable for the inorganic particles to have no electronic conductivity, a decomposition temperature or melting point of 500° C. or higher, and be chemically stable in the special environment inside the electricity storage device. By using inorganic particles having these characteristics, the safety of the electricity storage device can be maintained even if the inside of the electricity storage device becomes hot due to abnormal heat generation or the like.

The inorganic particles are particulate substances mainly composed of elements other than carbon, have no electronic conductivity, have a decomposition temperature or melting point of 500°C or higher, and are chemically stable inside the power storage device. There are no particular limitations on the inorganic particles, but examples include metal oxides such as silica, boehmite, alumina, titania, magnesium oxide, and zinc oxide; silicate minerals such as mica, clay, and kaolin; sulfates such as sodium sulfate, magnesium sulfate, calcium sulfate, barium sulfate, and aluminum sulfate; nitrates such as sodium nitrate, magnesium nitrate, and potassium nitrate; phosphates such as lithium phosphate, sodium phosphate, and magnesium phosphate; halides such as lithium fluoride, sodium chloride, and sodium bromide; carbonates such as lithium carbonate, sodium carbonate, and sodium bicarbonate; and hydroxides such as magnesium hydroxide and aluminum hydroxide.

Considering both the stability inside the power storage device and the cost, the inorganic particles are preferably boehmite, alumina, titania, magnesium oxide, zinc oxide, calcium sulfate, aluminum sulfate, barium sulfate, magnesium hydroxide, lithium phosphate, sodium phosphate, and magnesium phosphate, more preferably boehmite, alumina, titania, zinc oxide, barium sulfate, magnesium hydroxide, lithium phosphate, and sodium phosphate, further preferably boehmite, alumina, titania, and barium sulfate, and particularly preferably barium sulfate. One type of inorganic particle may be used alone, or two or more types may be used in combination.

The shape of the inorganic particles may be plate-like, scaly, needle-like, columnar, spherical, polyhedral, or block-like, and a combination of multiple types of inorganic particles having the above shapes may be used. The shape of the inorganic particles is not particularly limited, but from the viewpoint of suppressing rupture of the separator, columnar, spherical, or polyhedral particles are preferred, and spherical particles are more preferred.

The average particle size of the inorganic particles is preferably 31 nm or more, more preferably 40 nm or more, and even more preferably 50 nm or more. By having an average particle size of 31 nm or more, aggregation between inorganic particles is suppressed, and the separator can be prevented from breaking. On the other hand, the average particle size is preferably 1000 nm or less, more preferably 900 nm or less, and even more preferably 800 nm or less. By having an average particle size of 1000 nm or less, the formation of voids due to interfacial peeling during stretching of the porous layer B is suppressed, and the density of the porous layer B can be increased. The average particle size refers to the average value of the particle sizes of 30 randomly selected inorganic particles that can be confirmed in the field of view when the porous layer B is observed at 30,000 times magnification with a scanning electron microscope. When calculating the average particle size, the particle size when each particle exists alone is used, and when the particles are aggregated, the particle size of the primary particles that make up the aggregate is used.

Carbonates such as calcium carbonate have long been widely used as fillers for various polymers, and their performance as fillers has been proven in a variety of applications. However, in the special environment inside an electricity storage device, carbonates are not preferred because they may decompose and generate carbon dioxide.

In addition, depending on the conditions of use of the electricity storage device, silica and silicate minerals may react with hydrogen fluoride produced as a by-product inside the electricity storage device to generate silicon tetrafluoride gas. As a result, the electricity storage device may expand, and the safety and durability of the electricity storage device may be compromised, which is undesirable.

The inorganic particles may be surface-treated with a surface treatment agent. Specifically, the surface treatment agent may be a saturated fatty acid such as stearic acid and its salt, an unsaturated fatty acid such as oleic acid and its salt, polysiloxane, or a silane coupling agent. From the viewpoint of improving compatibility with polyolefin resins, the surface treatment agent for inorganic particles is preferably a saturated fatty acid and its salt, an unsaturated fatty acid and its salt, or a silane coupling agent, more preferably a saturated fatty acid having 8 or more carbon atoms and its salt, or an unsaturated fatty acid having 8 or more carbon atoms and its salt, or even more preferably a saturated fatty acid having 12 or more carbon atoms and its salt, or an unsaturated fatty acid having 12 or more carbon atoms and its salt. The surface treatment agent may be used alone or in combination.

<Laminated separator for power storage device>
The laminate separator for an electricity storage device according to the present embodiment has a high film resistance at high temperatures exceeding the melting point of the polyolefin resin, and has excellent insulating properties. Therefore, an electricity storage device using the laminate separator can maintain safety even if it becomes hot due to some factor.

The film resistance value (Ω·cm ² ) of the laminated separator for an electricity storage device is measured by the method described below. FIG. 1A shows a schematic diagram of the measurement device. 1 is a separator, and 2A and 2B are positive and negative electrodes prepared by the method described in the examples. 3 is an aramid film with a circular hole of 2 cm ² in area, which defines the effective electrode area. 4 is a silicone rubber sheet, which plays a role in reducing the variation in the surface pressure applied to the members 1 to 3. 5 is a ceramic plate with a thermocouple embedded therein. 6 is a heater, which heats the separator. 7 is a hydraulic press machine, which applies surface pressure to the members 1 to 4. 8 is an AC electrical resistance measuring device, which is connected to the current collector parts 2A and 2B and measures the resistance value between 2A and 2B. 9 is a thermometer, which is connected to the thermocouple of 5 and measures the temperature. Reference numeral 10 denotes a data collector, which is connected to the AC electrical resistance measuring device 8 and the thermometer 9 to record the resistance value and the temperature.

To explain in more detail, first, the separator 1, the positive electrode 2A, and the negative electrode 2B are sufficiently impregnated with an electrolyte (lithium borofluoride/propylene carbonate/ethylene carbonate/γ-butyrolactone/trioctyl phosphate = weight ratio 7.2/23.3/25.7/43.3/0.5) on the portions where the electrode active material is applied. Then, the separator 1, the positive electrode 2A, the negative electrode 2B, the aramid film 3, and the silicone rubber 4 are stacked as shown in Figure 1B and placed on the ceramic plate 5.

Next, the heater 6 is heated while applying a surface pressure of 4 MPa to the members 1 to 4 with a hydraulic press 7, and the temperature and resistance value are continuously measured. The temperature is raised from room temperature 23°C to 200°C at a rate of 10°C/min, and the resistance value is measured with an AC current of 1V and 1kHz. The obtained resistance value (Ω) is multiplied by the effective electrode area of ^{2 cm2} to calculate the film resistance value (Ω· ^cm2 ). The 170°C and 200°C film resistance values (Ω· ^cm2 ) refer to the film resistance values when the temperatures measured by a thermocouple embedded in the ceramic plate 5 are 170°C and 200°C, respectively.

From the viewpoint of improving the insulating properties of the laminated separator at high temperatures, the 170°C membrane resistance value (Ω· ^cm2 ) of the laminated separator for an electricity storage device is preferably ¹⁴⁰ Ω·cm2 or more, more preferably 160 Ω· ^cm2 or more, and even more preferably 180 Ω· ^cm2 or more. On the other hand, from the viewpoint of improving the productivity of the laminated separator, the 170°C membrane resistance value is preferably ^10,000 Ω·cm2 or less, more preferably 9,000 Ω· ^cm2 or less, and even more preferably 8,000 Ω· ^cm2 or less.

From the viewpoint of improving the insulating properties of the laminated separator at high temperatures, the 200°C membrane resistance value (Ω· ^cm2 ) of the laminated separator for an electricity storage device is 200 Ω· ^cm2 or more, preferably 250 Ω· ^cm2 or more, more preferably 300 Ω· ^cm2 or more, and even more preferably 350 Ω· ^cm2 or more. On the other hand, from the viewpoint of improving the productivity of the laminated separator, the 200°C membrane resistance value is preferably ^20,000 Ω·cm2 or less, more preferably 18,000 Ω· ^cm2 or less, and even more preferably 16,000 Ω· ^cm2 or less.

The membrane resistance value increase rate (Ω·cm ² /°C) of the laminated separator for an electricity storage device in the temperature range of 170°C to 200°C exceeds 0Ω·cm ² /°C, and is preferably 1Ω·cm ² /°C or more, more preferably 2Ω·cm ² /°C or more, and even more preferably 3Ω·cm ² /°C or more. The membrane resistance value increase rate in the temperature range of 170 ^{°C to 200°C can be calculated as the average increase in membrane resistance value per 1°C in the temperature range of 170°C to 200°C by subtracting the 170°C membrane resistance value (Ω·cm 2 )} from the 200°C membrane resistance value (Ω·cm 2 ) and dividing by the temperature difference of 30°C. A high membrane resistance value increase rate means that the insulating properties of the separator are improved as the temperature increases, and the use of a separator with a high membrane resistance value increase rate improves the safety of the electricity storage device.

The thickness of the laminated separator for an electricity storage device is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 50 μm or less, and even more preferably 3 μm or more and 25 μm or less. A thickness of 1 μm or more is preferable from the viewpoint of ensuring mechanical strength, and a thickness of 100 μm or less is preferable from the viewpoint of increasing the energy density of the electricity storage device. The thickness can be adjusted by controlling the die lip distance, the stretching ratio in the stretching process, etc.

From the viewpoint of achieving both ionic conductivity and voltage resistance, the average pore size of the laminated separator for an electric storage device is preferably 10 nm or more and 100 nm or less, more preferably 20 nm or more and 90 nm or less, and even more preferably 30 nm or more and 80 nm or less. The average pore size can be adjusted by controlling the composition ratio, the cooling rate of the extruded sheet, the stretching temperature, the stretching ratio, the heat setting temperature, the stretching ratio during heat setting, and the relaxation rate during heat setting, or by combining these.

The porosity of the laminated separator for an electricity storage device is preferably 25% or more and 95% or less, more preferably 30% or more and 90% or less, even more preferably 35% or more and 85% or less, and particularly preferably 40% or more and 80% or less. In order to improve ionic conductivity, a porosity of 25% or more is preferable, and in order to ensure voltage resistance, a porosity of 95% or less is preferable. The porosity of the separator can be adjusted by controlling the composition of the porous layer A and the porous layer B, the stretching temperature, the stretching ratio, the heat setting temperature, the stretching ratio during heat setting, and the relaxation rate during heat setting, or by combining these.

The air permeability of the laminated separator for an electricity storage device is preferably 30 sec/100 ml or more, more preferably 40 sec/100 ml or more, and even more preferably 50 sec/100 ml or more. If the air permeability is 30 sec/100 ml or more, self-discharge tends to be suppressed when used as a separator for an electricity storage device. On the other hand, the air permeability is preferably 150 sec/100 ml or less, more preferably 140 sec/100 ml or less, and even more preferably 130 sec/100 ml or less. If the air permeability is 150 sec/100 ml or less, good input/output characteristics tend to be obtained when used as a separator for an electricity storage device, which is preferable.

The reason why the laminate separator for a power storage device of this embodiment has high insulation at high temperatures is not necessarily clear, and is not limited to the action or principle described below, but it is believed that the separator of this embodiment has a good interface between the porous layer A, which is mainly made of polyolefin resin, and the porous layer B, which is mainly made of inorganic particles. When the inorganic layer is formed by coating, components such as binders and dispersants may segregate at the interface between the substrate and the inorganic layer when the medium is volatilized by drying. In that case, the interaction between the substrate and the inorganic layer is weakened, and under severe conditions such as high temperature conditions, the molten substrate and the inorganic layer may separate, the insulation may not be maintained, and the film resistance value may decrease. In the laminate separator for a power storage device of this embodiment, the porous layer A and the porous layer B are laminated by a co-extrusion method, so that it is believed that no components segregate at the interface between the porous layer A and the porous layer B, and the porous layer A and the porous layer B do not separate even at high temperatures.

One of the features of the laminated separator for an electricity storage device of this embodiment is that, at high temperatures where the polyolefin resin melts, the membrane resistance value increases as the temperature rises. In the separator of this embodiment, the inorganic particles contained in porous layer B are subjected to a specific surface treatment, which increases the affinity between the inorganic particles and the polyolefin resin contained in porous layer A, and the porous layers B and A are more likely to be combined at high temperatures, resulting in an increase in membrane resistance value.

Furthermore, the laminate separator for an electric storage device of this embodiment has a lower porosity of the porous layer B, i.e., a higher density of the porous layer B, compared to a laminate separator produced by a conventional co-extrusion method. Generally, the process of producing a separator includes a process of stretching the separator to an areal ratio of 50 times or more. After such a process, the porous layer B containing a large amount of inorganic particles is likely to break, and even if it can be stretched, the porosity of the porous layer B is likely to be higher than necessary. Since the density of the porous layer B of the laminate separator for an electric storage device of this embodiment is increased, it is assumed that the insulating properties are easily maintained at high temperatures and the membrane resistance value is high.

<<Method for producing laminated separator for power storage device>>
An example of a method for producing a laminated separator for an electric storage device according to the present invention will be described below, but the method is not necessarily limited to this example. The method for producing a separator may, for example, include the following steps:
(1) A step of melt-kneading a polyolefin resin and a plasticizer to obtain a melt for forming the porous layer A;
(2) A step of melt-kneading inorganic particles, a polyolefin resin, and a plasticizer to obtain a molten material for forming the porous layer B;
(3) A step of forming a sheet-like melt in which the porous layer A and the porous layer B are laminated;
(4) cooling and solidifying the sheet-like melt;
(5) stretching the sheet-like molded product at least uniaxially at an areal magnification of 20 times or more but less than 200 times;
(6) A step of extracting the plasticizer from the sheet-like molding.
may also include

In step (1), the polyolefin resin and the plasticizer are melt-kneaded. Examples of the melt-kneading method include a method in which the polyolefin resin, the plasticizer, and other additives as necessary are fed into a resin kneading device such as an extruder, kneader, lab plastomill, kneading roll, or Banbury mixer, and the plasticizer is introduced at any ratio while the resin components are heated and melted, and kneaded.

The plasticizer is not particularly limited, but it is preferable to use a non-volatile solvent that can form a homogeneous solution at or above the melting point of the polyolefin resin. Specific examples of such non-volatile solvents include, for example, hydrocarbons such as liquid paraffin and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; and higher alcohols such as oleyl alcohol and stearyl alcohol. After extraction, these plasticizers may be recovered and reused by operations such as distillation.

In addition, it is preferable to pre-mix the polyolefin resin and, if necessary, other additives in a predetermined ratio using a Henschel mixer or the like before feeding them into the resin kneading device. It is preferable not to add a plasticizer in the pre-kneading, and if a plasticizer is added, it is preferable to add only a portion of it, and knead the remaining plasticizer while heating it appropriately and side-feeding it into the resin kneading device. By using such a kneading method, the dispersibility of the plasticizer is increased, and when the sheet-shaped molded product is stretched in the subsequent process, it tends to be possible to stretch it at a high ratio without film rupture.

Among plasticizers, liquid paraffin is preferred because it has high compatibility with the polyolefin resin when it is polyethylene or polypropylene, and when the molten mixture is stretched, it is less likely to cause interfacial peeling between the resin and the plasticizer, making it easier to achieve uniform stretching.

There is no particular limitation on the ratio of the polyolefin resin and the plasticizer, so long as they can be uniformly melt-kneaded and molded into a sheet. For example, the mass fraction of the plasticizer in a composition consisting of the polyolefin resin and the plasticizer is preferably 10% by mass or more and 90% by mass or less, more preferably 20% by mass or more and 80% by mass or less. When the mass fraction of the plasticizer is 90% by mass or less, the melt tension during melt molding tends to be sufficient to improve moldability. On the other hand, when the mass fraction of the plasticizer is 10% by mass or more, even when the mixture of the polyolefin resin composition and the plasticizer is stretched at a high ratio, the polyolefin molecular chains do not break, and a uniform and fine pore structure tends to be formed.

In step (2), inorganic particles, polyolefin resin, and plasticizer are melt-kneaded. Examples of the melt-kneading method include a method in which inorganic particles, polyolefin resin, plasticizer, and other additives as necessary are fed into a resin kneading device such as an extruder, kneader, lab plastomill, kneading roll, or Banbury mixer, and the plasticizer is introduced at any ratio while the resin components are heated and melted, and kneaded.

The plasticizer is not particularly limited, but it is preferable to use the same plasticizer as that used in step (1). In addition, it is preferable to pre-mix the inorganic particles, polyolefin resin, and other additives as necessary in a predetermined ratio using a Henschel mixer or the like before feeding them into the resin kneading device. It is preferable not to feed the plasticizer in the pre-kneading, and even if a plasticizer is added, it is preferable to add only a part of it, and knead the remaining plasticizer while appropriately heating it and side-feeding it into the resin kneading device. Furthermore, it is preferable to thoroughly disintegrate the pre-kneaded mixture using a roller mill, jet mill, hammer mill, pin mill, planetary mill, vibration mill, bead mill, or the like. By using such a kneading method, the uniformity of the composition is increased, and when the sheet-shaped molded product is stretched in the subsequent step, it tends to be possible to stretch it at a high ratio without film breakage.

The ratio of inorganic particles, polyolefin resin, and plasticizer is not particularly limited as long as they can be uniformly melted and kneaded and molded into a sheet. For example, the mass fraction of the plasticizer in a composition consisting of inorganic particles, polyolefin resin, and plasticizer is preferably 1% by mass or more and 50% by mass or less, more preferably 5% by mass or more and 45% by mass or less, and even more preferably 10% by mass or more and 40% by mass or less. When the mass fraction of the plasticizer is 50% by mass or less, the melt tension during melt molding tends to be sufficient to improve moldability. On the other hand, when the mass fraction of the plasticizer is 1% by mass or more, flexibility is imparted to the melt-kneaded product, and a uniform and fine pore structure tends to be formed even when the kneaded product is stretched at a high ratio.

In step (3), a sheet-like molten material is produced in which porous layer A and porous layer B are laminated. Methods for forming the laminated sheet-like molten material include a method in which each layer is combined in a die and extruded into a sheet (intra-die bonding method), and a method in which each layer is extruded into a sheet from multiple dies and bonded outside the die (extra-die bonding method). From the viewpoint of increasing the adhesive strength of porous layer A and porous layer B, the intra-die bonding method is preferred.

For example, the flat die method and the inflation method can be used as an extrusion method for forming the laminated sheet-like melt. In either method, when bonding in the die, either the method of feeding the melts of the porous layer A and the porous layer B to separate manifolds of the lamination die and bonding them in layers at the die lip inlet (multiple manifold method) or the method of combining each melt in layers in advance and feeding them to the die (feed block method) can be used. The lip spacing of the flat die is preferably 200 μm or more and 3,000 μm or less, and more preferably 500 μm or more and 2,500 μm or less. If the die lip spacing is 200 μm or more, it is difficult for scum, streaks, defects, etc. to occur, and the quality of the separator is improved. On the other hand, if the die lip spacing is 3,000 μm or less, the cooling speed of the laminated sheet-like melt is increased, preventing uneven cooling and maintaining the thickness stability of the sheet.

In step (4), the sheet-shaped molten material is cooled and solidified to produce a sheet-shaped molded product. For example, the sheet-shaped molten material can be cooled and solidified by contacting it with a thermal conductor and cooling it to a temperature sufficiently lower than the crystallization temperature of the resin component. Examples of thermal conductors used for cooling and solidifying include metal, water, air, and plasticizers. Among these, it is preferable to use a metal roll because of its high thermal conductivity. In addition, it is more preferable to sandwich the extruded kneaded material between the metal rolls when contacting them, because this further increases the thermal conductivity, orients the sheet, increasing the film strength, and also tends to improve the surface smoothness of the sheet.

The sheet-shaped molded product may also be rolled. Rolling can be performed, for example, by a pressing method using a double belt press or the like. Rolling can increase the orientation, particularly in the surface layer portion. The rolling area ratio is preferably more than 1 and not more than 3, and more preferably more than 1 and not more than 2. If the rolling ratio exceeds 1, the surface orientation increases, and the membrane strength of the finally obtained separator tends to increase. On the other hand, if the rolling ratio is 3 or less, the difference in orientation between the surface layer portion and the central interior is small, and a uniform porous structure tends to be formed in the thickness direction of the membrane.

In step (5), the sheet-like molded product is stretched. The stretching may be performed after the plasticizer is extracted from the sheet-like molded product. Furthermore, the stretching may be performed both before and after the plasticizer is extracted from the sheet-like molded product.

As the stretching process, either uniaxial stretching or biaxial stretching can be suitably used, but biaxial stretching is preferred from the viewpoint of improving the strength of the resulting separator. When a sheet-like molded product is stretched in the biaxial direction at a high magnification, the molecules are oriented in the plane direction, and the finally obtained separator becomes less likely to tear and has high puncture strength. Examples of the stretching method include simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, and multiple stretching. Simultaneous biaxial stretching is preferred from the viewpoints of improving puncture strength, uniformity of stretching, and fuse characteristics. Furthermore, sequential biaxial stretching is preferred from the viewpoint of ease of control of plane orientation.

Simultaneous biaxial stretching refers to a stretching method in which stretching in MD (machine direction of separator continuous molding) and stretching in TD (direction crossing the separator MD at an angle of 90°) are performed simultaneously, and the stretching ratio in each direction may be different. Sequential biaxial stretching refers to a stretching method in which stretching in MD and TD is performed independently, and while stretching is performed in MD or TD, the other direction is in an unconstrained state or fixed at a fixed length.

The stretching ratio is preferably in the range of 20 to 200 times in area ratio, and more preferably in the range of 25 to 150 times. The stretching ratio in each axial direction is preferably in the range of 4 to 15 times in MD and 4 to 15 times in TD, and more preferably in the range of 5 to 12 times in MD and 5 to 12 times in TD. If the total area ratio is 25 times or more, the obtained separator tends to have sufficient strength, while if the total area ratio is 200 times or less, breakage of the separator during the stretching process tends to be prevented and high productivity tends to be obtained.

In step (6), the plasticizer is removed from the sheet-like molded product to form a separator. For example, the plasticizer may be removed by immersing the sheet-like molded product in an extraction solvent to extract the plasticizer, followed by thorough drying. The method for extracting the plasticizer may be either a batch method or a continuous method. In order to prevent the separator from shrinking, it is preferable to restrain the ends of the sheet-like molded product during the series of immersion and drying steps. In addition, it is preferable to adjust the amount of plasticizer remaining in the separator to less than 1% by mass relative to the mass of the entire separator.

The extraction solvent used to extract the plasticizer is preferably a poor solvent for the polyolefin resin and a good solvent for the plasticizer, with a boiling point lower than the melting point of the polyolefin resin. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; non-chlorine halogenated solvents such as hydrofluoroethers and hydrofluorocarbons; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; and ketones such as acetone and methyl ethyl ketone. These extraction solvents may be recovered and reused by operations such as distillation.

To suppress the shrinkage of the separator, a heat treatment can be performed for the purpose of heat fixing after the stretching process or after the separator is formed. The separator may also be subjected to post-treatment such as hydrophilization treatment using a surfactant or cross-linking treatment using ionizing radiation.

The separator is preferably heat-treated for the purpose of heat setting in order to suppress shrinkage. Heat treatment methods include a stretching operation carried out in a predetermined temperature atmosphere and at a predetermined stretching rate for the purpose of adjusting physical properties, and/or a relaxation operation carried out in a predetermined temperature atmosphere and at a predetermined relaxation rate for the purpose of reducing stretching stress. A relaxation operation may be carried out after the stretching operation. These heat treatments can be carried out using a tenter or roll stretching machine.

The stretching operation is preferably performed in the MD and/or TD of the membrane by 1.1 times or more, more preferably 1.2 times or more, from the viewpoint of obtaining a separator with even higher strength and higher porosity. The relaxation operation is a reduction operation in the MD and/or TD of the membrane. The relaxation rate is the value obtained by dividing the dimension of the membrane after the relaxation operation by the dimension of the membrane before the relaxation operation. When both the MD and TD are relaxed, the relaxation rate is the value obtained by multiplying the relaxation rate of the MD and the relaxation rate of the TD. The relaxation rate is preferably 1.0 or less, more preferably 0.97 or less, and even more preferably 0.95 or less. The relaxation rate is preferably 0.5 or more from the viewpoint of improving the quality of the separator. The relaxation operation may be performed in both the MD and TD directions, but may be performed in only one of the MD and TD directions.

The stretching and relaxation operations after the extraction of the plasticizer are preferably performed in TD. The temperature of the stretching and relaxation operations is preferably Tm-20°C or more and Tm+20°C or less, more preferably Tm-10°C or more and Tm+10°C or less, compared to the melting point (hereinafter also referred to as "Tm") of the polyolefin resin. If the temperature of the stretching and relaxation operations is in the above range, it is preferable from the viewpoint of the balance between the reduction of the thermal shrinkage rate and the porosity.

The laminated separator for an electricity storage device of this embodiment functions sufficiently as a separator for an electricity storage device by itself, but may be coated with an inorganic material and/or an organic material as necessary.

<Electricity storage device>
The electric storage device of the present embodiment includes a positive electrode, a negative electrode, and the laminated separator for an electric storage device according to the present embodiment. Specific examples of the electric storage device include lithium secondary batteries, lithium ion secondary batteries, sodium secondary batteries, sodium ion secondary batteries, potassium secondary batteries, potassium ion secondary batteries, magnesium secondary batteries, magnesium ion secondary batteries, calcium secondary batteries, calcium ion secondary batteries, aluminum secondary batteries, aluminum ion secondary batteries, nickel hydrogen batteries, nickel cadmium batteries, electric double layer capacitors, lithium ion capacitors, redox flow batteries, lithium sulfur batteries, lithium air batteries, and zinc air batteries. Among the above, from the viewpoint of practicality, lithium secondary batteries, lithium ion secondary batteries, potassium ion secondary batteries, nickel hydrogen batteries, or lithium ion capacitors are preferred, and lithium ion secondary batteries or potassium ion secondary batteries are more preferred.

The electricity storage device of this embodiment can be produced, for example, by stacking the above-mentioned positive and negative electrodes with a separator between them and winding them as necessary to form a laminated electrode body or a wound electrode body, which is then loaded into an exterior body, connecting the positive and negative electrodes to the positive and negative electrode terminals of the exterior body via a lead body or the like, and further injecting a nonaqueous electrolyte solution containing a nonaqueous solvent such as a chain or cyclic carbonate and an electrolyte such as a lithium salt into the exterior body, and then sealing the exterior body.

The exterior of the electricity storage device is not particularly limited, but a metal can container, a laminate container made of a metal foil laminate film, etc. can be suitably used. The shape of the electricity storage device is not particularly limited, and examples of shapes that can be used include a cylindrical shape, a square shape, a coin shape, a flat shape, and a sheet shape.

Positive pole
The positive electrode is prepared by adding an appropriate amount of a conductive assistant such as acetylene black and a binder such as polyvinylidene fluoride to a positive electrode active material to prepare a positive electrode mixture, which is then applied to a current collecting material such as aluminum foil, dried, and then pressed to fix a positive electrode mixture layer onto the current collecting material. However, the method for preparing the positive electrode is not limited to the above-mentioned example method.

The positive electrode active material is not particularly limited, but from the viewpoint of increasing the energy density and durability of the power storage device, preferred are layered oxide positive electrode active materials represented by the general formula LiNi _x Co _y Ma _1-xy O ₂ {wherein Ma represents one or more selected from the group consisting of Mn and Al, 0≦x≦1, 0≦y≦1, and x+y≦1.}; spinel-type oxide positive electrode active materials represented by the general formula LiMn _2-x Mb _x O ₄ {wherein Mb represents one or more selected from the group consisting of transition metals, and 0.2≦x≦0.7.}; Li ₂ McO ₃ and LiMdO ₂ {wherein Mc and Md each independently represent one or more selected from the group consisting of transition metals. }, a Li-excess layered oxide positive electrode active material represented by the general formula zLi ₂ McO ₃ -(1-z)LiMdO ₂ {wherein Mc and Md are independently one or more selected from the group consisting of transition metals, and 0.05≦z≦0.95.}; an olivine type positive electrode active material represented by LiMe _1-x Fe _x PO ₄ {wherein Me is one or more selected from the group consisting of Mn and Co, and 0≦x≦1.}; and one or more positive electrode active materials selected from the group consisting of Li ₂ MfPO ₄ F {wherein Mf is one or more selected from the group consisting of transition metals.}.

From the viewpoint of structural stability, the layered oxide positive electrode active material represented by the general formula LiNi _x Co _y Ma _1-xy O ₂ {wherein Ma represents one or more selected from the group consisting of Mn and Al, and 0≦x≦1, 0≦y≦1, and x+y≦1.} preferably has a composition represented by LiNi _x Co _y Mn _1-xy O ₂ {wherein 0≦x≦1, 0≦y≦1, and x+y≦1.} or LiNi _x Co _y Al _1-xy O ₂ {wherein 0.7≦x≦1, 0≦y≦0.3, and x+y≦1.}. More preferred compositions include _LiCoO2 , LiNi _x Co _y Mn1 _{-xy O2} (wherein 0.3≦x≦1, 0≦y≦0.4, and x+y _{≦ 1 )} , and _{LiNi0.85Co0.1Al0.05O2} .

Negative pole
The negative electrode can be produced by adding an appropriate amount of a thickener such as carboxymethyl cellulose and a binder such as styrene butadiene rubber to a negative electrode active material to prepare a negative electrode mixture, which is then applied to a current collecting material such as copper foil, dried, and then pressed to fix a negative electrode mixture layer onto the current collecting material. However, the method for producing the negative electrode is not limited to the above-mentioned example method.

In this embodiment, the negative electrode active material that can be used for the negative electrode is not particularly limited, but examples thereof include alloy compounds such as Al, Si, and Sn; metal oxides such as CuO and CoO; lithium-containing compounds such as lithium titanate; and carbon materials. From the viewpoint of improving the energy density of the power storage device, carbon materials that can absorb and release lithium ions at a relatively low potential are preferred. Examples of such carbon materials include, but are not limited to, hard carbon, soft carbon, artificial graphite, natural graphite, pyrolytic carbon, coke, glassy carbon, fired organic polymer compounds, fired organic natural products, carbon fibers, mesocarbon microbeads, and carbon black.

When a carbon material is used for the negative electrode active material, the interlayer distance d ₀₀₂ of the (002) plane of the carbon material is preferably 0.37 nm or less, more preferably 0.35 nm or less, and even more preferably 0.34 nm or less. The lower limit of d ₀₀₂ is not particularly limited, but is theoretically about 0.335 nm. The size of the crystallite in the c-axis direction of the carbon material is preferably 3 nm or more, more preferably 8 nm or more, and even more preferably 25 nm or more. The upper limit of the crystallite size is not particularly limited, but is usually about 200 nm. The average particle size of the carbon material is preferably 3 μm or more and 15 μm or less, and more preferably 5 μm or more and 13 μm or less. The purity of the carbon material is preferably 99.9% or more.

The energy storage device according to this embodiment is highly safe and easy to manufacture, and can therefore be widely used not only as a battery for electric vehicles, but also as a power source for a variety of devices, including batteries for consumer devices such as smartphones.

The present invention will be described in detail below using examples, but the present invention is not limited to the following examples as long as it does not deviate from the gist of the invention. The analytical and evaluation methods used in the examples and comparative examples are as follows.

(1) Film Thickness (μm)
The film thickness of the sample was measured at room temperature of 23° C. using a micro thickness gauge (Type KBM, Toyo Seiki Seisakusho Co., Ltd.). The thickness of each layer of the sample was measured by observing the cross section of the sample with a scanning electron microscope.

(2) Porosity (%)
A 10 cm×10 cm square sample was cut from the separator, and its volume (cm ³ ) and mass (g) were determined. From these and the membrane density (g/cm ³ ), the porosity was calculated using the following formula.
Porosity (%)=(volume−mass/film density)/volume×100
The film density was calculated from the respective densities and the mixing ratio of the polyolefin resin and inorganic particles used.

(3) Air permeability (sec/100ml)
The air permeability of the sample was measured using a Gurley air permeability meter (Toyo Seiki Seisakusho Co., Ltd.) conforming to JIS P-8117.

(4) Puncture strength (gf)
Using a handy compression tester "KES-G5" (Kato Tech Co., Ltd.), the separator was fixed with a sample holder having an opening diameter of 11.3 mm. The curvature of the needle tip with respect to the center of the fixed separator was The puncture test was carried out at room temperature of 23° C. under conditions of a radius of 0.5 mm and a puncture speed of 2 mm/sec, and the puncture strength (gf) was measured as the maximum puncture load.

(5) Film resistance (Ω·cm ² )
A 40 mm × 30 mm square sample was cut from the separator to prepare Sample 1. A 55 mm × 20 mm square positive electrode and negative electrode were cut from the positive electrode and negative electrode produced by the method described below to prepare a positive electrode 2A and a negative electrode 2B. Of the 55 mm length of 2A and 2B, 10 mm was not coated with the electrode active material, and the current collector was exposed.

Then, the parts of sample 1, positive electrode 2A, and negative electrode 2B where the electrode active material was applied were thoroughly impregnated with an electrolyte (lithium borofluoride/propylene carbonate/ethylene carbonate/γ-butyrolactone/trioctyl phosphate = weight ratio 7.2/23.3/25.7/43.3/0.5).

Then, a measuring device was prepared as shown in FIG. 1A, and a sample 1, a positive electrode 2A, a negative electrode 2B, an aramid film 3 (thickness 9 μm) having a circular hole with an area of ² cm2, and a silicone rubber 4 (thickness 4 mm) were overlapped as shown in FIG. 1B, and placed on a ceramic plate 5 in which a thermocouple was embedded.

Next, while applying a surface pressure of 4 MPa to members 1 to 4 with a hydraulic press 7, the heater 6 was heated, and the temperature and resistance were continuously measured using a thermometer 9 connected to the thermocouple 5 and an AC electrical resistance measuring device 8 "AG-4311" (Ando Electric Co., Ltd.) connected to the collector parts of the positive electrode 2A and negative electrode 2B. The temperature was raised from room temperature of 23°C to 200°C at a rate of 10°C/min, and the resistance was measured with an AC of 1V and 1kHz.

The film resistance (Ω· ^cm2 ) was calculated by multiplying the obtained resistance (Ω) by the effective electrode area of ² cm2. The film resistances at 170°C and 200°C were taken as the 170°C and 200°C film resistances (Ω· ^cm2 ), respectively. The film resistance increase rate (Ω· ^cm2 /°C) was calculated by subtracting the 170°C film resistance (Ω· ^cm2 ) from the 200°C film resistance (Ω· ^cm2 ) and dividing the result by the temperature difference of 30°C.

(6) Test using lithium ion secondary battery 《Preparation of positive electrode sheet》
LiNi _1/3 Mn _1/3 Co _1/3 O ₂ (Nippon Chemical Industry Co., Ltd.) as a positive electrode active material, acetylene black powder (Denka Co., Ltd.) as a conductive assistant, and polyvinylidene fluoride solution (Kureha Co., Ltd.) as a binder were mixed in a solid content mass ratio of 90:6:4, and N-methyl-2-pyrrolidone was added as a dispersion solvent so that the solid content was 40 mass%, and further mixed to prepare a slurry solution. This slurry solution was applied to both sides of an aluminum foil as a current collector material with a thickness of 20 μm, the solvent was dried and removed, and then the applied aluminum foil was rolled with a roll press to obtain a positive electrode sheet in which the porosity of the positive electrode mixture portion was adjusted to 38%.

<<Preparation of negative electrode sheet>>
Artificial graphite (Hitachi Chemical Co., Ltd.) was mixed as the negative electrode active material, and styrene butadiene rubber and a carboxymethyl cellulose aqueous solution were mixed as the binder in a solid content mass ratio of 100:1.5:1.8, and water was added as a dispersion solvent so that the solid content was 45 mass%, and further mixed to prepare a slurry solution. This slurry solution was applied to both sides of a copper foil as a current collector material with a thickness of 18 μm, the solvent was dried and removed, and then the applied copper foil was rolled with a roll press to obtain a negative electrode sheet in which the porosity of the negative electrode mixture portion was adjusted to 40%.

<Preparation of non-aqueous electrolyte>
A solution containing 1 mol/L of LiPF ₆ in a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1:2 as a non-aqueous solvent was prepared.

<<Preparation of lithium-ion secondary battery>>
The positive electrode sheet and the negative electrode sheet prepared as described above were laminated on both sides of the separator obtained in the examples and comparative examples described below to obtain a laminate. The obtained laminate was inserted into a bag (exterior body) made of an aluminum laminate film in which both sides of an aluminum foil (thickness 40 μm) were covered with a resin layer, with the positive and negative terminals protruding. Thereafter, the nonaqueous electrolyte prepared as described above was injected into the bag, and the bag was vacuum sealed to produce a pouch-type lithium ion secondary battery.

<<First charge/discharge of a lithium-ion secondary battery>>
The pouch-type lithium-ion secondary battery was placed in a thermostatic chamber "PLM-73S" (Futaba Scientific Co., Ltd.) set at 25°C and connected to a charge/discharge device "ACD-01" (Asuka Electronics Co., Ltd.). The secondary battery was then charged at a constant current of 0.1C until a voltage of 4.2V was reached, charged at a constant voltage of 4.2V for 1 hour, and then discharged at a constant current of 0.1C to 3.0V. This charge/discharge cycle was repeated three times. The battery was then charged at a constant current of 0.1C until a voltage of 4.2V was reached, bringing the lithium-ion secondary battery into a fully charged state. Note that 1C represents the current value when the entire capacity of the battery is discharged in 1 hour, and 0.1C represents 1/10 of that current value.

<20C discharge test of lithium-ion secondary battery>
The pouch-type lithium ion secondary battery after the initial charge and discharge was charged at a constant current of 1/3 C until a voltage of 4.2 V was reached, then charged at a constant voltage of 4.2 V for 1 hour, and discharged at a constant current of 1 C to 3.0 V. Thereafter, the battery was charged at a constant current of 1/3 C until a voltage of 4.2 V was reached, then charged at a constant voltage of 4.2 V for 1 hour, and discharged at a constant current of 20 C to 3.0 V. From the results of the above series of charge and discharge, the 20 C capacity retention rate (%) was calculated by the following formula.
20C capacity retention rate (%) = 20C discharge capacity/1C discharge capacity x 100

<Lithium-ion secondary battery cycle test>
The pouch-type lithium-ion secondary battery after the 20C discharge test was placed in a thermostatic chamber "PLM-73S" (Futaba Scientific Co., Ltd.) set at 50°C, and connected to a charge/discharge device "ACD-01" (Aska Electronics Co., Ltd.). The secondary battery was then charged at a constant current of 1C until a voltage of 4.2V was reached, and then charged at a constant voltage of 4.2V for 1 hour, and discharged at a constant current of 1C to 3.0V. The above series of charge/discharge cycles was counted as one cycle, and this cycle was repeated for 500 cycles, and the capacity retention rate (%) after 500 cycles was measured. The capacity retention rate (%) after 500 cycles was calculated using the following formula:
Capacity retention rate after 500 cycles (%)=discharge capacity at 500th cycle/discharge capacity at 1st cycle×100

<Safety testing of lithium-ion secondary batteries>
A pouch-type lithium-ion secondary battery in a fully charged state after the first charge and discharge was placed on a pedestal installed in an explosion-proof booth whose temperature was adjusted to 60°C. A nail (diameter 3 mm, made of SUS304) was then inserted into the battery at an angle of 90° to the flat surface of the battery, at a speed of 30 mm/sec, until the nail penetrated the battery. The behavior of the battery after the nail was inserted was observed, and a hazard level (HL) was assigned. The lower the HL value, the better the safety of the battery was maintained.
HL0: No change HL1: Protection mechanism activated HL2: Damage HL3: Leakage HL4: Eruption HL5: Ignition HL6: Rupture HL7: Explosion

[Example 1]
Composition A, which is the raw material for porous layer A, was prepared by mixing 47 parts by mass of high-density polyethylene (melting point 135°C) having a viscosity average molecular weight of 700,000, 46 parts by mass of high-density polyethylene (melting point 137°C) having a viscosity average molecular weight of 250,000, 7 parts by mass of polypropylene (melting point 163°C) having a viscosity average molecular weight of 400,000, and 0.3 parts by mass of tetrakis[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane as an antioxidant using a tumbler blender.

Next, 98 parts by mass of surface-treated barium sulfate with an average particle size of 300 nm (1 part by mass of sodium stearate was used for the surface treatment for 100 parts by mass of barium sulfate), 2 parts by mass of ultra-high molecular weight polyethylene (melting point 134°C) with a viscosity average molecular weight of 2 million, and 0.012 parts by mass of tetrakis[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane as an antioxidant were mixed in a Henschel mixer to prepare composition B, which is the raw material for porous layer B, and composition B was finely pulverized using a jet mill.

Composition A was fed to a twin-screw extruder, and liquid paraffin with a kinetic viscosity of 70.6 cSt at 40°C was side-fed into the twin-screw extruder cylinder so that the proportion of composition A in the melt-kneaded product was 35% by mass and the extrusion rate was 13 kg/h. The mixture was melt-kneaded while being heated to 180°C in the twin-screw extruder.

Composition B was fed to a twin-screw extruder, and liquid paraffin with a kinetic viscosity of 70.6 cSt at 40°C was side-fed into the twin-screw extruder cylinder so that the proportion of composition B in the melt-kneaded product was 82% by mass and the extrusion rate was 16 kg/h. The mixture was melt-kneaded while being heated to 180°C in the twin-screw extruder.

The two resulting molten mixtures were laminated in a T-die capable of co-extrusion of three layers of two kinds, with the outer layers being composition A and the middle layer being composition B, and extruded onto a cooling roll whose surface temperature was controlled at 90°C. The extrudate was brought into contact with the cooling roll, molded, and cooled to solidify, yielding a sheet-like molded product.

This sheet-like molded product was introduced into a simultaneous biaxial stretching machine and subjected to simultaneous biaxial stretching at a temperature of 122°C and a stretching ratio of 7x7. The sheet-like stretched product was then introduced into an extraction tank and immersed in methylene chloride to extract and remove the liquid paraffin, and the methylene chloride was dried. The resulting porous film was then introduced into a uniaxial stretching machine and stretched 1.7 times in the width direction at a temperature of 134°C, and then relaxed by approximately 20% in the width direction to obtain a laminated separator. The evaluation results of the resulting laminated separator are shown in Table 1 and Figure 2.

[Example 2]
A laminate separator was produced in the same manner as in Example 1, except that, instead of the lamination method in Example 1, both surface layers were laminated with composition B and the intermediate layer was laminated with composition A. The evaluation results of the obtained laminate separator are shown in Table 1 and FIG.

[Example 3]
A laminate separator was produced in the same manner as in Example 1, except that composition B was used, which was prepared by mixing 95 parts by mass of surface-treated barium sulfate having an average particle size of 300 nm (1 part by mass of sodium stearate was used for surface treatment for 100 parts by mass of barium sulfate), 5 parts by mass of ultra-high molecular weight polyethylene (melting point 134°C) having a viscosity average molecular weight of 2 million, and 0.03 parts by mass of tetrakis[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane as an antioxidant in a Henschel mixer. The evaluation results of the obtained laminate separator are shown in Table 1.

[Example 4]
A laminate separator was produced in the same manner as in Example 3, except that, instead of the lamination method in Example 3, both surface layers were laminated with composition B and the intermediate layer was laminated with composition A. The evaluation results of the obtained laminate separator are shown in Table 1.

[Example 5]
Instead of composition B in Example 1, 70 parts by mass of surface-treated barium sulfate having an average particle size of 300 nm (1 part by mass of sodium stearate was used for surface treatment for 100 parts by mass of barium sulfate), 30 parts by mass of high-density polyethylene (melting point 135°C) having a viscosity average molecular weight of 700,000, and 0.18 parts by mass of tetrakis [methylene-3- (3',5'-di-t-butyl-4'-hydroxyphenyl) propionate] methane as an antioxidant were mixed in a Henschel mixer to prepare composition B, and a laminate separator was produced in the same manner as in Example 1, except that the extrusion rate was set to 22 kg / h when composition B was supplied to an extruder and melt-kneaded. The evaluation results of the obtained laminate separator are shown in Table 1.

[Example 6]
A laminate separator was produced in the same manner as in Example 5, except that, instead of the lamination method in Example 5, both surface layers were laminated with composition B and the intermediate layer was laminated with composition A. The evaluation results of the obtained laminate separator are shown in Table 1.

[Example 7]
A laminated separator was produced in the same manner as in Example 1, except that composition B was prepared using surface-treated boehmite having an average particle size of 300 nm (1 part by mass of sodium stearate was used for the surface treatment per 100 parts by mass of boehmite) instead of the surface-treated barium sulfate having an average particle size of 300 nm in Example 1. The evaluation results of the obtained laminated separator are shown in Table 1.

[Example 8]
A laminate separator was produced in the same manner as in Example 7, except that, instead of the lamination method in Example 7, both surface layers were laminated with composition B and the intermediate layer was laminated with composition A. The evaluation results of the obtained laminate separator are shown in Table 2.

[Example 9]
A laminated separator was produced in the same manner as in Example 1, except that composition B was prepared using surface-treated titania having an average particle size of 300 nm (1 part by mass of sodium stearate was used for the surface treatment per 100 parts by mass of titania) instead of the surface-treated barium sulfate having an average particle size of 300 nm in Example 1. The evaluation results of the obtained laminated separator are shown in Table 2.

[Example 10]
A laminate separator was produced in the same manner as in Example 9, except that, instead of the lamination method of Example 9, both surface layers were laminated with composition B and the intermediate layer was laminated with composition A. The evaluation results of the obtained laminate separator are shown in Table 2.

[Example 11]
A laminated separator was produced in the same manner as in Example 1, except that composition B was prepared using surface-treated barium sulfate having an average particle size of 50 nm (3 parts by mass of sodium stearate was used for the surface treatment per 100 parts by mass of barium sulfate) instead of the surface-treated barium sulfate having an average particle size of 300 nm in Example 1. The evaluation results of the obtained laminated separator are shown in Table 2.

[Example 12]
A laminated separator was produced in the same manner as in Example 1, except that composition B was prepared using surface-treated barium sulfate having an average particle size of 600 nm (0.5 parts by mass of sodium stearate was used for the surface treatment per 100 parts by mass of barium sulfate) instead of the surface-treated barium sulfate having an average particle size of 300 nm in Example 1. The evaluation results of the obtained laminated separator are shown in Table 2.

[Comparative Example 1]
A laminated separator was produced according to the method described in Example 1 of Patent Document 1. The evaluation results of the obtained laminated separator are shown in Table 2 and Fig. 2. In the measurement of the membrane resistance value and various evaluations using a lithium ion secondary battery, the laminated separator was disposed so that the inorganic coating layer faced the positive electrode.

[Comparative Example 2]
A laminated separator was produced according to the method described in Example 5 of Patent Document 2. The evaluation results of the obtained laminated separator are shown in Table 2 and FIG.

[Comparative Example 3]
A laminated separator was produced according to the method described in Example 5 of Patent Document 3. The evaluation results of the obtained laminated separator are shown in Table 2 and FIG.

[Comparative Example 4]
An attempt was made to produce a laminated separator in the same manner as in Example 1, except that fine pulverization with a jet mill was not performed when preparing composition B of Example 1. However, breakage frequently occurred during the production of the laminated separator, and the separator could not be produced stably.

As can be seen from Tables 1 and 2, in comparison with Comparative Example 1, in which a laminated separator is produced by forming an inorganic layer by coating on a polyolefin substrate, Examples 1 to 12 have an excellent property of maintaining insulation at high temperatures, with the film resistance increasing as the temperature rises above the melting point of the polyolefin resin. In battery safety tests, when the laminated separators of Examples 1 to 12 are used, the safety of the battery is maintained better than in Comparative Example 1. Furthermore, Examples 1 to 12 are superior to Comparative Example 1 in terms of battery output characteristics.

Next, in comparison with Comparative Example 2, in which a laminated separator was produced by co-extrusion using silica as the inorganic particles, Examples 1 to 12 have a higher density of porous layer B containing inorganic particles, and are excellent in terms of maintaining insulation properties at high temperatures. In addition, the battery has high cycle characteristics and output characteristics, and is also excellent in safety.

In addition, in comparison with Comparative Example 3, in which a laminated separator was produced by co-extrusion using barium sulfate as the inorganic particles, Examples 1 to 12 have a higher density of porous layer B containing inorganic particles and are superior in the property of maintaining insulating properties at high temperatures. As a result, in a battery safety test, when the laminated separators of Examples 1 to 12 were used, the safety of the battery was maintained better than in Comparative Example 3.

From the above, it has become clear that a laminate separator for an electricity storage device, which has a porous layer A containing a polyolefin resin and a porous layer B containing a large amount of inorganic particles and exhibits specific resistance behavior at high temperatures, not only functions well as a separator, but also contributes to improving the safety of electricity storage devices at high temperatures, and furthermore, helps to improve the durability and output characteristics of electricity storage devices.

REFERENCE SIGNS LIST 1 Separator 2A, 2B Positive electrode, negative electrode 3 Aramid film with a circular hole of ² cm2 in area 4 Silicone rubber sheet 5 Ceramic plate with embedded thermocouple 6 Heater 7 Hydraulic press 8 AC electrical resistance measuring device 9 Thermometer 10 Data collector

Claims (10)

A laminated separator for an electricity storage device, comprising: a porous layer A containing a polyolefin resin ; and a porous layer B containing inorganic particles that have been surface-treated with one or more selected from saturated fatty acids and salts thereof, unsaturated fatty acids and salts thereof, polysiloxanes, and silane coupling agents, wherein the content of the inorganic particles in the porous layer B is 60% by mass or more and less than 100% by mass; the porosity of the porous layer B is 30% by mass or more and 70% by mass or less; and the membrane resistance at 200°C is 200 Ω· ^cm2 or more and exceeds a membrane resistance at 170°C, and at least one layer of the porous layer A and at least one layer of the porous layer B are laminated. 2 . The laminate separator for an electricity storage device according to claim 1 , wherein at least two of the porous layers A and at least one of the porous layers B are laminated. 2 . The laminate separator for an electricity storage device according to claim 1 , wherein at least one of the porous layers A and at least two of the porous layers B are laminated. 4. The laminate separator for an electricity storage device according to claim 1 , wherein both surface layers are the porous layer A or the porous layer B. The laminate separator for an electricity storage device according to any one of claims 1 to 4 , wherein the inorganic particles have an average particle size of 31 nm or more and 1000 nm or less. The laminate separator for an electricity storage device according to any one of claims 1 to 5, wherein the inorganic particles are one or more selected from the group consisting of boehmite, alumina, titania, magnesium oxide, zinc oxide, calcium sulfate, aluminum sulfate, barium sulfate, magnesium hydroxide, lithium phosphate, sodium phosphate, and magnesium phosphate. 7. The laminate separator for an electricity storage device according to claim 1, wherein the porous layer A contains polyethylene having a viscosity average molecular weight of 500,000 or more and 5,000,000 or less . 8. The laminate separator for an electricity storage device according to claim 1, wherein the porous layer B contains a polyolefin having a viscosity average molecular weight of 200,000 or more and 10,000,000 or less . 9. The laminated separator for an electricity storage device according to claim 1, wherein the laminated separator for an electricity storage device has an air permeability of 30 sec/100 ml or more and 150 sec/100 ml or less. An electricity storage device comprising a positive electrode, a negative electrode, and the laminate separator for an electricity storage device according to any one of claims 1 to 9 .

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