KR20240035448A - Electrode for electrochemical device and method for manufacturing electrode for electrochemical device - Google Patents

Abstract

It is an electrode for an electrochemical element including a current collector and a predetermined electrode mixture layer. The electrode mixture layer contains at least an electrode active material and thermally expandable particles with an expansion start temperature of 400°C or lower, and the exposed diameter of the electrode mixture layer surface is thermally expanded by 0.5 times or more and 5.0 times or less the volume average particle diameter D50 of the electrode active material. When the exposed particles are A, the area occupied by the exposed particles A on the surface of the electrode mixture layer is 0.5% or more and 20% or less.

Description

Electrode for electrochemical device and method for manufacturing electrode for electrochemical device

The present invention relates to an electrode for an electrochemical device and a method of manufacturing the electrode for an electrochemical device.

Electrochemical devices such as lithium ion secondary batteries have the characteristics of being small and lightweight, have high energy density, and can be repeatedly charged and discharged, and are used in a wide range of applications. Therefore, in recent years, improvements in battery members such as electrodes have been studied with the aim of further improving the performance of electrochemical elements.

Here, electrodes used in electrochemical devices such as lithium ion usually include a current collector and an electrode mixture layer formed on the current collector. Then, this electrode mixture layer is formed, for example, by applying a slurry composition containing an electrode active material, a binder composition containing a binder, etc., onto a current collector, and drying the applied slurry composition.

Electrochemical elements have the risk of causing thermal runaway due to internal short circuits and chains of various chemical reactions within them. In order to suppress the occurrence of such thermal runaway, it has been conventionally done to mix thermally expandable particles containing a material capable of inhibiting the chemical reaction when the temperature inside the electrochemical device rises into the electrode for the electrochemical device (e.g. For example, see patent documents 1 to 3).

International Publication No. 2015/133423 International Publication No. 2019/189865 Japanese Patent Publication No. 2003-031208

Here, electrodes for electrochemical devices are required to provide both good heat suppression performance to the resulting secondary battery and to reduce the IV resistance of the secondary battery. However, the conventional electrodes for electrochemical devices have room for improvement in that these properties can be achieved at a higher level.

Accordingly, the purpose of the present invention is to provide an electrode for an electrochemical device and a method for manufacturing the same, which can increase the heat suppression performance of the electrochemical device and reduce IV resistance.

The present inventor conducted intensive studies with the aim of solving the above problems. Furthermore, the present inventors have proposed that by using an electrode for an electrochemical device in which thermally expandable particles having an expansion start temperature in a predetermined temperature range are exposed at a predetermined ratio to the electrode surface, the heat generation suppression performance of the electrochemical device can be improved while IV By discovering a new way to reduce resistance, the present invention was completed.

That is, the purpose of this invention is to advantageously solve the above problems. [1] The electrode for an electrochemical device of the present invention is an electrode for an electrochemical device provided with a current collector and an electrode mixture layer, and the electrode The mixture layer includes at least an electrode active material and thermally expandable particles with an expansion start temperature of 400° C. or lower, and the exposed diameter present on the surface of the electrode mixture layer is 0.5 times or more and 5.0 times or less the volume average particle diameter D50 of the electrode active material. When the thermally expandable particles are exposed particles A, the area occupied by the exposed particles A on the surface of the electrode mixture layer is characterized in that it is 0.5% or more and 20% or less. By using an electrode for an electrochemical device having these characteristics, the heat generation suppression performance of the electrochemical device can be improved and the IV resistance can be reduced.

On the other hand, the expansion start temperature of the thermally expandable particles, the volume average particle diameter D50 of the electrode active material, and the occupied area ratio of the exposed particles A can be measured by the method described in the Examples of this specification.

Here, [2] in the electrode for an electrochemical device of the present invention according to [1] above, it is preferable that the volume average particle diameter D50 of the thermally expandable particles is 0.3 times to 5.0 times the volume average particle diameter D50 of the electrode active material. do. When an electrode for an electrochemical device is used in which the volume average particle diameter D50 of the thermally expandable particles and the volume average particle diameter D50 of the electrode active material satisfy the above relationship, the heat generation suppression performance of the electrochemical device is further improved and the IV resistance is further improved. It can be reduced further.

On the other hand, the value of the volume average particle diameter D50 can be measured by the method described in the Examples of this specification.

[3] In the electrode for an electrochemical device of the present invention according to [1] or [2], the number density of the exposed particles A on the surface of the electrode mixture layer is 10 particles/mm ² or more and 300 particles. /mm ² or less is preferable. If an electrode for an electrochemical device is used where the number density of exposed particles A on the electrode surface is within the above range, the heat generation suppression performance of the electrochemical device can be further improved and the IV resistance can be further reduced.

On the other hand, the number density of exposed particles A on the electrode surface can be measured by the method described in the Examples of this specification.

[4] In the electrode for an electrochemical device of the present invention according to any one of [1] to [3] above, the electrode mixture layer further includes a binder, and the binder includes a carboxylic acid group, It is preferable that it is a polymer having at least one type of functional group selected from the group consisting of hydroxyl group, nitrile group, amino group, epoxy group, oxazoline group, sulfonic acid group, ester group and amide group. If the electrode mixture layer further contains a predetermined binder, the adhesiveness of the electrode mixture layer can be improved.

[5] This invention aims to advantageously solve the above problems, and the method for manufacturing the electrode for an electrochemical device of the present invention according to any one of [1] to [4] above is as described above. A method for manufacturing an electrode for an electrochemical device, comprising the steps of applying and drying a slurry composition for a lower electrode layer on a current collector to form a lower electrode layer, and applying and drying a slurry composition for an upper electrode layer on the lower electrode layer. Thus, it includes a step of forming an upper electrode layer, wherein the slurry composition for the upper electrode layer and the slurry composition for the lower electrode layer each contain an electrode active material and thermally expandable particles, and the thermal expandability of the slurry composition for the upper electrode layer is The particle concentration is characterized in that it is higher than the thermally expandable particle concentration of the slurry composition for the electrode lower layer. According to this manufacturing method, the electrode for an electrochemical element of the present invention described above can be efficiently manufactured.

According to the present invention, it is possible to provide an electrode for an electrochemical device and a method for manufacturing the same, which can increase the heat suppression performance of the electrochemical device and reduce IV resistance.

Hereinafter, embodiments of the present invention will be described in detail.

Here, the electrode for an electrochemical device (hereinafter also simply referred to as “electrode”) of the present invention can be used when manufacturing an electrochemical device. The electrode for an electrochemical device of the present invention can be suitably used as a positive electrode for an electrochemical device, especially a secondary battery.

(Electrodes for electrochemical devices)

The electrode for an electrochemical element of the present invention includes a current collector and an electrode mixture layer. Specifically, the electrode, the electrode mixture layer, contains at least an electrode active material and thermally expandable particles with an expansion onset temperature of 400°C or lower, and the exposed diameter present on the surface of the electrode mixture layer is equal to the volume average particle diameter D50 of the electrode active material. When the thermally expandable particles of 0.5 to 5.0 times are used as exposed particles A, the area occupied by the exposed particles A on the surface of the electrode mixture layer is characterized in that it is 0.5% to 20%. By using an electrode for an electrochemical device in which thermally expandable particles whose expansion start temperature is in a predetermined temperature range are exposed to the electrode surface at a predetermined ratio, the heat generation suppression performance of the electrochemical device can be improved and the IV resistance can be reduced. . The reason is not clear, but it is assumed to be as follows.

In an abnormal situation where the exposed particles A having an exposed diameter within a predetermined range exist on the surface of the electrode mixture layer at an occupied area ratio within a predetermined range, the temperature inside the device rises above the expansion start temperature of the thermally expandable particles, and the electrode Since the spacing between adjacent members (for example, separators) can be effectively spaced apart, short circuiting during abnormalities is suppressed, and it is assumed that this can improve the heat generation suppression performance of the electrochemical element. In addition, if the occupied area of the exposed particles A whose exposed diameter is within a predetermined range is greater than or equal to the predetermined lower limit, the volume of the thermally expandable particles buried in the electrode mixture layer decreases, and the resistance in the electrode mixture layer can be reduced. If the occupied area of the exposed particle A is below the predetermined upper limit, the resistance between the electrode and a member (for example, a separator) adjacent to the electrode can be prevented from increasing in a normal state, and as a result, electricity It is speculated that the IV resistance of the chemical element can be reduced.

<Complete house>

As the current collector, a material that has electrical conductivity and is electrochemically durable is used. Specifically, as the current collector, for example, a current collector made of iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, platinum, etc. can be used. On the other hand, the above materials may be used individually, or two or more types may be used in combination at any ratio.

<Electrode compound layer>

The electrode compound layer provided in the electrode contains an electrode active material and thermally expandable particles with an expansion start temperature of 400°C or lower, and may optionally contain a binder, a conductive material, and other additives. In addition, the electrode mixture layer requires that the area occupied by the exposed particles A on the surface of the electrode mixture layer be 0.5% or more and 20% or less. As a result, the heat generation suppression performance of the resulting electrochemical device can be improved while the IV resistance can be reduced. Additionally, in the electrode mixture layer, it is desirable for thermally expandable particles to be distributed unevenly on the surface and in areas near the surface. If thermally expandable particles are distributed on and near the surface, the heat generation suppression performance of the resulting electrochemical device can be further improved and the IV resistance can be further reduced.

<<occupied area ratio of exposed particles A on the surface of the electrode mixture layer>>

First, exposed particles A refers to thermally expandable particles that exist on the surface of the electrode mixture layer and have an exposed diameter of 0.5 to 5.0 times the volume average particle diameter D50 of the electrode active material. By defining thermally expandable particles with an exposed diameter of 0.5 to 5.0 times the volume average particle diameter D50 of the electrode active material as exposed particles, the presence ratio of thermally expandable particles with an appropriate exposed size on the surface of the electrode mixture layer can be quantified. Thereby, it is possible to evaluate the presence of thermally expandable particles that effectively contribute to improvement of heat suppression performance.

<<Number density of exposed particles A on the surface of the electrode mixture layer>>

The number density of exposed particles A on the surface of the electrode mixture layer is preferably 10 particles/mm ² or more, more preferably 20 particles/mm ² or more, even more preferably 40 particles/mm ² or more, and 300 particles/mm 2 or more. It is preferable that it is ^mm2 or less, it is more preferable that it is 250 pieces/ ^mm2 or less, it is still more preferable that it is 150 pieces/ ^mm2 or less, and it is especially preferable that it is 110 pieces/ ^mm2 or less. If the number density of the exposed particles A on the surface of the electrode mixture layer is within the above range, the heat generation suppression performance of the resulting electrochemical device can be further improved, and the IV resistance can be further reduced. In particular, when the number density of the exposed particles A on the surface of the electrode mixture layer is more than the above lower limit, when the thermally expandable particles expand, the distance between the electrode and the member adjacent to it can be effectively reduced, so that the electrochemical element Heat suppression performance can be effectively improved. Moreover, if the number density of exposed particles A on the surface of the electrode mixture layer is below the above upper limit, the exposed thermally expandable particles are suppressed from increasing the internal resistance of the electrochemical element in normal times, and the IV resistance of the electrochemical element is reduced. can do.

Here, the occupied area ratio of the exposed particles A on the surface of the electrode mixture layer needs to be 0.5% or more, preferably 2.0% or more, needs to be 20% or less, preferably 10% or less, and 5.0% or less. It is more desirable. If the occupied area ratio of the exposed particles A is within the above range, the heat generation suppression performance of the resulting electrochemical element can be improved and the IV resistance can be reduced.

<<Thermally expandable particles>>

The thermally expandable particles need to have an expansion start temperature of 400°C or lower. The expansion start temperature is preferably 300°C or lower, preferably 130°C or higher, more preferably 150°C or higher, and even more preferably 170°C or higher. If the expansion start temperature is equal to or higher than the above lower limit, expansion of the thermally expandable particles can be suppressed during the manufacturing process of the electrochemical device, and an increase in the IV resistance of the obtained electrochemical device can be suppressed. Additionally, if the expansion start temperature is below the above upper limit, an increase in the internal temperature when an abnormality occurs in the electrochemical element can be quickly suppressed, and the occurrence of thermal runaway can be effectively suppressed.

In addition, the volume average particle diameter D50 of the thermally expandable particles is preferably 0.3 times or more, more preferably 0.5 times or more, preferably 5.0 times or less, and more preferably 3.0 times or less. . If the particle diameter of the thermally expandable particles and the electrode active material is more than the above lower limit, the thermally expandable particles can be suppressed from becoming resistance in the electrode mixture layer, and the performance of suppressing heat generation due to expansion can be improved. Additionally, if the particle diameter of the thermally expandable particles and the electrode active material is below the above upper limit, the IV resistance and heat suppression performance of the electrochemical device can be increased.

In addition, the thermally expandable particles preferably have a volume average particle diameter D50 of 0.1 μm or more, more preferably 1 μm or more, more preferably 5 μm or more, preferably 100 μm or less, and more preferably 80 μm or less, It is more preferable that it is 50 μm or less, and it is especially preferable that it is 30 μm or less. If the volume average particle diameter D50 of the thermally expandable particles is equal to or greater than the above lower limit, the thermally expandable particles become resistance and an increase in the internal resistance of the electrochemical element can be suppressed. If the volume average particle diameter D50 of the thermally expandable particles is below the above upper limit, the coatability of the slurry composition for electrodes obtained can be improved, and the heat generation suppression performance of the electrochemical element obtained can be improved.

Here, the amount of thermally expandable particles contained in the electrode is preferably 40 parts by mass or more, more preferably 50 parts by mass or more, with the total content (based on mass) of the binder and thermally expandable particles described later being 100 parts by mass, It is preferable that it is 97 parts by mass or less, and it is more preferable that it is 93 parts by mass or less. If the content of thermally expandable particles is more than the above lower limit, the heat generation suppression performance of the resulting electrochemical device can be further improved. If the content of thermally expandable particles is below the above upper limit, the IV resistance of the resulting electrochemical element can be further reduced.

Here, as the thermally expandable particles, any thermally expandable particle can be used as long as the expansion start temperature is 400°C or lower. For example, as the thermally expandable particles, commercially available thermally expandable particles such as Matsumoto Microsphere (registered trademark) and Expancel (manufactured by Nippon Philite Co., Ltd.), or thermally expandable particles satisfying the specific structure described later, can be used.

As thermally expandable particles, particles having a core-shell structure including a core and a shell covering the outer surface of the core are preferable. In this specification, the fact that the shell “covers the outer surface of the core” means that the shell exists on at least part of the outer surface of the core. The shell may cover part of the outer surface of the core, or may cover the entire outer surface of the core. Additionally, the shell is not particularly limited as long as it contains at least two types of polymers, and may include one layer or multiple layers. The layers constituting the shell may contain multiple types of polymers in one layer, or one layer may be made of one type of polymer and may contain two or more such layers.

The core of the thermally expandable particles is made of a gas-generating material that gasifies at 400°C or lower. In this specification, “gas-generating material” means a compound capable of generating gas when a predetermined temperature is reached; “Gasified” includes substances that change phase into gas. The core may optionally contain additives such as urea. The gas-generating substance gasified at 400°C or lower is gasified when the internal temperature of the electrochemical element rises to a predetermined temperature (temperature below 400°C), increases internal resistance, and initiates a chain of electrochemical reactions. By suppressing this, the occurrence of thermal runaway can be suppressed. Additionally, when the core is prepared according to the method for producing thermally expandable particles described later, it may contain a trace amount of metal oxide.

The gasification temperature of the gas-generating material needs to be 400°C or lower, preferably 300°C or lower, more preferably 150°C or lower, preferably 10°C or higher, more preferably 20°C or higher, and even more preferably 26°C or higher. desirable. If the gasification temperature is below the above upper limit, the internal temperature can be suppressed from rising when an abnormality occurs in the electrochemical element, and the occurrence of thermal runaway can be effectively suppressed. When the gasification temperature is equal to or higher than the above lower limit, the ease of producing thermally expandable particles increases.

Gas generating substances include isopentane (gasification temperature: 28°C), isooctane, n-pentane, n-hexane, isohexane, 2,2-dimethylbutane (gasification temperature: 50°C), and cyclohexane (gasification temperature: 81°C). ℃), hydrocarbon compounds such as heptane and petroleum ether, bicarbonate compounds such as sodium bicarbonate (gasification temperature: 150 ℃), guanidine compounds such as guanidine nitrate, nitroguanidine, and aminoguanidine nitrate, and azobisisobutyronitrile. Azo compounds such as (gasification temperature: 108°C) and azodicarbonamide (gasification temperature: 200°C), melamine, ammeline, ammelide, melamine cyanurate (gasification temperature: 280°C), trihydrazine and triazine (1 Triazine compounds such as ,3,5-triazine-2,4,6(1H,3H,5H)-triontrihydrazone), p,p'-oxybisbenzenesulfonylhydrazide (gasification temperature) : 160°C) and hydrazide compounds such as p-toluenesulfonylhydrazide, hydrazodicarbonamide, hydrazo compounds such as p-toluenesulfonylsemicarbazide, and dinitrosopentamethylenetetramine. and nitroamine compounds such as trimethylenetrinitroamine, tetrazole compounds such as 5-aminotetrazole and 5-phenyltetrazole, and bitetrazole compounds such as 5,5'-bitetrazolediammonium and bitetrazolepiperazine. You can. These can be used individually or in combination of multiple types. Among them, isopentane, 2,2-dimethylbutane, cyclohexane, azobisisobutyronitrile, and sodium bicarbonate are preferable from the viewpoint of improving the heat suppression performance of the resulting electrochemical device.

The content ratio of the core in the thermally expandable particles is preferably 0.1% by mass or more, more preferably 5% by mass or more, and preferably 90% by mass or less, assuming the total mass of the thermally expandable particles is 100% by mass, It is more preferable that it is 50 mass% or less, and even more preferably it is 30 mass% or less. If the content ratio of the core in the thermally expandable particles is more than the above lower limit, the heat generation suppression performance of the resulting electrochemical device can be further improved. Additionally, if the content ratio of the core in the thermally expandable particles is below the above upper limit, the thermally expandable particles become brittle and can be prevented from collapsing while the electrochemical element is operating normally. On the other hand, “content ratio of the core in thermally expandable particles” refers to the content ratio in the state in which the core is enclosed in the shell.

The shell of the thermally expandable particles is made of at least two types of polymers. The shell requires an electrolyte swelling degree of 500% by mass or less, preferably 350% by mass or less, and more preferably 300% by mass or less. If the swelling degree of the electrolyte solution of the shell is below the above upper limit, dissolution of the core into the electrolyte solution within the electrochemical element can be well suppressed, and the heat generation suppression performance of the electrochemical element can be improved. On the other hand, the lower limit of the degree of swelling of the electrolyte solution of the shell is not particularly limited, and is, for example, 100% by mass, and it does not need to be swollen at all. From the viewpoint of reducing the internal resistance of the resulting electrochemical element, it is preferable that the electrolyte swelling degree of the shell is 120% by mass or more.

In addition, the electrolyte swelling degree of at least two types of polymers constituting the shell is preferably 500 mass% or less, more preferably 350 mass% or less, and even more preferably 300 mass% or less. If the swelling degrees of the electrolyte solution of at least two types of polymers constituting the shell are respectively below the above upper limit, dissolution of the core into the electrolyte solution in the electrochemical element can be well suppressed, and the heat generation suppression performance of the electrochemical element can be improved. Additionally, the swelling degree of each electrolyte solution of at least two types of polymers constituting the shell may be 100% by mass, and is preferably 120% by mass or more from the viewpoint of reducing the internal resistance of the resulting electrochemical element.

On the other hand, the swelling degree of the electrolyte solution of at least two types of polymers constituting the shell can be measured by the method described in the Examples.

In addition, the shell preferably has a swelling degree with respect to N-methyl-2-pyrrolidone (hereinafter sometimes referred to as “NMP swelling degree”) of 500 mass% or less, more preferably 350 mass% or less, It is more preferable that it is 300% by mass or less. If the NMP swelling degree of the shell is below the above upper limit, when the electrode of the present invention is formed using a solution containing NMP as a solvent, dissolution of the core into NMP during the electrode manufacturing process is well suppressed, and the resulting electrochemical The heat suppression performance of the device can be improved. The lower limit of the NMP swelling degree of the shell is not particularly limited, for example, 100% by mass, and does not need to be swollen at all.

Meanwhile, the NMP swelling degree of the shell can be measured by the method described in the Examples described later.

In addition, the NMP swelling degree of at least two types of polymers constituting the shell is preferably 500 mass% or less, more preferably 350 mass% or less, and even more preferably 300 mass% or less. If the NMP swelling degree of at least two types of polymers constituting the shell is each less than the above upper limit, when the binder composition of the present invention is used when preparing a slurry composition for a positive electrode of a secondary battery using NMP as a solvent, in the slurry composition Elution of the core during NMP can be well suppressed, and the heat generation suppression performance of the resulting electrochemical device can be improved. Additionally, the NMP swelling degree of each of the at least two types of polymers constituting the shell is not particularly limited and may be 100% by mass.

On the other hand, the NMP swelling degree of at least two types of polymers constituting the shell can be measured by the method described in the Examples.

The at least two types of polymers constituting the shell must contain at least two types of polymers with a glass transition temperature difference of 10°C or more and 230°C or less. More specifically, the shell may contain only two types of polymers with different glass transition temperatures in the range of 10°C to 230°C, and in addition to these two types of polymers, other polymers (e.g., 2 above) It may contain a polymer whose glass transition temperature difference with at least one of the polymers is less than 10°C or more than 230°C. When the shell contains three or more types of polymers, the two types of polymers with high content (based on mass) just need to satisfy the above relative relationship regarding the glass transition temperature.

The temperature difference between the two types of glass transition temperatures is preferably 60°C or higher, more preferably 90°C or higher, preferably 150°C or lower, and more preferably 120°C or lower. If the temperature difference is more than the above lower limit, leakage of the shell out of the thermally expandable particles can be suppressed even when pressurized during the manufacturing process of the electrochemical element. Therefore, in particular, the electrode mixture layer is pressed at high pressure to obtain a high-density electrode, and the heat generation suppression performance of the electrochemical device provided with such a high-density electrode can be further improved. If the temperature difference is below the above upper limit, the high-temperature storage characteristics of the electrochemical device can be improved.

Additionally, it is preferable that the highest temperature among the glass transition temperatures of the polymers contained in the shell is higher than the gasification temperature of the gas-generating material forming the core. If the highest temperature among the glass transition temperatures of the polymer contained in the shell is higher than the gasification temperature of the gas-generating material, leakage of the gas-generating material when pressurized during the manufacture of the electrochemical element can be well suppressed, and the resulting electrochemical The heat suppression performance of the device can be further improved. Here, the difference between the highest glass transition temperature of the polymer contained in the shell and the gasification temperature of the gas-generating material is preferably 10°C or higher, more preferably 45°C or higher, and even more preferably 60°C or higher. Meanwhile, the upper limit of this difference is not particularly limited, but may be, for example, 200°C.

Preferably, one of the at least two types of polymers constituting the shell has a glass transition temperature of 60°C or higher, and the other has a glass transition temperature of less than 60°C. When the shell contains three or more types of polymers, it is preferable that the two types of polymers with the highest content (based on mass) are any of these polymers.

[Polymer with a glass transition temperature of 60°C or higher (polymer 1)]

The glass transition temperature of the polymer with a glass transition temperature of 60°C or higher (hereinafter sometimes referred to as “polymer 1”) is preferably 80°C or higher, preferably 180°C or lower, and more preferably 150°C or lower. It is more preferable that it is 130°C or lower. If the glass transition temperature of polymer 1 is equal to or higher than the above lower limit, the shell can be well protected even when pressurized in the manufacturing process of the electrochemical device, and the outflow of gas-generating substances out of the thermally expandable particles can be well suppressed. there is. Therefore, in particular, it becomes possible to press the electrode mixture layer at high pressure, making it possible to efficiently manufacture high-density electrodes. If the glass transition temperature is below the above upper limit, the polymerization stability during polymerization of polymer 1 can be increased, and the manufacturing efficiency of the electrode can be increased.

In addition, the solubility parameter (hereinafter sometimes referred to as SP value) of polymer 1 is preferably 23.0 MPa ^1/2 or more, more preferably 24.0 MPa ^1/2 or more, and 30.0 MPa ^1/2. It is preferable that it is ² or less, and it is more preferable that it is 29.5 MPa ^{1/2 or} less. More specifically, the SP value of polymer 1 is preferably higher than the SP value of NMP and electrolyte solution that can be used when manufacturing an electrochemical element. If the SP value of Polymer 1 is set to a value different from that of NMP and the electrolyte solution, it becomes difficult for Polymer 1 to swell and elute with respect to NMP and the electrolyte solution, and as a result, the electrochemical device containing thermally expandable particles does not operate normally. Therefore, it can be effective when fever is generated.

Meanwhile, the solubility parameter is the Hansen solubility parameter, the definition and calculation method of which are described in the following literature.

Charles M. Hansen, 「Hansen Solubility Parameters: A Users Handbook」, CRC Press, 2007.

Additionally, for substances for which the literature value of the Hansen solubility parameter is unknown, the Hansen solubility parameter can be easily estimated from its chemical structure by using computer software (Hansen Solubility Parameters in Practice (HSPiP)).

Here, the composition of polymer 1 is not particularly limited. As polymer 1, for example, it is preferable to use a polymer containing a monomer unit having a nitrile group. In this specification, the fact that a polymer "contains a monomer unit" means that "a repeating unit derived from the monomer is contained in the polymer obtained by using the monomer." In addition, in the present invention, the content ratio of various monomer units in the polymer can be measured using a nuclear magnetic resonance (NMR) method such as ¹ H-NMR.

Examples of the monomer unit having a nitrile group include α,β-ethylenically unsaturated nitrile monomer units. The monomer forming the α,β-ethylenically unsaturated nitrile monomer unit is not limited as long as it is an α,β-ethylenically unsaturated compound having a nitrile group, and includes acrylonitrile; α-halogenoacrylonitrile such as α-chloroacrylonitrile and α-bromoacrylonitrile; α-alkylacrylonitrile such as methacrylonitrile; etc. are mentioned, and acrylonitrile and methacrylonitrile are preferable. As an α,β-ethylenically unsaturated nitrile monomer, multiple types thereof may be used in combination.

The content ratio of the monomer unit having a nitrile group in Polymer 1 is preferably 70% by mass or more, more preferably 80% by mass or more, and 85% by mass, assuming that the total repeating units contained in Polymer 1 are 100% by mass. Above is more preferable, 98 mass% or less is preferable, and 97 mass% or less is more preferable. If the content ratio of the monomer unit having a nitrile group is more than the above lower limit, it is possible to suppress the swelling degree of the electrolyte solution of the polymer 1 from increasing. In addition, if the content ratio of the monomer unit having a nitrile group is below the above upper limit, the polymerization stability of Polymer 1 can be improved.

The polymer containing a monomer unit having a nitrile group may be a copolymer of a monomer that forms the monomer unit having a nitrile group and a copolymerizable monomer. Examples of the copolymerizable monomer include unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, and fumaric acid; Aromatic vinyl monomers such as styrene, chlorostyrene, vinyltoluene, t-butylstyrene, vinylbenzoic acid, methyl vinylbenzoate, vinylnaphthalene, chloromethylstyrene, hydroxymethylstyrene, and α-methylstyrene; Amide monomers such as acrylamide, N-methylolacrylamide, and acrylamido-2-methylpropanesulfonic acid; Olefins such as ethylene and propylene; Diene monomers such as butadiene and isoprene; halogen atom-containing monomers such as vinyl chloride and vinylidene chloride; Vinyl esters such as vinyl acetate, vinyl propionate, vinyl butyrate, and vinyl benzoate; Vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, and butyl vinyl ether; Vinyl ketones such as methyl vinyl ketone, ethyl vinyl ketone, butyl vinyl ketone, hexyl vinyl ketone, and isopropenyl vinyl ketone; Heterocycle-containing vinyl compounds such as N-vinylpyrrolidone, vinylpyridine, and vinylimidazole; Butyl acrylate, pentyl acrylate, hexyl acrylate, cyclohexyl acrylate, isopropyl acrylate, such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, and t-butyl acrylate. Acrylic acid alkyl esters such as octyl acrylate such as bornyl acrylate, heptyl acrylate, and 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, and stearyl acrylate. ; And butyl methacrylate, pentyl methacrylate, and hexyl such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, and t-butyl methacrylate. Methacrylate, cyclohexyl methacrylate, isobornyl methacrylate, heptyl methacrylate, 2-ethylhexyl methacrylate, etc., octyl methacrylate, nonyl methacrylate, decyl methacrylate, and lauryl methacrylate. Methacrylic acid alkyl esters such as methacrylate, n-tetradecyl methacrylate, and stearyl methacrylate can be mentioned. As the above-mentioned copolymerizable monomer, multiple types of these may be used together.

In addition, the polymer containing a monomer unit having a nitrile group as polymer 1 may have a crosslinkable monomer unit. Examples of the crosslinkable monomer that can form a crosslinkable monomer unit include polyfunctional monomers having two or more polymerization reactive groups in the monomer. Examples of the polyfunctional monomer include divinyl compounds such as allyl methacrylate and divinylbenzene; Di(meth)acrylic acid ester compounds such as diethylene glycol dimethacrylate, ethylene glycol dimethacrylate, diethylene glycol diacrylate, and 1,3-butylene glycol diacrylate; Tri(meth)acrylic acid ester compounds such as trimethylolpropane trimethacrylate and trimethylolpropane triacrylate; etc. can be mentioned. Among them, ethylene glycol dimethacrylate is preferable. On the other hand, these crosslinkable monomers may be used individually, or two or more types may be used in combination at an arbitrary ratio. Meanwhile, in this specification, (meth)acrylic means acrylic or methacrylic.

The content ratio of the crosslinkable monomer unit in Polymer 1 is not particularly limited, and for example, assuming the total repeating units contained in Polymer 1 as 100 mass%, it is preferably 0.05 mass% or more, and is 0.1 mass% or more. It is more preferable, it is more preferable that it is 0.5 mass % or more, it is preferable that it is 3.0 mass % or less, and it is more preferable that it is 2.0 mass % or less. If the content ratio of the crosslinkable monomer unit in the polymer 1 is more than the above lower limit, the core can be effectively prevented from flowing out of the thermally expandable particles by increasing the strength of the shell. If the content ratio of the crosslinkable monomer unit in the polymer 1 is below the above upper limit, the crosslinking density is suppressed from being excessively high and thus inhibits thermal expansion, and it becomes possible for the thermally expandable particles to expand at the desired temperature.

[Polymer with a glass transition temperature of less than 60°C (polymer 2)]

The glass transition temperature of a polymer with a glass transition temperature of less than 60°C (hereinafter sometimes referred to as “polymer 2”) needs to be less than 60°C, preferably 40°C or less, and more preferably 25°C or less. It is preferably -50°C or higher, more preferably -40°C or higher, and even more preferably -30°C or higher. If the glass transition temperature of polymer 2 is below the above upper limit, the adhesiveness of the binder composition can be further improved, and leakage of the shell out of the thermally expandable particles can be suppressed even when pressurized in the manufacturing process of the electrochemical device. . As a result, the heat generation suppression performance of the resulting electrochemical device can be further improved. Therefore, in particular, the electrode mixture layer is pressed at high pressure to obtain a high-density electrode, and the heat generation suppression performance of the electrochemical device provided with such a high-density electrode can be further improved. If the glass transition temperature of Polymer 2 is above the lower limit, the polymerization stability of Polymer 2 can be increased, and the productivity of the electrode can be increased.

In addition, polymer 2 preferably has an SP value of 16.0 MPa ^1/2 or more, more preferably 18.0 MPa ^1/2 or more, preferably 24.0 MPa ^1/2 or less, and more preferably 23.0 MPa ^1/2 or less. , it is more preferable that it is 21.0 MPa ^1/2 or less. More specifically, the SP value of polymer 2 is preferably lower than the SP value of NMP and electrolyte solution that can be used when manufacturing an electrochemical element. If the SP value of Polymer 2 is set to a value different from that of NMP and the electrolyte solution, it becomes difficult for Polymer 2 to swell and elute with respect to NMP and the electrolyte solution, and as a result, the electrochemical device containing thermally expandable particles does not operate normally. Therefore, it can be effective when fever is generated.

Here, the composition of polymer 2 is not particularly limited. Examples of polymer 2 include a polymer containing an aromatic vinyl monomer unit. Examples of the aromatic vinyl monomer unit include units formed using the aromatic vinyl monomers listed as monomers that can be used when preparing polymer 1. Among them, styrene is preferable. The content ratio of the aromatic vinyl monomer unit in Polymer 2 is preferably 40% by mass or more, more preferably 50% by mass or more, and 90% by mass or less, with the total repeating units contained in Polymer 2 being 100% by mass. It is preferable, and it is more preferable that it is 80 mass % or less. If the content ratio of the aromatic vinyl monomer unit in the polymer 2 is more than the above lower limit, it is possible to suppress the swelling degree of the electrolyte solution of the polymer 2 from increasing excessively.

Polymer 2 may contain a (meth)acrylic acid ester monomer unit instead of or in addition to the aromatic vinyl monomer unit. Examples of the (meth)acrylic acid ester monomer that can be used to form such a (meth)acrylic acid ester monomer unit include the various monomers listed as those that can be used when preparing polymer 1. Among them, 2-ethylhexyl acrylate is preferable.

Additionally, polymer 2 may contain other monomer units in addition to or instead of the aromatic vinyl monomer unit and (meth)acrylic acid ester monomer unit described above. Examples of such monomer units include units formed using the various monomers listed as monomers that can be used when preparing polymer 1.

Among these, it is preferable that the polymer 2 contains a crosslinkable monomer unit. Crosslinkable monomers that can be used to form crosslinkable monomer units in Polymer 2 include the various compounds listed above in relation to Polymer 1.

Among these, allyl methacrylate is preferable as a monomer used to form the crosslinkable monomer unit in Polymer 2. The content ratio of the crosslinkable monomer unit in Polymer 2 is not particularly limited, and is preferably 0.05 mass% or more, and more preferably 0.1 mass% or more, assuming that the total repeating units contained in Polymer 2 are 100 mass%. It is preferable, more preferably 0.2 mass% or more, preferably 3.0 mass% or less, and more preferably 2.0 mass% or less. If the content ratio of the crosslinkable monomer unit in the polymer 2 is more than the above lower limit, the core can be effectively prevented from flowing out of the thermally expandable particles by increasing the strength of the shell. If the content ratio of the crosslinkable monomer unit in polymer 2 is below the above upper limit, the ease of manufacturing the shell can be improved.

Additionally, polymer 2 may contain a monomer containing a carbon-carbon double bond and an epoxy group (epoxy group-containing unsaturated monomer) separately from these crosslinkable monomers. In this specification, “crosslinkable monomer” does not include monomers corresponding to epoxy group-containing unsaturated monomers.

Here, examples of the epoxy group-containing unsaturated monomer include unsaturated glycidyl ethers such as vinyl glycidyl ether, allyl glycidyl ether, butenyl glycidyl ether, and o-allylphenyl glycidyl ether; Dienes such as butadiene monoepoxide, chloroprene monoepoxide, 4,5-epoxy-2-pentene, 3,4-epoxy-1-vinylcyclohexene, 1,2-epoxy-5,9-cyclododecadiene, or Monoepoxides of polyenes; Alkenyl epoxides such as 3,4-epoxy-1-butene, 1,2-epoxy-5-hexene, and 1,2-epoxy-9-decene; Glycidyl acrylate, glycidyl methacrylate, glycidyl crotonate, glycidyl-4-heptenoate, glycidyl sorbate, glycidyl linoleate, glycidyl-4-methyl- Glycidyl esters of unsaturated carboxylic acids, such as 3-pentenoate, glycidyl ester of 3-cyclohexenecarboxylic acid, and glycidyl ester of 4-methyl-3-cyclohexenecarboxylic acid; I can hear it. In addition, these may be used individually by 1 type, and 2 or more types may be used in combination in arbitrary ratios.

The content ratio of the epoxy group-containing unsaturated monomer in Polymer 2 is preferably 0.5 mass% or more, more preferably 1.0 mass% or more, and 10.0 mass% or less, assuming that the total repeating units contained in Polymer 2 are 100 mass%. It is preferable, and it is more preferable that it is 8.0 mass % or less. If the content ratio of the epoxy group-containing unsaturated monomer in the polymer 2 is within this range, the heat generation suppression performance of the electrochemical device can be further improved.

[Composition of shell]

The composition of the shell is not particularly limited. For example, the shell may include various monomer units exemplified above as monomer units that may be included in Polymer 1 and Polymer 2. Among them, the content of aromatic vinyl monomer units in the shell is preferably 20% by mass or more, more preferably 30% by mass or more, preferably 50% by mass or less, and more preferably 40% by mass or less. In addition, the content of the nitrile group-containing monomer unit in the shell is preferably 20% by mass or more, more preferably 25% by mass or more, preferably 50% by mass or less, and more preferably 40% by mass or less. In addition, the content ratio of the (meth)acrylic acid ester monomer unit in the shell is preferably 20% by mass or more, more preferably 30% by mass or more, preferably 50% by mass or less, and more preferably 40% by mass or less. do. If the content ratio of the aromatic vinyl monomer unit, the content ratio of the nitrile group-containing monomer unit, and the content ratio of the (meth)acrylic acid ester monomer unit in the shell are each independently within the above range, the glass transition temperature of the shell, the electrolyte swelling degree, And it becomes possible to appropriately control the swelling degree of NMP. In addition, the content ratio of the crosslinkable monomer unit in the shell is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and 3.0% by mass or less from the viewpoint of shell strength, electrolyte swelling degree, and NMP swelling degree. It is preferable, and it is more preferable that it is 2.0 mass % or less. In addition, the content ratio of the epoxy group-containing unsaturated monomer unit in the shell is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, and preferably 10.0% by mass or less, from the viewpoint of electrolyte swelling degree and NMP swelling degree, It is more preferable that it is 8.0 mass% or less. These content ratios are the ratios when the total of repeating units contained in all polymers constituting the shell is 100% by mass.

The structure of the thermally expandable particles is not particularly limited as long as it has a structure in which a core made of a gas-generating material is covered with a shell containing Polymer 1 and Polymer 2. From the viewpoint of effectively increasing the heat suppression performance of the resulting electrochemical device, it is preferable that the shell includes a layer A made of polymer 1 and a layer B made of polymer 2. Additionally, from the viewpoint of improving the adhesiveness of the binder composition, it is preferable that layer A made of polymer 1 exists inside (core side) than layer B made of polymer 2. Additionally, the thermally expandable particles may have a conductive carbon material described later in the section <Conductive material> on its surface. In this case, the IV resistance of the electrochemical element can be further reduced.

It is preferable that the total area ratio α (%) of polymer 1 and the total area ratio β (%) of polymer 2 in the shell satisfy the relationship of α ≤ β. If the total area ratio of each polymer 1 and 2 contained in the shell satisfies the relationship of α ≤ β, the heat generation suppression performance of the resulting electrochemical device can be further improved. Here, from the viewpoint of further enhancing this effect, the difference between the total area ratio α (%) of polymer 1 and the total area ratio β (%) of polymer 2 is preferably 1% or more, and more preferably 20% or more. do.

In a shell composed of polymer 1 and polymer 2 forming layer A and layer B, respectively, the thickness a of layer A containing polymer 1 and the thickness b of layer B containing polymer 2 have the relationship a ≤ b. It is desirable to satisfy. If the thickness of each layer constituting the shell satisfies the relationship a ≤ b, the heat generation suppression performance of the resulting electrochemical device can be further improved. If the thickness of layer A, which contains polymer 1 whose glass transition temperature is higher than that of polymer 2, is thicker than the thickness of layer B, it becomes difficult for the thermally expandable particles to expand when the shell is gasified. Here, this effect is further enhanced. , the thickness b of layer B is preferably 1.2 times or more, and more preferably 1.4 times or more, the thickness a of layer A.

[Method for producing thermally expandable particles]

The thermally expandable particles can be prepared, for example, by polymerizing a monomer composition containing the above-mentioned monomers in a colloidal aqueous solution formed by dispersing a gas-generating substance. Here, the ratio of each monomer in the monomer composition is usually the same as the ratio of each monomer unit in the thermally expandable particles.

The polymerization method is not particularly limited, and for example, any method such as suspension polymerization, emulsion polymerization agglomeration, or pulverization can be used. Among them, the suspension polymerization method and the emulsion polymerization agglomeration method are preferable, and the suspension polymerization method is more preferable. Additionally, as the polymerization reaction, any reaction such as radical polymerization or living radical polymerization can be used.

In addition, the monomer composition used when preparing thermally expandable particles contains other compounding agents such as chain transfer agent, polymerization regulator, polymerization reaction retardant, reactive fluidizing agent, filler, flame retardant, anti-aging agent, and colorant in any mixing amount. can do.

Here, as an example, a method for preparing thermally expandable particles by a suspension polymerization method will be described.

-Preparation of thermally expandable particles by suspension polymerization method

(1) Preparation of monomer composition

First, prepare Monomer Composition 1 and Monomer Composition 2 whose compositions respectively correspond to the compositions of Polymer 1 and Polymer 2 constituting the shell. At this time, various monomers are blended according to the composition of Polymer 1 and Polymer 2, and other blending agents added as necessary are mixed.

(2) Formation of droplets

Next, the metal hydroxide as a dispersion stabilizer is dispersed in water to prepare a colloidal dispersion containing the metal hydroxide. Then, a gas-generating material capable of forming a core and either or both of Monomer Composition 1 and Monomer Composition 2 capable of forming a shell are added to this colloidal dispersion. Additionally, a polymerization initiator is added to obtain a mixed solution, and then droplets are formed. Here, the method of forming the droplets is not particularly limited, and for example, the liquid droplets can be formed by shearing and stirring them using a disperser such as an emulsification disperser. In addition, examples of the polymerization initiator include oil-soluble polymerization initiators such as t-butylperoxy-2-ethylhexanoate and azobisisobutyronitrile. Additionally, as the dispersion stabilizer, for example, metal hydroxides such as magnesium hydroxide and sodium dodecylbenzenesulfonate can be used.

(3) polymerization

Then, after forming the droplet, the temperature of the water containing the formed droplet is raised to initiate polymerization. In the case where only one of monomer composition 1 and monomer composition 2 was mixed into the droplet in step (2), at the stage when the polymerization conversion ratio was sufficiently high, 1/2 of the monomer composition not added in step (2) was added. Polymerization continues by adding. As a result, thermally expandable particles having a predetermined structure are formed in water. At that time, the polymerization reaction temperature is preferably 50°C or higher and 95°C or lower. Moreover, the duration of each polymerization reaction is preferably 1 hour or more and 10 hours or less, and preferably 8 hours or less.

(4) Washing, filtration, dewatering and drying process

After completion of polymerization, water containing thermally expandable particles can be washed, filtered, and dried according to a conventional method to obtain thermally expandable particles having a predetermined structure.

On the other hand, the ratio of the gas-generating substance to the monomer compositions 1 and 2 can be appropriately set to satisfy the suitable range of the above-mentioned “core content ratio in thermally expandable particles.” In addition, the amount ratio between monomer compositions 1 and 2 is appropriately set to satisfy the appropriate ranges of “area ratio of polymer 1 and polymer 2 in the shell” and “thickness ratio of layer A and layer B in the shell” described above. You can set it.

<<Binder>>

The electrode preferably further includes a binder. The binder is not particularly limited, and any polymer can be used as long as it is a polymer that can exhibit binder ability in the electrode mixture layer. Suitable examples of polymers used as binders include polymers and their hydrides (diene-based polymers) mainly containing aliphatic conjugated diene monomer units, polymers mainly containing (meth)acrylic acid ester monomer units (acrylic polymers), Examples include a polymer mainly containing (meth)acrylonitrile (nitrile-based polymer), a polymer mainly containing fluorine-containing monomer units (fluorine-based polymer), and a polymer mainly containing vinyl alcohol monomer units (vinyl alcohol-based polymer). . Among these, acrylic polymers, nitrile-based polymers, and fluorine-based polymers are more preferable.

On the other hand, one type of binder may be used individually, or two or more types may be used in combination in an arbitrary ratio. In addition, in this specification, "mainly containing" a certain monomer unit means that "when the total amount of repeating units contained in the polymer is 100% by mass, the content ratio of the monomer unit is 50% by mass. It means “exceeding.”

In addition, the binder does not contain the above-mentioned gas-generating substances and is at least one selected from the group consisting of carboxylic acid group, hydroxyl group, nitrile group, amino group, epoxy group, oxazoline group, sulfonic acid group, ester group, and amide group. It is preferable that it is a polymer having functional groups (hereinafter, these functional groups may be collectively referred to as “specific functional groups”). The polymer as a binder may have one type of the specific functional group described above, or may have two or more types of it.

If a polymer having these specific functional groups is used as a binder, the IV resistance of the electrochemical element can be further reduced. From the viewpoint of lowering the IV resistance of the electrochemical element, the polymer as the binder preferably has at least one selected from the group consisting of a carboxylic acid group, a hydroxyl group, and a nitrile group, and is comprised of a carboxylic acid group and a nitrile group. It is more preferable to have at least one of these, and it is even more preferable to have both a carboxylic acid group and a nitrile group.

Here, the method of introducing the specific functional groups described above into the polymer is not particularly limited. For example, a polymer containing a specific functional group-containing monomer unit may be obtained by preparing a polymer using a monomer (specific functional group-containing monomer) having the above-mentioned specific functional group, or by modifying an arbitrary polymer to form the above-mentioned specific functional group. Although an introduced polymer may be obtained, the former is preferred. That is, the polymer as a binder is 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, and a sulfonic acid group-containing monomer unit. , it is preferable to include at least one of an ester group-containing monomer unit and an amide group-containing monomer unit, and at least one of a carboxylic acid group-containing monomer unit, a hydroxyl group-containing monomer unit, and a nitrile group-containing monomer unit. It is more preferable that it contains at least one of a carboxylic acid group-containing monomer unit and a nitrile group-containing monomer unit, and it is particularly preferable that it contains both a carboxylic acid group-containing monomer unit and a nitrile group-containing monomer unit.

[Monomer unit containing carboxylic acid group]

Examples of carboxylic acid group-containing monomers that can form a carboxylic acid group-containing monomer unit include monocarboxylic acids and their derivatives, dicarboxylic acids and their acid anhydrides, and their derivatives.

Monocarboxylic acids include acrylic acid, methacrylic acid, and crotonic acid.

Monocarboxylic acid derivatives include 2-ethylacrylic acid, isocrotonic acid, α-acetoxyacrylic acid, β-trans-aryloxyacrylic acid, α-chloro-β-E-methoxyacrylic acid, etc.

Examples of dicarboxylic acids include maleic acid, fumaric acid, and itaconic acid.

Dicarboxylic acid derivatives include methylmaleic acid, dimethylmaleic acid, phenylmaleic acid, chloromaleic acid, dichloromaleic acid, fluoromaleic acid, nonyl maleate, decyl maleate, dodecyl maleate, and octadecyl maleate. and maleic acid monoesters such as fluoroalkyl maleic acid.

Examples of acid anhydrides of dicarboxylic acid include maleic anhydride, acrylic anhydride, methyl maleic anhydride, and dimethyl maleic anhydride.

Additionally, as the carboxylic acid group-containing monomer, an acid anhydride that generates a carboxylic acid group through hydrolysis can also be used. Among them, acrylic acid and methacrylic acid are preferable as carboxylic acid group-containing monomers. On the other hand, the carboxylic acid group-containing monomer may be used individually, or two or more types may be used in combination in an arbitrary ratio.

[Hydroxyl group-containing monomer unit]

Examples of the hydroxyl group-containing monomer that can form a hydroxyl group-containing monomer unit include ethylenically unsaturated alcohols such as (meth)allyl alcohol, 3-buten-1-ol, and 5-hexen-1-ol; -2-hydroxyethyl acrylate, -2-hydroxypropyl acrylate, -2-hydroxyethyl methacrylate, -2-hydroxypropyl methacrylate, di-2-hydroxyethyl maleate, di-4-maleate alkanol esters of ethylenically unsaturated carboxylic acids such as hydroxybutyl and di-2-hydroxypropyl itaconate; General formula: CH ₂ =CR ^a -COO-(C _q H _2q O) _p -H (wherein, p represents an integer of 2 to 9, q represents an integer of 2 to 4, and R ^a represents a hydrogen atom or a methyl group. esters of polyalkylene glycol and (meth)acrylic acid represented by ); Mono ( Meth)acrylic acid esters; Vinyl ethers such as 2-hydroxyethyl vinyl ether and 2-hydroxypropyl vinyl ether; (meth)allyl-2-hydroxyethyl ether, (meth)allyl-2-hydroxypropyl ether, (meth)allyl-3-hydroxypropyl ether, (meth)allyl-2-hydroxybutyl ether, (meth) ) Mono(meth)allyl ethers of alkylene glycols such as allyl-3-hydroxybutyl ether, (meth)allyl-4-hydroxybutyl ether, and (meth)allyl-6-hydroxyhexyl ether; polyoxyalkylene glycol mono(meth)allyl ethers such as diethylene glycol mono(meth)allyl ether and dipropylene glycol mono(meth)allyl ether; Halogens of (poly)alkylene glycols such as glycerin mono(meth)allyl ether, (meth)allyl-2-chloro-3-hydroxypropyl ether, and (meth)allyl-2-hydroxy-3-chloropropyl ether. and mono(meth)allyl ether of hydroxy substituent; Mono(meth)allyl ethers and halogen-substituted products thereof of polyhydric phenols such as eugenol and isoeugenol; (meth)allylthioethers of alkylene glycols such as (meth)allyl-2-hydroxyethylthioether and (meth)allyl-2-hydroxypropylthioether; Amides having a hydroxyl group such as N-hydroxymethyl acrylamide (N-methylol acrylamide), N-hydroxymethyl methacrylamide, N-hydroxyethyl acrylamide, and N-hydroxyethyl methacrylamide etc. can be mentioned. On the other hand, the hydroxyl group-containing monomer may be used individually, or two or more types may be used in combination in an arbitrary ratio.

Meanwhile, in this specification, “(meth)acryloyl” means acryloyl and/or methacryloyl.

[Nitrile group-containing monomer unit]

Examples of the nitrile group-containing monomer that can form a nitrile group-containing monomer unit include α,β-ethylenically unsaturated nitrile monomer. Specifically, those listed as examples of α,β-ethylenically unsaturated compounds having a nitrile group that can be used to form polymer 1 can be used. On the other hand, these compounds may be used individually, or two or more types may be used in combination at any ratio.

[Amino group-containing monomer unit]

Examples of amino group-containing monomers that can form amino group-containing monomer units include dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, aminoethyl vinyl ether, and dimethylaminoethyl vinyl ether. On the other hand, the amino group-containing monomer may be used individually, or two or more types may be used in combination in an arbitrary ratio.

[Epoxy group-containing monomer unit]

Examples of the epoxy group-containing monomer that can form the epoxy group-containing monomer unit include the various carbon-carbon double bonds and epoxy group-containing monomers listed as compounds that can be used to form the crosslinkable monomer unit in Polymer 2. there is. In addition, these monomers may be used individually by 1 type, or may be used in combination of 2 or more types in arbitrary ratios.

[Monomer unit containing oxazoline group]

Examples of oxazoline group-containing monomers that can form oxazoline group-containing monomer units include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, and 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. can be mentioned. On the other hand, the oxazoline group-containing monomer may be used individually, or two or more types may be used in combination at any ratio.

[Monomer unit containing sulfonic acid group]

Monomers containing a sulfonic acid group that can form a sulfonic acid group-containing monomer unit include vinylsulfonic acid, methylvinylsulfonic acid, (meth)allylsulfonic acid, styrenesulfonic acid, ethyl (meth)acrylic acid-2-sulfonate, and 2-acrylamido-2. -Methylpropanesulfonic acid, 3-allyloxy-2-hydroxypropanesulfonic acid, etc. are mentioned. On the other hand, the sulfonic acid group-containing monomer may be used individually, or two or more types may be used in combination in an arbitrary ratio.

[Ester group-containing monomer unit]

As an ester group-containing monomer that can form an ester group-containing monomer unit, for example, a (meth)acrylic acid ester monomer can be used. Examples of the (meth)acrylic acid ester monomer include various (meth)acrylic acid ester monomers listed as those that can be used to form the (meth)acrylic acid ester monomer unit in Polymer 2. In addition, these monomers may be used individually by 1 type, or may be used in combination of 2 or more types in arbitrary ratios.

In addition, in the present invention, when a certain monomer has a specific functional group other than an ester group, that monomer is not included in the ester group-containing monomer.

[Amide group-containing monomer unit]

Examples of amide group-containing monomers that can form amide group-containing monomer units include acrylamide, methacrylamide, and vinylpyrrolidone. On the other hand, the amide group-containing monomer may be used individually, or two or more types may be used in combination in an arbitrary ratio.

Here, when the total amount of repeating units contained in the polymer as a binder is 100% by mass, the content ratio of the specific functional group-containing monomer unit in the polymer is 10 from the viewpoint of further lowering the IV resistance of the electrochemical element. It is preferably % by mass or more, more preferably 20 % by mass or more, and even more preferably 30 % by mass or more. On the other hand, the upper limit of the content ratio of the specific functional group-containing monomer unit in the polymer as a binder is not particularly limited, and may be 100% by mass or less, for example, 99% by mass or less.

[Other repeat units]

The polymer as a binder may contain repeating units (other repeating units) other than the specific functional group-containing monomer units described above. These other repeating units are not particularly limited, but when the polymer is a diene-based polymer, aliphatic conjugated diene-based monomer units are included.

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. I can hear it. These may be used individually, or two or more types may be used in combination at any ratio.

Meanwhile, in the present invention, the “aliphatic conjugated diene monomer unit” further includes a structural unit (hydride unit) obtained by adding hydrogen to the monomer unit contained in the polymer obtained using the aliphatic conjugated diene monomer. do.

And among the above-mentioned aliphatic conjugated diene monomers, 1,3-butadiene and isoprene are preferable. In other words, the aliphatic conjugated diene monomer unit is preferably 1,3-butadiene unit, isoprene unit, 1,3-butadiene hydride unit, and isoprene hydride unit, and 1,3-butadiene hydride unit and isoprene water are preferred. Digestive units are more preferred.

Here, when the polymer as a binder contains an aliphatic conjugated diene monomer unit, when the total amount of repeating units contained in the polymer is 100% by mass, the content ratio of the diene monomer unit in the polymer is electrochemical element From the viewpoint of lowering the IV resistance, it is preferably more than 50 mass%, more preferably 60 mass% or more, preferably 90 mass% or less, preferably 80 mass% or less, and even more preferably 70 mass% or less. do.

[Method for preparing binder]

The preparation method of the binder is not particularly limited. A polymer binder is manufactured, for example, by polymerizing a monomer composition containing one or two or more types of monomers in an aqueous solvent, and optionally performing hydrogenation or modification. Meanwhile, the content ratio of each monomer in the monomer composition can be determined based on the content ratio of the desired monomer unit in the polymer.

Meanwhile, the polymerization method is not particularly limited, and any method such as solution polymerization, suspension polymerization, bulk polymerization, or emulsion polymerization can be used. Additionally, as the polymerization reaction, any reaction such as ionic polymerization, radical polymerization, living radical polymerization, various condensation polymerizations, or addition polymerization can be used. Also, during polymerization, a known emulsifier or polymerization initiator can be used as needed. In addition, hydrogenation and denaturation can be performed by known methods.

<<Ratio of the total content of thermally expandable particles and binder in the electrode mixture layer>>

On the other hand, the ratio of the total content of the thermally expandable particles and the binder in the electrode mixture layer is preferably 1% by mass or more, and more preferably 1.5% by mass or more, based on the total mass of the electrode mixture layer being 100% by mass. , Moreover, it is preferable that it is 10 mass % or less, and it is more preferable that it is 5 mass % or less.

[Other ingredients]

The electrode mixture layer may contain other components known as additives that can be blended into the electrode mixture layer of the electrochemical element. Other components include, for example, wetting agents, leveling agents, and electrolyte decomposition inhibitors.

<<Electrode active material>>

An electrode active material is a material that exchanges electrons in the electrode of an electrochemical element. Meanwhile, hereinafter, the 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 example below. And, as an electrode active material for a lithium ion secondary battery, a material that can occlude and release lithium is usually used. On the other hand, since the battery capacity is within the practical range, the electrode active material is preferably 90% by mass or more, more preferably 92% by mass or more, with the total mass of the electrode mixture layer being 100% by mass, and further preferably 99.5% by mass. It is preferable that it is % or less, and it is more preferable that it is 99 mass % or less.

[Positive active material]

Specifically, the positive electrode active material for lithium ion secondary batteries is not particularly limited and includes lithium-containing cobalt oxide (LiCoO ₂ ), lithium manganate (LiMn ₂ O ₄ ), lithium-containing nickel oxide (LiNiO ₂ ), and Co-Ni. Lithium-containing complex oxide of -Mn (Li(CoMnNi)O ₂ ), lithium-containing complex oxide of Ni-Mn-Al, lithium-containing complex oxide of Ni-Co-Al, olivine type lithium iron phosphate (LiFePO ₄ ), olivine type Lithium manganese phosphate (LiMnPO ₄ ), Li ₂ MnO ₃ -LiNiO ₂ based solid solution, spinel compound with excess lithium represented by Li _1+x Mn _2-x O ₄ (0 < X < 2), Li[Ni _0.17 Li _0.2 Known positive electrode active materials such as Co _0.07 Mn _0.56 ]O ₂ and LiNi _0.5 Mn _1.5 O ₄ can be mentioned.

On the other hand, the compounding amount or particle size of the positive electrode active material is not particularly limited and can be the same as that of the positive electrode active material conventionally used.

[Negative active material]

In addition, examples of negative electrode active materials for lithium ion secondary batteries include carbon-based negative electrode active materials, metal-based negative electrode active materials, and negative electrode active materials combining these.

Here, the carbon-based negative electrode active material refers to an active material whose main skeleton is carbon into which lithium can be inserted (also referred to as “dope”). Examples of the carbon-based negative electrode active material include carbonaceous materials and graphite materials. You can.

Examples of the carbonaceous material include easily graphitized carbon and non-graphitic carbon, which has a structure close to the amorphous structure represented by glassy carbon.

Here, examples of the graphitizing carbon include carbon materials made from tar pitch obtained from petroleum or coal. Specific examples include coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fiber, and thermally decomposed vapor-grown carbon fiber.

In addition, examples of non-graphitizing carbon include phenol resin sintered bodies, polyacrylonitrile-based carbon fibers, pseudoisotropic carbon, and furfuryl alcohol resin sintered bodies (PFA).

Additionally, examples of graphite materials include natural graphite, artificial graphite, and the like.

Here, artificial graphite includes, for example, artificial graphite obtained by heat-treating carbon containing easily graphitizing carbon at a temperature of 2800°C or higher, graphitized MCMB obtained by heat-treating MCMB at a temperature of 2000°C or higher, and mesophase pitch-based carbon fiber. Graphitized mesophase pitch-based carbon fiber heat-treated in .

In addition, a metallic negative electrode active material is an active material containing a metal, which usually contains an element capable of inserting lithium in its structure, and has a theoretical electric capacity per unit mass of 500 mAh/g or more when lithium is inserted. . Metal-based active materials include, for example, lithium metal and simple metals that can form lithium alloys (e.g., Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, Sr, Zn, Ti, etc.) and their alloys, and their oxides, sulfides, nitrides, silicides, carbides, phosphides, etc. are used. Among these, the metal-based negative electrode active material is preferably an active material containing silicon (silicon-based negative electrode active material). This is because the capacity of a lithium ion secondary battery can be increased by using a silicon-based negative electrode active material.

Examples of silicon-based negative electrode active materials include silicon (Si), alloys containing silicon, SiO, _SiO You can.

On the other hand, the compounding amount or particle size of the negative electrode active material is not particularly limited and can be the same as that of the negative electrode active material conventionally used.

<Challenging materials>

The conductive material is used to ensure electrical contact between electrode active materials. And, as the conductive material, carbon black (e.g., acetylene black, Ketjen black (registered trademark), furnace black, etc.), single-layer or multi-layer carbon nanotubes (multi-layer carbon nanotubes include cup stack type), Conductive carbon materials such as carbon nanohorns, vapor-grown carbon fibers, milled carbon fibers obtained by firing and pulverizing polymer fibers, single- or multi-layer graphene, and carbon non-woven fabric sheets obtained by firing non-woven fabrics made of polymer fibers; Fibers or foils of various metals can be used.

These can be used individually or in combination of two or more types. Moreover, among the above-mentioned materials, a conductive carbon material is preferable as a conductive material because it has excellent chemical stability.

The content ratio of the conductive material in the electrode mixture layer is preferably 0.1 mass% or more, preferably 3.0 mass% or less, and more preferably 2.5 mass% or less, assuming the total mass of the electrode mixture layer is 100 mass%. If the content ratio of the conductive material is more than the above lower limit, sufficient electrode contact between electrode active materials can be ensured. On the other hand, if the content ratio of the conductive material is below the above upper limit, the density of the electrode mixture layer can be maintained well, and the electrochemical element can be sufficiently increased in capacity.

(Method for manufacturing electrodes for electrochemical devices)

The electrode for an electrochemical device of the present invention includes, for example, a process of preparing a slurry composition containing an electrode active material and thermally expandable particles, and an optional binder, a conductive material, and other components (slurry composition preparation process) ), a process of applying a slurry composition on a current collector (application process), and a process of drying the slurry composition applied on the current collector to form an electrode mixture layer on the current collector (drying process).

<Slurry composition preparation process>

It can be prepared by dissolving or dispersing each of the above components in a solvent such as an organic solvent. In the mixing order, the ingredients may be added all at once, or they may be added in stages and mixed. Specifically, by mixing each of the above components and the solvent using a mixer such as a ball mill, sand mill, bead mill, pigment disperser, brain mill, ultrasonic disperser, homogenizer, planetary mixer, fill mix, etc., a slurry composition is formed. can be prepared.

<Application process>

The method of applying the slurry composition onto the current collector is not particularly limited and a known method can be used. Specifically, as an application method, a doctor blade method, a dip method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, a brush application method, etc. can be used. At this time, the slurry composition may be applied to only one side of the current collector or to both sides. The thickness of the slurry film on the current collector after application and before drying can be appropriately set according to the thickness of the electrode mixture layer obtained by drying.

<Drying process>

The method of drying the slurry composition on the current collector is not particularly limited and known methods can be used, for example, a drying method using warm air, hot air, or low humid air, a vacuum drying method, or a drying method using irradiation of infrared rays or electron beams. I can hear it. By drying the slurry composition on the current collector in this way, an electrode mixture layer can be formed on the current collector, and an electrode for a secondary battery including the current collector and the electrode mixture layer can be obtained. In the drying process, it is preferable to perform drying accompanied by a stepwise increase in temperature, such as drying at a low temperature followed by drying at a higher temperature. Specifically, it is preferable to dry under low temperature conditions of, for example, 110°C or lower, preferably 95°C or lower, more preferably 90°C or lower, and then dry under temperature conditions of 140°C or lower, preferably 120°C or lower. do.

Additionally, as a means for efficiently manufacturing the electrode for an electrochemical device of the present invention, it is preferable to carry out the following process in the above-mentioned coating process and drying process. That is, a process of applying and drying the slurry composition for the lower electrode layer on the current collector to form the lower electrode layer, and applying and drying the slurry composition for the upper electrode layer on the lower electrode layer to form the upper electrode layer. do. Here, the slurry composition for the upper electrode layer and the slurry composition for the lower electrode layer each contain an electrode active material and thermally expandable particles, and the concentration of the thermally expandable particles in the slurry composition for the upper electrode layer is the thermal expandability of the slurry composition for the lower electrode layer. higher than particle concentration. By carrying out this process in the electrode manufacturing process, it is possible to efficiently create a distribution pattern in which the frequency of thermally expandable particles in the surface and vicinity of the electrode mixture layer is higher than that in the lower part of the electrode mixture layer. If thermally expandable particles are distributed on and near the surface of the electrode mixture layer, the heat generation suppression performance of the electrochemical element can be improved and the IV resistance can be further reduced.

On the other hand, after the drying process, the electrode mixture layer may be subjected to pressure treatment using a mold press, roll press, etc. By pressure treatment, the adhesion between the electrode mixture layer and the current collector can be improved.

(Electrochemical device)

An electrochemical device can be provided using the electrode for an electrochemical device of the present invention described above. And, the electrochemical device provided with the electrode for electrochemical device of the present invention has excellent heat suppression performance.

Examples of the electrochemical element include secondary batteries using the electrode for electrochemical elements of the present invention as a positive electrode. In addition, below, the case where the secondary battery is a lithium ion secondary battery will be described as an example, but the present invention is not limited to the example below.

<Electrode>

Here, electrodes other than the above-described electrochemical device electrodes that can be used in the electrochemical device are not particularly limited, and known electrodes used in the manufacture of electrochemical devices can be used. Specifically, as an electrode other than the electrode for an electrochemical device described above, an electrode formed by forming an electrode mixture layer on a current collector using a known manufacturing method can be used.

<Separator>

The separator is not particularly limited, and for example, those described in Japanese Patent Application Publication No. 2012-204303 can be used. Among these, polyolefin-based (polyethylene, polypropylene, polybutene, polyvinyl chloride) can be used to reduce the film thickness of the entire separator, thereby increasing the ratio of electrode active materials in the secondary battery and increasing capacity per volume. ) A microporous membrane made of resin is preferable.

<Electrolyte>

As the electrolyte solution, an organic electrolyte solution in which a supporting electrolyte is dissolved in an organic solvent is usually used. As a supporting electrolyte for a lithium ion secondary battery, for example, lithium salt is used. Lithium salts include, for example, LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAlCl ₄ , LiClO ₄ , CF ₃ SO ₃ Li, C ₄ F ₉ SO ₃ Li, CF ₃ COOLi, (CF ₃ CO) ₂ NLi, (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) NLi, etc. can be mentioned. Among them, LiPF ₆ , LiClO ₄ , and CF ₃ SO ₃ Li are preferable because they are easily soluble in solvents and exhibit a high degree of dissociation, and LiPF ₆ is especially preferable. On the other hand, one type of electrolyte may be used individually, or two or more types may be used in combination in an arbitrary ratio. Usually, lithium ion conductivity tends to increase as a supporting electrolyte with a higher degree of dissociation is used, so lithium ion conductivity can be adjusted by the type of supporting electrolyte.

The organic solvent used in the electrolyte solution is not particularly limited as long as it can dissolve the supporting electrolyte, but examples include dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), and propylene carbonate (PC). , carbonates such as butylene carbonate (BC) and methyl ethyl carbonate (EMC); esters such as ethyl propionate, propyl propionate, γ-butyrolactone, and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; Sulfur-containing compounds such as sulfolane and dimethyl sulfoxide; etc. are conveniently used. Additionally, a mixed solution of these solvents may be used. Among them, it is preferable to use carbonates because they have a high dielectric constant and a wide stable potential region, and from the viewpoint of increasing electrochemical stability, it is preferable to use esters, and it is more preferable to use a mixture of these.

On the other hand, the concentration of the electrolyte in the electrolyte solution can be adjusted appropriately, for example, it is preferably 0.5 to 15 mass%, more preferably 2 to 13 mass%, and even more preferably 5 to 10 mass%. . Additionally, known additives such as vinylene carbonate, fluoroethylene carbonate, and ethylmethyl sulfone may be added to the electrolyte solution.

<Manufacturing method of secondary battery>

A secondary battery as an electrochemical device, for example, overlaps a positive electrode and a negative electrode with a separator interposed, wraps, folds, etc. according to the shape of the battery as needed, places it in a battery container, and seals it by injecting an electrolyte into the battery container. It can be manufactured by doing this. In order to prevent internal pressure rise and overcharge/discharge of the secondary battery, overcurrent prevention elements such as fuses and PTC elements, expanded metal, lead plates, etc. may be installed as needed. The shape of the secondary battery may be, for example, a coin shape, a button shape, a sheet shape, a cylindrical shape, a square shape, or a flat shape.

[Example]

Hereinafter, the present invention will be described in detail based on examples, but the present invention is not limited to these examples. Meanwhile, in the following description, “%” and “part” indicating amounts are based on mass, unless otherwise specified.

In addition, in a polymer manufactured by copolymerizing multiple types of monomers, unless otherwise specified, the ratio of the monomer units formed by polymerizing a certain monomer in the polymer is usually the total amount used for polymerization of the polymer. It is consistent with the ratio (input ratio) of the monomer in question.

In the examples and comparative examples, various measurements and evaluations were performed as follows.

<Gasification temperature of core>

The gas-generating material used to form the core in the Examples and Comparative Examples was prepared, and the temperature was raised from 25°C to 350°C at a temperature increase rate of 10°C/min using heat extraction GC-MS (PY-2020ID, manufactured by Frontier Labs). The core component was detected by doing this. The core component detected by this qualitative analysis was prepared alone, and subjected to thermogravimetric analysis using a thermogravimetric analysis device (“TG8110” manufactured by Rigaku) under a nitrogen atmosphere at a temperature increase rate of 10°C/min from 25°C to 500°C. The mass was measured while the temperature was raised to , and the temperature at which the measured mass was 95% of the mass at the start of measurement (25°C) was taken as the gasification temperature of the gas-generating material.

<Swelling degree of electrolyte of shell>

The thermally expandable particles manufactured in Examples and Comparative Examples were immersed in an electrolyte solution at a temperature of 60°C, and the thickness of the shell before and after was observed with an optical microscope (VHX-900, manufactured by Keyence). Observation was performed by randomly selecting 10 particles, and the average thickness was calculated. From the shell thickness before and after the immersion test, the expansion ratio of the shell was calculated using the following equation.

Meanwhile, as the electrolyte, LiPF is added to a mixed solvent consisting of ethylene carbonate (EC), ethyl propionate (EP), and propyl propionate (PP) mixed at EC:EP:PP = 3:5:2 (volume ratio at 20°C). A solution in which ₆ was dissolved at a concentration of 1 M was used.

Then, let the shell thickness before the immersion test be A and the shell thickness after the immersion test be B, according to the formula below:

Electrolyte swelling degree (%) = B/A × 100 (%)

<NMP swelling degree of shell>

The test was conducted by replacing the above electrolyte solution with NMP.

<Electrolyte swelling degree of polymer 1 and polymer 2>

Monomer compositions of the same composition as Monomer Composition 1 and Monomer Composition 2 prepared in Examples and Comparative Examples were polymerized under the same conditions as the polymerization conditions (including additives, etc.) in the preparation of thermally expandable particles, respectively, to obtain measurement samples. Aqueous dispersions containing the polymers were prepared.

The aqueous dispersion containing the polymer prepared as above was cast on a polytetrafluoroethylene sheet and dried to obtain a cast film. 4 cm ² of this cast film was cut out, its mass (mass A before immersion) was measured, and then it was immersed in an electrolyte solution at a temperature of 60°C. The immersed film was pulled up after 72 hours, wiped with towel paper, and its mass (mass B after immersion) was immediately measured. The swelling degree of the electrolyte solution of the polymer is calculated from the formula below and evaluated based on the following standards. Meanwhile, as the electrolyte, LiPF is added to a mixed solvent consisting of ethylene carbonate (EC), ethyl propionate (EP), and propyl propionate (PP) mixed at EC:EP:PP = 3:5:2 (volume ratio at 20°C). A solution in which ₆ was dissolved at a concentration of 1 M was used.

Swelling degree (%) = B/A × 100 (%)

<NMP swelling degree of polymer 1 and polymer 2>

Except that the electrolyte solution was changed to NMP and the immersion temperature was changed to 45°C, the same operation as the electrolyte swelling degree measurement method described above was performed to measure the NMP swelling degree of Polymer 1 and Polymer 2.

<Structure of thermally expandable particles, layer thickness, and glass transition temperature of polymers 1 and 2>

The obtained particles were embedded in an epoxy-based embedding resin and cut with a microtome to form a cross section. From the AFM image obtained in the contact mode of the atomic force microscope (AFM), it was confirmed that the shell had a two-layer structure, and the thickness of each layer was measured. Additionally, the glass transition temperature of each layer was measured at three points using nanoTA (nano thermal analysis), and the average value was taken as the glass transition temperature of each layer. Additionally, the thickness of each layer and the glass transition temperature of each layer were measured.

<Area ratio of polymer 1 and polymer 2 in the shell>

In the AFM image obtained in the contact mode of AFM, the particle diameter of the thermally expandable particles (10 randomly selected) to be measured was measured. The particle size was set as the particle size of the circumscribed circle containing the target thermally expandable particles. The particle diameter was set to a, the thickness of the layer close to the core constituting the shell was set to b, and the core diameter was set to c, and five points were measured for each thermally expandable particle to be measured. The average values were designated as A, B, and C, respectively.

Using these values A, B, and C, the area ratios of Polymer 1 and Polymer 2 in the shell were calculated, respectively.

Area ratio of polymer 1 = {(B + C/2) ² - (C/2) ² }/{(A/2) ² - (C/2) ² } × 100

Area ratio of polymer 2 = {(A/2) ² - (B + C/2) ² }/{(A/2) ² - (C/2) ² } × 100

The above was similarly calculated for any 10 particles, and the average value of the area ratio of Polymer 1 and Polymer 2 was obtained.

<SP value>

The solubility parameters of Polymer 1 and Polymer 2 were calculated using computer software (Hansen Solubility Parameters in Practice (HSPiP)).

<Expansion start temperature>

The prepared thermally expandable particles were heated at a temperature increase rate of 10°C/min using a heating stage (FTIR600 manufactured by Linckham). The particle diameter of the thermally expandable particles during heating was observed with an optical microscope (VHX-900, manufactured by Keyence), and the change in diameter with respect to temperature was observed. The point at which the change in diameter was 1.3 times or more than before heating was defined as the expansion start temperature.

<Volume average particle diameter>

The positive electrodes produced in Examples and Comparative Examples were subjected to cross-section observation processing using a cross-section polisher (IB-09020CP manufactured by Nippon Electronics). Next, cross-sectional observation was performed using FE-SEM (JSM-7800F manufactured by Nippon Electronics Co., Ltd.). The observation magnification was 1000 times, the irradiation voltage was 10 kV, and the irradiation current was 5.0 × 10 ^-8 A. On the obtained cross section, the diameter obtained by performing circumscribed circle fitting for each thermally expandable particle was defined as the diameter of each thermally expandable particle. Next, based on the obtained diameter, the volume of the thermally expandable particles assuming a spherical shape was calculated. For thermally expandable particles, the diameter of particles whose cumulative volume exceeds 50% when accumulating the volume from the side with the smaller diameter of each particle was taken as the volume average particle diameter D50 of the thermally expandable particles in the observation cross section. The above observation was performed in 5 views, and the average value of the obtained volume average particle diameter D50 was defined as the volume average particle diameter D50 of the thermally expandable particles. The same measurements and calculations were performed for the positive electrode active material, and the volume average particle diameter D50 was obtained.

<Particle ratio of thermally expandable particles and positive electrode active material>

Using the volume average particle diameter D50 of the positive electrode active material and the volume average particle diameter D50 of the thermally expandable particles measured as described above, the particle diameter ratio (times) was calculated according to the following formula.

Particle size ratio (times) = volume average particle diameter of thermally expandable particles D50/volume average particle diameter of positive electrode active material D50

<Surface exposure diameter of electrode mixture layer of thermally expandable particles>

A cross-section polisher (IB-09020CP, manufactured by Nippon Electronics) was used to form an observation cross-section on the surface of the electrode mixture layer of the positive electrode produced in Examples and Comparative Examples. Next, cross-sectional observation was performed using FE-SEM (JSM-7800F manufactured by Nippon Electronics Co., Ltd.). The observation magnification was 500 times, the irradiation voltage was 10 kV, and the irradiation current was 5.0 × 10 ^-8 A, and a cross-section was observed to obtain a surface image of the electrode mixture layer. On the obtained surface, each thermally expandable particle was fitted with a circumscribed circle, and the diameter was defined as the exposed diameter of the thermally expandable particle. And, thermally expandable particles with an exposed diameter of 0.5 to 5.0 times the volume average particle diameter D50 of the positive electrode active material were defined as exposed particles A.

<Exposed area ratio of thermally expandable particles on the surface of the electrode mixture layer>

On the surface obtained above, the areas of the exposed particles A were totaled, and the occupied area ratio of the exposed particles A on the surface of the electrode mixture layer was calculated according to the following formula.

Occupied area ratio (%) of exposed particles A = (total area of exposed particles A/area on surface) × 100

The above calculation was performed for the images in 5 fields of view, and the average value was taken as the occupied area ratio of the exposed particles A.

<Number density of exposed particles A>

On the surface obtained above, the number of exposed particles A was calculated, and the number density of exposed particles A was calculated according to the following formula.

Number density of exposed particles A (piece/mm ² ) = Number of exposed particles A (piece)/area on surface (mm ² )

The above calculation was performed for the images of 5 fields of view, and the average value was taken as the number density of exposed particles A.

<IV resistance measurement>

The lithium ion secondary battery as an electrochemical device manufactured in the Examples and Comparative Examples was left to stand at a temperature of 25° C. for 5 hours after the electrolyte solution was injected. Next, the cell was charged to a cell voltage of 3.65 V using a galvanostatic method at a temperature of 25°C and 0.2 C, and then an aging treatment was performed at a temperature of 60°C for 12 hours. Then, the cell was discharged to a voltage of 3.00 V using a galvanostatic method at a temperature of 25°C and 0.2C. After that, CC-CV charging (upper limit cell voltage 4.35 V) was performed using a 0.2 C galvanostatic method, and CC discharge was performed to 3.00 V using a 0.2 C galvanostatic method. This charging and discharging at 0.2 C was repeated three times. After that, in an environment of 25°C, charging was performed at 0.2 C to achieve a depth of charge (SOC; State of Charge) 50%, and left to stand for 600 seconds. The voltage at 600 seconds was set to V ₀ . After that, the discharge was performed for 10 seconds using a galvanostatic method at 0.5 C (=I _0.5 ), and the voltage at 10 seconds was set to V _0.5 . After that, the amount of electricity immediately before discharge was charged using a 0.2 C constant current method. Next, discharge was performed for 10 seconds using a galvanostatic method at 1.0 C (= I _1.0 ), and the voltage at 10 seconds was set to V _1.0 . Afterwards, the amount of electricity immediately before discharge was charged using a 0.2 C constant current method. Next, the discharge was performed for 10 seconds using a constant current method at 1.5 C (= I _1.5 ), and the voltage at the 10th second was set to V _1.5 . (I _0.5 , V _0.5 ), (I _1.0 , V _1.0 ), (I _1.5 , V _1.5 ) were plotted on the

[Equation 1]

The relative DCR value of each example was calculated when the DCR of Example 1 was set to 100, and evaluated according to the following criteria. The smaller the DCR relative value, the smaller the IV resistance of the lithium ion secondary battery.

A: DCR relative value is 103 or less

B: DCR relative value is greater than 103 and less than 105

C: DCR relative value is greater than 105 and less than 110

D: DCR relative value exceeds 110

<Suppression of heat generation during internal short circuit (forced internal short circuit test)>

The lithium ion secondary battery as an electrochemical device manufactured in the Examples and Comparative Examples was left to stand at a temperature of 25° C. for 5 hours after the electrolyte solution was injected. Next, the cell was charged to a cell voltage of 3.65 V using a galvanostatic method at a temperature of 25°C and 0.2 C, and then an aging treatment was performed at a temperature of 60°C for 12 hours. Then, the cell was discharged to a voltage of 3.00 V using a galvanostatic method at a temperature of 25°C and 0.2C. After that, CC-CV charging (upper limit cell voltage 4.35 V) was performed using a 0.2 C galvanostatic method, and CC discharge was performed to 3.00 V using a 0.2 C galvanostatic method. This charging and discharging at 0.2 C was repeated three times. Thereafter, in an atmosphere of 25°C, the battery was charged to 4.35 V (cutoff condition: 0.02 C) using a constant voltage constant current (CC-CV) method at a charge rate of 0.2 C. After that, an iron nail with a diameter of 3 mm and a length of 10 cm was passed through the vicinity of the center of the lithium ion secondary battery at a speed of 5 m/min to forcibly short-circuit it. This forced short circuit was performed on five lithium ion secondary batteries (test specimens) each produced by the same operation, and the number of test specimens in which neither rupture nor ignition occurred was evaluated according to the following criteria. The greater the number of test specimens in which neither rupture nor ignition occurs, indicating that the lithium ion secondary battery has excellent heat suppression performance during internal short circuit.

A: The number of test specimens that neither rupture nor ignite is 4 or 5.

B: The number of test specimens that neither rupture nor ignite is 3.

C: The number of test specimens that neither rupture nor ignite is 2.

D: The number of test specimens that neither rupture nor ignite is 1 or 0.

(Example 1)

<Preparation of thermally expandable particles>

[Preparation of monomer composition 1]

60.0 parts of acrylonitrile and 5.5 parts of methacrylonitrile as monomers having a nitrile group, 0.4 parts of styrene as an aromatic vinyl monomer, 0.6 parts of butylacrylate of (meth)acrylic acid ester, and ethylene glycol dimethacrylate as a crosslinkable monomer. (Kyoei Chemical Co., Ltd. “Light Ester EG”) was mixed with 0.2 parts to prepare monomer composition 1.

[Preparation of monomer composition 2]

Mix 34.1 parts of styrene as an aromatic vinyl monomer, 27.3 parts of 2-ethylhexyl acrylate as a (meth)acrylic acid ester monomer, 4.7 parts of glycidyl methacrylate as an epoxy group-containing unsaturated monomer, and 0.5 parts of allyl methacrylate as a crosslinkable monomer. , monomer composition 2 was prepared.

[Preparation of colloidal dispersion]

To an aqueous solution prepared by dissolving 8.0 parts of magnesium chloride in 200 parts of ion-exchanged water, an aqueous solution prepared by dissolving 5.6 parts of sodium hydroxide in 50 parts of ion-exchanged water was gradually added under stirring to prepare a colloidal dispersion containing magnesium hydroxide as a metal hydroxide. .

[Suspension polymerization method]

Thermal expandable particles were prepared by suspension polymerization. Specifically, 15.0 parts of isopentane as a gas-generating substance and monomer composition 1 obtained as described above were added to the colloidal dispersion containing magnesium hydroxide, and after further stirring, t-butylperoxy as a polymerization initiator was added. 2.0 parts of -2-ethylhexanoate (manufactured by Nichi Yu Co., Ltd., “Perbutyl O”) was added to obtain a mixed solution. The obtained mixed solution was stirred at high shear for 1 minute at a rotation speed of 15,000 rpm using an in-line emulsification disperser (“Cavitron” manufactured by Taiheiyo Mechanical Engineering Co., Ltd.), and the gas-generating substance and the monomer composition were added to the colloidal dispersion containing magnesium hydroxide. A dispersion containing 1 was obtained. Meanwhile, the stirring temperature was managed at 5-10°C.

A colloidal dispersion containing magnesium hydroxide, including the gas-generating substance and monomer composition 1, was placed in a 5 MPa pressure-resistant vessel equipped with a stirrer, and reacted at 70°C for 8 hours. The pressure during the reaction was 0.5 MPa.

To the aqueous dispersion containing the polymer obtained in this way, 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide] as a polymerization initiator dissolved in monomer composition 2 and 10 parts of ion-exchanged water ( 0.1 part of water-soluble initiator (VA-086, product name: VA-086, manufactured by Wako Pure Chemical Industries, Ltd.) was added and allowed to react at 90°C for 5 hours. After the polymerization reaction is continued, the reaction is stopped by water cooling, and the core containing the gas-generating substance is covered with a shell (including an inner layer made of polymer 1 and an outer layer made of polymer 2), resulting in thermal expansion. An aqueous dispersion containing aqueous particles was obtained.

Additionally, sulfuric acid was added dropwise at room temperature (25°C) while stirring the aqueous dispersion containing the thermally expandable particles, and acid washing was performed until the pH became 6.5 or lower. Next, filtration separation was performed, 500 parts of ion-exchanged water was added to the obtained solid content to re-slurry, and water washing treatment (washing, filtration, and dehydration) was repeated several times. Then, filtration separation was performed, the obtained solid content was placed in a dryer container, and drying was performed at 35°C for 48 hours to obtain dried thermally expandable particles.

As a result of analyzing the properties of the obtained thermally expandable particles according to the above, the gasification temperature of the core was 28°C and the shell had the following properties.

<Preparation of binder>

In an autoclave equipped with a stirrer, 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 t-dodecyl as a chain transfer agent. 0.25 part of mercaptan was added in this order, and the inside of the bottle was purged with nitrogen. After that, 65 parts of 1,3-butadiene as an aliphatic conjugated diene monomer was pressurized, 0.25 parts of ammonium persulfate was added, and polymerization was carried out at a reaction temperature of 40°C. Then, a polymer containing acrylonitrile, methacrylic acid, and 1,3-butadiene was obtained. The polymerization conversion rate was 85%.

A 400 mL (48 g total solids) solution of the obtained polymer whose total solids concentration was adjusted to 12% using water was placed in an autoclave with a volume of 1 L equipped with a stirrer, and nitrogen gas was flowed for 10 minutes to remove the dissolved substances in the solution. After removing oxygen, 75 mg of palladium acetate as a catalyst for hydrogenation reaction was dissolved in 180 mL of ion-exchanged water to which 4 times the mole of nitric acid relative to Pd was added and added. After replacing the system with hydrogen gas twice, the contents of the autoclave were heated to 50°C while pressurized with hydrogen gas to 3 MPa, and hydrogenation reaction (first stage hydrogenation reaction) was performed for 6 hours.

Next, the autoclave was returned to atmospheric pressure, and as a catalyst for hydrogenation reaction, 25 mg of palladium acetate was dissolved in 60 mL of water to which 4 times the mole of nitric acid relative to Pd was added, and further added. After replacing the system with hydrogen gas twice, the contents of the autoclave were heated to 50°C while pressurized with hydrogen gas up to 3 MPa, and hydrogenation reaction (second stage hydrogenation reaction) was performed for 6 hours to bind. An aqueous dispersion of ash was obtained. An appropriate amount of NMP was added to the obtained aqueous dispersion of the binder to obtain a mixture. Afterwards, reduced pressure distillation was performed at 90°C to remove water and excess NMP from the mixture, and an NMP solution as a binder (solid content concentration: 8%) was obtained.

<Preparation of binder composition>

A binder composition was prepared by mixing 10 parts by mass of the binder in terms of solid content and 90 parts by mass of the thermally expandable particles, adding NMP, and adjusting the solid content concentration to 30%.

<Preparation of slurry composition for positive electrode lower layer>

In a planetary mixer, 94.5 parts of lithium cobaltate (volume average particle diameter D50: 12 μm) as a positive electrode active material, 2.0 parts of PVDF (product name “Li-100” manufactured by Denka) as a conductive material equivalent to solid content, 2.0 parts of Solef5130) and 1.5 parts of the binder composition (equivalent to solid content) were added and mixed, NMP was gradually added, stirred and mixed at a temperature of 25 ± 3 ° C. and a rotation speed of 60 rpm, and a B-type viscometer, 60 rpm (rotor M4), a slurry composition for the positive electrode lower layer was obtained at 25 ± 3°C, viscosity was 3,500 mPa·s, and solid content concentration was 68%.

<Preparation of slurry composition for positive electrode upper layer>

In a planetary mixer, 93.5 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (Denka, brand name "Li-100") as a conductive material (equivalent to solid content), 2.0 parts of PVDF (Solef5130), and the above binder composition. 2.5 parts equivalent to the solid content were added and mixed, NMP was gradually added, and stirred and mixed at a temperature of 25 ± 3 ° C. and a rotation speed of 60 rpm. Using a B-type viscometer, 60 rpm (rotor M4), 25 ± 3 ° C. A slurry composition for the positive electrode upper layer was obtained with a viscosity of 3,600 mPa·s and a solid content concentration of 69%.

<Manufacture of positive electrode>

The slurry composition for the positive electrode lower layer was applied using a comma coater on an aluminum foil with a thickness of 20 μm as a current collector so that the mass per unit area was 10 ± 0.5 mg/cm ² . In addition, the slurry composition for the positive electrode lower layer on the aluminum foil is dried by transporting the slurry composition for the positive electrode lower layer on the aluminum foil at a speed of 0.5 m/min for 2 minutes in an oven at a temperature of 90°C for 2 minutes and in an oven at a temperature of 120°C for 2 minutes on the current collector. A positive electrode fabric (lower layer) on which a positive electrode mixture layer (lower layer) was formed was obtained. Next, the slurry composition for the positive electrode upper layer obtained above was applied onto the positive electrode fabric (lower layer) using a comma coater so that the mass per unit area of the lower layer + upper layer was 20 ± 0.5 mg/cm ² . In addition, the slurry composition for the positive electrode upper layer on the aluminum foil was dried by conveying the slurry composition for the positive electrode upper layer on the aluminum foil at a speed of 0.5 m/min for 2 minutes in an oven at a temperature of 90°C for 2 minutes and in an oven at a temperature of 120°C for 2 minutes on the current collector. A positive electrode fabric with a positive electrode mixture layer formed thereon was obtained. Afterwards, the positive electrode mixture layer side of the produced positive electrode fabric was roll pressed in an environment with a temperature of 25 ± 3°C and a load of 14 t (ton), to obtain a positive electrode with a density of the positive electrode mixture layer of 3.80 g/cm ³ . For the obtained positive electrode, the exposed area ratio of the thermally expandable particles, the volume average particle diameter D50 of the positive electrode active material, and the volume average particle diameter D50 of the thermally expandable particles were measured, and the particle diameter ratio of the thermally expandable particles and the positive electrode active material was calculated. The results are shown in Table 2.

<Manufacture of negative electrode>

In a 5 MPa pressure-resistant container equipped with a stirrer, 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, and acrylic acid-2 as a hydroxyl group-containing monomer. -Add 1 part of hydroxyethyl, 0.3 parts of t-dodecyl mercaptan as a molecular weight regulator, 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, and sufficiently After stirring, the mixture was heated to a temperature of 55°C to initiate polymerization. When the monomer consumption reached 95.0%, it was cooled and the reaction was stopped. To the aqueous dispersion containing the polymer obtained in this way, 5% aqueous sodium hydroxide solution was added, and the pH was adjusted to 8. Afterwards, unreacted monomers were removed by heating and distillation under reduced pressure. Furthermore, after that, the aqueous dispersion (binder composition for negative electrodes) containing the binder for negative electrodes was obtained by cooling to a temperature of 30°C or lower.

Into the planetary mixer, 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 carboxymethylcellulose as a thickener (equivalent to solid content) were added. did. Additionally, it was diluted with ion-exchanged water so that the solid content concentration was 60%, and then kneaded for 60 minutes at a rotation speed of 45 rpm. After that, 1.5 parts of the binder composition for negative electrodes obtained as described above (equivalent to solid content) was added, and kneaded for 40 minutes at a rotation speed of 40 rpm. Then, a slurry composition for a negative electrode was prepared by adding ion-exchanged water so that the viscosity was 3000 ± 500 mPa·s (measured with a B-type viscometer, 25°C, 60 rpm).

The slurry composition for negative electrodes was applied using a comma coater to the surface of a 15-μm-thick copper foil serving as a current collector so that the mass per unit area was 11 ± 0.5 mg/cm ² . Thereafter, the copper foil to which the slurry composition for negative electrodes was applied was transported at a speed of 400 mm/min in an oven at a temperature of 80°C for 2 minutes and further in an oven at a temperature of 110°C for 2 minutes, thereby forming a layer on the copper foil. The slurry composition for negative electrodes was dried to obtain a negative electrode fabric with a negative electrode mixture layer formed on the current collector. Thereafter, the negative electrode mixture layer side of the produced negative electrode fabric was roll pressed in an environment of a temperature of 25 ± 3°C and a linear pressure of 11 t (ton), to obtain a negative electrode with a density of the negative electrode mixture layer of 1.60 g/cm ³ .

<Preparation of separator>

A single-layer polypropylene separator (manufactured by Celgard, brand name “#2500”) was prepared.

<Production of lithium ion secondary battery>

Using the negative electrode, positive electrode, and separator described above, a laminated laminate cell (equivalent to an initial design discharge capacity of 3 Ah) was produced, placed in an aluminum packaging material, and vacuum dried under conditions of 60°C for 10 hours. Afterwards, LiPF ₆ was added to a mixed solvent consisting of mixing ethylene carbonate (EC), ethyl propionate (EP), and propyl propionate (PP) as an electrolyte at EC:EP:PP = 3:5:2 (volume ratio at 20°C). was dissolved at a concentration of 1M, and a solution containing 2% by volume (solvent ratio) of additive: vinylene carbonate was charged. Additionally, in order to seal the opening of the aluminum packaging material, heat sealing was performed at a temperature of 150°C to close the aluminum packaging material, and a lithium ion secondary battery was manufactured. The obtained lithium ion battery was evaluated for IV resistance and heat suppression performance during internal short circuit. The results are shown in Table 2.

(Example 2)

The procedure was the same as in Example 1 except that the following operations were performed when producing the positive electrode.

<Preparation of slurry composition for positive electrode mixture layer>

In a planetary mixer, 94.0 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (Denka, brand name "Li-100") as a conductive material (equivalent to solid content), 2.0 parts of PVDF (Solef5130), and the above binder composition. 2.0 parts equivalent to the solid content were added and mixed, NMP was gradually added, and stirred and mixed at a temperature of 25 ± 3 ° C. and a rotation speed of 60 rpm. Using a B-type viscometer, 60 rpm (rotor M4), 25 ± 3 ° C. A slurry composition for a positive electrode was obtained with a viscosity of 2,200 mPa·s and a solid content concentration of 66%.

<Manufacture of positive electrode>

The slurry composition for a positive electrode was applied using a comma coater onto an aluminum foil with a thickness of 20 μm as a current collector so that the mass per unit area was 20 ± 0.5 mg/cm ² . In addition, the slurry composition for positive electrodes on the aluminum foil was dried by conveying it at a speed of 0.5 m/min in an oven at a temperature of 110°C for 2 minutes and further in an oven at a temperature of 120°C for 2 minutes (drying conditions). A positive electrode fabric with a positive electrode mixture layer formed on the current collector was obtained. Afterwards, the positive electrode mixture layer side of the produced positive electrode fabric was roll pressed in an environment with a temperature of 25 ± 3°C and a load of 14 t (ton), to obtain a positive electrode with a density of the positive electrode mixture layer of 3.80 g/cm ³ . For the obtained positive electrode, various evaluations and measurements were performed in the same manner as in Example 1. The results are shown in Table 2.

(Example 3)

The procedure was the same as in Example 1 except that the following operations were performed when producing the positive electrode.

<Preparation of slurry composition for positive electrode lower layer>

In a planetary mixer, 93.5 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (Denka, brand name "Li-100") as a conductive material (equivalent to solid content), 2.0 parts of PVDF (Solef5130), and the above binder composition. 2.5 parts equivalent to the solid content were added and mixed, NMP was gradually added, and stirred and mixed at a temperature of 25 ± 3 ° C. and a rotation speed of 60 rpm. Using a B-type viscometer, 60 rpm (rotor M4), 25 ± 3 ° C. A slurry composition for a positive electrode lower layer was obtained with a viscosity of 3,600 mPa·s and a solid content concentration of 69%.

<Preparation of slurry composition for positive electrode upper layer>

In a planetary mixer, 94.5 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (Denka, brand name "Li-100") as a conductive material (equivalent to solid content), 2.0 parts of PVDF (Solef5130), and the above binder composition. 1.5 parts equivalent to the solid content were added and mixed, NMP was gradually added, stirred and mixed at a temperature of 25 ± 3 ° C. and a rotation speed of 60 rpm, using a B-type viscometer, 60 rpm (rotor M4), 25 ± 3 ° C. A slurry composition for the positive electrode upper layer was obtained with a viscosity of 3,400 mPa·s and a solid content concentration of 68%.

<Manufacture of positive electrode>

The slurry composition for the positive electrode lower layer obtained above was applied using a comma coater on an aluminum foil with a thickness of 20 μm as a current collector so that the mass per unit area was 10 ± 0.5 mg/cm ² . In addition, the slurry composition for the positive electrode lower layer on the aluminum foil is dried by transporting the slurry composition for the positive electrode lower layer on the aluminum foil at a speed of 0.5 m/min for 2 minutes in an oven at a temperature of 90°C for 2 minutes and in an oven at a temperature of 120°C for 2 minutes on the current collector. A positive electrode fabric (lower layer) on which a positive electrode mixture layer (lower layer) was formed was obtained. Next, the slurry composition for the positive electrode upper layer obtained above was applied onto the positive electrode fabric (lower layer) using a comma coater so that the mass per unit area of the lower layer + upper layer was 20 ± 0.5 mg/cm ² . In addition, the slurry composition for the positive electrode upper layer on the aluminum foil is dried by conveying the slurry composition for the positive electrode upper layer on the aluminum foil at a speed of 0.5 m/min for 2 minutes in an oven at a temperature of 90°C for 2 minutes and in an oven at a temperature of 120°C for 2 minutes on the current collector. A positive electrode fabric on which a positive electrode mixture layer was formed was obtained. Thereafter, the positive electrode mixture layer side of the produced positive electrode fabric was roll pressed in an environment of a temperature of 25 ± 3°C and a load of 14 t (ton), to obtain a positive electrode with a density of the positive electrode mixture layer of 3.80 g/cm ³ . For the obtained positive electrode, various evaluations and measurements were performed in the same manner as in Example 1. The results are shown in Table 2.

(Example 4)

The procedure was the same as in Example 1 except that the following operations were performed when producing the positive electrode.

<Preparation of slurry composition for positive electrode lower layer>

In a planetary mixer, 95.5 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (Denka, brand name "Li-100") as a conductive material in solid content equivalent, 2.0 parts of PVDF (Solef5130), and the above binder composition. 0.5 parts equivalent to the solid content were added and mixed, NMP was gradually added, and stirred and mixed at a temperature of 25 ± 3 ° C. and a rotation speed of 60 rpm. Using a B-type viscometer, 60 rpm (rotor M4), 25 ± 3 ° C. A slurry composition for a positive electrode lower layer was obtained with a viscosity of 3,600 mPa·s and a solid content concentration of 68%.

<Preparation of slurry composition for positive electrode upper layer>

In a planetary mixer, 92.5 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (Denka, brand name "Li-100") as a conductive material (equivalent to solid content), 2.0 parts of PVDF (Solef5130), and the above binder composition. 3.5 parts equivalent to the solid content were added and mixed, NMP was gradually added, and stirred and mixed at a temperature of 25 ± 3 ° C. and a rotation speed of 60 rpm. Using a B-type viscometer, 60 rpm (rotor M4), 25 ± 3 ° C. A slurry composition for the positive electrode upper layer was obtained with a viscosity of 3,400 mPa·s and a solid content concentration of 69%.

<Manufacture of positive electrode>

The slurry composition for the positive electrode lower layer obtained above was applied using a comma coater on an aluminum foil with a thickness of 20 μm as a current collector so that the mass per unit area was 10 ± 0.5 mg/cm ² . In addition, the slurry composition for the lower layer for the positive electrode on the aluminum foil is dried by conveying the slurry composition for the lower layer for the positive electrode on the aluminum foil by conveying the inside of the oven at a temperature of 90°C for 2 minutes and the oven at a temperature of 120°C for 2 minutes at a speed of 0.5 m/min, and the current collector A positive electrode fabric (lower layer) on which a positive electrode mixture layer (lower layer) was formed was obtained. Next, the slurry composition for the positive electrode upper layer obtained above was applied onto the positive electrode fabric (lower layer) using a comma coater so that the mass per unit area of the lower layer + upper layer was 20 ± 0.5 mg/cm ² . In addition, the slurry composition for the positive electrode upper layer on the aluminum foil is dried by conveying the slurry composition for the positive electrode upper layer on the aluminum foil at a speed of 0.5 m/min for 2 minutes in an oven at a temperature of 90°C for 2 minutes and in an oven at a temperature of 120°C for 2 minutes on the current collector. A positive electrode fabric on which a positive electrode mixture layer was formed was obtained. Thereafter, the positive electrode mixture layer side of the produced positive electrode fabric was roll pressed in an environment of a temperature of 25 ± 3°C and a load of 14 t (ton), to obtain a positive electrode with a density of the positive electrode mixture layer of 3.80 g/cm ³ . For the obtained positive electrode, various evaluations and measurements were performed in the same manner as in Example 1. The results are shown in Table 2.

(Example 5)

The procedure was the same as in Example 1 except that the following operations were performed when producing the positive electrode.

<Preparation of slurry composition for positive electrode lower layer>

In a planetary mixer, 95.8 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (manufactured by Denka, brand name "Li-100") as a conductive material in solid content equivalent, 2.0 parts of PVDF (Solef5130), and the above binder composition. 0.2 parts equivalent to the solid content were added and mixed, NMP was gradually added, and stirred and mixed at a temperature of 25 ± 3 ° C. and a rotation speed of 60 rpm. Using a B-type viscometer, 60 rpm (rotor M4), 25 ± 3 ° C. A slurry composition for a positive electrode lower layer was obtained with a viscosity of 3,800 mPa·s and a solid content concentration of 67%.

<Preparation of slurry composition for positive electrode upper layer>

In a planetary mixer, 92.2 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (manufactured by Denka, brand name "Li-100") as a conductive material in solid content equivalent, 2.0 parts of PVDF (Solef5130), and the above binder composition. 3.8 parts equivalent to the solid content were added and mixed, NMP was gradually added, and stirred and mixed at a temperature of 25 ± 3 ° C. and a rotation speed of 60 rpm. Using a B-type viscometer, 60 rpm (rotor M4), 25 ± 3 ° C. A slurry composition for the positive electrode upper layer was obtained with a viscosity of 3,500 mPa·s and a solid content concentration of 69%.

<Manufacture of positive electrode>

The slurry composition for the positive electrode lower layer obtained above was applied using a comma coater on an aluminum foil with a thickness of 20 μm as a current collector so that the mass per unit area was 10 ± 0.5 mg/cm ² . In addition, the slurry composition for the lower layer for the positive electrode on the aluminum foil is dried by conveying the slurry composition for the lower layer for the positive electrode on the aluminum foil by conveying the inside of the oven at a temperature of 90°C for 2 minutes and the oven at a temperature of 120°C for 2 minutes at a speed of 0.5 m/min, and the current collector A positive electrode fabric (lower layer) on which a positive electrode mixture layer (lower layer) was formed was obtained. Next, the slurry composition for the positive electrode upper layer obtained above was applied onto the positive electrode fabric (lower layer) using a comma coater so that the mass per unit area of the lower layer + upper layer was 20 ± 0.5 mg/cm ² . In addition, the slurry composition for the positive electrode upper layer on the aluminum foil is dried by conveying the slurry composition for the positive electrode upper layer on the aluminum foil at a speed of 0.5 m/min for 2 minutes in an oven at a temperature of 90°C for 2 minutes and in an oven at a temperature of 120°C for 2 minutes on the current collector. A positive electrode fabric on which a positive electrode mixture layer was formed was obtained. Thereafter, the positive electrode mixture layer side of the produced positive electrode fabric was roll pressed in an environment of a temperature of 25 ± 3°C and a load of 14 t (ton), to obtain a positive electrode with a density of the positive electrode mixture layer of 3.80 g/cm ³ . For the obtained positive electrode, various evaluations and measurements were performed in the same manner as in Example 1. The results are shown in Table 2.

(Example 6)

As the positive electrode active material, lithium cobaltate with a different volume average particle diameter D50 (25 μm) was used, and the same procedure as in Example 1 was performed, except that the following operation was performed during production of the positive electrode.

<Preparation of slurry composition for positive electrode lower layer>

In a planetary mixer, 94.5 parts of lithium cobaltate (volume average particle diameter D50: 25 μm) as a positive electrode active material, 2.0 parts of PVDF (product name: Li-100, manufactured by Denka) as a conductive material equivalent to solid content, were added to the planetary mixer. 2.0 parts of Solef5130) and 1.5 parts of the binder composition (equivalent to solid content) were added and mixed, NMP was gradually added, stirred and mixed at a temperature of 25 ± 3 ° C. and a rotation speed of 60 rpm, and a B-type viscometer, 60 rpm (rotor M4), a slurry composition for the positive electrode lower layer was obtained at 25 ± 3°C, viscosity was 3,600 mPa·s, and solid content concentration was 69%.

<Preparation of slurry composition for positive electrode upper layer>

In a planetary mixer, 93.5 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (Denka, brand name "Li-100") as a conductive material (equivalent to solid content), 2.0 parts of PVDF (Solef5130), and the above binder composition. 2.5 parts equivalent to the solid content were added and mixed, NMP was gradually added, and stirred and mixed at a temperature of 25 ± 3 ° C. and a rotation speed of 60 rpm. Using a B-type viscometer, 60 rpm (rotor M4), 25 ± 3 ° C. A slurry composition for the positive electrode upper layer was obtained with a viscosity of 3,600 mPa·s and a solid content concentration of 69%. The results are shown in Table 2.

(Example 7)

As the thermally expandable particles, commercially available thermally expandable particles (F260D, manufactured by Matsumoto Yuji Pharmaceutical Co., Ltd.) were used, and as the positive electrode active material, lithium cobaltate with a different volume average particle diameter D50 (7 μm) was used, and the following procedures were performed during the production of the positive electrode. It was the same as Example 1 except that this was carried out.

<Preparation of slurry composition for positive electrode lower layer>

Into a planetary mixer, 94.5 parts of lithium cobaltate (volume average particle diameter D50: 7 μm) as a positive electrode active material, 2.0 parts of PVDF (product name: Li-100, manufactured by Denka) as a conductive material equivalent to solid content, 2.0 parts of Solef5130) and 1.5 parts of F260D (manufactured by Matsumoto Yuji Pharmaceutical Co., Ltd., equivalent to solid content) were added and mixed, NMP was gradually added, stirred and mixed at a temperature of 25 ± 3° C. and a rotation speed of 60 rpm, and a B-type viscometer, 60 A slurry composition for a positive electrode lower layer was obtained at rpm (rotor M4) at 25 ± 3°C, viscosity at 3,200 mPa·s, and solid content concentration at 68%.

<Preparation of slurry composition for positive electrode upper layer>

In a planetary mixer, 93.5 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (Denka, brand name "Li-100") as a conductive material (equivalent to solid content), 2.0 parts of PVDF (Solef5130), Matsumoto Yuji Pharmaceutical Co., Ltd. Manufacture, 2.5 parts of F260D (equivalent to solid content) was added and mixed, NMP was gradually added, stirred and mixed at a temperature of 25 ± 3°C and a rotation speed of 60 rpm, using a B-type viscometer at 60 rpm (rotor M4), 25 ± A slurry composition for the positive electrode upper layer was obtained at 3°C, with a viscosity of 3,400 mPa·s, and a solid content concentration of 69%. The results are shown in Table 2.

(Comparative Example 1)

The drying conditions when manufacturing the positive electrode are transported 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. The drying temperature in the first stage is slightly different. Except for changing to lower conditions, various operations, evaluations, and measurements were performed in the same manner as in Example 2. The results are shown in Table 2.

(Comparative Example 2)

When preparing the positive electrode mixture layer slurry composition, various operations, evaluations, and measurements were performed in the same manner as in Example 2, except that the solid content concentration was set to 68% (viscosity was 3700 mPa·s). The results are shown in Table 2.

(Comparative Example 3)

The procedure was the same as in Example 1 except that the following operations were performed when producing the positive electrode. The results are shown in Table 2.

<Preparation of slurry composition for positive electrode lower layer>

In a planetary mixer, 92.5 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (Denka, brand name "Li-100") as a conductive material (equivalent to solid content), 2.0 parts of PVDF (Solef5130), and the above binder composition. 3.5 parts equivalent to the solid content were added and mixed, NMP was gradually added, and stirred and mixed at a temperature of 25 ± 3 ° C. and a rotation speed of 60 rpm. Using a B-type viscometer, 60 rpm (rotor M4), 25 ± 3 ° C. A slurry composition for a positive electrode lower layer was obtained with a viscosity of 3,400 mPa·s and a solid content concentration of 69%.

<Preparation of slurry composition for positive electrode upper layer>

In a planetary mixer, 95.5 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (Denka, brand name "Li-100") as a conductive material in solid content equivalent, 2.0 parts of PVDF (Solef5130), and the above binder composition. 0.5 parts equivalent to the solid content were added and mixed, NMP was gradually added, and stirred and mixed at a temperature of 25 ± 3 ° C. and a rotation speed of 60 rpm. Using a B-type viscometer, 60 rpm (rotor M4), 25 ± 3 ° C. A slurry composition for the positive electrode upper layer was obtained with a viscosity of 3,600 mPa·s and a solid content concentration of 68%.

(Comparative Example 4)

The procedure was the same as in Example 1 except that the following operations were performed when producing the positive electrode. The results are shown in Table 2.

<Preparation of slurry composition for positive electrode lower layer>

Into a planetary mixer, 96.0 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (Denka, brand name "Li-100") as a conductive material (equivalent to solid content), and 2.0 parts of PVDF (Solef5130) were added and mixed. , NMP was gradually added, stirred and mixed at a temperature of 25 ± 3 ° C. and a rotation speed of 60 rpm. Using a type B viscometer, 60 rpm (rotor M4), the results were 25 ± 3 ° C., the viscosity was 3,500 mPa·s, and the solid content concentration was At 67%, a slurry composition for a positive electrode lower layer was obtained.

<Preparation of slurry composition for positive electrode upper layer>

In a planetary mixer, 92.0 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (manufactured by Denka, brand name "Li-100") as a conductive material (equivalent to solid content), 2.0 parts of PVDF (Solef5130), and the above binder composition. 4.0 parts equivalent to the solid content were added and mixed, NMP was gradually added, and stirred and mixed at a temperature of 25 ± 3 ° C. and a rotation speed of 60 rpm. Using a B-type viscometer, 60 rpm (rotor M4), 25 ± 3 ° C. A slurry composition for the positive electrode upper layer was obtained with a viscosity of 3,600 mPa·s and a solid content concentration of 69%.

Meanwhile, in Table 2,

“D50 particle size” refers to the volume average particle size D50, and “HNBR” refers to hydrogenated nitrile rubber.

From Table 2, in Examples 1 to 7, which used electrode compound layers having a structure in which thermally expandable particles with an expansion start temperature of 400°C or lower are exposed to the surface at a predetermined ratio, heat suppression performance and low IV resistance were balanced. It can be seen that it was possible to provide an electrochemical device. On the other hand, it can be seen that in Comparative Examples 1 to 3, where the exposure of the thermally expandable particles was small, and in Comparative Example 4, where the exposure was excessive, these properties could not be improved at the same time.

[Industrial applicability]

According to the present invention, it is possible to provide an electrode for an electrochemical device and a method for manufacturing the same, which can increase the heat suppression performance of the electrochemical device and reduce IV resistance.

Claims (5)

An electrode for an electrochemical device comprising a current collector and an electrode mixture layer,
The electrode mixture layer includes at least an electrode active material and thermally expandable particles having an expansion onset temperature of 400° C. or lower,
When thermally expandable particles present on the surface of the electrode mixture layer and having an exposed diameter of 0.5 to 5.0 times the volume average particle diameter D50 of the electrode active material are used as exposed particles A, exposed particles A on the surface of the electrode mixture layer An electrode for an electrochemical device having an occupied area ratio of 0.5% to 20%. According to paragraph 1,
An electrode for an electrochemical device, wherein the volume average particle diameter D50 of the thermally expandable particles is 0.3 to 5.0 times the volume average particle diameter D50 of the electrode active material. According to paragraph 1,
An electrode for an electrochemical device, wherein the number density of the exposed particles A on the surface of the electrode mixture layer is 10 particles/mm ² or more and 300 particles/mm ^{2 or} less. According to paragraph 1,
The electrode mixture layer further includes a binder, and the binder is,
An electrode for an electrochemical device, which 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. A method for producing an electrode for an electrochemical device according to any one of claims 1 to 4, comprising:
A process of forming a lower electrode layer by applying and drying a slurry composition for a lower electrode layer on a current collector,
A process of forming an upper electrode layer by applying and drying a slurry composition for an upper electrode layer on the lower electrode layer,
The slurry composition for the upper electrode layer and the slurry composition for the lower electrode layer each contain an electrode active material and thermally expandable particles, and the concentration of the thermally expandable particles in the slurry composition for the upper electrode layer is the thermally expandable particles of the slurry composition for the lower electrode layer. higher than the concentration,
Method for manufacturing electrodes for electrochemical devices.