JP2024064853A - Composition for secondary battery negative electrode, negative electrode mixture layer, negative electrode for secondary battery, and lithium ion secondary battery - Google Patents

Abstract

[Problem] To provide a composition for a secondary battery negative electrode that can be used to produce a lithium ion secondary battery with excellent safety. [Solution] The composition for a secondary battery negative electrode disclosed herein contains a thermally expandable microcapsule (A) and polymer particles (B). The expansion start temperature of the thermally expandable microcapsule (A) is 100°C or higher. The thermally expandable microcapsule (A) contains a non-flammable gas generating material inside. The softening point of the polymer particles (B) is 70°C to 150°C. [Selected Figure] None

Description

The present disclosure relates to a composition for a secondary battery negative electrode, a negative electrode composite layer, a secondary battery negative electrode, and a lithium ion secondary battery.

In recent years, lithium-ion secondary batteries have been widely used as power sources for electronic devices, electric vehicles, and electrical storage. Although lithium-ion secondary batteries have the advantage of having a high energy density, they require sufficient safety measures because they use lithium metal and lithium ions.

Patent Document 1 discloses a binder composition for secondary batteries that can provide an electrode mixture layer that can improve the safety of lithium-ion secondary batteries. The composition disclosed in Patent Document 1 contains heat-sensitive gas expandable particles (hereinafter referred to as "microcapsules") and a binding resin, and the volume change of a cast film formed from the composition is within a specific range. The microcapsules expand due to gas when the temperature reaches or exceeds a predetermined temperature. The microcapsules have a core-shell structure that contains a shell material and a core material. A hydrocarbon having a boiling point of 10°C to 150°C is used as the core material.
The composition disclosed in Patent Document 1 is used in an electrode. The electrode comprises a current collector and an electrode mixture layer formed on the current collector. The electrode mixture layer contains the composition of Patent Document 1 and an electrode active material.

International Publication No. 2015/133423

However, if abnormal heat generation (e.g., 120°C to 250°C) occurs in a lithium ion secondary battery equipped with an electrode using the composition disclosed in Patent Document 1, the microcapsules may burst, causing leakage of hydrocarbon gas resulting from the hydrocarbon of the core material. Hydrocarbon gas is generally a flammable gas. In other words, if abnormal heat generation occurs in a lithium ion secondary battery, the leaked hydrocarbon gas may burn, causing the lithium ion secondary battery to ignite. Therefore, there is a demand for a composition that can be used to create a lithium ion secondary battery that is safer than conventional ones.

In view of the above circumstances, the present disclosure aims to provide a composition for a secondary battery negative electrode, a negative electrode mixture layer, and a secondary battery negative electrode that can be used to produce a lithium ion secondary battery with excellent safety, as well as a lithium ion secondary battery with excellent safety.

Means for solving the above problems include the following embodiments.
<1> A thermally expandable microcapsule (A) having an expansion start temperature of 100° C. or higher and containing a non-flammable gas-generating material therein;
and (B) polymer particles having a softening point of 70°C to 150°C.
<2> The composition for a secondary battery negative electrode according to <1>, wherein the non-flammable gas generating material contains a fluorine element-containing compound.
<3> The composition for a secondary battery negative electrode according to <2>, wherein the fluorine-containing compound includes at least one selected from the group consisting of hydrofluorocarbons, hydrofluoroethers, fluoroketones, perfluoroethers, perfluorocarbons, hydrofluoroolefins, hydrochlorofluoroolefins, carbonyl fluoride, and hydrogen fluoride.
<4> The composition for a secondary battery negative electrode according to any one of <1> to <3>, wherein the expansion starting temperature is 100° C. to 180° C.
<5> The composition for a secondary battery negative electrode according to any one of <1> to <4>, wherein a mass blending ratio (A/B) of the thermally expandable microcapsules (A) to the polymer particles (B) is 0.1 to 5.0.
<6> The composition for a secondary battery negative electrode according to any one of <1> to <5> above, further comprising a binder (C).
<7> The composition for a secondary battery negative electrode according to any one of <1> to <6>, wherein the thermally expandable microcapsules (A) have a maximum volume expansion temperature higher than the softening point.
<8> The average particle size of the thermally expandable microcapsules (A) before thermal expansion is 0.3 μm to 20 μm,
The composition for secondary battery negative electrode according to any one of <1> to <7>, wherein the polymer particles (B) have an average particle size of 0.1 μm to 5 μm.
<9> A negative electrode mixture layer comprising the secondary battery negative electrode composition according to any one of <1> to <8>.
<10> A negative electrode for a secondary battery, comprising the negative electrode mixture layer according to <9>.
<11> The negative electrode for a secondary battery according to <10>, further comprising: the negative electrode mixture layer further containing an active material (D); and a current collector.
<12> The negative electrode for secondary batteries according to <11>, wherein the content of the composition for secondary battery negative electrode is 0.5 parts by mass to 10 parts by mass per 100 parts by mass of the active material (D).
<13> The composition for secondary battery negative electrode further contains a conductive assistant (E),
The negative electrode for secondary batteries according to <11> or <12>, wherein the content of the conductive assistant (E) is 0.1 parts by mass to 10 parts by mass with respect to 100 parts by mass of the active material (D).
<14> A lithium ion secondary battery comprising the negative electrode for secondary battery according to any one of <10> to <13>, a positive electrode, a separator, and a non-aqueous electrolyte.

According to the present disclosure, a composition for a secondary battery negative electrode, a negative electrode mixture layer, and a secondary battery negative electrode that can be used to produce a lithium ion secondary battery with excellent safety, as well as a lithium ion secondary battery with excellent safety, are provided.

FIG. 1 is a schematic cross-sectional view showing a laminated battery, which is an example of a lithium ion secondary battery according to the present disclosure.

In this specification, a numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits.
In this specification, the term "process" refers not only to an independent process, but also to a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved.

Hereinafter, the composition, negative electrode, and lithium ion secondary battery will be described with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference symbols and will not be described repeatedly.

(1) Composition for secondary battery negative electrode The composition for secondary battery negative electrode of the present disclosure (hereinafter also simply referred to as the "composition") contains thermally expandable microcapsules (A) having an expansion start temperature of 100°C or higher and containing a non-flammable gas-generating material therein, and polymer particles (B) (hereinafter also simply referred to as "polymer particles (B)") having a softening point of 70°C to 150°C.

The term "thermally expandable microcapsule" refers to a microcapsule having an outer shell containing a thermoplastic resin and a non-flammable gas generating material encapsulated in the outer shell. Specifically, when the thermally expandable microcapsule (A) is exposed to a temperature (e.g., 70°C to 180°C) immediately before the onset of thermal runaway due to abnormal heat generation in a lithium ion secondary battery, it suddenly softens and foams, causing a volume expansion.
The "expansion initiation temperature" refers to the temperature at which at least a part of the non-flammable gas generating material encapsulated in the thermally expandable microcapsule (A) begins to gasify and the thermally expandable microcapsule (A) begins to expand in volume.
"Non-flammable gas generating material" refers to a substance that is a raw material for non-flammable gas, is in a liquid or solid state at room temperature (e.g., 23° C.), and changes into a gaseous state at a relatively high temperature (e.g., 110° C.). "Non-flammable gas" refers to a gas that is not a flammable gas that has a flash point, such as a hydrocarbon gas.
"Softening point" refers to a value measured according to JIS K 2207 (ring and ball method).
"For use in secondary battery negative electrodes" means that the material is used as a material for a negative electrode mixture layer contained in a negative electrode of a lithium ion secondary battery.

Since the composition of the present disclosure has the above-mentioned configuration, it is possible to provide a lithium ion secondary battery with excellent safety.
This effect is believed to be due to, but not limited to, the following reasons.
The composition of the present disclosure is added to the negative electrode mixture layer of a lithium ion secondary battery. When an internal short circuit occurs in a lithium ion secondary battery using the composition of the present disclosure, the polymer particles (B) melt and loosen the structure in the rigid negative electrode mixture layer. Furthermore, the thermally expandable microcapsules (A) expand due to heat, causing a morphological change inside the negative electrode mixture layer. As a result, when the positive and negative electrodes of the lithium ion secondary battery abnormally heat up due to the occurrence of an internal short circuit, the heat generation rate of the lithium ion secondary battery can be suppressed.
After that, when the thermally expandable microcapsule (A) continues to expand and bursts, the non-flammable gas mainly leaks from the thermally expandable microcapsule (A). In other words, unlike the conventional (e.g., Patent Document 1), the flammable gas is not easily released from the thermally expandable microcapsule (A). As a result, when abnormal heat generation occurs in the lithium ion secondary battery, oxygen is diluted in the atmosphere inside the lithium ion secondary battery by the supply of the non-flammable gas. Therefore, even if an internal short circuit occurs in the lithium ion secondary battery and abnormal heat generation occurs, the lithium ion secondary battery is less likely to ignite than the conventional one. As a result, it is presumed that the composition of the present disclosure can be made into a lithium ion secondary battery with excellent safety.

The form of the composition is not particularly limited, and may be a solid or liquid. The liquid form includes a suspension (i.e., a slurry liquid). The solvent for the suspension is, for example, water. From the viewpoint of the stability of the polymer particles (B), the form of the composition is preferably a liquid using an aqueous solvent.

(1.1) Use of Composition The composition of the present disclosure is used, for example, as an additive for the negative electrode of a lithium ion secondary battery.

(1.2) Thermally Expandable Microcapsules The composition of the present disclosure contains thermally expandable microcapsules (A).

The thermally expandable microcapsule (A) has an outer shell containing a thermoplastic resin and a non-flammable gas-generating material encapsulated in the outer shell.

In a lithium ion secondary battery using the composition of the present disclosure, when the thermally expandable microcapsules (A) are exposed to a temperature (e.g., 70°C to 180°C) immediately before the onset of thermal runaway due to abnormal heat generation in the lithium ion secondary battery, they suddenly soften and foam, causing a volume expansion. This is thought to increase the resistance of the negative electrode mixture layer and suppress heat generation in the lithium ion secondary battery.
Although the detailed mechanism is not entirely clear, it is speculated as follows.
In the negative electrode mixture layer during normal operation, a conductive path is formed between the active materials by the active material or the conductive assistant. For example, when the temperature of the lithium ion secondary battery reaches 120°C to 135°C due to an internal short circuit, the separator (e.g., made of polyethylene) melts and blocks the passage of lithium ions. Furthermore, the polymer particles (B) melt and loosen the structure in the rigid negative electrode mixture layer, and the thermally expandable microcapsules (A) expand due to heat, causing a morphological change inside the negative electrode mixture layer. It is presumed that this action increases the resistance of the negative electrode mixture layer, significantly reduces the short circuit current caused by an internal short circuit, and suppresses heat generation in the entire lithium ion secondary battery.

The expansion start temperature (Ts) of the thermally expandable microcapsule (A) is 100° C. or higher, and preferably 100° C. to 180° C. If the expansion start temperature (Ts) is within the range of 100° C. to 180° C., the chain reaction of heat generation inside the battery can be stopped.
The expansion starting temperature (Ts) is more preferably 105°C to 160°C, and further preferably 110°C to 150°C, from the viewpoint of the manufacturing conditions of the electrode.

The maximum volume expansion temperature (Tmax) of the thermally expandable microcapsules (A) is not particularly limited, and is preferably higher than the softening point of the polymer particles (B), thereby enabling the electrical resistance to be increased more efficiently in the negative electrode mixture layer.
The maximum volume expansion temperature (Tmax) is preferably 100°C to 180°C, more preferably 110°C to 180°C, and even more preferably 120°C to 170°C, from the viewpoint of the manufacturing conditions of the electrode.

"Maximum volume expansion temperature" refers to the temperature at which the expansion volume of the thermally expandable microcapsule (A) is maximized when the non-flammable gas-generating material contained in the thermally expandable microcapsule (A) is gasified and the thermally expandable microcapsule (A) expands in volume.

The expansion start temperature (Ts) and maximum volume expansion temperature (Tmax) of the thermally expandable microcapsule (A) are measured, for example, using a dynamic viscoelasticity measuring device (e.g., "DMA Q800" manufactured by TA Instruments). In detail, the thermally expandable microcapsule (A) is placed in an aluminum cup, and the obtained sample (i.e., the thermally expandable microcapsule (A)) is heated from 20°C to 300°C at a temperature increase rate of 10°C/min while a force of 0.01 N is applied from above with a pressure pin, and the displacement amount (D) in the vertical direction of the pressure pin is measured. The temperature at which the displacement in the forward direction starts is defined as the expansion start temperature (Ts), and the temperature at which the maximum displacement amount (Dmax) is shown is defined as the maximum volume expansion temperature (Tmax).

The average particle diameter of the thermally expandable microcapsules (A) before thermal expansion (hereinafter simply referred to as "average particle diameter") is not particularly limited, and is preferably 0.3 μm to 20 μm. If the average particle diameter of the thermally expandable microcapsules (A) is within the above range, the thermally expandable microcapsules (A) can be introduced into the negative electrode mixture layer without causing resistance inside the lithium ion secondary battery, and the battery performance can be ensured.
From the viewpoint of battery performance, the average particle size of the thermally expandable microcapsules (A) is more preferably 0.3 μm to 15 μm.
The average particle size is measured using a laser diffraction particle size distribution measuring device (for example, Microtrac 9320-HRA manufactured by Nikkiso Co., Ltd.) In detail, the average particle size is the D50 value by volume-based measurement.

The "average particle size of the thermally expandable microcapsule (A) before thermal expansion" refers to the average particle size of the thermally expandable microcapsule (A) that has no history of rupture in an atmosphere below the expansion start temperature (Ts) (e.g., 25°C).

(1.2.1) Non-flammable gas generating material There are no particular limitations on the non-flammable gas generating material, so long as it vaporizes when heated and generates a non-flammable gas.

The non-flammable gas may be any gas that is not flammable and does not have a flash point. Examples of the non-flammable gas include halogen-containing compounds and decomposition products of halogen-containing compounds. Examples of the halogen-containing compound include fluorine-containing compounds, chlorine-containing compounds, and bromine-containing compounds.
In particular, the non-flammable gas preferably contains a fluorine-containing compound, and is preferably a fluorine-containing compound. When the non-flammable gas contains a fluorine-containing compound, it is possible to prevent intensification of combustion when the lithium ion secondary battery experiences thermal runaway.

The non-flammable gas generating material is not particularly limited as long as it is a material that generates a non-flammable gas, and examples of the non-flammable gas generating material include halogen element-containing compounds (e.g., fluorine element-containing compounds, chlorine element-containing compounds, bromine element-containing compounds, etc.).
The non-flammable gas generating material preferably contains a fluorine element-containing compound, and more preferably is a fluorine element-containing compound.

_The fluorine-containing compound is not particularly limited, and examples thereof include hydrofluorocarbons (e.g., _CH2FCF3 , _CH3CHF2 , _{CHF2CF3 , etc.} ), _{hydrofluoroethers (} e.g., _{CH3OCH2CF2CHF2 , CH3OCH2CF2CF3 , CH3OCF2CHFCF3 , CH3OCF2CF2CF2CF3 , CHF2OCH2CF2CF3 , CH3OCH ( CF3} ) _{2 , CH3OCF ( CF3 ) 2 , CF3CH2OCF2CH2F , CF3CH2OCF2CHF2 , etc. ) , fluoroketones ( e.g. , CF3CF2} COCF( _CF3 ) _CF3 , etc.) _{, perfluoroethers} (e.g. _, CF3OCF3, CF3OCF2CF3, etc. ₎ , perfluorocarbons, hydrofluoroolefins (e.g., _{CF3CHCHCF3 ,} etc.), _{hydrochlorofluoroolefins} (e.g., _CF3CHCHCl , etc.), carbonyl fluoride ( _COF2 ), hydrogen fluoride (HF), etc.

The term "hydrofluorocarbon" refers to a compound consisting of carbon, hydrogen, and fluorine atoms. For example, hydrofluorocarbon refers to a compound in which some of the hydrogen atoms of an alkane are replaced with fluorine atoms.
The term "hydrofluoroether" refers to a compound that is composed of carbon atoms, hydrogen atoms, fluorine atoms, and oxygen atoms and has an ether (-O-). For example, a hydrofluoroether is a compound in which some of the hydrogen atoms of a dialkyl ether are substituted with fluorine atoms.
The term "fluoroketone" refers to a compound that is composed of carbon, hydrogen, fluorine, and oxygen atoms and has a ketone (-C(=O)-). For example, a fluoroketone is a compound in which some of the hydrogen atoms of a dialkyl ketone are substituted with fluorine atoms.
The term "perfluoroether" refers to a compound that is composed of carbon, fluorine, and oxygen atoms and has an ether (-O-). Perfluoroether is, for example, a compound in which all of the hydrogen atoms of an alkyl ether are substituted with fluorine atoms.
The term "perfluorocarbon" refers to a compound consisting of carbon atoms and fluorine atoms. For example, perfluorocarbon refers to a compound in which all of the hydrogen atoms of an alkane are replaced with fluorine atoms.
The term "hydrofluoroolefin" refers to a compound that is composed of carbon atoms, hydrogen atoms, and fluorine atoms and has a carbon-carbon double bond (-C=C-). For example, the hydrofluoroolefin refers to a compound in which some of the hydrogen atoms of an alkene are replaced with fluorine atoms.
The term "hydrochlorofluoroolefin" refers to a compound that is composed of carbon atoms, hydrogen atoms, chlorine atoms, and fluorine atoms and has a carbon-carbon double bond (-C=C-). For example, the hydrochlorofluoroolefin refers to a compound in which some of the hydrogen atoms of an alkene are replaced with chlorine atoms and fluorine atoms.

Among these, the fluorine-containing compound preferably contains at least one selected from the group consisting of hydrofluorocarbons, hydrofluoroethers, fluoroketones, perfluoroethers, perfluorocarbons, hydrofluoroolefins, hydrochlorofluoroolefins, carbonyl fluoride, and hydrogen fluoride (hereinafter also referred to as a "fluorine-based gas source"), and more preferably contains a hydrofluoroether. In addition, the _{hydrofluoroether} more preferably contains _{at least one of CH3OCH2CF2CHF2 , CH3OCH2CF2CF3 , CH3OCF2CHFCF3 , CH3OCF2CF2CF2CF3, CHF2OCH2CF2CF3, CH3OCH(CF3)2, CH3OCF(CF3)2 , CF3CH2OCF2CH2F , and CF3CH2OCF2CHF2 . The non - flammable gas vaporized by heating the fluorine -} based gas source contains a fluorine-element-containing compound. The _fluorine -element-containing compound may be _the fluorine-based gas source _itself , or may be a reactant or decomposition product of the fluorine _- based gas source. The fluorine-containing compound may be one type or two or more types.

The boiling point of the non-flammable gas generating material is not particularly limited, and is preferably 25°C to 150°C, more preferably 25°C to 100°C, even more preferably 25°C to 80°C, and particularly preferably 30°C to 70°C. As a result, in a lithium ion secondary battery using the composition of the present disclosure, the thermally expandable microcapsules (A) are likely to rapidly soften and foam and expand in volume when exposed to a temperature (e.g., 70°C to 180°C) just before the start of thermal runaway due to abnormal heat generation in the lithium ion secondary battery. As a result, the resistance of the negative electrode mixture layer increases, and heat generation in the lithium ion secondary battery can be further suppressed.
Examples of non-flammable gas generating materials having a boiling point of 25° C. to 150° C. include hydrofluorocarbons, hydrofluoroethers, perfluoroethers, perfluorocarbons, hydrofluoroolefins, hydrochlorofluoroolefins, and hydrogen fluoride.

The ratio of the mass of the non-flammable gas generating material (A1) to the total mass of the thermally expandable microcapsules (A) (hereinafter also referred to as the ratio "(A1/A)") is not particularly limited, and is preferably 2 mass% to 60 mass%, more preferably 5 mass% to 55 mass%, and even more preferably 10 mass% to 50 mass%.
When the ratio (A1/A) is within the above range, the expansion ratio of the thermally expandable microcapsule (A) is high, and the thickness of the outer shell relative to the particle diameter of the thermally expandable microcapsule (A) is sufficient, so that the thermally expandable microcapsule (A) is less likely to be broken.

(1.2.2) Outer Shell The thermoplastic resin contained in the outer shell is not particularly limited, and examples thereof include vinylidene chloride copolymers, unsaturated carboxylic acid-based copolymers such as (meth)acrylic acid-based copolymers, unsaturated carboxylic acid ester-based copolymers such as (meth)acrylic acid ester-based copolymers, nitrile-based copolymers such as (meth)acrylonitrile-based copolymers, and unsaturated carboxylic acid-nitrile-based copolymers such as (meth)acrylic acid-(meth)acrylonitrile-based copolymers.

Examples of polymerizable monomers that are used as raw materials for thermoplastic resins include vinylidene chloride; vinyl chloride; vinyl acetate; nitrile monomers such as acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, and fumaronitrile; unsaturated carboxylic acid (salt) monomers such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid, and metal salts thereof; (meth)acrylic acid ester monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobornyl (meth)acrylate, cyclohexyl (meth)acrylate, glycidyl (meth)acrylate, hydroxyethyl (meth)acrylate, and hydroxypropyl (meth)acrylate; amide monomers such as acrylamide, methacrylamide, substituted acrylamide, and substituted methacrylamide; imide monomers such as N-phenylmaleimide, N-(2-chlorophenyl)maleimide, and N-cyclohexylmaleimide; vinyl aromatic monomers such as styrene, α-methylstyrene, and chlorostyrene; isoprene; butadiene, etc. These polymerizable monomers may be used alone or in combination of two or more. If necessary, the polymerizable monomer may be used in combination with a monomer having two or more polymerizable double bonds (crosslinking agent).

(1.2.3) Manufacturing method of thermally expandable microcapsules The manufacturing method of the thermally expandable microcapsules (A) is not particularly limited and may be a known method, for example, a method of dispersing an oily mixture containing the above-mentioned polymerizable monomer, polymerization initiator, and non-flammable gas generating material in an aqueous dispersion medium containing a dispersant, etc., and carrying out suspension polymerization. The manufacturing method of the thermally expandable microcapsules (A) may refer to the manufacturing methods described in Japanese Patent No. 6944619, Japanese Patent No. 6735936, Japanese Patent No. 6587782, Japanese Patent No. 6619541, Japanese Patent No. 6283456, etc.

(1.3) Polymer Particles The composition of the present disclosure contains polymer particles (B) having a softening point of 70° C. to 150° C. By containing polymer particles (B) in the composition of the present disclosure, a lithium ion secondary battery using the composition of the present disclosure is less susceptible to thermal runaway, and the safety of the lithium ion secondary battery is improved.

(1.3.1) Material The material of the polymer particles (B) is not particularly limited as long as it is, for example, a thermoplastic resin having a softening point of 70° C. to 150° C. Examples of the material of the polymer particles (B) include olefin resins, thermoplastic elastomers, polyacetals, thermoplastic modified celluloses, polysulfones, etc. The polymer particles (B) may be used alone or in combination of two or more kinds.

"Olefin-based resin" refers to a resin containing structural units derived from olefins. In more detail, "olefin-based resin" refers to a homopolymer of an olefin, a copolymer of two or more types of olefins, or a copolymer of an olefin and another monomer.

Specific examples of olefin-based resin materials include polyethylene, polypropylene, ethylene-vinyl acetate copolymer (EVA), polyvinyl chloride, polyvinylidene chloride, polyvinyl fluoride, polyvinylidene fluoride, polyamide, polystyrene, polyacrylonitrile, polyethylene oxide, and polymethyl (meth)acrylate. Among these, the olefin-based resin material is preferably polyethylene or polypropylene, and more preferably polyethylene.

In particular, the material of the polymer particles (B) preferably contains an olefin-based resin, and more preferably contains a water-dispersible olefin-based resin. When the material of the polymer particles (B) contains a water-dispersible olefin-based resin and the composition of the present disclosure is used as a raw material for the negative electrode mixture layer, the lithium ion secondary battery is less susceptible to thermal runaway, and the safety of the lithium ion secondary battery is improved.

"Water-dispersible olefin resin" refers to fine particles of olefin resin that can be dispersed in water without the addition of either a surfactant or an organic solvent.

Examples of water-dispersible olefin resin materials include polyethylene, polyethylene elastomers, polyolefin ionomers, EVA, etc.

(1.3.2) Average Particle Diameter The average particle diameter of the polymer particles (B) is not particularly limited, and from the viewpoint of enabling the negative electrode mixture layer to be uniformly applied onto the current collector and further improving the battery characteristics, it is preferably 0.1 μm to 5 μm, more preferably 0.2 μm to 4.5 μm, and even more preferably 0.5 μm to 4 μm.
The average particle size of the polymer particles (B) is measured using a laser diffraction particle size distribution measuring device (for example, "HEROS &RODOS" manufactured by Sympatec).

It is preferable that the average particle diameter of the thermally expandable microcapsules (A) before thermal expansion is 0.3 μm to 20 μm, and the average particle diameter of the polymer particles (B) is 0.1 μm to 5 μm. This makes it easier to form a negative electrode mixture layer with a uniform thickness, and even if the thermally expandable microcapsules (A) are introduced inside the negative electrode mixture layer, they do not cause resistance inside the lithium ion secondary battery, and battery performance can be ensured.

(1.3.3) Softening Point The softening point of the polymer particles (B) is from 70°C to 150°C.
The softening point of the polymer particles (B) is preferably 70°C to 120°C, more preferably 90°C to 120°C, from the viewpoint of further suppressing the heat generation rate of the lithium ion secondary battery when the positive electrode and the negative electrode of the lithium ion secondary battery abnormally heat up due to the occurrence of an internal short circuit.
The softening point of the polymer particles (B) is preferably 90° C. to 140° C., more preferably 110° C. to 135° C., from the viewpoint of improving the handling property and safety of the lithium ion secondary battery.
The softening point of the polymer particles (B) is preferably 140° C. or lower, more preferably 135° C. or lower, from the viewpoint of more effectively exerting the shutdown function by melting the polymer particles (B) in a lower temperature range during a sudden temperature rise of the lithium ion secondary battery (hereinafter referred to as "effective exertion of the shutdown function"). The shutdown function includes preventing the progress of the battery reaction of the lithium ion secondary battery.
The softening point of the polymer particles (B) is preferably 70° C. or higher, more preferably 90° C. or higher, even more preferably 110° C. or higher, and particularly preferably 120° C. or higher, from the viewpoint of maintaining the shape of the polymer particles (B) before and after the drying treatment performed in the production process of the negative electrode (hereinafter referred to as "shape maintenance of the olefin-based resin (B) during the negative electrode drying step").

The polymer particles (B) may be commercially available. When the material of the polymer particles (B) is a water-dispersible olefin resin, commercially available products include the Chemipearl (registered trademark) series (polyolefin aqueous dispersions) manufactured by Mitsui Chemicals, Inc. Examples of finely divided aqueous dispersions of low molecular weight polyethylene include W300, W400, W410, W700, W4005, W401, W500, WF640, W900, W950, and WH201.

(1.3.4) Content The content of the polymer particles (B) is not particularly limited and is appropriately adjusted depending on the content of the thermally expandable microcapsules (A) and the like.
The mass blending ratio (A/B) of the thermally expandable microcapsules (A) to the polymer particles (B) is not particularly limited, and is preferably 0.1 to 5.0, more preferably 0.2 to 3.0, from the viewpoint of the internal resistance of the lithium ion secondary battery.

The "mass blending ratio (A/B)" refers to the ratio of the content of the thermally expandable microcapsules (A) to the content of the polymer particles (B).
In the present disclosure, the "content" and the "addition amount" (blended amount) are considered to be substantially the same.

(1.3.5) Relationship between polymer particles and thermally expandable microcapsules In the composition of the present disclosure, the maximum volume expansion temperature of the thermally expandable microcapsules (A) is preferably higher than the softening point of the polymer particles (B). As a result, when abnormal heat generation occurs in the lithium ion secondary battery, the polymer particles (B) melt first, and the negative electrode mixture layer begins to soften. Then, the thermally expandable microcapsules (A) expand, and the negative electrode mixture layer collapses. When the thermally expandable microcapsules (A) expand, the structural change of the negative electrode mixture layer is more actively expressed, and the resistance of the negative electrode mixture layer can be efficiently increased. As a result, the heat generation of the lithium ion secondary battery can be more suppressed.

One method for adjusting the maximum volume expansion temperature of the thermally expandable microcapsule (A) to be higher than the softening point of the polymer particles (B) is to select the material of the outer shell of the thermally expandable microcapsule (A) depending on the material of the polymer particles (B), etc.

The thermally expandable microcapsule (A) has an outer shell that contains a non-flammable gas generating material, and the outer shell of the thermally expandable microcapsule (A) preferably contains a resin that softens at a temperature higher than the softening point of the polymer particles (B). As a result, when abnormal heat generation occurs in the lithium ion secondary battery, the polymer particles (B) melt first, and the negative electrode mixture layer begins to collapse. Then, the thermally expandable microcapsule (A) expands, and the negative electrode mixture layer collapses. When the thermally expandable microcapsule (A) expands, the structural change of the negative electrode mixture layer is more actively expressed, and the resistance of the negative electrode mixture layer can be efficiently increased. As a result, heat generation in the lithium ion secondary battery can be further suppressed.

As a method for adjusting the outer shell of the thermally expandable microcapsule (A) to contain a resin that softens at a temperature higher than the softening point of the polymer particles (B), a method of selecting the material of the outer shell of the thermally expandable microcapsule (A) depending on the material of the polymer particles (B), etc., can be given.

(1.4) Binder The composition of the present disclosure may or may not further comprise a binder (C).

When the composition of the present disclosure contains the binder (C), in a lithium ion secondary battery using the composition of the present disclosure, the binder (C) can improve the physical properties of the negative electrode mixture layer (e.g., electrolyte permeability and peel strength) and can also improve the battery performance of the lithium ion secondary battery.

The binder (C) includes a water-soluble binder and a water-insoluble binder. The binder (C) may be one type or two or more types.

The term "water-soluble binder" refers to a binder that is soluble in an amount of 5 g or more in 100 g of water at 20°C.
The term "water-insoluble binder" refers to a binder that dissolves in an amount of less than 5 g per 100 g of water at 20°C.

Examples of the water-soluble binder include celluloses, alkali salts of celluloses, synthetic resins (e.g., polyvinyl alcohols, polyethylene oxide, acrylic acid polymers, etc.), starch, dextrin, casein, etc. Examples of the celluloses include methyl cellulose, ethyl cellulose, carboxymethyl cellulose (CMC), hydroxyethyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, etc. Examples of the alkali salts include lithium salts, sodium salts, ammonium salts, etc.
Examples of the non-water-soluble binder include rubbers, synthetic resins, etc. Examples of the rubbers include styrene butadiene rubber (SBR), acrylonitrile-butadiene rubber, acrylonitrile-butadiene-styrene rubber, etc. Examples of the synthetic resins include vinyl acetate copolymers, acrylic acid modified SBR resins (SBR-based latex), etc.
Among these, the binder (C) is preferably at least one selected from the group consisting of styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, acrylonitrile-butadiene-styrene rubber, carboxymethyl cellulose (CMC), hydroxypropyl methyl cellulose, polyvinyl alcohol, hydroxypropyl cellulose, and diacetyl cellulose. Among these, the binder (C) preferably contains styrene-butadiene rubber and carboxymethyl cellulose from the viewpoint of adhesiveness.

The content of the binder (C) is not particularly limited and is appropriately adjusted depending on the content of the thermally expandable microcapsules (A) and the like.
The mass blending ratio (A/C) of the thermally expandable microcapsules (A) to the binder (C) is not particularly limited, but is preferably 0.1 to 2.0, more preferably 0.5 to 1.5.

The average particle size of the binder (C) is not particularly limited, and from the viewpoint of battery performance (internal resistance), it is preferably 0.1 μm to 3 μm, more preferably 0.1 μm to 1 μm, and even more preferably 0.1 μm to 0.3 μm.

(1.4.1) Relationship between binder and polymer particles It is preferable that the average particle diameter of the binder (C) is 0.1 μm to 0.3 μm, and the average particle diameter of the polymer particles (B) is 0.1 μm to 5 μm. This makes it possible to ensure the battery characteristics without increasing the resistance of the negative electrode mixture layer. In addition, it is possible to maintain the life characteristics of the negative electrode without increasing the interface resistance between the negative electrode mixture layer and the current collector. Furthermore, the processability of the composition is better.

(1.5) Conductive Aid The composition of the present disclosure may or may not contain a conductive aid (E).

"Conductive additive" refers to a material that does not function as an active material in a lithium-ion secondary battery, but improves electrical conduction between active materials and between the active material and the current collector. "Active material" refers to a material that undergoes a battery reaction in the positive and negative electrodes. Therefore, among conductive additives, those that can function as active materials in the composite layer when the battery is charged and discharged are classified as active materials rather than conductive additives. "Battery reaction" refers to the insertion and desorption reaction of lithium ions that takes place between the positive and negative electrodes.

The conductive assistant (E) may be a known conductive assistant, and preferably contains a carbon material having conductivity. Examples of conductive carbon materials include graphite, carbon black, conductive carbon fiber, and fullerene. Examples of conductive carbon fibers include carbon nanotubes, carbon nanofibers, and carbon fibers. Examples of graphite include artificial graphite and natural graphite. Examples of natural graphite include flake graphite, lump graphite, and earthy graphite. The conductive assistant may be used alone or in combination of two or more types. The conductive assistant (E) may be a commercially available product. An example of a commercially available conductive assistant (E) is "Super-P" manufactured by TIMCAL.

The conductive assistant (E) has an average particle size of preferably 5 μm or less, more preferably 1 μm to 4 μm, and a primary particle size of preferably 0.5 μm or less, more preferably 0.1 μm to 0.4 μm.
The BET specific surface area of the conductive assistant (E) is preferably 30 m ² /g to 100 m ² /g, and more preferably 40 m ² /g to 80 m ² /g.

(1.6) Non-Solid Content The compositions of the present disclosure may or may not contain non-solid content.
For example, when the negative electrode mixture layer is formed from a negative electrode mixture slurry, the negative electrode mixture layer may contain various compounding components derived from the negative electrode mixture slurry. Examples of the non-solid content include various compounding components derived from the negative electrode mixture slurry (e.g., thickeners, surfactants, dispersants, wetting agents, antifoaming agents, etc.), water, etc.

(2) Negative electrode mixture layer The negative electrode mixture layer of the present disclosure contains the composition of the present disclosure. This makes it possible to provide a lithium ion secondary battery using the negative electrode mixture layer of the present disclosure with excellent safety.

The negative electrode composite layer may or may not contain active material (D) (hereinafter also referred to as "negative electrode active material (D)").

(2.1) Negative Electrode Active Material The negative electrode active material (D) is not particularly limited as long as it is a material capable of absorbing and releasing lithium ions. The negative electrode active material (D) is preferably at least one selected from the group consisting of, for example, metallic lithium, lithium-containing alloys, metals or alloys capable of alloying with lithium, oxides capable of doping and dedoping lithium ions, transition metal nitrides capable of doping and dedoping lithium ions, and carbon materials capable of doping and dedoping lithium ions. Among these, the negative electrode active material (D) is preferably a carbon material capable of doping and dedoping lithium ions (hereinafter referred to as "carbon material").

Examples of carbon materials include carbon black, activated carbon, graphite materials, and amorphous carbon materials. These carbon materials may be used alone or in combination of two or more. The form of the carbon material is not particularly limited, and examples of the form include fibrous, spherical, and flaky forms. The size of the carbon material is not particularly limited, and is preferably 5 μm to 50 μm, and more preferably 20 μm to 30 μm.
Examples of amorphous carbon materials include hard carbon, coke, mesocarbon microbeads (MCMB) calcined at 1500° C. or less, and mesophase pitch carbon fiber (MCF).
Examples of the graphite material include natural graphite and artificial graphite. Examples of the artificial graphite include graphitized MCMB and graphitized MCF. The graphite material may contain boron. The graphite material may be coated with a metal or amorphous carbon. Examples of the metal material that coats the graphite material include gold, platinum, silver, copper, and tin. The graphite material may be a mixture of amorphous carbon and graphite.

(2.2) Other Components The negative electrode mixture layer may contain other components in addition to the above-mentioned components. Examples of the other components include a thickener, a surfactant, a dispersant, a wetting agent, and a defoamer.

(2.3) Content When the negative electrode mixture layer of the present disclosure contains the negative electrode active material (D), the content of the composition of the present disclosure is not particularly limited, and is preferably 0.5 parts by mass to 10 parts by mass per 100 parts by mass of the negative electrode active material (D). This makes it possible to improve safety without affecting battery performance.
From the viewpoint of achieving both safety and battery performance, the content of the composition is more preferably 0.5 to 6.5 parts by mass, and even more preferably 0.5 to 5 parts by mass, per 100 parts by mass of the negative electrode active material (D).

The content of the thermally expandable microcapsules (A) is not particularly limited, and is preferably 0.1% by mass to 5.0% by mass, more preferably 0.3% by mass to 3.0% by mass, even more preferably 0.5% by mass to 2.5% by mass, particularly preferably 1.0% by mass to 2.5% by mass, even more preferably 1.3% by mass to 2.5% by mass, and even more preferably 1.4% by mass to 2.0% by mass, relative to the total amount of the negative electrode mixture layer. If the content of the thermally expandable microcapsules (A) is within the above range, the negative electrode internal resistance of the negative electrode mixture layer can be actively increased when the lithium ion secondary battery generates heat, and the lithium ion secondary battery can be made safer.

The content of the polymer particles (B) is not particularly limited, and is preferably 0.1% by mass to 5.0% by mass, more preferably 0.3% by mass to 2.0% by mass, even more preferably 0.3% by mass to 1.0% by mass, and particularly preferably 0.3% by mass to 0.7% by mass, relative to the total amount of the negative electrode mixture layer. If the content of the polymer particles (B) is within the above range, the negative electrode internal resistance of the negative electrode mixture layer can be actively increased when the lithium ion secondary battery generates heat, and the lithium ion secondary battery can be made safer.

When the composition of the present disclosure contains a binder (C) (i.e., when the composition contains a binder (C)), the content of the binder (C) is not particularly limited and is preferably 0.1% by mass to 5% by mass, more preferably 1% by mass to 5% by mass, and even more preferably 1% by mass to 3% by mass, relative to the total amount of the negative electrode mixture layer. If the content of the binder (C) is within the above range, the active material is less likely to be detached during the charge and discharge process of the lithium ion secondary battery, and a decrease in battery capacity is suppressed.

When the negative electrode mixture layer contains the negative electrode active material (D) (i.e., when the composition contains the negative electrode active material (D)), the content of the negative electrode active material (D) is not particularly limited, and is preferably 10% by mass to 99.9% by mass, more preferably 30% by mass to 98% by mass, even more preferably 50% by mass to 98% by mass, and particularly preferably 70% by mass to 97% by mass, relative to the total amount of the negative electrode mixture layer. If the content of the negative electrode active material (D) is within the above range, the battery performance is improved.

When the negative electrode mixture layer contains the conductive assistant (E) (that is, when the composition contains the conductive assistant (E)), the content of the conductive assistant (E) is preferably in the following range.
The content of the conductive assistant (E) is not particularly limited, but is preferably 0.1 to 10 parts by mass per 100 parts by mass of the negative electrode active material (D), thereby improving the battery performance.
From the viewpoint of the internal resistance of the lithium ion secondary battery, the content of the conductive assistant (E) is more preferably 0.1 parts by mass to 5 parts by mass, and even more preferably 0.1 parts by mass to 3 parts by mass, per 100 parts by mass of the negative electrode active material (D).

The total content of the thermally expandable microcapsules (A) and the polymer particles (B) is preferably 0.1 to 10 parts by mass per 100 parts by mass of the negative electrode active material (D). This makes it possible to actively increase the negative electrode internal resistance of the negative electrode mixture layer when the lithium ion secondary battery generates heat, thereby making the lithium ion secondary battery safer.
From the viewpoint of achieving both battery performance and safety, the total content of the thermally expandable microcapsules (A) and the polymer particles (B) is more preferably 0.1 to 5 parts by mass per 100 parts by mass of the negative electrode active material (D).

(3) Negative electrode for secondary battery The negative electrode for secondary battery (hereinafter also referred to as "negative electrode") of the present disclosure includes the negative electrode mixture layer of the present disclosure. This makes it possible to provide a lithium ion secondary battery using the negative electrode of the present disclosure with excellent safety.

The negative electrode of the present disclosure preferably includes a negative electrode composite layer further containing a negative electrode active material (D) and a current collector (hereinafter also referred to as a "negative electrode current collector"). In more detail, in the negative electrode of the present disclosure, it is preferable that the negative electrode composite layer further contains a negative electrode active material (D), and that the negative electrode composite layer is laminated on at least one surface of the current collector.

"Current collector" refers to a sheet-like material that collects electrons generated from active materials in lithium-ion secondary batteries and supplies electrons to the active materials.

(3.1) Negative Electrode Mixture Layer In the negative electrode of the present disclosure, the negative electrode mix layer is similar to the negative electrode mix layer described above, except that it contains a negative electrode active material (D).

In the negative electrode of the present disclosure, the content of the composition is not particularly limited, and is preferably 0.5 parts by mass to 10 parts by mass per 100 parts by mass of the negative electrode active material (D), which can improve safety without affecting the battery performance.
From the viewpoint of achieving both safety and battery performance, the content of the composition is more preferably 0.5 to 6.5 parts by mass, and even more preferably 0.5 to 5 parts by mass, per 100 parts by mass of the negative electrode active material (D).

In the negative electrode of the present disclosure, when the negative electrode mixture layer contains a conductive assistant (E) (i.e., when the composition contains a conductive assistant (E)), the content of the conductive assistant (E) is preferably in the following range.
The content of the conductive assistant (E) is not particularly limited, but is preferably 0.1 to 10 parts by mass per 100 parts by mass of the negative electrode active material (D), thereby improving the battery performance.
From the viewpoint of the internal resistance of the lithium ion secondary battery, the content of the conductive assistant (E) is more preferably 0.1 parts by mass to 5 parts by mass, and even more preferably 0.1 parts by mass to 3 parts by mass, per 100 parts by mass of the negative electrode active material (D).

(3.2) Negative Electrode Current Collector Examples of the material for the negative electrode current collector include copper, aluminum, nickel, stainless steel (SUS), and nickel-plated steel.

(4) Lithium-ion secondary battery The lithium-ion secondary battery of the present disclosure includes the negative electrode of the present disclosure, a positive electrode, a separator, and a non-aqueous electrolyte solution, which provides excellent safety for the lithium-ion secondary battery of the present disclosure.

(4.1) Positive Electrode The positive electrode preferably contains at least one active material (hereinafter also referred to as "positive electrode active material".) The positive electrode active material is capable of absorbing and releasing lithium ions.

The positive electrode comprises a current collector (hereinafter also referred to as the "positive electrode current collector") and a positive electrode composite layer. The positive electrode composite layer is provided on at least a portion of the surface of the positive electrode current collector.

"Positive electrode composite layer" refers to the composite layer used in the positive electrode.

The material of the positive electrode current collector can be, for example, a metal or an alloy. More specifically, the material of the positive electrode current collector can be aluminum, nickel, stainless steel (SUS), copper, etc. Among them, from the viewpoint of the balance between high conductivity and cost, it is preferable that the material of the positive electrode current collector is aluminum.

The positive electrode mixture layer contains a positive electrode active material and a binder.

There are no particular limitations on the positive electrode active material, so long as it is a material capable of absorbing and releasing lithium ions, and it can be adjusted appropriately depending on the application of the lithium ion secondary battery.

Examples of the positive electrode active material include a first oxide and a second oxide.
The first oxide contains lithium (Li) and nickel (Ni) as constituent metal elements.
The second oxide contains Li, Ni, and at least one metal element other than Li and Ni as a constituent metal element. Examples of metal elements other than Li and Ni include transition metal elements and typical metal elements. The second oxide preferably contains metal elements other than Li and Ni in an amount equal to or less than Ni in terms of atomic number. The metal elements other than Li and Ni may be, for example, at least one selected from the group consisting of Co, Mn, Al, Cr, Fe, V, Mg, Ca, Na, Ti, Zr, Nb, Mo, W, Cu, Zn, Ga, In, Sn, La, and Ce. These positive electrode active materials may be used alone or in a mixture of a plurality of them.

The positive electrode active material preferably contains a lithium-containing composite oxide (hereinafter also referred to as "NCM") represented by the following formula (C1). The lithium-containing composite oxide (C1) has the advantages of high energy density per unit volume and excellent thermal stability.

LiNi _a Co _b Mn _c O ₂ ... Formula (C1)

In formula (C1), a, b, and c are each independently greater than 0 and less than 1, and the sum of a, b, and c is 0.99 to 1.00.

Specific examples of NCM include _{LiNi0.33Co0.33Mn0.33O2 ( NCM333 ) , LiNi0.5Co0.3Mn0.2O2 ( NCM532 )} , _{LiNi0.5Co0.2Mn0.3O2 ( NCM523 ) , LiNi0.6Co0.2Mn0.2O2} ( _NCM622 ) _, and _{LiNi0.8Co0.1Mn0.1O2} ( _{NCM811 )} .

The positive electrode active material may contain a lithium-containing composite oxide (hereinafter also referred to as "NCA") represented by the following formula (C2):

Li _t Ni _1-x-y Co _x Al _y O ₂ ... Formula (C2)

In formula (C2), t is 0.95 to 1.15, x is 0 to 0.3, y is 0.1 to 0.2, and the sum of x and y is less than 0.5.

Specific examples _{of NCAs} include _{LiNi0.8Co0.15Al0.05O2 .}

The content of the positive electrode active material in the positive electrode mixture layer is preferably 10% by mass to 99.9% by mass, more preferably 30% by mass to 99% by mass, even more preferably 50% by mass to 99% by mass, and particularly preferably 70% by mass to 99% by mass, based on the total amount of the positive electrode mixture layer.

Examples of the binder include polyvinyl acetate, polymethyl methacrylate, nitrocellulose, fluororesin, and rubber particles.
Examples of the fluororesin include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), vinylidene fluoride-hexafluoropropylene copolymer, and the like.
Examples of the rubber particles include styrene-butadiene rubber particles and acrylonitrile rubber particles.
Among these, from the viewpoint of improving the oxidation resistance of the positive electrode mixture layer, the binder is preferably a fluororesin. One type of binder may be used alone, or two or more types may be used in combination as necessary.

The binder content in the positive electrode composite layer is preferably 0.1% to 4% by mass relative to the total amount of the positive electrode composite layer, from the viewpoint of achieving both the physical properties of the positive electrode composite layer (e.g., electrolyte permeability, peel strength, etc.) and battery performance. If the binder content is 0.1% by mass or more, the adhesion of the positive electrode composite layer to the positive electrode current collector and the binding between the positive electrode active materials are further improved. If the binder content is 4% by mass or less, the amount of positive electrode active material in the positive electrode composite layer can be increased, thereby further improving the capacity.

The positive electrode mixture layer preferably contains a conductive assistant.
The material of the conductive assistant may be a known conductive assistant, and is the same as the examples of the conductive assistant (D) contained in the composition of the present disclosure.

The positive electrode mixture layer may contain other components. Examples of the other components include a thickener, a surfactant, a dispersant, a wetting agent, and an antifoaming agent.

(4.2) Separator The separator may be, for example, a porous resin plate. The material of the porous resin plate may be a resin or a nonwoven fabric containing the resin. The resin may be a thermoplastic resin such as polyethylene (PE), polypropylene (PP), polymethylpentene (PMP), polyester, cellulose, or polyamide.

In particular, the separator is preferably a porous resin sheet having a single layer or multilayer structure. The material of the porous resin sheet is mainly made of one or more types of polyolefin resin. The thickness of the separator is preferably 5 μm to 30 μm. The separator is preferably disposed between the positive electrode and the negative electrode.

The separator is made of a thermoplastic resin,
The softening point of the thermoplastic resin is preferably higher than the softening point of the polymer particles (B) and lower than the volume expansion starting temperature of the thermally expandable microcapsules (A).
When the temperature of the separator reaches the softening point of the thermoplastic resin, the thermoplastic resin melts and clogs the pores of the separator, blocking the progress of the battery reaction and further suppressing heat generation in the lithium-ion secondary battery.

(4.3) Nonaqueous Electrolyte The nonaqueous electrolyte contains an electrolyte and a nonaqueous solvent.

(4.3.1) Electrolyte The electrolyte preferably contains at least one of a fluorine-containing lithium salt (hereinafter also referred to as a "fluorine-containing lithium salt") and a fluorine-free lithium salt.

Examples of the fluorine-containing lithium salt include inorganic acid anion salts and organic acid anion salts.
Examples of inorganic acid anion salts include lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), and lithium hexafluorotantalate (LiTaF ₆ ).
Examples of organic acid anion salts include lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ). Among them, LiPF ₆ is particularly preferred as the fluorine-containing lithium salt.
Examples of fluorine-free lithium salts include lithium perchlorate (LiClO ₄ ), lithium aluminum tetrachloride (LiAlCl ₄ ), and lithium decachlorodecaborate (Li ₂ B ₁₀ Cl ₁₀ ).

When the electrolyte contains a fluorine-containing lithium salt, the content of the fluorine-containing lithium salt is preferably 50% by mass to 100% by mass, more preferably 60% by mass to 100% by mass, and even more preferably 80% by mass to 100% by mass, based on the total amount of the electrolyte.

When the fluorine-containing lithium salt contains lithium hexafluorophosphate (LiPF ₆ ), the content of lithium hexafluorophosphate (LiPF ₆ ) is preferably 50% by mass to 100% by mass, more preferably 60% by mass to 100% by mass, and even more preferably 80% by mass to 100% by mass, based on the total amount of the electrolyte.

When the non-aqueous electrolyte contains an electrolyte, the concentration of the electrolyte in the non-aqueous electrolyte is preferably 0.1 mol/L to 3 mol/L, more preferably 0.5 mol/L to 2 mol/L.

When the non-aqueous electrolyte contains lithium hexafluorophosphate (LiPF ₆ ), the concentration of lithium hexafluorophosphate (LiPF ₆ ) in the non-aqueous electrolyte is preferably 0.1 mol/L to 3 mol/L, and more preferably 0.5 mol/L to 2 mol/L.

(4.3.2) Nonaqueous Solvent The nonaqueous electrolyte generally contains a nonaqueous solvent.

Examples of non-aqueous solvents include cyclic carbonates, fluorine-containing cyclic carbonates, chain carbonates, fluorine-containing chain carbonates, aliphatic carboxylic acid esters, fluorine-containing aliphatic carboxylic acid esters, γ-lactones, fluorine-containing γ-lactones, cyclic ethers, fluorine-containing cyclic ethers, chain ethers, fluorine-containing chain ethers, nitriles, amides, lactams, nitromethane, nitroethane, sulfolane, trimethyl phosphate, dimethyl sulfoxide, dimethyl sulfoxide phosphate, etc. The non-aqueous solvents may be used alone or in combination of two or more.

Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC).
Examples of fluorine-containing cyclic carbonates include fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoropropylene carbonate, and the like.
Examples of chain carbonates include dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), and dipropyl carbonate (DPC).
Examples of fluorine-containing chain carbonates include methyl 2,2,2-trifluoroethyl carbonate.
Examples of aliphatic carboxylate esters include methyl formate, methyl acetate, methyl propionate, methyl butyrate, methyl isobutyrate, methyl trimethylbutyrate, ethyl formate, ethyl acetate, ethyl propionate, ethyl butyrate, ethyl isobutyrate, and ethyl trimethylbutyrate.
Examples of the fluorine-containing aliphatic carboxylate esters include methyl difluoroacetate, methyl 3,3,3-trifluoropropionate, ethyl difluoroacetate, and 2,2,2-trifluoroethyl acetate.
Examples of γ-lactones include γ-butyrolactone and γ-valerolactone.
Examples of cyclic ethers include tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,3-dioxane, and 1,4-dioxane.
Examples of chain ethers include 1,2-ethoxyethane (DEE), ethoxymethoxyethane (EME), diethyl ether, 1,2-dimethoxyethane, and 1,2-dibutoxyethane.
_{Examples of fluorine - containing chain ethers include HCF2CF2CH2OCF2CF2H , CF3CF2CH2OCF2CF2H , HCF2CF2CH2OCF2CFHCF3 , CF3CF2CH2OCF2CFHCF3 , C6F13OCH3 , C6F13OC2H5 , C8F17OCH3 , C8F17OC2H5 , CF3CFHCF2CH ( CH3 ) OCF2CFHCF3 , and HCF2CF2OCH ( C2H5 ) . ​ ​ ​ ​ ​ ​ ​ 2 , HCF2CF2OC4H9 , HCF2CF2OCH2CH ( C2H5 ) 2 , HCF2CF2OCH2CH ( CH3 ) 2 ,} etc.
Examples of nitriles include acetonitrile, glutaronitrile, adiponitrile, methoxyacetonitrile, and 3-methoxypropionitrile.
The amides include, for example, N,N-dimethylformamide.
Examples of lactams include N-methylpyrrolidinone, N-methyloxazolidinone, and N,N'-dimethylimidazolidinone.

The non-aqueous solvent preferably contains at least one selected from the group consisting of cyclic carbonates, fluorine-containing cyclic carbonates, chain carbonates, and fluorine-containing chain carbonates. In this case, the total proportion of the cyclic carbonates, fluorine-containing cyclic carbonates, chain carbonates, and fluorine-containing chain carbonates is preferably 50% by mass to 100% by mass, more preferably 60% by mass to 100% by mass, and even more preferably 80% by mass to 100% by mass, based on the total amount of the non-aqueous solvent.

The non-aqueous solvent preferably contains at least one selected from the group consisting of cyclic carbonates and chain carbonates. In this case, the total proportion of the cyclic carbonates and chain carbonates in the non-aqueous solvent is preferably 50% by mass to 100% by mass, more preferably 60% by mass to 100% by mass, and even more preferably 80% by mass to 100% by mass, based on the total amount of the non-aqueous solvent.

The content of the nonaqueous solvent is preferably 60% by mass to 99% by mass, more preferably 70% by mass to 97% by mass, and further preferably 90% by mass to 97% by mass, based on the total amount of the nonaqueous electrolyte.

The intrinsic viscosity of the nonaqueous solvent is preferably 10.0 mPa·s or less at 25°C in order to further improve the dissociation property of the electrolyte and the mobility of the ions.

(4.3.3) Electrolyte Additive The non-aqueous solvent may contain an electrolyte additive. This makes it possible to make it difficult for side reactions that are not the original battery reaction to proceed during the charge/discharge cycle of the lithium ion secondary battery. The battery reaction refers to a reaction in which lithium ions enter and leave (intercalate) the positive electrode and the negative electrode. The side reactions include a reductive decomposition reaction of the non-aqueous electrolyte by the negative electrode, an oxidative decomposition reaction of the non-aqueous electrolyte by the positive electrode, and the elution of metal elements in the positive electrode active material.
The electrolyte additive is not particularly limited, and any known additive can be used. For example, the additives described in JP-A-2019-153443 can be used.

(4.4) Case A lithium ion secondary battery generally includes a case that houses a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte.

There are no particular limitations on the shape of the case, and it is selected appropriately depending on the application of the lithium-ion secondary battery. Examples of the case include a case including a laminate film and a case consisting of a battery can and a battery can lid.

(5) An Example of a Lithium-Ion Secondary Battery An example of a lithium-ion secondary battery according to an embodiment of the present disclosure will be specifically described with reference to Fig. 1. Fig. 1 is a cross-sectional view of a lithium-ion secondary battery 1 according to an embodiment of the present disclosure.

The lithium ion secondary battery 1 is a laminated type. As shown in FIG. 1, the lithium ion secondary battery 1 includes a battery element 10, a positive electrode lead 21, a negative electrode lead 22, and a case 30. The battery element 10 is sealed inside the case 30. The case 30 is formed of a laminate film. The positive electrode lead 21 and the negative electrode lead 22 are each attached to the battery element 10. The positive electrode lead 21 and the negative electrode lead 22 are each led out in opposite directions from the inside to the outside of the case 30.

As shown in FIG. 1, the battery element 10 is formed by stacking a positive electrode 11, a separator 13, and a negative electrode 12. The positive electrode 11 is formed by forming a positive electrode composite layer 11B on both main surfaces of a positive electrode collector 11A. The negative electrode 12 is formed by forming a negative electrode composite layer 12B on both main surfaces of a negative electrode collector 12A. The positive electrode composite layer 11B formed on one main surface of the positive electrode collector 11A of the positive electrode 11 and the negative electrode composite layer 12B formed on one main surface of the negative electrode collector 12A of the negative electrode 12 adjacent to the positive electrode 11 face each other via the separator 13.

A non-aqueous electrolyte is injected into the case 30 of the lithium ion secondary battery 1. The non-aqueous electrolyte permeates the positive electrode composite layer 11B, the separator 13, and the negative electrode composite layer 12B. In the lithium ion secondary battery 1, one single cell layer 14 is formed by the adjacent positive electrode composite layer 11B, the separator 13, and the negative electrode composite layer 12B. The positive electrode 11 may be formed by forming the positive electrode composite layer 11B on one main surface of the positive electrode collector 11A, and the negative electrode 12 may be formed by forming the negative electrode composite layer 12B on one main surface of the negative electrode collector 12A.

In this embodiment, the lithium ion secondary battery 1 is a stacked type, but the present disclosure is not limited to this, and the lithium ion secondary battery 1 may be, for example, a wound type. The wound type is formed by stacking a positive electrode, a separator, a negative electrode, and a separator in this order and winding them into layers. The wound type includes a cylindrical type and a rectangular type.

In this embodiment, as shown in FIG. 1, the directions in which the positive electrode lead 21 and the negative electrode lead 22 each protrude from the inside of the case 30 to the outside are opposite directions relative to the case 30, but the present disclosure is not limited thereto. For example, the directions in which the positive electrode lead and the negative electrode lead each protrude from the inside of the case 30 to the outside may be the same direction relative to the case 30.

The present disclosure will be explained in more detail below with reference to examples, but the invention of the present disclosure is not limited to these examples.

[1] Preparation of Composition [1.1] Preparation The products used in the Examples and Comparative Examples are as follows. The physical properties of each product are the catalog values.
<Thermal Expandable Microcapsules (X)>
- "FN-100SSD": Matsumoto Yushi Pharmaceutical Co., Ltd.'s "FN-100SSD" (thermally expandable microcapsules, encapsulated: liquid low-boiling point hydrocarbons)
<Polymer particles (B)>
W4005: "CHEMIPEARL (registered trademark) W405" manufactured by Mitsui Chemicals, Inc. (water dispersion of low molecular weight polyethylene, solid content concentration: 40 mass%, average particle size (Microtrack method): 0.6 μm, softening point (ring and ball method): 110° C.)
W900: "CHEMIPEARL (registered trademark) W900" manufactured by Mitsui Chemicals, Inc. (water dispersion of low molecular weight polyethylene, solid content concentration: 40 mass%, average particle size (Microtrack method): 0.6 μm, softening point (ring and ball method): 132° C.)
W950: "CHEMIPEARL (registered trademark) W405" manufactured by Mitsui Chemicals, Inc. (water dispersion of low molecular weight polyethylene, solid content concentration: 40 mass%, average particle size (Microtrack method): 0.6 μm, softening point (ring and ball method): 113° C.)
<Binder (C)>
SBR: Dispersion of styrene butadiene rubber particles (solvent: emulsifying solvent, solid content: 40% by mass)
CMC: "2200" manufactured by Daicel Miraize Co., Ltd. (sodium carboxymethylcellulose, solid content: 100% by mass)
<Negative Electrode Active Material (D)>
・Natural graphite: Natural graphite (commercially available)
<Conductive assistant (E)>
Super-P: "Super-P" manufactured by TIMCAL (main component: conductive carbon black, solid content concentration: 100% by mass, BET specific surface area: 62 m ² /g)

[1.2] Production of Thermally Expandable Microcapsules (A) Thermally expandable microcapsules (A1) (hereinafter also referred to as "MC(A1)") as thermally expandable microcapsules (A) were produced as follows.
To 600 g of ion-exchanged water, 150 g of sodium chloride, 70 g of a colloidal silica dispersion containing 20% by mass of active silica ingredient, and 0.5 g of ethylenediaminetetraacetic acid tetrasodium salt were added, and the pH of the resulting mixture was adjusted to 2.8 to 3.2 to prepare an aqueous dispersion medium.
Separately, 200 g of acrylonitrile, 70 g of methacrylonitrile, 5 g of methyl methacrylate, 1.2 g of ethylene glycol dimethacrylate, 150 g of hydrofluoroether (CH ₃ OCF ₂ CF ₂ CF ₃ ), and 2.0 g of azobisisobutyronitrile were mixed together to prepare an oily mixture.
The aqueous dispersion medium and the oil mixture were mixed, and the resulting mixture was dispersed using a homomixer (TK homomixer, manufactured by Tokushu Kika Kogyo Co., Ltd.) to prepare a suspension. The suspension was transferred to a 1.5-liter pressurized reactor and substituted with nitrogen, and then the initial reaction pressure was set to 0.5 MPa, and polymerization was performed for 20 hours at a polymerization temperature of 60° C. while stirring at 80 rpm. The polymerization liquid obtained after polymerization was filtered and dried to obtain MC (A1). The average particle size (D50 value by volume-based measurement) measured using a Microtrack particle size distribution meter (model 9320-HRA) manufactured by Nikkiso Co., Ltd. was 9 μm.

[1.2.1] Expansion initiation temperature (Ts) of thermally expandable microcapsules (A1)
A DMA (DMA Q800 type, manufactured by TA instruments) was used as a measuring device. 0.5 mg of MC (A1) was placed in an aluminum cup with a diameter of 6.0 mm (inner diameter 5.65 mm) and a depth of 4.8 mm, and an aluminum lid (diameter 5.6 mm, thickness 0.1 mm) was placed on the top of the microsphere layer to prepare a sample. The sample height was measured with a force of 0.01 N applied from above by a pressurizer. The sample was heated from 20°C to 300°C at a heating rate of 10°C/min with a force of 0.01 N applied by the pressurizer, and the displacement of the pressurizer in the vertical direction was measured. The temperature at which the displacement in the positive direction started was taken as the expansion start temperature (Ts) of MC (A1), and the temperature at which the maximum displacement was shown was taken as the maximum volume expansion temperature (Tm). The expansion start temperature (Ts) of MC (A1) was 122°C, and the maximum volume expansion temperature (Tm) was 155°C.

[1.3] Example 1
[1.3.1] Preparation of composition for secondary battery negative electrode A 5 L planetary dispenser was used to prepare the composition.
"CMC" (C) was dissolved in pure water so that the content of "CMC" (C) was 1.0 mass% relative to the total amount of "CMC aqueous solution" (C), to obtain "CMC aqueous solution" (C).
50 parts by mass of "thermally expandable microcapsules" (A1) and 125 parts by mass of "polymer particles (B)" aqueous dispersion solvent (solid content 40% by mass) were placed in a 300 ml container and mixed for 30 minutes using a planetary dispersion mixer. This prepared a secondary battery negative electrode composition (A1, B).
35 parts by mass of the secondary battery negative electrode composition (A1, B) and 400 parts by mass of the "CMC aqueous solution" (C) were mixed for 5 minutes to obtain secondary battery negative electrode compositions (A1, B, C).
To the obtained secondary battery negative electrode compositions (A1, B, C), 960 parts by mass of “negative electrode active material (natural graphite)” (D) and 10 parts by mass of “Super-P” (E) were added and mixed for 30 minutes, and then 133 parts by mass of “aqueous CMC solution” (C) was added and kneaded for 30 minutes.
Thereafter, 20 parts by mass of "SBR" (C) was added, and the mixture was degassed in vacuum for 30 minutes.
In this manner, a composition (negative electrode mixture slurry) with a solid content concentration of 45% was prepared.

[1.4] Examples 2 to 8, Comparative Examples 1 to 3
A composition (negative electrode mixture slurry) having a solid content concentration of 45% was prepared in the same manner as in Example 1, except that the types and amounts of the thermally expandable microcapsules (A), polymer particles (B), binder (C), active material (D), and conductive assistant (E) were changed as shown in Table 1.

[2] Preparation of Negative Electrodes Negative electrodes were prepared using the compositions (negative electrode mixture slurries) of Examples 1 to 8 and Comparative Examples 1 to 3 as described below.

[2.1] Coating and Drying A die coater was used to coat the negative electrode mixture slurry.
As a negative electrode current collector, a copper foil (thickness: 10 μm) was prepared.
The negative electrode composite slurry was applied to one main surface of the negative electrode current collector and dried so that the mass of the negative electrode composite layer (coating film after drying) was 11.0 mg/cm ^2. Next, the negative electrode composite slurry was similarly applied to the other main surface (uncoated surface) of the negative electrode current collector and dried so that the mass of the negative electrode composite layer (coating film after drying) was 11.0 mg/cm ² .
The negative electrode roll thus obtained, coated on both sides (total coating amount on both sides: 22.0 mg/cm ² ), was dried in a vacuum drying oven at 120° C. for 12 hours.

[2.2] Pressing The negative electrode roll was pressed using a small press machine. The gap between the upper and lower rolls was adjusted, and the negative electrode roll was pressed with the small press machine so that the press density was 1.5±0.05 g/ ^cm3 .

[2.3] Slitting The negative electrode roll was slit so as to obtain an area for the negative electrode composite layer (front surface: 58 mm × 372 mm, back surface: 58 mm × 431 mm) and an area for a tab welding margin, to obtain a negative electrode.

[3] Preparation of Lithium Ion Secondary Battery Using the negative electrodes made of the compositions (negative electrode composite slurries) of Examples 1 to 8 and Comparative Examples 1 to 3, lithium ion secondary batteries were prepared as described below.

[3.1] Fabrication of Positive Electrode [3.1.1] Preparation of Positive Electrode Mixture Slurry A 5 L planetary dispenser was used to prepare the positive electrode mix slurry.
A _mixture for a positive electrode was _obtained by mixing 1,520 parts by mass of “NCM622” (manufactured by Umicore, composition formula: _{LiNi0.6Co0.2Mn0.2O2} ) as a positive electrode active material, 30 _parts by mass of “Super-P” (manufactured by TIMCAL, conductive carbon) as a conductive additive, and 30 parts by mass of “KS-6” (manufactured by TIMREX, flake graphite) as a conductive additive for 10 minutes.
50 parts by mass of "NMP" was added to the mixture for the positive electrode, and the mixture was mixed for 20 minutes to obtain a first mixture for the positive electrode.
To the first positive electrode mixture, 350 parts by mass of the "PVDF solution" was added and kneaded for 30 minutes, then 260 parts by mass of the "PVDF solution" was added and kneaded for 15 minutes, and then 220 parts by mass of the "PVDF solution" was added and kneaded for 15 minutes to obtain a second positive electrode mixture. The "PVDF solution" was prepared by adding "PVDF" to "NMP" so that the content of "PVDF" was 8 mass% with respect to the total amount of the "PVDF solution".
To adjust the viscosity, 80 parts by mass of "NMP" was added to the second positive electrode mixture, and the mixture was mixed for 30 minutes, and then vacuum degassing was performed for 30 minutes.
In this way, a positive electrode mixture slurry having a solid content concentration of 65 mass % was prepared.

[3.1.2] Coating and Drying A die coater was used to coat the positive electrode mixture slurry.
As a positive electrode current collector, an aluminum foil (thickness: 20 μm, width: 200 mm) was prepared.
The positive electrode composite slurry was applied to one main surface of the positive electrode current collector and dried so that the mass of the positive electrode composite layer (coating film after drying) was 19.0 mg/cm ^2. Next, the positive electrode composite slurry was similarly applied to the other main surface of the positive electrode current collector and dried so that the mass of the positive electrode composite layer (coating film after drying) was 19.0 mg/cm ² .
The positive electrode roll thus obtained, coated on both sides (total coating amount on both sides: 38.0 mg/cm ² ), was dried in a vacuum drying oven at 130° C. for 12 hours.

[3.1.3] Pressing The positive electrode roll was pressed using a 35-ton press machine. The gap between the upper and lower rolls was adjusted, and the positive electrode was pressed with the 35-ton press machine so that the press density was 2.9±0.05 g/ ^cm3 .

[3.1.4] Slitting The positive electrode roll was slit so as to obtain an area for the positive electrode composite layer (front surface: 56 mm × 334 mm, back surface: 56 mm × 408 mm) and an area for a tab welding margin, thereby obtaining a positive electrode in which an undercoat layer was laminated on the aluminum foil.

[3.2] Preparation of a wound battery (design capacity: 1 Ah) [3.2.1] Winding For the separator, a polyethylene porous film (60.5 mm x 450 mm) with a porosity of 45 volume % and a thickness of 25 μm was used.
The negative electrode, separator, positive electrode, and separator obtained above were stacked and wound, and then press-molded. Next, an aluminum tab was bonded to the margin of the positive electrode using an ultrasonic bonding machine, and a nickel tab was bonded to the margin of the negative electrode using an ultrasonic bonding machine. This was sandwiched between laminate films, and three sides were heat-sealed. This resulted in a case with an opening (hereinafter referred to as the "first case").

[3.2.2] Injection of non-aqueous electrolyte Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of EC:EMC:DMC=3:3:4 to obtain a mixed solvent. _LiPF6 was dissolved in the mixed solvent to a concentration of 1.0 mol/L to prepare a non-aqueous electrolyte.
Before the injection of the non-aqueous electrolyte, the first case was dried under reduced pressure at 70° C. for 12 hours in a vacuum dryer. After 4.7±0.1 g of the electrolyte was injected into the first case, the opening of the first case was heat-sealed while drawing a vacuum. Thus, a lithium ion secondary battery precursor was obtained.

[3.2.3] Activation Treatment The lithium ion secondary battery precursor was kept at room temperature (25 ° C.) for 24 hours. Next, the lithium ion secondary battery precursor was charged at a constant current of 0.05 C (0.05 C-CC) for 4 hours, and then rested for 12 hours. Thereafter, it was charged at a constant current and constant voltage of 0.1 C to 4.2 V (0.1 C-CCCV), rested for 30 minutes, and then discharged at a constant current of 0.1 C to 2.8 V (0.1 C-CC). Furthermore, a charge-discharge cycle (charging to 4.2 V at 0.1 C-CCCV and discharging to 2.8 V at 0.1 C-CC) was repeated five times. After that, the lithium ion secondary battery precursor was stored at 25 ° C. for five days in a fully charged state of 4.2 V (SOC: 100%).
In this way, a wound type battery (lithium ion secondary battery) (design capacity: 1 Ah) was obtained.

[4] Safety Performance Evaluation The following nail penetration test and heat generation measurement test were carried out using the wound type battery.

[4.1] Nail Penetration Test A nail penetration test was carried out using a wound type battery (design capacity 1 Ah).
A nail with a diameter of 6 mm was inserted into the center of one of the main surfaces of the wound battery at a press speed of 20 mm/sec, and the positive and negative electrodes were short-circuited inside the case. Then, the wound battery was visually observed to see if it had ignited. The nail penetration test was performed five times in total using wound batteries manufactured with the same specifications. Table 1 shows the number of times that at least one of smoke generation and fire due to thermal runaway was observed in the wound battery during the five nail penetration tests (hereinafter referred to as the "number of smoke/fire occurrences").
The acceptable number of occurrences of smoke or fire is two or less.
In Table 1, in the item for the nail penetration test, "3/5" indicates that, out of five nail penetration tests, at least one of smoking and catching fire due to thermal runaway was observed in the wound battery three times.

[4.2] Heat generation rate measurement test A nail with a diameter of 6 mm was inserted into the center of one of the main surfaces of the wound battery at a press speed of 0.5 mm/sec to short-circuit the positive and negative electrodes inside the case. At this time, the surface temperature of the wound battery was measured over time using a thermocouple attached to the surface of the wound battery case.
The heat generation rate was calculated by the following formula (A) based on the surface temperature rise of the wound battery from the time when the voltage of the wound battery dropped until 5 seconds had elapsed. The allowable range of the heat generation rate is 90° C./sec or less.
Formula (A): Heat generation rate (°C/sec) = Surface temperature of the wound battery increased from the time when the voltage of the wound battery dropped until 5 seconds have elapsed (°C)/5 (sec)

In Table 1, "Ts (°C)" indicates the expansion start temperature of the thermally expandable microcapsule (A).

The compositions of Comparative Example 1 to Comparative Example 3 contained polymer particles (B) but did not contain thermally expandable microcapsules (A). Therefore, the number of times that the lithium ion secondary batteries of Comparative Example 1 to Comparative Example 3 emitted smoke and caught fire in the nail penetration test was 3 or more, which exceeded the allowable number of times that the lithium ion secondary batteries emitted smoke and caught fire. From this result, it was found that the compositions of Comparative Example 1 to Comparative Example 3 could not be used to produce lithium ion secondary batteries with excellent safety.
Furthermore, it was confirmed that the lithium ion secondary batteries of Comparative Examples 1 to 3 emitted severe smoke and caught fire. This is presumably because the hydrocarbon gas generated from the thermally expandable microcapsules (X) intensified the combustion.

The compositions of Examples 1 to 8 contain thermally expandable microcapsules (A) and polymer particles (B). Therefore, the number of smoke and fire occurrences in the nail penetration test of the lithium ion secondary batteries of Examples 1 to 8 was 2 or less, which was below the allowable number of smoke and fire occurrences. From this result, it was found that the compositions of Examples 1 to 8 can be used to make lithium ion secondary batteries with excellent safety.
Furthermore, it was confirmed that the smoke generation and fire of the lithium ion secondary batteries of Examples 1 to 8 were less severe than those of Comparative Examples 1 to 3. This is presumably because non-flammable gas was generated from the thermally expandable microcapsules (A) and oxygen was diluted by the supply of the non-flammable gas.

Claims (14)

A thermally expandable microcapsule (A) having an expansion start temperature of 100° C. or higher and containing a non-flammable gas generating material therein;
and (B) polymer particles having a softening point of 70°C to 150°C. The composition for a secondary battery negative electrode according to claim 1, wherein the non-flammable gas generating material includes a fluorine-containing compound. The composition for a secondary battery negative electrode according to claim 2, wherein the fluorine-containing compound includes at least one selected from the group consisting of hydrofluorocarbons, hydrofluoroethers, fluoroketones, perfluoroethers, perfluorocarbons, hydrofluoroolefins, hydrochlorofluoroolefins, carbonyl fluoride, and hydrogen fluoride. The composition for secondary battery negative electrodes according to claim 1, wherein the expansion start temperature is 100°C to 180°C. The composition for secondary battery negative electrodes according to claim 1, wherein the mass blending ratio (A/B) of the thermally expandable microcapsules (A) to the polymer particles (B) is 0.1 to 5.0. The secondary battery negative electrode composition according to claim 1, further comprising a binder (C). The secondary battery negative electrode composition according to claim 1, wherein the maximum volume expansion temperature of the thermally expandable microcapsules (A) is higher than the softening point. The average particle size of the thermally expandable microcapsules (A) before thermal expansion is 0.3 μm to 20 μm,
2. The composition for secondary battery negative electrode according to claim 1, wherein the average particle size of the polymer particles (B) is 0.1 μm to 5 μm. A negative electrode mixture layer comprising the secondary battery negative electrode composition according to any one of claims 1 to 8. A negative electrode for a secondary battery comprising the negative electrode mixture layer according to claim 9. The negative electrode for a secondary battery according to claim 10, comprising the negative electrode mixture layer further containing an active material (D) and a current collector. The negative electrode for a secondary battery according to claim 11, wherein the content of the composition for a secondary battery negative electrode is 0.5 parts by mass to 10 parts by mass per 100 parts by mass of the active material (D). The composition for secondary battery negative electrode further contains a conductive assistant (E),
12. The negative electrode for secondary batteries according to claim 11, wherein the content of the conductive assistant (E) is 0.1 parts by mass to 10 parts by mass per 100 parts by mass of the active material (D). A lithium ion secondary battery comprising the negative electrode for a secondary battery according to claim 10, a positive electrode, a separator, and a non-aqueous electrolyte.