JP2023151386A - Separator for storage battery and battery - Google Patents

Abstract

To provide a separator for a power storage device, the separator enabling reduction of resistance of the power storage device and improvement of the safety thereof to be made compatible with each other, and to provide a power storage device including the separator.SOLUTION: A separator for a power storage device is provided, including a polyolefin microporous membrane, and further including a porous layer containing lithium (Li) storage particles and inorganic particles on the polyolefin microporous membrane, and when the volume of the Li storage particles in the porous layer is A, the volume of the inorganic particles is B, the volume ratio (%) of the Li storage particles is A/(A+B)×100, and the volume ratio (%) of the inorganic particles is B/(A+B)×100, the separator for the power storage device has a volume ratio of the Li storage particles of 25% to 95%, and the volume ratio of the inorganic particles of 5% to 75%, and a power storage device or a non-aqueous electrolyte secondary battery including the separator for the power storage device is also provided.SELECTED DRAWING: Figure 1

Description

The present invention relates to an electricity storage device (hereinafter also simply referred to as a "battery") and a separator for an electricity storage device used in its power generation element.

Conventionally, in electricity storage devices, an electrolytic solution has been impregnated into a power generation element in which a battery separator is interposed between a positive electrode plate and a negative electrode plate.

In these power storage devices, under conditions such as charging the device with a relatively large current at a relatively low temperature, lithium (Li) is deposited on the negative electrode plate, and metal Li forms from this negative electrode plate into dendrites (dendritic crystals). ) may grow as If such dendrites continue to grow, they may break through or penetrate the separator to reach the positive electrode plate, or they may come close to the positive electrode plate, causing the dendrite itself to become a path and cause a short circuit. There is. For example, during a nail penetration test of a lithium ion secondary battery equipped with a conventional single-layer separator consisting of only a base material, when the nail penetrates the positive electrode, separator, and negative electrode, the electrons (e ^- ) in the nail are significantly transferred from the negative electrode to the positive electrode. Since it moves at high speed and Li ⁺ ions also move at extremely high speed from the negative electrode to the positive electrode, a large current flows and the amount of heat generated increases, which tends to impair safety.

Furthermore, in a battery, if the precipitated dendrite (metallic Li) is broken, highly reactive metallic Li may be present in a state where it is not electrically connected to the negative electrode plate. In this way, if there is a large amount of dendrite or metal Li caused by it in a battery, it may cause short circuits in other parts, short circuits between multiple parts, or when heat is generated due to overcharging, Even the highly reactive metal Li reacts, which tends to lead to further problems such as generation of heat, and also causes deterioration of battery capacity.

On the other hand, for example, Patent Document 1 discloses that dendrite growth is prevented by forming a layer of a compound capable of occluding Li and an insulating layer in an electricity storage device including positive and negative electrodes and a separator. It has been reported that it can be suppressed. Patent Document 2 reports that a material capable of occluding Li is disposed between electrodes of an electricity storage device.

Furthermore, in Patent Document 3, upper and lower porous insulating layers containing a mixture of a high-absorbency inorganic filler and a low-absorbency inorganic filler are arranged between electrodes or between an electrode and a separator, and a low-absorption inorganic filler is disposed on the electrode side. It has been reported that the layer containing a large amount of absorbent inorganic filler is oriented toward the separator, and the layer containing a large amount of superabsorbent inorganic filler is oriented toward the separator side.

International Publication No. 2020/230825 Japanese Patent Application Publication No. 2009-67657 Japanese Patent Application Publication No. 2011-25204

However, in the separators or power storage devices described in Patent Documents 1 to 3, there is room for further study on suppressing dendrite growth, and it is a challenge to achieve both lower resistance and improved safety of the constituent elements of power storage devices. there were. Further, in the conventional separator, there is room for consideration of achieving both low resistance and safety when a lithium (Li) metal negative electrode is used in an electricity storage device.

Therefore, an object of the present invention is to provide a separator for a power storage device that can achieve both lower resistance and improved safety of the power storage device, and a power storage device or non-aqueous electrolyte secondary battery including the separator. shall be.

As a result of extensive studies, the present inventors placed a porous layer on a polyolefin microporous membrane of a separator for electricity storage devices, and identified an appropriate volume ratio of lithium (Li) storage particles and inorganic particles in the porous layer. The inventors have discovered that the above problems can be solved by doing so, and have completed the present invention. Aspects of the present invention are listed below.
<1> A separator for a power storage device including a polyolefin microporous membrane,
A porous layer containing lithium (Li) storage particles and inorganic particles is provided on the polyolefin microporous membrane, and the volume A of the Li storage particles and the volume B of the inorganic particles in the porous layer are When the volume ratio (%) of the storage particles is A/(A+B)×100 and the volume ratio (%) of the inorganic particles is B/(A+B)×100, the volume ratio of the Li storage particles is 25% to 95. %, and a volume ratio of the inorganic particles is 5% to 75%.
<2> The separator for a power storage device according to item 1, wherein the average particle size of the inorganic particles is 0.30 to 1.5 times the average particle size of the Li storage particles.
<3> The separator for an electricity storage device according to item 1 or 2, wherein the porous layer has a porosity of 65.0% to 85.0%.
<4> According to any one of items 1 to 3, wherein the inorganic particles include at least one type selected from the group consisting of alumina, aluminum hydroxide, aluminum hydroxide oxide, aluminum silicate, barium sulfate, and zirconia. The separator for the electricity storage device described above.
<5> For the power storage device according to any one of items 1 to 4, wherein the Li storage particles include at least one type selected from the group consisting of tin oxide, silicon, tin, germanium, aluminum, and graphite. Separator.
<6> The separator for a power storage device according to any one of items 1 to 5,
A nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a nonaqueous electrolyte.
<7> The non-aqueous electrolyte secondary battery according to item 6, wherein the negative electrode is Li metal.

Advantageous Effects of Invention According to the present invention, there is provided a separator for an electricity storage device that can efficiently suppress dendrites and achieve both lower resistance and improved safety of an electricity storage device, and an electricity storage device or a non-aqueous electrolyte separator including the separator. It is possible to provide a secondary battery, and in turn, it is possible to achieve both low resistance and safety of a power storage device using a lithium (Li) metal negative electrode.

FIG. 1 is a schematic cross-sectional view of an electricity storage device including a separator for an electricity storage device for explaining the mechanism of the present invention, and contains lithium (Li) storage particles before expansion (a) and after expansion (b).

Hereinafter, an embodiment of the present invention (hereinafter referred to as "this embodiment") will be described in detail with reference to the drawings, but the present invention is not limited to this embodiment and the drawings. In this specification, the upper and lower limits of each numerical range can be arbitrarily combined. Furthermore, unless otherwise specified, each numerical value can be measured according to the method described in the Examples.

<Separator>
The separator for an electricity storage device (hereinafter referred to as "separator") according to the present embodiment includes a microporous polyolefin membrane, and a porous layer containing lithium (Li) storage particles and inorganic particles on the microporous polyolefin membrane. The volume ratio of lithium (Li) storage particles to inorganic particles in the porous layer has been specified.

In this embodiment, the volume of the Li storage particles in the porous layer of the separator is A, the volume of the inorganic particles is B, the volume ratio (%) of the Li storage particles is A/(A+B)×100, and the volume ratio of the inorganic particles (%) is When B/(A+B)×100, the volume ratio of Li storage particles is 25% to 95%, and the volume ratio of inorganic particles is 5% to 75%. The relationship of 25%≦A/(A+B)×100≦95% and 5%≦B/(A+B)×100≦75% means that in an electricity storage device, lithium or other metal dendrites are in contact with the porous layer of the separator. Then, the Li storage particles react with the dendrites, and voids are generated due to the difference in expansion between the inorganic particles and the Li storage particles, and the voids function as ion migration paths and pockets where the dendrites stay. be. Further, the above-mentioned generation of voids and the functions of ion movement paths or dendrite retention pockets become remarkable when a lithium (Li) metal negative electrode is used in the electricity storage device.

More specifically, when the porous layer of the separator according to the present embodiment comes into contact with the Li metal negative electrode, the Li storage particles expand, and the volume ratio and/or particle size ratio of the lithium (Li) storage particles and the inorganic particles change to the porous layer. Since the voids in the layer are adjusted to maintain a constant level, clogging of the porous layer is suppressed, and Li ion mobility during charging can also be ensured. Thereby, the separator according to this embodiment can achieve both the following two effects:
(I) Lower resistance: The porous layer containing Li storage particles functions similarly to the negative electrode in the electricity storage device, increasing the apparent specific surface area of the negative electrode and increasing the reaction rate of the negative electrode.
(II) Improved safety: It is believed that Li or other metal dendrites accumulate in the above-described voids of the porous layer and, without wishing to be bound by theory, dendrite growth is retarded.

Regarding the above effect (I), unlike the conventional separator which has a porous layer containing only Li storage particles and an insulating layer containing inorganic particles, the present embodiment has the advantage of achieving further lower resistance. In the separator, it is preferable that a porous layer containing lithium (Li) storage particles and inorganic particles be in contact with the negative electrode.

Regarding the above effect (II), in this embodiment, the relationships of 25%≦A/(A+B)×100≦95% and 5%≦B/(A+B)×100≦75% are optimized, so the dendrite The volume ratio of the inorganic particles is not so high that the particles pass through the porous layer and proceed casually. On the other hand, when Li storage particles are rich, the porous layer is too clogged and the reaction area decreases, causing electric field disturbance or local current flow. The volume ratio of Li storage particles is not so high that concentration occurs, dendrites segregate, and short circuits easily occur. Therefore, unlike conventional separators in which the short circuit resistance is lost after the Li occlusion particles occlude dendrites, the separator according to the present embodiment has a safe electric field stabilization and dendrite deposition due to voids rather than Li occlusion. Improve your sexuality.

Moreover, along with the above effects (I) and (II), the cycleability of the electricity storage device including the separator according to the present embodiment can also be ensured.

The configuration of the separator according to this embodiment will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view for explaining the mechanism of the separator according to this embodiment. In FIG. 1, in a power storage device, when lithium or other metal dendrites (Li-D or MD) grow from the negative electrode (1) and come into contact with the porous layer (2) of the separator (6), Li storage particles (3a) reacts with the dendrite and expands (3b), and the difference in expansion between the inorganic particles (4) and the Li storage particles (3b) generates voids, which secure a path for the movement of ions (i). , it has been shown that dendrites (Li-D or MD) are retained in the pocket (P).

In this embodiment, as long as the occlusion reaction between the Li occluding particles contained in the porous layer and the Li ions in the electricity storage device is sufficiently performed, the dendrites that trigger expansion or void formation are not only Li dendrites but also Li ions. Other metal dendrites may also be used.

In the porous layer of the separator according to the present embodiment, the volume ratio and/or particle size ratio of the lithium (Li) storage particles to the inorganic particles is set to one of the following from the viewpoint of the effects of the present invention explained above. relationship:
30%≦A/(A+B)×100≦90%;
10%≦B/(A+B)×100≦70%;
It is preferable to have one of the following relationships:
40%≦A/(A+B)×100≦80%;
20%≦B/(A+B)×100≦60%;
It is more preferable to have one of the following relationships:
42.0%≦A/(A+B)×100≦75.0%;
25.0%≦B/(A+B)×100≦58.0%;
The average particle size of the inorganic particles is within the range of 0.30 (times) to 1.5 (times) with respect to the average particle size of the Li storage particles;
It is more preferable to have one of the following relationships:
44.0%≦A/(A+B)×100≦70.0%;
30.0%≦B/(A+B)×100≦56.0%;
0.40 (times) ≦ Average particle size of inorganic particles / Average particle size of Li storage particles ≦ 1.40 (times)
It is even more preferable to have one of the following relationships:
44.0%≦A/(A+B)×100≦63.0%;
37.0%≦B/(A+B)×100≦56.0%;
0.70 (times) ≦ Average particle size of inorganic particles / Average particle size of Li storage particles ≦ 1.30 (times)
It is particularly preferable to have

The separator according to this embodiment may have a laminated structure including a microporous polyolefin membrane and a porous layer disposed thereon, or a structure in which the microporous polyolefin membrane is coated with a porous layer. Generally, a single-layer separator consists of a base material such as a microporous polyolefin membrane, and a laminated separator includes a base material and at least one layer laminated to the base material. At least one layer may have, for example, ion permeability, adhesiveness, thermoplasticity, inorganic porosity, etc., but from the viewpoint of the effects of the present invention, it is preferably not insulating. Moreover, at least one layer can be formed of a single membrane, or can constitute a porous layer having Li storage particles and inorganic particles as described above. The porous layer is not only formed as a single layer on the base material, but can also include a pattern such as dot coating or stripe coating formed on the base material.

The constituent elements, manufacturing method, physical properties, etc. of the separator according to this embodiment will be described below.

<Separator inclusions>
The separator includes a microporous polyolefin membrane, and has a porous layer containing lithium (Li) storage particles and inorganic particles on the microporous polyolefin membrane. If desired, the separator may contain a resin other than polyolefin, an organic filler, and the like in addition to the polyolefin, Li storage particles, and inorganic particles.

<Lithium (Li) storage particles>
In this specification, lithium (Li) storage particles refer to particulate materials capable of storing lithium (Li). As the Li storage particles, a compound capable of storing lithium (Li) (including intercalation, alloying, chemical conversion, etc.), such as a negative electrode active material of a lithium ion secondary battery, can be used in the form of particles. I can do it.

Specifically, materials for the Li storage particles include, for example, silicon, tin, germanium, aluminum, silicon monoxide, and lithium alloys (for example, lithium-aluminum, lithium-lead, lithium-tin, lithium-aluminum-tin, lithium metal-containing alloys such as lithium-gallium and wood alloys), carbon materials (e.g. graphite, hard carbon, low temperature fired carbon, amorphous carbon, etc.), metal oxides, lithium metal oxides (e.g. Li ₄ Ti ₅ O _12, etc.), polyphosphoric acid compounds, transition metal sulfides, and the like. Among these, it is preferable to use at least one selected from the group consisting of silicon, tin, germanium, aluminum, tin oxide, and graphite. Further, the Li storage material (A) may contain a compound that reacts with lithium, such as a compound that is reductively decomposed by lithium. By using such a material, it is possible to produce a power storage device with excellent safety, output, and cycle characteristics. More preferably, from the viewpoint of Li storage capacity per volume, the growth of Li dendrites can be suppressed over a long period of time by using tin, silicon, and/or tin oxide.

The shape of the Li storage particles is particulate from the viewpoint of heat resistance and permeability, and from the viewpoint of suppressing dendrites, it is preferably flaky, scaly, plate-like, block-like, or spherical, and more preferably, It is flaky, scaly, plate-like, or block-like.

The average particle size (Dp50 particle size) of the Li storage particles according to this embodiment is preferably 0.01 μm or more, more preferably 0.05 μm or more, still more preferably 0.1 μm or more, and even more preferably 0.3 μm. Above, it is particularly preferably 0.5 μm or more, and its upper limit is preferably 15.0 μm or less, more preferably 5.0 μm or less, and even more preferably 3.0 μm or less. It is preferable to adjust the average particle size to 0.01 μm or more from the viewpoint of reducing the water content of the separator. On the other hand, it is preferable to adjust the average particle size to 15.0 μm or less from the viewpoint of efficiently suppressing dendrite growth of lithium or other metals, and reducing the thermal shrinkage rate of the separator to make it difficult to rupture the membrane. . Further, it is preferable to adjust the average particle size to 3.0 μm or less from the viewpoint of forming a porous layer with a small layer thickness and from the viewpoint of dispersibility of inorganic particles in the porous layer.

The average particle size of the Li storage material is 1.5 to 50 times the average pore size of the layer that does not contain the Li storage material or the microporous polyolefin membrane, from the viewpoint of balancing the safety, electrical characteristics, and cycle characteristics of the power storage device. It is preferably 0 times, more preferably 3.0 times to 46.2 times, 5.0 times to 30.0 times, 10.0 times to 30.0 times, or 15.0 times to 30 times. .0 times.
In addition, in this specification, the average particle diameter of Li storage particles is a value measured according to the method using SEM in the measurement method of the Example mentioned later.

<Inorganic particles> Examples of inorganic particles include oxide ceramics such as alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide, and nitride ceramics such as silicon nitride, titanium nitride, and boron nitride. Ceramics, metal sulfates such as barium sulfate, silicon carbide, calcium carbonate, aluminum sulfate, aluminum hydroxide, aluminum hydroxide oxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite , mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, aluminum silicate, diatomaceous earth, ceramics such as silica sand, and glass fiber. These can be used alone or in combination of two or more. Among them, from the viewpoint of electrochemical stability, the inorganic particles preferably contain at least one type selected from the group consisting of alumina, aluminum hydroxide, aluminum hydroxide oxide, aluminum silicate, barium sulfate, and zirconia. or at least one type selected from the group consisting of these. In the present invention, lithium (Li) storage particles are excluded from the inorganic particles.

<Organic filler>
Examples of organic fillers include crosslinked polyacrylic acid, crosslinked polyacrylic ester, crosslinked polymethacrylic acid, crosslinked polymethacrylic ester, crosslinked polymethyl methacrylate, crosslinked polysilicone (polymethylsilsesquioxane, etc.), and crosslinked polystyrene. , various crosslinked polymer fine particles such as crosslinked polydivinylbenzene, crosslinked styrene-divinylbenzene copolymer, polyimide, melamine resin, phenol resin, benzoguanamine-formaldehyde condensate; polysulfone, polyacrylonitrile, aramid, polyacetal, thermoplastic polyimide Examples include heat-resistant polymer fine particles such as. The organic resin (polymer) constituting these organic fine particles may be a mixture, modified product, derivative, or copolymer (random copolymer, alternating copolymer, block copolymer, or graft copolymer) of the above-mentioned materials. polymer), or a crosslinked product (in the case of the above-mentioned heat-resistant polymer). Among them, 1 selected from the group consisting of crosslinked polyacrylic acid, crosslinked polyacrylic acid ester, crosslinked polymethacrylic acid, crosslinked polymethacrylic acid ester, crosslinked polymethyl methacrylate, and crosslinked polysilicone (polymethylsilsesquioxane, etc.) It is preferable that the resin be more than one species.

<Polyolefin and other resins>
As the resin contained in the separator, a thermoplastic resin is preferable from the viewpoint of moldability into a thin porous layer, a high-strength porous layer, and the like. Examples of thermoplastic resins include:
polyolefins such as polyethylene or polypropylene;
Fluorine-containing resins such as polyvinylidene fluoride or polytetrafluoroethylene;
Fluorine-containing rubber such as vinylidene fluoride-hexafluoropropylene copolymer or ethylene-tetrafluoroethylene copolymer;
Styrene-butadiene copolymer and its hydride, acrylonitrile-butadiene copolymer and its hydride, acrylonitrile-butadiene-styrene copolymer and its hydride, polymethacrylic acid, polyacrylic acid, methacrylic acid ester-acrylic acid ester Copolymers, styrene-acrylic ester copolymers, acrylonitrile-acrylic ester copolymers, ethylene propylene rubber, polyvinyl alcohol, polyvinyl acetate, polyoxyethylene, polyoxypropylene, ethylene-vinyl acetate copolymers, etc. Rubber;
Polyphenylene ether, polysulfone, polyether sulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyamide, aromatic polyamide, polyester, polycarbonate, polyethylene carbonate, polypropylene carbonate, polyacetal, poly(1-oxotrimethylene), poly(1- polyketones such as oxo-2methyltrimethylene);
Examples include.

From the viewpoint of shutdown properties, resins having a melting point and/or glass transition temperature of less than 180° C. are preferred, and polyolefin resins such as polyethylene or polypropylene are preferred. Furthermore, it is preferable that the polyolefin resin has polyethylene as its main component, from the viewpoint of ensuring shutdown at a lower temperature. From the viewpoint of suppressing meltdown, resins having a melting point and/or glass transition temperature of 180° C. or higher are preferred, and polyphenylene sulfide, polyamide, polyamideimide, aromatic polyamide, poly(1-oxotrimethylene), and the like are preferred. From the viewpoint of cycle characteristics, resins with good affinity with the electrolyte are preferred, and fluororesins such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, poly(1-oxotrimethylene), poly( Polyketone resins with a carbonyl group in the main skeleton such as 1-oxo-2methyltrimethylene), resins with an ether group in the main skeleton such as polyoxyether, or polymethacrylic acid, polyacrylic acid, methacrylic acid ester-acrylic A resin having an ester group in its main skeleton, such as an acid ester copolymer, or a resin having a carbonate group in its main skeleton, such as polyethylene carbonate or polypropylene carbonate, is preferable. These may be used alone or in combination of two or more.

Note that the lower limit of the viscosity average molecular weight of the polyolefin used in the resin is preferably 1,000 or more, more preferably 2,000 or more, and even more preferably 5,000 or more, from the viewpoint of molding processability. is preferably less than 12,000,000, preferably less than 2,000,000, and more preferably less than 1,000,000.

<Layer containing lithium (Li) storage particles and inorganic particles (porous layer)>
As explained above, the porous layer can contain Li storage particles and inorganic particles at a specific volume ratio and/or particle size ratio. Further, the Li storage particles may include a compound that reacts with lithium, such as a compound that is reductively decomposed by lithium. By using such a material, it is possible to manufacture a power storage device that can achieve both low resistance and improved safety. Furthermore, from the viewpoint of Li storage capacity per volume, it is preferable that the porous layer contains tin, silicon, and/or tin oxide as the material for the Li storage particles, and aluminum hydroxide oxide as the material for the inorganic particles. This is preferable because growth can be suppressed for a long period of time.

From the viewpoint of the effects of the present invention explained above, the porous layer preferably has a porosity of 60.0% to 85.0%, more preferably 65.0% to 85.0%. It is more preferably 66.0% to 80.0%, and particularly preferably 66.4% to 75.0%.

In the porous layer, the volume of the Li storage particles in the porous layer is A, the volume of the inorganic particles is B, the volume ratio (%) of the Li storage particles is A/(A+B)×100, and the volume ratio (%) of the inorganic particles is B/ When (A+B)×100, the relationships are 25%≦A/(A+B)×100≦95% and 5%≦B/(A+B)×100≦75%. From the perspective of the effects of the present invention, assuming that one layer has a constant thickness, this relationship is established between a layer consisting only of Li storage particles and a layer consisting only of inorganic particles in the thickness direction of the porous layer. It is preferred to exclude phase separation, accumulation of particles with higher specific gravity, and volume concentration gradients of lithium (Li) storage particles and inorganic particles.

The porous layer according to this embodiment may or may not contain a resin or a resin binder. If it does not contain a resin or resin binder, either no resin or resin binder is used in the process of forming the porous layer, or even if a resin or resin binder is used, the porous layer that is finally formed may be heated, for example. The resin or resin binder may be removed by means such as drying, sintering, peeling, removal, extraction, etc. When a resin is included, the content ratio (mass fraction) of Li storage particles to the resin in the porous layer is preferably 10% or more, more preferably 30% or more, and even more preferably is 50% or more, particularly preferably 90% or more, and the upper limit is preferably less than 100%, preferably 99.99% or less, more preferably 99.9% or less, particularly preferably 99% or less.

As the resin that can be contained in the porous layer, the resin described above as the separator-containing material can be used. The resin contained in the porous layer can bind Li storage particles and/or inorganic particles, and is It is preferable that it is insoluble in the electrolytic solution and electrochemically stable during use of the electricity storage device. From this point of view, fluororesins such as polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene copolymers; Polyketone resins having a carbonyl group; resins having an ether group in the main skeleton such as polyoxyether; resins having an ester group in the main skeleton such as polymethacrylic acid, polyacrylic acid, methacrylic acid ester-acrylic acid ester copolymer; Alternatively, a resin having a carbonate group in its main skeleton, such as polyethylene carbonate or polypropylene carbonate, may be used. In order to control the solubility in the electrolytic solution, it is also preferable to subject the raw resin or the separator containing the resin to physical crosslinking and/or chemical crosslinking.

The proportion of the resin according to this embodiment in the total amount of the porous layer is preferably 0.5% as a lower limit in terms of volume fraction from the viewpoint of binding between the resin and Li storage particles and/or inorganic particles. More preferably, it is 1.0% or more, still more preferably 3.0% or more, most preferably 5.0% or more, and the upper limit is preferably 80% or less, more preferably 60% or less. It is preferable to adjust the ratio to 0.5% or more from the viewpoint of sufficiently binding the particles and making it difficult for peeling, chipping, etc. to occur (that is, from the viewpoint of sufficiently ensuring good handling properties). . On the other hand, it is preferable to adjust the ratio to 80% or less from the viewpoint of realizing good ion permeability of the separator.

The lower limit of the thickness of the porous layer is preferably 0.5 μm or more, more preferably 1 μm or more, even more preferably 2 μm or more, even more preferably 3 μm or more, and particularly preferably from the viewpoint of dendrite suppression effect and heat resistance improvement. It is 4 μm or more or 5 μm or more. The upper limit of the layer thickness is preferably 100 μm or less, more preferably 50 μm or less, even more preferably 30 μm or less, particularly preferably 20 μm or less, and most preferably 11.5 μm or less, from the viewpoint of permeability or high battery capacity. be.

Regarding the porosity of the porous layer, it is preferable that the porosity of the porous layer after expansion exceeds 0%, taking into account the volumetric expansion coefficient of the Li storage particles when Li is stored. For example, when the volume expansion coefficient of the Li storage particles is twice, the porosity of the porous layer is preferably over 50%, and when the volume expansion coefficient is 3 times, it is preferably 66% or more. From the viewpoint of increasing the storage capacity and suppressing the growth of dendrites, the upper limit of the porosity is preferably 99% or less, more preferably 95% or less, and even more preferably 90% or less.

Regarding the basis weight of the porous layer, the lower limit is preferably 10 g/cm ² or more, more preferably 13 g/cm ² or more, and even more preferably 14.0 g/cm ² or more, from the viewpoint of dendrite suppression effect and heat resistance improvement. , and the upper limit is preferably 30 g/cm ² or less, more preferably 25 g/cm ² or less, still more preferably 20 g/cm ² or less, from the viewpoint of permeability or increasing the capacity of the electricity storage device.

<Polyolefin microporous membrane>
The microporous polyolefin membrane according to this embodiment may be a membrane containing the polyolefin described above as the separator-containing material as its main component. As used herein, a material containing a specific component "as a main component" means containing 50% by mass or more of the specific component based on the mass of the material.

If desired, the polyolefin microporous membrane may contain, in addition to the polyolefin, the above-described inorganic particles, an organic filler, a resin other than polyolefin, a resin for binding the inorganic particles or the organic filler, and the like.

Examples of resins that bind inorganic particles or organic fillers include the following 1) to 4):
1) Conjugated diene polymer,
2) Acrylic polymer,
3) polyvinyl alcohol resin or cellulose polymer, and 4) fluorine-containing resin.

1) A conjugated diene polymer is a polymer containing a conjugated diene compound as a monomer unit. Examples of the conjugated diene compound include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, styrene-butadiene, Examples include substituted linear conjugated pentadienes, substituted and side chain conjugated hexadienes, and these may be used alone or in combination of two or more. Among these, 1,3-butadiene is preferred from the viewpoint of high binding properties.

2) The acrylic polymer is a polymer containing a (meth)acrylic compound as a monomer unit. The above (meth)acrylic compound refers to at least one selected from the group consisting of (meth)acrylic acid and (meth)acrylic acid ester.
Examples of such compounds include compounds represented by the following formula (P1).
CH ₂ =CR ^Y1 -COO-R ^Y2 (P1)
{In the formula, R ^Y1 represents a hydrogen atom or a methyl group, and R ^Y2 represents a hydrogen atom or a monovalent hydrocarbon group. }
When R ^Y2 is a monovalent hydrocarbon group, it may have a substituent and/or a heteroatom in the chain. Examples of the monovalent hydrocarbon group include a linear or branched alkyl group, a cycloalkyl group, and an aryl group.
More specifically, the chain alkyl group which is one type of R ^Y2 includes a chain alkyl group having 1 to 3 carbon atoms, which is a methyl group, an ethyl group, an n-propyl group, and an isopropyl group; n- Examples include chain alkyl groups having 4 or more carbon atoms, such as a butyl group, an isobutyl group, a t-butyl group, an n-hexyl group, a 2-ethylhexyl group, and a lauryl group. Further, as the aryl group which is one type of R ^Y2 , for example, a phenyl group can be mentioned.
Furthermore, examples of substituents on the monovalent hydrocarbon group include hydroxyl and phenyl groups, and examples of heteroatoms in the chain include halogen atoms, oxygen atoms, and the like.
Examples of such (meth)acrylic compounds include (meth)acrylic acid, chain alkyl (meth)acrylate, cycloalkyl (meth)acrylate, (meth)acrylate having a hydroxyl group, (meth)acrylate containing a phenyl group, etc. can be mentioned. The (meth)acrylic compounds may be used alone or in combination of two or more.

3) Examples of the polyvinyl alcohol resin include polyvinyl alcohol, polyvinyl acetate, and the like. 3) Examples of cellulose polymers include carboxymethyl cellulose and carboxyethyl cellulose.

4) Examples of the fluororesin include polyvinylidene fluoride, polytetrafluoroethylene, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, and ethylene-tetrafluoroethylene. Examples include copolymers.

In order to ensure a laminated structure of the separator with excellent safety or low resistance, it is preferable that the lithium (Li) storage particles described above are not included in the microporous polyolefin membrane but are included in the porous layer. Furthermore, the separator may include a layer other than the microporous polyolefin membrane and the porous layer to the extent that the effects of the present invention are not impaired, and such a layer may include a resin other than polyolefin, inorganic particles, or a combination of organic filler. It may contain a resin for wearing.

<Separator manufacturing method and physical properties>
Methods for manufacturing the separator according to the present embodiment are not particularly limited, but include, for example, the following methods (i) to (iii):
(i) dispersing a molding precursor comprising the above-described lithium (Li) storage particles and inorganic particles, optionally the above-described resin, together with or on the polyolefin microporous membrane; , a method of obtaining a separator by molding by heating, melt-kneading, extrusion, stretching, relaxation, etc.;
(ii) The above-described lithium (Li) storage particles, inorganic particles, and optionally the above-described resin are dissolved or dispersed in a solvent, and the resulting dispersion is applied to at least one of the polyolefin microporous membranes. A method of obtaining a separator having a laminated structure or a coating structure by coating on one side; and (iii) Lithium (Li) storage particles, inorganic particles, and resin are mixed together with a plasticizer or the like as necessary in an extruder or the like. A separator having a laminate structure can be obtained by heating and mixing, co-extruding with the constituent materials of the polyolefin microporous membrane, forming the extruded laminate sheet, subjecting it to stretching or plasticizer extraction, and drying, or obtaining a separator having a laminate structure, or the above ( A method of obtaining a separator by forming another layer on the surface of the porous layer of the separator obtained in ii) or the microporous polyolefin membrane.

Note that the separator having a laminated structure can also be manufactured using a method different from the manufacturing method described above. For example, methods such as laminating the microporous membrane and the active layer together can be combined as appropriate. Alternatively, a dispersion liquid of Li storage particles and a dispersion liquid of inorganic particles may be prepared respectively, and both liquids may be mixed at desired timing.

As the solvent, it is preferable to use a solvent that can uniformly and stably dissolve or disperse the lithium (Li) storage particles, the inorganic particles, and, if desired, the resin. Examples of such solvents include N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, water, ethanol, toluene, hot xylene, and hexane.

In addition, in order to stabilize the solution containing Li-absorbing particles or inorganic particles, or to improve the coating properties on the polyolefin microporous membrane, a surfactant or the like is added to the dispersion of Li-absorbing particles. Various additives may be added, such as a thickening agent, a wetting agent, an antifoaming agent, and a pH adjusting agent containing acid or alkali. These additives are preferably those that can be removed during solvent removal or plasticizer extraction, but are electrochemically stable and do not inhibit battery reactions when used in lithium ion secondary batteries, and must be able to be heated at 200°C. If it is stable to some extent, it may remain in the battery (or the separator within the battery).

Examples of methods for dissolving or dispersing lithium (Li) storage particles, inorganic particles, and optionally a resin in a solvent include a ball mill, bead mill, planetary ball mill, vibrating ball mill, sand mill, colloid mill, attritor, roll mill, and high-speed impeller. Examples include dispersion, a disperser, a homogenizer, a high-speed impact mill, ultrasonic dispersion, and a mechanical stirring method using a stirring blade.

The method for applying the dispersion of lithium (Li) storage particles and inorganic particles to the surface of the microporous polyolefin membrane is not particularly limited as long as it can achieve a predetermined layer thickness or application area. Examples of such coating methods include gravure coater method, small-diameter gravure coater method, reverse roll coater method, transfer roll coater method, kiss coater method, dip coater method, knife coater method, air doctor coater method, blade coater method, and rod coater method. Examples include coater method, squeeze coater method, cast coater method, die coater method, screen printing method, and spray coating method. Furthermore, depending on the application, the dispersion liquid may be applied to only one side or both sides of the microporous polyolefin membrane, but it is preferable that the outermost surface that comes into contact with the negative electrode be a porous layer. This is preferable from the viewpoints of delaying dendrite growth, suppressing short circuits caused by dendrites, lowering resistance, and improving charge/discharge capacity.

Preferably, the solvent can be removed from the dispersion applied to the microporous polyolefin membrane. The method for removing the solvent is not particularly limited as long as it does not adversely affect the microporous polyolefin membrane or the separator. Methods for removing the solvent include, for example, a method of drying the polyolefin microporous membrane while fixing it at a temperature below the melting point of the polyolefin microporous membrane, a method of drying the polyolefin microporous membrane under reduced pressure at a temperature lower than the boiling point of the solvent, Examples include a method of coagulating the resin by immersing it in a poor solvent for the resin and simultaneously extracting the solvent.

The porous layer containing the Li storage particles and inorganic particles described above at a specific volume ratio and/or particle size ratio can be produced by, for example, the material selection of the Li storage particles and the inorganic particles, the mixing ratio, By adjusting the degree of dispersion, adjusting the mixing ratio of the individually prepared Li storage particle dispersion and inorganic particle dispersion, or optimizing the setting conditions of the porous layer on the polyolefin microporous membrane. can be formed.

If desired, a resin or the like having good affinity with the electrolytic solution may be added to the constituent materials of the separator. Examples of resins that have good affinity with the electrolytic solution include fluororesins such as polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene copolymer; poly(1-oxotrimethylene), poly(1-oxo-2methyltrimethylene); Polyketone (PK) resins that have a carbonyl group in their main skeleton such as ); resins that have an ether group in their main skeleton such as polyoxyether; polymethacrylic acid, polyacrylic acid, methacrylic ester-acrylic ester copolymers, etc. Resins having an ester group in their main skeleton; examples include resins having a carbonate group in their main skeleton, such as polyethylene carbonate and polypropylene carbonate.

In addition, when using microporous polyolefin membranes that are commonly used in lithium-ion battery separators, treatments such as corona discharge treatment, plasma discharge treatment, and fluorine/oxygen mixed gas treatment can add carboxylic acid groups and hydroxyl groups to the polyolefin structure. or by coating or impregnating the surface of the microporous polyolefin membrane with a solution in which a resin having high affinity with the electrolyte is dissolved or dispersed, and drying or impregnating it with a poor solvent. By reducing the solubility of the resin and causing the resin to precipitate, it is possible to modify the electrolyte affinity of the surface or interior (porous portion) of the polyolefin.

Regarding the film thickness (total layer thickness) of a single layer, multilayer, laminated or coated separator, the lower limit is preferably 2 μm or more, more preferably 5 μm or more, still more preferably 7 μm or more, even more preferably 10 μm or more. The upper limit thereof is preferably 200 μm or less, more preferably 100 μm or less, still more preferably 50 μm or less, even more preferably 30 μm or less. It is preferable to adjust the film thickness to 2 μm or more from the viewpoint of ensuring sufficient mechanical strength. On the other hand, adjusting the film thickness to 200 μm or less is preferable from the viewpoint of enabling a reduction in the volume occupied by the separator and increasing the capacity of the battery.

In a multilayer, laminated or coated separator, the ratio of the thickness of the thickest part of the porous layer to the thickness of the separator (total layer thickness) is preferably 10% or more, more preferably 20% or more as a lower limit. The upper limit is preferably 60% or less, more preferably 50% or less. Adjusting this ratio to 10% or more is preferable from the viewpoint of increasing the dendrite suppressing effect and short circuit temperature and achieving good heat resistance.On the other hand, adjusting this ratio to 50% or less is preferable. , is suitable from the viewpoint of suppressing a decrease in permeability of the separator.

The upper limit of the air permeability of the separator is preferably 600 seconds/100 cm ³ or less and 550 seconds/100 cm ³ from the viewpoint of electrical characteristics or cycle characteristics, regardless of whether it is a single layer structure, a multilayer structure, or a coated structure. Below, it is 500 seconds/100cm ^<3> or less, 450 seconds/100cm ^<3> or less, or 400 seconds/100cm ^<3> or less, and more preferably 300 seconds/100cm ^<3> . From the viewpoint of strength or safety, the lower limit of the air permeability of the separator is preferably 30 seconds/100cm3 ^or more, 60 seconds/100cm3 ^or more, 100 seconds/100cm3 ^or more, 120 seconds/100cm3 ^or more, 140 seconds /100cm3 or ^more , or 150 seconds/100cm3 ^or more.

The lower limit of the puncture strength of the separator is preferably 100 gf or more, more preferably 200 gf or more, even more preferably 300 gf or more, even more preferably 350 gf or more, particularly preferably 400 gf or more, from the viewpoint of safety of the electricity storage device including the separator. be. The upper limit of the puncture strength of the separator is preferably 950 gf or less, more preferably 900 gf or less, still more preferably 850 gf or less, and even more Preferably it is 800 gf or less, particularly preferably 750 gf or less.

In a multilayer or laminated separator, the peel strength between the layers is preferably 50 N/m or more. When the interlayer peel strength is 50 N/m or more, handling properties tend to improve when winding the separator onto a reel and when manufacturing an electricity storage device by winding it. In addition, when a multilayer or laminated separator is heated, the heat shrinkage rate is maintained as that of a layer with high heat resistance because the layers are bonded with a peel strength of 50 N/m or more, so the heat shrinkage rate is good. There is a tendency to The interlayer peel strength is more preferably 100 N/m or more, still more preferably 200 N/m or more.

The heat shrinkage rate at 150°C or the heat shrinkage rate at 130°C of the separator is preferably 0% or more and 15% or less, more preferably 0% or more and 10% or less, and 0% or more and 5% or less. It is particularly preferable that Adjusting the heat shrinkage rate to 15% or less effectively prevents separator membrane rupture even when the power storage device generates abnormal heat, and suppresses contact between the positive and negative electrodes (i.e., achieves better safety performance). preferred from the viewpoint of In addition, regarding the heat shrinkage rate, it is preferable to set both the machine direction (MD) and the transverse direction (TD) of the separator within the above range. Also, depending on the stretching conditions, it may shrink in one axis and slightly elongate in one axis perpendicular to that, and the shrinkage rate may show a negative value. Since suppressing shrinkage of the separator leads to suppressing short circuits between electrodes, it is important that the shrinkage rate is below a certain value, and the shrinkage rate may be a negative value.

The lower limit of the shutdown temperature of the separator is preferably 120°C or higher, and the upper limit is preferably 160°C or lower, more preferably 150°C or lower. Note that the shutdown temperature refers to the temperature at which the micropores of the separator are closed due to thermal melting or the like when the electricity storage device generates abnormal heat. Adjusting the shutdown temperature to 160° C. or lower is preferable from the viewpoint of quickly promoting current interruption and obtaining better safety performance even when the electricity storage device generates heat. On the other hand, it is preferable to adjust the shutdown temperature to 120° C. or higher, for example, from the viewpoint of usability of the separator at a high temperature of around 100° C., or from the viewpoint of being able to perform various heat treatments on the separator.

The lower limit of the short-circuit temperature of the separator is preferably 180°C or higher, more preferably 200°C or higher, and the upper limit thereof is preferably 1000°C or lower. It is preferable to adjust the short-circuit temperature to 180° C. or higher from the viewpoint of suppressing contact between the positive and negative electrodes until the heat is radiated even when the electricity storage device generates abnormal heat, and achieving better safety performance.

Note that the physical property values or measured values of the separator, active layer, or insulating layer explained above can be measured according to the measurement method of the Examples described later.

<Electricity storage device>
A power storage device according to another embodiment of the present invention includes a positive electrode, the separator described above, a negative electrode, and optionally an ion conductive medium. When the separator having a laminated structure has a porous layer containing lithium (Li) storage particles and inorganic particles on a microporous polyolefin membrane, the porous layer side of the separator is connected to at least a part of the negative electrode surface in the electricity storage device. It is preferable that they be arranged so as to face each other or to be in contact with at least a part of the negative electrode surface. From the perspective of lowering the resistance of electricity storage devices, it is better to position the porous layer of the separator in electricity storage devices so that it functions similarly to the negative electrode, increases the apparent specific surface area of the negative electrode, and increases the reaction rate of the negative electrode. preferable.

The ion conductive medium can be liquid, gel, or solid depending on the electrolyte of the power storage device. For example, a non-aqueous electrolyte may be used as the liquid ion conductive medium. Further, as the electricity storage device containing the non-aqueous electrolyte, for example, a non-aqueous electrolyte secondary battery or the like may be used. From the viewpoint of high cycle performance and safety, the negative electrode potential (vsLi ⁺ /Li) during charging of the electricity storage device is the potential of the material (A) capable of occluding lithium (Li) (vsLi ⁺ /Li) explained above. ), and more preferably 1.5V (vsLi ⁺ /Li) or less.

From the viewpoint of safety, capacity characteristics, output characteristics, and cycle characteristics of the power storage device, the positive electrode, the negative electrode, and the separator should have at least a gap between the positive electrode and the polyolefin microporous membrane of the separator and/or between the negative electrode and the polyolefin microporous membrane. It is preferable to arrange so that there is one porous layer, and more preferably to arrange so that at least one porous layer exists between the negative electrode and the polyolefin microporous membrane of the separator. Such an arrangement can be achieved, for example, by orienting the porous layer side of the separator toward the negative electrode side or the positive electrode side; or a laminated structure in which the porous layer is sandwiched between multiple microporous polyolefin membranes, or a microporous polyolefin membrane is sandwiched between multiple porous layers. It can also be achieved by the use of a separator with a sandwiched laminate structure; or by providing a porous layer, described as a component of the separator, separately from the separator, between the electrode and the separator.

The positive electrode of the electricity storage device is not particularly limited, but may have a known configuration including, for example, a positive electrode current collector and a positive electrode active material-containing layer. The negative electrode of the electricity storage device may have a known configuration including, for example, a negative electrode current collector and a negative electrode active material-containing layer. Preferably, the active material is lithium (Li) metal.

Specific examples of power storage devices include lithium batteries, lithium secondary batteries, lithium ion secondary batteries, sodium secondary batteries, sodium ion secondary batteries, magnesium secondary batteries, magnesium ion secondary batteries, and calcium secondary batteries. , calcium ion secondary battery, aluminum secondary battery, aluminum ion secondary battery, nickel metal hydride battery, nickel cadmium battery, electric double layer capacitor, lithium ion capacitor, redox flow battery, lithium sulfur battery, lithium air battery, zinc air battery Examples include. Among these, from the viewpoint of practicality, a lithium battery, a lithium secondary battery, a lithium ion secondary battery, a nickel-hydrogen battery, or a lithium ion capacitor is preferred, and a lithium battery or a lithium ion secondary battery is more preferred.

In the case of a lithium battery, a lithium secondary battery, a lithium ion secondary battery, or a lithium ion capacitor, the ion conductive medium is preferably a Li ion conductive medium.

In the electricity storage device, for example, a positive electrode and a negative electrode are overlapped with each other via the separator according to the present embodiment and wound as necessary to form a laminated electrode body or a wound electrode body, and then this is wrapped in an exterior body. It can be manufactured by loading the battery into the housing, connecting the positive and negative electrodes to the positive and negative electrode terminals of the exterior body through a lead body, and further injecting the ion conductive medium into the exterior body, and then sealing the exterior body. In the case of a lithium battery, lithium secondary battery, lithium ion secondary battery, or lithium ion capacitor, the Li ion conductive medium is a nonaqueous electrolyte containing a nonaqueous solvent such as a chain or cyclic carbonate and an electrolyte such as a lithium salt. or can be a solid electrolyte or a gel electrolyte.

Next, the present embodiment will be described in more detail with reference to Examples and Comparative Examples; however, the present invention is not limited to the following Examples unless the gist thereof is exceeded. In addition, the physical properties in Examples were measured or determined by the following method.

｛式中、ｔは、純度９８％以上のヘキサフルオロイソプロパノールの２５℃での粘度管の流過時間であり、Ｔは、ポリケトンがヘキサフルオロイソプロパノールに溶解している希釈溶液の２５℃での粘度管の流過時間であり、Ｃは、上記溶液１００ｍｌ中のグラム単位による溶質（すなわちポリケトン）質量値である。｝ (1) Intrinsic viscosity (η), viscosity average molecular weight (Mv)
The intrinsic viscosity [η] (dl/g) at 135° C. in decalin solvent is determined based on ASTM-D4020. Mv of polyethylene was calculated using the following formula.
[η]=6.77× ^10-4 Mv ^0.67
Regarding polypropylene, Mv was calculated using the following formula.
[η]=1.10×10 ⁻⁴ Mv ^0.80
The intrinsic viscosity [η] (unit: dl/g) of the polyketone was calculated using the following formula.

{where t is the flow time of hexafluoroisopropanol with a purity of 98% or higher through a viscosity tube at 25°C, and T is the viscosity at 25°C of a diluted solution of polyketone dissolved in hexafluoroisopropanol is the flow time of the tube and C is the mass value of the solute (ie polyketone) in grams in 100 ml of the solution. }

(2) Thickness (μm)
The film thickness of the sample was measured using a dial gauge (PEACOCK No. 25 (trademark) manufactured by Ozaki Seisakusho). A sample of MD 10 mm x TD 10 mm was cut out from the porous membrane, and the thickness was measured at 9 points (3 points x 3 points) in a grid pattern. The average value of the obtained measured values was calculated as the film thickness (μm) or layer thickness.
Note that the thickness of each single layer obtained in the present examples and comparative examples was measured in the state of a single layer obtained in each manufacturing process. In the case of a laminated state, the value was calculated by subtracting the value of the single layer measured above. For those in which a single layer state could not be obtained by coextrusion, the thickness of each layer was calculated from cross-sectional SEM.

(3) Volume ratio (%) of Li storage particles A and inorganic particles B
Planar images and cross-sectional images were taken with a scanning electron microscope (SEM) in a field of view of 10 μm x 10 μm, and EDX (HORIBA Elemental mapping of particle B is performed. After that, the elemental mapping image is loaded into an image analysis device, the area of each Li storage particle A or inorganic particle B present in the planar image and cross-sectional image is calculated, and the area ratio calculated from the average of the surface and cross-sectional area is calculated as the volume. It was compared. Note that the volume ratio was calculated by taking 10 planar images and 10 cross-sectional images, and using the average value thereof.

(4) Porosity (%)
A 10 cm x 10 cm square sample was cut from the film or layer to be measured, its volume (cm ³ ) and mass (g) were determined, and calculations were made from these and true density (g/cm ³ ) using the following formula.
Porosity (%) = (volume - (mass / true density of mixed composition)) / volume x 100
The density of the mixed composition was calculated from the density of each of the polyolefin resin and inorganic particles used and the mixing ratio.

(5) Average particle size (μm), particle size ratio A field of view of 10 μm x 10 μm magnified with a scanning electron microscope (SEM) is directly taken, or after being printed on a photograph from a negative, it is read into an image analysis device and then The number average value of the calculated circular diameter of each particle (the diameter of a circle when the particle is converted into a circle having the same area) was defined as the average particle diameter Dp50 (μm). However, if the grain boundaries were unclear when inputting from a photograph to the image analysis device, the photograph was traced and input to the image analysis device was performed using this diagram. In Examples, unless otherwise specified, "average particle size" is measured using a scanning electron microscope (SEM).
The equivalent circular diameter is calculated using the smallest unit particle that cannot be further loosened without applying excessive force. For example, the diameter of a particle in terms of a circle usually means the diameter of a primary particle in terms of a circle, but for particles that cannot be loosened without applying excessive force, such as granulated particles, the diameter of the secondary particle Means the converted diameter. Moreover, when a plurality of primary particles are connected by a weak force to form an amorphous structure, it means the diameter of the primary particle of the particle in terms of a circle.
Note that if it is difficult to determine the average particle size of the sample by SEM, measurement is performed using a laser particle size distribution measuring device. In this case, the sample was added to distilled water, a small amount of sodium hexametaphosphate aqueous solution was added, and the sample was dispersed for 1 minute using an ultrasonic homogenizer. , the particle size distribution can be measured and the number average value of each particle can be obtained as the average particle size of the inorganic filler.
Further, the particle size ratio can also be calculated from the average particle size obtained for a plurality of particles of different types.

(6) Density (g/cm ³ )
The bulk density of Li storage particles, inorganic particles, or porous layers was measured by a method according to JIS R-9301-2-4.

(7) Porosity (%)
The part where the porous layer of the separator is formed and the part where it is not formed (for example, the part consisting only of a microporous polyolefin membrane) were punched out to a diameter of 5 cm, and the film thickness (μm) and mass (g) were measured. The volume (cm ³ ) of each part of the separator was calculated from the measured film thickness (μm) and area (cm ² ). The porosity was calculated using the following formula from the film thickness, area, volume, and density (g/cm ³ ) of the porous layer and constituent materials measured above.
Porosity = (volume of porous layer - (mass of porous layer - mass of portion where porous layer is not formed) / density of porous layer) / volume of porous layer × 100
Note that if the separator cannot be punched out with a diameter of 5 cm, it may be measured with an appropriate size that can be punched out. Further, regarding the film thickness, the thickness of each layer may be calculated from a cross-sectional SEM.

(8) Volume fraction of resin binder (%)
The volume fraction (%) of the resin binder was calculated using the following formula.
Vb={(Wb/Db)/(Wb/Db+Wf/Df)}×100
Vb: Volume fraction of resin binder (%)
Wb: Weight of resin binder (g)
Wf: Weight of inorganic filler (g)
Db: Density of resin binder (g/cm ³ )
Df: Bulk density of inorganic filler (g/cm ³ )

(9) Air permeability (sec/ ^100cm3 )
The air permeability of the sample was measured using a Gurley air permeability meter (G-B2 (trademark) manufactured by Toyo Seiki) in accordance with JIS P-8117.

(10) Puncture strength (gf)
Using a handy compression tester KES-G5 (trademark) manufactured by Kato Tech, a microporous membrane or separator was fixed as a sample in a sample holder with an opening diameter of 11.3 mm. Next, a puncture test was performed on the central part of the fixed sample at a needle tip radius of 0.5 mm and a puncture speed of 2 mm/sec at a room temperature of 23°C and a humidity of 40% to determine the maximum puncture load. Puncture strength (gf) was measured. The measurement value of the puncture test is measured at 3 points along the TD of the membrane or separator, 2 points inside 10% of the total width from both ends toward the center and 1 point in the center, and the average value is calculated. This is the value.

(11) Battery safety evaluation (dendritic short circuit test) and rate characteristic evaluation Under the conditions shown below, the charge and discharge cycle of the electricity storage device in which Li dendrites are forcibly generated on the lithium (Li) foil was repeated, and the safety and evaluate the resistance.

a. Preparation of positive electrode 92.2% by mass of lithium cobalt composite oxide (LiCoO ₂ ) as a positive electrode active material, 4.6% by mass of acetylene black as a conductive material, 3.2% by mass of polyvinylidene fluoride (PVDF) as a binder, A slurry is prepared by dispersing in N-methylpyrrolidone (NMP). This slurry is applied to one side of a 20 μm thick aluminum foil serving as a positive electrode current collector using a die coater, dried at 130° C. for 3 minutes, and then compression molded using a roll press. At this time, the amount of active material applied to the positive electrode is 125 g/m ² , and the bulk density of the active material is 3.00 g/cm ³ .

b. Preparation of Laminate Using metallic lithium (Li) foils as the positive electrode and negative electrode, the positive electrode was punched out into a circular shape with an area of 2.00 cm ² , and the negative electrode was punched out into a circular shape with an area of 2.05 cm ² .

c. Non-aqueous electrolyte was prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate:ethyl methyl carbonate=1:2 (volume ratio) to a concentration of 1.0 mol/L.

d. Battery Assembly Lay the negative electrode, separator, and positive electrode vertically from bottom to top so that the positive and negative electrodes face each other, and store in a stainless steel container with a lid. The container and the lid are insulated, and the container is in contact with a metal lithium foil (Li) as a negative electrode, and the lid is in contact with an aluminum foil as a positive electrode current collector. In this container, item c. Inject the non-aqueous electrolyte prepared in step 1 and seal it.

e. Battery safety evaluation (dendritic short circuit test)
After storing the cell assembled as described above in an atmosphere at a temperature of 25°C for 12 hours, it was charged with CC-CV charging at 4.3V-40mA (6.6C), and the CUT OFF condition for CV charging was 0. It was set to 03mA. Thereafter, CC discharge was performed at 3.0V-0.6mA (0.1C), and the charge capacity due to precipitation of lithium (Li) due to charging was confirmed. To determine a short circuit, if the normal charging capacity is 7.0 to 8.5 mAh, when the charging capacity reaches 8.6 mAh or more, excessive lithium (Li) is deposited on the negative electrode side, causing a dendrite short circuit. It was judged. A plurality of samples were cut out from the same separator, and a plurality of cells were assembled and subjected to a dendrite short circuit test to evaluate the short circuit occurrence rate according to the following criteria.
[Evaluation criteria]
5: Short circuit suppression rate 100%
4: Short circuit suppression rate 75% or more and less than 100% 3: Short circuit suppression rate 50% or more and less than 75% 2: Short circuit suppression rate 25% or more and less than 50% 1: Short circuit suppression rate 0% and less than 25%

f. Rate characteristics d. Resistance evaluation was performed using the cell assembled in the following method.
In an atmosphere of 25°C, charge the battery to a voltage of 4.2V with a current value of 6.0mA (approximately 1.0C), then maintain the current value of 4.2V and start reducing the current value from 6.0mA. The battery was charged for about 3 hours, and then discharged to a battery voltage of 3.0 V at a current value of 3 mA, and the discharge capacity at that time was obtained as a 1C discharge capacity (mAh).
Next, charge the battery to a voltage of 4.2V at a current value of 6.0mA (approximately 1.0C) in an atmosphere of 25°C, and then start reducing the current value from 6.0mA while maintaining 4.2V. The battery was charged for a total of about 3 hours, and then discharged to a battery voltage of 3.0 V at a current value of 60.0 mA (about 10.0 C), and the discharge capacity at that time was obtained as a 5C discharge capacity (mAh).
The ratio of the 5C discharge capacity to the 1C discharge capacity was calculated, and this value was used as the rate characteristic, which was evaluated according to the following criteria.
Rate characteristics (%) = 5C discharge capacity/1C discharge capacity x 100
[Evaluation rank]
5: 50% or more 3: More than 30% to less than 50% 1: 30% or less

<Example 1>
1 part by mass of tetrakis-[methylene-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane was added as an antioxidant to 100 parts by mass of homopolymer high-density polyethylene (PE). A mixture was obtained by dry blending in a tumbler blender.
The resulting mixture was fed to a twin-screw extruder by a feeder under a nitrogen atmosphere.
Additionally, liquid paraffin (kinematic viscosity 7.59×10 ⁻⁵ m ² /s at 37.78° C.) was injected into the extruder cylinder using a plunger pump.
The operating conditions of the feeder and pump were adjusted so that the proportion of liquid paraffin was 65 parts by mass and the polymer concentration was 35 parts by mass out of 100 parts by mass of the total mixture to be extruded.
Next, they were melt-kneaded in a twin-screw extruder while heating to 200°C, and the obtained melt-kneaded product was extruded through a T-die onto a cooling roll whose surface temperature was controlled to 80°C, and the extrudate was A sheet-like molded product having a thickness of 1170 μm was obtained by bringing it into contact with a cooling roll, molding (casting), and cooling and solidifying it.

This sheet-like molded product was stretched at 122° C. to 7 times MD x 6.4 times TD using a simultaneous biaxial stretching machine, and then the stretched product was immersed in methylene chloride to extract and remove liquid paraffin, and then dried. Thereafter, the sheet was laterally stretched to 1.9 times at 128°C using a horizontal stretching machine, and then subjected to relaxation heat treatment at 135°C so that the width was 1.65 times the width when it was finally introduced into the horizontal stretching machine. A microporous polyolefin membrane was obtained. Hereinafter, the cooling sheet thickness, simultaneous biaxial stretching temperature, transverse stretching ratio and temperature, relaxation heat treatment ratio and temperature, etc. were adjusted to adjust the thickness of the resulting microporous polyolefin membrane.

Next, 27.8 parts by mass of SnO ₂ (average particle size 0.81 μm) as Li storage particles, 10.4 parts by mass of aluminum hydroxide oxide (average particle size 0.99 μm) as inorganic particles, and acrylic latex (solid content) were added. A coating solution was prepared by uniformly dispersing 2.1 parts by mass of 40% concentration) and 0.2 parts by mass of an aqueous ammonium polycarboxylate solution (ED001 manufactured by San Nopco Co., Ltd.) in 59.5 parts by mass of water. The coating liquid was applied onto the above-mentioned polyolefin microporous membrane using a gravure coater. The coating layer on the polyolefin microporous membrane was dried to remove water to form a separator having a porous layer with a thickness of 10.9 μm, a basis weight of 16.3 g/m ² and a porosity of 70.8%, and the above item (3) ), it was confirmed that the volume ratio of SnO ₂ was 46.0% and the volume ratio of aluminum hydroxide oxide was 54.0%. The separator had a microporous polyolefin membrane and a porous layer disposed thereon. During battery assembly, the separator was placed so that the porous layer faced a metal lithium (Li) foil, and a dendrite short circuit test was conducted.

<Examples 2 to 4, Comparative Examples 1 to 6>
As shown in Table 1 below, a separator was obtained in the same manner as in Example 1 except that the conditions of the Li storage particles, inorganic particles, or porous layer were changed.

The physical properties of the obtained separator and the electricity storage device containing the separator were measured and evaluated as described above. The results are also shown in Table 1 below.

1 Negative electrode 2 Porous layer 3a Lithium (Li) storage particles (before expansion, non-conductive)
3b Lithium (Li) storage particles (after expansion, conductive)
4 Inorganic particles 5 Polyolefin microporous membrane 6 Separator Li-D Li dendrite MD Metal dendrite i Ion P Pocket

Claims (7)

A separator for a power storage device including a microporous polyolefin membrane,
A porous layer containing lithium (Li) storage particles and inorganic particles is provided on at least the surface layer of the microporous polyolefin membrane, and the volume A of the Li storage particles and the volume B of the inorganic particles in the porous layer are , when the volume ratio (%) of the Li storage particles is A/(A+B)×100 and the volume ratio (%) of the inorganic particles is B/(A+B)×100, the volume ratio of the Li storage particles is 25. % to 95%, and the volume ratio of the inorganic particles is 5% to 75%. The separator for a power storage device according to claim 1, wherein the average particle size of the inorganic particles is 0.30 to 1.5 times the average particle size of the Li storage particles. The separator for an electricity storage device according to claim 1 or 2, wherein the porous layer has a porosity of 65.0% to 85.0%. The inorganic particles according to any one of claims 1 to 3, wherein the inorganic particles include at least one selected from the group consisting of alumina, aluminum hydroxide, aluminum hydroxide oxide, aluminum silicate, barium sulfate, and zirconia. Separator for power storage devices. The separator for an electricity storage device according to any one of claims 1 to 4, wherein the Li storage particles contain at least one type selected from the group consisting of tin oxide, silicon, tin, germanium, aluminum, and graphite. A separator for a power storage device according to any one of claims 1 to 5,
A nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a nonaqueous electrolyte. The non-aqueous electrolyte secondary battery according to claim 6, wherein the negative electrode is Li metal.

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