JP7822142B2 - Separator for power storage device and power storage device - Google Patents

Description

The present invention relates to separators for electricity storage devices, etc.

Microporous membranes, particularly polyolefin-based microporous membranes, are used in many technical fields, such as microfiltration membranes, battery separators, capacitor separators, and fuel cell materials, and are particularly used as separators for power storage devices, such as lithium-ion batteries. Lithium-ion batteries are used in a variety of applications, including small electronic devices such as mobile phones and notebook personal computers, as well as electric vehicles, including hybrid vehicles and plug-in hybrid vehicles.
In recent years, there has been a demand for lithium-ion batteries with high energy capacity, high energy density, and high power output characteristics, and this has led to an increasing demand for thin separators that are highly safe and have high strength (e.g., high puncture resistance).

Patent Document 1 describes a polyolefin microporous membrane containing a polypropylene-based resin, characterized in that the meltdown temperature is 195°C or higher and 230°C or lower, and that the weight-average molecular weight of the polypropylene-based resin may be 500,000 or higher and 800,000 or lower, and that the molecular weight distribution may be 7.5 or higher and 16 or lower.

Patent Document 2 describes a laminated microporous film comprising a first microporous film made of a first resin composition and a second microporous film made of a second resin composition, wherein the MFR of both the first and second resin compositions is 1.0 g/10 min or less, and the molecular weight distribution (Mw/Mn) of the resin contained in the first resin composition is 10 or more.

Patent Document 3 describes a laminated microporous film comprising a first microporous film containing a first resin composition having a melting point T _mA and a second microporous film containing a second resin composition having a melting point T _mB lower than the melting point T _mA , and having an extensional viscosity of 18,000 to 40,000 Pa s and a shear viscosity of 5,000 to 10,000 Pa s.

Patent Document 4 describes a polypropylene resin composition for forming a microporous membrane, containing a propylene homopolymer as an essential component, which has an intrinsic viscosity [η] of 1 dl/g or more but less than 7 dl/g, a mesopentad fraction in the range of 94.0 to 99.5%, an elution integrated amount up to 100°C during heating of 10% or less, a melting point of 153 to 167°C, and an elution temperature-elution amount curve in which the peak-top temperature of the maximum peak is between 105 and 130°C and the half-width of the peak is 7.0°C or less.

International Publication No. 2017/138512 JP 2016-22679 A JP 2016-022676 A International Publication No. 2010/079784

It is known that the use of high-molecular-weight polyolefins in microporous membranes can produce separators for energy storage devices that are both strong and safe. However, high-molecular-weight polyolefins have high viscosities when melted, making them difficult to thin. For example, the separator described in Patent Document 1 cited above is considered to be highly safe, but cannot be thinned due to its excessively high melt tension. According to the technology described in Patent Document 2, thinning is achieved through co-extrusion, but high-power characteristics are not achieved. Furthermore, the separators described in Patent Documents 1 and 2 tend to be unsafe for batteries at high temperatures. According to Patent Documents 3 and 4, the thermal shrinkage rate of the separator can be reduced, but both have low viscosities when melted, which is expected to result in low safety. For these reasons, there is room for improvement in obtaining separators that can be thinned while also achieving high strength, safety, and dimensional stability at high temperatures.

Therefore, the object of the present invention is to provide a separator for an electricity storage device that has high strength, high safety, and high dimensional stability at high temperatures, and that can be made thin.

As a result of extensive research into resolving the above-mentioned issues, the inventors discovered that using a polyolefin with a specific MFR (melt flow rate), controlling the short-circuit temperature during a fuse short test within a specific range, and further controlling the thermal shrinkage rate at 105°C within a specific range are advantageous in resolving the above-mentioned issues, leading to the completion of the present invention.

Examples of embodiments of the present invention are listed in the following items [1] to [11].
[1] A separator for an electricity storage device having a microporous layer,
the microporous layer comprises a polyolefin having a melt flow rate (MFR) of 0.7 g/10 min or less at a load of 2.16 kg and a temperature of 230°C;
the separator for an electricity storage device has a short-circuit temperature of 200°C or higher in a fuse short-circuit test;
A separator for an electric storage device, wherein when the separator for an electric storage device is heat-treated at 105°C for 1 hour, the heat shrinkage rate in the width direction (TD) and the heat shrinkage rate in the machine direction (MD) are TD≦1% and MD≦4%, respectively.
[2] The separator for an electricity storage device according to aspect 1, wherein the polyolefin is primarily composed of polypropylene.
[3] The separator for an electricity storage device according to aspect 1 or 2, wherein the polyolefin has a weight average molecular weight (Mw) divided by a number average molecular weight (Mn) (Mw/Mn) of 7 or less.
[4] The separator for an electricity storage device according to any one of aspects 1 to 3, wherein the fuse temperature in the fuse short circuit test is 150° C. or less.
[5] The separator for an electricity storage device according to any one of Aspects 1 to 4, which has a multilayer structure of a microporous layer containing polypropylene as a main component and a microporous layer containing polyethylene as a main component.
[6] The power storage device separator according to any one of Aspects 1 to 5, wherein, in wide-angle X-ray scattering measurement of the microporous layer, the ratio MD/TD of the orientation rate in the machine direction (MD) to the orientation rate in the width direction (TD) is 1.3 or greater.
[7] The separator for an electricity storage device according to any one of Aspects 1 to 6, having a thickness of 18 μm or less, a porosity of 42% or more, and an air resistance of 250 sec/100 cm or ^less .
[8] An electricity storage device comprising the separator for an electricity storage device according to any one of aspects 1 to 7.

The present invention provides a separator for an electricity storage device that has high strength, high safety, and high dimensional stability at high temperatures, and can be made thin. The above description should not be construed as disclosing all embodiments of the present invention or all advantages associated with the present invention. Further embodiments and advantages of the present invention will become apparent from the following description.

FIG. 1 is a diagram showing an example of the azimuthal angle distribution of scattering intensity to explain the measurement of the (110) crystal peak area ratio (MD/TD) in wide-angle X-ray scattering.

The following describes in detail an embodiment of the present invention (hereinafter referred to as the "present embodiment") for illustrative purposes, but the present invention is not limited to this embodiment.

<Separator for power storage device>
<Microporous layer>
The separator for an electricity storage device of this embodiment includes a microporous layer containing a polyolefin. The microporous layer preferably contains a polyolefin as a main component. In this embodiment, the microporous layer may be used as a single layer or as a multilayer formed by laminating two or more layers. A microporous layer containing a polyolefin as a main component refers to a film containing 50% by mass or more of polyolefin based on the total mass of the microporous layer. The lower limit of the polyolefin content in the microporous layer is preferably 55% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more, from the viewpoints of wettability of the film to an electrolyte, thinning, shutdown characteristics, etc. The upper limit of the polyolefin content in the microporous layer may be, for example, 60% by mass or less, 70% by mass or less, 80% by mass or less, 90% by mass or less, 95% by mass or less, 98% by mass or less, or 99% by mass or less, or may be 100% by mass.

<Polyolefin>
Polyolefins are polymers containing, as repeating units, monomers having a carbon-carbon double bond. Monomers constituting polyolefins include, but are not limited to, monomers having 1 to 10 carbon atoms and having a carbon-carbon double bond, such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Polyolefins are, for example, homopolymers, copolymers, or multi-stage polymers, and are preferably homopolymers.

Specific examples of polyolefins that are preferred from the standpoint of shutdown characteristics include polyethylene, polypropylene, ethylene-propylene copolymers, and mixtures thereof. Polypropylene is a more preferred polyolefin.

The polyolefin of this embodiment preferably contains polypropylene as a primary component. Here, "polyolefin containing polypropylene as a primary component" refers to a polyolefin containing 50% by mass or more of polypropylene, based on the total mass of the polyolefin. From the viewpoint of obtaining good shutdown characteristics, the lower limit of the polypropylene content in the polyolefin is preferably 55% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 95% by mass or more. The upper limit of the polypropylene content in the polyolefin may be 100% by mass, or may be 99% by mass or less, or 98% by mass or less.

The stereoregularity of the polypropylene is not limited, but examples include atactic, isotactic, or syndiotactic. The polypropylene is preferably an isotactic or syndiotactic highly crystalline homopolymer.

The polypropylene is preferably a homopolymer, but may also be a copolymer, such as a block polymer, in which a small amount of a comonomer other than propylene, such as an α-olefin comonomer, is copolymerized. The amount of propylene structures contained as repeating units in the polypropylene may be, for example, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, or 99 mol% or more. The amount of repeating units derived from comonomers other than the propylene structure contained in the polypropylene may be, for example, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, or 1 mol% or less. One type of polypropylene can be used alone, or two or more types can be used in combination.

From the viewpoint of the strength of the microporous layer, the weight average molecular weight (Mw) of the polyolefin is preferably 100,000 or more, more preferably 200,000 or more, even more preferably 300,000 or more, even more preferably 500,000 or more, and particularly preferably 550,000 or more, and is preferably 2,000,000 or less, more preferably 1,500,000 or less.

The upper limit of the value (Mw/Mn) obtained by dividing the weight average molecular weight (Mw) of the polyolefin by the number average molecular weight (Mn) is preferably 7 or less, 6.5 or less, 6 or less, 5.5 or less, or 5 or less. A narrow molecular weight distribution represented by Mw/Mn reduces entanglement between molecules, thereby reducing melt tension and achieving the effect of thinning the film. Furthermore, the Mw/Mn of the polyolefin is preferably 1 or more, 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. An Mw/Mn of 1 or more tends to maintain appropriate molecular entanglement and improve stability during film formation.

When the polyolefin is polypropylene, the weight average molecular weight (Mw) of the polypropylene is preferably 100,000 or more, more preferably 200,000 or more, even more preferably 300,000 or more, even more preferably 500,000 or more, and particularly preferably 550,000 or more, from the viewpoint of the strength of the microporous layer, and is preferably 2,000,000 or less, more preferably 1,500,000 or less.

The upper limit of the value (Mw/Mn) obtained by dividing the weight-average molecular weight (Mw) of polypropylene by the number-average molecular weight (Mn) is preferably 7 or less, 6.5 or less, 6 or less, 5.5 or less, or 5 or less. A narrow molecular weight distribution represented by Mw/Mn reduces entanglement between molecules, thereby reducing melt tension and enabling the film to be made thinner. Furthermore, Mw/Mn is preferably 1 or more, 1.3 or more, 1.5 or more, 2.0 or more, or 2.5 or more. Having an Mw/Mn ratio of 1 or more tends to maintain appropriate molecular entanglement and improve stability during film formation.

The weight average molecular weight, number average molecular weight, and Mw/Mn of the polyolefin of this embodiment are polystyrene-equivalent molecular weights obtained by GPC (gel permeation chromatography) measurement.

The density of the polyolefin is preferably 0.85 g/cm ^or more, 0.88 g/cm ^or more, 0.89 g/cm ^{or more} , or 0.90 g/cm or ^more . The density of the polyolefin is preferably 1.1 g/cm ^or less, 1.0 g/cm ^or less, 0.98 g/cm ^or less, 0.97 g/cm ^{or less} , 0.96 g/cm ^or less, 0.95 g/cm ^or less, 0.94 g/cm ^or less, 0.93 g/cm or less, or 0.92 g/cm ^or less. The density of the polyolefin is related to the crystallinity of the polyolefin, and a polyolefin density of 0.85 g/cm or ^more improves the productivity of the microporous layer, and is particularly advantageous in a method of making a resin raw sheet porous by a dry method (hereinafter referred to as a dry method).

<Melt tension of microporous layer>
From the viewpoint of formability of the microporous layer, the upper limit of the melt tension of the microporous layer (melt tension of a single layer) measured at a temperature of 230° C. is preferably 30 mN or less, more preferably 25 mN or less. From the viewpoint of strength of the microporous layer, the lower limit of the melt tension of the microporous layer (melt tension of a single layer) is preferably 10 mN or more, 15 mN or more, or 20 mN or more.

<Melt flow rate (MFR)>
The melt flow rate (MFR) of the polyolefin of this embodiment measured under a load of 2.16 kg at a temperature of 230°C (i.e., the MFR of a single layer) is, in one aspect, 0.7 g/10 min or less, for example, 0.6 g/10 min or less, 0.55 g/10 min or less, or 0.5 g/10 min or less, from the viewpoint of obtaining a microporous layer with higher strength and improved safety. The lower limit of the MFR of the microporous layer (the MFR of a single layer) may be, for example, 0.15 g/10 min or more, 0.2 g/10 min or more, 0.25 g/10 min or more, 0.3 g/10 min or more, 0.35 g/10 min or more, or 0.4 g/10 min or more, so that the melt tension of the microporous layer does not become too high and high safety and good formability of a thin-film microporous layer are obtained.
Even when the separator is made up of multiple microporous layers, each microporous layer can be easily peeled off and collected by applying adhesive tape to the edge of the separator and pulling it, and physical properties such as melt tension can be measured.

An MFR of 0.7 g/10 min or less, particularly 0.6 g/10 min or less, of the microporous layer means that the molecular weight of the polyolefin contained in the microporous layer is high. A high molecular weight polyolefin results in more tie molecules bonding crystalline materials together, which tends to result in a high-strength microporous layer. Furthermore, a high molecular weight results in a high viscosity when melted, which increases the short-circuit temperature during a fuse short test and improves the high-temperature safety of the battery. In one embodiment, a molecular weight distribution represented by Mw/Mn of 7 or less and an MFR of 0.7 g/10 min or less, particularly 0.6 g/10 min or less, is particularly advantageous in that it allows for the production of a highly safe, thin-film separator for an electricity storage device.

When the polyolefin is polypropylene, from the viewpoint of obtaining a highly safe and high-strength microporous layer, the MFR of the polypropylene, measured under a load of 2.16 kg and at a temperature of 230°C, is preferably 0.2 g/10 min or more and 0.7 g/10 min or less. From the viewpoint of obtaining a microporous layer with even higher strength, the upper limit of the MFR of the polypropylene may be, for example, 0.6 g/10 min or less, 0.55 g/10 min or less, 0.5 g/10 min or less, 0.45 g/10 min or less, 0.4 g/10 min or less, or 0.35 g/10 min or less. From the viewpoint of the formability of the microporous layer, the lower limit of the MFR of the polypropylene may be, for example, 0.25 g/10 min or more, 0.3 g/10 min or more, 0.35 g/10 min or more, 0.4 g/10 min or more, 0.45 g/10 min or more, or 0.5 g/10 min or more.

<Pentad fraction>
When the polyolefin is polypropylene, in this embodiment, the lower limit of the pentad fraction of the polypropylene measured by C-NMR (nuclear magnetic resonance) may be preferably 95.0% ^or more, 96.0% or more, 96.5% or more, 97.0% or more, 97.5% or more, 98.0% or more, 98.5% or more, or 99.0% or more, from the viewpoint of obtaining a microporous layer with low air permeability. The upper limit of the pentad fraction of the polypropylene may be, for example, 99.9% or less, 99.8% or less, or 99.5% or less.

When the pentad fraction of polypropylene is 95.0% or higher, the crystallinity of the polypropylene is high. Microporous layers obtained by the stretching method, particularly the dry method, are formed by stretching the amorphous portions between multiple crystalline portions, resulting in pores. Therefore, high crystallinity of polypropylene is preferred because it allows the air permeability to be controlled low.

<Additives>
In this embodiment, the microporous layer containing (preferably containing as a main component) a polyolefin may contain, in addition to the polyolefin, additives such as elastomers, fluorine-based flow modifiers, waxes, crystal nucleating agents, antioxidants, metal soaps such as metal salts of aliphatic carboxylic acids, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, coloring pigments, etc., either singly or in combination as needed.

<Elastomer>
Examples of elastomers include thermoplastic elastomers such as ethylene/α-olefin copolymers, ethylene/styrene copolymers, propylene/α-olefin copolymers, 1-butene/α-olefin copolymers, styrene and butadiene block copolymers (SBS) and their hydrogenated polymers (SEBS), and styrene and isoprene block copolymers (SIS) and their hydrogenated polymers (SEPS). Examples of α-olefins include α-olefins such as propylene, 1-butene, 1-hexene, 1-octene, and 4-methyl-1-pentene. Examples of suitable α-olefins include high molecular weight copolymers of ethylene and α-olefins, and high molecular weight copolymers in which long-chain branches are copolymerized by chain transfer during polymerization, such as linear low-density polyethylene or ultra-low-density polyethylene. These thermoplastic elastomers may be used alone or in combination of two or more.

<Fuse temperature and short circuit temperature>
In one aspect, the short-circuit temperature (sometimes simply referred to as the short-circuit temperature in this disclosure) of the separator for an electric storage device measured by the fuse short-circuit test of the present disclosure is controlled within a specific range, and in a preferred aspect, the fuse temperature (sometimes simply referred to as the fuse temperature in this disclosure) of the separator for an electric storage device measured by the fuse short-circuit test of the present disclosure is further controlled within a specific range.

The fuse short circuit test of the present disclosure is performed as follows. A separator sample is placed on a ceramic plate with an embedded thermocouple. A surface pressure of 1.5 MPa is applied using a hydraulic press while the heater is heated. The temperature and resistance value are continuously measured using an AC electrical resistance measuring device connected to the current collector portions of the positive and negative electrodes. The temperature is raised from room temperature (23°C in one embodiment) to 220°C at a rate of 15°C/min, and the impedance (resistance value) is measured using an AC of 1 kHz. This fuse short circuit test uses a positive electrode and a negative electrode and further applies surface pressure, making it possible to reflect the fuse short circuit behavior within the electricity storage device. The value calculated by multiplying the obtained impedance (Ω) by the effective positive electrode area is the impedance (Ω· ^cm2 ) converted to a unit area of the positive electrode. The temperature at which the impedance per unit area of the positive electrode reaches 100 Ω ^cm2 is defined as the fuse temperature (°C), and the temperature at which the impedance changes from increasing to decreasing (which means that the separator has reached a pore-blocked state) and then falls below 100 Ω ^cm2 again is defined as the short-circuit temperature (°C).

The short-circuit temperature corresponds to the temperature at which an electricity storage device short-circuits due to abnormally high temperatures during use. Controlling the short-circuit temperature of a separator for an electricity storage device to a predetermined value or higher is advantageous in terms of preventing short-circuiting of the electricity storage device at abnormally high temperatures. From the above perspective, the lower limit of the short-circuit temperature in one embodiment is 200°C or higher, and preferably 201°C or higher, 202°C or higher, 203°C or higher, or 204°C or higher. From the perspective of ease of manufacturing the separator, the upper limit of the short-circuit temperature may be, for example, 250°C or lower, 245°C or lower, 230°C or lower, or 225°C or lower.

The fuse temperature corresponds to the temperature at which the separator changes (e.g., melts) due to abnormally high temperatures during use of the electricity storage device, interrupting the current between the electrodes. Controlling the fuse temperature of the separator for an electricity storage device to a predetermined value or lower is advantageous in that it allows the fuse to function well at abnormally high temperatures in the electricity storage device, thereby achieving high safety. From the above perspective, the upper limit of the fuse temperature is preferably 150°C or lower, 145°C or lower, 140°C or lower, or 135°C or lower. From the perspective of good separator strength, the lower limit of the fuse temperature is preferably 105°C or higher, 110°C or higher, 115°C or higher, or 120°C or higher.

Orientation ratio (wide-angle X-ray scattering)
When the microporous layer in this embodiment is measured by wide-angle X-ray scattering, the ratio of the orientation ratio in the machine direction (MD) to the orientation ratio in the width direction (TD), MD/TD, is obtained as the (110) crystal peak area ratio (MD/TD). The lower limit of the ratio (MD/TD) is preferably 1.3 or more, 2 or more, 2.5 or more, 3 or more, 3.5 or more, 4 or more, 4.5 or more, or 5 or more. The upper limit of the orientation ratio ratio (MD/TD) may be, for example, 12 or less, 10 or less, 8 or less, 6 or less, 5.5 or less, 5 or less, 4.5 or less, or 4 or less. In this disclosure, "machine direction" and "MD" refer to the machine direction in continuous microporous layer molding, and "width direction" and "TD" refer to the direction crossing the MD at an angle of 90°.

A (110) crystal peak area ratio (MD/TD) of 1.3 or more in wide-angle X-ray scattering of the microporous layer means that the polymer molecular chains constituting the microporous layer are strongly oriented in the MD. When the microporous layer is produced by a method that subjects the microporous layer to strong MD stretching, particularly a dry MD stretching method, the orientation ratio ratio (MD/TD) tends to be 1.3 or more. The advantages of the microporous layer of this embodiment, that it is high in strength and can be made thin, are not limited to, but are particularly pronounced when the microporous layer is produced by a dry MD stretching method. For this reason, it is preferable that the orientation ratio ratio (MD/TD) of the microporous layer measured by wide-angle X-ray scattering be 1.3 or more.

<Multilayer structure>
The separator for an electric storage device of this embodiment may be composed of a single or multiple microporous layers, as long as it has at least one microporous layer containing the polyolefin of this embodiment. For example, the multilayer structure may contain all layers of a polyolefin, or may include a layer that does not contain a polyolefin. The multilayer structure may have a single microporous layer containing (preferably as a main component) the polyolefin of this embodiment described above, or a multilayer structure in which two or more such microporous layers are laminated. For example, the multilayer structure may be a multilayer structure in which a microporous layer containing polypropylene as a main component and a microporous layer containing polyethylene as a main component are laminated. The polyethylene-based microporous layer may be a polyethylene microporous layer having the properties described above for the microporous layer containing a polyolefin (e.g., one or more of a specific melt flow rate (MFR) and molecular weight distribution (Mw/Mn)), or may be a polyethylene microporous layer that does not have these properties. Since polyethylene has a melting point suitable for melt shutdown, it is preferable to include an additional microporous layer containing polyethylene as a main component from the viewpoint of shutdown characteristics.

The MFR of the polyethylene measured under a load of 2.16 kg at a temperature of 190°C is preferably 0.1 g/10 min or more, more preferably 0.15 g/10 min or more, even more preferably 0.18 g/10 min or more, and particularly preferably 0.2 g/10 min or more, from the viewpoints of shutdown characteristics, good pore opening, and clogging prevention. From the viewpoint of separator strength, it is preferably 2.0 g/10 min or less, more preferably 1.0 g/10 min or less, even more preferably 0.8 g/10 min or less, and particularly preferably 0.7 g/10 min or less.

The multilayer structure is preferably a multilayer structure in which three or more microporous layers are laminated. More preferably, the multilayer structure has at least two microporous layers (A) that contain the same constituent polymer and are primarily composed of the polyolefin of this embodiment, and at least one microporous layer (B) that contains a constituent polymer different from that of the microporous layer (A) and may or may not contain the polyolefin of this embodiment as the primary component. Even more preferably, the multilayer structure has at least two microporous layers (PP microporous layers) primarily composed of the polypropylene of this embodiment, and at least one further microporous layer (PE microporous layer) primarily composed of polyethylene.

While the advantages of this embodiment can be achieved regardless of the order in which the layers are laminated, a three-layer structure in which the layers are laminated in the order of PP microporous layer/PE microporous layer/PP microporous layer is particularly preferred. The three-layer structure of PP microporous layer/PE microporous layer/PP microporous layer allows the PE microporous layer to provide good shutdown properties while the PP microporous layer maintains good mechanical strength. The PP microporous layer is the microporous layer in this embodiment that has the specific melt tension and melt flow rate (MFR) described above for the microporous layer containing polyolefin. Meanwhile, the intermediate PE microporous layer may be a polyethylene microporous layer that has the specific melt tension and melt flow rate (MFR) described above, or it may be a polyethylene microporous layer that does not have these properties.

<Thickness of microporous layer>
The upper limit of the thickness of the microporous layer (thickness of a single layer) in the multilayer structure of this embodiment included in the separator for a power storage device of this embodiment is preferably 8 μm or less, 7.5 μm or less, 7 μm or less, 6.5 μm or less, 6 μm or less, 5.5 μm or less, or 5 μm or less, from the viewpoint of increasing the energy density of the power storage device, etc. The lower limit of the thickness of the microporous layer (thickness of a single layer) of this embodiment is preferably 1 μm or more, 2 μm or more, 3 μm or more, 3.5 μm or more, 4 μm or more, or 4.5 μm or more, from the viewpoint of strength, etc.

<Separator thickness>
The upper limit of the overall thickness of the separator for an electricity storage device, which may be single-layer or multi-layer, is preferably 18 μm or less, 17 μm or less, 16.5 μm or less, 16 μm or less, 15.5 μm or less, 15 μm or less, 14.5 μm or less, or 14 μm or less, from the viewpoint of increasing the energy density of the electricity storage device, etc. The lower limit of the overall thickness of the separator is preferably 3 μm or more, 5 μm or more, 8 μm or more, 10 μm or more, 11 μm or more, 11.5 μm or more, 12 μm or more, 12.5 μm or more, or 13 μm or more, from the viewpoint of strength, etc.

<Porosity of separator>
The porosity of the separator for an electricity storage device is preferably 20% or more, more preferably 25% or more, even more preferably 30% or more, and particularly preferably 35% or more from the viewpoint of suppressing clogging in the electricity storage device and obtaining good air permeability of the separator, and is preferably 70% or less, more preferably 65% or less, even more preferably 60% or less, and particularly preferably 55% or less from the viewpoint of maintaining the strength of the separator. The porosity is measured by the method described in the Examples.

<Air permeability (air resistance)>
The upper limit of the air permeability of the separator for an electricity storage device, when converted to a separator thickness of 14 μm, is preferably 300 sec/100 ^cm3 or less, 290 sec/100 ^cm3 or less, 280 sec/100 ^cm3 or less, 270 sec/100 ^cm3 or less, or 260 sec/100 ^cm3 or less. The lower limit of the air permeability of the separator for an electricity storage device, when converted to a separator thickness of 14 μm, may be, for example, 100 sec/100 ^cm3 or more, 110 sec/100 ^cm3 or more, or 120 sec/100 ^cm3 or more.

<Heat shrinkage rate>
The upper limit of the MD heat shrinkage rate when the separator for an electricity storage device of this embodiment is heat-treated at 105°C for 1 hour is 4% or less, preferably 3.5% or less, 3% or less, or 2.5% or less, from the viewpoint of safety at high temperatures of the electricity storage device. If the MD heat shrinkage rate is greater than 4%, the separator will shrink inside the electricity storage device at high temperatures, increasing the risk of deformation or short-circuiting of the stacked electrode body and wound electrode body. From the viewpoint of ease of manufacturing the separator for an electricity storage device, the lower limit of the MD heat shrinkage rate may be, for example, 0% or more, 0.5% or more, or 1% or more.

The upper limit of the TD heat shrinkage rate when the separator for an electricity storage device of this embodiment is heat-treated at 105°C for 1 hour is 1% or less, preferably 0.9% or less, and more preferably 0.8% or less, from the viewpoint of safety at high temperatures for the electricity storage device. While a lower TD heat shrinkage rate is desirable, from the viewpoint of ease of manufacturing the separator for an electricity storage device, it may be, for example, -1% or more, -0.5% or more, or 0% or more.

<Piercing strength>
The lower limit of the puncture strength of the separator for an electricity storage device, when converted to a thickness of 14 μm, is preferably 300 gf or more, 310 gf or more, 320 gf or more, 330 gf or more, 340 gf or more, 350 gf or more, or 360 gf or more. The upper limit of the puncture strength of the separator for an electricity storage device, when converted to a thickness of 14 μm, is preferably 450 gf or less, 440 gf or less, or 430 gf or less.

<Balance of thickness, breathability and puncture resistance>
The separator for an electric storage device of this embodiment uses a microporous layer containing a polyolefin having a specific melt flow rate (MFR), and can have low air permeability and high strength despite being thin. For example, in the separator for an electric storage device of this embodiment (particularly a separator including a multilayer structure of a microporous layer), it is more preferable that the thickness is 18 μm or less, the air permeability when converted to a thickness of 14 μm is 300 sec/100 cm ³ or less, and the pin puncture strength is 300 gf or more. It is even more preferable that the thickness of the multilayer structure is 18 μm or less, the thickness (thickness as a single layer) of the microporous layer of this embodiment included in the multilayer structure is 5 μm or less, the air permeability when converted to a thickness of 14 μm is 300 sec/100 cm ³ or less, and the pin puncture strength is 300 gf or more.

<Balance of thickness, porosity and air permeability>
In one aspect, the separator for an electricity storage device of this embodiment can have high porosity and low air permeability while being a thin film. In a preferred aspect, the separator for an electricity storage device has a thickness of 18 μm or less, a porosity of 42% or more, and an air permeation resistance of 250 sec/100 cm or ^less .

<<Method for manufacturing separator for electricity storage device>>
The method for producing the microporous layer of this embodiment generally includes a melt-extrusion step in which a resin composition containing the above-described polyolefin (hereinafter referred to as a polyolefin resin composition) is melt-extruded to obtain a resin film, and a hole-forming step in which the obtained resin film is perforated to make it porous, and may further include an annealing step, a stretching step, a heat-relaxing step, etc. after the hole-forming step, as necessary. The method for producing the microporous layer is broadly divided into a dry method in which no solvent is used in the hole-forming step, and a wet method in which a solvent is used. Examples of the melt-extrusion method include a T-die method and an inflation method.

In one embodiment, the polyolefin resin composition is a composition consisting of the components of the microporous layer. The polyolefin resin composition may optionally contain resins other than polyolefins, additives, etc., depending on the method for producing the microporous layer or the desired physical properties of the microporous layer. Examples of additives include pore-forming materials, fluorine-based flow modifiers, elastomers, waxes, crystal nucleating materials, antioxidants, metal soaps such as metal salts of aliphatic carboxylic acids, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and color pigments. Examples of pore-forming materials include plasticizers, inorganic fillers, and combinations thereof.

Examples of plasticizers include 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.

Examples of inorganic fillers include oxide ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide; nitride ceramics such as silicon nitride, titanium nitride, and boron nitride; ceramics such as silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; and glass fiber.

The extruder temperature during melt-kneading is preferably at least 20°C higher than the melting point of the polyolefin resin composition and not higher than 110°C higher than the melting point of the polyolefin resin composition. A temperature above the lower limit is preferable in that it ensures a uniform thickness for the resulting polyolefin resin film, thereby reducing the risk of the film breaking during extrusion. A temperature below the upper limit is preferable in that the polyolefin contained in the polyolefin resin composition is highly oriented, resulting in a favorable formation of a lamellar structure in the polyolefin. Therefore, a temperature below the upper limit is preferable in that it reduces the air permeability of the resulting separator for an electric storage device, thereby lowering the resistance of electric storage devices such as lithium-ion secondary batteries. When a polyolefin resin film has multiple layers formed using different polyolefin resin compositions, the melting point refers to the melting point of the polyolefin resin composition with the lowest melting point. In one embodiment, the melting point is evaluated as the peak top temperature of the endothermic peak with the greatest height observed when the temperature is increased at a rate of 10°C/min using a DSC (differential scanning calorimeter).

The draw ratio when the polyolefin resin composition is extruded into a film from the extruder is preferably 50 or more, more preferably 75 or more, and particularly preferably 100 or more, and preferably 400 or less, more preferably 350 or less, and particularly preferably 300 or less. The draw ratio refers to the value obtained by dividing the lip clearance of a T-die or inflation die by the thickness of the polyolefin resin film extruded from the die lip. By adjusting the draw ratio within the above range, the molecular orientation of the polyolefin is improved, lamellae are formed well, and the air permeability of the separator can be controlled to a low level. At the same time, the film formation stability of the film is not reduced, and the thickness and width precision of the resulting polyolefin resin film can be improved.

By thoroughly cooling the polyolefin resin composition extruded from the die with air, the polyolefin crystallizes to form lamellae, achieving good air permeability. The air flow rate is preferably 300 L/min or more to improve air permeability, and is preferably 600 L/min or less to improve film formation stability and the thickness and width precision of the resulting polyolefin resin film.

Methods for producing a multilayer electricity storage device separator having a laminated structure of multiple microporous layers, including the microporous layer of this embodiment, include, but are not limited to, coextrusion and lamination. In coextrusion, the resin compositions for each layer are coextruded and laminated into two or more layers to produce a raw film. The resulting raw film of two or more layers can be stretched and perforated to produce a microporous layer. It is easier to obtain a high-strength microporous layer by producing a raw film of two or more laminated layers and then perforating the resulting film, rather than by producing single microporous layers individually. In lamination, each layer is produced separately and then bonded together to adhere the layers to each other. Lamination methods include dry lamination, which uses an adhesive, and thermal lamination, which bonds multiple layers together by heating. However, thermal lamination is preferred from the perspective of further improving the air permeability and strength of the resulting electricity storage device separator.

Next, an annealing process is performed in which the resulting polyolefin resin film is heat-treated. This annealing process grows the lamellae formed in the polyolefin resin film during the extrusion process. This treatment improves the film's openability and reduces the air permeability of the resulting separator for an electricity storage device.

The annealing temperature of a polyolefin resin film is preferably set to at least "a temperature 40°C lower than the melting point of the polyolefin resin film" and not higher than "a temperature 1°C lower than the melting point of the polyolefin resin film." However, if the polyolefin resin film has a laminate structure, the annealing temperature is preferably set to at least "a temperature 40°C lower than the melting point of the resin film with the lowest melting point" and not higher than "a temperature 1°C lower than the melting point of the resin film with the lowest melting point." The annealing temperature refers to the temperature of the atmosphere inside the annealing device. If the annealing temperature is above the above lower limit, lamellae grow well and are easily opened during the film stretching process. If the annealing temperature is below the above upper limit, collapse of the lamellar structure due to relaxation of the orientation of the polyolefin in the polyolefin resin film is suppressed.

The annealing time for the polyolefin resin film is preferably 10 minutes or longer, from the viewpoint of favorable lamella growth and favorable pore opening in the polyolefin resin film during the stretching process. In one embodiment, the annealing time may be 180 minutes or shorter.

The annealing of the polyolefin resin film may be carried out while the polyolefin resin film is running, or may be carried out while the polyolefin resin film is wound up in a roll.

Next, a stretching process is carried out in which the annealed polyolefin resin film is stretched to form holes. This stretching process preferably includes a first stretching process and a second stretching process subsequent to the first stretching process.

In the first stretching process, the lamellae formed in the polyolefin resin film are separated, causing microcracks to form in the amorphous portions between the lamellae, and these cracks act as starting points to form numerous micropores. In the first stretching process, uniaxial stretching in the MD is performed.

In the first stretching step, the temperature of the polyolefin resin film is preferably -20°C or higher, more preferably 0°C or higher, and preferably 110°C or lower, more preferably 80°C or lower. If the temperature is above the above lower limit, breakage of the polyolefin resin film during stretching is suppressed. If the temperature is below the above upper limit, cracks are effectively generated in the amorphous portions between the lamellae, suppressing necking of the film.

In the first stretching step, the stretching ratio of the polyolefin resin film is preferably 1.02 times or more, more preferably 1.06 times or more, and preferably 1.5 times or less, more preferably 1.4 times or less. If the stretching ratio is equal to or greater than the above-mentioned lower limit, micropores are likely to form in the amorphous portions between the lamellae. If the stretching ratio is equal to or less than the above-mentioned upper limit, excessive micropores are not formed and the pore diameter does not become too small, thereby allowing for low air permeability. In the present disclosure, the stretching ratio of a polyolefin resin film refers to the value obtained by dividing the length of the polyolefin resin film after stretching by the length of the polyolefin resin film before stretching.

The stretching speed in the first stretching step of the polyolefin-based resin film is preferably 10%/min or more, more preferably 50%/min or more, and preferably 1000%/min or less, more preferably 600%/min or less. If the stretching speed is above the above lower limit, micropores are likely to be uniformly formed in the amorphous portions between the lamellae. If the stretching speed is below the above upper limit, breakage of the propylene-based resin film can be suppressed.

In this disclosure, the stretching speed of a polyolefin resin film refers to the rate of change in dimension of the polyolefin resin film in the stretching direction per unit time.

The method for stretching the polyolefin resin film in the first stretching step is not particularly limited as long as it can uniaxially stretch the polyolefin resin film. For example, a method in which the polyolefin resin film is stretched at a predetermined temperature using a uniaxial stretching device can be used.

Next, the polyolefin resin film after uniaxial stretching in the first stretching step is preferably subjected to a second stretching step in which the ambient temperature inside the apparatus is stretched to a temperature higher than the ambient temperature during uniaxial stretching in the first stretching step and lower by 1°C to 40°C below the melting point of the polyolefin resin film (or the resin film with the lowest melting point in the case of a multilayer structure). In the second stretching step, the polyolefin resin film is preferably uniaxially stretched only in the machine direction. By stretching the polyolefin resin film at an ambient temperature higher than the ambient temperature inside the apparatus in the first stretching step, the numerous micropores formed in the polyolefin resin film in the first stretching step can be grown. If the temperature is above the lower limit, the micropores formed in the polyolefin resin film in the first stretching step are more likely to grow, thereby reducing the air permeability of the resulting separator for a storage battery device. If the temperature is below the upper limit, the micropores formed in the polyolefin resin film in the first stretching step are less likely to clog, thereby reducing the air permeability of the resulting separator for a storage battery device.

In the second stretching step, the stretching ratio of the polyolefin resin film is preferably 1.5 times or more, more preferably 1.8 times or more, and preferably 3 times or less, more preferably 2.5 times or less. If the stretching ratio is above the above lower limit, the micropores formed in the polyolefin resin film during the first stretching step are likely to grow, resulting in a low air permeability of the resulting separator for an electricity storage device. If the stretching ratio is below the above upper limit, the micropores formed in the polyolefin resin film during the first stretching step are unlikely to be blocked, resulting in a low air permeability of the resulting separator for an electricity storage device.

In the second stretching step, the stretching speed of the polyolefin resin film is preferably 60%/min or less, or 30%/min or less, from the viewpoint of uniformly widening the micropores formed in the first stretching step. From the viewpoint of process efficiency, the stretching speed may be, for example, 2%/min or more, or 3%/min or more.

The method for stretching the polyolefin resin film in the second stretching step is not particularly limited as long as it can uniaxially stretch the polyolefin resin film. For example, a method of uniaxially stretching the polyolefin resin film at a predetermined temperature using a uniaxial stretching device can be used.

The polyolefin resin film uniaxially stretched in the second stretching step is then subjected to a heat-relaxing step in which it is heated to relieve residual stress. Residual stress may be generated in the polyolefin resin film due to the stretching in the second stretching step. The heat-relaxing step is performed to relieve the residual stress and prevent the resulting polyolefin resin microporous layer from thermally shrinking due to heating, thereby improving the safety of the resulting power storage device separator.

As mentioned above, to improve the dimensional stability of the polyolefin resin microporous layer during heating, it is necessary to relax the residual stress in the polyolefin resin film. To achieve this, the ambient temperature within the apparatus during the heat-relaxing step is preferably 40°C lower or higher, or 20°C lower or higher, than the melting point of the polyolefin resin film (the resin film with the lowest melting point in the case of a multilayer structure). Furthermore, from the perspective of preventing blockage of the micropores formed in the stretching step, the above temperature is preferably at least 1°C lower, or at least 4°C lower, than the melting point of the polyolefin resin film (the resin film with the lowest melting point in the case of a multilayer structure).

The lower limit of the heat shrinkage rate of the polyolefin resin film in the heat-relaxing step is preferably 25% or more, more preferably 30% or more. The upper limit of the heat shrinkage rate of the polyolefin resin film in the heat-relaxing step is preferably 60% or less, more preferably 50% or less. The heat shrinkage rate of the polyolefin resin film in the heat-relaxing step refers to the value obtained by dividing the shrinkage length of the polyolefin resin film in the stretching direction in the heat-relaxing step by the length of the polyolefin resin film in the stretching direction after the second stretching step, and multiplying the result by 100. When the heat shrinkage rate is at least the above lower limit, the residual stress in the polyolefin resin film is well relaxed, the resulting polyolefin resin microporous layer has good dimensional stability during heating, and the safety of electricity storage devices such as lithium-ion secondary batteries at high temperatures is good. When the heat shrinkage rate is at most the above upper limit, sagging of the polyolefin resin film is less likely to occur, and poor roll winding or reduced uniformity is suppressed.

In the second stretching step and the heat-relaxing step, it is preferable to adjust the stretching speed and conveying speed so that they do not become excessively high, in order to effectively relax stress and reduce the thermal shrinkage rate.

Furthermore, after the heat-relaxing step, it is preferable to apply heat-relaxing again at a temperature equal to or higher than the temperature used in the heat-relaxing step, but 20°C higher or lower than the temperature used in the heat-relaxing step. By applying this step, it is possible to obtain a separator for an electricity storage device with better heat shrinkage properties.

<Energy storage device>
The electricity storage device of this embodiment includes the electricity storage device separator of this embodiment. The electricity storage device of this embodiment has a positive electrode and a negative electrode. The electricity storage device separator is preferably laminated between the positive electrode and the negative electrode, or is located on the outside of the positive electrode or negative electrode within the battery exterior, or wraps the electrode. If desired, lead bodies may be connected to the positive electrode and the negative electrode, respectively, so that the positive electrode and the negative electrode can be connected to external equipment, etc.

Examples of power storage devices include, but are not limited to, lithium secondary batteries, lithium ion secondary batteries, sodium secondary batteries, sodium 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-metal hydride 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 these, lithium secondary batteries, lithium ion secondary batteries, nickel-metal hydride batteries, and lithium ion capacitors are preferred, with lithium ion secondary batteries being more preferred, from the standpoint of compatibility with the separator of this embodiment.

The electricity storage device can be produced, for example, by the following method:
The positive electrode and the negative electrode are stacked with the separator described above interposed therebetween and wound as necessary to form a stacked electrode body or a wound electrode body. The stacked electrode body or the wound electrode body is then loaded into an outer casing. At this time, lead bodies are connected to the positive and negative electrodes within the outer casing, and the ends of the lead bodies can be arranged so that they extend outside the outer casing. When there are multiple positive and negative electrodes, tabs of the same electrode may be joined by welding or the like to form a single lead body that extends outside the outer casing. The tabs of the same electrode may be formed from exposed portions of the current collectors, or may be formed by welding metal pieces to the exposed portions of the current collectors. The positive and negative electrodes are connected to the positive and negative electrode terminals of the outer casing via lead bodies or the like. At this time, portions of the lead body and portions of the outer casing may be joined by heat fusion or the like. Furthermore, a nonaqueous electrolyte solution containing a nonaqueous solvent such as a chain and/or cyclic carbonate and an electrolyte such as a lithium salt is injected into the outer casing. The outer casing is then sealed to produce an electricity storage device.

The present invention will be explained in detail below using examples and comparative examples, but the present invention is not limited to the following examples.

<<Measurement and Evaluation Methods>>
[Measurement of Melt Flow Rate (MFR)]
The melt flow rate (MFR) of the microporous layer was measured in accordance with JIS K 7210 at a temperature of 230°C and a load of 2.16 kg (unit: g/10 min). The MFR of polypropylene was measured in accordance with JIS K 7210 at a temperature of 230°C and a load of 2.16 kg. The melt flow rate (MFR) of polyethylene was measured in accordance with JIS K 7210 at a temperature of 190°C and a load of 2.16 kg. The MFR of elastomers was measured in accordance with JIS K 7210 at a temperature of 230°C and a load of 2.16 kg.

[Measurement of Mw and Mn by GPC (Gel Permeation Chromatography)]
A calibration curve was prepared by measuring standard polystyrene under the following conditions using an Agilent PL-GPC220 manufactured by Agilent Technologies, Inc. Chromatography was also performed on the sample polymer under the same conditions, and based on the calibration curve, the weight average molecular weight (Mw), number average molecular weight (Mn), and the value (Mw/Mn) obtained by dividing the weight average molecular weight (Mw) by the number average molecular weight (Mn) of the polymer were calculated under the following conditions.
Column: TSKgel GMHHR-H (20) HT (7.8 mm ID x 30 cm) x 2 Mobile phase: 1,2,4-trichlorobenzene Detector: RI
Column temperature: 160°C
Sample concentration: 1 mg/ml
Calibration curve: Polystyrene

[Melt Tension Measurement]
The melt tension (mN) of the microporous layer was measured under the following conditions using a Capillograph manufactured by Toyo Seiki Seisaku-sho, Ltd.
Capillary: diameter 1.0 mm, length 20 mm
Cylinder extrusion speed: 2 mm/min. Take-up speed: 60 m/min. Temperature: 230°C.

[Measurement of Pentad Fraction]
The pentad fraction of polypropylene was calculated by the peak height method from ^the C-NMR spectrum assigned based on the description in the Polymer Analysis Handbook (edited by the Japan Society for Analytical Chemistry). The C-NMR spectrum was measured using a ^JEOL -ECZ500 manufactured by JEOL Ltd., by dissolving the polypropylene in o-dichlorobenzene-d, at a measurement temperature of 145°C and 25,000 cycles.

[Measurement of wide-angle X-ray scattering (orientation ratio)]
The (110) crystal peak area ratio (MD/TD) of the polypropylene microporous layer was measured by transmission wide-angle X-ray scattering (WAXS) under the following conditions:
Device name: NANOPIX, manufactured by Rigaku Corporation X-ray wavelength λ: 0.154 nm
Optical system: Point collimation
1st slit: 0.55mmφ
2nd slit: Open
3rd slit: 0.35mmφ
Exposure time: 900 seconds Detector: HyPix-6000 (2D detector)
Camera length: 85.7 mm

X-rays were incident on a single microporous layer sample from the film normal direction, and transmitted and scattered light was detected. To minimize scattering from sources other than the sample, measurements were performed using a vacuum chamber in which the entire area from the sample to the beam stop was installed in a vacuum. Because the HyPix-6000 detector has a blind area, two measurements were taken by moving the detector vertically and combining the results to obtain two-dimensional data without the blind area. Transmittance correction and empty-cell scattering correction were then performed on the obtained two-dimensional WAXS pattern. Next, circular averaging was performed to convert the scattering data into one dimension, and the Bragg angles θs and θe, corresponding to the small-angle and wide-angle tails of the crystalline peak derived from the (110) plane of polypropylene, were determined. The azimuthal distribution of scattering intensity in the range 2θs < 2θ < 2θe (the azimuthal distribution of crystalline diffraction peak intensity derived from the (110) plane) was then calculated for the two-dimensional WAXS pattern after transmittance correction and empty-cell scattering correction. An example of the obtained azimuthal angle distribution of scattering intensity in the range of 2θs<2θ<2θe is shown in Figure 1. In the azimuthal angle distribution diagram of scattering intensity in the range of 2θs<2θ<2θe, a (110) peak derived from c-axis oriented crystals, in which the crystal c-axis is oriented in the MD, is observed in the TD, and a (110) peak derived from a-axis oriented crystals, in which the crystal a-axis is oriented in the MD, is observed near the MD. Peak separation was performed by approximating the peak derived from c-axis oriented crystals with one Gaussian function and the peak derived from a-axis oriented crystals with two Gaussian functions. An example is shown in Figure 1. WaveMetrics software IgorPro8 ver. 8.0.0.10 was used for peak separation. If the peak area derived from c-axis oriented crystals (crystals whose c-axis is oriented in the MD) obtained by this peak separation is defined as S_MD, and the peak area (sum of the areas of the two Gaussian functions) derived from a-axis oriented crystals (crystals whose c-axis is oriented in a direction close to TD) is defined as S_TD, then the (110) crystal peak area ratio (MD/TD) is defined as S_MD/S_TD. As shown in Figure 1, two peaks derived from c-axis oriented crystals and two peaks derived from a-axis oriented crystals are observed in the azimuthal angle distribution diagram of the scattering intensity. Therefore, the averages of the respective peak areas were defined as S_MD and S_TD.

[Measurement of thickness (μm)]
The thickness (μm) of the separator was measured at room temperature of 23±2° C. using a Digimatic Indicator IDC112 manufactured by Mitutoyo Corporation.

[Measurement of porosity (%)]
A sample measuring 10 cm x 10 cm square was cut from the separator, and its volume (cm ³ ) and mass (g) were determined. From these and the density (g/cm ³ ), the porosity was calculated using the following formula.
Porosity (%) = (volume - mass / density) / volume x 100

[Measurement of air permeability resistance (sec/100 cm ³ )]
The air resistance (seconds/100 ^cm3 ) of the separator was measured using a Gurley air permeability meter in accordance with JIS P-8117, divided by the thickness (unit: μm), and then multiplied by 14 to calculate the air resistance (air permeability) converted to a thickness of 14 μm.

[Measurement of puncture strength]
A needle with a hemispherical tip and a radius of 0.5 mm was prepared, and a separator was sandwiched between two plates with openings of 11 mm diameter (dia.), and the needle, separator, and plates were set. A puncture test was performed using an "MX2-50N" made by Imada Co., Ltd. under the following conditions: a needle tip curvature radius of 0.5 mm, an opening diameter of 11 mm in the separator holding plate, and a puncture speed of 25 mm/min. The needle and separator were brought into contact, and the maximum puncture load (i.e., puncture strength (gf)) was measured. The obtained puncture strength was divided by the thickness (unit: μm) and then multiplied by 14 to calculate the puncture strength equivalent to a thickness of 14 μm.

[Measurement of Heat Shrinkage Rate]
The separator was cut into 50 mm squares in both the MD and TD directions, and the resulting samples were placed in a hot air dryer (DF1032, manufactured by Yamato Scientific Co., Ltd.) heated to 105°C (atmospheric pressure, in the atmosphere). After one hour, the samples were removed from the hot air dryer, and the thermal shrinkage was determined. The samples were placed on copy paper before being placed in the hot air dryer, so as to prevent them from adhering to the inner walls of the dryer and to prevent the samples from fusing together.
Heat shrinkage rate (%): (dimension before heating (mm) - dimension after heating (mm)) / (dimension before heating (mm)) x 100

[Preparation of Positive Electrode]
LiNi _1/3 Mn _1/3 Co _1/3 O ₂ as the positive electrode active material, carbon black as a conductive additive, and polyvinylidene fluoride solution as a binder were mixed in a solids mass ratio of 91:5:4, and N-methyl-2-pyrrolidone was added as a dispersion solvent to a solids content of 68% by mass, and further mixed to prepare a slurry solution. This slurry solution was applied to one side of a 15 μm thick aluminum foil so that a portion of the aluminum foil was exposed, and then the solvent was dried and removed, resulting in a coating amount of 175 g / m ^2. The positive electrode mixture was then rolled with a roll press so that the density of the positive electrode mixture was 2.8 g / cm ^3. The coated portion was then cut to 20 mm x 20 mm and included the exposed aluminum foil portion to obtain a positive electrode.

[Fabrication of negative electrode]
Artificial graphite as the negative electrode active material, styrene butadiene rubber as a binder, and a carboxymethyl cellulose aqueous solution were mixed in a solids mass ratio of 96.4:1.9:1.7, and water was added as a dispersion solvent to a solids content of 50% by mass, followed by further mixing to prepare a slurry solution. This slurry solution was applied to one side of a 10 μm thick copper foil so that a portion of the copper foil was exposed, and the solvent was then dried and removed to a coating amount of 86 g/m ^2. The negative electrode mixture was then rolled with a roll press so that the density of the negative electrode mixture portion was 1.45 g/cm ^3. The coated portion was then cut to 25 mm x 25 mm and included the exposed copper foil portion to obtain a negative electrode.

[Preparation of non-aqueous electrolyte]
A non-aqueous electrolyte solution was prepared by dissolving _LiBF as a solute in a mixed solvent of propylene carbonate:ethylene carbonate:γ-butyl lactone = 1:1:2 (volume ratio) to a concentration of 1.0 mol/L, and further adding trioctyl phosphate to the solution to a concentration of 0.5 wt %.

[Fabrication of laminated electrode body]
A 30 mm × 30 mm square sample was cut from the separator and immersed in the nonaqueous electrolyte for 1 minute or more to prepare Sample 1. The negative electrode, Sample 1, positive electrode, Kapton film, and 4 mm thick silicone rubber were laminated in this order to prepare a laminated electrode body.

[Fuse short test]
This laminated electrode assembly was placed on a ceramic plate with an embedded thermocouple, and a surface pressure of 1.5 MPa was applied using a hydraulic press while the heater was heated. The temperature and resistance were continuously measured using an AC electrical resistance measuring device "AG-4311" (manufactured by Ando Electric Co., Ltd.) connected to the current collectors of the positive and negative electrodes. The temperature was raised from room temperature of 23°C to 220°C at a rate of 15°C/min, and the impedance was measured using an AC of 1 kHz. By using actual positive and negative electrodes and applying surface pressure, it is possible to reflect the fuse short behavior within the battery.
The value calculated by multiplying the obtained impedance (Ω) by the effective positive electrode area of 4 ^cm2 was taken as the impedance (Ω· ^cm2 ) converted into positive electrode unit area. The temperature at which the impedance converted into positive electrode unit area reached 100 Ω· ^cm2 was taken as the fuse temperature (°C), and the temperature at which the impedance converted into positive electrode unit area again fell below 100 Ω· ^cm2 after reaching the hole-blocked state was taken as the short-circuit temperature (°C).

[Fabrication of Laminated Cell (Lithium-ion Secondary Battery Sample)]
A positive electrode was obtained in the same manner as in the above [Preparation of Positive Electrode], except that the tab portion was removed and the positive electrode was punched out into a rectangular shape of 30 mm x 50 mm.
A negative electrode was obtained in the same manner as in the above [Preparation of negative electrode], except that the tab portion was removed and the blank was punched out into a rectangular shape of 32 mm x 52 mm.
A solution (LBG00069, manufactured by Kishida Chemical Co., Ltd.) containing 1 mol/L of LiPF ₆ salt as a lithium salt in a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1:2 as a nonaqueous solvent was used.
The separators prepared in the examples and comparative examples described below were punched into 34 mm x 54 mm rectangles, and the layer made of a mixture containing the positive electrode active material of the positive electrode prepared above and the layer made of a mixture containing the negative electrode active material of the negative electrode prepared above were placed facing the separator surface, and the positive electrode, separator, and negative electrode were stacked in this order. After inserting the positive and negative electrode terminals protruding into a bag made of a laminate film coated with a resin layer on both sides of aluminum foil (thickness 40 μm), 0.6 g of electrolyte was added, and the pressure was reduced to -90 kPa. This operation of reducing the pressure to -30 kPa was repeated twice, and then the pressure was held at -95 kPa for 5 minutes. After returning to normal pressure, an additional amount of electrolyte was added so that the total amount was 0.6 g. Thereafter, the pressure was reduced to -85 kPa, and sealing was performed to obtain a laminate cell.

[Heating test]
The obtained lithium ion secondary battery was placed in a thermostatic bath (manufactured by Futaba Scientific Co., Ltd., product name: PLM-73S) set to 25°C, connected to a charge/discharge device (manufactured by Asuka Electronics Co., Ltd., product name: ACD-01), and charged to 4.35 V at a constant current of 0.05 C. After the voltage reached 4.35 V, the battery was charged at a constant voltage of 4.35 V for 2 hours, and then discharged to 3.0 V at a constant current of 0.2 C.

The battery was then charged at a constant current of 0.33C until the voltage reached 4.35V, after which it was charged at a constant voltage of 4.35V for 1 hour, and then discharged at a constant current of 0.33C to 3.0V.

Then, charging and discharging were carried out in the same manner. Charging was carried out at 0.33C in the same manner, and after the voltage reached 4.35V, charging was carried out at a constant voltage of 4.35V for 1 hour. The charged cell was restrained at 1.5MPa and placed in an oven. The temperature was raised at 5°C/min to 200°C, and the voltage was monitored to see if a short circuit was observed. If no short circuit was observed, it was marked with an ◯, and if a short circuit was observed, it was marked with an ×. A short circuit here means that the cell voltage was below 2V.

Example 1
[Preparation of separator having microporous layer]
High molecular weight polypropylene (PP, MFR (230°C) = 0.51 g/10 min, density = 0.91 g/ ^cm3 , Mw = 900,000, Mw/Mn = 5.2, pentad fraction = 99.3%) was melted in a 2.5-inch extruder and fed using a gear pump to a T-die with a die width of 500 mm and a die lip clearance of 2.0 mm. The T-die temperature was set to 230°C, and the molten polymer was discharged from the T-die, sufficiently cooled by blowing in 25°C air at a rate of 400 L/min, and then wound up on a roll. Similarly, polyethylene (PE, MFR (190°C) = 0.38 g/10 min, density = 0.96 g/ ^cm3 ) was melted in a 2.5-inch extruder and fed using a gear pump to a T-die with a die lip width adjusted to 2.0 mm. The temperature of the T-die was set to 210 ° C. After the molten polymer was extruded from the T-die, it was thoroughly cooled by blowing air at 25 ° C. at a rate of 400 L / min and then wound up on a roll. The PP raw film and PE raw film wound up on the roll each had a thickness of 6 μm and a draw ratio of 330. Next, the PP raw film and the PE raw film were bonded to form a PP / PE / PP raw film, resulting in a raw film having a PP / PE / PP three-layer structure. The three-layer raw film was then annealed at 126 ° C. for 20 minutes. The annealed raw film was cold stretched to 8% at room temperature, then hot stretched to 185% at 116 ° C., and heat-relaxed to 45% at 126 ° C. to form micropores, resulting in a separator with a PP / PE / PP three-layer structure. The hot stretching and heat-relaxing rates were approximately 10% / min. After the stretching and perforation, the resulting three-layer structure separator was subjected to measurements of air permeability, puncture strength, fuse short circuit test, and heat shrinkage at 105° C. The results are shown in Table 1.

Examples 2 to 5
Separators having a microporous layer were obtained in the same manner as in Example 1, except that the raw materials and the shrinkage rate during heat relaxation were changed as shown in Tables 1 and 2, and the obtained separators were evaluated.

Comparative Examples 1 to 4
Separators having a microporous layer were produced and evaluated in the same manner as in Example 1, except that the raw materials and the shrinkage rates during heat relaxation were changed as shown in Table 2. However, samples for which a 6 μm-thick PP raw film could not be produced due to too high melt tension were deemed unproductionable.

Example 6
High molecular weight polypropylene (PP, MFR (230°C) = 0.51 g/10 min, density = 0.91 g/ ^cm3 , Mw = 900,000, Mw/Mn = 5.2, pentad fraction = 99.3%) was melted in a 2.5-inch extruder and fed using a gear pump to a T-die with a die width of 500 mm and a die lip clearance of 3.3 mm. The T-die temperature was set to 230°C, and after the molten polymer was extruded from the T-die, it was thoroughly cooled by blowing in 25°C air at a rate of 400 L/min and then wound up on a roll. The PP raw film wound up on the roll had a thickness of 15 μm and a draw ratio of 220. The three-layer raw film was then annealed at 130°C for 20 minutes. The annealed raw film was cold-stretched to 10% at room temperature, then hot-stretched to 185% at 130°C, and heat-relaxed to 45% at 130°C to form micropores, yielding a separator consisting of a single microporous PP layer. After the above stretching and perforation, the resulting three-layer separator was subjected to measurements of air permeability, puncture strength, fuse short circuit test, and heat shrinkage at 105°C. The results are shown in Table 3.

Comparative Example 5
Separators having a microporous layer were obtained in the same manner as in Example 6, except that the raw materials were changed as shown in Table 3, and the obtained separators were evaluated.

The separator for an electricity storage device of this embodiment is highly safe and can be made thin, making it suitable for use as a separator in electricity storage devices, such as lithium-ion secondary batteries.

Claims (7)

A separator for an electricity storage device having a microporous layer,
the microporous layer comprises a polyolefin having a melt flow rate (MFR) of 0.7 g/10 min or less at a load of 2.16 kg and a temperature of 230°C;
the separator for an electricity storage device has a short-circuit temperature of 200°C or higher in a fuse short-circuit test;
the heat shrinkage rate in the width direction (TD) and the heat shrinkage rate in the machine direction (MD) when the electricity storage device separator is heat-treated at 105°C for 1 hour are TD≦1% and MD≦4%, respectively;
The polyolefin is mainly composed of polypropylene, and
The pentad fraction of the polypropylene is 98.5% or more.
Separators for power storage devices. 2 . The electricity storage device separator according to claim 1 , wherein the polyolefin has a weight average molecular weight (Mw) divided by a number average molecular weight (Mn) (Mw/Mn) of 7 or less. 3. The separator for an electricity storage device according to claim 1 , wherein a fuse temperature in the fuse short circuit test is 150°C or less. The separator for an electricity storage device according to any one of claims 1 to 3 , having a multilayer structure of the microporous layer containing the polyolefin containing the polypropylene as a main component and a microporous layer containing polyethylene as a main component. The separator for an electricity storage device according to any one of claims 1 to 4, wherein, in wide-angle X-ray scattering measurement of the microporous layer containing the polyolefin containing the polypropylene as a main component , a ratio MD/ TD of the orientation rate in the machine direction (MD) to the orientation rate in the width direction (TD) is 1.3 or more. 6. The separator for an electricity storage device according to claim 1 , which has a thickness of 18 μm or less, a porosity of 42% or more, and an air resistance of 250 seconds/100 cm ³ or less. An electricity storage device comprising the electricity storage device separator according to any one of claims 1 to 6 .