Electrode for electrochemical element and electrochemical element

Abstract

The purpose of the present invention is to provide an electrode for electrochemical elements, said electrode being capable of sufficiently suppressing heat generation in the event of an internal short circuit of an electrochemical element, while being capable of decreasing the IV resistance. An electrode according to the present invention is provided with a collector and an electrode mixture layer. The electrode mixture layer contains an electrode active material, a binder and a foaming agent that has a thermal decomposition temperature of from 150°C to 400°C; in a cross-section of the electrode mixture layer in the thickness direction, there are thermally decomposable regions that are formed of the foaming agent, while having a circumscribed circle diameter of from 1.0 μm to 10.0 μm; and the number A of the thermally decomposable regions present per 50 μm2 in the surface region of the electrode mixture layer is larger than the number B of the thermally decomposable regions present per 50 μm2 in the deep region of the electrode mixture layer.

Description

[Technical Field]

[0001] The present invention relates to an electrode for an electrochemical device and an electrochemical device. [Background technology]

[0002] Electrochemical devices such as lithium ion secondary batteries, electric double layer capacitors, and lithium ion capacitors are small, lightweight, have high energy density, and can be repeatedly charged and discharged, and are therefore used in a wide range of applications.

[0003] The electrode used in the electrochemical element includes a current collector and an electrode mixture layer formed on the current collector. The electrode mixture layer is usually formed by binding electrode active materials with a binder.

[0004] However, electrochemical elements may experience thermal runaway due to an internal short circuit between electrodes, and therefore attempts have been made to suppress heat generation in electrochemical elements and ensure safety even when an internal short circuit occurs between electrodes. For example, Patent Document 1 discloses a positive electrode including a current collector and a positive electrode composite layer, the positive electrode composite layer containing a positive electrode active material and a melamine acid salt, which is a salt of melamine and an acid. According to Patent Document 1, the use of this positive electrode can improve input / output characteristics and charge / discharge efficiency while ensuring the safety of the non-aqueous electrolyte. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2014 / 119315 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the above-mentioned conventional techniques have room for further improvement in terms of further suppressing heat generation during an internal short circuit in an electrochemical element, while reducing the IV resistance and improving the element characteristics of the electrochemical element.

[0007] Therefore, an object of the present invention is to provide a new technique that can reduce the IV resistance of an electrochemical element while sufficiently suppressing heat generation when an internal short circuit occurs in the electrochemical element. [Means for solving the problem]

[0008] The present inventors have conducted extensive research to solve the above problems, and have found that by incorporating a foaming agent having a thermal decomposition temperature within a predetermined range in addition to an electrode active material and a binder into an electrode mixture layer formed on a current collector, and by arranging the foaming agent so as to satisfy predetermined conditions, heat generation during an internal short circuit in an electrochemical element can be sufficiently suppressed and the IV resistance of the electrochemical element can be reduced, thereby completing the present invention.

[0009] That is, the present invention has an object to advantageously solve the above-mentioned problems, and the electrode for an electrochemical element of the present invention is an electrode for an electrochemical element comprising a current collector and an electrode mixture layer formed on the current collector, wherein the electrode mixture layer contains an electrode active material, a binder, and a foaming agent having a thermal decomposition temperature of 150°C or more and 400°C or less, and in a cross section in the thickness direction of the electrode mixture layer, a thermally decomposable site having a circumscribed circle diameter of 1.0 μm or more and 10.0 μm or less and made of the foaming agent is present, and 50 μm of the thermally decomposable site is present in a surface layer region of 0% to 10% in the thickness direction of the electrode mixture layer, taking the surface opposite to the current collector as the base point. 2 The number A per 50 μm of thermally decomposable portions present in a deep layer region of 0% to 90% in the thickness direction of the electrode mixture layer, starting from the surface on the current collector side. 2The number of particles per electrode mixture layer is greater than the number B. In this way, if the electrode mixture layer contains a foaming agent having the above thermal decomposition temperature, if thermally decomposable sites made of the foaming agent and having the above size are present in the cross section of the electrode mixture layer (hereinafter sometimes abbreviated as "composite layer cross section"), and if more of the thermally decomposable sites are located in the surface region than in the deeper region of the cross section of the composite layer, it is possible to sufficiently suppress heat generation during an internal short circuit in the electrochemical device while reducing the IV resistance.

[0010] In the present invention, the term "blowing agent" refers to a compound that generates a non-flammable gas such as nitrogen, carbon dioxide, ammonia, or water vapor upon thermal decomposition. In the present invention, the "thermal decomposition temperature" of the blowing agent can be measured using the method described in the Examples. In the present invention, the cross section of the electrode mixture layer in the thickness direction can be analyzed by observing using a scanning electron microscope (SEM) and using an apparatus (SEM-EPMA) for performing elemental analysis using an electron probe microanalyzer (EPMA). Specifically, by using the SEM-EPMA method described in the examples, the presence or absence of "thermally decomposable portions having a circumscribed circle diameter of 1.0 μm or more and 10.0 μm or less and made of a foaming agent" in the cross section of the mixture layer is confirmed, and the "surface region" is examined for the presence or absence of "thermally decomposable portions within 50 μm of the thermally decomposable portions." 2 The number of particles per 50 μm of the thermally decomposable area in the deep layer is 2 The number of winning numbers "B" can be determined.

[0011] In the electrode for an electrochemical element of the present invention, the number A is preferably 3 or more and 50 or less. 2 If the number A of the contacts is within the above range, it is possible to achieve a good balance between suppressing heat generation and reducing the IV resistance when an internal short circuit occurs in the electrochemical element.

[0012] Additionally, in the electrode for electrochemical devices of the present invention, it is preferable that the ratio of the number A to the number B is more than 1.1 and less than 30. When the ratio of the number A to the number B (hereinafter sometimes abbreviated as "A / B") is within the above range, it is possible to achieve a good balance between suppressing heat generation and reducing IV resistance in the event of an internal short circuit in the electrochemical device.

[0013] In the electrode for electrochemical devices of the present invention, the binder is preferably a polymer having at least one functional group selected from the group consisting of a carboxylic acid group, a hydroxyl group, a nitrile group, an amino group, an epoxy group, an oxazoline group, a sulfonic acid group, an ester group, and an amide group. Use of a polymer having at least one of the above functional groups as the binder can further increase the peel strength of the electrode and further reduce the IV resistance of the electrochemical device.

[0014] In the electrode for electrochemical devices of the present invention, the foaming agent is preferably a nitrogen-based foaming agent, which can further reduce the IV resistance while further suppressing heat generation during an internal short circuit in the electrochemical device. In the present invention, the term "nitrogen-based blowing agent" refers to a blowing agent that generates nitrogen as a non-flammable gas by thermal decomposition.

[0015] The present invention aims to advantageously solve the above-mentioned problems, and the electrochemical device of the present invention is characterized by comprising any of the above-mentioned electrodes for electrochemical devices of the present invention. An electrochemical device comprising any of the above-mentioned electrodes of the present invention is excellent in safety because heat generation during an internal short circuit is sufficiently suppressed. Furthermore, the electrochemical device has low IV resistance and excellent device characteristics. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide an electrode for an electrochemical device that can sufficiently suppress heat generation during an internal short circuit in the electrochemical device and can also reduce the IV resistance. Furthermore, according to the present invention, it is possible to provide an electrochemical element in which heat generation during an internal short circuit is sufficiently suppressed and which has low IV resistance. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a cross-sectional view schematically showing an example of a cross section in the thickness direction of an electrode for an electrochemical element according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, embodiments of the present invention will be described in detail. The electrode for an electrochemical device of the present invention can be used as an electrode for electrochemical devices such as lithium ion secondary batteries, electric double layer capacitors, and lithium ion capacitors. The electrochemical device of the present invention comprises the electrode for an electrochemical device of the present invention.

[0019] (Electrodes for electrochemical elements) The electrode of the present invention comprises at least an electrode mixture layer and a current collector. The electrode of the present invention may comprise components other than the electrode mixture layer and the current collector. For example, the electrode of the present invention may comprise a known adhesive layer or heat-resistant layer on the electrode mixture layer. The electrode of the present invention may comprise an electrode mixture layer on only one side of the current collector, or may comprise electrode mixture layers on both sides. Furthermore, since the electrode of the present invention has a predetermined electrode mixture layer as described below, it is possible to sufficiently suppress heat generation in the event of an internal short circuit in the electrochemical device, and also to reduce the IV resistance. In addition, when the electrode of the present invention has electrode mixture layers on both sides of the current collector, if at least one of the electrode mixture layers is a specified electrode mixture layer described below, the above-mentioned effects regarding the electrochemical element can be fully achieved.

[0020] In order to achieve the above-mentioned effects, the electrode mixture layer of the electrode of the present invention has a cross section in which a circumscribed circle diameter is 1.0 μm or more and 10.0 μm or less, and thermally decomposable sites made of the foaming agent are present. When the cross section of the mixture layer is divided into a surface region on the side opposite the current collector (a region of 0% to 10% in the thickness direction of the electrode mixture layer, with the surface on the side opposite the current collector as the base point) and a deep region other than the surface region (a region of 0% to 90% in the thickness direction of the electrode mixture layer, with the surface on the current collector side as the base point), the number A of thermally decomposable sites in the surface region (number / 50 μm 2 ) is the number of thermally decomposable sites in the deep layer region B (number / 50 μm 2 ) is required.

[0021] The reason why the electrode composite layer contains a foaming agent, and in the cross section of the composite layer, the thermally decomposable portion made of the foaming agent has the above-mentioned specified size and is present in greater numbers in the surface region than in the deeper region, is that heat generation in the event of an internal short circuit in the electrochemical element can be sufficiently suppressed and the IV resistance can be reduced is not clear, but is presumed to be as follows.

[0022] First, in an electrochemical element using an electrode having an electrode mixture layer containing a foaming agent, when thermal runaway occurs and the temperature inside the element rises, the foaming agent in the electrode mixture layer decomposes (foams) and generates a non-flammable gas. The generation of this non-flammable gas dilutes flammable gases generated by the decomposition of the electrolyte solution due to high temperatures, preventing the spread of fire. Furthermore, the foaming of the foaming agent and the generation of non-flammable gas destroys the electrode structure (for example, the electrode active material is detached from the current collector), cutting off the conductive path. As a result, the generation of Joule heat is suppressed, and further temperature increases inside the electrochemical element can be suppressed. The present inventors conducted further studies and found that the size and arrangement of the foaming agent in the electrode mixture layer are important in order to foam the foaming agent at an appropriate timing during thermal runaway, while at the same time ensuring sufficient electrical contact between the electrode active materials during normal operation of the electrochemical device to exhibit good device characteristics. In the electrode mixture layer of the electrode of the present invention, the circumscribed circle diameter of the thermally decomposable portion consisting of the foaming agent observed in the cross section of the mixture layer is 1.0 μm or more and 10.0 μm or less. When the thermally decomposable portion has such a size, the foaming agent in the portion does not form excessive aggregates / fine particles, allowing it to foam at the appropriate time during thermal runaway while not significantly impeding electrical contact between the electrode active materials. Additionally, in the electrode mixture layer of the electrode of the present invention, there are more thermally decomposable portions in the surface region, which accounts for 10% of the thickness of the electrode mixture layer, than in the deep region, which accounts for 90% of the thickness. This means that the foaming agent is present at a higher density on the surface side (opposite the current collector) of the electrode mixture layer. In such an electrode mixture layer, the foaming agent, which could impede electrical contact between the electrode active materials, does not excessively cover the electrode active materials. For the reasons described above, it is believed that by using the electrode of the present invention having a predetermined electrode mixture layer, heat generation during an internal short circuit in an electrochemical device can be sufficiently suppressed and the IV resistance can be reduced.

[0023] An example of the structure of the electrode of the present invention will be described with reference to Fig. 1. In Fig. 1, the dimensions of some of the components are shown enlarged or reduced to facilitate understanding. FIG. 1 is a cross-sectional view schematically illustrating an example of a cross section in the thickness direction (in other words, the stacking direction of the electrode mixture layer 10 and the current collector 20) of an electrode 100 for electrochemical elements according to the present invention. In FIG. 1, the electrode 100 for electrochemical elements is formed by stacking the electrode mixture layer 10 on the current collector 20. The electrode mixture layer 10 contains an electrode active material 11 and a binder 12, as well as a foaming agent 13, and the foaming agent 13 forms a thermally decomposable site. In the example of FIG. 1, the electrode mixture layer 10 also contains a conductive material 14. When the electrode mixture layer 10 is divided into the surface region a and the deep region b, as described above, in the present invention, the number A (number / 50 μm 2 ) is the number B (number / 50 μm) of thermally decomposable sites in the deep layer b 2 ) is required. The electrode of the present invention is not limited to the example shown in Fig. 1. For example, the electrode mixture layer 10 may have a coating portion made of a surfactant that coats at least a portion of the periphery of the thermally decomposable portion 13. Furthermore, for example, the above-mentioned known adhesive layer or heat-resistant layer may be laminated on the electrode mixture layer 10.

[0024] <Electrode composite layer> The electrode mixture layer contains an electrode active material, a binder, and a foaming agent, and optionally contains a surfactant and / or other components. As described above, the electrode mixture layer has thermally decomposable sites with a circumscribed circle diameter of 1.0 μm to 10.0 μm, which are composed of a surfactant, on the cross section of the mixture layer, and the number A of thermally decomposable sites on the surface region of the cross section of the mixture layer (number / 50 μm 2 ) is the number B (number / 50 μm) of thermally decomposable sites in the deep layer of the composite layer cross section. 2 ) is required. In addition, the cross section of the composite layer may contain thermally decomposable portions with a circumscribed circle diameter of less than 1.0 μm (above the detection limit) and / or thermally decomposable portions with a circumscribed circle diameter of more than 10.0 μm, but the number of such portions (pieces / 50 μm 2 ) is preferably smaller than the number of thermally decomposable sites having a circumscribed circle diameter of 1.0 μm or more and 10.0 μm or less (i.e., the sum of number A and number B), and it is more preferable that the number of thermally decomposable sites having a circumscribed circle diameter of more than 10.0 μm is 0.

[0025] <<Quantity A and quantity B>>> Here, in the cross section of the composite layer, 50 μm in the surface layer region 2 The number A of thermally decomposable sites per 50 μm in the surface region is preferably 3 or more, more preferably 5 or more, even more preferably 10 or more, and particularly preferably 12 or more, and is preferably 50 or less, more preferably 40 or less, even more preferably 30 or less, and particularly preferably 20 or less. 2When the number A of thermally decomposable sites per unit area is within the above-mentioned range, it is possible to achieve a good balance between suppressing heat generation and reducing IV resistance in the event of an internal short circuit in the electrochemical device.

[0026] In addition, in the cross section of the composite layer, 50 μm in the deep layer area 2 The number B of thermally decomposable sites per 50 μm is not particularly limited as long as it is smaller than the number A, and may be 0, but for example, it is preferably 1 or more, more preferably 3 or more, even more preferably 7 or more, preferably 40 or less, more preferably 30 or less, even more preferably 20 or less, and particularly preferably 10 or less. 2 When the number B of thermally decomposable sites per unit area is within the above-mentioned range, it is possible to achieve a good balance between suppressing heat generation and reducing IV resistance in the event of an internal short circuit in the electrochemical device.

[0027] The ratio of the number A to the number B is preferably 1.1 or more, more preferably 1.3 or more, even more preferably 1.5 or more, and is preferably 30 or less, more preferably 20 or less, even more preferably 10 or less, and particularly preferably 5 or less. If the ratio of the number A to the number B is within the above-mentioned range, it is possible to achieve a good balance between suppressing heat generation and reducing the IV resistance during an internal short circuit in the electrochemical element.

[0028] <<Electrode active material>> Here, the electrode active material is a material that transfers electrons at the electrode of the electrochemical device. For example, when the electrochemical device is a lithium ion secondary battery, the electrode active material is usually a material that can absorb and release lithium. In the following, an example will be described in which the slurry composition for an electrochemical element electrode is a slurry composition for a lithium ion secondary battery electrode, but the present invention is not limited to the following example.

[0029] As the positive electrode active material for the lithium ion secondary battery, there is no particular limitation, and lithium-containing cobalt oxide (lithium cobalt oxide, LiCoO2), lithium manganate (LiMn2O4), lithium-containing nickel oxide (LiNiO2), Co-Ni-Mn lithium-containing composite oxide (Li(Co Mn Ni)O2), Ni-Mn-Al lithium-containing composite oxide, Ni-Co-Al lithium-containing composite oxide, olivine-type lithium iron phosphate (LiFePO4), olivine-type lithium manganese phosphate (LiMnPO4), Li2MnO3-LiNiO2-based solid solution, Li 1+x Mn 2-x excess lithium spinel compound represented by O4(0 < X < 2), Li[Ni 0.17 Li 0.2 Co 0.07 Mn 0.56 O2, LiNi 0.5 Mn 1.5 O4 and other known positive electrode active materials can be mentioned. In addition, the blending amount and particle diameter of the positive electrode active material are not particularly limited, and can be the same as those of the conventionally used positive electrode active material.

[0030] Further, as the negative electrode active material for the lithium ion secondary battery, for example, carbon-based negative electrode active materials, metal-based negative electrode active materials, and negative electrode active materials combining these can be mentioned.

[0031] Here, the carbon-based negative electrode active material refers to an active material having carbon as the main skeleton into which lithium can be inserted (also referred to as "doped"). Examples of the carbon-based negative electrode active material include carbonaceous materials and graphite materials.

[0032] And examples of the carbonaceous material include easily graphitizable carbon and hardly graphitizable carbon having a structure close to an amorphous structure typified by glassy carbon. Here, examples of the easily graphitizable carbon include carbon materials made from tar pitch obtained from petroleum or coal. Specific examples include coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fibers, pyrolytic gas-phase grown carbon fibers, and the like. Examples of non-graphitizable carbon include phenolic resin baked body, polyacrylonitrile carbon fiber, pseudo-isotropic carbon, furfuryl alcohol resin baked body (PFA), and hard carbon.

[0033] Furthermore, examples of graphite materials include natural graphite and artificial graphite. Examples of artificial graphite include artificial graphite obtained by heat-treating carbon containing easily graphitized carbon mainly at 2800°C or higher, graphitized MCMB obtained by heat-treating MCMB at 2000°C or higher, and graphitized mesophase pitch-based carbon fiber obtained by heat-treating mesophase pitch-based carbon fiber at 2000°C or higher.

[0034] Furthermore, a metal-based negative electrode active material is an active material containing a metal, typically an active material containing an element capable of intercalating lithium, and having a theoretical electrical capacity per unit mass of 500 mAh / g or more when lithium is intercalated. Examples of metal-based active materials include lithium metal, elemental metals capable of forming lithium alloys (e.g., Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, Sr, Zn, Ti, etc.), alloys thereof, and oxides, sulfides, nitrides, silicides, carbides, and phosphides thereof. Among these, silicon-containing active materials (silicon-based negative electrode active materials) are preferred as metal-based negative electrode active materials. This is because the use of silicon-based negative electrode active materials can increase the capacity of lithium-ion secondary batteries.

[0035] Examples of silicon-based negative electrode active materials include silicon (Si), silicon-containing alloys, SiO, and SiO x and a composite of a Si-containing material and conductive carbon, which is obtained by coating or compounding a Si-containing material with conductive carbon. These silicon-based negative electrode active materials may be used alone or in combination of two or more. The amount and particle size of the negative electrode active material are not particularly limited, and may be the same as those of conventionally used negative electrode active materials.

[0036] <<Binding material>> The binder is not particularly limited as long as it is a polymer that can exhibit binding ability inside the electrochemical element, and any polymer can be used. Suitable examples of polymers used as binders, from the viewpoint of increasing the peel strength of the electrode and further reducing the IV resistance of the electrochemical element, include polymers mainly containing aliphatic conjugated diene monomer units and their hydrogenated products (diene polymers), polymers mainly containing (meth)acrylic acid ester monomer units (acrylic polymers), polymers mainly containing (meth)acrylonitrile (nitrile polymers), and polymers mainly containing fluorine-containing monomer units (fluorine-containing polymers). Among these, diene polymers, acrylic polymers, and nitrile polymers are more preferred, and diene polymers are even more preferred. The binder may be used alone or in combination of two or more kinds in any ratio.

[0037] Here, in the present invention, "(meth)acrylic" means acrylic and / or methacrylic, and "(meth)acrylo" means acrylo and / or methacrylo. In the present invention, the phrase "containing a monomer unit" means that "a polymer obtained using the monomer contains a repeating unit derived from the monomer." Furthermore, in the present invention, the phrase "mainly comprising" a certain monomer unit means that "when the amount of all repeating units contained in the polymer is taken as 100% by mass, the content of the monomer unit exceeds 50% by mass." In the present invention, the content ratio of each monomer unit in the polymer is 1 It can be measured using nuclear magnetic resonance (NMR) techniques such as H-NMR.

[0038] [Specific functional group] The binder is preferably a polymer having a functional group. Examples of functional groups that the binder has include a carboxylic acid group, a hydroxyl group, a nitrile group, an amino group, an epoxy group, an oxazoline group, a sulfonic acid group, an ester group, and an amide group (hereinafter, these functional groups may be collectively referred to as "specific functional groups"). The polymer used as the binder may have one or more of the specific functional groups described above. The use of a polymer having these specific functional groups as a binder can increase the peel strength of the electrode and further reduce the IV resistance of the electrochemical device. From the viewpoint of further increasing the peel strength of the electrode and further reducing the IV resistance of the electrochemical device, the polymer used as a binder preferably has at least one selected from the group consisting of a carboxylic acid group, a hydroxyl group, and a nitrile group, more preferably has at least one of a carboxylic acid group and a nitrile group, and even more preferably has both a carboxylic acid group and a nitrile group.

[0039] Here, the method for introducing the specific functional group into the polymer is not particularly limited. For example, a polymer may be prepared using a monomer having the specific functional group (specific functional group-containing monomer) to obtain a polymer containing the specific functional group-containing monomer unit, or a polymer into which the specific functional group has been introduced may be obtained by modifying an arbitrary polymer, but the former is preferred. That is, the polymer used as a binder preferably contains at least one of a carboxylic acid group-containing monomer unit, a hydroxyl group-containing monomer unit, a nitrile group-containing monomer unit, an amino group-containing monomer unit, an epoxy group-containing monomer unit, an oxazoline group-containing monomer unit, a sulfonic acid group-containing monomer unit, an ester group-containing monomer unit, and an amide group-containing monomer unit, more preferably at least one of a carboxylic acid group-containing monomer unit, a hydroxyl group-containing monomer unit, and a nitrile group-containing monomer unit, even more preferably at least one of a carboxylic acid group-containing monomer unit and a nitrile group-containing monomer unit, and particularly preferably both a carboxylic acid group-containing monomer unit and a nitrile group-containing monomer unit.

[0040] [Carboxylic acid group-containing monomer unit] Examples of the carboxylic acid group-containing monomer capable of forming the carboxylic acid group-containing monomer unit include monocarboxylic acids and derivatives thereof, dicarboxylic acids and acid anhydrides thereof, and derivatives thereof. Examples of the monocarboxylic acid include acrylic acid, methacrylic acid, and crotonic acid. Examples of the monocarboxylic acid derivatives include 2-ethylacrylic acid, isocrotonic acid, α-acetoxyacrylic acid, β-trans-aryloxyacrylic acid, and α-chloro-β-E-methoxyacrylic acid. Examples of dicarboxylic acids include maleic acid, fumaric acid, and itaconic acid. Examples of dicarboxylic acid derivatives include methylmaleic acid, dimethylmaleic acid, phenylmaleic acid, chloromaleic acid, dichloromaleic acid, fluoromaleic acid, and maleic acid monoesters such as nonyl maleate, decyl maleate, dodecyl maleate, octadecyl maleate, and fluoroalkyl maleate. Examples of the acid anhydrides of dicarboxylic acids include maleic anhydride, acrylic anhydride, methyl maleic anhydride, and dimethyl maleic anhydride. Furthermore, as the carboxylic acid group-containing monomer, an acid anhydride that generates a carboxylic acid group upon hydrolysis can also be used. Among these, acrylic acid and methacrylic acid are preferred as the carboxylic acid group-containing monomer. Note that the carboxylic acid group-containing monomer may be used alone or in combination of two or more in any ratio.

[0041] [Hydroxyl group-containing monomer unit] Examples of hydroxyl group-containing monomers that can form hydroxyl group-containing monomer units include ethylenically unsaturated alcohols such as (meth)allyl alcohol, 3-butene-1-ol, and 5-hexene-1-ol; alkanol esters of ethylenically unsaturated carboxylic acids such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, di-2-hydroxyethyl maleate, di-4-hydroxybutyl maleate, and di-2-hydroxypropyl itaconate; and esters of ethylenically unsaturated carboxylic acids such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, di-4-hydroxybutyl maleate, and di-2-hydroxypropyl itaconate; a -COO-(C q H 2q O) p -H (wherein p is an integer of 2 to 9, q is an integer of 2 to 4, R arepresents a hydrogen atom or a methyl group) and (meth)acrylic acid esters; mono(meth)acrylic acid esters of dihydroxy esters of dicarboxylic acids such as 2-hydroxyethyl-2'-(meth)acryloyloxyphthalate and 2-hydroxyethyl-2'-(meth)acryloyloxysuccinate; vinyl ethers such as 2-hydroxyethyl vinyl ether and 2-hydroxypropyl vinyl ether; mono(meth)allyl ethers of alkylene glycols such as (meth)allyl-2-hydroxyethyl ether, (meth)allyl-2-hydroxypropyl ether, (meth)allyl-3-hydroxypropyl ether, (meth)allyl-2-hydroxybutyl ether, (meth)allyl-3-hydroxybutyl ether, (meth)allyl-4-hydroxybutyl ether, and (meth)allyl-6-hydroxyhexyl ether; diethylene glycol mono(meth)allyl ether, dipropylene glycol Examples of the hydroxyl group-containing monomer include polyoxyalkylene glycol mono(meth)allyl ethers such as glycerin mono(meth)allyl ether, (meth)allyl-2-chloro-3-hydroxypropyl ether, and (meth)allyl-2-hydroxy-3-chloropropyl ether; mono(meth)allyl ethers of halogen- and hydroxy-substituted (poly)alkylene glycols, such as glycerin mono(meth)allyl ether, (meth)allyl-2-chloro-3-hydroxypropyl ether; mono(meth)allyl ethers of polyhydric phenols such as eugenol and isoeugenol, and halogen-substituted derivatives thereof; (meth)allyl thioethers of alkylene glycols, such as (meth)allyl-2-hydroxyethyl thioether and (meth)allyl-2-hydroxypropyl thioether; and amides having a hydroxyl group, such as N-hydroxymethylacrylamide (N-methylolacrylamide), N-hydroxymethylmethacrylamide, N-hydroxyethylacrylamide, and N-hydroxyethylmethacrylamide. The hydroxyl group-containing monomer may be used alone or in combination of two or more in any ratio. In the present invention, "(meth)allyl" means allyl and / or methallyl, and "(meth)acryloyl" means acryloyl and / or methacryloyl.

[0042] [Nitrile group-containing monomer unit] Examples of nitrile group-containing monomers capable of forming nitrile group-containing monomer units include α,β-ethylenically unsaturated nitrile monomers. Specifically, the α,β-ethylenically unsaturated nitrile monomer is not particularly limited as long as it is an α,β-ethylenically unsaturated compound having a nitrile group, and examples thereof include acrylonitrile; α-halogenoacrylonitriles such as α-chloroacrylonitrile and α-bromoacrylonitrile; and α-alkylacrylonitriles such as methacrylonitrile and α-ethylacrylonitrile. The nitrile group-containing monomers may be used alone or in combination of two or more in any ratio.

[0043] [Amino group-containing monomer unit] Examples of amino group-containing monomers capable of forming amino group-containing monomer units include dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, aminoethyl vinyl ether, dimethylaminoethyl vinyl ether, etc. The amino group-containing monomers may be used alone or in combination of two or more in any ratio. In the present invention, the term "(meth)acrylate" means acrylate and / or methacrylate.

[0044] [Epoxy group-containing monomer unit] The epoxy group-containing monomer capable of forming the epoxy group-containing monomer unit includes a monomer containing a carbon-carbon double bond and an epoxy group. Examples of the monomer containing a carbon-carbon double bond and an epoxy group include unsaturated glycidyl ethers such as vinyl glycidyl ether, allyl glycidyl ether, butenyl glycidyl ether, and o-allylphenyl glycidyl ether; diene or polyene monoepoxides such as butadiene monoepoxide, chloroprene monoepoxide, 4,5-epoxy-2-pentene, 3,4-epoxy-1-vinylcyclohexene, and 1,2-epoxy-5,9-cyclododecadiene; and 3,4-epoxy-1- Examples of the epoxy group-containing monomer include alkenyl epoxides such as butene, 1,2-epoxy-5-hexene, and 1,2-epoxy-9-decene; and glycidyl esters of unsaturated carboxylic acids such as glycidyl acrylate, glycidyl methacrylate, glycidyl crotonate, glycidyl 4-heptenoate, glycidyl sorbate, glycidyl linoleate, glycidyl 4-methyl-3-pentenoate, glycidyl ester of 3-cyclohexenecarboxylic acid, and glycidyl ester of 4-methyl-3-cyclohexenecarboxylic acid. The epoxy group-containing monomer may be used alone or in combination of two or more in any ratio.

[0045] [Oxazoline group-containing monomer unit] Examples of oxazoline group-containing monomers capable of forming oxazoline group-containing monomer units include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-5-methyl-2-oxazoline, 2-isopropenyl-5-ethyl-2-oxazoline, etc. The oxazoline group-containing monomers may be used alone or in combination of two or more in any ratio.

[0046] [Sulfonic acid group-containing monomer unit] Examples of sulfonic acid group-containing monomers capable of forming sulfonic acid group-containing monomer units include vinyl sulfonic acid, methyl vinyl sulfonic acid, (meth)allyl sulfonic acid, styrene sulfonic acid, (meth)acrylic acid-2-ethyl sulfonate, 2-acrylamido-2-methylpropanesulfonic acid, 3-allyloxy-2-hydroxypropanesulfonic acid, etc. The sulfonic acid group-containing monomers may be used alone or in combination of two or more in any ratio.

[0047] [Ester group-containing monomer unit] Examples of the ester group-containing monomer capable of forming the ester group-containing monomer unit include (meth)acrylic acid ester monomers. Examples of the (meth)acrylic acid ester monomer include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, butyl acrylates such as n-butyl acrylate and t-butyl acrylate, octyl acrylates such as pentyl acrylate, hexyl acrylate, heptyl acrylate and 2-ethylhexyl acrylate, alkyl acrylates such as nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate and stearyl acrylate; and Examples of the ester group-containing monomer include methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, butyl methacrylates such as n-butyl methacrylate and t-butyl methacrylate, octyl methacrylates such as pentyl methacrylate, hexyl methacrylate, heptyl methacrylate and 2-ethylhexyl methacrylate, and methacrylic acid alkyl esters such as nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate and stearyl methacrylate. The ester group-containing monomer may be used alone or in combination of two or more at any ratio. In the present invention, when a certain monomer has a specific functional group other than an ester group, the monomer is not included in the ester group-containing monomer.

[0048] [Amide group-containing monomer unit] Examples of amide group-containing monomers capable of forming amide group-containing monomer units include acrylamide, methacrylamide, vinylpyrrolidone, etc. The amide group-containing monomers may be used alone or in combination of two or more in any ratio.

[0049] Here, when the amount of all repeating units contained in the polymer as a binder is taken as 100% by mass, the content of the specific functional group-containing monomer unit in the polymer is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, from the viewpoints of increasing the peel strength of the electrode and further reducing the IV resistance of the electrochemical device. Note that the upper limit of the content of the specific functional group-containing monomer unit in the polymer as a binder is not particularly limited, and can be 100% by mass or less, for example, 99% by mass or less.

[0050] [Other repeating units] The polymer serving as the binder may contain repeating units (other repeating units) other than the specific functional group-containing monomer units described above. Such other repeating units are not particularly limited, but when the polymer is a diene-based monomer, examples of such other repeating units include aliphatic conjugated diene-based monomer units. Examples of aliphatic conjugated diene monomers that can form aliphatic conjugated diene monomer units include 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene. These may be used alone or in combination of two or more in any ratio. In the present invention, the "aliphatic conjugated diene monomer unit" also includes a structural unit (hydride unit) obtained by further hydrogenating a monomer unit contained in a polymer obtained using an aliphatic conjugated diene monomer. Among the above-mentioned aliphatic conjugated diene monomers, 1,3-butadiene and isoprene are preferred. In other words, as the aliphatic conjugated diene monomer unit, a 1,3-butadiene unit, an isoprene unit, a 1,3-butadiene hydride unit, and an isoprene hydride unit are preferred, and a 1,3-butadiene hydride unit and an isoprene hydride unit are more preferred.

[0051] Here, when the polymer used as a binder contains an aliphatic conjugated diene monomer unit, the content of the diene monomer unit in the polymer, when the amount of all repeating units contained in the polymer is taken as 100% by mass, is preferably more than 50% by mass, more preferably 60% by mass or more, and is preferably 90% by mass or less, preferably 80% by mass or less, and even more preferably 70% by mass or less, from the viewpoint of increasing the peel strength of the electrode and further reducing the IV resistance of the electrochemical device.

[0052] [Method for preparing binder] The method for preparing the binder is not particularly limited. A polymeric binder is produced, for example, by polymerizing a monomer composition containing one or more monomers in an aqueous solvent, and optionally hydrogenating or modifying the monomer composition. The content of each monomer in the monomer composition can be determined based on the content of the desired monomer unit in the polymer. The polymerization method is not particularly limited, and any of solution polymerization, suspension polymerization, bulk polymerization, emulsion polymerization, etc. can be used. The polymerization reaction can be any of ionic polymerization, radical polymerization, living radical polymerization, various condensation polymerizations, addition polymerization, etc. Known emulsifiers and polymerization initiators can be used during polymerization, if necessary. Hydrogenation and modification can be carried out by known methods.

[0053] [Binder content] Here, the amount of binder contained in the electrode mixture layer is preferably 0.3 parts by mass or more, more preferably 0.7 parts by mass or more, and preferably 5 parts by mass or less, more preferably 4 parts by mass or less, and even more preferably 3 parts by mass or less, per 100 parts by mass of electrode active material. If the binder content is 0.3 parts by mass or more per 100 parts by mass of electrode active material, the peel strength of the electrode can be improved. On the other hand, if the binder content is 5 parts by mass or less per 100 parts by mass of electrode active material, the proportion of electrode active material in the electrode mixture layer can be ensured, and the capacity of the electrochemical device can be sufficiently increased.

[0054] <<Foaming agent>> The foaming agent contained in the electrode mixture layer and constituting the thermally decomposable portion of the cross section of the mixture layer is not particularly limited as long as it is a compound having a predetermined thermal decomposition temperature. However, from the viewpoint of further suppressing heat generation during an internal short circuit of the electrochemical device and further reducing the IV resistance, nitrogen-based foaming agents are preferred, compounds having at least one selected from the group consisting of an amino group, an azo group, a hydrazino group, a hydrazo group, and a nitroso group, derivatives thereof, and salts thereof are more preferred, compounds having at least one of an amino group and an azo group, derivatives thereof, and salts thereof are even more preferred, and melamine compounds are particularly preferred. The foaming agent may be used alone or in combination of two or more kinds in any ratio.

[0055] [Melamine compounds] The melamine compounds include melamine, melamine derivatives, and salts thereof. Examples of melamine and melamine derivatives include compounds represented by the following formula (I):

[0056] [ka]

[0057] In formula (I), each A is independently a hydroxyl group or NR 1 R 2 (R1 and R 2 each independently represents a hydrogen atom, a hydrocarbon group, or a hydroxyl group-containing hydrocarbon group. 1 If there are multiple R 1 may be the same or different, and R 2 If there are multiple R 2 may be the same or different.

[0058] where R 1 and R 2 When the hydrocarbon group and hydroxyl group-containing hydrocarbon group have two or more carbon atoms, one or more oxygen atoms (-O-) may be present between the carbon atoms (however, when two or more oxygen atoms are present, they are not adjacent to each other). 1 and R 2 The number of carbon atoms in the hydrocarbon group and the hydroxyl group-containing hydrocarbon group is not particularly limited, but is preferably 1 or more and 5 or less.

[0059] Furthermore, salts of melamine and melamine derivatives are not particularly limited, but include sulfates, cyanurates, and the like.

[0060] As the melamine compound, from the viewpoint of improving the peel strength of the electrode while further reducing the IV resistance of the electrochemical element, melamine, ammeline, ammelide, and their salts with cyanuric acid are preferred, melamine and melamine cyanurate (melamine cyanurate) are more preferred, and melamine cyanurate is even more preferred. The melamine compounds may be used singly or in combination of two or more kinds in any ratio.

[0061] [Other foaming agents] Examples of blowing agents other than the above-mentioned melamine compounds include azobisisobutyronitrile, p-toluenesulfonylhydrazide, 5-methyl-1H-benzotriazole, oxybisbenzenesulfonylhydrazide, trihydrazinetriazine, azodicarbonamide, hydrazodicarbonamide, dinitrosopentamethylenetetramine, p-toluenesulfonylsemicarbazide, p,p'-oxybisbenzenesulfonylsemicarbazide, and sodium hydrogen carbonate. These other blowing agents may be used alone or in combination of two or more in any ratio.

[0062] [Thermal decomposition temperature] The thermal decomposition temperature of the blowing agent must be 150°C or higher and 400°C or lower, preferably 200°C or higher, more preferably 300°C or higher, preferably 380°C or lower, more preferably 360°C or lower, and even more preferably 350°C or lower. If the thermal decomposition temperature of the blowing agent is lower than 150°C, the blowing agent may unexpectedly decompose during normal operation or storage of the electrochemical device, resulting in an increase in the IV resistance of the electrochemical device. On the other hand, if the thermal decomposition temperature of the blowing agent is higher than 400°C, it becomes difficult for the blowing agent to generate non-flammable gas at the appropriate time, and the expected heat generation suppression effect during an internal short circuit cannot be fully achieved by using the blowing agent.

[0063] [Foaming agent content] Here, the amount of foaming agent contained in the electrode mixture layer is preferably 0.3 parts by mass or more, more preferably 0.7 parts by mass or more, and preferably 5 parts by mass or less, more preferably 4 parts by mass or less, and even more preferably 3 parts by mass or less, per 100 parts by mass of electrode active material. If the foaming agent content is 0.3 parts by mass or more per 100 parts by mass of electrode active material, the peel strength of the electrode can be increased while further suppressing heat generation during an internal short circuit of the electrochemical device. On the other hand, if the foaming agent content is 5 parts by mass or less per 100 parts by mass of electrode active material, the IV resistance of the electrochemical device can be further reduced.

[0064] <<Surfactants>> The electrode mixture layer may contain a surfactant. The location of the surfactant in the electrode mixture layer is not particularly limited. For example, when a cross section of the mixture layer is observed, the surfactant may form a coating portion that covers at least a portion of the periphery of the thermally decomposable portion. Such a coating portion can be formed when, for example, a thermally decomposable material having a core-shell structure, which will be described later, is used in forming the electrode mixture layer.

[0065] [kinds] As the surfactant, any of anionic surfactants, nonionic surfactants and cationic surfactants can be used.

[0066] Examples of anionic surfactants include aliphatic carboxylic acids (salts) such as lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, and their metal salts (sodium salt, lithium salt, potassium salt, calcium salt, magnesium salt, aluminum salt, zinc salt); alkyl sulfates such as sodium 2-ethylhexyl sulfate and sodium lauryl sulfate; dialkyl sulfosuccinates such as sodium di-2-ethylhexyl-sulfosuccinate; and alkylbenzene sulfonates. Examples of nonionic surfactants include ether-type surfactants such as polyoxyethylene lauryl ether, polyoxyethylene stearyl ether, and polyoxyethylene-2-ethylhexyl ether; and ester-type surfactants such as polyoxyethylene monolaurate, polyoxyethylene monostearate, sorbitan monostearate, sorbitan monolaurate, sorbitan trioleate, and glycerin stearic acid ester. Examples of cationic surfactants include amine salt types such as tetradecylamine acetate and octadecylamine acetate; and trimethyl types such as dodecyltrimethyl-ammonium chloride and octadecyltrimethyl-ammonium chloride. The surfactant may be used alone or in combination of two or more in any ratio. From the viewpoints of increasing the peel strength of the electrode and further reducing the IV resistance of the electrochemical device, the surfactant is preferably an anionic surfactant, more preferably an aliphatic carboxylic acid (salt), further preferably sodium stearate or lithium stearate, and particularly preferably sodium stearate. In the present invention, the term "aliphatic carboxylic acid (salt)" refers to an aliphatic carboxylic acid and / or a salt of an aliphatic carboxylic acid.

[0067] [Molecular weight] The surfactant preferably has a molecular weight of 50 g / mol or more, more preferably 100 g / mol or more, and even more preferably 250 g / mol or more, and preferably 1,000 g / mol or less, more preferably 800 g / mol or less, and even more preferably 500 g / mol or less. If the surfactant has a molecular weight of 50 g / mol or more and 1,000 g / mol or less, the IV resistance of the electrochemical device can be further reduced.

[0068] [Melting point] In addition, the surfactant preferably has a melting point of 50°C or higher, more preferably 200°C or higher, even more preferably 250°C or higher, and preferably 350°C or lower, more preferably 330°C or lower, and even more preferably 320°C or lower. If the melting point of the surfactant is 50°C or higher, the IV resistance of the electrochemical device can be further reduced. On the other hand, if the melting point of the surfactant is 350°C or lower, heat generation in the event of an internal short circuit in the electrochemical device can be further suppressed.

[0069] [Surfactant content] The amount of surfactant contained in the electrode mixture layer is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, and even more preferably 1% by mass or more, and preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 4% by mass or less, based on 100% by mass of the total amount of the foaming agent and surfactant. If the proportion of the surfactant in the total amount of the foaming agent and surfactant is 0.01% by mass or more, the IV resistance of the electrochemical device can be further reduced. On the other hand, if the proportion of the surfactant in the total amount of the foaming agent and surfactant is 10% by mass or less, the peel strength of the electrode can be increased while further suppressing heat generation in the event of an internal short circuit in the electrochemical device. In the present invention, the "proportion of the surfactant in the total amount of the foaming agent and the surfactant" can be measured by thermogravimetric analysis, pyrolysis GC-MS, or the like.

[0070] <<Other ingredients>> In addition to the components described above, the electrode mixture layer may contain known components such as a conductive material, a crosslinking agent, a reinforcing material, an antioxidant, a dispersant, a rheology modifier, and an electrolyte additive having the function of suppressing decomposition of the electrolyte. The conductive material is not particularly limited, and examples thereof include conductive carbon materials such as carbon black (e.g., acetylene black, Ketjen Black (registered trademark), furnace black, etc.), single-walled or multi-walled carbon nanotubes (multi-walled carbon nanotubes include cup-stacked types), carbon nanohorns, vapor-grown carbon fibers, milled carbon fibers obtained by crushing polymer fibers after firing, single-walled or multi-walled graphene, and carbon nonwoven fabric sheets obtained by firing nonwoven fabric made of polymer fibers, as well as fibers or foils of various metals. The other components may be used singly or in combination of two or more in any ratio.

[0071] <<Thickness>> The thickness of the electrode mixture layer is not particularly limited, but is preferably 20 μm or more, more preferably 40 μm or more, and is preferably 200 μm or less, more preferably 150 μm or less, for example. In addition, when the electrode of the present invention has an electrode mixture layer on both sides of the current collector, the "thickness of the electrode mixture layer" means the thickness of each electrode mixture layer formed on each side (one side) of the current collector.

[0072] <Current collector> The current collector included in the electrode for an electrochemical device is not particularly limited as long as it is an electrically conductive and electrochemically durable material, and may be selected depending on the type of electrochemical device. When the electrode for an electrochemical device is an electrode for a lithium ion secondary battery, examples of materials constituting the current collector include iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, and platinum. Among these, copper foil is particularly preferred as the current collector used for the negative electrode. Furthermore, aluminum foil is particularly preferred as the material constituting the current collector used for the positive electrode. These materials may be used singly or in combination of two or more in any ratio. The thickness of the current collector is not particularly limited, but is preferably 5 μm or more, more preferably 10 μm or more, and is preferably 100 μm or less, more preferably 50 μm or less.

[0073] <Method of manufacturing an electrode for an electrochemical element> The method for producing the electrode of the present invention is not particularly limited as long as it is possible to form, on the current collector, the above-mentioned electrode mixture layer which contains many thermally decomposable sites of a predetermined size in its surface layer region. In producing the electrode of the present invention, the electrode mixture layer is preferably formed by drying a slurry composition for an electrochemical element electrode, which will be described later. That is, the electrode mixture layer is preferably a dried product of the slurry composition for an electrochemical element electrode.

[0074] <<Slurry composition for electrochemical element electrodes>> The slurry composition used to form the electrode mixture layer is a composition obtained by dissolving and / or dispersing in a solvent the above-mentioned electrode active material, the above-mentioned binder, the above-mentioned foaming agent, and optionally a thermally decomposable material containing a surfactant, and the above-mentioned other components that are optionally blended. The components contained in the electrode mixture layer formed using the slurry composition are those contained in the slurry composition, and the preferred abundance ratio of each of these components in the slurry composition can be the same as the preferred abundance ratio in the electrode mixture layer described above.

[0075] [solvent] Here, examples of the solvent contained in the slurry composition include water and organic solvents, with organic solvents being preferred. Examples of organic solvents that can be used include acetonitrile, N-methylpyrrolidone, acetylpyridine, cyclopentanone, N,N-dimethylacetamide, dimethylformamide, dimethyl sulfoxide, methylformamide, methyl ethyl ketone, furfural, and ethylenediamine. Among these, N-methyl-2-pyrrolidone (NMP) is particularly preferred from the viewpoints of ease of handling, safety, ease of synthesis, and the like. The solvent may be used alone or in combination of two or more kinds in any ratio.

[0076] [Thermal decomposition material containing a foaming agent] The thermally decomposable material used in preparing the slurry composition is a material containing at least a foaming agent, and may consist essentially of only a foaming agent, or may have a core-shell structure comprising a core consisting of a foaming agent and a shell consisting of a surfactant covering at least a portion of the outer surface of the core, as described below.

[0077] -Particle properties- The thermally decomposable material preferably has predetermined particle properties so as to form an electrode mixture layer having the above-mentioned predetermined properties. Specifically, the thermally decomposable material preferably has at least one property selected from the group consisting of a number average particle diameter of 0.01 μm or more and 10 μm or less, a volume average particle diameter of 0.01 μm or more and 10 μm or less, a ratio of the number average particle diameter to the volume average particle diameter (hereinafter sometimes abbreviated as "particle diameter ratio") of 0.05 or more and 1 or less, and a circularity of 0.05 or more and 0.95 or less, and more preferably has all of the properties in this group. When the thermally decomposable material has the above-mentioned particle properties, the behavior of the thermally decomposable material can be controlled during application and drying when forming an electrode mixture layer from a slurry composition containing a binder composition, and it is thought that this can prevent the thermally decomposable material (especially thermally decomposable materials with relatively small sizes) from excessively coating the electrode active material in the resulting electrode mixture layer, and that this particle properties make it easier for the thermally decomposable material to be arranged in the surface layer region.This can satisfactorily achieve effects such as reduced IV resistance while fully ensuring the effect of suppressing heat generation during internal short circuits caused by the foaming agent contained in the thermally decomposable material.

[0078] In the present invention, the "number average particle size" of a thermally decomposable material means the particle size at 50% of the cumulative value in the particle size distribution (number basis) measured using a laser diffraction particle size distribution analyzer. In the present invention, the "volume average particle size" of a thermally decomposable material means the particle size at an integrated value of 50% in the particle size distribution (volume basis) measured using a laser diffraction particle size distribution measuring device. In the present invention, the "circularity" of a thermally decomposable material is determined by the formula: circularity=4πS / L, where S is the area of ​​a two-dimensional image of the thermally decomposable material and L is the perimeter. 2 The circularity is calculated as follows. The circularity takes a value greater than 0 and less than or equal to 1, with a perfect circle having a circularity of 1. The more complex the two-dimensional image, the smaller the circularity value.

[0079] First, the number-average particle diameter of the thermally decomposable material is preferably 0.01 μm or more, more preferably 0.05 μm or more, and even more preferably 0.3 μm or more, and is preferably 10 μm or less, more preferably 5 μm or less, and even more preferably 4 μm or less. If the number-average particle diameter of the thermally decomposable material is 0.01 μm or more, heat generation during an internal short circuit in the electrochemical device can be further suppressed and the IV resistance can be further reduced. On the other hand, if the number-average particle diameter of the thermally decomposable material is 10 μm or less, the IV resistance of the electrochemical device can be further reduced.

[0080] Furthermore, the thermally decomposable material preferably has a volume average particle size of 0.01 μm or more, more preferably 0.1 μm or more, even more preferably 1 μm or more, and preferably 10 μm or less, more preferably 8 μm or less, and even more preferably 5 μm or less. If the volume average particle size of the thermally decomposable material is 0.01 μm or more, the IV resistance of the electrochemical device can be further reduced. On the other hand, if the volume average particle size of the thermally decomposable material is 10 μm or less, heat generation during an internal short circuit of the electrochemical device can be further suppressed and the IV resistance can be further reduced.

[0081] The ratio of the number-average particle size to the volume-average particle size of the thermally decomposable material is preferably 0.05 or more, more preferably 0.2 or more, even more preferably 0.3 or more, and is 1 or less, preferably 0.95 or less, and more preferably 0.8 or less. If the particle size ratio of the thermally decomposable material is 0.05 or more, the IV resistance of the electrochemical element can be further reduced. On the other hand, if the particle size ratio of the thermally decomposable material is 0.95 or less, the heat generation suppression effect during an internal short circuit, which is expected from the use of the thermally decomposable material, can be more fully obtained.

[0082] In addition, the thermally decomposable material preferably has a circularity of 0.05 or more, more preferably 0.5 or more, even more preferably 0.6 or more, particularly preferably 0.75 or more, and preferably 0.95 or less, more preferably 0.9 or less, and even more preferably 0.85 or less. If the circularity of the thermally decomposable material is 0.05 or more, the peel strength of the electrode can be increased and the IV resistance of the electrochemical element can be further reduced. On the other hand, if the circularity of the thermally decomposable material is 0.95 or less, the IV resistance of the electrochemical element can be further reduced.

[0083] The particle properties of the thermally decomposable material can be controlled by changing the preparation conditions of the thermally decomposable material. For example, when preparing a thermally decomposable material containing melamine cyanurate as a foaming agent, the number-average particle size and volume-average particle size can be controlled by changing the solid concentration in the reaction from melamine and cyanuric acid to obtain melamine cyanurate, or by subjecting the obtained melamine cyanurate to a pulverization treatment using a bead mill, etc. Furthermore, when preparing a thermally decomposable material containing melamine cyanurate as a foaming agent, the circularity can be controlled by changing the pH in the reaction from melamine and cyanuric acid to obtain melamine cyanurate. Furthermore, the particle properties of the thermally decomposable material can also be controlled by changing the granulation conditions when preparing the thermally decomposable material.

[0084] -Core-shell structure- The thermally decomposable material may have a core-shell structure including a core made of a foaming agent and a shell made of a surfactant that covers at least a part of the outer surface of the core. By using a thermally decomposable material having a core-shell structure in which the foaming agent is coated with the surfactant, the IV resistance of the electrochemical device can be further reduced. Although it is not clear why the thermally decomposable material having the above-mentioned core-shell structure can further reduce the IV resistance of the electrochemical element, it is presumed that this is because a thermally decomposable material having a structure in which the foaming agent is covered with a surfactant does not excessively adsorb to the electrode active material, thereby ensuring sufficient electrical contact between the electrode active materials.

[0085] -Method for preparing thermally decomposable materials- A thermally decomposable material having the above-described particulate properties can be prepared, for example, by granulating a composition containing at least a foaming agent and, optionally, a surfactant and a dispersant (hereinafter referred to as a "composition for thermally decomposable material"). The dispersion medium can be appropriately used depending on the granulation method, and the type of the dispersion medium can be appropriately selected from water and known organic solvents depending on the granulation method.

[0086] The granulation method for obtaining a thermally decomposable material from the above-mentioned composition for a thermally decomposable material is not particularly limited as long as it can obtain a thermally decomposable material having the desired particle properties, but examples include spray granulation, fluidized bed granulation, coagulant precipitation, pH precipitation, dry mixing, and a method of wet mixing followed by drying and granulation. Among these, spray granulation is preferred.

[0087] In the spray granulation method, a slurry composition containing a foaming agent and a dispersant, and optionally a surfactant, as a composition for a thermally decomposable material (hereinafter referred to as "slurry composition for a thermally decomposable material") is spray-dried to obtain a thermally decomposable material having predetermined particle properties.

[0088] The method for preparing the slurry composition for thermally decomposable materials is not particularly limited, and can be carried out by mixing the above-mentioned components using a known mixer. Known mixers include ball mills, sand mills, bead mills, pigment dispersers, crushers, ultrasonic dispersers, homogenizers, and planetary mixers. Mixing is usually carried out at a temperature ranging from room temperature to 80°C for 10 minutes to several hours.

[0089] The slurry composition for pyrolyzable materials obtained by the above-described mixing is sprayed using a spray dryer, and the sprayed droplets of the slurry composition for pyrolyzable materials are dried inside a drying tower. This allows for the production of a particulate pyrolyzable material. When the slurry composition for pyrolyzable materials contains a surfactant, the surfactant adheres physically and / or chemically to the outer surface of the foaming agent contained in the droplets, resulting in a pyrolyzable material in which at least a portion of the outer surface of the foaming agent is covered with the surfactant. The temperature of the sprayed slurry composition for pyrolyzable materials is typically room temperature, but it may be heated to a temperature higher than room temperature. The hot air temperature during spray drying is preferably below the thermal decomposition temperature of the pyrolyzable material, e.g., 80°C to 250°C, preferably 100°C to 200°C.

[0090] [Method for preparing slurry composition] The above-mentioned slurry composition can be prepared by mixing the above-mentioned components. Specifically, the slurry composition can be prepared by mixing the above-mentioned components using a mixer such as a ball mill, a sand mill, a bead mill, a pigment disperser, a crusher, an ultrasonic disperser, a homogenizer, a planetary mixer, or a Filmix.

[0091] <<Formation of electrode mixture layer>> To form an electrode mixture layer using the above-mentioned slurry composition, for example, the slurry composition may be supplied onto a current collector and then dried. Here, in order to control the number A of thermally decomposable sites in the surface region and the number B of thermally decomposable sites in the deep region in the obtained electrode composite layer, a method can be used in which the supply and drying of the slurry composition is repeated multiple times. For example, a first slurry composition and a second slurry composition having a higher concentration of thermally decomposable material than the first slurry composition are prepared. The first slurry composition is then applied to at least one surface of a current collector and dried. After that, the second slurry composition is applied onto the dried first slurry composition and the second slurry composition is dried. This method increases the number A of thermally decomposable sites in the surface layer region, making it possible to easily form an electrode mixture layer having desired properties.

[0092] The method for applying the slurry composition onto a current collector or the like is not particularly limited, and any known method can be used. Specific examples of the application method include a doctor blade method, a dipping method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, and a brush coating method. The thickness of the slurry film on the current collector after application and before drying can be appropriately set depending on the thickness of the electrode mixture layer obtained by drying.

[0093] The method for drying the slurry composition on the current collector or the like is not particularly limited, and any known method can be used, such as drying with warm air, hot air, or low-humidity air, vacuum drying, or drying by irradiation with infrared rays or electron beams.

[0094] After drying, the electrode mixture layer may be subjected to pressure treatment using a mold press, a roll press, etc. Pressure treatment can improve the peel strength of the electrode.

[0095] (electrochemical element) The electrochemical device of the present invention is characterized by comprising the above-described electrode for an electrochemical device. The electrochemical device of the present invention is not particularly limited and may be, for example, a lithium ion secondary battery, an electric double layer capacitor, or a lithium ion capacitor, and is preferably a lithium ion secondary battery. Since the electrochemical device of the present invention comprises the electrode of the present invention, heat generation during an internal short circuit is sufficiently suppressed, maintaining a high level of safety. Furthermore, the electrochemical device of the present invention has low IV resistance and excellent device characteristics such as output characteristics.

[0096] Hereinafter, a case where the electrochemical element is a lithium ion secondary battery will be described as an example, but the present invention is not limited to the following example. A lithium ion secondary battery as the electrochemical element of the present invention usually comprises electrodes (positive electrode and negative electrode), an electrolyte, and a separator, and uses the electrode for electrochemical elements of the present invention for at least one of the positive electrode and the negative electrode.

[0097] <Electrode> Here, the electrode other than the above-described electrode for electrochemical elements of the present invention that can be used in the lithium ion secondary battery as the electrochemical element of the present invention is not particularly limited, and any known electrode can be used. Specifically, the electrode other than the above-described electrode for electrochemical elements can be an electrode obtained by forming an electrode mixture layer on a current collector using a known manufacturing method.

[0098] <Electrolyte> As the electrolyte, an organic electrolyte solution in which a supporting electrolyte is dissolved in an organic solvent is typically used. For example, a lithium salt is used as the supporting electrolyte. Examples of lithium salts include LiPF6, LiAsF6, LiBF4, LiSbF6, LiAlCl4, LiClO4, CF3SO3Li, CF4F9SO3Li, CF3COOLi, (CF3CO)2NLi, (CF3SO2)2NLi, and (C2F5SO2)NLi. Among these, LiPF6, LiClO4, and CF3SO3Li are preferred, with LiPF6 being particularly preferred, because they are easily soluble in solvents and exhibit a high degree of dissociation. One type of electrolyte may be used alone, or two or more types may be used in combination at any ratio. Generally, the use of a supporting electrolyte with a higher degree of dissociation tends to result in higher lithium ion conductivity, so the lithium ion conductivity can be adjusted by the type of supporting electrolyte.

[0099] The organic solvent used in the electrolyte is not particularly limited as long as it can dissolve the supporting electrolyte. For example, carbonates such as dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate (BC), and methyl ethyl carbonate (EMC) are preferred. Other suitable solvents include γ-butyrolactone, methyl formate, and other esters. Ethers such as 1,2-dimethoxyethane and tetrahydrofuran are also suitable. Sulfur-containing compounds such as sulfolane and dimethyl sulfoxide are also suitable. Mixtures of these solvents may also be used. Among these, carbonates are preferred because of their high dielectric constant and wide stable potential range. A mixture of ethylene carbonate and ethyl methyl carbonate is even more preferred. The concentration of the electrolyte in the electrolytic solution can be adjusted appropriately, and is preferably 0.5 to 15 mass%, more preferably 2 to 13 mass%, and even more preferably 5 to 10 mass%. The electrolytic solution may also contain known additives, such as vinylene carbonate, fluoroethylene carbonate, and ethyl methyl sulfone.

[0100] <Separator> The separator is not particularly limited, and for example, the one described in JP 2012-204303 A can be used. Among these, a microporous membrane made of a polyolefin resin (polyethylene, polypropylene, polybutene, polyvinyl chloride) is preferred because it can reduce the overall separator thickness, thereby increasing the ratio of electrode active material particles in the lithium ion secondary battery and increasing the capacity per volume. Furthermore, a separator with a functional layer, in which a functional layer (porous membrane layer or adhesive layer) is provided on one or both sides of a separator substrate, may be used as the separator.

[0101] <Method of manufacturing lithium-ion secondary batteries> The lithium ion secondary battery according to the present invention can be produced, for example, by stacking a positive electrode and a negative electrode with a separator interposed therebetween, rolling or folding the resulting structure as necessary according to the battery shape, placing the resultant structure in a battery container, injecting an electrolyte into the battery container, and sealing the container. To prevent internal pressure buildup, overcharging and overdischarging, and the like, a fuse, an overcurrent protection element such as a PTC element, an expanded metal, a lead plate, or the like may be provided as necessary. The shape of the secondary battery may be any of a coin type, a button type, a sheet type, a cylindrical type, a rectangular type, a flat type, and the like. [Example]

[0102] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples. In the following description, "%" and "parts" representing amounts are based on mass unless otherwise specified. Furthermore, in a polymer produced by copolymerizing multiple types of monomers, the proportion of a monomer unit formed by polymerizing a certain monomer in the polymer usually coincides with the ratio (feed ratio) of the certain monomer to all the monomers used in the polymerization of the polymer, unless otherwise specified. In the examples and comparative examples, the thermal decomposition temperature of the foaming agent, the average particle size (number average particle size, volume average particle size, particle size ratio) of the thermally decomposable material containing the foaming agent, the circularity, the proportion of the amount of surfactant in the total amount of the foaming agent and surfactant in the thermally decomposable material, the number of thermally decomposable sites in the cross section of the electrode mixture layer (number A in the surface region, number B in the deep region), and the IV resistance of the lithium ion secondary battery and heat generation suppression during internal short circuit were measured or evaluated using the following methods.

[0103] <Thermal decomposition temperature> In thermogravimetric analysis using a thermogravimetric differential thermal analyzer (Hitachi High-Tech Science Corporation, product name "TG / DTA7200"), the weight of the blowing agent was measured while the temperature was raised from 25°C to 500°C at a rate of 10°C / min in a nitrogen atmosphere, and the temperature at which the measured weight became 95% of the mass at the start of the measurement (25°C) (5% weight loss temperature) was defined as the thermal decomposition temperature of the blowing agent. <Average particle size> The number-average particle size and volume-average particle size of the thermally decomposable material were measured using a laser diffraction particle size analyzer (Microtrac-Bell, product name "MT3000II"), and the particle size ratio was calculated. <Circularity> The circularity of 1,000 particles was measured using a particle image analyzer (Malvern Panalytical, product name "Morphologi (registered trademark) G3") and the average value was calculated to evaluate the circularity of the thermally decomposable material. <Proportion of surfactant amount in the total amount of foaming agent and surfactant> In thermogravimetric analysis using a thermogravimetric differential thermal analyzer (Hitachi High-Tech Science Corporation, product name "TG / DTA7200"), the weight of the thermally decomposable material was measured while the temperature was raised from 25°C to 500°C at a rate of 10°C / min in a nitrogen atmosphere, and the weight loss at temperatures other than the thermal decomposition temperature of the foaming agent was calculated as the surfactant content. <Number of thermally decomposable parts> The positive electrode was processed to expose a cross section for observation using a cross-section polisher (manufactured by JEOL Ltd., product name "IB-09020CP"). Next, nitrogen element mapping of the composite layer cross section was performed using a SEM-EPMA (manufactured by JEOL Ltd., product name "JXA-8530F Plus"). The observation magnification was 1300x (a magnification that allows observation of the entire thickness direction of the positive electrode composite layer), the irradiation voltage was 10 kV, and the irradiation current was 5.0 × 10 -8 Condition A was adopted. Nitrogen intensity was detected using a wavelength dispersive detector. The primary Ka line of X-rays was dispersed using a spectrometer (manufactured by JEOL, product name "LDH5E") to detect the intensity of nitrogen element. Measurements were performed while scanning at 0.4 μm x 0.4 μm intervals, and detection was performed for a length of 30 ms at each point. For the obtained mapping image, the number of thermally decomposable sites with a circumscribed circle diameter of 1.0 μm or more and 10.0 μm or less was counted using the method described below. Note that when counting, thermally decomposable sites located at the boundary of the analysis range were counted as being included in the analysis range. 1) The entire thickness direction of the positive electrode composite layer was included, and an analysis range with a width direction (direction perpendicular to the thickness direction) of 50 μm (i.e., a range of the thickness of the positive electrode composite layer × 50 μm in the width direction) was arbitrarily selected. Here, the area of this analysis range was designated as M (μm 2 ). 2) In the surface layer region of the selected analysis range (area: 0.1M μm 2 ), the number X (pieces) of sites where the intensity of the nitrogen element is 50 (CPS) or more and the circumscribed circle diameter is 1.0 μm or more and 10 μm or less was counted. The decimal part of the solution obtained by the formula: X / 0.1M × 50 was truncated, and the number A (pieces / 50 μm 2 ) was obtained. 3) In the deep layer region of the selected analysis range (area: 0.9M μm 2 ), the number Y (pieces) of sites where the intensity of the nitrogen element is 50 (CPS) or more and the circumscribed circle diameter is 1.0 μm or more and 10 μm or less was counted. The decimal part of the solution obtained by the formula: Y / 0.9M × 50 was truncated, and the number B (pieces / 50 μm 2 ) was obtained. <IV Resistance> The lithium-ion secondary battery was left standing at a temperature of 25°C for 5 hours after injecting the electrolyte. Next, it was charged up to a cell voltage of 3.65V by a constant current method at a temperature of 25°C and 0.2C, and then an aging treatment was performed at a temperature of 60°C for 12 hours. Then, it was discharged to a cell voltage of 3.00V by a constant current method at a temperature of 25°C and 0.2C. Then, CC-CV charging (upper limit cell voltage 4.35V) was performed by a constant current method of 0.2C, and CC discharge was performed up to 3.00V by a constant current method of 0.2C. This charge-discharge at 0.2C was repeated 3 times. Then, in an environment of 25°C, a charging operation was performed so that the state of charge (SOC) was 50% at 0.2C. It was left standing for 600 seconds. The voltage at the 600th second was designated as V0. Then, it was discharged for 10 seconds by a constant current method of 0.5C (= I 0.5 ), and the voltage at the 10th second was designated as V 0.5 . Then, it was charged with the same amount of electricity as the previous discharge by a constant current method of 0.2C. Next, it was discharged for 10 seconds by a constant current method of 1.0C (= I 1.0 ), and the voltage at the 10th second was designated as V 1.0Then, the battery was charged with the same amount of electricity as the previous discharge at a constant current of 0.2 C. Next, the battery was charged at 1.5 C (= I 1.5 ) constant current method for 10 seconds, and the voltage at the 10th second is V 1.5 (I 0.5 ,V 0.5 ),(I 1.0 ,V 1.0 ),(I 1.5 ,V 1.5 ) was plotted on an XY graph, and the slope b of the regression line was calculated using the following formula, which was taken as DCR (direct current resistance).

number

[0104] Example 1 <Preparation of binder> 240 parts of ion-exchanged water, 2.5 parts of sodium alkylbenzenesulfonate, 30 parts of acrylonitrile as a nitrile group-containing monomer, 5 parts of methacrylic acid as a carboxylic acid group-containing monomer, and 0.25 parts of t-dodecyl mercaptan as a chain transfer agent were placed in an autoclave equipped with a stirrer, in this order, and the inside of the bottle was purged with nitrogen. Then, 65 parts of 1,3-butadiene as an aliphatic conjugated diene monomer were pressurized, and 0.25 parts of ammonium persulfate were added, followed by polymerization at a reaction temperature of 40°C. A polymer containing acrylonitrile units, methacrylic acid units, and 1,3-butadiene units was obtained. The polymerization conversion was 85%. The resulting polymer was adjusted to a total solids concentration of 12% with water, and 400 mL of the resulting solution (48 g total solids) was placed in a 1 L autoclave equipped with a stirrer. After 10 minutes of nitrogen gas flow to remove dissolved oxygen from the solution, 75 mg of palladium acetate as a hydrogenation catalyst was dissolved in 180 mL of ion-exchanged water containing 4 times the molar amount of nitric acid relative to Pd and added. After the system was purged with hydrogen gas twice, the contents of the autoclave were heated to 50 °C under a pressure of 3 MPa with hydrogen gas, and the hydrogenation reaction (first-stage hydrogenation reaction) was carried out for 6 hours. Next, the autoclave was returned to atmospheric pressure, and 25 mg of palladium acetate was dissolved in 60 mL of water containing 4 times the molar amount of nitric acid relative to Pd as a hydrogenation catalyst and added. After the system was purged with hydrogen gas twice, the contents of the autoclave were heated to 50°C while pressurized with hydrogen gas to 3 MPa, and a hydrogenation reaction (second-stage hydrogenation reaction) was carried out for 6 hours to obtain an aqueous dispersion of hydrogenated nitrile rubber. An appropriate amount of NMP was added to the obtained aqueous dispersion of hydrogenated nitrile rubber to obtain a mixture. Thereafter, vacuum distillation was carried out at 90°C to remove water and excess NMP from the mixture, and an NMP solution of hydrogenated nitrile rubber (solid concentration: 8%) was obtained. <Preparation of thermally decomposable materials> Equimolar amounts of 63.0 g of melamine ground to a volume average particle size of 100 μm and 64.5 g of cyanuric acid were added to a reactor. Potassium hydroxide was then added at 1% relative to the total amount of melamine and cyanuric acid (100%), and ion-exchanged water was added to adjust the solids concentration to 55%. The mixture was then heated to 75°C with stirring and continued for 120 minutes to produce a slurry containing melamine cyanurate (foaming agent, thermal decomposition temperature: 350°C) as the core of the thermally decomposable material. To the resulting slurry, 97 parts of melamine cyanurate were added 3 parts of the anionic surfactant sodium stearate (molecular weight 306.5 g / mol, melting point: 305°C), and ion-exchanged water was added to adjust the solids concentration to 20%. The mixture was further stirred for 30 minutes to obtain a slurry composition for thermally decomposable materials. The resulting slurry composition was spray-dried at 140°C to obtain a thermally decomposable material. The average particle size and circularity of this thermally decomposable material were measured, as well as the ratio of the amount of surfactant to the total amount of foaming agent and surfactant, and the particle size ratio was calculated. The results are shown in Table 1. <Preparation of Binder Composition> 100 parts (solid content equivalent) of the NMP solution of the hydrogenated nitrile rubber and 100 parts of the thermally decomposable material were mixed, and NMP was further added to prepare a binder composition with a solid content concentration of 8%. <Preparation of Slurry Composition for Positive Electrode> 96 parts of lithium cobalt oxide as a positive electrode active material, 2.0 parts in solids equivalent of carbon black (manufactured by Denka, product name "Li-100") as a conductive material, and 2.0 parts in solids equivalent of the binder composition were charged into a planetary mixer and mixed, and NMP was gradually added thereto, followed by stirring and mixing at a temperature of 25±3°C and a rotation speed of 60 rpm. A positive electrode slurry composition was obtained by adjusting the viscosity to 3,600 mPa s at 25±3°C using a Brookfield viscometer at 60 rpm (rotor M4). <Production of positive electrode (single coating)> The positive electrode slurry composition was applied to a 20 μm thick aluminum foil current collector using a comma coater in an amount of 20±0.5 mg / cm 2The cathode slurry composition on the aluminum foil was then dried by transporting the aluminum foil at a speed of 0.5 m / min through an oven at 90°C for 2 minutes and then through an oven at 120°C for 2 minutes, thereby obtaining a cathode raw sheet with a cathode composite layer formed on the current collector. Thereafter, the cathode composite layer side of the prepared cathode raw sheet was roll-pressed under a temperature of 25±3°C and a load of 14 t, and the density of the cathode composite layer was determined to be 3.80 g / cm. 3 A positive electrode having a positive electrode mixture layer thickness of 53 μm was obtained. The number A of thermally decomposable sites of the obtained positive electrode was measured. The results are shown in Table 1. <Production of negative electrodes> A 5 MPa pressure vessel equipped with a stirrer was charged with 63 parts of styrene as an aromatic vinyl monomer, 34 parts of 1,3-butadiene as an aliphatic conjugated diene monomer, 2 parts of itaconic acid as a carboxylic acid group-containing monomer, 1 part of 2-hydroxyethyl acrylate as a hydroxyl group-containing monomer, 0.3 parts of t-dodecyl mercaptan as a molecular weight modifier, 5 parts of sodium dodecylbenzenesulfonate as an emulsifier, 150 parts of ion-exchanged water as a solvent, and 1 part of potassium persulfate as a polymerization initiator. After thorough stirring, the mixture was heated to 55°C to initiate polymerization. The reaction was stopped by cooling when the monomer consumption reached 95.0%. A 5% aqueous sodium hydroxide solution was added to the resulting aqueous dispersion containing the polymer to adjust the pH to 8. Unreacted monomer was then removed by heated vacuum distillation. The mixture was then cooled to a temperature of 30°C or below to obtain an aqueous dispersion containing a negative electrode binder (a negative electrode binder composition). A planetary mixer was charged with 48.75 parts of artificial graphite (theoretical capacity 360 mAh / g) as a negative electrode active material, 48.75 parts of natural graphite (theoretical capacity 360 mAh / g), and 1 part of carboxymethyl cellulose as a thickener (solids equivalent). The mixture was then diluted with ion-exchanged water to a solids concentration of 60% and kneaded for 60 minutes at a rotation speed of 45 rpm. Then, 1.5 parts (solids equivalent) of the negative electrode binder composition obtained above was added and kneaded for 40 minutes at a rotation speed of 40 rpm. Ion-exchanged water was then added to the mixture to a viscosity of 3000±500 mPa·s (measured with a Brookfield viscometer at 25°C and 60 rpm), thereby preparing a negative electrode slurry composition. The negative electrode slurry composition was applied to the surface of a 15 μm thick copper foil current collector using a comma coater in an amount of 11±0.5 mg / cm 2 The copper foil coated with the negative electrode slurry composition was then transported at a speed of 400 mm / min through an oven at 80°C for 2 minutes and then through an oven at 110°C for 2 minutes, thereby drying the negative electrode slurry composition on the copper foil and obtaining a negative electrode blank with a negative electrode composite layer formed on the current collector. The negative electrode composite layer side of the prepared negative electrode blank was then roll-pressed under a temperature of 25±3°C and a linear pressure of 11 t (tons), until the density of the negative electrode composite layer reached 1.60 g / cm. 3 A negative electrode of 1000 .mu.m was obtained. <Preparing the separator> A single-layer polypropylene separator (manufactured by Celgard, product name "#2500") was prepared. <Fabrication of lithium-ion secondary batteries> A laminated cell (with an initial design discharge capacity of 3 Ah) was fabricated using the negative electrode, positive electrode, and separator described above. It was then placed in an aluminum foil bag and vacuum dried at 60°C for 10 hours. A 1.0 M LiPF solution (solvent: ethylene carbonate (EC) / diethyl carbonate (DEC) = 3 / 7 (volume ratio), additive: vinylene carbonate 2 vol% (solvent ratio)) was then filled as the electrolyte. The aluminum foil was then heat-sealed at 150°C to seal the opening, completing the lithium-ion secondary battery. The resulting lithium-ion battery was evaluated for IV resistance and heat suppression during an internal short circuit. The results are shown in Table 1.

[0105] Example 2 Except for using the thermally decomposable material, binder composition, positive electrode slurry composition, and positive electrode prepared as follows, a binder, a negative electrode, a separator, and a lithium ion secondary battery were prepared and various evaluations were performed in the same manner as in Example 1. The results are shown in Table 1. <Preparation of thermally decomposable materials> Equimolar amounts of 63.0 g of melamine pulverized to a volume average particle size of 100 μm and 64.5 g of cyanuric acid were added to a reactor. Potassium hydroxide was then added at 1% relative to the total amount of melamine and cyanuric acid (100%), and ion-exchanged water was added to adjust the solids concentration to 55%, yielding a mixture. The mixture was then heated to 75°C with stirring and continued for 120 minutes to yield a slurry composition for a thermally decomposable material containing melamine cyanurate (a foaming agent). The resulting slurry composition was spray-dried at 140°C to yield a thermally decomposable material. <Preparation of Binder Composition> <<Preparation of First Binder Composition>> 100 parts (solid content equivalent) of the NMP solution of hydrogenated nitrile rubber obtained in the same manner as in Example 1 and 50 parts of the thermally decomposable material were mixed, and NMP was further added to prepare a first binder composition with a solid content concentration of 8%. <<Preparation of Second Binder Composition>> 100 parts (solid content equivalent) of the NMP solution of hydrogenated nitrile rubber obtained in the same manner as in Example 1 and 150 parts of the thermally decomposable material were mixed, and NMP was further added to prepare a second binder composition with a solid content concentration of 8%. <Preparation of Slurry Composition for Positive Electrode> <<Preparation of First Slurry Composition>> The planetary mixer was charged with 96.5% lithium cobalt oxide as the positive electrode active material. 1.5 parts of the first binder composition were added and mixed, and NMP was gradually added thereto. The mixture was stirred and mixed at a temperature of 25±3°C and a rotation speed of 60 rpm, and the viscosity was adjusted to 3,600 mPa·s at 25±3°C using a Brookfield viscometer at 60 rpm (rotor M4), to obtain a first slurry composition. <<Preparation of Second Slurry Composition>> 95.5 parts of lithium cobalt oxide as a positive electrode active material, 2.0 parts in solids equivalent of carbon black (manufactured by Denka, product name "Li-100") as a conductive material, and 2.5 parts in solids equivalent of the second binder composition were added to a planetary mixer and mixed, and NMP was gradually added thereto, followed by stirring and mixing at a temperature of 25±3°C and a rotation speed of 60 rpm. A second slurry composition was obtained by adjusting the viscosity to 3,600 mPa s at 25±3°C using a Brookfield viscometer at 60 rpm (rotor M4). <Production of positive electrode (double coating)> The first slurry composition was applied to a 20 μm thick aluminum foil current collector using a comma coater in an amount of 10±0.5 mg / cm 2 The first slurry composition on the aluminum foil was dried by transporting the aluminum foil at a speed of 0.5 m / min in an oven at a temperature of 90°C for 2 minutes and then in an oven at a temperature of 120°C for 2 minutes, thereby obtaining a laminate in which a dried product of the first slurry composition was formed on the current collector. Next, the second slurry composition was applied to the dried laminate using a comma coater in an amount of 10±0.5 mg / cm. 2The laminate was then transported at a speed of 0.5 m / min through an oven at 90°C for 2 minutes, and then through an oven at 120°C for 2 minutes to dry the second slurry composition on the laminate, thereby obtaining a positive electrode raw sheet having a positive electrode composite layer formed on the current collector. Thereafter, the positive electrode composite layer side of the prepared positive electrode raw sheet was roll-pressed under a temperature of 25±3°C and a load of 14 t, and the density of the positive electrode composite layer was adjusted to 3.80 g / cm. 3 A positive electrode having a positive electrode mixture layer thickness of 53 μm was obtained.

[0106] Example 3 Except for using a thermally decomposable material prepared in the same manner as in Example 2, a binder, a binder composition, a positive electrode slurry composition, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0107] Example 4 In preparing the thermally decomposable material, except that the solid content of the mixture for obtaining melamine cyanurate was changed from 55% to 60%, a binder, a thermally decomposable material, a binder composition, a slurry composition for a positive electrode, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0108] Example 5 Except for using the thermally decomposable material prepared as follows, a binder, a binder composition, a positive electrode slurry composition, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1. <Preparation of thermally decomposable materials> Equimolar amounts of 63.0 g of melamine pulverized to a volume average particle size of 100 μm and 64.5 g of cyanuric acid were added to a reactor. Potassium hydroxide was then added at 1% relative to the total amount of melamine and cyanuric acid (100%), and ion-exchanged water was added to adjust the solids concentration to 60%, yielding a mixture. The mixture was then heated to 75°C with stirring and continued for 120 minutes to yield a slurry composition for a thermally decomposable material containing melamine cyanurate (foaming agent, thermal decomposition temperature: 350°C). The resulting slurry composition was spray-dried at 140°C to yield a thermally decomposable material.

[0109] Example 6 Except for using the thermally decomposable material prepared as follows, a binder, a binder composition, a positive electrode slurry composition, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1. <Preparation of thermally decomposable materials> A pre-dispersion (premix) was prepared using a Three-One Motor (Shintokagaku Co., Ltd., product name "BL300") with 100.0 g of azodicarbonamide (thermal decomposition temperature: 150 °C) as a foaming agent, with ion-exchange water added to a solids concentration of 20%. The resulting pre-dispersion was milled for 5 minutes using a bead mill (Ashizawa Finetech Co., Ltd., product name "LMZ-015") at a bead diameter of 1.0 mm, a bead filling rate of 80%, and a peripheral speed of 8 m / s to produce a slurry containing azodicarbonamide as the core of the thermally decomposable material. To the resulting slurry, 3 parts of anionic surfactant sodium stearate (molecular weight: 306.5 g / mol, melting point: 305 °C) were added to 97 parts of azodicarbonamide and stirred for an additional 30 minutes to obtain a slurry composition for thermally decomposable materials. The resulting slurry composition was spray-dried at 140 °C to obtain a thermally decomposable material.

[0110] (Comparative Example 1) Except for using the thermally decomposable material prepared as follows, a binder, a binder composition, a positive electrode slurry composition, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1. <Preparation of thermally decomposable materials> Equimolar amounts of 63.0 g of melamine pulverized to a volume average particle size of 100 μm and 64.5 g of cyanuric acid were added to a reactor. Potassium hydroxide was then added at 1% relative to the total amount of melamine and cyanuric acid (100%), and ion-exchanged water was added to adjust the solids concentration to 55% to obtain a mixture. The mixture was then heated to 75°C with stirring and continued for 120 minutes to produce a slurry containing melamine cyanurate (foaming agent, thermal decomposition temperature: 350°C). The resulting slurry was dried at 80°C for 12 hours, and the resulting dried product was subjected to a spheronization treatment using a mechanical spheronizer (EarthTechnica, product name "Kryptron Orb CSH0"). Ion-exchanged water was added to the dried product after spheronization, adjusting the solids concentration to 55%, to obtain a slurry composition for a thermally decomposable material. The resulting slurry composition was dried by spray drying at 140°C to obtain a thermally decomposable material.

[0111] (Comparative Example 2) Except for using the thermally decomposable material prepared as follows, a binder, a binder composition, a positive electrode slurry composition, a positive electrode, a negative electrode, a separator, and a lithium ion secondary battery were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 1. <Preparation of thermally decomposable materials> Melamine cyanurate (manufactured by Nissan Chemical Industries, Ltd., product name "MC-6000") was mixed with ion-exchanged water to adjust the solid concentration to 55%, and a slurry composition for a thermally decomposable material containing melamine cyanurate (a foaming agent) was obtained. The obtained slurry composition was dried by spray drying at 140°C to obtain a thermally decomposable material.

[0112] In addition, in the following Table 1, "HNBR" indicates hydrogenated nitrile rubber; "MC" indicates melamine cyanurate; "ADCA" refers to azodicarbonamide.

[0113] [Table 1]

[0114] Table 1 shows that Examples 1 to 6, in which the electrode mixture layer formed on the current collector contains a foaming agent having a thermal decomposition temperature within a predetermined range in addition to the electrode active material and binder, and the thermally decomposable site formed by the foaming agent is arranged to satisfy predetermined conditions, can sufficiently suppress heat generation during an internal short circuit in the electrochemical element and can also reduce the IV resistance of the electrochemical element. On the other hand, Table 1 shows that in Comparative Examples 1 and 2, in which the thermally decomposable sites do not satisfy the specified conditions (number A is smaller than number B), heat generation during an internal short circuit in the electrochemical element can be sufficiently suppressed, but the IV resistance increases. [Industrial Applicability]

[0115] According to the present invention, it is possible to provide an electrode for an electrochemical device that can sufficiently suppress heat generation during an internal short circuit in the electrochemical device and can also reduce the IV resistance. Furthermore, according to the present invention, it is possible to provide an electrochemical element in which heat generation during an internal short circuit is sufficiently suppressed and which has low IV resistance. [Explanation of symbols]

[0116] 100 Electrode for electrochemical element 10 Electrode composite layer 11 Electrode active material 12 Binder 13. Foaming agent (thermally decomposable part) 14 Conductive materials 20 Current collector a Surface area b Deep region

Claims

1. An electrode for an electrochemical element comprising a current collector and an electrode mixture layer formed on the current collector, the electrode mixture layer contains an electrode active material, a binder, and a foaming agent having a thermal decomposition temperature of 150°C or higher and 400°C or lower, In a cross section in the thickness direction of the electrode mixture layer, a circumscribed circle diameter is 1.0 μm or more and 10.0 μm or less, and a thermally decomposable site made of the foaming agent is present; and 50 μm of the thermally decomposable portion present in a surface layer region of 0% to 10% in the thickness direction of the electrode mixture layer, starting from the surface opposite to the current collector. ２ The number A of thermally decomposable portions per 50 μm of the electrode mixture layer is 0% to 90% of the depth of the electrode mixture layer in the thickness direction, starting from the surface on the current collector side. ２ The number of electrodes for an electrochemical element is greater than the number B of electrodes per unit area.

2. 2. The electrode for an electrochemical element according to claim 1, wherein the number A is 3 or more and 50 or less.

3. 3. The electrode for an electrochemical element according to claim 1, wherein a ratio of the number A to the number B is greater than 1.1 and less than 30.

4. 4. The electrode for electrochemical elements according to claim 1, wherein the binder is a polymer having at least one functional group selected from the group consisting of a carboxylic acid group, a hydroxyl group, a nitrile group, an amino group, an epoxy group, an oxazoline group, a sulfonic acid group, an ester group, and an amide group.

5. 5. The electrode for an electrochemical element according to claim 1, wherein the foaming agent is a nitrogen-based foaming agent.

6. An electrochemical device comprising the electrode for an electrochemical device according to any one of claims 1 to 5.

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