CN117693835A

CN117693835A - Electrode for electrochemical element and method for manufacturing electrode for electrochemical element

Info

Publication number: CN117693835A
Application number: CN202280050827.6A
Authority: CN
Inventors: 宫崎明彦; 召田麻贵
Original assignee: Zeon Corp
Current assignee: Zeon Corp
Priority date: 2021-07-27
Filing date: 2022-07-19
Publication date: 2024-03-12
Also published as: KR20240035448A; WO2023008267A1; JPWO2023008267A1

Abstract

The invention provides an electrode for an electrochemical element, which comprises a current collector and a prescribed electrode composite material layer. The electrode composite layer contains at least an electrode active material and thermally expandable particles having an expansion initiation temperature of 400 ℃ or lower, and when the thermally expandable particles having an exposure diameter of 0.5 to 5.0 times the volume average particle diameter D50 of the electrode active material present on the surface of the electrode composite layer are used as the exposed particles A, the occupation area ratio of the exposed particles A on the surface of the electrode composite layer is 0.5 to 20%.

Description

Electrode for electrochemical element and method for manufacturing electrode for electrochemical element

Technical Field

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

Background

Electrochemical devices such as lithium ion secondary batteries have characteristics of being small in size, light in weight, high in energy density, and capable of repeated charge and discharge, and are used in a wide variety of applications. In recent years, therefore, improvements in battery members such as electrodes have been studied for the purpose of further improving the performance of electrochemical devices.

Here, an electrode used for an electrochemical element such as a lithium ion secondary battery generally includes a current collector and an electrode composite layer formed on the current collector. Further, the electrode composite layer is formed by, for example, the following means: a slurry composition including an electrode active material, a binder composition, and the like, which is a binder composition including a binder material, is applied to a current collector, and the applied slurry composition is dried.

The electrochemical element is at risk of thermal runaway due to occurrence of internal short-circuits, various internal chain chemical reactions, and the like. In order to suppress such occurrence of thermal runaway, there has been conventionally studied a method in which thermally expandable particles containing a substance capable of suppressing a chemical reaction when the temperature inside an electrochemical element increases are incorporated into an electrode for an electrochemical element (for example, refer to patent documents 1 to 3).

Prior art literature

Patent literature

Patent document 1: international publication No. 2015/133423;

patent document 2: international publication No. 2019/189865;

patent document 3: japanese patent laid-open No. 2003-031208.

Disclosure of Invention

Problems to be solved by the invention

Here, it is required that the electrode for an electrochemical element provide both good heat release suppressing performance and lower IV resistance of the secondary battery. However, the above-described conventional electrode for an electrochemical element has room for improvement in terms of achieving both of these properties at a higher level.

Accordingly, an object of the present invention is to provide an electrode for an electrochemical element capable of reducing IV resistance while improving heat release inhibition performance of the electrochemical element, and a method for manufacturing the same.

Solution for solving the problem

The present inventors have conducted intensive studies with a view to solving the above-mentioned problems. Then, the present inventors have newly found that: when an electrode for an electrochemical element is used in which thermally expandable particles having an expansion initiation temperature in a predetermined temperature range are exposed to the surface of the electrode at a predetermined ratio, the IV resistance can be reduced while improving the heat release suppressing performance of the electrochemical element, and the present invention has been completed.

Specifically, the present invention is directed to advantageously solve the above-mentioned problems, and [1] the electrode for an electrochemical element of the present invention is characterized by comprising a current collector and an electrode composite layer, wherein the electrode composite layer contains at least an electrode active material and thermally expandable particles having an expansion initiation temperature of 400 ℃ or lower, and wherein when thermally expandable particles having an exposure diameter of 0.5 to 5.0 times the volume average particle diameter D50 of the electrode active material present on the surface of the electrode composite layer are used as the exposed particles a, the area ratio of the exposed particles a on the surface of the electrode composite layer is 0.5 to 20%. If an electrode for an electrochemical element having such a feature is used, the IV resistance can be reduced while improving the heat release suppressing performance of the electrochemical element.

The expansion initiation 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 methods described in examples of the present specification.

In the electrode for an electrochemical element according to the invention as recited in item [1], the volume average particle diameter D50 of the thermally expandable particles is preferably 0.3 to 5.0 times the volume average particle diameter D50 of the electrode active material. When an electrode for an electrochemical element 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 IV resistance can be further reduced while the heat release suppressing performance of the electrochemical element is further improved.

The value of the volume average particle diameter D50 can be measured by the method described in examples of the present specification.

In addition, [3]Above [1]]Or [2]]The electrode for electrochemical element of the present invention preferably has a number density of the exposed particles A on the surface of the electrode composite layer of 10 particles/mm ² Above 300 pieces/mm ² The following is given. When the number density of the exposed particles a on the electrode surface is in the above range, the IV resistance can be further reduced while the heat release suppressing performance of the electrochemical element is further improved.

The number density of the exposed particles a on the electrode surface can be measured by the method described in the examples of the present specification.

Furthermore, [4]]The method of [1]]～[3]In the electrode for electrochemical element according to the present invention, the electrode composite layer preferably further comprises a binder, and the binder has a group selected from the group consisting of a carboxylic acid group, a hydroxyl group, a nitrile group, an amino group, an epoxy group, a hydroxyl group, a nitrile group, and an epoxy group,And (c) a polymer of at least one functional group of an oxazoline group, a sulfonic acid group, an ester group, and an amide group. If the electrode composite layer further contains a prescribed binder material, the adhesion of the electrode composite layer can be improved.

In addition, the present invention provides a method for producing an electrode for an electrochemical element according to any one of the above-mentioned items [1] to [4], which is advantageous in solving the above-mentioned problems, comprising the steps of: a step of forming an electrode lower layer by coating a current collector with the electrode lower layer slurry composition and drying the same; and a step of forming an electrode upper layer by applying and drying a slurry composition for an electrode upper layer on the electrode lower layer, wherein the slurry composition for an electrode upper layer and the slurry composition for an electrode lower layer each contain an electrode active material and thermally expandable particles, and the concentration of the thermally expandable particles of the slurry composition for an electrode upper layer is higher than the concentration of the thermally expandable particles of the slurry composition for an electrode lower layer. According to such a manufacturing method, the electrode for an electrochemical element of the present invention described above can be efficiently manufactured.

Effects of the invention

According to the present invention, it is possible to provide an electrode for an electrochemical element capable of reducing IV resistance while improving heat release inhibition performance of the electrochemical element, and a method for manufacturing the same.

Detailed Description

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

The electrode for an electrochemical element (hereinafter also simply referred to as "electrode") of the present invention can be used in manufacturing an electrochemical element. The electrode for an electrochemical element of the present invention can be preferably used as a positive electrode of an electrochemical element, particularly a secondary battery.

(electrode for electrochemical element)

The electrode for an electrochemical element of the present invention has a current collector and an electrode composite layer. Specifically, the electrode is characterized in that the electrode composite layer contains at least an electrode active material and thermally expandable particles having an expansion initiation temperature of 400 ℃ or lower, and that when the thermally expandable particles present on the surface of the electrode composite layer and having an exposure diameter of 0.5 to 5.0 times the volume average particle diameter D50 of the electrode active material are used as the exposed particles A, the area ratio of the exposed particles A on the surface of the electrode composite layer is 0.5 to 20%. When an electrode for an electrochemical element is used in which thermally expandable particles having an expansion initiation temperature in a predetermined temperature range are exposed to the surface of the electrode at a predetermined ratio, the IV resistance can be reduced while improving the heat release inhibition performance of the electrochemical element. The reason for this is not clear, but is presumed to be as follows.

It is presumed that, when an abnormality occurs in which the temperature in the element rises to a temperature equal to or higher than the expansion start temperature of the thermally expandable particles, the exposed particles a having an exposure diameter within a predetermined range are present on the surface of the electrode composite layer at an occupied area ratio within the predetermined range, the gap between the electrode and a member (for example, a separator) adjacent to the electrode can be effectively opened, and therefore, a short circuit at the time of occurrence of the abnormality can be suppressed, and the heat release suppressing performance of the electrochemical element can be improved. Further, it is estimated that if the occupied area of the exposed particles a in the predetermined range is equal to or larger than the predetermined lower limit, the volume of the thermally expandable particles buried in the electrode composite layer becomes smaller, and the resistance of the electrode composite layer can be reduced, and if the occupied area of the exposed particles a is equal to or smaller than the predetermined upper limit, the resistance between the electrode and the member (for example, the separator) adjacent to the electrode can be kept from being increased in a normal state, and as a result, the IV resistance of the electrochemical element can be reduced.

< Current collector >

As the current collector, a material having conductivity and electrochemical durability is used. Specifically, as the current collector, a current collector formed of, for example, iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, platinum, or the like can be used. The above materials may be used singly or in combination of two or more kinds in any ratio.

< electrode composite Material layer >

The electrode composite material layer of the electrode contains an electrode active material and thermally expandable particles having an expansion initiation temperature of 400 ℃ or lower, and may optionally contain a binder, a conductive material, and other additives. The electrode composite layer needs to have an area ratio of the exposed particles a on the surface of the electrode composite layer of 0.5% or more and 20% or less. This can improve the heat release suppressing performance of the obtained electrochemical element and reduce the IV resistance. Further, in the electrode composite layer, the thermally expandable particles are preferably biased to the regions existing on the surface and in the vicinity of the surface. If the thermally expandable particles are biased to exist on and near the surface, the IV resistance can be further reduced while the heat release suppressing performance of the obtained electrochemical element is further improved.

Area ratio of exposed particles A on surface of electrode composite layer

First, the exposed particles a are thermally expandable particles present on the surface of the electrode composite layer, the exposed diameter of which is 0.5 to 5.0 times the volume average particle diameter D50 of the electrode active material. By defining the thermally expandable particles having an exposure diameter of 0.5 to 5.0 times the volume average particle diameter D50 of the electrode active material as exposure particles, the presence ratio of the thermally expandable particles having a suitable exposure size on the surface of the electrode composite layer can be quantified, and thus the presence form of the thermally expandable particles which effectively contributes to improvement of the heat release inhibition performance can be evaluated.

Number density of exposed particles A on surface of electrode composite material layer

The number density of the exposed particles A on the surface of the electrode composite layer is preferably 10/mm ² The above is more preferably 20 pieces/mm ² The above is more preferably 40 pieces/mm ² Above, preferably 300 pieces/mm ² Hereinafter, more preferably 250 pieces/mm ² Hereinafter, 150 pieces/mm is more preferable ² Hereinafter, 110 pieces/mm is particularly preferable ² The following is given. When the number density of the exposed particles a on the surface of the electrode composite layer is within the above range, the IV resistance can be further reduced while further improving the heat release suppressing performance of the obtained electrochemical element. In particular, when the number density of the exposed particles a on the surface of the electrode composite layer is equal to or higher than the lower limit value, the distance between the electrode and the adjacent member can be effectively increased when the thermally expandable particles expand, and the heat release suppressing performance of the electrochemical element can be effectively improved. In addition, if the number density of the exposed particles a on the surface of the electrode composite layer is equal to or less than the upper limit value, the exposed thermally expandable particles can be suppressed in a normal state to increase the internal resistance of the electrochemical element and reduce the IV resistance of the electrochemical element.

Here, the occupation area ratio of the exposed particles a on the surface of the electrode composite layer is required to be 0.5% or more, preferably 2.0% or more, and 20% or less, preferably 10% or less, more preferably 5.0% or less. When the occupied area ratio of the exposed particles a is within the above range, the IV resistance can be reduced while improving the heat release suppressing performance of the obtained electrochemical element.

Thermal expansion particles

The thermally expandable particles need to have an expansion initiation temperature of 400 ℃ or lower. The expansion initiation temperature is preferably 300℃or less, more preferably 130℃or more, still more preferably 150℃or more, and still more preferably 170℃or more. If the expansion initiation temperature is not less than the lower limit, expansion of the thermally expandable particles in the process of manufacturing the electrochemical element can be suppressed, and an increase in IV resistance of the obtained electrochemical element can be suppressed. In addition, if the expansion start temperature is equal to or lower than the upper limit value, the internal temperature can be quickly suppressed from rising when abnormality occurs in the electrochemical element, and occurrence of thermal runaway can be effectively suppressed.

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, still more preferably 5.0 times or less, and still more preferably 3.0 times or less the volume average particle diameter D50 of the electrode active material. If the particle diameter ratio of the thermally expandable particles to the electrode active material is not less than the above lower limit value, the heat release suppressing performance due to expansion can be improved while suppressing the thermally expandable particles from becoming electric resistance in the electrode composite layer. Further, if the particle diameter ratio of the thermally expandable particles to the electrode active material is equal to or less than the upper limit value, the IV resistance and the heat release suppressing performance of the electrochemical element can be improved.

Further, the volume average particle diameter D50 of the thermally expandable particles is preferably 0.1 μm or more, more preferably 1 μm or more, further preferably 5 μm or more, preferably 100 μm or less, more preferably 80 μm or less, further preferably 50 μm or less, particularly preferably 30 μm or less. If the volume average particle diameter D50 of the thermally expandable particles is not less than the above lower limit value, the thermally expandable particles can be suppressed from becoming electric resistance, and the internal resistance of the electrochemical element can be increased. If the volume average particle diameter D50 of the thermally expandable particles is the above upper limit value or less, the heat release suppressing performance of the obtained electrochemical element can be improved while improving the coatability of the obtained electrode paste composition.

Here, the amount of the thermally expandable particles contained in the electrode is preferably 40 parts by mass or more, more preferably 50 parts by mass or more, still more preferably 97 parts by mass or less, and still more preferably 93 parts by mass or less, based on 100 parts by mass of the total content (mass basis) of the binder and the thermally expandable particles to be described later. If the content of the thermally expandable particles is not less than the above lower limit, the heat release suppressing performance of the obtained electrochemical element can be further improved. If the content of the thermally expandable particles is not more than the above-mentioned upper limit value, the IV resistance of the obtained electrochemical element can be further reduced.

Here, any thermally expandable particles can be used as long as the expansion start temperature is 400 ℃ or lower. For example, as the thermally expandable particles, commercially available thermally expandable particles such as MATSUMOTO MI CROSPHERE (registered trademark) and Expancel (manufactured by japan pellite corporation), or thermally expandable particles satisfying a specific structure, which will be described later, can be used.

As the thermally expandable particles, particles having a core-shell structure having a core and a shell covering the outer surface of the core are preferable. In this specification, the "covering the outer surface of the core" means that there is a shell on at least a portion of the outer surface of the core. The shell may cover a portion of the outer surface of the core or may cover the entirety of the outer surface of the core. In addition, the shell is not particularly limited as long as it contains at least two polymers, and contains one or more layers. The layers constituting the shell may contain a plurality of polymers in one layer, or may be formed of one polymer and contain two or more layers.

The core of the thermally expandable particles is formed of a gas generating substance that is gasified at 400 ℃ or lower. In the present specification, the "gas generating substance" refers to a compound capable of generating a gas when a predetermined temperature is reached; "gasification" refers to a substance that changes phase into a gas. The core may optionally contain an additive such as urea. The gas generating substance that vaporizes at 400 ℃ or lower vaporizes when the internal temperature of the electrochemical element increases to a predetermined temperature (400 ℃ or lower), and increases the internal resistance, thereby suppressing the interlocking of the electrochemical reaction, and thus suppressing the occurrence of thermal runaway. Further, when the core is produced by a method for producing thermally expandable particles described later, the core may contain a trace amount of metal oxide.

The gasification temperature of the gas generating substance is required to be 400 ℃ or less, preferably 300 ℃ or less, more preferably 150 ℃ or less, preferably 10 ℃ or more, more preferably 20 ℃ or more, and further preferably 26 ℃ or more. If the vaporization temperature is not higher than the upper limit, the increase in the internal temperature can be suppressed when abnormality occurs in the electrochemical element, and occurrence of thermal runaway can be effectively suppressed. If the vaporization temperature is not less than the lower limit, the ease of producing the thermally expandable particles is improved.

The gas generating substance may be: hydrocarbon compounds such as isopentane (vaporization temperature: 28 ℃), isooctane, n-pentane, n-hexane, isohexane, 2-dimethylbutane (vaporization temperature: 50 ℃), cyclohexane (vaporization temperature: 81 ℃), heptane and petroleum ether; bicarbonate compounds such as sodium bicarbonate (gasification temperature: 150 ℃ C.); guanidine compounds such as guanidine nitrate, nitroguanidine and aminoguanidine nitrate; azo compounds such as azobisisobutyronitrile (vaporization temperature: 108 ℃ C.) and azodicarbonamide (vaporization temperature: 200 ℃ C.); triazine compounds such as melamine, ammelide, melamine cyanurate (vaporization temperature: 280 ℃ C.), and trihydrazino-s-triazine (1, 3,5-triazine-2,4,6 (1H, 3H, 5H) -trione trihydrazone); hydrazide compounds such as p, p' -oxybisbenzenesulfonyl hydrazide (vaporization temperature: 160 ℃ C.) and p-toluenesulfonyl hydrazide; hydrazone compounds such as hydrazono-dimethylformamide and p-toluenesulfonyl semicarbazide; dinitroso pentamethylene tetramine, trimethylene trinitroamine and other nitramine compounds; tetrazole compounds such as 5-aminotetrazole and 5-phenyltetrazole; and 5,5' -bitetrazole diammonium salt, bitetrazole piperazine and other bitetrazole compounds. These can be used singly or in combination of plural kinds. Among them, isopentane, 2-dimethylbutane, cyclohexane, azobisisobutyronitrile, and sodium hydrogencarbonate are preferable from the viewpoint of improving the heat release inhibiting performance of the obtained electrochemical element.

The content of the core in the thermally expandable particles is preferably 0.1 mass% or more, more preferably 5 mass% or more, still more preferably 90 mass% or less, still more preferably 50 mass% or less, and still more preferably 30 mass% or less, based on 100 mass% of the total mass of the thermally expandable particles. If the content of the core in the thermally expandable particles is not less than the above lower limit, the heat release suppressing performance of the obtained electrochemical element can be further improved. Further, if the content of the core in the thermally expandable particles is not more than the above-described upper limit, the thermally expandable particles can be suppressed from becoming brittle and collapsing during normal operation of the electrochemical element. The "content of the core in the thermally expandable particles" means the content of the core in the state of being enclosed in the shell.

The shell of the thermally expandable particles is formed from at least two polymers. The electrolyte swelling degree of the shell is required to be 500 mass% or less, preferably 350 mass% or less, and more preferably 300 mass% or less. If the electrolyte swelling degree of the case is equal to or less than the upper limit, dissolution of the core into the electrolyte in the electrochemical element can be satisfactorily suppressed, and the heat release suppressing performance of the electrochemical element can be improved. The lower limit of the electrolyte swelling degree of the shell is not particularly limited, and may be, for example, 100 mass% and does not swell at all. From the viewpoint of reducing the internal resistance of the obtained electrochemical element, the electrolyte swelling degree of the case is preferably 120 mass% or more.

Further, the electrolyte swelling degree of at least two polymers constituting the shell is preferably 500% by mass or less, more preferably 350% by mass or less, and still more preferably 300% by mass or less, respectively. If the swelling degree of the electrolyte solutions of at least two polymers constituting the shell is not more than the above-mentioned upper limit value, dissolution of the core into the electrolyte solution in the electrochemical element can be favorably suppressed, and the heat release suppressing performance of the electrochemical element can be improved. The swelling degree of the electrolyte solution of each of the at least two polymers constituting the case may be 100 mass%, and is preferably 120 mass% or more from the viewpoint of reducing the internal resistance of the obtained electrochemical element.

The electrolyte swelling degree of at least two polymers constituting the shell can be measured by the method described in examples.

Further, the swelling degree of the shell with respect to N-methyl-2-pyrrolidone (hereinafter, sometimes referred to as "NMP swelling degree") is preferably 500 mass% or less, more preferably 350 mass% or less, and further preferably 300 mass% or less. When the NMP swelling degree of the shell is equal to or less than the above-described upper limit, the dissolution of the core into NMP in the electrode manufacturing process can be favorably suppressed and the heat release suppressing performance of the obtained electrochemical element can be improved in the case of forming the electrode of the present invention using a solution in which NMP is used as a solvent. The lower limit of the NMP swelling degree of the shell is not particularly limited, and may be, for example, 100 mass% and does not swell at all.

The NMP swelling degree of the shell can be measured by the method described in the following examples.

Further, the NMP swelling degree of at least two polymers constituting the shell is preferably 500 mass% or less, more preferably 350 mass% or less, and still more preferably 300 mass% or less, respectively. When the NMP swelling degree of at least two polymers constituting the shell is equal to or lower than the above upper limit value, the dissolution of the nuclei into NMP in the slurry composition can be favorably suppressed and the heat release suppressing performance of the obtained electrochemical element can be improved in the case of using the binder composition of the present invention in the preparation of the slurry composition for the positive electrode of the secondary battery using NMP as a solvent. The NMP swelling degree of each of the at least two polymers constituting the shell is not particularly limited, and may be 100 mass%.

The NMP swelling degree of at least two polymers constituting the shell can be measured by the method described in examples.

The at least two polymers constituting the shell need to contain at least two polymers having a difference in glass transition temperature of 10 ℃ or more and 230 ℃ or less. More specifically, the shell may contain only two polymers having different glass transition temperatures in a range of 10 ℃ or more and 230 ℃ or less, or may further contain other polymers (for example, a polymer having a difference in glass transition temperature from at least one of the two polymers of less than 10 ℃ or more than 230 ℃) in addition to the two polymers. In the case where the shell contains three or more polymers, it is sufficient that the two polymers in a large amount (mass basis) satisfy the above-described relative relationship with respect to the glass transition temperature.

The difference in the glass transition temperatures is preferably 60℃or more, more preferably 90℃or more, preferably 150℃or less, more preferably 120℃or less. If the temperature difference is not less than the lower limit, leakage of the case out of the thermally expandable particles can be suppressed even when pressure is applied in the manufacturing process of the electrochemical element. Therefore, the high-density electrode can be obtained by pressing the electrode composite layer at a high voltage, and the heat release suppressing performance of the electrochemical element having the high-density electrode can be further improved. If the temperature difference is not more than the upper limit value, the high-temperature storage characteristics of the electrochemical element can be improved.

Further, it is preferable that the highest temperature among the glass transition temperatures of the polymers contained in the shell is higher than the vaporization temperature of the gas generating substance forming the core. If the highest temperature of the glass transition temperatures of the polymers contained in the case is higher than the vaporization temperature of the gas generating substance, leakage of the gas generating substance during pressurization in the production of the electrochemical element can be favorably suppressed, and the heat release suppressing performance of the obtained electrochemical element can be further improved. Here, the difference between the highest temperature of the glass transition temperatures of the polymers contained in the shell and the vaporization temperature of the gas generating substance is preferably 10 ℃ or higher, more preferably 45 ℃ or higher, and still more preferably 60 ℃ or higher. The upper limit of the difference is not particularly limited, and may be, for example, 200 ℃.

It is preferable that one of at least two polymers constituting the shell has a glass transition temperature of 60 ℃ or more and the other has a glass transition temperature of less than 60 ℃. In the case where the shell contains three or more polymers, it is preferable that the two polymers are any one of the polymers in a large amount (mass basis).

[ Polymer (Polymer 1) having a glass transition temperature of 60 ℃ or higher ]

The glass transition temperature of the polymer having a glass transition temperature of 60℃or higher (hereinafter, sometimes referred to as "polymer 1") is preferably 80℃or higher, more preferably 180℃or lower, still more preferably 150℃or lower, and still more preferably 130℃or lower. If the glass transition temperature of the polymer 1 is equal to or higher than the lower limit value, the protective shell can be satisfactorily protected even when the pressure is applied in the process of manufacturing the electrochemical element, and the outflow of the gas generating substance out of the thermally expandable particles can be satisfactorily suppressed. Therefore, in particular, the electrode composite material layer can be pressed at a high pressure, and a high-density electrode can be efficiently manufactured. If the glass transition temperature is not higher than the upper limit, the polymerization stability at the time of polymerization of the polymer 1 can be improved, and the electrode manufacturing efficiency can be improved.

Furthermore, the solubility parameter (Solubility Parameter, hereinafter sometimes referred to as SP value) of the polymer 1 is preferably 23.0MPa ^1/2 The above is more preferably 24.0MPa ^1/2 The above is preferably 30.0MPa ^1/2 Hereinafter, it is more preferably 29.5MPa ^1/2 The following is given. More specifically, the SP value of the polymer 1 is preferably higher than the SP values of NMP and an electrolyte that can be used in manufacturing an electrochemical element. If the SP value of the polymer 1 is set to a value distant from the SP values of NMP and the electrolyte, the polymer 1 is less likely to swell and dissolve out with respect to NMP and the electrolyte, and as a result, the electrochemical element including the thermally expandable particles operates normally and can exert an effect in releasing heat.

The solubility parameter is a hansen solubility parameter which is a defined and calculated method described in the following document.

Charles M.Hansen, "Hansen Solubility Parameters: A Users Handbook", CRC publications, 2007.

In addition, the hansen solubility parameter can be easily calculated from its chemical structure by using computer software (Hansen Solubility Parameters in Practice (hsPIP)) for a substance whose literature value of hansen solubility parameter is unknown.

The composition of the polymer 1 is not particularly limited. As the polymer 1, for example, a polymer containing a monomer unit having a nitrile group is preferably used. In the present specification, the term "a polymer includes a monomer unit" means that "a polymer obtained by using the monomer includes a repeating unit derived from the monomer". In the present invention, the content ratio of each monomer unit in the polymer can be used ¹ The measurement is performed by a Nuclear Magnetic Resonance (NMR) method such as H-NMR.

Examples of the monomer unit having a nitrile group include an α, β -ethylenically unsaturated nitrile monomer unit and the like. 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 examples thereof include: acrylonitrile; alpha-halogenated acrylonitrile such as alpha-chloroacrylonitrile and alpha-bromoacrylonitrile; alpha-alkylacrylonitrile such as methacrylonitrile, etc., acrylonitrile and methacrylonitrile being preferred. As the α, β -ethylenically unsaturated nitrile monomer, a plurality of them may be used in combination.

The content of the monomer unit having a nitrile group in the polymer 1 is preferably 70% by mass or more, more preferably 80% by mass or more, further preferably 85% by mass or more, preferably 98% by mass or less, and further preferably 97% by mass or less, based on 100% by mass of the total repeating units contained in the polymer 1. If the content of the monomer unit having a nitrile group is not less than the above lower limit, the swelling degree of the electrolyte solution of the polymer 1 can be suppressed from increasing. In addition, if the content ratio of the monomer unit having a nitrile group is the above upper limit value or less, the polymerization stability of the polymer 1 can be improved.

The polymer containing a monomer unit having a nitrile group may also be a copolymer of a monomer forming a monomer unit having a nitrile group and a monomer capable of copolymerizing. The copolymerizable monomer may be: unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and the like; 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; heterocyclic vinyl compounds such as N-vinylpyrrolidone, vinylpyridine and vinylimidazole; butyl acrylate such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, and t-butyl acrylate, pentyl acrylate, hexyl acrylate, cyclohexyl acrylate, isobornyl acrylate, octyl acrylate such as heptyl acrylate, and 2-ethylhexyl acrylate, and alkyl acrylate such as nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, and stearyl acrylate; methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate and other butyl methacrylate, pentyl methacrylate, hexyl methacrylate, cyclohexyl methacrylate, isobornyl methacrylate, heptyl methacrylate, 2-ethylhexyl methacrylate and other octyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate, stearyl methacrylate and other alkyl methacrylates. The copolymerizable monomer may be used in combination of plural kinds thereof.

Further, the polymer containing a monomer unit having a nitrile group as the polymer 1 may have a crosslinkable monomer unit. Examples of the crosslinkable monomer capable of forming the crosslinkable monomer unit include polyfunctional monomers having 2 or more polymerizable reactive groups in the monomer. Examples of the polyfunctional monomer include: divinyl compounds such as allyl methacrylate and divinylbenzene; di (meth) acrylate compounds such as diethylene glycol dimethacrylate, ethylene glycol dimethacrylate, diethylene glycol diacrylate and 1, 3-butanediol diacrylate; and tri (meth) acrylate compounds such as trimethylolpropane trimethacrylate and trimethylolpropane triacrylate. Among them, ethylene glycol dimethacrylate is preferable. These crosslinkable monomers may be used alone or in combination of two or more thereof in any ratio. In the present specification, (meth) acrylic acid means acrylic acid or methacrylic acid.

The content ratio of the crosslinkable monomer unit in the polymer 1 is not particularly limited, and is, for example, preferably 0.05 mass% or more, more preferably 0.1 mass% or more, still more preferably 0.5 mass% or more, preferably 3.0 mass% or less, and more preferably 2.0 mass% or less, based on 100 mass% of the total repeating units contained in the polymer 1. If the content of the crosslinkable monomer unit in the polymer 1 is not less than the above lower limit, the outflow of the core out of the thermally expandable particles can be effectively suppressed by increasing the strength of the shell. If the content of the crosslinkable monomer units in the polymer 1 is not more than the above-mentioned upper limit, the heat-expandable particles can be expanded at a desired temperature while suppressing an excessive increase in the crosslinking density to inhibit thermal expansion.

[ Polymer (Polymer 2) having a glass transition temperature of less than 60 ]

The polymer having a glass transition temperature of less than 60 ℃ (hereinafter sometimes referred to as "polymer 2") needs to have a glass transition temperature of less than 60 ℃, preferably 40 ℃ or less, more preferably 25 ℃ or less, preferably-50 ℃ or more, more preferably-40 ℃ or more, and still more preferably-30 ℃ or more. If the glass transition temperature of the polymer 2 is equal to or lower than the upper limit value, the adhesiveness of the adhesive composition can be further improved, and leakage of the shell out of the thermally expandable particles can be suppressed even when the pressure is applied in the manufacturing process of the electrochemical element. As a result, the heat release suppressing performance of the obtained electrochemical element can be further improved. Therefore, the high-density electrode can be obtained by pressing the electrode composite layer at a high voltage, and the heat release suppressing performance of the electrochemical element having the high-density electrode can be further improved. If the glass transition temperature of the polymer 2 is not less than the above lower limit, the polymerization stability of the polymer 2 can be improved, and the productivity of the electrode can be improved.

Further, the SP value of the polymer 2 is preferably 16.0MPa ^1/2 The above is more preferably 18.0MPa ^1/2 The above is preferably 24.0MPa ^1/2 Hereinafter, it is more preferably 23.0MPa ^1/2 Hereinafter, it is more preferably 21.0MPa ^1/2 The following is given. More specifically, the SP value of the polymer 2 is preferably lower than the SP values of NMP and electrolyte that can be used in manufacturing the electrochemical element. If the SP value of the polymer 2 is far from the SP value of NMP and the electrolyte, the polymer 2 is less likely to swell and dissolve out with respect to NMP and the electrolyte, and as a result, the electrochemical element including the thermally expandable particles operates normally and can exert an effect in releasing heat.

The composition of the polymer 2 is not particularly limited. Examples of the polymer 2 include polymers containing an aromatic vinyl monomer unit. Examples of the aromatic vinyl monomer unit include a unit formed using an aromatic vinyl monomer exemplified as a monomer usable in the preparation of the polymer 1. Among them, styrene is preferable. The content of the aromatic vinyl monomer unit in the polymer 2 is preferably 40 mass% or more, more preferably 50 mass% or more, still more preferably 90 mass% or less, and still more preferably 80 mass% or less, based on 100 mass% of all the repeating units contained in the polymer 2. If the content of the aromatic vinyl monomer unit in the polymer 2 is not less than the above lower limit, an excessive increase in the electrolyte swelling degree of the polymer 2 can be suppressed.

The polymer 2 may contain a (meth) acrylate monomer unit instead of the aromatic vinyl monomer unit, or may contain a (meth) acrylate monomer unit in addition to the aromatic vinyl monomer unit. Examples of the (meth) acrylate monomer that can be used for forming the (meth) acrylate monomer unit include various monomers that can be used for preparing the polymer 1. Among them, 2-ethylhexyl acrylate is preferable.

Further, the polymer 2 may contain other monomer units in addition to the above-mentioned aromatic vinyl monomer units and (meth) acrylate monomer units, or may contain other monomer units instead of the aromatic vinyl monomer units and (meth) acrylate monomer units, and as the monomer units, there may be mentioned units formed using various monomers listed as monomers that can be used in the production of the polymer 1.

Among them, the polymer 2 preferably contains a crosslinkable monomer unit. As the crosslinkable monomer that can be used to form the crosslinkable monomer unit in the polymer 2, various compounds listed above can be cited in association with the polymer 1.

Among them, allyl methacrylate is preferable as the monomer used for forming the crosslinkable monomer unit in the polymer 2. The content ratio of the crosslinkable monomer unit in the polymer 2 is not particularly limited, and is preferably 0.05 mass% or more, more preferably 0.1 mass% or more, still more preferably 0.2 mass% or more, still more preferably 3.0 mass% or less, and still more preferably 2.0 mass% or less, based on 100 mass% of the total repeating units contained in the polymer 2. If the content of the crosslinkable monomer unit in the polymer 2 is not less than the above lower limit, the outflow of the core out of the thermally expandable particles can be effectively suppressed by increasing the strength of the shell. If the content of the crosslinkable monomer unit in the polymer 2 is not more than the above upper limit, the ease of manufacturing the shell can be improved.

Further, the polymer 2 may contain a monomer having a carbon-carbon double bond and an epoxy group (epoxy group-containing unsaturated monomer) separately from the crosslinkable monomer. In the present specification, the "crosslinkable monomer" does not include a monomer corresponding to an epoxy group-containing unsaturated monomer.

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-allyl phenyl glycidyl ether; diene or polyene monoepoxide such as butadiene monoepoxide, chloroprene monoepoxide, 4, 5-epoxy-2-pentene, 3, 4-epoxy-1-vinylcyclohexene, 1, 2-epoxy-5, 9-cyclododecadiene, and the like; alkenyl epoxides such as 3, 4-epoxy-1-butene, 1, 2-epoxy-5-hexene, and 1, 2-epoxy-9-decene; glycidyl esters of unsaturated carboxylic acids such as glycidyl acrylate, glycidyl methacrylate, glycidyl crotonate, glycidyl 4-heptenoate, glycidyl sorbate, glycidyl linoleate, glycidyl 4-methyl-3-pentenoate, glycidyl 3-cyclohexene carboxylate, and glycidyl 4-methyl-3-cyclohexene carboxylate. One kind of these may be used alone, or two or more kinds may be used in combination in any ratio.

The content of the epoxy group-containing unsaturated monomer in the polymer 2 is preferably 0.5 mass% or more, more preferably 1.0 mass% or more, preferably 10.0 mass% or less, and even more preferably 8.0 mass% or less, based on 100 mass% of all the repeating units contained in the polymer 2. If the content ratio of the epoxy group-containing unsaturated monomer in the polymer 2 is within this range, the heat release inhibiting performance of the electrochemical element can be further improved.

[ composition of Shell ]

The composition of the shell is not particularly limited. For example, the shell can contain various monomer units listed in the examples above as monomer units that can be contained in polymer 1 and polymer 2. The content of the aromatic vinyl monomer unit in the shell is preferably 20 mass% or more, more preferably 30 mass% or more, preferably 50 mass% or less, more preferably 40 mass% or less. The content of the nitrile group-containing monomer unit in the shell is preferably 20 mass% or more, more preferably 25 mass% or more, preferably 50 mass% or less, more preferably 40 mass% or less. Further, the content ratio of the (meth) acrylate 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, more preferably 40% by mass or less. 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-described ranges, the glass transition temperature, the electrolyte swelling degree, and the NMP swelling degree of the shell can be appropriately controlled. Further, from the viewpoints of the shell strength, the electrolyte swelling degree, and the NMP swelling degree, the content ratio of the crosslinkable monomer unit in the shell is preferably 0.1 mass% or more, more preferably 0.2 mass% or more, preferably 3.0 mass% or less, and still more preferably 2.0 mass% or less. Further, from the viewpoint of the electrolyte swelling degree and NMP swelling degree, the content ratio of the epoxy group-containing unsaturated monomer unit in the shell is preferably 0.5 mass% or more, more preferably 1.0 mass% or more, preferably 10.0 mass% or less, and still more preferably 8.0 mass% or less. The content ratio is a ratio of 100 mass% of the total of the repeating units contained in all the polymers constituting the shell.

The structure of the thermally expandable particles is not particularly limited as long as it is a structure in which a shell containing the polymer 1 and the polymer 2 is covered with a core formed of a gas generating substance. From the viewpoint of effectively improving the heat release suppressing performance of the obtained electrochemical element, it is preferable that the case has a layer a formed of the polymer 1 and a layer B formed of the polymer 2. Further, from the viewpoint of improving the adhesiveness of the adhesive composition, it is preferable that the layer a formed of the polymer 1 is present further inside (core side) than the layer B formed of the polymer 2. The thermally expandable particles may have a conductive carbon material described below in the item < conductive material > on the surface thereof. In this case, the IV resistance of the electrochemical element can be further reduced.

It is preferable that the total area ratio α (%) of the polymer 1 and the total area ratio β (%) of the polymer 2 in the shell satisfy the relationship of α.ltoreq.β. If the total area ratio of the polymers 1 and 2 contained in the case satisfies the relationship of α.ltoreq.β, the heat release suppressing performance of the obtained electrochemical device can be further improved. Here, from the viewpoint of further improving the effect, the difference between the total area ratio α (%) of the polymer 1 and the total area ratio β (%) of the polymer 2 is preferably 1% or more, more preferably 20% or more.

In the case where polymer 1 and polymer 2 form layers A and B, respectively, it is preferable that the thickness a of layer A containing polymer 1 and the thickness B of layer B containing polymer 2 satisfy the relationship of a.ltoreq.b. If the thicknesses of the layers constituting the case satisfy the relationship of a.ltoreq.b, the heat release suppressing performance of the resulting electrochemical element can be further improved. If the thickness of layer a containing polymer 1 having a higher glass transition temperature than polymer 2 is thicker than the thickness of layer B, the thermally expandable particles are less likely to expand when the shell is gasified, and therefore, from the viewpoint of further improving the effect, the thickness B of layer B is preferably 1.2 times or more, more preferably 1.4 times or more the thickness a of layer a.

[ method for producing thermally-expansive particles ]

The thermally expandable particles can be produced, for example, by polymerizing a monomer composition containing the above monomer in an aqueous colloidal solution in which a gas generating substance is dispersed. The ratio of each monomer in the monomer composition is generally the same as the ratio of each monomer unit in the thermally expandable particles.

The polymerization method is not particularly limited, and any of suspension polymerization, emulsion polymerization coagulation, pulverization, and the like can be used. Among them, the suspension polymerization method and the emulsion polymerization coagulation method are preferable, and the suspension polymerization method is more preferable. Further, as the polymerization reaction, any of radical polymerization, living radical polymerization, and the like can be used.

The monomer composition used in the production of the thermally expandable particles may contain other compounding agents such as chain transfer agents, polymerization regulators, polymerization retarders, reactive fluidizers, fillers, flame retardants, antioxidants, and colorants in any amount.

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

Preparation of thermally expandable particles by suspension polymerization

(1) Preparation of the monomer composition

First, a monomer composition 1 and a monomer composition 2 having compositions corresponding to the compositions of a polymer 1 and a polymer 2 constituting a shell, respectively, are prepared. At this time, various monomers are blended according to the compositions of the polymer 1 and the polymer 2, and further, other compounding agents added as needed are mixed.

(2) Droplet formation

Next, a metal hydroxide as a dispersion stabilizer is dispersed in water to prepare a colloidal dispersion liquid containing the metal hydroxide. Then, to the colloidal dispersion, a gas generating substance capable of forming a core, and either or both of the monomer composition 1 and the monomer composition 2 capable of forming a shell are added. Further, a polymerization initiator was added to obtain a mixed solution, and droplets were formed. The method for forming the droplets is not particularly limited, and the droplets can be formed by shearing and stirring the mixed liquid using a dispersing machine such as an emulsifying and dispersing machine. Further, examples of the polymerization initiator include oil-soluble polymerization initiators such as t-butyl peroxy-2-ethylhexanoate and azobisisobutyronitrile. Further, as the dispersion stabilizer, for example, a metal hydroxide such as magnesium hydroxide, sodium dodecylbenzenesulfonate, or the like can be used.

(3) Polymerization

Then, after forming the droplets, the water containing the formed droplets is heated to initiate polymerization. In the case where only either one of the monomer composition 1 and the monomer composition 2 is blended in the liquid droplets in the step (2), the monomer composition 1/2 which is not added in the step (2) is added at a stage where the polymerization conversion is sufficiently high, and the polymerization is continued. As a result, thermally expandable particles having a predetermined structure are formed in water. In this case, the reaction temperature of the polymerization is preferably 50℃or more and 95℃or less. The duration of each polymerization reaction is preferably 1 hour or more and 10 hours or less, and more preferably 8 hours or less.

(4) Washing, filtering, dehydrating and drying

After the polymerization, the water containing the thermally expandable particles is washed, filtered and dried according to a conventional method, whereby thermally expandable particles having a predetermined structure can be obtained.

The ratio of the gas generating substance to the monomer compositions 1 and 2 is appropriately set so as to satisfy the preferable range of the "content ratio of the core in the thermally expandable particles" described above. Further, the ratio of the amount of the monomer compositions 1 and 2 can be appropriately set so as to satisfy the above-described preferable ranges of "the area ratio of the polymer 1 and the polymer 2 in the shell" and "the thickness ratio of the layer a and the layer B in the shell".

< adhesive Material >

The electrode preferably further comprises a binder material. The binder is not particularly limited as long as it is a polymer capable of exhibiting adhesive performance in the electrode composite layer, and any polymer can be used. Preferable examples of the polymer that can be used as the adhesive material include a polymer mainly containing an aliphatic conjugated diene monomer unit and a hydrogenated product thereof (diene polymer), a polymer mainly containing a (meth) acrylic acid ester monomer unit (acrylic polymer), a polymer mainly containing (meth) acrylonitrile (nitrile polymer), a polymer mainly containing a fluorine-containing monomer unit (fluorine-containing polymer), and a polymer mainly containing a vinyl alcohol monomer unit (vinyl alcohol polymer). Among these, acrylic polymers, nitrile polymers and fluorine polymers are more preferable.

In addition, the binder may be used alone, or two or more kinds may be used in combination in any ratio. In the present specification, the term "mainly contains" a certain monomer unit means that the content of the monomer unit is more than 50% by mass when the amount of all the repeating units contained in the polymer is 100% by mass.

The binder preferably contains no gas generating substance and is selected from the group consisting of carboxylic acid groups, hydroxyl groups, nitrile groups, amino groups, epoxy groups, and combinations thereof,A polymer having at least one functional group selected from the group consisting of an oxazoline group, a sulfonic acid group, an ester group and an amide group (hereinafter, these functional groups may be collectively referred to as "specific functional groups"). The polymer used as the adhesive material may have one kind of the above specific functional group or may have two or more kinds.

If a polymer having these specific functional groups is used as the binder, the IV resistance of the electrochemical element can be further reduced. Further, from the viewpoint of reducing 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, more preferably has at least one of a carboxylic acid group and a nitrile group, and still more preferably has both a carboxylic acid group and a nitrile group.

Here, the polymer is polymerizedThe method for introducing the above specific functional group into the compound is not particularly limited. For example, a polymer may be produced using a monomer having the above specific functional group (a monomer having the specific functional group) to obtain a polymer containing a monomer unit having the specific functional group, or a polymer having the above specific functional group introduced thereto may be obtained by modifying an arbitrary polymer, and the former is preferable. That is, the polymer as the adhesive material preferably contains 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, and an epoxy group-containing monomer unit At least one of the oxazoline group-containing monomer unit, the sulfonic acid group-containing monomer unit, the ester group-containing monomer unit, and the amide group-containing monomer unit, more preferably at least one of the carboxylic acid group-containing monomer unit, the hydroxyl group-containing monomer unit, and the nitrile group-containing monomer unit, still more preferably at least one of the carboxylic acid group-containing monomer unit and the nitrile group-containing monomer unit, and particularly preferably both the carboxylic acid group-containing monomer unit and the nitrile group-containing monomer unit.

[ Carboxylic acid group-containing monomer Unit ]

Examples of the carboxylic acid group-containing monomer capable of forming a carboxylic acid group-containing monomer unit include monocarboxylic acids and derivatives thereof, dicarboxylic acids and anhydrides thereof, and derivatives thereof.

Examples of the monocarboxylic acid include acrylic acid, methacrylic acid, and crotonic acid.

Examples of the monocarboxylic acid derivative include 2-ethyl acrylic acid, isocrotonic acid, α -acetoxy acrylic acid, β -trans-aryloxy acrylic acid, α -chloro- β -E-methoxy acrylic acid, and the like.

Examples of the dicarboxylic acid include maleic acid, fumaric acid, itaconic acid, and the like.

Examples of the dicarboxylic acid derivative include: methyl maleic acid, dimethyl maleic acid, phenyl maleic acid, chloro maleic acid, dichloro maleic acid, fluoro maleic acid; maleic monoesters such as nonylmaleate, decylmaleate, dodecylmaleate, octadecylmaleate and fluoroalkyl maleate.

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

Further, as the carboxylic acid group-containing monomer, an acid anhydride which generates a carboxylic acid group by hydrolysis can also be used. Among them, acrylic acid and methacrylic acid are preferable as the carboxylic acid group-containing monomer. The carboxylic acid group-containing monomer may be used alone or in combination of two or more kinds in any ratio.

[ hydroxyl group-containing monomer Unit ]

Examples of the hydroxyl group-containing monomer capable of forming a hydroxyl group-containing monomer unit include: ethylenically unsaturated alcohols such as (meth) allyl alcohol, 3-buten-1-ol, 5-hexen-1-ol, and the like; alkyl esters of ethylenically unsaturated carboxylic acids such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, di-2-hydroxyethyl maleate, di-4-hydroxybutyl maleate, and di-2-hydroxypropyl itaconate; a general formula: CH (CH) ₂ ＝CR ^a -COO-(C _q H _2q O) _p -H (wherein p is an integer of 2 to 9, q is an integer of 2 to 4, R) ^a An ester of a polyalkylene glycol represented by a hydrogen atom or a methyl group) and (meth) acrylic acid; mono (meth) acrylates of dihydroxyesters of dicarboxylic acids such as 2-hydroxyethyl-2 '- (meth) acryloyloxy phthalate and 2-hydroxyethyl-2' - (meth) acryloyloxy succinate; vinyl ethers such as 2-hydroxyethyl vinyl ether and 2-hydroxypropyl vinyl ether; mono (meth) allyl ethers of alkylene glycols such as (meth) allyl-2-hydroxyethyl ether, (meth) allyl-2-hydroxypropyl ether, (meth) allyl-3-hydroxypropyl ether, (meth) allyl-2-hydroxybutyl ether, (meth) allyl-3-hydroxybutyl ether, (meth) allyl-4-hydroxybutyl ether, and (meth) allyl-6-hydroxyhexyl ether; polyoxyalkylene glycol mono (meth) allyl ethers such as diethylene glycol mono (meth) allyl ether and dipropylene glycol mono (meth) allyl ether; halogen and hydroxy substituents of (poly) alkylene glycol such as glycerol mono (meth) allyl ether, (meth) allyl-2-chloro-3-hydroxypropyl ether, (meth) allyl-2-hydroxy-3-chloropropyl ether, and the like Mono (meth) allyl ether of (a); mono (meth) allyl ethers of polyhydric phenols such as eugenol and isoeugenol, and halogen substituents thereof; (meth) allyl thioethers of alkylene glycols such as (meth) allyl-2-hydroxyethyl sulfide and (meth) allyl-2-hydroxypropyl sulfide; amides having a hydroxyl group such as N-methylolacrylamide (N-methylolacrylamide), N-methylolmethacrylamide, N-hydroxyethyl acrylamide, and N-hydroxyethyl methacrylamide. The hydroxyl group-containing monomers may be used alone or in combination of two or more kinds in any ratio.

In the present specification, "(meth) acryl" means acryl and/or methacryl.

[ monomer unit containing nitrile group ]

Examples of the nitrile group-containing monomer capable of forming a nitrile group-containing monomer unit include α, β -ethylenically unsaturated nitrile monomers. Specifically, monomers exemplified as the α, β -ethylenically unsaturated compound having a nitrile group that can be used to form the polymer 1 can be used. These compounds may be used singly or in combination of two or more kinds in any ratio.

[ comprising amino group-containing monomer units ]

Examples of the amino group-containing monomer capable of forming an amino group-containing monomer unit include dimethylaminoethyl (meth) acrylate, diethylaminoethyl (meth) acrylate, aminoethyl vinyl ether, dimethylaminoethyl vinyl ether, and the like. The amino group-containing monomer may be used alone, or two or more kinds may be used in combination in any ratio.

[ epoxy group-containing monomer Unit ]

Examples of the epoxy group-containing monomer capable of forming an epoxy group-containing monomer unit include various monomers containing a carbon-carbon double bond and an epoxy group, which are exemplified as compounds that can be used to form the crosslinkable monomer unit in the polymer 2. These monomers may be used alone or in combination of two or more at an arbitrary ratio.

[ containingOxazolinyl monomer units]

As a means for forming a composition containingContent of oxazolinyl monomer units>As oxazolinyl monomers, 2-vinyl-2-/is mentioned>Oxazoline, 2-vinyl-4-methyl-2->Oxazoline, 2-vinyl-5-methyl-2->Oxazoline, 2-isopropenyl-2->Oxazoline, 2-isopropenyl-4-methyl-2->Oxazoline, 2-isopropenyl-5-methyl-2->Oxazoline, 2-isopropenyl-5-ethyl-2->Oxazolines, and the like. In addition, contain->The oxazoline group monomer may be used alone or in combination of two or more kinds in any ratio.

[ sulfonic acid group-containing monomer Unit ]

Examples of the sulfonic acid group-containing monomer capable of forming a sulfonic acid group-containing monomer unit include vinylsulfonic acid, methylvinylsulfonic acid, (meth) allylsulfonic acid, styrenesulfonic acid, ethyl (meth) acrylate-2-sulfonate, 2-acrylamido-2-methylpropanesulfonic acid, and 3-allyloxy-2-hydroxypropanesulfonic acid. The carboxylic acid group-containing monomer may be used alone or in combination of two or more kinds in any ratio.

[ ester group-containing monomer Unit ]

As the ester group-containing monomer capable of forming an ester group-containing monomer unit, for example, a (meth) acrylate monomer can be used. Examples of the (meth) acrylate monomer include various (meth) acrylate monomers listed as monomers that can be used to form the (meth) acrylate monomer unit in the polymer 2. These monomers may be used alone or in combination of two or more at an arbitrary ratio.

In addition, in the present invention, in the case where a certain monomer has a specific functional group other than an ester group, the monomer is not contained in the ester group-containing monomer.

[ amide group-containing monomer Unit ]

Examples of the amide group-containing monomer capable of forming an amide group-containing monomer unit include acrylamide, methacrylamide, and vinylpyrrolidone. The amide group-containing monomer may be used alone or in combination of two or more kinds in any ratio.

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

[ other repeating units ]

The polymer as the adhesive material may also contain a repeating unit (other repeating unit) other than the above-mentioned monomer unit containing a specific functional group. The other repeating unit is not particularly limited, and in the case where the polymer is a diene polymer, an aliphatic conjugated diene monomer unit is exemplified.

Examples of the aliphatic conjugated diene monomer capable of forming an aliphatic conjugated diene monomer unit include 1, 3-butadiene, isoprene, 2, 3-dimethyl-1, 3-butadiene, and 1, 3-pentadiene. One kind of these may be used alone, or two or more kinds may be used in combination in any ratio.

In the present invention, the "aliphatic conjugated diene monomer unit" also includes a structural unit (hydride unit) obtained by further hydrogenation of a monomer unit contained in a polymer obtained by using an aliphatic conjugated diene monomer.

Among the aliphatic conjugated diene monomers, 1, 3-butadiene and isoprene are preferable. In other words, the aliphatic conjugated diene monomer unit is preferably a 1, 3-butadiene unit or an isoprene unit, and the 1, 3-butadiene hydride unit or the isoprene hydride unit is more preferably a 1, 3-butadiene hydride unit or an isoprene hydride unit.

Here, when the polymer as the binder contains aliphatic conjugated diene monomer units, the content of diene monomer units in the polymer is preferably greater than 50 mass%, more preferably 60 mass% or more, preferably 90 mass% or less, still more preferably 80 mass% or less, and even more preferably 70 mass% or less, based on 100 mass% of the total repeating units contained in the polymer, from the viewpoint of reducing the IV resistance of the electrochemical element.

[ method for producing adhesive Material ]

The method for preparing the adhesive material is not particularly limited. The binder for the polymer is produced, for example, by polymerizing a monomer composition containing one or two or more monomers in an aqueous solvent, optionally followed by hydrogenation and modification. The content ratio of each monomer in the monomer composition can be determined according to the desired content ratio of the monomer unit in the polymer.

The polymerization method is not particularly limited, and any of solution polymerization, suspension polymerization, bulk polymerization, emulsion polymerization, and the like can be used. Further, as the polymerization reaction, any of ion polymerization, radical polymerization, living radical polymerization, various polycondensation, addition polymerization, and the like can be used. In addition, a known emulsifier and a known polymerization initiator can be used as needed in the polymerization. Furthermore, the hydrogenation and modification can be carried out by known methods.

Ratio of the total content of the thermally-expansive particles and the binder in the electrode composite layer

The ratio of the total content of the thermally expandable particles and the binder in the electrode composite layer is preferably 1 mass% or more, more preferably 1.5 mass% or more, and preferably 10 mass% or less, more preferably 5 mass% or less, based on 100 mass% of the total mass of the electrode composite layer.

[ other Components ]

The electrode composite layer may contain other components known as additives capable of being blended with the electrode composite layer of the electrochemical element. Examples of the other component include a wetting agent, a leveling agent, and an electrolyte decomposition inhibitor.

Electrode active material

The electrode active material is a material that performs electron transfer in an electrode of an electrochemical element. In the following, a case where the electrochemical device is a lithium ion secondary battery will be described as an example, but the present invention is not limited to the following example. As an electrode active material for a lithium ion secondary battery, a material capable of inserting and extracting lithium is generally used. In addition, from the viewpoint that the battery capacity is in a practical range, the electrode active material is preferably 90 mass% or more, more preferably 92 mass% or more, and preferably 99.5 mass% or less, more preferably 99 mass% or less, based on 100 mass% of the total mass of the electrode composite layer.

[ Positive electrode active Material ]

Specifically, the positive electrode active material for a lithium ion secondary battery is not particularly limited, and examples thereof include: lithium-containing cobalt oxide (LiCoO) ₂ ) Lithium manganate (LiMn) ₂ O ₄ ) Lithium-containing nickel oxide (LiNiO) ₂ ) Co-Ni-Mn lithium-containing composite oxide (Li (CoMnNi) O) ₂ ) Lithium-containing composite oxide of Ni-Mn-Al, lithium-containing composite oxide of Ni-Co-Al, olivine-type lithium iron phosphate (LiFePO) ₄ ) Olivine lithium manganese phosphate (LiMnPO) ₄ )、Li ₂ MnO ₃ -LiNiO ₂ Is a solid solution made of Li _1+x Mn _2-x O ₄ (0<X<2) Represented lithium excess spinel compound, li [ Ni ] _0.17 Li _0.2 Co _0.07 Mn _0.56 ]O ₂ 、LiNi _0.5 Mn _1.5 O ₄ And the like.

The amount of the positive electrode active material to be mixed and the particle diameter of the positive electrode active material are not particularly limited, and may be the same as those of the positive electrode active material used in the prior art.

[ negative electrode active material ]

Examples of the negative electrode active material for lithium ion secondary batteries include carbon-based negative electrode active materials, metal-based negative electrode active materials, and a negative electrode active material obtained by combining these materials.

The carbon-based negative electrode active material refers to an active material having carbon as a main skeleton capable of intercalating (also referred to as "doping") lithium, and examples thereof include carbonaceous materials and graphite materials.

Examples of the carbonaceous material include easily graphitizable carbon and hardly graphitizable carbon having a structure close to an amorphous structure typified by glassy carbon.

Here, examples of the graphitizable carbon include carbon materials using tar pitch obtained from petroleum or coal as a raw material. When a specific example is mentioned, there may be mentioned: coke, mesophase Carbon Microbeads (MCMB), mesophase pitch-based carbon fibers, thermally decomposed vapor grown carbon fibers, and the like.

Examples of the hardly graphitizable carbon include a phenolic resin sintered body, polyacrylonitrile-based carbon fiber, pseudo-isotropic carbon, furfuryl alcohol resin sintered body (PFA), and the like.

Examples of the graphite material include natural graphite and artificial graphite.

Examples of the artificial graphite include artificial graphite in which carbon containing graphitizable carbon is mainly heat-treated at 2800 ℃ or higher, graphitizable MCMB in which MCMB is heat-treated at 2000 ℃ or higher, graphitized mesophase pitch carbon fibers in which mesophase pitch carbon fibers are heat-treated at 2000 ℃ or higher, and the like.

The metal-based negative electrode active material is an active material containing a metal, and generally means an active material having a structure containing an element capable of intercalating lithium and having a theoretical capacitance per unit mass of 500mAh/g or more when lithium is intercalated. As the metal-based active material, for example, can be used: lithium metal, elemental metals capable of forming lithium alloys (e.g., ag, al, ba, bi, cu, ga, ge, in, ni, P, pb, sb, si, sn, sr, zn, ti, etc.) and alloys thereof, and oxides, sulfides, nitrides, silicides, carbides, phosphides, and the like thereof. Among these, as the metal-based anode active material, an active material containing silicon (silicon-based anode active material) is preferable. This is because the use of the silicon-based negative electrode active material can increase the capacity of the lithium ion secondary battery.

Examples of the silicon-based negative electrode active material include: silicon (Si), alloys containing silicon, siO _x And a composite of a Si-containing material and conductive carbon, which is obtained by coating or compositing a Si-containing material with conductive carbon.

The amount of the negative electrode active material to be mixed and the particle diameter of the negative electrode active material are not particularly limited, and may be the same as those of the negative electrode active material conventionally used.

< conductive Material >

The conductive material is a material that ensures electrical contact between electrode active materials. Further, as the conductive material, it is possible to use: conductive carbon materials such as carbon black (for example, acetylene black, ketjen black (registered trademark), furnace black, and the like), single-walled or multi-walled carbon nanotubes (including stacked cup type carbon nanotubes), carbon nanohorns, vapor grown carbon fibers, ground carbon fibers obtained by sintering and pulverizing polymer fibers, single-layer or multi-layer graphene, and carbon nonwoven fabric sheets obtained by sintering nonwoven fabrics formed from polymer fibers; fibers or foils of various metals, and the like.

These can be used singly or in combination of two or more. Among the above, conductive carbon materials are preferable as the conductive materials in view of excellent chemical stability.

The content of the conductive material in the electrode composite layer is preferably 0.1 mass% or more, preferably 3.0 mass% or less, and more preferably 2.5 mass% or less, based on 100 mass% of the total mass of the electrode composite layer. If the content ratio of the conductive material is not less than the above lower limit, the electrode contact between the electrode active materials can be sufficiently ensured. On the other hand, if the content ratio of the conductive material is not more than the upper limit value, the density of the electrode composite material layer can be maintained satisfactorily, and the electrochemical element can be made sufficiently high in capacity.

(method for manufacturing electrochemical element electrode)

The electrode for an electrochemical element of the present invention is produced, for example, by the following steps: a step of preparing a slurry composition containing an electrode active material and thermally expandable particles, and optionally containing a binder material, a conductive material, and other components (slurry composition preparation step); a step (coating step) of coating a slurry composition on a current collector; and a step (drying step) of drying the slurry composition applied to the current collector to form an electrode composite layer on the current collector.

< procedure for preparing slurry composition >

The above-mentioned components can be prepared by dissolving or dispersing them in a solvent such as an organic solvent. The components may be added together or may be added and mixed in stages as a mixing procedure. Specifically, the slurry composition can be prepared by mixing the above components and the solvent using a mixer such as a ball mill, a sand mill, a bead mill, a pigment disperser, a kneader, an ultrasonic disperser, a homogenizer, a planetary mixer, or a filemix.

< coating Process >

The method of applying the slurry composition to the current collector is not particularly limited, and a known method can be used. Specifically, as the coating method, a doctor blade method, dipping method, reverse roll coating method, direct roll coating method, gravure method, extrusion method, brush coating method, or the like can be used. In this case, the slurry composition may be applied to only one side of the current collector, or may be applied to both sides. The thickness of the slurry film on the current collector before drying after coating can be appropriately set according to the thickness of the electrode composite material layer obtained by drying.

< drying Process >

The method for drying the slurry composition on the current collector is not particularly limited, and known methods can be used, and examples thereof include: drying by hot air, or low humidity air, vacuum drying, and drying by irradiation with infrared rays, electron beams, or the like. By drying the slurry composition on the current collector in this manner, an electrode composite layer can be formed on the current collector, and an electrode for a secondary battery having the current collector and the electrode composite layer can be obtained. In the drying step, the following drying is preferably performed: drying at a higher temperature is performed after drying at a low temperature, with stepwise temperature increase. Specifically, for example, it is preferable to dry at a low temperature of 110℃or less, preferably 95℃or less, more preferably 90℃or less, and then dry at a temperature of 140℃or less, preferably 120℃or less.

Further, as one embodiment for effectively manufacturing the electrode for an electrochemical element of the present invention, it is preferable to perform the following steps in the coating step and the drying step. Namely, the following steps are performed: a step of forming an electrode lower layer by applying a slurry composition for an electrode lower layer on a current collector and drying the slurry composition; and a step of forming an electrode upper layer by applying the electrode upper layer slurry composition to the electrode lower layer and drying the electrode upper layer slurry composition. The electrode upper layer slurry composition and the electrode lower layer slurry composition contain an electrode active material and thermally expandable particles, respectively, and the concentration of the thermally expandable particles in the electrode upper layer slurry composition is higher than that in the electrode lower layer slurry composition. By performing such a step in the electrode manufacturing step, it is possible to effectively create a distribution pattern in which the frequency of occurrence of thermally expandable particles in the surface and the region near the surface of the electrode composite material layer is higher than that in the lower portion of the electrode composite material layer. If the thermally expandable particles are biased to exist on and near the surface in the electrode composite layer, the IV resistance can be further reduced while improving the heat release suppressing performance of the electrochemical element.

After the drying step, the electrode composite layer may be subjected to a pressing treatment using a die press, a roll press, or the like. The adhesion between the electrode composite material layer and the current collector can be improved by the pressure treatment.

(electrochemical element)

The electrochemical element can be provided by using the electrode for electrochemical element of the present invention. The electrochemical element having the electrode for an electrochemical element of the present invention is excellent in heat release inhibition performance.

Examples of the electrochemical element include a secondary battery using the electrode for an electrochemical element of the present invention as a positive electrode. In the following, a 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 following example.

< electrode >

Here, the electrode other than the electrode for an electrochemical element that can be used in an electrochemical element is not particularly limited, and a known electrode used for manufacturing an electrochemical element can be used. Specifically, as an electrode other than the above-described electrode for an electrochemical element, an electrode in which an electrode composite layer is formed on a current collector by a known manufacturing method can be used.

< spacer >

The spacer is not particularly limited, and for example, a spacer described in japanese patent application laid-open No. 2012-204303 can be used. Among these, from the viewpoint that the film thickness of the entire separator can be made thin, and thus the ratio of the electrode active material in the secondary battery can be increased to increase the capacity per unit volume, a microporous film formed of a polyolefin-based (polyethylene, polypropylene, polybutylene, polyvinyl chloride) resin is preferable.

< electrolyte solution >

As the electrolyte solution, an organic electrolyte solution in which a supporting electrolyte is dissolved in an organic solvent is generally used. As the supporting electrolyte of the lithium ion secondary battery, for example, a lithium salt can be used. Examples of the lithium salt include 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. Among them, liPF is preferable because it is easily dissolved in a solvent and shows a high dissociation degree ₆ 、LiClO ₄ 、CF ₃ SO ₃ Li, particularly preferred is LiPF ₆ . The electrolyte may be used alone or in combination of two or more kinds in any ratio. In general, the lithium ion conductivity tends to be higher as a supporting electrolyte having a higher dissociation degree is used, so that the lithium ion conductivity can be adjusted according to the type of the supporting electrolyte.

The organic solvent used in the electrolyte solution is not particularly limited as long as it is an organic solvent capable of dissolving the supporting electrolyte, and for example, it is preferable to use: carbonates such as dimethyl carbonate (DMC), ethylene Carbonate (EC), diethyl carbonate (DEC), propylene Carbonate (PC), butylene Carbonate (BC), and ethylmethyl carbonate (EMC); esters such as ethyl propionate, propyl propionate, gamma-butyrolactone, and methyl formate; ethers such as 1, 2-dimethoxyethane and tetrahydrofuran; sulfolane, dimethyl sulfoxide and other sulfur-containing compounds. In addition, a mixture of these solvents may be used. Among them, carbonates are preferably used because of their high dielectric constant and wide stable potential range, and esters are preferably used, and a mixture of these is more preferably used from the viewpoint of improving electrochemical stability.

The concentration of the electrolyte in the electrolyte solution can be appropriately adjusted, and is preferably 0.5 to 15 mass%, more preferably 2 to 13 mass%, and even more preferably 5 to 10 mass%, for example. In addition, known additives such as vinylene carbonate, fluoroethylene carbonate, methyl ethyl sulfone, and the like may be added to the electrolyte.

< method for producing Secondary Battery >

The secondary battery as an electrochemical element can be manufactured by, for example, stacking a positive electrode and a negative electrode with a separator interposed therebetween, winding the stacked materials in a battery shape, folding the stacked materials as necessary, and placing the stacked materials in a battery container, and injecting an electrolyte into the battery container to seal the stacked materials. In order to prevent pressure rise, overcharge and discharge in the secondary battery, overcurrent prevention elements such as fuses and PTC elements, porous metal mesh, and guide plates may be provided as needed. The shape of the secondary battery may be, for example, any one of coin-shaped, button-shaped, sheet-shaped, cylindrical, square, flat-shaped, and the like.

Examples

The present invention will be specifically described below based on examples, but the present invention is not limited to these examples. In the following description, unless otherwise specified, "%" and "parts" indicating amounts are based on mass.

In addition, unless otherwise specified, in a polymer produced by copolymerizing a plurality of monomers, the proportion of monomer units formed by polymerizing a certain monomer in the above-described polymer generally coincides with the proportion (feed ratio) of the certain monomer in all monomers used in the polymerization of the polymer.

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

< gasification temperature of core >

The gas generating substances used for forming the nuclei in the examples and comparative examples were prepared, and the nuclear component was detected by heating at a heating rate of 10℃per minute from 25℃to 350℃in a thermal extraction GC-MS (PY-2020 ID manufactured by FRONTIER LAB Co., ltd.). In the thermogravimetric analysis using a thermogravimetric analyzer (TG 8110, manufactured by Rigaku corporation), the nuclear component monomer detected by the qualitative analysis was prepared, the mass was measured while being heated from 25 ℃ to 500 ℃ at a heating rate of 10 ℃/min under a nitrogen atmosphere, and the temperature at which the measured mass became 95% of the mass at the time of the start of the measurement (25 ℃) was used as the vaporization temperature of the gas generating substance.

< electrolyte swelling degree of Shell >

The thermally expandable particles produced in examples and comparative examples were immersed in an electrolyte solution at a temperature of 60℃and the thicknesses of the shells before and after observation were observed by an optical microscope (VHX-900, manufactured by Kabushiki Kaisha). 10 particles were randomly drawn for observation and the average thickness calculated. The expansion ratio of the shell was calculated from the following formula based on the thickness of the shell before and after the dipping test.

As the electrolyte, liPF was dissolved at a concentration of 1M in a mixed solvent in which Ethylene Carbonate (EC), ethyl Propionate (EP), and Propyl Propionate (PP) were mixed at EC: EP: pp=3:5:2 (volume ratio at 20 ℃) ₆ And a solution obtained.

Then, the shell thickness before the impregnation test was designated as a, the shell thickness after the impregnation test was designated as B, and the electrolyte swelling degree of the shell was calculated according to the following formula.

Electrolyte swelling degree (%) =b/a×100 (%)

< NMP swelling degree of Shell >

The above electrolyte was replaced with NMP, and the test was performed.

< electrolyte swelling degree of Polymer 1 and Polymer 2 >

The same monomer compositions as the monomer composition 1 and the monomer composition 2 prepared in examples and comparative examples were polymerized under the same polymerization conditions (including additives and the like) as those used in the preparation of the thermally expandable particles, respectively, to prepare aqueous dispersions containing the polymers to be prepared into measurement samples.

Casting the aqueous dispersion containing the polymer prepared as described above on a polytetrafluoroethylene sheet, followed byDrying to obtain the casting film. Cutting 4cm ² The casting film was measured for mass (mass a before dipping) and then immersed in an electrolyte at 60 ℃. The impregnated film was lifted up after 72 hours, and the mass (mass B after impregnation) was measured immediately after wiping with a wiping paper. The swelling degree of the polymer electrolyte was calculated by the following formula, and evaluated based on the following criteria. As the electrolyte, liPF was dissolved at a concentration of 1M in a mixed solvent in which Ethylene Carbonate (EC), ethyl Propionate (EP), and Propyl Propionate (PP) were mixed at EC: EP: pp=3:5:2 (volume ratio at 20 ℃) ₆ And a solution obtained.

Swelling degree (%) =b/a×100 (%)

< NMP swelling degree of Polymer 1 and Polymer 2 >

The same procedure as in the above method for measuring swelling degree of electrolyte was carried out except that the electrolyte was changed to NMP and the impregnation temperature was changed to 45℃to measure swelling degree of NMP of the polymer 1 and the polymer 2.

< Structure, layer thickness of thermally-expansive particles > glass transition temperatures of polymers 1 and 2

The obtained pellets were embedded in an epoxy-based embedding resin, and the pellets were cut with a microtome to expose the cross section. The thickness of each layer was measured by confirming that the shell had a double layer structure from an Atomic Force Microscope (AFM) image obtained in Contact Mode (Contact Mode). The glass transition temperatures of three points of each layer were measured by nanoTA (nanothermal analysis), and the average value was used as the glass transition temperature of each layer. Further, the thickness of each layer and the glass transition temperature of each layer were measured.

< area ratio of Polymer 1 and Polymer 2 in Shell >

In an AFM image obtained by the contact mode of the AFM, the particle diameters of thermally expandable particles (10 particles selected randomly) to be measured were measured. The particle diameter is the diameter of a circumscribed circle containing the target thermally expandable particles. Five points were measured for each thermally expandable particle to be measured, with the particle diameter denoted as a, the thickness of the layer on the side of the core constituting the shell denoted as b, and the diameter of the core denoted as c. The average values thereof were designated A, B, C, respectively.

Using these values A, B, 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 an average value of the area ratios of the polymer 1 and the 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 (hsppip)).

< expansion initiation temperature >

The prepared thermally expandable particles were heated at a temperature rising rate of 10℃per minute using a heating table (FTIR 600 manufactured by Linkam Co., ltd.). The particle diameter of the thermally expandable particles during heating was observed by an optical microscope (VHX-900, manufactured by Kihn Kabushiki Kaisha), and the change in diameter with respect to temperature was observed. The point at which the diameter was changed by 1.3 times or more before heating was defined as the expansion initiation temperature.

< volume average particle diameter >

The anodes produced in examples and comparative examples were subjected to observation cross-section exposure processing using a cross-section polisher (IB-09020 CP, manufactured by japan electronics corporation). Then, a cross-sectional view was taken by FE-SEM (JSM-7800F manufactured by Japanese electronics Co., ltd.). The observation magnification is 1000 times, the irradiation voltage is 10kV, and the irradiation current is 5.0X10 ^-8 The conditions of A are carried out. In the obtained sectional image, the diameter obtained by fitting a circumscribed circle to each thermally expandable particle is defined as the diameter of each thermally expandable particle. Then, the volume of the thermally expandable particles when the spherical shape is assumed is calculated from the obtained diameter. For the thermally expandable particles, the diameter of particles exceeding 50% of the cumulative volume when the volume is accumulated from the smaller diameter side of each particle was taken as the heat in the observation cross sectionVolume average particle diameter D50 of the expandable particles. The above observation was performed under 5 fields of view, and the average value of the obtained volume average particle diameter D50 was used as the volume average particle diameter D50 of the thermally expandable particles. The same measurement and calculation were also performed for the positive electrode active material, to obtain a volume average particle diameter D50.

< particle diameter ratio of thermally-expansive particles to positive electrode active Material >

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 were used to calculate the particle diameter ratio (times) from the following formula.

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

< diameter of exposed surface of electrode composite layer of thermally-expansive particles >

The surface of the electrode composite layer of the positive electrode produced in examples and comparative examples was subjected to observation cross-section exposure processing using a cross-section polisher (IB-09020 CP, manufactured by japan electronics corporation). Then, a cross-sectional view was taken by FE-SEM (JSM-7800F manufactured by Japanese electronics Co., ltd.). Setting the observation magnification to 500 times, the irradiation voltage to 10kV and the irradiation current to 5.0X10 ^- ⁸ And A, performing cross-sectional observation to obtain a surface image of the electrode composite material layer. In the obtained surface image, each thermally expandable particle was fitted with a circumscribed circle, and the diameter thereof was defined as the exposure diameter of the thermally expandable particle. Then, the thermally expandable particles having an exposure diameter of 0.5 to 5.0 times the volume average particle diameter D50 of the positive electrode active material are defined as exposure particles a.

< exposed area ratio of surface of electrode composite layer of thermally-expansive particles >

In the surface image obtained above, the areas of the exposed particles a were summed up, and the occupied area ratio of the exposed particles a on the surface of the electrode composite layer was calculated according to the following formula.

Occupancy area (%) = (total area of exposed particles a/area of surface image) ×100 of exposed particles a

The above calculation was performed on the 5-field images, and the average value was used as the occupied area ratio of the exposed particles a.

< number Density of exposed particles A >

In the surface image obtained as described 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 (number/mm of exposed particles A ² ) Number of exposed particles a/machine product of surface image (mm) ² )

The above calculation was performed on the 5-field images, and the average value was used as the number density of the exposed particles a.

< IV resistance measurement >

The lithium ion secondary batteries as electrochemical elements fabricated in examples and comparative examples were injected with an electrolyte solution, and then allowed to stand at 25℃for 5 hours. Then, the battery was charged to a cell voltage of 3.65V by a constant current method at 25℃and 0.2℃and then aged at 60℃for 12 hours. Then, the cell voltage was discharged to 3.00V by a constant current method at 25℃and 0.2 ℃. Then, CC-CV charging was performed by a constant current method of 0.2C (the upper limit cell voltage was 4.35V), and CC discharging was performed by a constant current method of 0.2C to 3.00V. This charge and discharge at 0.2C was repeated 3 times. Then, in an environment of 25 ℃, an operation of charging was performed at 0.2C so that the State of Charge (SOC; state of Charge) became 50%. Standing for 600 seconds. The 600 th second voltage was designated as V ₀ . Then, at 0.5C (=i _0.5 ) Is discharged for 10 seconds by the constant current method, and the voltage of 10 th second is expressed as V _0.5 . Then, the amount of electricity just discharged was charged by a constant current method of 0.2C. Subsequently, 1.0C (=i) _1.0 ) Is discharged for 10 seconds by the constant current method, and the voltage of 10 th second is expressed as V _1.0 . Then, the amount of electricity just discharged was charged by a constant current method of 0.2C. Subsequently, 1.5C (=i) _1.5 ) Is discharged for 10 seconds by the constant current method, and the voltage of 10 th second is expressed as V _1.5 . Will (I) _0.5 ,V _0.5 ),(I _1.0 ,V _1.0 ),(I _1.5 ,V _1.5 ) The slope b of the regression line was obtained by the following equation and plotted on the XY chart, and was designated as DCR (direct current resistance).

[ mathematics 1]

The relative values of the DCR of each example were calculated when the DCR of example 1 was designated as 100, and evaluation was performed on the basis of the following criteria. The smaller the DCR relative value, the smaller the IV resistance of the lithium ion secondary battery.

A: the DCR relative value is below 103

B: a DCR relative value of greater than 103 and less than 105

C: a DCR relative value of greater than 105 and less than 110

D: DCR relative value is greater than 110

< suppression of heat emission at internal short-circuiting (forced internal short-circuiting test) >

The lithium ion secondary batteries as electrochemical elements fabricated in examples and comparative examples were injected with an electrolyte solution, and then allowed to stand at 25℃for 5 hours. Then, the battery was charged to a cell voltage of 3.65V by a constant current method at 25℃and 0.2℃and then aged at 60℃for 12 hours. Then, the cell voltage was discharged to 3.00V by a constant current method at 25℃and 0.2 ℃. Then, CC-CV charging was performed by a constant current method of 0.2C (the upper limit cell voltage was 4.35V), and CC discharging was performed by a constant current method of 0.2C to 3.00V. This charge and discharge at 0.2C was repeated 3 times. Then, the battery was charged to 4.35V (termination condition: 0.02C) by constant voltage and constant current (CC-CV) at a charging rate of 0.2C under an atmosphere of 25 ℃. Next, a nail made of iron having a diameter of 3mm and a length of 10cm was inserted at a speed of 5 m/min near the center of the lithium ion secondary battery, and the nail was forcibly short-circuited. The forced short circuit was performed on 5 lithium ion secondary batteries (test pieces) fabricated in the same manner, and the number of test pieces having no rupture or ignition was evaluated based on the following criteria. The larger the number of test pieces that did not generate cracks and did not catch fire, the more excellent the heat release inhibition performance of the lithium ion secondary battery at the time of internal short circuit was shown.

A: the number of test bodies which did not generate a crack nor catch fire was 4 or 5

B: the number of test bodies which did not generate cracks nor catch fire was 3

C: the number of test bodies which did not generate cracks nor catch fire was 2

D: the number of test bodies which did not generate a crack nor catch fire was 1 or 0

Example 1

< preparation of thermally-expansive particles >

[ preparation of monomer composition 1 ]

A monomer composition 1 was prepared by mixing 60.0 parts of acrylonitrile and 5.5 parts of methacrylonitrile as monomers having nitrile groups, 0.4 parts of styrene as an aromatic vinyl monomer, 0.6 parts of butyl acrylate as a (meth) acrylate monomer, and 0.2 parts of ethylene glycol dimethacrylate (Light Ester EG, kyowa chemical Co., ltd.) as a crosslinking monomer.

[ preparation of monomer composition 2 ]

A monomer composition 2 was prepared by mixing 34.1 parts of styrene as an aromatic vinyl monomer, 27.3 parts of 2-ethylhexyl acrylate as a (meth) acrylate 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.

[ preparation of colloidal Dispersion ]

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

Suspension polymerization method

The thermally expandable particles were produced 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 above colloidal dispersion containing magnesium hydroxide, followed by stirring, and then 2.0 parts of t-butyl peroxy-2-ethylhexanoate (manufactured by Nikko corporation, "PERBUTYL O") as a polymerization initiator was added and mixed to obtain a mixed solution. The obtained mixed solution was stirred with high shear at a rotation speed of 15000rpm for 1 minute using a pipeline type emulsion disperser (manufactured by pacific corporation, "CAVITRON"), and a dispersion liquid containing a gas generating substance and a monomer composition 1 was obtained in a colloidal dispersion liquid containing magnesium hydroxide. The stirring temperature is controlled to be 5 to 10 ℃.

The colloidal dispersion containing the above gas generating substance and monomer composition 1, and containing magnesium hydroxide was charged into a 5MPa pressure-resistant vessel having a stirrer, and reacted at 70 ℃ for 8 hours. The pressure at the time of the reaction was 0.5MPa.

To the thus obtained aqueous dispersion containing a polymer, monomer composition 2 and 0.1 part of 2,2' -azobis [ 2-methyl-N- (2-hydroxyethyl) propionamide ] (trade name: VA-086, water-soluble initiator, manufactured by Wako pure chemical industries, ltd.) dissolved in 10 parts of ion-exchanged water as a polymerization initiator were added, and reacted at 90℃for 5 hours. After the polymerization reaction was continued, a water-cooling termination reaction was performed to obtain an aqueous dispersion of thermally expandable particles comprising a core containing a gas generating substance and a shell (comprising an inner layer formed of polymer 1 and an outer layer formed of polymer 2).

Further, while stirring the above aqueous dispersion containing thermally expandable particles, sulfuric acid was added dropwise at room temperature (25 ℃) and the resulting mixture was acid-washed until the pH became 6.5 or less. Next, filtration and separation were performed, and 500 parts of ion-exchanged water was added to the obtained solid component, and the mixture was slurried again, and water washing treatment (washing, filtration, and dehydration) was repeated a plurality of times. Then, the solid component obtained was separated by filtration, and dried at 35℃for 48 hours in a vessel of a dryer to obtain dried thermally expandable particles.

As a result of the analysis of the obtained thermally expandable particles, the vaporization temperature of the core was 28 ℃.

TABLE 1

< preparation of adhesive Material >

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 0.25 part of t-dodecyl mercaptan as a chain transfer agent were successively added, and the inside of the flask was subjected to nitrogen substitution. Then, 65 parts of 1, 3-butadiene as an aliphatic conjugated diene monomer was introduced thereinto, and 0.25 part of ammonium persulfate was added thereto to carry out a polymerization reaction at a reaction temperature of 40 ℃. Then, a polymer containing acrylonitrile, methacrylic acid and 1, 3-butadiene was obtained. The polymerization conversion was 85%.

400mL (48 g) of a solution having a total solid content concentration of 12% was added to the polymer obtained by using water, the solution was put into a 1L autoclave equipped with a stirrer, oxygen was dissolved in the solution was removed by introducing nitrogen for 10 minutes, and then 75mg of palladium acetate as a catalyst for hydrogenation was dissolved in 180mL of ion-exchanged water to which nitric acid was added in an amount of 4 times by mol relative to Pd, and the solution was added to the autoclave. After the hydrogen was replaced 2 times in the system, the content of the autoclave was heated to 50℃under pressure of 3MPa with hydrogen, and hydrogenation was carried out for 6 hours (hydrogenation in the first stage).

Subsequently, the autoclave was returned to atmospheric pressure, and 25mg of palladium acetate as a catalyst for hydrogenation reaction was further dissolved in 60mL of water to which 4-fold molar amount of nitric acid was added to Pd, and the solution was added to the autoclave. After the hydrogen was replaced 2 times in the system, the content of the autoclave was heated to 50℃under pressure of 3MPa with hydrogen, and hydrogenation reaction (hydrogenation reaction in the second stage) was carried out for 6 hours to obtain an aqueous dispersion of the binder material. To the aqueous dispersion of the obtained adhesive material, NMP was added in an appropriate amount to obtain a mixture. Then, distillation under reduced pressure was carried out at 90℃to remove water and an excess of NMP from the mixture, to obtain an NMP solution (solid content concentration: 8%) of the binder material.

< preparation of adhesive composition >

The binder composition was prepared by mixing 10 parts by mass of the binder material and 90 parts by mass of the thermally expandable particles in terms of solid content, and adding NMP thereto to adjust the solid content to 30%.

< preparation of slurry composition for Positive electrode underlayer >

94.5 parts of lithium cobaltate (volume average particle diameter D50:12 μm) as a positive electrode active material, 2.0 parts of carbon black (trade name "Li-100" manufactured by electric Co., ltd.) as a conductive material in terms of a solid content equivalent, 2.0 parts of PVDF (Solef 5130) and 1.5 parts of the binder composition in terms of a solid content equivalent were charged into a planetary mixer, and then, NMP was slowly added thereto, and the mixture was stirred and mixed at a temperature of 25.+ -. 3 ℃ and a rotational speed of 60rpm, and a viscosity of 3500 mPa.S was set at 60rpm (rotor M4) and 25.+ -. 3 ℃ in terms of a type B viscosimeter, thereby obtaining a solid content concentration of 68% to obtain a slurry composition for a positive electrode lower layer.

< preparation of slurry composition for Positive electrode upper layer >

93.5 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (trade name "Li-100" manufactured by electric Co., ltd.) as a conductive material in terms of a solid content amount, 2.0 parts of PVDF (Solef 5130) in terms of a solid content amount, and 2.5 parts of the binder composition were mixed in a planetary mixer, NMP was further slowly added, and the mixture was stirred and mixed at a temperature of 25.+ -. 3 ℃ at a rotational speed of 60rpm, and a viscosity of 3600 mPa.S was set at 60rpm (rotor M4) and 25.+ -. 3 ℃ by a type B viscometer, whereby a solid content concentration of 69% was obtained.

< production of Positive electrode >

The paste composition for the lower layer of the positive electrode was applied in an amount of 10.+ -. 0.5mg/cm by using a corner-roll coater ² Is coated on an aluminum foil having a thickness of 20 μm as a current collector. Further, the slurry composition for the lower layer of the positive electrode on the aluminum foil was dried at a rate of 0.5 m/min by carrying the slurry composition for the lower layer of the positive electrode in an oven at a temperature of 90℃for 2 minutes, and further in an oven at a temperature of 120℃for 2 minutes, to obtain a positive electrode having a positive electrode composite material layer (lower layer) formed on a current collectorRaw material (lower layer). Next, the slurry composition for positive electrode upper layer obtained above was applied in an amount of 20.+ -. 0.5mg/cm by using a corner-roll coater ² Is coated on the positive electrode raw material (lower layer). Further, the slurry composition for the positive electrode upper layer on the aluminum foil was dried at a rate of 0.5 m/min by carrying the slurry composition in an oven at a temperature of 90℃for 2 minutes, and further in an oven at a temperature of 120℃for 2 minutes, to obtain a positive electrode material having a positive electrode composite material layer formed on a current collector. Then, the positive electrode composite material layer side of the prepared positive electrode raw material was rolled under the condition of a load of 14t (ton) at a temperature of 25+ -3 ℃ to obtain a positive electrode composite material layer having a density of 3.80g/cm ³ Is a positive electrode of (a). 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 to the positive electrode active material was calculated. The results are shown in Table 2.

< production of negative electrode >

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 carboxyl group-containing monomer, 1 part of 2-hydroxyethyl acrylate as a hydroxyl group-containing monomer, 0.3 part of t-dodecyl mercaptan as a molecular weight regulator, 5 parts of sodium dodecyl benzene sulfonate as an emulsifier, 150 parts of ion exchange water as a solvent, and 1 part of potassium persulfate as a polymerization initiator were charged into a pressure-resistant vessel of 5MPa equipped with a stirrer, and after sufficiently stirring, polymerization was initiated by heating to a temperature of 55 ℃. The reaction was terminated by cooling at the point when the monomer consumption reached 95.0%. To the thus obtained aqueous dispersion containing a polymer, a 5% aqueous sodium hydroxide solution was added to adjust the pH to 8. Then, unreacted monomers were removed by distillation under reduced pressure with heating. Then, the mixture was cooled to 30℃or lower to obtain an aqueous dispersion containing a binder for negative electrode (binder composition for negative electrode).

In a planetary mixer, 48.75 parts of artificial graphite (theoretical capacity: 360 mAh/g) and 48.75 parts of natural graphite (theoretical capacity: 360 mAh/g) were added as a negative electrode active material, and then 1 part of carboxymethyl cellulose in terms of a solid content equivalent was added as a thickener. Further, the mixture was diluted with ion-exchanged water to a solid content of 60%, and then kneaded at a rotation speed of 45rpm for 60 minutes. Then, 1.5 parts by weight of the binder composition for negative electrode obtained above was added in terms of a solid content, and the mixture was kneaded at a rotational speed of 40rpm for 40 minutes. Then, ion-exchanged water was added so that the viscosity became 3000.+ -.500 mPas (type B viscometer, measured at 25 ℃ C., 60 rpm), whereby a slurry composition for negative electrode was prepared.

The above-mentioned slurry composition for negative electrode was applied in an amount of 11.+ -. 0.5mg/cm using a corner-roll coater ² Is coated on the surface of a copper foil having a thickness of 15 μm as a current collector. Then, the copper foil coated with the paste composition for negative electrode was transported in an oven at a temperature of 80 ℃ for 2 minutes and further in an oven at a temperature of 110 ℃ for 2 minutes at a speed of 400 mm/min, whereby the paste composition for negative electrode on the copper foil was dried to obtain a negative electrode raw material having a negative electrode composite material layer formed on a current collector. Then, the negative electrode composite material layer side of the prepared negative electrode raw material was rolled under the conditions of a line pressure of 11t (ton) at a temperature of 25.+ -. 3 ℃ to obtain a negative electrode composite material layer having a density of 1.60g/cm ³ Is a negative electrode of (a).

< preparation of spacer >

A single-layer polypropylene spacer (trade name "# 2500") was prepared.

< production of lithium ion Secondary Battery >

Using the above negative electrode, positive electrode and separator, a laminated battery cell (initial design discharge capacity: 3 Ah) was produced, and the laminated battery cell was placed in an aluminum packaging material and vacuum-dried at 60 ℃ for 10 hours. Then, the following solutions were filled as electrolytes: in a mixed solvent in which Ethylene Carbonate (EC), ethyl Propionate (EP) and Propyl Propionate (PP) were mixed in a ratio of EC: EP: PP=3:5:2 (volume ratio at 20 ℃ C.) LiPF was dissolved at a concentration of 1M ₆ A solution of an additive and vinylene carbonate was blended at a ratio of 2% by volume (solvent ratio). Furthermore, in order toSealing the opening of the aluminum packaging material, and performing heat sealing at 150 ℃ to seal the aluminum packaging material to manufacture the lithium ion secondary battery. The obtained lithium ion battery was evaluated for IV resistance and heat release inhibition performance at the time of internal short circuit. The results are shown in Table 2.

Example 2

The production of the positive electrode was performed in the same manner as in example 1, except that the following operations were performed.

< preparation of slurry composition for Positive electrode composite layer >

94.0 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (trade name "Li-100" manufactured by Kagaku Co., ltd.) as a conductive material in terms of a solid content amount, 2.0 parts of PVDF (Solef 5130) in terms of a solid content amount, and 2.0 parts of the binder composition were mixed in a planetary mixer, NMP was further slowly added, and the mixture was stirred and mixed at a temperature of 25.+ -. 3 ℃ at a rotational speed of 60rpm, and a viscosity of 2200 mPa.S was set at 60rpm (rotor M4) and 25.+ -. 3 ℃ by a type B viscometer, whereby a solid content concentration was 66% to obtain a positive electrode slurry composition.

< production of Positive electrode >

The positive electrode slurry composition was applied to a coating amount of 20.+ -. 0.5mg/cm using a corner-roll coater ² Is coated on an aluminum foil having a thickness of 20 μm as a current collector. Further, the positive electrode slurry composition on the aluminum foil was dried (drying condition) at a rate of 0.5 m/min by carrying the positive electrode slurry composition in an oven at a temperature of 110 ℃ for 2 minutes, and further in an oven at a temperature of 120 ℃ for 2 minutes, to obtain a positive electrode material having a positive electrode composite material layer formed on a current collector. Then, the positive electrode composite material layer side of the prepared positive electrode raw material was rolled under the condition of a load of 14t (ton) at a temperature of 25+ -3 ℃ to obtain a positive electrode composite material layer having a density of 3.80g/cm ³ Is a positive electrode of (a). 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

< preparation of slurry composition for Positive electrode underlayer >

< preparation of slurry composition for Positive electrode upper layer >

94.5 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (trade name "Li-100" manufactured by electric Co., ltd.) as a conductive material in terms of a solid content amount, 2.0 parts of PVDF (Solef 5130) in terms of a solid content amount, and 1.5 parts of the binder composition were mixed in a planetary mixer, NMP was further slowly added, and the mixture was stirred and mixed at a temperature of 25.+ -. 3 ℃ at a rotational speed of 60rpm, and a viscosity of 3400 mPa.S was set at 60rpm (rotor M4) and 25.+ -. 3 ℃ by a type B viscometer, whereby a solid content concentration was 68% to obtain a slurry composition for a positive electrode upper layer.

< production of Positive electrode >

The slurry composition for the lower layer of the positive electrode obtained above was applied in an amount of 10.+ -. 0.5mg/cm using a corner-roll coater ² Is coated on an aluminum foil having a thickness of 20 μm as a current collector. Further, the slurry composition for the lower layer of the positive electrode on the aluminum foil was dried at a rate of 0.5 m/min by carrying the slurry composition for the lower layer of the positive electrode in an oven at a temperature of 90℃for 2 minutes, and further in an oven at a temperature of 120℃for 2 minutes, to obtain a positive electrode material (lower layer) having a positive electrode composite material layer (lower layer) formed on a current collector. Next, the slurry composition for positive electrode upper layer obtained above was applied in an amount of 20.+ -. 0.5mg/cm by using a corner-roll coater ² Is coated on the positive electrode raw material (lower layer). Further, the slurry composition for the positive electrode upper layer on the aluminum foil was dried at a rate of 0.5 m/min by carrying the slurry composition in an oven at a temperature of 90℃for 2 minutes, and further in an oven at a temperature of 120℃for 2 minutes, to obtain a positive electrode material having a positive electrode composite material layer formed on a current collector. Then, the positive electrode composite material layer side of the prepared positive electrode raw material was rolled under the condition of a load of 14t (ton) at a temperature of 25+ -3 ℃ to obtain a positive electrode composite material layer having a density of 3.80g/cm ³ Is a positive electrode of (a). 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

< preparation of slurry composition for Positive electrode underlayer >

To a planetary mixer, 95.5 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (trade name "Li-100" manufactured by electric Co., ltd.) as a conductive material in terms of a solid content equivalent, 2.0 parts of PVDF (Solef 5130) in terms of a solid content equivalent, and 0.5 parts of the binder composition were added and mixed, and further NMP was slowly added, and the mixture was stirred and mixed at a temperature of 25.+ -. 3 ℃ at a rotational speed of 60rpm, and a viscosity of 3600 mPa.S was set at 60rpm (rotor M4) and 25.+ -. 3 ℃ by a type B viscometer, whereby a solid content concentration was 68%, to obtain a slurry composition for a positive electrode lower layer.

< preparation of slurry composition for Positive electrode upper layer >

To a planetary mixer, 92.5 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (trade name "Li-100" manufactured by electric Co., ltd.) as a conductive material in terms of a solid content equivalent, 2.0 parts of PVDF (Solef 5130) in terms of a solid content equivalent, and 3.5 parts of the binder composition were added and mixed, and further NMP was slowly added, and the mixture was stirred and mixed at a temperature of 25.+ -. 3 ℃ at a rotational speed of 60rpm, and a viscosity of 3400 mPa.S was set at 60rpm (rotor M4) and 25.+ -. 3 ℃ by a type B viscometer, whereby a solid content concentration of 69% was obtained.

< production of Positive electrode >

The slurry composition for the lower layer of the positive electrode obtained above was applied in an amount of 10.+ -. 0.5mg/cm using a corner-roll coater ² Is coated on an aluminum foil having a thickness of 20 μm as a current collector. Further, the slurry composition for the lower layer for the positive electrode on the aluminum foil was dried at a rate of 0.5 m/min in an oven at a temperature of 90 ℃ for 2 minutes, and further in an oven at a temperature of 120 ℃ for 2 minutes, to obtain a positive electrode material (lower layer) having a positive electrode composite material layer (lower layer) formed on a current collector. Next, the slurry composition for positive electrode upper layer obtained above was applied in an amount of 20.+ -. 0.5mg/cm by using a corner-roll coater ² Is coated on the positive electrode raw material (lower layer). Further, the slurry composition for the positive electrode upper layer on the aluminum foil was dried at a rate of 0.5 m/min by carrying the slurry composition in an oven at a temperature of 90℃for 2 minutes, and further in an oven at a temperature of 120℃for 2 minutes, to obtain a positive electrode material having a positive electrode composite material layer formed on a current collector. Then, the positive electrode composite material layer side of the prepared positive electrode raw material was rolled under the condition of a load of 14t (ton) at a temperature of 25+ -3 ℃ to obtain a positive electrode composite material layer having a density of 3.80g/cm ³ Is a positive electrode of (a). 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

< preparation of slurry composition for Positive electrode underlayer >

To a planetary mixer, 95.8 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (trade name "Li-100" manufactured by electric Co., ltd.) as a conductive material in terms of a solid content equivalent, 2.0 parts of PVDF (Solef 5130) in terms of a solid content equivalent, and 0.2 parts of the binder composition were added and mixed, and further NMP was slowly added, and the mixture was stirred and mixed at a temperature of 25.+ -. 3 ℃ at a rotation speed of 60rpm, and a viscosity of 3800 mPa.S was set at 60rpm (rotor M4) and 25.+ -. 3 ℃ by a type B viscometer, whereby a solid content concentration was 67%, to obtain a slurry composition for a positive electrode lower layer.

< preparation of slurry composition for Positive electrode upper layer >

To a planetary mixer, 92.2 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (trade name "Li-100" manufactured by electric Co., ltd.) as a conductive material in terms of a solid content equivalent, 2.0 parts of PVDF (Solef 5130) in terms of a solid content equivalent, and 3.8 parts of the binder composition were added and mixed, and further NMP was slowly added, and the mixture was stirred and mixed at a temperature of 25.+ -. 3 ℃ at a rotational speed of 60rpm, and a viscosity of 3500 mPa.S was set at 60rpm (rotor M4) and 25.+ -. 3 ℃ by a type B viscometer, whereby a solid content concentration was 69%, to obtain a positive electrode slurry composition.

< production of Positive electrode >

Example 6

The procedure of example 1 was repeated except that lithium cobaltates having different volume average particle diameters D50 (25 μm) were used as the positive electrode active material, and the following operations were performed in the production of the positive electrode.

< preparation of slurry composition for Positive electrode underlayer >

94.5 parts of lithium cobaltate (volume average particle diameter D50:25 μm) as a positive electrode active material, 2.0 parts of carbon black (trade name "Li-100" manufactured by electric Co., ltd.) as a conductive material in terms of a solid content equivalent, 2.0 parts of PVDF (Solef 5130) and 1.5 parts of the binder composition in terms of a solid content equivalent were charged into a planetary mixer, and then, NMP was slowly added thereto, and the mixture was stirred and mixed at a temperature of 25.+ -. 3 ℃ and a rotational speed of 60rpm, and a viscosity of 3600 mPa.S was set at 60rpm (rotor M4) and 25.+ -. 3 ℃ in terms of a B-type viscosimeter, thereby obtaining a solid content concentration of 69%.

< preparation of slurry composition for Positive electrode upper layer >

93.5 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (trade name "Li-100" manufactured by electric Co., ltd.) as a conductive material in terms of a solid content amount, 2.0 parts of PVDF (Solef 5130) in terms of a solid content amount, and 2.5 parts of the binder composition were mixed in a planetary mixer, NMP was further slowly added, and the mixture was stirred and mixed at a temperature of 25.+ -. 3 ℃ at a rotational speed of 60rpm, and a viscosity of 3600 mPa.S was set at 60rpm (rotor M4) and 25.+ -. 3 ℃ by a type B viscometer, whereby a solid content concentration of 69% was obtained. The results are shown in Table 2.

Example 7

The same procedure as in example 1 was carried out except that commercially available heat-expandable particles (manufactured by Sorbon fat and oil Co., ltd., F260D) were used as the heat-expandable particles, lithium cobaltates having different volume average particle diameters D50 (7 μm) were used as the positive electrode active material, and the following procedure was carried out at the time of producing the positive electrode.

< preparation of slurry composition for Positive electrode underlayer >

94.5 parts of lithium cobaltate (volume average particle diameter D50:7 μm) as a positive electrode active material, 2.0 parts of carbon black (trade name "Li-100" manufactured by electric Co., ltd.) as a conductive material in terms of a solid content equivalent, 2.0 parts of PVDF (Solef 5130) and 1.5 parts of F260D manufactured by Sonblack oil pharmaceutical Co., ltd.) in terms of a solid content equivalent were charged into a planetary mixer, and then NMP was slowly added thereto, and stirred and mixed at a temperature of 25.+ -. 3 ℃ and a rotational speed of 60rpm, with a B-type viscosimeter, at 60rpm (rotor M4) and 25.+ -. 3 ℃ to give a viscosity of 3200 mPa.S, and a solid content concentration was 68%, to obtain a positive electrode lower slurry composition.

< preparation of slurry composition for Positive electrode upper layer >

93.5 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (trade name "Li-100" manufactured by Kagaku Co., ltd.) as a conductive material in terms of a solid content, 2.0 parts of PVDF (Solef 5130) in terms of a solid content and 2.5 parts of Songshen oil pharmaceutical Co., ltd were charged into a planetary mixer, and then mixed with each other, and further, NMP was slowly added, and stirred and mixed at a temperature of 25.+ -. 3 ℃ and a rotational speed of 60rpm, and a viscosity of 3400 mPa.S was set at 60rpm (rotor M4) and 25.+ -. 3 ℃ by a type B viscometer, so that the solid content concentration was 69%, to obtain a slurry composition for a positive electrode layer. The results are shown in Table 2.

Comparative example 1

The drying conditions at the time of manufacturing the positive electrode were changed to: the same operations, evaluations and measurements as in example 2 were carried out except that the conditions of slightly lower drying temperature in the first stage were carried out at a rate of 0.5 m/min in an oven at a temperature of 90℃for 2 minutes and further in an oven at a temperature of 120℃for two minutes. The results are shown in Table 2.

Comparative example 2

The same operations, evaluations and measurements as in example 2 were carried out except that the solid content concentration was set to 68% (viscosity: 3700mpa·s) in the preparation of the positive electrode composite material layer slurry composition. The results are shown in Table 2.

Comparative example 3

The procedure of example 1 was repeated except that the following procedure was performed in the production of the positive electrode. The results are shown in Table 2.

< preparation of slurry composition for Positive electrode underlayer >

< preparation of slurry composition for Positive electrode upper layer >

To a planetary mixer, 95.5 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (trade name "Li-100" manufactured by electric Co., ltd.) as a conductive material in terms of a solid content equivalent, 2.0 parts of PVDF (Solef 5130) in terms of a solid content equivalent, and 0.5 parts of the binder composition were added and mixed, and further NMP was slowly added, and the mixture was stirred and mixed at a temperature of 25.+ -. 3 ℃ at a rotational speed of 60rpm, and a viscosity of 3600 mPa.S was set at 60rpm (rotor M4) and 25.+ -. 3 ℃ by a type B viscometer, whereby a solid content concentration was 68%, to obtain a slurry composition for a positive electrode upper layer.

Comparative example 4

< preparation of slurry composition for Positive electrode underlayer >

To a planetary mixer, 96.0 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (trade name "Li-100" manufactured by electric Co., ltd.) as a conductive material in terms of a solid content equivalent, and 2.0 parts of PVDF (Solef 5130) were charged and mixed, and NMP was further slowly added, and the mixture was stirred and mixed at a temperature of 25.+ -. 3 ℃ and a rotational speed of 60rpm, and a viscosity of 3500 mPa.S was set at 60rpm (rotor M4) and 25.+ -. 3 ℃ by a type B viscometer to give a solid content concentration of 67%, to obtain a slurry composition for a positive electrode underlayer.

< preparation of slurry composition for Positive electrode upper layer >

To a planetary mixer, 92.0 parts of lithium cobaltate as a positive electrode active material, 2.0 parts of carbon black (trade name "Li-100" manufactured by electric Co., ltd.) as a conductive material in terms of a solid content equivalent, 2.0 parts of PVDF (Solef 5130) in terms of a solid content equivalent, and 4.0 parts of the binder composition were added and mixed, and further NMP was slowly added, and the mixture was stirred and mixed at a temperature of 25.+ -. 3 ℃ at a rotational speed of 60rpm, and a viscosity of 3600 mPa.S was set at 60rpm (rotor M4) and 25.+ -. 3 ℃ by a type B viscometer, whereby a solid content concentration of 69% was obtained.

In addition, in Table 2,

"D50 particle diameter" means a volume average particle diameter D50, and "HNBR" means a hydrogenated nitrile rubber.

TABLE 2

As is clear from table 2, in examples 1 to 7 using the electrode composite material layers having the structure in which the thermally expandable particles having an expansion initiation temperature of 400 ℃ or lower were exposed to the surface at a predetermined ratio, it was possible to provide electrochemical elements having a balance between the heat release suppressing performance and the IV resistance lowering performance. On the other hand, it was found that these properties could not be improved simultaneously in comparative examples 1 to 3 in which the exposure of the thermally expandable particles was small and in comparative example 4 in which the exposure was excessive.

Industrial applicability

Claims

1. An electrode for an electrochemical element having a current collector and an electrode composite layer,

the electrode composite layer contains at least an electrode active material and thermally expandable particles having an expansion initiation temperature of 400 ℃ or lower,

when thermally expandable particles having an exposure diameter of 0.5 to 5.0 times the volume average particle diameter D50 of the electrode active material, which are present on the surface of the electrode composite material layer, are used as the exposure particles A, the occupation area ratio of the exposure particles A on the surface of the electrode composite material layer is 0.5 to 20%.

2. The electrode for electrochemical element according to claim 1, 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.

3. The electrode for electrochemical element according to claim 1, wherein the number density of the exposed particles a on the surface of the electrode composite layer is 10 pieces/mm ² Above 300 pieces/mm ² The following is given.

4. The electrode for electrochemical element according to claim 1, wherein the electrode composite layer further comprises a binder material,

the adhesive material is selected from carboxylic acid group, hydroxyl group, nitrile group, amino group, epoxy group,And (c) a polymer of at least one functional group of an oxazoline group, a sulfonic acid group, an ester group, and an amide group.

5. A method for producing an electrode for an electrochemical element according to any one of claims 1 to 4, comprising the steps of:

a step of forming an electrode underlayer by applying and drying a slurry composition for an electrode underlayer on a current collector, and

a step of forming an electrode upper layer by applying the electrode upper layer slurry composition onto the electrode lower layer and drying the electrode upper layer slurry composition,

the electrode upper layer slurry composition and the electrode lower layer slurry composition contain an electrode active material and thermally expandable particles, respectively, and the concentration of the thermally expandable particles in the electrode upper layer slurry composition is higher than that in the electrode lower layer slurry composition.